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

Anteroposterior patterning in the zebrafish, Danio rerio: an explant assay reveals inductive and suppressive cell interactions

We report the first extended culture system for analysing zebrafish (Danio rerio) embryogenesis with which we demonstrate neural induction and anteroposterior patterning. Explants from the animal pole region of blastula embryos ('animal caps') survived for at least two days and increased in cell number. Mesodermal and neural-specific genes were not expressed in cultured animal caps, although low levels of the dorsoanterior marker otx2 were seen. In contrast, we observed strong expression of gta3, a ventral marker and cyt1, a novel type I cytokeratin expressed in the outer enveloping layer. Isolated ‘embryonic shield’, that corresponds to the amphibian organizer and amniote node, went on to express the mesodermal genes gsc and ntl, otx2, the anterior neural marker pax6, and posterior neural markers eng3 and krx20. The expression of these genes defined a precise anteroposterior axis in shield explants. When conjugated to animal caps, the shield frequently induced expression of anterior neural markers. More posterior markers were rarely induced, suggesting that anterior and posterior neural induction are separable events. Mesodermal genes were also seldom activated in animal caps by the shield, demonstrating that neural induction did not require co-induction of mesoderm in the caps. Strikingly, ventral marginal zone explants suppressed the low levels of otx2 in animal caps, indicating that ventral tissues may play an active role in axial patterning. These data suggest that anteroposterior patterning in the zebrafish is a multi-step process.

Establishment and characterization of conditionally immortalized cells from the mouse urogenital ridge

Cell cultures from the urogenital ridge have been established to facilitate the study of the regulation and downstream interactions of Sry in mammalian sex determination. Cells have been explanted from transgenic mice carrying a temperature sensitive SV40 large T-antigen, and established in ongoing cultures. Analysis of the cells in these cultures at the electron microscope level reveals multiple cell types that compare to the cell types found in vivo during this period of development. Primordial germ cells, that are simultaneously explanted in the course of these experiments, also survive in culture. The explants undergo a morphogenetic organization into branching cord-like structures when cells are trypsinized and plated in extracellular matrix (Matrigel). We analyzed the expression of a number of molecular markers of the fetal gonad during monolayer culture, during in vitro morphogenesis in Matrigel, and in clonal lines derived from the complex explants. This analysis included Sry which is found to be expressed in some cultures from XY urogenital ridges that have been maintained for as long as 8 months.

Checklist: vertebrate homeobox genes

Up to now around 170 different homeobox genes have been cloned from vertebrate genomes. A compilation of the various isolates from mouse, chick, frog, fish and man is presented in the form of a concise checklist, including the designations from the original publications. Putative homologs from different species are aligned, and key characteristics of embryonic or adult expression domains, as well as mutant phenotypes are briefly indicated.

Localization of mRNAs for Rlim-1, the rat Xlim-1 homolog, in the developing rat brain

We studied the localization of Rlim-1 mRNAs, the rat Xlim-1 homolog, in the developing rat brain using in situ hybridization histochemistry. On embryonic day 13 (E13), strong signals were observed in the most superficial layer of the telencephalon, the zonalimitans intrathalamica, the ventral thalamus, some nuclei of the hypothalamus, the tectum, the cerebellum, the lower brainstem and the spinal cord. In the above-mentioned regions except the cerebellum, the distribution pattern remained almost the same from embryonic stage to adulthood but the intensity of expression gradually decreased after birth. In the cerebellum, the distribution pattern changed. during development; all the primordium of cerebellum in E13, the external granular and the Purkinje cell layers in postnatal day 7 (P7), and only the Purkinje cell layer in the adult expressed positive signals. These results suggest that Rlim-1 may be involved in region specification.

A targeted mouse Otx2 mutation leads to severe defects in gastrulation and formation of axial mesoderm and to deletion of rostral brain

Mouse Otx2 is a bicoid-class homeobox gene, related to the Drosophila orthodenticle (otd) gene. Expression of this gene is initially widespread in the epiblast at embryonic day 5.5 but becomes progressively restricted to the anterior end of the embryo at the headfold stage. In flies, loss of function mutations in otd result in deletion of pre-antennal and antennal segments; which leads to the absence of head structures derived from these segments. To study the function of Otx2 in mice, we have generated a homeobox deletion mutation in this gene. Mice homozygous for this mutation show severe defects in gastrulation and in formation of axial mesoderm and loss of anterior neural tissues. These results demonstrate that Otx2 is required for proper development of the epiblast and patterning of the anterior brain in mice, and supports the idea of evolutionary conservation of the function of Otd/Otx genes in head development in flies and mice.

Rpx: a novel anterior-restricted homeobox gene progressively activated in the prechordal plate, anterior neural plate and Rathke's pouch of the mouse embryo

We have isolated a new murine homeobox gene, Rpx (for Rathke's pouch homeobox), that is dynamically expressed in the prospective cephalic region of the embryo during gastrulation. Early expression is seen in the anterior midline endoderm and prechordal plate precursor. Expression is subsequently activated in the overlying ectoderm of the cephalic neural plate, suggesting that inductive contact with Rpx-expressing mesendoderm is required for this expression. Subsequently, Rpx expression is extinguished in the mesendoderm while remaining in the prospective prosencephalic region of the neural plate ectoderm. Ultimately, transcripts become restricted to Rathke's pouch, the primordium of the pituitary, which is known to be derived from the most anterior ectoderm of the early embryo. Down regulation of Rpx in the pouch coincides with the differentiation of pituitary-specific cell types. Rpx is the earliest known marker for the pituitary primordium, suggestive of a role in the early determination or differentiation of the pituitary. Since Rpx is expressed so dynamically and so early in the anterior region of the embryo, and since its early expression domain is much more extensive than the region fated to form the pituitary, it is likely that Rpx is involved in the initial determination of the anterior (prechordal) region of the embryo.

The enigma of LIM domains

LIM domains are two-zinc-finger structures found in proteins that have diverse functions. They are proposed to be protein dimerization motifs that assemble protein complexes necessary for growth, development and adaptive responses.

Bmpr encodes a type I bone morphogenetic protein receptor that is essential for gastrulation during mouse embryogenesis

Bone morphogenetic proteins (BMPs) are secreted proteins that interact with cell-surface receptors and are believed to play a variety of important roles during vertebrate embryogenesis. Bmpr, also known as ALK-3 and Brk-1, encodes a type I transforming growth factor-beta (TGF-beta) family receptor for BMP-2 and BMP-4. Bmpr is expressed ubiquitously during early mouse embryogenesis and in most adult mouse tissues. To study the function of Bmpr during mammalian development, we generated Bmpr-mutant mice. After embryonic day 9.5 (E9.5), no homozygous mutants were recovered from heterozygote matings. Homozygous mutants with morphological defects were first detected at E7.0 and were smaller than normal. Morphological and molecular examination demonstrated that no mesoderm had formed in the mutant embryos. The growth characteristics of homozygous mutant blastocysts cultured in vitro were indistinguishable from those of controls; however, embryonic ectoderm (epiblast) cell proliferation was reduced in all homozygous mutants at E6.5 before morphological abnormalities had become prominent. Teratomas arising from E7.0 mutant embryos contained derivatives from all three germ layers but were smaller and gave rise to fewer mesodermal cell types, such as muscle and cartilage, than controls. These results suggest that signaling through this type I BMP-2/4 receptor is not necessary for preimplantation or for initial postimplantation development but may be essential for the inductive events that lead to the formation of mesoderm during gastrulation and later for the differentiation of a subset of mesodermal cell types.

The murine LIM-kinase gene (limk) encodes a novel serine threonine kinase expressed predominantly in trophoblast giant cells and the developing nervous system

Throughout vertebrate embryogenesis, membrane bound and intracellular protein kinases govern the fundamental decisions necessary for coordinated cell growth and differentiation. Here we have characterized limk, a novel protein kinase with serine threonine substrate specificity which also contains two LIM domains. We used Northern blot and in situ hybridization techniques to determine its pattern of expression in early mouse development. Between 7.5 and 8.5 d.p.c., limk is expressed in three broad domains within the embryo, the neuroectodermal of the prospective forebrain and mid-brain regions, the cardiac mesoderm, and the newly formed definitive endodermal derivatives the foregut and hindgut. By 10.0 d.p.c. limk remains prominently expressed in the ventromedial regions of the developing forebrain and midbrain, with continued expression in the hindgut. In adults limk is expressed most prominently in the brain. Additionally we have shown that limk is most abundantly expressed in the trophoblast giant cells, from 4.5 d.p.c. onwards. Moreover, high levels of limk expression is associated with the overt formation of giant cells from diploid progenitors, suggesting an involvement for limk in the differentiation of this highly specialized extra-embryonic cell type.

Sonic hedgehog induces the differentiation of ventral forebrain neurons: a common signal for ventral patterning within the neural tube

The vertebrate hedgehog-related gene Sonic hedgehog (Shh) is expressed in ventral domains along the entire rostrocaudal length of the neural tube, including the forebrain. We show here that SHH induces the differentiation of ventral neuronal cell types in explants derived from prospective forebrain regions of the neural plate. Neurons induced in explants derived from both diencephalic and telencephalic levels of the neural plate express the LIM homeodomain protein Isl-1, and these neurons possess distinct identities that match those of the ventral neurons generated in these two subdivisions of the forebrain in vivo. A single inducing molecule, SHH, therefore appears to mediate the induction of distinct ventral neuronal cell types along the entire rostrocaudal extent of the embryonic central nervous system.

Control of neuronal pathway selection by the Drosophila LIM homeodomain gene apterous

The Drosophila apterous gene encodes a LIM homeodomain protein expressed embryonically in a small subset of differentiating neurons. To establish the identity of these neurons and to study the role of apterous in their development, we made apterous promoter fusions to an axon-targeted reporter gene. We found that all apterous-expressing neurons are interneurons that choose a single pathway within the developing central nervous system. In apterous mutants, these neurons choose incorrect pathways and fail to fasciculate with one another. Our results indicate that apterous functions to control neuronal pathway selection and suggest that other vertebrate and invertebrate members of the LIM homeodomain class of proteins may serve similar functions.

Spatially and temporally regulated expression of the LIM class homeobox gene Hrlim suggests multiple distinct functions in development of the ascidian, Halocynthia roretzi

Hrlim is a LIM class homeobox gene that was first isolated from the ascidian Halocynthia roretzi. To assess its roles in early development of the ascidian, spatial and temporal expression of Hrlim was examined by whole mount in situ hybridization. This revealed that transcription of Hrlim is activated at the 32-cell stage specifically in the endoderm lineage. Hrlim is also transiently expressed in all notochord precursor cells. Expression in the endoderm lineage continues through to the middle of gastrulation. After gastrulation, Hrlim is expressed in certain lineages that give rise to subsets of cells in the brain and spinal cord. Based on these observations, it is suggested that Hrlim plays multiple distinct roles in ascidian embryogenesis.

Specifying the target identity of motoneurons

In the vertebrate spinal cord, motoneurons are clustered into longitudinal columns in agreement with the targets they innervate. Motoneurons within each column acquire properties early in development that ensure their axons navigate to appropriate targets, but how this target identity is specified is unknown. Recently, Tsuchida et al. described the expression of putative regulatory genes within motor columns in the chicken spinal cord. Combinations of LIM-family homeobox genes differentially mark columns that project to distinct target groups. Expression precedes column formation and axon outgrowth, making these genes candidates for specifying the target identity of motoneuron groups.

Expression pattern of the murine LIM class homeobox gene Lhx3 in subsets of neural and neuroendocrine tissues

Murine Lhx3 cDNA isolated from the mouse pituitary cDNA library encodes a LIM-type homeodomain protein that contains two tandemly repeated LIM domains and the homeodomain. The identities of predicted amino acid sequences between the mouse of Lhx3 and Xenopus Xlim-3 genes are 80, 95, and 97% in the LIM domains 1 and 2, and the homeodomain, respectively, and 84% in the entire protein. 5'-RACE procedures and genomic cloning revealed that two distinct N-terminal sequences arise from two different exons 1a and 1b. Exon 1a encodes a sequence similar to that of Xlim-3, whereas exon 1b encodes a different N-terminus. It is likely that there are two transcription initiation sites in the Lhx3 gene. The Lhx3 transcripts were detected by whole mount in situ hybridization as early as day E9.5 post coitum in Rathke's pouch and the closing neural tube. During subsequent development, Lhx3 expression was observed in the anterior and intermediate but not in the posterior lobes of the pituitary, and in the ventral hindbrain and spinal cord. Northern blot analysis of adult tissues showed that Lhx3 mRNA persists in the pituitary. The expression pattern of Lhx3 is well conserved between Xenopus and mouse, underscoring the functional importance of this gene as a regulator of development. A number of established cell lines of pituitary origin express Lhx3 and therefore constitute a useful tool for further study of Lhx3 gene function.

Requirement for Lim1 in head-organizer function

Lim1 is a homeobox gene expressed in the organizer region of mouse embryos. To investigate the role of Lim1 during embryogenesis, a targeted deletion of the Lim1 gene was generated in embryonic stem cells. Embryos homozygous for the null allele lacked anterior head structures but the remaining body axis developed normally. A partial secondary axis developed anteriorly in some mutant embryos. Lim1 is thus an essential regulator of the vertebrate head organizer.

P-Lim, a LIM homeodomain factor, is expressed during pituitary organ and cell commitment and synergizes with Pit-1

A pituitary LIM homeodomain factor, P-Lim, is expressed as Rathke's pouch forms and as specific pituitary cell phenotypes are established, suggesting functional roles throughout pituitary development. While selectively expressed in both anterior and intermediate pituitary in mature mice, P-Lim is also transiently expressed in the developing ventral neural cord and brainstem. P-Lim binds to and activates the promoter of the alpha-glycoprotein subunit gene, a marker of early pituitary development, and synergizes with Pit-1 in transcriptional activation of genes encoding terminal differentiation markers. The LIM domain of P-Lim specifically interacts with the Pit-1 POU domain and is required for synergistic interactions with Pit-1, but not for basal transcriptional activation events.

Expression of zfh-4, a new member of the zinc finger-homeodomain family, in developing brain and muscle

We have identified zfh-4, a new member of a recently recognized zinc finger-homeodomain (zfh) family of putative transcription factors. Zfh-4 expression is prominent in developing muscle and brain. In both tissues, zfh-4 RNA levels are highest embryonically, then decrease gradually to barely detectable levels in adults. In myogenic cell lines, far more zfh-4 is expressed in proliferating myoblasts than in myotubes, suggesting a cellular basis for the developmental regulation observed in vivo. In contrast, zfh-4 RNA in brain is more abundant in postmitotic cells of the marginal zone than in proliferating cells of the ventricular zone. Within the brain, zfh-4 RNA is regionally localized: expression is highest in midbrain, readily detectable in hindbrain, and very low in cerebral cortex. Its patterns of expression, and its homology to known DNA binding proteins, support the idea that zfh-4 may be a regulator of gene expression in developing brain and muscle.

Nodal induces ectopic goosecoid and lim1 expression and axis duplication in zebrafish

One of the first intercellular signalling events in the vertebrate embryo leads to mesoderm formation and axis determination. In the mouse, a gene encoding a new member of the TGF-beta superfamily, nodal, is disrupted in a mutant deficient in mesoderm formation (Zhou et al., 1993, Nature 361, 543). nodal mRNA is found in prestreak mouse embryos, consistent with a role in the development of the dorsal axis. To examine the biological activities of nodal, we have studied the action of this factor in eliciting axis determination in the zebrafish, Danio rerio. Injection of nodal mRNA into zebrafish embryos caused the formation of ectopic axes that included notochord and somites. Axis duplication was preceded by the generation of an apparent ectopic shield (organizer equivalent) in nodal-injected embryos, as indicated by the appearance of a region over-expressing gsc and lim1; isolation and expression in the shield of the lim1 gene is reported here. These results suggest a role for a nodal-like factor in pattern formation in zebrafish.

Topographic organization of embryonic motor neurons defined by expression of LIM homeobox genes

Motor neurons located at different positions in the embryonic spinal cord innervate distinct targets in the periphery, establishing a topographic neural map. The topographic organization of motor projections depends on the generation of subclasses of motor neurons that select specific paths to their targets. We have cloned a family of LIM homeobox genes in chick and show here that the combinatorial expression of four of these genes, Islet-1, Islet-2, Lim-1, and Lim-3, defines subclasses of motor neurons that segregate into columns in the spinal cord and select distinct axonal pathways. These genes are expressed prior to the formation of distinct motor axon pathways and before motor columns appear. Our results suggest that LIM homeobox genes contribute to the generation of motor neuron diversity and may confer subclasses of motor neurons with the ability to select specific axon pathways, thereby initiating the topographic organization of motor projections.

Pancreatic beta cells express a diverse set of homeobox genes

Homeobox genes, which are found in all eukaryotic organisms, encode transcriptional regulators involved in cell-type differentiation and development. Several homeobox genes encoding homeodomain proteins that bind and activate the insulin gene promoter have been described. In an attempt to identify additional beta-cell homeodomain proteins, we designed primers based on the sequences of beta-cell homeobox genes cdx3 and lmx1 and the Drosophila homeodomain protein Antennapedia and used these primers to amplify inserts by PCR from an insulinoma cDNA library. The resulting amplification products include sequences encoding 10 distinct homeodomain proteins; 3 of these proteins have not been described previously. In addition, an insert was obtained encoding a splice variant of engrailed-2, a homeodomain protein previously identified in the central nervous system. Northern analysis revealed a distinct pattern of expression for each homeobox gene. Interestingly, the PCR-derived clones do not represent a complete sampling of the beta-cell library because no inserts encoding cdx3 or lmx1 protein were obtained. Beta cells probably express additional homeobox genes. The abundance and diversity of homeodomain proteins found in beta cells illustrate the remarkable complexity and redundancy of the machinery controlling beta-cell development and differentiation.

HNF-3 beta is essential for node and notochord formation in mouse development

HNF-3 beta, a member of the HNF-3/fork head family of transcription factors, is expressed in the node, notochord, floor plate, and gut in mouse embryos. A null mutation of this gene leads to embryonic lethality. The primary defect of HNF-3 beta -/- embryos is an absence of organized node and notochord formation, which leads to secondary defects in dorsal-ventral patterning of the neural tube. In contrast, patterning along the anterior-posterior axis was surprisingly little affected. Although HNF-3 beta is required for node and notochord formation, some organizer activity persists in the absence of these structures. HNF-3 beta is not required for the development of definitive endoderm cells, but foregut morphogenesis is severely affected in HNF-3 beta -/- embryos.

The winged-helix transcription factor HNF-3 beta is required for notochord development in the mouse embryo

HNF-3 beta, a transcription factor of the winged-helix family, is expressed in embryonic and adult endoderm and also in midline cells of the node, notochord, and floor plate in mouse embryos. To define the function of HNF-3 beta, a targeted mutation in the HNF-3 beta locus was generated by homologous recombination in embryonic stem cells. Mice lacking HNF-3 beta die by embryonic day (E) 10-11. Mutant embryos examined from E6.5 to E9.5 do not form a distinct node and lack a notochord. In addition, mutant embryos show marked defects in the organization of somites and neural tube that may result from the absence of the notochord. The neural tube of mutant embryos exhibits overt anteroposterior polarity but lacks a floor plate and motor neurons. Endodermal cells are present but fail to form a gut tube in mutant embryos. These studies indicate that HNF-3 beta has an essential role in the development of axial mesoderm in mouse embryos.