Specification and segmentation of the paraxial mesoderm

ENC-1: a novel mammalian kelch-related gene specifically expressed in the nervous system encodes an actin-binding protein

We have identified and characterized a novel murine gene, Ectoderm-Neural Cortex-1 (ENC-1), that is an early and highly specific marker of neural induction in vertebrates. ENC-1, which encodes a kelch family related protein, is expressed during early gastrulation in the prospective neuroectodermal region of the epiblast and later in development throughout the nervous system (NS). ENC-1 expression is highly dynamic and, after neurulation, preferentially defines prospective cortical areas. The only apparent expression of ENC-1 outside the NS is restricted to the rostral-most somitomere of the presomitic mesoderm, at the times corresponding to the epithelialization that precedes somite formation. Cellular expression of epitope-tagged ENC-1 shows extensive co-localization of ENC-1 with the actin cytoskeleton, and immunoprecipitation studies demonstrate a physical association between ENC-1 and actin. ENC-1 functions as an actin-binding protein that may be important in the organization of the actin cytoskeleton during neural fate specification and development of the NS.

The allocation of epiblast cells to the embryonic heart and other mesodermal lineages: the role of ingression and tissue movement during gastrulation

The cardiogenic potency of cells in the epiblast of the early primitive-streak stage (early PS) embryo was tested by heterotopic transplantation. The results of this study show that cells in the anterior and posterior epiblast of the early PS-stage embryos have similar cardiogenic potency, and that they differentiated to heart cells after they were transplanted directly to the heart field of the late PS embryo. That the epiblast cells can acquire a cardiac fate without any prior act of ingression through the primitive streak or movement within the mesoderm suggests that neither morphogenetic event is critical for the specification of the cardiogenic fate. The mesodermal cells that have recently ingressed through the primitive streak can express a broad cell fate that is characteristic of the pre-ingressed cells in the host when they were returned to the epiblast. However, mesoderm cells that have ingressed through the primitive streak did not contribute to the lateral plate mesoderm after transplantation back to the epiblast, implying that some restriction of lineage potency may have occurred during ingression. Early PS stage epiblast cells that were transplanted to the epiblast of the mid PS host embryos colonised the embryonic mesoderm but not the extraembryonic mesoderm. This departure from the normal cell fate indicates that the allocation of epiblast cells to the mesodermal lineages is dependent on the timing of their recruitment to the primitive streak and the morphogenetic options that are available to the ingressing cells at that instance.

Sclerotome development and peripheral nervous system segmentation in embryonic zebrafish

Vertebrate embryos display segmental patterns in many trunk structures, including somites and peripheral nervous system elements. Previous work in avian embryos suggests a role for somite-derived sclerotome in segmental patterning of the peripheral nervous system. We investigated sclerotome development and tested its role in patterning motor axons and dorsal root ganglia in embryonic zebrafish. Individual somite cells labeled with vital fluorescent dye revealed that some cells of a ventromedial cell cluster within each somite produced mesenchymal cells that migrated to positions expected for sclerotome. Individual somites showed anterior/posterior distinctions in several aspects of development: (1) anterior ventromedial cluster cells produced only sclerotome, (2) individual posterior ventromedial cluster cells produced both sclerotome and muscle, and (3) anterior sclerotome migrated earlier and along a more restricted path than posterior sclerotome. Vital labeling showed that anterior sclerotome colocalized with extending identified motor axons and migrating neural crest cells. To investigate sclerotome involvement in peripheral nervous system patterning, we ablated the ventromedial cell cluster and observed subsequent development of peripheral nervous system elements. Primary motor axons were essentially unaffected by sclerotome ablation, although in some cases outgrowth was delayed. Removal of sclerotome did not disrupt segmental pattern or development of dorsal root ganglia or peripheral nerves to axial muscle. We propose that peripheral nervous system segmentation is established through interactions with adjacent paraxial mesoderm which develops as sclerotome in some vertebrate species and myotome in others.

Chicken Pax-1 gene: structure and expression during embryonic somite development

Recent mouse genetic studies have implicated Pax-1, a paired-box-containing gene, in sclerotomal differentiation and vertebral body formation. To investigate Pax-1 function in somitic sclerotomal differentiation in the chick embryo, we have cloned the chicken Pax-1 gene, and its full length cDNA, and characterized its temporal and spatial expression pattern during somite development. Sequence analysis shows that chicken Pax-1 is highly homologous to murine and human Pax-1 genes with respect to the putative DNA-binding paired-box domain and the octapeptide domain. Northern analysis using probes derived from the paired-box domain and a unique non-paired box sequence of chicken Pax-1 detected 2-kb mRNA transcript. The expression profiles of Pax-1 were examined by in situ hybridization and Northern analysis. The first detectable expression of Pax-1 is seen in the most caudal epithelial somite. As the somite matures, Pax-1 expression takes on a medial distribution, thus corresponding to but preceding the emergence of the sclerotome. In the more mature, rostral somites (stage V and older), Pax-1 expression is found to be progressively localized first to the ventral-medial regions, and then to the caudal-ventral-medial quadrant of the mature somite. This pattern strongly supports the notion that Pax-1 expression is involved in somitogenesis and sclerotomal differentiation, and that it is subsequently a characteristic of the caudal half of the sclerotome, the presumptive precursor of vertebral cartilage. Northern analysis substantiated this expression profile and further revealed that the level of somitic Pax-1 expression increases as a function of embryonic development. Finally, we subjected chicken embryos to controlled heat shock treatment to perturb somite formation and segmentation. The pattern of Pax-1 expression in the anomalous somitic structures generated by controlled heat shock further supports a functional role for Pax-1 in somite development.

I-mf, a novel myogenic repressor, interacts with members of the MyoD family

During embryogenesis, cells from the ventral and dorsal parts of the somites give rise to sclerotome and dermomyotome, respectively. Dermomyotome contains skeletal muscle precursors that are determined by the MyoD family of myogenic factors. We have isolated a novel myogenic repressor, I-mf (Inhibitor of MyoD family), which is highly expressed in the sclerotome. In contrast, MyoD family members are concentrated in the dermomyotome. We demonstrate that I-mf inhibits the transactivation activity of the MyoD family and represses myogenesis. I-mf associates with MyoD family members and retains them in the cytoplasm by masking their nuclear localization signals. I-mf can also interfere with the DNA binding activity of MyoD family members. We postulate that I-mf plays an important role in the patterning of the somite early in development.

Valproic acid-induced somite teratogenesis in the chick embryo: relationship with Pax-1 gene expression

The repeated pattern of the axial skeleton results from the segmentation and re-segmentation of the mesodermally derived somites. During these early events of somite development, the vertebrate embryonic axial skeleton is most susceptible to the teratogenic effects of a variety of pharmaceutical and environmental agents. One example is the anticonvulsant drug valproic acid (VPA), which has been shown to cause craniofacial and minor and major skeletal defects in human and animal embryos. We hypothesize that a candidate set of molecular targets of teratogens are the Pax family of pattern-forming genes, specifically Pax-1, which has been previously demonstrated to be an important regulator of axial skeletal patterning at the somite level. In this study, early developmental stage chick embryos were treated with VPA and dose-dependent malformations in somite development were observed. Two classes of anomalies were evident: class I included discrete sites of somitic fusions or mis-segmentation, and Class II included large areas of disorganized somite patterning. Northern blot analysis revealed a decreased level of Pax-1 expression in VPA-treated embryos. Whole mount in situ hybridization analysis showed that somite anomalies correlate spatially with regions of decreased Pax-1 expression. Finally, comparison of the VPA-induced somitic anomalies with those caused by gene-specific perturbation of Pax-1 gene expression through the use of an antisense oligonucleotide revealed significant similarities. Taken together, these results support the hypothesis that Pax-1 is a molecular target in VPA axial skeletal teratogenicity.

Conserved regulatory element involved in the early onset of Hoxb6 gene expression

We have identified a 338 bp DNA fragment, the lateral plate mesoderm (LPM) enhancer, that is highly conserved between mouse and human. The LPM enhancer directs gene expression into the posterior lateral plate mesoderm and hindgut endoderm at early stages of development. By reporter gene analysis in transgenic mice, we demonstrate that both mouse and human DNA sequences possess similar enhancer activity. The expression patterns of the transgene and Hoxb6 during early stages of mouse development are identical, suggesting that the LPM enhancer is involved in the initial activation of Hoxb6 gene expression in posterior regions of mammalian embryos.

Altered Hox expression and segmental identity in Mll-mutant mice

The mixed-lineage leukaemia gene (MLL/HRX/ALL-1) is disrupted by chromosomal translocation in human acute leukaemias that often display mixed lymphoid-myeloid phenotypes and present in infancy. MLL possesses a highly conserved SET domain also found in Drosophila trithorax (trx) and Polycomb group (Pc-G) genes, which are known to regulate homeotic genes (HOM-C) in a positive or negative fashion, respectively. Mll was targeted in mice by homologous recombination in embryonic stem (ES) cells to assess its role in pattern development. Mll heterozygous (+/-) mice had retarded growth, displayed haematopoietic abnormalities, and demonstrated bidirectional homeotic transformations of the axial skeleton as well as sternal malformations. Mll deficiency (-/-) was embryonic lethal. Anterior boundaries of Hoxa-7 and Hoxc-9 expression were shifted posteriorly in Mll +/- embryos, but their expression was abolished in Mll -/- embryos. Thus Mll is required for proper segment identity in mammals, displays haplo-insufficiency, and positively regulates Hox gene expression.

TLE expression correlates with mouse embryonic segmentation, neurogenesis, and epithelial determination

The TLE proteins are the mammalian homologues of Groucho, a member of the Drosophila Notch signaling pathway. Notch signaling controls the differentiation of a variety of tissues in invertebrates and vertebrates. We are investigating the role of the TLE genes during mammalian development. We show that TLE 1 and TLE 3 are expressed during a number of cell-determination events, including embryonic segmentation, central and peripheral neurogenesis, and epithelial differentiation. This expression pattern is in agreement with the involvement of Groucho in similar fate choices in Drosophila and suggests that Groucho and TLE proteins perform similar developmental roles. Our results also show that TLE genes are co-expressed during a variety of cell-fate choices with several vertebrate homologues of genes implicated in the Drosophila Notch cascade, suggesting a role for the TLE proteins in mammalian Notch signaling.

Cranial paraxial mesoderm and neural crest cells of the mouse embryo: co-distribution in the craniofacial mesenchyme but distinct segregation in branchial arches

The spatial distribution of the cranial paraxial mesoderm and the neural crest cells during craniofacial morphogenesis of the mouse embryo was studied by micromanipulative cell grafting and cell labelling. Results of this study show that the paraxial mesoderm and neural crest cells arising at the same segmental position share common destinations. Mesodermal cells from somitomeres I, III, IV and VI were distributed to the same craniofacial tissues as neural crest cells of the forebrain, the caudal midbrain, and the rostral, middle and caudal hindbrains found respectively next to these mesodermal segments. This finding suggests that a basic meristic pattern is established globally in the neural plate ectoderm and paraxial mesoderm during early mouse development. Cells from these two sources mixed extensively in the peri-ocular, facial, periotic and cervical mesenchyme. However, within the branchial arches a distinct segregation of these two cell populations was discovered. Neural crest cells colonised the periphery of the branchial arches and enveloped the somitomere-derived core tissues on the rostral, lateral and caudal sides of the arch. Such segregation of cell populations in the first three branchial arches is apparent at least until the 10.5-day hindlimb bud stage and could be important for the patterning of the skeletal and myogenic derivatives of the arches.

Tissue-specific and differential expression of alternatively spliced alpha 1(II) collagen mRNAs in early human embryos

Expression of the alpha 1(II) procollagen gene is not confined to chondrogenic tissues during vertebrate development. Transcripts of the human gene (COL2A1) are alternatively spliced to give mRNAs which either exclude (type IIB mRNA) or include (type IIA mRNA) an exon encoding a cysteine-rich domain in the amino-propeptide. The distribution of COL2A1 mRNAs in 27- to 44-day human embryos and 8- to 24-week fetuses was studied by in situ hybridization and RNase protection analyses. Type IIA mRNAs were expressed in prechondrogenic cells and were also preferentially expressed in chondrogenic tissues at regions of chondrocyte commitment and cartilage growth. During maturation of chondrocytes, there is a switch to expression of type IIB mRNAs. In non-chondrogenic tissues of early embryos, type IIA mRNA expression was associated with active tissue remodeling, epithelial organization, and sites of tissue interaction. Type IIA mRNAs were also expressed in some non-chondrogenic tissues where expression had previously been undetected, such as the tooth bud, liver, adrenal cortex, apical ectodermal ridge, and indifferent gonad. In older fetuses type IIA mRNAs were the sole or major transcript in most non-chondrogenic tissues except the choroid plexus and tendon. In the meninges there was a unique switch from type IIB to type IIA expression. The expression pattern of COL2A1 transcripts suggests that, in addition to contributing to the structural integrity of the cartilage extracellular matrix, type II procollagen may serve a morphogenetic role in embryonic development. Our findings clearly show that the pattern of expression of type II procollagen mRNAs is largely conserved between man and mouse. However, some differences exist, and these should be taken into consideration when animal models are used to study human diseases associated with COL2A1.

Neuroectodermal fate of epiblast cells in the distal region of the mouse egg cylinder: implication for body plan organization during early embryogenesis

The developmental fate of cells in the distal region (distal cap) of the epiblast was analysed by fate mapping studies. The displacement and differentiation of cells labelled in situ with carbocyanine dyes and lacZ-expressing cells grafted to the distal cap were studied over a 48-hour period of in vitro development. The distal cap epiblast differentiates predominantly into neurectodermal cells. Cells at the anterior site of the distal cap colonise the fore-, mid- and hindbrain and contribute to non-neural ectoderm cells of the amnion and craniofacial surface ectoderm. Those cells in the most distal region of the epiblast contribute to all three brain compartments as well as the spinal cord and the posterior neuropore. Cells at the posterior site of the distal cap are mainly localised to the caudal parts of the neural tube. A minor contribution to the embryonic (paraxial and lateral) and extraembryonic (allantoic and yolk sac) mesoderm is also found. Epiblast cells located outside the distal cap give rise to surface ectoderm and other non-ectodermal derivatives, with only a minor contribution to the neuroectoderm. Results of this study provide compelling evidence that the precursor population of the neural tube is contained in the distal cap epiblast of the early-primitive-streak-stage embryo. Furthermore, the regionalisation of cell fate within this small population suggest that a preliminary craniocaudal patterning may have occurred in the neural primordium before neurulation.

Regionalisation of cell fate and morphogenetic movement of the mesoderm during mouse gastrulation

The developmental fate of cells in the epiblast of early-primitive-streak-stage mouse embryos was assessed by studying the pattern of tissue colonisation displayed by lac Z-expressing cells grafted orthotopically to nontransgenic embryos. Results of these fate-mapping experiments revealed that the lateral and posterior epiblast contain cells that will give rise predominantly to mesodermal derivatives. The various mesodermal populations are distributed in overlapping domains in the lateral and posterior epiblast, with the embryonic mesoderm such as heart, lateral, and paraxial mesoderm occupying a more distal position than the extraembryonic mesoderm. Heterotopic grafting of presumptive mesodermal cells results in the grafted cells adopting the fate appropriate to the new site, reflecting a plasticity of cell fate determination before ingression. The first wave of epiblast cells that ingress through the primitive streak are those giving rise to extraembryonic mesoderm. Cells that will form the mesoderm of the yolk sac and the amnion make up a major part of the mesodermal layer of the midprimitive-streak-stage embryo. Cells that are destined for embryonic mesoderm are still found within the epiblast, but some have been recruited to the distal portion of the mesoderm. By the late-primitive-streak-stage, the mesodermal layer contains only the precursors of embryonic mesoderm. This suggests that there has been a progressive displacement of the midstreak mesoderm to extraembryonic sites, which is reminiscent of that occurring in the overlying endodermal tissue. The regionalisation of cell fate in the late-primitive-streak mesoderm bears the same spatial relationship as their ancestors in the epiblast prior to cell ingression. This implies that both the position of the cells in the proximal-distal axis and their proximity to the primitive streak are major determinants for the patterning of the embryonic mesoderm.

Expression of HGF/SF, HGF1/MSP, and c-met suggests new functions during early chick development

We report the cloning of fulllength cDNAs for a plasminogen-related growth factor, hepatocyte growth factor/scatter factor (HGF/SF), its tyrosine kinase receptor, c-met, and a close member of the same family, hepatocyte growth factor-like/macrophage stimulating protein (HGF1/MSP), from the chick. We have used these cDNAs to provide the first report of the expression of this family of growth factors and the c-met receptor at early stages of vertebrate development. RNAase protection and wholemount in situ hybridization were used on chick embryos between formation of the primitive streak and early organogenesis. We find patterns of expression for HGF/SF and its receptor c-met consistent with their known roles in epithelial-mesenchymal transformation and angiogenesis. In addition, these genes and HGF1/MSP are expressed in discrete locations within developing somites, suggesting a role in paraxial mesodermal development. Very strong and early expression of HGF/SF in the elevating limb buds suggests its involvement in limb outgrowth. HGF1/MSP is expressed in the notochord and then in the prospective floor plate region and could play a role in development of the neural tube. Interestingly, c-met is often more closely associated with HGF1/MSP than with its known ligand, HGF/SF, raising the possibility that c-met expression may be induced by HGF1/MSP.

Ectopic expression of Sonic hedgehog alters dorsal-ventral patterning of somites

Differentiation of somites into sclerotome, dermatome, and myotome is controlled by a complex set of inductive interactions. The ability of axial midline tissues, the notochord and floor plate, to induce sclerotome has been well documented and has led to models in which ventral somite identity is specified by signals derived from the notochord and floor plate. Herein, we provide evidence that Sonic hedgehog, a vertebrate homolog of the Drosophila segment polarity gene hedgehog, is a signal produced by the notochord and floor plate that directs ventral somite differentiation. Sonic hedgehog is expressed in ventral midline tissues at critical times during somite specification and has the ability, when ectopically expressed, to enhance the formation of sclerotome and antagonize the development of dermatome.

Expression of an X-linked HMG-lacZ transgene in mouse embryos: implication of chromosomal imprinting and lineage-specific X-chromosome activity

X-chromosome activity in female mouse embryos was studied at the cellular level using an X-linked lacZ transgene which encodes beta-galactosidase (beta-Gal). Translation of maternal RNA in oocytes is seen as beta-Gal activity that persists into early cleavage-stages. Zygotic transcription of the transgene from the maternal X chromosome (Xm) is first found at about the 8-cell stage. By contrast, expression of the lacZ transgene on the paternal X chromosome (Xp) is not seen until later at the 16-32-cell stage. Preferential inactivation of Xp occurs in the mural trophectoderm, the primitive endoderm, and derivatives of the polar trophectoderm, but a small number of cells in these lineages may still retain an active paternal X chromosome. X inactivation begins at 3.5 days in the inner cell mass but contrary to previous findings the process is not completed in the embryonic ectoderm by 5.5 to 6.0 days. Regional variation in beta-Gal activity is also observed in the embryonic ectoderm during gastrulation which may be related to the specification of cell fates. Random inactivation of Xp and Xm ensues in all somatic tissues but the process is completed at different times in different tissues. The slower progression of X inactivation in tissues such as the notochord, the heart, and the embryonic gut is primarily due to the persistent maintenance of two active X chromosomes in a significant fraction of cells in these tissues. Recent findings on the methylation of endogenous X-linked genes suggest that the prolonged expression of beta-Gal might also be due to the different rate of spreading of inactivation along the X chromosome to the lacZ transgene locus in different tissues.