Mox-1 and Mox-2 define a novel homeobox gene subfamily and are differentially expressed during early mesodermal patterning in mouse embryos

Maintenance of somite borders in mice requires the Delta homologue DII1

During vertebrate embryonic development, the paraxial mesoderm is subdivided into metameric subunits called somites. The arrangement and cranio-caudal polarity of the somites governs the metamerism of all somite-derived tissues and spinal ganglia. Little is known about the molecular mechanisms underlying somite formation, segment polarity, maintenance of segment borders, and the interdependency of these processes. The mouse Delta homologue Dll1, a member of the DSL gene family, is expressed in the presomitic mesoderm and posterior halves of somites. Here we report that, in Dll1-deficient mouse embryos, a primary metameric pattern is established in mesoderm, and cytodifferentiation is apparently normal, but the segments have no cranio-caudal polarity, and no epithelial somites form. Caudal sclerotome halves do not condense, and the pattern of spinal ganglia and nerves is perturbed, indicating loss of segment polarity. Myoblasts span segment borders, demonstrating that these borders are not maintained. These results show that Dll1 is involved in compartmentalization of somites, that dermomyotome and sclerotome differentiation are independent of formation of epithelia and subdivision of somites in cranial and caudal halves, and that compartmentalization is essential for the maintenance of segment borders in paraxial mesoderm-derived structures.

Requirement of the transcription factor GATA4 for heart tube formation and ventral morphogenesis

The zinc finger transcription factor GATA4 has been implicated in heart development based on its early expression in precardiogenic splanchnic mesoderm and its ability to activate the expression of a number of cardiac-specific genes. To determine the role of GATA4 in embryogenesis, we generated mice homozygous for a GATA4 null allele. Homozygous GATA4 null mice arrested in development between E7.0 and E9.5 because of severe developmental abnormalities. Mutant embryos most notably lacked a primitive heart tube and foregut and developed partially outside the yolk sac. In the mutants, the two bilaterally symmetric promyocardial primordia failed to migrate ventrally but instead remained lateral and generated two independent heart tubes that contained differentiated cardiomyocytes. We show that these deformities resulted from a general loss in lateral to ventral folding throughout the embryo. GATA4 is most highly expressed within the precardiogenic splanchnic mesoderm at the posterior lip of the anterior intestinal portal, corresponding to the region of the embryo that undergoes ventral fusion. We propose that GATA4 is required for the migration or folding morphogenesis of the precardiogenic splanchnic mesodermal cells at the level of the AIP.

The winged helix transcription factor MFH1 is required for proliferation and patterning of paraxial mesoderm in the mouse embryo

The gene mfh1, encoding a winged helix/forkhead domain transcription factor, is expressed in a dynamic pattern in paraxial and presomitic mesoderm and developing somites during mouse embryogenesis. Expression later becomes restricted to condensing mesenchyme of the vertebrae, head, limbs, and kidney. A targeted disruption of the gene was generated by homologous recombination in embryonic stem cells. Most homozygous mfh1 null embryos die prenatally but some survive to birth, with multiple craniofacial and vertebral column defects. Using molecular markers, we show that the initial formation and patterning of somites occurs normally in mutants. Differentiation of sclerotome-derived cells also appears unaffected, although a reduction of the level of some markers [e.g., mtwist, mf1, scleraxis, and alpha1(II) collagen] is seen in the anterior of homozygous mutants. The most significant difference, however, is a marked reduction in the proliferation of sclerotome-derived cells, as judged by BrdU incorporation. This proliferation defect was also seen in micromass cultures of somite-derived cells treated with transforming growth factor beta1 and fibroblast growth factors. Our findings establish a requirement for a winged helix/forkhead domain transcription factor in the development of the paraxial mesoderm. A model is proposed for the role of mfh1 in regulating the proliferation and differentiation of cell lineages giving rise to the axial skeleton and skull.

Embryonic expression of the Gax homeodomain protein in cardiac, smooth, and skeletal muscle

Gax is a homeobox-containing gene that has been detected in adult cardiovascular tissues and exhibits a growth arrest-specific pattern of expression in cultured vascular myocytes. To study the regulation of gax during development, we performed immunohistochemistry and in situ hybridization on mouse embryos. Gax was present in mesodermally and, as with other homeobox genes, neuroectodermally derived tissues. Early mesodermal protein expression was limited to the lateral plate and somitic mesoderm. Gax in the cardiac muscle lineage exhibited a biphasic pattern of expression. Expression was prominent in the heart tube of the earliest cardiomyocytes and remained prominent through the looping stage (day 12.5 post coitum [pc]) but fell below the threshold of detection in atria and ventricles by day 13.5 pc. At day 15.5 pc, Gax protein was again detectable but restricted to cells within the compact layer of the ventricular myocardium. Gax expression was also noted in smooth muscle cells as early as day 9.5 pc. In the skeletal muscle lineage, Gax protein was expressed at the onset of somitogenesis before the expression of the myogenic basic helix-loop-helix and MEF2/RSRF family proteins. Subsequently, it was noted at day 9.5 pc in premyogenic cells migrating into head, trunk, and limb buds. Gax was detected in myotomes, premuscle masses, and mature muscle groups. These data suggest an important developmental role for Gax in all muscle lineages.

Evidence that absence of Wnt-3a signaling promotes neuralization instead of paraxial mesoderm development in the mouse

Wnt-3a mutant embryos show defects caudal to the forelimb level; somites are absent, the notochord is disrupted, and the central nervous system has a pronounced dysmorphology. Previous studies revealed that the primary defects of the mutant embryos are likely to be in the process of paraxial mesoderm formation. In this study, we analyzed the phenotype of Wnt-3a mutant embryos at early somite stages (8.0 days post coitum), when somite formation is initiated. In Wnt-3a mutants, cells which have ingressed through the primitive streak do not migrate laterally but remain under the streak and form an ectopic tubular structure. Several neural-specific molecular markers, but no paraxial mesoderm markers, are expressed in this structure, suggesting that the ectopic tube is an additional neural tube. In normal embryos, Wnt-3a is expressed in the primitive ectoderm, including the cells which are fated to give rise to the paraxial mesoderm and neurectoderm, but expression is absent in migrating mesoderm cells. These results suggest that Wnt-3a signaling may play a role in regulating paraxial mesodermal fates, at the expense of neurectodermal fates, within the primitive ectoderm of the gastrulating mouse embryo.

Isolation and identification of homeobox genes from the human placenta including a novel member of the Distal-less family, DLX4

We have carried out a DNA binding site screen of a 32-week human placental cDNA library using a consensus homeodomain binding site as a probe. This study represents the first library screen carried out to isolate homeobox genes from the human placenta. We have shown that three homeobox genes known to be expressed in the embryo, HB24, GAX and MSX2 are also expressed in the placenta. We have also identified a novel homeobox gene, DLX4, that shows 85% sequence identity with the homeodomain encoded by the Drosophila Distal-less (Dll) gene. DLX4 therefore represents a new member of the Distal-less family of homeobox genes. This is the first evidence that members of the Distal-less family of homeobox genes are expressed in the placenta. Using fluorescence in situ hybridisation (FISH), DLX4 has been assigned to human chromosome 17q21-q22. This places DLX4 in the same region of chromosome 17 as another member of the Distal-less family, DLX3 (Scherer et al., 1995), and the HOX-B homeobox gene cluster (Acampora et al., 1989: Boncinelli et al., 1991). Members of the Distal-less family (DLX1 and DLX2; DLX5 and DLX6) are found as closely linked pairs on human chromosomes (Simeone et al., 1994). We predict that DLX3 and DLX4 are closely linked and have arisen through gene duplication and divergence from a common ancestral precursor.

Rbt (Rabo torcido), a new mouse skeletal mutation involved in anteroposterior patterning of the axial skeleton, maps close to the Ts (tail-short) locus and distal to the Sox9 locus on chromosome 11

Rbt (Rabo torcido) is a new semidominant mouse mutant with a variety of skeletal abnormalities. Heterozygous Rbt mutants display homeotic anteroposterior patterning problems along the axial skeleton that resemble Polycomb group and trithorax gene mutations. In addition, the Rbt mutant displays strong similarities to the phenotype observed in Ts (Tail-short), indicating also a homeotically transformed phenotype in these mice. We have mapped the Rbt locus to an interval of approximately 6 cM on mouse Chromosome (Chr) 11 between microsatellite markers D11Mit128 and D11Mit103. The Ts locus was mapped within a shorter interval of approximately 3 cM between D11Mit128 and D11Mit203. This indicates that Rbt and Ts may be allelic mutations. Sox9, the human homolog of which is responsible for the skeletal malformation syndrome campomelic dysplasia, was mapped proximal to D11Mit128. It is, therefore, unlikely that Ts and Rbt are mouse models for this human skeletal disorder.

Mesodermal development in mouse embryos mutant for fibronectin

Three independent mutations were made by homologous recombination in two different regions of the fibronectin (FN) gene; all three appeared to be functional null mutations. The embryonic lethal phenotypes of these mutations were indistinguishable; all three FN mutant strains show mesodermal defects and fail to develop notochord or somites. Nevertheless analysis with lineage markers (Brachyury, sonic hedgehog, Notch-1, and mox-1) showed that both the notochord and the somite lineages were induced at the correct times and places. Furthermore, notochord precursor cells showed extensive cell migration in the absence of FN. However, neither notochord nor somites condensed properly in the absence of FN. These results show that specification of notochordal and somitic mesodermal lineages and significant cell migration are independent of fibronectin but that correct morphogenesis of these structures is FN-dependent.

A role for cadherins in tissue formation

We have produced null mutant mouse embryonic stem cells for the cell adhesion molecule E-cadherin. Such E-cadherin−/− ES cells are defective in cell aggregation; this defect can be corrected by transfection with cDNA for either E-cadherin or N-cadherin driven by a constitutive promoter. The presence (or absence) of E-cadherin regulates the expression of the transcription factor T-brachyury, indicating that cadherins play a role in linking cell surface receptors and gene expression. Comparative analysis of the parental and the genetically altered ES cell lines was performed to examine cell differentiation and the capability to form organized tissues. While differentiating E-cadherin−/− ES cells are still able to express various early and late differentiation markers, they show a clear-cut deficiency in forming organized structures. This phenotype can be rescued by constitutive expression of E-cadherin, which results exclusively in formation of epithelia. In contrast, rescue transfectants expressing N-cadherin show no epithelial structures, instead forming neuroepithelium and cartilage. These results provide the first evidence that specific cadherins directly stimulate differentiation into certain types of tissues.

The Xvent-2 homeobox gene is part of the BMP-4 signalling pathway controlling [correction of controling] dorsoventral patterning of Xenopus mesoderm

We describe a novel Xenopus homeobox gene, Xvent-2, which together with the previously identified homeobox gene Xvent-1, defines a novel class of homeobox genes. vent genes are related by sequence homology, expression pattern and gain-of-function phenotype. Evidence is presented for a role of Xvent-2 in the BMP-4 pathway involved in dorsoventral patterning of mesoderm. (1) Xvent-2 is expressed in regions that also express BMP-4. (2) Xvent-2 and BMP-4 interact in a positive feedback loop. (3) Xvent-2 ventralizes dorsal mesoderm in a dose-dependent manner resulting in phenoytpes ranging from microcephaly to Bauchstuck pieces, as does BMP-4. (4) Like BMP-4 and gsc, Xvent-2 and gsc are able to interact in a crossregulatory loop to suppress each other. (5) Microinjection of Xvent-2 mRNA can rescue dorsalization by a dominant-negative BMP-4 receptor. The results suggest that Xvent-2 functions in the BMP-4 signalling pathway that antagonizes the Spemann organizer.

The caudal-type homeobox protein Cdx-2 binds to the colon promoter of the carbonic anhydrase 1 gene

Carbonic anhydrase 1 (CA1) is an abundant enzyme in colon epithelia. In the gastrointestinal tract, carbonic anhydrase is vital for NaCl resorption, alkalinization of gut contents, and absorption of short-chain fatty acids. The CA1 gene has two promoters, one of which is specifically active in colon epithelia and the other in erythroid cells. We are investigating the factors that regulate CA1 expression from the colon-specific promoter. Colon-specific deoxyribonuclease I hypersensitive sites (DHS) have been mapped close to the colon transcription initiation site (DHS6c) and in the upstream intron (DHS5c). Using electrophoretic mobility-shift assays to search the 650-bp region which contains DHS6c, we have identified sequences that bind a colon-specific factor (COF1) and by deletion analysis we have narrowed down the COF1-binding motif to a 17-bp sequence. A comparison of this motif with a protein-binding motif in the sucrase-isomaltase gene promoter, competition assays, and antibody studies indicate that COF1 is identical to the homeodomain protein Cdx-2. We propose that Cdx-2 plays an important role in the intestine-specific expression of CA1.

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.

The role of positive and negative signals in somite patterning

Signals from the axial tissues, neural tube and notochord play a crucial role in patterning cell fates in adjacent somitic tissue. Work over the past four decades has indicated how signals from the axial tissues, as well as the surface ectoderm and lateral plate mesoderm, together act to pattern somitic cell fate. Furthermore, recent results have shed light on how some of these molecules control the specification and migratory behaviour of somitic cells.

Analysis of the vestigial tail mutation demonstrates that Wnt-3a gene dosage regulates mouse axial development

Mice homozygous for the recessive mutation vestigial tail (vt), which arose spontaneously on Chromosome 11, exhibit vertebral abnormalities, including loss of caudal vertebrae leading to shortening of the tail. Wnt-3a, a member of the wingless family of secreted glycoproteins, maps to the same chromosome. Embryos homozygous for a null mutation in Wnt-3a (Wnt-3a(neo)) have a complete absence of tail bud development and are truncated rostral to the hindlimbs. Several lines of evidence reveal that vt is a hypomorphic allele of Wnt-3a. We show that Wnt-3a and vt cosegregate in a high-resolution backcross and fail to complement, suggesting that Wnt-3a(neo) and vt are allelic. Embryos heterozygous for both alleles have a phenotype intermediate between that of Wnt-3a(neo) and vt homozygotes, lacking a tail, but developing thoracic and a variable number of lumbar vertebrae. Although no gross alteration in the Wnt-3a gene was detected in vt mice and the Wnt-3a coding region was normal, Wnt-3a expression was markedly reduced in vt/vt embryos consistent with a regulatory mutation in Wnt-3a. Furthermore, the analysis of allelic combinations indicates that Wnt-3a is required throughout the period of tail bud development for caudal somitogenesis. Interestingly, increasing levels of Wnt-3a activity appear to be necessary for the formation of more posterior derivatives of the paraxial mesoderm.

Severe defects in the formation of epaxial musculature in open brain (opb) mutant mouse embryos

The differentiation of somite derivatives is dependent on signals from neighboring axial structures. While ventral signals have been described extensively, little is known about dorsal influences, especially those from the dorsal half of the neural tube. Here, we describe severe phenotypic alterations in dorsal somite derivatives of homozygous open brain (opb) mutant mouse embryos which suggest crucial interactions between dorsal neural tube and dorsal somite regions. At Theiler stage 17 (day 10.5 post coitum) of development, strongly altered expression patterns of Pax3 and Myf5 were observed in dorsal somite regions indicating that the dorsal myotome and dermomyotome were not differentiating properly. These abnormalities were later followed by the absence of epaxial (dorsal) musculature; whereas, body wall and limb musculature formed normally. Analysis of Mox1 and Pax1 expression in opb embryos revealed additional defects in the differentiation of the dorsal sclerotome. The observed abnormalities coincided with defects in differentiation of dorsal neural tube regions. The implications of our findings for interactions between dorsal neural tube, surface ectoderm and dorsomedial somite regions in specifying epaxial musculature are discussed.

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.

Disruption of the mouse RBP-J kappa gene results in early embryonic death

The RBP-J kappa protein is a transcription factor that recognizes the sequence C(T)GTGGGGA. The RBP-J kappa gene is highly conserved in a wide variety of species and the Drosophila homologue has been shown to be identical to Suppressor of Hairless [Su(H)] which plays important roles in the development of the peripheral nervous system. To explore the function of the RBP-J kappa gene in mouse embryogenesis, a mutation was introduced into the functional RBP-J kappa gene in embryonic stem (ES) cells by homologous recombination. Null mutant ES cells survived but null mutant mice showed embryonic lethality before 10.5 days of gestation. The mutant mice showed severe growth retardation as early as 8.5 days of gestation. Developmental abnormalities, including incomplete turning of the body axis, microencephaly, abnormal placental development, anterior neuropore opening and defective somitogenesis, were observed in the mutant mice at 9.5 days of gestation. RBP-J kappa mutant embryos expressed a posterior mesodermal marker FGFR1. Their irregularly shaped somites expressed a somite marker gene Mox 1 but failed to express myogenin. The RBP-J kappa gene was revealed to be essential for postimplantation development of mice.

Malformations of the heart, kidney, palate, and skeleton in alpha-MHC-Hoxb-7 transgenic mice

To begin to define the genetic network involved in cardiogenesis, we generated mice bearing the alpha-myosin heavy chain (MHC)-Hoxb-7 transgene. We hypothesized that using the cardiac-specific alpha-MHC promoter, we can direct ectopic expression of Hoxb-7 in the heart and perturb its normal development. Both whole mount in situ hybridization and northern analyses showed that this alpha-MHC promoter resulted in transgene expression in the developing heart. Severe ventricular septal defects (VSD) were found in several mutant mice. Interestingly, transgenic mice were observed to have other malformations as well, including cleft palate, renal anomalies, and skeletal abnormalities in the craniocervical and costosternal regions. The kidney defect consisted of double ureter and pelvis. In summary, we have shown that a dominant gain-of-function mutation of Hoxb-7 using the murine alpha-MHC promoter results in perturbation of the genetic circuitry underlying multiple developmental processes, including cardiogenesis. Misexpression of Hoxb-7 during heart development may be involved in the pathogenesis of VSD.

The expression pattern of Xenopus Mox-2 implies a role in initial mesodermal differentiation

We have isolated a Xenopus homolog of the murine Mox-2 gene. As is the case for the mouse homolog, mesoderm specific expression of Xenopus Mox-2 (X. Mox-2) expression begins during gastrulation. Using whole mount in situ hybridization, we show that X. Mox-2 is expressed in undifferentiated dorsal, lateral and ventral mesoderm in the posterior of neurula/tailbud embryos, with expression more anteriorly detected in the dermatomes. In the tailbud tadpole, X. Mox-2 is expressed in tissues of the tailbud itself that represent a site of continued gastrulation-like processes resulting in mesoderm formation. X. Mox-2 is not expressed in the marginal zone of blastula, nor in the dorsal lip of gastrula, nor midline tissues (i.e. prospective notochord). Treatments that affect mesodermal patterning during embryonic development, including LiCl and ultraviolet light, and injection of mRNAs encoding BMP-4, or dominant negative activin and FGF receptors, produce changes in X. Mox-2 expression consistent with the types of tissues affected by these manipulations. X. Mox-2 expression is induced more in animal caps treated with FGF than those treated with activin. Together with the fact that X. Mox-2 activation in animal caps requires protein synthesis, our data suggest that X. Mox-2 is involved in initial mesodermal differentiation, downstream of molecules affecting mesoderm induction and determination such as Brachyury and goosecoid, and upstream of factors controlling terminal differentiation such as MyoD and myf5. X. Mox-2, therefore, is another useful marker for understanding the formation of mesoderm in amphibian development.

Molecular regulation of atrioventricular valvuloseptal morphogenesis

The majority of congenital heart defects arise from abnormal development of valvuloseptal tissue. The primordia of the valve leaflets and membranous septa of the heart are the cardiac cushions. Remodeling of the cushions is associated with a transitional extracellular matrix that includes sulfated proteoglycans and the microfibrillar proteins fibulin and fibrillin. Cushion formation is restricted to the AV canal and ventricular outflow tract regions of the primary heart tube. The proper placement of the cushions may be the result of the development of the primary heart tube as a segmented organ, as well as the subsequent looping of the heart. Segmentation of the heart tube may be demonstrated by the alternating molecular expression pattern along the longitudinal axis. In support of this hypothesis is the restricted expression of BMP-4 and msx-2 to the AV canal and ventricular outflow tract. The importance of looping for cushion positioning may imply that the iv and inv genes and retinoic acid are important for the proper patterning of the heart. The cells of the cushions evolve from endocardial cells that undergo an epithelial-to-mesenchymal transformation. This developmental event is regulated by the myocardium and is probably due to the production of protein complexes, present within the cardiac jelly of the cushion-forming regions, that consist of fibronectin and the ES proteins. Both the cushion mesenchyme and its endocardial cell antecedents express JB3, an ECM protein. JB3 expression is also featured within the heart-forming fields of the primary mesoderm, from which the endocardial progenitors of the cushion cells originate.(ABSTRACT TRUNCATED AT 250 WORDS)

Detection of planarian Antennapedia-like homeobox genes expressed during regeneration

Using the polymerase chain reaction with degenerate oligo primers we identified six new Antennapedia-like homeobox sequences expressed in a regenerating planarian. cDNA fragments containing the entire homeobox sequences for four of these genes were obtained. Two of them, Dutarh-3 and Dutarh-4, belong to the classical Antennapedia type and display extensive identity with Antennapedia- and Deformed-type homeodomains respectively. The third homeodomain, Dutarh-1, exhibits some similarity to the Hox1.5-, AbdB- and Dfd-type homeodomains and a fourth gene, Dutarh-6, is very similar to Mox2 in the mouse. Our results suggest that, in spite of their simple body plan, planarians contain a number of Antennapedia-like homeobox genes, probably enough to fill a cluster such as found in higher animals.

Notch1 is required for the coordinate segmentation of somites

Members of the Notch family of transmembrane receptors mediate a number of developmental decisions in invertebrates. In order to study Notch function in a vertebrate organism, we have mutated the Notch1 gene of the mouse. Notch1 gene function is required for embryonic survival in the second half of gestation. In the first half of gestation, we have found no effect of the mutation on the normal programs of neurogenesis, myogenesis or apoptosis. We conclude that Notch1 function is not essential for these processes, at least in early postimplantation development. However, we have found that somitogenesis is delayed and disorganized in Notch1 mutant embryos. We propose that Notch1 normally coordinates the process of somitogenesis, and we provide a model of how this might occur.

Isolation of the human MOX2 homeobox gene and localization to chromosome 7p22.1-p21.3

We have isolated and characterized cDNA clones encoding a novel human homeobox gene, MOX2, the homologue of the murine mox-2 gene. The MOX2 protein contains all of the characteristic features of Mox-2 proteins of other vertebrate species, namely the homeobox, the polyhistidine stretch, and a number of potential serine/threonine phosphorylation sites. The homeodomain of MOX2 protein is identical to all other vertebrate species reported so far (rodents and amphibians). Outside the homeodomain, Mox-2 proteins share a high degree of identity, except for a few amino acid differences encountered between the human and the rodent polypeptides. A polyhistidine stretch of 12 amino acids in the N terminal region of the protein is also conserved among humans, rodents, and (only partly) amphibians. The chromosomal position of MOX2 was assigned to 7p22.1-p21.3.

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.

Mechanisms of cell transformation in the embryonic heart

The process of cell transformation in the heart is a complex one. By use of the invasion bioassay, we have been able to identify several critical components of the cell transformation process in the heart. TGF beta 3 can be visualized as a switch in the environment that contributes to the initial process of cell transformation. Our data show that it is a critical switch in the transformation process. Even so, it is apparently only one of the factors involved. Others may include other TGF beta family members, the ES antigens described by Markwald and co-workers and additional unknown substances. Observing the sensitivity of the process to pertussis toxin, there is likely to be a G-protein-linked receptor involved, yet we have not identified a known ligand for this type of receptor. Clearly, there are several different signal transduction processes involved. The existence of multiple pathways is consistent with the idea that the target endothelial cells receive a variety of environmental imputs, the sum of which will produce cell transformation at the correct time and place. Adjacent endothelial cells of the ventricle that do not undergo cell transformation are apparently refractory to one or more of the stimuli. Figure 4 depicts a summary diagram of this invasion process with localization of most of the molecules mentioned in this narrative. As hypothesized here, elements of the transformation process may recapitulate aspects of gastrulation. Since some conservation of mechanism is expected in cells, it is not surprising that cells undergoing phenotypic change might reutilize mechanisms used previously to produce mesenchyme from the blastodisk. Though we have preliminary data to suggest this point, confirmation of the hypothesis by perturbation of genes such as brachyury, msx-1, etc. will be required to establish this point. The advantage of this hypothesis is that it provides, from the work of others in the area of gastrulation, a ready source of molecules and mechanisms that can be tested in the transforming heart. Whereas, perturbation of such mechanisms at gastrulation may be lethal to the embryo, such molecules and mechanisms may be responsible for the high incidence of birth defects in the heart.

Effects of the TWis mutation on notochord formation and mesodermal patterning

The mouse T (Brachyury) gene is required for early mesodermal patterning. Mice homozygous for mutations in T die at midgestation and display defects in mesodermal tissues such as the notochord, the allantois and the somitic mesoderm. To examine the role of T in patterning of somitic and posterior mesoderm along the anterior-posterior axis, we have examined the expression of a panel of molecular markers normally localized to the sub-set of cell types affected in TWis mutant mice. Through the use of whole-mount antibody double labelling techniques, we have analysed the spatial relationships of distinct mesodermal populations relative to cells expressing the T protein. We have also examined the consequences of the TWis mutation on mesodermal populations recognised by these markers. We demonstrate that TWis homozygous mutants retain the ability to form notochordal precursor cells, as identified both by the T antibody and the expression of sonic hedgehog/vertebrate homolog of hedgehog 1 (Shh/vhh-1) and goosecoid, however, these cells fail to proliferate or differentiate. These early notochordal defects appear to result in aberrant somitic differentiation as revealed by the distribution of mox-1 protein and twist RNA expression. Moreover, twist expression in paraxial mesoderm appears to be dependent on normal T activity, while Shh/vhh-1, goosecoid, mox-1 and cdx-4 are not T dependent. We propose that T is required for the maintenance of notochordal tissue and subsequent signals required for somite differentiation.

The major locus for multifactorial nonsyndromic cleft lip maps to mouse chromosome 11

Cleft lip with or without cleft palate, CL(P), a common human birth defect, has a genetically complex etiology. An animal model with a similarly complex genetic basis is established in the A/WySn mouse strain, in which 20% of newborns have CL(P). Using a newly created congenic strain, AEJ.A, and SSLP markers, we have mapped a major CL(P)-causing gene derived from the A/WySn strain. This locus, here named clf1 (cleft lip) maps to Chromosome (Chr) 11 to a region having linkage homology with human 17q21-24, supporting reports of association of human CL(P) with the retinoic acid receptor alpha (RARA) locus.

Dlx and other homeobox genes in the morphological development of the dentition

The dentition is a segmental system whose evolution and morphology bears analogy to the evolution of segmentation in the vertebral column and limb. Combinatorial expression of members of the large "Hox" class of homeobox regulatory genes has been shown to play an important role in positional specification in these skeletal systems. This raises the possibility that homeobox genes are also used for positional specification in the dentition, and several homeobox genes are known to be expressed in developing teeth. To identify additional dentally expressed homeobox genes, cDNA from from murine tooth germs at 9.5, 14.5, and 17.5 days gestational age was amplified by PCR using sets of degenerate primers to the homeodomains of 18 different classes of homeobox genes. Amplification products were cloned and sequenced and compared to known gene sequences. To date this approach has confirmed the presence of Msx1, Msx2, Dlx1, and Dlx2, and identified several other homeobox genes not previously known to be expressed in teeth: Dbx, MHox, and Mox2A, plus an a additional Dlx gene, Dlx7. The Msx and Dlx genes are the best current candidates for a combinatorial mechanism that controls the differentiation of structures within and between teeth, and perhaps also the evolution of those structures.

fgfr-1 is required for embryonic growth and mesodermal patterning during mouse gastrulation

Experiments in amphibians have implicated fibroblast growth factors (FGFs) in the generation and patterning of mesoderm during embryogenesis. We have mutated the gene for fibroblast growth factor receptor 1 (fgfr-1) in the mouse to genetically dissect the role of FGF signaling during development. In the absence of fgfr-1 signaling, embryos displayed early growth defects; however, they remained capable of gastrulating and generating mesoderm. The nascent mesoderm of fgfr-1 homozygous mutant embryos differentiated into diverse mesodermal subtypes, but mesodermal patterning was aberrant. Somites were never generated and axial mesoderm was greatly expanded at the expense of paraxial mesoderm. These results suggest that FGFR-1 transduces signals that specify mesodermal cell fates and regional patterning of the mesoderm during gastrulation.

The novel homeodomain gene buttonless specifies differentiation and axonal guidance functions of Drosophila dorsal median cells

We have identified a novel homeodomain gene, buttonless (btn), that is specifically expressed in 20 cells of a single type during Drosophila embryonic development. These cells, the dorsal median (DM) cells, are arranged as a single pair within each segment along the dorsal midline of the CNS. Distinctive features of the DM cells include a large cell body and a long thick process extending laterally to the muscle attachment site. In the absence of btn gene function the initial commitment to the DM cell fate is made but differentiation fails to occur and the DM cells are lost. The btn mutation thus specifically eliminates the DM cells, and this genetic ablation in turn reveals a requirement for DM cells as cellular cues for axonal guidance during transverse nerve outgrowth and bifurcation of the median nerve.

Distal-less and other homeobox genes in the development of the dentition

The mammalian tooth develops through an interaction between two tissue layers of different embryologic origin. A number of transcription factors and as well as two members of the Msx class of homeobox genes have been shown to be involved in the histogenesis of the mammalian tooth. This raised the possibility that other homeobox genes might be involved in dental morphogenesis. We have amplified mouse tooth germ cDNA from three different gestational ages by the polymerase chain reaction with degenerate primers for 18 classes of homeobox genes. Members of several classes have been isolated, including the Msx genes, two Dlx genes, and the Dbx, MHox, Mox2A genes. One of the Dlx genes, Dlx-7, had not previously been reported in mammals, and some details are presented of its cDNA sequence. This work plus that of other investigators has shown that at least six Dlx genes are expressed in developing teeth or in first branchial arches, suggesting the possibility that these genes are involved in specifying complexity within or between teeth. The screening approach with degenerate primers is a successful way to identify new as well as previously known regulatory genes expressed in developing tooth embryos.

Disruption of the HNF-4 gene, expressed in visceral endoderm, leads to cell death in embryonic ectoderm and impaired gastrulation of mouse embryos

Expression of HNF-4, a transcription factor in the steroid hormone receptor superfamily, is detected only in the visceral endoderm of mouse embryos during gastrulation and is expressed in certain embryonic tissues from 8.5 days of gestation. To examine the role of HNF-4 during embryonic development, we disrupted the gene in embryonic stem cells and found that the homozygous loss of functional HNF-4 protein was an embryonic lethal. Cell death was evident in the embryonic ectoderm at 6.5 days when these cells normally initiate gastrulation. As assessed by expression of Brachyury and HNF-3 beta, primitive streak formation and initial differentiation of mesoderm do occur, but with a delay of approximately 24 h. Development of embryonic structures is severely impaired. These results demonstrate that the expression of HNF-4 in the visceral endoderm is essential for embryonic ectoderm survival and normal gastrulation.

Towards a genetic basis for kidney development

Although it is not easy to investigate the regulatory basis of developmental processes in most mammalian tissues, the mouse kidney has several distinct advantages as a model system. Its development involves a wide variety of developmental processes that include induction, stem-cell regulation, a mesenchyme-to-epithelium transition, epithelial morphogenesis and pattern formation. Further, there are several genetic disorders associated with its development, much of nephrogenesis will take place in vitro and a significant start has been made in elucidating the regulatory molecules involved in its ontogeny. Here, we summarise current knowledge on how the various aspects of kidney development are controlled at the genetic level. For this, we have compiled a table showing when and where the more than forty regulatory genes thus far identified are expressed during nephrogenesis (this table being a subset of a database also containing information on structural and functional proteins expressed during nephrogenesis). The data on the regulatory genes demonstrate, in particular, the importance of the Wilms' tumour gene, WT1, in nephrogenesis, the growth-stimulating interaction between the hepatocyte growth factor and its receptor, c-met, and the differences between uninduced and induced metanephric mesenchyme. In an attempt to highlight those stable developmental pathways which underpin the formation of the kidney and to facilitate future work, we have identified possible checkpoints occurring during nephrogenesis (stages at which a positive signal is needed for development to continue). The data to hand suggest that such checkpoints occur when metanephric mesenchyme is established in the intermediate mesoderm, when induction takes place, when stem cells are activated and before mesenchyme aggregates to form nephrogenic condensations.(ABSTRACT TRUNCATED AT 250 WORDS)

Selected Aspects of Homeobox Gene Function during Mammalian Development

The genetic pathways of development are only beginning to be revealed. But the tools now exist to allow the rapid isolation of genes that carry sequence motifs such as the homeobox, zinc finger or basic-helix loop helix that can mark genes of special developmental significance. Expression patterns are readily determined by in situ hybridization and in vivo developmental functions can be analyzed by generating mice with targeted mutations. Upstream regulators of genes can be identified by finding proteins that bind to cis-regulatory elements. Downstream targets are more difficult to find but there are polymerase chain reaction approaches to define sequences bound by transcription factors and subtractive library approaches to finding specific targets. Although an enormous amount of work remains to be done it is clear that the basic techniques necessary to understand the genetic program of mammalian development are now available. As these techniques are applied and refined we will elaborate the genetic regulatory pathways of organogenesis. This will be deeply satisfying from an intellectual perspective. It will also lead to a better understanding of birth defects and to better treatments of a variety of diseases that involve organ malformation or deterioration. Copyright 1994 S. Karger AG, Basel

Programming gene expression in developing epidermis

As the major proteins of adult keratinocytes, keratins provide biochemical markers for exploring mouse epidermal embryogenesis. Here, we used a modified method of whole-mount in situ hybridization to track skin-specific expression of endogenous keratin mRNAs throughout embryogenesis. To monitor transcriptional regulation, we coupled this with beta-galactosidase expression of a human epidermal keratin promoter-driven transgene. These studies have radically changed our perception of how the program of gene expression becomes established during epidermal development. Specifically, we have discovered that (1) basal keratin (K5 and K14) genes are first detected at E9.5 in a highly regional fashion, and surprisingly as early as the single layered ectodermal stage; (2) the early patterns do not correlate with morphogenesis per se, but rather with regional variations in the embryonic origin of underlying mesenchyme, supporting morphogenetic criteria that early inductive cues are mesenchymal; (3) epidermal keratin genes are expressed in periderm, supporting the notion that this layer arises from ectodermal stratification, even though it is simple epithelial-like in morphology and is subsequently sloughed during development; (4) later embryonic patterns of K5 and K14 gene expression parallel proliferative capacity and not stratification; and (5) K1 and K10 mRNAs are first detected as early as E13.5, and their patterns correlate with differentiation and not stratification. These patterns of epidermal gene expression led us to explore whether potential transcriptional regulators of these genes are expressed similarly. We show that AP2 (but not Sp1) cRNAs hybridize in a pattern similar to, but preceding that of basal keratin cRNAs. Finally, using gene expression in cultured cells, we demonstrate that AP2 has a strong inductive effect on basal keratin expression in a cellular environment that does not normally possess AP2 activity.

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.

Identification of homeobox genes expressed in human haemopoietic progenitor cells

Homeodomain (HD)-containing proteins have been shown to regulate cellular commitment and differentiation in fungal, invertebrate and vertebrate systems. Bone marrow cells synthesizing the CD34 antigen are a complex mix of early, stem and progenitor cells at various stages of commitment to the many haemopoietic lineages. Here, we report the cloning and sequencing of 31 homeobox (HB) sequences, identified using degenerate oligodeoxyribonucleotide primers, in a polymerase chain reaction with cDNA derived from a purified CD34+ population of human haemopoietic cells. Of these sequences, 16 correspond to previously identified genes, and 13 are located within the HOX A, B and C clusters. Ten of the clones most likely represent human homologues of genes identified previously in other species. Five of the clones reported here represent novel HD sequences. The identification of five new genes using a subclass-specific 5' primer, designed from the engrailed and Xanf1 sequences, suggests that there still remain several uncharacterized HB genes in the human genome. Haemopoietic cells purified on the basis of CD34 antigen synthesis are a rich source of regulatory genes consistent with their ability to differentiate into diverse haemopoietic cell types.

A primary requirement for nodal in the formation and maintenance of the primitive streak in the mouse

The 413.d insertional mutation arrests mouse development shortly after gastrulation. nodal, a novel TGF beta-related gene, is closely associated with the locus. The present study provides direct evidence that the proviral insertion causes a loss of function mutation. nodal RNA is initially detected at day 5.5 in the primitive ectoderm. Concomitant with gastrulation, expression becomes restricted to the proximal posterior regions of the embryonic ectoderm. nodal RNA is also expressed in the primitive endoderm overlying the primitive streak. A few hours later, expression is strictly confined to the periphery of the mature node. Interestingly 413.d mutant embryos show no morphological evidence for the formation of a primitive streak. Nonetheless, about 25% of mutant embryos do form randomly positioned patches of cells of a posterior mesodermal character. Data presented in this report demonstrate the involvement of a TGF beta-related molecule in axis formation in mammals.

Cardiovascular development. Prospects for a genetic approach

Genetics is a powerful tool, especially when used in combination with embryology, in the seeking of genes necessary for assembly of the cardiovasculature. The first questions must address the types of cellular decisions that are made during development. As for simpler systems in C elegans and D melanogaster, the lineage and cell-fate decisions of the cardiovascular progenitors need to be assessed. In addition it is likely that new paradigms will emerge for multicellular assembly. The study of cardiovascular mutations will define individual genetic steps that define organotypic decisions. A genetic approach is a natural extension of embryology, physiology, and anatomy, fields of great sophistication with regard to the cardiovasculature, because, like them, it focuses on integrative biology and on the intact organism. The zebrafish is particularly well suited to a combination genetic-embryologic study of the fashioning of the cardiovasculature.

Wnt-3a regulates somite and tailbud formation in the mouse embryo

Amphibian studies have implicated Wnt signaling in the regulation of mesoderm formation, although direct evidence is lacking. We have characterized the expression of 12 mammalian Wnt-genes, identifying three that are expressed during gastrulation. Only one of these, Wnt-3a, is expressed extensively in cells fated to give rise to embryonic mesoderm, at egg cylinder stages. A likely null allele of Wnt-3a was generated by gene targeting. All Wnt-3a-/Wnt-3a- embryos lack caudal somites, have a disrupted notochord, and fail to form a tailbud. Thus, Wnt-3a may regulate dorsal (somitic) mesoderm fate and is required, by late primitive steak stages, for generation of all new embryonic mesoderm. Wnt-3a is also expressed in the dorsal CNS. Mutant embryos show CNS dysmorphology and ectopic expression of a dorsal CNS marker. We suggest that dysmorphology is secondary to the mesodermal and axial defects and that dorsal patterning of the CNS may be regulated by inductive signals arising from surface ectoderm.

Primitive streak mesoderm-like cell lines expressing Pax-3 and Hox gene autoinducing activities

Differentiating P19 embryonal carcinoma (EC) cells transiently express an endogenous activity capable of inducing Pax-3 expression in adjacent P19 stem cells (Pruitt, Development 116, 573–583, 1992). In the present study, expression of this activity in mesodermal cell lineages is demonstrated. First, expression of the mesodermal marker Brachyury correlates with expression of Pax-3-inducing activity. Second, the ability of leukemia inhibitory factor (LIF) to block mesoderm differentiation at two different points is demonstrated and correlated with the inhibition of Pax-3-inducing activity. Finally, two mesodermal cell lines that express Pax-3-inducing activity were derived from P19 EC cells. Each of these lines expresses high levels of the mesodermal marker Brachyury and high levels of Oct-3/4 (which is down-regulated at early times during mesoderm differentiation) suggesting that these lines are early mesodermal derivatives. Unlike EC or embryonic stem cell lines, each of the two mesodermal derivatives autoinduces Hox gene expression on aggregation even in the presence of LIF. Following aggregation, anterior-specific genes are expressed more rapidly than more posterior genes. These observations directly demonstrate the ability of murine mesodermal derivatives to autoinduce Hox gene expression in the absence of signals from other cell lineages. Similar to the Pax-3-inducing activity, signals from mesodermal cell lines were sufficient to induce HOX expression in adjacent P19 stem cells in cell mixing assays. These observations are consistent with the previous suggestion (Blum, M., Gaunt, S. J., Cho, K. W. Y., Steinbeisser, H., Blumberg, B., Bittner, D. and De Robertis, E. M. (1992) Cell 69, 1097–1106) that signals responsible for anterior-posterior organizer activity are localized to the anterior primitive streak mesoderm of the mouse embryo.

The HOX complex neighbored by the EVX gene, as well as two other homeobox-containing genes, the GBX-class and the EN-class, are located on the same chromosomes 2 and 7 in humans

Two newly identified human homeobox-containing genes, GBX1 and GBX2, are closely related genes, as are members of the other homeobox genes, EN-1 and EN-2. GBX1 and EN-2 have been mapped to chromosome 7q36. The present study shows that GBX2 was mapped to chromosome 2q37. EN-1 was mapped to chromosome 2q14. Moreover, two HOX complexes neighbored by the EVX gene, HOXA and HOXD, are located at chromosome 7p15-p14 and 2q31-q37, respectively. Thus, it is possible that these homeobox genes were linked to each other on an ancestral genome and that the ancestral chromosome segment was duplicated during evolution.

Presence of eight distinct homeobox-containing genes in cnidarians

Using the polymerase chain reaction, we identified four different homeobox-containing genes in Hydra magnipapillata. Three of them, cnox1-Hm, cnox2-Hm and cnox4-Hm, were equivalent to homeobox genes that had already been identified in other species of cnidarians. cnox5-Hm was a new homeobox gene and was very similar to Mox1 in the mouse. Together with the published data, our results indicate that there are at least eight distinct classes of homeobox genes in cnidarians. These homeobox genes show a maximum of 60 to 77% identity in terms of the amino acid residues in their homeodomains to certain classes of homeobox genes that have been identified in Drosophila.

Murine Cdx-4 bears striking similarities to the Drosophila caudal gene in its homeodomain sequence and early expression pattern

A third member of the murine caudal-like gene family, Cdx-4, has been isolated. In situ hybridization and immunohistochemistry have been used to study the localization of mRNA and protein during murine embryogenesis. Cdx-4 is expressed transiently from 7.0 d.p.c. (days post coitum) until 10 d.p.c., starting at the beginning of gastrulation (7.0-7.5 d.p.c.) in the allantois and posterior tip of the primitive streak. At the mid-streak stage, Cdx-4 expression moves rostrally, and protein and mRNA are detected in all cells over the posterior half of the primitive streak. As development proceeds, Cdx-4 gene products continue to be restricted to the posterior of the embryo, including the remnants of the primitive streak. Cdx-4 is expressed in neurectoderm, presomitic and lateral plate mesoderm, and hindgut endoderm, but the anterior boundary in the paraxial mesoderm is staggered with respect to the other germ layers. At all stages analyzed, Cdx-4 exhibits a graded expression pattern with a posterior maximum, a distribution highly reminiscent of the Drosophila caudal gene. These data add to the recently described distributions of other vertebrate caudal-like genes, and further support the idea that members of this homeobox gene subfamily have regulatory roles in the specification of anteroposterior axial polarity in early embryos.

Genetic control of gastrulation in the mouse

In the past, understanding of the process of gastrulation in the mouse has primarily been based on morphological analyses. Recently, a number of molecules have been implicated in mesoderm induction and axis formation in Xenopus, and several of these exhibit unique patterns of expression during mouse gastrulation. These gene-expression data, together with fate mapping, ectopic expression experiments and mutational analysis, will now facilitate studies on the functional aspects of gastrulation in the mouse.

Differential expression of multiple fork head related genes during gastrulation and axial pattern formation in the mouse embryo

Four genes encoding fork-head-domain-containing proteins (FD genes) have been isolated from a mouse 8.5 days post coitum (p.c.) embryo cDNA library. Two are mouse homologues of rat HNF-3 beta and HNF-3 alpha. The other two are novel and have been named MF-1 and MF-2 (for mesoderm/mesenchyme fork head). Wholemount in situ hybridization of embryos between 6.5 and 9.5 days p. c. shows that each gene has a unique expression pattern. HNF-3 beta is expressed in the node, notochord, floor plate and gut, while HNF-3 alpha is mainly in the definitive endoderm and gut, but also in the floor plate of the midbrain. These results suggest that HNF-3 beta and HNF-3 alpha, in addition to their known functions as transcriptional activators in adult liver, play a role in body axis formation, neural tube patterning and definitive endoderm formation during gastrulation. MF-1 RNA is present in non-notochordal mesoderm, and in neural-crest-derived head mesenchyme, while MF-2 transcripts are found in the sclerotomes of the somites and in head mesenchyme, including that from neural crest. Studies on gastrulation stage embryos suggest that the early temporal and spatial patterns of HNF-3 beta, MF-1 and HNF-3 alpha correlate with populations of cells undergoing commitment to different developmental fates. A model is proposed linking FD gene expression with gastrulation events in the mouse.