Differential utilization of the same reading frame in a Xenopus homeobox gene encodes two related proteins sharing the same DNA-binding specificity

Two Hoxc6 transcripts are differentially expressed and regulate primary neurogenesis in Xenopus laevis

Hox genes are key players in defining positional information along the main body axis of vertebrate embryos. In Xenopus laevis, Hoxc6 was the first homeobox gene isolated. It encodes two isoforms. We analyzed in detail their spatial and temporal expression pattern during early development. One major expression domain of both isoforms is the spinal cord portion of the neural tube. Within the spinal cord and its populations of primary neurons, Hox genes have been found to play a crucial role for defining positional information. Here we report that a loss-of-function of either one of the Hoxc6 products does not affect neural induction, the expression of general neural markers is not modified. However, Hoxc6 does widely affect the formation of primary neurons within the developing neural tissue. Manipulations of Hoxc6 expression severly changes the expression of the neuronal markers N-tubulin and Islet-1. Formation of primary neurons and formation of cranial nerves are affected. Hence, Hoxc6 functions are not restricted to the expected role in anterior-posterior pattern formation, but they also regulate N-tubulin, thereby having an effect on the initial formation of primary neurons in Xenopus laevis embryos.

Xenopus Teashirt1 regulates posterior identity in brain and cranial neural crest

We have isolated two related Xenopus homologues of the homeotic zinc finger protein Teashirt1 (Tsh1), XTsh1a and XTsh1b. While Drosophila teashirt specifies trunk identity in the fly, the developmental relevance of vertebrate Tsh homologues is unknown. XTsh1a/b are expressed in prospective trunk CNS throughout early neurula stages and later in the migrating cranial neural crest (CNC) of the third arch. In postmigratory CNC, XTsh1a/b is uniformly activated in the posterior arches. Gain- and loss-of-function experiments reveal that reduction or increase of XTsh1 levels selectively inhibits specification of the hindbrain and mid/hindbrain boundary in Xenopus embryos. In addition, both overexpression and depletion of XTsh1 interfere with the determination of CNC segment identity. In transplantation assays, ectopic XTsh1a inhibits the routing of posterior, but not of mandibular CNC streams. The loss of function phenotype could be rescued with low amounts either of XTsh1a or murine Tsh3. Our results demonstrate that proper expression of XTsh1 is essential for segmentally restricted gene expression in the posterior brain and CNC and suggest for the first time that teashirt genes act as positional factors also in vertebrate development.

Glypican 4 modulates FGF signalling and regulates dorsoventral forebrain patterning in Xenopus embryos

Heparan sulphate proteoglycans such as glypicans are essential modulators of intercellular communication during embryogenesis. In Xenopus laevis embryos, the temporal and spatial distribution of Glypican 4 (Gpc4) transcripts during gastrulation and neurulation suggests functions in early development of the central nervous system. We have functionally analysed the role of Xenopus Gpc4 by using antisense morpholino oligonucleotides and show that Gpc4 is part of the signalling network that patterns the forebrain. Depletion of GPC4 protein results in a pleiotropic phenotype affecting both primary axis formation and early patterning of the anterior central nervous system. Molecular analysis shows that posterior axis elongation during gastrulation is affected in GPC4-depleted embryos, whereas head and neural induction are apparently normal. During neurulation, loss of GPC4 disrupts expression of dorsal forebrain genes, such as Emx2, whereas genes marking the ventral forebrain and posterior central nervous system continue to be expressed. This loss of GPC4 activity also causes apoptosis of forebrain progenitors during neural tube closure. Biochemical studies establish that GPC4 binds FGF2 and modulates FGF signal transduction. Inhibition of FGF signal transduction, by adding the chemical SU5402 to embryos from neural plate stages onwards, phenocopies the loss of gene expression and apoptosis in the forebrain. We propose that GPC4 regulates dorsoventral forebrain patterning by positive modulation of FGF signalling.

Antisense inhibition of Xbrachyury impairs mesoderm formation in Xenopus embryos

Expression of the Xbrachyury (Xbra) gene was inhibited by antisense RNA synthesized in situ from an expression vector read by RNA polymerase III, injected into the fertilized egg or the 2-cell stage embryo of Xenopus laevis. Antisense-treated embryos had markedly reduced levels of Xbra mRNA and protein, and showed deficiencies in mesodermal derivatives and axis formation. In particular, organization of the posterior axis was affected, but often the anterior axis was also reduced. Some embryos failed to form mesoderm altogether and remained amorphous. The antisense effect is dose-dependent and may be "rescued" by overexpression of Xbra. In Xbra-deficient embryos, expression of several mesodermal genes (Xvent, pintallavis, Xlim, Xwnt-8 and noggin) was reduced to varying degrees, whereas goosecoid levels remained normal. The modified expression levels were partly normalized when Xbra deficiency was rescued. The observation that antisense inhibition yields slightly different phenotypes from dominant-negative inhibition suggests the recommendation of using several surrogate genetic approaches to determine the functional role of a gene in Xenopus development.

Increased XRALDH2 activity has a posteriorizing effect on the central nervous system of Xenopus embryos

Retinoic acid (RA) metabolizing enzymes play important roles in RA signaling during vertebrate embryogenesis. We have previously reported on a RA degrading enzyme, XCYP26, which appears to be critical for the anteroposterior patterning of the central nervous system (EMBO J. 17 (1998) 7361). Here, we report on the sequence, expression and function of its counterpart, XRALDH2, a RA generating enzyme in Xenopus. During gastrulation and neurulation, XRALDH2 and XCYP26 show non-overlapping, complementary expression domains. Upon misexpression, XRALDH2 is found to reduce the forebrain territory and to posteriorize the molecular identity of midbrain and individual hindbrain rhombomeres in Xenopus embryos. Furthermore, ectopic XRALDH2, in combination with its substrate, all-trans-retinal (ATR), can mimic the RA phenotype to result in microcephalic embryos. Taken together, our data support the notion that XRALDH2 plays an important role in RA homeostasis by the creation of a critical RA concentration gradient along the anteroposterior axis of early embryos, which is essential for proper patterning of the central nervous system in Xenopus.

The HOXC6 homeodomain-containing proteins

The HOXC6 homeodomain-containing proteins act as transcription factors in the genetic control of multiple genes involved in development and cell differentiation. Two HOXC6 polypeptides are encoded by a single homeobox ('HOX') gene described as 'master gene' for the crucial role it plays in the patterning and axial morphogenesis of multiple species. Transcription of the HOXC6 gene is initiated from two promoters and generates two proteins that share the same DNA-binding domain but harbor a distinct N-terminal region. Recent studies have demonstrated that both HOXC6 products can activate or repress transcription, depending on the cellular context. Functional in vivo specificity of HOXC6 proteins may be achieved through combinatorial interactions with other members of the HOX family as well as with co-factors whose identities are largely unknown. Disruption of this 'HOX code' may lead to pathology such as developmental defects.

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

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

Inhibition of gastrulation in Xenopus embryos by an antibody against a cathepsin L-like protease

An antibody against cathepsin L-like protease (AACLP) was injected into one cell of 2-celled Xenopus embryos. The blastopores of AACLP-injected embryos either did not invaginate or failed to complete invagination. As a result of this failure to complete gastrulation, the body axes could not form normally and tail bud stage embryos were bent dorsally. Embryos injected with a control antibody (CA) developed normally through the tadpole stage. Mesodermal induction was not inhibited in embryos exhibiting this AACLP-induced gastrulation defect, but the mesodermal structure of these embryos was organized incorrectly due to the defective gastrulation during the early stages.

Negative regulation of dorsal patterning in early embryos by overexpression of XrelA, a Xenopus homologue of NF-kappa B

Recent results by Richardson et al. (Mech. Dev., 52 (1995) 165-177) suggest that the Xenopus Rel gene XrelA may be involved in the formation of the head and tail of the early embryo. We present evidence to suggest that wild-type XrelA also has a role in dorsoventral development. XrelA overexpression in the dorsal side of embryos reduces dorsal development and attenuates in vitro dorsal morphogenetic movements. XrelA also strongly reduces axis duplication caused by overexpression of a dominant negative mutant of Xenopus glycogen synthase kinase-3 beta. Our results indicate that XrelA may have a role in dorsoventral patterning in early embryos.

Analysis of Hox gene expression in the chick limb bud

The vertebrate Hox genes have been shown to be important for patterning the primary and secondary axes of the developing vertebrate embryo. The function of these genes along the primary axis of the embryo has been generally interpreted in the context of positional specification and homeotic transformation of axial structures. The way in which these genes are expressed and function during the development of the secondary axes, particularly the limb, is less clear. In order to provide a reference for understanding the role of the Hox genes in limb patterning, we isolated clones of 23 Hox genes expressed during limb development, characterized their expression patterns and analyzed their regulation by the signalling centers which pattern the limb. The expression patterns of the Abd-B-related Hoxa and Hoxd genes have previously been partially characterized; however, our study reveals that these genes are expressed in patterns more dynamic and complex than generally appreciated, only transiently approximating simple, concentric, nested domains. Detailed analysis of these patterns suggests that the expression of each of the Hoxa and Hoxd genes is regulated in up to three independent phases. Each of these phases appears to be associated with the specification and patterning of one of the proximodistal segments of the limb (upper arm, lower arm and hand). Interestingly, in the last of these phases, the expression of the Hoxd genes violates the general rule of spatial and temporal colinearity of Hox gene expression with gene order along the chromosome. In contrast to the Abd-B-related Hoxa and Hoxd genes, which are expressed in both the fore and hind limbs, different sets of Hoxc genes are expressed in the two limbs. There is a correlation between the relative position of these genes along the chromosome and the axial level of the limb bud in which they are expressed. The more 3′ genes are expressed in the fore limb bud while the 5′ genes are expressed in the hind limb bud; intermediate genes are transcribed in both limbs. However, there is no clear correlation between the relative position of the genes along the chromosome and their expression domains within the limb. With the exception of Hoxc-11, which is transcribed in a posterior portion of the hind limb, Hoxc gene expression is restricted to the anterior/proximal portion of the limb bud. Importantly, comparison of the distributions of Hoxc-6 RNA and protein products reveals posttranscriptional regulation of this gene, suggesting that caution must be exercised in interpreting the functional significance of the RNA distribution of any of the vertebrate Hox genes. To understand the genesis of the complex patterns of Hox gene expression in the limb bud, we examined the propagation of Hox gene expression relative to cell proliferation. We find that shifts in Hox gene expression cannot be attributed to passive expansion due to cell proliferation. Rather, phase-specific Hox gene expression patterns appear to result from a context-dependent response of the limb mesoderm to Sonic hedgehog. Sonic hedgehog (the patterning signal from the Zone of Polarizing Activity) is known to be able to activate Hoxd gene expression in the limb. Although we find that Sonic hedgehog is capable of initiating and polarizing Hoxd gene expression during both of the latter two phases of Hox gene expression, the specific patterns induced are not determined by the signal, but depend upon the temporal context of the mesoderm receiving the signal. Misexpression of Sonic hedgehog also reveals that Hoxb-9, which is normally excluded from the posterior mesenchyme of the leg, is negatively regulated by Sonic hedgehog and that Hoxc-11, which is expressed in the posterior portion of the leg, is not affected by Sonic hedgehog and hence is not required to pattern the skeletal elements of the lower leg.

Theoretical approaches to the analysis of homeobox gene evolution

The homeobox gene system presents a unique model for experimental and theoretical analyses of gene evolution. Homeobox genes play a role in patterning the embryonic development of diverse organisms and as such are likely to have been fundamental to the evolution of the specialized body plans of many animal species. The organization of Hox-genes in chromosomal, clusters in many species implicates gene duplication as a prominent mechanism in the evolution of this multigene family. I review here various theoretical analyses that have contributed to our understanding of the molecular evolution of this class of developmental control genes. This article also illustrates relationships between theoretical predictions and experimental studies and outlines future avenues for the evolutionary analysis of developmental systems.

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.

Inhibition of Xhox1A gene expression in Xenopus embryos by antisense RNA produced from an expression vector read by RNA polymerase III

Antisense inhibition of gene expression during Xenopus development was obtained by injecting, into the zygote, an expression vector carrying the adenovirus VAI gene read by RNA polymerase III. This vector yields high levels of antisense RNA in most embryonic cells between mid-blastula transition and tailbud stage. As a target we chose the Xenopus homeobox gene Xhox1A. A 26 bp long oligonucleotide, including the initiation codon of this gene, was inserted in opposite polarity into the vector. Antisense treatment reduces Xhox1A mRNA in embryos up to stage 22 and Xhox1A protein expression up to stage 30. Half of the antisense-treated embryos develop a characteristic phenotype with disorganized somites in the anterior trunk and delayed development of the intestinal tract.

The gene of chicken axonin-1. Complete structure and analysis of the promoter

We have isolated and characterised the gene encoding the chicken axonal cell adhesion molecule axonin-1. This gene comprises 23 exons distributed over approximately 40 kb. Each of the six immunoglobulin-like domains and the four fibronectin-type-III-like domains of axonin-1 is encoded by two exons. The introns between two domains are exclusively phase I. Their exon/intron borders correspond to the domain borders of the protein, suggesting that the gene of axonin-1 had been generated by exon shuffling. Three transcripts with a length of 4.3 kb, 5 kb, and 8 kb are found, and we provide evidence that they result from alternative use of polyadenylation signals. In situ hybridization revealed co-localisation of these transcripts in time and space in the developing chicken retina. Several identical transcription initiation sites were found in retina, brain, and cerebellum by RNase protection assay and anchored polymerase chain reaction. By transfection of HeLa cells, rat PC-12 phaeochromocytoma cells, and chicken embryonic fibroblasts with serially truncated segments of the 5'-flanking region linked to a luciferase reporter gene, we have found that the sequence from -91 to +56 relative to the transcription initiation site is sufficient to promote efficient gene expression. Tissue-specific expression of the axonin-1 gene seems to be regulated in part by sequences more than 1 kb upstream of the transcription initiation site. As revealed by computer analysis, the sequence immediately upstream of exon 1 contains an AP-2 binding site, a tumor phorbol-ester-responsive element, and a homeodomain protein binding site, but no canonical TATA box. A second AP-2 binding site and a homeodomain protein binding site are located within exon 1.

Transcription factor binding sites in the matrix attachment region (MAR) of the chicken alpha-globin gene

Nuclear matrix is a nuclear protein-DNA superstructure believed to be the exclusive site of DNA replication, transcription, repair, and recombination. The attachment regions of chromatin loops to the nuclear matrix, called MARs, nest origins of replication, have transcriptional enhancer activity, and via their interaction with protein transcription factors may govern gene switch during development and tissue-specific gene expression. In this study the 967 bp MAR of the chicken alpha-globin gene is analyzed for the presence of hexanucleotides from a number (83 in total) of vertebrate protein transcription factors and core origins of replication. A total number of 760 hexanucleotides from factor sites or origins of replication were used for this search. We found that: (1) The occurrence of protein transcription factor binding sites overall on the MAR fragment as well as on the enhancer and promoter regions of other genes is only about 1.2-1.5 times higher than in random DNA, something consistent for all MAR and enhancer sequences examined. However, a high concentration (up to 2.7 times over random sequences) of hexanucleotide factor sites is observed on small stretches of the alpha-globin gene MAR. (2) Some regulatory protein binding sites are underrepresented whereas others are overrepresented, giving to an MAR a particular transcription factor flavor. (3) The DNA curvature map of the MAR sequence and the potential sites of positioned nucleosomes suggest the sites where a competition between core histone octamers and protein transcription factors for DNA might be found. This approach might provide a novel technique to diagnose for the regulatory or nonregulatory function of a stretch of DNA. Furthermore, MARs are proposed to constitute important regulatory elements of genes in addition to enhancers, promoters, silencers, locus control regions, and origins of replication. Additional parameters such as interaction of a transcription factor with other transcription factors fixed at vicinal sites, DNA methylation, intrinsic DNA curvature torsional strain, and nucleosome positioning might also determine the high-affinity binding of a transcription factor to its functional sites and its exclusion from or low affinity binding to other nonregulatory regions.

Vertebrate homeobox genes

In the former part of the review the principal available data about Hox genes, their molecular organisation and their expression in vertebrate embryos, with particular emphasis for mammals, are briefly summarized. In the latter part we analysed the expression of four mouse homeobox genes related to two Drosophila genes expressed in the developing head of the fly: Emx1 and Emx2, related to ems, and Otx1 and Otx2, related to otd.

Identification and promoter activity of DNase I hypersensitive sites in the region of the Hox-1.3 gene

The Hox genes encode transcriptional regulatory proteins that play a critical role in rostrocaudal specification in the developing embryo. The genes lie in four clusters in the mouse and human genome and are arranged such that a colinear relation exists between a gene's position in the cluster and the time of activation of the gene's expression. We have analyzed the Hox-1.3 region within the Hox-1 gene cluster for DNase I hypersensitive sites to identify putative regulatory sequences. Fragments identified in this way were then analyzed for transcriptional activity using gene transfer experiments in embryonal carcinoma (EC) cells. Three DNase I hypersensitive sites were identified, one of which includes the Hox-1.3 promoter and another, located 550 bp upstream, which enhances the Hox-1.3 promoter activity. The third occurs in the intron and may represent a Hox binding site. Significantly, the DNase I hypersensitive site pattern of this region of the Hox-1 cluster is not altered when F9 stem cells are differentiated with retinoic acid, suggesting that sequential activation of Hox genes by retinoic acid is not due to a sequential opening of the chromatin structure in the Hox gene region.

Spatial localisation of transcripts of the Hox-C6 gene

The Hox-C6 gene, in common with its Xenopus and human homologues, is organised as 3 exons with 2 distinct promoters (PRI and PRII) producing 2 different transcripts. The PRII promoter produces a 'typical' Hox transcript by splicing 2 exons separated by a 700 bp intron. The PRI promoter initiates transcription from a 3rd exon, 9 kbp 5' of PRII. This exon is spliced into the 1st exon of the PRII transcript at a point 3' of the translation start codon. This means that the predicted protein product of the PRI protein contains the Hox-C6 homeodomain but is truncated by 82 amino acids compared with the PRII product. In situ hybridisation using probes specific for PRI or PRII showed them to have essentially the same distribution in the developing nervous system and prevertebrae of 12.5 d embryos. However, the distribution of transcripts in the CNS was distinctly different from that reported previously for a probe containing the Hox-C6 homeobox.

Antennapedia-class homebox genes define diverse neuronal sets in the embryonic CNS of the leech

Studies of the Antennapedia-class homeobox genes suggest that specific combinations of these transcription factors play a role in defining neuronal identities. We examined the expression of these genes in the leech Hirudo medicinalis, an organism well-suited for neurobiological research at the level of identified neurons. Leeches contain at least as many Antennapedia-class and related genes as insects do, despite the apparently lower complexity of the leech body plan. The CNS expression patterns of two Antennapedia-class leech homeobox genes (Lox genes) were examined in detail. Lox1 is expressed during early gangliogenesis in only one pair of transient neurons present in every segment (the Bipolar cells) and, at later stages of embryonic development, in 15-20 pairs of central neurons repeated in most segments. The monoclonal antibody Laz1-1 identified two pairs of Lox1-expressing neurons as the Bipolar cells and the L1 neurons. The Bipolar cells extended processes in the primordia of the longitudinal connective nerves and later degenerated. The L1 neurons were detected late in gangliogenesis and became stable neurons. Lox2 is expressed in an iterated set of neurons in the posterior two-thirds of the CNS. On the basis of cell body position and relative size, two pairs of Lox2-expressing cells were identified as the RPE-like neurons and the CV motor neurons. Other Lox genes are also expressed in segmentally repeated subpopulations of neurons. These neuronal subpopulations appear to be different from one another but partially overlapping. Different combinations of Lox genes that may be expressed in individual cells could in theory generate enough variability to specify all central neurons in a leech ganglion.

A universal target sequence is bound in vitro by diverse homeodomains

To determine the number of DNA binding proteins capable of binding a consensus Engrailed binding site, this consensus sequence was used to screen a library of Drosophila cDNA clones in a bacteriophage expression vector. We retrieved clones encoding 20 distinct DNA binding domains, 17 of which are homeodomains. Binding to a variety of oligonucleotides confirms the related sequence specificity of the retrieved binding domains. Nonetheless, the homeodomains have remarkably diverse amino acid sequences. We conclude that during the evolutionary divergence of homeodomains, the specificity of DNA binding has been much more highly conserved than the amino acid sequence.

Protein product of the somatic-type transcript of the Hoxa-4 (Hox-1.4) gene binds to homeobox consensus binding sites in its promoter and intron

The murine Hoxa-4 gene encodes a protein with a homeodomain closely related to those produced by the Antennapedia-like class of Drosophila genes. Drosophila homeodomain proteins can function as transcription factors, binding to several specific DNA sequences. One sequence that is frequently encountered contains a core ATTA motif within a larger consensus sequence, such as CAATTAA. The in vitro synthesized protein product of Hoxa-4 was shown to bind to a subset of restriction fragments of the Hoxa-4 gene itself as determined by gel retardation experiments. Direct examination of the sequences of the fragments bound by Hoxa-4 protein revealed the presence of four regions containing the core ATTA motif. Two regions contained sequences of the CAATTAA class and were located approximately 1 kb upstream from the putative somatic Hoxa-4 promoter and within the intron. Two additional binding sites containing the consensus target sequence involved in autoregulation of Drosophila Deformed gene were identified: one immediately downstream of the putative embryonic transcription start site and one within the intron, respectively. Specific binding of the in vitro produced Hoxa-4 protein to oligonucleotides corresponding to these sequences was observed in gel retardation assays. The same results were obtained with Hoxa-4 protein produced in a Baculovirus expression system. Experiments using oligonucleotides containing base substitutions in positions 1, 3, 4, and 5 in the sequence CAATTAA showed severely reduced binding. The use of truncated mutant Hoxa-4 proteins in gel retardation assays and in transient co-transfection experiments revealed that the intact homeodomain was required for the binding. These results also suggested that the Hoxa-4 gene has the potential to auto-regulate its expression by interacting with the homeodomain binding sites present in the promoter as well as in the intron.

Hoxb-4 (Hox-2.6) mutant mice show homeotic transformation of a cervical vertebra and defects in the closure of the sternal rudiments

Two Hoxb-4 (Hox-2.6) mutations were introduced into the mouse germline. The overt phenotype caused by one of the mutations was assayed on two different genetic backgrounds, an inbred 129SvEv and a hybrid 129SvEv-C57BL/6J. The allele hoxb-4' is a disruption of the first exon and causes two obvious skeletal changes: a partial homeotic transformation of the second cervical vertebra from axis to atlas and a defective morphogenesis of the sternum. Both phenotypes have incomplete penetrance and variable expressivity when assayed in the hybrid genetic background, but the sternum defect is completely penetrant in the inbred background. The mutant allele hoxb-4s has a premature stop codon, introduced by the "hit and run" method in the second exon, that disrupts the third helix of the homeodomain. This allele also causes the partial homeotic transformation of axis to atlas, but it does not affect the sternum.

Characterization of the Xenopus Hox 2.4 gene and identification of control elements in its intron

We report on the Xenopus homolog of the Hox 2.4 gene. This gene occupies the next to 5'-most position in the Xenopus Hox2 complex. Hox 2.4 RNA is first detected at the early neurula stage, reaching a peak at the early tailbud stage, and is localized in the middle and posterior portions of the embryos. Antibodies raised against a fusion protein show expression of Hox 2.4 protein in Xenopus embryos in a band located in the mid spinal cord. Thus, the protein is expressed in a narrower domain than that of Hox 2.4 mRNA. The Xenopus Hox 2.4 antibody cross-reacts readily with mouse embryonic tissue, where the protein is detected in migrating neural crest cells, the dorsal portion of the spinal cord, somites, lateral plate mesoderm, and in the forelimb bud. The Xenopus Hox 2.4 intron shares considerable sequence identity with the intron in the mouse homolog. A reporter gene containing an element from this intron which can bind homeodomain proteins is activated following microinjection into Xenopus embryos. The short distance between the end of the Hox 2.4 cDNA and the start site of the neighboring gene in the complex raises the possibility that this transcriptional element might be shared by two Hox genes.

A Hox 3.3-lacZ transgene expressed in developing limbs

We describe transgenic mouse lines that express lacZ under the control of the Hox 3.3 Promoter II. The correct anterior boundary can be fixed by 3.6 kb of promoter DNA (plus 1.6 kb of 5' transcribed sequences), both in tissues of ectodermal and mesodermal origin. The posterior border, however, is not respected, and lacZ expression continues into the tail region. One line has particularly strong graded expression in the anterior proximal limb bud. Other lines, containing a shorter promoter fragment (0.6 kb), have ectopic expression in the head region, including one line that has expression in the anterior half of the retina. Such mouse lines make it possible to molecularly distinguish cells in regions of the embryo that look otherwise identical and may be useful in studying the establishment of molecular differences in the mouse embryo.

Homeotic transformations in the mouse induced by overexpression of a human Hox3.3 transgene

A permanent transgenic mouse line was generated carrying 40 copies of the human Hox3.3 gene. The resulting mice express large amounts of Hox3.3 protein in posterior regions of the embryo where this homeodomain protein is normally not expressed. The transgene causes homeotic transformations of the skeleton, in particular the appearance of an extra pair of ribs in the lumbar region, transformation of the shape of posterior ribs into that of more anterior ones, and the joining of an additional pair of ribs to the sternum. The phenotype of this line resembles that obtained by the targeted loss-of-function mutation of Hox3.1 (Le Mouellic et al., 1992). In transient assays, the human Hox3.3 transgene leads to the formation of additional ribs in more posterior vertebrae as well.

Homeotic protein binding sites, origins of replication, and nuclear matrix anchorage sites share the ATTA and ATTTA motifs

Nuclear matrix organizes the mammalian chromatin into loops. This is achieved by binding of nuclear matrix proteins to characteristic DNA landmarks in introns as well as proximal and distal sites flanking the 5' and 3' ends of genes. Matrix anchorage sites (MARs), origins of replication (ORIs), and homeotic protein binding sites share common DNA sequence motifs. In particular, the ATTA and ATTTA motifs, which constitute the core elements recognized by the homeobox domain from species as divergent as flies and humans, are frequently occurring in the matrix attachment sites of several genes. The human apolipoprotein B 3' MAR and a stretch of the Chinese hamster DHFR gene intron and human HPRT gene intron shown to anchor these genes to the nuclear matrix are mosaics of ATTA and ATTTA motifs. Several origins of replication also share these elements. This observation suggests that homeotic proteins which control the expression level of many genes and pattern formation during development are components of the nuclear matrix. Thus, the nuclear matrix, known as the site of DNA replication, might sculpture the crossroads of the differential activation of origins during development and S-phase and the control of gene expression and pattern formation in embryogenesis.

Cell-surface changes induced by ectopic expression of the murine homeo☐ gene Hox-3.3

Murine homeobox-containing genes (Hox genes) are postulated as playing key roles in the establishment of the anterior-posterior embryonic body axis, possibly providing cells with positional cues. Little is known, however, concerning how cells might respond to homeobox gene expression to interpret these cues. Since changes in the cell-surface are central to many processes in early development we reasoned that cells expressing different complements of Hox genes might have different surface properties. In order to investigate this we have used the sensitive, non-disruptive technique of multiple two-phase aqueous partition, which is able to detect small differences on the surface of intact cells. Using this technique we have found that ectopic expression of the murine Hox-3.3 gene in cultured cells induces reproducible changes in the cell surface. Changes only occurred above a threshold level of gene expression, but above this level a correlation between surface change and gene expression was seen. The implications for the establishment of a 'Hox' code of homeobox genes acting to specifically change cell-surface properties are discussed.

DNA-binding activity of the murine homeodomain protein Hox-2.3 produced by a hybrid phage T7/vaccinia virus system

Homeobox-containing genes encode transcription factors that, via the homeodomain, bind specifically to DNA. To study the DNA-binding properties of the murine homeodomain-containing protein, Hox-2.3, a hybrid expression system was used, combining gene expression by recombinant vaccinia virus (reVV) with bacteriophage T7 transcription. Expression was achieved by co-infecting HeLa cells with two reVVs, one expressing the T7-RNA polymerase-encoding gene directed by the VV promoter, P7.5, and another containing the Hox-2.3 coding sequence under control of a T7 promoter [Fuerst et al., Mol. Cell. Biol. 7 (1987) 2538-2544]. Co-infected HeLa cells produced large amounts of full-length Hox-2.3 protein. Cytoplasmic and nuclear extracts from these cells were used to examine DNA-binding specificity in vitro. reVV-produced Hox-2.3 protein bound to oligos that contained one or several copies of the common homeodomain-binding site, 5'-TCA-ATTAAAT, and to a lesser extent to multiple (TAA) repeats. Using Southwestern blot analysis, no Hox-2.3-binding sites were detected in a region of the Hox-2 cluster containing the Hox-2.3, Hox-2.4 and Hox-2.5 genes.

The homeodomain: a new face for the helix-turn-helix?

The discovery of conserved protein domains found in many Drosophila and mammalian developmental gene products suggests that fundamental developmental processes are conserved throughout evolution. Our understanding of development has been enhanced by the discovery of the widespread role of the homeodomain (HD). The action of HD-containing proteins as transcriptional regulators is mediated through a helix-turn-helix motif which confers sequence specific DNA binding. Unexpectedly, the well conserved structural homology between the HD and the prokaryotic helix-turn-helix proteins contrasts with their divergent types of physical interaction with DNA. A C-terminal extension of the HD recognition helix has assumed the role that the N-terminus of the prokaryotic helix plays for specification of DNA binding preference. However, the HD appears also capable of recognizing DNA in an alternative way and its specificity in vivo may be modified by regions outside the helix-turn-helix motif. We propose that this intrinsic complexity of the HD, as well as its frequent association with other DNA binding domains, explains the functional specificity achieved by genes encoding highly related HDs.

DVR-4 (bone morphogenetic protein-4) as a posterior-ventralizing factor in Xenopus mesoderm induction

Establishment of mesodermal tissues in the amphibian body involves a series of inductive interactions probably elicited by a variety of peptide growth factors. Results reported here suggest that mesodermal patterning involves an array of signalling molecules including DVR-4, a TGF-beta-like molecule. We show that ectopic expression of DVR-4 causes embryos to develop with an overall posterior and/or ventral character, and that DVR-4 induces ventral types of mesoderm in animal cap explants. Moreover, DVR-4 overrides the dorsalizing effects of activin. DVR-4 is therefore the first molecule reported both to induce posteroventral mesoderm and to counteract dorsalizing signals such as activin. Possible interactions between these molecules resulting in establishment of the embryonic body plan are discussed.

Retinoic acid induces changes in the localization of homeobox proteins in the antero-posterior axis of Xenopus laevis embryos

We have studied the localization of the proteins of Xeb1 and Xeb2, two homeobox (hbx)-containing genes that are expressed during the early development of Xenopus laevis. Both proteins are expressed in juxtaposed and partially overlapping domains along the antero-posterior axis of Xenopus laevis embryos, with clearly defined anterior boundaries. Xeb2 is predominantly expressed in the caudal region of the hindbrain, whereas the Xeb1 protein is located in the most rostral region of the spinal cord. Furthermore, both proteins are expressed in single cells dispersed in the lateral flanks of the embryo in positions that correlate with the expression domains in the neural tube. We suggest that these cells are migratory neural crest cells that have acquired positional information in the neural tube prior to migration. The Xeb2 protein was also detected in the most posterior branchial arches and the pronephros. In stage 45 embryos, nuclei of the IX-X cranial ganglia, the lung buds and cells spreading into the forelimb rudiment express the Xeb2 antigen. The Xeb1 protein was also detected in the lung buds and the forelimb rudiment. To examine the effect of retinoic acid on expression, gastrula embryos were treated with all-trans retinoic acid (RA). Increasing concentrations of RA caused progressive truncation of anterior structures. The most severely affected embryos lacked eyes, nasal pits, forebrain, midbrain and otic vesicles, and the anterior boundary of the hindbrain seemed to be displaced rostrally. This alteration correlates with a progressive displacement of the anterior boundary of the expression domain of Xeb2. On the other hand, 10(-6) M RA induces an ectopic site of Xeb1 expression at the anterior end of the central nervous system, located just anterior to the extended domain of Xeb2 whereas expression in the spinal cord remains unaffected.

Expression and modification of Hox 2.1 protein in mouse embryos

A polyclonal antibody, alpha Hox 2.1a, has been generated and used to immunolocalize Hox 2.1 protein in mouse embryos. Protein is present in nuclei of all tissues previously shown to express Hox 2.1 RNA. In addition, protein is seen in somites and proximal regions of the limb buds, tissues in which Hox 2.1 RNA expression was not clearly detected previously by in situ hybridization. At the 7 somite stage, protein is detectable in the neural tube up to the level of somite 1, but later retracts to a more posterior position. Immunoblot, in vitro translation, and immunoprecipitation experiments were carried out to characterize the Hox 2.1 protein. The results show that the Hox 2.1 gene produces at least two related phosphorylated proteins present in different proportions in different tissues.

Specific activation of mammalian Hox promoters in mosaic transgenic zebrafish

Homeo box-containing genes (Hox) are expressed in restricted regions of vertebrate embryos and may specify positional information. The organization and expression patterns of these genes are highly conserved among different species, suggesting that their regulation may also have been conserved. We developed a transient expression system, using mosaically transgenic zebrafish, which allows rapid analysis of transgene expression, and examined the activities of two mammalian Hox genes, mouse Hox-1.1 and human HOX-3.3. We found that these Hox promoters are activated in specific regions and tissues of developing zebrafish embryos and that this specificity depends upon the same regulatory elements within the promoters that specify the spatial expression of these genes in mice. Our results suggest that the promoter activities have been remarkably conserved from fish to mammals. To study the regulation of Hox expression in the developing nervous system, we analyzed the promoter activities in spt-1 mutants that have a mesodermal deficiency. Our results suggest that interactions, probably with the paraxial mesoderm, differentially regulate the activities of Hox promoters in the developing nervous system.

Cell type dependent transcription regulation by chick homeodomain proteins

Five chick homeodomain proteins (CHOXs), CHOX-1.7, -1.1, -1.4, -4.2 and -2.6, had different transcription-regulating activities in a chick cultured cell line, LMH. In particular, CHOX-1.7 highly activated transcription when NP6 was used as the target site whereas CHOX-1.4 did not. This was mainly due to differences in the activation domains since both proteins bound to NP with almost the same affinities in vitro. In LMH cells, they competitively acted on target gene transcription. Moreover, the strength of the CHOX-1.4 activation domain depended on the cell type. These findings suggest that the effect on a target gene is determined by a combination of CHOXs and cell types.

Activation of a C. elegans Antennapedia homologue in migrating cells controls their direction of migration

Anterior-posterior patterning in insects, vertebrates and nematodes involves members of conserved Antennapedia-class homeobox gene clusters (HOM-C) that are thought to give specific body regions their identities. The effects of these genes on region-specific body structures have been described extensively, particularly in Drosophila, but little is known about how HOM-C genes affect the behaviours of cells that migrate into their domains of function. In Caenorhabditis elegans, the Antennapedia-like HOM-C gene mab-5 not only specifies postembryonic fates of cells in a posterior body region, but also influences the migration of mesodermal and neural cells that move through this region. Here we show that as one neuroblast migrates into this posterior region, it switches on mab-5 gene expression; mab-5 then acts as a developmental switch to control the migratory behaviour of the neuroblast descendants. HOM-C genes can therefore not only direct region-specific patterns of cell division and differentiation, but can also act within migrating cells to programme region-specific migratory behaviour.

Molecular nature of Spemann's organizer: the role of the Xenopus homeobox gene goosecoid

This study analyzes the function of the homeobox gene goosecoid in Xenopus development. First, we find that goosecoid mRNA distribution closely mimics the expected localization of organizer tissue in normal embryos as well as in those treated with LiCl and UV light. Second, goosecoid mRNA accumulation is induced by activin, even in the absence of protein synthesis. It is not affected by bFGF and is repressed by retinoic acid. Lastly, microinjection of goosecoid mRNA into the ventral side of Xenopus embryos, where goosecoid is normally absent, leads to the formation of an additional complete body axis, including head structures and abundant notochordal tissue. The results suggest that the goosecoid homeodomain protein plays a central role in executing Spemann's organizer phenomenon.

Vertebrate homeobox genes

Recent highlights in vertebrate homeobox gene research include the discovery of new genes with novel expression patterns, observations that peptide growth factors and retinoic acid influence homeobox gene expression, and the generation of mutant phenotypes of embryos homozygous for null mutations. These combined studies reinforce the idea that homeobox genes function near the top of the gene hierarchies controlling vertebrate embryogenesis.

A dominant negative form of transcription activator mTFE3 created by differential splicing

Transcription factor E3 (mTFE3) is a murine transcription activator that binds to the intronic enhancer of the immunoglobulin heavy chain gene. A naturally occurring splice product of mTFE3 messenger RNA (mRNA) lacked 105 nucleotides that encode an activation domain; both absolute and relative amounts of long and truncated mRNAs varied in different tissues. Cells were cotransfected with complementary DNAs that encoded the two mRNA forms in amounts that corresponded to the amounts of each mRNA found in different cells. Small changes in substoichiometric amounts of the truncated form of mRNA effected trans-dominant negative modulation of mTFE3 activity. These findings identify a function for differential splicing in the regulation of transcription factor activity.

HOX gene activation by retinoic acid

Vertebrate homeobox genes of the Hox family are, like Drosophila homeotic genes, organized in gene clusters and show a strict correspondence, or collinearity, between the order of the genes (3' to 5') within the chromosomal cluster and that of their expression domains (anterior to posterior) in the embryo. Recent data obtained from embryonal carcinoma cells induced to differentiate by retinoic acid cast some light on the molecular mechanisms underlying the collinear expression of the Hox genes.

Characterisation of the murine Hox-3.3 gene and its promoter

The murine Hox-3.3 homeobox containing gene and its Xenopus homologue (XlHbox 1) produce two embryonic transcripts from two distinct promoters located approximately 9 kb apart. In order to begin to characterise one of these promoter regions (PRII), we have sequenced 3 kb of DNA immediately upstream of the transcription start site of the PRII transcript and analyzed the sequence for sequences known to bind transcription factors. Within this region are located a number of sequences that match known cis-elements. We have analysed the ability of two of these sequences that match to the Drosophila hunchback and Antennapedia/fushi-tarazu consensus binding sequences to specifically bind proteins extracted from embryos and from adult tissues. Using gel retention assays with oligonucleotides derived from these sequences, we show that both sequences specifically bind proteins present in extracts of mouse embryos and some, but not all extracts of various adult tissues. Protein binding cannot, however, be correlated with the known spatial domains of Hox-3.3 expression, suggesting that binding to these sequences is not simply related to activation of Hox-3.3 expression. A two base pair change in the most conserved region of the hunchback-like binding sequence completely abolishes protein binding. The presence of these highly conserved cis-acting elements that are known to be involved in regulation of the hunchback, even-skipped and engrailed genes in Drosophila suggests that these sequences may also be involved in the regulation of expression of Hox-3.3 and furthermore that regulation may in part at least involve binding of hunchback-like proteins (i.e. zinc-finger proteins) and Antennapedia-like homeobox-containing proteins.

Organizer-specific homeobox genes in Xenopus laevis embryos

The dorsal blastopore lip of the early Xenopus laevis gastrula can organize a complete secondary body axis when transplanted to another embryo. A search for potential gene regulatory components specifically expressed in the organizer was undertaken that resulted in the identification of four types of complementary DNAs from homeobox-containing genes that fulfill this criterion. The most abundant of these encodes a DNA-binding specificity similar to that of the Drosophila melanogaster anterior morphogen bicoid. The other three are also homologous to developmentally significant Drosophila genes. These four genes may participate in the regulation of the developmental potential of the organizer.