Gradients and forward spreading of vertebrate Hox gene expression detected by using a Hox/lacZ transgene.

Elucidation of the kinetics with which vertebrate Hox expression patterns develop may help us to choose between various models already proposed to explain this process. The chick Hoxa-7/lacZ transgene, expressed in mouse embryos, changes over time in the distribution of its activity along the developing posterior to anterior axis. During an establishment (E) phase (lasting at least up to 10 days) expression is graded from highest levels posteriorly, to low levels anteriorly. Within the graded domain, the overall level of expression spreads forward with time along both neurectoderm and paraxial mesoderm. Spreading in expression is not due to movement of cells, but to increases in both the proportion of lacZ expressing cells and the intensity of expression per cell. By 10.8 days, embryos have reached a late (L) phase in which an anterior up-regulation in expression, together with a posterior down-regulation, cause the graded nature of the expression to be lost. E and L phases are also seen for Hox gene expression detected by in situ hybridization. The switch from E to L occurs at progressively later times as we move 3' to 5' along the Hox cluster. The results are in keeping with models in which Hox genes become differentially expressed according to a graded concentration of an inducer. Binding motifs for the caudal (cdx) proteins, already proposed as such inducers, are conserved in mouse and chick Hoxa-7 enhancer elements.

Evolutionary shifts of vertebrate structures and Hox expression up and down the axial series of segments: a consideration of possible mechanisms.

The term 'transposition' describes how, during vertebrate evolution, anatomical structures have shifted up or down the axial series of segments. For example, the neck/thorax junction and the position of the forelimb in the chicken have shifted posteriorly, relative to mouse, by a distance of seven somites or vertebrae. By examining the expression boundaries of some chick Hox genes not previously described, we provide new evidence that axial shifts in anatomical structures correspond with shifts in Hox expression domains. These shifts occur both in mesodermal components (somites, vertebrae, and lateral plate mesoderm) and neural components (spinal ganglia). We discuss morphogen gradient, timing, spreading, and growth models for the setting of Hoxexpression boundaries, and consider how evolutionary shifts in boundary positions might have been effected in terms of these models.

Evidence that Hoxa expression domains are evolutionarily transposed in spinal ganglia, and are established by forward spreading in paraxial mesoderm.

Transposition of anatomical structures along the anteroposterior axis has been a commonly used mechanism for changing body proportions during the course of evolutionary time. Earlier work (Gaunt, S.J., 1994. Conservation in the Hox code during morphological evolution. Int. J. Dev. Biol. 38, 549-552; Burke, A.C., Nelson, C.E., Morgan, B.A., Tabin, C., 1995. Hox genes and the evolution of vertebrate axial morphology. Development 121, 333-346) showed how transposition in mesodermal derivatives (vertebrae) could be attributed to transposition in the expression of Hox genes along the axial series of somites. We now show how transposition in the segmental arrangement of the spinal nerves can also be correlated with shifts in the expression domains of Hox genes. Specifically, we show how the expression domains of Hoxa-7, a-9 and a-10 in spinal ganglia correspond similarly in both mouse and chick with the positions of the brachial and lumbosacral plexuses, and that this is true even though the brachial plexus of chick is shifted posteriorly, relative to mouse, by seven segmental units. In spite of these marked species differences in the boundaries of Hoxa-7 expression, cis regulatory elements located up to 5 kb upstream of the chick Hoxa-7 gene showed much functional and structural conservation with those described in the mouse (Puschel, A.W., Balling, R., Gruss, P., 1991. Separate elements cause lineage restriction and specify boundaries of Hox-1.1 expression. Development 112, 279-287; Knittel, T., Kessel, M., Kim, M.H., Gruss, P., 1995. A conserved enhancer of the human and murine Hoxa-7 gene specifies the anterior boundary of expression during embryonal development. Development 121, 1077-1088). We also show that chick Hoxa-7 and a-10 expression domains spread forward into regions of somites that are initially negative for the expression of these genes. We discuss this as evidence that Hox expression in paraxial mesoderm spreads forward, as earlier found for neurectoderm and lateral plate mesoderm, in a process that occurs independently of cell movement.

Altered cellular proliferation and mesoderm patterning in Polycomb-M33-deficient mice.

In Drosophila, the trithorax-group and the Polycomb-group genes are necessary to maintain the expression of the homeobox genes in the appropriate segments. Loss-of-function mutations in those groups of genes lead to misexpression of the homeotic genes resulting in segmental homeotic transformations. Recently, mouse homologues of the Polycomb-group genes were identified including M33, the murine counterpart of Polycomb. In this report, M33 was targeted in mice by homologous recombination in embryonic stem (ES) cells to assess its function during development. Homozygous M33 (-/-) mice show greatly retarded growth, homeotic transformations of the axial skeleton, sternal and limb malformations and a failure to expand in vitro of several cell types including lymphocytes and fibroblasts. In addition, M33 null mutant mice show an aggravation of the skeletal malformations when treated to RA at embryonic day 7.5, leading to the hypothesis that, during development, the M33 gene might play a role in defining access to retinoic acid response elements localised in the regulatory regions of several Hox genes.

Temporal colinearity in expression of anterior Hox genes in developing chick embryos.

Temporal colinearity describes a correspondence between the spatial ordering of Hox genes within their clusters (in the direction 3' to 5') and the time of their first expression (earlier to later) during embryonic development (Izpisúa-Belmonte et al. [1991] EMBO J. 10:2279-2289). It suggests that activation of each Hox gene might be controlled in some way by its position within the cluster. So far, in situ hybridization experiments on vertebrate embryos have provided clear evidence of temporal colinearity only for "posterior" Hox genes (5' located, AbdB related). We now describe a search in the chick embryo for evidence of temporal colinearity in the expression of some anterior Hox genes (Hoxb-1, b-3, b-4, b-6, and a-9). Clear evidence for temporal colinearity was seen in neural tube tissue adjacent to the first few somites. Here, there were delays in the expression of Hoxb-3 following b-1, Hoxb-4 following b-3, and Hoxb-6 following b-4. Temporal colinearity was also detected in anterior primitive streak tissue. Hox gene expression reached both the neural tube and the anterior streak following forward spreading from posteriormost parts of the primitive streak. Overall, therefore, temporal colinearity was seen as sequential waves of Hox genes expression that proceeded forward (3' genes before 5' genes) along the developing chick embryo. Within posterior primitive streak tissue, there was only limited evidence for temporal colinearity. We discuss these results in terms of possible models for the establishment of Hox gene expression patterns.

Assignment of a Polycomb-like chromobox gene (CBX2) to human chromosome 17q25.

A human clone corresponding to the homologue of the murine Polycomb-like gene M33 has been used to map this gene (CBX2) to human chromosomes. Both somatic cell hybrid panels and FISH on metaphase chromosomes have been used. These techniques gave a consistent localization, at the tip of the long arm of chromosome 17 (17q25). This localization, as well as the potential role of a mammalian Polycomb-like protein, suggests a potential involvement in two different pathologies: the campomelic syndrome, an inherited disorder, and neoplastic disorders linked to allele loss already described in this region.

Conservation in the Hox code during morphological evolution.

The expression domains in paraxial mesoderm of the chicken embryo are described for Hoxb-3, a-4 and c-6 genes, and these are compared with published expression data for the corresponding genes in the mouse. In both species, it is found that the anterior limits of Hoxb-3 and a-4 expression lie in the upper cervical region, and the anterior limits of Hoxc-6 expression lie in the upper thoracic region. This finding is remarkable because the cervical region, or neck, of the chicken (with fourteen cervical vertebrae) is much longer than that of the mouse (seven cervical vertebrae). The results suggest that the Hox code, at least in the development of homologous axial structures, is conserved between species (Hoxb-3 and a-4, for example, being associated with an anterior cervical phenotype; Hoxc-6 being associated with an anterior thoracic phenotype). The results also suggest that an evolutionary change in body proportions is accomplished by a shift in the relative positions of Hox expression domains during embryonic development.

Forward spreading in the establishment of a vertebrate Hox expression boundary: the expression domain separates into anterior and posterior zones, and the spread occurs across implanted glass barriers.

By use of wholemount in situ hybridization, we show how expression of the chicken homeobox gene Hoxd-4 commences in the posterior part of the primitive streak and then spreads forward, covering most of the primitive streak by the 2 somite stage, covering the entire primitive streak by the 5 somite stage, reaching the somite 1/somite 2 level of the neural tube by the 9 somite stage, and reaching the rhombomere 6/rhombomere 7 junction of the hindbrain by the 15 somite stage. Forward spreading does not depend upon cell migration, as was evidenced by vital dye (DiI) cell marking experiments. Furthermore, forward spreading does not apparently require tissue continuity since it could not be blocked by impermeable (glass) barriers surgically implanted to divide embryonic tissues. As forward spreading of chick Hoxd-4 proceeds, the domain of expression separates, at late primitive streak stages, into "anterior" and "posterior zones," with an intervening "intermediate zone" of weak or non-expression. Clear anterior and posterior zones were also found for Hoxa-3 and a-4 expression in late primitive streak stage mouse embryos. We present evidence that the anterior zone corresponds with the "definitive" domain of Hox gene expression, as has earlier been extensively characterized in midgestation embryos. The posterior zone is transitory, probably persisting only for the duration of the primitive streak, and it is a region of intense Hox expression in primitive streak tissue, Hensen's node, and adjacent regions of neurectoderm and mesoderm. We suggest that the posterior zone marks the source of a morphogen which is the primary activator of Hox gene expression, and we discuss possible models for the mechanism of forward spreading in expression.

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.

Expression of the mouse goosecoid gene during mid-embryogenesis may mark mesenchymal cell lineages in the developing head, limbs and body wall.

After an earlier, transient phase of expression in the developing primitive streak of 6.4- to 6.8-day mouse embryos, the homeobox gene goosecoid is now shown to be expressed in a later phase of mouse development, from 10.5 days onwards. The later, spatially restricted domains of goosecoid expression are detected in the head, limbs and ventrolateral body wall. At all sites, the domains of expression are first detected in undifferentiated tissue, and then expression persists as these tissues undergo subsequent morphogenesis. For example, goosecoid expression is noted in the first branchial arch at 10.5 days, and then expression persists as this tissue undergoes morphogenesis to form the lower jaw and the body of the tongue. Expression in tissues around the first branchial cleft persists as these undergo morphogenesis to form the base of the auditory meatus and eustachian tube. Expression in tissues around the newly formed nasal pits persists as these elongate to form the nasal chambers. Expression in the ventral epithelial lining of the otic vesicle persists as this eventually gives rise to the non-sensory epithelium of the cochlea. Expression in the proximal limb buds and ventrolateral body wall persists as these tissues undergo morphogenesis to form proximal limb structures and ventral ribs respectively. Our findings lead us to suggest that the goosecoid gene product plays a role in spatial programming within discrete embryonic fields, and possibly lineage compartments, during organogenesis stages of mouse development.

Sequence and embryonic expression of the murine Hox-3.5 gene.

The murine Hox-3.5 gene has been mapped and linked genomically to a position 18 kb 3' of its most 5' locus neighbour, Hox-3.4, on chromosome 15. The sequence of the Hox-3.5 cDNA, together with the position of the gene within the locus, show it to be a paralogue of Hox-2.6, Hox-1.4 and Hox-4.2. The patterns of embryonic expression for the Hox-3.5 gene are examined in terms of three rules, proposed to relate a Hox gene's expression pattern to its position within the locus. The anterior boundaries of Hox-3.5 expression in the hindbrain and prevertebral column lie anterior to those of Hox-3.4 and all other, more 5'-located Hox-3 genes. Within the hindbrain, the Hox-3.5 boundary is seen to lie posterior to that of its paralogue, Hox-2.6, by a distance equal to about the length of one rhombomere. Patterns of Hox-3.5 expression within the oesophagus and spinal cord, but not the testis, are similar to those of other Hox-3 genes, Hox-3.3 and Hox-3.4.

Gastrulation in the mouse: the role of the homeobox gene goosecoid.

Mouse goosecoid is a homeobox gene expressed briefly during early gastrulation. Its mRNA accumulates as a patch on the side of the epiblast at the site where the primitive streak is first formed. goosecoid-expressing cells are then found at the anterior end of the developing primitive streak, and finally in the anteriormost mesoderm at the tip of the early mouse gastrula, a region that gives rise to the head process. Treatment of early mouse embryos with activin results in goosecoid mRNA accumulation in the entire epiblast, suggesting that a localized signal induces goosecoid expression during development. Transplantation experiments indicate that the tip of the murine early gastrula is the equivalent of the organizer of the amphibian gastrula.

The mouse has a Polycomb-like chromobox gene.

The Drosophila gene Polycomb (Pc) has been implicated in the clonal inheritance of determined states and is a trans-regulator of the Antennapedia-like homeobox genes. Pc shares a region of homology (the chromobox) with the Drosophila gene Heterochromatin Protein 1 (HP1), a component of heterochromatin. The Pc chromobox has been used to isolate a mouse chromobox gene, M33, which encodes a predicted 519 amino acid protein. The M33 chromodomain is more similar to that in the Pc protein, than that in the HP1 protein. In addition to the chromodomain, the M33 and Pc proteins also share a region of homology at their C termini. The temporal and spatial expression patterns of M33 have been studied by in situ hybridization and northern analysis. During the final 10 days of embryonic development, M33 expression mirrors that of the cell-cycle-specific cyclin B gene. It is therefore suggested that the rate of cellular proliferation controls M33 expression. From comparisons of the characteristics of M33 with those of Pc it is proposed that M33 is a Pc-like chromobox gene. The roles of M33 and Pc in models of cellular memory are examined and implications of the memory models addressed.

Mapping of a mouse homolog of a heterochromatin protein gene the X chromosome.

Modifiers of position-effect-variegation in Drosophila are thought to encode proteins that are either structural components of heterochromatin or enzymes that modify these components. We have recently shown that a sequence motif found in one Drosophila modifier gene, Heterochromatin protein 1 (HP1), is conserved in a wide variety of animal and plant species (Singh et al. 1991). Using this motif, termed chromo box, we have cloned a mouse candidate modifier gene, M31, that also shows considerable sequence homology to Drosophila HP1. Here we report evidence of at least four independently segregating loci in the mouse homologous to the M31 cDNA. One of these loci--Cbx-rs1--maps to the X Chromosome (Chr), 1 cM proximal to Amg and outside the X-inactivation center region.

Expression patterns of mouse Hox genes: clues to an understanding of developmental and evolutionary strategies.

Expression patterns of Antennapedia-like homeogenes in the mouse embryo show many similarities o those of their homologues in Drosophila. It is argued here that homeogenes may regulate development of the body plan in mouse by mechanisms similar to those used in Drosophila. In particular, they may differentially specify positional address of cell groups within lineage compartments along the body axes. In vertebrates, a single ancestral homeogene cluster has become duplicated to give four separate clusters. Comparisons of homeogene expression patterns between different clusters of the mouse suggest ways in which duplication has permitted development of a more complex body plan. Cluster duplication may therefore have provided a selective advantage during vertebrate evolution.

A sequence motif found in a Drosophila heterochromatin protein is conserved in animals and plants.

Modifiers of position-effect-variegation in Drosophila encode proteins that are thought to modify chromatin, rendering it heritably changed in its expressibility. In an attempt to identify similar modifier genes in other species we have utilized a known sequence homology, termed chromo box, between a suppressor of position-effect-variegation, Heterochromatin protein 1 (HP1), and a repressor of homeotic genes, Polycomb (Pc). A PCR generated probe encompassing the HP1 chromo box was used to clone full-length murine cDNAs that contain conserved chromo box motifs. Sequence comparisons, in situ hybridization experiments, and RNA Northern blot analysis suggest that the murine and human sequences presented in this report are homologues of the Drosophila HP1 gene. Chromo box sequences can also be detected in other animal species, and in plants, predicting a strongly conserved structural role for the peptide encoded by this sequence. We propose that epigenetic (yet heritable) changes in gene expressibility, characteristic of chromosomal imprinting phenomena, can largely be explained by the action of such modifier genes. The evolutionary conservation of the chromo box motif now enables the isolation and study of putative modifier genes in those animal and plant species where chromosomal imprinting has been described.

Homeogene expression patterns and chromosomal imprinting.

In both mouse and Drosophila, Antennapedia-like homeobox-containing genes (homeogenes) display a strict correspondence between the order of genes (3' to 5') along the chromosome and the order of their expression domains (anterior to posterior) in the developing embryo. We show here how this, and other points of similarity, may indicate that both species use a common mechanism of chromosomal imprinting in order to retain cellular memory of homeogene expression patterns throughout embryonic development.

Mouse Hox-3.4: homeobox sequence and embryonic expression patterns compared with other members of the Hox gene network.

A putative mouse homeobox gene (Hox-3.4) was previously identified 4kb downstream of the Hox-3.3 (Hox-6.1)* gene (Sharpe et al. 1988). We have now sequenced the Hox-3.4 homeobox region. The predicted amino acid sequence shows highest degree of homology in the mouse with Hox-1.3 and -2.1. This, together with similarities in the genomic organisation around these three genes, suggests that they are comembers of a subfamily, derived from a common ancestor. Hox-3.4 appears to be a homologue of the Xenopus Xlhbox5 and human cp11 genes (Fritz and De Robertis, 1988; Simeone et al. 1988). Using a panel of mouse-hamster somatic cell hybrids we have mapped the Hox-3.4 gene to chromosome 15. From the results of in situ hybridization experiments, we describe the distribution of Hox-3.4 transcripts within the 12 1/2 day mouse embryo, and we compare this with the distributions of transcripts shown by seven other members of the Hox gene network. We note three consistencies that underlie the patterns of expression shown by Hox-3.4. First, the anterior limits of Hox-3.4 transcripts in the embryo are related to the position of the Hox-3.4 gene within the Hox-3 locus. Second, the anterior limits of Hox-3.4 expression within the central nervous system are similar to those shown by subfamily homologues Hox-2.1 and Hox-1.3, although the tissue-specific patterns of expression for these three genes show many differences. Third, the patterns of Hox-3.4 expression within the spinal cord and the testis are very similar to those shown by a neighbouring Hox-3 gene (Hox-3.3), but they are quite different from those shown by Hox-1 genes (Hox-1.2, -1.3 and -1.4).

Antigens on rat spermatozoa with a potential role in fertilization.

Eight monoclonal antibodies (McAbs), directed against antigens on rat cauda epididymal spermatozoa, were tested for their capacity to interfere with fertilization in vitro as a means of identifying molecules with a potential role in sperm-egg recognition and fusion. Antigens recognized by the McAbs were visualized on live spermatozoa by indirect immunofluorescence (IIF) and characterized by immunoblotting. Five McAbs (designated 1B5, 2C4, 4B5, 5B1, and 8C4) recognized antigens specifically on the sperm acrosome and three (designated 2B1, 2D6, and 6B2) bound to the flagellum. Of the eight McAbs investigated, three (2B1, 2C4, and 6B2) were effective in blocking fertilization in vitro when added as culture supernatants to mixtures of sperm and eggs. McAb 6B2 was inhibitory due to its ability to agglutinate spermatozoa. McAbs 2B1 and 2C4 did not agglutinate capacitated spermatozoa, had no observable effect on motility, and yet blocked fertilization in a dose-dependent manner. McAb 2C4 did not give a reaction on immunoblots, but the 2B1 antigen was identified as an Mr 40 kD glycoprotein. McAb 2B1 appeared to block fertilization at the level of zona binding, whereas the effects of 2C4 were directed more against zona penetration and/or fusion with the vitellus. When sperm-egg complexes were stained with 2C4 or 2B1 McAbs and viewed by IIF, all spermatozoa that were attached to the zona showed fluorescence on the head. These results suggest that different antigens on the rat sperm head participate in different aspects of the fertilization process and that during capacitation there is either exposure of these antigens or else they migrate to their site of action from the flagellum.