Paralogous Hox genes: function and regulation

Dynamic interactions and the evolutionary genetics of dental patterning

The mammalian dentition is a segmental, or periodically arranged, organ system whose components are arrayed in specific number and in regionally differentiated locations along the linear axes of the jaws. This arrangement evolved from simpler dentitions comprised of many single-cusp teeth of relatively indeterminate number. The different types of mammalian teeth have subsequently evolved as largely independent units. The experimentally documented developmental autonomy of dental primordia shows that the basic dental pattern is established early in embryogenesis. An understanding of how genetic patterning processes may work must be consistent with the different modes of development, and partially independent evolution, of the upper and lower dentition in mammals. The periodic nature of the location, number, and morphological structure of teeth suggests that processes involving the quantitative interaction of diffusible signaling factors may be involved. Several extracellular signaling molecules and their interactions have been identified that may be responsible for locating teeth along the jaws and for the formation of the incisor field. Similarly, the wavelike expression of signaling factors within developing teeth suggests that dynamic interactions among those factors may be responsible for crown patterns. These factors seem to be similar among different tooth types, but the extent to which crown differences can be explained strictly in terms of variation in the parameters of interactions among the same genes, as opposed to tooth-type-specific combinatorial codes of gene expression, is not yet known. There is evidence that combinatorial expression of intracellular transcription factors, including homeobox gene families, may establish domains within the jaws in which different tooth types are able to develop. An evolutionary perspective can be important for our understanding of dental patterning and the designing of appropriate experimental approaches, but dental patterns also raise basic unresolved questions about the nature of the evolutionary assumptions made in developmental genetics.

Sequence characterisation and expression of homeobox HOX A7 in the multi-potential erythroleukaemic cell line TF-1

Homeobox gene expression was examined in the erythroleukaemic cell line TF-1. Expression of a number of HOX A, B and C genes, including HOX A7 was detected. Expression of this gene has not previously been reported in erythroleukaemic cell lines. A 2.1 kb full length cDNA of the HOX A7 gene was cloned. The predicted amino acid sequence C-terminal to the homeodomain consists of an alanine-rich region and a strongly negatively charged domain consisting entirely of aspartic and glutamic acid residues.

How to build a vertebrate hindbrain. Lessons from genetics

During vertebrate embryogenesis, the hindbrain is the site of a segmentation process which leads to the formation, along the anterior-posterior axis, of 7-8 metameres called rhombomeres. This phenomenon plays an essential role in early hindbrain regionalisation and in the specification of the pattern of developing structures in this region of the brain. Data accumulated during the last 10 years have also shown that rhombomeres are units of gene expression and of cell lineage. Hence, a number of regulatory genes are expressed according to segment-specific patterns in the hindbrain and have been implicated in the pattern formation process. In this review, we focus on the analysis of the function and regulation of these genes along the different steps of hindbrain segmentation, from segment delimitation to acquisition of positional identity. On this basis, we propose a model for the control of early hindbrain development.

Analysis of Hox gene expression during early avian heart development

The anteroposterior (A-P) patterning of the developing heart underlies atrial and ventricular lineage specification and heart chamber morphogenesis. The posteriorization of cardiomyogenic phenotype with retinoic acid (RA) treatment of primitive streak stage chicken embryos is suggestive of a role for the clustered homeobox (Hox) genes in early heart patterning (Yutzey et al. [1994] Development 120:871-873; [1995] Dev. Biol. 170:531-541). A screen for Hox genes expressed in chick heart primordia and primitive heart led to the isolation of anterior genes of the Hox clusters expressed during cardiogenesis. Specific hoxd-3, hoxa-4, and hoxd-4 transcripts were detected at the early stages of heart formation and full-length cDNA clones were isolated. Expression of hoxd-3 was detected in the heart forming region of embryos prior to heart tube formation. Expression of hoxa-4, hoxd-3, and hoxb-5 was increased in cardiogenic tissue treated with RA in culture conditions that also produced changes in positionally restricted cardiomyogenic phenotypes. Hox genes expressed in cardiac explants exhibited distinct sensitivities to RA and ouabain treatment when compared to genes, such as nkx-2.5, that are involved in cardiac commitment and differentiation. These studies support a role for Hox genes in early heart patterning and suggest that positional information in the cardiogenic region is established by regulatory mechanisms distinct from early heart lineage specification.

Hox proteins meet more partners

The Hox genes are clustered sets of homeobox-containing genes that play a central role in animal development. Recent genetic and molecular data suggest that Hox proteins interact with pre-existing homeodomain protein complexes. These complexes may help to regulate Hox activity and Hox specificity, and help cells to interpret signaling cascades during development.

Mouse Alx3: an aristaless-like homeobox gene expressed during embryogenesis in ectomesenchyme and lateral plate mesoderm

Mouse Alx3 is a homeobox gene that is related to the Drosophila aristaless gene and to a group of vertebrate genes including Prx1, Prx2, Cart1, and Alx4. The protein encoded contains a diverged variant of a conserved peptide sequence present near the carboxyl terminus of at least 15 different paired-class-homeodomain proteins. Alx3 is expressed in mouse embryos from 8 days of gestation onward in a characteristic pattern, predominantly in neural crest-derived mesenchyme and in lateral plate mesoderm. We detected prominent expression in frontonasal head mesenchyme and in the first and second pharyngeal arches and some of their derivatives. High expression was also seen in the tail and in many derivatives of the lateral plate mesoderm including the limbs, the body wall, and the genital tubercle. aristaless-related genes like Alx3, Cart1, and Prx2 are expressed in overlapping proximodistal patterns in the pharyngeal arches. Similar, but more lateral patterns have been described for the Distal-less-related (Dlx) genes. Intriguingly, expression and to some extent function of aristaless and Distal-less in Drosophila also have overlapping as well as complementary aspects. Alx3 was localized to chromosome 3, near the droopy-ear (de) mutation.

Expression of the murine Hoxa4 gene requires both autoregulation and a conserved retinoic acid response element

Analysis of the regulatory regions of the Hox genes has revealed a complex array of positive and negative cis-acting elements that control the spatial and temporal pattern of expression of these genes during embryogenesis. In this study we show that normal expression of the murine Hoxa4 gene during development requires both autoregulatory and retinoic acid-dependent modes of regulation. When introduced into a Hoxa4 null background, expression of a lacZ reporter gene driven by the Hoxa4 regulatory region (Hoxa4/lacZ) is either abolished or significantly reduced in all tissues at E10. 5-E12.5. Thus, the observed autoregulation of the Drosophila Deformed gene is conserved in a mouse homolog in vivo, and is reflected in a widespread requirement for positive feedback to maintain Hoxa4 expression. We also identify three potential retinoic acid response elements in the Hoxa4 5′ flanking region, one of which is identical to a well-characterized element flanking the Hoxd4 gene. Administration of retinoic acid to Hoxa4/lacZ transgenic embryos resulted in stage-dependent ectopic expression of the reporter gene in the neural tube and hindbrain. When administered to Hoxa4 null embryos, however, persistent ectopic expression was not observed, suggesting that autoregulation is required for maintenance of the retinoic acid-induced expression. Finally, mutation of the consensus retinoic acid response element eliminated the response of the reporter gene to exogenous retinoic acid, and abolished all embryonic expression in untreated embryos, with the exception of the neural tube and prevertebrae. These data add to the evidence that Hox gene expression is regulated, in part, by endogenous retinoids and autoregulatory loops.

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.

Homeotic gene action in embryonic brain development of Drosophila

Studies in vertebrates show that homeotic genes are involved in axial patterning and in specifying segmental identity of the embryonic hindbrain and spinal cord. To gain further insights into homeotic gene action during CNS development, we here characterize the role of the homeotic genes in embryonic brain development of Drosophila. We first use neuroanatomical techniques to map the entire anteroposterior order of homeotic gene expression in the Drosophila CNS, and demonstrate that this order is virtually identical in the CNS of Drosophila and mammals. We then carry out a genetic analysis of the labial gene in embryonic brain development. Our analysis shows that loss-of-function mutation and ubiquitous overexpression of labial results in ectopic expression of neighboring regulatory genes. Furthermore, this analysis demonstrates that mutational inactivation of labial results in regionalized axonal patterning defects which are due to both cell-autonomous and cell-nonautonomous effects. Thus, in the absence of labial, mutant cells are generated and positioned correctly in the brain, but these cells do not extend axons. Additionally, extending axons of neighboring wild-type neurons stop at the mutant domains or project ectopically, and defective commissural and longitudinal pathways result. Immunocytochemical analysis demonstrates that cells in the mutant domains do not express neuronal markers, indicating a complete lack of neuronal identity. An alternative glial identity is not adopted by these mutant cells. Comparable effects are seen in Deformed mutants but not in other homeotic gene mutants. Our findings demonstrate that the action of the homeotic genes labial and Deformed are required for neuronal differentiation in the developing brain of Drosophila.

Genetic interactions between Hoxa1 and Hoxb1 reveal new roles in regulation of early hindbrain patterning

In the developing vertebrate hindbrain Hoxa1 and Hoxb1 play important roles in patterning segmental units (rhombomeres). In this study, genetic analysis of double mutants demonstrates that both Hoxa1 and Hoxb1 participate in the establishment and maintenance of Hoxb1 expression in rhombomere 4 through auto- and para-regulatory interactions. The generation of a targeted mutation in a Hoxb1 3′ retinoic acid response element (RARE) shows that it is required for establishing early high levels of Hoxb1 expression in neural ectoderm. Double mutant analysis with this Hoxb1(3′RARE) allele and other targeted loss-of-function alleles from both Hoxa1 and Hoxb1 reveals synergy between these genes. In the absence of both genes, a territory appears in the region of r4, but the earliest r4 marker, the Eph tyrosine kinase receptor EphA2, fails to be activated. This suggests a failure to initiate rather than maintain the specification of r4 identity and defines new roles for both Hoxb1 and Hoxa1 in early patterning events in r4. Our genetic analysis shows that individual members of the vertebrate labial-related genes have multiple roles in different steps governing segmental processes in the developing hindbrain.

Inhibition of retinoic acid receptor-mediated signalling alters positional identity in the developing hindbrain

Retinoids regulate gene expression via nuclear retinoic acid receptors, the RARs and RXRs. To investigate the functions of retinoid receptors during early neural development, we expressed a dominant negative RARbeta in early Xenopus embryos. We obtained evidence that dominant negative RARbeta specifically inhibits RAR/RXR heterodimer-mediated, but not RXR homodimer-mediated, transactivation. Both all-trans- and 9-cis-RA-induced teratogenesis were, however, efficiently opposed by ectopic expression of dominant negative RARbeta, indicating that only RAR/RXR transactivation is required for retinoid teratogenesis by each of these ligands. Experiments with two RXR-selective ligands confirmed that activation of RXR homodimers does not cause retinoid teratogenesis. Dominant negative RARbeta thus specifically interferes with the retinoid signalling pathway that is responsible for retinoid teratogenesis. Dominant negative RARbeta-expressing embryos had a specific developmental phenotype leading to disorganization of the hindbrain. Mauthner cell multiplications in the posterior hindbrain, and (both anteriorly and posteriorly) expanded Krox-20 expression domains indicated (partial) transformation of a large part of the hindbrain into (at least partial) rhombomere 3, 4 and/or 5 identity. In contrast, the fore- and midbrain and spinal cord appeared to be less affected. These data indicate that RARs play a role in patterning the hindbrain.

An evolutionary conserved element is essential for somite and adjacent mesenchymal expression of the Hoxa1 gene

The murine Hoxa1 gene is a member of the vertebrate Hox complex and plays a role in defining the body plan during development. At day 8.0-9.0 post coitus, Hoxa1 transcripts are detected extensively throughout the embryo in the neural tube, adjacent mesenchyme, paraxial mesoderm, somites and gut epithelium; expression extends from the most caudal region of the embryo to the rhombomere 3/4 border. This spatiotemporal expression of Hoxa1 mRNA is critical for normal embryonic development. We have previously identified a 10 bp element, called CE2, which is located approximately 3 kilobases 3' of the Hoxa1 coding region in the RAIDR5 enhancer, and which binds to an approximately 170 kd protein in retinoic acid treated P19 embryonal carcinoma cells. CE2 elements were also identified 3' of the murine Hoxb1 gene, the chicken Hoxb1 gene and the human Hoxa1 gene. To examine the role of this CE2 element in regulating Hoxa1 expression in vivo, transgenic mice were generated which express a Hoxa1 beta-galactosidase reporter gene that contains a mutation in the CE2 element. Relative to transgenic mice bearing a wild type CE2 element, the mutant CE2 construct recapitulated rhombomeric, neural, and gut epithelium expression but failed to show beta-galactosidase expression in somites and adjacent mesenchymal tissue. Gel shift analysis showed that binding activity similar to that detected in extracts prepared from retinoic acid treated P19 cells was present in nuclear extracts prepared from day 9.0 embryos. However, an additional binding complex not detected in P19 cells was also observed. These results indicate that in transgenic animals, the evolutionary conserved CE2 element is a somite and adjacent mesenchymal enhancer of Hoxa1 expression.

Oncogenic transcription factors in the human acute leukemias

Chromosomal translocations in the human acute leukemias rearrange the regulatory and coding regions of a variety of transcription factor genes. The resultant protein products can interfere with regulatory cascades that control the growth, differentiation, and survival of normal blood cell precursors. Support for this interpretation comes from the results of gene manipulation studies in mice, as well as the sequence homology of oncogenic transcription factors with proteins known to regulate embryonic development in primitive organisms, including the nematode Caenorhabditis elegans and the fruit fly Drosophila melanogaster. Many of these genetic alterations have important prognostic implications that can guide the selection of therapy. The insights gained from studies of translocation-generated oncogenes and their protein products should hasten the development of highly specific, and hence less toxic, forms of leukemia therapy.

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.

Exploring the role of homeobox and zinc finger proteins in pancreatic cell proliferation, differentiation, and apoptosis

Transcription factors are DNA binding proteins that regulate gene expression in response to a large variety of extracellular stimuli, and thereby act as key molecular switches for controlling cell differentiation, proliferation, and apoptosis. During the last decade, a myriad of these proteins have been identified and classified into different structural families, including homeobox, zinc finger, leucine zipper, and helix-loop-helix transcription factors. Members of the homeobox and zinc finger superfamilies are among the best-characterized transcription factors known to act as potent regulators of normal development in organisms ranging from insects to humans. In addition, mutations or aberrant expression in genes encoding these proteins can result in neoplastic transformation in several different cell types, further supporting their role as "guardians" of normal cell growth and differentiation. Therefore, the purpose of this article is to review this field of research with a particular emphasis on the role of homeobox- and zinc finger-containing transcription factors in pancreatic cell growth, cell differentiation, and apoptosis. The potential participation of these proteins in neoplastic transformation is also discussed.

The chicken caudal genes establish an anterior-posterior gradient by partially overlapping temporal and spatial patterns of expression

The caudal genes in vertebrates as in invertebrates assume a posterior position along the anterior-posterior axis and they appear to regulate the expression of the Hox genes. The third chicken caudal gene, Cdx-C, was cloned. Extensive comparisons of the sequence of this protein to the other known members of this homeobox family has lead to the suggestion that vertebrate genomes contain three members of the caudal homeobox family. A comparative study of the chicken Cdx-A and Cdx-C genes during gastrulation and neurulation revealed the differences between the genes. The caudal genes exhibit sequential activation in the newly formed neural plate and sequential extinction in axial midline structures during the primitive streak regression along the anterior-posterior axis. This pattern of expression suggests that the number and identity of caudal genes expressed along the anterior-posterior axis changes dynamically.