Patterning of Caenorhabditis elegans posterior structures by the Abdominal-B homolog, egl-5

The Caenorhabditis elegans body axis, like that of other animals, is patterned by the action of Hox genes. In order to examine the function of one C. elegans Hox gene in depth, we determined the postembryonic expression pattern of egl-5, the C. elegans member of the Abdominal-B Hox gene paralog group, by means of whole-mount staining with a polyclonal antibody. A major site of egl-5 expression and function is in the epithelium joining the posterior digestive tract with the external epidermis. Patterning this region and its derived structures is a conserved function of Abd-B paralog group genes in other animals. Cells that initiate egl-5 expression during embryogenesis are clustered around the presumptive anus. Expression is initiated postembryonically in four additional mesodermal and ectodermal cell lineages or tissues. Once initiated in a lineage, egl-5 expression continues throughout development, suggesting that the action of egl-5 can be regarded as defining a positional cell identity. A variety of cross-regulatory interactions between egl-5 and the next more anterior Hox gene, mab-5, help define the expression domains of their respective gene products. In its expression in a localized body region, function as a marker of positional cell identity, and interactions with another Hox gene, egl-5 resembles Hox genes of other animals. This suggests that C. elegans, in spite of its small cell number and reproducible cell lineages, may not differ greatly from other animals in the way it employs Hox genes for regional specification during development.     

Mice doubly deficient for the Polycomb Group genes Mel18 and Bmi1 reveal synergy and requirement for maintenance but not initiation of Hox gene expression

Polycomb group genes were identified as a conserved group of genes whose products are required in multimeric complexes to maintain spatially restricted expression of Hox cluster genes. Unlike in Drosophila, in mammals Polycomb group (PcG) genes are represented as highly related gene pairs, indicative of duplication during metazoan evolution. Mel18 and Bmi1 are mammalian homologs of Drosophila Posterior sex combs. Mice deficient for Mel18 or Bmi1 exhibit similar posterior transformations of the axial skeleton and display severe immune deficiency, suggesting that their gene products act on overlapping pathways/target genes. However unique phenotypes upon loss of either Mel18 or Bmi1 are also observed. We show using embryos doubly deficient for Mel18 and Bmi1 that Mel18 and Bmi1 act in synergy and in a dose-dependent and cell type-specific manner to repress Hox cluster genes and mediate cell survival of embryos during development. In addition, we demonstrate that Mel18 and Bmi1, although essential for maintenance of the appropriate expression domains of Hox cluster genes, are not required for the initial establishment of Hox gene expression. Furthermore, we show an unexpected requirement for Mel18 and Bmi1 gene products to maintain stable expression of Hox cluster genes in regions caudal to the prospective anterior expression boundaries during subsequent development.     

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.     

Acquisition of Hox codes during gastrulation and axial elongation in the mouse embryo

Early sequential expression of mouse Hox genes is essential for their later function. Analysis of the relationship between early Hox gene expression and the laying down of anterior to posterior structures during and after gastrulation is therefore crucial for understanding the ontogenesis of Hox-mediated axial patterning. Using explants from gastrulation stage embryos, we show that the ability to express 3' and 5' Hox genes develops sequentially in the primitive streak region, from posterior to anterior as the streak extends, about 12 hours earlier than overt Hox expression. The ability to express autonomously the earliest Hox gene, Hoxb1, is present in the posterior streak region at the onset of gastrulation, but not in the anterior region at this stage. However, the posterior region can induce Hoxb1 expression in these anterior region cells. We conclude that tissues are primed to express Hox genes early in gastrulation, concomitant with primitive streak formation and extension, and that Hox gene inducibility is transferred by cell to cell signalling. Axial structures that will later express Hox genes are generated in the node region in the period that Hox expression domains arrive there and continue to spread rostrally. However, lineage analysis showed that definitive Hox codes are not fixed at the node, but must be acquired later and anterior to the node in the neurectoderm, and independently in the mesoderm. We conclude that the rostral progression of Hox gene expression must be modulated by gene regulatory influences from early on in the posterior streak, until the time cells have acquired their stable positions along the axis well anterior to the node.     

Position-specific activity of the Hox1.1 promoter in transgenic mice

During development, positional values have to be assigned to groups of cells. The murine Hox genes are a class of genes that are predicted to be involved at some stage in this process. During embryogenesis they are expressed in distinct overlapping region- and stage-specific patterns and therefore must be regulated in response to positional information. In this study, we have analysed the activity of Hox1.1 promoter sequences in transgenic mice. The use of lacZ as a marker allows a detailed analysis of expression at the single cell level during early embryonic development. We show that 3.6 kbp of promoter and 1.7 kbp of 3' sequences provide sufficient regulatory information to express a transgene in a spatial and temporal manner indistinguishable from the endogenous Hox1.1 gene during the period of development when Hox1.1 expression is established. The activation occurs in a strict order in specific ectodermal and mesodermal domains. Within each of these domains the transgene is activated over a period of four hours apparently randomly in single cells. In a following second period, Hox1.1 and transgene expression patterns diverge. In this period, transgene expression persists in many mesodermally derived cells that do not express Hox1.1 indicating the absence of a negative regulatory element in the transgene. The anterior boundary of transgene expression is identical to that of Hox1.1. However, no posterior boundary of transgene expression is set, suggesting that a separate element absent from the transgene specifies this boundary.     

HOX Pro: a specialized database for clusters and networks of homeobox genes

It is now clear that the homeobox motif is well conserved across metazoan phyla. It has been established experimentally that a subset of genes containing this motif plays key roles in the orchestration of gene expression during development. Auto- and cross-regulatory functional interactions join homeobox genes into genetic networks. We have developed a specialized database HOX-Pro in order to arrange all available data on structure, function, phylogeny and evolution of Hox genes, Hox clusters and Hox networks. Its primary location is http://www.iephb.nw.ru/hoxpro. The database is also mirrored at http://www.mssm.edu/molbio/hoxpro. The HOX-Pro database is aimed at: (i) analysis and classification of regulatory and coding regions in diverse homeobox and related genes; (ii) comparative analysis of organization of 'Hox-based' genetic networks in the sea urchin Strongylocentrotus purpuratus, the fruit fly Drosophila melanogaster and the mouse Mus musculus; and (iii) analysis of phylogeny and evolution of homeobox genes and clusters.