Separate elements cause lineage restriction and specify boundaries of Hox-1.1 expression.

The Hox genes are a class of putative developmental control genes that are thought to be involved in the specification of positional identity along the anteroposterior axis of the vertebrate embryo. It is apparent from their expression pattern that their regulation is dependent upon positional information. In a previous analysis of the Hox-1.1 promoter in transgenic mice, we identified sequences that were sufficient to establish transgene expression in a specific region of the embryo. The construct used, however, did not contain enough regulatory sequences to reproduce all aspects of Hox-1.1 expression. In particular, neither a posterior boundary nor a restriction of expression to prevertebrae was achieved. Here we show correct regulation by Hox-1.1 sequences in transgenic mice and identify the elements responsible for different levels of control. Concomitant with the subdivision of mesodermal cells into different lineages during gastrulation and organogenesis, Hox-1.1 expression is restricted to successively smaller sets of cells. Distinct elements are required at different stages of development to execute this developmental programme. One position-responsive element (130 bp nontranslated leader) was shown to be crucial for the restriction of expression not only along the anteroposterior axis of the embryo, setting the posterior border, but also along the dorsoventral axis of the neural tube and to the lineage giving rise to the prevertebrae. Thus, Hox-1.1 expression is established in a specific region of the embryo and in a specific lineage of the mesoderm by restricting the activity of the promoter by the combined effect of several regulatory elements.     

Primary structure and embryonic expression pattern of the mouse Hox-4.3 homeobox gene

We report the cloning, genomic localization, primary structure and developmental expression pattern of the novel mouse Hox-4.3 gene. This gene is located within the HOX-4(5) complex, at a position which classifies it as a member of the Hox-3.1 and -2.4 subfamily, the DNA and predicted protein sequences further confirmed this classification. Hox-4.3 has a primary structure characteristic of a Hox gene but, in addition, contains several monotonic stretches of amino acids, one of the 'paired'-like type. As expected from its presence and position within the complex. Hox-4.3 is developmentally expressed in structures of either mesodermal or neurectodermal origin located or derived from below a precise craniocaudal level. However, a very important offset between anteroposterior boundaries within neuroectoderm versus mesoderm derivatives is observed. Like other genes of the HOX-4(5) complex, Hox-4.3 is expressed in developing limbs and gonads, suggesting that 'cluster specificity' could be a feature of the HOX network.     

Isolation and analysis of embryonic expression of Hox-4.9, a member of the murine labial-like gene family.

Two members of the murine labial (lab) subfamily of Antennapedia-like homeobox-containing genes, Hox-1.6 and Hox-2.9, have been identified previously. Here we describe a third member genetically linked to the Hox-4 cluster on chromosome 2. This gene, designated Hox-4.9, is similar in structure to the other lab subfamily members. However, little coding sequence other than the homeobox and sequences immediately upstream of it have been conserved. By in situ hybridization analysis, Hox-4.9 mRNA is first detected at the end of the late streak stage (E7.75) in presumptive lateral and extraembryonic mesoderm. During early neurogenesis (E8.0-8.5), Hox-4.9 is detected solely in lateral mesoderm; its lack of expression in somitic mesoderm and the neural tube makes it unique among the Hox genes. By late neurogenesis and through mid-gestation (E9.0-E11.5), Hox-4.9 is no longer detected in lateral mesoderm but is found instead in a restricted region of presumed trunk neural crest and in the dermatome. These data are discussed in comparison with what is known about expression of the other members of the lab subfamily.     

Analysis of mouse Evx genes: Evx-1 displays graded expression in the primitive streak.

Two mouse genes, Evx-1 and Evx-2, each encoding a homeodomain closely related to that of the Drosophila even-skipped gene were isolated using a PCR-based strategy. The structure and sequence of these genes are described. Mapping studies localized Evx-1 to chromosome 6, near the Hox-1 gene cluster, and Evx-2 to chromosome 2, near the Hox-4 cluster. The evolutionary implications of these linkages are discussed. RNA in situ hybridization analysis of Evx expression in embryos demonstrated a striking pattern of Evx-1 expression during gastrulation, whereas Evx-2 RNA could not be detected at any stage by this technique. Evx-1 RNA is first detected shortly before the onset of gastrulation in a region of ectoderm containing cells that will soon be found in the primitive streak. This localized expression of Evx-1 provides the first molecular evidence for regional differences in the mouse embryonic ectoderm before gastrulation. Throughout gastrulation, Evx-1 expression is limited to cells near and within the streak and that expression is graded, with a posterior-to-anterior decrease in the level of RNA. Based on fate-mapping studies indicating that different types of mesoderm emerge from different regions of the primitive streak and our observation that high levels of expression are localized to the region that will give rise to extraembryonic and ventral mesoderm, we speculate that Evx-1 plays a role in the dorsoventral specification of mesodermal cell fate.