Expression of the homeobox gene,Hox 2.1, during mouse embryogenesis

This article reviews recent studies on the expression of the homeobox gene, Hox 2.1, during mouse embryogenesis, using the technique of in situ hybridization. Differential hybridization of radiolabelled antisense versus sense strand RNA is first clearly detected in sections of 8.5 day post coitum (p.c.) early somite embryos. At 12.5 days p.c., higher levels of Hox 2.1 expression are seen in the spinal cord, extending into the base of the hind brain. Hybridization of antisense Hox 2.1 RNA is also seen in the spinal ganglia, in the nodose ganglia of the Xth cranial nerve (which contains derivatives of the neural crest arising from the posterior hind brain), and in the myenteric plexus. Mesodermal cells of certain visceral organs also express Hox 2.1 RNA, in particular the mesoderm of the lung, stomach and meso- and meta-nephric kidney. Comparison of the spatial domains of expression of mouse homeobox genes reveals a pattern consistent with the idea that they play a role in anteroposterior positional specification during embryogenesis.     

Spatially and temporally regulated expression of N-acetylglucosamine-6-O-sulfotransferase during mouse embryogenesis.

GlcNAc-6-O-sulfotransferase is involved in formation of 6-sulfo-N -acetyllactosamine-containing structures such as 6-sulfo sialyl Lewis x. We investigated the mode of expression of GlcNAc-6-O-sulfotransferase during postimplantation embryogenesis in the mouse by in situ hybridization. Sulfotransferase mRNA was not detected on embryonic day (E) 6.5, while on E7.5 it was detected in the mesoderm, ectoderm, and ectoplacental cone. On E10.5, the sulfotransferase signals were mainly observed in the nervous tissue. On E12.5 and 13.5, various tissues in the process of differentiation expressed this mRNA. Several epithelial and mesenchymal tissues undergoing epithelial-mesenchymal interactions strongly expressed the mRNA. For example, in the developing tooth strong sulfotransferase mRNA expression was found only in the condensing mesenchyme on E13.5. On E13.5 and 15.5, the sites showing intense expression of the sulfotransferase again became restricted. In the brain, sulfotransferase mRNA was frequently found as discrete signals in narrow regions. These results suggest that 6-sulfo-N-acetyllactosamine structures have important roles in development. On E13.5 and 15.5, G152 (6-sulfo sialyl Lewis x antigen) was expressed in the neocortex, and AG223 (6-sulfo Lewis x antigen) in the thalamus and neocortex where the sulfotransferase signal was detected. However, in other organs, expression of these antigens did not correlate with the sulfotransferase mRNA, implicating complex nature of regulation of expression of the fucosyl 6-sulfo antigens.     

Expression of the mouse labial-like homeobox-containing genes, Hox 2.9 and Hox 1.6, during segmentation of the hindbrain.

The sequence of a mouse Hox 2.9 cDNA clone is presented. The predicted homeodomain is similar to that of the Drosophila gene labial showing 80% identity. The equivalent gene in the Hox 1 cluster is Hox 1.6 which shows extensive similarity to Hox 2.9 both within and outside the homeodomain. Hox 2.9 and Hox 1.6 are the only two mouse members of the labial-like family of homeobox-containing genes as yet identified. Hox 2.9 has previously been shown to be expressed in a single segmental unit of the developing hindbrain (rhombomere) and has been predicted to be involved in conferring rhombomere identity. To analyse further the function of Hox 2.9 during development and to determine if the other mouse labial-like gene Hox 1.6, displays similar properties, we have investigated the expression patterns of these two genes and an additional rhombomere-specific gene, Krox 20, on consecutive embryonic sections at closely staged intervals. This detailed analysis has enabled us to draw the following conclusions: (1) There are extensive similarities in the temporal and spatial expression of Hox 2.9 and Hox 1.6, throughout the period that both genes are expressed in the embryo (7 1/2 to 10 days). At 8 days the genes occupy identical domains in the neuroectoderm and mesoderm with the same sharp anterior boundary in the presumptive hindbrain. These similarities indicate a functional relationship between the genes and further suggest that the labial-like genes are responding to similar signals in the embryo. (2) By 9 days the neuroectoderm expression of both genes retreats posteriorly along the anteroposterior (AP) axis. The difference at this stage between the expression patterns is the persistence of Hox 2.9 in a specific region of the hindbrain, illustrating the capacity of Hox 2.9 to respond to additional positional regulatory signals and indicating a unique function for this gene in the hindbrain. (3) The restriction of Hox 2.9 expression in the hindbrain occurs at 8 1/2 days, approximately the same time as Krox 20 is first detected in the posterior adjoining domain. The mutually exclusive expression of Hox 2.9 and Krox 20 demarcated by sharp expression boundaries suggest that compartmentalisation of cells within the hindbrain has occurred up to 6 h before rhombomeres (morphological segments) are clearly visible. (4) Hox 2.9 expression is confined to the region of rhombomere 4 that shows cell lineage restriction and, unlike Krox 20, is expressed throughout the period that rhombomeres are visible (to 11 1/2 days).(ABSTRACT TRUNCATED AT 400 WORDS)     

Three Drosophila Hox complex microRNAs do not have major effects on expression of evolutionarily conserved Hox gene targets during embryogenesis

The discovery of microRNAs has resulted in a major expansion of the number of molecules known to be involved in gene regulation. Elucidating the functions of animal microRNAs has posed a significant challenge as their target interactions with messenger RNAs do not adhere to simple rules. Of the thousands of known animal microRNAs, relatively few microRNA:messenger RNA regulatory interactions have been biologically validated in an normal organismal context. Here we present evidence that three microRNAs from the Hox complex in Drosophila (miR-10-5p, miR-10-3p, miR-iab-4-5p) do not have significant effects during embryogenesis on the expression of Hox genes that contain high confidence microRNAs target sites in the 3' untranslated regions of their messenger RNAs. This is significant, in that it suggests that many predicted microRNA-target interactions may not be biologically relevant, or that the outcomes of these interactions may be so subtle that mutants may only show phenotypes in specific contexts, such as in environmental stress conditions, or in combinations with other microRNA mutations.     

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.     

Developmentally regulated expression of the LRRTM gene family during mid-gestation mouse embryogenesis

We have analysed the expression during mid-gestation mouse development of the four member LRRTM gene family which encodes type 1 transmembrane proteins containing 10 extracellular leucine rich repeats and a short intracellular tail. Each family member has a developmentally regulated pattern of expression distinct from all other members. LRRTM1 is expressed in the neural tube, otic vesicle, apical ectodermal ridge, forebrain and midbrain up to a sharp central boundary. LRRTM2 is expressed in a subset of progenitors in the neural tube. LRRTM3 is expressed in a half somite wide stripe in the presomitic mesoderm adjacent to the boundary with the most recently formed somite. Additional expression is seen in the neural tube, forebrain and hindbrain. LRRTM4 is expressed in the limb mesenchyme, neural tube, caudal mesoderm and in three distinct regions of the head. Later expression occurs in a subset of the developing sclerotome. Each family member has a unique expression domain within the neural tube.     

Tissue-specific expression of the mouse Q10 H-2 class-I gene during embryogenesis

We have studied the pattern of expression of the Q10 gene, a H-2 class-I gene located in the major histocompatibility complex which encodes a soluble class-I molecule, in the mid-gestation mouse embryo, and compared it to those of two other class-I genes, namely Kd and 37, the latter gene located in the thymus leukemia region. We found that the steady-state amount of these different mRNAs gradually increased from day 13 to day 18. By comparison with the level of expression of these genes in adult liver, the increase during gestation was fairly more marked for Q10 mRNA than for the others. Furthermore, we found that the Q10 gene is transiently expressed in the endoderm layer of the visceral yolk sac and in the fetal heart. Expression in the latter tissue decreases abruptly while increasing in the liver. It has been proposed that the Q10 protein is involved in immune tolerance. However, the time course of expression of Q10 mRNA and its tissue distribution during embryogenesis suggest that the Q10 protein could play a role in the differentiation of hematopoietic stem cells.