Loss of Eph-receptor expression correlates with loss of cell adhesion and chondrogenic capacity in Hoxa13 mutant limbs.

Mesenchymal patterning is an active process whereby genetic commands coordinate cell adhesion, sorting and condensation, and thereby direct the formation of morphological structures. In mice that lack the Hoxa13 gene, the mesenchymal condensations that form the autopod skeletal elements are poorly resolved, resulting in missing digit, carpal and tarsal elements. In addition, mesenchymal and endothelial cell layers of the umbilical arteries (UAs) are disorganized, resulting in their stenosis and in embryonic death. To further investigate the role of Hoxa13 in these phenotypes, we generated a loss-of-function allele in which the GFP gene was targeted into the Hoxa13 locus. This allele allowed FACS isolation of mesenchymal cells from Hoxa13 heterozygous and mutant homozygous limb buds. Hoxa13(GFP) expressing mesenchymal cells from Hoxa13 mutant homozygous embryos are defective in forming chondrogenic condensations in vitro. Analysis of pro-adhesion molecules in the autopod of Hoxa13 mutants revealed a marked reduction in EphA7 expression in affected digits, as well as in micromass cell cultures prepared from mutant mesenchymal cells. Finally, antibody blocking of the EphA7 extracellular domain severely inhibits the capacity of Hoxa13(GFP) heterozygous cells to condense and form chondrogenic nodules in vitro, which is consistent with the hypothesis that reduction in EphA7 expression affects the capacity of Hoxa13(-/-) mesenchymal cells to form chondrogenic condensations in vivo and in vitro. EphA7 and EphA4 expression were also decreased in the mesenchymal and endothelial cells that form the umbilical arteries in Hoxa13 mutant homozygous embryos. These results suggest that an important role for Hoxa13 during limb and UA development is to regulate genes whose products are required for mesenchymal cell adhesion, sorting and boundary formation.

Roles of Hoxa1 and Hoxa2 in patterning the early hindbrain of the mouse.

Early in its development, the vertebrate hindbrain is transiently subdivided into a series of compartments called rhombomeres. Genes have been identified whose expression patterns distinguish these cellular compartments. Two of these genes, Hoxa1 and Hoxa2, have been shown to be required for proper patterning of the early mouse hindbrain and the associated neural crest. To determine the extent to which these two genes function together to pattern the hindbrain, we generated mice simultaneously mutant at both loci. The hindbrain patterning defects were analyzed in embryos individually mutant for Hoxa1 and Hoxa2 in greater detail and extended to embryos mutant for both genes. From these data a model is proposed to describe how Hoxa1, Hoxa2, Hoxb1, Krox20 (Egr2) and kreisler function together to pattern the early mouse hindbrain. Critical to the model is the demonstration that Hoxa1 activity is required to set the anterior limit of Hoxb1 expression at the presumptive r3/4 rhombomere boundary. Failure to express Hoxb1 to this boundary in Hoxa1 mutant embryos initiates a cascade of gene misexpressions that result in misspecification of the hindbrain compartments from r2 through r5. Subsequent to misspecification of the hindbrain compartments, ectopic induction of apoptosis appears to be used to regulate the aberrant size of the misspecified rhombomeres.

Detection of targeted GFP-Hox gene fusions during mouse embryogenesis.

The ability to use a vital cell marker to study mouse embryogenesis will open new avenues of experimental research. Recently, the use of transgenic mice, containing multiple copies of the jellyfish gene encoding the green fluorescent protein (GFP), has begun to realize this potential. Here, we show that the fluorescent signals produced by single-copy, targeted GFP in-frame fusions with two different murine Hox genes, Hoxa1 and Hoxc13, are readily detectable by using confocal microscopy. Since Hoxa1 is expressed early and Hoxc13 is expressed late in mouse embryogenesis, this study shows that single-copy GFP gene fusions can be used through most of mouse embryogenesis. Previously, targeted lacZ gene fusions have been very useful for analyzing mouse mutants. Use of GFP gene fusions extends the benefits of targeted lacZ gene fusions by providing the additional utility of a vital marker. Our analysis of the Hoxc13(GFPneo) embryos reveals GFP expression in each of the sites expected from analysis of Hoxc13(lacZneo) embryos. Similarly, Hoxa1(GFPneo) expression was detected in all of the sites predicted from RNA in situ analysis. GFP expression in the foregut pocket of Hoxa1(GFPneo) embryos suggests a role for Hoxa1 in foregut-mediated differentiation of the cardiogenic mesoderm.

Phylogenetic conservation and physical mapping of members of the H6 homeobox gene family.

Homeobox genes represent a class of transcription factors that play key roles in the regulation of embryogenesis and development. Here we report the identification of a homeobox-containing gene family that is highly conserved at both the nucleotide and amino acid levels in a diverse number of species. These species encompass both vertebrate and invertebrate phylogenies, ranging from Homo sapiens to Drosophila melanogaster. In humans, at least two homeobox sequences from this family were identified representing a previously reported member of this family as well as a novel homeobox sequence that we physically mapped to the 10q25.2-q26.3 region of human Chromosome (Chr) 10. Multiple members of this family were also detected in three additional vertebrate species including Equus caballus (horse), Gallus gallus (Chicken), and Mus musculus (mouse), whereas only single members were detected in Tripneustes gratilla (sea urchin), Petromyzon marinus (lamprey), Salmo salar (salmon), Ovis aries (sheep), and D. melanogaster (fruit fly).

Characterization of the homeobox-containing gene GH6 identifies novel regions of homeobox gene expression in the developing chick embryo.

Homeobox genes are a major group of genes involved in regulating, embryogenesis. Here we describe the identification of GH6, a novel chicken homeobox-containing gene and its spatial and temporal expression pattern in the developing chick embryo. Identity comparisons of the GH6 homeodomain suggest that it is closely related to the human homeobox gene H6, with 93% amino acid conservation. Temporally, GH6 expression is highest between embryonic stages 23 and 26; however, some expression is also detectable as early as stage 13. In situ hybridization of stage 23 embryos indicates that GH6 expression occurs at high levels in discrete craniofacial regions including the second branchial arch, the neural retina, the lens epithelium, the optic nerve, and the infundibulum. GH6 expression was also seen in the developing ventricular myocardium, representing the first report of homeobox gene expression in the developing ventricle. GH6 is also expressed in sensory spinal and cranial ganglia, suggesting that GH6 plays several roles not only in the development of craniofacial structures such as the eye and ear, but also in formation of functionally defined ganglia and myocardial structures.

Identification and genetic mapping of a homeobox gene to the 4p16.1 region of human chromosome 4.

A human craniofacial cDNA library was screened with a degenerate oligonucleotide probe based on the conserved third helix of homeobox genes. From this screening, we identified a homeobox gene, H6, which shared only 57-65% amino acid identity to previously reported homeodomains. H6 was physically mapped to the 4p16.1 region by using somatic cell hybrids containing specific deletions of human chromosome 4. Linkage data from a single-stranded conformational polymorphism derived from the 3' untranslated region of the H6 cDNA placed this homeobox gene more than 20 centimorgans proximal of the previously mapped HOX7 gene on chromosome 4. Identity comparisons of the H6 homeodomain with previously reported homeodomains reveal the highest identities to be with the Nk class of homeobox genes in Drosophila melanogaster.

Characterization of the human HOX 7 cDNA and identification of polymorphic markers.

cDNA clones for a human HOX 7 gene obtained with homologous clones of Drosophila were used in human gene mapping studies. The human cDNA clone was isolated from a library constructed from human embryonic craniofacial material. The sequence of the cDNA demonstrates significant homology with mouse HOX 7. A search for RFLPs identified MboII and BstEII variants. A CA dinucleotide repeat with 5 alleles was also identified and allowed placement of HOX 7 into a defined linkage map. Evidence for linkage disequilibrium was found with markers tested. These results place the human HOX 7 gene in a defined position on 4p.