Segregation of myogenic lineages in Drosophila requires numb

Terminal divisions of myogenic lineages in the Drosophila embryo generate sibling myoblasts that found larval muscles or form precursors of adult muscles. Alternative fates adopted by sibling myoblasts are associated with distinct patterns of gene expression. Genes expressed in the progenitor cell are maintained in one sibling and repressed in the other. These differences depend on an asymmetric segregation of Numb between sibling cells. In numb mutants, muscle fates associated with repression are duplicated and alternative muscles are lost. If numb is overexpressed the reverse transformation occurs. Numb acts to block Notch-mediated repression of genes expressed in muscle progenitor cells. Thus asymmetric cell divisions are essential determinants of muscle fates during myogenesis in Drosophila     

Mammalian CD2 is an effective heterologous marker of the cell surface in Drosophila

Ectopic expression of neutral proteins, such as beta-galactosidase, in developing embryos has been an invaluable tool for studies of gene expression and embryonic development. However, expression of beta-galactosidase does not reveal the shape of the cells containing it. We have examined the suitability of rat CD2, a small transmembrane protein of the immunoglobulin superfamily, as a marker of cell morphology in Drosophila. We selected the regulatory sequences of the Drosophila mesoderm-specific gene twist to express CD2 and prepared a chimeric gene, twi-CD2. Embryos containing twi-CD2 faithfully express CD2 in the same pattern as Twist. Expression of CD2 on the surface of cells reveals the shape of cells when stained with existing monoclonal antibodies. We have also constructed a CD2 gene that can be used with the GAL4 system and show that CD2 can be expressed on the surface of epithelial cells and along the length of axons.     

Kakapo, a novel cytoskeletal-associated protein is essential for the restricted localization of the neuregulin-like factor, vein, at the muscle-tendon junction site

In the Drosophila embryo, the correct association of muscles with their specific tendon cells is achieved through reciprocal interactions between these two distinct cell types. Tendon cell differentiation is initiated by activation of the EGF-receptor signaling pathway within these cells by Vein, a neuregulin-like factor secreted by the approaching myotube. Here, we describe the cloning and the molecular and genetic analyses of kakapo, a Drosophila gene, expressed in the tendons, that is essential for muscle-dependent tendon cell differentiation. Kakapo is a large intracellular protein and contains structural domains also found in cytoskeletal-related vertebrate proteins (including plakin, dystrophin, and Gas2 family members). kakapo mutant embryos exhibit abnormal muscle-dependent tendon cell differentiation. A major defect in the kakapo mutant tendon cells is the failure of Vein to be localized at the muscle-tendon junctional site; instead, Vein is dispersed and its levels are reduced. This may lead to aberrant differentiation of tendon cells and consequently to the kakapo mutant deranged somatic muscle phenotype.     

The role of lin-22, a hairy/enhancer of split homolog, in patterning the peripheral nervous system of C. elegans

In C. elegans, six lateral epidermal stem cells, the seam cells V1-V6, are located in a row along the anterior-posterior (A/P) body axis. Anterior seam cells (V1-V4) undergo a fairly simple sequence of stem cell divisions and generate only epidermal cells. Posterior seam cells (V5 and V6) undergo a more complicated sequence of cell divisions that include additional rounds of stem cell proliferation and the production of neural as well as epidermal cells. In the wild type, activity of the gene lin-22 allows V1-V4 to generate their normal epidermal lineages rather than V5-like lineages. lin-22 activity is also required to prevent additional neurons from being produced by one branch of the V5 lineage. We find that the lin-22 gene exhibits homology to the Drosophila gene hairy, and that lin-22 activity represses neural development within the V5 lineage by blocking expression of the posterior-specific Hox gene mab-5 in specific cells. In addition, in order to prevent anterior V cells from generating V5-like lineages, wild-type lin-22 gene activity must inhibit (directly or indirectly) at least five downstream regulatory gene activities. In anterior body regions, lin-22(+) inhibits expression of the Hox gene mab-5. It also inhibits the activity of the achaete-scute homolog lin-32 and an unidentified gene that we postulate regulates stem cell division. Each of these three genes is required for the expression of a different piece of the ectopic V5-like lineages generated in lin-22 mutants. In addition, lin-22 activity prevents two other Hox genes, lin-39 and egl-5, from acquiring new activities within their normal domains of function along the A/P body axis. Some, but not all, of the patterning activities of lin-22 in C. elegans resemble those of hairy in Drosophila.     

Modulation of the expression of the rabbit galectin-3 gene by p53 and c-Ha-ras proteins and PMA

Galectin-3 is a galactose-binding lectin that has been found in several mammalian tissues. Galectin-3 gene is expressed in a wide range of normal and tumoral cells. In the case of myeloid cells, its expression correlates with the differentiation of monocytes to macrophages. In the case of cancer cell lines, its expression correlates with tumorigenicity and metastatic potential. The regulation of the expression of this gene is still largely unknown. The rabbit galectin-3 gene has been isolated and characterized. Its structure revealed an organization similar to that of the murine galectin-3 gene. The genomic sequences located upstream from its 5' end, upon insertion upstream from a promoter-free reporter gene, exhibited a strong promoter activity. This activity was upregulated upon treatment of transfected smooth muscle cells with phorbol 12-myristate 13-acetate (PMA) as well as upon transfection with a EJ/ras encoding plasmid. Conversely, it was downmodulated upon transfection with wild-type p53 but not with mutated p53. The regulatory sequences involved in the positive regulation of the gene were located upon serial deletion experiments.     

Analysis of a Caenorhabditis elegans Twist homolog identifies conserved and divergent aspects of mesodermal patterning

Mesodermal development is a multistep process in which cells become increasingly specialized to form specific tissue types. In Drosophila and mammals, proper segregation and patterning of the mesoderm involves the bHLH factor Twist. We investigated the activity of a Twist-related factor, CeTwist, during Caenorhabditis elegans mesoderm development. Embryonic mesoderm in C. elegans derives from a number of distinct founder cells that are specified during the early lineages; in contrast, a single blast cell (M) is responsible for all nongonadal mesoderm formation during postembryonic development. Using immunofluorescence and reporter fusions, we determined the activity pattern of the gene encoding CeTwist. No activity was observed during specification of mesodermal lineages in the early embryo; instead, the gene was active within the M lineage and in a number of mesodermal cells with nonstriated muscle fates. A role for CeTwist in postembryonic mesodermal cell fate specification was indicated by ectopic expression and genetic interference assays. These experiments showed that CeTwist was responsible for activating two target genes normally expressed in specific subsets of nonstriated muscles derived from the M lineage. In vitro and in vivo assays suggested that CeTwist cooperates with the C. elegans E/Daughterless homolog in directly activating these targets. The two target genes that we have studied, ceh-24 and egl-15, encode an NK-2 class homeodomain and an FGF receptor (FGFR) homolog, respectively. Twist activates FGFR and NK-homeodomain target genes during mesodermal patterning of Drosophila and similar target interactions have been proposed to modulate mesenchymal growth during closure of the vertebrate skull. These results suggest the possibility that a conserved pathway may be used for diverse functions in mesodermal specification.