Left-right asymmetry in Drosophila

Seminal studies of left-right (L/R) patterning in vertebrate models have led to the discovery of roles for the nodal pathway, ion flows and cilia in this process. Although the molecular mechanisms underlying L/R asymmetries seen in protostomes are less well understood, recent work using Drosophila melanogaster as a novel genetic model system to study this process has identified a number of mutations affecting directional organ looping. The genetic analysis of this, the most evolutionary conserved feature of L/R patterning, revealed the existence of a L/R pathway that involves the actin cytoskeleton and an associated type I myosin. In this review, we describe this work in the context of Drosophila development, and discuss the implications of these results for our understanding of L/R patterning in general.

The Drosophila JNK pathway controls the morphogenesis of the egg dorsal appendages and micropyle

During Drosophila oogenesis, the formation of the egg respiratory appendages and the micropyle require the shaping of anterior and dorsal follicle cells. Prior to their morphogenesis, cells of the presumptive appendages are determined by integrating dorsal-ventral and anterior-posterior positional information provided by the epidermal growth factor receptor (EGFR) and Decapentaplegic (Dpp) pathways, respectively. We show here that another signaling pathway, the Drosophila Jun-N-terminal kinase (JNK) cascade, is essential for the correct morphogenesis of the dorsal appendages and the micropyle during oogenesis. Mutant follicle cell clones of members of the JNK pathway, including DJNKK/hemipterous (hep), DJNK/basket (bsk), and Djun, block dorsal appendage formation and affect the micropyle shape and size, suggesting a late requirement for the JNK pathway in anterior chorion morphogenesis. In support of this view, hep does not affect early follicle cell patterning as indicated by the normal expression of kekkon (kek) and Broad-Complex (BR-C), two of the targets of the EGFR pathway in dorsal follicle cells. Furthermore, the expression of the TGF-beta homolog dpp, which is under the control of hep in embryos, is not coupled to JNK activity during oogenesis. We show that hep controls the expression of puckered (puc) in the follicular epithelium in a cell-autonomous manner. Since puc overexpression in the egg follicular epithelium mimics JNK appendages and micropyle phenotypes, it indicates a negative role of puc in their morphogenesis. The role of the JNK pathway in the morphogenesis of follicle cells and other epithelia during development is discussed.

Survival of Escherichia coli during long-term starvation: effects of aeration, NaCl, and the rpoS and osmC gene products

The survival of Escherichia coli was investigated during long-term starvation in rich media. In aerated cultures, E. coli lost the ability to form colonies earlier in NaCl-free Luria broth than in LB medium containing NaCl. Improved survival at low aeration and the sensitivity to hydrogen peroxide in aging cultures indicated a major role for oxidative stress in cell mortality. Mutants in rpoS, lacking the sigmaS subunit of RNA polymerase, showed altered survival in salt-containing media. However, in the absence of NaCl, although these mutants exhibited a massive loss of viability during the first 2 days, this was followed by a stabilization of the number of survivors. The starved culture contained survivors until at least day 9, long after a wild-type strain had completely lost viability. This peculiar behavior suggests that, in rich media of low osmotic pressure, sigmaS helps in short-term survival but hampers long-term survival. Mutants in osmC, a member of the rpoS regulon, also exhibited reduced survival and increased sensitivity to oxidative stress. The biochemical function of the envelope protein OsmC remains unknown, but present data indicated that it participates, directly or indirectly, in the defense against oxidative compounds.

The Drosophila JNK pathway controls the morphogenesis of imaginal discs during metamorphosis

In Drosophila, the Jun-N-terminal Kinase-(JNK) signaling pathway is required for epithelial cell shape changes during dorsal closure of the embryo. In the absence of JNK pathway activity, as in the DJNKK/hemipterous (hep) mutant, the dorsolateral ectodermal cells fail both to elongate and move toward the dorsal midline, leading to dorsally open embryos. We show here that hep and the JNK pathway are required later in development, for correct morphogenesis of other epithelia, the imaginal discs. During metamorphosis, the imaginal discs undergo profound morphological changes, giving rise to the adult head and thoracic structures, including the cuticle and appendages. hep mutant pupae and pharate adults show severe defects in discs morphogenesis, especially in the fusion of the two lateral wing discs. We show that these defects are accompanied by a loss of expression of puckered (puc), a JNK phosphatase-encoding gene, in a subset of peripodial cells that ultimately delineates the margins of fusing discs. In further support of a role of puc in discs morphogenesis, pupal and adult hep phenotypes are suppressed by reducing puc function, indicative of a negative role of puc in disc morphogenesis. Furthermore, we show that the small GTPase Dcdc42, but not Drac1, is an activator of puc expression in a hep-dependent manner in imaginal discs. Altogether, these results demonstrate a new role for the JNK pathway in epithelial morphogenesis, and provide genetic evidence for a role of the peripodial membrane in disc morphogenesis. We discuss a general model whereby the JNK pathway regulates morphogenesis of epithelia with differentiated edges.

The Drosophila p38 MAPK pathway is required during oogenesis for egg asymmetric development

In mammalian cells, the p38 mitogen-activated protein kinase (MAPK) pathway is activated in response to a variety of environmental stresses and inflammatory stimuli. However, the role of p38 MAPK signaling in unchallenged conditions remains largely unknown. We have isolated mutations in a Drosophila p38 MAPKK gene homolog, licorne (lic), and show that during oogenesis, lic is required in the germ line for correct asymmetric development of the egg. In lic mutant egg chambers, oskar mRNA posterior localization is not properly maintained, resulting in anteroposterior patterning defects in the embryo. Furthermore, lic loss-of-function in the germ line leads to reduced EGF receptor activity in dorsal follicle cells and ventralization of the egg shell. Both these defects are associated with a diminution of gurken protein levels in the oocyte. Our phenotypic data argue for a role of lic in a post-transcriptional regulation of the grk gene. Furthermore, they show that in addition to the well-characterized Ras/Raf/ERK MAPK pathway acting in the follicle cells, another related signaling cascade, the p38 MAPK pathway, is required in the germ line for correct axes determination. These results provide the first genetic demonstration of an essential function for a p38 pathway during development.

MKK7 is a stress-activated mitogen-activated protein kinase kinase functionally related to hemipterous

Exposure of mammalian cells to stressful stimuli results in activation of the c-Jun NH2-terminal kinase (JNK)/stress-activated protein kinases (SAPKs), a family of protein kinases related to mitogen-activated protein (MAP) kinase. JNK/SAPKs are activated by specific MAP kinase kinases (MKKs), one of which, MKK4/SEK1, has been characterized extensively. In Drosophila, the JNK/SAPK Basket (Bsk) and the MKK Hemipterous (Hep), are important for embryonic development. Loss of function of either gene inhibits dorsal closure, a morphogenetic movement in which the edges of the embryonic ectoderm move together over the amnioserosa. There is evidence that the Rho GTPases Rac and Cdc42 are also required for dorsal closure, suggesting that Rac or Cdc42 may regulate Hep and Bsk. We have identified MKK7, a murine homolog of Hep. MKK7 functionally rescues hep mutant flies. In fibroblasts, MKK7 is activated by stress and by the GTPase Rac1. MKK7 directly phosphorylates and activates JNK/SAPK. Thus, MKK7 is a homolog of hep and functions in a conserved signaling pathway involving JNK/SAPK and the GTPase Rac1.