EGF-dependent and independent activation of MAP kinase during Drosophila oogenesis

A comparative study of Pointed and Yan expression reveals new complexity to the transcriptional networks downstream of receptor tyrosine kinase signaling

The biochemical regulatory network downstream of receptor tyrosine kinase (RTK) signaling is controlled by two opposing ETS family members: the transcriptional activator Pointed (Pnt) and the transcriptional repressor Yan. A bistable switch model has been invoked to explain how pathway activation can drive differentiation by shifting the system from a high-Yan/low-Pnt activity state to a low-Yan/high-Pnt activity state. Although the model explains yan and pnt loss-of-function phenotypes in several different cell types, how Yan and Pointed protein expression dynamics contribute to these and other developmental transitions remains poorly understood. Toward this goal we have used a functional GFP-tagged Pnt transgene (Pnt-GFP) to perform a comparative study of Yan and Pnt protein expression throughout Drosophila development. Consistent with the prevailing model of the Pnt-Yan network, we found numerous instances where Pnt-GFP and Yan adopt a mutually exclusive pattern of expression. However we also observed many examples of co-expression. While some co-expression occurred in cells where RTK signaling is presumed low, other co-expression occurred in cells with high RTK signaling. The instances of co-expressed Yan and Pnt-GFP in tissues with high RTK signaling cannot be explained by the current model, and thus they provide important contexts for future investigation of how context-specific differences in RTK signaling, network topology, or responsiveness to other signaling inputs, affect the transcriptional response.

The pea aphid uses a version of the terminal system during oviparous, but not viviparous, development

Background

In most species of aphid, female nymphs develop into either sexual or asexual adults depending on the length of the photoperiod to which their mothers were exposed. The progeny of these sexual and asexual females, in turn, develop in dramatically different ways. The fertilized oocytes of sexual females begin embryogenesis after being deposited on leaves (oviparous development) while the oocytes of asexual females complete embryogenesis within the mother (viviparous development). Compared with oviparous development, viviparous development involves a smaller transient oocyte surrounded by fewer somatic epithelial cells and a smaller early embryo that comprises fewer cells. To investigate whether patterning mechanisms differ between the earliest stages of the oviparous and viviparous modes of pea aphid development, we examined the expression of pea aphid orthologs of genes known to specify embryonic termini in other insects.

Results

Here we show that pea aphid oviparous ovaries express torso-like in somatic posterior follicle cells and activate ERK MAP kinase at the posterior of the oocyte. In addition to suggesting that some posterior features of the terminal system are evolutionarily conserved, our detection of activated ERK in the oocyte, rather than in the embryo, suggests that pea aphids may transduce the terminal signal using a mechanism distinct from the one used in Drosophila. In contrast with oviparous development, the pea aphid version of the terminal system does not appear to be used during viviparous development, since we did not detect expression of torso-like in the somatic epithelial cells that surround either the oocyte or the blastoderm embryo and we did not observe restricted activated ERK in the oocyte.

Conclusions

We suggest that while oviparous oocytes and embryos may specify posterior fate through an aphid terminal system, viviparous oocytes and embryos employ a different mechanism, perhaps one that does not rely on an interaction between the oocyte and surrounding somatic cells. Together, these observations provide a striking example of a difference in the fundamental events of early development that is both environmentally induced and encoded by the same genome.

Arctigenin anti-tumor activity in bladder cancer T24 cell line through induction of cell-cycle arrest and apoptosis

Bladder cancer is the most common neoplasm in the urinary system. This study assesses arctigenin anti-tumor activity in human bladder cancer T24 cells in vitro and the underlying molecular events. The flow cytometry analysis was used to detect cell-cycle distribution and apoptosis. Western blotting was used to detect changes in protein expression. The data showed that arctigenin treatment reduced viability of bladder cancer T24 cells in a dose- and time-dependent manner after treatment with arctigenin (10, 20, 40, 80, and 100 μmol/L) for 24 hr and 48 hr. Arctigenin treatment clearly arrested tumor cells in the G1 phase of the cell cycle. Apoptosis was detected by hoechst stain and flow cytometry after Annexin-V-FITC/PI double staining. Early and late apoptotic cells were accounted for 2.32-7.01% and 3.07-7.35%, respectively. At the molecular level, arctigenin treatment decreased cyclin D1 expression, whereas CDK4 and CDK6 expression levels were unaffected. Moreover, arctigenin selectively altered the phosphorylation of members of the MAPK superfamily, decreasing phosphorylation of ERK1/2 and activated phosphorylation of p38 significantly in a dose-dependent manner. These results suggest that arctigenin may inhibit cell viability and induce apoptosis by direct activation of the mitochondrial pathway, and the mitogen-activated protein kinase pathway may play an important role in the anti-tumor effect of arctigenin. The data from the current study demonstrate the usefulness of arctigenin in bladder cancer T24 cells, which should further be evaluated in vivo before translation into clinical trials for the chemoprevention of bladder cancer.

Feedback control of the EGFR signaling gradient: superposition of domain-splitting events in Drosophila oogenesis

The morphogenesis of structures with repeated functional units, such as body segments and appendages, depends on multi-domain patterns of cell signaling and gene expression. We demonstrate that during Drosophila oogenesis, the two-domain expression pattern of Broad, a transcription factor essential for the formation of the two respiratory eggshell appendages, is established by a single gradient of EGFR activation that induces both Broad and Pointed, which mediates repression of Broad. Two negative-feedback loops provided by the intracellular inhibitors of EGFR signaling, Kekkon-1 and Sprouty, control the number and position of Broad-expressing cells and in this way influence eggshell morphology. Later in oogenesis, the gradient of EGFR activation is split into two smaller domains in a process that depends on Argos, a secreted antagonist of EGFR signaling. In contrast to the previously proposed model of eggshell patterning, we show that the two-domain pattern of EGFR signaling is not essential for specifying the number of appendages. Thus, the processes that define the two-domain patterns of Broad and EGFR activation are distinct; their actions are separated in time and have different effects on eggshell morphology.

Awd, the homolog of metastasis suppressor gene Nm23, regulates Drosophila epithelial cell invasion

Border cell migration during Drosophila melanogaster oogenesis is a highly pliable model for studying epithelial to mesenchymal transition and directional cell migration. The process involves delamination of a group of 6 to 10 follicle cells from the epithelium followed by guided migration and invasion through the nurse cell complex toward the oocyte. The guidance cue is mainly provided by the homolog of platelet-derived growth factor/vascular endothelial growth factor family of growth factor, or Pvf, emanating from the oocyte, although Drosophila epidermal growth factor receptor signaling also plays an auxiliary role. Earlier studies implicated a stringent control of the strength of Pvf-mediated signaling since both down-regulation of Pvf and overexpression of active Pvf receptor (Pvr) resulted in stalled border cell migration. Here we show that the metastasis suppressor gene homolog Nm23/awd is a negative regulator of border cell migration. Its down-regulation allows for optimal spatial signaling from two crucial pathways, Pvr and JAK/STAT. Its overexpression in the border cells results in stalled migration and can revert the phenotype of overexpressing constitutive Pvr or dominant-negative dynamin. This is a rare example demonstrating the relevance of a metastasis suppressor gene function utilized in a developmental process involving cell invasion.

Drosophila awd, the homolog of human nm23, regulates FGF receptor levels and functions synergistically with shi/dynamin during tracheal development

Human nm23 has been implicated in suppression of metastasis in various cancers, but the underlying mechanism of such activity has not been fully understood. Using Drosophila tracheal system as a genetic model, we examined the function of the Drosophila homolog of nm23, the awd gene, in cell migration. We show that loss of Drosophila awd results in dysregulated tracheal cell motility. This phenotype can be suppressed by reducing the dosage of the chemotactic FGF receptor (FGFR) homolog, breathless (btl), indicating that btl and awd are functionally antagonists. In addition, mutants of shi/dynamin show similar tracheal phenotypes as in awd and exacerbate those in awd mutant, suggesting defects in vesicle-mediated turnover of FGFR in the awd mutant. Consistent with this, Btl-GFP chimera expressed from a cognate btl promoter-driven system accumulate at high levels on tracheal cell membrane of awd mutants as well as in awd RNA duplex-treated cultured cells. Thus, we propose that awd regulates tracheal cell motility by modulating the FGFR levels, through a dynamin-mediated pathway.