The Drosophila forkhead transcription factor FOXO mediates the reduction in cell number associated with reduced insulin signaling.

BACKGROUND: Forkhead transcription factors belonging to the FOXO subfamily are negatively regulated by protein kinase B (PKB) in response to signaling by insulin and insulin-like growth factor in Caenorhabditis elegans and mammals. In Drosophila, the insulin-signaling pathway regulates the size of cells, organs, and the entire body in response to nutrient availability, by controlling both cell size and cell number. In this study, we present a genetic characterization of dFOXO, the only Drosophila FOXO ortholog. RESULTS: Ectopic expression of dFOXO and human FOXO3a induced organ-size reduction and cell death in a manner dependent on phosphoinositide (PI) 3-kinase and nutrient levels. Surprisingly, flies homozygous for dFOXO null alleles are viable and of normal size. They are, however, more sensitive to oxidative stress. Furthermore, dFOXO function is required for growth inhibition associated with reduced insulin signaling. Loss of dFOXO suppresses the reduction in cell number but not the cell-size reduction elicited by mutations in the insulin-signaling pathway. By microarray analysis and subsequent genetic validation, we have identified d4E-BP, which encodes a translation inhibitor, as a relevant dFOXO target gene. CONCLUSION: Our results show that dFOXO is a crucial mediator of insulin signaling in Drosophila, mediating the reduction in cell number in insulin-signaling mutants. We propose that in response to cellular stresses, such as nutrient deprivation or increased levels of reactive oxygen species, dFOXO is activated and inhibits growth through the action of target genes such as d4E-BP.

A genetic screen for hedgehog targets involved in the maintenance of the Drosophila anteroposterior compartment boundary.

The development of multicellular organisms requires the establishment of cell populations with different adhesion properties. In Drosophila, a cell-segregation mechanism underlies the maintenance of the anterior (A) and posterior (P) compartments of the wing imaginal disc. Although engrailed (en) activity contributes to the specification of the differential cell affinity between A and P cells, recent evidence suggests that cell sorting depends largely on the transduction of the Hh signal in A cells. The activator form of Cubitus interruptus (Ci), a transcription factor mediating Hh signaling, defines anterior specificity, indicating that Hh-dependent cell sorting requires Hh target gene expression. However, the identity of the gene(s) contributing to distinct A and P cell affinities is unknown. Here, we report a genetic screen based on the FRT/FLP system to search for genes involved in the correct establishment of the anteroposterior compartment boundary. By using double FRT chromosomes in combination with a wing-specific FLP source we screened 250,000 mutagenized chromosomes. Several complementation groups affecting wing patterning have been isolated, including new alleles of most known Hh-signaling components. Among these, we identified a class of patched (ptc) alleles exhibiting a novel phenotype. These results demonstrate the value of our setup in the identification of genes involved in distinct wing-patterning processes.