The morphogen Decapentaplegic employs a two-tier mechanism to activate target retinal determining genes during ectopic eye formation in Drosophila

CtBP represses Dpp-dependent Mad activation during Drosophila eye development

Complex networks of signaling pathways maintain the correct balance between positive and negative growth signals, ensuring that tissues achieve proper sizes and differentiation pattern during development. In Drosophila, Dpp, a member of the TGFβ family, plays two main roles during larval eye development. In the early eye primordium, Dpp promotes growth and cell survival, but later on, it switches its function to induce a developmentally-regulated cell cycle arrest in the G1 phase and neuronal photoreceptor differentiation. To advance in the identification and characterization of regulators and targets of Dpp signaling required for retinal development, we carried out an in vivo eye-targeted double-RNAi screen to identify punt (Type II TGFβ receptor) interactors. Using a set of 251 genes associated with eye development, we identified CtBP, Dad, Ago and Brk as punt genetic interactors. Here, we show that downregulation of Ago, or conditions causing increased tissue growth including overexpression of Myc or CyclinD-Cdk4 are sufficient to partially rescue punt-dependent growth and photoreceptor differentiation. Interestingly, we show a novel role for the transcriptional co-repressor CtBP in inhibiting Dpp-dependent Mad activation by phosphorylation, downstream or in parallel to Dad, the inhibitory Smad. Furthermore, CtBP downregulation activates JNK signaling pathway, implying a complex regulation of signaling pathways by CtBP during eye development.

Noncanonical Decapentaplegic Signaling Activates Matrix Metalloproteinase 1 To Restrict Hedgehog Activity and Limit Ectopic Eye Differentiation in Drosophila

One of the pertinent issues associated with cellular plasticity is to understand how the delicate balance between the determined state of cells and the extent to which they can transdetermine is maintained. Employing the well-established model of generating ectopic eyes in developing wing discs of Drosophila by ectopic eyeless expression, we provide evidence for the genetic basis of this mechanism. By both loss-of-function and gain-of-function genetic analyses, we demonstrate that Matrix metalloproteinase 1 (Mmp1) plays an important role in regulating the extent of ectopic ommatidial differentiation. Transcriptional activation of ectopic Mmp1 by the morphogen Decapentaplegic (Dpp) is not triggered by its canonical signaling pathway which involves Mad. Rather, Dpp activates an alternate cascade involving dTak1 and JNK, to induce ectopic Mmp1 expression. Mutational analyses reveal that Mmp1 negatively regulates ectopic eye differentiation by restricting the rate of proliferation and the levels of expression of retinal-determining genes dachshund and eyes absent This is primarily achieved by restricting the range of Hedgehog (Hh) signaling. Importantly, the increase in proliferation and upregulation of target retinal-determining genes, as observed upon attenuating Mmp1 activity, gets significantly rescued when ectopic eyes are generated in wing discs of hh heterozygous mutants. In conjunction with the previously established instructive and permissive roles of Dpp in facilitating ectopic eye differentiation in wing discs, the outcome of this study sheds light on a mechanism by which Dpp plays a dual role in modulating the delicate balance between the determined state of cells and the extent they can transdetermine.