Activation and function of TGF signalling during Drosophila wing development and its interactions with the BMP pathway. Covadonga F. Hevia, Jose F. de Celis. Centro de Biología Molecular Severo Ochoa CSIC-UAM, Madrid, Spain.

   The development of the Drosophila wing disc requires the activities of the BMP and TGF signaling pathways. BMP signaling is critical for the growth and patterning of the disc, whereas the related TGF pathway is mostly required for growth. The BMP and TGF pathways share a common co-receptor (Punt) and a nuclear effector (Med), and consequently it is likely that these pathways can interfere with each other during normal development. Here, we analyze the requirements of TGF signaling during wing disc development and identify possible mechanisms linking TGF and BMP activities. We found that the phosphorylation of Smad2, the specific transducer for TGF signaling, occurs in a generalized manner in the wing disc and that Smad2 influences cell division rates and cell growth. The expression in the wing disc of the four candidate TGF ligands (activin, dawdle, maverick and myoglianin) is required to obtain normal levels of TGF signaling. We confirm that Baboon, the specific receptor of the TGF pathway, can phosphorylate Mad, the specific transducer of the BMP pathway, but we find that this activation only occurs when the receptor is constitutively activated in a background of reduced expression of Smad2. In the presence of Smad2, the normal situation during wing disc development, high levels of activated Baboon lead to a depletion in Mad phosphorylation and to BMP loss-of-function phenotypes. Although cross- interactions between TGF and BMP signaling based in molecular competition for common components of the pathways seem irrelevant to determine each pathway signaling outcome in the wing disc, they could be critical in other developmental systems and in pathological conditions.