Regulation of Axonal Branch Refinement by EGF-Receptor Signaling. Marlen Zschaetzsch, Bassem Hassan. VIB, Leuven, Belgium.

   Neuronal circuit function requires correct development of appropriate connectivity. In mammalian systems a common mechanism to establish circuits is refinement of excessive projections during development. However, mechanisms of exuberant neurite refinement are largely unknown. Invertebrate nervous systems are thought not to require developmental refinement. Contrary to this view, we show that an adult specific neuronal circuit in the central visual system of Drosophila shows excessive axon branch formation and subsequent pruning during development. We find that dosage critical activation of asymmetrically distributed Epidermal Growth Factor Receptor (EGFR) by ligand released from retinal axons is necessary for branch pruning. Live imaging of the developing brain shows that EGFR signaling is required for branch growth and retraction dynamics during, likely through regulation of the actin cytoskeleton. These observations establish the first Drosophila model for developmental neuronal circuit refinement and identify non-canonical localized EGFR signaling as a novel refinement mechanism.