Use of time-lapse imaging and dominant negative receptors to dissect the steroid receptor control of neuronal remodeling in Drosophila

During metamorphosis, the reorganization of the nervous system of Drosophila melanogaster proceeds in part through remodeling of larval neurons. In this study, we used in-vitro imaging techniques and immunocytochemistry to track the remodeling of the thoracic ventral neurosecretory cells. Axons of these neurons prune their larval arbors early in metamorphosis and a larger, more extensive adult arbor is established via branch outgrowth. Expression of EcR dominant negative constructs and an EcR inverted repeat construct resulted in pruning defects of larval axon arbors and a lack of filopodia during pruning, but showed variable effects on outgrowth depending on the construct expressed. Cells expressing either UAS-EcR-B1(W650A) or UAS-EcR-A(W650A) lacked filopodia during the outgrowth period and formed a poorly branched, larval-like arbor in the adult. Cells expressing UAS-EcR-B1(F645A), UAS-EcR-B2(W650A) or UAS-IR-EcR (core) showed moderate filopodial activity and normal, albeit reduced, adult-like branching during outgrowth. These results are consistent with the role of activation versus derepression via EcR for successive phases of neuronal remodeling and suggest that functional ecdysone receptor is necessary for some, but not all, remodeling events.