ftz-f1 and Hr39 opposing roles on EcR expression during Drosophila mushroom body neuron remodeling

Developmental axon pruning is a general mechanism that is required for maturation of neural circuits. During Drosophila metamorphosis, the larval-specific dendrites and axons of early γ neurons of the mushroom bodies are pruned and replaced by adult-specific processes. We found that the nuclear receptor ftz-f1 is required for this pruning, activates expression of the steroid hormone receptor EcR-B1, whose activity is essential for γ remodeling, and represses expression of Hr39, an ftz-f1 homologous gene. If inappropriately expressed in the γ neurons, HR39 inhibits normal pruning, probably by competing with endogenous FTZ-F1, which results in decreased EcR-B1 expression. EcR-B1 was previously identified as a target of the TGFβ signaling pathway. We found that the ftz-f1 and Hr39 pathway apparently acts independently of TGFβ signaling, suggesting that EcR-B1 is the target of two parallel molecular pathways that act during γ neuron remodeling.