Pruning of exuberant neuronal connections is a fundamental and widespread mechanism to develop the diversity and specificity of neuronal connections evident in both vertebrate and invertebrate nervous systems. Mechanisms used for developmental pruning of axons and dendrites could also be used for structural plasticity of adult neurons in response to experience, learning, and injuries of the nervous system. Misregulation of such mechanisms may contribute to pathogenesis of neurodegenerative diseases. We are using axon pruning of Drosophila mushroom body (MB) y neurons during metamorphosis as a model to investigate the mechanisms of developmental axon pruning. During the past grant period, we found that MB 7 axon pruning utilizes a degenerative mechanism to eliminate specific axon branches, and that degenerating axons are engulfed for endosome-lysosome degradation by nearby glial cells. We showed that axon pruning requires cell autonomous transcriptional regulation by steroid hormone ecdysone receptor (EcR), and cell-autonomous action of the ubiquitin-proteasome system (UPS). We have also initiated several screens to identify other genes involved in MB y axon pruning. These include a forward genetic screen using chemical mutagen EMS that identified two new pruning mutants so far, a gain-of-function screen for genes that when over-expressed would block axon pruning, and a microarray-based screen that identified hundreds of genes whose transcription is up- or down-regulated by EcR in MB neurons during metamorphosis. The last two screens converged on the identification of a protein whose expression in MB neurons is down-regulated 17 fold by EcR during metamorphosis, and when over-expressed blocks axon pruning. Here we propose a series of experiments to follow the existing evidence, and to continue our investigation of molecular mechanisms of developmental axon pruning. We have provided evidence that MB y axon pruning shares striking similarities to axon degeneration in injury and neurological diseases. Thus, we believe these studies will contribute to our understanding of axon degeneration in diseases