| Developmental axon pruning is broadly used for the refinement of neural circuits in both vertebrates and invertebrates. Mechanisms of axon pruning may also contribute to the pathogenesis of neurodegenerative diseases. However, little is known about the cellular and molecular mechanisms of axon pruning. We use the stereotyped pruning of g neurons of the Drosophila mushroom bodies (MB) during metamorphosis to investigate these mechanisms. Specifically, MB gamma neurons initially extend axon branches into both the dorsal and medial MB axon lobes in larvae. Through a well-orchestrated set of developmental events during metamorphosis, axon branches to both lobes prune prior to the formation of adult connections. Detailed time-course analyses indicate that MB axon pruning is mediated by local degeneration rather than retraction, and that the disruption of the microtubule cytoskeleton precedes axon pruning. We also demonstrate an intrinsic role of the ubiquitin-proteasome system in axon pruning. Using a genetically encoded electron microscopic (EM) marker to selectively label gamma neurons, ultrastructural changes underlying axon pruning were analyzed. Events observed include changes in axon ultrastructure, synaptic degeneration, and engulfment of degenerating axon fragments by glia for their subsequent breakdown via the endosomal-lysosomal pathway. Interestingly, glia selectively invade MB axon lobes prior to and independent of axon fragmentation. Taken together, these studies provide an essential framework to pursue further cellular and molecular questions about developmental axon pruning. In addition, our findings suggest that some forms of axon pruning during development may share similarities with degeneration of axons in response to injury. |
