Axon degeneration is a hallmark of neurodegenerative disease and neural injury. Axotomy activates an intrinsic pro-degenerative axon death signaling cascade involving loss of the NAD biosynthetic enzyme Nmnat/Nmnat2 in axons, activation of dSarm/Sarm1, and subsequent Sarm-dependent depletion of NAD. Here we identify Axundead (Axed) as a mediator of axon death. axed mutants suppress axon death in several types of axons for the lifespan of the fly and block the pro-degenerative effects of activated dSarm in vivo. Neurodegeneration induced by loss of the sole fly Nmnat ortholog is also fully blocked by axed, but not dsarm, mutants. Thus, pro-degenerative pathways activated by dSarm signaling or Nmnat elimination ultimately converge on Axed. Remarkably, severed axons morphologically preserved by axon death pathway mutations remain integrated in circuits and able to elicit complex behaviors after stimulation, indicating that blockade of axon death signaling results in long-term functional preservation of axons.

Axons damaged by acute injury, toxic insults, or neurodegenerative diseases execute a poorly defined autodestruction signaling pathway leading to widespread fragmentation and functional loss. Here, we describe an approach to study Wallerian degeneration in the Drosophila L1 wing vein that allows for analysis of axon degenerative phenotypes with single-axon resolution in vivo. This method allows for the axotomy of specific subsets of axons followed by examination of progressive axonal degeneration and debris clearance alongside uninjured control axons. We developed new Flippase (FLP) reagents using proneural gene promoters to drive FLP expression very early in neural lineages. These tools allow for the production of mosaic clone populations with high efficiency in sensory neurons in the wing. We describe a collection of lines optimized for forward genetic mosaic screens using MARCM (mosaic analysis with a repressible cell marker; i.e., GFPlabeled, homozygous mutant) on all major autosomal arms (∼95% of the fly genome). Finally, as a proof of principle we screened the X chromosome and identified a collection eight recessive and two dominant alleles of highwire, a ubiquitin E3 ligase required for axon degeneration. Similar unbiased forward genetic screens should help rapidly delineate axon death genes, thereby providing novel potential drug targets for therapeutic intervention to prevent axonal and synaptic loss.neurodegeneration | glial response W idespread axonal degeneration and synapse loss occurs during neurodegenerative disease and after neural trauma. These degradative events result in disruption of neural circuit connectivity and ultimately functional impairment of the nervous system. Identifying molecular cascades that actively promote axonal self-destruction is a key goal. However, despite decades of work, remarkably little is known about the molecular pathways that drive the degeneration of neurites or synapses in any context (1, 2).Axotomy-induced axon degeneration (termed Wallerian degeneration, WD) serves as a useful model to study the mechanisms of axonal self-destruction. When axons are severed, the portion of the axon distal to the injury site and its synapses undergo catastrophic fragmentation after a defined latent phase, and the resulting debris is eventually cleared by surrounding glial cells. The discovery of the spontaneous Wallerian degeneration slow (Wld S ) mouse revealed, surprisingly, that severed axons can in fact survive for weeks in the absence of a cell body (3). It also led to the proposal that "axon death" signaling cascades might exist, akin to apoptotic cell death programs, which actively drive the destruction of the axon (4).Interestingly, Wld S provides significant suppression in mouse models of progressive motor neuron disease and glaucoma (5-8), and moderate protection from chemotherapy-induced axon degeneration (9). These observations argue that defining the molecular mechanisms of axon degeneration in the context of WD could have an important therapeutic impact on the treat...

The elimination of large portions of axons is a widespread event in the developing and diseased nervous system. Subsets of axons are selectively destroyed to help fine tune neural circuit connectivity during development. Axonal degeneration is also an early feature of nearly all neurodegenerative diseases, occurs after most neural injuries, and is a primary driver of functional impairment in patients. In this review, we discuss the diversity of cellular mechanisms by which axons degenerate. Initial molecular characterization highlights some similarities in their execution, but also argues that unique genetic programs modulate each mode of degeneration. Defining these pathways rigorously will provide new targets for therapeutic intervention after neural injury or in neurodegenerative disease.