Axon degeneration is a common feature of all neurodegenerative diseases and neuronal injuries. Defining the signaling pathways that control this process should provide new therapeutic strategies to block axon loss in disease. Identifying ?axon death? molecules?endogenous signaling molecules that signal to actively promote axon destruction?had proven extremely challenging for the field and such molecules were only speculated to exist. However, using axotomy as a simple model and unbiased forward genetic screens in Drosophila, our lab discovered the axon death gene dSarm (fly)/Sarm1 (mouse). Loss of dSarm/Sarm1 resulted in a complete blockade of axon degeneration after axotomy in both flies and mice in vivo. Sarm1 null mutants mice were also highly neuroprotective in models of traumatic brain injury and peripheral neuropathy, suggesting the dSarm/Sarm1 pathway is widely engaged after neural trauma to drive axon degeneration. Understanding how the axon death pathway is activated and drives axon loss is now the central focus for the field. Here I describe a new assay for in vivo activation of the dSarm pathway: single-cell depletion of Nmnat. Loss of Nmnat in flies or mice activates axon degeneration that is dSarm/Sarm1-dependent. We have developed a high throughput system for imaging single neuronal clones in the Drosophila wing that express reduced levels of Nmnat. Using this a high-throughput forward genetic screen, I have begun to use an unbiased approach to knockout genes in the Drosophila genome, and screen for new genes required for dSarm-dependent destruction of axons. In preliminary work we have found one new allele of dSarm (demonstrating the screen works), and two new lines that strongly suppresses axon degeneration after Nmnat depletion. We have identified a novel molecule in Drosophila NAD metabolic pathway that has not been previously characterized, but can protect neurons from degeneration induced by Nmnat depletion. I will work to characterize the function of this molecule using genetic and biochemical approaches in vivo. This work holds promise to identify new axon death genes, each of which will help us understand the molecular basis of axon degeneration, and provide exciting new targets to alleviate human disease.
Neuronal death is a common characteristic of all neurodegenerative diseases as well as acute brain trauma. Surprisingly, we know very little about how neurons trigger axonal death. The experiments described here will help identify novel genes involved in axonal destruction, with the hope of identifying genes that can be therapeutically targeted to combat neurodegenerative disease and brain trauma.
