Regenerative and degenerative responses to axonal injury

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Project Summary/Abstract: Axons form connections between neurons over great distances in the brain and body, hence are vulnerable to damage and stress. This project studies an evolutionarily conserved stress response pathway that becomes activated in multiple scenarios of damage and stress, including nerve damage, traumatic axonal injury, disruption of axonal cytoskeleton, axonal transport, and mouse models of ALS and Alzheimer's Disease. The pathway, governed by the dileucine zipper kinase DLK, known as Wallenda (Wnd) in Drosophila, promotes dichotomous outcomes of neuronal death, degeneration and regeneration in these diverse scenarios. The long term goals of this project are (1) to understand the mechanisms that lead to DLK signaling activation, and (2) to understand the cellular pathways that are regulated by DLK. The project combines studies in both Drosophila and mice, focusing on motoneuron (MN) responses to peripheral nerve injury (PNI). For the first goal, Aim 1 tests a hypothesis, built upon observation in Drosophila, that DLK/Wnd signaling is restrained by the presence of an intact synaptic connection, hence becomes activated following synapse loss. Subaim 1a probes the mechanism of synaptic restraint using genetic tools at Drosophila neuromuscular junction (NMJ). Subaim 1b tests whether the principle is conserved for mammalian neurons, taking advantage of the accessibility of the mouse NMJ to surgical manipulations and imaging. For the second goal, Aim 2 builds from translatome profiling (using RiboTag technology) of DLK-dependent responses in mouse MNs, which identified many secreted proteins and immune molecules as targets of DLK regulation following PNI. Subaim 2a establishes a descriptive characterization of the glial and immune responses gated by DLK in injured MNs and in DRG neurons, which are also injured by PNI. Subaim 2b tests a specific hypothesis that `synaptic stripping' in which upstream synaptic inputs to damaged motoneurons are removed concomitant with a recruitment of microglia to the MN cell body, is controlled by DLK activation in the injured MN. While Aim 1 identifies specific circumstances that activate DLK signaling, Aim 2 identifies downstream cellular functions of DLK that may be broadly relevant in many paradigms of injury and stress. Together the work in these two aims is expected to shed light on the relationship of synapse loss with other responses in the nervous system (neuronal death, inflammation and plasticity) in diverse contexts of injury, neuropathies and neurodegenerative disease.