Traumatic Axonopathy in the CNS as Wallerian degeneration

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PROJECT SUMMARY In this application we propose that traumatic axonal injury (TAI) leads to an active axonopathy with the molecular features of Wallerian degeneration and that targeting some of the relevant signals may protect axons as well as brain systems. This hypothesis is based on recent findings in our laboratory showing that axonal breakdown after diffuse TAI depends on the activation of Sterile Alpha and TIR Motif Containing 1 (SARM1) signaling and that molecular interventions to block SARM1 lead to significant gains in preserving axons and rescuing functions/behaviors that rely on axonal integrity. The case of TAI is unique in the sense that that many axons are only partially injured and are potentially salvageable, therefore blocking Wallerian-type self-destruction may afford long-term neuroprotection and change the prognosis of traumatic brain injury. Our proposal is organized in three specific aims. In Aim 1 we establish that TAI in an index CNS tract, i.e. the corticospinal system, leads to progressive axonopathy with the molecular signatures of Wallerian degeneration, i.e. activation of SARM1. In Aim 2 we ask whether axonal protection by genetic or pharmacological blockade of SARM1 signaling in the injured corticospinal tract translate into protection at the systems level, i.e. prevention of retrograde atrophy of corticospinal neurons, preservation of corticospinal connectivity and rescue of CST-dependent motor skills. In Aim 3 we explore the synergistic role of the mitogen-activated protein kinase (MAPK) pathway, specifically signaling by the dual leucine zipper kinase (DLK) and related leucine zipper kinase (LZK), in corticospinal axonal degeneration following TAI. The MAPK pathway signals general neuronal responses to injury and there is evidence that specific members of the pathway cooperate with SARM1-related signals in triggering or affecting the outcome of Wallerian degeneration. To achieve the previous aims, we use a complement of molecular genetic tools including knockout mice, dominant negative strategies and genome editing with CRISPR-Cas9, metabolomic assessments, CLARITY-based high-resolution neuropathology, structural and functional connectivity markers, behavioral testing, and small molecules as probes for molecular targets and also as therapeutic agents (the NAMPT inhibitor FK866 that serves as indirect inhibitor of SARM1 and the pan-Aurora inhibitor tozasertib that blocks DLK/LZK signaling). In summary, here we explore specific molecular mechanisms related to Wallerian degeneration and, in the course of doing this, we establish molecular targets for potential pharmacological interventions in traumatic brain injury.