SARM1-mediated mechanisms of axon degeneration

Oregon Health & Science University, Portland OR

Investigators

Abstract

Project summary Nervous system injury activates a complex series of events including axon degeneration, reactive glial responses, and nervous system inflammation. These processes conspire in poorly-defined ways to drive the destruction of axons and synaptic connections, and neuronal death, which ultimately result in neural circuit dysfunction. Our goal is to define molecular pathways that drive neurodegeneration and use that information to block axon and synapse loss in patients. In previous work we identified the TIR domain molecule dSarm/SARM1 (flies/mammals) as a key regulator of axon degeneration after axotomy. In dSarm/SARM1 mutant flies or mice, distal severed axons survive and remain functionally intact for weeks after axotomy without attachment to a cell body. This work identified dSarm/SARM1 as the first “axon death” gene, an endogenous gene whose normal function was to promote axon and synapse destruction. Since this discovery, we and others have shown that the SARM1 pathway is required to drive axon degeneration in a number of preclinical models of human neurological disease. SARM1 is now a therapeutic target for many pharmaceutical companies to treat neurodegenerative disorders, and the first Phase 1 trials with SARM1 inhibitors began in 2022. We sought to identify new molecules that drive neurodegeneration with dSarm/SARM1. A key step in activating dSarm/Sarm1 in both flies and mice is loss of the primary NAD+ biosynthetic enzyme in axons, Nmnat. Nmnat is a labile survival factor that is transported down axons from the cell body. After injury or in neurological disease, loss of Nmnat supplies depletes axonal NAD+ and activates dSarm/SARM1-dependent neurodegeneration. In a screen for genes required for neurodegeneration after depletion of Nmnat, we identified the Drosophila with-no-lysine kinase (dWnk) gene. dWnk is a conserved (WNK1-4 in mammals) kinase that is best known for acting as an osmosensor in kidney, where it activates the downstream kinases Spak/OSR1, and ultimately modulates blood pressure through activation of Na+/K+/Cl- co- transporters. In preliminary work we show that loss of dWnk or fray (fly Spak/OSR1) can suppress neurodegeneration induced by depletion of Nmnat, and provide strong genetic evidence that dWnk/Fray act in parallel with dSarm to drive neurodegeneration through the downstream BTB/Back domain molecule Axundead. In Aim1, we will define the key domains required for dWnk to drive neurodegeneration, explore its signaling relationship with dSarm, assay dynamic changes in localization of dWnk during neurodegeneration, and whether it is required for neurodegeneration in two simple fly models of chemotherapy-induced neuropathay and ALS. In Aim 2 we will perform a similar study with Fray, and explore how its activation is sufficient to bypass the requirements for dSarm in neurodegeneration, as our preliminary data strongly suggests. In Aim 3 we will use candidate gene, forward screens and proximity labeling approaches to identify key downstream signaling molecules required for Fray to promote neurodegeneration. Together, our work will define the role of several new pro-degenerative molecules that work with dSarm in vivo and determine how they drive neurodegeneration. Given the strong conservation of dSarm/Sarm1 and dWnk/WNK signaling, we anticipate our work will also provide fundamental new insights into the mechanisms of axon degeneration in human disease.