Sarm1-Mediated Axon Degeneration Requires Both SAM and TIR Interactions

Abstract

Axon degeneration is an evolutionarily conserved pathway that eliminates damaged or unneeded axons. Manipulation of this poorly understood pathway may allow treatment of a wide range of neurological disorders. In an RNAi-based screen performed in cultured mouse DRG neurons, we observed strong suppression of injury-induced axon degeneration upon knockdown of Sarm1 [SARM (sterile ␣-motif-containing and armadillo-motif containing protein)]. We find that a SARM-dependent degeneration program is engaged by disparate neuronal insults: SARM ablation blocks axon degeneration induced by axotomy or vincristine treatment, while SARM acts in parallel with a soma-derived caspase-dependent pathway following trophic withdrawal. SARM is a multidomain protein that associates with neuronal mitochondria. Deletion of the N-terminal mitochondrial localization sequence disrupts SARM mitochondrial localization in neurons but does not alter its ability to promote axon degeneration. In contrast, mutation of either the SAM (sterile ␣ motif) or TIR (Toll-interleukin-1 receptor) domains abolishes the ability of SARM to promote axonal degeneration, while a SARM mutant containing only these domains elicits axon degeneration and nonapoptotic neuronal death even in the absence of injury. Protein-protein interaction studies demonstrate that the SAM domains are necessary and sufficient to mediate SARM-SARM binding. SARM mutants lacking a TIR domain bind full-length SARM and exhibit strong dominant-negative activity. These results indicate that SARM plays an integral role in the dismantling of injured axons and support a model in which SAM-mediated multimerization is necessary for TIR-dependent engagement of a downstream destruction pathway. These findings suggest that inhibitors of SAM and TIR interactions represent therapeutic candidates for blocking pathological axon loss and neuronal cell death.

SARM1-specific motifs in the TIR domain enable NAD + loss and regulate injury-induced SARM1 activation

“…Tandem sterile alpha motif (SAM) domains mediate SARM1 multimerization, and hence are required for SARM1 activity. The SARM1 N-terminal domain contains Heat/Armadillo repeats and restrains SAM-TIR activity, maintaining SARM1 in an autoinhibited state (3,11,12). Although SARM1 could possess constitutive activity that is countered in a healthy axon downstream of the pathway, our current model is that axon injury triggers a release of SARM1 autoinhibition, thus enabling TIR-dependent neuronal destruction (1).…”

“…Genetic screens in invertebrate and vertebrate model systems identified the Sterile alpha and Toll/interleukin receptor (TIR) motif-containing protein 1 (SARM1) as a conserved, fundamental executioner of pathological axon destruction (2,3). After nerve injury, SARM1 is required for the precipitous loss of the metabolite NAD + (4).…”

“…Hence, SARM1 exerts a major role in pathological neuron destruction, so a deeper understanding of the molecular mechanism of SARM1 function may identify novel targets for the inhibition of this prodestructive factor. Domain analysis of the SARM1 protein generated a model of SARM1 activation and function (3,11,12). SARM1 possesses a Toll/interleukin receptor (TIR) domain that is required for SARM1-dependent axon degeneration and cell death.…”

“…SARM1 possesses a Toll/interleukin receptor (TIR) domain that is required for SARM1-dependent axon degeneration and cell death. Recently, others and we demonstrated that enforced dimerization of the SARM1 TIR domain is sufficient to induce local axon degeneration (3,4,13). Tandem sterile alpha motif (SAM) domains mediate SARM1 multimerization, and hence are required for SARM1 activity.…”

Innate immune responses regulate morphogenesis and degeneration: roles of Toll-like receptors and Sarm1 in neurons

“…The TIR domain of Sarm1 is important for activation of the downstream destruction pathway, while multimerization mediated by the SAM domain of Sarm1 is also essential for the function of Sarm1 to trigger axon degeneration [56] .…”