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

Abstract

Axon injury in response to trauma or disease stimulates a selfdestruction program that promotes the localized clearance of damaged axon segments. Sterile alpha and Toll/interleukin receptor (TIR) motif-containing protein 1 (SARM1) is an evolutionarily conserved executioner of this degeneration cascade, also known as Wallerian degeneration; however, the mechanism of SARM1-dependent neuronal destruction is still obscure. SARM1 possesses a TIR domain that is necessary for SARM1 activity. In other proteins, dimerized TIR domains serve as scaffolds for innate immune signaling. In contrast, dimerization of the SARM1 TIR domain promotes consumption of the essential metabolite NAD + and induces neuronal destruction. This activity is unique to the SARM1 TIR domain, yet the structural elements that enable this activity are unknown. In this study, we identify fundamental properties of the SARM1 TIR domain that promote NAD + loss and axon degeneration. Dimerization of the TIR domain from the Caenorhabditis elegans SARM1 ortholog TIR-1 leads to NAD + loss and neuronal death, indicating these activities are an evolutionarily conserved feature of SARM1 function. Detailed analysis of sequence homology identifies canonical TIR motifs as well as a SARM1-specific (SS) loop that are required for NAD + loss and axon degeneration. Furthermore, we identify a residue in the SARM1 BB loop that is dispensable for TIR activity yet required for injury-induced activation of full-length SARM1, suggesting that SARM1 function requires multidomain interactions. Indeed, we identify a physical interaction between the autoinhibitory N terminus and the TIR domain of SARM1, revealing a previously unrecognized direct connection between these domains that we propose mediates autoinhibition and activation upon injury.A xon degeneration in response to injury or disease is a hallmark of neurological disorders of the peripheral and central nervous systems. Akin to programmed cell death pathways, such as apoptosis, axon injury in response to disease or trauma stimulates a local signaling cascade that executes destruction of the injured axon segment (1). Despite growing attention, the molecular details of this prodegenerative cascade are still unclear. A more substantial understanding of the molecular factors that participate in this axon destructive process will likely provide new therapeutic strategies for peripheral neuropathies and other neurodegenerative conditions.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). Augmenting NAD + biosynthetic pathways protects injured axons from degeneration, suggesting this step is crucial in axonal breakdown (5, 6). In addition to local axon degeneration, SARM1 promotes neuronal cell death in response to mitochondrial toxins, oxygen gluc...

The SARM1 Toll/Interleukin-1 Receptor Domain Possesses Intrinsic NAD + Cleavage Activity that Promotes Pathological Axonal Degeneration

“…Both TLR4 and MyD88 TIR containing complexes showed no NADase activity (Figure S1B and S1C). These results support the previously reported unique roles of SARM1 among TIR adaptor proteins (Gerdts et al, 2015; O’Neill et al, 2013, Summers et al, 2016) in promoting axonal degeneration and neuronal NAD + depletion.…”

“…In contrast, no decrease in NAD + was observed if beads exposed to lysates were prepared from either non-transfected NRK1-HEK293T cells or from NRK1-HEK293T cells expressing SARM1-TIR lacking the StrepTag II (Figure 1D). We also tested a non-functional TIR domain mutant [SARM1(E596K)] that we recently identified in a structure/function analysis of SARM1 (Summers et al, 2016). Magnetic beads loaded with complexes assembled on this SARM1-TIR(E596K) mutant failed to degrade NAD + in this in vitro assay (Figure 1D).…”

Death Receptor 6 Promotes Wallerian Degeneration in Peripheral Axons

“…Although first described as a Toll like receptor adaptor protein, a primary receptor for Sarm1 in Wallerian degeneration has yet to be identified. It is known that axonal injury results in the release of autoinhibition by the N-terminal domain of Sarm1 although the precise mechanism that triggers this remains obscure [39]. Nevertheless, it is apparent that activation of Sarm1 downstream of axonal injury leads to JNK activation [15,29,40].…”

Therapeutic Potential of NAD-Boosting Molecules: The In Vivo Evidence

“…PARP1 inhibitors are marketed as adjuncts to chemotherapy or monotherapies for cancer (Brown et al, 2016). SARM1 is another NADase, which initiates a local axonal degeneration program after nerve injury that involves the rapid breakdown of NAD + to ADPR, cADPR and NAM (Essuman et al, 2017; Gerdts et al, 2015; Summers et al, 2016). XAV939, a putative SARM1 inhibitor that also inhibits PARP5a (TNKS) and 5b (TNKS2), was identified in a chemical genetic screen nearly a decade ago (Huang et al, 2009), though its potential to boost NAD + has only recently become evident (Gerdts et al, 2015).…”