SARM1 (sterile alpha and TIR motif containing 1) is responsible for depletion of nicotinamide adenine dinucleotide in its oxidized form (NAD+) during Wallerian degeneration associated with neuropathies. Plant nucleotide-binding leucine-rich repeat (NLR) immune receptors recognize pathogen effector proteins and trigger localized cell death to restrict pathogen infection. Both processes depend on closely related Toll/interleukin-1 receptor (TIR) domains in these proteins, which, as we show, feature self-association–dependent NAD+ cleavage activity associated with cell death signaling. We further show that SARM1 SAM (sterile alpha motif) domains form an octamer essential for axon degeneration that contributes to TIR domain enzymatic activity. The crystal structures of ribose and NADP+ (the oxidized form of nicotinamide adenine dinucleotide phosphate) complexes of SARM1 and plant NLR RUN1 TIR domains, respectively, reveal a conserved substrate binding site. NAD+ cleavage by TIR domains is therefore a conserved feature of animal and plant cell death signaling pathways.

Toll-like receptor (TLR) signaling is a key innate immunity response to pathogens. Recruitment of signaling adapters such as MAL (TIRAP) and MyD88 to the TLRs requires Toll/interleukin-1 receptor (TIR)-domain interactions, which remain structurally elusive. Here we show that MAL TIR domains spontaneously and reversibly form filaments in vitro. They also form cofilaments with TLR4 TIR domains and induce formation of MyD88 assemblies. A 7-Å-resolution cryo-EM structure reveals a stable MAL protofilament consisting of two parallel strands of TIR-domain subunits in a BB-loop-mediated head-to-tail arrangement. Interface residues that are important for the interaction are conserved among different TIR domains. Although large filaments of TLR4, MAL or MyD88 are unlikely to form during cellular signaling, structure-guided mutagenesis, combined with in vivo interaction assays, demonstrated that the MAL interactions defined within the filament represent a template for a conserved mode of TIR-domain interaction involved in both TLR and interleukin-1 receptor signaling.

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Degeneration of axons eliminates unwanted or damaged nerves from an organism as part of normal neuronal development and injury, but is also a common feature in neurodegenerative disease and neuropathies. Recently, a Toll-like receptor (TLR) adaptor protein, sterile-alpha and armadillo motif-containing protein (SARM), has shown to promote axon degeneration after injury (Wallerian degeneration) and promote cell death. The protein comprises three domains: two central tandem sterile-alpha motifs (SAM) flanked by an N-terminal armadillo repeat motif (ARM) and a C-terminal Toll/interleukin-1 receptor (TIR) domain. We have solved the crystal structure of the tandem SAM domains of human SARM at 2.8Å resolution which form an octameric ring. Using small-angle X-ray scattering (SAXS) and multi-angle light scattering (MALS) we can determine this ring structure is conserved across species. We have also solved the crystal structure of the C-terminal TIR domain at 1.8Å resolution. The TIR domains are responsible for transmitting signal in TLR signalling and requires a clustering event to bring TIR domains in close proximity. In relation to SARM promoting axon degeneration and cell-death, we hypothesise that the tandem SAM domains act as the "clustering mechanism" to bring the TIR domains together, which then recruits downstream proteins to cause axon degeneration and cell death. We are currently testing mutants that knock out the oligomeric ring of the tandem SAM domains and residues in the TIR domain to analyse the role these have in axon degeneration and cell-death, which could provide new targets for therapeutic drugs in neurodegenerative disease and neuropathies. This concept of forming complex assemblies in immunity and inflammation are seen in other immune pathways, such as the inflammasome pathway, effector-triggered immunity in plants and apoptosis.

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