Structural and biochemical characterisation of SARM in axon degeneration, cell-death and TLR signalling

S Horsefield1, T Ve1,2, X Zhang4, L Casey1 and B Kobe1,3

  1. School of Chemistry and Molecular Biology and Australian Infectious Disease Research Centre, University of Queensland, Queensland 4072, Australia
  2. Institute for Glycomics, Griffith University, Southport, Queensland 4222, Australia
  3. Institute for Molecular Biosciences, University of Queensland, Brisbane, Queensland 4072, Australia
  4. Commonwealth Scientific and Industrial Research Organisation, Canberra, Australian Capital Territory 2061, Australia

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). The SARM protein also inhibits TLR signalling and promotes 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 2.8 Å 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. 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. We are currently testing mutants that knock out the oligomeric ring to confirm whether this prevents 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.