Cell Survival and Cytokine Release after Inflammasome Activation Is Regulated by the Toll-IL-1R Protein SARM.

Assembly of inflammasomes after infection or injury leads to the release of interleukin-1ß (IL-1ß) and to pyroptosis. After inflammasome activation, cells either pyroptose or enter a hyperactivated state defined by IL-1ß secretion without cell death, but what controls these different outcomes is unknown. Here, we show that removal of the Toll-IL-1R protein SARM from macrophages uncouples inflammasome-dependent cytokine release and pyroptosis, whereby cells displayed increased IL-1ß production but reduced pyroptosis. Correspondingly, increasing SARM in cells caused less IL-1ß release and more pyroptosis. SARM suppressed IL-1ß by directly restraining the NLRP3 inflammasome and, hence, caspase-1 activation. Consistent with a role for SARM in pyroptosis, Sarm1-/- mice were protected from lipopolysaccharide (LPS)-stimulated sepsis. Pyroptosis-inducing, but not hyperactivating, NLRP3 stimulants caused SARM-dependent mitochondrial depolarization. Thus, SARM-dependent mitochondrial depolarization distinguishes NLRP3 activators that cause pyroptosis from those that do not, and SARM modulation represents a cell-intrinsic mechanism to regulate cell fate after inflammasome activation.

SARM1 regulates NAD+-linked metabolism and select immune genes in macrophages.

Sterile alpha and HEAT/armadillo motif-containing protein (SARM1) was recently described as a NAD+-consuming enzyme and has previously been shown to regulate immune responses in macrophages. Neuronal SARM1 is known to contribute to axon degeneration due to its NADase activity. However, how SARM1 affects macrophage metabolism has not been explored. Here, we show that macrophages from Sarm1-/- mice display elevated NAD+ concentrations and lower cyclic ADP-ribose, a known product of SARM1-dependent NAD+ catabolism. Further, SARM1-deficient macrophages showed an increase in the reserve capacity of oxidative phosphorylation and glycolysis compared to WT cells. Stimulation of macrophages to a proinflammatory state by lipopolysaccharide (LPS) revealed that SARM1 restricts the ability of macrophages to upregulate glycolysis and limits the expression of the proinflammatory gene interleukin (Il) 1b, but boosts expression of anti-inflammatory Il10. In contrast, we show macrophages lacking SARM1 induced to an anti-inflammatory state by IL-4 stimulation display increased oxidative phosphorylation and glycolysis, and reduced expression of the anti-inflammatory gene, Fizz1. Overall, these data show that SARM1 fine-tunes immune gene transcription in macrophages via consumption of NAD+ and altered macrophage metabolism.

CRISPR/Cas9-mediated SARM1 knockout and epitope-tagged mice reveal that SARM1 does not regulate nuclear transcription, but is expressed in macrophages.

SARM1 is a toll/interleukin-1 receptor -domain containing protein, with roles proposed in both innate immunity and neuronal degeneration. Murine SARM1 has been reported to regulate the transcription of chemokines in both neurons and macrophages; however, the extent to which SARM1 contributes to transcription regulation remains to be fully understood. Here, we identify differential gene expression in bone-marrow-derived macrophages (BMDMs) from C57BL/6 congenic 129 ES cell-derived Sarm1-/- mice compared with wild type (WT). However, we found that passenger genes, which are derived from the 129 donor strain of mice that flank the Sarm1 locus, confound interpretation of the results, since many of the identified differentially regulated genes come from this region. To re-examine the transcriptional role of SARM1 in the absence of passenger genes, here we generated three Sarm1-/- mice using CRISPR/Cas9. Treatment of neurons from these mice with vincristine, a chemotherapeutic drug causing axonal degeneration, confirmed SARM1's function in that process; however, these mice also showed that lack of SARM1 has no impact on transcription of genes previously shown to be affected such as chemokines. To gain further insight into SARM1 function, we generated an epitope-tagged SARM1 mouse. In these mice, we observed high SARM1 protein expression in the brain and brainstem and lower but detectable levels in macrophages. Overall, the generation of these SARM1 knockout and epitope-tagged mice has clarified that SARM1 is expressed in mouse macrophages yet has no general role in macrophage transcriptional regulation and has provided important new models to further explore SARM1 function.