A TIR-1/SARM1 phase transition underlies p38 immune pathway activation in theC. elegansintestine

Abstract

Intracellular signaling regulators can be concentrated into membrane-free, higher-ordered protein assemblies to initiate protective responses during stress — a process known as phase transition. Here, we show that a phase transition of the C. elegans Toll/interleukin-1 receptor domain protein (TIR-1), a homolog of the mammalian sterile alpha and TIR motif-containing 1 (SARM1), primes host immune defenses when dietary sterols are limited to handle subsequent bacterial infection. TIR-1/SARM1 is an upstream component of the p38 PMK-1 pathway in intestinal cells, an innate immune defense and stress response pathway in metazoans. Under conditions of low cholesterol availability, multimerization and precipitation of TIR-1/SARM1 potentiates the intrinsic NAD+ glycohydrolase activity of this protein complex, increases p38 PMK-1 phosphorylation, and promotes pathogen clearance from the intestine. Dietary cholesterol is required for C. elegans to survive infection with pathogenic bacteria and to support development, fecundity, and lifespan. Thus, activation of the p38 PMK-1 pathway in sterol-deficient animals is an adaptive response that allows a metazoan host to anticipate environmental threats under conditions of essential metabolite scarcity.SIGNIFICANCE STATEMENTThe nematode C. elegans must consume dietary sterols to support growth and reproduction. However, access to dietary sterols in its natural habitat is not guaranteed and thus, nematodes have evolved mechanisms to promote survival in sterol-poor environments. Here, we demonstrate that activation of the p38 PMK-1 innate immune pathway in response to low sterol availability promotes clearance of a bacterial pathogen from the intestine. Pre-emptive activation of innate immune defenses occurs through a phase transition of TIR-1/SARM1, an upstream component of the p38 PMK-1 pathway, which activates its intrinsic NAD+ glycohydrolase activity. Thus, nematodes anticipate threats from infectious pathogens during a time when the animal is relatively susceptible to infection.

Activation of intestinal immunity by pathogen effector-triggered aggregation of lysosomal TIR-1/SARM1

“…Previously, we used clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9 to engineer a C. elegans strain that expresses TIR-1/SARM1 protein tagged with the fluorophore wrmScarlet at its endogenous genomic locus. 12 We observed that pathogen infection induces TIR-1::wrmScarlet multimerization into large protein assemblies, or puncta, in the intestinal epithelium. 12 Here, we sought to characterize the mechanism by which infection promoted TIR-1/SARM1 assembly to activate the p38 PMK-1 innate immune pathway.…”

“…12 We observed that pathogen infection induces TIR-1::wrmScarlet multimerization into large protein assemblies, or puncta, in the intestinal epithelium. 12 Here, we sought to characterize the mechanism by which infection promoted TIR-1/SARM1 assembly to activate the p38 PMK-1 innate immune pathway.…”

“…We therefore performed immunostaining on fixed animals to visualize TIR-1/SARM1 in intestinal tissues. For these studies, a C. elegans strain that we previously generated using CRISPR-Cas9, which contains TIR-1/SARM1 tagged at its endogenous locus with a 3xFLAG sequence, 12 was probed with an anti-FLAG antibody in fixed samples. Immunostaining of TIR-1/SARM1 confirmed that this protein formed puncta in intestinal epithelial cells (Figure 1A).…”

“…[6][7][8][9][10][11] We previously showed TIR-1/SARM1 multimerizes into protein assemblies (puncta) in intestinal epithelial cells. 12 Functionally, concentration of TIR-1/SARM1 in this manner engages its intrinsic NADase activity, which activates p38 PMK-1 signaling to promote resistance to pathogen infection. 12 Intriguingly, multimerization of plant TIR is also a prerequisite for its NADase activity, 13,14 as it is for human SARM1, which promotes axonal degeneration following neuronal injury.…”

“…12 Functionally, concentration of TIR-1/SARM1 in this manner engages its intrinsic NADase activity, which activates p38 PMK-1 signaling to promote resistance to pathogen infection. 12 Intriguingly, multimerization of plant TIR is also a prerequisite for its NADase activity, 13,14 as it is for human SARM1, which promotes axonal degeneration following neuronal injury. 15,16 In this study, we sought to determine how pathogen infection initiates TIR-1/SARM1 multimerization and its intrinsic NADase activity to promote host defense against bacterial infection in the animal intestine.…”