Browsing by keyword "phase transition"
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A phase transition enhances the catalytic activity of SARM1, an NAD(+) glycohydrolase involved in neurodegenerationSterile alpha and toll/interleukin receptor (TIR) motif-containing protein 1 (SARM1) is a neuronally expressed NAD(+) glycohydrolase whose activity is increased in response to stress. NAD(+) depletion triggers axonal degeneration, which is a characteristic feature of neurological diseases. Notably, loss of SARM1 is protective in murine models of peripheral neuropathy and traumatic brain injury. Herein, we report that citrate induces a phase transition that enhances SARM1 activity by ~2000-fold. This phase transition can be disrupted by mutating a residue involved in multimerization, G601P. This mutation also disrupts puncta formation in cells. We further show that citrate induces axonal degeneration in C. elegans that is dependent on the C. elegans orthologue of SARM1 (TIR-1). Notably, citrate induces the formation of larger puncta indicating that TIR-1/SARM1 multimerization is essential for degeneration in vivo. These findings provide critical insights into SARM1 biology with important implications for the discovery of novel SARM1-targeted therapeutics.
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Pathogen infection and cholesterol deficiency activate the C. elegans p38 immune pathway through a TIR-1/SARM1 phase transitionIntracellular 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 Caenorhabditis elegans Toll/interleukin-1 receptor domain protein (TIR-1), an NAD(+) glycohydrolase homologous to mammalian sterile alpha and TIR motif-containing 1 (SARM1), underlies p38 PMK-1 immune pathway activation in C. elegans intestinal epithelial cells. Through visualization of fluorescently labeled TIR-1/SARM1 protein, we demonstrate that physiologic stresses, both pathogen and non-pathogen, induce multimerization of TIR-1/SARM1 into visible puncta within intestinal epithelial cells. In vitro enzyme kinetic analyses revealed that, like mammalian SARM1, the NAD(+) glycohydrolase activity of C. elegans TIR-1 is dramatically potentiated by protein oligomerization and a phase transition. Accordingly, C. elegans with genetic mutations that specifically block either multimerization or the NAD(+) glycohydrolase activity of TIR-1/SARM1 fail to induce p38 PMK phosphorylation, are unable to increase immune effector expression, and are dramatically susceptible to bacterial infection. Finally, we demonstrate that a loss-of-function mutation in nhr-8, which alters cholesterol metabolism and is used to study conditions of sterol deficiency, causes TIR-1/SARM1 to oligomerize into puncta in intestinal epithelial cells. Cholesterol scarcity increases p38 PMK-1 phosphorylation, primes immune effector induction in a manner that requires TIR-1/SARM1 oligomerization and its intrinsic NAD(+) glycohydrolase activity, and reduces pathogen accumulation in the intestine during a subsequent infection. These data reveal a new adaptive response that allows a metazoan host to anticipate pathogen threats during cholesterol deprivation, a time of relative susceptibility to infection. Thus, a phase transition of TIR-1/SARM1 as a prerequisite for its NAD(+) glycohydrolase activity is strongly conserved across millions of years of evolution and is essential for diverse physiological processes in multiple cell types.
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Surveillance of Host and Pathogen Derived Metabolites Activates Intestinal ImmunityIntestinal epithelial cells function, in part, to detect infection with pathogenic organisms and are key regulators of intestinal immune homeostasis. However, it is not fully understood how intestinal epithelial cells sense pathogen infection and coordinate the induction of protective immune defenses. Here, we define two new mechanisms of innate immune regulation in a metazoan host. First, we characterize the first bacterial pattern recognition receptor and its natural ligand in Caenorhabditis elegans. We show that the C. elegans nuclear hormone receptor NHR-86/HNF4 directly senses phenazine-1-carboxamide (PCN), a metabolite produced by pathogenic strains of Pseudomonas aeruginosa. PCN binds to the ligand-binding domain of NHR-86/HNF4, a ligand-gated transcription factor, and activates innate immunity in intestinal epithelial cells. In addition, we show that C. elegans NHR-86 senses PCN, and not other phenazine metabolites, as a marker of pathogen virulence to engage protective anti-pathogen defenses. Second, we show that a phase transition of the C. elegans Toll/interleukin-1 receptor domain protein (TIR-1) controls signaling by the C. elegans p38 PMK-1 MAPK pathway. Physiologic stress, both P. aeruginosa infection and sterol scarcity, induce multimerization of TIR-1 within intestinal epithelial cells. Like the mammalian homolog of TIR-1, SARM1, oligomerization and phase transition of C. elegans TIR-1 dramatically potentiate its NAD+ glycohydrolase activity. TIR-1/SARM1 multimerization and NAD+ glycohydrolase activity are required for activation of C. elegans p38 PMK-1 pathway signaling and pathogen resistance. These data uncover a mechanism by which nematodes interpret environmental conditions to prime innate immune defenses and promote survival in microbe rich environments. C. elegans animals augment these immune defenses by surveying for ligands specifically associated with toxigenic pathogens that are poised to cause disease. These findings define a new paradigm of intestinal immune control that informs the evolution of innate immunity in all metazoans.
