Pathogen infection and cholesterol deficiency activate the C. elegans p38 immune pathway through a TIR-1/SARM1 phase transition

Evolutionarily ancient functions of enzymatic TIR proteins in innate immunity

Proteins with a Toll/interleukin-1 receptor/resistance (TIR) domain are among the most ancient immune regulators and include well-known pattern recognition receptors (PRRs). A specialized subset of TIR domain proteins are enzymes that predominantly use nicotinamide adenine dinucleotide (NAD+) to generate second messenger metabolites. These enzymatic TIR proteins have essential roles in bacteria, plant, and animal immunity. The mechanism of activation of these TIR proteins, conserved across Kingdoms, involves oligomerization into higher-ordered structures, which activates their intrinsic enzymatic activity. Here, we review the functions of enzymatic TIR proteins in innate immunity in bacteria, plants, and animals. This work offers insights into the evolutionary origins of immunity itself and defines fundamental principles of immune surveillance across the Tree of Life.

Modulation of Pseudomonas aeruginosa-induced avoidance behavior by Shen Qi pills via mitogen-activated protein kinase PMK-1 and forkhead box protein O DAF-16 in Caenorhabditis elegans

BACKGROUND

Avoidance behavior is one of the core features of anxiety and related in-depth study can help to reveal the biological basis of these disorders. In recent years, traditional Chinese medicine has incorporated Shen Qi pills to treat neuropsychiatric disorders, such as depression and post-traumatic stress, and has achieved significant therapeutic effects. However, its specific mechanism of action is still unclear.

PURPOSE

The aim of this study was to link the avoidance phenotype to psychiatric disorders by utilizing the Caenorhabditis elegans as a biological model, revealing the potential common mechanisms underlying the treatment of these disorders with Shen Qi pills.

METHODS

Avoidance behavior and immunity of C. elegans as a phenotypic entry point to explore the molecular mechanisms by which Shen Qi pills affects avoidance behavior with the help of pmk-1 and daf-16 mutants and RNA interference techniques.

RESULTS

We found that the intervention of Shen Qi pills can delay the avoidance behavior of C. elegans to P. aeruginosa, improve the immunity level, and reduce the up-regulation of pmk-1 and daf-16 genes induced by P. aeruginosa. Shen Qi pills did not improve the immunity of pmk-1 mutant but could still enhance the immunity of daf-16 mutant. After daf-16 knockout, Shen Qi pills could not delay its avoidance behavior, which was consistent with the results shown in the neuron-specific silencing of daf-16 C. elegans.

CONCLUSION

These findings reveal the conclusion that Shen Qi pills regulate the avoidance behavior of C. elegans induced by P. aeruginosa via PMK-1 and DAF-16, with the latter acting directly on neurons independent of immune pathways.

Patterns of pathogenesis in innate immunity: insights from C. elegans

The cells in barrier tissues can distinguish pathogenic from commensal bacteria and target inflammatory responses only in the context of infection. As such, these cells must be able to identify pathogen infection specifically and not just the presence of an infectious organism, because many innocuous bacteria express the ligands that activate innate immunity in other contexts. Unravelling the mechanisms that underly this specificity, however, is challenging. Free-living nematodes, such as Caenorhabditis elegans, are faced with a similar dilemma, as they live in microorganism-rich habitats and eat bacteria as their source of nutrition. Nematodes lost canonical mechanisms of pattern recognition during their evolution and have instead evolved mechanisms to identify specific ligands or symptoms in the host that indicate active infection with an infectious microorganism. Here we review how C. elegans surveys for these patterns of pathogenesis to activate innate immune defences. Collectively, this work demonstrates that using C. elegans as an experimental platform to study host-pathogen interactions at barrier surfaces reveals primordial and fundamentally important principles of innate immune sensing in the animal branch of the tree of life.

Endosomal catabolism of phosphatidylinositol 4,5-bisphosphate is fundamental in building resilience against pathogens

Endosomes are characterized by the presence of various phosphoinositides that are essential for defining the membrane properties. However, the interplay between endosomal phosphoinositides metabolism and innate immunity is yet to be fully understood. Here, our findings highlight the evolutionary continuity of RAB-10/Rab10's involvement in regulating innate immunity. Upon infection of Caenorhabditis elegans with Pseudomonas aeruginosa, an increase in RAB-10 activity was observed in the intestine. Conversely, when RAB-10 was absent, the intestinal diacylglycerols (DAGs) decreased, and the animal's response to the pathogen was impaired. Further research revealed that UNC-16/JIP3 acts as an RAB-10 effector, facilitating the recruitment of phospholipase EGL-8 to endosomes. This leads to a decrease in endosomal phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) and an elevation of DAGs, as well as the activation of the PMK-1/p38 MAPK innate immune pathway. It is noteworthy that the dimerization of UNC-16 is a prerequisite for its interaction with RAB-10(GTP) and the recruitment of EGL-8. Moreover, we ascertained that the rise in RAB-10 activity, due to infection, was attributed to the augmented expression of LET-413/Erbin, and the nuclear receptor NHR-25/NR5A1/2 was determined to be indispensable for this increase. Hence, this study illuminates the significance of endosomal PI(4,5)P2 catabolism in boosting innate immunity and outlines an NHR-25-mediated mechanism for pathogen detection in intestinal epithelia.

Biopeptide-rich fermented hemp seeds: Boosting anti-inflammatory and immune responses through Lactiplantibacillus plantarum probiotic fermentation

Cannabis sativa L. (hemp) seeds are increasingly recognized as a promising food source rich in phytochemicals that support inflammatory and immunological reactions. This study investigates whether fermentation with Lactiplantibacillus plantarum can further enhance these functional properties, paving the way for hemp seeds to be developed into potent functional food ingredients. Aqueous, 70 % ethanol, and ethyl acetate extracts from both L. plantarum-fermented (FHS) and unfermented hemp seeds (HS) were evaluated for their anti-inflammatory activities using cell-based assays. The 70 % ethanol extract of FHS exhibited marked inhibitory effects on cytokines, including TNF-α, IL-1β, and IL-10, with fermentation significantly enhancing these effects by 25 %, 39.3 %, and 29.6 %, respectively, compared to the unfermented extracts. Additionally, mRNA expression analysis confirmed the strong immunomodulatory potential of the fermented extracts. Intracellular metabolomic analysis revealed that the 'antifolate resistance', 'nicotine addiction', 'aminoacyl-tRNA biosynthesis', and 'D-amino acid metabolism' are highlighted in the reasons for this enhancement. Furthermore, FHS significantly prolonged the survival of C. elegans exposed to pathogens, with gene expression analysis indicating modulation of the innate immune system via regulation of genes such as gcs-1, lys-1, dbl-1, pmk-1, elt-2, and dod-22. A comprehensive metabolomic and correlation analysis identified five novel bioactive peptides (AAELIGVP, AAVPYPQ, VFPEVAP, DVIGVPLG, PVPKVL) and bioactive acids (indoleacetic acid and homovanillic acid) that were enriched during fermentation, which are strongly linked to the enhanced anti-inflammatory and immunomodulatory effects observed. These findings suggest that L. plantarum-fermented hemp seeds hold significant promise as functional ingredients in anti-inflammatory and immunomodulatory food products, with potential applications in health and wellness industries.

Di(2-ethylhexyl) phthalate disrupts circadian rhythm associated with changes in metabolites and cytochrome P450 gene expression in Caenorhabditis elegans

The plasticizer di(2-ethylhexyl) phthalate (DEHP) is a widespread environmental pollutant due to its extensive use. While circadian rhythms are inherent in most living organisms, the detrimental effects of DEHP on circadian rhythm and the underlying mechanisms remain largely unknown. This study investigated the influence of early developmental exposure to DEHP on circadian rhythm and explored the possible relationship between circadian disruption and DEHP metabolism in the model organism Caenorhabditis elegans. We observed that DEHP disrupted circadian rhythm in a dose-dependent fashion. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis revealed that DEHP-induced circadian disruption accompanies with altered proportions of DEHP metabolites in C. elegans. RNA sequencing data demonstrated that DEHP-induced circadian rhythm disruption caused differential gene expression. Moreover, DEHP-induced circadian disruption coincided with attenuated inductions of DEHP-induced cytochrome P450 genes, cyp-35A2, cyp-35A3, and cyp-35A4. Notably, cyp-35A2 mRNA exhibited circadian rhythm with entrainment, but DEHP exposure disrupted this rhythm. Our findings suggest that DEHP exposure disrupts circadian rhythm, which is associated with changes in DEHP metabolites and cytochrome P450 gene expression in C. elegans. Given the ubiquitous nature of DEHP pollution and the prevalence of circadian rhythms in living organisms, this study implies a potential negative impact of DEHP on circadian rhythm and DEHP metabolism in organisms.

Comamonas aquatica inhibits TIR-1/SARM1 induced axon degeneration

Emerging evidence suggests the microbiome critically influences the onset and progression of neurodegenerative diseases; however, the identity of neuroprotective bacteria and the molecular mechanisms that respond within the host remain largely unknown. We took advantage of Caenorhabditis elegans' well characterized nervous system and ability to eat uni-bacterial diets to determine how metabolites and neuroprotective molecules from single species of bacteria suppress degeneration of motor neurons. We found Comamonas aquatica significantly protects against degeneration induced by overexpressing a key regulator of axon degeneration, TIR-1/SARM1. Genetic analyses and metabolomics reveal Comamonas protects against neurodegeneration by providing sufficient Vitamin B12 to activate METR-1/MTR methionine synthase in the intestine, which then lowers toxic levels of homocysteine in TIR-1-expressing animals. Defining a molecular pathway between Comamonas and neurodegeneration adds significantly to our understanding of gut-brain interactions and, given the prominent role of homocysteine in neurodegenerative disorders, reveals how such a bacterium could protect against disease.

Translation initiation or elongation inhibition triggers contrasting effects on Caenorhabditis elegans survival during pathogen infection

Diverse microbial pathogens are known to attenuate host protein synthesis. Consequently, the host mounts a defense response against protein translation inhibition, leading to increased transcript levels of immune genes. The seemingly paradoxical upregulation of immune gene transcripts in response to blocked protein synthesis suggests that the defense mechanism against translation inhibition may not universally benefit host survival. However, a comprehensive assessment of host survival on pathogens upon blockage of different stages of protein synthesis is currently lacking. Here, we investigate the impact of knockdown of various translation initiation and elongation factors on the survival of Caenorhabditis elegans exposed to Pseudomonas aeruginosa. Intriguingly, we observe opposing effects on C. elegans survival depending on whether translation initiation or elongation is inhibited. While translation initiation inhibition enhances survival, elongation inhibition decreases it. Transcriptomic studies reveal that translation initiation inhibition activates a bZIP transcription factor ZIP-2-dependent innate immune response that protects C. elegans from P. aeruginosa infection. In contrast, inhibiting translation elongation triggers both ZIP-2-dependent and ZIP-2-independent immune responses that, while effective in clearing the infection, are detrimental to the host. Thus, our findings reveal the opposing roles of translation initiation and elongation inhibition in C. elegans survival during P. aeruginosa infection, highlighting distinct transcriptional reprogramming that may underlie these differences.

IMPORTANCE

Several microbial pathogens target host protein synthesis machinery, potentially limiting the innate immune responses of the host. In response, hosts trigger a defensive response, elevating immune gene transcripts. This counterintuitive response can have either beneficial or harmful effects on host survival. In this study, we conduct a comprehensive analysis of the impact of knocking down various translation initiation and elongation factors on the survival of Caenorhabditis elegans exposed to Pseudomonas aeruginosa. Intriguingly, inhibiting initiation and elongation factors has contrasting effects on C. elegans survival. Inhibiting translation initiation activates immune responses that protect the host from bacterial infection, while inhibiting translation elongation induces aberrant immune responses that, although clear the infection, are detrimental to the host. Our study reveals divergent roles of translation initiation and elongation inhibition in C. elegans survival during P. aeruginosa infection and identifies differential transcriptional reprogramming that could underlie these differences.

6-PPD quinone at environmentally relevant concentrations induces immunosenescenece by causing immunosuppression during the aging process

6-PPD quinone (6-PPDQ) could accelerate aging process. However, the underlying mechanism for the acceleration in aging process remains largely unclear. We aimed to examine the role of immunosuppression in 6-PPDQ in causing accelerated aging process in Caenorhabditis elegans. 6-PPDQ (0.1-10 μg/L) could decrease locomotion and increase reactive oxygen species (ROS) generation at both adult day-8 and day-12. 6-PPDQ at adult day-12 induced more severe immunosuppression reflected by decrease in expression of antimicrobial genes (lys-1, lys-7, spp-1, and dod-6) compared to that at adult day-8. Meanwhile, 6-PPDQ (10 μg/L) affected expressions of some transcriptional factor genes during the aging. Among them, at adult day-8, susceptibility to 6-PPDQ toxicity was caused by RNAi of daf-16, bar-1, elt-2, atf-7, skn-1, and nhr-8, and resistance to 6-PPDQ toxicity was induced by RNAi of daf-5, daf-3, and daf-12. Additionally, RNAi of daf-16, bar-1, elt-2, atf-7, skn-1, and nhr-8 caused more severe decrease in lys-1 and lys-7 expressions in 6-PPDQ exposed nematodes, whereas decrease in lys-1 and lys-7 expressions in 6-PPDQ exposed nematodes was inhibited by RNAi of daf-5, daf-3, and daf-12. The 6-PPDQ toxicity and 6-PPDQ induced decrease in lys-1 and lys-7 expressions were further suppressed by RNAi of insulin ligand genes (ins-6, ins-7, and daf-28) and receptor gene daf-2. Therefore, immunosuppression-caused immunosenescenece mediated the acceleration in aging process in 6-PPDQ exposed nematodes, which was under the control of certain transcriptional factors.

Intestinal immunity in C. elegans is activated by pathogen effector-triggered aggregation of the guard protein TIR-1 on lysosome-related organelles

Toll/interleukin-1/resistance (TIR)-domain proteins with enzymatic activity are essential for immunity in plants, animals, and bacteria. However, it is not known how these proteins function in pathogen sensing in animals. We discovered that the lone enzymatic TIR-domain protein in the nematode C. elegans (TIR-1, homolog of mammalian sterile alpha and TIR motif-containing 1 [SARM1]) was strategically expressed on the membranes of a specific intracellular compartment called lysosome-related organelles. The positioning of TIR-1 on lysosome-related organelles enables intestinal epithelial cells in the nematode C. elegans to survey for pathogen effector-triggered host damage. A virulence effector secreted by the bacterial pathogen Pseudomonas aeruginosa alkalinized and condensed lysosome-related organelles. This pathogen-induced morphological change in lysosome-related organelles triggered TIR-1 multimerization, which engaged its intrinsic NAD+ hydrolase (NADase) activity to activate the p38 innate immune pathway and protect the host against microbial intoxication. Thus, TIR-1 is a guard protein in an effector-triggered immune response, which enables intestinal epithelial cells to survey for pathogen-induced host damage.

Lysosome-related organelle integrity suppresses TIR-1 aggregation to restrain toxic propagation of p38 innate immunity

Innate immunity in bacteria, plants, and animals requires the specialized subset of Toll/interleukin-1/resistance gene (TIR) domain proteins that are nicotinamide adenine dinucleotide (NAD+) hydrolases. Aggregation of these TIR proteins engages their enzymatic activity, but it is unknown how this protein multimerization is regulated. Here, we discover that TIR oligomerization is controlled to prevent immune toxicity. We find that p38 propagates its own activation in a positive feedback loop, which promotes the aggregation of the lone enzymatic TIR protein in the nematode C. elegans (TIR-1, homologous to human sterile alpha and TIR motif-containing 1 [SARM1]). We perform a forward genetic screen to determine how the p38 positive feedback loop is regulated. We discover that the integrity of the specific lysosomal subcompartment that expresses TIR-1 is actively maintained to limit inappropriate TIR-1 aggregation on the membranes of these organelles, which restrains toxic propagation of p38 innate immunity. Thus, innate immunity in C. elegans intestinal epithelial cells is regulated by specific control of TIR-1 multimerization.

Regulation of TIR-1/SARM-1 by miR-71 Protects Dopaminergic Neurons in a C. elegans Model of LRRK2-Induced Parkinson's Disease

Parkinson's disease (PD) is a common neurodegenerative disorder characterized by symptoms such as bradykinesia, resting tremor, and rigidity, primarily driven by the degradation of dopaminergic (DA) neurons in the substantia nigra. A significant contributor to familial autosomal dominant PD cases is mutations in the LRRK2 gene, making it a primary therapeutic target. This study explores the role of microRNAs (miRNAs) in regulating the proteomic stress responses associated with neurodegeneration in PD using C. elegans models. Our focus is on miR-71, a miRNA known to affect stress resistance and act as a pro-longevity factor in C. elegans. We investigated miR-71's function in C. elegans models of PD, where mutant LRRK2 expression correlates with dopaminergic neuronal death. Our findings reveal that miR-71 overexpression rescues motility defects and slows dopaminergic neurodegeneration in these models, suggesting its critical role in mitigating the proteotoxic effects of mutant LRRK2. Conversely, miR-71 knockout exacerbates neuronal death caused by mutant LRRK2. Additionally, our data indicate that miR-71's neuroprotective effect involves downregulating the toll receptor domain protein tir-1, implicating miR-71 repression of tir-1 as vital in the response to LRRK2-induced proteotoxicity. These insights into miR-71's role in C. elegans models of PD not only enhance our understanding of molecular mechanisms in neurodegeneration but also pave the way for potential research into human neurodegenerative diseases, leveraging the conservation of miRNAs and their targets across species.

Modeling Host-Pathogen Interactions in C. elegans: Lessons Learned from Pseudomonas aeruginosa Infection

Infections, such as that by the multiresistant opportunistic bacterial pathogen Pseudomonas aeruginosa, may pose a serious health risk, especially on vulnerable patient populations. The nematode Caenorhabditis elegans provides a simple organismal model to investigate both pathogenic mechanisms and the emerging role of innate immunity in host protection. Here, we review the virulence and infection strategies of P. aeruginosa and host defenses of C. elegans. We summarize the recognition mechanisms of patterns of pathogenesis, including novel pathogen-associated molecular patterns and surveillance immunity of translation, mitochondria, and lysosome-related organelles. We also review the regulation of antimicrobial and behavioral defenses by the worm's neuroendocrine system. We focus on how discoveries in this rich field align with well-characterized evolutionary conserved protective pathways, as well as on potential crossovers to human pathogenesis and innate immune responses.

SARM1 regulates pro-inflammatory cytokine expression in human monocytes by NADase-dependent and -independent mechanisms

SARM1 is a Toll-IL-1 receptor (TIR) domain-containing protein with roles in innate immunity and neuronal death in diverse organisms. Unlike other innate immune TIR proteins that function as adaptors for Toll-like receptors (TLRs), SARM1 has NADase activity, and this activity regulates murine neuronal cell death. However, whether human SARM1, and its NADase activity, are involved in innate immune regulation remains unclear. Here, we show that human SARM1 regulates proinflammatory cytokine expression in both an NADase-dependent and -independent manner in monocytes. SARM1 negatively regulated TLR4-dependent TNF mRNA induction independently of its NADase activity. In contrast, SARM1 inhibited IL-1β secretion through both NADase-dependent inhibition of pro-IL-1β expression, and NADase-independent suppression of the NLRP3 inflammasome and hence processing of pro-IL-1β to mature IL-1β. Our study reveals multiple mechanisms whereby SARM1 regulates pro-inflammatory cytokines in human monocytes and shows, compared to other mammalian TIR proteins, a distinct NADase-dependent role for SARM1 in innate immunity.

Glucose-fed microbiota alters C. elegans intestinal epithelium and increases susceptibility to multiple bacterial pathogens

Overconsumption of dietary sugar can lead to many negative health effects including the development of Type 2 diabetes, metabolic syndrome, cardiovascular disease, and neurodegenerative disorders. Recently, the human intestinal microbiota, strongly associated with our overall health, has also been known to be affected by diet. However, mechanistic insight into the importance of the human intestinal microbiota and the effects of chronic sugar ingestion has not been possible largely due to the complexity of the human microbiome which contains hundreds of types of organisms. Here, we use an interspecies C. elegans/E. coli system, where E. coli are subjected to high sugar, then consumed by the bacterivore host C. elegans to become the microbiota. This glucose-fed microbiota results in a significant lifespan reduction accompanied by reduced healthspan (locomotion), reduced stress resistance, and changes in behavior and feeding. Lifespan reduction is also accompanied by two potential major contributors: increased intestinal bacterial density and increased concentration of reactive oxygen species. The glucose-fed microbiota accelerated the age-related development of intestinal cell permeability, intestinal distention, and dysregulation of immune effectors. Ultimately, the changes in the intestinal epithelium due to aging with the glucose-fed microbiota results in increased susceptibility to multiple bacterial pathogens. Taken together, our data reveal that chronic ingestion of sugar, such as a Western diet, has profound health effects on the host due to changes in the microbiota and may contribute to the current increased incidence of ailments including inflammatory bowel diseases as well as multiple age-related diseases.

The TWK-26 potassium channel governs nutrient absorption in the C. elegans intestine

Ion channels are necessary for proper water and nutrient absorption in the intestine, which supports cellular metabolism and organismal growth. While a role for Na + co-transporters and pumps in intestinal nutrient absorption is well defined, how individual K + uniporters function to maintain ion homeostasis is poorly understood. Using Caenorhabditis elegans , we show that a gain-of-function mutation in twk-26 , which encodes a two-pore domain K + ion channel orthologous to human KCNK3, facilitates nutrient absorption and suppresses the metabolic and developmental defects displayed by impaired intestinal MAP Kinase (MAPK) signaling. Mutations in drl-1 and flr-4, which encode two components of this MAPK pathway, cause severe growth defects, reduced lipid storage, and a dramatic increase in autophagic lysosomes, which mirror dietary restriction phenotypes. Additionally, these MAPK mutants display structural defects of the intestine and an impaired defecation motor program. We find that activation of TWK-26 reverses the dietary restriction-like state of the MAPK mutants by restoring intestinal nutrient absorption without correcting the intestinal bloating or defecation defects. This study provides unique insight into the mechanisms by which intestinal K + ion channels support intestinal metabolic homeostasis.

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

TIR-domain proteins with enzymatic activity are essential for immunity in plants, animals, and bacteria. However, it is not known how these proteins function in pathogen sensing in animals. We discovered that a TIR-domain protein (TIR-1/SARM1) is strategically expressed on the membranes of a lysosomal sub-compartment, which enables intestinal epithelial cells in the nematode C. elegans to survey for pathogen effector-triggered host damage. We showed that a redox active virulence effector secreted by the bacterial pathogen Pseudomonas aeruginosa alkalinized and condensed a specific subset of lysosomes by inducing intracellular oxidative stress. Concentration of TIR-1/SARM1 on the surface of these organelles triggered its multimerization, which engages its intrinsic NADase activity, to activate the p38 innate immune pathway and protect the host against microbial intoxication. Thus, lysosomal TIR-1/SARM1 is a sensor for oxidative stress induced by pathogenic bacteria to activate metazoan intestinal immunity.

Germline mitotic quiescence and cell death are induced in Caenorhabditis elegans by exposure to pathogenic Pseudomonas aeruginosa

The impact of exposure to microbial pathogens on animal reproductive capacity and germline physiology is not well understood. The nematode Caenorhabditis elegans is a bacterivore that encounters pathogenic microbes in its natural environment. How pathogenic bacteria affect host reproductive capacity of C. elegans is not well understood. Here, we show that exposure of C. elegans hermaphrodites to the Gram-negative pathogen Pseudomonas aeruginosa causes a marked reduction in brood size with concomitant reduction in the number of nuclei in the germline and gonad size. We define 2 processes that are induced that contribute to the decrease in the number of germ cell nuclei. First, we observe that infection with P. aeruginosa leads to the induction of germ cell apoptosis. Second, we observe that this exposure induces mitotic quiescence in the proliferative zone of the C. elegans gonad. Importantly, these processes appear to be reversible; when animals are removed from the presence of P. aeruginosa, germ cell apoptosis is abated, germ cell nuclei numbers increase, and brood sizes recover. The reversible germline dynamics during exposure to P. aeruginosa may represent an adaptive response to improve survival of progeny and may serve to facilitate resource allocation that promotes survival during pathogen infection.

Immunity-linked genes are stimulated by a membrane stress pathway linked to Golgi function and the ARF-1 GTPase

Infection response and other immunity-linked genes (ILGs) were first named in Caenorhabditis elegans-based expression after pathogen challenge, but many are also up-regulated when lipid metabolism is perturbed. Why pathogen attack and metabolic changes both increase ILGs is unclear. We find that ILGs are activated when phosphatidylcholine (PC) levels change in membranes of secretory organelles in C. elegans. RNAi targeting of the ADP-ribosylation factor arf-1, which disrupts the Golgi and secretory function, also activates ILGs. Low PC limits ARF-1 function, suggesting a mechanism for ILG activation via lipid metabolism, as part of a membrane stress response acting outside the ER. RNAi of selected ILGs uncovered defects in the secretion of two GFP reporters and the accumulation of a pathogen-responsive complement C1r/C1s, Uegf, Bmp1 (CUB) domain fusion protein. Our data argue that up-regulation of some ILGs is a coordinated response to changes in trafficking and may act to counteract stress on secretory function.

Mitochondrial p38 Mitogen-Activated Protein Kinase: Insights into Its Regulation of and Role in LONP1-Deficient Nematodes

p38 Mitogen-Activated Protein Kinase (MAPK) cascades are central regulators of numerous physiological cellular processes, including stress response signaling. In C. elegans, mitochondrial dysfunction activates a PMK-3/p38 MAPK signaling pathway (MAPKmt), but its functional role still remains elusive. Here, we demonstrate the induction of MAPKmt in worms deficient in the lonp-1 gene, which encodes the worm ortholog of mammalian mitochondrial LonP1. This induction is subjected to negative regulation by the ATFS-1 transcription factor through the CREB-binding protein (CBP) ortholog CBP-3, indicating an interplay between both activated MAPKmt and mitochondrial Unfolded Protein Response (UPRmt) surveillance pathways. Our results also reveal a genetic interaction in lonp-1 mutants between PMK-3 kinase and the ZIP-2 transcription factor. ZIP-2 has an established role in innate immunity but can also modulate the lifespan by maintaining mitochondrial homeostasis during ageing. We show that in lonp-1 animals, ZIP-2 is activated in a PMK-3-dependent manner but does not confer increased survival to pathogenic bacteria. However, deletion of zip-2 or pmk-3 shortens the lifespan of lonp-1 mutants, suggesting a possible crosstalk under conditions of mitochondrial perturbation that influences the ageing process. Furthermore, loss of pmk-3 specifically diminished the extreme heat tolerance of lonp-1 worms, highlighting the crucial role of PMK-3 in the heat shock response upon mitochondrial LONP-1 inactivation.

A phase transition reduces the threshold for nicotinamide mononucleotide-based activation of SARM1, an NAD(P) hydrolase, to physiologically relevant levels

Axonal degeneration is a hallmark feature of neurodegenerative diseases. Activation of the NAD(P)ase sterile alpha and toll-interleukin receptor motif containing protein 1 (SARM1) is critical for this process. In resting neurons, SARM1 activity is inhibited, but upon damage, SARM1 is activated and catalyzes one of three NAD(P)+ dependent reactions: (1) NAD(P)+ hydrolysis to form ADP-ribose (ADPR[P]) and nicotinamide; (2) the formation of cyclic-ADPR (cADPR[P]); or (3) a base exchange reaction with nicotinic acid (NA) and NADP+ to form NA adenine dinucleotide phosphate. Production of these metabolites triggers axonal death. Two activation mechanisms have been proposed: (1) an increase in the nicotinamide mononucleotide (NMN) concentration, which leads to the allosteric activation of SARM1, and (2) a phase transition, which stabilizes the active conformation of the enzyme. However, neither of these mechanisms have been shown to occur at the same time. Using in vitro assay systems, we show that the liquid-to-solid phase transition lowers the NMN concentration required to activate the catalytic activity of SARM1 by up to 140-fold. These results unify the proposed activation mechanisms and show for the first time that a phase transition reduces the threshold for NMN-based SARM1 activation to physiologically relevant levels. These results further our understanding of SARM1 activation and will be important for the future development of therapeutics targeting SARM1.

Transcriptional suppression of sphingolipid catabolism controls pathogen resistance in C. elegans

Sphingolipids are required for diverse biological functions and are degraded by specific catabolic enzymes. However, the mechanisms that regulate sphingolipid catabolism are not known. Here we characterize a transcriptional axis that regulates sphingolipid breakdown to control resistance against bacterial infection. From an RNAi screen for transcriptional regulators of pathogen resistance in the nematode C. elegans, we identified the nuclear hormone receptor nhr-66, a ligand-gated transcription factor homologous to human hepatocyte nuclear factor 4. Tandem chromatin immunoprecipitation-sequencing and RNA sequencing experiments revealed that NHR-66 is a transcriptional repressor, which directly targets sphingolipid catabolism genes. Transcriptional de-repression of two sphingolipid catabolic enzymes in nhr-66 loss-of-function mutants drives the breakdown of sphingolipids, which enhances host susceptibility to infection with the bacterial pathogen Pseudomonas aeruginosa. These data define transcriptional control of sphingolipid catabolism in the regulation of cellular sphingolipids, a process that is necessary for pathogen resistance.

Structure-function analysis of ceTIR-1/hSARM1 explains the lack of Wallerian axonal degeneration in C. elegans

Wallerian axonal degeneration (WD) does not occur in the nematode C. elegans, in contrast to other model animals. However, WD depends on the NADase activity of SARM1, a protein that is also expressed in C. elegans (ceSARM/ceTIR-1). We hypothesized that differences in SARM between species might exist and account for the divergence in WD. We first show that expression of the human (h)SARM1, but not ceTIR-1, in C. elegans neurons is sufficient to confer axon degeneration after nerve injury. Next, we determined the cryoelectron microscopy structure of ceTIR-1 and found that, unlike hSARM1, which exists as an auto-inhibited ring octamer, ceTIR-1 forms a readily active 9-mer. Enzymatically, the NADase activity of ceTIR-1 is substantially weaker (10-fold higher Km) than that of hSARM1, and even when fully active, it falls short of consuming all cellular NAD+. Our experiments provide insight into the molecular mechanisms and evolution of SARM orthologs and WD across species.

Opposing action of the FLR-2 glycoprotein hormone and DRL-1/FLR-4 MAP kinases balance p38-mediated growth and lipid homeostasis in C. elegans

Animals integrate developmental and nutritional signals before committing crucial resources to growth and reproduction; however, the pathways that perceive and respond to these inputs remain poorly understood. Here, we demonstrate that DRL-1 and FLR-4, which share similarity with mammalian mitogen-activated protein kinases, maintain lipid homeostasis in the C. elegans intestine. DRL-1 and FLR-4 function in a protein complex at the plasma membrane to promote development, as mutations in drl-1 or flr-4 confer slow growth, small body size, and impaired lipid homeostasis. To identify factors that oppose DRL-1/FLR-4, we performed a forward genetic screen for suppressors of the drl-1 mutant phenotypes and identified mutations in flr-2 and fshr-1, which encode the orthologues of follicle stimulating hormone and its putative G protein-coupled receptor, respectively. In the absence of DRL-1/FLR-4, neuronal FLR-2 acts through intestinal FSHR-1 and protein kinase A signaling to restrict growth. Furthermore, we show that opposing signaling through DRL-1 and FLR-2 coordinates TIR-1 oligomerization, which modulates downstream p38/PMK-1 activity, lipid homeostasis, and development. Finally, we identify a surprising noncanonical role for the developmental transcription factor PHA-4/FOXA in the intestine where it restricts growth in response to impaired DRL-1 signaling. Our work uncovers a complex multi-tissue signaling network that converges on p38 signaling to maintain homeostasis during development.

Germline mitotic quiescence and programmed cell death are induced in C. elegans by exposure to pathogenic P. aeruginosa

The impact of exposure to microbial pathogens on animal reproductive capacity and germline physiology is not well understood. The nematode Caenorhabditis elegans is a bacterivore that encounters pathogenic microbes in its natural environment. How pathogenic bacteria affect host reproductive capacity of C. elegans is not well understood. Here, we show that exposure of C. elegans hermaphrodites to the Gram-negative pathogen Pseudomonas aeruginosa causes a marked reduction in brood size with concomitant reduction in the number of nuclei in the germline and gonad size. We define two processes that are induced that contribute to the decrease in the number of germ cell nuclei. First, we observe that infection with P. aeruginosa leads to the induction of programmed germ cell death. Second, we observe that this exposure induces mitotic quiescence in the proliferative zone of the C. elegans gonad. Importantly, these processes appear to be reversible; when animals are removed from the presence of P. aeruginosa, germ cell death is abated, germ cell nuclei numbers increase, and brood sizes recover. The reversible germline dynamics during exposure to P. aeruginosa may represent an adaptive response to improve survival of progeny and may serve to facilitate resource allocation that promotes survival during pathogen infection.

Modulating p38 MAPK signaling by proteostasis mechanisms supports tissue integrity during growth and aging

The conserved p38 MAPK family is activated by phosphorylation during stress responses and inactivated by phosphatases. C. elegans PMK-1 p38 MAPK initiates innate immune responses and blocks development when hyperactivated. Here we show that PMK-1 signaling is enhanced during early aging by modulating the stoichiometry of non-phospho-PMK-1 to promote tissue integrity and longevity. Loss of pmk-1 function accelerates progressive declines in neuronal integrity and lysosome function compromising longevity which has both cell autonomous and cell non-autonomous contributions. CED-3 caspase cleavage limits phosphorylated PMK-1. Enhancing p38 signaling with caspase cleavage-resistant PMK-1 protects lysosomal and neuronal integrity extending a youthful phase. PMK-1 works through a complex transcriptional program to regulate lysosome formation. During early aging, the absolute phospho-p38 amount is maintained but the reservoir of non-phospho-p38 diminishes to enhance signaling without hyperactivation. Our findings show that modulating the stoichiometry of non-phospho-p38 dynamically supports tissue-homeostasis during aging without hyper-activation of stress response.

Hirudomacin: a Protein with Dual Effects of Direct Bacterial Inhibition and Regulation of Innate Immunity

Hirudomacin (Hmc) belongs to the Macin family of antimicrobial peptides, which can be used for bactericidal purposes in vitro by cleaving cell membranes. Although the Macin family has broad-spectrum antibacterial properties, few studies have been reported on bacterial inhibition by enhancing innate immunity. To further investigate the mechanism of Hmc inhibition, we chose the classical innate immune model organism Caenorhabditis elegans as the study subject. In this investigation, we found that Hmc treatment directly reduced the number of Staphylococcus aureus and Escherichia coli in the intestine of infected wild-type nematodes and infected pmk-1 mutant nematodes. Hmc treatment significantly prolonged the life span of infected wild-type nematodes and increased the expression of antimicrobial effectors (clec-82, nlp-29, lys-1, lys-7), and Hmc treatment still significantly increased the expression of antimicrobial effectors (clec-82, nlp-29, lys-7) in wild-type nematodes in the absence of bacterial stimulation. In addition, Hmc treatment significantly increased the expression of key genes of the pmk-1/p38 MAPK pathway (pmk-1, tir-1, atf-7, skn-1) under both infected and uninfected conditions but failed to increase the life span of infected pmk-1 mutant nematodes as well as the expression of antimicrobial effector genes. Western blot results further demonstrated that Hmc treatment significantly elevated pmk-1 protein expression levels in infected wild-type nematodes. In conclusion, our data suggest that Hmc has both direct bacteriostatic and immunomodulatory effects and may upregulate antimicrobial peptides in response to infection via the pmk-1/p38 MAPK pathway. It has the potential to serve as a new antibacterial agent and immune modulator. IMPORTANCE In today's world, bacterial drug resistance is becoming increasingly serious, and natural antibacterial proteins are attracting attention because of advantages such as their diverse and complex antibacterial modes, lack of residue, and harder-to-develop drug resistance. Notably, there are few antibacterial proteins with multiple effects such as direct antibacterial and innate immunity enhancement at the same time. We believe that an ideal antimicrobial agent can be developed only through a more comprehensive and in-depth study of the bacteriostatic mechanism of natural antibacterial proteins. The significance of our study is that based on the known in vitro bacterial inhibition of Hirudomacin (Hmc), we further clarified its mechanism in vivo, which can be subsequently developed as a natural bacterial inhibitor for various applications in medicine, food, farming, and daily chemicals.

SARM1, an Enzyme Involved in Axon Degeneration, Catalyzes Multiple Activities through a Ternary Complex Mechanism

Sterile alpha and toll/interleukin receptor (TIR) motif containing protein 1 (SARM1) is an NAD+ hydrolase and cyclase involved in axonal degeneration. In addition to NAD+ hydrolysis and cyclization, SARM1 catalyzes a base exchange reaction between nicotinic acid (NA) and NADP+ to generate NAADP, which is a potent calcium signaling molecule. Herein, we describe efforts to characterize the hydrolysis, cyclization, and base exchange activities of TIR-1, the Caenorhabditis elegans ortholog of SARM1; TIR-1 also catalyzes NAD(P)+ hydrolysis and/or cyclization and regulates axonal degeneration in worms. We show that the catalytic domain of TIR-1 undergoes a liquid-to-solid phase transition that regulates not only the hydrolysis and cyclization reactions but also the base exchange reaction. We define the substrate specificities of the reactions, demonstrate that cyclization and base exchange reactions occur within the same pH range, and establish that TIR-1 uses a ternary complex mechanism. Overall, our findings will aid drug discovery efforts and provide insight into the mechanism of recently described inhibitors.

Paeoniflorin increases the survival of Pseudomonas aeruginosa infected Caenorhabditis elegans at the immunosuppression stage by activating PMK-1, BAR-1, and EGL-1 signals

Paeoniflorin is the major active compound of total glycoside of paeony in Paeonia lactiflora Pall. Although several aspects of beneficial effects of paeoniflorin have been described, whether the paeoniflorin treatment is helpful for inhibiting the pathogen infection-induced immunosuppression remains largely unclear. Using the immunosuppression model in Caenorhabditis elegans induced by Pseudomonas aeruginosa infection, we here examined the beneficial effect of paeoniflorin treatment against the immunosuppression induced by bacterial pathogen infection. In this immunosuppression model, we observed that the survival rate of P. aeruginosa infected nematodes at the immunosuppression stage could be significantly increased by 25-100 mg/L paeoniflorin treatment. P. aeruginosa accumulation in intestinal lumen of nematodes at the immunosuppression stage was reduced by paeoniflorin treatment. Paeoniflorin could activate the expressions of antimicrobial genes (lys-1 and lys-8) in nematodes at the immunosuppression stage. Moreover, at the immunosuppression stage, paeoniflorin treatment increased the expressions of bar-1, pmk-1, and egl-1 required for the control of innate immunity against bacterial infection. Meanwhile, RNAi of bar-1, pmk-1, and egl-1 inhibited the beneficial effect of paeoniflorin treatment in increasing the survival, reducing the P. aeruginosa accumulation in intestinal lumen, and activating the expressions of antimicrobial genes (lys-1 and lys-8) in nematodes at the immunosuppression stage. Therefore, paeoniflorin treatment could effectively inhibit the immunosuppression induced by bacterial pathogen infection in the hosts.

Exposure to polystyrene nanoparticles at predicted environmental concentrations enhances toxic effects of Acinetobacter johnsonii AC15 infection on Caenorhabditis elegans

Nanoplastics and microbial pathogens are both widely distributed in the environment; however, their combined toxicity remains largely unclear. Using Caenorhabditis elegans as an animal model, we examined the possible effect of exposure to polystyrene nanoparticle (PS-NP) in Acinetobacter johnsonii AC15 (a bacterial pathogen) infected animals. Exposure to PS-NP at the concentrations of 0.1-10 μg/L significantly enhanced the toxicity of Acinetobacter johnsonii AC15 infection on lifespan and locomotion behaviors. In addition, after exposure to 0.1-10 μg/L PS-NP, the accumulation of Acinetobacter johnsonii AC15 in body of nematodes was also increased. Meanwhile, the innate immune response indicated by the increase of antimicrobial gene expressions in Acinetobacter johnsonii AC15 infected nematodes was suppressed by exposure to 0.1-10 μg/L PS-NP. Moreover, expressions of egl-1, dbl-1, bar-1, daf-16, pmk-1, and elt-2 governing the bacterial infection and immunity in Acinetobacter johnsonii AC15 infected nematodes were further inhibited by exposure to 0.1-10 μg/L PS-NP. Therefore, our data suggested the possible exposure risk of nanoplastic at predicted environmental concentrations in enhancing the toxic effects of bacterial pathogens on environmental organisms.

Non-canonical pattern recognition of a pathogen-derived metabolite by a nuclear hormone receptor identifies virulent bacteria in C. elegans

Distinguishing infectious pathogens from harmless microorganisms is essential for animal health. The mechanisms used to identify infectious microbes are not fully understood, particularly in metazoan hosts that eat bacteria as their food source. Here, we characterized a non-canonical pattern-recognition system in Caenorhabditis elegans (C. elegans) that assesses the relative threat of virulent Pseudomonas aeruginosa (P. aeruginosa) to activate innate immunity. We discovered that the innate immune response in C. elegans was triggered by phenazine-1-carboxamide (PCN), a toxic metabolite produced by pathogenic strains of P. aeruginosa. We identified the nuclear hormone receptor NHR-86/HNF4 as the PCN sensor in C. elegans and validated that PCN bound to the ligand-binding domain of NHR-86/HNF4. Activation of NHR-86/HNF4 by PCN directly engaged a transcriptional program in intestinal epithelial cells that protected against P. aeruginosa. Thus, a bacterial metabolite is a pattern of pathogenesis surveilled by nematodes to identify a pathogen in its bacterial diet.

TIR-1/SARM1 inhibits axon regeneration and promotes axon degeneration

Growth and destruction are central components of the neuronal injury response. Injured axons that are capable of repair, including axons in the mammalian peripheral nervous system and in many invertebrate animals, often regenerate and degenerate on either side of the injury. Here we show that TIR-1/dSarm/SARM1, a key regulator of axon degeneration, also inhibits regeneration of injured motor axons. The increased regeneration in tir-1 mutants is not a secondary consequence of its effects on degeneration, nor is it determined by the NADase activity of TIR-1. Rather, we found that TIR-1 functions cell-autonomously to regulate each of the seemingly opposite processes through distinct interactions with two MAP kinase pathways. On one side of the injury, TIR-1 inhibits axon regeneration by activating the NSY-1/ASK1 MAPK signaling cascade, while on the other side of the injury, TIR-1 simultaneously promotes axon degeneration by interacting with the DLK-1 mitogen-activated protein kinase (MAPK) signaling cascade. In parallel, we found that the ability to cell-intrinsically inhibit axon regeneration is conserved in human SARM1. Our finding that TIR-1/SARM1 regulates axon regeneration provides critical insight into how axons coordinate a multidimensional response to injury, consequently informing approaches to manipulate the response toward repair.

C. elegans orphan nuclear receptor NHR-42 represses innate immunity and promotes lipid loss downstream of HLH-30/TFEB

In recent years, transcription factors of the Microphthalmia-TFE (MiT) family, including TFEB and TFE3 in mammals and HLH-30 in Caenorhabditis elegans, have emerged as important regulators of innate immunity and inflammation in invertebrates and vertebrates. Despite great strides in knowledge, the mechanisms that mediate downstream actions of MiT transcription factors in the context of innate host defense remain poorly understood. Here, we report that HLH-30, which promotes lipid droplet mobilization and host defense, induces the expression of orphan nuclear receptor NHR-42 during infection with Staphylococcus aureus. Remarkably, NHR-42 loss of function promoted host infection resistance, genetically defining NHR-42 as an HLH-30-controlled negative regulator of innate immunity. During infection, NHR-42 was required for lipid droplet loss, suggesting that it is an important effector of HLH-30 in lipid immunometabolism. Moreover, transcriptional profiling of nhr-42 mutants revealed wholesale activation of an antimicrobial signature, of which abf-2, cnc-2, and lec-11 were important for the enhanced survival of infection of nhr-42 mutants. These results advance our knowledge of the mechanisms by which MiT transcription factors promote host defense, and by analogy suggest that TFEB and TFE3 may similarly promote host defense via NHR-42-homologous nuclear receptors in mammals.

Exploring a diverse world of effector domains and amyloid signaling motifs in fungal NLR proteins

NLR proteins are intracellular receptors constituting a conserved component of the innate immune system of cellular organisms. In fungi, NLRs are characterized by high diversity of architectures and presence of amyloid signaling. Here, we explore the diverse world of effector and signaling domains of fungal NLRs using state-of-the-art bioinformatic methods including MMseqs2 for fast clustering, probabilistic context-free grammars for sequence analysis, and AlphaFold2 deep neural networks for structure prediction. In addition to substantially improving the overall annotation, especially in basidiomycetes, the study identifies novel domains and reveals the structural similarity of MLKL-related HeLo- and Goodbye-like domains forming the most abundant superfamily of fungal NLR effectors. Moreover, compared to previous studies, we found several times more amyloid motif instances, including novel families, and validated aggregating and prion-forming properties of the most abundant of them in vitro and in vivo. Also, through an extensive in silico search, the NLR-associated amyloid signaling was identified in basidiomycetes. The emerging picture highlights similarities and differences in the NLR architectures and amyloid signaling in ascomycetes, basidiomycetes and other branches of life.

The Nucleotide Revolution: Immunity at the Intersection of Toll/Interleukin-1 Receptor Domains, Nucleotides, and Ca2

The discovery of the enzymatic activity of the toll/interleukin-1 receptor (TIR) domain protein SARM1 five years ago preceded a flood of discoveries regarding the nucleotide substrates and products of TIR domains in plants, animals, bacteria, and archaea. These discoveries into the activity of TIR domains coincide with major advances in understanding the structure and mechanisms of NOD-like receptors and the mutual dependence of pattern recognition receptor- and effector-triggered immunity (PTI and ETI, respectively) in plants. It is quickly becoming clear that TIR domains and TIR-produced nucleotides are ancestral signaling molecules that modulate immunity and that their activity is closely associated with Ca²⁺ signaling. TIR domain research now bridges the separate disciplines of molecular plant- and animal-microbe interactions, neurology, and prokaryotic immunity. A cohesive framework for understanding the role of enzymatic TIR domains in diverse organisms will help unite the research of these disparate fields. Here, we review known products of TIR domains in plants, animals, bacteria, and archaea and use context gained from animal and prokaryotic TIR domain systems to present a model for TIR domains, nucleotides, and Ca²⁺ at the intersection of PTI and ETI in plant immunity. [Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

Multifaceted roles of SARM1 in axon degeneration and signaling

Axons are considered to be particularly vulnerable components of the nervous system; impairments to a neuron’s axon leads to an effective silencing of a neuron’s ability to communicate with other cells. Nervous systems have therefore evolved plasticity mechanisms for adapting to axonal damage. These include acute mechanisms that promote the degeneration and clearance of damaged axons and, in some cases, the initiation of new axonal growth and synapse formation to rebuild lost connections. Here we review how these diverse processes are influenced by the therapeutically targetable enzyme SARM1. SARM1 catalyzes the breakdown of NAD+, which, when unmitigated, can lead to rundown of this essential metabolite and axonal degeneration. SARM1’s enzymatic activity also triggers the activation of downstream signaling pathways, which manifest numerous functions for SARM1 in development, innate immunity and responses to injury. Here we will consider the multiple intersections between SARM1 and the injury signaling pathways that coordinate cellular adaptations to nervous system damage.

Shared TIR enzymatic functions regulate cell death and immunity across the tree of life

In the 20th century, researchers studying animal and plant signaling pathways discovered a protein domain shared across diverse innate immune systems: the Toll/Interleukin-1/Resistance-gene (TIR) domain. The TIR domain is found in several protein architectures and was defined as an adaptor mediating protein-protein interactions in animal innate immunity and developmental signaling pathways. However, studies of nerve degeneration in animals, and subsequent breakthroughs in plant, bacterial and archaeal systems, revealed that TIR domains possess enzymatic activities. We provide a synthesis of TIR functions and the role of various related TIR enzymatic products in evolutionarily diverse immune systems. These studies may ultimately guide interventions that would span the tree of life, from treating human neurodegenerative disorders and bacterial infections, to preventing plant diseases.

The chemical biology of NAD+ regulation in axon degeneration

During axon degeneration, NAD⁺ levels are largely controlled by two enzymes: nicotinamide mononucleotide adenylyltransferase 2 (NMNAT2) and sterile alpha and toll interleukin motif containing protein 1 (SARM1). NMNAT2, which catalyzes the formation of NAD⁺ from NMN and ATP, is actively degraded leading to decreased NAD⁺ levels. SARM1 activity further decreases the concentration of NAD⁺ by catalyzing its hydrolysis to form nicotinamide and a mixture of ADPR and cADPR. Notably, SARM1 knockout mice show decreased neurodegeneration in animal models of axon degeneration, highlighting the therapeutic potential of targeting this novel NAD⁺ hydrolase. This review discusses recent advances in the SARM1 field, including SARM1 structure, regulation, and catalysis as well as the identification of the first SARM1 inhibitors.