Mechanisms of innate immunity in C. elegans epidermis

TMEM39A and TMEM131 facilitate bulk transport of ECM proteins through large COPII vesicle formation

The growth of Caenorhabditis elegans involves multiple molting processes, during which old cuticles are shed and new cuticles are rapidly formed. This process requires the regulated bulk secretion of cuticle components. The transmembrane protein-39 (TMEM-39) mutant exhibits distinct dumpy and ruptured phenotypes characterized by notably thin cuticles. TMEM-39 primarily co-localizes with the coat protein II complex (COPII) in large vesicles rather than small COPII vesicles. These TMEM-39-associated large vesicles (TMEM-39-LVs) form robustly during the molting period and co-localize with various extracellular matrix components, including BLI-1 collagen, BLI-3 dual oxidase, and carboxypeptidases. Through immunoprecipitation using TMEM39A-FLAG and proteomics analysis in human sarcoma cells, we identify TMEM39A-associated proteins, including TMEM131. Knockdown of TMEM131 results in reduced TMEM39A-LV formation and collagen secretion in both C. elegans and human sarcoma cells, indicating a cooperative role between TMEM39A and TMEM131 in the secretion of extracellular components through the formation of large COPII vesicles. Given the conservation of TMEM39A and its associated proteins between C. elegans and humans, TMEM39A-LVs may represent a fundamental machinery for rapid and extensive secretion across metazoans.

Structural and physiological functions of Caenorhabditis elegans epidermis

Research on the skin is continuously evolving, and it is imperative to select a streamlined and efficient research model. Caenorhabditis elegans is a free-leaving nematode whose epidermis serves as the primary barrier epithelium, composed of a collagen matrix. Differentiation of the epidermis begins in the middle of embryonic development, including polarization of the cytoskeleton and formation of cell junctions. Cuticle secretion is one of the main developmental and physiological features of the epidermis. Mutations in the collagen genes of individual worms lead to cuticle defects, thereby changing the shape of the animals. The complete genome sequence of C. elegans indicates that more than 170 different collagen genes may be related to this structure. Collagen is a structural protein that plays an important role in the development of extracellular matrix. Different collagen genes are expressed at different stages of matrix synthesis, which may help form specific interactions between different collagens. The differentiated epidermis also plays a key role in the transmission of hormonal signals, fat storage, and ion homeostasis and is closely related to the development and function of the nervous system. The epidermis also provides passive and active defenses against pathogens that penetrate the skin and can repair wounds. In addition, age-dependent epidermal degeneration is a prominent feature of aging and may affect aging and lifespan. This review we highlight recent findings of the structure and related physiological functions of the cuticle of C. elegans. In contrast to previous studies, we offer novel insights into the utilization of C. elegans as valuable models for skin-related investigations. It also encourages the use of C. elegans as a skin model, and its high-throughput screening properties facilitate the acceleration of fundamental research in skin-related diseases.

Single-cell phylotranscriptomics of developmental and cell type evolution

Single-cell phylotranscriptomics is an emerging tool to reveal the molecular and cellular mechanisms of evolution. We summarize its utility in studying the hourglass pattern of ontogenetic evolution and for understanding the evolutionary history of cell types. The developmental hourglass model suggests that the mid-embryonic stage is the most conserved period of development across species, which is supported by morphological and molecular studies. Single-cell phylotranscriptomic analysis has revealed previously underappreciated heterogeneity in transcriptome ages among lineages and cell types throughout development, and has identified the lineages and tissues that drive the whole-organism hourglass pattern. Single-cell transcriptome age analyses also provide important insights into the origin of germ layers, the different selective forces on tissues during adaptation, and the evolutionary relationships between cell types.

The MAB-5/Hox family transcription factor is important for Caenorhabditis elegans innate immune response to Staphylococcus epidermidis infection

Innate immunity functions as a rapid defense against broad classes of pathogenic agents. While the mechanisms of innate immunity in response to antigen exposure are well-studied, how pathogen exposure activates the innate immune responses and the role of genetic variation in immune activity is currently being investigated. Previously, we showed significant survival differences between the N2 and the CB4856 Caenorhabditis elegans isolates in response to Staphylococcus epidermidis infection. One of those differences was expression of the mab-5 Hox family transcription factor, which was induced in N2, but not CB4856, after infection. In this study, we use survival assays and RNA-sequencing to better understand the role of mab-5 in response to S. epidermidis. We found that mab-5 loss-of-function (LOF) mutants were more susceptible to S. epidermidis infection than N2 or mab-5 gain-of-function (GOF) mutants, but not as susceptible as CB4856 animals. We then conducted transcriptome analysis of infected worms and found considerable differences in gene expression profiles when comparing animals with mab-5 LOF to either N2 or mab-5 GOF. N2 and mab-5 GOF animals showed a significant enrichment in expression of immune genes and C-type lectins, whereas mab-5 LOF mutants did not. Overall, gene expression profiling in mab-5 mutants provided insight into MAB-5 regulation of the transcriptomic response of C. elegans to pathogenic bacteria and helps us to understand mechanisms of innate immune activation and the role that transcriptional regulation plays in organismal health.

The Emerging Role of 3-Hydroxyanthranilic Acid on C. elegans Aging Immune Function

3-hydroxyanthranilic acid (3HAA) is considered to be a fleeting metabolic intermediate along tryptophan catabolism through the kynurenine pathway. 3HAA and the rest of the kynurenine pathway have been linked to immune response in mammals yet whether it is detrimental or advantageous is a point of contention. Recently we have shown that accumulation of this metabolite, either through supplementation or prevention of its degradation, extends healthy lifespan in C. elegans and mice, while the mechanism remained unknown. Utilizing C. elegans as a model we investigate how 3HAA and haao-1 inhibition impact the host and the potential pathogens. What we find is that 3HAA improves host immune function with aging and serves as an antimicrobial against gram-negative bacteria. Regulation of 3HAA's antimicrobial activity is accomplished via tissue separation. 3HAA is synthesized in the C. elegans hypodermal tissue, localized to the site of pathogen interaction within the gut granules, and degraded in the neuronal cells. This tissue separation creates a new possible function for 3HAA that may give insight to a larger evolutionarily conserved function within the immune response.

Body stiffness is a mechanical property that facilitates contact-mediated mate recognition in Caenorhabditis elegans

Physical contact is prevalent in the animal kingdom to recognize suitable mates by decoding information about sex, species, and maturity. Although chemical cues for mate recognition have been extensively studied, the role of mechanical cues remains elusive. Here, we show that C. elegans males recognize conspecific and reproductive mates through short-range cues, and that the attractiveness of potential mates depends on the sex and developmental stages of the hypodermis. We find that a particular group of cuticular collagens is required for mate attractiveness. These collagens maintain body stiffness to sustain mate attractiveness but do not affect the surface properties that evoke the initial step of mate recognition, suggesting that males utilize multiple sensory mechanisms to recognize suitable mates. Manipulations of body stiffness via physical interventions, chemical treatments, and 3D-printed bionic worms indicate that body stiffness is a mechanical property for mate recognition and increases mating efficiency. Our study thus extends the repertoire of sensory cues of mate recognition in C. elegans and provides a paradigm to study the important roles of mechanosensory cues in social behaviors.

C. elegans Hemidesmosomes Sense Collagen Damage to Trigger Innate Immune Response in the Epidermis

The collagens are an enormous family of extracellular matrix proteins that play dominant roles in cell adhesion, migration and tissue remodeling under many physiological and pathological conditions. However, their function mechanisms in regulating innate immunity remain largely undiscovered. Here we use C. elegans epidermis as the model to address this question. The C. elegans epidermis is covered with a collagen-rich cuticle exoskeleton and can produce antimicrobial peptides (AMPs) against invading pathogens or physical injury. Through an RNAi screen against collagen-encoding genes, we found that except the previously reported six DPY collagens and the BLI-1 collagen, the majority of collagens tested appear unable to trigger epidermal immune defense when damaged. Further investigation suggests that the six DPY collagens form a specific substructure, which regulates the interaction between BLI-1 and the hemidesmosome receptor MUP-4. The separation of BLI-1 with MUP-4 caused by collagen damage leads to the detachment of the STAT transcription factor-like protein STA-2 from hemidesmosomes and the induction of AMPs. Our findings uncover the mechanism how collagens are organized into a damage sensor and how the epidermis senses collagen damage to mount an immune defense.

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.

Cisplatin toxicity is counteracted by the activation of the p38/ATF-7 signaling pathway in post-mitotic C. elegans

Cisplatin kills proliferating cells via DNA damage but also has profound effects on post-mitotic cells in tumors, kidneys, and neurons. However, the effects of cisplatin on post-mitotic cells are still poorly understood. Among model systems, C. elegans adults are unique in having completely post-mitotic somatic tissues. The p38 MAPK pathway controls ROS detoxification via SKN-1/NRF and immune responses via ATF-7/ATF2. Here, we show that p38 MAPK pathway mutants are sensitive to cisplatin, but while cisplatin exposure increases ROS levels, skn-1 mutants are resistant. Cisplatin exposure leads to phosphorylation of PMK-1/MAPK and ATF-7 and the IRE-1/TRF-1 signaling module functions upstream of the p38 MAPK pathway to activate signaling. We identify the response proteins whose increased abundance depends on IRE-1/p38 MAPK activity as well as cisplatin exposure. Four of these proteins are necessary for protection from cisplatin toxicity, which is characterized by necrotic death. We conclude that the p38 MAPK pathway-driven proteins are crucial for adult cisplatin resilience.

Treatment with paeoniflorin increases lifespan of Pseudomonas aeruginosa infected Caenorhabditis elegans by inhibiting bacterial accumulation in intestinal lumen and biofilm formation

Paeoniflorin is one of the important components in Paeoniaceae plants. In this study, we used Caenorhabditis elegans as a model host and Pseudomonas aeruginosa as a bacterial pathogen to investigate the possible role of paeoniflorin treatment against P. aeruginosa infection in the host and the underlying mechanisms. Posttreatment with 1.25–10 mg/L paeoniflorin could significantly increase the lifespan of P. aeruginosa infected nematodes. After the infection, the P. aeruginosa colony-forming unit (CFU) and P. aeruginosa accumulation in intestinal lumen were also obviously reduced by 1.25–10 mg/L paeoniflorin treatment. The beneficial effects of paeoniflorin treatment in increasing lifespan in P. aeruginosa infected nematodes and in reducing P. aeruginosa accumulation in intestinal lumen could be inhibited by RNAi of pmk-1, egl-1, and bar-1. In addition, paeoniflorin treatment suppressed the inhibition in expressions of pmk-1, egl-1, and bar-1 caused by P. aeruginosa infection in nematodes, suggesting that paeoniflorin could increase lifespan of P. aeruginosa infected nematode by activating PMK-1, EGL-1, and BAR-1. Moreover, although treatment with 1.25–10 mg/L paeoniflorin did not show obvious anti-P. aeruginosa activity, the P. aeruginosa biofilm formation and expressions of related virulence genes (pelA, pelB, phzA, lasB, lasR, rhlA, and rhlC) were significantly inhibited by paeoniflorin treatment. Treatment with 1.25–10 mg/L paeoniflorin could further decrease the levels of related virulence factors of pyocyanin, elastase, and rhamnolipid. In addition, 2.5–10 mg/L paeoniflorin treatment could inhibit the swimming, swarming, and twitching motility of P. aeruginosa, and treatment with 2.5–10 mg/L paeoniflorin reduced the cyclic-di-GMP (c-di-GMP) level. Therefore, paeoniflorin treatment has the potential to extend lifespan of P. aeruginosa infected hosts by reducing bacterial accumulation in intestinal lumen and inhibiting bacterial biofilm formation.

Juxtaposing Caenorhabditis elegans-Pathogenic Mould Model with Other Models; How Reliable Is This Nematode Model? A Mini Review

The application of Caenorhabditis elegans as a pathogenic model has spanned decades. Its use for pathogenic mould modeling has been attracting some attention lately, though not without some reservations. Several studies have shown C. elegans to be a reliable model for evaluating moulds' virulence factors and patterns as well as for screening the pathogenicity of mutant strains alongside their parental/wild type and revertant/complementary strains. There is a very high degree of reported similarities between the virulence patterns demonstrated in C. elegans and those of other invertebrate and vertebrate models. We have here presented several works in which this nematode model was adopted for virulence evaluation, and other comparative research in which virulence in C. elegans model were juxtaposed with other models. We have further presented possible reasons why there might have been variations of virulence in a few cases, thereby validating C. elegans to be an effective and reliable tool in the study of pathogenic moulds.

New insights into the regulation mechanism of red claw crayfish (Cherax quadricarinatus) hepatopancreas under air exposure using transcriptome analysis

Red claw crayfish (Cherax quadricarinatus) is an important freshwater shrimp species worldwide with enormous economic value. Waterless transportation is an inherent feature of red claw crayfish transportation. However, the high mortality of red claw crayfish is a severe problem in the aquaculture of crayfish after waterless transportation. In this study, we investigated the responses of the hepatopancreas from the red claw crayfish undergoing air exposure stress and normal conditions on transcriptome levels. We used Illumina-based RNA sequencing (RNA-Seq) to perform a transcriptome analysis from the hepatopancreas of red claw crayfish challenged by air exposure. An average of 57,148,800 clean reads per library was obtained, and 33,567 unigenes could be predicted and classified according to their homology with matches in the National Center for Biotechnology Information (NCBI) non-redundant protein sequences (Nr), Gene Ontology (GO), a manually annotated and reviewed protein sequence database (Swiss-Prot), protein families (Pfam), Clusters of Orthologous Groups (COG) of proteins, and Kyoto Encyclopedia of Genes and Genomes (KEGG) databases. 690 and 3407 differentially expressed genes (DEGs) were identified between the two stress stages of the red claw crayfish. More DEGs were identified in 12 h, indicating that gene expressions were largely changed at 12 h. Some immune-related pathways and genes were identified according to KEGG and GO enrichment analysis. A total of 12 DEGs involved in immune response and trehalose mechanism were verified by quantitative real-time-polymerase chain reaction (qRT-PCR). The results indicated that the red claw crayfish might counteract the stress of air exposure at the transcriptomic level by increasing expression levels of antioxidant-, immune-, and trehalose metabolism-related genes. These transcriptome results from the hepatopancreas provide significant insights into the influence mechanism of air exposure to the trehalose mechanism and immune response in the red claw crayfish.

TENT5 cytoplasmic noncanonical poly(A) polymerases regulate the innate immune response in animals

Innate immunity is the first line of host defense against pathogens. Here, through global transcriptome and proteome analyses, we uncover that newly described cytoplasmic poly(A) polymerase TENT-5 (terminal nucleotidyltransferase 5) enhances the expression of secreted innate immunity effector proteins in Caenorhabditis elegans. Direct RNA sequencing revealed that multiple mRNAs with signal peptide-encoding sequences have shorter poly(A) tails in tent-5-deficient worms. Those mRNAs are translated at the endoplasmic reticulum where a fraction of TENT-5 is present, implying that they represent its direct substrates. Loss of tent-5 makes worms more susceptible to bacterial infection. Notably, the role of TENT-5 in innate immunity is evolutionarily conserved. Its orthologs, TENT5A and TENT5C, are expressed in macrophages and induced during their activation. Analysis of macrophages devoid of TENT5A/C revealed their role in the regulation of secreted proteins involved in defense response. In summary, our study reveals cytoplasmic polyadenylation to be a previously unknown component of the posttranscriptional regulation of innate immunity in animals.

Beneficial effect of Xuebijing against Pseudomonas aeruginosa infection in Caenorhabditis elegans

In the clinical intensive care units (ICU), the traditional Chinese medicine (TCM) formulation of Xuebijing has been frequently used for treating sepsis. Nevertheless, the underlying pharmacological mechanisms of Xuebijing remain largely unclear. Caenorhabditis elegans is an important experimental host for bacterial infections. Using C. elegans as an animal model, we here examined the potential of Xuebijing treatment against bacterial infection and the underlying mechanisms. Xuebijing treatment could inhibit the reduction tendency of lifespan caused by Pseudomonas aeruginosa infection. For the cellular mechanisms of this antibacterial infection property, we found that Xuebijing treatment rescued C. elegans lifespan to be against P. aeruginosa infection by inhibiting Pseudomonas colonization in the intestinal lumen. Meanwhile, the increase in the expression of antimicrobial genes induced by Pseudomonas infection was also suppressed by Xuebijing treatment. Moreover, the beneficial effect of Xuebijing against Pseudomonas infection depended on insulin, p38 MAPK, Wnt, DBL-1/TGF-β, ELT-2, and programmed cell death (PCD)-related signals. Although Xuebijing did not show obvious antibacterial activity, Xuebijing (100%) treatment could inhibit the Pseudomonas biofilm formation and decrease the expression of virulence genes (lasA, lasB, rhlA, rhlC, phzA, phzM, phzH, and phzS) and quorum sensing (QS)-related genes (lasI, lasR, rhlI, rhlR, pqsA, and pqsR). Our results support the potential role of Xuebijing treatment against bacterial infection in hosts.

Wild-type Caenorhabditis elegans isolates exhibit distinct gene expression profiles in response to microbial infection

The soil-dwelling nematode Caenorhabditis elegans serves as a model system to study innate immunity against microbial pathogens. C. elegans have been collected from around the world, where they, presumably, adapted to regional microbial ecologies. Here we use survival assays and RNA-sequencing to better understand how two isolates from disparate climates respond to pathogenic bacteria. We found that, relative to N2 (originally isolated in Bristol, UK), CB4856 (isolated in Hawaii), was more susceptible to the Gram-positive microbe, Staphylococcus epidermidis, but equally susceptible to Staphylococcus aureus as well as two Gram-negative microbes, Providencia rettgeri and Pseudomonas aeruginosa. We performed transcriptome analysis of infected worms and found gene-expression profiles were considerably different in an isolate-specific and microbe-specific manner. We performed GO term analysis to categorize differential gene expression in response to S. epidermidis. In N2, genes that encoded detoxification enzymes and extracellular matrix proteins were significantly enriched, while in CB4856, genes that encoded detoxification enzymes, C-type lectins, and lipid metabolism proteins were enriched, suggesting they have different responses to S. epidermidis, despite being the same species. Overall, discerning gene expression signatures in an isolate by pathogen manner can help us to understand the different possibilities for the evolution of immune responses within organisms.

Multi-omics analysis identifies essential regulators of mitochondrial stress response in two wild-type C. elegans strains

The mitochondrial unfolded protein response (UPRmt) is a promising pharmacological target for aging and age-related diseases. However, the integrative analysis of the impact of UPRmt activation on different signaling layers in animals with different genetic backgrounds is lacking. Here, we applied systems approaches to investigate the effect of UPRmt induced by doxycycline (Dox) on transcriptome, proteome, and lipidome in two genetically divergent worm strains, named N2 and CB4856. From the integrated omics datasets, we found that Dox prolongs lifespan of both worm strains through shared and strain-specific mechanisms. Specifically, Dox strongly impacts mitochondria, upregulates defense response, and lipid metabolism, while decreasing triglycerides. We further validated that lipid genes acs-2/20 and fat-7/6 were required for Dox-induced UPRmt and longevity in N2 and CB4856 worms, respectively. Our data have translational value as they indicate that the beneficial effects of Dox-induced UPRmt on lifespan are consistent across different genetic backgrounds through different regulators.

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Dox extends lifespan of N2 and CB4856 via shared and strain-specific mechanisms

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Dox controls mitochondria, defense responses, and lipid metabolism in both strains

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Dox-mediated longevity requires acs-2/20 in N2 and fat-7/6 in CB4856 worms

Loss of Function of Scavenger Receptor SCAV-5 Protects C. elegans Against Pathogenic Bacteria

Scavenger receptors play a critical role in innate immunity by acting as the pattern-recognition receptors. There are six class B scavenger receptors homologs in C. elegans. However, it remains unclear whether they are required for host defense against bacterial pathogens. Here, we show that, of the six SCAV proteins, only loss of function scav-5 protect C. elegans against pathogenic bacteria S. typhimurium SL1344 and P. aeruginosa PA14 by different mechanism. scav-5 mutants are resistant to S. typhimurium SL1344 due to dietary restriction. While scav-5 acts upstream of or in parallel to tir-1 in conserved PMK-1 p38 MAPK pathway to upregulate the innate immune response to defend worms against P. aeruginosa PA14. This is the first demonstration of a role for SCAV-5 in host defense against pathogenic bacteria. Our results provide an important basis for further elucidating the underlying molecular mechanism by which scav-5 regulates innate immune responses.

Take a Walk to the Wild Side of Caenorhabditis elegans-Pathogen Interactions

Microbiomes form intimate functional associations with their hosts. Much has been learned from correlating changes in microbiome composition to host organismal functions. However, in-depth functional studies require the manipulation of microbiome composition coupled with the precise interrogation of organismal physiology-features available in few host study systems. Caenorhabditis elegans has proven to be an excellent genetic model organism to study innate immunity and, more recently, microbiome interactions. The study of C. elegans-pathogen interactions has provided in depth understanding of innate immune pathways, many of which are conserved in other animals. However, many bacteria were chosen for these studies because of their convenience in the lab setting or their implication in human health rather than their native interactions with C. elegans In their natural environment, C. elegans feed on a variety of bacteria found in rotting organic matter, such as rotting fruits, flowers, and stems. Recent work has begun to characterize the native microbiome and has identified a common set of bacteria found in the microbiome of C. elegans While some of these bacteria are beneficial to C. elegans health, others are detrimental, leading to a complex, multifaceted understanding of bacterium-nematode interactions. Current research on nematode-bacterium interactions is focused on these native microbiome components, both their interactions with each other and with C. elegans We will summarize our knowledge of bacterial pathogen-host interactions in C. elegans, as well as recent work on the native microbiome, and explore the incorporation of these bacterium-nematode interactions into studies of innate immunity and pathogenesis.

Specific collagens maintain the cuticle permeability barrier in Caenorhabditis elegans

Collagen-enriched cuticle forms the outermost layer of skin in nematode Caenorhabditis elegans. The nematode's genome encodes 177 collagens, but little is known about their role in maintaining the structure or barrier function of the cuticle. In this study, we found six permeability determining (PD) collagens. Loss of any of these PD collagens-DPY-2, DPY-3, DPY-7, DPY-8, DPY-9, and DPY-10-led to enhanced susceptibility of nematodes to paraquat (PQ) and antihelminthic drugs- levamisole and ivermectin. Upon exposure to PQ, PD collagen mutants accumulated more PQ and incurred more damage and death despite the robust activation of antioxidant machinery. We find that BLMP-1, a zinc finger transcription factor, maintains the barrier function of the cuticle by regulating the expression of PD collagens. We show that the permeability barrier maintained by PD collagens acts in parallel to FOXO transcription factor DAF-16 to enhance survival of insulin-like receptor mutant, daf-2. In all, this study shows that PD collagens regulate cuticle permeability by maintaining the structure of C. elegans cuticle and thus provide protection against exogenous toxins.

Innate immunity in C. elegans

In its natural habitat, C. elegans encounters a wide variety of microbes, including food, commensals and pathogens. To be able to survive long enough to reproduce, C. elegans has developed a complex array of responses to pathogens. These activities are coordinated on scales that range from individual organelles to the entire organism. Often, the response is triggered within cells, by detection of infection-induced damage, mainly in the intestine or epidermis. C. elegans has, however, a capacity for cell non-autonomous regulation of these responses. This frequently involves the nervous system, integrating pathogen recognition, altering host biology and governing avoidance behavior. Although there are significant differences with the immune system of mammals, some mechanisms used to limit pathogenesis show remarkable phylogenetic conservation. The past 20 years have witnessed an explosion of host-pathogen interaction studies using C. elegans as a model. This review will discuss the broad themes that have emerged and highlight areas that remain to be fully explored.

AP2M1 Supports TGF-β Signals to Promote Collagen Expression by Inhibiting Caveolin Expression

The extracellular matrix (ECM) is important for normal development and disease states, including inflammation and fibrosis. To understand the complex regulation of ECM, we performed a suppressor screening using Caenorhabditis elegans expressing the mutant ROL-6 collagen protein. One cuticle mutant has a mutation in dpy-23 that encodes the μ2 adaptin (AP2M1) of clathrin-associated protein complex II (AP-2). The subsequent suppressor screening for dpy-23 revealed the lon-2 mutation. LON-2 functions to regulate body size through negative regulation of the tumor growth factor-beta (TGF-β) signaling pathway responsible for ECM production. RNA-seq analysis showed a dominant change in the expression of collagen genes and cuticle components. We noted an increase in the cav-1 gene encoding caveolin-1, which functions in clathrin-independent endocytosis. By knockdown of cav-1, the reduced TGF-β signal was significantly restored in the dpy-23 mutant. In conclusion, the dpy-23 mutation upregulated cav-1 expression in the hypodermis, and increased CAV-1 resulted in a decrease of TβRI. Finally, the reduction of collagen expression including rol-6 by the reduced TGF-β signal influenced the cuticle formation of the dpy-23 mutant. These findings could help us to understand the complex process of ECM regulation in organism development and disease conditions.

Microtubule plus-end dynamics link wound repair to the innate immune response

The skin protects animals from infection and physical damage. In Caenorhabditis elegans, wounding the epidermis triggers an immune reaction and a repair response, but it is not clear how these are coordinated. Previous work implicated the microtubule cytoskeleton in the maintenance of epidermal integrity (Chuang et al., 2016). Here, by establishing a simple wounding system, we show that wounding provokes a reorganisation of plasma membrane subdomains. This is followed by recruitment of the microtubule plus end-binding protein EB1/EBP-2 around the wound and actin ring formation, dependent on ARP2/3 branched actin polymerisation. We show that microtubule dynamics are required for the recruitment and closure of the actin ring, and for the trafficking of the key signalling protein SLC6/SNF-12 toward the injury site. Without SNF-12 recruitment, there is an abrogation of the immune response. Our results suggest that microtubule dynamics coordinate the cytoskeletal changes required for wound repair and the concomitant activation of innate immunity.

Avoidance behavior independent of innate-immune signaling seen in Caenorhabditis elegans challenged with Bacillus anthracis

Small organisms, like the nematode C. elegans, are emerging as insightful models in which to study host/pathogen interactions and the evolving interplay between host defenses and microbial offenses. In C. elegans the innate immune response has been shown to be connected to the DAF-2 insulin/insulin-like growth factor 1 (IGF-1) signal pathway, a critical transduction pathway that mediates stress response in the worms via the DAF-16 FOXO/forkhead transcription factor. Our studies of the C. elegans' phenotypes that are associated with behavioral innate immune response (avoidance behavior) and IGF-1 signaling perturbations (lifespan effects) led us to question the cause of the avoidance behavior observed when C. elegans are challenged with B. anthracis. While worms indeed avoid B. anthracis, and this behavior seems to be partly tied to IGF-1 signaling, the bacteria have neither nematocidal nor visible pathogenic effects on the worms. In fact, worms fed B. anthracis alone exhibit extended lifespans. We demonstrate that the extended lifespan phenotype seen in worms fed B. anthracis is likely the result of calorie restriction, and that worms do not eat B. anthracis even when avoidance behaviors have been suppressed. We further demonstrate a large time lag between the onset of avoidance behavior (which occurs upon contact with B. anthracis), and the induction of IGF-1 signaling (which occurs much later) in worms fed B. anthracis. Taken together, our data demonstrate behavioral avoidance that does not appear to be linked to a measurable immune response. We propose that, in some situations, avoidance behaviors categorized as immunological might be more accurately described as broad foraging behaviors induced in worms presented with a non-preferred food choice, or with a food choice that is either difficult or impossible for the worms to ingest.

Intercellular communication is required for trap formation in the nematode-trapping fungus Duddingtonia flagrans

Nematode-trapping fungi (NTF) are a large and diverse group of fungi, which may switch from a saprotrophic to a predatory lifestyle if nematodes are present. Different fungi have developed different trapping devices, ranging from adhesive cells to constricting rings. After trapping, fungal hyphae penetrate the worm, secrete lytic enzymes and form a hyphal network inside the body. We sequenced the genome of Duddingtonia flagrans, a biotechnologically important NTF used to control nematode populations in fields. The 36.64 Mb genome encodes 9,927 putative proteins, among which are more than 638 predicted secreted proteins. Most secreted proteins are lytic enzymes, but more than 200 were classified as small secreted proteins (< 300 amino acids). 117 putative effector proteins were predicted, suggesting interkingdom communication during the colonization. As a first step to analyze the function of such proteins or other phenomena at the molecular level, we developed a transformation system, established the fluorescent proteins GFP and mCherry, adapted an assay to monitor protein secretion, and established gene-deletion protocols using homologous recombination or CRISPR/Cas9. One putative virulence effector protein, PefB, was transcriptionally induced during the interaction. We show that the mature protein is able to be imported into nuclei in Caenorhabditis elegans cells. In addition, we studied trap formation and show that cell-to-cell communication is required for ring closure. The availability of the genome sequence and the establishment of many molecular tools will open new avenues to studying this biotechnologically relevant nematode-trapping fungus.

Nematode-trapping fungi are fascinating microorganisms, because they are able to switch from saprotrophic growth to a predatory lifestyle. Duddingtonia flagrans forms adhesive trap systems and conidia and resistant chlamydospores. Chlamydospores are ideal for dissemination in the environment to control nematode populations in the field. We show that D. flagrans is able to catch C. elegans but also the very large wine-pathogenic nematode Xiphinema index. We sequenced the D. flagrans genome and show that it encodes about 10,000 genes with a large proportion of secreted proteins. We hypothesize that virulence effector proteins are involved in the interkingdom organismic interaction and identified more than 100 candidates. In order to investigate the molecular biology of D. flagrans and its interaction with nematodes, we established a transformation system and several molecular tools. We show that cell-to-cell communication and hyphal fusion are required for trap formation. Finally, we show that one putative virulence effector protein targets nuclei when expressed in C. elegans.

Functional disruption in epidermal barrier enhances toxicity and accumulation of graphene oxide

In Caenorhabditis elegans, mutation of mlt-7 causes the deficits in epidermal barrier. Using the nematodes with epidermal-specific RNA interference (RNAi) knockdown of mlt-7 as a genetic tool, we found that epidermal-specific RNAi knockdown of mlt-7 resulted in a susceptibility to graphene oxide (GO) toxicity, and enhanced GO accumulation in the body. Epidermal-development related proteins of BLI-1 and IFB-1 acted as downstream targets of MLT-7, and mediated the function of MLT-7 in maintaining the epidermal barrier. Antimicrobial proteins of NLP-30 and CNC-2 also acted as downstream targets of MLT-7 in the regulation of GO toxicity. Epidermal-specific RNAi knockdown of nlp-30 or cnc-2 enhanced GO toxicity and accumulation in bli-1(RNAi) or ifb-1(RNAi) nematodes. Our data highlights the importance of maintaining normal epidermal barrier for nematodes against the GO toxicity.

Evaluation of Caenorhabditis elegans as a host model for Paracoccidioides brasiliensis and Paracoccidioides lutzii

Paracoccidioidomycosis is a systemic fungal infection affecting mainly Latin American countries that is caused by Paracoccidioides brasiliensis and Paracoccidioides lutzii. During the study of fungal pathogenesis, in vivo studies are crucial to understand the overall mechanisms involving the infection as well as to search for new therapeutic treatments and diagnosis. Caenorhabditis elegans is described as an infection model for different fungi species and a well-characterized organism to study the innate immune response. This study evaluates C. elegans as an infection model for Paracoccidioides spp. It was observed that both species do not cause infection in C. elegans, as occurs with Candida albicans, and one possible explanation is that the irregular size and shape of Paracoccidioides spp. difficult the ingestion of these fungi by the nematode. Besides this difficulty in the infection, we could observe that the simple exposition of C. elegans to Paracoccidioides species was able to trigger a distinct pattern of expression of antimicrobial peptide genes. The expression of cnc-4, nlpl-27 and nlp-31 was superior after the exposure to P. brasiliensis in comparison to P. lutzii (P < 0.05), and these findings demonstrate important differences regarding innate immune response activation caused by the two species of the Paracoccidioides genus.

Silencing of a Meloidogyne incognita selenium-binding protein alters the cuticular adhesion of Pasteuria penetrans endospores

Pasteuria penetrans is an endospore forming hyperparasitic bacterium of the plant-pathogenic root-knot nematode, Meloidogyne incognita. For successful parasitization, the first step is adherence of bacterial endospores onto the cuticle surface of nematode juveniles. The knowledge of molecular intricacies involved during this adherence is sparse. Here, we identified a M. incognita selenium-binding protein (Mi-SeBP-1) differentially expressed during the initial interaction of M. incognita and P. penetrans, and show that it is involved in modulating parasitic adhesion of bacterial endospores onto nematode cuticle. Selenium-binding proteins (SeBPs) are selenium associated proteins important for growth regulation, tumor prevention and modulation of oxidation/reduction in cells. Although reported to be present in several nematodes, the function of SeBPs is not known in Phylum Nematoda. In situ hybridization assay localized the Mi-SeBP-1 mRNA to the hypodermal cells. RNAi-mediated silencing of Mi-SeBP-1 significantly increased the adherence of P. penetrans endospores to the nematode juvenile cuticle. Silencing of Mi-SeBP-1 did not change the nematode's ability to parasitize plants and reproduction potential within the host. These results suggest that M. incognita Mi-SeBP-1 might be involved in altering the attachment of microbial pathogens on the nematode cuticle, but is not involved in nematode-host plant interaction. This is the first report for a function of SeBP in Phylum Nematoda.

Functional Annotation of Caenorhabditis elegans Genes by Analysis of Gene Co-Expression Networks

Caenorhabditis elegans (C. elegans) is a well-characterized metazoan, whose transcriptome has been profiled in different tissues, development stages, or other conditions. Large-scale transcriptomes can be reused for gene function annotation through systematic analysis of gene co-expression relationships. We collected 2101 microarray data from National Center for Biotechnology Information Gene Expression Omnibus (NCBI GEO), and identified 48 modules of co-expressed genes that correspond to tissues, development stages, and other experimental conditions. These modules provide an overview of the transcriptional organizations that may work under different conditions. By analyzing higher-order module networks, we found that nucleus and plasma membrane modules are more connected than other intracellular modules. Module-based gene function annotation may help to extend the candidate cuticle gene list. A comparison with other published data validates the credibility of our result. Our findings provide a new source for future gene discovery in C. elegans.

Transcriptome analysis of adult Caenorhabditis elegans cells reveals tissue-specific gene and isoform expression

The biology and behavior of adults differ substantially from those of developing animals, and cell-specific information is critical for deciphering the biology of multicellular animals. Thus, adult tissue-specific transcriptomic data are critical for understanding molecular mechanisms that control their phenotypes. We used adult cell-specific isolation to identify the transcriptomes of C. elegans' four major tissues (or "tissue-ome"), identifying ubiquitously expressed and tissue-specific "enriched" genes. These data newly reveal the hypodermis' metabolic character, suggest potential worm-human tissue orthologies, and identify tissue-specific changes in the Insulin/IGF-1 signaling pathway. Tissue-specific alternative splicing analysis identified a large set of collagen isoforms. Finally, we developed a machine learning-based prediction tool for 76 sub-tissue cell types, which we used to predict cellular expression differences in IIS/FOXO signaling, stage-specific TGF-β activity, and basal vs. memory-induced CREB transcription. Together, these data provide a rich resource for understanding the biology governing multicellular adult animals.

iTRAQ-based quantitative proteomic analysis of Procambarus clakii hemocytes during Spiroplasma eriocheiris infection

As a new-found aquaculture pathogen, Spiroplasma eriocheiris, has resulted in inconceivable economic losses in aquaculture. In the infection of S. eriocheiris, the Procambarus clakii hemocytes have indicated to be major target cells. What was designed to examine in our study is the hemocytes' immune response at the protein levels. Before the pathogen was injected and after 192 h of post-injection, the differential proteomes of the crayfish hemocytes were analyzed immediately by isobaric tags for relative and absolute quantization (iTRAQ) labeling, followed by liquid chromatogramphytandem mass spectrometry (LC-MS/MS). This research had identified a total of 285 differentially expressed proteins. Eighty-three and 202 proteins were up-regulated and down-regulated, respectively, caused by the S. eriocheiris infection. Up-regulated proteins included alpha-2-macroglobulin (α2M), vitellogenin, ferritin, etc. Down-regulated proteins, involved with serine protease, peroxiredoxin 6, 14-3-3-like protein, C-type lectin, cdc42 homolog precursor, etc. The prophenoloxidase-activating system, antimicrobial action involved in the immune responses of P. clarkii is considered to be damaged due to S. eriocheiris infection. The present work could lay the foundation for future research on the proteins related to the susceptibility/resistance of P. clarkii to S. eriocheiris. In addition, it is helpful for our understanding molecular mechanism of disease processes in crayfishes.

A Damage Sensor Associated with the Cuticle Coordinates Three Core Environmental Stress Responses in Caenorhabditis elegans

Although extracellular matrices function as protective barriers to many types of environmental insult, their role in sensing stress and regulating adaptive gene induction responses has not been studied carefully...

Extracellular matrix barriers and inducible cytoprotective genes form successive lines of defense against chemical and microbial environmental stressors. The barrier in nematodes is a collagenous extracellular matrix called the cuticle. In Caenorhabditis elegans, disruption of some cuticle collagen genes activates osmolyte and antimicrobial response genes. Physical damage to the epidermis also activates antimicrobial responses. Here, we assayed the effect of knocking down genes required for cuticle and epidermal integrity on diverse cellular stress responses. We found that disruption of specific bands of collagen, called annular furrows, coactivates detoxification, hyperosmotic, and antimicrobial response genes, but not other stress responses. Disruption of other cuticle structures and epidermal integrity does not have the same effect. Several transcription factors act downstream of furrow loss. SKN-1/Nrf and ELT-3/GATA are required for detoxification, SKN-1/Nrf is partially required for the osmolyte response, and STA-2/Stat and ELT-3/GATA for antimicrobial gene expression. Our results are consistent with a cuticle-associated damage sensor that coordinates detoxification, hyperosmotic, and antimicrobial responses through overlapping, but distinct, downstream signaling.

Wound healing, cellular regeneration and plasticity: the elegans way

Regeneration and wound healing are complex processes that allow organs and tissues to regain their integrity and functionality after injury. Wound healing, a key property of epithelia, involves tissue closure that in some cases leads to scar formation. Regeneration, a process rather limited in mammals, is the capacity to regrow (parts of) an organ or a tissue, after damage or amputation. What are the properties of organs and the features of tissue permitting functional regrowth and repair? What are the cellular and molecular mechanisms underlying these processes? These questions are crucial both in fundamental and applied contexts, with important medical implications. The mechanisms and cells underlying tissue repair have thus been the focus of intense investigation. The last decades have seen rapid progress in the domain and new models emerging. Here, we review the fundamental advances and the perspectives that the use of C. elegans as a model have brought to the mechanisms of wound healing and cellular plasticity, axon regeneration and transdifferentiation in vivo.

Transcriptome analysis of hepatopancraes of Procambarus clarkii challenged with polyriboinosinic polyribocytidylic acid (poly I:C)

Crustacean hepatopancreas regulates metabolic processes, biogenesis and innate immune processes, and the knowledge on its immune genes are crucial to understand antimicrobial mechanisms. In this study, we reported the transcriptomic profile of Procambarus clarkii hepatopancreas after poly I:C administration using high-throughput sequencing. Following de novo assembly 56,716 unigene sequences with an average length of 810 bp was obtained. The unigene sequences were annotated to three ontologies including cellular components, biological processes and molecular functions, further 56,716 unigene sequences were mapped to 25 COG categories. A total of 2497 differentially expressed genes (DEGs) were identified following the comparative analysis between poly I:C treated and control group, and then KEGG enrichment analysis were performed to detect immune related pathways. Quantitative real time polymerase chain reaction showed that the selected DEGs significantly up-regulated following poly I:C administration in comparison to control group. The transcriptomic sequence information will improve the knowledge of this economically important crustacean, and will shed light on its antiviral immune mechanisms.

Novel Insights into Antiviral Gene Regulation of Red Swamp Crayfish, Procambarus clarkii, Infected with White Spot Syndrome Virus

White spot syndrome virus (WSSV), one of the major pathogens of Procambarus clarkii, has caused severe disruption to the aquaculture industry of P. clarkii in China. To reveal the gene regulatory mechanisms underlying WSSV infection, a comparative transcriptome analysis was performed among WSSV-infected susceptible individuals (GS), viral resistant individuals (GR), and a non-infected control group (GC). A total of 61,349 unigenes were assembled from nine libraries. Subsequently, 515 and 1033 unigenes exhibited significant differential expression in sensitive and resistant crayfish individuals compared to the control group (GC). Many differentially expressed genes (e.g., C-type lectin 4, Peroxinectin, Prophenoloxidase, and Serine/threonine-protein kinase) observed in GR and GS play critical roles in pathogen recognition and viral defense reactions after WSSV infection. Importantly, the glycosaminoglycan biosynthesis-chondroitin sulfate/dermatan sulfate pathway was identified to play critical roles in defense to WSSV infection for resistant crayfish individuals by upregulating the chondroitin sulfate related genes for the synthesis of WSSV-sensitive, functional chondroitin sulfate chains containing E units. Numerous genes and the key pathways identified between resistant and susceptible P. clarkii individuals provide valuable insights regarding antiviral response mechanisms of decapoda species and may help to improve the selective breeding of P. clarkii WSSV-resistance.

An enhanced C. elegans based platform for toxicity assessment

There is a well-defined regulatory framework governing the approval of chemicals for use as pharmaceuticals or release into the environment. Toxicity assessment is thus a major hurdle in the compound discovery pipeline, currently involving large scale animal testing. The search for alternative testing platforms is therefore an important priority. We have developed a convenient, low cost assay utilising the nematode Caenorhabditis elegans, to rapidly assess both acute toxicity and developmental and reproductive toxicity (DART). However the worm is protected by a robust cuticle that forms a barrier to chemical uptake. We assessed mutants with altered cuticle properties to identify sensitized strains optimized for toxicity assays. Evaluating the trade-off between increased permeability and reduced fitness identifies bus-5(br19) as the most suitable strain for chemical exposure. We demonstrate the applicability of this assay for a range of chemicals with differing properties, including a modified exposure protocol for volatile or less soluble compounds. This work enhances the effectiveness of C. elegans for convenient toxicity assessment, which could contribute to a reduction in the use of vertebrates particularly at the crucial early stages of product development. Strains identified in this work will also enhance the sensitivity of C. elegans based drug discovery platforms.

Comparative transcriptome and potential antiviral signaling pathways analysis of the gills in the red swamp crayfish, Procambarus clarkii infected with White Spot Syndrome Virus (WSSV)

Red swamp crayfish is an important model organism for research of the invertebrate innate immunity mechanism. Its excellent disease resistance against bacteria, fungi, and viruses is well-known. However, the antiviral mechanisms of crayfish remain unclear. In this study, we obtained high-quality sequence reads from normal and white spot syndrome virus (WSSV)-challenged crayfish gills. For group normal (GN), 39,390,280 high-quality clean reads were randomly assembled to produce 172,591 contigs; whereas, 34,011,488 high-quality clean reads were randomly assembled to produce 182,176 contigs for group WSSV-challenged (GW). After GO annotations analysis, a total of 35,539 (90.01%), 14,931 (37.82%), 28,221 (71.48%), 25,290 (64.05%), 15,595 (39.50%), and 13,848 (35.07%) unigenes had significant matches with sequences in the Nr, Nt, Swiss-Prot, KEGG, COG and GO databases, respectively. Through the comparative analysis between GN and GW, 12,868 genes were identified as differentially up-regulated DEGs, and 9,194 genes were identified as differentially down-regulated DEGs. Ultimately, these DEGs were mapped into different signaling pathways, including three important signaling pathways related to innate immunity responses. These results could provide new insights into crayfish antiviral immunity mechanism.

Glucose or Altered Ceramide Biosynthesis Mediate Oxygen Deprivation Sensitivity Through Novel Pathways Revealed by Transcriptome Analysis in Caenorhabditis elegans

Individuals with type 2 diabetes display metabolic abnormalities, such as hyperglycemia, increased free fatty acids, insulin resistance, and altered ceramide levels, that contribute to vascular dysfunctions and compromised oxygen delivery. Caenorhabditis elegans fed a glucose-supplemented diet or with altered ceramide metabolism, due to a hyl-2 mutation, are sensitive to oxygen deprivation (anoxia). Our experiments showed that the combination of these factors further decreased the anoxia survival. RNA-sequencing analysis was performed to assess how a glucose-supplemented diet and/or a hyl-2 mutation altered the transcriptome. Comparison analysis of transcripts associated with anoxia-sensitive animals [hyl-2(tm2031) mutation or a glucose diet] revealed 199 common transcripts encoded by genes with known or predicted functions involving innate immunity, cuticle function (collagens), or xenobiotic and endobiotic phase I and II detoxification system. Use of RNA interference (RNAi) to target gene products of the xenobiotic and endobiotic phase I and II detoxification system (UDP-glycosyltransferase and Cytochrome p450 genes; ugt-15, ugt-18, ugt-19, ugt-41, ugt-63, cyp-13A12, cyp-25A1, and cyp-33C8) increased anoxia survival in wild-type animals fed a standard diet. Anoxia sensitivity of the hyl-2(tm2031) animals was suppressed by RNAi of cyp-25A1 or cyp-33C8 genes. A glucose diet fed to the P0 hermaphrodite decreased the anoxia survival of its F1 embryos; however, the RNAi of ugt-63 and cyp-33C8 suppressed anoxia sensitivity. These studies provide evidence that the detoxification system impacts oxygen deprivation responses and that C. elegans can be used to model the conserved detoxification system.

In-depth comparative transcriptome analysis of intestines of red swamp crayfish, Procambarus clarkii, infected with WSSV

Crayfish has become one of the most important farmed aquatic species in China due to its excellent disease resistance against bacteria and viruses. However, the antiviral mechanism of crayfish is still not very clear. In the present study, many high-quality sequence reads from crayfish intestine were obtained using Illumina-based transcriptome sequencing. For the normal group (GN), 44,600,142 high-quality clean reads were randomly assembled to produce 125,394 contigs. For the WSSV-challenged group (GW), 47,790,746 high-quality clean reads were randomly assembled to produce 148,983 contigs. After GO annotation, 39,482 unigenes were annotated into three ontologies: biological processes, cellular components, and molecular functions. In addition, 15,959 unigenes were mapped to 25 different COG categories. Moreover, 7,000 DEGs were screened out after a comparative analysis between the GN and GW samples, which were mapped into 250 KEGG pathways. Among these pathways, 36 were obviously changed (P-values < 0.05) and 28 pathways were extremely significantly changed (P-values < 0.01). Finally, five key DEGs involved in the JAK-STAT signaling pathway were chosen for qRT-PCR. The results showed that these five DEGs were obviously up-regulated at 36 h post WSSV infection in crayfish intestine. These results provide new insight into crayfish antiviral immunity mechanisms.