Genetic Screen Reveals Link between the Maternal Effect Sterile Gene mes-1 and Pseudomonas aeruginosa-induced Neurodegeneration in Caenorhabditis elegans

Dendrite morphogenesis in Caenorhabditis elegans

Since the days of Ramón y Cajal, the vast diversity of neuronal and particularly dendrite morphology has been used to catalog neurons into different classes. Dendrite morphology varies greatly and reflects the different functions performed by different types of neurons. Significant progress has been made in our understanding of how dendrites form and the molecular factors and forces that shape these often elaborately sculpted structures. Here, we review work in the nematode Caenorhabditis elegans that has shed light on the developmental mechanisms that mediate dendrite morphogenesis with a focus on studies investigating ciliated sensory neurons and the highly elaborated dendritic trees of somatosensory neurons. These studies, which combine time-lapse imaging, genetics, and biochemistry, reveal an intricate network of factors that function both intrinsically in dendrites and extrinsically from surrounding tissues. Therefore, dendrite morphogenesis is the result of multiple tissue interactions, which ultimately determine the shape of dendritic arbors.

Decoding lifespan secrets: the role of the gonad in Caenorhabditis elegans aging

The gonad has become a central organ for understanding aging in C. elegans, as removing the proliferating stem cells in the germline results in significant lifespan extension. Similarly, when starvation in late larval stages leads to the quiescence of germline stem cells the adult nematode enters reproductive diapause, associated with an extended lifespan. This review summarizes recent advancements in identifying the mechanisms behind gonad-mediated lifespan extension, including comparisons with other nematodes and the role of lipid signaling and transcriptional changes. Given that the gonad also mediates lifespan regulation in other invertebrates and vertebrates, elucidating the underlying mechanisms may help to gain new insights into the mechanisms and evolution of aging.

C. elegans as an Animal Model to Study the Intersection of DNA Repair, Aging and Neurodegeneration

Since its introduction as a genetic model organism, Caenorhabditis elegans has yielded insights into the causes of aging. In addition, it has provided a molecular understanding of mechanisms of neurodegeneration, one of the devastating effects of aging. However, C. elegans has been less popular as an animal model to investigate DNA repair and genomic instability, which is a major hallmark of aging and also a cause of many rare neurological disorders. This article provides an overview of DNA repair pathways in C. elegans and the impact of DNA repair on aging hallmarks, such as mitochondrial dysfunction, telomere maintenance, and autophagy. In addition, we discuss how the combination of biological characteristics, new technical tools, and the potential of following precise phenotypic assays through a natural life-course make C. elegans an ideal model organism to study how DNA repair impact neurodegeneration in models of common age-related neurodegenerative diseases.

Caenorhabditis elegans Neurotoxicity Testing: Novel Applications in the Adverse Outcome Pathway Framework

Neurological hazard assessment of industrial and pesticidal chemicals demands a substantial amount of time and resources. Caenorhabditis elegans is an established model organism in developmental biology and neuroscience. It presents an ideal test system with relatively fewer neurons (302 in hermaphrodites) versus higher-order species, a transparent body, short lifespan, making it easier to perform neurotoxic assessment in a time and cost-effective manner. Yet, no regulatory testing guidelines have been developed for C. elegans in the field of developmental and adult neurotoxicity. Here, we describe a set of morphological and behavioral assessment protocols to examine neurotoxicity in C. elegans with relevance to cholinergic and dopaminergic systems. We discuss the homology of human genes and associated proteins in these two signaling pathways and evaluate the morphological and behavioral endpoints of C. elegans in the context of published adverse outcome pathways of neurodegenerative diseases. We conclude that C. elegans neurotoxicity testing will not only be instrumental to eliminating mammalian testing in neurological hazard assessment but also lead to new knowledge and mechanistic validation in the adverse outcome pathway framework.

Myotubularin-related protein protects against neuronal degeneration mediated by oxidative stress or infection

Microbial infections have been linked to the onset and severity of neurodegenerative diseases such as amyotrophic lateral sclerosis, multiple sclerosis, Alzheimer's disease, but the underlying mechanisms remain largely unknown. Here, we used a genetic screen for genes involved in protection from infection-associated neurodegeneration and identified the gene mtm-10. We then validated the role of the encoded myotubularin-related protein, MTM-10, in protecting the dendrites of Caenorhabditis elegans from degeneration mediated by oxidative stress or Pseudomonas aeruginosa infection. Further experiments indicated that mtm-10 is expressed in the AWC neurons of C. elegans, where it functions in a cell-autonomous manner to protect the dendrite degeneration caused by pathogen infection. We also confirm that the changes observed in the dendrites of the animals were not because of premature death or overall sickness. Finally, our studies indicated that mtm-10 functions in AWC neurons to preserve chemosensation after pathogen infection. These results reveal an essential role for myotubularin-related protein 10 in the protection of dendrite morphology and function against the deleterious effects of oxidative stress or infection.

G-Protein-Coupled Receptor SRBC-48 Protects against Dendrite Degeneration and Reduced Longevity Due to Infection

Increasing evidence suggests that deficient immune modulation and microbial infections underline neurodegeneration, but the mechanisms remain obscure. Here, we show that the G-protein-coupled receptor (GPCR) SRBC-48, which belongs to the class BC serpentine receptors, has a protective role in Caenorhabditis elegans dendrite degeneration caused by Pseudomonas aeruginosa infection. Our results indicate that SRBC-48 functions in a cell-autonomous manner in AWC neurons to protect against infection-associated dendrite degeneration. The absence of SRBC-48 results in a reduced lifespan caused by a pathogen infection early in life that induces dendrite degeneration. The decreased longevity in animals deficient in SRBC-48 is due to uncontrolled activation of immune genes, particularly those regulated by the FOXO family transcription factor DAF-16 that is part of the insulin/insulin-like growth factor (IGF)-1 receptor homolog DAF-2. These results reveal how an infection early in life can not only induce dendrite degeneration but also reduce lifespan.

Colonization of the Caenorhabditis elegans gut with human enteric bacterial pathogens leads to proteostasis disruption that is rescued by butyrate

Protein conformational diseases are characterized by misfolding and toxic aggregation of metastable proteins, often culminating in neurodegeneration. Enteric bacteria influence the pathogenesis of neurodegenerative diseases; however, the complexity of the human microbiome hinders our understanding of how individual microbes influence these diseases. Disruption of host protein homeostasis, or proteostasis, affects the onset and progression of these diseases. To investigate the effect of bacteria on host proteostasis, we used Caenorhabditis elegans expressing tissue-specific polyglutamine reporters that detect changes in the protein folding environment. We found that colonization of the C. elegans gut with enteric bacterial pathogens disrupted proteostasis in the intestine, muscle, neurons, and the gonad, while the presence of bacteria that conditionally synthesize butyrate, a molecule previously shown to be beneficial in neurodegenerative disease models, suppressed aggregation and the associated proteotoxicity. Co-colonization with this butyrogenic strain suppressed bacteria-induced protein aggregation, emphasizing the importance of microbial interaction and its impact on host proteostasis. Further experiments demonstrated that the beneficial effect of butyrate depended on the bacteria that colonized the gut and that this protective effect required SKN-1/Nrf2 and DAF-16/FOXO transcription factors. We also found that bacteria-derived protein aggregates contribute to the observed disruption of host proteostasis. Together, these results reveal the significance of enteric infection and gut dysbiosis on the pathogenesis of protein conformational diseases and demonstrate the potential of using butyrate-producing microbes as a preventative and treatment strategy for neurodegenerative disease.

Ubiquitin-related processes and innate immunity in C. elegans

Innate immunity is an evolutionary ancient defence strategy that serves to eliminate infectious agents while maintaining host health. It involves a complex network of sensors, signaling proteins and immune effectors that detect the danger, then relay and execute the immune programme. Post-translational modifications relying on conserved ubiquitin and ubiquitin-like proteins are an integral part of the system. Studies using invertebrate models of infection, such as the nematode Caenorhabditis elegans, have greatly contributed to our understanding of how ubiquitin-related processes act in immune sensing, regulate immune signaling pathways, and participate to host defence responses. This review highlights the interest of working with a genetically tractable model organism and illustrates how C. elegans has been used to identify ubiquitin-dependent immune mechanisms, discover novel ubiquitin-based resistance strategies that mediate pathogen clearance, and unravel the role of ubiquitin-related processes in tolerance, preserving host fitness during pathogen attack. Special emphasis is placed on processes that are conserved in mammals.

Stressful development: integrating endoderm development, stress, and longevity

In addition to performing digestion and nutrient absorption, the intestine serves as one of the first barriers to the external environment, crucial for protecting the host from environmental toxins, pathogenic invaders, and other stress inducers. The gene regulatory network (GRN) governing embryonic development of the endoderm and subsequent differentiation and maintenance of the intestine has been well-documented in C. elegans. A key regulatory input that initiates activation of the embryonic GRN for endoderm and mesoderm in this animal is the maternally provided SKN-1 transcription factor, an ortholog of the vertebrate Nrf1 and 2, which, like C. elegans SKN-1, perform conserved regulatory roles in mediating a variety of stress responses across metazoan phylogeny. Other key regulatory factors in early gut development also participate in stress response as well as in innate immunity and aging and longevity. In this review, we discuss the intersection between genetic nodes that mediate endoderm/intestine differentiation and regulation of stress and homeostasis. We also consider how direct signaling from the intestine to the germline, in some cases involving SKN-1, facilitates heritable epigenetic changes, allowing transmission of adaptive stress responses across multiple generations. These connections between regulation of endoderm/intestine development and stress response mechanisms suggest that varying selective pressure exerted on the stress response pathways may influence the architecture of the endoderm GRN, thereby leading to genetic and epigenetic variation in early embryonic GRN regulatory events.

Notch receptor GLP-1 regulates toxicity of simulated microgravity stress by activating germline-intestine communication of insulin signaling in C. elegans

We here investigated molecular basis of notch receptor GLP-1 in controlling simulated microgravity stress in Caenorhabditis elegans. glp-1 expression was decreased by simulated microgravity. Meanwhile, glp-1 mutation caused resistance to toxicity of simulated microgravity. GLP-1 acted in germline cells to control toxicity of simulated microgravity. In germline cells, RNAi knockdown of glp-1 increased daf-16 expression. RNAi knockdown of daf-16 suppressed resistance to toxicity of simulated microgravity in glp-1 mutant. In simulated microgravity treated worms, germline RNAi knockdown of glp-1 decreased expressions of daf-28, ins-39, and ins-8 encoding insulin peptides, and resistance to simulated microgravity toxicity could be detected in daf-28(RNAi), ins-39(RNAi), and ins-8(RNAi) worms. In simulated microgravity treated worms, RNAi knockdown of daf-28, ins-39, or ins-8 in germline cells further increased expression and nucleus localization of transcriptional factor DAF-16 in intestinal cells. Therefore, the GLP-1-activated germline-intestine communication of insulin signaling is required for control of simulated microgravity toxicity in C. elegans.

Regulation of Innate Immune Response to Fungal Infection in Caenorhabditis elegans by SHN-1/SHANK

In Caenorhabditis elegans, SHN-1 is the homologue of SHANK, a scaffolding protein. In this study, we determined the molecular basis for SHN-1/SHANK in the regulation of innate immune response to fungal infection. Mutation of shn-1 increased the susceptibility to Candida albicans infection and suppressed the innate immune response. After C. albicans infection for 6, 12, or 24 h, both transcriptional expression of shn-1 and SHN-1::GFP expression were increased, implying that the activated SHN-1 may mediate a protection mechanism for C. elegans against the adverse effects from fungal infection. SHN-1 acted in both the neurons and the intestine to regulate the innate immune response to fungal infection. In the neurons, GLR-1, an AMPA ionotropic glutamate receptor, was identified as the downstream target in the regulation of innate immune response to fungal infection. GLR-1 further positively affected the function of SER-7-mediated serotonin signaling and antagonized the function of DAT-1-mediated dopamine signaling in the regulation of innate immune response to fungal infection. Our study suggests the novel function of SHN-1/SHANK in the regulation of innate immune response to fungal infection. Moreover, our results also denote the crucial role of neurotransmitter signals in mediating the function of SHN-1/SHANK in regulating innate immune response to fungal infection.

Effects of ecologically relevant concentrations of cadmium on locomotor activity and microbiota in zebrafish

Cadmium (Cd) is widely spread in the aquatic environment, and its impact on humans and the ecosystem is an important issue in public health. However, its effects on zebrafish microbiota are still poorly understood. In this study, the potential developmental neurotoxicity and microbiota dysbiosis of ecologically relevant concentrations of Cd (0, 1.25, 2.5 and 5 μg/L) was evaluated by waterborne exposure for 7 days. The data showed that exposure to 5 μg/L of Cd significantly decreased survival rates and impaired locomotor activities. Uptake of Cd was enhanced with the increase of the concentration and duration of exposure. High-throughput sequencing analysis revealed a significant change in the richness and diversity of the microbiota of Cd-treated zebrafish. At the phylum level, the abundance of Proteobacteria increased, while that Firmicutes was significantly decreased after exposure to 5 μg/L Cd. At the genus level, there were significant changes in the abundances of several bacteria involved in the regulation of neurodegenerative diseases (Pseudomonas, Ruminococcaceae, Blautia, Bacteroides, Lactobacillus, Lachnospiraceae, and Phascolarctobacterium) in the Cd-treatment groups, as compared to the control group. In addition, the mRNA expression profiles of bdnf and genes involved in serotonin signaling and metabolism were changed in the Cd exposure groups. Together, these data suggest that Cd could be harmful to zebrafish health by inducing the microbiota changes, and the microbiota could serve as a potential target to protect against the adverse effects of Cd toxicity.

Harnessing the power of genetics: fast forward genetics in Caenorhabditis elegans

Forward genetics is a powerful tool to unravel molecular mechanisms of diverse biological processes. The success of genetic screens primarily relies on the ease of genetic manipulation of an organism and the availability of a plethora of genetic tools. The roundworm Caenorhabditis elegans has been one of the favorite models for genetic studies due to its hermaphroditic lifestyle, ease of maintenance, and availability of various genetic manipulation tools. The strength of C. elegans genetics is highlighted by the leading role of this organism in the discovery of several conserved biological processes. In this review, the principles and strategies for forward genetics in C. elegans are discussed. Further, the recent advancements that have drastically accelerated the otherwise time-consuming process of mutation identification, making forward genetic screens a method of choice for understanding biological functions, are discussed. The emphasis of the review has been on providing practical and conceptual pointers for designing genetic screens that will identify mutations, specifically disrupting the biological processes of interest.

Long-term and low-dose exposure to nanopolystyrene induces a protective strategy to maintain functional state of intestine barrier in nematode Caenorhabditis elegans

Functional state of intestinal barrier plays an important role for environmental animals in being against various toxicants. We investigated GATA transcriptional factor ELT-2-mediated intestinal response to nanopolystyrere in Caenorhabditis elegans. Prolonged exposure to nanopolystyrene (≥1 μg/L) induced an increase in expression of ELT-2, and intestinal RNA interference (RNAi) knockdown of elt-2 caused enhancement in intestinal permeability. Meanwhile, mutation of elt-2 resulted in susceptibility to nanopolystyrene toxicity, and ELT-2 functioned in intestine to regulate the nanopolystyrene toxicity. ERM-1, CLEC-63, and CLEC-85 were identified as targets of ELT-2 in regulating the nanopolystyrene toxicity. ERM-1 was required for maintaining functional state in intestinal barrier, and functioned synergistically with CLEC-63 or CLEC-85 to regulate nanopolystyrene toxicity. Therefore, activation of intestinal ELT-2 by nanopolystyrere could mediate a protective strategy to maintain the functional state of intestinal barrier. During this process, intestinal ELT-2 activated two different molecular signals (ERM-1 signal and CLEC-63/85 signal) for nematodes against the nanopolystyrene toxicity.

Biology is the root of variability: cautionary tales in Caenorhabditis elegans biology

Reproducibility is critical for the standardization, interpretation, and progression of research. However, many factors increase variability and reduce reproducibility. In Caenorhabditis elegans research, there are many possible causes of variability that may explain why experimental outcomes sometimes differ between laboratories and between experiments. Factors contributing to experimental variability include the genetic background of both C. elegans and its bacterial diet, differences in media composition, intergenerational and transgenerational effects that may be carried over for generations, and the use of chemicals or reagents that may have unexpected consequences. This review summarizes sources of variability in C. elegans research and serves to identify laboratory practices that could influence reproducibility.

mir-355 Functions as An Important Link between p38 MAPK Signaling and Insulin Signaling in the Regulation of Innate Immunity

We performed a systematic identification of microRNAs (miRNAs) involved in the control of innate immunity. We identified 7 novel miRNA mutants with altered survival, colony forming in the body, and expression pattern of putative antimicrobial genes after Pseudomonas aeruginosa infection. Loss-of-function mutation of mir-45, mir-75, mir-246, mir-256, or mir-355 induced resistance to P. aeruginosa infection, whereas loss-of-function mutation of mir-63 or mir-360 induced susceptibility to P. aeruginosa infection. DAF-2 in the insulin signaling pathway acted as a target for intestinal mir-355 to regulate innate immunity. mir-355 functioned as an important link between p38 MAPK signaling pathway and insulin signaling pathway in the regulation of innate immunity. Our results provide an important molecular basis for further elucidation of the functions of various miRNAs in the regulation of innate immunity.

Graphene Oxide Dysregulates Neuroligin/NLG-1-Mediated Molecular Signaling in Interneurons in Caenorhabditis elegans

Graphene oxide (GO) can be potentially used in many medical and industrial fields. Using assay system of Caenorhabditis elegans, we identified the NLG-1/Neuroligin-mediated neuronal signaling dysregulated by GO exposure. In nematodes, GO exposure significantly decreased the expression of NLG-1, a postsynaptic cell adhesion protein. Loss-of-function mutation of nlg-1 gene resulted in a susceptible property of nematodes to GO toxicity. Rescue experiments suggested that NLG-1 could act in AIY interneurons to regulate the response to GO exposure. In the AIY interneurons, PKC-1, a serine/threonine protein kinase C (PKC) protein, was identified as the downstream target for NLG-1 in the regulation of response to GO exposure. LIN-45, a Raf protein in ERK signaling pathway, was further identified as the downstream target for PKC-1 in the regulation of response to GO exposure. Therefore, GO may dysregulate NLG-1-mediated molecular signaling in the interneurons, and a neuronal signaling cascade of NLG-1-PKC-1-LIN-45 was raised to be required for the control of response to GO exposure. More importantly, intestinal RNAi knockdown of daf-16 gene encoding a FOXO transcriptional factor in insulin signaling pathway suppressed the resistant property of nematodes overexpressing NLG-1 to GO toxicity, suggesting the possible link between neuronal NLG-1 signaling and intestinal insulin signaling in the regulation of response to GO exposure.

Molecular Control of Innate Immune Response to Pseudomonas aeruginosa Infection by Intestinal let-7 in Caenorhabditis elegans

The microRNA (miRNA) let-7 is an important miRNA identified in Caenorhabditis elegans and has been shown to be involved in the control of innate immunity. The underlying molecular mechanisms for let-7 regulation of innate immunity remain largely unclear. In this study, we investigated the molecular basis for intestinal let-7 in the regulation of innate immunity. Infection with Pseudomonas aeruginosa PA14 decreased let-7::GFP expression. Intestine- or neuron-specific activity of let-7 was required for its function in the regulation of innate immunity. During the control of innate immune response to P. aeruginosa PA14 infection, SDZ-24 was identified as a direct target for intestinal let-7. SDZ-24 was found to be predominantly expressed in the intestine, and P. aeruginosa PA14 infection increased SDZ-24::GFP expression. Intestinal let-7 regulated innate immune response to P. aeruginosa PA14 infection by suppressing both the expression and the function of SDZ-24. Knockout or RNA interference knockdown of sdz-24 dampened the resistance of let-7 mutant to P. aeruginosa PA14 infection. Intestinal overexpression of sdz-24 lacking 3'-UTR inhibited the susceptibility of nematodes overexpressing intestinal let-7 to P. aeruginosa PA14 infection. In contrast, we could observed the effects of intestinal let-7 on innate immunity in P. aeruginosa PA14 infected transgenic strain overexpressing sdz-24 containing 3'-UTR. In the intestine, certain SDZ-24-mediated signaling cascades were formed for nematodes against the P. aeruginosa PA14 infection. Our results highlight the crucial role of intestinal miRNAs in the regulation of the innate immune response to pathogenic infection.

The Mitochondria-Regulated Immune Pathway Activated in the C. elegans Intestine Is Neuroprotective

Immunological mediators that originate outside the nervous system can affect neuronal health. However, their roles in neurodegeneration remain largely unknown. Here, we show that the p38MAPK-mediated immune pathway activated in intestinal cells of Caenorhabditis elegans upon mitochondrial dysfunction protects neurons in a cell-non-autonomous fashion. Specifically, mitochondrial complex I dysfunction induced by rotenone activates the p38MAPK/CREB/ATF-7-dependent innate immune response pathway in intestinal cells of C. elegans. Activation of p38MAPK in the gut is neuroprotective. Enhancing the p38MAPK-mediated immune pathway in intestinal cells alone suppresses rotenone-induced dopaminergic neuron loss, while downregulating it in the intestine exacerbates neurodegeneration. The p38MAPK/ATF-7 immune pathway modulates autophagy and requires autophagy and the PTEN-induced putative kinase PINK-1 for conferring neuroprotection. Thus, mitochondrial damage induces the clearance of mitochondria by the immune pathway, protecting the organism from the toxic effects of mitochondrial dysfunction. We propose that mitochondria are subject to constant surveillance by innate immune mechanisms.

Chikka et al. find that mitochondrial complex I damage activates the p38MAPK/ATF-7 signaling pathway in the intestine of C. elegans. Activation of the p38MAPK/ATF-7 immune pathway in the intestine is neuroprotective and sufficient to prevent rotenone-induced degeneration of dopaminergic neurons.

Local and long-range activation of innate immunity by infection and damage in C. elegans

The nematode worm Caenorhabditis elegans lends itself naturally to investigation of innate immunity, from the scale of molecules to the whole animal. Numerous studies have begun to reveal the complex interplay of signalling mechanisms that underlie host defence in C. elegans. We discuss here research that illustrates the connection between cell and tissue-level homeostatic mechanisms and the activation of innate immune signalling pathways. These are woven together to provide a comprehensive organismal protection against perceived threats.