The WD40 repeat protein WDR-23 functions with the CUL4/DDB1 ubiquitin ligase to regulate nuclear abundance and activity of SKN-1 in Caenorhabditis elegans

SQST-1/p62-regulated SKN-1/Nrf mediates a phagocytic stress response via transcriptional activation of lyst-1/LYST

Cells may be intrinsically fated to die to sculpt tissues during development or to maintain homeostasis. Cells can also die in response to various stressors, injury or pathological conditions. Additionally, cells of the metazoan body are often highly specialized with distinct domains that differ both structurally and with respect to their neighbors. Specialized cells can also die, as in normal brain development or pathological states and their different regions may be eliminated via different programs. Clearance of different types of cell debris must be performed quickly and efficiently to prevent autoimmunity and secondary necrosis of neighboring cells. Moreover, all cells, including those programmed to die, may be subject to various stressors. Some largely unexplored questions include whether predestined cell elimination during development could be altered by stress, if adaptive stress responses exist and if polarized cells may need compartment-specific stress-adaptive programs. We leveraged Compartmentalized Cell Elimination (CCE) in the nematode C. elegans to explore these questions. CCE is a developmental cell death program whereby three segments of two embryonic polarized cell types are eliminated differently. We have previously employed this in vivo genetic system to uncover a cell compartment-specific, cell non-autonomous clearance function of the fusogen EFF-1 in phagosome closure during corpse internalization. Here, we introduce an adaptive response that serves to aid developmental phagocytosis as a part of CCE during stress. We employ a combination of forward and reverse genetics, CRISPR/Cas9 gene editing, stress response assays and advanced fluorescence microscopy. Specifically, we report that, under heat stress, the selective autophagy receptor SQST-1/p62 promotes the nuclear translocation of the oxidative stress-related transcription factor SKN-1/Nrf via negative regulation of WDR-23. This in turn allows SKN-1/Nrf to transcribe lyst-1/LYST (lysosomal trafficking associated gene) which subsequently promotes the phagocytic resolution of the developmentally-killed internalized cell even under stress conditions.

The Ancestral KEAP1-NRF Pathway in Amphioxus Branchiostoma japonicum: Implications for the Evolution of Antioxidant Defense System

The Kelch-like ECH-associated protein 1 (KEAP1)/Nuclear factor E2-related factor 2 (NRF2) pathway is a key mechanism that responds to oxidative stress and xenobiotic stimuli in vertebrates. However, knowledge of its evolutionary origins remains limited. In this study, we identify the ancestral homologues of KEAP1 and NRF (BjKEAP1 and BjNRF) in cephalochordate amphioxus (Branchiostoma japonicum). BjNRF uniquely combines the feature domains of vertebrates NRF1 and NRF2, marking it as an evolutionary intermediate. High expression levels of Bjkeap1 and Bjnrf in the gill, hepatic cecum, and intestine highlight their roles in environmental defense at key interface tissues. Functional studies reveal that BjKEAP1 regulates the cytoplasmic localization of BjNRF. Typical NRF2 activator sulforaphane (SFN) induces its nuclear translocation and significantly elevates the transcriptional expression of BjNRF and phase II detoxification enzymes. Moreover, exposure to the environmental toxin Benzo[a]pyrene (BaP) activates this stress response system. These findings bridge critical gaps in our understanding of this pathway in basal chordates and offer new insights into the evolutionary trajectory of the KEAP1-NRF system. Furthermore, this study highlights crucial implications for the conservation of amphioxus in deteriorating marine environments.

Unraveling the functional dynamics of Caenorhabditis elegans stress-responsive omega class GST-44

Glutathione transferases from the omega class are notable for their roles in redox regulation and cellular stress response. In this study, we conducted a comprehensive functional characterization of GST-44, an omega-class glutathione S-transferase (GSTO), in Caenorhabditis elegans, focusing on its role in cellular defense mechanisms against stress. Biochemical analysis revealed GSTO-specific enzymatic activities of recombinant GST-44, including dehydroascorbate reductase, thioltransferase, and arsenate reductase activities. Using transgenic GFP reporter strains, we identified predominant expression of GST-44 in the intestine and excretory H-cell, with significant upregulation observed under diverse stress conditions. Induction of GST-44 was particularly pronounced in the intestine in response to pathogen-, oxidative-, and endoplasmic reticulum stress. Notably, under arsenic stress, the expression of gst-44 was significantly upregulated in the excretory system of the worm, underscoring its critical role in mediating arsenic detoxification. Moreover, we demonstrated the induction of GST-44 using dimethyl fumarate, a highly specific mammalian Nrf-2 activator. The upregulation of GST-44 during arsenic stress was dependent not only on the oxidative stress response transcription factor SKN-1/Nrf2 but also on PHA-4. The deletion mutant strain gst-44(tm6133) exhibited reduced stress resistance and a shortened lifespan, with a highly diminished survival rate under arsenic stress compared to other CRISPR-generated C. elegans GSTO deletion mutants. Our findings highlight the essential role of GST-44 in mediating arsenic detoxification, as well as in stress adaptation and defense mechanisms in C. elegans.

Norharmane prevents muscle aging via activation of SKN-1/NRF2 stress response pathways

Sarcopenia, the age-related decline in muscle mass and function, is a significant contributor to increased frailty and mortality in the elderly. Currently, no FDA-approved treatment exists for sarcopenia. Here, we identified norharmane (NR), a β-carboline alkaloid, as a potential therapeutic agent for mitigating muscle aging. We aimed to determine the ability of NR to delay muscle aging in Caenorhabditis elegans (C. elegans), mouse, and muscle cells in mice and humans. NR treatment improved swimming ability and increased the maximum velocity in aged C. elegans. Transcriptomic analysis revealed that NR upregulated detoxification genes in C. elegans, including cytochrome P450, UGT, and GST enzymes. NR-induced benefits were dependent on the SKN-1/Nrf2 stress response pathway. In mammalian models, NR delayed cellular senescence in human skeletal muscle myoblasts and enhanced myogenesis in C2C12 cells and primary aged myoblasts. NR supplementation in aged mice prevented muscle loss, improved muscle function, and reduced markers of cellular senescence. We found that the p38 MAPK pathway mediated NR activation of Nrf2 by disrupting the Nrf2-Keap1 interaction. NR also improved oxygen consumption rates and promoted mitochondrial biogenesis. These findings suggest that NR is a promising candidate for preventing sarcopenia and improving muscle health.

Tolerance to a Diet of Toxic Microcystis aeruginosa in Caenorhabditis elegans

Reported incidences of cyanobacterial harmful algal blooms (CHABs) are increasing across the world due to climate change and nutrient loading, dominating freshwater ecosystems and producing dangerous cyanotoxins that cause ecological damage. Microcystis aeruginosa is one of the most common species of cyanobacteria; it produces hepatotoxic and neurotoxic microcystin-LR. The ecological and human impact of algal blooms is immense, and traditional CHAB remediation methods are not always adequate in eutrophic regions such as Lake Erie in North America. As a result, a proactive, targeted approach is needed to bioremediate cyanobacteria in their pre-colonial stages. Nematodes, such as the model organism Caenorhabditis elegans, are potential candidates for bioremediating cyanobacteria such as M. aeruginosa. C. elegans have metabolic pathways that could detoxify microcystin-LR and enable tolerance to cyanobacteria in nature. We analyzed C. elegans health and fat accumulation on a diet of toxic M. aeruginosa and found that C. elegans can ingest, digest, metabolize, and survive off of this diet. The mean lifespans of the worm populations were only slightly different at 20.68 ± 0.35 (mean ± S.E.M) and 17.89 ± 0.40 when fed E. coli and toxic M. aeruginosa, respectively. In addition, a diet of toxic M. aeruginosa compared to E. coli did not have any significant impact on C. elegans pharyngeal pumping (304.2 ± 9.3 versus 330.0 ± 10.4 pumps/min), dauer response (86.3 ± 1.0 versus 83.65 ± 1.0% in dauer), mobility (209.25 ± 7.0 versus 210.15 ± 4.4 thrashes/min), or SKN-1 expression based on SKN1::GFP fluorescence measurements. Overall, a diet of toxic M. aeruginosa was able to sustain C. elegans development, and C. elegans was tolerant of it. These results suggest that C. elegans and similar nematodes could be viable candidates for cyanobacterial bioremediation.

Model organisms for investigating the functional involvement of NRF2 in non-communicable diseases

Non-communicable chronic diseases (NCDs) are most commonly characterized by age-related loss of homeostasis and/or by cumulative exposures to environmental factors, which lead to low-grade sustained generation of reactive oxygen species (ROS), chronic inflammation and metabolic imbalance. Nuclear factor erythroid 2-like 2 (NRF2) is a basic leucine-zipper transcription factor that regulates the cellular redox homeostasis. NRF2 controls the expression of more than 250 human genes that share in their regulatory regions a cis-acting enhancer termed the antioxidant response element (ARE). The products of these genes participate in numerous functions including biotransformation and redox homeostasis, lipid and iron metabolism, inflammation, proteostasis, as well as mitochondrial dynamics and energetics. Thus, it is possible that a single pharmacological NRF2 modulator might mitigate the effect of the main hallmarks of NCDs, including oxidative, proteostatic, inflammatory and/or metabolic stress. Research on model organisms has provided tremendous knowledge of the molecular mechanisms by which NRF2 affects NCDs pathogenesis. This review is a comprehensive summary of the most commonly used model organisms of NCDs in which NRF2 has been genetically or pharmacologically modulated, paving the way for drug development to combat NCDs. We discuss the validity and use of these models and identify future challenges.

Protein sequence editing defines distinct and overlapping functions of SKN-1A/Nrf1 and SKN-1C/Nrf2

The Nrf/NFE2L family of transcription factors regulates redox balance, xenobiotic detoxification, metabolism, proteostasis, and aging. Nrf1/NFE2L1 is primarily responsible for stress-responsive upregulation of proteasome subunit genes and is essential for adaptation to proteotoxic stress. Nrf2/NFE2L2 is mainly involved in activating oxidative stress responses and promoting xenobiotic detoxification. Nrf1 and Nrf2 contain very similar DNA binding domains and can drive similar transcriptional responses. In C. elegans, a single gene, skn-1, encodes distinct protein isoforms, SKN-1A and SKN-1C, that function analogously to mammalian Nrf1 and Nrf2, respectively, and share an identical DNA binding domain. Thus, the extent to which SKN-1A/Nrf1 and SKN-1C/Nrf2 functions are distinct or overlapping has been unclear. Regulation of the proteasome by SKN-1A/Nrf1 requires post-translational conversion of N-glycosylated asparagine residues to aspartate by the PNG-1/NGLY1 peptide:N-glycanase, a process we term 'sequence editing'. Here, we reveal the consequences of sequence editing for the transcriptomic output of activated SKN-1A. We confirm that activation of proteasome subunit genes is strictly dependent on sequence editing. In addition, we find that sequence edited SKN-1A can also activate genes linked to redox homeostasis and xenobiotic detoxification that are also regulated by SKN-1C, but the extent of these genes' activation is antagonized by sequence editing. Using mutant alleles that selectively inactivate either SKN-1A or SKN-1C, we show that both isoforms promote optimal oxidative stress resistance, acting as effectors for distinct signaling pathways. These findings suggest that sequence editing governs SKN-1/Nrf functions by tuning the SKN-1A/Nrf1 regulated transcriptome.

CUL4-Based Ubiquitin Ligases in Chromatin Regulation: An Evolutionary Perspective

Ubiquitylation is a post-translational modification that modulates protein function and stability. It is orchestrated by the concerted action of three types of enzymes, with substrate specificity governed by ubiquitin ligases (E3s), which may exist as single proteins or as part of multi-protein complexes. Although Cullin (CUL) proteins lack intrinsic enzymatic activity, they participate in the formation of active ubiquitin ligase complexes, known as Cullin-Ring ubiquitin Ligases (CRLs), through their association with ROC1 or ROC2, along with substrate adaptor and receptor proteins. Mammalian genomes encode several CUL proteins (CUL1-9), each contributing to distinct CRLs. Among these CUL proteins, CUL1, CUL3, and CUL4 are believed to be the most ancient and evolutionarily conserved from yeast to mammals, with CUL4 uniquely duplicated in vertebrates. Genetic evidence strongly implicates CUL4-based ubiquitin ligases (CRL4s) in chromatin regulation across various species and suggests that, in vertebrates, CRL4s have also acquired a cytosolic role, which is facilitated by a cytosol-localizing paralog of CUL4. Substrates identified through biochemical studies have elucidated the molecular mechanisms by which CRL4s regulate chromatin and cytosolic processes. The substantial body of knowledge on CUL4 biology amassed over the past two decades provides a unique opportunity to explore the functional evolution of CRL4. In this review, we synthesize the available structural, genetic, and biochemical data on CRL4 from various model organisms and discuss the conserved and novel functions of CRL4s.

The Rich Evolutionary History of the Reactive Oxygen Species Metabolic Arsenal Shapes Its Mechanistic Plasticity at the Onset of Metazoan Regeneration

Regeneration, the ability to restore body parts after injury, is widespread in metazoans; however, the underlying molecular and cellular mechanisms involved in this process remain largely unknown, and its evolutionary history is consequently unresolved. Recently, reactive oxygen species (ROS) have been shown in several metazoan models to be triggers of apoptosis and cell proliferation that drive regenerative success. However, it is not known whether the contribution of ROS to regeneration relies on conserved mechanisms. Here we performed a comparative genomic analysis of ROS metabolism actors across metazoans, and carried out a comparative study of the deployment and roles of ROS during regeneration in two different metazoan models: the annelid Platynereis dumerilii and the cnidarian Nematostella vectensis. We established that the vast majority of metazoans encode a core redox kit allowing for the production and detoxification of ROS, and overall regulation of ROS levels. However, the precise composition of the redox arsenal can vary significantly from species to species, suggesting that evolutionary constraints apply to ROS metabolism functions rather than precise actors. We found that while ROS are necessary for regeneration in both Platynereis and Nematostella, the two species deploy different enzymatic activities controlling ROS dynamics, and display distinct effects of ROS signaling on injury-induced apoptosis and cell proliferation. We conclude that, while ROS are a common feature of metazoan regeneration, their production and contribution to this phenomenon may depend on different molecular mechanisms highlighting the overall plasticity of the machinery.

Hepatic WDR23 proteostasis mediates insulin homeostasis by regulating insulin-degrading enzyme capacity

Maintaining insulin homeostasis is critical for cellular and organismal metabolism. In the liver, insulin is degraded by the activity of the insulin-degrading enzyme (IDE). Here, we establish a hepatic regulatory axis for IDE through WDR23-proteostasis. Wdr23KO mice have increased IDE expression, reduced circulating insulin, and defective insulin responses. Genetically engineered human cell models lacking WDR23 also increase IDE expression and display dysregulated phosphorylation of insulin signaling cascade proteins, IRS-1, AKT2, MAPK, FoxO, and mTOR, similar to cells treated with insulin, which can be mitigated by chemical inhibition of IDE. Mechanistically, the cytoprotective transcription factor NRF2, a direct target of WDR23-Cul4 proteostasis, mediates the enhanced transcriptional expression of IDE when WDR23 is ablated. Moreover, an analysis of human genetic variation in WDR23 across a large naturally aging human cohort in the US Health and Retirement Study reveals a significant association of WDR23 with altered hemoglobin A1C (HbA1c) levels in older adults, supporting the use of WDR23 as a new molecular determinant of metabolic health in humans.

Increased expression of metabolism and lysosome-associated genes in a C. elegans dpy-7 cuticle furrow mutant

The collagen-based epidermal 'cuticle' of Caenorhabditis elegans functions as an extracellular sensor for damage that regulates genes promoting osmotic balance, innate immunity, and detoxification. Prior studies demonstrate that SKN-1 , an ortholog of the mammalian Nrf transcription factors, activates core detoxification genes downstream from cuticle damage. Prior RNAseq data suggested that expression of five genes with functions in redox balance, ATP homeostasis, and lysosome function ( gst-15 , gst-24 , cyts-1 , argk-1 , and mfsd-8.4 ) were increased in a cuticle collagen mutant; this study employed RT-qPCR to verify this observation and to test the role of SKN-1 . Activation of all five genes was verified in dpy-7 mutants, but none were reduced by skn-1 (RNAi) suggesting parallel or distinct regulatory mechanisms.

Chemical Genetics in C. elegans Identifies Anticancer Mycotoxins Chaetocin and Chetomin as Potent Inducers of a Nuclear Metal Homeostasis Response

C. elegans numr-1/2 (nuclear-localized metal-responsive) is an identical gene pair encoding a nuclear protein previously shown to be activated by cadmium and disruption of the integrator RNA metabolism complex. We took a chemical genetic approach to further characterize regulation of this novel metal response by screening 41,716 compounds and extracts for numr-1p::GFP activation. The most potent activator was chaetocin, a fungal 3,6-epidithiodiketopiperazine (ETP) with promising anticancer activity. Chaetocin activates numr-1/2 strongly in the alimentary canal but is distinct from metal exposure, because it represses canonical cadmium-responsive metallothionine genes. Chaetocin has diverse targets in cancer cells including thioredoxin reductase, histone lysine methyltransferase, and acetyltransferase p300/CBP; further work is needed to identify the mechanism in C. elegans as genetic disruption and RNAi screening of homologues did not induce numr-1/2 in the alimentary canal and chaetocin did not affect markers of integrator dysfunction. We demonstrate that disulfides in chaetocin and chetomin, a dimeric ETP analog, are required to induce numr-1/2. ETP monomer gliotoxin, despite possessing a disulfide linkage, had almost no effect on numr-1/2, suggesting a dimer requirement. Chetomin inhibits C. elegans growth at low micromolar levels, and loss of numr-1/2 increases sensitivity; C. elegans and Chaetomiaceae fungi inhabit similar environments raising the possibility that numr-1/2 functions as a defense mechanism. There is no direct orthologue of numr-1/2 in humans, but RNaseq suggests that chaetocin affects expression of cellular processes linked to stress response and metal homeostasis in colorectal cancer cells. Our results reveal interactions between metal response gene regulation and ETPs and identify a potential mechanism of resistance to this versatile class of preclinical compounds.

Disrupting the SKN-1 homeostat: mechanistic insights and phenotypic outcomes

The mechanisms that govern maintenance of cellular homeostasis are crucial to the lifespan and healthspan of all living systems. As an organism ages, there is a gradual decline in cellular homeostasis that leads to senescence and death. As an organism lives into advanced age, the cells within will attempt to abate age-related decline by enhancing the activity of cellular stress pathways. The regulation of cellular stress responses by transcription factors SKN-1/Nrf2 is a well characterized pathway in which cellular stress, particularly xenobiotic stress, is abated by SKN-1/Nrf2-mediated transcriptional activation of the Phase II detoxification pathway. However, SKN-1/Nrf2 also regulates a multitude of other processes including development, pathogenic stress responses, proteostasis, and lipid metabolism. While this process is typically tightly regulated, constitutive activation of SKN-1/Nrf2 is detrimental to organismal health, this raises interesting questions surrounding the tradeoff between SKN-1/Nrf2 cryoprotection and cellular health and the ability of cells to deactivate stress response pathways post stress. Recent work has determined that transcriptional programs of SKN-1 can be redirected or suppressed to abate negative health outcomes of constitutive activation. Here we will detail the mechanisms by which SKN-1 is controlled, which are important for our understanding of SKN-1/Nrf2 cytoprotection across the lifespan.

Proteasome inhibition triggers tissue-specific immune responses against different pathogens in C. elegans

Protein quality control pathways play important roles in resistance against pathogen infection. For example, the conserved transcription factor SKN-1/NRF up-regulates proteostasis capacity after blockade of the proteasome and also promotes resistance against bacterial infection in the nematode Caenorhabditis elegans. SKN-1/NRF has 3 isoforms, and the SKN-1A/NRF1 isoform, in particular, regulates proteasomal gene expression upon proteasome dysfunction as part of a conserved bounce-back response. We report here that, in contrast to the previously reported role of SKN-1 in promoting resistance against bacterial infection, loss-of-function mutants in skn-1a and its activating enzymes ddi-1 and png-1 show constitutive expression of immune response programs against natural eukaryotic pathogens of C. elegans. These programs are the oomycete recognition response (ORR), which promotes resistance against oomycetes that infect through the epidermis, and the intracellular pathogen response (IPR), which promotes resistance against intestine-infecting microsporidia. Consequently, skn-1a mutants show increased resistance to both oomycete and microsporidia infections. We also report that almost all ORR/IPR genes induced in common between these programs are regulated by the proteasome and interestingly, specific ORR/IPR genes can be induced in distinct tissues depending on the exact trigger. Furthermore, we show that increasing proteasome function significantly reduces oomycete-mediated induction of multiple ORR markers. Altogether, our findings demonstrate that proteasome regulation keeps innate immune responses in check in a tissue-specific manner against natural eukaryotic pathogens of the C. elegans epidermis and intestine.

A dicer-related helicase opposes the age-related pathology from SKN-1 activation in ASI neurons

Coordination of cellular responses to stress is essential for health across the lifespan. The transcription factor SKN-1 is an essential homeostat that mediates survival in stress-inducing environments and cellular dysfunction, but constitutive activation of SKN-1 drives premature aging thus revealing the importance of turning off cytoprotective pathways. Here, we identify how SKN-1 activation in two ciliated ASI neurons in Caenorhabditis elegans results in an increase in organismal transcriptional capacity that drives pleiotropic outcomes in peripheral tissues. An increase in the expression of established SKN-1 stress response and lipid metabolism gene classes of RNA in the ASI neurons, in addition to the increased expression of several classes of noncoding RNA, define a molecular signature of animals with constitutive SKN-1 activation and diminished healthspan. We reveal neddylation as a unique regulator of the SKN-1 homeostat that mediates SKN-1 abundance within intestinal cells. Moreover, RNAi-independent activity of the dicer-related DExD/H-box helicase, drh-1, in the intestine, can oppose the effects of aberrant SKN-1 transcriptional activation and delays age-dependent decline in health. Taken together, our results uncover a cell nonautonomous circuit to maintain organism-level homeostasis in response to excessive SKN-1 transcriptional activity in the sensory nervous system.

Comparative analysis of the molecular and physiological consequences of constitutive SKN-1 activation

Molecular homeostats play essential roles across all levels of biological organization to ensure a return to normal function after responding to abnormal internal and environmental events. SKN-1 is an evolutionarily conserved cytoprotective transcription factor that is integral for the maintenance of cellular homeostasis upon exposure to a variety of stress conditions. Despite the essentiality of turning on SKN-1/NRF2 in response to exogenous and endogenous stress, animals with chronic activation of SKN-1 display premature loss of health with age, and ultimately, diminished lifespan. Previous genetic models of constitutive SKN-1 activation include gain-of-function alleles of skn-1 and loss-of-function alleles of wdr-23 that impede the turnover of SKN-1 by the ubiquitin proteasome. Here, we define a novel gain-of-function mutation in the xrep-4 locus that results in constitutive activation of SKN-1 in the absence of stress. Although each of these genetic mutations results in continuously unregulated transcriptional output from SKN-1, the physiological consequences of each model on development, stress resistance, reproduction, lipid homeostasis, and lifespan are distinct. Here, we provide a comprehensive assessment of the differential healthspan impacts across multiple models of constitutive SKN-1 activation. Although our results reveal the universal need to reign in the uncontrolled activity of cytoprotective transcription factors, we also define the unique signatures of each model of constitutive SKN-1 activation, which provides innovative solutions for the design of molecular "off-switches" of unregulated transcriptional homeostats.

A dicer-related helicase opposes the age-related pathology from SKN-1 activation in ASI neurons

Coordination of cellular responses to stress are essential for health across the lifespan. The transcription factor SKN-1 is an essential homeostat that mediates survival in stress-inducing environments and cellular dysfunction, but constitutive activation of SKN-1 drives premature aging thus revealing the importance of turning off cytoprotective pathways. Here we identify how SKN-1 activation in two ciliated ASI neurons in C. elegans results in an increase in organismal transcriptional capacity that drives pleiotropic outcomes in peripheral tissues. An increase in the expression of established SKN-1 stress response and lipid metabolism gene classes of RNA in the ASI neurons, in addition to the increased expression of several classes of non-coding RNA, define a molecular signature of animals with constitutive SKN-1 activation and diminished healthspan. We reveal neddylation as a novel regulator of the SKN-1 homeostat that mediates SKN-1 abundance within intestinal cells. Moreover, RNAi-independent activity of the dicer-related DExD/H-box helicase, drh-1 , in the intestine, can oppose the e2ffects of aberrant SKN-1 transcriptional activation and delays age-dependent decline in health. Taken together, our results uncover a cell non-autonomous circuit to maintain organism-level homeostasis in response to excessive SKN-1 transcriptional activity in the sensory nervous system.

SIGNIFICANCE STATEMENT

Unlike activation, an understudied fundamental question across biological systems is how to deactivate a pathway, process, or enzyme after it has been turned on. The irony that the activation of a transcription factor that is meant to be protective can diminish health was first documented by us at the organismal level over a decade ago, but it has long been appreciated that chronic activation of the human ortholog of SKN-1, NRF2, could lead to chemo- and radiation resistance in cancer cells. A colloquial analogy to this biological idea is a sink faucet that has an on valve without a mechanism to shut the water off, which will cause the sink to overflow. Here, we define this off valve.

Biological resilience and aging: Activation of stress response pathways contributes to lifespan extension

While aging was traditionally viewed as a stochastic process of damage accumulation, it is now clear that aging is strongly influenced by genetics. The identification and characterization of long-lived genetic mutants in model organisms has provided insights into the genetic pathways and molecular mechanisms involved in extending longevity. Long-lived genetic mutants exhibit activation of multiple stress response pathways leading to enhanced resistance to exogenous stressors. As a result, lifespan exhibits a significant, positive correlation with resistance to stress. Disruption of stress response pathways inhibits lifespan extension in multiple long-lived mutants representing different pathways of lifespan extension and can also reduce the lifespan of wild-type animals. Combined, this suggests that activation of stress response pathways is a key mechanism by which long-lived mutants achieve their extended longevity and that many of these pathways are also required for normal lifespan. These results highlight an important role for stress response pathways in determining the lifespan of an organism.

Casein kinase 1 gamma regulates oxidative stress response via interacting with the NADPH dual oxidase complex

Oxidative stress response is a fundamental biological process mediated by conserved mechanisms. The identities and functions of some key regulators remain unknown. Here, we report a novel role of C. elegans casein kinase 1 gamma CSNK-1 (also known as CK1γ or CSNK1G) in regulating oxidative stress response and ROS levels. csnk-1 interacted with the bli-3/tsp-15/doxa-1 NADPH dual oxidase genes via genetic nonallelic noncomplementation to affect C. elegans survival in oxidative stress. The genetic interaction was supported by specific biochemical interactions between DOXA-1 and CSNK-1 and potentially between their human orthologs DUOXA2 and CSNK1G2. Consistently, CSNK-1 was required for normal ROS levels in C. elegans. CSNK1G2 and DUOXA2 each can promote ROS levels in human cells, effects that were suppressed by a small molecule casein kinase 1 inhibitor. We also detected genetic interactions between csnk-1 and skn-1 Nrf2 in oxidative stress response. Together, we propose that CSNK-1 CSNK1G defines a novel conserved regulatory mechanism for ROS homeostasis.

Targeting WDxR motif reprograms immune microenvironment and inhibits hepatocellular carcinoma progression

The WD-repeat (WDR) family affects carcinogenesis, but its role in the immune microenvironment is poorly characterized. Although functional loss or gain of WDR6 does not markedly change in vitro proliferative and invasive capacity of HCC cells, its deficiency in hepa1-6 cells drastically inhibits the growth and lung metastasis of orthotopically implanted tumors in immune-competent C57BL/6J mice. Mechanistically, WDR6 targets tumor suppressor UVRAG to the CUL4A-DDB1-ROC1 E3 ubiquitin ligase complex through a unique WDxR motif and promotes its degradation. This upregulates chromatin accessibility at the TNFα locus by blocking autophagic degradation of p65, elevates intratumoral myeloid-derived suppressor cell (MDSC) number, and reduces CD8⁺ T cell infiltration, thereby promoting HCC progression. These immunosuppressive effects are reversed by TNFα blockade. TNFα recruits NF-κB to activate the transcription of WDR6, establishing a WDR6-TNFα loop. Clinically, the WDR6/UVRAG/NF-κB pathway is hyperactivated in HCC, predicting a poor prognosis. Importantly, a WDxR-like peptide disrupts the WDR6/UVRAG complex and enhances the efficiency of anti-PD-L1 against HCC with WDR6 dysregulation.

Effects of Vegetal Extracts and Metabolites against Oxidative Stress and Associated Diseases: Studies in Caenorhabditis elegans

Oxidative stress is a natural physiological process where the levels of oxidants, such as reactive oxygen species (ROS) and nitrogen (RNS), exceed the strategy of antioxidant defenses, culminating in the interruption of redox signaling and control. Oxidative stress is associated with multiple pathologies, including premature aging, neurodegenerative diseases, obesity, diabetes, atherosclerosis, and arthritis. It is not yet clear whether oxidative stress is the cause or consequence of these diseases; however, it has been shown that using compounds with antioxidant properties, particularly compounds of natural origin, could prevent or slow down the progress of different pathologies. Within this context, the Caenorhabditis elegans (C. elegans) model has served to study the effect of different metabolites and natural compounds, which has helped to decipher molecular targets and the effect of these compounds on premature aging and some diseases such as neurodegenerative diseases and dyslipidemia. This article lists the studies carried out on C. elegans in which metabolites and natural extracts have been tested against oxidative stress and the pathologies associated with providing an overview of the discoveries in the redox area made with this nematode.

Regulation and Functions of the ER-Associated Nrf1 Transcription Factor

Nrf1 is a member of the nuclear erythroid 2-like family of transcription factors that regulate stress-responsive gene expression in animals. Newly synthesized Nrf1 is targeted to the endoplasmic reticulum (ER) where it is N-glycosylated. N-glycosylated Nrf1 is trafficked to the cytosol by the ER-associated degradation (ERAD) machinery and is subject to rapid proteasomal degradation. When proteasome function is impaired, Nrf1 escapes degradation and undergoes proteolytic cleavage and deglycosylation. Deglycosylation results in deamidation of N-glycosylated asparagine residues to edit the protein sequence encoded by the genome. This truncated and "sequence-edited" form of Nrf1 enters the nucleus where it induces up-regulation of proteasome subunit genes. Thus, Nrf1 drives compensatory proteasome biogenesis in cells exposed to proteasome inhibitor drugs and other proteotoxic insults. In addition to its role in proteasome homeostasis, Nrf1 is implicated in responses to oxidative stress, and maintaining lipid and cholesterol homeostasis. Here, we describe the conserved and complex mechanism by which Nrf1 is regulated and highlight emerging evidence linking this unusual transcription factor to development, aging, and disease.

RNAi screening for modulators of an osmo-sensitive gene response to extracellular matrix damage reveals negative feedback and interactions with translation inhibition

In epidermal tissues, extracellular matrices (ECMs) function as barriers between the organism and environment. Despite being at the interface with the environment, little is known about the role of animal barrier ECMs in sensing stress and communicating with cytoprotective gene pathways in neighboring cells. We and others have identified a putative damage sensor in the C. elegans cuticle that regulates osmotic, detoxification, and innate immune response genes. This pathway is associated with circumferential collagen bands called annular furrows; mutation or loss of furrow collagens causes constitutive activation of osmotic, detoxification, and innate immune response genes. Here, we performed a genome-wide RNAi screen for modulators of osmotic stress response gene gpdh-1 in a furrow collagen mutant strain. RNAi of six genes identified in this screen were tested under other conditions and for effects on other stress responses. The functions of these genes suggest negative feedback within osmolyte accumulation pathways and interactions with ATP homeostasis and protein synthesis. Loss of these gpdh-1 modulators had distinct effects on canonical detoxification and innate immune response genes.

Extracellular proteins OSM-7 and OSM-8 are required for stress response gene regulation at all post-embryonic stages

Nematode cuticles are extracellular matrices (ECMs) that function as structural support and permeability barriers. Genetic disruption of specific cuticle collagen structures or secreted epidermal proteins in C. elegans activates stress response genes in epithelial cells suggesting the presence of an extracellular damage signaling mechanism. Cuticles are replaced during development via molting but investigations of extracellular signaling to stress responses have focused on adults. In our current study, we measured cuticle phenotypes and stress response gene expression in all post-embryonic stages of mutant strains for a collagen and two secreted epidermal proteins to gain insights into developmental patterns.

Caenorhabditis elegans as a Model for the Effects of Phytochemicals on Mitochondria and Aging

The study of aging is an important topic in contemporary research. Considering the demographic changes and the resulting shifts towards an older population, it is of great interest to preserve youthful physiology in old age. For this endeavor, it is necessary to choose an appropriate model. One such model is the nematode Caenorhabditis elegans (C. elegans), which has a long tradition in aging research. In this review article, we explore the advantages of using the nematode model in aging research, focusing on bioenergetics and the study of secondary plant metabolites that have interesting implications during this process. In the first section, we review the situation of aging research today. Conventional theories and hypotheses about the ongoing aging process will be presented and briefly explained. The second section focuses on the nematode C. elegans and its utility in aging and nutrition research. Two useful genome editing methods for monitoring genetic interactions (RNAi and CRISPR/Cas9) are presented. Due to the mitochondria's influence on aging, we also introduce the possibility of observing bioenergetics and respiratory phenomena in C. elegans. We then report on mitochondrial conservation between vertebrates and invertebrates. Here, we explain why the nematode is a suitable model for the study of mitochondrial aging. In the fourth section, we focus on phytochemicals and their applications in contemporary nutritional science, with an emphasis on aging research. As an emerging field of science, we conclude this review in the fifth section with several studies focusing on mitochondrial research and the effects of phytochemicals such as polyphenols. In summary, the nematode C. elegans is a suitable model for aging research that incorporates the mitochondrial theory of aging. Its living conditions in the laboratory are optimal for feeding studies, thus enabling bioenergetics to be observed during the aging process.

Extracellular matrix regulation of stress response genes during larval development in Caenorhabditis elegans

Mutation or loss of 6 extracellular matrix collagen genes disrupts annular furrows in adult C. elegans cuticles, causes a wide “Dumpy” body morphology, and activates osmotic, detoxification, and antimicrobial defense genes. High environmental osmolarity reduces internal turgor pressure, physically distorts the epidermis, and activates the same stress responses. Collagen gene mutations that cause Dumpy without furrow disruption do not activate stress responses. These results are consistent with an extracellular damage sensor associated with furrows in the adult cuticle that regulates environmental stress responses in adjacent cells. Several cuticle characteristics change between molts, but all stages have annular furrows and express furrow collagen genes. We compared body shape, furrow organization imaged with differential interference contrast microscopy, and stress response gene expression in furrow collagen gene mutants at all postembryonic stages. We find that most body shape and furrow disorganization phenotypes start at the L3 stage and increase in severity with each molt afterwards. Stress response genes were induced the strongest in adults, correlating with the greatest Dumpy and furrow phenotypes. Although weaker than in adults, osmolyte transporter gene hmit-1.1 and antimicrobial gene nlp-29 were also induced in some early larvae that had weak or undetectable cuticle phenotypes. Our data are consistent with progressive cuticle phenotypes in which each new cuticle is at least partially directed by organization of the former cuticle. Gene expression and cuticle data support the role of furrow disruption as a signal in L4 larvae and adults, but also suggest a role for other cuticle organization or epidermal cell effects in early larvae.

Sexual dimorphism in Caenorhabditis elegans stress resistance

Physiological responses to the environment, disease, and aging vary by sex in many animals, but mechanisms of dimorphism have only recently begun to receive careful attention. The genetic model nematode Caenorhabditis elegans has well-defined mechanisms of stress response, aging, and sexual differentiation. C. elegans has males, but the vast majority of research only uses hermaphrodites. We found that males of the standard N2 laboratory strain were more resistant to hyperosmolarity, heat, and a natural pro-oxidant than hermaphrodites when in mixed-sex groups. Resistance to heat and pro-oxidant were also male-biased in three genetically and geographically diverse C. elegans strains consistent with a species-wide dimorphism that is not specific to domestication. N2 males were also more resistant to heat and pro-oxidant when keep individually indicating that differences in resistance do not require interactions between worms. We found that males induce canonical stress response genes by similar degrees and in similar tissues as hermaphrodites suggesting the importance of other mechanisms. We find that resistance to heat and pro-oxidant are influenced by the sex differentiation transcription factor TRA-1 suggesting that downstream organ differentiation pathways establish differences in stress resistance. Environmental stress influences survival in natural environments, degenerative disease, and aging. Understanding mechanisms of stress response dimorphism can therefore provide insights into sex-specific population dynamics, disease, and longevity.

1-Mesityl-3-(3-Sulfonatopropyl) Imidazolium Protects Against Oxidative Stress and Delays Proteotoxicity in C. elegans

Due to the increase in life expectancy worldwide, age-related disorders such as neurodegenerative diseases (NDs) have become more prevalent. Conventional treatments comprise drugs that only attenuate some of the symptoms, but fail to arrest or delay neuronal proteotoxicity that characterizes these diseases. Due to their diverse biological activities, imidazole rings are intensively explored as powerful scaffolds for the development of new bioactive molecules. By using C. elegans, our work aims to explore novel biological roles for these compounds. To this end, we have tested the in vivo anti-proteotoxic effects of imidazolium salts. Since NDs have been largely linked to impaired antioxidant defense mechanisms, we focused on 1-Mesityl-3-(3-sulfonatopropyl) imidazolium (MSI), one of the imidazolium salts that we identified as capable of improving iron-induced oxidative stress resistance in wild-type animals. By combining mutant and gene expression analysis we have determined that this protective effect depends on the activation of the Heat Shock Transcription Factor (HSF-1), whereas it is independent of other canonical cytoprotective molecules such as abnormal Dauer Formation-16 (DAF-16/FOXO) and Skinhead-1 (SKN-1/Nrf2). To delve deeper into the biological roles of MSI, we analyzed the impact of this compound on previously established C. elegans models of protein aggregation. We found that MSI ameliorates β-amyloid-induced paralysis in worms expressing the pathological protein involved in Alzheimer’s Disease. Moreover, this compound also delays age-related locomotion decline in other proteotoxic C. elegans models, suggesting a broad protective effect. Taken together, our results point to MSI as a promising anti-proteotoxic compound and provide proof of concept of the potential of imidazole derivatives in the development of novel therapies to retard age-related proteotoxic diseases.

Oxidation and Antioxidation of Natural Products in the Model Organism Caenorhabditiselegans

Natural products are small molecules naturally produced by multiple sources such as plants, animals, fungi, bacteria and archaea. They exert both beneficial and detrimental effects by modulating biological targets and pathways involved in oxidative stress and antioxidant response. Natural products’ oxidative or antioxidative properties are usually investigated in preclinical experimental models, including virtual computing simulations, cell and tissue cultures, rodent and nonhuman primate animal models, and human studies. Due to the renewal of the concept of experimental animals, especially the popularization of alternative 3R methods for reduction, replacement and refinement, many assessment experiments have been carried out in new alternative models. The model organism Caenorhabditis elegans has been used for medical research since Sydney Brenner revealed its genetics in 1974 and has been introduced into pharmacology and toxicology in the past two decades. The data from C. elegans have been satisfactorily correlated with traditional experimental models. In this review, we summarize the advantages of C. elegans in assessing oxidative and antioxidative properties of natural products and introduce methods to construct an oxidative damage model in C. elegans. The biomarkers and signaling pathways involved in the oxidative stress of C. elegans are summarized, as well as the oxidation and antioxidation in target organs of the muscle, nervous, digestive and reproductive systems. This review provides an overview of the oxidative and antioxidative properties of natural products based on the model organism C. elegans.

Chlorogenic acid activates Nrf2/SKN-1 and prolongs the lifespan of Caenorhabditis elegans via the Akt-FOXO3/DAF16a-DDB1 pathway and activation of DAF16f

Chlorogenic acid (CGA) is the most abundant polyphenol in coffee. It has been widely reported to exhibit antioxidant activity by activating nuclear factor erythroid 2-related factor 2 (Nrf2) potentially via the canonical Keap1-Nrf2 pathway. We herein demonstrated that the knockdown of WDR23, but not Keap1, abolished the effects of CGA on the activation of Nrf2. CGA decreased the expression of DDB1, an adaptor for WDR23-Cullin 4A-RING ligase (CRL4A WDR23). FOXO3, a major target for inactivation by the PI3K/Akt pathway, was identified as the transcription factor responsible for the basal and CGA-inhibited expression of the DDB1 gene. CGA blocked FOXO3 binding to importin-7 (IPO7), thereby inhibiting the nuclear accumulation of FOXO3, down-regulating the expression of DDB1, inhibiting the activity of CRL4 WDR23, and ultimately increasing that of Nrf2. This pathway was conserved in Caenorhabditis elegans, and CGA extended the lifespan partly through this pathway. We found that in C. elegans, the isoform DAF-16a, but not DAF-16f, regulated the expression levels of ddb-1 mRNA and SKN-1 protein. CGA prolonged the mean lifespan of DAF-16a- and DAF-16f-rescued worms by 24% and 9%, respectively, suggesting that both isoforms involve in lifespan-extending effects of CGA, with DAF-16a being more important than DAF-16f. Based on these results, we established a novel Akt-FOXO3/DAF16a-DDB1 axis that links nutrient sensing and oxidative stress response pathways. Our results also provide a novel molecular mechanism for Nrf2/SKN-1 activation by CGA and the increased lifespan of C. elegans by CGA via this pathway.

An extracellular matrix damage sensor signals through membrane-associated kinase DRL-1 to mediate cytoprotective responses in Caenorhabditis elegans

We and others previously identified circumferential bands of collagen named annular furrows as key components of a damage sensor in the cuticle of Caenorhabditis elegans that regulates cytoprotective genes. Mutation or loss of noncollagen secreted proteins OSM-7, OSM-8, and OSM-11 activate the same cytoprotective responses without obvious changes to the cuticle indicating that other extracellular proteins are involved. Here, we used RNAi screening to identify protein kinase DRL-1 as a key modulator of cytoprotective gene expression and stress resistance in furrow and extracellular OSM protein mutants. DRL-1 functions downstream from furrow disruption and is expressed in cells that induce cytoprotective genes. DRL-1 is not required for the expression of cytoprotective genes under basal or oxidative stress conditions consistent with specificity to extracellular signals. DRL-1 was previously shown to regulate longevity via a "Dietary Restriction-Like" state, but it functions downstream from furrow disruption by a distinct mechanism. The kinase domain of DRL-1 is related to mammalian MEKK3, and MEKK3 is recruited to a plasma membrane osmosensor complex by a scaffold protein. In C. elegans, DRL-1 contains an atypical hydrophobic C-terminus with predicted transmembrane domains and is constitutively expressed at or near the plasma membrane where it could function to receive extracellular damage signals for cells that mount cytoprotective responses.

Probiotics Interactions and the Modulation of Major Signalling Pathways in Host Model Organism Caenorhabditis elegans

Microorganisms live in the human digestive system and the gut microbiome constitutes part of our prime determining component for healthy aging and wellness. Gut microbiota has broad influences on its host, beginning from the digestion of food and nutrients absorption to protective roles against invading pathogens and host immune system regulation. Dysbiosis of the gut microbial composition has been linked to numerous diseases and there is a need to have a better grasp on what makes a 'good' gut microbiome. Caenorhabditis elegans (C. elegans) model organism is considered as a well-suited in-vivo model system and, is at the frontline of probiotic research because of its well-defined characteristics and prolific nature. Most importantly, C. elegans feeds on bacteria, which speeds up manipulations and investigations in probiotics research tremendously. With its unique salient features of short lifespan, and ease of propagation, different unknown probiotics biological roles can be measured at an organism level with precision in the form of worm's stress responses, survivability, and lifespan. In this review, new insights on the different mechanisms underlying the establishment of probiotics regulations of conserved signalling pathways such as p38 MAPK/SKN-1, DAF-2/DAF-16, and JNK-1/DAF-16 is highlighted based on information obtained from C. elegans studies. Along with the current state of knowledge and the uniqueness of C. elegans as a model organism, explorations of its future contribution and scope in synthetic biology and probiotics engineering strains are also addressed. This is expected to strengthen our understanding of probiotics roles and to facilitate novel discovery and applications, for specific therapeutics against age-related disorders and various pathophysiological conditions.

Tribbles pseudokinase NIPI-3 regulates intestinal immunity in Caenorhabditis elegans by controlling SKN-1/Nrf activity

In Caenorhabditis elegans, ROS generated in response to intestinal infection induces SKN-1, a protective transcription factor homologous to nuclear factor erythroid 2-related factor 1 or 2 (NRF1/2) in mammals. Many factors regulate SKN-1, including the p38 mitogen-activated protein kinase (MAPK) cascade that activates SKN-1 by phosphorylation. In this work, another positive regulator of SKN-1 is identified: NIPI-3, a Tribbles pseudokinase. NIPI-3 has been reported to protect against intestinal infection by negatively regulating the CCAT enhancer binding protein (C/EBP) bZIP transcription factor CEBP-1. Here we demonstrate that CEBP-1 positively regulates the vhp-1 transcript, which encodes a phosphatase that dephosphorylates the p38 MAPK called PMK-1. The increased levels of VHP-1 caused by CEBP-1 transcriptional enhancement result in less PMK-1 phosphorylation, affecting SKN-1 activity and intestinal resistance to the pathogen. The data support a model in which NIPI-3's negative regulation of CEBP-1 decreases VHP-1 phosphatase activity, allowing increased stimulation of SKN-1 activity by the p38 MAPK phosphorylation cascade in the intestine.

Dietary thiols accelerate aging of C. elegans

Glutathione (GSH) is the most abundant cellular antioxidant. As reactive oxygen species (ROS) are widely believed to promote aging and age-related diseases, and antioxidants can neutralize ROS, it follows that GSH and its precursor, N-acetyl cysteine (NAC), are among the most popular dietary supplements. However, the long- term effects of GSH or NAC on healthy animals have not been thoroughly investigated. We employed C. elegans to demonstrate that chronic administration of GSH or NAC to young or aged animals perturbs global gene expression, inhibits skn-1-mediated transcription, and accelerates aging. In contrast, limiting the consumption of dietary thiols, including those naturally derived from the microbiota, extended lifespan. Pharmacological GSH restriction activates the unfolded protein response and increases proteotoxic stress resistance in worms and human cells. It is thus advantageous for healthy individuals to avoid excessive dietary antioxidants and, instead, rely on intrinsic GSH biosynthesis, which is fine-tuned to match the cellular redox status and to promote homeostatic ROS signaling.

Sp1 is a substrate of Keap1 and regulates the activity of CRL4AWDR23 ubiquitin ligase toward Nrf2

Nuclear factor erythroid 2–related factor 2 (Nrf2) is a critical transcription factor that orchestrates cellular responses to oxidative stress. Because the dysregulation of Nrf2 has been implicated in many diseases, precise regulation of its protein level is crucial for maintaining homeostasis. Kelch-like-ECH-associated protein 1 (Keap1) and WD40 repeat protein 23 (WDR23) directly regulate Nrf2 levels via similar but distinct proteasome-dependent pathways. WDR23 forms a part of the WDR23-Cullin 4A-RING ubiquitin ligase complex (CRL4A^WDR23), whereas Keap1 serves as a substrate adaptor for the Cullin 3–containing ubiquitin ligase complex. However, the mechanisms underlying crosstalk between these Keap1 and WDR23 pathways for the regulation of Nrf2 levels have not been investigated. Here, we showed that knockdown (KD) of Keap1 upregulated the expression of Cullin4A (CUL4A) in a specificity protein 1 (Sp1)–dependent manner. We also revealed that Sp1 interacted with Keap1, leading to ubiquitination of Sp1. Increases in Sp1 by Keap1 KD triggered Sp1 binding to the fourth Sp1 binding site (Sp1_M4) within the −230/+50 region of the CUL4A gene. We also demonstrated that the overexpression and KD of Sp1 reduced and increased Nrf2 protein levels, respectively. These effects were abrogated by the WDR23 KD, suggesting that Sp1 also regulates Nrf2 levels via the ubiquitin ligase complex CRL4A^WDR23. In conclusion, we discovered Sp1 as a novel substrate of Keap1 and provided evidence that Sp1 regulates the expression of CUL4A. We revealed a novel role for Sp1 in mediating crosstalk between two independent regulators of Nrf2 protein levels.

The C. elegans Hypertonic Stress Response: Big Insights from Shrinking Worms

Cell volume is one of the most aggressively defended physiological set points in biology. Changes in intracellular ion and water concentrations, which are induced by changes in metabolism or environmental exposures, disrupt protein folding, enzymatic activity, and macromolecular assemblies. To counter these challenges, cells and organisms have evolved multifaceted, evolutionarily conserved molecular mechanisms to restore cell volume and repair stress induced damage. However, many unanswered questions remain regarding the nature of cell volume 'sensing' as well as the molecular signaling pathways involved in activating physiological response mechanisms. Unbiased genetic screening in the model organism C. elegans is providing new and unexpected insights into these questions, particularly questions relating to the hypertonic stress response (HTSR) pathway. One surprising characteristic of the HTSR pathway in C. elegans is that it is under strong negative regulation by proteins involved in protein homeostasis and the extracellular matrix (ECM). The role of the ECM in particular highlights the importance of studying the HTSR in the context of a live organism where native ECM-tissue associations are preserved. A second novel and recently discovered characteristic is that the HTSR is regulated at the post-transcriptional level. The goal of this review is to describe these discoveries, to provide context for their implications, and to raise outstanding questions to guide future research.

Toxic stress-specific cytoprotective responses regulate learned behavioral decisions in C. elegans

BACKGROUND

Recognition of stress and mobilization of adequate "fight-or-flight" responses is key for survival and health. Previous studies have shown that exposure of Caenorhabditis elegans to pathogens or toxins simultaneously stimulates cellular stress and detoxification responses and aversive behavior. However, whether a coordinated regulation exists between cytoprotective stress responses and behavioral defenses remains unclear.

RESULTS

Here, we show that exposure of C. elegans to high concentrations of naturally attractive food-derived odors, benzaldehyde and diacetyl, induces toxicity and food avoidance behavior. Benzaldehyde preconditioning activates systemic cytoprotective stress responses involving DAF-16/FOXO, SKN-1/Nrf2, and Hsp90 in non-neuronal cells, which confer both physiological (increased survival) and behavioral tolerance (reduced food avoidance) to benzaldehyde exposure. Benzaldehyde preconditioning also elicits behavioral cross-tolerance to the structurally similar methyl-salicylate, but not to the structurally unrelated diacetyl. In contrast, diacetyl preconditioning augments diacetyl avoidance, weakens physiological diacetyl tolerance, and does not induce apparent molecular defenses. The inter-tissue connection between cellular and behavioral defenses is mediated by JNK-like stress-activated protein kinases and the neuropeptide Y receptor NPR-1. Reinforcement of the stressful experiences using spaced training forms stable stress-specific memories. Memory retrieval by the olfactory cues leads to avoidance of food contaminated by diacetyl and context-dependent behavioral decision to avoid benzaldehyde only if there is an alternative, food-indicative odor.

CONCLUSIONS

Our study reveals a regulatory link between conserved cytoprotective stress responses and behavioral avoidance, which underlies "fight-or-flight" responses and facilitates self-protection in real and anticipated stresses. These findings imply that variations in the efficiency of physiological protection during past episodes of stress might shape current behavioral decisions.

Modeling paraquat-induced lung fibrosis in C. elegans reveals KRIT1 as a key regulator of collagen gene transcription

Paraquat poisoning causes lung fibrosis, which often results in long-term pulmonary dysfunction. Lung fibrosis has been attributed to collagens accumulation, but the underlying regulatory pathway remains unclear. Here we use the genetically tractable C. elegans as a model to study collagen gene transcription in response to paraquat. We find that paraquat robustly up-regulates collagen gene transcription, which is dependent on KRI-1, a poorly studied protein homologous to human KRIT1/CCM1. KRI-1 knockdown prevents paraquat from activating the oxidative stress response transcription factor SKN-1/Nrf2, resulting in reduced collagen transcription and increased paraquat sensitivity. Using human lung fibroblasts (MRC-5), we confirm that both KRIT1 and Nrf2 are required for collagen transcription in response to paraquat. Nrf2 hyper-activation by KEAP1 knockdown bypasses KRIT1 to up-regulate collagen transcription. Our findings on the regulation of collagen gene transcription by paraquat could suggest potential strategies to treat pulmonary fibrosis caused by paraquat poisoning.

The KEAP1-NRF2 System as a Molecular Target of Cancer Treatment

Simple Summary

Nuclear factor erythroid-derived 2-like 2 (encoded by the Nfe2l2 gene; NRF2) is a transcription factor that regulates a variety of cytoprotective genes, including antioxidant enzymes, detoxification enzymes, inflammation-related proteins, drug transporters and metabolic enzymes. NRF2 is regulated by unique molecular mechanisms that stem from Kelch-like ECH-associated protein 1 (KEAP1) in response to oxidative and electrophilic stresses. It has been shown that disturbance or perturbation of the NRF2 activation causes and/or exacerbates many kinds of diseases. On the contrary, aberrant activations of NRF2 also provoke intriguing pathologic features, especially in cancers. Cancer cells with high NRF2 activity have been referred to as NRF2-addicted cancers, which are frequently found in lung cancers. In this review, we summarize the current accomplishments of the KEAP1–NRF2 pathway analyses in special reference to the therapeutic target of cancer therapy. The concept of synthetic lethality provides a new therapeutic approach for NRF2-addicted cancers.

Abstract

The Kelch-like ECH-associated protein 1 (KEAP1)—Nuclear factor erythroid-derived 2-like 2 (encoded by the Nfe2l2 gene; NRF2) system attracts extensive interest from scientists in basic and clinical cancer research fields, as NRF2 exhibits activity as both an oncogene and tumor suppressor, depending on the context. Especially unique and malignant, NRF2-addicted cancers exhibit high levels of NRF2 expression. Somatic mutations identified in the NRF2 or KEAP1 genes of NRF2-addicted cancers cause the stabilization and accumulation of NRF2. NRF2-addicted cancers hijack the intrinsic roles that NRF2 plays in cytoprotection, including antioxidative and anti-electrophilic responses, as well as metabolic reprogramming, and acquire a marked advantage to survive under severe and limited microenvironments. Therefore, NRF2 inhibitors are expected to have therapeutic effects in patients with NRF2-addicted cancers. In contrast, NRF2 activation in host immune cells exerts significant suppression of cancer cell growth, indicating that NRF2 inducers also have the potential to be therapeutics for cancers. Thus, the KEAP1–NRF2 system makes a broad range of contributions to both cancer development and suppression. These observations thus demonstrate that both NRF2 inhibitors and inducers are useful for the treatment of cancers with high NRF2 activity.

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.

WDR23 regulates the expression of Nrf2-driven drug-metabolizing enzymes

Nrf2 plays a central role in the response to xenobiotics and oxidative stress. The activation of Nrf2 induces the expression of drug-metabolizing enzymes (DMEs) and is important for cytoprotection. Keap1 is a widely accepted proteasome-dependent regulator of Nrf2. Keap1 was reported to be absent in Caenorhabditis elegans, and the level of the Nrf2 ortholog SKN-1 was mainly regulated by WDR23. The WDR23 locus is highly conserved from C. elegans to humans. We investigated whether WDR23 regulates Nrf2 activity in mammalian cells, hepatocellular carcinoma cells (Hep3B) and human cervical carcinoma cells (HeLa). We found that WDR23 has two isoforms (1 and 2) and that knockdown of WDR23 was sufficient to stabilize Nrf2 and alter the expression of several DMEs. Keap1 knockdown resulted in higher Nrf2 levels than WDR23 knockdown, and their effects on DMEs differed. These results were consistent with Keap1 being a canonical regulator of Nrf2, and that WDR23 may assist in Nrf2 regulation. We confirmed that WDR23 physically interacted with Nrf2, suggesting that WDR23 directly regulates Nrf2-dependent DMEs. In immunostaining experiments, human WDR23 isoform 1 was localized to the cytoplasm, whereas isoform 2 mainly resided in the nucleus. Taken together, our results suggested WDR23 is a novel regulator of DME expression.

ER-Resident Transcription Factor Nrf1 Regulates Proteasome Expression and Beyond

Protein folding is a substantively error prone process, especially when it occurs in the endoplasmic reticulum (ER). The highly exquisite machinery in the ER controls secretory protein folding, recognizes aberrant folding states, and retrotranslocates permanently misfolded proteins from the ER back to the cytosol; these misfolded proteins are then degraded by the ubiquitin–proteasome system termed as the ER-associated degradation (ERAD). The 26S proteasome is a multisubunit protease complex that recognizes and degrades ubiquitinated proteins in an ATP-dependent manner. The complex structure of the 26S proteasome requires exquisite regulation at the transcription, translation, and molecular assembly levels. Nuclear factor erythroid-derived 2-related factor 1 (Nrf1; NFE2L1), an ER-resident transcription factor, has recently been shown to be responsible for the coordinated expression of all the proteasome subunit genes upon proteasome impairment in mammalian cells. In this review, we summarize the current knowledge regarding the transcriptional regulation of the proteasome, as well as recent findings concerning the regulation of Nrf1 transcription activity in ER homeostasis and metabolic processes.

Measurements of Physiological Stress Responses in C. Elegans

Organisms are often exposed to fluctuating environments and changes in intracellular homeostasis, which can have detrimental effects on their proteome and physiology. Thus, organisms have evolved targeted and specific stress responses dedicated to repair damage and maintain homeostasis. These mechanisms include the unfolded protein response of the endoplasmic reticulum (UPR^ER), the unfolded protein response of the mitochondria (UPR^MT), the heat shock response (HSR), and the oxidative stress response (OxSR). The protocols presented here describe methods to detect and characterize the activation of these pathways and their physiological consequences in the nematode, C. elegans. First, the use of pathway-specific fluorescent transcriptional reporters is described for rapid cellular characterization, drug screening, or large-scale genetic screening (e.g., RNAi or mutant libraries). In addition, complementary, robust physiological assays are described, which can be used to directly assess sensitivity of animals to specific stressors, serving as functional validation of the transcriptional reporters. Together, these methods allow for rapid characterization of the cellular and physiological effects of internal and external proteotoxic perturbations.

SKN-1 Is a Negative Regulator of DAF-16 and Somatic Stress Resistance in Caenorhabditis elegans

The transcription factor SKN-1, the C. elegans ortholog of mammalian Nrf protein, is a well-known longevity factor, and its activation is observed in several long-lived models. SKN-1 also plays essential roles in xenobiotic and oxidative stress responses. Here, we report deleterious functions of SKN-1 in somatic stress resistance that may impair lifespan. Constitutive SKN-1 activation impairs animal resistance to several stresses, including heat, ER stress and mitochondrial stress, which result from the suppression of DAF-16, another master regulator of longevity. SKN-1 activation abrogates DAF-16 nuclear import and downregulates DAF-16 target genes under stress conditions, while SKN-1 inhibition promotes the expression of DAF-16 targets, even in long-lived mutants. Further, SKN-1 activation induces the expression of vitellogenin proteins, which are required for SKN-1-mediated suppression of DAF-16 and stress resistance. Together, these findings identify detrimental roles for SKN-1 activation in animal health, and more importantly, inspire the rethinking of the complex roles for SKN-1 in aging regulation.

FSHR-1/GPCR Regulates the Mitochondrial Unfolded Protein Response in Caenorhabditis elegans

The mitochondrial unfolded protein response (UPRmt) is an evolutionarily conserved adaptive response that functions to maintain mitochondrial homeostasis following mitochondrial damage. In Caenorhabditis elegans, the nervous system plays a central role in responding to mitochondrial stress by releasing endocrine signals that act upon distal tissues to activate the UPRmt The mechanisms by which mitochondrial stress is sensed by neurons and transmitted to distal tissues are not fully understood. Here, we identify a role for the conserved follicle-stimulating hormone G protein-coupled receptor, FSHR-1, in promoting UPRmt activation. Genetic deficiency of fshr-1 severely attenuates UPRmt activation and organism-wide survival in response to mitochondrial stress. FSHR-1 functions in a common genetic pathway with SPHK-1/sphingosine kinase to promote UPRmt activation, and FSHR-1 regulates the mitochondrial association of SPHK-1 in the intestine. Through tissue-specific rescue assays, we show that FSHR-1 functions in neurons to activate the UPRmt, to promote mitochondrial association of SPHK-1 in the intestine, and to promote organism-wide survival in response to mitochondrial stress. We propose that FSHR-1 functions cell nonautonomously in neurons to activate UPRmt upstream of SPHK-1 signaling in the intestine.

Protein Sequence Editing of SKN-1A/Nrf1 by Peptide:N-Glycanase Controls Proteasome Gene Expression

The proteasome mediates selective protein degradation and is dynamically regulated in response to proteotoxic challenges. SKN-1A/Nrf1, an endoplasmic reticulum (ER)-associated transcription factor that undergoes N-linked glycosylation, serves as a sensor of proteasome dysfunction and triggers compensatory upregulation of proteasome subunit genes. Here, we show that the PNG-1/NGLY1 peptide:N-glycanase edits the sequence of SKN-1A protein by converting particular N-glycosylated asparagine residues to aspartic acid. Genetically introducing aspartates at these N-glycosylation sites bypasses the requirement for PNG-1/NGLY1, showing that protein sequence editing rather than deglycosylation is key to SKN-1A function. This pathway is required to maintain sufficient proteasome expression and activity, and SKN-1A hyperactivation confers resistance to the proteotoxicity of human amyloid beta peptide. Deglycosylation-dependent protein sequence editing explains how ER-associated and cytosolic isoforms of SKN-1 perform distinct cytoprotective functions corresponding to those of mammalian Nrf1 and Nrf2. Thus, we uncover an unexpected mechanism by which N-linked glycosylation regulates protein function and proteostasis.

Glucose negatively affects Nrf2/SKN-1-mediated innate immunity in C. elegans

High glucose levels negatively affect immune response. However, the underlying mechanisms are not well understood. Upon infection, the round worm C. elegans induces multiple gene transcription programs, including the Nrf2/SKN-1-mediated detoxification program, to activate the innate immunity. In this study, we find that high glucose conditions inhibit the SKN-1-mediated immune response to Salmonella typhimurium, exacerbate the infection and greatly decrease survival. The effect of glucose shows specificity to SKN-1 pathway, as UPR^mit and UPR^ER that are known to be induced by infection, are not affected. Hyper-activation of SKN-1 by wdr-23 RNAi restores partly the immune response and increases the survival rate in response to S. typhimurium. In all, our study reveals a molecular pathway responsible for glucose’s negative effect on innate immunity, which could help to better understand diseases associated with hyperglycemia.

A Novel Mechanism To Prevent H2S Toxicity in Caenorhabditis elegans

Hydrogen sulfide (H2S) is an endogenously produced signaling molecule that can be cytoprotective, especially in conditions of ischemia/reperfusion injury. However, H2S is also toxic, and unregulated accumulation or exposure to environmental H2S can be lethal. In Caenorhabditis elegans, the hypoxia inducible factor (hif-1) coordinates the initial transcriptional response to H2S, and is essential to survive exposure to low concentrations of H2S. We performed a forward genetic screen to identify mutations that suppress the lethality of hif-1 mutant animals in H2S. The mutations we recovered are specific for H2S, as they do not suppress embryonic lethality or reproductive arrest of hif-1 mutant animals in hypoxia, nor can they prevent the death of hif-1 mutant animals exposed to hydrogen cyanide. The majority of hif-1 suppressor mutations we recovered activate the skn-1/Nrf2 transcription factor. Activation of SKN-1 by hif-1 suppressor mutations increased the expression of a subset of H2S-responsive genes, consistent with previous findings that skn-1 plays a role in the transcriptional response to H2S. Using transgenic rescue, we show that overexpression of a single gene, rhy-1, is sufficient to protect hif-1 mutant animals in H2S. The rhy-1 gene encodes a predicated O-acyltransferase enzyme that has previously been shown to negatively regulate HIF-1 activity. Our data indicate that RHY-1 has novel, hif-1 independent, function that promotes survival in H2S.

Mitochondrial Reactive Oxygen Species Generated at the Complex-II Matrix or Intermembrane Space Microdomain Have Distinct Effects on Redox Signaling and Stress Sensitivity in Caenorhabditis elegans

Aims: How mitochondrial reactive oxygen species (ROS) impact physiological function may depend on the quantity of ROS generated or removed, and the subcellular microdomain in which this occurs. However, pharmacological tools currently available to alter ROS production in vivo lack precise spatial and temporal control. Results: We used CRISPR/Cas9 to fuse the light-sensitive ROS-generating protein, SuperNova to the C-terminus of mitochondrial complex II succinate dehydrogenase subunits B (SDHB-1::SuperNova) and C (SDHC-1::SuperNova) in Caenorhabditis elegans to localize SuperNova to the matrix-side of the inner mitochondrial membrane, and to the intermembrane space (IMS), respectively. The presence of the SuperNova protein did not impact complex II activity, mitochondrial respiration, or C. elegans development rate under dark conditions. ROS production by SuperNova protein in vitro in the form of superoxide (O₂˙^-) was both specific and proportional to total light irradiance in the 540-590 nm spectra, and was unaffected by varying the buffer pH to resemble the mitochondrial matrix or IMS environments. We then determined using SuperNova whether stoichiometric ROS generation in the mitochondrial matrix or IMS had distinct effects on redox signaling in vivo. Phosphorylation of PMK-1 (a p38 MAPK homolog) and transcriptional activity of SKN-1 (an Nrf2 homolog) were each dependent on both the site and duration of ROS production, with matrix-generated ROS having more prominent effects. Furthermore, matrix- but not IMS-generated ROS attenuated susceptibility to simulated ischemia reperfusion injury in C. elegans. Innovation and Conclusion: Overall, these data demonstrate that the physiological output of ROS depends on the microdomain in which it is produced. Antioxid. Redox Signal. 31, 594-607.

GABA receptors differentially regulate life span and health span in C. elegans through distinct downstream mechanisms

GABA, a prominent inhibitory neurotransmitter, is best known to regulate neuronal functions in the nervous system. However, much less is known about the role of GABA signaling in other physiological processes. Interestingly, recent work showed that GABA signaling can regulate life span via a metabotropic GABA_B receptor in Caenorhabditis elegans. However, the role of other types of GABA receptors in life span has not been clearly defined. It is also unclear whether GABA signaling regulates health span. Here, using C. elegans as a model, we systematically interrogated the role of various GABA receptors in both life span and health span. We find that mutations in four different GABA receptors extend health span by promoting resistance to stress and pathogen infection and that two such receptor mutants also show extended life span. Different GABA receptors engage distinct transcriptional factors to regulate life span and health span, and even the same receptor regulates life span and health span via different transcription factors. Our results uncover a novel, profound role of GABA signaling in aging in C. elegans, which is mediated by different GABA receptors coupled to distinct downstream effectors.