A conserved transcription motif suggesting functional parallels between Caenorhabditis elegans SKN-1 and Cap'n'Collar-related basic leucine zipper proteins

Differential squamous cell fates elicited by NRF2 gain of function versus KEAP1 loss of function

Clinical evidence has revealed that high-level activation of NRF2 caused by somatic mutations in NRF2 (NFE2L2) is frequently detected in esophageal squamous cell carcinoma (ESCC), whereas that caused by somatic mutations in KEAP1, a negative regulator of NRF2, is not. Here, we aspire to generate a mouse model of NRF2-activated ESCC using the cancer-derived NRF2L30F mutation and cancer driver mutant TRP53R172H. Concomitant expression of NRF2L30F and TRP53R172H results in formation of NRF2-activated ESCC-like lesions. In contrast, while squamous-cell-specific deletion of KEAP1 induces similar NRF2 hyperactivation, the loss of KEAP1 combined with expression of TRP53R172H does not elicit the formation of ESCC-like lesions. Instead, KEAP1-deleted cells disappear from the esophageal epithelium over time. These findings demonstrate that, while cellular NRF2 levels are similarly induced, NRF2 gain of function and KEAP1 loss of function elicits distinct fates of squamous cells. The NRF2L30F mutant mouse model developed here will be instrumental in elucidating the mechanistic basis leading to NRF2-activated ESCC.

Hypsizygus marmoreus extract exhibited antioxidant effects to promote longevity and stress resistance in Caenorhabditis elegans

In this study, we explored the lifespan extension effect of a popular edible mushroom, Hypsizygus marmoreus, using the model organism Caenorhabditis elegans (C. elegans). The results showed that Hypsizygus marmoreus extract (HME) could increase the lifespan of C. elegans and ameliorate the healthspan by improving motility, attenuating lipofuscin accumulation, and enhancing the ability to withstand oxidative and heat stress. Then, we found noteworthy enhancements in SOD and CAT activities and reactive oxygen species (ROS) scavenging activity in vivo. Combined with the up-regulation of the expression of antioxidant genes (skn-1, sod-1, sod-3, mev-1, and gst-4), HME may function as an antioxidant in nematodes, which may be closely related to its phenolic compounds. Furthermore, we found that HME promoted the transfer of the transcription factor SKN-1 to the nucleus but had no impact on the lifespan of skn-1 mutants, indicating that SKN-1 was essential for Hypsizygus marmoreus to exert beneficial biological effects in C. elegans. Our findings elucidated that dietary supplementation with Hypsizygus marmoreus might have beneficial anti-aging effects and contribute to exploring the lifespan extension and underlying mechanisms of Hypsizygus marmoreus in C. elegans.

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.

Transient inhibition of mitochondrial function by chrysin and apigenin prolong longevity via mitohormesis in C. elegans

Mild inhibition of mitochondrial function leads to longevity. Genetic disruption of mitochondrial respiratory components either by mutation or RNAi greatly extends the lifespan in yeast, worms, and drosophila. This has given rise to the idea that pharmacologically inhibiting mitochondrial function would be a workable strategy for postponing aging. Toward this end, we used a transgenic worm strain that expresses the firefly luciferase enzyme widely to evaluate compounds by tracking real-time ATP levels. We identified chrysin and apigenin, which reduced ATP production and increased the lifespan of worms. Mechanistically, we discovered that chrysin and apigenin transiently inhibit mitochondrial respiration and induce an early ROS, and the lifespan-extending effect is dependent on transient ROS formation. We also show that AAK-2/AMPK, DAF-16/FOXO, and SKN-1/NRF-2 are required for chrysin or apigenin-mediated lifespan extension. Temporary increases in ROS levels trigger an adaptive response in a mitohormetic way, thereby increasing oxidative stress capacity and cellular metabolic adaptation, finally leading to longevity. Thus, chrysin and apigenin represent a class of compounds isolated from natural products that delay senescence and improve age-related diseases by inhibiting mitochondrial function and shed new light on the function of additional plant-derived polyphenols in enhancing health and delaying aging. Collectively, this work provides an avenue for pharmacological inhibition of mitochondrial function and the mechanism underlining their lifespan-extending properties.

SGPL1 stimulates VPS39 recruitment to the mitochondria in MICU1 deficient cells

Objective

Mitochondrial “retrograde” signaling may stimulate organelle biogenesis as a compensatory adaptation to aberrant activity of the oxidative phosphorylation (OXPHOS) system. To maintain energy-consuming processes in OXPHOS deficient cells, alternative metabolic pathways are functionally coupled to the degradation, recycling and redistribution of biomolecules across distinct intracellular compartments. While transcriptional regulation of mitochondrial network expansion has been the focus of many studies, the molecular mechanisms promoting mitochondrial maintenance in energy-deprived cells remain poorly investigated.

Methods

We performed transcriptomics, quantitative proteomics and lifespan assays to identify pathways that are mechanistically linked to mitochondrial network expansion and homeostasis in Caenorhabditis elegans lacking the mitochondrial calcium uptake protein 1 (MICU-1/MICU1). To support our findings, we carried out biochemical and image analyses in mammalian cells and mouse-derived tissues.

Results

We report that micu-1(null) mutations impair the OXPHOS system and promote C. elegans longevity through a transcriptional program that is independent of the mitochondrial calcium uniporter MCU-1/MCU and the essential MCU regulator EMRE-1/EMRE. We identify sphingosine phosphate lyase SPL-1/SGPL1 and the ATFS-1-target HOPS complex subunit VPS-39/VPS39 as critical lifespan modulators of micu-1(null) mutant animals. Cross-species investigation indicates that SGPL1 upregulation stimulates VPS39 recruitment to the mitochondria, thereby enhancing mitochondria-lysosome contacts. Consistently, VPS39 downregulation compromises mitochondrial network maintenance and basal autophagic flux in MICU1 deficient cells. In mouse-derived muscles, we show that VPS39 recruitment to the mitochondria may represent a common signature associated with altered OXPHOS system.

Conclusions

Our findings reveal a previously unrecognized SGPL1/VPS39 axis that stimulates intracellular organelle interactions and sustains autophagy and mitochondrial homeostasis in OXPHOS deficient cells.

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micu-1(null) nematodes are long-lived mitochondrial mutants.

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MICU-1/MICU1 deficiency stimulates VPS-39/VPS39 and SPL-1/SGPL1 upregulation.

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VPS-39 sustains mitochondrial network expansion in micu-1(null) nematodes.

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VPS39 and SGPL1 expression influences mitochondria-lysosome contact sites in MICU1 deficient cells.

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VPS39/SGPL1 signaling may be a common signature of mitochondrial deficient cells.

Organismal and Cellular Stress Responses upon Disruption of Mitochondrial Lonp1 Protease

Cells engage complex surveillance mechanisms to maintain mitochondrial function and protein homeostasis. LonP1 protease is a key component of mitochondrial quality control and has been implicated in human malignancies and other pathological disorders. Here, we employed two experimental systems, the worm Caenorhabditis elegans and human cancer cells, to investigate and compare the effects of LONP-1/LonP1 deficiency at the molecular, cellular, and organismal levels. Deletion of the lonp-1 gene in worms disturbed mitochondrial function, provoked reactive oxygen species accumulation, and impaired normal processes, such as growth, behavior, and lifespan. The viability of lonp-1 mutants was dependent on the activity of the ATFS-1 transcription factor, and loss of LONP-1 evoked retrograde signaling that involved both the mitochondrial and cytoplasmic unfolded protein response (UPR^mt and UPR^cyt) pathways and ensuing diverse organismal stress responses. Exposure of worms to triterpenoid CDDO-Me, an inhibitor of human LonP1, stimulated only UPR^cyt responses. In cancer cells, CDDO-Me induced key components of the integrated stress response (ISR), the UPR^mt and UPR^cyt pathways, and the redox machinery. However, genetic knockdown of LonP1 revealed a genotype-specific cellular response and induced apoptosis similar to CDDO-Me treatment. Overall, the mitochondrial dysfunction ensued by disruption of LonP1 elicits adaptive cytoprotective mechanisms that can inhibit cancer cell survival but diversely modulate organismal stress response and aging.

Rice bran peptide KF-8 extends the lifespan and improves healthspan of Caenorhabditis elegans via skn-1 and daf-16

With the increased aging of the population, the extension of lifespan and the improvement of healthspan have become important. Our previous studies showed that the rice bran peptide KF-8 exerts...

NGLY1: Insights from C. elegans

Peptide:N-glycanase is an evolutionarily conserved deglycosylating enzyme that catalyzes the removal of N-linked glycans from cytosolic glycoproteins. Recessive mutations that inactivate this enzyme cause NGLY1 deficiency, a multisystemic disorder with symptoms including developmental delay and defects in cognition and motor control. Developing treatments for NGLY1 deficiency will require an understanding of how failure to deglycosylate NGLY1 substrates perturbs cellular and organismal function. In this review, I highlight insights into peptide:N-glycanase biology gained by studies in the highly tractable genetic model animal C. elegans. I focus on the recent discovery of SKN-1A/Nrf1, an N-glycosylated transcription factor, as a peptide:N-glycanase substrate critical for regulation of the proteasome. I describe the elaborate post-translational mechanism that culminates in activation of SKN-1A/Nrf1 via NGLY1-dependent 'sequence editing' and discuss the implications of these findings for our understanding of NGLY1 deficiency.

Crosstalk in oxygen homeostasis networks: SKN-1/NRF inhibits the HIF-1 hypoxia-inducible factor in Caenorhabditis elegans

During development, homeostasis, and disease, organisms must balance responses that allow adaptation to low oxygen (hypoxia) with those that protect cells from oxidative stress. The evolutionarily conserved hypoxia-inducible factors are central to these processes, as they orchestrate transcriptional responses to oxygen deprivation. Here, we employ genetic strategies in C. elegans to identify stress-responsive genes and pathways that modulate the HIF-1 hypoxia-inducible factor and facilitate oxygen homeostasis. Through a genome-wide RNAi screen, we show that RNAi-mediated mitochondrial or proteasomal dysfunction increases the expression of hypoxia-responsive reporter Pnhr-57::GFP in C. elegans. Interestingly, only a subset of these effects requires hif-1. Of particular importance, we found that skn-1 RNAi increases the expression of hypoxia-responsive reporter Pnhr-57::GFP and elevates HIF-1 protein levels. The SKN-1/NRF transcription factor has been shown to promote oxidative stress resistance. We present evidence that the crosstalk between HIF-1 and SKN-1 is mediated by EGL-9, the prolyl hydroxylase that targets HIF-1 for oxygen-dependent degradation. Treatment that induces SKN-1, such as heat or gsk-3 RNAi, increases expression of a Pegl-9::GFP reporter, and this effect requires skn-1 function and a putative SKN-1 binding site in egl-9 regulatory sequences. Collectively, these data support a model in which SKN-1 promotes egl-9 transcription, thereby inhibiting HIF-1. We propose that this interaction enables animals to adapt quickly to changes in cellular oxygenation and to better survive accompanying oxidative stress.

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.

The dietary triterpenoid 18α-Glycyrrhetinic acid protects from MMC-induced genotoxicity through the ERK/Nrf2 pathway

18α-Glycyrrhetinic acid (18α-GA) is a bioactive triterpenoid that has been shown to activate the nuclear factor (erythroid-derived-2)-like 2 (Nrf2), the main transcription factor that orchestrates the cellular antioxidant response, in both cellular and organismal context. Although various beneficial properties of 18α-GA have been revealed, including its anti-oxidation and anti-aging activity, its possible protective effect against DNA damage has never been addressed. In this study, we investigated the potential beneficial properties of 18α-GA against DNA damage induced by mitomycin C (MMC) treatment. Using human primary fibroblasts exposed to MMC following pre-treatment with 18α-GA, we reveal an Nrf2-mediated protective effect against MMC-induced cell death that depends on extracellular signal-regulated kinase (ERK) signaling. In total, our results reveal an additional beneficial effect of the Nrf2 activator 18α-GA, suggesting that this important phytochemical compound is a potential candidate in preventive and/or therapeutic schemes against conditions (such as aging) or diseases that are characterized by both oxidative stress and DNA damage.

CBP-1/p300 acetyltransferase regulates SKN-1/Nrf cellular levels, nuclear localization, and activity in C. elegans

SKN-1/Nrf transcription factors regulate diverse biological processes essentially stress defense, detoxification, and longevity. Studies in model organisms have identified a broad range of regulatory processes and mechanisms that profoundly influence SKN-1/Nrf functions. Defining the mechanisms how SKN-1 is regulated will provide insight how cells defend against diverse stressors contributing to aging and disease. In this study, we demonstrate a crucial role for the acetyltransferase CBP-1, the C. elegans homolog of mammalian CREB-binding protein CBP/p300 in the activation of SKN-1. cbp-1 is essential for tolerance of oxidative stress and normal lifespan. CBP-1 directly interacts with SKN-1 and increases SKN-1 protein abundance. In particular CBP-1 modulates SKN-1 nuclear translocation under basal conditions and in response to stress and promotes SKN-1-dependent transcription of protective genes. Moreover, CBP-1 is required for SKN-1 nuclear recruitment, transcriptional activity, and longevity due to reduced insulin/IGF-1-like signaling, mTOR-, and GSK-3 signaling. Our findings establish the acetyltransferase CBP-1 as a critical activator of SKN-1 that directly modulates SKN-1 protein stability, nuclear localization, and function to ascertain normal stress response and lifespan.

Tyrosine aminotransferase is involved in the oxidative stress response by metabolizing meta-tyrosine in Caenorhabditis elegans

Under oxidative stress conditions, hydroxyl radicals can oxidize the phenyl ring of phenylalanine, producing the abnormal tyrosine isomer meta-tyrosine (m-tyrosine). m-Tyrosine levels are commonly used as a biomarker of oxidative stress, and its accumulation has recently been reported to adversely affect cells, suggesting a direct role for m-tyrosine in oxidative stress effects. We found that the Caenorhabditis elegans ortholog of tyrosine aminotransferase (TATN-1)-the first enzyme involved in the metabolic degradation of tyrosine-is up-regulated in response to oxidative stress and directly activated by the oxidative stress-responsive transcription factor SKN-1. Worms deficient in tyrosine aminotransferase activity displayed increased sensitivity to multiple sources of oxidative stress. Biochemical assays revealed that m-tyrosine is a substrate for TATN-1-mediated deamination, suggesting that TATN-1 also metabolizes m-tyrosine. Consistent with a toxic effect of m-tyrosine and a protective function of TATN-1, tatn-1 mutant worms exhibited delayed development, marked reduction in fertility, and shortened lifespan when exposed to m-tyrosine. A forward genetic screen identified a mutation in the previously uncharacterized gene F01D4.5-homologous with human transcription factor 20 (TCF20) and retinoic acid-induced 1 (RAI1)-that suppresses the adverse phenotypes observed in m-tyrosine-treated tatn-1 mutant worms. RNA-Seq analysis of F01D4.5 mutant worms disclosed a significant reduction in the expression of specific isoforms of genes encoding ribosomal proteins, suggesting that alterations in protein synthesis or ribosome structure could diminish the adverse effects of m-tyrosine. Our findings uncover a critical role for tyrosine aminotransferase in the oxidative stress response via m-tyrosine metabolism.

Arginine methylation of SKN-1 promotes oxidative stress resistance in Caenorhabditis elegans

Caenorhabditis elegans NRF (NF-E2-related factor)/CNC (Cap'n'collar) transcription factor, Skinhead-1 (SKN-1), is conservatively critical for promoting phase II detoxification gene expressions in response to oxidative stress. SKN-1 activity is controlled by well-known phosphorylation and recently-reported O-GlcNAcylation. Whether other kinds of posttanslational modifications of SKN-1 occur and influence its function remains elusive. Here, we found arginines 484 and 516 (R484/R516) of SKN-1 were asymmetrically dimethylated by PRMT-1. Oxidative stress enhanced the binding of PRMT-1 to SKN-1. Consequently, asymmetrical dimethylation of arginines on SKN-1 was elevated. Loss of prmt-1 or disruption of R484/R516 dimethylation decreased the enrichment of SKN-1 on the promoters of SKN-1-driven phase II detoxification genes, including gamma-glutamine cysteine synthetase gcs-1, glutathione S-transferases gst-7 and gst-4, which resulted in reduced ability of worms to defense against oxidative stress. These findings have important implications for investigating the physiological and pathological functions of arginine methylation on conserved NRF/CNC transcription factors in human diseases related to oxidative stress response.

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Arg 484/516 of SKN-1 are asymmetrically dimethylated by PRMT-1 in C. elegans.

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Oxidative stress enhances the binding of PRMT-1 to SKN-1.

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Oxidative stress elevates asymmetrical dimethylation of arginines on SKN-1.

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Arg 484/516 methylation increases SKN-1 binding to detoxification gene promoters.

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Arg 484/516 methylation promotes function of SKN-1 in oxidative stress resistance.

WDR-23 and SKN-1/Nrf2 Coordinate with the BLI-3 Dual Oxidase in Response to Iodide-Triggered Oxidative Stress

Animals utilize conserved mechanisms to regulate oxidative stress. The C. elegans SKN-1 protein is homologous to the vertebrate Nrf (NF-E2-related factor) family of cap 'n' collar (CnC) transcription factors and functions as a core regulator of xenobiotic and oxidative stress responses. The WD40 repeat-containing protein WDR-23 is a key negative regulator of SKN-1 activity. We previously found that the oxidative stress induced by excess iodide can be relieved by loss of function in the BLI-3/TSP-15/DOXA-1 dual oxidase complex. To further understand the molecular mechanism of this process, we screened for new mutants that can survive in excess iodide and identified gain-of-function mutations in skn-1 and loss-of-function mutations in wdr-23 The SKN-1C isoform functions in the hypodermis to affect animal's response to excess iodide, while the SKN-1A isoform appears to play a minor role. wdr-23(lf) can interact with bli-3 mutations in a manner different from skn-1(gf) Transcriptome studies suggest that excess iodide causes developmental arrest largely independent of changes in gene expression, and wdr-23(lf) could affect the expression of a subset of genes by a mechanism different from SKN-1 activation. We propose that WDR-23 and SKN-1 coordinate with the BLI-3/TSP-15/DOXA-1 dual oxidase complex in response to iodide-triggered oxidative stress.

East Indian sandalwood (Santalum album L.) oil confers neuroprotection and geroprotection in Caenorhabditis elegans via activating SKN-1/Nrf2 signaling pathway

East Indian Sandalwood Oil (EISO) has diverse beneficial effects and has been used for thousands of years in traditional folk-medicine for treatment of different human ailments. However, there has been no in-depth scientific investigation to decipher the neuroprotective and geroprotective mechanism of EISO and its principle components, α- and β-santalol. Hence the current study was undertaken to assess the protective effects of EISO, and α- and β-santalol against neurotoxic (6-OHDA/6-hydroxydopamine) and proteotoxic (α-synuclein) stresses in a Caenorhabditis elegans model. Initially, we found that EISO and its principle components exerted an excellent antioxidant and antiapoptotic activity as it was able to extend the lifespan, and inhibit the ROS generation, and germline cell apoptosis in 6-OHDA-intoxicated C. elegans. Further, we showed that supplementation of EISO, and α- and β-santalol reduced the 6-OHDA and α-synuclein-induced Parkinson's disease associated pathologies and improved the physiological functions. The genetic and reporter gene expression analysis revealed that an EISO, or α- and β-santalol-mediated protective effect does not appear to rely on DAF-2/DAF-16, but selectively regulates SKN-1 and its downstream targets involved in antioxidant defense and geroprotective processes. Together, our findings indicated that EISO and its principle components are worth exploring further as a candidate redox-based neuroprotectant for the prevention and management of age-related neurological disorders.

Functional crosstalk among oxidative stress and O-GlcNAc signaling pathways

In metazoans, thousands of intracellular proteins are modified with O-linked β-N-acetylglucosamine (O-GlcNAc) in response to a wide range of stimuli and stresses. In particular, a complex and evolutionarily conserved interplay between O-GlcNAcylation and oxidative stress has emerged in recent years. Here, we review the current literature on the connections between O-GlcNAc and oxidative stress, with a particular emphasis on major signaling pathways, such as KEAP1/NRF2, FOXO, NFκB, p53 and cell metabolism. Taken together, this work sheds important light on the signaling functions of protein glycosylation and the mechanisms of stress responses alike and illuminates how the two are integrated in animal cell physiology.

Redox stress and signaling during vertebrate embryonic development: Regulation and responses

Vertebrate embryonic development requires specific signaling events that regulate cell proliferation and differentiation to occur at the correct place and the correct time in order to build a healthy embryo. Signaling pathways are sensitive to perturbations of the endogenous redox state, and are also susceptible to modulation by reactive species and antioxidant defenses, contributing to a spectrum of passive vs. active effects that can affect redox signaling and redox stress. Here we take a multi-level, integrative approach to discuss the importance of redox status for vertebrate developmental signaling pathways and cell fate decisions, with a focus on glutathione/glutathione disulfide, thioredoxin, and cysteine/cystine redox potentials and the implications for protein function in development. We present a tissue-specific example of the important role that reactive species play in pancreatic development and metabolic regulation. We discuss NFE2L2 (also known as NRF2) and related proteins, their roles in redox signaling, and their regulation of glutathione during development. Finally, we provide examples of xenobiotic compounds that disrupt redox signaling in the context of vertebrate embryonic development. Collectively, this review provides a systems-level perspective on the innate and inducible antioxidant defenses, as well as their roles in maintaining redox balance during chemical exposures that occur in critical windows of development.

Expression patterns of members of the ethylene signaling-related gene families in response to dehydration stresses in cassava

Drought is the one of the most important environment stresses that restricts crop yield worldwide. Cassava (Manihot esculenta Crantz) is an important food and energy crop that has many desirable traits such as drought, heat and low nutrients tolerance. However, the mechanisms underlying drought tolerance in cassava are unclear. Ethylene signaling pathway, from the upstream receptors to the downstream transcription factors, plays important roles in environmental stress responses during plant growth and development. In this study, we used bioinformatics approaches to identify and characterize candidate Manihot esculenta ethylene receptor genes and transcription factor genes. Using computational methods, we localized these genes on cassava chromosomes, constructed phylogenetic trees and identified stress-responsive cis-elements within their 5’ upstream regions. Additionally, we measured the trehalose and proline contents in cassava fresh leaves after drought, osmotic, and salt stress treatments, and then it was found that the regulation patterns of contents of proline and trehalose in response to various dehydration stresses were differential, or even the opposite, which shows that plant may take different coping strategies to deal with different stresses, when stresses come. Furthermore, expression profiles of these genes in different organs and tissues under non-stress and abiotic stress were investigated through quantitative real-time PCR (qRT-PCR) analyses in cassava. Expression profiles exhibited clear differences among different tissues under non-stress and various dehydration stress conditions. We found that the leaf and tuberous root tissues had the greatest and least responses, respectively, to drought stress through the ethylene signaling pathway in cassava. Moreover, tuber and root tissues had the greatest and least reponses to osmotic and salt stresses through ethylene signaling in cassava, respectively. These results show that these plant tissues had differential expression levels of genes involved in ethylene signaling in response to the stresses tested. Moreover, after several gene duplication events, the spatiotemporally differential expression pattern of homologous genes in response to abiotic and biotic stresses may imply their functional diversity as a mechanism for adapting to the environment. Our data provide a framework for further research on the molecular mechanisms of cassava resistance to drought stress and provide a foundation for breeding drought-resistant new cultivars.

WDR23 regulates NRF2 independently of KEAP1

Cellular adaptation to stress is essential to ensure organismal survival. NRF2/NFE2L2 is a key determinant of xenobiotic stress responses, and loss of negative regulation by the KEAP1-CUL3 proteasome system is implicated in several chemo- and radiation-resistant cancers. Advantageously using C. elegans alongside human cell culture models, we establish a new WDR23-DDB1-CUL4 regulatory axis for NRF2 activity that operates independently of the canonical KEAP1-CUL3 system. WDR23 binds the DIDLID sequence within the Neh2 domain of NRF2 to regulate its stability; this regulation is not dependent on the KEAP1-binding DLG or ETGE motifs. The C-terminal domain of WDR23 is highly conserved and involved in regulation of NRF2 by the DDB1-CUL4 complex. The addition of WDR23 increases cellular sensitivity to cytotoxic chemotherapeutic drugs and suppresses NRF2 in KEAP1-negative cancer cell lines. Together, our results identify WDR23 as an alternative regulator of NRF2 proteostasis and uncover a cellular pathway that regulates NRF2 activity and capacity for cytoprotection independently of KEAP1.

Chronic exposure to environmental stressors throughout life (“the exposome”) has been tied to several cancers in humans. Cellular adaptation to stress is essential to ensure organismal survival, and NRF2 is an exceptionally well-studied and key determinant of cellular stress responses that plays complex roles in cancer biology and responses to xenobiotics, including chemotherapies. Our studies have established a functional and evolutionarily conserved role for WDR23 as a substrate receptor for the Cullin4(CUL4)-DDB1 E3-ubiquitin ligase, which regulates NRF2 protein levels and activity, and which operates independently of the canonical KEAP1-CUL3 pathway. KEAP1 has been the most highly studied regulator of NRF2, as mutations in KEAP1, which result in uncontrolled activation of NRF2 and chemo-resistance, are found in many aggressive cancers. Importantly, increased expression of WDR23 in KEAP1(-/-) cancer cells restores aberrant NRF2 regulation. In the absence of a KEAP1-like system, C. elegans WDR-23 has been shown to regulate the worm cytoprotective transcription factor SKN-1. We have leveraged C. elegans genetic approaches to identify conserved regulatory mechanisms of mammalian cytoprotection by NRF2. Collectively, our studies suggest control of NRF2 homeostasis is much more sophisticated than previously appreciated.

Baicalein modulates stress-resistance and life span in C. elegans via SKN-1 but not DAF-16

The flavonoid baicalein has been demonstrated to be an activator of the transcription factor Nrf2 in mammalian cell lines. We show that it further modulates the Nrf2 homolog SKN-1 in Caenorhabditis elegans and by this pathway mediates beneficial effects in the nematode: baicalein enhances the resistance of C. elegans against lethal thermal and sodium arsenite stress and dose-dependently prolongs the life span of the nematode. Using RNA interference against SKN-1 we were able to show that the induction of longevity and the enhanced stress-resistance were dependent on this transcription factor. DAF-16 (homolog to mammalian FOXO) is another pivotal aging-related transcription factor in the nematode. We demonstrate that DAF-16 does not participate in the beneficial effects of baicalein: since baicalein causes no increase in the nuclear translocation of DAF-16 (DAF-16::GFP expressing strain, incubation time: 1h) and it still induces longevity even in a DAF-16 loss-of-function strain, we conclude, that baicalein increases stress-resistance and life span in C. elegans via SKN-1 but not DAF-16.

Molecular and cellular basis for the unique functioning of Nrf1, an indispensable transcription factor for maintaining cell homoeostasis and organ integrity

The consensuscis-regulatory AP-1 (activator protein-1)-like AREs (antioxidant-response elements) and/or EpREs (electrophile-response elements) allow for differential recruitment of Nrf1 [NF-E2 (nuclear factor-erythroid 2)-related factor 1], Nrf2 and Nrf3, together with each of their heterodimeric partners (e.g. sMaf, c-Jun, JunD or c-Fos), to regulate different sets of cognate genes. Among them, NF-E2 p45 and Nrf3 are subject to tissue-specific expression in haemopoietic and placental cell lineages respectively. By contrast, Nrf1 and Nrf2 are two important transcription factors expressed ubiquitously in various vertebrate tissues and hence may elicit putative combinational or competitive functions. Nevertheless, they have de facto distinct biological activities because knockout of their genes in mice leads to distinguishable phenotypes. Of note, Nrf2 is dispensable during development and growth, albeit it is accepted as a master regulator of antioxidant, detoxification and cytoprotective genes against cellular stress. Relative to the water-soluble Nrf2, less attention has hitherto been drawn to the membrane-bound Nrf1, even though it has been shown to be indispensable for embryonic development and organ integrity. The biological discrepancy between Nrf1 and Nrf2 is determined by differences in both their primary structures and topovectorial subcellular locations, in which they are subjected to distinct post-translational processing so as to mediate differential expression of ARE-driven cytoprotective genes. In the present review, we focus on the molecular and cellular basis for Nrf1 and its isoforms, which together exert its essential functions for maintaining cellular homoeostasis, normal organ development and growth during life processes. Conversely, dysfunction of Nrf1 results in spontaneous development of non-alcoholic steatohepatitis, hepatoma, diabetes and neurodegenerative diseases in animal models.

TRX-1 Regulates SKN-1 Nuclear Localization Cell Non-autonomously in Caenorhabditis elegans

The Caenorhabditis elegans oxidative stress response transcription factor, SKN-1, is essential for the maintenance of redox homeostasis and is a functional ortholog of the Nrf family of transcription factors. The numerous levels of regulation that govern these transcription factors underscore their importance. Here, we add a thioredoxin, encoded by trx-1, to the expansive list of SKN-1 regulators. We report that loss of trx-1 promotes nuclear localization of intestinal SKN-1 in a redox-independent, cell non-autonomous fashion from the ASJ neurons. Furthermore, this regulation is not general to the thioredoxin family, as two other C. elegans thioredoxins, TRX-2 and TRX-3, do not play a role in this process. Moreover, TRX-1-dependent regulation requires signaling from the p38 MAPK-signaling pathway. However, while TRX-1 regulates SKN-1 nuclear localization, classical SKN-1 transcriptional activity associated with stress response remains largely unaffected. Interestingly, RNA-Seq analysis revealed that loss of trx-1 elicits a general, organism-wide down-regulation of several classes of genes; those encoding for collagens and lipid transport being most prevalent. Together, these results uncover a novel role for a thioredoxin in regulating intestinal SKN-1 nuclear localization in a cell non-autonomous manner, thereby contributing to the understanding of the processes involved in maintaining redox homeostasis throughout an organism.

Nrf2 and Nrf2-related proteins in development and developmental toxicity: Insights from studies in zebrafish (Danio rerio)

Oxidative stress is an important mechanism of chemical toxicity, contributing to developmental toxicity and teratogenesis as well as to cardiovascular and neurodegenerative diseases and diabetic embryopathy. Developing animals are especially sensitive to effects of chemicals that disrupt the balance of processes generating reactive species and oxidative stress, and those anti-oxidant defenses that protect against oxidative stress. The expression and inducibility of anti-oxidant defenses through activation of NFE2-related factor 2 (Nrf2) and related proteins is an essential process affecting the susceptibility to oxidants, but the complex interactions of Nrf2 in determining embryonic response to oxidants and oxidative stress are only beginning to be understood. The zebrafish (Danio rerio) is an established model in developmental biology and now also in developmental toxicology and redox signaling. Here we review the regulation of genes involved in protection against oxidative stress in developing vertebrates, with a focus on Nrf2 and related cap'n'collar (CNC)-basic-leucine zipper (bZIP) transcription factors. Vertebrate animals including zebrafish share Nfe2, Nrf1, Nrf2, and Nrf3 as well as a core set of genes that respond to oxidative stress, contributing to the value of zebrafish as a model system with which to investigate the mechanisms involved in regulation of redox signaling and the response to oxidative stress during embryolarval development. Moreover, studies in zebrafish have revealed nrf and keap1 gene duplications that provide an opportunity to dissect multiple functions of vertebrate NRF genes, including multiple sensing mechanisms involved in chemical-specific effects.

SKN-1/Nrf, stress responses, and aging in Caenorhabditis elegans

The mammalian Nrf/CNC proteins (Nrf1, Nrf2, Nrf3, p45 NF-E2) perform a wide range of cellular protective and maintenance functions. The most thoroughly described of these proteins, Nrf2, is best known as a regulator of antioxidant and xenobiotic defense, but more recently has been implicated in additional functions that include proteostasis and metabolic regulation. In the nematode Caenorhabditis elegans, which offers many advantages for genetic analyses, the Nrf/CNC proteins are represented by their ortholog SKN-1. Although SKN-1 has diverged in aspects of how it binds DNA, it exhibits remarkable functional conservation with Nrf/CNC proteins in other species and regulates many of the same target gene families. C. elegans may therefore have considerable predictive value as a discovery model for understanding how mammalian Nrf/CNC proteins function and are regulated in vivo. Work in C. elegans indicates that SKN-1 regulation is surprisingly complex and is influenced by numerous growth, nutrient, and metabolic signals. SKN-1 is also involved in a wide range of homeostatic functions that extend well beyond the canonical Nrf2 function in responses to acute stress. Importantly, SKN-1 plays a central role in diverse genetic and pharmacologic interventions that promote C. elegans longevity, suggesting that mechanisms regulated by SKN-1 may be of conserved importance in aging. These C. elegans studies predict that mammalian Nrf/CNC protein functions and regulation may be similarly complex and that the proteins and processes that they regulate are likely to have a major influence on mammalian life- and healthspan.

Direct interaction between the WD40 repeat protein WDR-23 and SKN-1/Nrf inhibits binding to target DNA

SKN-1/Nrf transcription factors activate cytoprotective genes in response to reactive small molecules and strongly influence stress resistance, longevity, and development. The molecular mechanisms of SKN-1/Nrf regulation are poorly defined. We previously identified the WD40 repeat protein WDR-23 as a repressor of Caenorhabditis elegans SKN-1 that functions with a ubiquitin ligase to presumably target the factor for degradation. However, SKN-1 activity and nuclear accumulation are not always correlated, suggesting that there could be additional regulatory mechanisms. Here, we integrate forward genetics and biochemistry to gain insights into how WDR-23 interacts with and regulates SKN-1. We provide evidence that WDR-23 preferentially regulates one of three SKN-1 variants through a direct interaction that is required for normal stress resistance and development. Homology modeling predicts that WDR-23 folds into a β-propeller, and we identify the top of this structure and four motifs at the termini of SKN-1c as essential for the interaction. Two of these SKN-1 motifs are highly conserved in human Nrf1 and Nrf2 and two directly interact with target DNA. Lastly, we demonstrate that WDR-23 can block the ability of SKN-1c to interact with DNA sequences of target promoters identifying a new mechanism of regulation that is independent of the ubiquitin proteasome system, which can become occupied with damaged proteins during stress.

Transcriptional regulation of gene expression in C. elegans

Protein coding gene sequences are converted to mRNA by the highly regulated process of transcription. The precise temporal and spatial control of transcription for many genes is an essential part of development in metazoans. Thus, understanding the molecular mechanisms underlying transcriptional control is essential to understanding cell fate determination during embryogenesis, post-embryonic development, many environmental interactions, and disease-related processes. Studies of transcriptional regulation in C. elegans exploit its genomic simplicity and physical characteristics to define regulatory events with single-cell and minute-time-scale resolution. When combined with the genetics of the system, C. elegans offers a unique and powerful vantage point from which to study how chromatin-associated proteins and their modifications interact with transcription factors and their binding sites to yield precise control of gene expression through transcriptional regulation.

A cytoprotective perspective on longevity regulation

There are many mechanisms of lifespan extension, including the disruption of insulin/insulin-like growth factor 1 (IGF-1) signaling, metabolism, translation, and feeding. Despite the disparate functions of these pathways, inhibition of each induces responses that buffer stress and damage. Here, emphasizing data from genetic analyses in Caenorhabditis elegans, we explore the effectors and upstream regulatory components of numerous cytoprotective mechanisms activated as major elements of longevity programs, including detoxification, innate immunity, proteostasis, and oxidative stress response. We show that their induction underpins longevity extension across functionally diverse triggers and across species. Intertwined with the evolution of longevity, cytoprotective pathways are coupled to the surveillance of core cellular components, with important implications in normal and aberrant responses to drugs, chemicals, and pathogens.

Cap'n'collar differentiates the mandible from the maxilla in the beetle Tribolium castaneum

BACKGROUND

The biting mandible of the arthropods is thought to have evolved in the ancestor of the insects, crustaceans and myriapods: the Mandibulata. A unique origin suggests a common set of developmental genes will be required to pattern the mandible in different arthropods. To date we have functional studies on patterning of the mandibular segment of Drosophila melanogaster showing in particular the effects of the gene cap'n'collar (cnc), however, the dipteran head is far from representative of insects or of more distantly related mandibulates; Drosophila does not even possess a mandibular appendage. To study the development of a more representative insect mandible, we chose the red flour beetle Tribolium castaneum and investigated the function of the Tribolium orthologs of cap'n'collar (Tc-cnc) and the Hox gene Deformed (Tc-Dfd). In order to determine the function of Tc-cnc and Tc-Dfd, transcripts were knocked down by maternal RNA interference (RNAi). The effects of gene knockdown were examined in the developing embryos and larvae. The effect of Tc-cnc and Tc-Dfd knockdown on the expression of other genes was determined by using in situ hybridization on Tribolium embryos.

RESULTS

Our analyses show that Tc-cnc is required for specification of the identity of the mandibular segment of Tribolium and differentiates the mandible from maxillary identity. Loss of Tc-cnc function results in a transformation of the mandible to maxillary identity as well as deletion of the labrum. Tc-Dfd and the Tribolium homolog of proboscipedia (Tc-mxp = maxillopedia), Hox genes that are required to pattern the maxillary appendage, are expressed in a maxilla-like manner in the transformed mandible. Tribolium homologs of paired (Tc-prd) and Distal-less (Tc-Dll) that are expressed in the endites and telopodites of embryonic appendages are also expressed in a maxilla-like manner in the transformed mandible.We also show that Tc-Dfd is required to activate the collar of Tc-cnc expression in the mandibular segment but not the cap expression in the labrum. Tc-Dfd is also required for the activation of Tc-prd in the endites of the mandible and maxillary appendages.

CONCLUSIONS

Tc-cnc is necessary for patterning the mandibular segment of Tribolium. Together, Tc-cnc and Tc-Dfd cooperate to specify mandibular identity, as in Drosophila. Expression patterns of the homologs of cnc and Dfd are conserved in mandibulate arthropods suggesting that the mandible specifying function of cnc is likely to be conserved across the mandibulate arthropods.

Mitochondrial SKN-1/Nrf mediates a conserved starvation response

SKN-1/Nrf plays multiple essential roles in development and cellular homeostasis. We demonstrate that SKN-1 executes a specific and appropriate transcriptional response to changes in available nutrients, leading to metabolic adaptation. We isolated gain-of-function (gf) alleles of skn-1, affecting a domain of SKN-1 that binds the transcription factor MXL-3 and the mitochondrial outer membrane protein PGAM-5. These skn-1(gf) mutants perceive a state of starvation even in the presence of plentiful food. The aberrant monitoring of cellular nutritional status leads to an altered survival response in which skn-1(gf) mutants transcriptionally activate genes associated with metabolism, adaptation to starvation, aging, and survival. The triggered starvation response is conserved in mice with constitutively activated Nrf and may contribute to the tumorgenicity associated with activating Nrf mutations in mammalian somatic cells. Our findings delineate an evolutionarily conserved metabolic axis of SKN-1/Nrf, further establishing the complexity of this pathway.

Molecular basis of electrophilic and oxidative defense: promises and perils of Nrf2

Induction of drug-metabolizing enzymes through the antioxidant response element (ARE)-dependent transcription was initially implicated in chemoprevention against cancer by antioxidants. Recent progress in understanding the biology and mechanism of induction revealed a critical role of induction in cellular defense against electrophilic and oxidative stress. Induction is mediated through a novel signaling pathway via two regulatory proteins, the nuclear factor erythroid 2-related factor 2 (Nrf2) and the Kelch-like erythroid cell-derived protein with CNC homology-associated protein 1 (Keap1). Nrf2 binds to Keap1 at a two site-binding interface and is ubiquitinated by the Keap1/cullin 3/ring box protein-1-ubiquitin ligase, resulting in a rapid turnover of Nrf2 protein. Electrophiles and oxidants modify critical cysteine thiols of Keap1 and Nrf2 to inhibit Nrf2 ubiquitination, leading to Nrf2 activation and induction. Induction increases stress resistance critical for cell survival, because knockout of Nrf2 in mice increased susceptibility to a variety of toxicity and disease processes. Collateral to diverse functions of Nrf2, genome-wide search has led to the identification of a plethora of ARE-dependent genes regulated by Nrf2 in an inducer-, tissue-, and disease-dependent manner to control drug metabolism, antioxidant defense, stress response, proteasomal degradation, and cell proliferation. The protective nature of Nrf2 could also be hijacked in a number of pathological conditions by means of somatic mutation, epigenetic alteration, and accumulation of disruptor proteins, promoting drug resistance in cancer and pathologic liver features in autophagy deficiency. The repertoire of ARE inducers has expanded enormously; the therapeutic potential of the inducers has been examined beyond cancer prevention. Developing potent and specific ARE inducers and Nrf2 inhibitors holds certain new promise for the prevention and therapy against cancer, chronic disease, and toxicity.

Induction of cytoprotective pathways is central to the extension of lifespan conferred by multiple longevity pathways

Many genetic and physiological treatments that extend lifespan also confer resistance to a variety of stressors, suggesting that cytoprotective mechanisms underpin the regulation of longevity. It has not been established, however, whether the induction of cytoprotective pathways is essential for lifespan extension or merely correlated. Using a panel of GFP-fused stress response genes, we identified the suites of cytoprotective pathways upregulated by 160 gene inactivations known to increase Caenorhabditis elegans longevity, including the mitochondrial UPR (hsp-6, hsp-60), the ER UPR (hsp-4), ROS response (sod-3, gst-4), and xenobiotic detoxification (gst-4). We then screened for other gene inactivations that disrupt the induction of these responses by xenobiotic or genetic triggers, identifying 29 gene inactivations required for cytoprotective gene expression. If cytoprotective responses contribute directly to lifespan extension, inactivation of these genes would be expected to compromise the extension of lifespan conferred by decreased insulin/IGF-1 signaling, caloric restriction, or the inhibition of mitochondrial function. We find that inactivation of 25 of 29 cytoprotection-regulatory genes shortens the extension of longevity normally induced by decreased insulin/IGF-1 signaling, disruption of mitochondrial function, or caloric restriction, without disrupting normal longevity nearly as dramatically. These data demonstrate that induction of cytoprotective pathways is central to longevity extension and identify a large set of new genetic components of the pathways that detect cellular damage and couple that detection to downstream cytoprotective effectors.

Many mutations that increase animal lifespan also confer stress tolerance, suggesting that cytoprotective mechanisms underpin the regulation of longevity. It has not been established, however, whether the induction of individual cytoprotective pathways is essential for lifespan extension, or merely correlated. To establish whether the regulatory pathways for the induction of cytoprotective responses are key in the extension of lifespan, we performed an RNAi screen for gene inactivations that decouple the activation of cytoprotective pathways from xenobiotic stimuli that normally induce them. The screen identified 29 genes that constitute the regulatory cascades of the unfolded protein response, oxidative stress response, and detoxification. These upstream regulatory genes are critical to stress tolerance and the extension of lifespan conferred by decreased insulin/IGF-1 signaling, disruption of mitochondrial function, or caloric restriction, but have little effect on normal longevity.

CO(2)-cAMP-responsive cis-elements targeted by a transcription factor with CREB/ATF-like basic zipper domain in the marine diatom Phaeodactylum tricornutum

Expression controls of the carbon acquisition system in marine diatoms in response to environmental factors are an essential issue to understand the changes in marine primary productivity. A pyrenoidal β-carbonic anhydrase, PtCA1, is one of the most important candidates to investigate the control mechanisms of the CO(2) acquisition system in the marine diatom Phaeodactylum tricornutum. A detailed functional assay was carried out on the putative core regulatory region of the ptca1 promoter using a β-glucuronidase reporter in P. tricornutum cells under changing CO(2) conditions. A set of loss-of-function assays led to the identification of three CO(2)-responsive elements, TGACGT, ACGTCA, and TGACGC, at a region -86 to -42 relative to the transcription start site. Treatment with a cyclic (c)AMP analog, dibutyryl cAMP, revealed these three elements to be under the control of cAMP; thus, we designated them, from 5' to 3', as CO(2)-cAMP-Responsive Element1 (CCRE1), CCRE2, and CCRE3. Because the sequence TGACGT is known to be a typical target of human Activating Transcription Factor6 (ATF6), we searched for genes containing a basic zipper (bZIP) region homologous to that of ATF6 in the genome of P. tricornutum. Gel-shift assays using CCRE pentamers as labeled probes showed that at least one candidate of bZIP proteins, PtbZIP11, bound specifically to CCREs. A series of gain-of-function assays with CCREs fused to a minimal promoter strongly suggested that the alternative combination of CCRE1/2 or CCRE2/3 at proper distances from the minimal promoter is required as a potential target of PtbZIP11 for an effective CO(2) response of the ptca1 gene.

Caloric restriction and antiaging effects

Caloric restriction (CR) is widely used to study aging processes. It is a simple and highly reproducible method for delaying the aging process, preventing the onset of aging-related diseases and extending average or maximum lifespan. However, the mechanism underlying these effects of CR is still not clear. CR can inhibit growth, reduce body size and maintain a low body temperature. At the same time, there is a measurable decrease in the volume of adipose tissue, hyperglycemia and hyperinsulinemia, accompanied by modifications of lipid and energy metabolism and increased resistance to endogenous and extraneous stress. The metabolic changes induced by dietary restriction, the inhibition of fat deposition in nonadipose tissue and the effects on signal transduction are considered the most likely candidates for mechanisms underlying the effects of CR.

Discovery of the negative regulator of Nrf2, Keap1: a historical overview

An antioxidant response element (ARE) or an electrophile responsive element (EpRE) regulate the transcriptional induction of a battery of drug-detoxifying enzymes that are protective against electrophiles. Based on the high similarity of the ARE consensus sequence to an erythroid gene regulatory element NF-E2 binding site, we have found that the transcription factor Nrf2 is indispensable for the ARE-mediated induction of drug-metabolizing enzymes. Recent genome-wide analysis demonstrated that Nrf2 regulates hundreds of genes that are involved in the cytoprotective response against oxidative stress. In-depth analysis of Nrf2 regulatory mechanisms has led us to the discovery of a novel protein, which we have named Keap1. Keap1 suppresses Nrf2 activity by specifically binding to its evolutionarily conserved N-terminal Neh2 regulatory domain. In this review article, we summarize the findings and observations that have lead to the discovery of the Nrf2-Keap1 system. Furthermore, we briefly discuss the function of the Nrf2-Keap1 system under the regulation of the endogenous electrophilic compound 15-deoxy-Δ¹²(,)¹⁴-prostaglandin J₂. We propose that Nrf2-Keap1 plays a significant physiological role in the response to endogenous, environmental, and pharmacological electrophiles.

Variability in gene expression underlies incomplete penetrance

The phenotypic differences between individual organisms can often be ascribed to underlying genetic and environmental variation. However, even genetically identical organisms in homogenous environments vary, suggesting that randomness in developmental processes such as gene expression may also generate diversity. In order to examine the consequences of gene expression variability in multicellular organisms, we studied intestinal specification in the roundworm Caenorhabditis elegans in which wild-type cell fate is invariant and controlled by a small transcriptional network. Mutations in elements of this network can have indeterminate effects: some mutant embryos fail to develop intestinal cells, while others produce intestinal precursors. By counting transcripts of the genes in this network in individual embryos, we show that the expression of an otherwise redundant gene becomes highly variable in the mutants and that this variation is thresholded to produce an ON/OFF expression pattern of the master regulatory gene of intestinal differentiation. Our results demonstrate that mutations in developmental networks can expose otherwise buffered stochastic variability in gene expression, leading to pronounced phenotypic variation.

Condition-adapted stress and longevity gene regulation by Caenorhabditis elegans SKN-1/Nrf

Studies in model organisms have identified regulatory processes that profoundly influence aging, many of which modulate resistance against environmental or metabolic stresses. In Caenorhabditis elegans, the transcription regulator SKN-1 is important for oxidative stress resistance and acts in multiple longevity pathways. SKN-1 is the ortholog of mammalian Nrf proteins, which induce Phase 2 detoxification genes in response to stress. Phase 2 enzymes defend against oxygen radicals and conjugate electrophiles that are produced by Phase 1 detoxification enzymes, which metabolize lipophilic compounds. Here, we have used expression profiling to identify genes and processes that are regulated by SKN-1 under normal and stress-response conditions. Under nonstressed conditions SKN-1 upregulates numerous genes involved in detoxification, cellular repair, and other functions, and downregulates a set of genes that reduce stress resistance and lifespan. Many of these genes appear to be direct SKN-1 targets, based upon presence of predicted SKN-binding sites in their promoters. The metalloid sodium arsenite induces skn-1-dependent activation of certain detoxification gene groups, including some that were not SKN-1-upregulated under normal conditions. An organic peroxide also triggers induction of a discrete Phase 2 gene set, but additionally stimulates a broad SKN-1-independent response. We conclude that under normal conditions SKN-1 has a wide range of functions in detoxification and other processes, including modulating mechanisms that reduce lifespan. In response to stress, SKN-1 and other regulators tailor transcription programs to meet the challenge at hand. Our findings reveal striking complexity in SKN-1 functions and the regulation of systemic detoxification defenses.

Increased age reduces DAF-16 and SKN-1 signaling and the hormetic response of Caenorhabditis elegans to the xenobiotic juglone

Cells adapt to stressors by activating mechanisms that repair damage and protect them from further injury. Stress-induced damage accumulates with age and contributes to age associated diseases. Increased age attenuates the ability to mount a stress response, but little is known about the mechanisms by which this occurs. To begin addressing this problem, we studied hormesis in the nematode Caenorhabditis elegans. When exposed to a low concentration of the xenobiotic juglone, young worms mount a robust hormetic stress response and survive a subsequent exposure to a higher concentration of juglone that is normally lethal to naïve animals. Old worms are unable to mount this adaptive response. Microarray and RNAi analyses demonstrate that an altered transcriptional response to juglone is responsible in part for the reduced adaptation of old worms. Many genes differentially regulated in young versus old animals are known or postulated to be regulated by the FOXO homologue DAF-16 and the Nrf2 homologue SKN-1. Activation of these pathways is greatly reduced in juglone stressed old worms. DAF-16- and SKN-1-like transcription factors play highly conserved roles in regulating stress resistance and longevity genes. Our studies provide a foundation for developing a molecular understanding of how age affects cytoprotective transcriptional pathways.

The Nrf1 CNC/bZIP protein is a nuclear envelope-bound transcription factor that is activated by t-butyl hydroquinone but not by endoplasmic reticulum stressors

In rat liver RL-34 cells, endogenous Nrf1 (nuclear factor-erythroid 2 p45 subunit-related factor 1) is localized in the ER (endoplasmic reticulum) where it exists as a glycosylated protein. Electron microscopy has demonstrated that ectopic Nrf1 in COS-1 cells is located in the ER and the NE (nuclear envelope). Subcellular fractionation, together with a membrane proteinase protection assay, revealed that Nrf1 is an integral membrane protein with both luminal and cytoplasmic domains. The N-terminal 65 residues of Nrf1 direct its integration into the ER and NE membranes and tether it to a Triton X-100-resistant membrane microdomain that is associated with lipid rafts. The activity of Nrf1 was increased by the electrophile tBHQ (t-butyl hydroquinone) probably through an N-terminal domain-dependent process. We found that the NST (Asn/Ser/Thr-rich) domain, along with AD1 (acidic domain 1), contributes positively to the transactivation activity of full-length Nrf1. Furthermore, the NST domain contains seven putative -Asn-Xaa-Ser/Thr- glycosylation sites and, when glycosylation was prevented by replacing all of the seven asparagine residues with either glutamine (Nrf1(1-7xN/Q)) or aspartic acid (Nrf1(1-7xN/D)), the former multiple point mutant possessed less activity than the wild-type factor, whereas the latter mutant exhibited substantially greater activity. Lastly, the ER stressors tunicamycin, thapsigargin and Brefeldin A were found to inhibit basal Nrf1 activity by approximately 25%, and almost completely prevented induction of Nrf1-mediated transactivation by tBHQ. Collectively, these results suggest that the activity of Nrf1 critically depends on its topology within the ER, and that this is modulated by redox stressors, as well as by its glycosylation status.

Utility of Caenorhabditis elegans in high throughput neurotoxicological research

Caenorhabditis elegans is a nematode that has been used as a valuable research tool in many facets of biological research. Researchers have used the many tools available to investigate this well-studied nematode, including a cell lineage map, sequenced genome, and complete wiring diagram of the nervous system, making in-depth investigation of the nervous system practical. These tools, along with other advantages, such as its small size, short life cycle, transparency, and ability to generate many progeny, have made C. elegans an attractive model for many studies, including those investigating toxicological paradigms and those using high throughput techniques. Researchers have investigated a number of endpoints, such as behavior and protein expression using a green fluorescent protein (GFP) marker following toxicant exposure and have explored the mechanisms of toxicity using techniques such as microarray, RNA interference (RNAi), and mutagenesis. This review discusses the benefits of using C. elegans as a model system and gives examples of the uses of C. elegans in toxicological research. High throughput techniques are discussed highlighting the advantages of using an in vivo system that has many advantageous characteristics of an in vitro system while emphasizing endpoints relating to developmental and adult neurotoxicity.

Knockdown of SKN-1 and the Wnt effector TCF/POP-1 reveals differences in endomesoderm specification in C. briggsae as compared with C. elegans

In the nematode, C. elegans, the bZIP/homeodomain transcription factor SKN-1 and the Wnt effector TCF/POP-1 are central to the maternal specification of the endomesoderm prior to gastrulation. The 8-cell stage blastomere MS is primarily a mesodermal precursor, giving rise to cells of the pharynx and body muscle among others, while its sister E clonally generates the entire endoderm (gut). In C. elegans, loss of SKN-1 results in the absence of MS-derived tissues all of the time, and loss of gut most of the time, while loss of POP-1 results in a mis-specification of MS as an E-like cell, resulting in ectopic gut. We show that in C. briggsae, RNAi of skn-1 results in a stronger E defect but no apparent MS defect, while RNAi of pop-1 results in loss of gut and an apparent E to MS transformation, the opposite of the pop-1 knockdown phenotype seen in C. elegans. The difference in pop-1(-) phenotypes correlates with changes in how the endogenous endoderm-specifying end genes are regulated by POP-1 in the two species. Our results suggest that integration of Wnt-dependent and Wnt-independent cell fate specification pathways within the Caenorhabditis genus can occur in different ways.

Aging and survival: the genetics of life span extension by dietary restriction

Reducing food intake to induce undernutrition but not malnutrition extends the life spans of multiple species, ranging from single-celled organisms to mammals. This increase in longevity by dietary restriction (DR) is coupled to profound beneficial effects on age-related pathology. Historically, much of the work on DR has been undertaken using rodent models, and 70 years of research has revealed much about the physiological changes DR induces. However, little is known about the genetic pathways that regulate the DR response and whether or not they are conserved between species. Elucidating these pathways may facilitate the design of targeted pharmaceutical treatments for a range of age-related pathologies. Here, we discuss how recent work in nonmammalian model organisms has revealed new insight into the genetics of DR and how the discovery of DR-specific transcription factors will advance our understanding of this phenomenon.

Nrf2 mediates cancer protection but not prolongevity induced by caloric restriction

Caloric restriction (CR) is the most potent intervention known to both protect against carcinogenesis and extend lifespan in laboratory animals. A variety of anticarcinogens and CR mimetics induce and activate the NF-E2-related factor 2 (Nrf2) pathway. Nrf2, in turn, induces a number of antioxidative and carcinogen-detoxifying enzymes. Thus, Nrf2 offers a promising target for anticarcinogenesis and antiaging interventions. We used Nrf2-disrupted (KO) mice to examine its role on the biological effects of CR. Here, we show that Nrf2 is responsible for most of the anticarcinogenic effects of CR, but is dispensable for increased insulin sensitivity and lifespan extension. Nrf2-deficient mice developed tumors more readily in response to carcinogen exposure than did WT mice, and CR was ineffective in suppressing tumors in the KO mice. However, CR extended lifespan and increased insulin sensitivity similarly in KO and WT mice. These findings identify a molecular pathway that dissociates the prolongevity and anticarcinogenic effects of CR.

Activation of SKN-1 by novel kinases in Caenorhabditis elegans

Here we use a large-scale RNAi suppression screen to identify additional kinases playing a role in the activation of SKN-1 in response to oxidative stress. The SKN-1 transcription factor specifies cell fate of the EMS blastomere at the four-cell stage in the nematode Caenorhabditis elegans and also directs transcription of many genes responding to oxidative stress, including glutathione S-transferase, NAD(P)H:quinone oxidoreductase, and superoxide dismutase. SKN-1 localizes to the nucleus and directs transcription following exposure to paraquat, heat, hyperbaric oxygen, and sodium azide. Previous studies have identified GSK-3 as an inhibitor of SKN-1 nuclear localization, in the absence of stress, and PMK-1 as an activator of SKN-1 during periods of oxidative stress. Through this screen we have identified four kinases, MKK-4, IKK epsilon-1, NEKL-2, and PDHK-2, which are necessary for the nuclear localization of SKN-1 in response to oxidative stress. Inhibition of two of these kinases results in shorter life span and increased sensitivity to stress.

The chemical defensome: environmental sensing and response genes in the Strongylocentrotus purpuratus genome

Metazoan genomes contain large numbers of genes that participate in responses to environmental stressors. We surveyed the sea urchin Strongylocentrotus purpuratus genome for homologs of gene families thought to protect against chemical stressors; these genes collectively comprise the 'chemical defensome.' Chemical defense genes include cytochromes P450 and other oxidases, various conjugating enzymes, ATP-dependent efflux transporters, oxidative detoxification proteins, and transcription factors that regulate these genes. Together such genes account for more than 400 genes in the sea urchin genome. The transcription factors include homologs of the aryl hydrocarbon receptor, hypoxia-inducible factor, nuclear factor erythroid-derived 2, heat shock factor, and nuclear hormone receptors, which regulate stress-response genes in vertebrates. Some defense gene families, including the ABCC, the UGT, and the CYP families, have undergone expansion in the urchin relative to other deuterostome genomes, whereas the stress sensor gene families do not show such expansion. More than half of the defense genes are expressed during embryonic or larval life stages, indicating their importance during development. This genome-wide survey of chemical defense genes in the sea urchin reveals evolutionary conservation of this network combined with lineage-specific diversification that together suggest the importance of these chemical stress sensing and response mechanisms in early deuterostomes. These results should facilitate future studies on the evolution of chemical defense gene networks and the role of these networks in protecting embryos from chemical stress during development.

An updated catalogue of salivary gland transcripts in the adult female mosquito, Anopheles gambiae

Salivary glands of blood-sucking arthropods contain a variety of compounds that prevent platelet and clotting functions and modify inflammatory and immunological reactions in the vertebrate host. In mosquitoes, only the adult female takes blood meals, while both sexes take sugar meals. With the recent description of the Anopheles gambiae genome, and with a set of approximately 3000 expressed sequence tags from a salivary gland cDNA library from adult female mosquitoes, we attempted a comprehensive description of the salivary transcriptome of this most important vector of malaria transmission. In addition to many transcripts associated with housekeeping functions, we found an active transposable element, a set of Wolbachia-like proteins, several transcription factors, including Forkhead, Hairy and doublesex, extracellular matrix components and 71 genes coding for putative secreted proteins. Fourteen of these 71 proteins had matching Edman degradation sequences obtained from SDS-PAGE experiments. Overall, 33 transcripts are reported for the first time as coding for salivary proteins. The tissue and sex specificity of these protein-coding transcripts were analyzed by RT-PCR and microarray experiments for insight into their possible function. Notably, two gene products appeared to be differentially spliced in the adult female salivary glands, whereas 13 contigs matched predicted intronic regions and may include additional alternatively spliced transcripts. Most An. gambiae salivary proteins represent novel protein families of unknown function, potentially coding for pharmacologically or microbiologically active substances. Supplemental data to this work can be found at http://www.ncbi.nlm.nih.gov/projects/omes/index.html#Ag2.

Coordination of ER and oxidative stress signaling: the PERK/Nrf2 signaling pathway

In the broadest sense, cellular stress describes conditions wherein cells encounter and react to a 'non-normal' state. Perturbations may originate through both extracellular and intracellular means. Whereas transient levels of stress are expected to occur on a regular basis, a series of checks and balances ensures that cells are well equipped to maintain a homeostatic state. In the case of supra-physiological stress signaling, cellular challenges are more severe, and programmed cell death may be the best option for the organism. The ability of a cell, and by extension, an organism, to adequately manage cellular stress is fundamental--a question of life or death. The endoplasmic reticulum (ER) is exquisitely poised to sense and respond to cellular stresses including those that result from metabolic and/or protein folding imbalances. In response to stress originating from within the ER, the PERK and Ire1 protein kinases, along with other proximal signaling molecules, initiate a program of transcriptional and translational regulation termed the unfolded protein response. A consequence of ER stress is the accumulation of reactive oxygen species that promotes a state of oxidative stress. PERK signaling, via activation of the Nrf2 and ATF4 transcription factors, coordinates the convergence of ER stress with oxidative stress signaling. Here we discuss progress regarding the signaling pathways involved in these cellular stresses and the implications of the intersection between the two signaling pathways.

Regulation of the Caenorhabditis elegans oxidative stress defense protein SKN-1 by glycogen synthase kinase-3

Oxidative stress plays a central role in many human diseases and in aging. In Caenorhabditis elegans the SKN-1 protein induces phase II detoxification gene transcription, a conserved oxidative stress response, and is required for oxidative stress resistance and longevity. Oxidative stress induces SKN-1 to accumulate in intestinal nuclei, depending on p38 mitogen-activated protein kinase signaling. Here we show that, in the absence of stress, phosphorylation by glycogen synthase kinase-3 (GSK-3) prevents SKN-1 from accumulating in nuclei and functioning constitutively in the intestine. GSK-3 sites are conserved in mammalian SKN-1 orthologs, indicating that this level of regulation may be conserved. If inhibition by GSK-3 is blocked, background levels of p38 signaling are still required for SKN-1 function. WT and constitutively nuclear SKN-1 comparably rescue the skn-1 oxidative stress sensitivity, suggesting that an inducible phase II response may provide optimal stress protection. We conclude that (i) GSK-3 inhibits SKN-1 activity in the intestine, (ii) the phase II response integrates multiple regulatory signals, and (iii), by inhibiting this response, GSK-3 may influence redox conditions.

The C. elegans p38 MAPK pathway regulates nuclear localization of the transcription factor SKN-1 in oxidative stress response

The evolutionarily conserved p38 mitogen-activated protein kinase (MAPK) cascade is an integral part of the response to a variety of environmental stresses. Here we show that the Caenorhabditis elegans PMK-1 p38 MAPK pathway regulates the oxidative stress response via the CNC transcription factor SKN-1. In response to oxidative stress, PMK-1 phosphorylates SKN-1, leading to its accumulation in intestine nuclei, where SKN-1 activates transcription of gcs-1, a phase II detoxification enzyme gene. These results delineate the C. elegans p38 MAPK signaling pathway leading to the nucleus that responds to oxidative stress.