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1.  Juvenile Hormone-Receptor Complex Acts on Mcm4 and Mcm7 to Promote Polyploidy and Vitellogenesis in the Migratory Locust 
PLoS Genetics  2014;10(10):e1004702.
Juvenile hormone (JH), a sesquiterpenoid produced by the corpora allata, coordinates insect growth, metamorphosis, and reproduction. While JH action for the repression of larval metamorphosis has been well studied, the molecular basis of JH in promoting adult reproduction has not been fully elucidated. Methoprene-tolerant (Met), the JH receptor, has been recently shown to mediate JH action during metamorphosis as well as in vitellogenesis, but again, the precise mechanism underlying the latter has been lacking. We have now demonstrated using Met RNAi to phenocopy a JH-deprived condition in migratory locusts, that JH stimulates DNA replication and increases ploidy in preparation for vitellogenesis. Mcm4 and Mcm7, two genes in the DNA replication pathway were expressed in the presence of JH and Met. Depletion of Mcm4 or Mcm7 inhibited de novo DNA synthesis and polyploidization, and resulted in the substantial reduction of vitellogenin mRNA levels as well as severely impaired oocyte maturation and ovarian growth. By using luciferase reporter and electrophoretic mobility shift assays, we have shown that Met directly regulates the transcription of Mcm4 and Mcm7 by binding to upstream consensus sequences with E-box or E-box-like motifs. Our work suggests that the JH-receptor complex acts on Mcm4 and Mcm7 to regulate DNA replication and polyploidy for vitellogenesis and oocyte maturation.
Author Summary
Vitellogenesis, a hormonally-regulated process for the synthesis of yolk proteins by the fat body or liver and their sequestration in developing oocytes, takes place in all oviparous animals except mammals. Polyploidy has also been implicated in hormone-regulated development and reproduction. Although juvenile hormone (JH) is known to regulate polyploidy in insect models and also plays a pivotal role in stimulating insect vitellogenesis, the molecular mechanisms remain poorly understood. In the migratory locust, Locusta migratoria, vitellogenesis is dependent on JH, and JH stimulates DNA replication and increases ploidy in the fat body. Here, we show that a JH-receptor complex comprised of Methoprene-tolerant (Met) and a steroid receptor co-activator activates the transcription of two mini-chromosome maintenance (Mcm) genes, Mcm4 and Mcm7. Knockdown of Mcm4 or Mcm7 via RNAi can phenocopy JH-deprivation and Met-depletion, resulting in reduced ploidy, blocked vitellogenin (Vg) expression, as well as arrested oocyte maturation and ovarian growth. This study provides evidence that JH acts through its receptor on the Mcm machinery to replicate the genome of fat body cells in preparation for the massive synthesis of Vg and possibly other proteins required for oocyte maturation and egg production.
PMCID: PMC4207617  PMID: 25340846
2.  Genome-Wide Analysis of Germline Signaling Genes Regulating Longevity and Innate Immunity in the Nematode Pristionchus pacificus 
PLoS Pathogens  2012;8(8):e1002864.
Removal of the reproductive system of many animals including fish, flies, nematodes, mice and humans can increase lifespan through mechanisms largely unknown. The abrogation of the germline in Caenorhabditis elegans increases longevity by 60% due to a signal emitted from the somatic gonad. Apart from increased longevity, germline-less C. elegans is also resistant to other environmental stressors such as feeding on bacterial pathogens. However, the evolutionary conservation of this pathogen resistance, its genetic basis and an understanding of genes involved in producing this extraordinary survival phenotype are currently unknown. To study these evolutionary aspects we used the necromenic nematode Pristionchus pacificus, which is a genetic model system used in comparison to C. elegans. By ablation of germline precursor cells and subsequent feeding on the pathogen Serratia marcescens we discovered that P. pacificus shows remarkable resistance to bacterial pathogens and that this response is evolutionarily conserved across the Genus Pristionchus. To gain a mechanistic understanding of the increased resistance to bacterial pathogens and longevity in germline-ablated P. pacificus we used whole genome microarrays to profile the transcriptional response comparing germline ablated versus un-ablated animals when fed S. marcescens. We show that lipid metabolism, maintenance of the proteasome, insulin signaling and nuclear pore complexes are essential for germline deficient phenotypes with more than 3,300 genes being differentially expressed. In contrast, gene expression of germline-less P. pacificus on E. coli (longevity) and S. marcescens (immunity) is very similar with only 244 genes differentially expressed indicating that longevity is due to abundant gene expression also involved in immunity. By testing existing mutants of Ppa-DAF-16/FOXO and the nuclear hormone receptor Ppa-DAF-12 we show a conserved function of both genes in resistance to bacterial pathogens and longevity. This is the first study to show that the influence of the reproductive system on extending lifespan and innate immunity is conserved in evolution.
Author Summary
Removal of the germline in the nematode Caenorhabditis elegans can increase lifespan and resistance to bacterial pathogens. Currently there is no information on what genes are regulated to produce this resistance phenotype in other nematodes and whether they are the same as genes involved in lifespan regulation. We used the necromenic nematode, Pristionchus pacificus, a species that diverged from C. elegans 250–400 MYA, ablated its germline and found increased resistance to the pathogens Serratia marcescens and Xenorhabdus nematophila. In a novel manner we performed cell ablation of the germline, exposure to bacterial pathogens and used whole genome microarrays of the same animals to find that this resistance is due to expression of genes involved in insulin signaling, nuclear pore complexes, ribosomal translation and lipid production. Furthermore, we see little difference between germline ablated lifespan and immunity leading us to believe that living longer is due to an abundance of genes also being involved with immunity. We could also show that, similar to C. elegans, the transcription factor DAF-16/FOXO and nuclear hormone receptor DAF-12, are integral for this response. Our study is the first to understand how the reproductive system regulates both lifespan and innate immunity transcriptionally and offers insights into the signaling cascades involved with resisting pathogen attack.
PMCID: PMC3415453  PMID: 22912581
3.  The Somatic Reproductive Tissues of C. elegans Promote Longevity through Steroid Hormone Signaling 
PLoS Biology  2010;8(8):e1000468.
Removal of the germ cells of C. elegans extends lifespan in part because signals from the somatic reproductive tissues activate the nuclear hormone receptor DAF-12.
In Caenorhabditis elegans and Drosophila melanogaster, removing the germline precursor cells increases lifespan. In worms, and possibly also in flies, this lifespan extension requires the presence of somatic reproductive tissues. How the somatic gonad signals other tissues to increase lifespan is not known. The lifespan increase triggered by loss of the germ cells is known to require sterol hormone signaling, as reducing the activity of the nuclear hormone receptor DAF-12, or genes required for synthesis of the DAF-12 ligand dafachronic acid, prevents germline loss from extending lifespan. In addition to sterol signaling, the FOXO transcription factor DAF-16 is required to extend lifespan in animals that lack germ cells. DAF-12/NHR is known to assist with the nuclear accumulation of DAF-16/FOXO in these animals, yet we find that loss of DAF-12/NHR has little or no effect on the expression of at least some DAF-16/FOXO target genes. In this study, we show that the DAF-12-sterol signaling pathway has a second function to activate a distinct set of genes and extend lifespan in response to the somatic reproductive tissues. When germline-deficient animals lacking somatic reproductive tissues are given dafachronic acid, their expression of DAF-12/NHR-dependent target genes is restored and their lifespan is increased. Together, our findings indicate that in C. elegans lacking germ cells, the somatic reproductive tissues promote longevity via steroid hormone signaling to DAF-12.
Author Summary
Reproductive tissues are known to generate important intercellular signals. For example, in mammals, the reproductive tissues produce steroid hormones such as estrogen and testosterone that have profound effects on development and physiology. Studies of the nematode C. elegans and other organisms have shown that the reproductive system can also affect the rate at which an animal ages. Removal of C. elegans' germ cells extends lifespan but this effect is not simply due to sterility, as removal of both the somatic reproductive tissues and the germ cells does not extend lifespan. Instead, loss of the germ cells extends lifespan by activating a pathway that requires input from the somatic gonad. In this study, we demonstrate that the somatic reproductive tissues promote longevity by controlling the activity of a steroid signaling pathway that regulates the DAF-12 nuclear hormone receptor.
PMCID: PMC2930862  PMID: 20824162
4.  A Transcription Elongation Factor That Links Signals from the Reproductive System to Lifespan Extension in Caenorhabditis elegans 
PLoS Genetics  2009;5(9):e1000639.
In Caenorhabditis elegans and Drosophila melanogaster, the aging of the soma is influenced by the germline. When germline-stem cells are removed, aging slows and lifespan is increased. The mechanism by which somatic tissues respond to loss of the germline is not well-understood. Surprisingly, we have found that a predicted transcription elongation factor, TCER-1, plays a key role in this process. TCER-1 is required for loss of the germ cells to increase C. elegans' lifespan, and it acts as a regulatory switch in the pathway. When the germ cells are removed, the levels of TCER-1 rise in somatic tissues. This increase is sufficient to trigger key downstream events, as overexpression of tcer-1 extends the lifespan of normal animals that have an intact reproductive system. Our findings suggest that TCER-1 extends lifespan by promoting the expression of a set of genes regulated by the conserved, life-extending transcription factor DAF-16/FOXO. Interestingly, TCER-1 is not required for DAF-16/FOXO to extend lifespan in animals with reduced insulin/IGF-1 signaling. Thus, TCER-1 specifically links the activity of a broadly deployed transcription factor, DAF-16/FOXO, to longevity signals from reproductive tissues.
Author Summary
The reproductive status and longevity of animals are strongly interlinked. Increasing age influences the reproductive capacities of most animals. However, little is known about how reproductive status might affect lifespan. Experiments in worms and flies have shown that removing cells that give rise to gametes, the “germ cells”, makes them live longer. We know very little about the genes and molecules that are involved in this process. In this study, we have identified a gene called tcer-1 that promotes the longevity of the roundworm Caenorhabditis elegans when its germ cells are removed. The gene tcer-1 codes for a protein, TCER-1, that is predicted to function as a “transcription elongation factor” (it allows the completion of RNA synthesis during the process of gene expression). Our experiments imply that when the germ cells of worms are removed, TCER-1 collaborates with a transcription factor called DAF-16/FOXO to express genes that contribute to increased longevity. DAF-16/FOXO can extend lifespan in response to other physiological cues besides loss of germ cells. However, TCER-1 specifically helps this widely used longevity protein to respond to signals that reflect the reproductive status. Counterparts of DAF-16/FOXO are known to control aging in other organisms, including humans, so the identification of TCER-1 may lead to a better understanding of the relationship between reproduction and aging in other species, too.
PMCID: PMC2729384  PMID: 19749979
5.  Revealing genes associated with vitellogenesis in the liver of the zebrafish (Danio rerio) by transcriptome profiling 
BMC Genomics  2009;10:141.
In oviparous vertebrates, including fish, vitellogenesis consists of highly regulated pathways involving 17β-estradiol (E2). Previous studies focused on a relatively small number of hepatic expressed genes during vitellogenesis. This study aims to identify hepatic genes involved in vitellogenesis and regulated by E2, by using zebrafish microarray gene expression profiling, and to provide information on functional distinctive genes expressed in the liver of a vitellogenic female, using zebrafish as a model fish.
Genes associated with vitellogenesis were revealed by the following paired t-tests (SAM) comparisons: a) two-month old vitellogenic (Vit2) females were compared with non-vitellogenic (NV) females, showing 825 differentially expressed transcripts during early stages of vitellogenesis, b) four-month old vitellogenic (Vit4) females were compared with NV females, showing 1,046 differentially expressed transcripts during vitellogenesis and c) E2-treated males were compared with control males, showing 1,828 differentially expressed transcripts regulated by E2. A Venn diagram revealed 822 common transcripts in the three groups, indicating that these transcripts were involved in vitellogenesis and putatively regulated by E2. In addition, 431 transcripts were differentially expressed in Vit2 and Vit4 females but not in E2-treated males, indicating that they were putatively not up-regulated by E2. Correspondence analysis showed high similarity in expression profiles of Vit2 with Vit4 and of NV females with control males. The E2-treated males differed from the other groups. The repertoire of genes putatively regulated by E2 in vitellogenic females included genes associated with protein synthesis and reproduction. Genes associated with the immune system processes and biological adhesion, were among the genes that were putatively not regulated by E2. E2-treated males expressed a large array of transcripts that were not associated with vitellogenesis.
The study revealed several genes that were not reported before as being regulated by E2. Also, the hepatic expression of several genes was reported here for the first time.
Gene expression profiling of liver samples revealed 1,046 differentially expressed transcripts during vitellogenesis of which at least ~64% were regulated by E2. The results raise the question on the regulation pattern and temporal pleiotropic expression of hepatic genes in vitellogenic females.
PMCID: PMC2678157  PMID: 19335895
6.  daf-31 Encodes the Catalytic Subunit of N Alpha-Acetyltransferase that Regulates Caenorhabditis elegans Development, Metabolism and Adult Lifespan 
PLoS Genetics  2014;10(10):e1004699.
The Caenorhabditis elegans dauer larva is a facultative state of diapause. Mutations affecting dauer signal transduction and morphogenesis have been reported. Of these, most that result in constitutive formation of dauer larvae are temperature-sensitive (ts). The daf-31 mutant was isolated in genetic screens looking for novel and underrepresented classes of mutants that form dauer and dauer-like larvae non-conditionally. Dauer-like larvae are arrested in development and have some, but not all, of the normal dauer characteristics. We show here that daf-31 mutants form dauer-like larvae under starvation conditions but are sensitive to SDS treatment. Moreover, metabolism is shifted to fat accumulation in daf-31 mutants. We cloned the daf-31 gene and it encodes an ortholog of the arrest-defective-1 protein (ARD1) that is the catalytic subunit of the major N alpha-acetyltransferase (NatA). A daf-31 promoter::GFP reporter gene indicates daf-31 is expressed in multiple tissues including neurons, pharynx, intestine and hypodermal cells. Interestingly, overexpression of daf-31 enhances the longevity phenotype of daf-2 mutants, which is dependent on the forkhead transcription factor (FOXO) DAF-16. We demonstrate that overexpression of daf-31 stimulates the transcriptional activity of DAF-16 without influencing its subcellular localization. These data reveal an essential role of NatA in controlling C. elegans life history and also a novel interaction between ARD1 and FOXO transcription factors, which may contribute to understanding the function of ARD1 in mammals.
Author Summary
The development of a living organism is influenced by the environmental conditions such as nutrient availability. Under starvation conditions, the C. elegans larvae will enter a special developmental stage called dauer larva. An insulin-like signaling pathway controls dauer formation as well as adult lifespan by inhibiting the activity of FOXO transcription factor DAF-16 that regulates expression of stress-resistant genes. Here we isolate a new gene called daf-31; this gene encodes a protein that regulates C. elegans larval development, metabolism and adult lifespan. This protein has been found in other species to be part of an enzyme that functions to modify other proteins. We show that overexpression of our newly discovered protein stimulates the transcriptional activity of DAF-16. Interestingly, abnormal regulation of human proteins similar to DAF-31 results in tumor formation. It is known that human FOXO proteins prevent tumorigenesis. Therefore, it is possible that abnormal DAF-31 activity may lead to tumor growth by reducing DAF-16 activity. Thus, the present study may not only contribute to understanding the role of a universal enzyme in controlling development, metabolism and lifespan in other organisms besides worms but may also shed light on the mechanisms of tumorigenesis in humans.
PMCID: PMC4199510  PMID: 25330189
7.  Fatty Acid Desaturation Links Germ Cell Loss to Longevity Through NHR-80/HNF4 in C. elegans 
PLoS Biology  2011;9(3):e1000599.
Lifespan extension induced by germline ablation in C. elegans is regulated by the nuclear hormone receptor NHR-80 in a process that requires the production of oleic acid by activation of the lipid desaturase FAT-6/SCD1.
Preventing germline stem cell proliferation extends lifespan in nematodes and flies. So far, studies on germline-longevity signaling have focused on daf-16/FOXO and daf-12/VDR. Here, we report on NHR-80/HNF4, a nuclear receptor that specifically mediates longevity induced by depletion of the germ line through a mechanism that implicates fatty acid monodesaturation.
Methods and Findings
nhr-80/HNF4 is induced in animals lacking a germ line and is specifically required for their extended longevity. Overexpressing nhr-80/HNF4 increases the lifespan of germline-less animals. This lifespan extension can occur in the absence of daf-16/FOXO but requires the presence of the nuclear receptor DAF-12/VDR. We show that the fatty acid desaturase, FAT-6/SCD1, is a key target of NHR-80/HNF4 and promotes germline-longevity by desaturating stearic acid to oleic acid (OA). We find that NHR-80/HNF4 and OA must work in concert to promote longevity.
Taken together, our data indicate that the NHR-80 pathway participates in the mechanism of longevity extension through depletion of the germ line. We identify fat-6 and OA as essential downstream elements although other targets must also be present. Thus, NHR-80 links fatty acid desaturation to lifespan extension through germline ablation in a daf-16/FOXO independent manner.
Author Summary
Reproduction and aging are two processes that seem to be closely intertwined. Experiments in Caenorhabditis elegans and Drosophila have shown that depletion of the germ line increases lifespan and that this process depends on insulin and lipophilic-hormone signaling. Recently, it was demonstrated that when germline stem cells (GSCs) cease to proliferate, fat metabolism is altered and this affects longevity. In this study, we have identified a nuclear hormone receptor, NHR-80, that mediates longevity through depletion of the germ line by promoting fatty acid desaturation. The nhr-80 gene is up-regulated at the mRNA and protein levels in germline-less animals, leading to the transcription of the gene, fat-6, and the production of oleic acid (OA). Our experiments also show that the NHR-80/FAT-6/OA pathway does not require the presence of DAF-16 but instead, depends fully on the presence of DAF-12, a steroid receptor that affects lifespan. We provide evidence that other NHR-80 targets must be present concomitantly. Our results reinforce the notion that fat metabolism is profoundly altered in response to GSC proliferation, and the data contribute to a better understanding of the molecular relationship between reproduction, fat metabolism, and aging.
PMCID: PMC3057950  PMID: 21423649
8.  DAF-16 and TCER-1 Facilitate Adaptation to Germline Loss by Restoring Lipid Homeostasis and Repressing Reproductive Physiology in C. elegans 
PLoS Genetics  2016;12(2):e1005788.
Elimination of the proliferating germline extends lifespan in C. elegans. This phenomenon provides a unique platform to understand how complex metazoans retain metabolic homeostasis when challenged with major physiological perturbations. Here, we demonstrate that two conserved transcription regulators essential for the longevity of germline-less adults, DAF-16/FOXO3A and TCER-1/TCERG1, concurrently enhance the expression of multiple genes involved in lipid synthesis and breakdown, and that both gene classes promote longevity. Lipidomic analyses revealed that key lipogenic processes, including de novo fatty acid synthesis, triglyceride production, desaturation and elongation, are augmented upon germline removal. Our data suggest that lipid anabolic and catabolic pathways are coordinately augmented in response to germline loss, and this metabolic shift helps preserve lipid homeostasis. DAF-16 and TCER-1 also perform essential inhibitory functions in germline-ablated animals. TCER-1 inhibits the somatic gene-expression program that facilitates reproduction and represses anti-longevity genes, whereas DAF-16 impedes ribosome biogenesis. Additionally, we discovered that TCER-1 is critical for optimal fertility in normal adults, suggesting that the protein acts as a switch supporting reproductive fitness or longevity depending on the presence or absence of the germline. Collectively, our data offer insights into how organisms adapt to changes in reproductive status, by utilizing the activating and repressive functions of transcription factors and coordinating fat production and degradation.
Author Summary
The balance between production and breakdown of fats is critical for health, especially during reproduction-related changes such as onset of puberty or menopause. However, little is known about how animals retain a balanced metabolism when undergoing major life events. Here, we have used a C. elegans mutant that successfully adapts to loss of reproductive cells to address this question. Our data suggest that the conserved proteins DAF-16/FOXO3A and TCER-1/TCERG1 mediate a coordinated increase in fat synthesis and degradation when the reproductive cells are lost. This coupling likely helps the animal to manage the lipids that would have been deposited in eggs as yolk, thus preventing metabolic disarray. These proteins also inhibit processes that would have normally supported reproduction. Together the activities of these transcription regulators allow the mutant to convert a debilitating loss of fertility into improved health and longevity. We also report that TCER-1 promotes reproductive health in normal adults, whereas when procreation is impeded, it switches roles to repress fertility and enhance lipid equilibrium. These observations offer insights into how complex organisms coordinate their metabolism to suit their reproductive needs.
PMCID: PMC4749232  PMID: 26862916
9.  Co-chaperone p23 Regulates C. elegans Lifespan in Response to Temperature 
PLoS Genetics  2015;11(4):e1005023.
Temperature potently modulates various physiologic processes including organismal motility, growth rate, reproduction, and ageing. In ectotherms, longevity varies inversely with temperature, with animals living shorter at higher temperatures. Thermal effects on lifespan and other processes are ascribed to passive changes in metabolic rate, but recent evidence also suggests a regulated process. Here, we demonstrate that in response to temperature, daf-41/ZC395.10, the C. elegans homolog of p23 co-chaperone/prostaglandin E synthase-3, governs entry into the long-lived dauer diapause and regulates adult lifespan. daf-41 deletion triggers constitutive entry into the dauer diapause at elevated temperature dependent on neurosensory machinery (daf-10/IFT122), insulin/IGF-1 signaling (daf-16/FOXO), and steroidal signaling (daf-12/FXR). Surprisingly, daf-41 mutation alters the longevity response to temperature, living longer than wild-type at 25°C but shorter than wild-type at 15°C. Longevity phenotypes at 25°C work through daf-16/FOXO and heat shock factor hsf-1, while short lived phenotypes converge on daf-16/FOXO and depend on the daf-12/FXR steroid receptor. Correlatively daf-41 affected expression of DAF-16 and HSF-1 target genes at high temperature, and nuclear extracts from daf-41 animals showed increased occupancy of the heat shock response element. Our studies suggest that daf-41/p23 modulates key transcriptional changes in longevity pathways in response to temperature.
Author Summary
Temperature is a critical environmental factor that affects ageing in both cold-blooded and warm-blooded species. In invertebrate animals, lifespan varies inversely with temperature, with higher temperature resulting in faster development but shorter lifespan. This phenomenon has been usually attributed to passive changes in metabolic rate, but recent work suggests that this process is regulated. In this study, we identify the co-chaperone protein p23 in the nematode C. elegans as an important modulator of longevity in response to temperature. Co-chaperones bind to client proteins to assist in their folding or stabilize their shape, thereby regulating their activity. Remarkably, deletion of p23 results in animals that are long lived at high temperatures and short lived at low temperatures relative to normal wild type animals. Our experiments indicate that p23 regulates lifespan through the neurosensory apparatus. These in turn impinge on key longevity regulators that mediate the transcriptional outputs of insulin/IGF, heat shock response and steroidal signaling. These studies suggest that complexes formed by p23 play a central role in regulating longevity in response to temperature.
PMCID: PMC4382338  PMID: 25830239
10.  Insulin/IGF-1-mediated longevity is marked by reduced protein metabolism 
Quantitative proteomics, lifespan analysis, and biochemical assays were utilized to show that Insulin/IGF-1-mediated longevity in C. elegans is strongly associated with a daf-16 dependent global reduction in protein metabolism.
A daf-16 dependent global reduction in protein translation is observed in daf-2 long-lived mutant.The reduction in active translation is independent of germline activityA role for protein metabolism is identified in the Insulin/IGF-1-mediated extension of life.
Mutations in the daf-2 gene of the conserved Insulin/Insulin-like Growth Factor (IGF-1) pathway double the lifespan of the nematode Caenorhabditis elegans. This phenotype is completely suppressed by deletion of Forkhead transcription factor daf-16. To uncover regulatory mechanisms coordinating this extension of life, we employed a quantitative proteomics strategy with daf-2 mutants in comparison with N2 and daf-16; daf-2 double mutants. This revealed a remarkable longevity-specific decrease in proteins involved in mRNA processing and transport, the translational machinery, and protein metabolism. Correspondingly, the daf-2 mutants display lower amounts of mRNA and 20S proteasome activity, despite maintaining total protein levels equal to that observed in wild types. Polyribosome profiling in the daf-2 and daf-16;daf-2 double mutants confirmed a daf-16-dependent reduction in overall translation, a phenotype reminiscent of Dietary Restriction-mediated longevity, which was independent of germline activity. RNA interference (RNAi)-mediated knockdown of proteins identified by our approach resulted in modified C. elegans lifespan confirming the importance of these processes in Insulin/IGF-1-mediated longevity. Together, the results demonstrate a role for the metabolism of proteins in the Insulin/IGF-1-mediated extension of life.
PMCID: PMC3734508  PMID: 23820781
ageing; high-throughput analysis; metabolism; protein metabolism; translation
11.  Non-autonomous DAF-16/FOXO activity antagonizes age-related loss of C. elegans germline stem/progenitor cells 
Nature Communications  2015;6:7107.
Stem cells maintain tissues and organs over the lifespan of individuals. How aging influences this process is unclear. Here we investigate the effects of aging on C. elegans germline stem/progenitor cells and show that the progenitor pool is depleted over time in a manner dependent on inhibition of DAF-16/FOXO by insulin/IGF-1 signalling (IIS). Our data indicate that DAF-16/FOXO activity in certain somatic gonad cells is required for germline progenitor maintenance, and that this role is separable from the effect of DAF-16/FOXO on organismal aging. In addition, blocking germ cell flux, similar to reducing IIS, maintains germline progenitors. This effect is partially dependent on gonadal DAF-16/FOXO activity. Our results imply that (1) longevity pathways can regulate aging stem cells through anatomically separable mechanisms, (2) stem cell maintenance is not necessarily prioritized and (3) stem cell regulation can occur at the level of an entire organ system such as the reproductive system.
The number of germline stem/progenitor cells in C. elegans declines with age. Here the authors show this cell loss is mediated by the transcription factor DAF-16/FOXO acting in specific somatic gonad cells, demonstrating that stem cell aging can be anatomically uncoupled from organismal aging.
PMCID: PMC4432587  PMID: 25960195
12.  CAMKII and Calcineurin regulate the lifespan of Caenorhabditis elegans through the FOXO transcription factor DAF-16 
eLife  2013;2:e00518.
The insulin-like signaling pathway maintains a relatively short wild-type lifespan in Caenorhabditis elegans by phosphorylating and inactivating DAF-16, the ortholog of the FOXO transcription factors of mammalian cells. DAF-16 is phosphorylated by the AKT kinases, preventing its nuclear translocation. Calcineurin (PP2B phosphatase) also limits the lifespan of C. elegans, but the mechanism through which it does so is unknown. Herein, we show that TAX-6•CNB-1 and UNC-43, the C. elegans Calcineurin and Ca2+/calmodulin-dependent kinase type II (CAMKII) orthologs, respectively, also regulate lifespan through DAF-16. Moreover, UNC-43 regulates DAF-16 in response to various stress conditions, including starvation, heat or oxidative stress, and cooperatively contributes to lifespan regulation by insulin signaling. However, unlike insulin signaling, UNC-43 phosphorylates and activates DAF-16, thus promoting its nuclear localization. The phosphorylation of DAF-16 at S286 by UNC-43 is removed by TAX-6•CNB-1, leading to DAF-16 inactivation. Mammalian FOXO3 is also regulated by CAMKIIA and Calcineurin.
eLife digest
Although aging might seem to be a passive process—resulting simply from wear and tear over a lifetime—it can actually be accelerated or slowed down by genetic mutations. This phenomenon has been most thoroughly studied in the nematode worm, Caenorhabditis elegans. Normally, this worm lives for just two or three weeks, but genetic mutations that reduce the activity of certain enzymes in a series of biochemical reactions known as the insulin/IGF-1 signalling pathway can extend its lifespan by up to a factor of ten, and similar effects have been seen in flies and mice. Lifespans can also be increased by blocking other signalling pathways or restricting the intake of calories.
This increase in lifespan associated with the insulin/IGF-1 signalling pathway is known to involve a protein called DAF-16 and two kinases called AKT-1 and AKT-2. Under normal conditions the AKT kinases add several phosphate groups to the DAF-16, which prevents it from travelling to the nucleus of the cell. However, when genetic techniques are used to block the insulin/IGF-1 signalling pathway, the AKT kinases are unable to add the phosphate groups; this leaves the DAF-16 free to enter the nucleus, where it activates a network of genes that promotes longevity.
In addition to kinases, the insulin/IGF-1 signalling pathway also involves enzymes called phosphatases that remove the phosphate groups from other proteins. In particular, a phosphatase called calcineurin is known to be involved in the regulation of lifespan, but the details of this process are not fully understood.
Now, Tao et al. have carried out a series of genetic and biochemical experiments to determine how phosphatases exert their influence on aging. The results show that calcineurin targets DAF-16, the same protein that is targeted by the AKT kinases. Moreover, another kinase also targets DAF-16 when the worm is exposed to heat, starvation or some other form of stress: this kinase, which is not involved in the insulin/IGF-1 signalling pathway, is called CAMKII.
Tao et al. show that these kinases act on DAF-16 in different ways: CAMKII activates it by adding the phosphate group at a specific site known as S286, whereas the AKT kinases deactivate DAF-16 because they add phosphate groups at different sites, thereby preventing it from entering the nucleus. Calcineurin neutralizes the effect of CAMKII by removing the phosphate group at S286 to deactivate the DAF-16.
In addition to shedding new light on the regulation of lifespan in C. elegans, the new results could improve our understanding of aging in humans, and also the development of diabetes and other age-related diseases, because the equivalent molecules in mammalian cells are regulated in similar ways.
PMCID: PMC3691573  PMID: 23805378
aging; lifespan; FOXO; CAMKII; calcineurin; DAF-16; C. elegans
13.  DAF-2/Insulin-Like Signaling in C. elegans Modifies Effects of Dietary Restriction and Nutrient Stress on Aging, Stress and Growth 
PLoS ONE  2007;2(11):e1240.
Dietary restriction (DR) and reduced insulin/IGF-I-like signaling (IIS) are two regimens that promote longevity in a variety of organisms. Genetic analysis in C. elegans nematodes has shown that DR and IIS couple to distinct cellular signaling pathways. However, it is not known whether these pathways ultimately converge on overlapping or distinct targets to extend lifespan.
Principal Findings
We investigated this question by examining additional effects of DR in wildtype animals and in daf-2 mutants with either moderate or severe IIS deficits. Surprisingly, DR and IIS had opposing effects on these physiological processes. First, DR induced a stress-related change in intestinal vesicle trafficking, termed the FIRE response, which was suppressed in daf-2 mutants. Second, DR did not strongly affect expression of a daf-2- and stress-responsive transcriptional reporter. Finally, DR-related growth impairment was suppressed in daf-2 mutants.
These findings reveal that an important biological function of DAF-2/IIS is to enhance growth and survival under nutrient-limited conditions. However, we also discovered that levels of DAF-2 pathway activity modified the effects of DR on longevity. Thus, while DR and IIS clearly affect lifespan through independent targets, there may also be some prolongevity targets that are convergently regulated by these pathways.
PMCID: PMC2080776  PMID: 18043747
14.  Cell-Nonautonomous Signaling of FOXO/DAF-16 to the Stem Cells of Caenorhabditis elegans 
PLoS Genetics  2012;8(8):e1002836.
In Caenorhabditis elegans (C. elegans), the promotion of longevity by the transcription factor DAF-16 requires reduced insulin/IGF receptor (IIR) signaling or the ablation of the germline, although the reason for the negative impact of germ cells is unknown. FOXO/DAF-16 activity inhibits germline proliferation in both daf-2 mutants and gld-1 tumors. In contrast to its function as a germline tumor suppressor, we now provide evidence that somatic DAF-16 in the presence of IIR signaling can also result in tumorigenic activity, which counteracts robust lifespan extension. In contrast to the cell-autonomous IIR signaling, which is required for larval germline proliferation, activation of DAF-16 in the hypodermis results in hyperplasia of the germline and disruption of the surrounding basement membrane. SHC-1 adaptor protein and AKT-1 kinase antagonize, whereas AKT-2 and SGK-1 kinases promote, this cell-nonautonomous DAF-16 function. Our data suggest that a functional balance of DAF-16 activities in different tissues determines longevity and reveals a novel, cell-nonautonomous role of FOXO/DAF-16 to affect stem cells.
Author Summary
Previous studies have shown that DAF–16/FOXO transcription factor promotes longevity and stress resistance and inhibits tumor progression in the absence of insulin signaling. Here we show that active DAF-16 in the epidermis can shorten lifespan by promoting a tumorous germline phenotype. In contrast to the known inhibitory effect of insulin signaling upon DAF-16, an active insulin and PI3K signaling are required for DAF-16–mediated signaling to the germline. In addition, AKT-1– and SHC-1–mediated JNK signaling antagonize AKT-2 and SGK-1 to affect the reproductive system. This is to our knowledge the first report about a detrimental effect of DAF-16 on lifespan. Furthermore it emphasizes that DAF-16 activity is highly dependent on the cellular context and communication between different tissues.
PMCID: PMC3420913  PMID: 22916022
15.  Positive Feedback between Transcriptional and Kinase Suppression in Nematodes with Extraordinary Longevity and Stress Resistance 
PLoS Genetics  2009;5(4):e1000452.
Insulin/IGF-1 signaling (IIS) regulates development and metabolism, and modulates aging, of Caenorhabditis elegans. In nematodes, as in mammals, IIS is understood to operate through a kinase-phosphorylation cascade that inactivates the DAF-16/FOXO transcription factor. Situated at the center of this pathway, phosphatidylinositol 3-kinase (PI3K) phosphorylates PIP2 to form PIP3, a phospholipid required for membrane tethering and activation of many signaling molecules. Nonsense mutants of age-1, the nematode gene encoding the class-I catalytic subunit of PI3K, produce only a truncated protein lacking the kinase domain, and yet confer 10-fold greater longevity on second-generation (F2) homozygotes, and comparable gains in stress resistance. Their F1 parents, like weaker age-1 mutants, are far less robust—implying that maternally contributed trace amounts of PI3K activity or of PIP3 block the extreme age-1 phenotypes. We find that F2-mutant adults have <10% of wild-type kinase activity in vitro and <60% of normal phosphoprotein levels in vivo. Inactivation of PI3K not only disrupts PIP3-dependent kinase signaling, but surprisingly also attenuates transcripts of numerous IIS components, even upstream of PI3K, and those of signaling molecules that cross-talk with IIS. The age-1(mg44) nonsense mutation results, in F2 adults, in changes to kinase profiles and to expression levels of multiple transcripts that distinguish this mutant from F1 age-1 homozygotes, a weaker age-1 mutant, or wild-type adults. Most but not all of those changes are reversed by a second mutation to daf-16, implicating both DAF-16/ FOXO–dependent and –independent mechanisms. RNAi, silencing genes that are downregulated in long-lived worms, improves oxidative-stress resistance of wild-type adults. It is therefore plausible that attenuation of those genes in age-1(mg44)-F2 adults contributes to their exceptional survival. IIS in nematodes (and presumably in other species) thus involves transcriptional as well as kinase regulation in a positive-feedback circuit, favoring either survival or reproduction. Hyperlongevity of strong age-1(mg44) mutants may result from their inability to reset this molecular switch to the reproductive mode.
Author Summary
Insulin/IGF-1 signaling (IIS) impacts development, metabolism, and longevity in Caenorhabditis elegans. It has been viewed as a cascade of kinase reactions, chiefly phosphorylation of other kinases, leading to inactivation of the DAF-16/FOXO transcription factor. PI3K, a phosphatidylinositol kinase at the center of this pathway, converts PIP2 to PIP3, instrumental to kinase docking and activation. Here we show that PI3K deficiency elicits transcriptional inhibition of many kinases, including those of IIS itself. This creates a positive-feedback loop, wherein DAF-16/FOXO silences expression of the very kinases that would have inactivated it. In the resulting “flip-flop” genetic switch, either kinase signaling or transcriptional silencing may predominate. We discovered the transcriptional arm of this switch in infertile age-1(mg44) mutants, defective for PI3K activity. The absence of PIP3 and PIP3-dependent kinase activity gives free rein to gene silencing by DAF-16/FOXO. This two-tiered response could scarcely have evolved for the benefit of a sterile mutant; some components presumably serve regulatory functions in normal animals, reinforcing a switch responsive to environmental and internal signals. In age-1(mg44) mutants, complete inactivation of PI3K “fuses” the switch, locking worms into longevity mode. With signaling profoundly silenced, they cannot resume reproduction, but instead acquire a remarkable capacity for individual survival.
PMCID: PMC2661368  PMID: 19360094
16.  DAF-16/FoxO Directly Regulates an Atypical AMP-Activated Protein Kinase Gamma Isoform to Mediate the Effects of Insulin/IGF-1 Signaling on Aging in Caenorhabditis elegans  
PLoS Genetics  2014;10(2):e1004109.
The DAF-16/FoxO transcription factor controls growth, metabolism and aging in Caenorhabditis elegans. The large number of genes that it regulates has been an obstacle to understanding its function. However, recent analysis of transcript and chromatin profiling implies that DAF-16 regulates relatively few genes directly, and that many of these encode other regulatory proteins. We have investigated the regulation by DAF-16 of genes encoding the AMP-activated protein kinase (AMPK), which has α, β and γ subunits. C. elegans has 5 genes encoding putative AMP-binding regulatory γ subunits, aakg-1-5. aakg-4 and aakg-5 are closely related, atypical isoforms, with orthologs throughout the Chromadorea class of nematodes. We report that ∼75% of total γ subunit mRNA encodes these 2 divergent isoforms, which lack consensus AMP-binding residues, suggesting AMP-independent kinase activity. DAF-16 directly activates expression of aakg-4, reduction of which suppresses longevity in daf-2 insulin/IGF-1 receptor mutants. This implies that an increase in the activity of AMPK containing the AAKG-4 γ subunit caused by direct activation by DAF-16 slows aging in daf-2 mutants. Knock down of aakg-4 expression caused a transient decrease in activation of expression in multiple DAF-16 target genes. This, taken together with previous evidence that AMPK promotes DAF-16 activity, implies the action of these two metabolic regulators in a positive feedback loop that accelerates the induction of DAF-16 target gene expression. The AMPK β subunit, aakb-1, also proved to be up-regulated by DAF-16, but had no effect on lifespan. These findings reveal key features of the architecture of the gene-regulatory network centered on DAF-16, and raise the possibility that activation of AMP-independent AMPK in nutritionally replete daf-2 mutant adults slows aging in C. elegans. Evidence of activation of AMPK subunits in mammals suggests that such FoxO-AMPK interactions may be evolutionarily conserved.
Author Summary
Aging is an important problem for human health and is regulated by complex gene regulatory networks. In a simple nematode worm (Caenorhabditis elegans) mutation of the insulin/IGF-1 receptor daf-2 dramatically extends lifespan. This is due to the increased activity of DAF-16, a FoxO transcription factor, leading to altered expression of genes, many encoding other regulatory proteins. We have focused on one such protein, AMP-activated protein kinase (AMPK), that is important for regulating cellular homeostasis under conditions of low energy availability (e.g. starvation). We find that DAF-16 binds to the promoter of aakg-4 (a gene encoding an atypical γ subunit of AMPK) and increases its expression. Inhibition of aakg-4 leads to down-regulation of multiple DAF-16 target genes and shortens the life of daf-2 mutants. Taken together with a previous report showing that AMPK activates DAF-16, this suggests that AAKG-4 and DAF-16 are involved in a positive feedback loop which accelerates effects of DAF-16 on gene expression, and could contribute to longevity. This study defines a new part of the complex gene regulatory network in which DAF-16 acts to control aging. FoxO-AMPK interactions are present in higher animals, where they could potentially also influence aging.
PMCID: PMC3916255  PMID: 24516399
17.  Long-term starvation and ageing induce AGE-1/PI 3-kinase-dependent translocation of DAF-16/FOXO to the cytoplasm 
BMC Biology  2006;4:1.
The provision of stress resistance diverts resources from development and reproduction and must therefore be tightly regulated. In Caenorhabditis elegans, the switch to increased stress resistance to promote survival through periods of starvation is regulated by the DAF-16/FOXO transcription factor. Reduction-of-function mutations in AGE-1, the C. elegans Class IA phosphoinositide 3-kinase (PI3K), increase lifespan and stress resistance in a daf-16 dependent manner. Class IA PI3Ks downregulate FOXOs by inducing their translocation to the cytoplasm. However, the circumstances under which AGE-1 is normally activated are unclear. To address this question we used C. elegans first stage larvae (L1s), which when starved enter a developmentally-arrested diapause stage until food is encountered.
We find that in L1s both starvation and daf-16 are necessary to confer resistance to oxidative stress in the form of hydrogen peroxide. Accordingly, DAF-16 is localised to cell nuclei after short-term starvation. However, after long-term starvation, DAF-16 unexpectedly translocates to the cytoplasm. This translocation requires functional age-1. H2O2 treatment can replicate the translocation and induce generation of the AGE-1 product PIP3. Because feeding reduces to zero in ageing adult C. elegans, these animals may also undergo long-term starvation. Consistent with our observation in L1s, DAF-16 also translocates to the cytoplasm in old adult worms in an age-1-dependent manner.
DAF-16 is activated in the starved L1 diapause. The translocation of DAF-16 to the cytoplasm after long-term starvation may be a feedback mechanism that prevents excessive expenditure on stress resistance. H2O2 is a candidate second messenger in this feedback mechanism. The lack of this response in age-1(hx546) mutants suggests a novel mechanism by which this mutation increases longevity.
PMCID: PMC1403811  PMID: 16457721
18.  p38 MAPK Regulates Expression of Immune Response Genes and Contributes to Longevity in C. elegans 
PLoS Genetics  2006;2(11):e183.
The PMK-1 p38 mitogen-activated protein kinase pathway and the DAF-2–DAF-16 insulin signaling pathway control Caenorhabditis elegans intestinal innate immunity. pmk-1 loss-of-function mutants have enhanced sensitivity to pathogens, while daf-2 loss-of-function mutants have enhanced resistance to pathogens that requires upregulation of the DAF-16 transcription factor. We used genetic analysis to show that the pathogen resistance of daf-2 mutants also requires PMK-1. However, genome-wide microarray analysis indicated that there was essentially no overlap between genes positively regulated by PMK-1 and DAF-16, suggesting that they form parallel pathways to promote immunity. We found that PMK-1 controls expression of candidate secreted antimicrobials, including C-type lectins, ShK toxins, and CUB-like genes. Microarray analysis demonstrated that 25% of PMK-1 positively regulated genes are induced by Pseudomonas aeruginosa infection. Using quantitative PCR, we showed that PMK-1 regulates both basal and infection-induced expression of pathogen response genes, while DAF-16 does not. Finally, we used genetic analysis to show that PMK-1 contributes to the enhanced longevity of daf-2 mutants. We propose that the PMK-1 pathway is a specific, indispensable immunity pathway that mediates expression of secreted immune response genes, while the DAF-2–DAF-16 pathway appears to regulate immunity as part of a more general stress response. The contribution of the PMK-1 pathway to the enhanced lifespan of daf-2 mutants suggests that innate immunity is an important determinant of longevity.
The innate immune system provides the first line of defense against pathogen infection and relies upon pathways conserved across mammals, insects, and nematodes. Here, the authors have analyzed the transcriptional response of the nematode Caenorhabditis elegans to infection by the human pathogen Pseudomonas aeruginosa. They investigated this transcriptional response in the context of two conserved pathways involved in pathogen defense: the PMK-1 p38 mitogen-activated protein kinase (p38 MAPK) pathway and the DAF-2–DAF-16 insulin-signaling pathway. Specifically, the authors found that the p38 MAPK pathway plays a critical role in the infection-induced expression of secreted immune response genes. These genes include C-type lectins, lysozymes, and antimicrobial peptides that fight off infection in many species. In contrast, they found that the DAF-16 pathway is not required for immune response gene expression and may regulate immunity as part of a general stress response that functions in parallel to p38 MAPK. In addition, the authors observed that p38 MAPK contributes to the enhanced longevity of daf-2 mutants, implicating p38 MAPK signaling in the regulation of longevity, possibly through its role in immunity.
PMCID: PMC1635533  PMID: 17096597
19.  Genes That Act Downstream of Sensory Neurons to Influence Longevity, Dauer Formation, and Pathogen Responses in Caenorhabditis elegans 
PLoS Genetics  2012;8(12):e1003133.
The sensory systems of multicellular organisms are designed to provide information about the environment and thus elicit appropriate changes in physiology and behavior. In the nematode Caenorhabditis elegans, sensory neurons affect the decision to arrest during development in a diapause state, the dauer larva, and modulate the lifespan of the animals in adulthood. However, the mechanisms underlying these effects are incompletely understood. Using whole-genome microarray analysis, we identified transcripts whose levels are altered by mutations in the intraflagellar transport protein daf-10, which result in impaired development and function of many sensory neurons in C. elegans. In agreement with existing genetic data, the expression of genes regulated by the transcription factor DAF-16/FOXO was affected by daf-10 mutations. In addition, we found altered expression of transcriptional targets of the DAF-12/nuclear hormone receptor in the daf-10 mutants and showed that this pathway influences specifically the dauer formation phenotype of these animals. Unexpectedly, pathogen-responsive genes were repressed in daf-10 mutant animals, and these sensory mutants exhibited altered susceptibility to and behavioral avoidance of bacterial pathogens. Moreover, we found that a solute transporter gene mct-1/2, which was induced by daf-10 mutations, was necessary and sufficient for longevity. Thus, sensory input seems to influence an extensive transcriptional network that modulates basic biological processes in C. elegans. This situation is reminiscent of the complex regulation of physiology by the mammalian hypothalamus, which also receives innervations from sensory systems, most notably the visual and olfactory systems.
Author Summary
The senses provide animals with information about their environment, which affects not only their behavior but also their internal state and physiological outputs. How this information is processed is still unclear. In this study, we used mutant C. elegans roundworms that had defective sensory neurons to investigate how changes in sensation alter the expression of genes and regulate physiology, specifically the worms' choice to hibernate during growth and their longevity as fully-grown adults. We showed that defects in sensory neurons change the pattern of gene expression and regulate these outputs through known hormonal pathways, including insulin/IGF-1 and steroid pathways. We also identified a new regulator of longevity, MCT-1, that is predicted to transport small metabolites and hormones in the body. Unexpectedly, we found that sensory impairment altered yet another physiological output, the response to infectious agents. It prevented the worms from avoiding infectious bacteria and reduced the expression of potentially protective factors, but also increased the worms' resistance to infection, suggesting a complex network of responses to environmental stimuli. Understanding how sensory information is relayed in this relatively simple organism may inform our understanding of sensory processing in higher organisms like mammals.
PMCID: PMC3527274  PMID: 23284299
20.  C. elegans SIRT6/7 Homolog SIR-2.4 Promotes DAF-16 Relocalization and Function during Stress 
PLoS Genetics  2012;8(9):e1002948.
FoxO transcription factors and sirtuin family deacetylases regulate diverse biological processes, including stress responses and longevity. Here we show that the Caenorhabditis elegans sirtuin SIR-2.4—homolog of mammalian SIRT6 and SIRT7 proteins—promotes DAF-16–dependent transcription and stress-induced DAF-16 nuclear localization. SIR-2.4 is required for resistance to multiple stressors: heat shock, oxidative insult, and proteotoxicity. By contrast, SIR-2.4 is largely dispensable for DAF-16 nuclear localization and function in response to reduced insulin/IGF-1-like signaling. Although acetylation is known to regulate localization and activity of mammalian FoxO proteins, this modification has not been previously described on DAF-16. We find that DAF-16 is hyperacetylated in sir-2.4 mutants. Conversely, DAF-16 is acetylated by the acetyltransferase CBP-1, and DAF-16 is hypoacetylated and constitutively nuclear in response to cbp-1 inhibition. Surprisingly, a SIR-2.4 catalytic mutant efficiently rescues the DAF-16 localization defect in sir-2.4 null animals. Acetylation of DAF-16 by CBP-1 in vitro is inhibited by either wild-type or mutant SIR-2.4, suggesting that SIR-2.4 regulates DAF-16 acetylation indirectly, by preventing CBP-1-mediated acetylation under stress conditions. Taken together, our results identify SIR-2.4 as a critical regulator of DAF-16 specifically in the context of stress responses. Furthermore, they reveal a novel role for acetylation, modulated by the antagonistic activities of CBP-1 and SIR-2.4, in modulating DAF-16 localization and function.
Author Summary
Sensing and responding appropriately to environmental insults is a challenge facing all organisms. In the roundworm C. elegans, the FoxO protein DAF-16 moves to the nucleus in response to stress, where it regulates gene expression and plays a key role in ensuring organismal survival. In this manuscript, we characterize SIR-2.4 as a novel factor that promotes DAF-16 function during stress. SIR-2.4 is a member of a family of proteins called sirtuins, some of which promote increased lifespan in model organisms. Worms lacking SIR-2.4 show impaired DAF-16 nuclear recruitment, DAF-16–dependent gene expression, and survival in response to a variety of stressors. SIR-2.4 regulates DAF-16 by indirectly affecting levels of a modification called acetylation on DAF-16. Overall, our work has revealed SIR-2.4 to be a key new factor in stress resistance and DAF-16 regulation in C. elegans. Future studies will address whether mammalian SIR-2.4 homologs SIRT6 and SIRT7 act similarly towards mammalian FoxO proteins.
PMCID: PMC3441721  PMID: 23028355
21.  Insulin/IGF-1 and Hypoxia Signaling Act in Concert to Regulate Iron Homeostasis in Caenorhabditis elegans 
PLoS Genetics  2012;8(3):e1002498.
Iron plays an essential role in many biological processes, but also catalyzes the formation of reactive oxygen species (ROS), which can cause molecular damage. Iron homeostasis is therefore a critical determinant of fitness. In Caenorhabditis elegans, insulin/IGF-1 signaling (IIS) promotes growth and reproduction but limits stress resistance and lifespan through inactivation of the DAF-16/FoxO transcription factor (TF). We report that long-lived daf-2 insulin/IGF-1 receptor mutants show a daf-16–dependent increase in expression of ftn-1, which encodes the iron storage protein H-ferritin. To better understand the regulation of iron homeostasis, we performed a TF–limited genetic screen for factors influencing ftn-1 gene expression. The screen identified the heat-shock TF hsf-1, the MAD bHLH TF mdl-1, and the putative histone acetyl transferase ada-2 as activators of ftn-1 expression. It also revealed that the HIFα homolog hif-1 and its binding partner aha-1 (HIFβ) are potent repressors of ftn-1 expression. ftn-1 expression is induced by exposure to iron, and we found that hif-1 was required for this induction. In addition, we found that the prolyl hydroxylase EGL-9, which represses HIF-1 via the von Hippel-Lindau tumor suppressor VHL-1, can also act antagonistically to VHL-1 in regulating ftn-1. This suggests a novel mechanism for HIF target gene regulation by these evolutionarily conserved and clinically important hydroxylases. Our findings imply that the IIS and HIF pathways act together to regulate iron homeostasis in C. elegans. We suggest that IIS/DAF-16 regulation of ftn-1 modulates a trade-off between growth and stress resistance, as elevated iron availability supports growth but also increases ROS production.
Author Summary
Iron plays a role in many biological processes, including energy generation and DNA replication. But to maintain health, levels of cellular iron must be just right: too much or too little iron can cause illnesses, such as anemia and hemochromatosis, respectively. Animals therefore carefully control their iron levels by regulating of iron uptake, transport, and storage within protein capsules called ferritins. But how do they coordinate this? Using the model organism C. elegans, we have discovered a network of genes and pathways that control iron homeostasis. We find that ferritin is regulated by insulin/IGF-1 signaling, which also controls growth and resistance to oxidative stress in response to harsh environmental conditions. Ferritin is also regulated by the hypoxia signaling pathway, which responds to oxygen and iron levels as well as to metabolic cues. We find that the hypoxia pathway acts as an iron sensor, a role it may also play in humans. Our work defines a network of signaling pathways that can adjust iron availability in response to a range of environmental cues. Understanding this network in C. elegans can help us to understand the causes of iron dyshomeostasis in humans, which can profoundly affect health.
PMCID: PMC3291539  PMID: 22396654
22.  dbl-1/TGF-β and daf-12/NHR Signaling Mediate Cell-Nonautonomous Effects of daf-16/FOXO on Starvation-Induced Developmental Arrest 
PLoS Genetics  2015;11(12):e1005731.
Nutrient availability has profound influence on development. In the nematode C. elegans, nutrient availability governs post-embryonic development. L1-stage larvae remain in a state of developmental arrest after hatching until they feed. This “L1 arrest” (or "L1 diapause") is associated with increased stress resistance, supporting starvation survival. Loss of the transcription factor daf-16/FOXO, an effector of insulin/IGF signaling, results in arrest-defective and starvation-sensitive phenotypes. We show that daf-16/FOXO regulates L1 arrest cell-nonautonomously, suggesting that insulin/IGF signaling regulates at least one additional signaling pathway. We used mRNA-seq to identify candidate signaling molecules affected by daf-16/FOXO during L1 arrest. dbl-1/TGF-β, a ligand for the Sma/Mab pathway, daf-12/NHR and daf-36/oxygenase, an upstream component of the daf-12 steroid hormone signaling pathway, were up-regulated during L1 arrest in a daf-16/FOXO mutant. Using genetic epistasis analysis, we show that dbl-1/TGF-β and daf-12/NHR steroid hormone signaling pathways are required for the daf-16/FOXO arrest-defective phenotype, suggesting that daf-16/FOXO represses dbl-1/TGF-β, daf-12/NHR and daf-36/oxygenase. The dbl-1/TGF-β and daf-12/NHR pathways have not previously been shown to affect L1 development, but we found that disruption of these pathways delayed L1 development in fed larvae, consistent with these pathways promoting development in starved daf-16/FOXO mutants. Though the dbl-1/TGF-β and daf-12/NHR pathways are epistatic to daf-16/FOXO for the arrest-defective phenotype, disruption of these pathways does not suppress starvation sensitivity of daf-16/FOXO mutants. This observation uncouples starvation survival from developmental arrest, indicating that DAF-16/FOXO targets distinct effectors for each phenotype and revealing that inappropriate development during starvation does not cause the early demise of daf-16/FOXO mutants. Overall, this study shows that daf-16/FOXO promotes developmental arrest cell-nonautonomously by repressing pathways that promote larval development.
Author Summary
Animals must cope with feast and famine in the wild. Environmental fluctuations require a balancing act between development in favorable conditions and survival during starvation. Disruption of the pathways that govern this balance can lead to cancer, where cells proliferate when they should not, and metabolic diseases, where nutrient sensing is impaired. In the roundworm Caenorhabditis elegans, larval development is controlled by nutrient availability. Larvae are able to survive starvation by stopping development and starting again after feeding. Stopping and starting development in this multicellular animal requires signaling to coordinate development across tissues and organs. How such coordination is accomplished is poorly understood. Insulin/insulin-like growth factor (IGF) signaling governs larval development in response to nutrient availability. Here we show that insulin/IGF signaling activity in one tissue can affect the development of other tissues, suggesting regulation of additional signaling pathways. We identified two pathways that promote development in fed larvae and are repressed by lack of insulin/IGF signaling in starved larvae. Repression of these pathways is crucial to stopping development throughout the animal during starvation. These three pathways are widely conserved and associated with disease, suggesting the nutrient-dependent regulatory network they comprise is important to human health.
PMCID: PMC4676721  PMID: 26656736
23.  Germline Signals Deploy NHR-49 to Modulate Fatty-Acid β-Oxidation and Desaturation in Somatic Tissues of C. elegans 
PLoS Genetics  2014;10(12):e1004829.
In C. elegans, removal of the germline extends lifespan significantly. We demonstrate that the nuclear hormone receptor, NHR-49, enables the response to this physiological change by increasing the expression of genes involved in mitochondrial β-oxidation and fatty-acid desaturation. The coordinated augmentation of these processes is critical for germline-less animals to maintain their lipid stores and to sustain de novo fat synthesis during adulthood. Following germline ablation, NHR-49 is up-regulated in somatic cells by the conserved longevity determinants DAF-16/FOXO and TCER-1/TCERG1. Accordingly, NHR-49 overexpression in fertile animals extends their lifespan modestly. In fertile adults, nhr-49 expression is DAF-16/FOXO and TCER-1/TCERG1 independent although its depletion causes age-related lipid abnormalities. Our data provide molecular insights into how reproductive stimuli are integrated into global metabolic changes to alter the lifespan of the animal. They suggest that NHR-49 may facilitate the adaptation to loss of reproductive potential through synchronized enhancement of fatty-acid oxidation and desaturation, thus breaking down some fats ordained for reproduction and orchestrating a lipid profile conducive for somatic maintenance and longevity.
Author Summary
Much is known about how increasing age impairs fertility but we know little about how reproduction influences rate of aging in animals. Studies in model organisms such as worms and flies have begun to shed light on this relationship. In worms, removing germ cells that give rise to sperm and oocytes extends lifespan, increases endurance and elevates fat. Fat metabolism and hormonal signals play major roles in this lifespan augmentation but the genetic mechanisms involved are poorly understood. We show that a gene, nhr-49, enhances worm lifespan following germ-cell removal. NHR-49 is increased in animals that lack germ cells by conserved longevity proteins, DAF-16 and TCER-1. NHR-49, in turn, increases levels of genes that help burn fat and convert saturated fats into unsaturated forms. Through synchronized enhancement of these processes, NHR-49 helps eliminate excess fat delegated for reproduction and converts lipids into forms that favor a long life. NHR-49 impacts these processes during aging in normal animals too, but using different regulatory mechanisms. Our data helps understand how normal lipid metabolic processes can be harnessed to adapt to physiological fluctuations brought on by changes in the reproductive status of animals.
PMCID: PMC4256272  PMID: 25474470
24.  DamID in C. elegans reveals longevity-associated targets of DAF-16/FoxO 
Insulin/IGF-1 signaling controls metabolism, stress resistance and aging in Caenorhabditis elegans by regulating the activity of the DAF-16/FoxO transcription factor (TF). However, the function of DAF-16 and the topology of the transcriptional network that it crowns remain unclear. Using chromatin profiling by DNA adenine methyltransferase identification (DamID), we identified 907 genes that are bound by DAF-16. These were enriched for genes showing DAF-16-dependent upregulation in long-lived daf-2 insulin/IGF-1 receptor mutants (P=1.4e−11). Cross-referencing DAF-16 targets with these upregulated genes (daf-2 versus daf-16; daf-2) identified 65 genes that were DAF-16 regulatory targets. These 65 were enriched for signaling genes, including known determinants of longevity, but not for genes specifying somatic maintenance functions (e.g. detoxification, repair). This suggests that DAF-16 acts within a relatively small transcriptional subnetwork activating (but not suppressing) other regulators of stress resistance and aging, rather than directly regulating terminal effectors of longevity. For most genes bound by DAF-16∷DAM, transcriptional regulation by DAF-16 was not detected, perhaps reflecting transcriptionally non-functional TF ‘parking sites'. This study demonstrates the efficacy of DamID for chromatin profiling in C. elegans.
PMCID: PMC2950082  PMID: 20706209
aging; C. elegans; DAF-16/FoxO; DamID chromatin profiling; transcriptional networks
25.  The DAF-16 FOXO Transcription Factor Regulates natc-1 to Modulate Stress Resistance in Caenorhabditis elegans, Linking Insulin/IGF-1 Signaling to Protein N-Terminal Acetylation 
PLoS Genetics  2014;10(10):e1004703.
The insulin/IGF-1 signaling pathway plays a critical role in stress resistance and longevity, but the mechanisms are not fully characterized. To identify genes that mediate stress resistance, we screened for C. elegans mutants that can tolerate high levels of dietary zinc. We identified natc-1, which encodes an evolutionarily conserved subunit of the N-terminal acetyltransferase C (NAT) complex. N-terminal acetylation is a widespread modification of eukaryotic proteins; however, relatively little is known about the biological functions of NATs. We demonstrated that loss-of-function mutations in natc-1 cause resistance to a broad-spectrum of physiologic stressors, including multiple metals, heat, and oxidation. The C. elegans FOXO transcription factor DAF-16 is a critical target of the insulin/IGF-1 signaling pathway that mediates stress resistance, and DAF-16 is predicted to directly bind the natc-1 promoter. To characterize the regulation of natc-1 by DAF-16 and the function of natc-1 in insulin/IGF-1 signaling, we analyzed molecular and genetic interactions with key components of the insulin/IGF-1 pathway. natc-1 mRNA levels were repressed by DAF-16 activity, indicating natc-1 is a physiological target of DAF-16. Genetic studies suggested that natc-1 functions downstream of daf-16 to mediate stress resistance and dauer formation. Based on these findings, we hypothesize that natc-1 is directly regulated by the DAF-16 transcription factor, and natc-1 is a physiologically significant effector of the insulin/IGF-1 signaling pathway that mediates stress resistance and dauer formation. These studies identify a novel biological function for natc-1 as a modulator of stress resistance and dauer formation and define a functionally significant downstream effector of the insulin/IGF-1 signaling pathway. Protein N-terminal acetylation mediated by the NatC complex may play an evolutionarily conserved role in regulating stress resistance.
Author Summary
What are the mechanisms used by animals to cope with stressful environments that inflict damage or restrict essential processes such as growth, development, and reproduction? One strategy is changes in physiology that increase stress resistance, and an extreme version of this strategy is diapause, an alternative developmental state that is enduring and stress resistant. In the nematode C. elegans, stress tolerance and entry into a diapause state called dauer larvae are mediated by the conserved insulin/IGF-1 pathway. Specifically, the FOXO transcription factor DAF-16 promotes stress tolerance and dauer larvae development. However, the targets of DAF-16 that mediate these processes remain largely elusive. Using an unbiased forward genetic screen to discover new mediators of stress tolerance, we identified natc-1, a novel target of DAF-16 and the insulin/IGF-1 pathway. natc-1 encodes a conserved subunit of the N-terminal acetyltransferase C (NAT) complex. The NatC complex modifies target proteins by acetylating the N-terminus. We demonstrated that natc-1 mediates diapause entry and stress tolerance. Furthermore, we elucidated regulation of NatC by demonstrating that natc-1 is a direct transcriptional target that is repressed by DAF-16. These findings may be relevant to other animals because both the insulin/IGF-1 signaling pathway and the NAT system are conserved during evolution.
PMCID: PMC4199503  PMID: 25330323

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