Regulation of Caenorhabditis elegans vitellogenesis by DAF-2/IIS through separable transcriptional and posttranscriptional mechanisms
© DePina et al; licensee BioMed Central Ltd. 2011
Received: 22 June 2011
Accepted: 12 July 2011
Published: 12 July 2011
Evolutionary theories of aging propose that longevity evolves as a competition between reproduction and somatic maintenance for a finite pool of resources. Reproduction is thought to shorten lifespan by depleting resources from processes promoting somatic maintenance. Maternal yolk production, vitellogenesis, represents a significant maternal cost for reproduction and is suppressed under genetic and environmental conditions that extend lifespan. However, little is known about the pathways regulating vitellogenesis in response to prolongevity cues.
In order to identify mechanisms that suppress vitellogenesis under prolongevity conditions, we studied factors regulating vitellogenesis in C. elegans nematodes. In C. elegans, vitellogenesis is depressed in the absence of insulin-like signaling (IIS). We found that the C. elegans daf-2/IIS pathway regulates vitellogenesis through two mechanisms. vit-2 transcript levels in daf-2 mutants were indirectly regulated through a germline-dependent signal, and could be rescued by introduction of daf-2(+) sperm. However, yolk protein (YP) levels in daf-2 mutants were also regulated by germline-independent posttranscriptional mechanisms.
C. elegans vitellogenesis is regulated transcriptionally and posttranscriptionally in response to environmental and reproductive cues. The daf-2 pathway suppressed vitellogenesis through transcriptional mechanisms reflecting reproductive phenotypes, as well as distinct posttranscriptional mechanisms. This study reveals that pleiotropic effects of IIS pathway mutations can converge on a common downstream target, vitellogenesis, as a mechanism to modulate longevity.
According to evolutionary theories of aging, lifespan evolves as a trade-off between the metabolic costs of somatic maintenance with those of reproduction. Reproductive processes such as egg production and progeny rearing are energy-intense and drain resources away from processes that promote somatic maintenance. Studies have provided evidence for a trade-off between reproduction and survival. For example, experimentally increased egg production in wild seabirds is associated with lower rates of postmigratory return to breeding grounds [1, 2]. One mechanism by which organisms can regulate the relative burdens of reproduction and somatic maintenance in response to environmental conditions is through phenotypic plasticity of life-history traits, such as growth and reproduction . Plasticity in life-history traits can affect lifespan directly, such as to delay reproduction until environmental conditions improve, or indirectly, as a consequence of elevated stress resistance.
One highly plastic reproductive process is vitellogenesis, the process of maternal yolk production that provides the major nutrient source for developing embryos. Vitellogenin genes are expressed in adult females within tissues specialized for yolk production, such as the insect fat body and avian liver. Vitellogenesis has been well-studied in birds and insects and is hormonally regulated in response to environmental conditions. In insects, vitellogenesis is induced by the coordinate action of 20-hydroxyecdysone and juvenile hormone (JH). Insect JH is regulated, in turn, by insulin-like signaling (IIS). Eggs in Drosophila insulin receptor mutants fail to become vitellogenic and this phenotype can be rescued by methoprene, a JH analog [4, 5]. In Aedes aegypti mosquitoes, vitellogenin gene expression is synergistically enhanced by ecdysone and insulin signaling, via the nutrient sensing protein, TOR .
IIS is also required for vitellogenesis in C. elegans nematodes [7–9]. In C. elegans, IIS is mediated by the insulin receptor-like protein (IR), DAF-2 which transduces signals via AGE-1/PI3K to antagonize the activity of DAF-16, a FoxO transcription factor [10–13]. Under conditions that reduce DAF-2/IR signaling, levels of vit gene transcripts and yolk proteins (YP) are both reduced [7, 14]. In addition to suppressing vitellogenesis, defective IIS prolongs adult lifespan in C. elegans and Drosophila. The mechanism by which IIS modulates longevity is likely to be multifactorial and complex . Vitellogenesis appears to be one target of the IIS prolongevity mechanism, possibly for the purpose of reallocating resources to somatic maintenance. Consistently, RNAi knockdown of vitellogenin gene expression could extend lifespan of wildtype adults . Fecundity has been linked to longevity in daf-2 mutants. Sixteen daf-2 alleles were associated with reduced brood size at nonpermissive temperatures . However, certain alleles, termed class 1, had brood sizes ranging from 85-100% that of wildtype, while class 2 alleles had brood sizes ranging from 60-93% of wildtype. In insects, IIS regulates vitellogenesis hormonally through JH. However, it is not known whether C. elegans IIS also regulates vitellogenesis through hormonal effectors or by direct action in the intestine, the site of vitellogenin production.
To address this question, we identified factors modulating C. elegans vitellogenesis and then examined whether these factors were modified by IIS or were IIS-independent. The C. elegans genome contains six vitellogenin (vit) genes. The genes, vit-1, -2, -3, -4 and -5, all contribute to the pool of YP170, the major yolk protein. vit-6 encodes a protein that is processed into YP115 and YP88. Our analyses show that IIS stimulates vitellogenin gene transcription through a sperm-dependent signal, implying the existence of germline-dependent hormonal regulators of vitellogenesis in C. elegans. However, yolk protein levels correlated poorly with vit-2 mRNA and were regulated by IIS independently of sperm and nutrients. Based on these findings, we propose that vit genes may be transcribed in excess and titrated to reflect reproductive needs. Furthermore, IIS deficits appear to suppress vitellogenesis convergently through germline-dependent transcriptional mechanisms and a separate posttranscriptional mechanism.
Differential regulation of vit-2 transcription and translation
Vitellogenin transcriptional regulation in daf-2 pathway mutants
To determine the concordance between vit message and protein levels during this period, we also examined yolk proteins directly in wildtype hermaphrodites. Yolk proteins (YP) are the most abundant proteins in the worm and are easily detected in Coomassie-stained crude lysates after polyacrylamide gel electrophoresis . We confirmed the identities of bands corresponding to YP170, YP115, YP88, actin (42 kDa) and myosin (200 kDa) by mass spectrometry. We quantified the levels of each protein by densitometry and calculated abundance relative to myosin, as an internal standard, as well as an independent loading standard run on the same gel. In contrast to the progressive decline observed for vit-2 and -5 transcripts, YP levels increased in abundance nearly 4-fold by adult day 4 (p ≤ 0.0001, T-test) (Figure 1C,D). Actin levels did not change over this period. Furthermore, the absolute levels of myosin relative to a loading standard were constant, showing there was no overall change in protein levels. Rather, the increased level of YP170 reflected specific increases in this pool.
Sperm stimulate vit mRNA levels in hermaphrodites
Levels of vit-2, vit-5 and sod-3 mRNA in sterile strains
0.33 (0.12, 7)*
0.041 (0.09, 7)*
0.93 (0.1, 6)
0.15 (0.08, 4)*
0.34 (0.18, 4)*
3.0 (3.7, 2)
0.092 (0.03, 3)
0.20 (0.09, 3)
1.2 (1.4, 2)
0.094 (0.03, 7)
0.13 (0.08, 7)
0.63 (0.5, 6)
1.1 (0.76, 4)
1.3 (1.1, 4)
0.98 (1.2, 4)
1.1 (0.60, 4)
1.1 (0.54, 4)
4.4 (6.8, 4)
0.53 (0.22, 4)
0.43 (0.18, 4)
1.69 (1.2, 4)
0.16 (0.12, 3)
0.05 (0.04, 3)*
2.72 (2.31, 3)
fem-1(hc17) × wildtype males
0.75 (0.22, 4)^
0.71 (0.23, 4)
0.53 (0.10, 4)
fem-1(hc17) × wildtype males
0.38 (0.14, 2)
0.55 (0.17, 2)
0.12 (0.15, 2)
fem-1(hc17) × wildtype males
0.082 (1 trial)
0.16 (1 trial)
0.24 (1 trial)
fem-1(hc17) × wildtype males
0.357 (0.37, 3)
0.41 (0.4, 3)
0.20 (0.12, 3)
To test this hypothesis, we examined whether wildtype sperm could stimulate vit-2 mRNA levels in sperm-deficient fem-1(hc17) hermaphrodites by measuring mRNA after allowing them to mate with wildtype males. We confirmed that mating rescued fem-1(hc17) sterility by the appearance of fertile heterozygous progeny (not shown). Sterile fem-1 hermaphrodites co-cultured with wildtype males exhibited a broad range of vit-2 mRNA levels which, on average, were elevated compared to unmated fem-1 hermaphrodites (Figure 3C, Table 2). On adult day 1, we observed a statistically significant elevation in vit-2 mRNA levels from mating (p = 0.02; T-test). Mating had no effect on sod-3 mRNA (p = 0.81). The progressive decline in vit-2 mRNA between adult days 1 and 6 was also abrogated by coculture with wildtype males (vit-2 mRNA days 1-6: p < 0.001 selfed, p = 0.17 mated; ANOVA). These results demonstrate that vit transcriptional suppression in sterile fem-1(hc17) hermaphrodites could be rescued by the introduction of wildtype sperm from mating.
To assess the impact of reduced vit transcripts on overall yolk protein production, we examined YP170 levels in sterile fem-1(hc17) adult hermaphrodites. In comparison to wildtype adults, YP levels relative to myosin were normal or elevated in fem-1(hc17) adults (Figure 3D,E). Yolk protein levels were also elevated in germline-masculinized fem-3(q20gf) hermaphrodites. This indicates that the sperm-dependent stimulation in vit-2/5 mRNA levels is likely to be dispensable for YP production under normal conditions.
Deficits in IIS suppress vitellogenesis through two separable effects
Cis-acting requirements for vitellogenin repression in daf-2(e1370) adults
Effect of daf-1 6 RNAi on pvit-2:gfp expression in daf-2(e1370) adult hermaphrodites
26% (0.2, 5)*
-3% (0, 1)
Using GFP reporters expressed from subfragments of the 2-kb vit-2 promoter, we identified a cis-regulatory region responsive to daf-16 activity. Deletions of 0.5, 1.2 and 1.7 kb upstream of the vit-2 translational start sequence did not disrupt daf-16-dependent repression of pvit-2:GFP expression in daf-2(e1370) hermaphrodites (Figure 5 Table 3). Thus, daf-16 regulation of YP requires only a short 300-bp vit-2 promoter, but not additional upstream sequences. The 300-bp vit-2 promoter contains binding sites for three transcription factors that direct vitellogenenin expression in the adult hermaphrodite intestine: the ELT-2 GATA factor, the male-specific transcriptional repressor MAB-3, and an unidentified factor which binds to VPE2 repeat sequences in the vit-2 promoter [28–30]. DAF-16 recognizes its transcriptional targets through a binding site with consensus GTAAAc/tA . Although the vit-2 promoter contains this sequence within the 5-kb upstream of the translational start , we did not detect any matches within the 300-bp promoter fragment sufficient for daf-16-mediated repression.
Levels of pvit-2:gfp mRNA in wildtype and daf-2(e1370) adults
This study's goal was to characterize the effect of daf-2 pathway activity on vitellogenesis in C. elegans. Our findings corroborate previous reports showing that daf-16 activity suppresses vitellogenesis at both the transcriptional and translational levels [7–9, 14]. However, our temporal analysis of this suppression do not support a simple model where DAF-16 acts as a direct transcriptional repressor of vit genes. First, YP levels were reduced in a daf-16-dependent manner in young adults, although vit mRNA levels were normal or elevated at this stage. Second, wildtype sperm stimulated vit-2 mRNA non-autonomously in daf-2 hermaphrodites, but failed to increase YP levels. Third, we observed daf-16-dependent suppression of pvit-2(0.3 kb):gfp expression, although this promoter fragment lacks DBE-like DAF-16 binding sites, with the caveat that DAF-16 might bind to non-DBE sites in the vit-2 promoter . Finally, we note that vit genes were not represented among 103 direct DAF-16 targets identified by chromatin immunoprecipitation .
In contrast to the progressive decline of vit mRNA in daf-2(e1370) hermaphrodites, yolk protein production was suppressed constitutively throughout adulthood. This observation suggests a shift of translation regulation. Consistently, GFP expression from the pvit-2(2.0):gfp transcriptional reporter was suppressed in a daf-16-dependent manner, although gfp mRNA levels were comparable in wildtype and daf-2(e1370) hermaphrodites. When under stress, cells respond by upregulating translation of stress-resistance proteins, such as heat-shock proteins and chaperones. At the same time, cells repress translation of non-essential proteins to conserve energy supplies . One interpretation of our results is that daf-2(e1370) adults undergo a similar translational switch to repress YP synthesis in early adulthood. Genetic results indicate that such a shift in translational regulation should be regulated by one or more DAF-16 target genes. It is not currently known if specific DAF-16 targets may regulate translation. However, MS analysis of 1685 proteins in daf-2(e1370) and wildtype adults identified 86 proteins with differential abundance, only 35 of which had been previously identified as DAF-16 target genes . While the remaining 51 proteins might be novel DAF-16 targets, as proposed , it is also possible that these might represent additional secondary targets of a daf-16-dependent shift in translational regulation. We note that global translational reprogramming in response to thermal stress (35°C, 2 hours) occurs normally in daf-2(e1370) adults . Thus, daf-2/daf-16 pathway activity may regulate translational reprogramming at 25°C, a relatively nonstressful condition, but not under severe stress, such as at 35°C.
We were surprised to observe significant discordance between vit transcript and YP levels under the conditions we examined in the course of this study. In wildtype adults, YP170 increased during adulthood although vit-2 and -5 mRNA decreased. Furthermore, YP170 was reduced in daf-2(e1370) day 1 adults which contained normal levels of vit-2 and -5 mRNA. We acknowledge the caveat that other vit loci (vit-1, -3 and -4), which contribute to the YP170 pool, may have increased expression to make up for the decline in vit-2 and -5. However, other studies have observed these 5 loci to be expressed at similar levels  (S.K. Rao & C.A.Wolkow, unpublished results). In addition, alternative regulation of these loci could not account for the discrepancy between vit mRNA and YP170 in day 1 daf-2(e1370) adults.
Discrepancies between mRNA and protein levels have been observed under other conditions. Studies in yeast have examined the correlation between transcript and protein abundance on a large scale. With the exception of highly expressed genes, these studies report little correlation between relative levels of mRNA and protein, concluding that transcript levels may be inadequate reflections of the true cellular composition . One factor that could contribute to discordance between vit transcript and YP levels is the potential accumulation of YP within embryos in the hermaphrodite uterus or the pseudocoelomic body cavity. Thus, older hermaphrodites containing more eggs in the uterus might also contain more yolk in bulk lysates. However, this explanation does not account for our finding that fem-3(q20) hermaphrodites, which fail to make oocytes, contained normal levels of YP170. In this strain, YP are likely to accumulate in the pseudocoelom. Rather, we hypothesize that vit genes are transcribed at high levels on the first day of adulthood, in anticipation of reproductive needs. Then, as reproduction proceeds over the next 3-5 days, vit mRNA levels are titrated downward in response to actual reproductive demand. Providing excess vit transcripts in early adulthood may ensure adequate YP synthesis capacity for maximum reproductive rate.
Vitellogenins have been repeatedly identified as targets repressed by daf-16 activity and downregulated under prolongevity conditions [7–9, 14]. Furthermore, vitellogenin downregulation by RNAi was itself sufficient to extend adult C. elegans lifespan . These findings indicate that longevity and somatic maintenance in daf-2 mutants is likely to reflect combinatorial effects of protective enzyme induction and reproductive suppression. Our studies show that daf-2 pathway deficits suppress vitellogenesis through two separable mechanisms. This reveals new insight that longevity assurance in daf-2 mutants can result from convergent regulation of vitellogenesis by pleitropic phenotypes in these mutants.
Strains and growth conditions
The strains used in this study were N2 Bristol [wildtype], DR1572 [daf-2(e1368)], CB1370 [daf-2(e1370)], CY312 [daf-16(mgDf50); daf-2(e1370)], BA17 [fem-1(hc17)], JK816 [fem-3(q20)], CY625 [daf-2(e1370); bvIs7 (pvit-2:gfp; gcy-7:gfp)]. Strains were obtained from Caenoharbditis Genetics Center at the University of Minnesota. All strains were maintained at 15°C on nematode growth medium (NGM) agar plates seeded with OP50 E. coli strain following standard protocols . For mating assays, animals were cultured from embryos at 15°C for 4-5 days to the young adult (day 0) stage. Mating assays were carried out using at a ratio of 1 hermaphrodite to 2 males and then shifted to 25°C. 30 hermaphrodites were harvested from mating plates on days 1, 2, 4 and 6 for Q-PCR analysis. For nutrient deprivation experiments, young adult animals (day 0) raised at 15°C were shifted to 25°C for 24 hours (day 1). Day 1 animals were kept on NGM plates with OP50 bacteria (fed) or transferred to plates without food for 6 hours (fasted). Worms were then harvested and processed for Q-PCR analysis or protein gel analysis.
Quantitative real time RT-PCR
Worms were allowed to lay eggs on NGM agar plates and grown at 15°C for 4-5 days to obtain synchronized populations. Young adult (YA) worms were then shifted to 25°C and harvested on day 1 (fertile adult), day 2, day 4 and day 6 (post-reproductive adult) by picking 30 worms into M9 Buffer. Total RNA was isolated using Absolutely RNA Miniprep Kit (Stratagene, La Jolla, CA). cDNA was synthesized using Stratascript First-Strand Synthesis System (Stratagene). Triplicate 25 μl quantitative real time RT-PCR (qPCR) reactions were set-up in 96-well plates using 2 × SYBRGreen master mix (Applied Biosystems, Foster City, CA), and reactions were run on MJ Research Opticon thermal cycler (BioRad, Hercules, CA). Data was analyzed using the Ct method and relative mRNA levels is reported as the mRNA abundance of each gene relative to the mRNA abundance of the control gene, act-1 (β-actin). The following qPCR primers were used: vit-2, 5'-GACACCGAGCTCATCCGCCCA and 5'-TTCCTTCTCTCCATTGACCT; vit- 5, 5'-GGCAATTTGTTAAGCCACAA and 5'-CCTCCTTTGGTCCAGAAACCT; sod-3, 5'-CCAACCAGCGCTGAAATTCAATGG and 5'-GGAACCGAAGTCGCGCTTAATAGT; act-1, 5'-CCAGGAATTGCTGATCGTATGCAGAA and 5'-TGGAGAGGGGAAGCGAGGATAGA; gfp, 5' CTGGAGTTGTCCCAATTCTTG and 5'- AAGCATTGAACACCATAACAGAAA. Results of individual trials were statistically analysed using JMP 7.0 (SAS, Cary, NC) to obtain p-values of significance using one-way ANOVA and T-test.
Worms were allowed to lay eggs on NGM agar plates and grown at 15°C for 4-5 days to obtain synchronized populations. Young adult (YA) worms were then shifted to 25°C and harvested on adult days 1 and 4 by picking 100 worms into M9 Buffer, diluted in 2 × Laemmli Sample Buffer (Bio-Rad) containing beta-mercaptoethanol and incubated in 70°C water bath for 15 min. The worm mixture was vortexed every 5 min during the incubation and spun for 5 min to remove insoluble precipitates prior to loading on a NuPAGE 4-12% Bis-Tris gel (Invitrogen). Gels were stained and destained following standard protocols. Protein bands were quantified using ImageJ (NIH Image).
For protein identification, Coomassie-stained protein bands were excised, micro-dissected and destained with 50% acetonitrile (ACN) with 50 mM ammonium bicarbonate. Proteins were reduced with 10 mM DTT for 30 minutes at room temperature (RT) and alkylated with 10 mg/mL iodoacetamide (Sigma) for 20 min at RT. After alkylation, gel pieces were treated with trypsin (Sequencing grade, Promega) in 50 mM ammonium bicarbonate overnight at 37°C. The resultant peptides were extracted from the gel with 0.1% TFA in water and then 0.1% TFA in 70% ACN for 10 minutes, collected in eppendorf tubes and dried using a vacuum drier (Savant, USA). Peptide samples were desalted with ZipTip (Millipore), dried again, and stored at -80°C for LC-MS/MS analysis.
LC-MS/MS analyses were performed using an LXQ linear ion trap mass spectrometer coupled to a Surveyor LC system (Thermo Fisher Scientific). Peptide samples were loaded onto pre-equilibrated analytical columns using a pressure-loader. The capillary-tip-columns were affixed to the MS ion-source system and reverse-phase chromatographic separations were carried out using the following gradient setting; 100% A (0.1% formic acid in water) for 5 minutes; 0-50% B (0.1% formic acid in ACN) for 60 minutes; 50-70% B for 5 minutes; 70% B for 5 minutes. Pump flow rates were controlled to deliver 200 nL/min to the analytical column, which was achieved by splitting pump flow at a 1:1000 ratio. LXQ settings were as follows: spray voltage, 1.6 kV; 1 microscan for MS scans at maximum inject time 10 ms with mass range 400-1400 m/z, 3 microscans for MS/MS at maximum inject time100 ms with automatic mass range. The LXQ was operated in a data-dependent mode, that is, one MS scan for precursor ions followed by four data-dependent MS/MS scans for precursor ions above a threshold ion count of 500 with normalized collision energy value of 35%.
The MS/MS data were analyzed with Mascot software v2.3.0 (Matrix Sciences). MS/MS peak lists were generated using lcq_dta.exe from the BioworksBrowser v3.3.1 SP1 (Thermo Fisher Scientific) with the following options applied: grouping tolerance, 0; intermediate scans, 0; minimal scans per group, 0; precursor charge state analysis, auto. The generated lists were searched against Sprot C. elegans database with the following criteria: enzyme, trypsin (KR/P); missed cleavage sites, 3; peptide tolerance, 1.0 amu; fragment ions tolerance, 0.5 amu; variable modifications, carbamidomethylation (+57Da) and methionine oxidation (+16Da); decoy database, on. To ensure the false discovery rate (FDR) below 1%, the search results were further filtered with various significant p-values.
pvit-2:gfp reporter analysis
The pvit-2:gfp reporter was constructed using a 2-kb fragment upstream of the vit-2 presumptive ATG start site, which was PCR amplified with primers containing unique BamHI and KpnI restriction sites and ligated to pPD95.75 vector (gift of A. Fire, Stanford University). Animals were transformed by standard microinjection-mediated transformation using pvit-2:gfp plasmid DNA and gcy-7:gfp coinjection marker at a final concentration of 25 ng/μL and 100 ng/μL, respectively. For vit-2 promoter deletion analysis, the 2-kb vit-2 promoter was digested and re-ligated to remove 5' regions of the promoter and generate smaller fragments. Animals were photographed under epi-fluorescence using a 10 × objective on a Nikon E800 with a Hamamatsu Orca ER camera using Openlab software (Improvision). Exposure times were constant for each trial, although exposure times varied between some trials. For GFP quantification, ImageJ (NIH) was used to determine the maximum pixel intensity for a region-of-interest encompassing the intestine. Individual GFP intensity values were measured for an average of 14 animals/trial (range, 3-23 animals) and the values for daf-16 RNAi versus L4440 were compared for each trial of each transgene by T-test (Excel). Percentage changes in GFP fluorescence were calculated from the average maximum GFP intensity measured in each experiment after normalization for exposure time, if necessary.
RNA-interference by feeding
RNA-interference (RNAi) knockdown of daf-16 in C. elegans was achieved by feeding animals E. coli strain HT115 expressing daf-16 dsRNA or contained a control plasmid (L4440) . To obtain synchronous populations, 5-10 adult wildtype or daf-2(e1370) adults carrying pvit-2:gfp transgenic arrays were allowed to lay eggs for 4-6 hrs on NGM agar plates supplemented with ampicillin (100 μg/mL) and IPTG (1 mM). Animals were exposed to daf-16 RNAi from hatching, and grown at 15°C for 4-5 days to YA stage. Animals were then shifted to 25°C and images were captured on days 1, 2, and 4 for GFP fluorescence analysis. GFP fluorescence intensity was measured using ImageJ.
This work was carried out with funds from the Intramural Research Program of the National Institute on Aging, NIH. We are grateful to members of the NIA Laboratory of Neuroscience and Baltimore/DC area worm club for helpful comments.
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