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. 2008 Feb 29;4(2):e1000021.
doi: 10.1371/journal.pgen.1000021.

The Mediator subunit MDT-15 confers metabolic adaptation to ingested material

Stefan Taubert  1 , Malene Hansen, Marc R Van Gilst, Samantha B Cooper, Keith R Yamamoto
Affiliations

The Mediator subunit MDT-15 confers metabolic adaptation to ingested material

Stefan Taubert et al. PLoS Genet. .

Abstract

In eukaryotes, RNA polymerase II (Pol(II)) dependent gene expression requires accessory factors termed transcriptional coregulators. One coregulator that universally contributes to Pol(II)-dependent transcription is the Mediator, a multisubunit complex that is targeted by many transcriptional regulatory factors. For example, the Caenorhabditis elegans Mediator subunit MDT-15 confers the regulatory actions of the sterol response element binding protein SBP-1 and the nuclear hormone receptor NHR-49 on fatty acid metabolism. Here, we demonstrate that MDT-15 displays a broader spectrum of activities, and that it integrates metabolic responses to materials ingested by C. elegans. Depletion of MDT-15 protein or mutation of the mdt-15 gene abrogated induction of specific detoxification genes in response to certain xenobiotics or heavy metals, rendering these animals hypersensitive to toxin exposure. Intriguingly, MDT-15 appeared to selectively affect stress responses related to ingestion, as MDT-15 functional defects did not abrogate other stress responses, e.g., thermotolerance. Together with our previous finding that MDT-15:NHR-49 regulatory complexes coordinate a sector of the fasting response, we propose a model whereby MDT-15 integrates several transcriptional regulatory pathways to monitor both the availability and quality of ingested materials, including nutrients and xenobiotic compounds.

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Conflict of interest statement

The authors have declared that no competing interests exist.

Figures

Figure 1
Figure 1. MDT-15 depletion enhances toxin sensitivity, but does not affect thermotolerance.
(A) N2 worms were grown on the indicated RNAi bacteria on plates harboring fluoranthene at various concentrations. After four days, animals were scored for morphological defects. Micrographs show representative animals grown on toxin-RNAi combinations, as indicated. The size bar represents 64.5 µm. Exposure of mdt-15(RNAi) worms to fluoranthene results in small, scrawny adults; on DMSO, mdt-15(RNAi) animals are only slightly thinner than control(RNAi) worms. (B) Quantification of adult arrest. Each bar graph represents the fraction of worms scored as arrested; bars represent the average of three individual biological repeats, and errors bars represent the SEM. Fluoranthene was present at 0.4, 2, and 10 µg/ml, respectively, with medium and high concentrations resulting in adult arrest of mdt-15(RNAi) but not control(RNAi) worms. (C) Survival rate of L4 stage control(RNAi) and mdt-15(RNAi) worms following exposure to 35°C (starting temperature 20°C). Mean survival time was 11.0 hr for control(RNAi) worms (n = 92), and 10.9 hr for mdt-15(RNAi) worms (n = 87); P-value = 0.22 (log-rank test). The data represent one of five independent experiments with similar outcome, two performed at the L4 stage, and three with day two old adult worms (all exposed to RNAi bacteria from L1 stage on).
Figure 2
Figure 2. MDT-15 is essential for mRNA induction of heavy metal-induced genes.
(A–C) Relative mRNA accumulation (determined by qPCR quantification) from select metal responsive genes in worms grown in the presence of metal ions (as indicated, either 25 µM Cd2+, 100 µM Zn2+, or 10 µM Cu2+). Each bar graph represents the average relative mRNA level from three or more independent worm growths and mRNA isolations; mRNA levels are normalized to act-1 mRNA. The error bars represent SEM. Note that the scale is logarithmic. Tables above or next to the panels indicate the fold-induction by Cd2+ vs. no metal in the same genetic condition. RNAi depletion (A, C) or mutation of mdt-15 (B) compromised mRNA expression of select metal detoxification genes, whereas expression of the control genes hmt-1 (heavy metal tolerance factor), pyc-1 (pyruvate carboxylase), cdr-6, cdr-7 (cadmium responsive genes), ama-1 (PolII, large subunit), and nhr-23 (nuclear hormone receptor) were unaffected by mdt-15 depletion or mutation. (A) mRNA abundance in N2 L4 stage animals fed control RNAi (blue) or mdt-15 RNAi (red). (B) mRNA abundance in L4 stage N2 (blue) or mdt-15(tm2182) (red) animals. (C) mRNA abundance in day two old sterile adult animals (CF512) exposed to control RNAi (blue) or mdt-15 RNAi (red). (D) Fluorescence micrographs of adult worms carrying an mtl-2::GFP promoter fusion. Five hr exposure to Cd2+ increased the GFP signal worms fed with control RNAi, but not with mdt-15 RNAi bacteria. The size bar represents 32.3 µm.
Figure 3
Figure 3. MDT-15 is dispensable for the heat-shock response.
mRNA accumulation from select heat-shock protein genes in L4-stage control(RNAi) worms (blue) and mdt-15(RNAi) worms (red) exposed to 35°C for various times, as indicated. Each bar represents the average relative mRNA level of the indicated gene (average from three independent worm growths and RNA isolations from N2 L4 stage animals). Relative mRNA levels are normalized to act-1 mRNA levels; the error bars represent SEM.
Figure 4
Figure 4. A model for MDT-15 function.
Model depicting the role of MDT-15 in monitoring availability and quality of ingested material. Note that in each case, we propose that MDT-15 confers regulatory effects both in “unchallenged” (sufficient energy supply, no xenobiotics or metals, no pathogens) and “challenged” (fasting, xenobiotic or metal exposure, pathogenic infection) conditions. For further explanations, see text.
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