Figure 1.

HSF-1 represses daf-7 expression, and is negatively regulated by DAF-11 and DAF-21. A, A signaling network of the C. elegans insulin/IGF-1, cGMP and TGF-β neuroendocrine systems. The model relies on previously published data. TGF-β signaling promotes reproductive growth. Under adverse environmental conditions, daf-7 becomes downregulated, resulting in the activation of the nuclear hormone receptor DAF-12 that eventually induces dauer development. Crowding pheromone-dependent cGMP signaling mediated by the receptor guanylate cyclase DAF-11 acts upstream of DAF-7 to inhibit dauer larva formation. Insulin/IGF-1 signaling hampers dauer development through the TGF-β pathway. In addition, activity of the IGF-1 receptor DAF-2, which is inhibited or activated by different insulin-like peptides, such as DAF-28, accelerates aging by inhibiting nuclear translocation of the forkhead transcription factor DAF-16. HSF-1 is negatively regulated by insulin/IGF-1 signaling, and required for longevity response triggered by DAF-16. The inhibitory effect of DAF-2/IGF-1 signaling on HSF-1 occurs through the activation of the DDL-1/2 proteins, two negative regulators of HSF-1. DAF-11 upregulates daf-28 (the dotted arrow), while DAF-3 influences DAF-2 activity through its regulation of the ins-7 agonist and ins-18 antagonist (the dotted green bar). Arrows indicate activations, bars represent inhibitory interactions. C. elegans proteins are in blue; their mammalian counterparts are in black. Dotted lines show known signaling links between these neuroendocrine systems. FoXO: Forkhead box O transcription factor; PDK1: 3-phosphoinositide-dependent kinase 1; Akt: AKT8 virus protooncogene; PKB: protein kinase B; SGK: serum- and glucocorticoid-inducible kinase; PTEN: phosphatase and tensin homolog; PI3K: phosphoinositide 3-kinase; IGF-1: insulin-like growth factor receptor-1; HSF-1: heat shock factor-1; NHR: nuclear hormone receptor; SMAD: Caenorhabditis elegans protein small (SMA) and Drosophila protein mothers against decapentaplegic (MAD); TGF-β: transforming growth factor-beta; HSP90: heat shock protein 90; GC: guanylate cyclase. B, Both DAF-11 and DAF-21 activate daf-7 expression via inhibiting HSF-1. DAF-7 abundantly accumulates in the two ASI neurons (white arrows) in wild-type L1 larvae. In contrast, daf-7 is strongly downregulated in daf-11(m47) and daf-21(p673) mutants (only a faint daf-7 expression is visible in the ASIs). HSF-1 deficiency suppresses daf-7 repression in daf-11(m47) and daf-21(p673) mutant genetic backgrounds. At 20°C, mutations in hsf-1 do not significantly alter daf-7 expression. This implies that HSF-1 has no or a weak activity at this temperature. C, Quantification (mean value) of daf-7::gfp expression in the ASIs in wild-type and mutant genetic backgrounds. *indicates p<0.0001. D, Dauer development in both daf-11(m47) and daf-21(p673) mutants requires HSF-1 activity. Mutational inactivation of hsf-1 largely protects daf-11(m47) and daf-21(p673) mutant animals from developing as dauer larvae. In each single mutant vs. double mutant comparison, p<0.0001. E, Transcriptional activity of daf-7 depends on the ambient temperature and HSF-1 activity. qRT-PCR analysis shows that daf-7 transcript levels decrease at 27°C (the left panel; p<0.001), as compared with those measured at 20°C, and this response requires HSF-1 activity (the right panel; p=0.993). In agreement with these results, hyperactivation of HSF-1 decreases daf-7 mRNA levels at 20°C (bottom; p<0.001). hsf-1(gf) represents a hyperactivating effect of an integrated hsf-1 transgene (hsf-1 cDNA). F, Epistasis model showing that DAF-11 and DAF-21 stimulate DAF-7 activity via inhibition of HSF-1. Thus, HSF-1 is an upstream component of the TGF-β cascade; it represses daf-7, thereby promoting dauer development at high temperatures. In fluorescent figures, images were captured with the same exposure time, and animals were examined at the L1 stage. N indicates number of animals tested, bars represent S.E.M. For statistics: Students t-test.

Barna et al. BMC Developmental Biology 2012 12:32   doi:10.1186/1471-213X-12-32
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