Role of Histone Methyltransferase SET-4 in Developmental Plasticity and Longevity

Abstract

FoxO transcription factors promote longevity across diverse organisms through upregulation of parallel stress responses. Therefore, understanding how FoxO activity is regulated may provide insights for improving overall vitality in an aging population. Insulin and insulin-like growth factor signaling (IIS) is highly conserved across metazoans and antagonizes FoxO function. Decreasing IIS pathway activity increases lifespan in model organisms and may be conserve in humans. This association between FoxO transcription factors and longevity was first discovered in studies using the free living nematode Caenorhabditis elegans. In C. elegans, the DAF-2/IIS pathway also influences dauer diapause. Dauer is a state of larval developmental diapause that occurs in response to stressful environmental conditions including population stress perceived through elaboration of a complex pheromone mixture. Mutations that reduce DAF-2/IIS pathway activity arrest as dauers in a manner requiring the FoxO transcription factor DAF-16. When AKT activity is reduced, DAF-16/FoxO translocates to the nucleus, where it is inhibited by EAK- 7/TLDC1, a conserved protein of unknown function. eak-7;akt-1 double mutants always arrest as dauers. We designed a forward genetic screen to discover novel regulators of DAF-16/FOXO by mutagenizing eak-7;akt-1 worms and looking for suppressors of eak-7;akt-1 dauer arrest (seak mutants). Whole genome sequencing revealed one seak mutant strain harbored a mutation in set- 4, which encodes a conserved histone H4 lysine 20 (H4K20) methyltransferase. SET-4 is required for dauer arrest in both reduced IIS mutant backgrounds and in response to dauer inducing pheromone. SET-4 promotes dauer arrest only in XX hermaphrodites, not XO males, consistent with existing data that SET-4 participates in dosage compensation of X chromosomes. We found that neuron-specific overexpression of set-4 rescued dauer arrest in set-4 mutants to a similar extent as the native promoter. SET-4 acts together with DAF-16/FoxO in specific sensory neurons to silence expression of the X-linked insulin-like peptide ins-9. Loss of ins-9 rescues the xvii dauer-defective phenotype observed in set-4 mutants in response to pheromone. ins-9 itself is also sensitive to dauer pheromone. Taken together, these data implicate chromatin remodeling as a novel mechanism that regulates FoxO transcription factor activity. Since epigenetic marks like histone methylation are sensitive to environmental interventions, these findings may lead to new therapies for chronic diseases associated with aging.