The Role And Mechanism Of SET-18 In Regulating C.elegans's Lifespan

Histones methylation at lysines is catalyzed by histone lysine methyltransferases(HKMTs),which contain SET(Suppressor of variegation,Enhancer of zeste,Trithorax)domain(excluding DOT1-like protein).SET domain contributes to the enzymes binding to histones and transferring methyl onto the lysines of these substrates.Amounting evidence has shown that histone modifications play a key role in regulating aging,including trimethylation of histone H3 at Lys 4,Lys 27 and Lys 36.Caenorhabditis elegans(C.elegans)is classical model for studying aging.Aging is subjected to regulation by typical signaling pathways conserved from worms to mammals,including insulin/IGF-1 signaling(IIS)pathway.In C.elegans,the insulin-like receptor DAF-2 is activated by insulin-like ligands.DAF-2 recruits a PI3 kinase,AGE-1/AAP-1,which induces phosphorylation of downstream serine/threonine kinases AKT-1,AKT-2,SGK-1,and PDK-1.This cascade phosphorylates forkhead transcription factor(FoxO)DAF-16 and prevents it from entering nuclei and activating target genes including antiaging and stress-resistance genes.By RNAi screening,some HKMTs have been discovered to influence lifespan,including ASH-2,SET-2,SET-4,SET-6,SET-9,SET-15,and SET-26.C.elegans SET-18 has high homology to the mammalian histone methyltransferase SMYD(SET and MYND Domain Containing)family(SMYD1-3).However,the roles of SET-18 remain unclear to date.To discover new HKMTs regulating lifespan,we performed RT-qPCR screening for 38 genes that encode known histone methyltransferases and proteins containing SET domain by detecting the changes of their mRNA expression levels in aging process.We found that the mRNA level of set-18 is up-regulated in the aged worms(day 11),suggesting that it functions in lifespan control.By utilizing set-18(gk334)mutants,we found that loss of set-18 extended the lifespan of wild-type worms,but not daf-2 mutants,indicating that SET-18 regulated lifespan in insulin/IGF-1-like pathway.SET-18 homologous SMYD family proteins have histone methyltransferase activity.In vivo and in vitro histone methylation assays showed that SET-18 predominantly di-methylate H3 at Lys36.Loss of set-18 decreased the H3K36me2 level and increased H3 acetylation(H3Ac)level on daf-16 a promoter,resulting in the up-regulation of daf-16 a expression.Importantly,we observed in the wild-type worms,the muscle-specific expression of SET-18 was activated in aged stages(day 7 and day 11),which lead to the increase of global H3K36me2 level and the decrease of daf-16 a expression.These findings suggest that H3K36me2 modification catalyzed by muscle-specific histone methyltransferase SET-18 might be a new driver of aging,which will have key implications for studying the roles of H3K36me2 methyltransferases on human age-related diseases.