C.elegans Histone Methyltransferase SET-18 Regulates ER Unfolded Protein Response(UPR^ER) Via XBP-1

Endoplasmic reticulum（ER）is an important site for protein processing in eukaryotic cells.The accumulation of a large number of unfolded proteins in ER will lead to ER stress and activate UPR^ER.The regulation of UPR^ERR in eukaryotes mainly includes three pathways IRE-1/XBP-1、ATF-6、PEK-1,among which IRE-1/XBP-1 is considered the most important and conservative.When ER stress occurs,ire-1dissociates from chaperone molecule GRP78 and activates its endonuclease activity,thereby splicing XBP-1 mRNA to produce sXBP-1.sXBP-1 as a transcription factor activates the expression of UPR^ERR related genes.In addition to UPR^ER,endoplasmic reticulum-associated protein degradation（ERAD）and endoplasmic reticulum stress-mediated autophagy are also considered to be two ways to regulate the quality control of ER protein.Moreover,studies have found that the above two methods are also closely related to the regulation of UPR^ER.Studying the regulation mechanism of UPR^ERR by using model biology is the main way to understand UPR^ERR activity.C.elegans is one of the model organisms that study the regulation of UPR^ER.HSP-4 is a homologous of mammalian chaperone GRP78.It has been found that treating nematodes with Tunicamycin（TM）and infecting wild-type nematodes with pseudomonas aeruginosa（PA14）can induce ER stress,increase the expression of hsp-4 in nematodes,and activate the nematodes UPR^ER.Furthermore,the decrease in the rate of nematode development from eggs to adults under the TM treatment conditions due to gene mutations（adult rate）,and the shortened survival rate of nematodes under PA14 infection conditions,reflected the inhibition of UPR^ERR activation.Recently,we reported that histone H3K36 dimethyltransferase SET-18 can regulate the aging of nematodes.Since there have been reports in the literature that UPR^ERR is involved in the regulation of nematode senescence,we conducted a preliminary study on the relationship between SET-18 and UPR^ER.Observation by inverted fluorescence microscopy revealed that the mutation of set-18 gene can inhibit the upregulation of HSP-4::GFP expression level under TM treatment,indicating that SET-18 may have an activation effect on ER stress induced UPR^ER.In which way does SET-18 activate ER stress induced UPR^ER?Whether this activation process is related to ER stress-mediated ERAD and autophagy processes.In order to answer the above questions,we investigated the possible ways of set-18 activating UPRER by analyzing the adult rate under TM treatment and the survival rate under PA14infection.The experimental results show that,compared with the wild-type nematode,3μg/ml TM treatment resulted in a significant decrease in the adult rate of the set-18（gk334）mutant nematode;moreover,the decline process of adult rate of set-18（gk334）mutant nematodes was inhibited by xbp-1 mutation,but was not affected by mutations of pek-1 and atf-6 genes,indicating that under TM induced ER stress,SET-18activated UPR^ERR through XBP-1.In addition,we found that after PA14 infection,there was no significant difference in survival rate and median lifespan between the set-18 mutant nematode and the wild-type nematode;but the mutation of set-18 gene inhibited the decline of survival rate of xbp-1 mutant nematodes due to PA14 infection.The above results show that under the condition of ER stress induced by PA14infection,SET-18 cannot directly activate UPR^ER,but SET-18 participates in the process of XBP-1 regulating UPR^ER.It is known that sXBP-1 after splicing has an important activation effect on the expression of UPRER target gene.Therefore,we analyzed the mRNA expression of sxbp-1 and its target gene by fluorescence quantitative PCR,and found that SET-18did not affect the changes in the splicing level of xbp-1 and the mRNA level of sXBP-1 target gene caused by TM treatment.In addition,we found that set-18 gene mutation inhibited mRNA expression level of ERAD gene ufd-1 under TM treatment,but had no effect on mRNA expression level of autophagy-related genes.The above results indicate that under the condition of TM induced ER stress,SET-18 activates UPR^ERR through XBP-1 and affects the mRNA expression of ERAD gene ufd-1.In the process of PA14 infection inducing ER stress,although SET-18could not directly activate UPR^ER,it also participated in the process of XBP-1regulating UPR^ER.The above study laid a foundation for further analysis of the mechanism of histone H3K36 dimethylation modification in regulating UPR^ER.