Unmasking Transposable Element Transcriptional Heterogeneity In Senescent Cells

In the lifespan of a cell,they continually experience exogenous and endogenous damage and stress.When the damage exceeds the limits of repair capacity,some cells will enter a state of irreversible arrest of proliferation which we called cellular senescence.Cellular senescence can be triggered by telomere erosion,DNA damage and oncogene activation.Senescent cells in vitro exhibit changes in cellular morphology and chromatin organization.Senescent cells secrete a plenty of factors which called senescence associated secretory phenotype(SASP).Cellular senescence plays important roles in tumor suppression,development,aging and age-related disease.DNA repetitive sequences occupy about 50%of the human genome and are mainly composed of transposable elements including retrotransposons and DNA transposons,which are widely distributed in heterochromatin-enriched regions of normal cells.Numerous findings have shown an association between activation of DNA repetitive sequences and senescence.After cellular senescence occurs,the accessibility of DNA repetitive sequences changes due to chromatin remodeling.However,the heterogeneity of different types of DNA repetitive sequences expressed between different types of cellular senescence and the heterogeneity of their expression within senescent cells is not known,and the question of how the heterogeneity of DNA repetitive sequence expression is associated with senescent cells and whether there are key transcription factors that regulate DNA repetitive sequence heterogeneity remains to be addressed.Therefore,this project is to investigate the heterogeneity of DNA repetitive sequences in senescent cells and to find the key transcription factors that regulate them.The main findings are as follows:(1)Analysis of transcriptome data from different senescent cells,different tissue and different time points during cellular senescence revealed that DNA repetitive elements were generally up-regulated after the onset of cellular senescence,with the L1 being the most significantly up-regulated family.In addition,differentially expressed DNA repetitive elements are mainly derived from heterochromatin,Poly comb repressed regions and Quiescent regions.(2)Analysis of oncogene-induced senescence single-cell transcriptome data revealed that the expression of DNA repetitive elements was also heterogeneous within senescent cells and highly correlated with the heterogeneity of senescent cells.By constructing a co-expression regulatory network,we found that the L1 family dominates the network and is highly correlated with the expression of genes in cellular senescence-associated pathways.(3)By examining the protein coding gens adjacent to common up-regulated DNA repeat element across different senescent cells.We found that these genes are enriched in cellular senescence associated pathway:type Ⅰ interferon signaling,cytokine response,negative regulation of apoptosis,growth factor response,and angiogenesis pathways.Suggesting that DNA repeat elementss may play a cis-regulatory role in regulating the expression of protein coding genes associated with cellular senescence,thereby promoting cellular senescence.(4)By performing transcription factor motif enrichment analysis in common differentially expressed DNA repetitive elements in senescent cells,we screened for transcription factors enriched in common differentially expressed DNA repetitive elements in senescent cells.Candidate genes were screened using luciferase reporter vectors,and finally PAX5 was found to significantly promote LINE-1 promoter activity.Chromatin immunoprecipitation experiments confirmed that PAX5 binds to the first 100-300 bp of the LINE-1 5’-UTR region.Finally,PAX5 overexpressed stable lines were irradiated with IR to induce stress-induced senescence cells,and a series of experiments revealed that PAX5 overexpression promoted increased LINE-1 mRNA expression and PAX5 overexpression contributed to increased expression of cellular senescence markers such as β-galactosidase activity,p21 protein expression and cellular senescence-related secreted phenotypic factors.The above results tentatively confirm that the screened transcription factor PAX5 promotes the transcription of LINE-1 and its overexpression also promotes cellular senescence.The present results provide a good target for the future development of anti-aging drugs to improve the health of the elderly.