Studies found that sympathetic nerves showed germination growth and increased density in the tissues of some disease animal models and old mouse models. These studies suggest that sympathetic nerves are closely related to aging. In this study, the expression of tyrosine hydroxylase (TH), marker for sympathetic nerve was detected by immunohistochemistry in liver, lung, spleen tissues of older mice and human colorectal adenoma tissues. The results confirmed that the density of sympathetic nerve fibers increased significantly in aging tissues. However, the mechanism of this biological change in aging tissues is not yet clear. In this paper, the human fibroblast lines 2BS were used as the experimental materials, through transcriptome sequencing we found that the Netrin-1, a secretory axon guidance protein, was significantly upregulated in senescent cells. ELISA and Real-time PCR results showed that the level of Netrin-1 proteins and mRNAs were higher in premature senescence induced by bleomycin, ionizing radiation, cancer protein RasV12 overexpression and replicative senescence of 2BS cells than that of young control groups. DNA damage response (DDR) is an important regulatory pathway for senescent cell secretion proteins. To determine whether up-regulated expression of Netrin-1 in senescent cells is related to DDR, small molecule compounds were used to inhibit the activity of ataxia telangiectasia mutated (ATM) and checkpoint kinase (CHK2), which are important effector molecules of DDR, in bleomycin-induced senescent 2BS cells. ELISA and Real-time quantitative PCR results showed that: blocking DDR activity in senescent cells did not change the level of Netrin-1 compared with the bleomycin-induced senescence group, suggesting that Netrin-1 in senescent cells is not dependent on the DDR pathway. Western blotting revealed that histone methyltransferase EZH2 was significantly less expressed in senescent cells than in younger cells. Real-time PCR results showed that inhibition of EZH2 activity in young cells by small molecule compounds resulted in increased expression of Netrin-1. CHIP assays showed that EZH2 significantly enriched the DNA fragments in the promoter region of Netrin-1 in young cells, but not in senescent cells. The results above suggest that the down-regulated expression of EZH2 in senescent cells is an important reason for the increased expression of Netrin-1. The co-culture results of DRG in rats and senescent cells confirmed that senescent cells have a guiding effect on sympathetic nerves and this phenomenon is dependent on the secretion of Netrin-1. Based on the results above, this paper concluded that the up-regulated expression of Netrin-1 in senescent cells is independent of DDR pathway and related to the down-regulated expression of EZH2. Senescent cells promote sympathetic axonal directional growth by secreting Netrin-1. This provides a new clue for the in-depth study about the mechanism and prevention and treatment of senile diseases, which could be of potential application value.
周晓佳,于爱清,毛泽斌. EZH2在衰老细胞中表达下调引起Netrin-1表达增加进而促进交感神经轴突生长[J]. 中国生物化学与分子生物学报, 2019, 35(5): 517-527.
ZHOU Xiao-Jia, YU Ai-Qing, Mao Ze-Bin. Down-regulated Expression of EZH2 in Senescent Cells Leads to Increased Netrin-1 and thereby Promotes Sympathetic Axon Growth. Chinese Journal of Biochemistry and Molecular Biol, 2019, 35(5): 517-527.
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