Identification of DEGs
A total of 631 DEGs, including 341 up-regulated genes and 290 down-regulated genes, were identified from the GSE45233 dataset by comparing the gerontic and young meniscus samples. The heatmap and volcano plot of total DEGs were shown in Figure. 1A and B, respectively.
GO function and KEGG pathway enrichment analysis of the DEGs
The results of the GO analysis showed that DEGs related to CC were notably concentrated in extracellular region, extracellular space, integral component of plasma membrane, and proteinaceous extracellular matrix (Figure. 2A). Variations in DEGs connected with BP were mainly concentrated in signal transduction, regulation of transcription from RNA polymerase II promoter, and regulation of cell proliferation (Figure. 2B). Regarding MF, DEGs were prominently gathered in calcium ion binding, chromatin binding, sequence-specific DNA binding, and heparin-binding (Figure. 2C). KEGG pathway analysis manifested that the top typical pathways correlated with DEGs were PI3K-Akt signaling pathway, transcriptional misregulation in cancers, and Jak-STAT signaling pathway (Figure. 2D).
Construction of PPI network and hub genes analysis
The PPI network of the DEGs was constructed by employing STRING (Figure. 3A). The Degree, MCC, MNC, and EPC algorithms were applied to filtrate hub genes shown in Figure. 3B, C, D, and E, respectively. The mutual hub genes (RRM2, AURKB, CDK1, and TIMP1) were discerned by the Venn diagram (Figure. 3F). According to Metascape analysis, GO enrichment showed that the significant hub genes related to BP were mainly concentrated in aging and regulation of the cell cycle process (Figure. 3G).
Construction of a lncRNA–miRNA–mRNA network and molecular docking of potential drugs with hub genes
As for the screening of miRNAs, since there were no miRNAs that could act on all Hub genes, miRNAs working on three Hub genes were selected as ideal targets. Through the miRWalk website predictive analysis, we screened 5 miRNAs (hsa-miR-6810-5p, hsa-miR-4676-5p, hsa-miR-6877-5p, hsa-miR-8085, and hsa-miR-6133) (Figure. 4A, Table 2). As illustrated in Figure. 4B, these miRNAs are referred to some pathways that include glycosaminoglycans biosynthesis. To further understand the function of the hub genes in meniscal degeneration, we employed Cytoscape to combine lncRNA/miRNA interactions with miRNA/mRNA interactions, thus constructing a lncRNA-miRNA-mRNA network (Figure. 4C).
Patients undergoing meniscal resection may have difficulty regaining previous motion abilities.[19, 20] Therefore, preventing meniscus aging is one of the treatment strategies to reduce the incidence of meniscus tearing. Through probing the DGIdb website, we selected the above hub genes for molecular docking analysis in order to ascertain potential drugs that could act on these genes (Figure. 5D). The predicted results showed the high-affinity hydrogen binding events between drugs, including Cladribine, Danusertib, Barasertib, Riviciclib, and Dinaciclib, and residues of hub genes, so it was necessary to consider the possible senescence and degeneration of meniscus caused by these drugs in clinical application.
Table 2
Information of the 5 miRNAs most relevant to the common hub genes.
|
Target genes
|
Position
|
Energy
|
Binding region length
|
P-value
|
hsa-miR-6810-5p
|
RRM2,CDK1,TIMP1
|
3UTR
|
-24.1
|
29
|
0.01664477
|
hsa-miR-4676-5p
|
RRM2,CDK1,TIMP1
|
3UTR
|
-22.9
|
18
|
0.007139029
|
hsa-miR-6877-5p
|
RRM2,CDK1,TIMP1
|
3UTR
|
-21.8
|
24
|
0.000873123
|
hsa-miR-8085
|
RRM2,CDK1,TIMP1
|
3UTR
|
-22.6
|
25
|
0.006883728
|
hsa-miR-6133
|
RRM2,AURKB,TIMP1
|
3UTR
|
-22.4
|
17
|
0.001472693
|
RRM2, ribonucleotide reductase regulatory subunit M2; AURKB, aurora kinase B; CDK1, cyclin dependent kinase 1; TIMP1, tissue inhibitor of metalloproteinases 1.
The expression of AURKB, RRM2, CDK1, and TIMP1 were decreased in senescent meniscus tissues
SafraninO-Fast green and Toloniumchloride Blue staining results of the meniscus of 18-month-old aging mice revealed more severe degenerative histopathological changes than the meniscus in 3-month-old young mice due to decreased collagen fiber content (Figure.5A). Immunohistochemistry showed that the expression levels of AURKB, RRM2, CDK1, and TIMP1 protein were higher in the meniscus of young mice than that of the senescent mice (Figure.5B and C). In order to further explore the expression levels of hub genes in senescent meniscus, we applied human meniscus for immunohistochemical detection. The results illustrated that the Hub genes were also poorly expressed in senescent human meniscus (Figure.5D and E). Moreover, the mRNA levels of AURKB, RRM2, CDK1, and TIMP1 were also decreased both in human and mouse menisci, respectively (Figure.5F). These founds suggested that the decreased gene expressions of AURKB, RRM2, CDK1, and TIMP1 might be a trigger or indicator that participating in meniscus aging and degeneration.
Hub genes expression in cultured mouse meniscus cells
As an additional method to further investigate the expressions of hub genes in senescent meniscus tissues, we treated normal mouse meniscus cells with IL-1β (10ng/ml) for 48h to induce degeneration in mouse meniscus cells in vitro. Take into consideration that the overexpression and accumulation of endogenous lysosomal β-galactosidase are one of the characteristics of senile cells, SA-β-gal is often taken as a biomarkers of senile cells.[21, 22] More positive stained cells, the blue ones, were observed in the IL-1β group than the control by the SA-β-gal staining (Figure. 6A). Meanwhile, more apoptotic cells were detected in the IL-1β group than the control by flow cytometry (Figure. 6B and D). In addition, the suppressed mRNAs expressions of AURKB, RRM2, CDK1, and TIMP1 were detected in meniscus cells treated with IL-1β (10ng/ml) when compared with the control group (Figure. 6C). Then, the immunofluorescence assay further supported that these Hub genes were poor expressions in senescent meniscus cells (Figure. 6E). These consequences further confirmed the decreased AURKB, RRM2, CDK1, and TIMP1 expressions in the senescent meniscus.