In the present study, we explored the hub genes and pathways in damaged cartilage tissues of knee by bioinformatics methods. By comparing gene expression profiles between 10 knee cartilage from damaged side and 10 knee non-weight-bearing healthy cartilage tissues, we found 182 DEGs, including 123 up-regulated and 59 down-regulated genes in damaged cartilage tissues. Subsequently, GO enrichment analysis found that DEGs were associated with various biological processes and molecular functions, such as extracellular matrix organization, collagen catabolic process, antigen processing and presentation of peptide or polysaccharide antigen via MHC class II, and endocytic vesicle membrane. In addition, a PPI network with these DEGs was constructed, and 10 hub genes, including POSTN, FBN1, LOX, IGFBP3, C3AR1, MMP2, ITGAM, CDKN2A, COL1A1, COL5A1, were identified as the hub genes with higher connectivity degree in damaged cartilage tissues.
In view of the results of GO terms enrichment analysis, we linked the up-regulated gene DEGs with extracellular matrix organization and collagen catabolic process. The structure and organization of cartilage’s extracellular matrix are the primary determinants of normal function. Most diseases involving cartilage lead to dramatic changes in the extracellular matrix. Well organized elastic networks have been found in the superficial zone of articular cartilage using immunohistochemistry or multiphoton microscopy together with histochemistry. Fibrillin-1(FBN1) as the major component of elastic networks, mainly present in the uppermost superficial zone of articular cartilage . Collagens such as COL1A1 and COL5A1 and FBN1 were detectable in damaged cartilage tissues compared to undamaged controls in this study. The significant difference in expression levels of COL1A1, COL5A1, and FBN1 between damaged and undamaged sites of cartilage suggesting the attempt of chondrocytes for recovery of the damaged cartilage. Meanwhile, several matrix degrading proteases previously described to be linked to cartilage degeneration were up-regulated in damaged cartilage, such as MMP2 and POSTN. Study considered POSTN as a catabolic protein that promotes cartilage degeneration through collagen and proteoglycan degradation in OA by up-regulating MMP-13 and ADAMTS4 . These differentially-expressed genes demonstrated the balance between synthetic and catabolic activities of chondrocytes is struggling after cartilage injuries.
Lysyl oxidase (LOX), is a copper-dependent amine oxidase, that catalyzes cross-linking of collagen and elastin, which plays an important role in biological functions of extracellular matrix . LOXL2 was visualized in tissues from human knee and hip joints by immunofluorescence. LOXL2 is upregulated in cartilage affected by OA, may be a protective response that promotes anabolism while inhibiting specific catabolic responses in the pathophysiology of OA . LOXL3, a downstream of leptin, stimulates the apoptosis, but inhibits the autophagy of chondrocytes, is a potential therapy target for osteoarthritis . In this study, LOX is identified as an up-regulated gene in damaged cartilage tissues, which could promote cartilage maintenance.
IGFBP3 is one of six insulin-like growth factor binding proteins (IGFBPs) in humans. IGFBP3 modulates IGF signaling by binding to the two ligands (IGF-I and IGF-II) that primarily signal through IGF-I receptor . In healthy articular cartilage from individuals without OA, IGF-I activates anabolism and inhibits catabolism of cartilage. Higher levels of IGFBP3 have been observed in OA cartilage, leading to the decrease of IGF-I’s anabolic activity and could be partially responsible for the reduced responsiveness of OA cartilage to IGF-I [28, 29]. And a meta-analysis of genome-wide association studies of hip osteoarthritis found suggestive links between IGFBP3 and hip osteoarthritis and IGFBP3 overexpression induced cartilage catabolism and osteogenic differentiation . In this study, IGFBP3 is identified as an up-regulated gene in damaged cartilage tissues, which could promote cartilage degeneration.
Cells release into the extracellular environment different types of membrane vesicles called exosomes or microvesicles. Kim at al. found dendritic cells or vesicles derived from the dendritic cells expressing either secreted IL-4 or membrane-bound IL-4 are able to modulate the activity of APC and T cells in vivo through a MHC class II and partly Fas ligand/Fas-dependent mechanism, resulting in effective treatment of established collagen-induced arthritis . And Kim at al. also found exosomes derived from IL-10-treated dendritic cells can suppress inflammation and collagen-induced arthritis . In this study, we linked the up-regulated gene DEGs in antigen processing and presentation of peptide or polysaccharide antigen via MHC class II and endocytic vesicle membrane through GO terms enrichment analysis, which suggests extracellular vesicle and inflammation implicated in the pathogenesis of cartilage injuries.
In conclusion, our data provide a comprehensive bioinformatics analysis of DGEs and pathways, which may be involved in synthetic and catabolic metabolisms of chondrocytes after cartilage injuries. Nevertheless, additional experiments are needed to further validate the identifed genes and pathways.