The circRNAs were originally discovered in plant viroid and yeast mitochondrial RNAs [7, 21] and have been rarely tested by transcriptomic polyadenylated RNA profiling owing to the lack of polyadenylation at their 3' ends [22–25]. With the development of novel technology including calculation procedure and high-throughput sequencing , non-polyadenylated human RNA sequencing analysis and the signals of RNA sequencing can be obtained to assess back-spliced exons  or stabilized introns . With their covalently closed structure, circRNAs are highly stable non-coding RNAs that play a critical role in cell and tissue development and several disease processes, including cancer, IDD, and neurodegeneration [19, 28–30].
Although the molecular mechanisms of circRNAs in mammalian cells remain mostly unclear, circRNA regulation of the level of mRNAs by a circRNA-miRNA-mRNA network may be an important mechanism in the initiation and development of IDD. Wang et al. showed that the circular RNA SEMA4B could weaken the impact of IL-1β on NPC senescence, ECM and aggrecan degradation in IDD via inhibition of miR-431 (a targeted miRNA of circRNA SEMA4B) . Moreover, circRNA_104670 suppressed the expression level of collagen II and the proliferation of NPCs, and promoted NPCs' apoptosis by circRNA_104670/miR-17-3p/MMP-2. The circRNA VMA21, regarded as a sponge of mir-200c, plays a role in regulating the level of inflammatory cytokines by targeting mir200c and XIAP . To research the role of circRNAs in IDD, we identified 620 DEcircRNAs by comparing normal and degenerated groups' expression profiles. Similarly, we discovered 13 DEmiRNAs and 273 DEmRNAs. To further understand the underlying mechanisms of these RNA molecules, we constructed a circRNA-miRNA-mRNA network for IDD. Our research provided several novel targets and molecular biomarkers for IDD. The hsa_circ_0091570, a sponge of miR-1307 that acted to regulate ISM1 expression in hepatocellular cancer, was screened from the ceRNA network constructed by 8 circRNAs, 13 miRNAs and 15 genes . Three additional genes, CHEK1, TSGA10, and GSTCD, have been reported to downregulate genes that inhibit cellular proliferation or reduce cell numbers [33–35]. what's more, NPCs' proliferation and apoptosis play a key role in IDD . Meanwhile, we confirmed that the level of these genes was downregulated in IDD. Thus, Hsa_circ_0091570 may suppress NPC proliferation by downregulating the expression of CHEK1, TSGA10, and GSTCD. The regulated network of hsa_circ_0091570-hsa-miR-508-5p-CHEK1/TSGA10/GSTCD was constructed and the RNAs (circRNAs and miRNAs) and genes in the network may all be novel targets for IDD. In parallel, Li et al. reported that miR-183-3p regulates autophagy in gastric cancer cells by the PI3K/AKT/mTOR pathway . According to previous research, the PI3K/AKT/mTOR pathway has a significant effect on cell growth, survival, metabolism, proliferation, and angiogenesis . Moreover, overexpressed PLG can promote the proliferation of keratinocytes . We constructed a network with downregulated hsa_circ_0004565, upregulated hsa-miR-183-3p, and downregulated PLG. However, whether hsa_circ_0004565 depresses the proliferation of NPCs via downregulating the expression of PLG which further regulates PI3K/AKT/mTOR signaling is unclear yet. Some studies have shown that S100B regulates inflammation by increasing the expression of TNF‑α and IL‑1β during osteoarthritis . Overexpression of TNF‑α and IL‑1β is also significant in the breaking of intervertebral discs . However, we found that the level of S100B expression was decreased in IDD. The S100B-related ceRNA network (hsa_circ_0003183/hsa_circ_0032253/hsa_circ_0001293-hsa-miR-4534- S100B) was constructed. We discover that circRNA (hsa_circ_0003183/hsa_circ_0032253/hsa_circ_0001293) may act as an independent protective factor to inhibit IDD by decreasing the expression of S100B. Many miRNAs are involved in different diseases, such as hsa-miR-1827 and hsa-miR-508-5p[41, 42]. In our research, we found that hsa-miR-1827 regulated the seven upregulated target genes, including GBP6, CEACAM8, C7orf34, KCNV2, OLFM4, NR5A1, and ZNF488. Studies have demonstrated that hsa-miR-1827 and its target(s) are closely associated with breast cancer, cervical cancer, and tongue squamous cell carcinoma [42–48]; we also explored whether these genes or miRNAs are involved in IDD, and then we further verify the connection of circRNA and IDD by constructing an integrated circRNA-miRNA-mRNA network. For example, the high expression of target genes CEACAM8 and OLFM4 plays a crucial role in the inflammatory process by increasing the level of inflammatory cytokines [49, 50]. A great number of inflammatory cytokines promote extracellular matrix degradation and disorder of NPCs' inner or outer environment, which plays a significant role in IDD . We also constructed the hsa_circ_0057552-hsa-miR-1827-GBP6/CEACAM8/C7orf34/KCNV2/OLFM4/NR5A1/ZNF488 network. We find that hsa_circ_0057552 may take part in the inflammatory process and degeneration in IDD by enhancing the expression of CEACAM8 and OLFM4. Additionally, it was reported that ZNF488-overexpressing cells could increase the expression of collagen IV  which obviously decreased in IDD . Hence, hsa_circ_0057552 may be a protective factor to resist IDD by increasing the expression of ZNF488. Although different miRNAs or genes involved in the network have been reported in different studies, this ceRNA-regulated network has not yet been reported in IDD. Moreover, the molecular circadian function of the hsa_circ_0057552-hsa-miR-1827-GBP6/CEACAM8/C7orf34/KCNV2/OLFM4/NR5A1/ZNF488 network remains unclear and will require further research.
KEGG analysis showed that the majority of genes were enriched in the p53 signaling pathway. Although p53 is a nuclear transcription factor that plays a vital role in regulating cell cycle progression, senescence, and cell death , its role in IDD will require further study. GO analysis revealed genes mostly focused on the regulation of the nervous system process, myelination, and steroid hormone receptor activity.
In conclusion, this research identified several circRNA-miRNA-mRNA interaction axes, such as hsa_circ_0091570-hsa-miR-508-5p- CHEK1, hsa_circ_0057552-hsa-miR-1827-GBP6, and hsa_circ_0003183-hsa-miR-4534- S100B, which provided novel insight into the pathological mechanism and clinical treatment of IDD.
This study has some restricted conditions. At first, the number of samples is not enough. We need an additional cohort to further validate the expressed level of circRNAs, miRNAs and mRNAs in IDD. Secondly, based on our preliminary screening study, the interactions of identified ceRNA should be confirmed in future studies.