Circular RNA circ-4099 is induced by TNF-α and regulates ECM synthesis by blocking miR-616-5p inhibition of Sox9 in intervertebral disc degeneration

Circular RNAs (circRNAs) play important roles in the initiation and development of different diseases. Here, we detected their role in intervertebral disc (IVD) degeneration. An Arraystar human circular RNA microarray assay was used to detect circRNAs in normal and degenerated human IVD nucleus pulposus (NP) tissues. The role of circ-4099 in IVDD and its mechanism were evaluated by qRT-PCR and gain-of-function/loss-of-function studies. Interaction networks for competing endogenous RNAs (ceRNAs), miRNAs, and miRNA target gene were detected by bioinformatics analysis, RNA immunoprecipitation and luciferase assay. Expression of seventy-two circRNAs were increased by more than twofold in degenerated NP tissues. qRT-PCR showed that the expression of circ-4099 in NP tissues was consistent with that of the array screening. Over-expression of circ-4099 increased the expression of Collagen II and Aggrecan and decreased the secretion of the pro-inflammatory factors IL-1β, TNF-α, and PGE2. TNF-α treatment increased circ-4099 expression in NP cells. NF-κB/MAPK inhibitors or shRNAs abolished the inductive effects of TNF-α on circ-4099 expression. We further demonstrated that circ-4099 was able to function as a “sponge” by competitively binding miR-616-5p, which reversed the suppression of Sox9 by miR-616-5p. We used DNA pull-down and spectrometry experiments to show that TNF-α can promote circ-4099 transcription through upregulation of GRP78. We provide the first evidence that shows circRNAs are differentially expressed in degenerated and normal NP tissues. Circ-4099 may play a role in a protective mechanism and be part of a compensatory response that maintains the synthesis and secretion of the extracellular matrix in NP cells and might be a protective factor in IVD degeneration as well as restore NP cell function. Spinal disease: Circular RNA linked to disc degeneration A circular RNA molecule helps protect against degenerative disc disease. Hua Wang and coworkers from Sun Yat-Sen University in Guangzhou, China, examined whether circular RNAs, regulatory molecules that take the form of closed RNA loops, contribute to intervertebral disc degeneration, a condition in which connective tissue in the spine breaks down over time, causing back pain and weakness. They found 72 circular RNAs that were either significantly over- or under-expressed in the inner core tissue of intervertebral discs from patients with this condition. They showed that one of these circular RNAs, ci