Objectives : Intervertebral disc degeneration (IDD) is widely accepted as a cause of low back pain and related degenerative musculoskeletal disorders. Nucleus pulposus (NP) cell apoptosis which is related to excessive endoplasmic reticulum (ER) stress in the intervertebral disc (IVD) could aggravate IDD progression. Many studies have shown the therapeutic potential of exosomes derived from bone marrow mesenchymal stem cells (MSC-exos) in degenerative diseases. We hypothesized that the delivery of MSC-exos could modulate ER stress and inhibit excessive NP cell apoptosis during IDD. Methods : The ER stress levels were measured in normal or degenerative NP tissues for contrast. The effects of MSC-exos were testified in advanced glycation end products (AGEs) -induced ER stress in human NP cells. The mechanism involving AKT and ERK signaling pathways was investigated using RNA interference or signaling inhibitors. Histological or immunohistochemical analysis and TUNEL staining were used for evaluating MSC-exos therapeutic effects in vivo . Results : The ER stress level and apoptotic rate was elevated in degenerative IVD tissues. MSC-exos could attenuate ER stress-induced apoptosis by activating AKT and ERK signaling. Moreover, delivery of MSC-exos in vivo modulated ER stress-related apoptosis and retarded IDD progression in a rat tail model. Conclusions : These results highlight the therapeutic effects of exosomes in preventing IDD progression. Our work is the first to demonstrate that MSC-exos could modulate ER stress-induced apoptosis during AGEs-associated IVD degeneration.

Oxidative stress-induced mitochondrial dysfunction and nucleus pulposus (NP) cell apoptosis play crucial roles in the development of intervertebral disc degeneration (IDD). Increasing studies have shown that interventions targeting impaired autophagic flux can maintain cellular homeostasis by relieving oxidative damage. Here, we investigated the effect of curcumin (CUR), a known autophagy activator, on IDD in vitro and in vivo. CUR suppressed tert-butyl hydroperoxide- (TBHP-) induced oxidative stress and mitochondrial dysfunction and thereby inhibited human NP cell apoptosis, senescence, and ECM degradation. CUR treatment induced autophagy and enhanced autophagic flux in an AMPK/mTOR/ULK1-dependent manner. Notably, CUR alleviated TBHP-induced interruption of autophagosome-lysosome fusion and impairment of lysosomal function and thus contributed to the restoration of blocked autophagic clearance. These protective effects of CUR in TBHP-stimulated human NP cells resembled the effects produced by the autophagy inducer rapamycin, but the effects were partially eliminated by 3-methyladenine- and compound C-mediated inhibition of autophagy initiation or chloroquine-mediated obstruction of autophagic flux. Lastly, CUR also exerted a protective effect against puncture-induced IDD progression in vivo. Our results showed that suppression of excessive ROS production and mitochondrial dysfunction through enhancement of autophagy coupled with restoration of autophagic flux ameliorated TBHP-induced human NP cell apoptosis, senescence, and ECM degradation. Thus, maintenance of the proper functioning of autophagy represents a promising therapeutic strategy for IDD, and CUR might serve as an effective therapeutic agent for IDD.

Background: Intervertebral disc degeneration (IDD) is a major contributor to lower back pain, however, the molecular and pathogenetic mechanisms underlying IDD are poorly understood. As a high-risk factor for IDD, compression stress was reported to induce apoptosis of nucleus pulposus (NP) cells and extracellular matrix (ECM) degradation during IDD progression. Circular RNA (circRNA) is a class of endogenous non-coding RNA (ncRNA) and has been reported to function in several diseases. However, whether and how circRNA regulates compression-induced damage of NP cells remains vague. Here, we aimed to investigate the key role of circRNA in compression loading-induced IDD. Methods: We analysed the circRNA expression of three samples from compression-treated NP cells and three control samples using circRNA microarray assays and further investigated the circRNA involved in compression-induced damage of NP cells (circRNA-CIDN). We investigated the effects of circRNA-CIDN on compression-induced cell apoptosis and NP ECM degradation in vitro and ex vivo. We observed that circRNA-CIDN bound to miRNAs as a miRNA sponge based on luciferase and RNA immunoprecipitation (RIP) assays. Findings:: CircRNA-CIDN was significantly downregulated in compression-treated human NP cells, as validated by circRNA microarray and qRT-PCR analysis, and overexpressing circRNA-CIDN inhibited compression-induced apoptosis and NP ECM degradation. Further studies demonstrated that circRNA-CIDN served as a sponge for miR-34a-5p, an important miRNA that enhanced compression-induced damage of NP cells via repressing the silent mating type information regulation 2 homolog 1 (SIRT1). CircRNA-CIDN was also verified to contain IDD development in an ex vivo IDD model. Interpretation: Our results revealed that circRNA-CIDN binding to miR-34a-5p played an important role in mitigating compression loading-induced nucleus pulposus cell damage via targeting SIRT1, providing a potential therapeutic strategy for IDD treatment.

Objective: Lactate accumulation is an important factor in the intervertebral disc degeneration (IVDD). Currently, the effect and underlying mechanism of action of lactate on nucleus pulposus (NP) cell inflammation during IVDD are unclear. Previous studies have found that the NLRP3 inflammasome plays an important role in the regulation of NP inflammation. This study focused on the regulation of acid-sensitive ion channels (ASICs) in relation to inflammation and the effect of NLRP3 on pyroptosis levels in NP cells under acidic conditions.Design: For the in vitro experiments, human NP cells were exposed to 6 mM lactate solution; different groups were either treated with NLRP3 inhibitor or transfected with siRNA against NLRP3, siRNA against ASC or a mix of these, and mRNA and protein expression levels were then assessed. For the in vivo experiment, varying concentrations of lactate were injected into rat intervertebral discs and examined via magnetic resonance imaging (MRI) and histological staining. Results: Extracellular lactate promoted NLRP3 inflammasome activation and degeneration of the NP extracellular matrix; furthermore, it increased the levels of inflammation and pyroptosis in the NP. Lactate-induced NLRP3 inflammasome activation was blocked by ASIC inhibitors and NLRP3 siRNA. Conclusions: Extracellular lactate regulates levels of intercellular reactive oxygen species (ROS) through ASIC1 and ASIC3. ROS activate the NF-κB signalling pathway, thus promoting NLRP3 inflammasome activation and IL-1β release, both of which promote NP degeneration. How to cite this article: Zhao K, An R, Xiang Q, et al. Acidsensing ion channels regulate nucleus pulposus cell inflammation and pyroptosis via the NLRP3 inflammasome in intervertebral disc degeneration.

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Low back pain (LBP) is a major musculoskeletal disorder and the socioeconomic problem with a high prevalence that mainly involves intervertebral disc (IVD) degeneration, characterized by progressive nucleus pulposus (NP) cell death and the development of an inflammatory microenvironment in NP tissue. Excessively accumulated cytosolic DNA acts as a damage-associated molecular pattern (DAMP) that is monitored by the cGAS-STING axis to trigger the immune response in many degenerative diseases. NLRP3 inflammasome-dependent pyroptosis is a type of inflammatory programmed death that promotes a chronic inflammatory response and tissue degeneration. However, the relationship between the cGAS-STING axis and NLRP3 inflammasome-induced pyroptosis in the pathogenesis of IVD degeneration remains unclear. Here, we used magnetic resonance imaging (MRI) and histopathology to demonstrate that cGAS, STING, and NLRP3 are associated with the degree of IVD degeneration. Oxidative stress induced cGAS-STING axis activation and NLRP3 inflammasome-mediated pyroptosis in a STING-dependent manner in human NP cells. Interestingly, the canonical morphological and functional characteristics of mitochondrial permeability transition pore (mPTP) opening with the cytosolic escape of mitochondrial DNA (mtDNA) were observed in human NP cells under oxidative stress. Furthermore, the administration of a specific pharmacological inhibitor of mPTP and self-mtDNA cytosolic leakage effectively reduced NLRP3 inflammasome-mediated pyroptotic NP cell death and microenvironmental inflammation in vitro and degenerative progression in a rat disc needle puncture model. Collectively, these data highlight the critical roles of the cGAS-STING-NLRP3 axis and pyroptosis in the progression of IVD degeneration and provide promising therapeutic approaches for discogenic LBP.

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The present findings suggest that icariin may have a protective effect on NP cells. The antiinflammatory effect may contribute to the therapeutic action of icariin in IDD.