Cytosolic escape of mitochondrial DNA triggers cGAS-STING-NLRP3 axis-dependent nucleus pulposus cell pyroptosis

Abstract

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.

Identification of SMIM1 and SEZ6L2 as Potential Biomarkers for Genes Associated with Intervertebral Disc Degeneration in Pyroptosis

“…The mechanism of IDD caused by pyroptosis is definite [ 41 ]; therefore, this study found that inflammatory factors IL1A, IL1B, NOD2, GBP1, IL6, AK1, EEF2K, and PYCARD were significantly altered in IVD after the intervention. Subsequently, WGCNA analysis was performed to identify the modules associated with cell scorching, and ultimately 11 candidate genes were retrieved from the intersection, namely, SMIM1, FBLN2, ZFP2, B4GALT5, HCRT, SLC6A17, MUSK, SLC26A8, CRHR2, SEZ6L2, and KCNJ15.…”

The cGAS-STING Pathway Affects Vertebral Bone but Does Not Promote Intervertebral Disc Cell Senescence or Degeneration

“…This DNA-sensing pathway promotes SASP and mediates type-I interferon inflammatory responses to foreign viral and bacterial DNA as well as self-DNA ( 22 ). Cursory studies of NP cells and in vivo injury models have implicated the cGAS-STING pathway as an active inducer of senescence in the disc; however, this pathway in the disc is yet to be explored independent of chemical or mechanical insult ( 27 , 28 , 53 – 55 ). In this study, we clearly show STING is not a critical mediator of senescence onset and consequent degeneration in the disc.…”

“…Recently, a rat model of vertebral inflammation-induced intervertebral disc degeneration, where lipopolysaccharide (LPS) was introduced into vertebral defects, showed a degenerative phenotype marked by cartilaginous endplate defects, immune cell infiltration, and higher levels of cGAS, STING, TBK1, and IRF3 ( 27 ). Likewise, an acute injury model of disc herniation was used by Zhang et al to show that inhibition of STING results in the partial rescue of a degenerative phenotype by modulating NLRP3 signaling ( 28 ).…”

The cGAS-STING pathway affects vertebral bone but does not promote intervertebral disc cell senescence or degeneration

“…Recent work has demonstrated cGAS-STING activation in a LPS vertebral injury-induced disc degeneration model; however, this activation was only achieved with the inflammatory stimulus of LPS and by breaching the integrity of the EP, enabling immune cell infiltration into the disc compartment (27). Similarly, a recent study showed the degenerative effects of needle puncture injury to rat discs were reduced when STING activation was attenuated by limiting mitochondrial DNA release from damaged mitochondria resulting from calcium overload (28). It is likely, however, that in the context of this acute herniation injury, infiltrating scavenging immune cells such as neutrophils and macrophages were the primary effectors of the cGAS-STING response and responsible for promoting inflammation; a similar immune response has been documented in the Tg197 mouse model, which overexpresses hTNF and is predisposed to spontaneous disc herniations (61).…”

“…This DNAsensing pathway promotes SASP and mediates type-I interferon inflammatory responses to foreign viral and bacterial DNA as well as self-DNA (22). Cursory studies of NP cells and in vivo injury models have implicated the cGAS-STING pathway as an active inducer of senescence in the disc; however, this pathway in the disc is yet to be explored independent of chemical or mechanical insult (27,28,(53)(54)(55). In this study, we clearly show STING is not a critical mediator of senescence onset and consequent degeneration in the disc.…”