Low back pain is a global health concern, with intervertebral disc degeneration (IVDD) being a major contributor. IVDD is a progressive condition characterized by structural damage and cellular dysfunction within the disc, leading to instability and chronic inflammation. Among the disc's components, the annulus fibrosus (AF) is crucial for maintaining disc integrity. During IVDD, AF cells undergo significant changes, including matrix degradation, increased fibrosis, and inflammatory activation. These alterations compromise the disc's structure and function, accelerating tissue breakdown. Despite the central role of AF cells in IVDD, the underlying mechanisms driving their degeneration are not fully understood.
PANoptosis is a recently discovered form of programmed cell death that simultaneously activates pyroptosis, apoptosis, and necroptosis. It is regulated by the PANoptosome complex, which recruits key molecules like ASC, RIPK3, and Caspase-8. PANoptosis has gained attention in IVDD research due to its role in excessive cell death and inflammation. Studies have shown that PANoptosis occurs in nucleus pulposus cells (NPCs), and its inhibition can promote ECM synthesis. However, the relationship between PANoptosis and AF cells remains unexplored.
Toll-like receptor 4 (TLR4) is a key molecule linking inflammation and cell death, and its involvement in various degenerative diseases has been established. NLRP12, a negative regulatory factor in the NLR family, plays a crucial role in balancing inflammatory signal transduction and cell death. A study found that NLRP12 is induced by heme and PAMPs through the TLR2/4-IRF1-ROS axis, leading to PANoptosis and the release of inflammatory factors. However, its role in the inflammatory microenvironment of IVDD is unclear.
This study aims to explore the regulatory role of the TLR4 signaling pathway in PANoptosis of AF cells and clarify the mechanism of NLRP12 as a key molecule. By analyzing single-cell RNA sequencing data, we identified cellular remodeling and subtype shifts in degenerative AF tissue. We found an increased proportion of fibroblast-like AF cells and transcriptional enrichment of pyroptosis and apoptosis pathways. Functional experiments confirmed the activation of PANoptosis in AF cells upon TNF-α stimulation. Transcriptomic analysis revealed TLR4 as a key upstream regulator, acting via NLRP12. In vivo, TLR4 knockdown mitigated disc degeneration, further supporting its role in AF PANoptosis during IVDD.
Our findings provide novel insights into the degenerative process of the disc. By identifying PANoptosis as a relevant mode of cell death in AF cells, we highlight the potential of targeting the TLR4-NLRP12 axis for therapeutic interventions in IVDD. Further validation in human IVD tissues and exploration of additional regulatory pathways are warranted to fully understand the complexity of disc degeneration.
