Changes in Nucleus Pulposus Cell Atlas and the Role of SPP1 During Intervertebral Disc Degeneration: Single-Cell Sequencing Analysis.

Background: The nucleus pulposus (NP) plays a central role in the pathogenesis of intervertebral disc degeneration (IVDD); however, its internal cellular heterogeneity and molecular mechanisms have not yet been elucidated. Methods: ScRNA-seq was used to evaluate the structure of the NP at different degenerative stages in the same individual with IVDD. Unsupervised clustering of cells based on gene expression profiles was performed using the Seurat package and passed to uniform manifold approximation and projection (UMAP) for cluster visualization. A rat disc degeneration model and an in vitro human NP cell degeneration model were established to validate the scRNA-seq identification results. Results: Six NP subclusters and immune cells were identified and their distribution and functional differences between healthy and degenerative states were investigated. Immune cells were present only in degenerated NPs and may trigger NP degeneration. Cellular communication within the NP was altered by the intervention of immune cells. Secreted phosphorylated protein 1 (SPP1), secreted by immune cells, plays a major role and is a key molecule in NP degeneration. The results of in vivo animal experiments and in vitro cellular experiments showed that the expression of SPP1 was increased in degenerating NPs. High expression of SPP1 promoted NP degeneration, whereas inhibition of its expression attenuated degeneration. Conclusions: Cytoarchitectural changes in NP were revealed by scRNA-seq. SPP1 is involved in the pathogenesis of disc degeneration and may be a new target for intervention in IVDD.