Fibro-NPC: a pathogenic subtype identified at single-cell resolution with secreted SFRP4 as a biomarker in intervertebral disc degeneration

Intervertebral disc degeneration (IDD) is a primary cause of low back pain and, in severe cases, can lead to disability. Current treatments for low back pain remain limited in efficacy, underscoring the need for a deeper understanding of the molecular mechanisms driving IDD. The degeneration process is primarily driven by an imbalance in the extracellular matrix, largely due to the senescence of nucleus pulposus cells (NPCs). Through single-cell sequencing of degenerated nucleus pulposus tissue from five intervertebral discs, we identified five distinct NPC subtypes. Notably, fibrosis-associated NPCs (Fibro-NPC) were predominantly observed at the terminal stage of cell differentiation, identifying Fibro-NPC as a pathogenic subtype in IDD. To further explore intercellular interactions, we used the CellChat algorithm to construct a cell communication network encompassing the diverse cell types in the nucleus pulposus. Mass spectrometry analysis of normal and degenerated tissue subsequently identified seven core proteins associated with IDD. Among these, WGCNA and machine learning highlighted SFRP4 as a central pathogenic protein, highly expressed in Fibro-NPC. Advanced differentiation of nucleus pulposus cells, particularly the Fibro-NPC subtype, is associated with the secretion of SFRP4, which accelerates cellular senescence. This senescence contributes to fibrosis within the nucleus pulposus, along with angiogenesis and inflammatory infiltration in the nucleus pulposus microenvironment. Collectively, these processes drive intervertebral disc degeneration. Our findings position SFRP4 as a biomarker for IDD, presenting a novel target for its diagnosis and treatment.