Excessive mechanical stress is a major cause of intervertebral disc degeneration (IVDD). Macrophages can sense physical signals, but their role in responding to mechanical stress within the disc to maintain homeostasis is unclear. This study investigates the function of macrophage-derived legumain (LGMN) in IVDD. Single-cell RNA sequencing data of human disc samples were analyzed. Macrophage-specific Lgmn knockout (LgmnF/F;LysMCre) and nucleus pulposus cell (NPC)-specific Yap1 knockin (Yap1LSL/LSL; Col2a1Cre) mice were generated to study IVDD progression in vivo using a lumbar spine instability model. In vitro, NPCs and macrophages were cultured under mechanical compression. Molecular interactions were predicted with AlphaFold3 and validated by coimmunoprecipitation and mass spectrometry. Signaling pathways were analyzed via RNA sequencing, western blot, and chromatin immunoprecipitation. Engineered LGMN-overexpressing small extracellular vesicles (sEVs) were tested therapeutically in a rat compression model. LGMN was significantly upregulated in human and animal degenerate discs, primarily in macrophages. Conditional knockout in macrophages accelerated IVDD in mice. Mechanistically, macrophage-derived LGMN bound to integrin αvβ3 on NPCs, inhibiting RhoA activity and activating the Hippo pathway. This led to phosphorylation and cytoplasmic retention of YAP1, which suppressed mechanical stress-induced ferroptosis in NPCs. Mechanical stress promoted STAT3 nuclear translocation in macrophages, directly enhancing LGMN transcription. Intradiscal delivery of LGMN-enriched sEVs alleviated IVDD in rats. Macrophage-derived LGMN is a key mechanosensitive regulator that ameliorates IVDD by inhibiting NPC ferroptosis via the integrin αvβ3–Hippo pathway, revealing a novel endogenous protective mechanism and a potential therapeutic strategy.
Wang et al. (Mon,) studied this question.