Mitofusin 1 in mitochondrial quality control and anti-inflammatory responses in nucleus pulposus cells during disc degeneration

Mitochondrial dysfunction drives intervertebral disc degeneration, and mitochondrial dynamics are regulated by mitofusins (MFNs). In this study, we evaluated the roles of MFN1 and MFN2 in mitochondrial quality control and their responses to inflammation and antioxidant treatment in grade I and III disc nucleus pulposus cells (NPCs). Human NPCs were isolated from intervertebral disc tissues of patients. Tumor necrosis factor-α (TNF-α)-induced inflammation was treated with vitamin E (Vit E) or saponin. Mitochondrial quality control was evaluated via quantitative polymerase chain reaction, western blotting, and immunocytochemistry. Sulfated glycosaminoglycan levels were quantified to assess extracellular matrix (ECM) integrity. Mitochondrial morphology and function were assessed via transmission electron microscopy and a xenograft model using MFN1-knockout NPCs. TNF-α significantly upregulated MFN1 and MFN2, with MFN1 showing heightened sensitivity in grade III disc NPCs, leading to mitochondrial fragmentation and ECM degradation. Antioxidants mitigated these effects, with Vit E proving more effective than saponin in reducing MFN1 expression, preserving mitochondrial structure, and stabilizing ECM composition. Vit E maintained mitochondrial integrity, whereas TNF-α induced mitochondrial swelling. In vivo, MFN1-knockout NPCs exhibited reduced ECM proteoglycan levels, reinforcing its role in disc homeostasis. These findings suggest that although MFN1 and MFN2 respond to TNF-α, MFN1 reacts more robustly, making it a more promising target under inflammatory stress.