Impaired Chaperone-Mediated Autophagy Accelerates Intervertebral Disc Degeneration by Inducing MIDN Accumulation to Target TSC2 for Proteasomal Degradation.

Intervertebral disc degeneration (IDD) is a leading cause of low back pain with incompletely understood mechanisms. Although autophagy dysfunction is a documented contributor to IDD, the precise pathobiological role of chaperone-mediated autophagy (CMA) remains poorly understood. Here, we demonstrate that CMA activity is downregulated in nucleus pulposus cells (NPCs) from IDD patients and IL-1β-induced rat intervertebral disc cell models, causing cytoplasmic accumulation of a novel CMA substrate, Midnolin (MIDN). Accumulated MIDN bypasses the ubiquitin-proteasome system and directly binds to Tuberous Sclerosis Complex 2 (TSC2), mediating its degradation. TSC2 loss relieves mechanistic target of rapamycin complex 1 (mTORC1) inhibition, resulting in mTORC1 hyperactivation, which drives cellular senescence, senescence-associated secretory phenotype (SASP), and extracellular matrix (ECM) degradation in NPCs. In vitro and in a rat caudal needle puncture model, MIDN knockdown (shRNA), CMA activation (LAMP2A overexpression), or mTORC1 inhibition (Rapamycin) significantly attenuated IL-1β or MIDN overexpression-induced senescence and disc degeneration. Our findings reveal an "Impaired CMA-MIDN accumulation-TSC2 degradation-mTORC1 activation" axis central to IDD pathogenesis, offering potential therapeutic targets.