Reduced cartilage matrix stiffness in temporomandibular joint osteoarthritis impairs the functions of superficial zone chondrocytes via downregulation of CREB5-PLPP3 signaling

Temporomandibular joint (TMJ) plays critical roles in the movement of mandible. TMJ osteoarthritis (TMJOA) leads to pain and limited jaw function. Histologically, TMJOA causes cartilage degradation and a reduction in extracellular matrix (ECM) stiffness. The superficial zone chondrocytes (SZC) contribute in the regeneration of the condylar fibrocartilage in TMJ, while their responses to the softened ECM remains unclear. Here, we showed that the ECM stiffness was decreased in the superficial zone cartilage of TMJOA patients and rat models. Single-cell RNA sequencing demonstrated the diminished phospholipid phosphatase 3 (Plpp3) expression, impaired migration, and ECM secretion, as well as the down-regulated PI3K-AKT pathway of SZC in rat TMJOA. Such alternations were also revealed by mRNA sequencing of SZC cultured on the softened ECM. Further studies disclosed that reduced ECM stiffness induced decreased PLPP3 on the endoplasmic reticulum (ER), which inhibited mitochondrial fission and respiration via increasing phosphatidic acid (PA) in the mitochondria. Meanwhile, deactivated PI3K-AKT pathway reduced the intra-nuclear translocation of transcriptional factor Cyclic AMP responsive element binding protein 5 (CREB5), which limited PLPP3 expression. Overexpression of PLPP3 alleviated the functional damage of SZC in vitro and the cartilage ECM degradation in vivo. This work displayed the functional impairment of SZC on the softened ECM and the underlying mechanism, as well as suggested PLPP3 as a potential target in the regenerative treatments for TMJOA.