YAP in synovial macrophages mitigates mechanically-induced osteoarthritis by preserving mitochondrial fusion

Osteoarthritis (OA) is a prevalent and progressive degenerative joint disease resulting from abnormal loading; however, the mechanisms by which load induces OA remain incompletely understood. The Hippo/YAP signaling pathway is crucial for cellular mechanosensation, yet its effects on synovial macrophages-key players in joint inflammation-have not been documented. This study aims to investigate whether YAP regulates mitochondrial homeostasis to influence macrophage function in the context of mechanically induced OA.

We generated anterior cruciate ligament transection (ACLT) mice to induce OA and administered bone marrow-derived macrophages (BMDMs) subjected to cyclic tensile strain (CTS). We employed micro-computed tomography (micro-CT), histological analysis, immunofluorescence, and Western blotting to assess joint damage and molecular alterations. DCFH-DA and JC-1 staining were conducted to investigate mitochondrial function, followed by transmission electron microscopy for further analysis. Functional enhancement and deficiency studies were performed using Yap1 overexpression and small interfering RNAs (siRNAs), with rescue experiments conducted using AAV5-YAP1 in macrophages.

ACLT induced joint instability, activated the Hippo pathway, and inhibited YAP in synovial macrophages, which is associated with OA progression. Pathological mechanical stress (12% CTS) directly inhibited YAP's nuclear localization and promoted M1 polarization in BMDMs. Mechanistically, YAP maintained mitochondrial homeostasis through TEAD1-dependent transcriptional regulation of Mfn1, a crucial mitochondrial fusion protein. Inhibition of YAP disrupted mitochondrial dynamics, leading to a decrease in mitochondrial membrane potential, an increase in mitochondrial ROS, and a reduction in ATP content. Overexpression of YAP1 rescued mitochondrial dysfunction, suppressed M1 polarization, and protected chondrocytes from catabolic effects. In vivo, specific macrophage overexpression of YAP1 significantly alleviated OA progression by restoring the YAP-MFN1 axis.

This study identifies YAP as a mechanosensitive regulator of synovial macrophages that modulates mitochondrial homeostasis to inhibit inflammatory polarization. The YAP-MFN1 axis emerges as a novel therapeutic target for mechanical stress-induced osteoarthritis and elucidates the interplay between cellular mechanosensing and joint inflammation.