CircPAFAH1B2 induces chondrocytes mitochondrial dysfunction and promotes cartilage degeneration through binding molecular chaperone ClpB.

INTRODUCTION: This study explores the role of circPAFAH1B2 in osteoarthritis (OA) by investigating its influence on nuclear-mitochondrial communication, a largely unexplored area in OA progression. By uncovering how circPAFAH1B2 regulates mitochondrial function, the study aims to identify novel therapeutic targets for OA prevention and treatment. OBJECTIVES: This study aimed to identify the regulatory role of circPAFAH1B2 in nuclear-mitochondrial communication within chondrocytes and cartilage homeostasis. METHODS: circPAFAH1B2 expression was determined via quantitative real-time polymerase chain reaction (qRT-PCR) and in situ hybridization. RNA pulldown experiments, proteomic analyses, and RNA immunoprecipitation were conducted to identify the downstream targets of circPAFAH1B2. Gain- and loss-of-function assays were performed to evaluate the regulatory roles of circPAFAH1B2 and the molecular chaperone caseinolytic peptidase B protein homolog (ClpB) in mitochondrial function and chondrocyte homeostasis in cartilage. Cross-linking immunoprecipitation and sequencing were performed to identify binding sites between circPAFAH1B2 and ClpB. RESULTS: circPAFAH1B2 was upregulated in OA and localized to the cytoplasm of chondrocytes. In vivo and in vitro experiments demonstrated that increased levels of circPAFAH1B2 induced mitochondrial dysfunction and promoted cartilage degeneration. Mechanistic investigations revealed that circPAFAH1B2 bound to and restricted the mitochondrial import of the molecular chaperone ClpB, which disaggregates misfolded mitochondrial proteins, stabilizes mitochondrial homeostasis, and maintains chondrocyte homeostasis. We characterized the binding sites of circPAFAH1B2 and ClpB, and demonstrated that mutation of these sites effectively suppressed circPAFAH1B2-mediated OA phenotypes. CONCLUSIONS: Our findings indicate that circPAFAH1B2 acts as a molecular decoy blocking ClpB mitochondrial translocation, driving mitochondria-dependent cartilage degradation, which may provide novel therapeutic targets for OA.