m⁷G-modified mt-tRF3b-LeuTAA regulates mitophagy and metabolic reprogramming via SUMOylation of SIRT3 in chondrocytes

N7-methylguanosine (m⁷G) modification is one of the most prevalent RNA modifications, and methyltransferase-like protein-1 (METTL1) is a key component of the m⁷G methyltransferase complex. METTL1-catalyzed m⁷G as a new RNA modification pathway that regulates RNA structure, biogenesis, and cell migration. Increasing evidence indicates that m⁷G modification has been implicated in the pathophysiological process of osteoarthritis (OA). However, the underlying molecular mechanisms of m⁷G modification remains incompletely elucidated during the progression of OA. Here we found that METTL1 and m⁷G levels were markedly increased in OA chondrocytes. In addition, METTL1-mediated m⁷G modification upregulated mt-tRF3b-LeuTAA expression to exacerbate chondrocyte degeneration. Mechanistically, mt-tRF3b-LeuTAA decreased the SUMO-specific protease 1 (SENP1) protein expression and upregulated the level of sirtuin 3 (SIRT3) SUMOylation to inhibit PTEN induced kinase 1 (PINK1)/Parkin-mediated mitochondrial mitophagy. Intra-articular injection of PMC-tRF3b-LeuTAA inhibitor (Polyamidoamine-polyethylene glycol surface-modified with Minimal self-peptides and Chondrocyte-affinity peptides, PMC) attenuated destabilization of the medial meniscus (DMM) mouse cartilage degeneration in vivo. Our study demonstrates that METTL1/m⁷G/mt-tRF3b-LeuTAA axis accelerate cartilage degradation by inhibiting mitophagy and promoting mitochondrial dysfunction through SIRT3 SUMOylation, and suggest that targeting METTL1 and its downstream signaling axis could be a promising therapeutic target for OA treatment.