NMN-primed exosomes derived from infrapatellar fat pad mesenchymal stem cells exert synergistic anti-inflammatory and cartilage-protective effects via MERTK pathway activation in knee osteoarthritis

Osteoarthritis (OA) is a chronic degenerative joint disease lacking effective disease-modifying therapies. This study evaluated the therapeutic potential of nicotinamide mononucleotide (NMN)–primed exosomes derived from infrapatellar fat pad mesenchymal stem cells (IPFP-MSCs), termed Exo^NMN(+), under the hypothesis that NMN priming enhances their anti-inflammatory, antioxidant, and chondroprotective efficacy through MERTK-dependent signaling. Exo^NMN(+) were isolated and characterized by nanoparticle tracking analysis and Western blotting, showing enrichment of exosomal markers (CD9, CD63, ALIX) without size alteration, indicating enhanced cargo loading. In IL-1β-stimulated human chondrocytes, Exo^NMN(+) improved viability, migration, and proliferation (increased Ki-67 and EdU expression), suppressed catabolic factors (MMP2, MMP9, VEGF), and upregulated anabolic markers (COL2A1, MERTK). Transcriptomic profiling identified 428 uniquely regulated genes, primarily involved in PI3K–AKT and small-GTPase signaling. Intersection analysis with single-cell OA datasets revealed five shared differentially expressed genes forming the basis of the GAS6–MERTK–PI3K/AKT axis. Exo^NMN(+) also reduced intracellular reactive oxygen species (by 51 %) and apoptosis (by 60 %), effects abrogated upon MERTK knockdown, confirming pathway specificity. In a papain-induced OA mouse model, intra-articular administration of Exo^NMN(+) preserved cartilage architecture, improved subchondral bone integrity, reduced synovial inflammation, and significantly decreased IL-6, TNF-α, IL-18, IFN-γ, and MMP13 expression, without detectable systemic toxicity. Histological and OARSI assessments corroborated marked attenuation of disease severity. Collectively, these results demonstrate that NMN priming metabolically enhances the therapeutic efficacy of IPFP-MSC-derived exosomes. Exo^NMN(+) exert potent anti-inflammatory, antioxidant, and regenerative effects through activation of the GAS6–MERTK–PI3K/AKT signaling network, supporting their development as a safe, mechanism-driven, and disease-modifying nanotherapeutic for OA management.