MSC exosomes alleviate osteoarthritis through restoration of matrix homeostasis

Abstract

Background & Aim The efficacy of mesenchymal stromal cells (MSC) therapies is increasingly attributed to paracrine secretion, particularly exosomes. This study aims to elucidate the mechanisms of action of MSC exosomes in an immunocompetent rat model of temporomandibular joint osteoarthritis (TMJ-OA). Methods, Results & Conclusion Exosomes were isolated from conditioned medium of human MSCs. OA of bilateral TMJs was induced in rats by injection of monosodium iodoacetate. Thereafter, weekly intra-articular injections of exosomes (100µg/50µl) or 50µl of vehicle (PBS) were given over 2, 4 and 8 weeks. Analyses were performed by head withdrawal threshold (HWT) measurement, micro-computed tomography, histology and immunohistochemistry, and Mankin scoring. Gene expression changes were measured by transcriptomic analysis of the condylar cartilage tissue. Cell-based assays were performed to determine the cellular processes and signalling pathways activated by MSC exosomes. Our results showed that exosome-mediated repair of the osteoarthritic TMJs was characterized by early suppression of pain and degeneration with reduced inflammation, followed by sustained proliferation and gradual improvements in matrix expression and subchondral bone architecture, leading to overall joint restoration and regeneration by the end of treatment. By 8 weeks, exosome-treated rats had reduced pain with HWT comparable to that of sham rats, and displayed cartilage and subchondral bone restoration including appropriate cartilage thickness, cellularity, matrix and subchondral bone architecture that closely resembled that of the naive rats. Using an in-vitro chondrocyte model of OA, MSC exosomes increased sulphated glycosaminoglycan synthesis and reduced nitric oxide production in interleukin-1β-treated chondrocytes through exosomal CD73-mediated adenosine activation of AKT and AMPK signalling. These effects were partially abrogated by theophylline, wortmannin or compound C, which inhibited adenosine receptor activation, AKT and AMPK phosphorylation, respectively. Together, our observations suggest that MSC exosomes promote TMJ repair and regeneration in OA through a well-orchestrated mechanism of action that involved multiple cellular processes to restore the matrix and overall joint homeostasis.