Integrin β1, PDGFRβ, and type II collagen are essential for meniscus regeneration by synovial mesenchymal stem cells in rats

Synovial mesenchymal stem cells (MSCs) injected into the knee promote meniscus regeneration in several animal models; however, the mode of action is unknown. Our purpose was to identify the molecules responsible for this meniscus regeneration. Rat synovial MSCs were treated with neutralizing antibodies for integrin β1, PDGFRβ, or CD44 or with the CRISPR/Cas9 system to delete Vcam1, Tnfr1, or Col2a1 genes. After partial meniscectomy, rat knees were injected with MSCs, and the regenerated meniscus area was quantified three weeks later. The in vivo and in vitro functions were compared between the treated and control MSCs. Anti-integrin β1 neutralizing antibody inhibited in vitro MSC adhesion to collagen-coated chambers, anti-PDGFRβ neutralizing antibody inhibited proliferation in culture dishes, and Col2a1 deletion inhibited in vitro chondrogenesis. In vivo, the regenerated meniscus area was significantly smaller after injection of MSCs treated with integrin β1 and PDGFRβ neutralizing antibodies or lacking type II collagen gene than after control MSC injection. By contrast, the regenerated areas were similar after injection of control, CD44-, Vcam1-, or Tnfr1 treated MSCs (n = 12–16) MSCs. Synovial MSCs injected into the knee joint promoted meniscus regeneration by adhesion to integrin β1 in the meniscectomized region, proliferation by PDGFRβ, and cartilage matrix production from type II collagen.