Deletion of NFKB1 enhances canonical NF-κB signaling and increases macrophage and myofibroblast content during tendon healing

Flexor tendon injuries heal with excessive scar tissue that limits range of motion and increases incidence of re-rupture. The molecular mechanisms that govern tendon healing are not well defined. Both the canonical nuclear factor kappa B (NF-kappa B) and mitogen activated protein kinase (MAPK) pathways have been implicated in tendon healing. The gene NFKB1 (proteins p105/p50) is involved in both NF-kappa B and MAPK signaling cascades. In the present study, we tested the hypothesis that global NFKB1 deletion would increase activation of both NF-kappa B and MAPK through loss of signaling repressors, resulting in increased matrix deposition and altered biomechanical properties. As hypothesized, NFKB1 deletion increased activation of both NF-kappa B and MAPK signaling. While gliding function was not affected, NFKB1 deletion resulted in tendons that were significantly stiffer and trending towards increased strength by four weeks post-repair. NFKB1 deletion resulted in increased collagen deposition, increase macrophage recruitment, and increased presence of myofibroblasts. Furthermore, NFKB1 deletion increased expression of matrix-related genes (Col1a1, Col3a1), macrophage-associated genes (Adgre1, Ccl2), myofibroblast markers (Acta2), and general inflammation (Tnf). Taken together, these data suggest that increased activation of NF-kappa B and MAPK via NFKB1 deletion enhance macrophage and myofibroblast content at the repair, driving increased collagen deposition and biomechanical properties.