Fig. 4

Schematic of NF-κB signalling during chondrogenesis in endochondral ossification. A General scheme of chondrogenesis and related molecules and signalling pathways. I. Condensation: MSC condenses into proliferating chondrocytes. TGF-β and BMP signalling regulate this stage, with BMP positively modulating NF-κB signalling. II. Proliferation and Differentiation: Proliferating chondrocytes transition to prehypertrophic chondrocytes. BMP and IGF-1/GH positively regulate NF-κB signalling, while RSPO2 counteracts it, with reduced expression during this stage. III. Terminal Differentiation: Prehypertrophic chondrocytes mature into hypertrophic chondrocytes. BMP and Wnt signalling positively regulate NF-κB signalling, while RSPO2 antagonizes it. Sox9 expression is regulated by NF-κB and is present in proliferating chondrocytes but suppresses their differentiation into prehypertrophic chondrocytes and is absent in the final maturation stage. B Detailed molecular mechanism of NF-κB regulation during chondrogenesis. Inflammatory macrophage-derived IL-1β and growth factors (IGF-1, GH, BMP2) activate PI3K/AKT signalling. PI3K converts phosphatidylinositol - 4,5 - bisphosphate (PIP2) into phosphatidylinositol [3,4,5]-trisphosphate (PIP3), activating AKT via mTORC2 and pyruvate dehydrogenase kinase 1 (PDK1). mTORC1 signalling may also stimulate NF-κB-65. Active P50/P65 translocates to the nucleus, inducing Sox9 promoter activity, chondrogenic gene expression, and B-cell lymphoma-extra large (Bcl-XL) expression for chondrocyte survival, proliferation, and differentiation. Mechanical overload stimulates Rac1, activating NF-κB signalling to promote RSPO2 expression, which inhibits chondrogenesis (Created with BioRender.com)