Runt-related transcription factor 2 (Runx2) is a fundamental transcription factor for bone development. In endochondral ossification, Runx2 induces chondrocyte maturation, enhances chondrocyte proliferation through Indian hedgehog (Ihh) induction, and induces the expression of vascular endothelial growth factor A (Vegfa), secreted phosphoprotein 1 (Spp1), integrin-binding sialoprotein (Ibsp), and matrix metallopeptidase 13 (Mmp13) in the terminal hypertrophic chondrocytes. Runx2 inhibits the apoptosis of the terminal hypertrophic chondrocytes and induces their transdifferentiation into osteoblasts and osteoblast progenitors. The transdifferentiation is required for trabecular bone formation during embryonic and newborn stages but is dispensable for acquiring normal bone mass in young and adult mice. Runx2 enhances the proliferation of osteoblast progenitors and induces their commitment to osteoblast lineage cells through the direct regulation of the expressions of a hedgehog, fibroblast growth factor (Fgf), Wnt, and parathyroid hormone-like hormone (Pthlh) signaling pathway genes and distal-less homeobox 5 (Dlx5), which all regulate Runx2 expression and/or protein activity. Runx2, Sp7, and Wnt signaling further induce osteoblast differentiation. In immature osteoblasts, Runx2 regulates the expression of bone matrix protein genes, including Col1a1, Col1a2, Spp1, Ibsp, and bone gamma carboxyglutamate protein (Bglap)/Bglap2, and induces osteoblast maturation. Osteocalcin (Bglap/Bglap2) is required for the alignment of apatite crystals parallel to the collagen fibers; however, it does not physiologically work as a hormone that regulates glucose metabolism, testosterone synthesis, or muscle mass. Thus, Runx2 exerts multiple functions essential for skeletal development.
Keywords: Fgfr; Ihh; Runx2; Sp7; Wnt; bone matrix proteins; chondrocyte differentiation; osteoblast differentiation; osteoblast proliferation; transdifferentiation.