Whole Aspect of Runx2 Functions in Skeletal Development

Abstract

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.

Figure 1

Regulation of chondrocyte maturation and proliferation in the growth plate and osteoblastogenesis in the bone marrow: Runt-related transcription factor 2 (Runx2) expression is upregulated in prehypertrophic chondrocytes and Runx2 induces chondrocyte maturation. Runx3 is also involved in chondrocyte maturation. Runx2 induces Indian hedgehog (Ihh) expression in the prehypertrophic chondrocytes, and Ihh enhances the proliferation of chondrocytes in the proliferating layer. Ihh also induces the expression of parathyroid hormone like hormone (Pthlh), which inhibits Runx2 expression through parathyroid hormone 1 receptor (Pth1r), and inhibits chondrocyte maturation, forming a negative feedback loop of chondrocyte maturation. Ihh induces Runx2 expression and osteoblast differentiation in the perichondrium. Runx2 induces the expressions 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 terminal hypertrophic chondrocytes and induces the transdifferentiation of the terminal hypertrophic chondrocytes into osteoblast lineage cells. Osteoblasts in the bone marrow are derived from chondrocytes in the growth plate and osteoblast progenitors in the perichondrium/periosteum.

Figure 2

Regulation of proliferation, differentiation, and bone matrix protein gene expression by Runx2 during osteoblast differentiation: Runx2 induces the commitment of multipotent mesenchymal cells to preosteoblasts and enhances the proliferation of osteoblast progenitors by inducing the expressions of hedgehog, fibroblast grow factor (Fgf), Wnt, and Pthlh signaling pathway genes and distal-less homeobox 5 (Dlx5), which also enhance Runx2 expression and Runx2 protein activity. Runx2 also induces Sp7 expression. Runx2, Sp7 and Wnt signaling induce the differentiation of preosteoblasts into immature osteoblasts. Sp7 and Wnt signaling activate the osteoblast-specific Runx2 enhancer. In immature osteoblasts, Runx2 induces the expressions of bone matrix protein genes, including Col1a1, Col1a2, Spp1, Ibsp, and bone gamma carboxyglutamate protein (Bglap)/Bglap2, and the osteoblasts maturate and produce abundant bone matrix proteins. Runx2 also induces tumor necrosis factor superfamily member 11 (Tnfsf11) expression and enhances bone resorption. Osteocalcin (Bglap/Bglap2) aligns the apatite crystals parallel to collagen fibers. The collagen fibers run along the longitudinal direction in the dialysis of long bone, which is formed by mature osteoblasts, whereas collagen fibers run irregularly in metaphyseal cortical bone in young mice, which is formed by immature osteoblasts. However, apatite crystals are always aligned parallel to the collagen fibers by osteocalcin [63].