CCN2: a master regulator of the genesis of bone and cartilage

Abstract

CCN family member 2 (CCN2), also known as connective tissue growth factor (CTGF), has been suggested to be an endochondral ossification genetic factor that has been termed “ecogenin”, because in vitro studies revealed that CCN2 promotes the proliferation and differentiation of growth-plate chondrocytes, osteoblasts, and vascular endothelial cells, all of which play important roles in endochondral ossification. In addition to its action toward these three types of cells, CCN2 was recently found to promote the formation of osteoclasts in vitro, which cells play an important role in the replacement of cartilage by bone during endochondral ossification, thus strengthening the “ecogenin” hypothesis. For confirmation of this hypothesis, transgenic mice over-expressing CCN2 in cartilage were generated. The results proved the hypothesis; i.e., the over-expression of CCN2 in cartilage stimulated the proliferation and differentiation of growth-plate chondrocytes, resulting in the promotion of endochondral ossification. In addition to its “ecogenin” action, CCN2 had earlier been shown to promote the differentiation of various cartilage cells including articular cartilage cells. In accordance with these findings, cartilage-specific overexpression of CCN2 in the transgenic mice was shown to protect against the development of osteoarthritic changes in aging articular cartilage. Thus, CCN2 may also play a role as an anti-aging (chondroprotective) factor, stabilizing articular cartilage. CCN2 also had been shown to promote intramembranous ossification, regenerate cartilage and bone, and induce angiogenesis in vivo. For understanding of the molecular mechanism underlying such multifunctional actions, yeast two-hybrid analysis, protein array analysis, solid-phase binding assay, and surface plasmon resonance (SPR) analysis have been used to search for binding partners of CCN2. ECMs such as fibronectin and aggrecan, growth factors including BMPs and FGF2 and their receptors such as FGFR1 and 2 and RANK, as well as CCN family members themselves, were shown to bind to CCN2. Regarding the interaction of CCN2 with some of them, various binding modules in the CCN2 molecule have been identified. Therefore, the numerous biological actions of CCN2 would depend on what kinds of binding partners and what levels of them are present in the microenvironment of different types of cells, as well as on the state of differentiation of these cells. Through this mechanism, CCN2 would orchestrate various signaling pathways, acting as a signal conductor to promote harmonized skeletal growth and regeneration.

Physical interaction of CCN2 with diverse growth factors involved in chondrocyte differentiation during endochondral ossification

“…PTH and PTHrP stimulate chondrocyte differentiation and chondrogenesis of mesenchymal stem cells (Takigawa et al 1980;Kim et al 2008).However, in the late stage of the differentiation of growth-plate chondrocytes, PTHrP inhibits their terminal differentiation (Lee et al 1996).In this study, we found that CCN2 did not bind to PTHrP ( Table 1). Considering that CCN2 accelerates not only the maturation but also the hypertrophy of growth-plate chondrocytes (Takigawa 2013),at least physical interaction between CCN2 and PTHrP would not seem to be involved in the multi-functionality of CCN2. However, in light of the case of IGFs, further investigation including studies on regulation of gene expression is needed to discuss the functional relevance of interaction between them.…”

Extracellular signaling molecules to promote fracture healing and bone regeneration

“…CCN2 or connective tissue growth factor has been shown to promote proliferation and differentiation of chondrocytic vessels and osteoblasts and the fusion of pre-osteoclasts (122 …”

WNT1-induced Secreted Protein-1 (WISP1), a Novel Regulator of Bone Turnover and Wnt Signaling

“…This insertion was predicted to cause a frameshift in the coding sequence of the WISP1 protein. The construct was transfected into murine embryonic stem (ES) 2 cells, and 400 G418-resistant clones were expanded and analyzed. Two ES clones, 3H4 and 4C5, were specifically targeted by the construct, determined by digestion of their DNA with ApaI, followed by Southern analysis using a 5Ј-external probe.…”

“…All six CCN members are found in the skeleton but with unique locations. For example, during normal skeletal development, CCN2 and CCN3 are highly expressed in cartilage (2), whereas CCN4 is largely confined to newly forming bone (3,4). Considering that many of the CCN family members bind and regulate TGF-␤, it is not surprising that they are being considered as potential therapeutic targets for diseases such as fibrosis, cancer, and osteoarthritis, in which TGF-␤ plays a crucial role in tissue pathology (1).…”