Overlapping and Discrete Functions of the Matricellular Proteins CCN1 and CCN2 during Endochondral Bone Formation

The CCN (Cyr61/Ctgf/Nov) family of matricellular proteins integrates the extracellular environment with the intracellular environment, with involvement in many cellular processes such as adhesion, proliferation, mechanical stretch, fibrosis, and more. A large question lurks regarding how these CCNs interact with each other to regulate many normal physiological events, as well as disease pathogenesis. While CCN1 and CCN2 are two of the best-studied CCN family members, it remains elusive whether they have similar roles in endochondral bone formation despite similar patterns of mRNA and protein expression within developing bones. Furthermore, in vivo data have already shown the significance of CCN2 for endochondral bone formation, but a role for CCN1 in chondrogenesis has only been shown in vitro. In order to study whether CCN1 and CCN2 have overlapping functions in endochondral bone formation, we utilized a collagen II/cartilage-selective deletion of Ccn1 and/or Ccn2 to analyze the phenotypes of Ccn1 and Ccn2 single and double mutant mice. A generally more severe skeletal phenotype was evident in the double mutant axial skeletons, implying the possibility that CCN1 and CCN2 may have overlapping functions. Surprisingly, in the appendicular skeletons there was not a more severe double mutant phenotype but rather a phenotype closely resembling that of Ccn2 single mutants, indicating that CCN2 plays a more dominant role within the appendicular skeleton. This supports the new idea that CCN2 is epistatic to CCN1. In addition, CCN1 and CCN2 may work antagonistically to regulate chondrocyte proliferation in the resting zone of tibial growth plates. Also, a novel phenotype was discovered as the double mutant exhibited increased apoptosis in cells at the perichondrial border. These studies will facilitate the understanding of embryonic bone development and how the proteins CCN1 and CCN2 are involved in this process