Background Chondrocytes in growth plate is known to respond to hydrostatic loading by increasing Indian hedgehog (Ihh) signalling, and that the primary cilium is required for this mechano-biological signal transduction to occur. Dicam (dual Ig domain containing cell adhesion molecule) was originally cloned from human chondrocyte cell-line, HCS-2/8 cells, but the role during endochondral bone formation and osteoarthritis has not been elucidated. This study reveals that Dicam has a novel function as a modulator of primary cilia-mediated Ihh signalling in chondrocytes.

Materials and methods Primary chondrocytes and tibia were isolated from limbs of C57BL/6 embryo (E15.5) and used in vitro study. Cartilage-specific Dicam transgenic (Col2-Dicam-Tg) mice were constructed and the phenotype of E15.5 long bone was compared with their wild type-littermates.

Results Dicam mainly expressed in resting and proliferating chondrocytes in growth plate and it was increased by Pthrp and BMP2 in primary chondrocytes. Gain-of function study with Col2-Dicam-Tg revealed that Dicam increased length of long bones. Col2-Dicam-Tg showed an increased expression of chondrogenic, Col2a1 and proliferating marker, PCNA in immunostaining analysis. In addition, early and late hypertrophic chondrocyte marker, Col10a1 and MMP13, respectively, also increased in Col2-Dicam-Tg compared to wild-type. To elucidate a molecular mechanism of Dicam, we checked the major signalling targets in chondrogenesis, which showed an increased expression of Hhip and Zfp521, the target molecule of Ihh and Pthrp signalling, respectively. Other Ihh signalling molecules such as Ptch1, Gli2, and Gli3 and Ihh itself were also increased by Dicam overexpression in primary chondrocytes. Mechanistically, Dicam was localised to primary cilia of chondrocytes and increased a number of primary cilia and their assembly molecule, IFT88/polaris. Knock-down of IFT88/polaris attenuated the Dicam-mediated increase of length in primary tibia organ culture. In next step, we investigated whether Dicam influences the severity of cartilage degradation in surgically-induced osteoarthritis (OA) model. Dicam was expressed in articular cartilage and its expression level was attenuated in OA cartilage.

Conclusions Dicam was localised to primary cilia and increased proliferation and hypertrophic differentiation of chondrocytes through Ihh signalling resulting in an increase of bone length In Vivo.

Acknowledgement This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2013R1A2A2A01069204)