Nerve injury induced protein 1 (Ninjurin1) was originally described in neuroscience in which the expression of Ninjurin1 was regulated by Schwann cells and dorsal root ganglion neuronal cells of damaged nerve tissues. After the first discovery of Ninjurin1, the widespread expression of Ninjurin1 in adult and embryonic tissues have been observed including bone marrow, peripheral blood lymphocytes, thymus, and heart. Currently, the Ninjurin1 mediated positive regulation of pre-osteoclasts fusion and osteoclast development was reported. The bone homeostasis is dynamically balanced by bone-resorbing activity of osteoclast and bone-forming activity of osteoblast. Until now, the role of Ninjurin1 was never been described in osteoblastogenesis. Therefore, in this study, we have evaluated the expression and function of Ninjurin1 in osteoblast. The ample expression of Ninjurin1 was observed in bone marrow of mouse tibia sections but it was barely expressed in osteocytes. And also the expression levels of Ninjurin1 were gradually increased during osteoblast differentiation of calvarial pre-osteoblast, C2C12, and MC3T3-E1 cells. Importantly, the expression of Ninjurin1 was increased in the absence of osteogenic stimulus, BMP2, which suggests the cell density-dependent regulation of Ninjurin1. The controlled expression of Ninjurin1 by cell-density was evidently shown in not only pre-osteogenic osteoblast lineage cells but also in non-osteogenic cancer cells such as HeLa and A549 cells. In addition, the isoform-specific knockdown of Ninjurin1 remarkably reduced the alkaline phosphatase (ALP)-positive osteoblast differentiation. Thus, our results suggest a previously unappreciated mechanism of Ninjurin1 expression and also suggest its role on osteoblastogenesis.

• 오주환(부산대학교 치의학전문대학원 구강생리학교실) | Juhwan Oh (Department of Oral Physiology, School of Dentistry, Pusan National University)
• 최윤정(부산대학교 치의학전문대학원 구강생리학교실, BK21플러스 사업단) | YunJeong Choi (Department of Oral Physiology, School of Dentistry, Pusan National University, BK21 PLUS Project, School of Dentistry, Pusan National University)
• 유미현(구강병리학교실) | Mi Heon Ryu (Department of Oral Pathology, School of Dentistry, Pusan National University)
• 배문경(부산대학교 치의학전문대학원 구강생리학교실, BK21플러스 사업단) | Moon-Kyoung Bae (Department of Oral Physiology, School of Dentistry, Pusan National University, BK21 PLUS Project, School of Dentistry, Pusan National University)
• 김형준(부산대학교 치의학전문대학원 구강생리학교실, BK21플러스 사업단) | Hyung Joon Kim (Department of Oral Physiology, School of Dentistry, Pusan National University, BK21 PLUS Project, School of Dentistry, Pusan National University) Correspondence