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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.
Key words : Ninjurin1, Osteoblast, Cell density, Alkaline phosphatase