The P75 neurotrophin receptor regulates proliferation of the human MG63 osteoblast cell line

• NgR expression was detected in MG63 cells.
• Exogenous p75NTR mediated osteogenic differentiation in MG63 cells.
• Overexpression of p75NTR did not mediate cell proliferation in MG63 cells.
• An MG63 cell line with the GDI domain deleted in p75NTR was generated (p75NTR Del-MG63).
• The cell proliferation potential of p75NTR Del-MG63 was enhanced compared to p75NTR-MG63.

The 75 kDa transmembrane protein, p75NTR, is a marker of mesenchymal stem cells (MSCs). Isolated MSCs are capable of differentiating into osteoblasts, but the molecular function of p75NTR in MSCs and osteoblasts is poorly understood. The aim of this study was to examine the function of p75NTR in the human MG63 osteoblast cell line compared to the murine MC3T3E-1 pre-osteoblast cell line.MG63 cells and MC3T3-E1 cells expressing exogenous p75NTR protein (denoted as p75-MG63 and p75GFP-E1, respectively) were generated to compare osteogenic differentiation and cell proliferation abilities. Overexpression of p75NTR induced alkaline phosphatase activity and the mRNA expression of osteoblast-related genes such as osterix and bone sialoprotein in both p75-MG63 and p75GFP-E1. Interestingly, exogenous p75NTR stimulated cell proliferation and cell cycle progression in p75GFP-E1, but not in p75-MG63.To elucidate any different effects of p75NTR expression on osteogenic differentiation and cell proliferation, we examined the mRNA expression of tropomyosin receptor kinase (trk) genes (trkA, trkB, trkC) and Nogo receptor (NgR), which are binding partners of p75NTR. Although trkA, trkB, and trkC were detected in both p75-MG63 and p75GFP-E1, only NgR was detected in p75-MG63. We then used the K252a inhibitor of the trks to identify the signaling pathway for osteogenic differentiation and cell proliferation. Inhibition of trks by K252a suppressed p75NTR-mediated osteogenic differentiation of p75GFP-E1, whereas deletion of the GDI domain in P75NTR from the p75-MG63 produced enhanced cell proliferation compared to p75-MG63. These results suggest that p75NTR signaling associated with trk receptors promotes both cell proliferation and osteoblast differentiation, but that p75NTR-mediated proliferation may be suppressed by signaling from the p75NTR/NgR complex.