Department of Orthodontics and Pediatric Dentistry, School of Dentistry, 1011 N University Avenue, University of Michigan, Ann Arbor, MI 48109-1078, USA.
Department of Orthodontics and Pediatric Dentistry, School of Dentistry, 1011 N University Avenue, University of Michigan, Ann Arbor, MI 48109-1078, USA.
Bone. 2019 Mar;120:125-136. doi: 10.1016/j.bone.2018.10.013. Epub 2018 Oct 17.
Bone growth is dependent upon the presence of self-renewing progenitor cell populations. While the contribution of Tissue Nonspecific Alkaline Phosphatase (TNAP) enzyme activity in promoting bone mineralization when expressed in differentiated bone forming cells is well understood, little is known regarding the role of TNAP in bone progenitor cells. We previously found diminished proliferation in the calvarial MC3T3E1 cell line upon suppression of TNAP by shRNA, and in calvarial cells and tissues of TNAP mice. These findings indicate that TNAP promotes cell proliferation. Here we investigate how TNAP mediates this effect. Results show that TNAP is essential for calvarial progenitor cell cycle progression and cytokinesis, and that these effects are mediated by inorganic phosphate and Erk1/2. Levels of active Erk1/2 are significantly diminished in TNAP deficient cranial cells and tissues even in the presence of inorganic phosphate. Moreover, in the absence of TNAP, FGFR2 expression levels are high and FGF2 rescues phospho-Erk1/2 levels and cell cycle abnormalities to a significantly greater extent than inorganic phosphate. Based upon the data we propose a model in which TNAP stimulates Erk1/2 activity via both phosphate dependent and independent mechanisms to promote cell cycle progression and cytokinesis in calvarial bone progenitor cells. Concomitantly, TNAP feeds back to inhibit FGFR2 expression. These results identify a novel mechanism by which TNAP promotes calvarial progenitor cell renewal and indicate that converging pathways exist downstream of FGF signaling and TNAP activity to control craniofacial skeletal development.
骨生长依赖于自我更新祖细胞群体的存在。虽然组织非特异性碱性磷酸酶（TNAP）酶活性在表达于分化的成骨细胞中促进骨矿化方面的作用已得到充分理解，但对于 TNAP 在骨祖细胞中的作用知之甚少。我们之前发现，在通过 shRNA 抑制 TNAP 后，颅骨 MC3T3E1 细胞系中的增殖减少，并且在 TNAP 小鼠的颅骨细胞和组织中也是如此。这些发现表明 TNAP 促进细胞增殖。在这里，我们研究了 TNAP 如何介导这种作用。结果表明，TNAP 对于颅骨祖细胞的细胞周期进程和胞质分裂是必不可少的，并且这些作用是由无机磷酸盐和 Erk1/2 介导的。即使存在无机磷酸盐，TNAP 缺陷型颅细胞和组织中的活性 Erk1/2 水平也显著降低。此外，在没有 TNAP 的情况下，FGFR2 表达水平较高，并且 FGF2 可挽救磷酸化-Erk1/2 水平和细胞周期异常，其程度远大于无机磷酸盐。基于这些数据，我们提出了一个模型，其中 TNAP 通过依赖于磷酸盐的和独立的机制刺激 Erk1/2 活性，以促进颅骨骨祖细胞的细胞周期进程和胞质分裂。同时，TNAP 反馈抑制 FGFR2 的表达。这些结果确定了 TNAP 促进颅骨祖细胞更新的新机制，并表明 FGF 信号和 TNAP 活性下游存在收敛途径来控制颅面骨骼发育。
