Dental and Craniofacial Research Institute, University of California Los Angeles, Los Angeles, CA, USA.
J Bone Miner Res. 2011 Apr;26(4):777-91. doi: 10.1002/jbmr.267.
Mesenchymal stem cell commitment to an osteoprogenitor lineage requires the activity of Runx2, a molecule implicated in the etiopathology of multiple congenital craniofacial anomalies. Through promoter analyses, we have recently identified a new direct transcriptional target of Runx2, Nell-1, a craniosynostosis (CS)-associated molecule with potent osteogenic properties. This study investigated the mechanistic and functional relationship between Nell-1 and Runx2 in regulating osteoblast differentiation. The results showed that spatiotemporal distribution and expression levels of Nell-1 correlated closely with those of endogenous Runx2 during craniofacial development. Phenotypically, cross-mating Nell-1 overexpression transgenic (CMV-Nell-1) mice with Runx2 haploinsufficient (Runx2(+/-)) mice partially rescued the calvarial defects in the cleidocranial dysplasia (CCD)-like phenotype of Runx2(+/-) mice, whereas Nell-1 protein induced mineralization and bone formation in Runx2(+/-) but not Runx2(-/-) calvarial explants. Runx2-mediated osteoblastic gene expression and/or mineralization was severely reduced by Nell-1 siRNA oligos transfection into Runx2(+/+) newborn mouse calvarial cells (NMCCs) or in N-ethyl-N-nitrosourea (ENU)-induced Nell-1(-/-) NMCCs. Meanwhile, Nell-1 overexpression partially rescued osteoblastic gene expression but not mineralization in Runx2 null (Runx2(-/-)) NMCCs. Mechanistically, irrespective of Runx2 genotype, Nell-1 signaling activates ERK1/2 and JNK1 mitogen-activated protein kinase (MAPK) pathways in NMCCs and enhances Runx2 phosphorylation and activity when Runx2 is present. Collectively, these data demonstrate that Nell-1 is a critical downstream Runx2 functional mediator insofar as Runx2-regulated Nell-1 promotes osteoblastic differentiation through, in part, activation of MAPK and enhanced phosphorylation of Runx2, and Runx2 activity is significantly reduced when Nell-1 is blocked or absent.
间质干细胞向成骨前体细胞谱系的分化需要转录因子 Runx2 的活性，该分子与多种先天性颅面畸形的病因学有关。通过启动子分析，我们最近鉴定了 Runx2 的一个新的直接转录靶标 Nell-1，这是一种与颅缝早闭（CS）相关的具有强大成骨特性的分子。本研究探讨了 Nell-1 和 Runx2 在调节成骨细胞分化中的机制和功能关系。结果表明，Nell-1 的时空分布和表达水平与颅面部发育过程中内源性 Runx2 的分布和表达水平密切相关。表型上，Nell-1 过表达转基因（CMV-Nell-1）与 Runx2 杂合子缺失（Runx2(+/-)）小鼠交配，部分挽救了 Runx2(+/-)小鼠的颅缝早闭样表型中的颅骨缺陷，而 Nell-1 蛋白诱导 Runx2(+/-)而不是 Runx2(-/-)颅骨外植体的矿化和骨形成。Nell-1 siRNA 寡核苷酸转染到 Runx2(+/+)新生小鼠颅骨细胞（NMCCs）或 N-乙基-N-亚硝脲（ENU）诱导的 Nell-1(-/-) NMCCs 中，严重降低了 Runx2 介导的成骨基因表达和/或矿化。同时，Nell-1 过表达部分挽救了 Runx2 缺失（Runx2(-/-)）NMCCs 中的成骨基因表达，但不能挽救矿化。从机制上讲，无论 Runx2 基因型如何，Nell-1 信号在 NMCCs 中激活 ERK1/2 和 JNK1 丝裂原激活蛋白激酶（MAPK）通路，并增强 Runx2 磷酸化和活性，前提是存在 Runx2。总之，这些数据表明 Nell-1 是 Runx2 功能的关键下游介质，因为 Runx2 调节的 Nell-1 通过部分激活 MAPK 和增强 Runx2 的磷酸化来促进成骨细胞分化，并且当 Nell-1 被阻断或不存在时，Runx2 活性会显著降低。
