Xing Weirong, Godwin Catrina, Pourteymoor Sheila, Mohan Subburaman
1Musculoskeletal Disease Center, VA Loma Linda Healthcare System, Loma Linda, CA 92357 USA.
2Department of Medicine, Loma Linda University, Loma Linda, CA 92357 USA.
Bone Res. 2019 Aug 5;7:24. doi: 10.1038/s41413-019-0064-9. eCollection 2019.
In our previous studies, we have found that the prepubertal increase in thyroid hormone levels induces osterix (Osx) signaling in hypertrophic chondrocytes to transdifferentiate them into osteoblasts. To test if expressed in chondrocytes directly contributes to transdifferentiation and secondary ossification, we generated ; mice and knocked out with a single injection of tamoxifen at postnatal day (P) 3 prior to evaluation of the epiphyseal bone phenotype by µCT, histology, and immunohistochemistry (IHC) at P21. Vehicle (oil)-treated ; and tamoxifen-treated, -negative mice were used as controls. µCT analysis of tibial epiphyses revealed that trabecular bone mass was reduced by 23% in the conditional knockout (cKO) compared with control mice. Trabecular number and thickness were reduced by 28% and 8%, respectively, while trabecular separation was increased by 24% in the cKO mice. Trichrome staining of longitudinal sections of tibial epiphyses showed that bone area and bone area adjusted for total area were decreased by 22% and 18%, respectively. IHC studies revealed the presence of abundant -expressing prehypertrophic chondrocytes in the epiphyses of control mice at P10, but not in the cKO mice. Furthermore, expression levels of MMP13, COL10, ALP, and BSP were considerably reduced in the epiphyses of cKO mice. We also found that overexpression in ATDC5 chondrocytes increased expression of , , , and . Our data indicate that Osx expressed in chondrocytes plays a significant role in secondary ossification by regulating expression of genes involved in chondrocyte hypertrophy and osteoblast transdifferentiation.
在我们之前的研究中，我们发现青春期前甲状腺激素水平的升高会诱导肥大软骨细胞中的osterix（Osx）信号，使其转分化为成骨细胞。为了测试软骨细胞中表达的 是否直接促成转分化和次级骨化，我们构建了 ；小鼠，并在出生后第3天（P3）单次注射他莫昔芬敲除 ，然后在P21通过μCT、组织学和免疫组织化学（IHC）评估骨骺骨表型。用载体（油）处理的 ；和用他莫昔芬处理的、 阴性的 小鼠作为对照。对胫骨骨骺的μCT分析显示，与对照小鼠相比，条件性敲除（cKO）小鼠的小梁骨量减少了23%。cKO小鼠的小梁数量和厚度分别减少了28%和8%，而小梁间距增加了24%。胫骨骨骺纵切面的三色染色显示，骨面积和调整为总面积的骨面积分别减少了22%和18%。免疫组织化学研究显示，P10时对照小鼠骨骺中有大量表达 的前肥大软骨细胞，但cKO小鼠中没有。此外，cKO小鼠骨骺中MMP13、COL10、ALP和BSP的表达水平显著降低。我们还发现，在ATDC5软骨细胞中过表达 会增加 、 、 和 的表达。我们的数据表明，软骨细胞中表达的Osx通过调节参与软骨细胞肥大和成骨细胞转分化的基因表达，在次级骨化中起重要作用。
