Targeting Runx2 expression in hypertrophic chondrocytes impairs endochondral ossification during early skeletal development.
Runx2 is a known master transcription factor for osteoblast differentiation, as well as an essential regulator for chondrocyte maturation. Recently, more and more data has shown that Runx2 regulates hypertrophic chondrocyte-specific type X collagen gene (Col10a1) expression in different species. However, how Runx2 regulation of Col10a1 expression impacts chondrocyte maturation, an essential step of endochondral bone formation, remains unknown. We have recently generated transgenic mice in which Flag-tagged Runx2 was driven by a cell-specific Col10a1 control element. Significantly increased level of Runx2 and Col10a1 mRNA transcripts were detected in transgenic mouse limbs at both E17.5 (embryonic day 17.5) and P1 (post-natal day1) stages, suggesting an in vivo correlation of Runx2 and Col10a1 expression. Surprisingly, skeletal staining suggested delayed ossification in both the axial and the appendicular skeleton of transgenic mice from E14.5 until P6. Histological analysis showed elongated hypertrophic zones in transgenic mice, with less von Kossa and TUNEL staining in long bone sections at both E17.5 and P1 stages, suggesting defective mineralization due to delayed chondrocyte maturation or apoptosis. Indeed, we detected increased level of anti-apoptotic genes B-cell leukemia/lymphoma 2, Osteopontin, and Sox9 in transgenic mice by real-time RT-PCR. Moreover, immunohistochemistry and Western blotting analysis also suggested increased Sox9 expression in hypertrophic chondrocytes of transgenic mice. Together, our data suggest that targeting Runx2 in hypertrophic chondrocytes upregulates expression of Col10a1 and other marker genes (such as Sox9). This will change the local matrix environment, delay chondrocyte maturation, reduce apoptosis and matrix mineralization, and eventually, lead to impaired endochondral ossification.
Department of Anatomy and Cell Biology, Rush University Medical Center, Chicago, IL, USA.
Ming
M
Ding
Yaojuan
Y
Lu
Sam
S
Abbassi
Feifei
F
Li
Xin
X
Li
Yu
Y
Song
Valérie
V
Geoffroy
Hee-Jeong
HJ
Im
Qiping
Q
Zheng
D000818:Animals; D017209:Apoptosis / Q000235:genetics; D001402:B-Lymphocytes / Q000378:metabolism; D001842:Bone and Bones / Q000502:physiology; D002454:Cell Differentiation / Q000502:physiology; D019902:Chondrocytes / Q000166:cytology / Q000378:metabolism*; D024121:Collagen Type X / Q000235:genetics; D050659:Core Binding Factor Alpha 1 Subunit / Q000235:genetics*; D018507:Gene Expression Regulation, Developmental; D051379:Mice; D008822:Mice, Transgenic; D010012:Osteogenesis / Q000502:physiology*; D053495:Osteopontin / Q000235:genetics / Q000378:metabolism; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000235:genetics; D012333:RNA, Messenger / Q000235:genetics; D055758:SOX9 Transcription Factor / Q000235:genetics; D012863:Skeleton; D015854:Up-Regulation
C570620:Col10a1 protein, mouse; D024121:Collagen Type X; D050659:Core Binding Factor Alpha 1 Subunit; D019253:Proto-Oncogene Proteins c-bcl-2; D012333:RNA, Messenger; C493716:Runx2 protein, mouse; D055758:SOX9 Transcription Factor; C527932:SOX9 protein, human; D053495:Osteopontin