Ablation of Runx2 in Ameloblasts Suppresses Enamel Maturation in Tooth Development

Qing Chu,Qing Chu,Xianhua Gao,Xianhua Gao,Zhenzhen Xu,Mingyu Li,Mingyu Li,Wenying Song,Yumin Wang,Zhiheng Dong,Zhiheng Dong,Lili Xiang,Yuguang Gao,Yuguang Gao,Lili Xiang,Li Zhang,Li Zhang,Yan Gao,Yan Gao,Yumin Wang

doi:10.1038/s41598-018-27873-5

Abstract

Runt-related transcription factor 2 (Runx2) is involved in the early stage of tooth development. However, only few studies have reported the role of Runx2 in enamel development, which may be attributed to that Runx2 full knockout mice cannot survive after birth. In the present study, we successfully established a Runx2-deficient mouse model using a conditional knockout (cKO) method. We observed a significant reduction in the degree of mineralization and the decreased size of enamel rods in cKO mice. Histological analysis showed the retained enamel proteins in enamel layer at maturation stage in cKO molars. Further analysis by qRT-PCR revealed that the expressions of genes encoding enamel structure proteins, such as amelogenin (AMELX), ameloblastin (AMBN) and enamelin (ENAM), were increased in cKO enamel organs. On the other hand, the expression of kallikrein-related peptidase-4 (KLK4) at the mRNA and protein levels was dramatically decreased from late secretory stage to maturation stage in cKO enamel organs, while the expression of matrix metalloproteinase-20 (MMP-20) was not significantly altered. Finally, immunohistochemistry indicated that the uptake of amelogenins by ameloblasts was significantly decreased in cKO mice. Taken together, Runx2 played critical roles in controlling enamel maturation by increasing synthesis of KLK4 and decreasing synthesis of AMELX, AMBN and ENAM.

Highlights

IntroductionColumnar-shaped ameloblasts secrete enamel proteins, such as amelogenin (AMELX), ameloblastin (AMBN) and enamelin (ENAM), into the enamel matrix (em), which act as scaffolds for enamel crystallization
Enamel development consists of two major stages: secretory and maturation stages[2]
A conditional knockout mouse model was successfully established, and we found that ameloblast-specific ablation of Runx[2] in mice resulted in enamel hypomaturation phenotypes

Summary

Introduction

Columnar-shaped ameloblasts secrete enamel proteins, such as amelogenin (AMELX), ameloblastin (AMBN) and enamelin (ENAM), into the enamel matrix (em), which act as scaffolds for enamel crystallization. Bone tissue contains hydroxyapatite crystals and several types of extracellular matrix (ECM) proteins, including type I collagen, bone sialoprotein, osteonectin, osteopontin and osteocalcin[20,21] These genes are regulated via Runx[2] binding sites in the proximal promoter of the respective genes[22]. Genetic disorder of Runx[2] has been reported to cause cleidocranial dysplasia (CCD) and dental anomalies[25] These genetic findings highlight the critical roles of Runx[2] in the development of mineralized tissues, but its role in enamel development remains largely unexplored. Our findings revealed the biological functions of Runx[2] in amelogenesis and provided insights into the molecular mechanisms underlying enamel maturation

Objectives

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Results

Conclusion