Functional Consequences of Interactions between Pax9 and Msx1 Genes in Normal and Abnormal Tooth Development

Takuya Ogawa,Hitesh Kapadia,Jian Q Feng,Rajendra Raghow,Heiko Peters,Rena N D'Souza

doi:10.1074/jbc.m601543200

Abstract

Pax9 and Msx1 encode transcription factors that are known to be essential for the switch in odontogenic potential from the epithelium to the mesenchyme. Multiple lines of evidence suggest that these molecules play an important role in the maintenance of mesenchymal Bmp4 expression, which ultimately drives morphogenesis of the dental organ. Here we demonstrate that Pax9 is able to directly regulate Msx1 expression and interact with Msx1 at the protein level to enhance its ability to transactivate Msx1 and Bmp4 expression during tooth development. In addition, we tested how a missense mutation (T62C) in the paired domain of PAX9 that is responsible for human tooth agenesis (1) affects its functions. Our data indicate that although the mutant Pax9 protein (L21P) can bind to the Msx1 protein, it fails to transactivate the Msx1 and Bmp4 promoter, presumably because of its inability to bind cognate paired domain recognition sequences. In addition, synergistic transcriptional activation of the Bmp4 promoter was lost with coexpression of mutant Pax9 and wild-type Msx1. This suggests that Pax9 is critical for the regulation of Bmp4 expression through its paired domain rather than Msx1. Our findings demonstrate the partnership of Pax9 and Msx1 in a signaling pathway that involves Bmp4. Furthermore, the regulation of Bmp4 expression by the interaction of Pax9 with Msx1 at the level of transcription and through formation of a protein complex determines the fate of the transition from bud to cap stage during tooth development.

Highlights

Pax9 and Msx1 encode transcription factors that are known to be essential for the switch in odontogenic potential from the epithelium to the mesenchyme
Pax9 Is Needed for the Expression of Msx1 in Mesenchyme during Tooth Morphogenesis—To determine whether Pax9 is involved in the regulation of Msx1, the expression of Msx1 was analyzed in tooth primordia of Pax9-deficient mice
Despite the wealth of information available on the molecular signals required for tooth morphogenesis, there is little known about how the independent and coordinated activities of key dental mesenchymal transcription factors like Msx1 and Pax9 affect the progress of the tooth organ from the bud to cap stage of development

Summary

Introduction

Pax and Msx encode transcription factors that are known to be essential for the switch in odontogenic potential from the epithelium to the mesenchyme. Synergistic transcriptional activation of the Bmp promoter was lost with coexpression of mutant Pax and wild-type Msx1 This suggests that Pax is critical for the regulation of Bmp expression through its paired domain rather than Msx. Studies of tooth development in mice indicate a molecular relationship between Pax and Msx1 Both genes are coexpressed in dental mesenchyme and appear critical for tooth morphogenesis, because in Msx and Pax homozygous null mutants, tooth organs arrest at the bud stage [14, 15]. Clues about a potential downstream effector gene of the Pax9-Msx pathway come from the observation that in Msx and Pax single homozygous mutant mice, Bmp expression is markedly reduced in dental mesenchyme [15, 16].

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Results

Conclusion