A New Wnt/beta-catenin Signaling Mechanism Controlling Orofacial Development

Homeodomain transcription factors are required for early patterning of maxillary and mandibular processes and require protein interactions for their spatial and temporal activation. Objectives: The chromatin-associated high mobility group protein (HMG-17) is associated with transcriptionally active chromatin, however its role in regulating gene expression is unclear. This report reveals a unique strategy in which, HMG-17 acts a molecular switch regulating homeodomain transcriptional activity. The switch utilizes the Wnt/beta-catenin signaling pathway and adds to the diverse functions of beta-catenin. Methods: We use transient transfections, real-time PCR, mutant mice, chromatin immunoprecipitation (ChIP) assays, electrophoretic mobility shift assays, immunochemical and biochemical assays. Results: HMG-17 is specifically expressed within the epithelial component of the tooth germ at early embryonic stages and is down-regulated at later post-natal stages. It is expressed in the external and internal enamel epithelia being continuous at the cervical loop and enamel knot. A high affinity HMG-17 interaction with the PITX2 homeodomain protein inhibits PITX2 DNA binding activity. The HMG-17/PITX2 inactive complex is concentrated to specific nuclear regions primed for active transcription. Beta-catenin forms a ternary complex with PITX2/HMG-17 to switch it from a repressor to an activator complex. Without beta-catenin, HMG-17 can physically remove PITX2 from DNA to inhibit its transcriptional activity. Conclusions: The PITX2/HMG-17 regulatory complex acts independently of promoter targets and is a general mechanism for the control of homeodomain transcriptional activity. HMG-17 is developmentally regulated and has a unique role during embryogenesis to regulate the activity of transcription factors. This mechanism provides a new role for canonical Wnt/beta-catenin signaling in regulating homeodomain transcriptional activity during craniofacial development using HMG-17 as a molecular switch. These data demonstrate a new role for HMG-17 in tooth morphogenesis. Support for this research was provided from grant DE13941 from the National Institute of Dental and Craniofacial Research.