<i>Six1</i> Regulates Mouse Incisor Development by Promoting <i>Dlx1/2/5</i> Expression

Tooth development is a complex process orchestrated by intricate gene regulatory networks, involving both odontogenic epithelium and ectomesenchyme. <i>Six1</i>, a pivotal transcription factor (TF), is involved in the development of the lower incisor. However, its precise role during incisor development and the molecular mechanisms underpinning its regulatory functions remain poorly understood. This study employs <i>Six1</i> deletion mouse models to elucidate the critical regulatory role of <i>Six1</i> in governing dental mesenchyme development. By performing single-cell RNA sequencing, we constructed a comprehensive transcriptome atlas of tooth germ development from the bud to bell stage. Our analyses suggest that the dental follicle and the dental papilla (DP) are differentiated from dental ectomesenchyme (DEM) and identify the key TFs underlying these distinct states. Notably, we show that <i>Dlx1</i>, <i>Dlx2</i>, and <i>Dlx5</i> (<i>Dlx1</i>/<i>2</i>/<i>5</i>) may function as the key TFs that promote the formation of DP. We further show that the deletion of <i>Six1</i> perturbs dental mesenchyme development by impeding the transitions from DEM to DP states. Importantly, SIX1 directly binds to the promoters of <i>Dlx1</i>/<i>2</i>/<i>5</i> to promote their co-expression, which subsequently leads to widespread epigenetic and transcriptional remodeling. In summary, our findings unveil <i>Six1</i>'s indispensable role in incisor development, offering key insights into TF-driven regulatory networks that govern dental mesenchyme cell fate transitions during tooth development.