Runx2 function in cells of neural crest origin during intramembranous ossification

Abstract

Runt-related transcription factor 2 (Runx2), also known as core binding factor 1 (Cbfa1), is a multifunctional transcription factor and an essential master gene controlling osteoblast differentiation. We previously demonstrated the in vivo functions of Runx2 in mesoderm-derived cells. However, no studies have been conducted on Runx2 function during the differentiation of neural crest (NC)-derived cells in vivo. Wingless-type MMTV integration site family member 1 (Wnt1) is expressed in the NC, and Wnt1-Cre efficiently targets craniofacial NC-derived cells. Runx2 deficiency in cells of the Wnt1 lineage (referred henceforth as Runx2wnt1−/− within mice) resulted in defective ossification in certain regions, primarily in the anterior half of the craniofacial bones, including the frontal bone, jugal bone, squamous temporal bone, mandible, maxilla, and nasal bone. The skeletal analysis also revealed that heterozygous Runx2wnt1+/− embryos had an impaired closure of the frontal bone at the metopic suture and lacked the secondary palate in spite of otherwise normal ossification. This result suggests that ossification at the central part of the frontal bone is more dependent on Runx2 expression in comparison to other areas. These results indicate that Runx2 is indispensable not only for mesoderm-derived cells but also for NC-derived cells to differentiate during intramembranous ossification after migration to their destination from the neural plate border. Moreover, this implies that there are different levels of dependency on Runx2 expression for successful ossification between NC-derived cells that have migrated to different locations.