Wnt/β-Catenin Pathway in Cranial Bone Progenitor Specification

Abstract

The skull is comprised of flat bones that protect the brain and are separated by fibrous suture joints, which accommodate brain growth. The mammalian skull has complex cellular origins and arises through direct ossification, an atypical skeletogenic pathway. Congenital skull defects are relatively common, yet only a handful of causative mutations have been identified. Skull morphology exhibits tremendous variation across species, and is nearly synonymous with vertebrate evolution. In spite of the importance of skull bones, our understanding of the initial genetic events that pattern the skull is limited. Here I demonstrate in the mouse embryo that ß-catenin signal transduction in the supraorbital mesenchyme comprises a definitive signal fate selection of cranial bone progenitors. The Wnt/ß-catenin pathway is active in cranial bone progenitors undergoing fate specification. Removal of ß-catenin from the cranial mesenchyme results in transformation of skull ossification to ectopic chondrogenesis. ß-catenin directly induces Twist1 expression, which is required to prevent chondrogenesis in the skull, where it binds to the Sox9 gene. These data implicate a ß-catenin-Twist1 pathway in the repression of alternate cell fates during cranial bone progenitor cell fate specification. A suite of Wnt ligands, along with the specific Wnt ligand trafficking regulator, Wntless (Wls), is expressed throughout cranial surface ectoderm and mesenchyme during skull patterning. Removal of Wls from surface ectoderm results in loss of ossification and skin patterning due to defects in cranial bone and dermal progenitor specification. Ectoderm Wnt ligand secretion also initiates expression of a subset of mesenchyme Wnt ligands via ß-catenin. Mesenchymal Wnt ligands, as shown by removal of Wls, are required for downstream differentiation of bone and dermis effects. Collectively, our data implicate the cranial surface ectoderm as the source of a patterning Wnt signal to the supraorbital mesenchyme, initiating cranial bone specification and Wnt ligand expression. Characterization of the signaling sequences that initiate skull morphogenesis has implications for understanding evolutionary variation of skull morphology as well as the pathogenesis of congenital skull defects in humans, and for improving our approach to engineering skeletal cells.