Opposing activities of Twist and Msx2 regulate the development of the coronal suture and the neural crest-mesoderm boundary in the murine skull vault

Heterozygous loss of TWIST function in humans causes Saethre-Chotzen syndrome, characterized by defects in the skull vault. These defects include coronal synostosis, a premature fusion of the frontal and parietal bones at the coronal suture. A targeted mutation in the murine Twist gene mimics the features of Saethre-Chotzen syndrome. Although much is known about the genetics of craniosynostosis, there is little information on its cellular mechanisms. Prevalent views, based largely on ex vivo approaches, explain craniosynostosis in terms of the timing of differentiation or survival of osteoblast populations. Our results suggest that understanding the fundamental causes of craniosynostosis requires an analysis at a higher level of biological organization. The coronal suture marks the boundary between the frontal and parietal bones, which are derived from neural crest and mesoderm respectively. By using a genetic lineage marker for neural crest, we show mesodermal and neural crest cells undergo inappropriate mixing in the developing coronal sutures of Twist mutant mice. The boundary defect is accompanied by changes in expression of Eph-ephrin pathway members, which have been implicated in repulsive cell-cell interactions that mediate boundary formation during hindbrain segmentation and neural crest migration. We showed previously that the homeobox gene, Msx2, cooperates with Twist in the control of frontal bone development, and that a gain of function mutation in human MSX2 causes Boston Type craniosynostosis. We found expansion of Msx2 expression in the frontal and parietal bone mesenchyme of Twist+/- mutants. Genetic inactivation of Msx2 in the Twist mutant rescues the cellular mixing phenotype, restores complementary expression of Eph-ephrin, and rescues craniosynostosis. BMP signaling regulates Msx2 and coronal suture development. Noggin is upregulated in the Twist+/- mutant coronal suture. Genetic inactivation of Bmp4 and overexpression of Noggin causes coronal synostosis in mice. This suggests that Twist, Msx2, Eph-ephrins, and the BMP pathway may together regulate the neural crest-mesoderm boundary and coronal suture patency.