The bones of the mammalian skull vault form through intramembranous ossification. Skull bones ossify directly, in a process regulated by β-catenin, instead of passing through a cartilage intermediate. We tested whether β-catenin is necessary for fate selection of intramembranous bone progenitors in the skull. Here, we show in mice that removal of β-catenin from skull bone progenitors results in the near complete transformation of the skull bones to cartilage, whereas constitutive β-catenin activation inhibits skull bone fate selection. β-catenin directly activated Twist1 expression in skull progenitors, conditional Twist1 deletion partially phenocopied the absence of β-catenin, and Twist1 deletion partially restored bone formation in the presence of constitutive β-catenin activation. Finally, Twist1 bound robustly to the 3′UTR of Sox9, the central initiator of chondrogenesis, suggesting that Twist1 might directly repress cartilage formation through Sox9. These findings provide insight into how β-catenin signaling via Twist1 actively suppresses the formation of cartilage and promotes intramembranous ossification in the skull.
Funding
This work was supported in part by Case Startup funds from Case Western Reserve University (R.P.A.); by the National Institutes of Dental and Craniofacial Research [grants F31 DE020220-02 to L.H.G. and R01-DEO1870 to R.P.A.]; a Pilot and Feasibility Grant from the Case Skin Disease Research Center (R.P.A.); an Howard Hughes Medical Institute SPUR Fellowship (C.T.); by the National Institute of Child Health and Development [R01 HD056369 to P.C.S.]; by the National Human Genome Research Institute [R01 HG004722 to P.C.S.]; by the National Institutes of Health Director's New Innovator Award [1 DP2 OD002420-01 to J.Y.]; and by The Mary Kay Ash Foundation Cancer Research Grant [096-09 to J.Y.]. Deposited in PMC for release after 12 months.
