Differential regulation of coronal and lambdoid suture patency by PTHLH and HHIP activity in mice

Craniofacial development depends on the formation of fibrous joints, or sutures, between skull bones. Premature fusion of sutures, or craniosynostosis, is a common human pathology. Ectopic Hedgehog (HH) signaling is one cause of craniosynostosis. Hhip encodes an inhibitor of HH ligands, and we previously identified coronal suture dysgenesis in embryonic Hhip-/- mice, in which suture mesenchyme was depleted between closely opposed but unfused osteogenic fronts at E18.5. Here, we report that the lambdoid suture fuses in Hhip-/- mice by E18.5. RNA-seq analysis of the Hhip-/- coronal and lambdoid sutures show that HH target gene expression, including Pthlh, is upregulated. Paradoxically, expression of Ihh is downregulated. We hypothesized that PTHLH, a negative regulator of Ihh expression, may reduce HH signaling to promote coronal suture patency and prevent fusion of the Hhip-/- coronal suture. We generated Hhip-/-;Pthlh-/- embryos and found that coronal sutures are fusing by E18.5. Our results reveal a previously undescribed role for Pthlh in suture development and demonstrate suture-specific roles for HH inhibitors in maintaining suture patency.