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

## TL;DR

This study shows how PTHLH and HHIP genes control the development of skull sutures in mice, preventing premature fusion.

## Contribution

The study reveals a new role for PTHLH in preventing suture fusion and highlights suture-specific roles of HHIP in craniofacial development.

## Key findings

- Hhip deletion causes lambdoid suture fusion in mice by E18.5.
- Deleting Pthlh in Hhip−/− mice leads to coronal suture fusion.
- PTHLH reduces HH signaling to maintain coronal suture patency.

## Abstract

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.

Summary: Hhip deletion causes lambdoid suture fusion in mice and additional deletion of Pthlh causes coronal suture fusion, demonstrating the suture-specific requirements of restricting Hedgehog signaling and maintaining suture patency during calvarial development.