Fgfr3–Wnt signaling crosstalk is involved in maintaining cranial suture integrity

Cranial suture formation is a dynamic process that requires precise cellular and molecular coordination to regulate bone growth and maintain suture homeostasis. The Fibroblast Growth Factor Receptor 3 (FGFR3) signaling pathway is among the major pathways disrupted in craniosynostosis; however, its precise role during cranial suture formation is still unknown. Using a relevant fgfr3 LoF zebrafish model exhibiting abnormal suture morphology, we demonstrated for the first time that Fgfr3 plays a pleiotropic role in both the formation and maintenance of cranial sutures. Transmission electron microscopy and second harmonic generation imaging revealed that Fgfr3 is essential for the proper organization of the collagen network within the suture. Using specific transgenic reporter lines, we showed that Fgfr3 is crucial for regulating osteogenesis in this region. Specifically, Fgfr3 limits the number of osteoprogenitors at the osteogenic front and promotes osteoblast maturation at the suture edge. RNAscope analyses further revealed that loss of Fgfr3 led to significant upregulation of fgf18 expression. Finally, our findings show that loss of Fgfr3 results in the activation of the canonical Wnt pathway, and possibly the BMP pathway, within cranial sutures. Pharmacological inhibition of canonical Wnt signaling during suture development using the β-catenin inhibitor XAV939 restored fgf18 expression, partially normalized levels of the BMP antagonist grem1, reduced SMAD1/5 phosphorylation, and produced a significant improvement in cranial suture morphology. In conclusion, these findings position Fgfr3 as a central regulator of cranial suture formation and homeostasis, acting through intricate cross-talk between the FGF, canonical Wnt, and possibly BMP signaling pathways. These data offer new insights into the biology of cranial suture and FGFR3-related craniosynostoses.