BMP-dependent cellular dynamics during cranial suture establishment in zebrafish

Cranial sutures separate neighboring skull bones and contain skeletal stem cells that drive bone growth. A key question is how osteogenic activity is controlled to promote bone growth while preventing aberrant bone fusions during skull expansion. Here we integrate single-cell transcriptomics, in vivo expression validation, photoconversion-based lineage tracing, and a zebrafish craniosynostosis model to uncover key developmental transitions regulating bone formation during skull expansion. In addition to conservation of meninges and osteoblast lineage cells between zebrafish and mouse, single-cell transcriptomic analysis of the zebrafish skull reveals distinct subpopulations of suture mesenchyme that undergo transcriptomic changes during suture establishment. While lineage tracing with an osteoblast-specific nlsEOS reporter shows that bone formation largely occurs at suture edges, a subset of mesenchyme cells in the mid-suture region upregulate a suite of genes including BMP antagonists (e.g. grem1a) and pro-angiogenic factors. Further, lineage tracing with grem1a:nlsEOS reveals that this mid-suture subpopulation is largely non-osteogenic. In twist1b; tcf12 mutant zebrafish, a model for the coronal synostosis of Saethre-Chotzen Syndrome, reduction of grem1a+ mid-suture cells correlates with misregulated bone formation and reduced blood vessels at the coronal suture. In addition, combinatorial mutation of BMP antagonists enriched in the mid-suture subpopulation results in increased BMP signaling in the suture, misregulated bone formation, and abnormal suture morphology. These data support roles of a subset of mid-suture mesenchyme in locally promoting BMP antagonism that ensures proper suture morphology. Dissected calvaria of zebrafish were dissociated from juvenvile and adult zebrafish and live cells were identified by sorting for the absence of fluoresence (sp7:GFP or ecad:YFP) and the absence of DAPI using Fluorescence-activated cell sorting (FACS). Sorted cells were processed using 10X Genomics