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. Overall design: 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