Transcriptome mapping of bone fracture identifies Notch signaling as an important regulator of skeletal repair

We present a transcriptomic analysis that provides a better understanding of regulatory mechanisms within the healthy and injured periosteum leading to novel translational approaches for bone healing. The focus of this work is on the early regulatory control of bone healing by completing a transcriptomic analysis of forming periosteal callus cells on day 3 post fracture. Based on our previous work indicating that induced Notch1 signaling in osteoprogenitors leads to better healing, we contrasted samples in which Notch 1 intracellular domain (NICD1) is overexpressed by periosteal stem/progenitor cells with control unperturbed periosteum. We determined molecular mechanisms and changes in skeletal stem/progenitor cells (SSPC) and other cell populations within callus including hematopoietic lineages. Notch ligands were differentially expressed in endothelial and mesenchymal populations, with Dll4 restricted to endothelial cells while Jag1 expressed by various mesenchymal populations. When targeting deletion of Dll4 in SSPCs using a-smooth muscle actin (aSMACreER) in mesenchymal cells there was no phenotype, while deletion in EC using Cdh5CreER exhibited negative effects on the early fracture healing. Translation of these observations into clinically relevant model of bone healing revealed the positive effects of combination of Notch ligands delivery with currently used osteogenic inducer BMP2.