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

ABSTRACT: 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.

ORGANISM(S): Mus musculus

PROVIDER: GSE260749 | GEO | 2025/03/01

REPOSITORIES: GEO

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Project description:The phases of fracture healing have been well characterized. However, the exact source and genetic profile of the skeletal progenitors that participate in bone repair is somewhat unclear. Sox9 expression in skeletal elements precedes bone and cartilage formation and a Sox9+ cell type is retained in the adult periosteum. We hypothesized that Sox9+ periosteal cells are multipotent skeletal progenitors normally participating in fracture repair. To test this hypothesis we used tamoxifen (TM)-mediated lineage tracing of Sox9+ cells in Sox9CreErt2:Td-Tomato mice. TM injection indelibly labels Sox9+ cells and all their descendants with the fluorescent reporter protein Td-Tomato. Intact mouse femora were harvested 2 weeks after TM injection and analyzed by histology and immunostaining and RNA sequencing, to evaluate the skeletal distribution and gene expression profile of Td-Tomato positive cells in the adult femur. To assess the role of these cells in fracture repair, mice underwent a closed mid-diaphyseal femoral fracture and their hind limbs were harvested at 3, 9 or 56 days post-fracture to assess the contribution of Sox9+ and Td-tomato+ cells in the fracture healing process. In the intact adult mouse femur, Td-Tomato-labelled cells were observed in articular and growth plate cartilage where Sox9 is known to be expressed, but also in the primary spongiosa, periosteum, and endosteum. RNA sequencing and subsequent analysis showed that Td-Tomato positive periosteal cells were enriched in Sox9 transcripts, and transcripts for various osteogenic and chondrocyte specific genes. In a femoral fracture model, we showed that pre-labeled Td-Tomato positive descendent cells were mobilized during the fracture repair process, expanding and migrating towards the fracture site 3 days post-fracture. Here, Td-Tomato positive cells differentiate into chondrocytes and osteoblasts in the soft and hard callus, respectively, 9 days post-fracture. By 2 months post-fracture, descendants of the original Sox9 labelled cell population were prevalent in the cortex and periosteum, and amongst the differentiated osteocyte population embedded within the cortical bone. Thus, a Sox9+ progenitor population resides in the adult periosteum. Fate tracing shows that these cells likely play a substantial role in repair of the fractured femur giving rise to chondrocytes, osteoblasts and mature cortical osteocytes. To our knowledge this is the first report of this Sox9+ cell population in the periosteum of the adult long bone. Taken together with developmental studies on skeletal formation, our data suggest that Sox9+ osteochondroprogenitors play a continuous role in skeletal formation throughout life.

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

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