Our recent work has shown that smooth muscle alpha actin (?SMA) is a marker of mesenchymal progenitor cells that expand rapidly following fracture, and show significant contribution to fibrous tissue, osteoblast, and chondrocyte lineages within a fracture callus. Gene expression analysis of isolated ?SMA- labeled progenitor cells revealed that the Notch signaling pathway is significantly decreased during the early stages of fracture healing. Previous studies have shown that Notch signaling exhibits different effects dependent on the stage of osteoprogenitor maturation. We hypothesize that decreases in Notch signaling regulate periosteal progenitor cells expansion, migration and differentiation into mature mesenchymal lineages in the fracture callus. We propose to evaluate the effects of Notch using stage specific genetic Notch gain- and loss-of-function models during fracture healing. We will also evaluate the inhibition of Notch using small peptide SAHM1 that directly interferes with the Notch transcriptional complex. This approach will provide evidence for potential future application to accelerate or improve fracture healing. In Aim 1 we will evaluate the effects of Notch overexpression. Overexpression will be achieved by directing forced Notch 1 intracellular domain (NICD1) expression to different stages of the osteogenic lineage. For timed activation of the NICD1 following generation of fractures, we propose to use stage- specific inducible-Cre transgenes. ?SMACreERT2 mice will be used to target Notch overexpression to progenitor stage while overexpression in osteoblasts/osteocytes will be achieved by using DMP1-CreERT2 mice. Effects of Notch modulation will be assessed by evaluating progress of callus formation, and changes in bone strength and stiffness during fracture healing. We will also examine the mechanisms of effects of Notch overexpression on PPCs using in vitro and in vivo approaches to study effects on proliferation, migration and differentiation. In Aim 2 we will determine the effects of stage-specific Notch inhibition on fracture healing. To disrupt Notch signaling, we will use a transgenic model in which a direct transcriptional effector of Notch signaling, Rbpj?, is deleted (Rbpj?flox) following generation of fracture. In vitro and in vivo evaluation of Notch inhibition using PPCs will be evaluated. We will extend the inhibition studies to evaluate the treatment with Notch transcription factor complex inhibitor SAHM1 (stapled a-helical peptides derived from MAML1) on fracture healing. Our results will provide a better understanding of the role of Notch signaling during fracture healing and will evaluate the future therapeutic modulation of the healing process.