PROJECT SUMMARY/ABSTRACT Hox genes are a group of evolutionarily conserved transcription factors important for several developmental processes, including patterning of the anterior-posterior axis of the skeleton. The Hox11 paralogous gene group, which is expressed in the zeugopod region (radius/ulna and fibula/tibia), are necessary for proper patterning of the zeugopod. In the past few years, work from the Wellik laboratory has shown that these developmentally important Hox transcription factors remain expressed in the skeleton throughout life, specifically in progenitor-enriched mesenchymal stem cells (MSCs). Rigorous genetic lineage labeling from the lab demonstrated that these cells give rise to all three mesenchymal lineages, osteoblasts, chondrocytes and adipocytes, and exhibit life-long self-renewal, providing strong evidence that this population of cells are skeletal stem cells. A key question based on this information is whether Hox gene function is important in these stem cells throughout life. We recently reported that temporal deletion of Hox11 at adult stages results in defects in osteoblastogenesis, wherein differentiation is initiated, but osteoblasts and osteocytes fail to mature. Adult conditional loss of Hox11 function results in a progressively weakened bone matrix where collagen does not properly assemble in remodeling bone. In this study, I will use a temporally-controlled, conditional loss-of- function model to assess defects in response to fracture repair (Aim 1). Preliminary data shows that temporally-deleted, ROSACreERT2/+;Hoxa11eGFP/-;Hoxd11LoxP/LoxP mice are unable to repair after fracture. Additionally, preliminary data suggest that the populations of osteoblasts and chondrocytes appear to be in abnormal in mutants. Using Hoxa11CreERT2 to enact both deletion and lineage labeling, I can mark the cells that have undergone recombination for isolation and transcriptomic analyses (Hoxa11eGFP/CreERT2;Hoxd11LoxP/LoxP; ROSAtd-Tomato/+, Aim 2). Fracture injury induces an acute response in which stem/progenitor expansion and differentiation to both skeletal lineages is occurring simultaneously, providing an excellent model to isolate single cells and identify the pathways and targets Hox genes regulate in these processes. Preliminary data shows that a large proportion of GFP+ cells are available for collection from the fracture callus, making single cell sequencing not only possible, but a highly effective tool to investigate transcriptomic change in Hox- expressing and Hox-lineage cells. The overall goal of this project is to define Hox genetic function in fracture repair and to identify the molecular mechanisms by which Hox genes regulate skeletal behavior in this process.