Cellular and molecular coordinators of large-scale bone repair

ABSTRACT The challenge of repairing extensive bone injuries remains an unmet clinical challenge. Our research is dedicated to exploring a novel mouse model of large-scale bone regeneration to unravel the intricate cellular and molecular events necessary for the large-scale regeneration. While the outer periosteum and the inner endosteum are recognized as sources of new osteoblasts during normal bone homeostasis, the precise mechanisms governing the initiation and coordination of repair are not well-understood. We hypothesize a two- phase repair process following large-scale injury. Initially, a sentinel-type cell population marked by the expression of Sox9 transitions from quiescent to activated state. In the second phase, we propose that this subpopulation orchestrates the recruitment of other cells, including bone marrow-derived subpopulations, and culminating in a callus spanning the injury site. Building on our previous work, which has shown the requirement for Sox9-lineage cells in large-scale repair, in Aim 1 we investigate the mechanisms that trigger activation of these cells in response to injury. Specifically we test that the gene Fos is required for their activation and for the transcription of genes important for coordinating repair. In Aim 2 we determine if these Sox9-lineage cells recruit bone marrow stromal cells expressing Cxcl12, with this interaction being vital for robust bone callus generation. Finally, In Aim 3, we determine if Sox9-lineage cells originating from the rib possess distinctive features that enable them to facilitate the healing of critical-sized femur injuries, possibly through recruiting Cxcl12-lineage cells from the femur bone-marrow niche. The successful completion of these experiments will provide invaluable insights into the critical cell types and mechanisms driving large-scale repair in a mammalian model. These insights could lay the groundwork for future pre-clinical studies targeting challenging skeletal injuries in patients.