Approximately 8 million bone fractures occur annually and 10% of these are delayed and nonunion fractures which fail to heal despite therapeutic intervention. Current therapeutic strategies are severely hindered by the limited supplies of autograft bone and the poor mechanical properties of artificial materials. Zebrafish are masters of regeneration, and I have found that adult zebrafish can rapidly regenerate up to half of its lower jawbone after resection. Craniofacial bone development is highly conserved between zebrafish and humans, and hence understanding large-scale bone regeneration in zebrafish may lead to novel treatments in patients. A unique feature of zebrafish jawbone regeneration is the involvement of an unusual bone-producing chondrocyte population. In this application, I aim to identify the progenitor population that generates these ossifying chondrocytes during large-scale bone regeneration, as well as the role of macrophages in stimulating this progenitor population. Utilizing Cre-based transgenic lineage tracing strategies, I will test that a Runx2+/Sp7- population of pre-osteolasts in the periosteum generate the cartilage callus during regeneration. Using a genetic ablation strategy, I will then test that macrophages are required for the earliest events in bone repair, namely the shift of periosteal cells from making bone during homeostasis to making cartilage during repair. Lastly, I propose to utilize a RUNX2:GFP transgene to isolate these periosteal cells for deep sequencing of expressed mRNAs, which will help identify macrophage-dependent genes activated in periosteal cells after injury. Together, positive findings will illuminate the role of the immune system in shifting the fate of periosteal cells from bone to cartilage during bone repair, with knowledge gained in the zebrafish model being used in the future to develop new bone repair strategies in patients.
Often traumatic bone fractures in patients fail to heal on their own, and current therapeutic approaches are limited by the supply of bone grafts and poor mimicry by artificial materials. The robust regenerative ability of zebrafish to repair large scale jawbone removal in combination with a high genetic conservation with humans during bone development make zebrafish an optimal model to identify genes which may enhance the innate bone repair ability in humans. My findings aim to provide insight into the genetic differences in the heightened bone regeneration of zebrafish for augmenting existing cell based therapies of bone repair in human patients.
