The Interplay Between Erythropoiesis and Bone Fracture Repair

Long bone fractures not only disrupt the bone matrix but also injure the bone marrow. Fractures compromise the vasculature, and since bone marrow is a low-oxygen tissue, it has long been thought that disruption of arterial oxygen transport creates a severely hypoxic environment at the fracture site. However, in a previous study, we discovered that the early fracture gap is not hypoxic but instead exhibits high oxygen tension. In this study, we aimed to elucidate the underlying mechanism and leverage this new understanding to develop novel therapeutic approaches. Red blood cells (RBCs), which transport oxygen through the bloodstream, are produced in the bone marrow through erythropoiesis. RBC development begins with megakaryocytic-erythroid progenitors (MEPs), which undergo several stages of erythroid maturation, including the proerythroblast (ProE) and erythroblast (EryB) stages, before entering the circulation as enucleated reticulocytes. We performed femoral fracture (osteotomy) surgeries on adult female C57BL/6J mice using external fixation. All procedures were conducted in accordance with IACUC regulations (protocol no: 806482). Mice were euthanized three days post-fracture, and tissue samples were either embedded or cells were isolated by flushing the fracture site (between the middle pins). Intact contralateral bone marrow was used as a comparison. Tissues and cells were analyzed using multiplex histology, flow cytometry, and scRNAseq. Our findings revealed an increased abundance of CD71+ ProE at the fracture site. These cells are capable of locally regulating oxygen levels through the transcriptional regulation of mitochondrial heme synthesis for hemoglobin production. CD71 (transferrin receptor 1) is a marker for erythrocyte precursors and is essential for iron import into erythroid cells. CD71 blockade significantly reduced local oxygen tension while enhancing osteoprogenitor activation, angiogenesis, and bone formation in the callus and fracture gap. In this study, we demonstrate that erythroid progenitors are locally activated in the injured bone marrow and play a critical role in concentrating oxygen at the fracture site, which can be targeted to promote bone repair. Our findings establish that bone fractures also disrupt bone marrow homeostasis activating local erythropoiesis, providing new opportunities for future research and therapeutic strategies.