Fracture Repair by Mouse and Human Hematopoietic Stem Cells

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DESCRIPTION (provided by applicant): ABSTRACT Military personnel are at a substantially increased risk of bone fracture. A major complication of fracture repair, especially in cases of high-energy combat trauma, is delayed or non-union, meaning that bone does not heal in a timely manner or does not heal at all. Bone repair requires the recruitment of stem cells with the capacity to differentiate to functional osteoblasts that mineralize. Given the close association of bone and bone marrow (BM), it has been suggested that BM may serve as a source of these progenitors. Preliminary in vivo studies of mice whose BM was reconstituted by a clonal population of cells derived from a single enhanced green fluorescent protein positive (EGFP+) hematopoietic stem cell (HSC) show EGFP+ cells with morphological characteristics of osteoblasts, osteocytes, and chondrocytes. Data also demonstrate an increase in these cells during non-stabilized fracture repair. These findings support our hypothesis that osteoblasts, osteocytes, and chondrocytes are derived from HSCs. We propose to elucidate the contribution of HSCs to fracture healing, identify the human stem cell that possess this oste-chondrogenic potential, and exploit this unique source of osteoprogenitor cells to augment fracture healing. This hypothesis will be tested using both clonal cell mouse-to-mouse and xenograft transplantation methods in conjunction with stabilized, non-stabilized and non-union fracture models through two Specific Aims: 1) To examine the temporal and functional contribution of HSCs to oste-chondrogenic lineages during fracture repair. This Aim will utilize our established murine transplantation model in conjunction with stabilized, non-stabilized fracture and non-union fractures to histochemically, biochemically and morphometrically examine the HSC-derived cell types in the fracture callus. This aim will also examine the effects of HSC mobilization via granulocyte-colony stimulating factor (G-CSF) administration alone or in combination with administration of bone morphogenic protein (BMP) at the fracture site. 2) To examine the contribution of human stem cells to osteo-chondrogenic lineages during fracture repair. FACS-sorting based on surface antigens and dye exclusion will be used to identify and enrich for human stem cells that participate in fracture repair in a xenograft model. The ability of G-CSF to augment their participation will also be examined. These studies are significant in that they suggest a novel HSC origin for bone and cartilage cells during fracture repair. Methods to enhance and accelerate fracture healing based on mobilization of this unique osteo- chondrogenic HSC source would have far-reachingbenefits for military personnel. Given that high-impact trauma have increased risk of forming non-union, these findings have great relevance to the VA mission and have potential to impact Veterans Health Care by identifying unique targets to improve fracture recovery. PUBLIC HEALTH RELEVANCE: RELEVANCE Military personnel are at a substantially increased risk of bone fracture, especially those resulting from high- energy combat-related trauma. A major complication of fracture repair in such cases is delayed union or non- union, meaning that the bone does not heal in a timely manner or does not heal at all. Repair of skeletal bone requires the recruitment and proliferation of stem cells with the capacity to differentiate to functional bone cells. Our studies demonstrate that the hematopoietic stem cell (HSC) is a novel source of stem cells for this repair process. Our data also demonstrate an increase in these cells during non-stabilized fracture repair. The goal of these studies is to elucidate the contribution of HSCs to fracture healing, identify the human stem cell that possess this oste-chondrogenic potential, and exploit this unique source of osteoprogenitor cells to augment fracture healing. Methods to enhance and accelerate the fracture healing process based on mobilization of this particular osteo-chondrogenic stem cell source would have far-reachingbenefits for military personnel. Decreasingthe time to return to complete mobility and range of function would reduce medicalcosts, decrease the time for military recruits toenter active duty after injuries incurred in basic training, and accelerate the return of professional soldiers to combat. Moreover, augmenting the healing process may enhance quality of life by decreasing pain, increasingmobility and preventing the long-termdisability caused by fracture non- union.