BCCMA:Foundational Research to Act Upon and Resist Conditions unfavorable to bone (FRACTURECURB):Role of abaloparatide for fracture healing

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The guiding research strategy of the collaborative projects is to use pre-clinical models of disease that weaken bone or delay fracture repair of high relevance to the VA population, employing in vivo/in vitro models to identify ‘druggable’ mechanisms enabling the testing of promising therapies directed at those mechanisms, while incorporating best methods and outcome measures utilizing the combined expertise of the group. Fractures, and in particular delayed or non union of fractures, is a major cause of morbidity and lost productivity in our Veterans. Better treatment is required. Parathyroid hormone (PTH) treatment shows promise in accelerating fracture repair and closing non unions. Our previous studies demonstrated that IGF1 signaling is required for the anabolic actions of PTH, and that the actions of IGF1 are in turn dependent on ephrinB2/EphB4 bidirectional signaling. Abaloparatide is an analog of PTH that appears even more anabolic to bone than PTH. But how these anabolic agents actually promote fracture repair is unclear, and the fracture repair process is itself complex. We have focused on three sites at which fracture repair takes place within the callus in fractured long bones, each with unique characteristics. 1) Intramembranous bone formation occurs on the periosteal surface of the fractured cortex involving direct differentiation of periosteal osteoprogenitors to osteoblasts. 2) Endochondral bone formation bridges the gap between the broken ends of bone involving differentiation of periosteal osteochondroal progenitors into chondrocytes which then transdifferentiate into osteoblasts forming the bony callus. 3) Intramedullary bone formation occurs within the marrow of the broken bone pieces initiated by a unique set of osteocytes and potentially transcortical vessel perivascular cells expressing stem cell markers that move from the cortex into the marrow to differentiate (dedifferentiate) into osteoblasts forming bone within the marrow. We hypothesize that abaloparatide, acting on these different cells, all of which express the PTH/PTHrP receptor through which abaloparatide acts, will promote the coordinated repair process via mechanisms that include IGF1 regulated ephrinB2/EphB4 coupling between the osteoprogenitors, chondrocytes, osteoblasts, and perivascular cells essential for fracture repair. We will use a variety of mechanical, molecular, and imaging means established in our own laboratory and that of our collaborators in the collaborative VAMR proposal to examine the response to abaloparatide at each of these sites. We will then test this response in mouse models in which the IGF1 receptor and ephrinB2 have been deleted from the cells involved with fracture repair to test the role of these signaling mechanisms in the response to abaloparatide. Our project has three aims. Aim 1 will demonstrate whether abaloparatide accelerates fracture repair via IGF1/ephrinB2/EphB4 signaling at each site of regenerative bone formation by measuring the rate of bone formation using microcomputed tomography to assess structure, Xray computed tomography to assess mineral composition, histomorphometry to evaluate cellular changes, mRNA levels to assess genomic changes, and finite element analysis and cantilever bending to evaluate the strength of the callus. Aim 2 will examine the contribution of intramedullary bone to fracture repair and the role of osteocytes in its formation with the use cell depletion strategies. Aim 3 will use lineage tracing to demonstrate the activation of osteochondral progenitors in the periosteum and cortical bone by abaloparatide and the subsequent involvement of their progeny in bone formation at the different sites of repair. Aim 4 will determine the molecular changes at each site during the repair process induced by abaloparatide using spatial transcriptomics to evaluate the genomic changes occurring in the different site of fracture repair.