Impaired Angiogenesis, Early Callus Formation, and Late Stage Remodeling in Fracture Healing of Osteopontin-Deficient Mice

Authors

  • Craig L Duvall,

    1. Wallace H. Coulter Department of Biomedical Engineering, Georgia Institute of Technology and Emory University, Atlanta, Georgia, USA
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  • W Robert Taylor,

    1. Wallace H. Coulter Department of Biomedical Engineering, Georgia Institute of Technology and Emory University, Atlanta, Georgia, USA
    2. Division of Cardiology, Department of Medicine, Emory University, Atlanta, Georgia, USA
    3. Veterans Affairs Medical Center, Atlanta, Georgia, USA
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  • Daiana Weiss,

    1. Division of Cardiology, Department of Medicine, Emory University, Atlanta, Georgia, USA
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  • Abigail M Wojtowicz,

    1. Wallace H. Coulter Department of Biomedical Engineering, Georgia Institute of Technology and Emory University, Atlanta, Georgia, USA
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  • Robert E Guldberg PhD

    Corresponding author
    1. Wallace H. Coulter Department of Biomedical Engineering, Georgia Institute of Technology and Emory University, Atlanta, Georgia, USA
    2. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia, USA
    • 315 Ferst Drive, Institute for Bioengineering and Bioscience, Atlanta, GA 30332, USA
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  • The authors state that they have no conflicts of interest.

  • This work was supported by NIH Grants R01AR051336, RO1HL70531, PO1HL58000, the Georgia Tech/Emory Center for the Engineering of Living Tissues (GTEC) NSF Grant EEC-9731643, and the NSF Graduate Research Fellowship Program (CLD).

Abstract

OPN is an ECM protein with diverse localization and functionality. The role of OPN during fracture healing was examined using wildtype and OPN−/− mice. Results showed that OPN plays an important role in regulation of angiogenesis, callus formation, and mechanical strength in early stages of healing and facilitates late stage bone remodeling and ECM organization.

Introduction: Osteopontin (OPN) is an extracellular matrix (ECM) protein with diverse localization and functionality that has been reported to play a regulatory role in both angiogenesis and osteoclastic bone remodeling, two vital processes for normal bone healing.

Materials and Methods: Bone repair in wildtype and OPN−/− mice was studied using a femoral fracture model. μCT was used for quantitative angiographic measurements at 7 and 14 days and to assess callus size and mineralization at 7, 14, 28, and 56 days. Biomechanical testing was performed on intact bones and on fracture specimens at 14, 28, and 56 days. Histology and quantitative RT-PCR were used to evaluate cellular functions related to ECM formation and bone remodeling.

Results: OPN deficiency was validated in the OPN−/− mice, which generally displayed normal levels of related ECM proteins. Intact OPN−/− bones displayed increased elastic modulus but decreased strength and ductility. Fracture neovascularization was reduced at 7 but not 14 days in OPN−/− mice. OPN−/− mice exhibited smaller fracture calluses at 7 and 14 days, as well as lower maximum torque and work to failure. At 28 days, OPN−/− mice had normal callus size but a persistent reduction in maximum torque and work to failure. Osteoclast differentiation occurred normally, but mature osteoclasts displayed reduced functionality, decreasing late stage remodeling in OPN−/− mice. Thus, at 56 days, OPN−/− fractures possessed increased callus volume, increased mechanical stiffness, and altered collagen fiber organization.

Conclusions: This study showed multiple, stage-dependent roles of OPN during fracture healing. We conclude that OPN deficiency alters the functionality of multiple cell types, resulting in delayed early vascularization, altered matrix organization and late remodeling, and reduced biomechanical properties. These findings contribute to an improved understanding of the role of OPN in vivo and provide new insight into mechanistic control of vascularization and bone regeneration during fracture repair.

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