| Summary: | Tissue engineering has emerged as a promising alternative to conventional orthopaedic grafting therapies. The general paradigm for this approach, in which phenotype-specific cells and/or bioactive growth factors are integrated into polymeric matrices, has been successfully applied in recent years toward the development of bone, ligament, and cartilage tissues in vitro and in vivo. Despite these advances, an optimal cell source for skeletal tissue repair and regeneration has not been identified. Furthermore, the lack of robust, functional orthopaedic tissue interfaces, such as the bone-ligament enthesis, severely limits the integration and biological performance of engineered tissue substitutes. This works aims to address these limitations by spatially controlling the genetic modification and differentiation of fibroblasts into a mineralizing osteoblastic phenotype within three-dimensional polymeric matrices. The overall objective of this project was to investigate transcription factor-based gene therapy strategies for the differentiation of fibroblasts into a mineralizing cell source for orthopaedic tissue engineering applications. Our central hypothesis was that fibroblasts genetically engineered to express Runx2 via conventional and biomaterial-mediated ex vivo gene transfer approaches will differentiate into a mineralizing osteoblastic phenotype. We have demonstrated that a combination of retroviral Runx2 overexpression and glucocorticoid hormone treatment synergistically induces osteoblastic differentiation and biological mineral deposition in primary dermal fibroblasts cultured in monolayer. We report for the first time that glucocorticoids induce osteoblastic differentiation in this model system by modulating the phosphorylation state of a negative regulatory serine residue (Ser125) on Runx2 through an MKP-1-dependent mechanism. Furthermore, we utilized these Runx2-genetically engineered fibroblasts to create mineralized templates for bone repair in vitro and in vivo. Finally, we engineered a heterogeneous bone-soft tissue interface with a novel biomaterial-mediated gene transfer approach. Overall, these results are significant toward the ultimate goal of regenerating complex, higher-order orthopaedic grafting templates which mimic the cellular and microstructural characteristics of native tissue. Cellular therapies based on primary dermal fibroblasts would be particularly beneficial for patients with a compromised ability to recruit progenitors to the sight of injury as result of traumatic injury, radiation treatment, or osteodegenerative disease.
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