Investigating the Role of PDGF-BB/PDGFR-Β in Periosteum-Mediated Bone Regeneration

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URL: https://opencommons.uconn.edu/dissertations/1927

OCLC: 1196368486

Abstract

Fracture repair involves complex interactions between cell lineages under the spatiotemporal control of growth factors and cytokines. Understanding the mechanisms that regulate the commitment and differentiation of mesenchymal progenitor cells is critical for the advancement of regenerative medicine. While bone marrow stromal cells (BMSCs) have been well characterized over the years as an experimental model for osteoblast precursors, increasing evidence suggests a critical role of the periosteum during fracture. Platelet derived growth factor (PDGF) is a potent mitogen for mesenchymal cells and an important mediator during fracture healing. The role of PDGF signaling on periosteum-derived cells is still unknown. Therefore, the objective of this study is to evaluate the effects of PDGFBB/PDGFR-β signaling on the periosteal derived cells in vitro and fracture healing in vivo.

We detected broad expression of PDGFR-b within the intact periosteum and periosteal callus. Isolated periosteum-derived progenitor cells were highly responsive to PDGF-BB as demonstrated by increased proliferation, migration and decreased apoptosis. However, PDGF-BB blocked bone morphogenetic protein 2 (BMP2)-induced osteogenesis by inhibiting the canonical BMP2/Smad pathway and downstream target gene expression. This effect is mediated via PDGFR-b and involves ERK1/2 MAPK and PI3K/AKT signaling pathways.

Alpha smooth muscle actin (aSMA) is a marker for mesenchymal stem cells that contribute to fracture healing. To further understand the role of PDGFR-β signaling in periosteum derived cells in vivo, we conditionally deleted PDGFR-β using aSMA-CreERT2 crossed with PDGFRβ^flox/flox mice. Differentiation assay showed that PDGFR-β deficient cells exhibited significantly enhanced osteogenic differentiation in vitro. Following femoral fracture, we observed significant decrease of EdU+ periosteal cells in aSMA-PDGFRβ^del/del mice. Cartilage area/total area decreased after 7 days post fracture. On day 14, we detected a smaller callus with increased mineralized area/total area upon conditionally deleting PDGFR-β.

Our studies show that PDGF-BB/PDGFR-β signaling as an important regulator for periosteum derived cells. Therapeutic targeting of the PDGFRb pathway in periosteum-mediated bone repair might have profound implications in the treatment of bone disease in the future. Further investigations in vivo will help comprehensively understand the role of PDGF during fracture.

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