Date of Completion

8-15-2018

Embargo Period

2-11-2019

Keywords

Periosteum, Bone morphogenetic protein 2, Platelet derived growth factor, Stem cells, Fracture healing

Major Advisor

Ivo Kalajzic

Associate Advisor

Ernesto Canalis

Associate Advisor

Marja Hurley

Associate Advisor

Mina Mina

Associate Advisor

Liisa Kuhn

Field of Study

Biomedical Science

Degree

Doctor of Philosophy

Open Access

Open Access

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 PDGF-BB/PDGFR-β signaling on the periosteal derived cells in vitro and fracture healing in vivo.

We detected broad expression of PDGFR-β 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-β and involves ERK1/2 MAPK and PI3K/AKT signaling pathways.

Alpha smooth muscle actin ( αSMA) 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 αSMA-CreERT2 crossed with PDGFRβflox/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 αSMA-PDGFRβdel/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 PDGFRβ 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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