The role of the A2B adenosine receptor in the differentiation of mesenchymal stem cells to osteoblasts and chondrocytes: implications for bone development and fracture repair
Carroll, Shannon H
MetadataShow full item record
The development, maintenance and repair of the skeletal system are dependent on the differentiation of both chondrocytes and osteoblasts from their common progenitor, the mesenchymal stem cell (MSC). The A2B adenosine receptor (A2BAR) is a G-protein-coupled receptor that signals by increasing cAMP and/or activating phospholipase C signaling. Considering the published roles of cAMP on MSC differentiation, and our finding that the expression of the A2BAR is induced following injury, we hypothesized that ablation or activation of the A2BAR impacts the differentiation of osteoblasts and chondrocytes and that this would manifest as changes in skeletal development and bone fracture repair. Activation of the A2BAR increased the differentiation of bone marrow-derived MSCs to osteoblasts by increasing mRNA expression of the transcription factors runt-related transcription factor 2 (Runx2) and Sp7 transcription factor (Osterix), which are essential for osteoblast differentiation. To examine the effect of the A2BAR on bone formation in vivo, we subjected wild type (WT) and A2BAR knockout (KO) mice to bone fracture. A2BAR KO mice had impaired bone formation during fracture repair with increased cartilage volume. As fracture repair recapitulates the events that occur during endochondral ossification, we compared the growth plates of WT and A2BAR KO mice. In comparison to WT, A2BAR KO mice had a shorter growth plate initially, but a taller growth plate at a later age. These results suggest that initiation of endochondral ossification may be delayed in the A2BAR KO mice. Finally, we investigated whether the A2BAR is involved in chondrocyte differentiation. A2BAR activation decreased mRNA expression of the key transcription factor for chondrocyte differentiation, SRY (sex-determining region Y)-box 9 (Sox9) and decreased the mRNA expression of the hypertrophic chondrocyte marker Collagen X. Taken together, these data demonstrate a previously unidentified role of the A2BAR receptor in regulating MSC differentiation to both osteoblast and chondrocyte lineages. Further, we showed that mice null for the A2BAR have dysregulated bone formation during development and after injury. The importance of this receptor during bone formation and fracture repair could have implications for A2BAR-based therapies for bone maintenance and repair.
Thesis (Ph.D.)--Boston University