Abstract
The differentiation of osteoblasts from their precursors, mesenchymal stem cells, is an important component of bone homeostasis as well as fracture healing. The A2B adenosine receptor (A2BAR) is a Gα(s)/α(q)-protein-coupled receptor that signals via cAMP. cAMP-mediated signaling has been demonstrated to regulate the differentiation of mesenchymal stem cells (MSCs) into various skeletal tissue lineages. Here, we studied the role of this receptor in the differentiation of MSCs to osteoblasts. In vitro differentiation of bone marrow-derived MSCs from A2BAR KO mice resulted in lower expression of osteoblast differentiation transcription factors and the development of fewer mineralized nodules, as compared with WT mice. The mechanism of effect involves, at least partially, cAMP as indicated by experiments involving activation of the A2BAR or addition of a cAMP analog during differentiation. Intriguingly, in vivo, microcomputed tomography analysis of adult femurs showed lower bone density in A2BAR KO mice as compared with WT. Furthermore, A2BAR KO mice display a delay in normal fracture physiology with lower expression of osteoblast differentiation genes. Thus, our study identified the A2BAR as a new regulator of osteoblast differentiation, bone formation, and fracture repair.
Highlights
mesenchymal stem cells (MSCs) Differentiation to Osteoblasts Is Impaired in A2BAR KO Mice—Because of the known effect that A2BAR activation has on cAMP levels and the effect that cAMP levels have on osteoblast differentiation (10 –15), we examined whether the loss of A2BAR would alter MSC differentiation to osteoblasts
MSC differentiation to osteoblasts plays a necessary role in development and postnatal growth and homeostasis, as well as skeletal tissue repair after injuries such as fracture [38]
The origin of the osteoblasts involved in fracture healing has not been demonstrated unequivocally, experimental evidence is accumulating to support the hypothesis that some of the MSCs that contribute to repair are harbored in the bone perivasculature [39] and upon receiving a signal (one of which is stromal derived factor-1 [38]) travel through the marrow capillaries [37] or neovasculature [40] to the site of injury or stress
Summary
In vitro differentiation of bone marrow-derived MSCs from A2BAR KO mice resulted in lower expression of osteoblast differentiation transcription factors and the development of fewer mineralized nodules, as compared with WT mice. In rat bone marrow-derived MSCs, activation of the A2BAR increased Runx and alkaline phosphatase expression, as well as the staining of mineralized nodules with alizarin red [21]. These studies, relied on pharmacological approaches with a focus on in vitro systems and no verification of bone homeostasis in vivo. Base-line bone density and bone fracture physiology are compromised in A2BAR KO mice
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