Abstract
Understanding the molecular events that regulate osteoblast differentiation is essential for the development of effective approaches to bone regeneration. In this study, we analysed the osteoinductive properties of extracellular calcium in bone marrow-derived mesenchymal stem cell (BM-MSC) differentiation. We cultured BM-MSCs in 3D gelatin scaffolds with Ca2+ and BMP-2 as osteoinductive agents. Early and late osteogenic gene expression and bone regeneration in a calvarial critical-size defect model demonstrate that extracellular Ca2+ enhances the effects of BMP-2 on Osteocalcin, Runx2 and Osterix expression and promotes bone regeneration in vivo. Moreover, we analysed the molecular mechanisms involved and observed an antagonistic effect between Ca2+ and BMP-2 on SMAD1/5, ERK and S6K signalling after 24 hours. More importantly, a cooperative effect between Ca2+ and BMP-2 on the phosphorylation of SMAD1/5, S6, GSK3 and total levels of β-CATENIN was observed at a later differentiation time (10 days). Furthermore, Ca2+ alone favoured the phosphorylation of SMAD1, which correlates with the induction of Bmp2 and Bmp4 gene expression. These data suggest that Ca2+ and BMP-2 cooperate and promote an autocrine/paracrine osteogenic feed-forward loop. On the whole, these results demonstrate the usefulness of calcium-based bone grafts or the addition of exogenous Ca2+ in bone tissue engineering.
Highlights
The combination of calcium-containing biomaterials and mesenchymal progenitors is regarded as a potential bone graft substitute in tissue engineering
In order to evaluate the influence of the culture system on the osteoblast differentiation of bone marrow-derived mesenchymal stem cell (BM-MSC), we compared three different culture models: untreated plastic surface, 2D gelatincoated surface and 3D gelatin scaffold
Cells seeded on 3D gelatin scaffolds showed greater upregulation of all osteogenic markers evaluated (Alpl p< 0.001; Osteocalcin p< 0.001 and Osterix p< 0.001) than monolayers on plastic surfaces or 2D gelatin coated plates (S1 Fig)
Summary
The combination of calcium-containing biomaterials and mesenchymal progenitors is regarded as a potential bone graft substitute in tissue engineering. Bone is a heterogeneous biocomposite tissue that consists of an inorganic phase (essentially hydroxyapatite), an organic phase (mainly type I collagen) and cells [1] Physiological bone remodelling occurs when osteoblast precursors attach, differentiate and replace the bone previously resorbed by osteoclasts. Osteoblast progenitors differentiate after exposure to multiple degradation products released by osteoclasts. The bone components released presented high extracellular calcium concentrations, collagen fragments and growth factors.
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