BMP2 and mechanical loading cooperatively regulate immediate early signalling events in the BMP pathway

Here, we use a three-dimensional bioreactor system to describe how mechanical forces are integrated into the BMP pathway. Time-dependent phosphorylation of Smad, mitogen-activated protein kinases and Akt in human fetal osteoblasts was investigated under loading and/or BMP2 stimulation conditions. The phosphorylation of R-Smads is increased both in intensity and duration under BMP2 stimulation with concurrent mechanical loading. Interestingly, the synergistic effect of both stimuli on immediate early Smad phosphorylation is reflected in the transcription of only a subset of BMP target genes, while others are differently affected. Together this results in a cooperative regulation of osteogenesis that is guided by both signalling pathways.Mechanical signals are integrated into the BMP signalling pathway by enhancing immediate early steps within the Smad pathway, independent of autocrine ligand secretion. This suggests a direct crosstalk of both mechanotransduction and BMP signalling, most likely at the level of the cell surface receptors. Furthermore, the crosstalk of both pathways over longer time periods might occur on several signalling levels.Despite considerable advances in regenerative medicine and orthopaedic surgery, delayed fracture healing or non-unions of fractures still represent an important clinical concern [1]. Bone morphogenetic proteins (BMPs) are major and indispensable players during bone repair [2,3]. After approval by the Food and Drug Administration in 2001 and 2002, recombinant human BMP2 and recombinant human BMP7 have been used clinically in different applications. However, roughly 1,000 times the normal physiological concentration has to be administered, and in many cases treatment is not superior to autologous bone grafting [4,5]. If BMPs are to be widely used as powerful stimuli, a molecular understanding of their functionality under physiological and diseased conditions appears mandatory.BMPs belong to the transforming growth factor-β (TGF-