Abstract
The recruitment of mesenchymal stem cells (MSCs) is a crucial process in the development, maintenance and repair of tissues throughout the body. Transforming growth factor-β1 (TGFβ1) is a potent chemokine essential for the recruitment of MSCs in bone, coupling the remodelling cycle. The primary cilium is a sensory organelle with important roles in bone and has been associated with cell migration and more recently TGFβ signalling. Dysregulation of TGFβ signalling or cilia has been linked to a number of skeletal pathologies. Therefore, this study aimed to determine the role of the primary cilium in TGFβ1 signalling and associated migration in human MSCs. In this study we demonstrate that low levels of TGFβ1 induce the recruitment of MSCs, which relies on proper formation of the cilium. Furthermore, we demonstrate that receptors and downstream signalling components in canonical TGFβ signalling localize to the cilium and that TGFβ1 signalling is associated with activation of SMAD3 at the ciliary base. These findings demonstrate a novel role for the primary cilium in the regulation of TGFβ signalling and subsequent migration of MSCs, and highlight the cilium as a target to manipulate this key pathway and enhance MSC recruitment for the treatment of skeletal diseases.
Highlights
Have demonstrated an important role for the primary cilium in adult bone where the organelle is required for loading-induced osteogenesis[14,15,16,17,18,19,20]
As there are a limited number of often conflicting studies on the effect of TGFβ1 on Human bone marrow mesenchymal stem cells (hMSCs) behaviour[28,43], we first examined the effect of TGFβ1 concentration on human mesenchymal stem cell (MSC) migration, proliferation and osteogenic differentiation (Fig. 1)
With regards to hMSC recruitment, the effect of TGFβ1 concentration revealed a bell shaped profile centred on low concentrations promoting a 2.1 and 2-fold increase in hMSC migration for 0.1 and 1 pg/ml TGFβ1 respectively when compared to unstimulated control (p < 0.01 and p < 0.05, N = 4) (Fig. 1a)
Summary
Have demonstrated an important role for the primary cilium in adult bone where the organelle is required for loading-induced osteogenesis[14,15,16,17,18,19,20]. Chen et al demonstrated that bone formation, in response to loading, was significantly inhibited in mice subjected to a conditional knock-out of the primary cilium within the bone marrow, including the stem cell population[25] This occurred due to a decrease in the area of active mineralizing surface rather than the rate of mineralization suggesting that the cilium may be involved in osteoprogenitor cell recruitment. A number of skeletal pathologies such as osteopenia, osteoarthritis or Camurati-Engelmann disease have been associated with a deficiency or overexpression of TGFβ1 and associated signalling[29,33,34,35,36] These studies highlight the critical role of TGFβ1 in maintaining bone homeostasis, and importantly, the need for tight regulation of this pathway. This study demonstrates that signalling through the primary cilium is necessary for the sensing of a TGFβ1 chemotactic gradient in human bone mesenchymal stem cells demonstrating a novel mechanism of TGFβsignal regulation and recruitment in stem cells, and highlights the primary cilium as a potential target to enhance MSC recruitment and bone formation in orthopaedic disease
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