Abstract
Bone remodelling is mediated through the synchronism of bone resorption (catabolism) by osteoclasts and bone formation (anabolism) by osteoblasts. Imbalances in the bone remodelling cycle represent an underling cause of metabolic bone diseases such as osteoporosis, where bone resorption exceeds formation (1). Current therapeutic strategies to repair osteoporotic bone fractures focus solely in targeting anabolism or supressing catabolism (2). However, these therapeutics do not reverse the structural damage present at the defect site, ultimately leading to impaired fracture healing, making the repair of osteoporotic fractures particularly challenging in orthopaedics. Herein, we focus on investigating a combined versatile pro-anabolic and anti-catabolic effect of Magnesium (Mg2+) to modulate bone cell behaviour (3), to develop an engineered biomimetic bio-instructive biomaterial scaffold structurally designed to enhance bone formation while impeding pathological osteoclast resorption activities to facilitate better bone healing and promote repair.
Pre-osteoblasts MC3T3-E1 (OBs) and osteoclasts progenitors RAW 264.7 (OCs) cell lines were cultured in growth media exposed to increasing concentrations of MgCl2 (0, 0.5, 1, 10, 25 and 50mM) and the optimal concentration to concurrently promote the differentiation of OBs and inhibit the differentiation or funtion of RANKL-induced OCs was assessed. We next used Fluorescence Lifetime Imaging Microscopy to investigate changes in the metabolic pathways during OBs and OCs differentiation when exposed to increasing MgCl2 concentrations. We developed a range of magnesium-incorporated collagen scaffolds to permit the spatiotemporal release of Mg2+ within the established therapeutic window, and to investigate the behaviour of bone cells in a 3D environment.
In our results, we reported an increase in the expression of the bone formation markers osteocalcin and osteopontin for OBs exposed to 10mM MgCl2, and a significant downregulation of the osteoclast-specific markers TRAP and cathepsin K in RANKL-induced OCs differentiation when exposed to 25mM MgCl2. Moreover, 25mM MgCl2 induced changes in the energy metabolism of OCs from a predominantly oxidative phosphorylation towards a more glycolytic pathway suggesting a regulatory effect of Mg2+ in the underlying mechanisms of osteoclasts formation and function. The developed porous collagen-magnesium scaffolds significantly reduced the expression of early osteoclastogenic markers RANK and NFkB, and an elevated expression of the osteogenic markers Runx2 and Col1A1 was reported after 7 days.
Our research to date has provided evidences to demonstrate the potential of Mg2+ to concurrently enhance osteogenesis while inhibiting osteoclastogenesis in vitro, potentially introducing new targets for developing therapies to repair osteoporotic bone fractures.
