High magnesium prevents matrix vesicle-mediated mineralization in human bone marrow-derived mesenchymal stem cells via mitochondrial pathway and autophagy.

Magnesium, as a physiological calcium antagonist, plays a vital role in the bone metabolism and the balance between magnesium and calcium is crucial in bone physiology. We recently demonstrated that matrix mineralization in human bone marrow-derived mesenchymal stem cells (hBMSCs) can be suppressed by high Mg . However, a complete understanding of the mechanisms involved still remains to be elucidated. As mitochondrial calcium phosphate granules depletion manifests concurrently with the appearance of matrix vesicles (MVs) and autophagy are associated with matrix mineralization, we studied the effect of high extracellular Mg on these pathways. Our results first demonstrated that high Mg has a significant inhibitory effect on the generalization of extracellular mineral aggregates and the expression of collagen 1 along which the mineral crystals grow. Transmission electron microscope results showed that less amount of MVs were observed inside hBMSCs treated with high Mg and high Mg inhibited the release of MVs. In addition, high Mg significantly suppressed mitochondrial Ca accumulation. Autophagy is promoted as a response to osteogenesis of hBMSCs. High Mg inhibited the level of autophagy upon osteogenesis and autophagy inhibitor 3-MA significantly suppressed mineralization. Exogenous ATP can reverse the inhibitory effect of high Mg by increasing the level of autophagy. Taken together, our results indicate that high Mg may modulate MVs-mediated mineralization via suppressing mitochondrial Ca intensity and regulates autophagy of hBMSCs upon osteogenesis, resulting in decreased extracellular mineralized matrix deposition. Our results contribute to the understanding of the role of magnesium homeostasis in osteoporosis and the design of magnesium alloys.