Cyclic ADP-Ribose Increases Human Mesenchymal Stem Cells Proliferation through the Modulation of Spontaneous Ca²⁺ Oscillation.

Human mesenchymal stem cells (hMSCs) play an important role in the regulation of hematopoietic microenvironment. Cyclic ADP-ribose (cADPR), a universal Ca²⁺ mobilizer produced by hMSCs in the bone marrow, has been demonstrated to be an important factor in the regulation of hematopoietic stem cells proliferation. However, its biological effects on hMSCs are not understood. The present study was designed to investigate the effects of cADPR on Ca²⁺ signaling, cell proliferation and differentiation in hMSCs. Ca²⁺ activity in hMSCs was measured with a confocal microscope. It was found that cADPR increased the frequency of spontaneous Ca²⁺ oscillatory transient, and the effect was abolished by pretreatment with the specific cADPR antagonist 8-Br-cADPR, but not by ryanodine, though the ryanodine receptors are the classic targets of cADPR in other types of cells. RT-PCR showed no gene expression for ryanodine receptors in hMSCs. Interestingly, the cADPR-induced increase of spontaneous Ca²⁺ oscillation was abolished by the TRPM2 channel inhibitors econazole and clotrimazole. The downregulation of TRPM2 channels with siRNA technique had no significant effects on spontaneous Ca²⁺ oscillation; however, it abrogated the cADPR-induced increase of spontaneous Ca²⁺ oscillation. The effects of cADPR on cell proliferation and differentiation were then investigated in hMSCs. We found that cADPR increased hMSCs proliferation, but had no significant effects on osteogeneic and adipogeneic differentiation. Importantly, the effect of cADPR on hMSCs proliferation was antagonized by 8-Br-cADPR or by the selectively knockdown of TRPM2 channels. In conclusion, our results demonstrate the novel information that cADPR regulate cell proliferation by increasing spontaneous Ca²⁺ oscillation mediated by TRPM2 channels, but has no effects on osteogeneic and adipogeneic differentiation in hMSCs.