Purinergic responses of chondrogenic stem cells to dynamic loading
In habitually loaded tissues, dynamic loading can trigger ATP (adenosine 5?- triphosphate) release to extracellular environment, and result in calcium signaling via ATP binding to purine P2 receptors1. In the current study we have compared purinergic responses (ATP release) of two types of cells: bovine chondrocytes (bCHs) and human mesenchymal stem cells (hMSC) that were encapsulated in agarose and subjected to dynamic loading. Both cell types were cultured under chondrogenic conditions, and their responses to loading were evaluated by ATP release assay in combination with connexin (Cx)-sensitive fluorescent dye (Lucifer Yellow - LY) and a Cx-hemichannel blocker (Flufenamic acid - FFA). In response to dynamic loading, chondrogenic hMSCs released significantly higher amounts of ATP (5-fold) in comparison to the bCHs early in culture (day 2). Triggering of LY uptake in the bCHs and hMSCs by dynamic loading implies opening of the Cx-hemichannels. However, the number of LY-positive cells in hMSC-constructs was 2.5-fold lower compared to the loaded bCH-constructs, suggesting utilization of additional mechanisms of ATP release. Cx-reactive sites were detected in both bCHs and hMSCs-constructs. FFA application led to reduced ATP release both in bCHs and hMSCs, which confirms the involvement of connexin hemichannels, with more prominent effects in bCHs than in hMSCs, further implying the existence of additional mechanism of ATP release in chondrogenic hMSCs. Taken together, these results indicate stronger purinergic response to dynamic loading of chondrogenic hMSCs than primary chondrocytes, by activation of connexin hemichannels and additional mechanisms of ATP release.