Tunneling nanotubes mediate the expression of senescence markers in mesenchymal stem/stromal cell spheroids

The therapeutic potential of mesenchymal stem/stromal cells (MSCs) is limited by acquired senescence following prolonged culture expansion and high-passage numbers. However, the degree of cell senescence is dynamic, and cell-cell communication is critical to promote cell survival. MSC spheroids exhibit improved viability compared with monodispersed cells, and actin-rich tunneling nanotubes (TNTs) may mediate cell survival and other functions through the exchange of cytoplasmic components. Building upon our previous demonstration of TNTs bridging MSCs within these cell aggregates, we hypothesized that TNTs would influence the expression of senescence markers in MSC spheroids. We confirmed the existence of functional TNTs in MSC spheroids formed from low-passage, high-passage, and mixtures of low- and high-passage cells using scanning electron microscopy, confocal microscopy, and flow cytometry. The contribution of TNTs toward the expression of senescence markers was investigated by blocking TNT formation with cytochalasin D (CytoD), an inhibitor of actin polymerization. CytoD-treated spheroids exhibited decreases in cytosol transfer. Compared with spheroids formed solely of high-passage MSCs, the addition of low-passage MSCs reduced p16 expression, a known genetic marker of senescence. We observed a significant increase in p16 expression in high-passage cells when TNT formation was inhibited, establishing the importance of TNTs in MSC spheroids. These data confirm the restorative role of TNTs within MSC spheroids formed with low- and high-passage cells and represent an exciting approach to use higher-passage cells in cell-based therapies.