Tunneling Nanotube-Mediated Mitochondrial Transfer Rescues Nucleus Pulposus Cells from Mitochondrial Dysfunction and Apoptosis

Stem cell-based therapy has been indicated to be beneficial for intervertebral disc regeneration. However, the underlying mechanisms have not been fully identified. The present study showed that bone marrow mesenchymal stem cells (BMSCs) donated mitochondria to adjacent nucleus pulposus cells (NPCs) in a coculture system. The mode of mitochondrial transfer between these cells was intercellular tunneling nanotube (TNT), which acted as a transportation expressway for mitochondria. NPCs acquired additional mitochondria from BMSCs in a concentration-dependent manner after rotenone-induced mitochondrial dysfunction in NPCs. Further research demonstrated that TNT-mediated mitochondrial transfer rescued NPCs from mitochondrial dysfunction and apoptosis, which was indicated by the recovery of the mitochondrial respiratory chain, the increase in mitochondrial membrane potential, and the decreases in reactive oxygen species (ROS) levels and apoptosis rates. Furthermore, Miro1, a critical protein that regulates mitochondrial movement, was knocked down in BMSCs and significantly reduced mitochondrial transfer from BMSCs to NPCs. These results suggested that Miro1 depletion inhibited the rescue of NPCs with mitochondrial dysfunction. Taken together, our data shed light on a novel mechanism by which BMSCs rescue impaired NPCs, providing a concrete foundation to study the critical role of intercellular interactions in disc regeneration.