Melatonin, tunneling nanotubes, mesenchymal cells, and tissue regeneration.

Mesenchymal stem cells are multipotent stem cells that reside in many human tissues and organs. Mesenchymal stem cells are widely used in experimental and clinical regenerative medicine due to their capability to transdifferentiate into various lineages. However, when transplanted, they lose part of their multipotency and immunomodulatory properties, and most of them die after injection into the damaged tissue. In this review, we discuss the potential utility of melatonin in preserving mesenchymal stem cells' survival and function after transplantation. Melatonin is a pleiotropic molecule regulating critical cell functions including apoptosis, endoplasmic reticulum stress, and autophagy. Melatonin is also synthesized in the mitochondria where it reduces oxidative stress, the opening of the mitochondrial permeability transition pore and the downstream caspase activation, activates uncoupling proteins, and curtails the proinflammatory response. In addition, recent findings showed that melatonin also promotes the formation of tunneling nanotubes and the transfer of mitochondria between cells through the connecting tubules. As mitochondrial dysfunction is a primary cause of mesenchymal stem cells death and senescence and a critical issue for survival after transplantation, we propose that melatonin by favoring mitochondria functionality and their transfer through tunneling nanotubes from healthy to suffering cells could improve mesenchymal stem cell-based therapy in a large number of diseases for which basic and clinical trials are underway.