From threads to strings of pearls
Endosymbiosis caused ancient bacteria to gradually evolve into mitochondria, the organelles that are responsible for energy production in today’s complex cells. In human cells, mitochondria form a dynamic, threadlike network. “The threads respond to negative pressure and transform into something like a string of pearls, from which individual mitochondria subsequently pinch off,” says Christoph Gäbelein, lead author of the paper.
Using cylindrical nanosyringes that were specially developed for this study, the researchers pierced the cell membrane and sucked up the spherical mitochondria. They then pierced the membrane of a different cell and pumped the mitochondria back out of the nanosyringe into the recipient cell.
The position of the nanosyringe is controlled by laser light from a converted atomic force microscope. A pressure regulator adjusts the flow, allowing scientists to transfer incredibly small volumes of fluid in the femtolitre range (millionths of a millionth of a millilitre) during organelle transplants. “Both the donor and acceptor cells survive this minimally invasive procedure,” Gäbelein says.
The transplanted mitochondria also have a high survival rate – more than 80 percent. In most cells, the injected mitochondria begin to fuse with the filamentous network of the new cell 20 minutes after transplantation. “The host cell accepts them,” Vorholt says. This holds true in most cases, though in a few cells they fall victim to the host cell’s quality control system and are degraded.
In their paper, the researchers write that “the technique presented in this paper will facilitate applications in various areas of research in the future”. It is conceivable that it could be used to rejuvenate stem cells, which exhibit a decline in metabolic activity as they age. But Vorholt’s team is currently pursuing other plans: “We want to understand the processes that control how different cell compartments cooperate – and we hope to unravel how endosymbioses develop over evolutionary time,” Vorholt says.