Once again, I apologize to my readers for the week-long hiatus, but I was at Roberts Wesleyan College for the 60th Free Methodist National Bible Quiz. My teams had some successes and failure, but I am exceedingly proud of them.
Now on to our news for the day.
Takanori Takebe, a stem-cell biologist at Yokohama City University in Japan, and his colleagues have transplanted small liver buds constructed from human stem cells into mice that restored liver function in mice. Even though this is a preliminary study, these results offer a potential path towards developing treatments for the thousands of patients who are awaiting liver transplants every year.
Takebe and others reported their data in the journal Nature recently. The liver buds constructed by Takebe and co-workers are about 4 milliliters in diameter and were able to prevent death in mice suffering from liver failure. The transplanted liver buds also differentiated into cells that assumed a range of liver functions that ranged from secreting liver-specific proteins and producing human-specific metabolites. Most notably, these buds quickly made connections with nearby blood vessels and continued to grow after transplantation.
Valerie Gouon-Evans, who studies liver development and regeneration at Mount Sinai Hospital in New York, noted that even these results are preliminary but promising. “This is a very novel thing,” she said. Because these liver buds are supported by the host’s blood system, transplanted cells continue to proliferate and perform liver functions.
However, Gouon-Evans cautioned, the transplanted animals must be placed under observation for several more months in order to determine if the transplanted cells begin to degenerate or form tumors.
Globally and even in the United States, there is a dismal scarcity of human livers for transplant. In 2011, 5,805 adult liver transplants were done in the United States. That same year, 2,938 people died waiting for new livers or became too sick to remain on waiting lists.
However, attempts to create complex organs in the laboratory have been challenging. Takebe believes that his study represents the first time that people have made a solid organ using induced pluripotent stem cells (iPSCs), which are created by reprogramming mature adult cells into an embryonic stem cell-like state.
Unfortunately, determining whether or not these liver buds could help sick patients is years away, according to Takebe. Apart from the need for longer-term experiments in animals, it is not yet possible to make liver buds in quantities sufficient for human transplantation.
In the current work, Takebe surgically transplanted liver buds at sites in the cranium or the abdomen, but in future work, Takebe hopes to generate liver buds small enough to be delivered intravenously in mice and, eventually, in humans. Takebe also hopes to transplant the buds to the liver itself, where he hopes they will form bile ducts, which are crucial important for proper digestion and were not observed in the latest study.
The researchers make the liver buds from three types of human cells. First, they induce iPSCs to differentiate into a cell type that expresses liver genes. To these cells, they add endothelial progenitor cells (EPCs; endothelial line blood vessels) from umbilical cord blood, and mesenchymal stem cells, which can make bone, cartilage and fat. These cell types also come together as the liver begins to form in the developing embryo.
“It’s a great day for developmental biology,” says Kenneth Zaret, who studies regenerative medicine and liver development at the University of Pennsylvania in Philadelphia. “By reconstituting cell interactions that we know are important for natural liver progression, they get what appears to be robust, mature tissue.”
The project began with an unexpected phenomenon, says Takebe. He initially hoped to design ways of to make vascularized liver tissues. Therefore, he tried culturing multiple cell types together and noticed that they began to self-organize into three-dimensional structures. After this, the process for making liver buds took hundreds of trials to adjust various experimental parameters (e.g., the maturity and ratios of the different cell types).
This strategy takes a middle path between two common strategies in regenerative medicine. For simple, hollow organs such as the bladder and trachea, researchers seed scaffolds with living cells and then transplant the entire organ into patients. Researchers have also worked to create pure cultures of functional cells in the laboratory, hoping that cells could be infused into patients, where they would establish themselves. But even if the cells work perfectly in the laboratory, says Gouon-Evans, the process of harvesting cells can damage them and destroy their function.
Zaret thinks that the liver buds work might encourage an intermediate approach. “Basically, put the cells in a room together and let them talk to each other and make the organ.”
Self-organizing structures from stem cells have also been observed for other organ systems, such as the optic cup, an early structure in eye development. And “mini-guts” have been grown in culture from single human stem cells.
Takebe believes that the self-organizing approach might also be applicable to other organs, such as lung, pancreas and kidney.