Kidney illustration
In a bid to improve organ transplant options in the future, a team led by researchers at the Chinese Academy of Sciences has shown that it’s possible to produce pig embryos containing “humanized” pre-kidney structures.
To do so, the researchers turned to a chimeric embryo approach, introducing human induced pluripotent stem cells (iPSCs) into a pig embryo engineered to have kidney formation defects — work they reported in the journal Cell Stem Cell today.
The human cells, which began as donated human umbilical cord blood, were coaxed back to an earlier stage of development using established iPSC techniques. But there were a few key differences.
In particular, the activity of two survival-enhancing genes known as MYCN and BCL2 was enhanced in the stem cell-like cells. The human cells were also “tagged” with a protein that fluoresces red when exposed to specific wavelengths of light, making it possible to track the destination and spread of the cells within the developing pig.
After nurturing the human iPSCs under optimized growth culture conditions, the team injected them into early-stage pig embryos that had been genetically edited to remove pig genes related to kidney development.
The investigators ultimately transferred more than 1,800 chimeric pig embryos into 13 surrogate sow mothers, halting development of the embryos after 25 days or 28 days to examine human cell integration in the burgeoning kidneys.
“We found that if you create a niche in the pig embryo, then the human cells naturally go into these spaces,” co-corresponding author Zhen Dai, a regenerative biology researcher affiliated with the Chinese Academy of Sciences and the Hainan Provincial Research Centre of Laboratory Animals, said in a statement.
When they took a closer look at three of the 28-day post-implantation embryos and two of the 25-day embryos, the investigators saw signs of normal kidney development with a key difference: at least half of the cells in the kidney appeared human.
Based on these and other findings, the investigators argued that it may one day be possible to produce more extensively humanized organs in pigs, potentially dialing down the risk of rejection in organ transplant recipients.
“Our study demonstrates the generation of a solid organ primordium inside large animals by inter-species embryo complementation,” the investigators wrote.
“Although the gestation was terminated rather early, before the [early kidney] metanephros effectively appeared,” they added, “the results indicate that it might be possible to generate a functional human kidney inside newborn pigs, offering an attractive alternative to overcome the shortage of human organs for transplantation.”
On that front, the team pointed to the short-term success that’s been achieved through “xenotransplantation” efforts, including transplants involving genetically modified pig hearts or kidneys.
Still, the use of human-animal chimeras in particular has long been fraught and controversial.
In a review article published in the journal Stem Cell Research and Therapy in 2016, for example, French researchers noted that “worst-case scenarios” involving the formation of human consciousness, reproductive cells, or other recognizably human features “are obviously unacceptable and must be strictly monitored by careful risk assessment, and, if necessary, technically prevented.”
Authors of the new study attempted to address such concerns by searching for human cells beyond the kidney site using the red fluorescent protein tag specific to these cells.
“We saw only very few human neural cells in the brain and spinal cord,” Dai maintained, noting that human cells were also missing from the genital ridge region of the developing pig embryo, suggesting human cells would not have had the opportunity to develop into sperm or egg cells in the pig.
The authors also noted that parents who donated the cord blood samples used to make the iPSCs used in the study were informed — albeit after the formal consent process — about the nature of the experiments.
The ability to produce fully functional human organs in other animals will require more extensive genetic engineering and ethical study, according to Miguel Esteban, a co-corresponding author on the study. In a statement, he said the type of work presented in the new Cell Stem Cell study “provides a window for studying human development.”
“You can trace the human cells you’re injecting and manipulate them so that you can study diseases and how cell lineages are formed,” said Esteban, who is affiliated with regenerative biology, integrative biology, animal embryo engineering, and stem cell and regeneration programs at the Guangzhou Institutes of Biomedicine and Health and the Chinese Academy of Sciences.