by Francisco Galdos
For centuries, humans have marveled at the ancient myths of chimeras—from Homeric references to half lion-half goat beasts, to Kafka’s frightening tale of the metamorphosis of a man into an insect, and to the ancient Greek legend of the horse Pegasus, whose winged horse body allowed the hero Bellerophon to kill the wretched chimera (ironic, since Pegasus himself was a chimeric animal!). Indeed, if one looks at animals in nature, it does not seem all that unreasonable to believe that similarities between animals could indeed make way for fantastic chimeric beasts such as Homer’s chimera. Those very similarities between species was what eventually led Charles Darwin to draft his own theory of evolution, and it is how evolutionary biologists today are able to find molecular evidence towards identifying common ancestors of various species.
In biology, the term “chimera” refers to an individual that has cells or tissues from a different individual. Today, even human chimeras exist. Patients with failing heart valves often have their human valves replaced by pig valves, a process known as xenotransplantation (1). For patients with failing organs, scientists and physicians have even proposed to go a step further toward transplanting whole organs from pigs into humans with organ failure. A wide range of complications comes with this idea, however. For one, even transplants between genetically non-identical humans pose risks of organ rejection by the immune system, so having transplants between species has an even greater risk of immune rejection According to the United States Organ Procurement and Transplantation Network, more than 123,000 men, women, and children currently need lifesaving transplants, with another name being added to the list every 10 minutes. Moreover, organ demand surpasses organ availability, putting at risk the lives of many patients in need of these lifesaving organs (2).
With the rise of the era of regenerative medicine, scientists have begun to ask questions as to whether it will be possible to use a patient’s own cells to regenerate entire organs. In 2006, Nobel Prize winner Shinya Yamanaka discovered a way to make pluripotent stem cells from differentiated cells, enabling scientists to use a patient’s own adult cells to make pluripotent cells that can differentiate into any cell of the body (3). With the rise of this technology, scientists have begun exploring the possibility of generating entire organs for transplantation, a task that aims to recapitulate the way the body makes our organs during embryonic development. With this goal in mind, however, Hiro Nakauchi’s group at Stanford University is taking a different approach from trying to build organs outside of the context of normal development. Each organ in the body fulfills a particular role and develops within a highly regulated environment, which makes the engineering of organs an enormous and perhaps impractical feat. To overcome the barrier that making whole organs outside of a developmental context may pose, Nakauchi’s ultimate goal is to grow human organs in an animal that is anatomically quite similar to us—the pig.
Several questions arise from Nakauchi’s work. How do you grow a human organ in a pig? Don’t pigs have organs for themselves? Importantly, how do you solve issues of pig cells getting into the human organ? Wouldn’t this pose the same problems as xenotransplantation? To answer these questions, Nakauchi’s group published a paper in 2013 that aimed to make chimeras between two genetically distinct pigs. During embryonic development, a fertilized egg begins to divide into exponentially increasing numbers of cells. Eventually, a structure known as a blastocyst forms, where a small mass of pluripotent cells inside this spherical blastocyst go on to develop into any cell of the body. For organs to develop, certain genes must be turned on to allow for these pluripotent cells to start changing their identity into the specific cells needed to make particular organs. Take for example, the pancreas. A protein known as Pdx1 is required in order to turn on genes that allow for the development of the whole pancreas. Without Pdx1, an animal can be born without a pancreas (2). Importantly, some genetic programs override others; for example, the protein Hes1 turns on the genes that make the biliary system, and this developmental path overrides the pancreatic path that Pdx1 specifies. To take advantage of this system, Nakauchi’s team generated pig embryos that had a gene that enabled the Pdx1 protein, itself, to turn on the production of Hes1. This caused the embryo to be unable to produce a pancreas, since Hes1 overrides the pancreatic program in order to produce the biliary system. The next step was to see whether the injection of pluripotent cells from a normal pig without this Pdx1-Hes1 system could complement an embryo that was unable to produce a pancreas. They injected the normal pig’s pluripotent cells into the embryo of the pancreas-defective pigs and saw that the normal pluripotent cells were able to give rise to a fully functioning pancreas (4). This process, known as blastocyst complementation, occurs because each organ in the body has a specific function and, thus, occupies a unique niche. Since normal pluripotent cells can develop into any cell and, thus, fill any niche in the body, transplanting pluripotent cells from a normal embryo into an organ-deficient embryo can help replace the missing organ (2).
This remarkable process produces chimeric pigs, since the cells of the pig are composed of two different individual pigs. As Nakauchi reports, they have even gone on to do this with rats and mice, where mice pluripotent cells are transplanted into the embryos of rats that are unable to form a pancreas. In their review, they report that they were able to see the full development of the mouse pancreas in the rat (2). The cross-species boundary is one that, if crossed, could allow for human pluripotent cells to be transplanted into pancreas-deficient pig blastocysts. Since all of the cells of the pancreas would come from the human cells, this would enable embryonic development to guide these human cells towards making a fully functional pancreas, which could perhaps be transplanted into patients with diabetes to effectively cure their disease. Moreover, if this system can be made for organs such as the heart, it would be possible to generate fully functional human hearts in pigs that could then be transplanted into patients with heart failure (2).
As Nakauchi’s group reports, there are various problems that still need to be solved. One problem includes the possible mixing of pig cells with the human organs grown in the pig, which may cause immune problems once the organ is transplanted into humans. Another possible problem is that these transplanted human pluripotent cells could give rise to cells other than the organ of interest, which may cause the pigs to have human neurons or other human cells, raising ethical questions about whether this process “humanizes” the pig’s brain or body. If these problems are addressed, it may be possible to generate fully functional organs from patient-derived pluripotent cells that could allow for these organs to be transplanted from pigs back into these patients, one day providing a solution to the organ shortage problem. If this is achieved, then these pigs would be acting much like Pegasus—as chimeras majestically conquering disease in the human body.
Francisco Galdos ’15 is a Human Developmental and Regenerative Biology concentrator in Quincy House.
- Manji, R. A., Menkis, A. H., Ekser, B. & Cooper, D. K. Porcine bioprosthetic heart valves: The next generation. American heart journal 164, 177-185, doi:10.1016/j.ahj.2012.05.011 (2012).
- Rashid, T., Kobayashi, T. & Nakauchi, H. Revisiting the Flight of Icarus: Making Human Organs from PSCs with Large Animal Chimeras. Cell stem cell 15, 406-409, doi:10.1016/j.stem.2014.09.013 (2014).
- Takahashi, K. & Yamanaka, S. Induction of pluripotent stem cells from mouse embryonic and adult fibroblast cultures by defined factors. Cell 126, 663-676, doi:10.1016/j.cell.2006.07.024 (2006).
- Matsunari, H. et al. Blastocyst complementation generates exogenic pancreas in vivo in apancreatic cloned pigs. Proceedings of the National Academy of Sciences of the United States of America 110, 4557-4562, doi:10.1073/pnas.1222902110 (2013).