A human brain can be grown in animals – stem cell biologist 

SS: All right. So I'd like to start from the following. Chimera is a monstrous creature from Greek mythology. I personally associate it with something grotesque, evil. Yet in modern microbiology, genetics, it's used for something that can potentially be used to save human lives. Isn't a choice of words somewhat misleading in this regard? Can there be a better name?

HN: Yes, I do agree. Many people, you know, just imagine, when I say ‘chimera’, as you know, a monster in Greek mythology, you know, consisting of beast parts of animals like lion’s head, sheep body and snake tail. So, that gives many people a very bad impression of what chimera is. But actually, it's very useful. Chimera in biology is just a mixture of two different cells of different genetic origin. So it's just a mixture of cells, of two genetically different cells.

SS: So chimera research is needed in order to basically grow human organs inside animals for transplants. But an animal, be it a chimp or a pig, already has a heart. So how do you make it have a human one instead?

HN: If we just mix human stem cells in early embryo, we’ll just make a chimera, in which all the tissues, cells, organs, are a mixture of both human and animal cells. But the one trick I introduced is to use a host animal that has been genetically modified so that this animal cannot form a heart, for example, one organ, that was organogenesis disabled. And so when they grow up, then make a chimera, only human IPS, only human stem cell-derived cells can make a heart but host cells cannot. So although it's basically a mixture of two different cell types, of two different origins, a heart should be entirely made up of human stem cell-derived cells. So the body's mixture of human and animal cells but the heart is only from the human stem cells. That is a trick that I introduced.

SS: All right, so, basically, we inject the embryo with cells from a human patient, and then the heart grows and is transplanted back into the patient's body. What if, and it's likely, the patient has really bad heart genetics, like heart disease for ten generations back, is this patient stuck with bad genetics or can that be fixed as well, growing the new heart?

HN: Ideally, you know, with a technology called CRISPR, now, it is relatively easy to genetically correct the genetic defects that this patient has. So after correcting this genetic defect at the level of stem cells, then we can grow organs and put them back to the patient. The patient's would be normal because the genetic defect has been fixed by genetic engineering. The alternative is we may be able to simply transplant this newly created generated heart to the patient because, you know, with some diseases, it takes time to develop phenotype, symptoms, so, it might help just delaying the onset of the disease, depending on the disease and patient's condition, you know. There’s several alternatives.

SS: Is it possible, theoretically, at least, to grow several organs at the same time in the same animal?

HN: Yes, that's possible, we have recently found a way to do that. Not just one single organ, but basically, we may be able to make several internal organs from one animal, that is also a possible approach.

SS: Why do we need host animals to grow organs? Can't we do it without having to use embryos, just in a lab. I mean, scientists are able to grow meat in the lab, why not human organs?

HN: You know, an organ is composed of many different cell types, such as vessels, blood, nerves and many other components are required, and also it has a 3D dimension. So it's not quite practical to make those organs in a test tube, in vitro. So, I came up with an idea of using animal body, animal developing environment, as a sort of a bioreactor, instead of, you know, making organs in a test tube. So that is how we started this project.

SS: You know, one of the reasons for this line of research is a long line of those waiting for available transplants. So, if we learn to grow hearts, people won't, in theory, have to wait for a heart transplant if they need one. But, you know, what you're working on isn't an instant solution since it takes at least a year before an animal can grow to a full size to be used as a donor. I mean, this means a person waiting for a transplant won’t probably last to this moment all the same, right? How could this timeframe be reduced even more?

HN: Right. So, again, several ways to answer this question. So, yeah, it will probably take about a year to generate a human functional transplantable organ, say, in pigs, in livestock animals. However, you know, some organs we have – an artificial organ, for example, a heart or a kidney – patients can survive over a year easily with the help of these artificial organs. So, this is one approach. Also, we may be able to prepare off-the-shelf organs, if we, you know, prepare organs of different HLA type for wider HLA matching, we may be able to wait sort of the order from the patients for different HLA types, – this is one approach. I think this is possible eventually. The third alternative is we may be able to generate organs that can match to different people, you know, universal matching to different HLA types. People are working on these, what we call, universal organs, universal cells. So, with this approach, we may be able to prepare off-the-shelf organs for any people with different HLA types.

SS: You know, so far growing human organs in pigs isn't really working. The anatomy is quite similar. But the species are too far apart biologically for this to work. You call it the ‘xenogenic barrier’ – a barrier that prevents cells from one species from growing in another. How do you go around that barrier theoretically?

HN: Of course, we do not have an answer to that, we are working on it. Theoretically, I think the major reason for this xenogenic barrier is the evolutionary distance or genetic distance between human and animals because although we evolved from the same probably ancestor, it's been many, many years since we diverged. So, some of the molecules important for the development of, say, embryo, they could differ, the receptor and hormone may not have good affinity to bind together because both animals and us diverged genetically. So one potential approach is we have to probably humanize some of the molecules necessary for the development of, say, pig embryos to match with human cytokines, hormones, or some of the things. So that is the approach that we are taking. We're trying to humanize some of the molecules that pig has.

SS: And here’s a question from a skeptic, I mean, do you think the xenogenetic barrier might be there for, I don't know, good, evolutionary reason? So, for instance, we don't mess with crossbreeding chimera species?

HN: Yeah. I don't know if there's any biological reason for that or just by chance, we have diverged based on maybe the outside environment. So it may be a bit difficult. Just like livestock animals, they can grow very fast because they have been domesticated. So, you know, it may not be that easy, but I think there should be a way to do that. That is our challenge.

SS: You know, a few years back another team of scientists, they announced that they're making human-monkey chimeras. Is a human-monkey chimera going to have more chances of successfully growing an organ?

HN: Yes, I think so. We also have some preliminary data. As I said, I think this xenogenetic barrier is very much based on the evolutionary distance or genetic distance between the two species. Obviously, non-human primates, they're much closer in terms of evolutionary distance. So I'm pretty sure that, you know, if we try to generate human organs in non-human primates, monkeys, it would work much better than between human and pigs. But it has some problems, you know. In addition to the ethical issues, non-human primates, monkeys tend to grow slower and it may take more than a year, maybe four or five years to get to a certain size. And also I really worry about zoonosis, you know, the infection of certain viruses or, you know, bacteria that monkeys have. Because we're so close, they may, indeed, infect us. So I think there are many issues with the use of monkeys. It's not like using pigs or sheep. They're very much more difficult to do some developmental biology to make chimeras, for example.

SS: At least we know that the size of an animal is crucial. But what about other genetic factors? For instance, pigs grow, age and die sooner than humans. Is there a risk that a heart grown in a pig will wear out faster than a human heart, for instance?

HN: Now that's another interesting point. We have very interesting data using rodent studies that is when we generated rat pancreas in mouse – rat and mouse, you know, are different species, a rat is 10 times bigger than a mouse, – but when we generated rat pancreas in mouse, the size of pancreas was mouse-sized. So, somehow the size of the organ is determined by the environment, not by the cells making that organ. So, we also tried the reverse experiment, we tried to generate mouse pancreas in rats, and then we found huge, rat-sized mouse pancreas. So, again, you know, these experiments suggest that, you know, the size of the organ is determined not by the cells but the environment. So, if it's too big, it may be difficult, but if the size is suitable, not too big, once transplanted, I think, the size of the organ is determined by the environment. That's what we think.

SS: I don't know if you read it, but there's this immensely popular old Russian novel, ‘The Heart of a Dog’ where a doctor transplants a human organ into a dog, which then grows into a human but keeps a lot of dog’s habits, like chasing cats and such. It sounds silly, but in the same sci-fi way, will a monkey-grown organ inside me make me behave more like a monkey? Even a little bit maybe?

HN: Well, if you transplant, say, a monkey's brain into you, then you may get some behaviour of monkeys, or a consciousness monkeys have. But still, that's something we do not know. But I do not think that transplantations of your heart generated in monkeys will change your behaviour or thinking. I need to say, it's still your own heart, it's not monkey’s heart that we're transplanting, they're just generated within monkeys. I think that makes a difference.

SS: So what organs theoretically could be grown this way? Liver, heart, pancreas, what else? Can a brain be worked out using this method?

HN: We have this rodent study, – we're not trying to create pigs with human brain, – just between mice and rats. We have made in this case a rat brain in a mouse and a mouse brain in a rat. So it works. Essentially, all the organs we tested. The basic principle applies to any organ. I think any transplantable organs, tissues, cells, we can use, you know, those generated in pigs, or in animals, it has to be transplantable. Brain transplantation is not possible. So it has to be a transplantable organ.

SS: Well, I know it took you 10 years to get the approval to do whatever you're doing, the current research. How long before this technology can be fully implemented on patients – like, a century?

HN: I hope much sooner.

SS: Ok, what are your predictions?

HN: Maybe five to ten years. It depends. On an experimental basis, hopefully within five years, but if we want to bring it to the clinic, we have to do a lot of safety many other studies that are necessary before actually bringing this to the clinic. So I would say hopefully, within 10 years, we're able to use these organs generated in animals in the clinic.

SS: You know, the health benefits, they appear to be enormous. However, really, the whole thing is, it's a little gory to create an animal that can’t grow a heart first, then you put a human heart in that animal, and then you cut the heart out. I mean, poor animal, right? How do you deal with these kinds of concerns? I'd say ethical concerns. I know your research is government-approved and is monitored by ethics commissions and all of that. But personally, do you have reservations about this? I mean the whole methodology thing.

HN: Yeah, of course, you know, I like animals. So I feel sorry for them. But you know, on the other hand, I see patients, you know, dying waiting for donor organs. And if you think we're sacrificing maybe billions of pigs for food, and what we need probably is pigs in the order of thousands, to help those patients at the end stage of organ failure. So I know, this is not a perfect solution for the treatment of organ failure patients but we need to help those patients. And since we are eating many, many pigs, I think this –

SS: Yeah, I mean, I agree, we eat bacon, we eat piglets. But does the fact that we farm pigs justify the use of their embryos like that?

HN: I think it's justified.

SS: Another concern is that the human cells in an animal can stray beyond the targeted organs into the animal's brain and then create some form of consciousness. Is this even possible?

HN: Well, at this moment the presence, the contribution of human cells is very small. So it's unlikely. But as a scientist, I cannot say it would never happen. So what we are trying to do is we have already generated iPS cells, human iPS cells, mouse iPS cells, and that cannot differentiate into brain cells or … cells. So there's no chance of having those iPS cell, human stem cell-derived human cells in the brain of the animals. So that is, I think, a very clear solution to this ethical concern.

SS: You know, the team that announced their experiments on monkey-human chimeras a little while back, they said that they did the research in China to avoid legal complications. I'm just wondering, are government regulations around the world making chimera research difficult in general?

HN: Well, that's a difficult question because, you know, I could not do this kind of research in Japan years ago, I moved to Stanford, so it’s more or less the same thing, I moved to the country where this kind of research was possible. So I think, you know, we need international guidelines for this type of research, or many other researches that probably require some sort of restrictions. So indeed, we have International Society of Stem Cell Research, and they recently revised the ethical guidelines to do this kind of research. So I think, hopefully, soon, the funding agencies for journals, like many major science journals, so they should require proof of the approval by the IRB or ethical committee, that scientists did that experiment, otherwise they shouldn't accept grant proposal or publication of the papers. So this could be a good enforcement to stop doing some ethically problematic researches.

SS: Dr. Nakauchi, thank you very much for this talk. It was very informative. We wish you all the best of luck with your research. Let's see where it can bring us and hopefully can really help humans suffer less, not stand lines for human organs and find some sort of ethical guidelines where animals don't suffer that much. But anyways, it's been too interesting talking to you. Thanks a lot for your time.

HN: Thank you so much for having me. I enjoyed it.

SS: Take care. Bye-bye.