The Decibel - How organ transplants could be changed by ... frozen frogs

Episode Date: January 3, 2023

Some animals survive harsh winter conditions by completely freezing and thawing in spring, like the wood frog. Researchers are looking to harness these abilities for humans – particularly for organ transplants.Shannon Tessier is an assistant professor at Harvard Medical School and an investigator at the Center for Engineering in Medicine and Surgery at Massachusetts General Hospital. She tells us how animals freeze over and how it might be harnessed for organ transplants.Questions? Comments? Ideas? Email us at [e-mail].

Before we start the show, I just want to wish you a happy new year. And we're looking to hear about your New Year's resolutions for an upcoming episode. So you can send us a voice memo or an email to thedecibel at globeandmail.com. Okay, but on with the show. Because today, we're looking to the future. A future where frozen frogs could save lives by radically changing organ transplants. Yeah, you heard that correctly. Frozen frogs. To explain this connection and how it plays into her research, today we're talking to Shannon Tessier.

She's an investigator at the Center for Engineering in Medicine and Surgery at Massachusetts General Hospital. And she's also an assistantgery at Massachusetts General Hospital. And she's also an assistant professor at Harvard Medical School. I'm Mainika Raman-Wilms, and this is The Decibel from The Globe and Mail. Shannon, thank you so much for talking to me today. Thank you so much for having me. It really is a pleasure to be part of the show. So just start by helping me understand here, what does a frog have to do with organ transplant? Well, in many ways for medicine, we always try to look to nature to help try to solve some complex problems, because nature has really resolved many of the problems that we face in medicine. And the wood frog and freeze tolerance is just one of those applications. So the wood frog does a process where it can freeze completely solid in the winter.

And by doing that, it enters what we call the state of suspended animation. And suspended animation has many applications in medicine. One of the sort of more direct applications or sort of one that could be realized, you know, closer in time is applying suspended animation to whole organs for transplant. And that's because of the many challenges that transplantation faces in transporting an organ from its donor to its recipient. Wow. Okay, so let's talk about this wood frog here then first. Just at a very high level, for someone who's never heard of this before, how does it freeze over in winter? Like, how is that possible?

So the wood frog basically, it sort of hides under the leaf litter in actually regions surrounding Toronto and Canada and even the northern parts of the U.S. And it hides under the leaf litter. And as the area around it freezes, it has these what we call ice nucleators that are on its skin that help to initiate ice that's kind of creeping from the external environment. And that ice kind of starts to basically move through the wood frog, trying to stay mostly in the external spaces and in the vasculature and in the bloodstream and things like that. some of the sort of more sensitive cellular structures, but also restrict ice to sort of these regions that can survive that kind of expansion of ice. Yeah, actually, the expansion

of ice is like a big question I have with this too, right? Because I know like if there's a water bottle that freezes, it expands, ice expands. So how does this work? Why doesn't the frog explode? Yeah, it's a great question. So that water expansion, as it freezes, you need to make room and space for that. So one of the things that they think the wood frog can do that, you know, many human organs, of course, could not do is to actually be able to engorge, to be able to facilitate or accommodate that sort of expansion of the ice. So that's one of the key adaptations that humans don't have. And so when we're thinking about translating some of these lessons from nature,

we need to think about how we can accommodate that expansion of ice in a system that wouldn't be able to naturally overcome that type of stress. Yeah, because it sounds like the frog is expanding then essentially when it is freezing. The vascular system can engorge, yes. But there is also ways that the wood frog can also control the amount of ice that's present. So one of the other key survival mechanisms that the wood frog uses is it synthesizes massive amounts of glucose, really, really high amounts of glucose. And what happens is when you pump a really high concentration of any sort of analyte, including like a sugar like glucose, what you do is you actually lower the freezing point of that solution. So what that

means, if you have just pure water, and you try to freeze it, there's going to be more ice in that pure water than if you had a really sugary drink, because the sugar sort of suppresses the amount of ice that's going to be present because it lowers the freezing point. So what the frog does is it synthesizes these, like I said, these massive amounts of sugar. And that also helps control how much ice is actually present. And we use a lot of those sort of same tricks in a slightly different way in the field of cryopreservation, for example. So we use these molecules that we call cryoprotectant agents. They can be sugars, they can be other types of sort of antifreeze molecules that you hear about to try and control

the total amount of ice that's formed so that the tissue or the organ or whatever you're trying to preserve can actually live at those colder temperatures without as much damaging ice. Wow. Huh. Okay. And so when we're talking about the wood frog vent, is it actually freezing solid? Like the entire animal is like a block of ice? Yeah.

So it's estimated that about 65% of the wood frog's water becomes trapped as ice. And if you sort of touch it or feel it, it feels like an icy ice cube. It's hard. And then you can actually, you know, like if you tap it, you can actually hear the clicking of the ice. Like it's a hard, icy ice cube. That is fascinating. Wow. So I kind of want to compare this to, I guess, what happens to humans. Like, so what does our body do then when we get cold? So the main thing is your cell needs energy to be able to perform its normal cell functions. So all of these systems that are in place to enable your cell to make energy in the form of something that we call ATP

requires them to function at 37. So those enzymes and all those proteins have been adapted to only work at 37 degrees. So all of these things start to break down and then the cell just can't support itself. Okay, so maybe you can just connect the dots here for me, Shannon. What is it about this type of adaptation of being able to freeze, essentially, that is so interesting for researchers like you? What I find the most fascinating, well, there's, of course, many reasons why it's fascinating. But I think it's just amazing to me that we have such a diverse system of animals that can, you know, survive these extreme conditions. Whereas for humans, our cells are really not adaptive at all. You know, we can only survive very limited temperature fluctuations. But we have this diverse amount of species that can, you know, have really just

dominated these cold adapted regions in a way that we just could never imagine. So by taking all of those lessons from nature, and of course nature has had so much more time to adapt and figure these systems out, and try to pull that into medicine in a unique way to try and basically give human cells more flexibility to be able to survive these sort of more extreme conditions. I think there's lots of promise, not just in transplantation, but in medicine broadly. Okay. You mentioned transplantation specifically there, though. So let me ask you about that. When we do an organ transplant for humans, how long do you typically have to transplant an organ?

Yeah, it really depends on the type of organ. And there are some slight differences across transplant centers. But in general, for something like a liver, you're looking at about 12 hours. Actually, specifically here at Mass General, we have a limit of nine hours. For heart, it's closer to four hours. So it's very short amount of time. But then if you're looking at something like kidney, you have a little bit more time, so closer to 24 hours. So you have to imagine that if you're, of course, going to take, you know, this life-saving organ, very generously donated, you have to be able to transport it. You have to be able to bring in the recipient, prep them. You have to try and find the best match for the organ. And all of this needs to happen,

you know, in the context of a heart in less than four hours. So you can imagine the scramble that must occur to be able to facilitate these types of life-saving procedures. We'll be right back. And this technology, of course, this is all, this is based on the idea that we could freeze these organs, keep them cooler for longer. But as we talked about, humans don't have the same abilities as the wood frog. So this is the big question here, Shannon.

How do you actually do this? Like, how do you apply what the frog can do to human organs? I think the main thing is we're not trying to perfectly recapitulate what the frog's doing. So we definitely want to, you know, borrow lessons from the wood frog as best we can, but we try to also enhance those with guiding principles in cryobiology, new engineering technologies that are coming up that enable us to do this. So it's sort of this bringing together of many of these sort of interdisciplinary approaches that includes these wood frogs to try and make this a realistic and feasible way forward.

So even though we're not doing exactly what nature is doing, we're tricking the system and adapting in a way to try and make it work for mammalian human cells that can't normally survive that condition. That's very cool. And I know that you and some of the researchers at Harvard and Massachusetts General Hospital came up with a new way of freezing a liver. Can you tell me about that? Yeah. So we call this approach partial freezing. And again, takes, you know, many of the sort of influences from nature, but also mix and matches them with sort of current technologies that are coming up in transplantation, both on the engineering side, but also brings in principles from cryobiology as well. So this is an approach that we basically aim to freeze whole livers. So we actually can sort of like, just like the wood frog, you can kind of

feel the ice. If you touch it, it's not, you know, it's actually hard. And then we use these principles that I mentioned, the ice nucleators that the wood frog uses is one of them. We also basically, you know, perfuse that into the tissue and provide that as a mechanism to overcome the freezing stress. But there are many other things that we do. We use a platform called machine perfusion to be able to deliver all of these protective molecules. We allow the organ to freeze completely solid. And in this case, in this specific study, we froze them, the livers, for five-fold longer than the clinical standard. So for how long, like in terms of hours, what are we talking about?

We were using rodent, and they have slightly different, what we call cold ischemic injury pathways. So there, that was a five-day study, but the equivalent would have been two and a half days for a human liver. So two and a half days compared to what, like less than 10 hours otherwise? Yeah, nine to 12 hours kind of thing for a human liver. Exactly. And then of course, after we freeze it for this extended period of time, we reanimate it, we thaw it, we remove a lot of these sort of molecules that we've added in to try and protect

the liver. And then we do very detailed assessment to try and see, you know, is the liver still functioning properly? Are there any sort of, you know, did we injure it at all? And we do all of these sort of extensive studies to try and evaluate the liver after the freezing stress. And what did those find? Like, are there any risks to the liver from doing that? So far, we've done all of these studies, most of these studies with rodent livers. So there's certainly much more that needs to be done to sort of scale this up and make it feasible for human organs. But it definitely looks promising in terms of the clinical criteria that what you would use to deem an organ transplantable. We sort of, you know, tick all the boxes. That being said, one of the really important things to consider is the only

true way that you can know if an organ is transplantable is if you actually transplant it. And then how close, I mean, it's kind of hard to say here, but I guess how close are we actually from doing this with human patients? In terms of, you know, cryopreserving or preserving the whole complex, you know, liver, for example, and being able to sort of realize that clinically, so transplanting that into another human, we're definitely far. However, this partial freezing approach, we are freezing down to minus 15 degrees Celsius. However, there are other approaches that are a little bit higher in temperature and that don't actually initiate ice formation.

So we also have some efforts where we showed some promise for super cooling. And these was also shown to be promising for whole human organs. So these were not ultimately transplanted, but we did super cool whole organs, whole human organs for threefold longer. So, you know, up to about 30 hours approximately, and then assessed them after and showed that there was some promise that we could recover liver function and et cetera. Of course, we need far more extensive studies to show that those organs are actually still transplantable. But my point is that there are these other technologies that still achieve longer storage durations,

but not to the extremes that partial freezing could, that might also enter into the clinic faster than these ones. So in terms of the pathway for seeing changes in this field in the near future, I think they're real. In terms of realizing true banking or long-term storage of organs, that's certainly going to take much longer. So in terms of, I guess, practical terms for patients and for doctors, having a bit more time, a few more hours, maybe a couple days with the organ before you need to transplant it, what kind of difference could that actually make? So even, I mean, a couple more hours would be great. But if we could achieve, you know, something on the order of three to five days, that would completely change the landscape of organ sharing.

So, for example, right now, when you only have a couple of hours, you can't do something that's called HLA matching. So this is a type of form of matching the organ with its recipient. And you can't do that type of matching with only a limited number of hours. However, if you were able to extend that past a couple of days, you could do that type of matching. In fact, even further than that, not only could you improve matching over sort of a shorter distance, but we could actually even start thinking about potentially achieving global matching and actually organ sharing internationally rather than in these very small geographic regions. So I really like to think about extending this timeframe.

It doesn't just impact one part of the system. It really is an enabling technology that would change many aspects of the transplantation and allocation process. Shannon, this is all very fascinating, and the work you're doing is very cool. Thank you so much for taking the time to talk to me today. Oh, thank you so much for inviting me and always love to talk about the research. So I really appreciate the opportunity and thanks so much. Thank you.

That's it for today. I'm Mainika Raman-Wilms. Our producers are Madeline White, Cheryl Sutherland, and Rachel Levy-McLaughlin. David Crosby edits the show. Kasia Mihailovic is our senior producer, and Angela Pichenza is our executive editor. Thanks so much for listening, and I'll talk to you tomorrow.