Nature Podcast 29 June 2006

Introduction

Gareth Mitchell: Welcome to this Nature podcast special accompanying June's Nature insight on stem cells. Hello, I'm Gareth Mitchell, presenter of BBC World Service technology programme, Digital Planet.

Allan Coukell: Hello, I am Allan Coukell, science reporter at WBUR in Boston.

Gareth Mitchell: From repairing tissue to growing new organs, and from offering new treatments for cancer to extending our natural lifespan. Research into stem cells has the potential to change the way we understand and treat human disease forever.

Allan Coukell: Yet significant hurdles and years of research lie ahead, today we'll explore some of the current ideas, the technical questions and the controversies in this very promising area of biology.

Gareth Mitchell: And to that end I've been talking to three authors of stem cell related reviews. You'll hear short clips of our conversations in this programme, and we've put the full versions up on the website at http://www.nature.com/podcast/stemcells, and we'll be hearing from Jo Marchant who has been to the annual meeting of the European Society of Human Reproduction and Embryology in Prague. But before we head east to hear from Jo, let's first head west to Boston with Allan, who is in the company of some leaders in this important and exciting field.

Allan Coukell: Thanks Gareth. With me in the WBUR studio are three leaders in this field, Brad Bernstein from the Massachusetts General Hospital and Harvard Medical School, welcome.

Brad Bernstein: Thank you Alan.

Allan Coukell: And Amy Wagers, also Harvard Medical School and the Joslin Diabetes Institute, welcome to you.

Amy Wagers: Thank you.

Allan Coukell: And Bob Lanza from the Biotech Company Advanced Cell Technology in Worcester Massachusetts, welcome Dr Lanza.

Bob Lanza: Thank you.

Allan Coukell: All cells come from cells, it's one of the fundamental principles of biology, and that's one thing when you're talking about bacteria, quite another in organisms like ourselves that begin as a single cell and end up with liver and lungs, blood and brain. How does that happen and what would it mean for medicine if we could control it, and how close is that future? Brad Bernstein, some of our listeners are biologists, many are not, so let's begin with a simple definition, what is a stem cell?

Brad Bernstein: Well, stem cells are cells that both have the capacity to self-renew and also have the ability or the potency to then form multiple differentiated tissues or tissue types or organ types.

Allan Coukell: Meaning they can keep growing and keep growing, and at some point they can divide in such a way that sends a cell down a new line to become, say, a neuron?

Brad Bernstein: Sure, down a particular pathway.

Allan Coukell: And Amy Wagers, what good are these cells? What do you see as their potential?

Amy Wagers: Yes, I think the key reason that stem cells are useful is that they are both able to make these very specialised cells that are important for the function of a tissue or an organ, and they are also a renewable source of those cells. And so when you are thinking about using stem cells for transplantation, it's not just an initial engraftment or initial production of those specialised cells that you need, but potentially a source that would continually produce those cells.

Allan Coukell: Do you see that as the big application, the actual cells themselves as treatment, or is it what we learn about biology from the cells?

Amy Wagers: I think there's two ways to go and I think transplantation is one really strong application for stem cells, and that's certainly the application that's clinically useful today. But I think another equally and perhaps even more beneficial route to go with stem cell research is the use of those cells to understand the development of tissues so that you can develop strategies for intervening in that development in the cases of disease or to promote sort of recruitment of the endogenous stem cell pool to promote tissue regeneration.

Allan Coukell: So let's start to take this apart a little bit. One of the divisions that we hear about is embryonic stem cells versus adult stem cells, and Bob Lanza, what's the difference there?

Bob Lanza: Well, embryonic stem cells are the master cells and they are undifferentiated.

Allan Coukell: Meaning?

Bob Lanza: Well, they have the potential to basically become any type of cell in the body. They are immortalised, which means they can grow indefinitely, and again, these cells we do know can virtually become every cell in the body and all the organ systems. By contrast, adult stem cells are less versatile. They are more lineage specified, in other words a bone marrow stem cell can only do certain tricks or nerve stem cells can only become certain types of neurons. So again, I think there are distinct differences between the embryonic and the adult.

Allan Coukell: So there is a lot of focus on embryonic cells. Is that because the thinking is they are just more powerful because they could be directed down any road we choose?

Bob Lanza: Absolutely. I think anyone working in the lab who sees embryonic stem cells, you'll see that they have a mind of their own and in one part of the dish they become bone marrow, others you see beating heart cells, whereas adult stem cells are more limited. In other words, they are only able to do certain tricks.

Amy Wagers: And if I could just add to that, I think another issue is that it's not clear that there are adult stem cells for all tissues and for all cell types, and so one of the attractive aspects of embryonic stem cells is that it's clear that they can make every kind of cell in the body.

Allan Coukell: So that's a little bit about the potential of embryonic stem cells. What is it that makes adult stem cells attractive?

Amy Wagers: I would say one promise and also issue with embryonic stem cells is their ability to generate all types of cell types. So, a real push in the embryonic stem cell field is trying to direct the differentiation of an embryonic stem cell to a particular cell type, whereas in the adult tissues where stem cells have been identified, often we find they are already directed towards that lineage. So, you want to think about perhaps using the stem cell that's most suited to the type of tissue you want to regenerate. And if a blood forming stem cell is very suited for generating blood, it's probably a good idea to use that to generate blood not to generate skeletal muscle, and conversely where you have a population of cells in a skeletal muscle that's very capable of regenerating muscle tissue, that's perhaps where effort can be focussed in the adult stem cell field in generating that type of tissue rather than trying to divert those stem cells to a new lineage.

Allan Coukell: And what's the attraction of nuclear transfer of cloning to create new stem cell lines?

Bob Lanza: Well, in transplant medicine there are two major hurdles. One is, is there is a very serious shortage of tissues and cells. On the other hand, you need to transplant these cells without immune rejection. So, cloning actually comes in once you have your cell. So we certainly hope in the next few years we're going to be able for instance to create insulin producing cells or new heart cells, but we can't just plonk them back in the body, your body will reject them. Now, we do have immunosuppressor drugs, these powerful drugs, but unfortunately they have serious side effects such as cancer and malignancies. And so what we hope is that through using therapeutic cloning we can actually generate your own cells. We're able to basically take a cell from your body and turn that into embryonic stem cells, which then can be used to repair any part of your body.

Allan Coukell: And this is done in a number of mammalian species; it seems to be tough in humans. Why are humans so difficult?

Bob Lanza: Well, I think that for every new species that has been cloned, we've had to go up a learning curve and it's taken many years to achieve that. I think that unlike humans we have a problem. We know when we're working with mice or cows, you know, you can get hundreds, if not thousands of eggs. With humans it's very difficult even just to get a handful of these eggs. But fundamentally I don't think there's any biological reason why we shouldn't be able to clone a human embryo. In fact, we and other groups have gotten human embryos to the advanced stage. It's simply a matter of time before someone is actually able to derive stem cells.

Allan Coukell: Your company was in there early, but are some species intrinsically easier than others, or is it just a matter of practice?

Bob Lanza: Well, I think there's been a trick that's been required for every new species. So for instance, for pigs you need multiple embryos that need to implant, or for some of the rodents, they spontaneously start to fertilise. So I think that for each new species we run into a unique set of different problems so that if we apply for instance the techniques we've learnt in animals to humans, you basically kill your eggs.

Allan Coukell: And what makes a stem cell a stem cell? What is the essence of stem cell-ness? All cells in our bodies have the same genes, so Brad Bernstein, what is it that keeps a stem cell in this undifferentiated, perpetually dividing state?

Bob Bernstein: Well, you know, we study an area called epigenetics and exactly as you just suggested, how the genes are regulated, and clearly in different cell types you have completely different regulatory processes controlling the genes in different ways. During differentiation, so as you move from this pluripotent uncommitted stem cell to a differentiated neuron for example, you sort of progressively restrict or turn off genes or you know, permanently silence regions of the genome in a particular fashion. Now, what's emerging about embryonic stem cells in particular is these cells have the potential, or maintain the potential of essentially all the genes in the genome, and this makes a lot of sense because depending on which particular pathway the stem cell differentiates or embarks upon, they will need any of these genes. So what we are learning is that there's a very special genome organisation or control of these genes in embryonic stem cell that is really distinct from what we've seen in any other cell type.

Allan Coukell: So is it something that sits on the genes and says "hey whoa, don't you go specialising yet"?

Bob Bernstein: Yes, that's exactly right. So there's a structure called chromatin that forms the chromosomes and it sort of wraps up the DNA. The fashion in which the DNA is wrapped up is actually quite distinct in the embryonic stem cell and very special, and we think what that means is basically it's keeping these cells in a state, or keeping the nucleus in a state that keeps its pluripotency alive and the potential to activate any of the genes, depending on which particular pathway the cell embarks upon during differentiation.

Allan Coukell: Amy Wagers, that's embryonic stem cells. Is the picture the same in adult stem cell?

Amy Wagers: Yes, so understanding what defines a stem cell in different tissues I think is going to be some overlapping and some distinct aspects because stem cells certainly in adult tissues are fulfilling different roles. So, in my lab we work on stem cells that make blood cells and stem cells that make skeletal muscle cells, and these are distinct stem cells, and they really have different responses that they need to carry out. So, blood production has to happen continuously, and you are making billions of blood cells every day, every week. Whereas in the skeletal muscle the stem cells have to remain inert for long periods of time, but they have to be able to be recruited to regenerate the muscle if there's damage. And so there are probably common properties that define a cell as unspecialised, but because they are fulfilling different properties, they have to also have different regulations. So, a muscle stem cell for instance has to learn how to remember that it's a stem cell and remember that it needs to respond to damage for long periods of inactivity, and a blood stem cell has to be able to continuously renew both itself and the production of blood cells without losing or depleting the stem cell pool. So I think in adult stem cells and also in comparison to embryonic stem cells there will be both common and distinct mechanisms.

Allan Coukell: And I suppose adult stem cell, unlike embryonic stem cells have to have the ability somehow to be told turn on now?

Amy Wagers: Yes, so as I was saying, in some instances these cells are sort of constitutively homeostatically replacing cells, and this is the case in the skin, in the blood. Although that production can certainly be ramped up in the case of injury or a haematopoietic need for instance, and in other situations those stem cells could be considered more facultative and they are recruited in response to damage.

Allan Coukell: Until now we have been talking mostly about stem cells in isolation, but at least adult stem cells, as we were saying, don't exist in isolation. They are surrounded by other cells and these other cells have an important function. I talked with David Scadden of the Massachusetts General Hospital Centre for Regenerative Medicine about the concept of a stem cell niche, and asked him just what exactly is a stem cell niche?

Bob Scadden: Well, the niche is a place where the stem cell resides, but more than that, it is a context that really governs how the stem cell behaves. So, there are some places where stem cells actually probably simply transit through, and there are other places where the stem cell spends its time and it is receiving instructions to determine whether or not it will start to regenerate, generate new offspring, try to repair tissue or build new tissue. And in that sense it's a combination of a functional and an anatomical definition. These have been best defined in the invertebrates, and the same thing is true in mammalian systems. So, for example the bone marrow where the blood stem cell resides, it has an interface with the bone itself, and the bone seems to provide key elements that tell the stem cell when to turn on, how much new production of stem cells should be, and whether or not the pool should expand or decrease.

Allan Coukell: So we're talking about a physical location but also a sort of set of chemical signals at that location, and how powerful are they? If you took an ordinary cell that had some function and stuck it into a niche, would it turn into a stem cell?

Bob Scadden: That's a great question. I think there are some examples in which you can take a cell that is a more mature cell, not a fully mature cell, but a more mature cell from the stem cell itself and now if you place it ectopically into the setting of the niche, that the niche has a capacity to actually determine what that cell will become. It isn't restricted to stem cells, it actually provides signals for other cell types, and some of those signals can be to induce the stem cell state. So it does seem that a more mature cell has the capacity to undergo something of a de-differentiation.

Allan Coukell: How possible is it to study stem cell biology without studying stem cell niches?

Bob Scadden: I think it's very difficult to study one without the other. It is really a little bit like looking at a skeleton without the sinews that connect it.

Allan Coukell: And when it comes to actually doing this in the lab, how challenging is it to grow not just the cells but the cells' natural environment, as it were?

Bob Scadden: You know, I think that's a real challenge that we have in adult stem cells because we often take them out of the tissue in which they reside and then try to manipulate them in the laboratory, often without those heterologous cell types that usually surround them. That's very complicated. If you work with more immature cells, stem cells that might come from the embryo say, they actually start to create their own niche. And so there are elements in which the niche can be created ex vivo, but I don't think we've figured out quite enough of those elements yet.

Allan Coukell: Break it down for me just a little bit more. Give me a picture of the stem cell and the cells around it and how they are interacting.

Bob Scadden: You know, that's a great question. We understand this in relatively limited detail, but we do know that for example the blood stem cell, as it circulates, it must engage the wall of a blood vessel. Now, where it does so appears to be not just random. There are these discontinuous areas in the micro vessels of say the bone marrow where the stem cells can engage the wall and actually crawl through the wall of that vessel, and then it seems to make its path, either staying around the blood vessel or finding its way to the edge of the bone, and at that position it seems to be resident for a period of time in close association with the osteoblast, the bone forming cell. Now, usually close by is the osteoclast, and the osteoclast has recently been defined as also playing a role, and has some impact as to whether or not the stem cell will stay in the bone or will move out into the circulation again. And how this is all finely tuned is at this point still unclear, but we do know that certain hormonal influences matter, certain neural inputs matter, that the kinds of rates of exchange of the cell types will be changed by physiologic settings, and in the setting of this particular stem cell we know we can exploit that. We actually can use that in the use of stem cell therapies for patients with blood disease and cancer.

Allan Coukell: That's David Scadden of the Massachusetts General Hospital talking about stem cell niches, a physical location and also a chemical environment in which the stem cell lives. Amy Wagers, how important is this idea of a niche and how well do we understand it?

Amy Wagers: I think it's very important, and one of the hurdles in adult stem cell biology has been the ability to culture these, or propagate adult stem cells outside the body, and probably that has to do with the fact that these cells require a very complex three-dimensional chemical and physical association with niche cells. And so the more we understand about the niche the better able we will be able to recreate that outside the body and potentially expand stem cells.

Allan Coukell: What do you do in a lab, Brad Bernstein, to get these cells to grow?

Brad Bernstein: So in the case of embryonic stem cells there are established protocols that can grow the cells and actually grow a large number of cells. You know, the particular work that we're doing is – as I mentioned earlier – we're interested in how the genes are regulated and how the chromatin structure or the structure of the chromosomes regulate the genes. We focussed on these chemical modifications that occur either directly on the DNA or on the proteins that sort of form the scaffold of the genome. Now, these chemical modifications actually regulate, not only regulate whether the genes are active or not, but they actually have an influence on whether the genes could potentially be turned on at some later stage. We have been able to look at embryonic stem cells just for the reason that these culturing methods have been developed and we can look and we've identified actually particular signatures of these modifications that are indicative of this embryonic stem cell state or this pluripotent state. They make a lot of sense because the signatures suggest that the master regulator genes, these genes that are important for development, are all sitting in a poised state, where they're not actually expressed in the embryonic stem cell, but they have the ability to be turned on, sort of like the finger on the trigger, that once the signal has come along and the embryonic stem cell is induced to differentiate, those genes can be rapidly induced. We've looked a little bit at adult stem cells, or at least tissue stem cells in the form of neural progenitors. I mean in that case we actually see a very different signature. Most of that special pluripotent signature that we see in embryonic stem cells actually goes away when the cells commit to a neural fate.

Allan Coukell: So, actually looking at embryonic cells and adult cells suggest very different biology going on at the level of gene expression?

Brad Bernstein: At the level of the special sort of chemical modifications and how they regulate both gene expression and sort of the potential for gene expression, does suggest some critical differences.

Allan Coukell: And what do we know about how locked in these adult cells are? Could you for instance take a bone marrow derived stem cell and nudge it down the line to forming some completely different cell type forming an epithelial cell?

Amy Wagers: I think that's an interesting question and there has been a lot of interest in that possibility and in part because bone marrow stem cells are so easily accessible. I would say that the consensus of the field now is that it's not easy or not an intrinsic property of the bone marrow stem cell to do that, to generate other types of, and I should say by bone marrow stem cell I mean the haematopoietic or blood forming stem cells to generate tissues of non-blood cell types. But as we understand more and more about what defines pluripotency in embryonic stem cells, it's possible that we might be able to construct artificially through regulation of gene expression a mechanism where we could dedifferentiate a cell to allow it them to acquire that pluripotency. But it doesn't appear to be an intrinsic property of those cells to generate other tissue types.

Allan Coukell: But is there agreement that it's at least possible?

Amy Wagers: I think as I said, as we define what makes up pluripotency and if one could come up with a set of genes, and certainly these have now begun to be reported in the literature, that promote or enhance or promote pluripotency, promote dedifferentiation and nuclear reprogramming, it's possible that we could push an adult lineage committed cell back to a pluripotent stage.

Allan Coukell: And in a way Bob Lanza, this is exactly what you are doing when you are cloning a stem cell line, you are taking an adult cell, putting it into an egg and pushing it all the way back to the beginning.

Bob Lanza: Exactly, you are starting with a terminally differentiated cell, and then by placing it into the egg you are actually able to totally dedifferentiate it back to the embryonic state, at which point it has the capacity to create virtually all the cells in the body.

Allan Coukell: How does the egg do that?

Bob Lanza: That's a good question. I don't think anyone knows. We know the magic is in the egg, but it's much like having a seed and putting it in the soil and it just grows. We really don't understand.

Allan Coukell: Are there other ways to re-programme these cells, to kick them back up so they become more able to differentiate?

Bob Lanza: Well, I think there are a number of tricks that some groups are trying. For instance, you confuse an adult cell with an embryonic stem cell and we know that that does have the capacity to re-programme the cell. I think that there are a number of groups that are for instance permeabilising cells and exposing them to ooplasm or to extracts from embryonic stem cells, and there's some very tantalising data that in fact, be it the cytoplasm or the nucleus are able to modify the programme. So I think that you can either de-differentiate or trans-differentiate cells through any one of several different tricks.

Allan Coukell: And how complete is the process? I know that Rudolph Jaenisch, a stem cell biologist at the Whitehead Institute argues that you can't make normal clones. Even animals that appear normal, he says, still have abnormalities. Does that suggest you can't actually completely re-programme a cell?

Bob Lanza: Well, I think that almost any manoeuvre does influence a cell. For instance we know the methylation patterns are altered simply by changing your culture media. So, I think it's really a matter of at what level are you looking. I think certainly we know that we can create stem cells using nuclear transfer that are normal in all therapeutic aspects.

Allan Coukell: Brad Bernstein, what do you think?

Brad Bernstein: I talked a little bit about these chemical modifications that sort of restrict the genes and restrict the genome as the cells differentiate or terminally differentiate and commit to a neuron for example. Actually, we think that a lot of the genome becomes permanently silenced and turned off by these chemical modifications to the DNA and to the carrier structures. Clearly a key element of nuclear transfer and of the reprogramming of the nucleus is that these chemical modifications have to be essentially erased and then reset in a particular fashion that defines the embryonic stem cell, and I think what is an open question now is whether that can be done precisely, whether that resetting and the fact that that may only be done efficiently in some small fraction of the case, this may account for why only a few of the attempts at nuclear transfer actually are successful, why only you only actually get a couple of clones when you try many, many trials. And so there is some sort of stochastic element in the reprogramming of the nucleus and in the resetting of these modifications.

Allan Coukell: Stochastic element, what do you mean?

Brad Bernstein: Well, I don't think that 100% of the time you get the precise effect you are looking for. Sometimes the modifications might not be correctly erased, sometimes the modification may not be put back in the appropriate fashion, and therefore you wouldn't get the proper genome structure that is associated with the embryonic stem cell.

Allan Coukell: So are you thinking that that's something fundamental, or do we just need to learn how to more effectively press the reset button?

Brad Bernstein: I think we don't know how to press the reset button, but I suspect that we are going to learn a lot more; we're going to get a lot better at doing this over time as we learn about precisely what's happening during the reprogramming process.

Allan Coukell: Well, we've been talking a little bit about adult stem cells, but if we have these populations of self-renewing cells in our bodies, why then do we get old? Thomas Rando directs an ageing research centre at Stanford University in California. He's written a Nature review on stem cells ageing and the quest for immortality. Gareth Mitchell asked him about ageing. Does it happen because the stem cells themselves get old, or because of changes around the cells?

Thomas Rando: That's really fundamentally the issue of the article. I think that certainly it's manifested everywhere. The question is to what extent does the changing of a tissue reflect the effect of the ageing process on the stem cell compartment or the rest of the tissue, and are there ways in which the tissue talked back and forth to the stem cell? I think some of the work that's reviewed in this article suggests that stem cells exist within tissues, even as one gets older, and those stem cells actually possess a dormant quality to participate in tissue repair. In a sense, they are getting signals from the environment in which they live that is keeping them somewhat quiescent. If one can harness that potential, that dormant potential, there's the possibility I think of enhancing tissue repair throughout the lifespan.

Gareth Mitchell: And in your review article, I mean really you raise two questions right at the beginning of the article, the first question being what the effect is of ageing on the stem cells themselves, and the second question is the extent to which the way tissues decline can be attributed to the ability of stem cells to keep going basically to structure and function. And it seems as if that first question, the effect of ageing on stem cells themselves, has already been partially answered.

Thomas Rando: I think that it's been partially answered in the sense that if one looks at stem cells they can be isolated from different tissues. When one takes stem cells from an older animal or an older person, they behave differently. And so there clearly is an effect of ageing on stem cells, but I think fundamentally there's another question that emerges from that answer, and the other question is are those changes reversible or irreversible. That's where it gets quite interesting in terms of recent research. In some sense one thinks of the ageing process in general as unidirectional and irreversible, and yet it seems as if the stem cell behaviour that changes with age may be at least partially, if not completely reversible, so those cells which are almost by definition young in some tissues, are able to restore their regenerate potential as if they were young again. And so in that case the ageing effect on the stem cells may be manifested in behaviour but may also reversible.

Gareth Mitchell: So it's almost as if the clock gets reset then. So you take a stem cell out of a relatively aged organism and introduce into a new one and somehow its biological clock – to put it like that – gets reset, gets turned back.

Thomas Rando: That's exactly right. So taking a tissue from an old animal, transplanting it into a young animal, and finding that in that case the transplanted tissue behaves or regenerates very much as if it's a young tissue. So it's really the environment in which those stem cells are acting in the animal that really seems to matter. The other question that is raised in this review is not just maintaining healthy tissues throughout life, but living longer, and I think that gets to a very different issue is, is there really any evidence that enhancing stem cell function will actually allow maximum lifespan to be increased? I think, at least to my reading, there's very little evidence that that's the case.

Allan Coukell: Amy Wagers, you've worked with Thomas Rando on this question of ageing. Talk a little bit more about what you learnt from the experiments you were doing.

Amy Wagers: In the experiments that Tom and I and Irena Conboy were taking on, we were interested in whether there was an environmental effect on stem cells in the skeletal muscle in particular. What we found was that it had been known for some time that skeletal muscle regenerates more poorly in old age, but we found that providing a younger environment to those stem cells could actually rejuvenate their capacity to respond to regenerative queues. So it appears that throughout ageing, at least in the skeletal muscle as a system, there's an accumulation of damage or a loss of stem cell responsiveness that's associated with an environmental factor that seems to inhibit the stem cell activity. This is not to say that this mechanism would be in action in all tissues, but it did seem to be a systemic effect that could potentially impact on a variety of different tissue stem cells.

Allan Coukell: And when you said there's an accumulation of damage, what kind of damage, do you know?

Amy Wagers: This is a hypothesis that the underlying cause would be exposure to environmental toxins or oxidative damage, these types of stresses that might accumulate in tissues and change the fundamental physiology of the tissue that would then be read out on the stem cell activity.

Allan Coukell: So actually causing DNA breaks, or affecting the systems that Brad Bernstein works on that sit on top of the DNA and control gene expression?

Amy Wagers: Exactly, so either of those mechanisms could be at play and I think understanding exactly the mechanisms by which stem cells are affected during ageing will help us to design strategies to intervene to reverse or prevent those changes.

Allan Coukell: So given what you've seen, how do you assess the potential to slow down or reverse age related changes? Do you think it's a possibility?

Amy Wagers: I hope it's a possibility. I mean it's certainly the area in which a lot of labs are pursuing these sorts of findings, and certainly in the ageing field, particular genetic regulators of ageing have been identified that promote longer healthy lifespan. And so some of those same factors may be at work in maintaining appropriate stem cell pools for tissue maintenance.

Allan Coukell: Bob Lanza, I would think in the biotech industry there has to be a great deal of interest in possible anti-ageing therapies. That would be a huge market.

Bob Lanza: Well, I think certainly what we have found is we can generate certain progenitors from these stem cells that do have the capacity to be home to worn out and damaged tissues throughout the body. So say for instance we've created these haematopoietic cells known as hemangioblasts and it turns out that they are able to home to say for instance if you have an injury in the eye, within 24 hours and repair that damaged vasculature. We also know that the same haematopoietic stem cells have the capacity to fix say worn out joints. We know from a paper that we published that we are able to actually inject them into the heart and they are able to restore the infarcted tissue as well as replace the vasculature. So I think that there is the capacity in the coming years to generate early progenitor populations that will have the ability because they are quite smart to pick up the environmental queues in the body and just basically repair damaged tissue.

Allan Coukell: We'll talk more about some of those applications in a moment, but first let's talk about cancer. It might be somehow closely related to ageing, and certainly there are many cancers that show up only when we are well advanced in life, and there seem to be some similarities between cancer cells and stem cells. As we've already said, stem cells have to have the ability to divide both to make more stem cells, this is called symmetric division, and also to sometimes divide in a way that one daughter cell begins to become specialised, and this is asymmetric division. Gareth Mitchell spoke with Sean Morrison of the Howard Hughes Medical Institute and University of Michigan. He asked him, is cancer a product of symmetric division gone awry?

Sean Morrison: I think we can't say that directly, but that's a hypothesis that's based on a number of different observations. For example, as laboratories have identified mechanisms that promote asymmetric cell division in model systems like fruit flies for example, the genes that they've identified that promote asymmetric cell division turn out consistently to be tumour suppressors in mammals, and the genes that promote symmetric division in model systems can act as oncogenes when over expressed, both in mammals and in fruit flies. But I guess what makes me believe that there could be a symmetric component in cancer is that there's a number of reasons to believe that the whole division process might be much more highly regulated during asymmetric cell divisions where you have to asymmetrically distribute cellular components between two different cells, perhaps even ensuring that certain chromosomes go to one cell and other chromosomes go to other cells. And there may be a lot of checkpoints that make sure that this process occurs in a highly regulated and appropriate way, whereas many of those kinds of checkpoints may not exist during symmetric divisions. And that may make symmetric divisions much more permissive for the process of generating aneuploidy when cells acquire abnormal numbers of chromosomes, which is a characteristic of cancer cells.

Gareth Mitchell: So you are saying basically that if there's this symmetric component to cancer, you talk about permissiveness. It's almost as if the biological system is primed in some way, genetically possibly, for that cancer to occur. Is that it?

Sean Morrison: I think the idea would be that a cell that has to divide asymmetrically may be much less likely to progress to cancer because there may be many checkpoints that are making sure that that cell is dividing properly, that the asymmetric division is occurring as it's supposed to, and that if something goes wrong that would otherwise create aneuploidy in the cells, then the division of those cells may be shut down by triggering these unknown checkpoints. Whereas the hypothesis is that these checkpoints are not engaged during symmetric division, and that creating a circumstance where cells can divide and divide, despite aneuploidy.

Gareth Mitchell: Right and so that might explain this very aggressively proliferative way in which cells divide in tumours?

Sean Morrison: Yes, and how they seem to at an abnormally rapid rate acquire all kinds of changes in their genetic material.

Gareth Mitchell: So I suppose onto a very obvious question then, having understood this, or beginning to understand this, or at least hypothesising it, how might this help us begin to understand cancer a bit better, and then I suppose most obviously find some way of curing or treating certain forms of cancer?

Sean Morrison: Right, so there are two specific predictions that the hypothesis would make. The first is that as we better understand the mechanisms that promote asymmetric cell division, we will identify new tumour suppressor genes that act in perhaps new ways, and that as we study the mechanisms that promote symmetric division, we'll identify new kinds of oncogenes that may act in new ways. We spoke earlier about the switch, the hypothetical switch that allows stem cells to go back and forth between asymmetric and symmetric modes of division. The prediction would be that if we could understand how that switch works and we had drugs that would flip the switch and push it toward an asymmetric mode of division that we could delay the process by which cancer cells become aneuploid and therefore delay the process of carcinogenesis.

Allan Coukell: That's Sean Morrison of the University of Michigan Centre for Stem Cell Biology. Amy Wagers, is it well established now, at least for some cancers, that there really is such a thing as a cancer stem cell?

Amy Wagers: Yes, I think to start out with we have to talk first a little bit about definitions in cancer stem cells. So, there's really sort of two activities you could be referring to when you talk about cancer stem cells, and one is the cell in which the cancer originates or the tumour initiating cell. That is potentially distinct from what would be a tumour propagating cell, which often people think of as the cancer stem cell, that the cell which you can isolate in order to transfer or continue production of the tumour. But as far as looking at tumour propagating cells, they have been defined in several types of cancer. Early on in leukaemia in haematopoietic cancers, but more recently evidence that one can use service markers to separate out a population that propagates solid tumours like breast cancer or glioblastoma.

Allan Coukell: And if you actually look at a tumour, what are the characteristics of tumours that look a lot like stem cells?

Amy Wagers: One of the characteristics of tumours is often heterogeneity, that there are cells of different cells of different stages of differentiation. This gave rise to the question is every cell within a tumour capable of producing more tumour cells, or is there only a select sub set of cells that is capable of doing that, and that would be the cancer stem cell hypothesis. So using service markers that distinguish different components of the tumour, one can test for the ability of particular sub sets of tumour cells to regenerate the tumour, and it's been found, at least in certain tumours, that only a sub set of cells is capable of doing that which is consistent with this being a self-renewing stem cell population that promotes sort of a dis-regulated organogenesis that would be a cancer.

Allan Coukell: So the implication of that is what, you can kill most of the cancer, but if you don't get those key cells, it don't matter?

Amy Wagers: That's exactly it. The implication is that you want to be very sure that you are targeting the tumour propagating population within the tumour.

Allan Coukell: Brad Bernstein, thinking about getting those cancer stem cells, what, from a molecular point of view comes to mind? How could you look at attacking those cells?

Brad Bernstein: Well, I think it would be critical to identify the cells first. There is actually some good evidence that epigenetic mechanisms such as what I talked about, the modifications to the DNA, the modifications to the scaffold of the DNA, are defective in many forms of cancer. This makes a lot of sense because these are the mechanisms that sort of restrict a cell's potential fate, and in a differentiated cell it's really well restricted to just being a neuron or a particular cell type. But earlier on when I talked about the stem cell and that the stem cell has a genome that is more permissive, these cells then may in certain cases be more susceptible to going along the steps towards malignancy, and indeed there is examples for epigenetic dis-regulation or dis-regulation of the genome structure in malignancy. Now, an open question is whether some of the signatures that have been identified in embryonic stem cells in terms of the modifications to the DNA and so on may also be signatures of the cancer stem cell, and although the methodology is not quite there yet, we can't develop enough cells, and do the analysis, but we hope to soon be able to use these techniques to define signatures of first the tissue stem cells and then ultimately of cancer stem cells to learn more about their biology.

Allan Coukell: Bob Lanza it sounds like here what we are talking about is not using the cells themselves therapeutically but using them as a biological tool to develop perhaps drugs?

Bob Lanza: Absolutely. I think that one of the main strengths I think of this research is that we will be able to create different cell types that could be used for screening instead of needing animals in the laboratory.

Allan Coukell: Do you think that the focus, certainly in the popular mind, I think the focus has been on re-infusing one's own cells to cure disease. When you look at it in terms of looking at the near future, what do you see coming first?

Bob Lanza: Well, I think certainly we know that one of the default mechanisms are the neuronal progenitors. So I think that you are likely to see some of the first applications to be in the central nervous system. I think that we already know how to create for instance various types of neuronal tissues. Also, we have the advantage in the central nervous system of it being immune privileged so that we can transplant those cells with a lesser risk of rejection. I think soon after the neuronal applications, I think you'll be seeing, for instance we've created populations of retinal pigment epithelium that I think could be used for macular degeneration. I think there's a considerable amount of effort underway right now for the haematopoietic lineages. Like I said earlier, we have created some of these hemangioblasts that we think now can be used for various vascular repair disorders. So I think that you're going to be seeing several of these applications entering into the clinic in the next two or three years.

Allan Coukell: On the topic of neural stem cells, let's hear from Professor Fred Gage of the Salk Institute for Biological Sciences. He talked with Gareth Mitchell about the potential for stem cells to regenerate brain tissue. It might not be straightforward in this disease he says, because neural cells are so complex.

Fred Gage: Well, one of the principle difficulties is that there is a, it's called a blood brain barrier, so that the brain is protected from all other tissues by a physical barrier made up of cells that make it impermeable to many, many molecules, and it's much more difficult to deliver therapeutic drugs and chemicals to the brain than other organs. So that's one level of complexity. Another is that the cells that exist within the adult nervous system, in the developing nervous system, are quite diverse. So there are estimated something in the order of 10,000 different types of neurons that exist in the brain, and this complexity of neural types is expanded in its complexity by virtue of how these cells are interconnected with each other. When you think about what the demands are in the brain, which is the central organ for organising our behaviours, this tissue is responsible for our speech, for our movements, for our thoughts, and really coordinating all the different activities that mammals undergo during their normal processes and during development. Interestingly, it's estimated that each single neuron in the brain has as many as between 5,000 and 200,000 connections with other cells in its environment. So it's any single neuron, which then when you do the numbers on that, gives you an estimate of something in the order of 150 trillion individual connections between cells in the human brain.

Gareth Mitchell: So you people are dealing with absolutely mind boggling complexity, and let's see then how stem cells come into all this, because you have this incredible diversity of neurons. How do stem cells figure in all that?

Fred Gage: Well, one way to think about it is that a single cell, which is a fertilised egg, it's a sperm and the egg merge to form a fertilised egg, that single cell can give rise to all the different cells of the body. Once one of those cells that that very early primitive cell can give rise to, are what are called neural stem cells or neural blasts, and these are cells that emerge during the initial phases of development that are the early forms of the brain cells. Those primitive neural stem cells then in and of themselves give rise to all the different complexity of cells that exit in the brain. So, what you have to imagine is a single cell that is undifferentiated that contains within it the capacity to differentiate into all the different cell types, and can make connections between those cell types in such a way that it's a functioning brain.

Gareth Mitchell: The stem cells themselves have a role, not just in how the brain cells are made, how the neurons are made, but also in how they interconnect?

Fred Gage: Absolutely, and this is actually a very important part that we are now getting a better understanding of, and that is how does, and when does, the diversity that is expressed as the mature brain, when does that diversity begin, and we're beginning to understand that some of those choices, some of those events that result in a very mature cell of one type or another, those changes occur very early in the primitive stem cell. So, early choices have a very dramatic effect on the consequences of what the cell will be later on.

Gareth Mitchell: And so could this be the key then, because it's interesting, isn't it, that you have a very uniform family of neural stem cells, and yet they, if you like, are the precursors to this incredible diversity of neurons? So, if this all-important activity is taking place so early on, I mean is that a challenge for you, or actually more of an opportunity?

Fred Gage: I personally feel this is a great opportunity and part of the opportunity comes with the ability now to be able to examine, harvest these cells in an experimental setting. Because we can purify now or enrich for these stem cells, and examine how these immature cells begin to make their choices down these different cell lineages, and I should say by virtue of being able to do this in an experimental setting, we can learn much more about the process. Importantly, we can either isolate the cells with genes that are indicative of the disease, or from patients that have the disease, or we can make the cells express genes that are diseased, and then we can look at how that mutant gene or diseased gene can influence the early events in how these genes ultimately affect the process of the disease.

Gareth Mitchell: And does that mean that we can intervene, you know, sort of quite early on, at the very least we can understand more about how the disease forms?

Fred Gage: Precisely that. With the ability to grow the cells in a closed environment, and experimental environment, and to examine the process by which they make their choices to become different types of cell types, and by virtue of being able to see how they do that with the mutated gene, we can use that cellular process, that experimental setting to screen for drugs that may protect the cell against that aberrant gene, and this is really where the technology and the drug development can utilise the basic biology to understand and develop new therapeutics which will target very specific molecular processes which are defined as being crucial and are interrupted by diseased genes.

Allan Coukell: Bob Lanza, how do you see it? Nervous system particularly complex, more challenging than other tissues?

Bob Lanza: Well, I think absolutely. I think that for many of these diseases it's going to be quite a challenge. For instance Alzheimer's Disease I don't think we really have a clue how we would use these cells, but for a disorder such as Parkinson's Disease, I think we know how to readily generate this is for dopanergic neurons. So I think it's really going to be very much disease dependent.

Allan Coukell: Another application that exists now in, at least mice, is spinal cord regeneration.

Bob Lanza: Absolutely. We do know now that we are able to inject various neuronal progenitors to the sight of injury and we can have a quite significant effect. We have known that we are able to restore function to partially paralysed animals that have limbs where again, they were unable to walk and then eventually were able to at least restore partial function.

Allan Coukell: And of course some of these things, Amy Wagers, are already in the clinic and blood stem cells you've mentioned have been in use for a long time. What's coming? Is there a tidal wave of new stem cell based therapies around the corner, or is it further away? What do you think?

Amy Wagers: I think it's going to depend in large part on the tissue that you are trying to regenerate and the ability to transplant those cells and the effectiveness with which they can be derived. I think we shouldn't discount the possibility of using stem cells for drug screening mechanisms that would recruit then endogenous stem cell activity or progenitor activity, sort of making use of your body's own regenerative capacity that might be impaired for instance in the case of an ageing tissue where an environmental signal is perhaps blocking the activation of that stem cell population. So I think the two uses for stem cells clinically right now, in bone marrow transplant for the reconstitution of the haematopoietic system and in skin grafting for the regeneration of the epidermis, both rely on cell transplantation. But this is not the only mechanism by which stem cells could be clinically useful.

Allan Coukell: When we think about the use of stem cells for tissue transplantation, how big an issue is rejection likely to be, Bob Lanza?

Bob Lanza: I think it's likely to be a very significant problem. I think that in the first applications we are likely to have the advantage of implanting cells - as I mentioned previously – into the central nervous system, or into the eye, which is also immune privileged, but I think as soon as you get into peripheral applications you're very much going to run into the immune system head on. I think that there is some data out there that perhaps embryonic stem cell derived derivatives may be less immunogenic, but there's certainly going to be some immunogenicity. So I think we're going to need some strategies to overcome immune rejection, be it through SCNT or through creating banks of reduced HLA complexity.

Allan Coukell: SCNT is Somatic Cell Nuclear Transfer, also called cloning. When you look ahead, do you see an individualised stem cell line for every man, woman and child, or is it something in between, maybe a bank of tissue matched cells?

Bob Lanza: Well, I don't think it will be very practical to have a custom therapy for every patient. Certainly if you look at the numbers, 200 million people who have cardiovascular disease, another 200 million who have diabetes, that would require literally billions of eggs. That's just not going to happen. So, I think the next best strategy would be to create banks of embryonic stem cells that have a reduced HLA complexity, and I think there are some strategies that some groups are proceeding with. One for instance would be parthenogenesis where you have a homozygous HLA so that you could create a bank of say a few dozen or a few hundred lines that would match most of the population.

Allan Coukell: Of course creating these new lines is controversial on an ethical level. Many, many people, perhaps especially in the United States object to the destruction of embryos to create stem cell lines. Here are some voices recorded recently in Prague at the meeting of the European Society of Human Reproduction and Embryology.

Jo Marchant: I'm Jo Marchant, news editor of Nature and I'm at the annual meeting of the European Society of Human Reproduction and Embryology here in Prague, where some of the world's leading stem cell researchers have gathered to discuss the latest in stem cell research. I'm here with Professor Luca Gianaroli. There are considerable ethical concerns around the fact that to create a human embryonic stem cell line at the moment you need to destroy a viable human embryo. What ways are researchers trying to get around that at the moment?

Luca Gianaroli: Well, first of all there is a big mistake in this concept because we are not destroying most of the time human viable embryos, because if you think that most of the embryos regenerate in our laboratories for infertility treatment and they represent the same story in vivo conception, they are not viable at all. They have chromosomal abnormalities so these embryos will never implant, and the reason why so many patients are infertile, so many people are infertile, these embryos are chromosomally abnormal so they would never implant. Secondly, the embryos are discharged, that are not escaped from the patient, they have only one possibility left – to be thrown in a bin. So, why should we allow this waste and not to use them for research?

Jo Marchant: So why do you think that message isn't getting across to politicians and the public?

Luc Gianaroli: This is the big question, and you are right. It doesn't go to the public because some of the population, some of the politicians, are driven from moral convincement, and doing this they cannot see the reality of the thing. Biology cannot be mistaken. I mean it's very simple. What we see in the lab is what is happening inside the womb of any woman. So, most of the time an embryo is not even a project of life.

Stephen Minger: I am Dr Stephen Minger, I direct the stem cell biology lab at Kings College London.

Jo Marchant: There has been a lot of media attention recently around the case of Woo-Suk Hwang, the South Korean researcher who claimed to have derived stem cell lines from cloned human embryos. We subsequently found out that most of that work was faked. Could you just tell me what effects you think that's had on the field?

Stephen Minger: I don't think it's had a tremendous adverse effect. Somatic cell nuclear transfer or therapeutic cloning really represents sort of a fringe element of a much broader field of stem cell biology, and while it's obviously quite distressing that this has taken place, I think its long term impact on the field is negligible really. I mean this could be any area of science, stem cell science or astrophysics. You don't like to see people who feel so much pressure that they resort to fabricating data. The long term implication – if there is one – is it really does put us back to square one with regards to is somatic cell nuclear transfer possible using human cells, and if so, what is the true efficiency, and I think that's something we still don't really know.

Allan Coukell: Bob Lanza, there you had one issue saying this issue of destroying embryos really shouldn't be an issue. What do you think?

Bob Lanza: Well, I think we have some new strategies that some groups have been working on that may help solve part of that problem. There is a procedure known as PGD, pre-implantation genetic diagnosis, where they remove one cell for genetic testing, and we've been working on some work in mice and in humans, and it appears as though we can use that cell, not only to do the genetic testing but to actually derive a new stem cell line, and this of course does not require the destruction of the embryo. So I think we now have a technology before us that will allow us to create new lines without destroying the embryo.

Allan Coukell: So you are taking that ethical concern seriously. You are not saying get past it folks, you're saying that we can do this work without destroying embryos?

Bob Lanza: Well, I think that in our society it's going to be a problem, and even the President has stated that he is opposed to destroying life to save life, and I know that a lot of large pharmaceutical companies, which really will be important to move these therapies to the public on a wide scale will be very reluctant to move ahead if embryos are destroyed, and I know they have a very keen interest in this. So I think it's very important, and I think it's also important for many public institutions and universities which rely on federal funding and would not want to jeopardise that funding by using cells that would violate the agreement.

Allan Coukell: Brad Bernstein, the case of Woo-Suk Hwang, his creation of new stem cell lines now shown to have been fraudulent, what effect did that have in your lab and the people that you work with?

Brad Bernstein: Well, I'd look at it in very broad terms in that it's kind of a frustration that scientists in the United States are not able to use federal funding to pursue this line of work. I fully recognise that there are critical ethical issues that need to be handled and I think they should be handled and I would applaud people for pushing them. At the same time, when you put a blanket restriction and absolutely rule out the use of federal funds for doing any of this research, you make it very difficult for people to obtain the scientific knowledge in this country, you put this country at a disadvantage, the research in this country at a disadvantage relative to the rest of the world, and also I think you make the science susceptible to fraudulent work, like what was done in Korea, because the scientists certainly in the United States have less of a capacity to review this work when they are not actively engaged in similar studies.

Allan Coukell: How important is it to you to have new lines?

Brad Bernstein: Our own research and model systems in mice at this point is not hampered by this work, but the potential, the work that we are doing now, we like to think of it as a tremendous potential for regenerative medicine down the road. If access to new lines is going to be prohibited, then the work is not going to move to its fruition.

Amy Wagers: I was going to say I think it's very important to have new lines and it's clear just from studying the lines that are in existence that there are differences in their epigenetics that could potentially give you different results, and you need to be able to compare those sorts of issues. In addition, in the creation of disease specific embryonic stem cells for studying disease development, that can't be done with the existing lines.

Allan Coukell: Public interest in stem cells is very high, not to use the word hype, but there are big expectations. Can the field live up? I'll ask each of you – Amy Wagers.

Amy Wagers: I think there is a huge amount of promise in the stem cell field and I think we've made an enormous number of advances in even the last few years. I think the question is the timeline, obviously which everyone always asks is how many years until we have a therapy for muscular dystrophy, these sorts of questions, and these are very difficult to predict, and I think it's informative to think about the progress of bone marrow transplant, which is arguably the most successful application of stem cell therapy, which took decades, and is still being optimised. And so we have to look at this in a realistic standpoint of every step is an important one, and we are moving closer and closer to the goal.

Allan Coukell: Brad Bernstein?

Brad Bernstein: I have to agree with Amy on that point. There's no question that our scientific knowledge about the stem cells is exponentially expanding, but the complexities of applying these to human disease is just immense and it's dangerous to put timelines in place when you promise this fruition.

Allan Coukell: And Bob Lanza?

Bob Lanza: Well, I think stem cells definitely have enormous potential. I think that in addition to treating a number of diseases that we're going to have the ability to reconstitute the stem cells into more complex tissues and structures such as blood vessels, and even entire organs. So I think we're going to see these therapies move into the clinic in the next few years.

Allan Coukell: My guests in the studio have been Robert Lanza from the biotech company Advanced Cell Technology, thank you for being here.

Bob Lanza: Thank you.

Allan Coukell: Amy Wagers from Harvard Medical School and the Joslin Diabetes Institute.

Amy Wagers: Thank you.

Allan Coukell: And Brad Bernstein from Massachusetts General Hospital and Harvard Medical School, thank you.

Brad Bernstein: Thanks.

Allan Coukell: Thanks to all my guests. I am Allan Coukell in the WBUR studio in Boston.

Gareth Mitchell: Fascinating stuff Allan, thanks very much. Well, that's it for this Nature podcast special, but if you want to hear the full versions of those short interviews, just go to http://www.nature.com/podcast/stemcells, and if you have any comment about this podcast, we'd love to hear your thoughts. Just email us, the address is mailto:podcast@nature.com. Well, I'm Gareth Mitchell, you've also been hearing from co-presenter Allan Coukell, and this podcast has been produced by Yada Yada Productions in association with Nature Publishing Group. Well, from all of us on the team, thanks very much for listening, and goodbye.