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Moving Stem Cell Research Forward: The Need for Standardization
[0:00:00] Sean Sanders: Hello and welcome to this Science/AAAS webinar. My name is Sean
Sanders and I’m the commercial editor and webinar editor at Science. Slide 1 In today’s webinar, we’ll be covering the very popular and sometimes
controversial topic of stem cells. Stem cell research has the potential to significantly impact a broad range of life science endeavors but faces challenges in the handling of cells and the lack of automation and standardization. The ability to control differentiational stem cells into specialized cell types with high yield and precision is a key success factor that will to a great extent determine the ultimate utility of such research. In this hour, our panelists will discuss the need for and progress towards a new level of standardization and automation in the management and handling of stem cell cultures and their differentiated progeny.
With me today I have three top scientists who all have extensive
experience in this field. Firstly, to my left is Dr. Ron McKay from the National Institutes of Health in Bethesda, Maryland; next, Dr. Amy Wagers from Harvard University in Boston, Massachusetts; and finally, Dr. Mark Noble with us from the University of Rochester Medical Center in Rochester, New York. Very warm welcome to all of you and thanks for taking the time to be here.
A reminder to everyone watching that you can see an enlarged version of
the slides by clicking on the enlarge slides button located underneath the slide window of your web console. You can also download a PDF copy of all of the slides by using the download slides button. If you're joining us live, you can submit a question to the panel at any time by typing it into the ask‐a‐question box on the bottom left of your viewing console below the video screen and clicking the submit button. As always, please do keep your questions short and to the point and we’ll do our best to get to as many of them as possible in the Q&A session following the presentations.
Finally, thank you to Cyntellect for their sponsorship of today’s webinar. Slide 2
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It now gives me great pleasure to introduce our first speaker for this
webinar, Dr. Ron McKay. Dr. McKay received a B.Sc. degree and Ph.D. from the University of Edinburgh in the United Kingdom, where he studied under the tutelage of Edwin Southern, examining DNA organization and chromosome structure. He received postdoctoral training at the University of Oxford and in 1978 he became a senior staff investigator at Cold Spring Harbor Laboratory. Dr. McKay was part of the Massachusetts Institute of Technology faculty for nine years before joining the National Institutes of Neurological Disorders and Stroke at the National Institutes of Health in Bethesda, Maryland as Chief of the Laboratory of Molecular Biology. His laboratory currently studies contact‐dependent and soluble signals that control the proliferation and differentiation of stem cells.
Welcome, Dr. McKay. Dr. Ronald McKay: Thank you. Slide 3 Well, what I’d like to do is I’d like to start by introducing two examples
where pluripotent cells have been used to generate the functional somatic cells, a cell of the body, and to demonstrate that these cells work in animal models of disease. And in the first one, I’m showing you a simple example taken from mouse embryonic stem cells where we make dopamine neurons in the laboratory and put these dopamine neurons into an animal where the dopamine neurons have been lesioned to model Parkinson’s disease.
And on the left hand side of the image, you can see what is now
considered to be a classic experiment setting up this model by Steve Dunnett when he was in Cambridge in the UK, and what Steve did here was he took lesioned animals and you can see in the green lines in that left hand graph that if you graft cells that the behavior of the animals corrected; and in one group of animals you can see that that four months after the graft, when the grafted neurons were again lesioned with a toxin that kills dopamine neurons, that the behavior of the animal again becomes pathological.
So it’s quite clear in this example that this grafting experiment and this
behavioral measurement is measuring the function of neurons in the body, but the neurons have been derived entirely in the laboratory and has had remarkable technical achievement. But to succeed in this kind of
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goal, we have to show that this kind of approach can also be used with human cells and we have to show that we can take human pluripotent cells and grow them in a completely controlled way from the beginning, and that’s the subject that I want to discuss with you today.
Slide 4 So in the next image here, what you can see is an example of taking
human embryonic stem cells and we will make the assumption now that ES cells and IPS cells, reprogrammed cells, are very similar. So in this case, it’s human embryonic stem cells differentiated in the laboratory to make functional pancreatic cells and demonstrating that function in a very simple way.
So here, the animals are again lesioned. The lesion makes them diabetic.
If you look at the red line, you can see that the blood glucose levels go up and stay up after the lesion. But if you transplant these human cells into the animal, then you can see blood glucose levels recover. And again, in a very similar way, when you take out the graft in this case, the blood glucose levels again become pathological.
[0:05:09] So here’s an example showing that human ES cells can be controlled to
generate functional progeny, but what’s the problem? And the problem is that many of these cell types, these early cell types are hard to grow, and I’m just going to spend a couple of minutes showing you examples of what the problem is and showing you some examples of how the solutions might be generated.
So if you look at the bottom panels on the slide, you can just immediately
see that these human ES cells grow as colonies and the colonies have different morphologies. And one initially learns in an empirical kind of way what’s a good colony and a bad colony.
Now, if you look at different lines and compare different lines, you can
show as is shown here that they have quite different properties. Slide 5 Here we’re using the expression of a gene which controls p53, one of the
key stress response pathways in cells and you can see that these two different lines have very different levels of expression of MDM2. And there’s a consequence of these pathways and these cells so cells which
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have low levels of MDM2 have much greater tendency in our experience to acquire genetic changes which is showing in the middle of this image, which is a whole genome scan, and you can see that the BG01 cells quite commonly acquire chromosome changes characteristic of the changes seen in tumors of these early human cells, teratocarcinomas.
Slide 6 Now, it’s also true that within a single line, you can have variations in
colonies and that’s shown here. And again, we’re using the p53 pathway and the transcription factor NANOG which is involved in the pluripotency control systems to demonstrate that within one dish and looking at two colonies that you have very different conditions.
So there’s clearly going to be many different strategies that we’ve
developed to overcome this kind of heterogeneity and control it, but one of the things that we clearly need to know is we clearly need to know more about the cell types in the early embryo and how many different types of pluripotent cell there are.
Slide 7 And one interesting approach to this is being to use mouse early
development to identify a pluripotent cell which appears to be the appropriate model for the human ES cell, which is actually rather distinct from the classic mouse ES cell. And one of the ways it’s distinct, it has completely different pathways regulating the renewal and the differentiation of the cell from the mouse ES cell, and those pathways are much more similar to the human cell.
So one other thing that you can sort of gain from an image like this is that
we’re increasingly getting control of the different cell types in the early embryo. Even though this is a simplified version, we’re clearly making progress in that direction.
Slide 8 And this kind of control is leading us to develop very new and more
precise ways of monitoring the differentiated states. So on the left you can see what’s the sort of classic teratoma assay where you can empirically see these cells are generating many different cell types. But on the right, we’re using global gene expression to monitor very precisely the transitions between the different cells, and we’re using our
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understanding of the pathways controlling renewal and differentiation to move the cells very precisely through these states.
Slide 9 Now, if you take this kind of an idea and you apply it to human embryonic
stem cells, what you can see in this image is first of all, the human embryonic stem cells shown in the bottom in this clustered diagram can be grown and analyzed by whole genome transcription studies and then just in the undifferentiated state, you can see that one cell line ‐‐ let’s focus on TE03 as it’s red ‐‐ is more similar to itself by this measurement than it is to other lines. So that’s a step in the right direction.
And in addition in this image, what you see is that TE03 is homozygous
for a G/G mutation in MDM2, the p53 regulator that I introduced earlier; and this homozygosity is known from epidemiological studies that are referenced here in this paper from a group I think now at Princeton that this mutation is known to regulate cancer risk. So you can see two things here, first of all that we can control the growth of the cells, and secondly that these cells in this state are potentially important tools to understand these cancer risk factors.
Slide 10 And I want to end with this image here. So what we’re doing here is
we’re taking the same kind of approach, whole genome transcriptome, but now, you’re looking at the way ‐‐ our analysis of cells grown here, human ES cells grown here in Bethesda at NIH over many, many passages and that’s indicated in the table on the left, and these cells have been analyzed in many different ways, which is shown on the diagram on the left.
[0:10:15] But let’s just focus on the data, so the data, our principle components 1,
2, and 3 of the whole transcriptome of these human ES cells. And you can see in these green dots that when we grow the cells here in a standard way that this analysis shows that all the cells are behaving very similar to one another and we’ve included in this image data generated from other laboratories on the same cells where presumably the conditions differ, and you can see here that it’s very important to grow the cells under one controlled set of conditions to make further progress.
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The work is done by my colleagues at NIH as was shown here, and the main point I want to make is that if we’re going to use these cells in functional assays of the type that I introduced you to at the beginning, then it’s very important to be able to control the starting point and that’s what’s indicated here. So I appreciate this opportunity to show you this.
Sean Sanders: Great. Thank you so much, Dr. McKay. Slide 11 Our next speaker today is Dr. Amy Wagers. Dr. Wagers completed her
undergraduate degree in biological sciences at Northwestern University in Evanston, Illinois followed by a Ph.D. at the Northwestern Medical School in the area of immunology and microbial pathogenesis. She is currently an associate professor in the Department of Stem Cell and Regenerative Biology at Harvard University in Boston, Massachusetts and investigating the section on developmental and stem cell biology at the Joslin Diabetes Center, and a principal faculty member of the Harvard Stem Cell Institute in Cambridge. Dr. Wagers directs a research laboratory that focuses on defining the factors and mechanisms regulating the migration, expansion, and regenerative potential of blood‐forming and muscle‐forming stem cells.
Welcome, Dr. Wagers. Dr. Amy Wagers: Thank you very much. So we’ve just heard a discussion of the challenges
faced by attempts to direct the differentiation of pluripotent stem cells and I’m going to spend my time talking about a different subset of stem cells and these are stem cells that are specific for the differentiation of particular tissue types.
Slide 12 These cells may in some ways address some of these problems because
they have within them intrinsic programs that direct them along the development of particular types of cells. These tissue‐specific stem cells also address sort of classical problem in development which is that the adoption of a multicellular body plan requires that cells become specialized.
Slide 13 This specialization of cell types in many cases causes cells to lose the
capacity to reproduce themselves.
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Slide 14 And what this means is that when these cells are lost due to injury or
disease, if the body is going to replace them, they have to be replaced from unspecialized precursor cells that retain this capacity to reproduce themselves.
Slide 15 And in many tissues, though probably not all, stem cells fulfill this
function of serving as replacements for cells that are lost or exhausted during the normal process of tissue function.
Slide 16‐17 And this is because of course because they retain the two very important
properties of self‐renewal and differentiation. Slide 18 And these properties then makes themselves interesting and promising
targets for experimental approaches and clinical approaches in regenerative medicine, not only through the classical sense of cell transplantation to replace cells that are lost, but also by providing a model for understanding the normal process of development and the pathological processes of disease. And then finally, an interest that has become particularly compelling in my laboratory, trying to understand how these cells normally maintain regenerative processes in the body and being able to manipulate those cells that are actually still present endogenously in tissues in order to tailor regenerative response or boost the regenerative response when one is lacking.
Slide 19‐23 So all stem cell types, both embryonic, fetal, and adult are likely to be
important in these types of regenerative medicine approaches. Slide 24 What’s important again to consider about tissue‐specific stem cells in
particular is that as depicted here in this chart, which gives a really still incomplete listing of known and studied tissue‐specific stem cell types, is
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that they are in fact restricted in the types of cells they can produce upon differentiation.
Slide 25 In specific, unlike embryonic or induced pluripotent stem cells which are
pluripotent and can give rise to any cell in the body, these tissue‐specific stem cells tend to differentiate along certain lines and specializing to tissue types that are often the same as the tissue from which they originate. So a blood stem cell gives rise to all of the blood cells in the body. A skeletal muscle stem cell gives rise to skeletal muscle fibers.
[0:15:00] Slide 26 And in the sense of experimental study or clinical application, it’s likely
that the best source for a stem cell for a particular treatment or a particular investigation is going to vary on the specific questions that are being asked.
Slide 27 But it’s critical in all of these cases to be able to specifically identify and
isolate the stem cell populations in tissues and in culture in order to definitively study their stem cell properties.
Slide 28‐29 This can be accomplished and has been accomplished in fact in a number
of different ways, both retrospective and prospective. Slide 30 And the really critical aspect of this is that direct functional test to verify
stem cell properties are absolutely essential. Slide 31 Now, the development of these functional tests is actually in some ways
problematic and certainly the standardization of those is. Slide 32
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And that is because by being able to isolate stem cells, what we’ve learned is that stem cell function is regulated in a very highly complex way and stem cells are constantly receiving and interpreting signals from their environment depicter here as a niche or micro environment, which regulates their ability to proliferate, to differentiate, to survive, et cetera. And these signals from the environment can change in very significant ways in response to physiologic and pathologic signals.
Slide 33 One of the areas that we’ve particularly been interested in studying
recently is how these signals from the environment that regulate stem cells change with the normal physiologic process of aging, and what we have found in both the blood system and the skeletal muscle system, which are the systems we’re particularly interested in, is that the signals that stem cells are receiving from their environment with age actually act to in many cases deregulate their function such that in aging tissues, the regenerative potential of those stem cells becomes depressed. What’s exciting, however, is that we know that we can, by identifying those signals, begin manipulate them and in some cases restore their regenerative potential of these tissue stem cells in aged tissues as well as in youthful tissues.
Slide 34‐38 And so by understanding not only how stem cells are regulated
intrinsically but also how they receive and interpret signals from their environment, we’re hopeful that we can develop strategies that actually target the endogenous populations, control stem cell number, alter their activity, and this knowledge could lead to better mechanisms for expanding stem cell populations outside of the body, which is currently an important challenge faced by tissue‐specific stem cell researchers, ways to manipulate them within the body to boost regenerative potential, and also to develop better systems for transplantation to introduce these cells into complex tissues in the appropriate manner.
Slide 39 And these applications of course are important for combating
degenerative diseases, both genetic and age‐related, as well as in malignancy.
Slide 40‐43
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So I’ll leave you just with a summary here of the key points that I've tried to address, and thank you for attending the webinar. Thank you.
Sean Sanders: Great. Thank you so much, Dr. Wagers. Slide 44 Our final speaker today is Dr. Mark Noble. Dr. Noble obtained his Ph.D.
from Stanford University before carrying out his postdoctoral training at the University College London in the United Kingdom. He is the member of the team of researchers in London who discovered the first precursor cell isolated from the central nervous system. Dr. Noble currently holds professorships in genetics, neurology, and neurobiology and anatomy at the University of Rochester School of Medicine and is Director of the University of Rochester Stem Cell and Regenerative Medicine Institute and Co‐Director of the New York State Center of Research Excellence for Spinal Injury Research. Dr. Noble and his associates currently work in a broad range of areas related to sickling within the central nervous system in normal and diseased states including cancer and damage and repair of central nervous system injury.
Dr. Noble. Dr. Mark Noble: Thank you so much. So I’m going to focus on the next level of development and the next level
of specificity, which are the lineage‐restricted progenitor cells that lie between stem cells and the differentiated cells of the body.
Slide 45 I’m going to talk about work conducted in collaboration with the
laboratories of Margot Mayer‐Pröschel and Chris Pröschel, and for spinal cord injury, the laboratories of Stephen and Jeannette Davies in Denver.
Slide 46 In the central nervous system, over the course of the past 25 years, we
have identified a number of lineage‐restricted progenitor cells. At the top of the screen you see the neuroepithelial stem cells that give rise to all the cell types of the nervous system, but they don’t do so directly. Margot Mayer‐Pröschel and Mahendra Rao discovered that these cells first give rise to restricted precursors that either give rise to neurons on the left called neuron‐restricted precursor cells or nerve cells, or cells
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that give rise only to the support cells of the nervous system. Let me call these glial‐restricted precursor cells.
[0:19:58] Working together, we next showed that the glial‐restricted precursor cell
gives rise to the cell that Martin Raff, Bob Miller, and I discovered back in 1983, the oligodendrocyte type‐2 astrocyte progenitor cell. The GRP cell gives rise to two different kinds of astrocytes and oligodendrocytes. The O‐2A progenitor cell only gives rise to oligodendrocytes and one type of astrocyte.
Critically, when we talk about different cell types, the definition that we
use is that we have isolated these cells from the animal, we have studied them at the clonal level, we have demonstrated in hundreds or thousands of clones that every clone undergoes identical patterns of differentiation, and that when the cells are transplanted back into the animal, the differentiation we see is the same that we saw on a tissue culture dish. And this level of standardization is critical to obtain proof of homogeneity in one’s cultures.
Slide 47 Because we have these cell types to work with and we have a significant
control over this system, we’ve been able to work on many different problems. So with one area of our work, we’ve been able to discover that many developmental maladies are actually diseases of precursor cells. And this was first shown in our work on hypothyroidism, then Margot Mayer‐Pröschel’s work on iron deficiency, Chris Pröschel’s work on vanishing white matter diseases. All these different kinds of diseases appeared to be disruptions of precursor cell function. So we cannot only use these cells for repair, but we can use them to understand how disease processes work.
Slide 48 In the context of repair, I’m going to focus on our work on spinal cord
injury. Here we have to pay attention to these details. The GRP cell gives rise to two antigenically distinct population of astrocytes. Exposed to bone morphogenetic protein or BMP, it gives rise to a cell within the antigenic phenotype that we many years ago designated as a type‐1 astrocyte. Exposed to ciliary neurotrophic factor or interleukin‐6 or other members of the GP‐130 agonist families, it gives to rise to cells with the antigenic phenotype of type‐2 astrocytes. O‐2A progenitor cells in
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contrast exposed to BMP only give rise to type‐2 astrocytes. This distinction between these populations is critical in terms of repairing the damaged spinal cord as we have seen.
Slide 49 What you see on the left are images of spinal cords that have been
transected and then transplanted in the lower panel with the BMP generated type‐1 like astrocytes or in the upper panel, the CNTF generated type‐2 like astrocytes. The green lines that you see are axons and in the center you see the axons growing into the lesion site and growing beyond the lesion site. In the CNTF‐generated astrocytes, there is no entry into the lesion site.
The quantification for this is shown on the upper right where the upper
blue bar shows that when we transplant the type‐1 like astrocytes, 60% to 70% of the intradorsal column axons enter the lesion and two‐thirds of these exit the lesion and grow back into normal tissue within eight days. In contrast, there’s very little regeneration with transplantation of either the type‐2 like astrocytes or the precursor cells themselves. The bottom panel shows you behavioral recovery and an analysis of the lesion on full splice spot.
Animals are trained to carry out a particular task. When they are injured
in the black line, they start making mistakes. If they receive a transplant of the type‐2 like astrocytes or the GRP cells, they don’t get better. Transplanted with the BMP‐generated astrocytes, they get so much better that after 4 weeks you cannot statistically distinguish them from the animals that had never been injured.
Slide 50 And it’s not simply a matter that one population works and other
populations don’t. If you transplant the wrong populations, you actually have adverse outcomes. What we’ve discovered in these studies is that the transplantation of either the precursor cells or the CNTF‐generated astrocytes causes neuropathic pain syndromes. If you have someone with a spinal cord injury who you want to try and make better, neuropathic pain syndromes are one of the worst quality of life issues in spinal cord injuries. So the idea that you could transplant cells that make individuals worse is something that one has to be very concerned about, and Stephen and Jeanette Davies, looking at this at the tissue level, discovered that the BMP‐generated astrocytes, which are so good at promoting regeneration, do not cause any sprouting of the CGRP fibers
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that are involved in the pain response, whereas the precursor GRP cells and the CNTF‐generated astrocytes do cause sprouting of these very fibers.
Slide 51 The O‐2A progenitor cell in contrast is probably not going to be any good
for this repair because it only generates type‐2 like astrocytes, and the problem here is that the markers we have are not what we need to reliably generate GRP cells from O‐2A progenitor cells as derived from embryonic stem cells for example. We need better markers. The only way we can analyze these different populations assuredly is to analyze differentiation at the clonal level.
[0:25:15] Slide 52 These distinctions are also critical in our studies of cancer. For example, if
you take the GRP cells and the O‐2A progenitor cells and express exactly the same oncogenes in them, you get very different kinds of tumors. The GRP cells give rise to benign diffuse infiltrative astrocytoma, whereas the O‐2A progenitor cells with the same oncogenes give rise to malignant oligodendrogliomas, again a critical difference on the distinctions.
Slide 53 These nuances also extend not just to the differences between different
progenitor cells but even within progenitor cells. And here, to finish, I will focus on our attempts to understand the integration of cell regulation and integrating metabolic status with protein activation status and then genetic components of cell control.
Slide 54 Over the course of our work, we’ve discovered that small changes in
oxidative status are vital to controlling cell function, and these small changes of 10% to 15% can turn on or off entire signaling systems. When we studied this developmentally, we found that right out of the animal different O‐2A progenitor populations from different regions of the nervous system have different redox states. So what this graph shows is the degree of oxidation of cells that are freshly isolated from the animal. The cells on the extreme left, the optic nerve cells, are much more oxidized than the ones on the right, the cortical progenitor cells. Even
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now at the clonal level, they’re identical. Our work indicates that these differences in oxidative state may be regulating the timing of differentiation and vulnerability to other stressors. It turns out that these nuances are critical if you want to use progenitor cells for things like drug discovery or toxicology.
Slide 55 I’ll finish with the challenge of toxicology. I think it’s very important that
stem cell biologists become interested in toxicology. These agents cause developmental maladies. They contribute to many different diseases and the problem we have is that there are 80,000 to 150,000 registered chemicals for which we have no information and they are therefore assumed to be safe and are released into the environment. And what our work has shown is that the target of these toxicants appears to be the progenitor cells themselves. So if you want to do toxicology studies to find responses environmentally relevant to exposure ranges, it turns out you have to work on progenitor cells. Even beyond that, you have to work on the right progenitor cells.
Slide 56 By studying the O‐2A progenitor cells that are from a more oxidized
region of the nervous system, we were able to discover a new regulatory pathway that converts changes in oxidative status into degradation, a specific receptor, tyrosine kinase, and this was discovered by Zibo Li. The way this pathway works is when a cell becomes oxidized in the top right, that activates Fyn kinase. That activates the Cbl ubiquitin ligase which attaches ubiquitin to its target’s proteins which are degraded. Many of the target proteins are things like this PDGF receptor, the EGF receptor, c‐Met receptor for hepatocyte growth factor. You can’t do these experiments if you work with the cortical progenitor cells because they are intrinsically so reduced that they protect against these changes.
This also applies to the standardization of tissue culture conditions. There
is a great use of tissue culture media that is enriched in antioxidants in order to promote cell survival. These drive cells into what appears to be a reduced status that does not resemble lack of normal development and makes it literally impossible to discover the nuances of oxidative cell regulation.
Slide 57
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Sean Sanders: Thank you very much, Dr. Noble, and thank you all for the very informative presentations. I’m very impressed that everyone kept to time so that gives us a good half hour for our Q&A portion of the webinar. A reminder to everyone that you can submit your questions simply by typing them into the ask‐a‐question box and clicking the submit button; and once again, please do keep them as short as possible.
So I’m going to start with the first question, which is maybe more of a
philosophical one before we start digging into the technical side, and that is somebody asks, instead of talking about taking stem cells out of the body, expanding, and then transplanting them, when will we have a better understanding of how to administer agents that activate endogenous stem cells and is this a realistic goal? So let’s start with Dr. McKay.
Dr. Ronald McKay: Yes, it is a realistic goal and there’s a lot of work currently that shows that
this is not only an object of people’s research attention but it happens and it’s an important regulator of health and disease.
[0:30:00] And I’ll add one more point to this which is I think that it will require new
technology and it will require new technology that allows us to be more precise in characterizing the properties of cells in vivo in space and time.
Sean Sanders: Dr. Wagers? Dr. Amy Wagers: Yes, I absolutely agree. This is a realistic goal and one that we’re working
towards also in my own laboratory. We are already starting to understand the ways that stem cells are responding within tissues to their environment and how that changes in diseased states, and those targets will be I think really critical ways or pathways into regulating tissue regenerative function. I actually think of it not even as an either‐or question because these same pathways that might target endogenous cells could be targeted to enhance the transplant ability or engraft ability of transferred stem cells as well. And so, these are really convergent types of investigations.
Sean Sanders: Okay. Dr. Mark Noble: And I’ll just add that in fact, we’ve been doing that for a long time.
Erythropoietin and GCSF which are used in the after‐treatment of cancer are targeting hematopoietic stem cells and lineage‐restricted progenitor cells. There’s great interest now in parathyroid hormone mimetics in
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their ability to target perhaps mesenchymal stem cells to enhance fracture repair. So what we are finding is that this is possible and this contributes to actual benefit. And I also very much agree with what Amy said that when we combine these technologies of transplantation and manipulation of these molecules, we may greatly enhance the value with the transplants.
Sean Sanders: Excellent. So a specific question. I think Dr. Wagers might be the best
person to answer this. My group is working with mesenchymal stem cells and we want to know which are the best molecular markers used to set differentiation potential? They named a few that they’re using but maybe you can help them out.
Dr. Amy Wagers: Right, right. I guess ‐‐ I mean, my strong argument and a point that I often
make is that markers are just markers, and the real way to define a population of cells is to correlate those markers very closely with function, but the function is really critical. So if you're designing your mesenchymal stromal cell compartment as differentiating into osteocytes, chondrocytes, and adipocytes, then you have to demonstrate that the cells that you're isolating actually have that differentiation capacity as well as renewal capacity.
Sean Sanders: Great. Dr. McKay, you have something to add? Dr. Ronald McKay: Well, yeah. I mean, it’s true what Amy says but it’s important in this use
of the word mesenchymal to be very precise about what tissue you're talking about. And you noticed that in Amy’s response, she talked about the classic definition of a stromal cell from the bone marrow; and if you're talking about another tissue out there, you might want to redefine the relationship carefully.
Sean Sanders: Okay. A little cryptic but I’m sure the message got across. So talking about
markers, somebody has asked are there any morphological aspects of stem cells that would let us recognize them under the microscope without using biochemical markers.
Dr. Ronald McKay: Well, one property they have is they grow and a lot of the success in this
field has essentially taken advantage of that property of the cells. And actually, one interesting story which is a success story is the use of expanded stem cells or expanded cells I should say from the skin where that’s now clinically used. And the reason that I’m talking about this is because that’s a functional property and different cells in the lineage, it grew to different extents which is an extremely interesting feature of these compartments. They’re not all the same. So yeah, I think that’s
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right. You can grow cells, and as Mark said in his presentation, how you grow them is going to make a lot of difference.
Sean Sanders: And Dr. Noble, what assays do you use to assess the stem cell properties
of your cultured cells? Dr. Mark Noble: Well remember, we work on progenitor cells so progenitor cell
properties. I’m going to say go to our webpage for details, but we look at many, many different properties. We look at the ability to generate specific cell types. We look at migration. We look at response to different kinds of mitogens. We look at response to different kinds of toxic agents, whether those be environmental toxicants or chemotherapeutic agents. So we study them from a large variety of different perspectives.
Sean Sanders: And if you compared progenitor cells to undifferentiated stem cells for
your treatments or you only work on progenitor cells? Dr. Mark Noble: No. We actually have and it makes a critical point I think. [0:34:55] So in the area of our toxicology research and in our work to try and
understand why some patients who are treated with chemotherapeutic agents develop cognitive problems, what we have found is that the lineage‐restricted progenitor cells of the central nervous system are particularly vulnerable to these kinds of toxic agents, whereas the stem cells are not. So if you want to look at the toxicity of cancer drugs or methylmercury, lead, paraquat and so on, and you study the stem cells, you get the wrong answer. If you study the lineage‐restricted progenitor cells, then you see vulnerabilities that you can readily detect with in vivo application.
Sean Sanders: A question for you, Dr. McKay. You said that in your talk, you were
making assumptions that ES cells and IPS cells are very similar. How similar are they and what are the differences if any?
Dr. Ronald McKay: So there’s a huge amount of interest in this at the moment, and probably
the most important point to make is that they are very similar. So there’s a number of different assays you can use, but what people are mostly looking at at the moment are gene expression assays and differentiation assays which are currently quite general. We haven’t got really down to the details yet. So that’s an important thing to say because it’s also so surprising that you can take a cell from the adult and you can readily turn
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it into a cell which is very similar to the cell from this very early stage of development.
Now, there are likely to be differences, but I think the point that I want to
stress is that those differences, many of them may not be the differences between ES cells and IPS cells but the differences between different genomes. And currently, we don’t have enough examples to really make strong distinctions in most of the published studies between those two possibilities.
Sean Sanders: Dr. Noble, and if you want to react to that. I know we were talking last
night about proteomics and genomics and you had some views on the importance of the genomes and if you want to respond.
Dr. Mark Noble: The cells are not just about genetics. The cell is a physiologically
regulated entity and our current understanding of evolution is that it was the interactions between chemicals that created the fundamental rules of the game for how cells work ‐‐ genes and proteins, very interesting, very important, but evolutionary‐related events. So we’ve been pursing for some time the hypothesis that if we focused attention on metabolic regulation of cell function, we could get general principles that told us a lot about different kinds of precursor cells in both normal development and disease, and it turns out that that has been a very heuristic hypothesis and we’ve been actually able to succeed at least in some areas to make this integration.
But I think that this metabolic issue is ‐‐ one of the demonstrations of
how critical it is is just whether you grow cells in atmospheric oxygen or in normoxic conditions. Atmospheric oxygen, 21% oxygen, is not seen by most cells of the body with the exception of skin cells, lung cells, corneal cells. Cells in deep structure never see this. So if you want to get closer to the normal properties of cells, you need to go to these 4% or 5% oxygen conditions and then you actually in many cases see different behaviors of the cells as shown by us and also I think Ron, by you in the distant past.
Dr. Ronald McKay: Yeah, but I mean, but they’re not opposing statements. I mean, you do
need to control the environment very carefully, but you also ‐‐ and if we’re talking about human cells, you have to know what the genome is. So for 30% of us for example, we’re deleted in one of the key enzymes that’s controlling responses to toxic chemicals, and it’s kind of remarkable that this gene is missing in 30% of us. So if you don’t know this, then you're comparing apples and oranges. So I mean, the point that I was trying is that you can control them very well and so then, these differences between the different lines becomes hugely interesting.
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Dr. Mark Noble: Yes, I totally agree with you. Sean Sanders: So I’m going to come back to you, Dr. Noble. A question for you, they’re
asking is there an optimal way to expand stem cells while maintaining the stem cell phenotype or the progenitor phenotype?
Dr. Mark Noble: This appears to be cell type specific. The first expansion condition that we
discovered was growth factor cooperation where we’ve discovered that progenitor cells, O‐2A progenitor cells exposed to platelet‐derived growth factor undergo a limited number of divisions, and then we’ve discovered that those exposed to platelet‐derived growth factor plus fibroblast growth factor will divide extensively without differentiating.
[0:40:06] There are multiple other cell types where growth factor cooperation
enables this extensive expansion, but that’s not necessary for all cell types. For example, the GRP cells expand very well with fibroblast growth factor alone. So you have to figure this out for each cell type individually, and the general principles appear to be simply that many times you're going to need combinations of factors. And it appears that some of those factors that enhance self‐renewal, maybe many of them actually work by making cells slightly more reduced as a key part of their action.
Dr. Amy Wagers: If I could just add to that. Sean Sanders: Sure. Dr. Amy Wagers: Because I think a lot of the focus often is on growth factors, but your
point about cell type specificity is really important. And for some cell types, it may not just be an issue of growth factors. They may be issues of mechanical support, other kinds of electrical stimulation that are needed in order to maintain those particular stem cells, and so that would be very surprising to me if there were universal culture conditions to maintain.
Dr. Ronald McKay: Absolutely, some substrate requirements. Dr. Amy Wagers: Right, right. Sean Sanders: So that comes to the question of standardization and a question that
came in about how one would compare stem cell types and I guess experiments as well done in different labs. So when you work with other
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labs, do you have a way of standardizing your conditions? So let’s start with Dr. McKay and we’ll work our way down.
Dr. Ronald McKay: Well, I mean I think that’s a very important question as you can probably
gather from my comments here. So I mean currently, there isn’t a standard for it. I’ll just focus on pluripotent human cells and it’s one of the important things that we need to do is generate such a standard and there are several of us who are putting a lot of effort into this, who are talking right now about what such a standard would be. And it will have multiple different components in it, but one of them for sure is going to be to have an assay which looks quite broadly at the cell. So Amy made this point about markers and that the problem with markers of course is that they’re individual proteins or genes, and I think what’s very interesting about systems biology and our new sort of global tools that we have is that you can actually define the cell much more broadly. And I think that’s very exciting and I think those are going to be absolutely important in generating powerful standards.
Sean Sanders: Dr. Wagers? Dr. Amy Wagers: I absolutely agree. It’s a challenge. I think perhaps even a more critical
challenge in the study of tissue‐specific stem cells is where people do use different strategies for isolating those cells and ultimately even for testing their function, and where the definition of the cells actually continues to change and become refined and we continue to appreciate a greater and greater heterogeneity among the types of cells that are in a population that we refer to as stem cells.
And so the best that we can do in our lab is actually to exchange
researchers between the labs and try to really have a good understanding of exactly what it means in my lab when I say I isolate those skeletal muscle stem cell and, you know, for instance with that approach, we’ve had more than 30 different labs that have come and spent time with us to learn those techniques. And that sort of really close conversation I think is maybe one way forward for achieving standardization although probably not a rapid one.
Sean Sanders: Great. Dr. Noble? Dr. Mark Noble: So I got a couple of comments saying first that I agree with everything
that Ron and Amy have said. First from an evolutionary perspective, I think it’s critical that people are trying different things because that’s how discoveries get made about how to enhance cell growth, but it is absolutely essential that people provide in their materials and methods
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sections the details of what they’re doing. Because over and over again we find that when it is ‐‐ there’s a lack of replication between layups, it comes down to this nuances. Did they expose the cells to fetal cancer? Did they use trypsin instead of papain? And these detailed protocols I think should be up on people’s websites, should be described in detail in literature, and those who were trying to replicate results have got to realize that these nuances are there for a reason, that if you try to cut corners, you're going to get a different result.
Sean Sanders: So that brings me on to the question about passaging these types of cells
and whether ‐‐ the person asks whether mechanical passage is better than trypsinization and also whether genetic abnormalities can result from trypsinization. Who wants to grab that one? Dr. Wagers?
[0:45:09] Dr. Amy Wagers: We actually don’t grow our cells in culture. All the studies that we do, we
work with freshly isolated cells because the challenge we have with the populations we’re studying is that we can’t expand them in culture and have them maintain their properties. So…
Dr. Mark Noble: So cell type specific again. The GRP cell expanded for multiple passages
remains able to make these type‐1 and type‐2 astrocytes, but I’d have to say that critical in the success of those experiments has been that Jeanette Davies is also trained in precursor cell biology so we could keep the conditions same in those experiments.
Other cells like the O‐2A progenitor cell derived from certain parts of the
nervous system, if you expand it for three weeks in tissue culture, it actually loses its response to platelet‐derived growth factor. This property has changed. And this is, as we move to medical applications, it’s essential that we move from poetry to engineering. We have to be able to supply products that are the same. That is the standard for drug analysis. We can’t have situations where this time the cell is 75% pure and this time they’re 95% pure and this time they’re 80% pure because the experimental outcomes may change and we won’t understand why.
Sean Sanders: Dr. McKay? Dr. Ronald McKay: So I mean, I think that, you know, that all of these questions are sort of
driving at something which is at the heart of our work, even whether we’re doing it in the animal or in an experimental system outside the animal, which is to try and understand the signals that control the state of these different cells in our body. And in the ES and IPS field, when we
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showed that you could get a pluripotent cell from the post‐implantation mouse embryo, we’re immediately taking a cell out of its normal context, and of course, we’re looking very carefully to see whether it’s a rare cell. And in that case, these cells grew very rapidly and it looked like there was not a selection.
But then in the question asks are there conditions where you have a
selection? And I just want to stress to the person asking the question that that’s a really interesting question because pretty soon we’re going to be able to answer it with the right tools. You're going to know in all of these systems when you passage cells how many cells die, whether you're picking a very particular cell, and if you're interested in change, which is the criticism of course of putting cells into an experimental setting, you will know exactly when the event took place, and that I think is going to be a big change in the way we sort of view these technologies.
Dr. Mark Noble: I guess we’d also better actually answer the question itself which is that
yes, there are some cells that you can passage mechanically with a sharp tap on the side on the flask. There are others where you need to use a digestive enzyme, and for some cell types you might use trypsin, for some cell types you might use papain, for some cell types you might use dispase. Every cell has its best glycosidase.
Sean Sanders: Great. Have any of you succeeded in sorting stem cells using flow
cytometry and so do you have enough cells to culture them viably? So Dr. Wagers, you’re nodding.
Dr. Amy Wagers: Yes, we do that every day. Sean Sanders: Every day? Okay. Dr. Amy Wagers: Yeah. So it is a challenge and it’s certainly again cell type specific which
cells can endure that type of shear stress and so flow cytometry really was worked out around blood cells which are quite comfortable in high shear. And so in our experience, when we tried to apply those same kinds of approaches to skeletal muscle stem cells which are much less likely to encounter such forces there, there was a lot of optimization of the equipment that had to happen.
And so there are even more sensitive cell types that I think will probably
be resistant to cell sorting with the canonical flow cytometer, and that’s one of the reasons we’re going to need new technologies that allow isolation of cells by different types of methods that don’t have to subject them to those kinds of shear forces.
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Sean Sanders: Excellent. Okay. Our next question actually relates to this and it’s going
broaden the question a little bit, and whether any of the speakers envisaged the use of microfluidic devices, lab ownership technology, and nanotechnologies for the fundamental study of stem cell properties.
[0:49:57] Dr. Ronald McKay: I could answer that and I could make it an extension of the previous
comments. So when we grow neural stem cells under a system where we’re constantly observing them, then of course we have to pass medium across them. And what we’ve found was that the cells are extraordinarily sensitive to flow rates, and initially, it took us quite a while to figure this out. So microfluidics is a very interesting approach and there’s a number of results out there that just started moving in this direction and some companies are developing these new tools.
And one interesting comment, general comments I’d make about it is
that microfluidics and nanotechnology offer a kind of conceptual advantage which is that you can be more precise. It’s not just that you're smaller. It’s that you can also get more data which is going to be a very important feature of making progress.
Sean Sanders: Dr. Noble? Dr. Mark Noble: Remember I commented that there is 80,000 to 150,000 chemicals that
we don’t know anything about. What we know from the work of many people that the outcome of adding these chemicals together is very different from looking at one alone. The toxicity that you can get just from adding two things together is different from exposure to a single agent. So think of all the combinations that we want to look at in all the cell types, in all the different conditions, and you rapidly see that the standard approaches of growing things in an incubator and looking down the microscope are not going to enable us to address this challenge.
Sean Sanders: So a question has come in about growing stem cells in on feeder layers
and I’m not sure how much this is still used, but what is the impact of using feeder layers and how this tie into eventually using this cell for clinical use? And I’m talking feeder layers and maybe we can expand it also to other substrates.
Dr. Ronald McKay: Well, in the ES/IPS field, feeder layers are widely used and it’s, I mean,
the next thing I’m going to say is sort of slightly empirical. So the reason they’re used is because they work. But you have to be extremely careful
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with the feeder. And so by controlling the density of the feeder layer you can move the cells up and down a kind of a stress pathway. And we’re trying to develop ways now where we don’t use feeder layers and there are protocols out there. But again, it comes back to the standardization issue which is are those really the same? And so it’s a slightly messy response, but the answer of course is that in a way, the cells themselves are not always completely homogenous so they’re talking to each other. So you can’t avoid the issue of cell interaction here. In fact, that sort of is the issue, but at an empirical level, feeder layers are very important still in the growth of human ES and IPS cells.
Sean Sanders: Dr. Wagers, anything to add? Dr. Amy Wagers: No, I think Ron said everything. Sean Sanders: Great. Dr. Mark Noble: And in the nervous system, we’ve been extraordinarily fortunate that we
very rapidly were able to discover not just feeder‐free culture conditions but chemically defined medium. So in working in the nervous system, cells never see serum from any species, and for whatever fortunate reason, the nervous system works like that. It has been a tremendous meaningful work in this tissue.
Dr. Ronald McKay: Sean, can I make a comment on this? Sean Sanders: Sure. Dr. Ronald McKay: Because let’s say you've gotten rid of feeders completely and everything
is defined and you're feeling very relaxed, right? But you do have to have something there. I mean, cells don’t just, you know, live on H2O, right? So where the growth factors come from could turn out to be very important, and everybody sort of says that, you know, recombinant factors are where it’s at. But actually, these proteins are often glycosylated, and as we get more precise, it might be important to sort of bear in mind that these issues don’t sort of suddenly go away; they just change. Exactly. Yeah.
Dr. Mark Noble: They change and we’re currently dealing within an experience in the
layup where the identical growth factor from two different companies has an order of magnitude difference, and it’s proteins, issues that we constantly have to pay attention to.
Dr. Ronald McKay: Why is that?
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Dr. Mark Noble: Why is that? Sean Sanders: I’ll share with you one of my favorite quotes that confidence is that quite
assured feeling you get just before you fall flat on your face. So I guess we always have to be careful.
So I’m going to shoot a question to Dr. Wagers. While treating a disease
related to a particular organ, do you think it is better to use stem cells from that particular organ or an embryonic stem cell?
[0:55:00] Dr. Amy Wagers: So again, I think it depends. It’s too early to say and in an abstract way to
place your bet if you will on one or the other. I think it looks past all of the synergy that’s evolving in those two fields. So tissue‐specific stem cells in formal studies of the differentiation of embryonic stem cells, embryonic stem cells in formal studies about how stem cells in tissues renew and are maintained and serve as models for the production of stem cells that one would never be able to study because they are a population that happens in development and is killed by disease before you ever have a chance to study it.
So where I think there are potential issues is when there’s attempts to
push a cell that’s intrinsically interested in differentiating into one lineage into another lineage and appropriate markers and functional assays aren’t applied. So I think changing the fate of a cell while we clearly are in a really exciting time for doing that right now, there has to be stringency in understanding what type of cell you've produced at the end of that process. So I’m sorry for the vague answer that, you know, but I really do feel strongly it depends, and the answer today may not be the answer five years from now.
Sean Sanders: Question for you Dr. McKay. Do you believe that human embryonic stem
cells are inherently heterogeneous and that they require this feature to some or retain maximum developmental capacity?
Dr. Ronald McKay: Yeah, I’d think I’d say yes. But the reason I’d say yes is because I think
that may actually be true for essentially all cells. So there’s a very interesting sort of change in our view of cells as our tools become better and we understand that cells can occupy different states. So a cell is not a single thing. It’s a device which is designed to occupy different states, and actually that ‐‐ so I’m very confident that we’ve got very good conditions to grow human ES cells in standard ways and human IPS cells in standard
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ways, but in the middle of that, the cell is a sophisticated device which is moving through different states and I think it’s important to remember that both as a sort of practical sort of precaution but also because it reflects the fundamental property we’re interested in. Yeah.
Sean Sanders: So in the last couple of minutes that we have, I’m going to touch on an
area that we haven’t really discussed and that is cancer stem cells. I had a couple of questions come in that we didn’t have time to get to, but I guess my overall question is do you think these exist and if so, what are their properties and characteristics and how would they differ and how will we be able to use them in therapy? So I see Dr. Noble I think is itching to answer this question.
Dr. Mark Noble: Sure. It’s one of the problems we work on. It seems that for some
cancers, there are some cells that are better at tumor initiation than other cells. I have to say that I think that the cancer stem cell hypothesis is a bit exaggerated, that there’s repeated studies that show that if you look at the putative non‐cancer stem cell compartment, those cells have capacity to make tumors with high frequency. I think we also have to make it absolutely clear that this misuse of language is a problem. Cancer stem cells doesn’t mean cancer comes from stem cells. It means that cells that become transformed have the ability to make lots of cells, and it doesn’t matter whether that transformed cells is a stem cell or a lineage‐restricted progenitor cell or even a more differentiated cell.
Sean Sanders: Dr. McKay? Dr. Ronald McKay: Well, I mean it seems to me that what we need to know here is we need
to know where every cell in our body comes from and what controls its numbers. And if you know that, that’s to say the lineage, then you've got to place your cancer in the lineage and cancer has multiple types of cells in it and you need to understand where the disruptions occurred. And so cancer is a developmental problem and it won’t really help to say it comes from somewhere and I don’t know where it comes from. Yeah.
Sean Sanders: Dr. Wagers, any last words? Dr. Amy Wagers: I say this really gets to the issue of cancer stem cells in terminology or are
you talking about the cell of origin or are you talking about the tumor propagating cell, and those are not necessarily the same type of cell and so segregating out those two notions I think will help clarify that issue.
Sean Sanders: Well, I think we’re going to have to leave it there 'cause we are out of
time. So please join me in thanking our panelists for being with us today
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and for generously sharing their expertise: Dr. Ron McKay from NIH, Dr. Amy Wagers from Harvard University, and Dr. Mark Noble from the University of Rochester Medical Center.
[1:00:03] Thank you also to the viewers for your questions. As usual, there were
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email at the address now up in your slide viewer; [email protected]. And again, thank you to our panel for being with us and thank you to
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