64 M A X P L A N C K R E S E A R C H 3 /2 0 0 1 FIRST-HAND Knowledge 3 /2 0 0 1 M A X P L A N C K R E S E A R C H 65 Stem Cells: Capital Stock of a New Medicine?CELL Biology Let me start with a quote from an article which Federal Chancellor Schröder recently wrote for the SÜDDEUTTSCH ZEITUNG: Under the heading “Knowledge Comes First” one could read the following, “Decoding of the human genome and the legalisation of therapeutic cloning in Great Britain have dramatically demonstrrate that genetic engineering is no longer a Utopia but rather a part of our present. Up to now our society has not undertaken a candid discussiio of the opportunities and risks of genetic engineering processes. For the questions connected to these processes touch the very core of our self-image. We have to decide here about matters that move in the field between conceivability and feasibili-On February 15 this year the Scientific Board of the Max Planck Society held a meeting in Berlin. The agenda included two lectures on the subject of stem cells and therapeutic cloning. First, PROF. PETER GRUSS, director at the MAX PLANCK INSTITUTE OF BIOPHYSICAL CHEMISTRY in Göttingen, presented an overview of the status at research and the possible use of stem cells in a future “regenerative medicine”. The second lecture was given by Prof. Rüdiger Wolfrum, director of the Max Planck Institute for Comparative Public Law and International Law in Heidelberg. The topic was the legal principles and bounds of scientific work with stem cells and their therapeutic utilisation. The following article presents an edited version of Prof. Gruss’ talk. Prof. Wolfrum’s lecture will be published in the next edition of MaxPlanckResearch. ty, accountability and responsibility not least in respect of future generatioons. At a later point the term “participattion pops up – and initially this means self-determination and co-determinnation However, this requires “co-knowledge” – and that is the main point here: only a society that is fully informed can express a view on such momentous future questioons In this context, I would like to explain the scientific aspects of the “stem cells” topic – in the hope of thus contributing to a firmer basis for future public discussion. What are stem cells? Some researrcher once answered this questiio in an oversimplified manner and said that one might apply an “uncertainty principle” to stem cells because they can only be defined and discovered by their function. Basically a stem cell is understood to be any undifferentiated cell of an organism that can not only multiply but also produce more mature daughter cells. This means that one must consider two processes, two cell functions: on In therapeutic cloning the nucleus of a skin cell (1) for example is transferrre into an enucleated egg cell (2 and 3) that then begins to divide and forms a clone (4). An embryonic blastocyst emerges (5). In cell culture specific cell types such as muscle or nerve cells (6) are obtained from this blastocyst that are then implanted in the patient’s body. one hand the proliferation of these cells into homogeneous, undifferentiatte daughter cells, and on the other the process of differentiation during which a cell with completely different, new characteristics emerges from a stem cell. First of all, the discernible potentiia for therapy that is intrinsic to stem cells: every person, and this is no revolutionary statement, originaate from a single cell, the fertilised egg. This means that the fertilised egg embodies a totipotent cell, i.e. one that is “capable of everything”. And this original totipotence is retaiine in the early stages of mammallia embryo development up to the eight-cell stage before becoming increasingly restricted. ❿ According to Prof. Peter Gruss’ lecture on stem cells and therapeuuti cloning, codetermminatio requires co-knowledge, “Only a society that is fully informed can express a view on such momentoou future questions.” PHOTOS: WOLFGANG FILSER, ILLUSTRATION: MPI FOR BIOPHYSICAL CHEMISTRY3 /2 0 0 1 M A X P L A N C K R E S E A R C H 67 This means that one could obtain “dopaminergic neurons” by way of embryonic stem cells and then impllan them into people suffering from Parkinson’s disease. In this case the use of embryonic stem cells in the brain offers the advanntag that the immune system does not attack and reject these cells since it is practically “locked out” by the blood-brain barrier. Thus, implaant from any available cell line could be used. However, this is quite different if one is considering the replacement of organs and tissues outside the central nervous system: in this case immune reactions occur and rejectiio has to be suppressed. THERAPY ACCORDING TO THE “DOLLY SYSTEM” Therapeutic cloning would offer new routes. Initially it would functiio as with Dolly: for example one takes a skin cell and transfers its nuclleu into an enucleated egg cell. This cell starts to divide and grows into a blastocyst that is placed in tisssu culture. The blastocyst is then not re-implanted but caused to produuc specific cell types in tissue cultuur such as muscle or nerve cells for. And there is no anticipation of rejection reactions with cells obtaiine by therapeutic cloning. So how can somatic stem cells – that are less ethically “loaded” – be used? One example is the pancreas which is also of scientific interest to our laboratory. The pancreas consists on one hand of so-called exocrine cells that produce enzymes which break down our food as “gastric juices”. On the other hand it contains two types of cells that are to be found in the “islets of Langerhans”: the beta cells that produce insulin and the alpha cells that generate glucagon. Approximately five percent of the world’s population is known to sufffe from diabetes and this number is growing. Could the body’s own stem cells be induced to produce new islet cells? Furthermore: does the pancrrea have stem cells of its own? The answer is “yes”. However, somatic stem cells are always hidden in recessse where they interact with the surrounding tissues and factors. This makes it difficult to track down somaati stem cells and get hold of them. Nevertheless, with the help of specific developmental control genes we succeeded in generating insulinprodducin cells. Thus we are at the beginning of a development that might end in a therapy: in the possibillit of making the body’s own stem cells differentiate specifically into either insulin-producing beta cells or glucagon-producing alpha cells. Parallel to this, other groups have successfully obtained insulin-produccin cells from mouse embryonic stem cells in tissue culture. Thus one can imagine being able in this way to obtain a made-to-measure therapp for type I diabetes – assuming of course that it is possible to “encap-CELL Biology 66 M A X P L A N C K R E S E A R C H 3 /2 0 0 1 Up to the eight-cell stage of the mammalian embryo it would in fact be possible for a complete organism to emerge from each individual cell. In the course of continued embryogeneesis during the so-called pre-implanttatio stage in the fallopian tube, the outer cells are packed densely together and form a socallle blastocyst as the last stage of pre-implantation. A union of ten cells, the embryonic mass, is added on at one point of this blastocyst, – and the complete infant organism develops in time from these ten cells. These cells of the embryonic mass are termed embryonic stem cells: they proliferate almost indefinitely in vitro (in tissue culture), provided that no differentiation takes place. And this is where these cells differ from the so-called somatic stem cells: factors have been discovered that prevent their differentiation in vitro. Their pluripotence is retained if these substances are added to the culture medium thus preventing differenttiatio of the cells. The early embryo offers a second opportunity for obtaining more or less pluripotent stem cells, the socallle primordial germ cells. These cells are less pluripotent than the embryonic stem cells; at this point it should only be noted that the Germma legislation permits primordial germ cells to be generated whilst the production of embryonic stem cells is prohibited. Another way of obtaining early stem cells was illustrated in Dolly, probably the world’s most famous sheep. And this process also documeent the principle of therapeutic cloning. The clone Dolly was generatte by first taking egg cells from a ewe and enucleating them, i. e. only the egg plasma was left. Then nuclei were removed from body cells – in this case taken from udder tissue – and introduced into the enucleated egg cells. Each of these modified egg cells developed in vitro into a kind of early embryo. One such embryo, implanted into a ewe, finally produuce the sheep Dolly. What is scientifically revolutionaar about Dolly is the evidence that the cytoplasm of the egg cell is able to re-program the nucleus of a body cell in such a way that this cell nuclleu becomes totipotent. Let us now turn to the stem cells of the adult body, the somatic stem cells. In the human organism, tissue must constantly be renewed and cells replaced. The skin for example is “completely renewed” once every fourteen days – which is extremely helpful following sunburn. Within 24 hours several billion cells in the blood are replaced with new ones. Anyone who has ever had a broken leg knows that the muscle degeneratte but it is built up again afterwarrds Other phenomena are less wellknoown such as the ability of partialll regenerating the pancreas or the fact that new nerve cells are formed from stem cells in the brain even in adults. TISSUE REPLACEMENT INSTEAD OF ORGAN TRANSPLANTATION What is the difference between embryonic and somatic stem cells? The embryonic stem cells possess a totipotence or omnipotence which is restricted in the course of further develoopmen – to a pluripotence or multipotence in somatic stem cells whose function consists of regenerattin specific tissues to which they belong. How does one get to a therapy and what can this therapy offer? Throughout the world there are too few donors available for organ transplants. It would be possible to redress this deficiency with tissue replaccemen therapies, with a – let me call this Utopia – “regenerative mediciine” Parkinson’s disease, an illness from which one percent of the populattio over 60 suffers, provides one example for this. This disease initialll manifests itself in a delayed motor response as well as in passive tremor, and eventually one loses the ability to move spontaneously and consciously. The cause is a progressiiv degeneration of specific nerve cells in the brain that produce the messenger substance dopamine which is essential for the transmissiio of nerve signals. Initially, the loss of these cells may be compensaate to some extent by administeriin a precursor substance of dopamine as a drug. But as the illnees progresses this therapy no longer has an effect. A Swedish group has transplanted tissue from the brains of aborted foetuses into the brain of a patient with Parkinson’s disease. And this treatment was successful: the transplaante tissue was still producing dopamine ten years later. Thus in principle embryonic materiia is able to replace damaged tissuues Nevertheless the ethical evaluattio of such a therapy is open to question – and for this reason alternattive are sought that also have to be discussed. Amongst these are the embryonic stem cells that can generate all the cells of the body. We have experimennte with embryonic stem cells of the mouse and made them differentiaat into muscle or nerve cells. And there are substances with which one can specifically obtain dopamineprodducin nerve cells from embryonni stem cells. These cells, as animma experiments showed, can be used to treat Parkinson’s disease. FIRST-HAND Knowledge Top: Dolly, the cloned sheep, was generated from enucleated egg cells of a ewe into which nuclei from udder cells were introduced. These egg cells developed in vitro into early embryos, one of which was implanted in the ewe and in the end produced Dolly. Bottom: The development of a fertilised egg cell into two-cell, four-cell and eight-cell organism to morula, the “mulberry mass”, and finally to blastocyst with the embryonic mass (top right) – the potential reservoir of embryonic stem cells.68 M A X P L A N C K R E S E A R C H 3 /2 0 0 1 sulate” the islet cells and thus to shield them against auto-aggression on the part of the immune system (the most frequent cause of type I diabettes) Even in type II diabetes – if such a patient requires insulin – these encapsuulate islet cells might help. However, a “more elegant” and scientifficall more accurate therapy would be to stimulate somatic stem cells of the pancreas to differentiatiio and endogenous production of new islet cells by means of developmennta control genes. Animal experimeent have already provided very promising results on this front. The objective would be to generate from somatic stem cells not only those cells they were originally intennde to generate but also to lend them greater plasticity by way of socallle transdifferentiation and dedifferenttiation Then it would actually be possible to obtain a large number of different tissues from endogenous stem cells. Although, in principle it is possible to do this, experimentally it is extremely difficult and timeconsuuming An extraordinary amount of research will be needed to make this route practicable. I would like to add another comparriso of the various processes of stem cell therapy (see also the table on the left): Somatic stem cells by their nature only have a limited potential for differentiattion And in vitro they can only be cultivated with difficullty there are still no known factors for suppressing their differentiation, that is they lose their stem cell character in cultuure Their targeted differentiatiio for therapeutic purposes necessitates a detailed understanndin of the relevant “switching processes” – somethhin that we are still a very long way from. Studies on the dedifferenttiatio of somatic cells, that is their transformation back into cells with a broader potential for developmeent are still in their infancy. Parallel to this is the pragmatic alternnativ of using embryonic stem cells. These cells are pluripotent, and there is a whole series of known methods for bringing them to a targeete differentiation in tissue culturres Embryonic stem cells and therapeutic cloning are immunologicaall neutral, somatic stem cells on the other hand provoke immunoreactiions Somatic stem cells would have to be generated individually, embryonic stem cells would be universally available – and theoretically one single stem cell line, that is one “relinquuishe embryo”, would be sufficiien to supply science and medicine with embryonic stem cells for an unlimmite time. Even therapeutic cloning requires the individual manufacturing of cells. And in this case, as in the case with somatic stem cells, the problem of repairing possible genetic defects has not yet been solved – a problem which does not arise with embryonic stem cells. Neither has the risk of cancer been ruled out yet. If one Prof. Peter Gruss, born in 1949, is Directto of the Department of Molecular Cell Biology at the Max Planck Institute of Biophysical Chemistry in Göttingen. His research field, the developmeenta biology of mammals, may be sharply defined by one question: which genes and mechanisms control the chronologgica and spatial “choreography”, accordiin to which a multicellular organism with all its different tissues and organs is finally generated from a single fertilised egg cell? FIRST-HAND Knowledge starts with embryonic stem cells and fails to separate them cleanly from the differentiated cells, this may lead to tumours that are in principle beniig but which consist of numerous different types of cells. FEASIBILITY PRESUPPOSES ACCOUNTABILITY So much for the rough overview on the topic of stem cell therapy from a scientific point of view with its possibilities and its “practical” problems. Science alone cannot and will not answer the ethical questions brought up by these techniques. Sociiet as a whole must be included in the decision as to whether and to what extent these possibilities should or may be used. In this case moral, ethical and legal aspects must still rank above all purely technical or financial considerations. For this applies more strictly for modern biollog and medicine than for any other science: feasibility must at any rate be based on accountability. Or in other words: the Hippocratic oath, once coined for the individual person, the patient, must now be extennde to the welfare of a communitt – human society. STEMCELL THERAPY: ASPECTS OF COMPARISON Properties Problems Ressource Ethical aspect SOMATIC EMBRYONIC THERAPEUTIC STEM CELLS STEM CELLS CLONING multipotent no immune barrier individually generated repair of possible genetic defects necessary identification isolation, cultivation access to therapy one´s own boby pluripotent immune reaction general availability risk of cancer cell culture human embryo used once, then “disposed of” totipotent no immune barrier individually generated repair of possible genetic defects necessary risk of cancer one´s own body + foreign egg cell human embryo generated, used and “disposed of”