Stem Cells Capital Stock of a New Medicine

FIRST-HAND Knowledge CELL Biology On February 15 this year the Scientific Board of the Max Planck Society held a meeting in Berlin. GRUSS, The agenda included two lectures on the subject of stem cells and therapeutic cloning. First, PROF. PETER director at the MAX PLANCK INSTITUTE OF Stem Cells: Capital Stock of a New Medicine? in Göttingen, presented BIOPHYSICAL CHEMISTRY 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. According to Prof. Peter Gruss’ lecture on stem cells and therapeutic cloning, codetermination requires co-knowledge, “Only a society that is fully informed can express a view on such momentous future questions.” In therapeutic cloning the nucleus of a skin cell (1) for example is transferred 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. 64 PHOTOS: WOLFGANG FILSER, ILLUSTRATION: MPI FOR BIOPHYSICAL CHEMISTRY L et me start with a quote from an article which Federal Chancellor Schröder recently wrote for the SÜDDEUTSCHE 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 demonstrated that genetic engineering is no M A X longer a Utopia but rather a part of our present. Up to now our society has not undertaken a candid discussion 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- ty, accountability and responsibility not least in respect of future generations.” At a later point the term “participation” pops up – and initially this means self-determination and co-determination. 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 questions. 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 researchers once answered this question 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 one hand the proliferation of these cells into homogeneous, undifferentiated 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 potential for therapy that is intrinsic to stem cells: every person, and this is no revolutionary statement, originates 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 retained in the early stages of mammalian embryo development up to the eight-cell stage before becoming ❿ increasingly restricted. M A X P L A N C K R E S E A R C H 3/2001 3/2001 P L A N C K R E S E A R C H 65 FIRST-HAND Knowledge CELL Biology 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 embryogenesis, during the so-called pre-implantation stage in the fallopian tube, the outer cells are packed densely together and form a socalled 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 differentiation of the cells. The early embryo offers a second opportunity for obtaining more or less pluripotent stem cells, the socalled primordial germ cells. These cells are less pluripotent than the embryonic stem cells; at this point it should only be noted that the German 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 documents the principle of therapeutic cloning. The clone Dolly was generated 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 produced the sheep Dolly. What is scientifically revolutionary 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 nucleus 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 degenerates but it is built up again afterwards. Other phenomena are less wellknown, such as the ability of partially 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 development – to a pluripotence or multipotence in somatic stem cells whose function consists of regenerating 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 replacement therapies, with a – let me call this Utopia – “regenerative medicine”. Parkinson’s disease, an illness from which one percent of the population over 60 suffers, provides one example for this. This disease initially 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 progressive degeneration of specific nerve cells in the brain that produce the messenger substance dopamine which is essential for the transmission of nerve signals. Initially, the loss of these cells may be compensated to some extent by administering a precursor substance of dopamine as a drug. But as the illness 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 transplanted tissue was still producing dopamine ten years later. Thus in principle embryonic material is able to replace damaged tissues. Nevertheless the ethical evaluation of such a therapy is open to question – and for this reason alternatives 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 experimented with embryonic stem cells of the mouse and made them differentiate into muscle or nerve cells. And there are substances with which one can specifically obtain dopamineproducing nerve cells from embryonic stem cells. These cells, as animal experiments showed, can be used to treat Parkinson’s disease. This means that one could obtain “dopaminergic neurons” by way of embryonic stem cells and then implant them into people suffering from Parkinson’s disease. In this case the use of embryonic stem cells in the brain offers the advantage that the immune system does not attack and reject these cells since it is practically “locked out” by the blood-brain barrier. Thus, implants 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 rejection has to be suppressed. THERAPY ACCORDING TO THE “DOLLY SYSTEM” Therapeutic cloning would offer new routes. Initially it would function as with Dolly: for example one takes a skin cell and transfers its nucleus into an enucleated egg cell. This cell starts to divide and grows into a blastocyst that is placed in tissue culture. The blastocyst is then not re-implanted but caused to produce specific cell types in tissue culture such as muscle or nerve cells for. And there is no anticipation of rejection reactions with cells obtained 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 suffer from diabetes and this number is growing. Could the body’s own stem cells be induced to produce new islet cells? Furthermore: does the pancreas have stem cells of its own? The answer is “yes”. However, somatic stem cells are always hidden in recesses where they interact with the surrounding tissues and factors. This makes it difficult to track down somatic stem cells and get hold of them. Nevertheless, with the help of specific developmental control genes we succeeded in generating insulinproducing cells. Thus we are at the beginning of a development that might end in a therapy: in the possibility 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-producing cells from mouse embryonic stem cells in tissue culture. Thus one can imagine being able in this way to obtain a made-to-measure therapy for type I diabetes – assuming of course that it is possible to “encap- 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. 66 M A X P L A N C K R E S E A R C H 3/2001 3/2001 M A X P L A N C K R E S E A R C H 67 FIRST-HAND Knowledge STEMCELL THERAPY: ASPECTS OF COMPARISON THERAPEUTIC CLONING SOMATIC Properties STEM CELLS EMBRYONIC STEM CELLS stem cell therapy (see also the table on the left): Somatic stem cells by their nature only have a limited potential for differentiation. And in vitro they can only be cultivated with diffirisk of cancer risk of cancer culty: there are still no known identification factors for suppressing their isolation, differentiation, that is they lose cultivation their stem cell character in culaccess to therapy ture. Their targeted differentiaone´s own body Ressource one´s own boby cell culture tion for therapeutic purposes + foreign egg cell necessitates a detailed underEthical aspect human embryo human embryo used once, then generated, used standing of the relevant “disposed of” and “disposed of” “switching processes” – something that we are still a very sulate” the islet cells and thus to long way from. Studies on the dedifshield them against auto-aggression ferentiation of somatic cells, that is on the part of the immune system their transformation back into cells (the most frequent cause of type I diwith a broader potential for developabetes). ment, are still in their infancy. Even in type II diabetes – if such a Parallel to this is the pragmatic alpatient requires insulin – these enternative of using embryonic stem capsulated islet cells might help. cells. These cells are pluripotent, and However, a “more elegant” and scithere is a whole series of known entifically more accurate therapy methods for bringing them to a tarwould be to stimulate somatic stem geted differentiation in tissue culcells of the pancreas to differentiatures. Embryonic stem cells and tion and endogenous production of therapeutic cloning are immunologinew islet cells by means of developcally neutral, somatic stem cells on mental control genes. Animal experthe other hand provoke immunoreiments have already provided very actions. promising results on this front. Somatic stem cells would have to The objective would be to generate be generated individually, embryonic from somatic stem cells not only stem cells would be universally those cells they were originally inavailable – and theoretically one tended to generate but also to lend single stem cell line, that is one “rethem greater plasticity by way of solinquished embryo”, would be sufficalled transdifferentiation and dedifcient to supply science and medicine ferentiation. Then it would actually with embryonic stem cells for an unbe possible to obtain a large number limited time. of different tissues from endogenous Even therapeutic cloning requires stem cells. Although, in principle it the individual manufacturing of is possible to do this, experimentally cells. And in this case, as in the case it is extremely difficult and timewith somatic stem cells, the problem consuming. An extraordinary of repairing possible genetic defects amount of research will be needed to has not yet been solved – a problem make this route practicable. which does not arise with embryonic I would like to add another comstem cells. Neither has the risk of parison of the various processes of cancer been ruled out yet. If one multipotent no immune barrier individually generated Problems repair of possible genetic defects necessary pluripotent immune reaction general availability totipotent no immune barrier individually generated repair of possible genetic defects necessary starts with embryonic stem cells and fails to separate them cleanly from the differentiated cells, this may lead to tumours that are in principle benign 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. Society 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 biology 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 extended to the welfare of a community – human society. G Prof. Peter Gruss, born in 1949, is Director of the Department of Molecular Cell Biology at the Max Planck Institute of Biophysical Chemistry in Göttingen. His research field, the developmental biology of mammals, may be sharply defined by one question: which genes and mechanisms control the chronological and spatial “choreography”, according to which a multicellular organism with all its different tissues and organs is finally generated from a single fertilised egg cell? 68 M A X P L A N C K R E S E A R C H 3/2001