When Can Neural Tissue Be Transplanted?
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WHEN CAN NEURAL TISSUE BE TRANSPLANTED? C. N. SVENDSEN and S. B. DUNNETT Cambridge Although corneal transplantation is now a widely neurons of the substantia nigra slowly begin to die used technique in ophthalmology, there have been no for unknown reasons leaving the patient trapped in a attempts to clinically transplant retinal ganglion body which has lost control of movement. During the (RGCs) which are damaged through trauma or early stages of the disease, patients respond well to L disease. This is due to a number of factors, the DOPA which allows them freedom of movement most important of which is that axons growing out again for short periods of time. However, the drug from transplanted tissue would have to grow long becomes less and less effective as the disease distances to reach their normal targets within the progresses and has a number of serious side effects. central nervous system (CNS). The major problem is The most obvious way to treat Parkinson's disease that the adult CNS is known to support such long would be to somehow replace the dopamine neurons distance axon growth and regeneration only rather with new ones - a strategy that has been used very poorly. However, recent experimental studies in the successfully for kidney, heart and corneal diseases rat have shown that axotomised RGCs can regener where new tissues taken from recently deceased ate axons over long distances through environments donors have been transplanted to replace old or more conducive to growth. For example, RGC axons diseased ones in the affected patient. Unfortunately, will grow through a bridge of peripheral nerve to it is not possible to use nerve cells from the adult make appropriate connections within the superior after death as they are no longer functional following colliculus from both a functional and anatomical removal of the brain. However, in animal experi perspective. 1 Furthermore, other studies have shown ments fetal CNS tissue has been shown to survive that axons from embryonic retinal transplants placed dissociation and transplantation into the adult brain. over the brainstem of neonatal rats can innervate the These neural grafts differentiate and can send out 4 pretectal nucleus and superior colliculus and are axons over fairly long distances. There have also capable of driving a pupillary response pathway in been a number of clinical trials where dopaminergic 2 3 the host rat. , These results suggest that under neurons taken from aborted human fetal tissue have certain conditions embryonic neural transplants or been transplanted into the brains of patients with induced neuronal regeneration in remaining RGCs Parkinson's disease. Although these trials are still can restore some visual function. Although massive under way, preliminary results suggest that in many practical and experimental issues lie ahead before cases these cells can survive for extended periods of these grafting methods can be considered in a clinical time and often lead to significant clinical improve s n setting, there is at least potential for future neural ment. , replacement therapy in patients where the connec The major limitations to these transplantation tions between the eye and the brain are destroyed. therapies for Parkinson's disease, or other diseases But when can neural tissue be transplanted? where neurons may be required to repair a specific Perhaps the most extensive literature, both clinical deficit, revolve around the ethics associated with the and experimental, with regard to neural transplants use of human fetal tissue and the practical problems comes from studies involving transplantation in of achieving a reliable high level of graft survival and Parkinson's disease. In this disorder, dopaminergic growth. Certain countries do not allow fetal tissue to be used in any human transplantation studies while From: MRC Cambridge Centre for Brain Repair and Department of Experimental Psychology, University of Cam others do not allow elective abortions to occur under bridge, England, any circumstances. For example, during the Reagan Correspondence to: C N, Svendsen, MRC Cambridge Centre and Bush administrations in the United States, for Brain Repair and Department of Experimental Psychology, University of Cambridge, Cambridge CB2 3EB, UK. Fax: 01223 government funding for all work on human aborted 331174. fetal tissue for transplantation was terminated. Eye (1995) 9, 233-235 © 1995 Royal College of Ophthalmologists 234 C. N. SVENDSEN AND S. B. DUNNETT However, the recent Clinton administration has control the phenotypic development of precursor reversed this ruling and several multi-centre clinical cells are still only poorly understood and are the trials of dopamine nerve cell transplants for patients subject of intense investigation. with Parkinson's disease are now under way in the Although we have found that human CNS United States. In addition to the complex moral precursor cells can be expanded in culture while issues, there are further practical concerns. Using the retaining the ability to survive and mature into nerve present techniques, only about 5-10% of dopamine cells when transplanted into the adult rat brain, a cells survive transplantation. Consequently, in order current major limitation of this method with regard to achieve good functional response, it has been to using these cells for treating Parkinson's disease is found necessary to use donor tissue from four or five the small number of dopamine neurons which fetuses implanted into each side of the brain in normally develop from the grafted precursors. patients with Parkinson's disease. The additional However, it may be possible to modify these cells requirements that the donor tissue must be fresh and in culture prior to transplantation by using gene undamaged raises substantial problems for obtaining transfer methods or specific growth factors to direct a reliable supply of donor tissue other than in very the division of neural precursors into specific lineages particular research contexts. In most hospitals for their differentiation into dopamine neurons. In around the world this is to all intents and purposes support of the viability of this strategy, recent studies impractical if not impossible. Even in those few have found that pure neuronal progenitors can be centres where it is attempted, the quality of the fetal expanded from EGF-generated precursors or pri tissue varies enormously and patients from the mary embryonic brain cultures using high concentra different centres show inconsistent levels of improve tions of fibroblast growth factor (FGF: Svendsen and ment. 10 11 Rossor, unpublished observations , ). One strategy to address these problems of In parallel to these studies on EGF-generated availability of suitable donor tissue is to use new precursors other laboratories have been using methods of growing, selecting and expanding cells in various types of immortalised cell lines which retain the test tube for subsequent transplantation. We have neuronal characteristics upon differentiation. The found that precursor cells located within the lines are generated by inserting a viral oncogene into developing rat and human embryonic brain can be developing neuronal precursors which induce the· isolated and induced to proliferate rapidly in culture cells to continue dividing indefinitely provided when exposed to epidermal growth factor (EGF) permissive conditions exist. Under non-permissive under suitable culture conditions. These pluripotent conditions these cells differentiate into neuronal-like precursors are predominantly stem cells and give rise cells and have been shown to integrate well with host to astrocytes, oligodendrocytes and neurons. 12 13 tissue following grafting. , Like the expanded Although EGF-responsive CNS precursor cells precursor cell populations, immortalised cells may have been described previously by Weiss and 7 provide an alternative source of tissue for transplan colleagues in Canada using mouse tissue, rat and tation instead of primary neuronal tissue from human precursor cells have proved more difficult to aborted fetuses. However, one caveat with immorta grow in the test tube. However, using a modified culture medium we have been able to expand human lised cell lines is the threat of tumour formation as it and rat precursor cells from the rat and human is difficult to exclude completely the possibility that S embryonic brain and have found that a proportion the viral insert may overcome its non-permissive of the cells survive transplantation into the adult rat environment in some way and return to the 9 brain and develop into mature neurons. In the proliferative state, leading to tumour formation in search for alternative neural tissue for transplanta the host brain. tion, these results may hold some promise for the In conclusion, the feasibility of neural transplanta future since these EGF-driven precursor cells can be tion to replace populations of neurons lost in human grown and proliferated in culture such that one neurological disease has been well demonstrated in human fetus can potentially give rise to millions of the early clinical trials in Parkinson's disease. The pluripotent cells which can be frozen, stored in small limitations of grafting neuronal tissue at present stem batches and distributed to hospitals where patients in large part from the need to use only embryonic are being treated. As these precursors cells are able donors, resulting in substantial moral, ethical and to develop into many different