Cell Therapy for Neurological Disorders : a Comprehensive Review

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Acta neurol. belg., 2005, 105, 158-170

Cell therapy for neurological disorders : a comprehensive review

Robrecht RAEDT and Paul BOON Laboratory for Clinical and Experimental Neurophysiology, Department of Neurology, Ghent University Hospital, Ghent, Belgium

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Abstract system has weak capabilities for both endogenous Neurodegenerative diseases are characterized by the cell replacement and pattern repair. The reason for irreversible loss of neurons involved in networks, impor- this defective self repair is that adult neuronal cells tant for specific physiological functions. At present, cannot regenerate after being damaged and that several renewable cell sources stand in line to replace endogenous neural stem cells have only a very fetal brain cells as potential cell source for transplanta- limited potential to generate new neuronal cells to tion in the damaged brain. Recent developments raise replace degenerated neurons. Therefore there is the hope that selective populations of different neuronal great interest in restoring the damaged nervous sys- phenotypes could be made “on demand”. However, for tem by stimulating endogenous repair or by trans- every potential cell source there are still a lot of ques- planting new cells into the damaged brain. These tions and drawbacks, which need to be resolved before a cells can be selected on the base of their phenotype, cell source could become the standard for clinical neu- the neurotransmitter they release or by the way ronal transplantation. The recent finding that the brain responds to damage by increased endogenous neuro- they are genetically engineered. Before cell thera- genesis could prelude new “neurothrophic therapies”, py can be a routinely done practice in the clinic, a based on stimulating this endogenous repair. From pre- lot of questions will have to be answered by preclinical studies it is evident that different disease mech- clinical research. At this moment different cell anisms require different cell therapy approaches, sources are tested for their potential to mediate depending on the underlying factor of the disease, the functional repair of brain damage. The goal of this identity of neuronal systems that are involved and the review is to critically evaluate the potential of dif- complexity of networks that are affected. In this review ferent candidate cell sources for transplantation. the potential of different cell sources, including the The possibility of stimulating endogenous self endogenous progenitor cells, are discussed. Also results repair will be discussed. Three selected neurode- of preclinical and clinical transplantation studies in generative diseases will be presented and the three different disease models are critically evaluated. progress and possibilities of cell therapy will be Key words : Neurodegenerative diseases ; fetal brain ; discussed. stem cell ; cell therapy ; stroke ; Parkinson’s disease ; epilepsy. Cell sources

Introduction FETAL BRAIN TISSUE

Current therapies for neurodegenerative diseases Most studies in neurodegenerative diseases have provide effective symptomatic relief, particularly used fetal brain tissue for implantation. Cells are in early stages of the disease. However, there are isolated at a time point on which the cells, that have too few therapies, if any, that affect the underlying to be implanted, are already fully differentiated in disease processes. Therefore disease-modifying the appropriate cell type. There is a critical time therapies that halt, slow down or reverse disease window for the isolation of the population of neu- progression are sorely needed. Some of the possi- rons for implantation. If the relevant neurons are ble treatment options would be : immunological too young, they are not yet differentiated. If they responses, neurotrophic or anti-apoptotic treat- are too old, they have developed extensive connec- ment, gene therapy and cell therapy. Replacement tions so that dissection involves axotomy and trau- of the lost cells seems to be a vital step for func- ma. This optimal time window, however, varies tional repair of the brain damage, since in most between different neuronal populations (Dunnett cases the spared systems cannot replace the and Bjorklund, 1992). This implies that a lot of function of the lost cells. In contrast to other fetal brains are necessary to obtain sufficient tissue mammalian tissues the adult mammalian nervous to be implanted and mostly only one neuronal CELL THERAPY FOR NEURODEGENERATIVE DISEASES 159 phenotype can be isolated from one single fetus. 2003 ; Yandava et al., 1999). However, precursors isolated from adult telencephalon and propagated STEM CELLS as neurospheres generate disappointingly few neurons, both in transplantation paradigms as well as The ethical and practical problems around fetal in differentiating conditions in vitro (Fricker et al., tissue transplantation have led to the search for 1999 ; Song et al., 2002a). Also the kind of differ- alternative cell sources. Stem cells seem to be ideal entiated cell types that they can generate is limited candidates for transplantation. Stem cells are depending upon the developmental stage and broadly defined as progenitor cells which produce region from which they are isolated and the in vitro differentiated progeny and are capable to self- conditions in which they are grown thereafter renew (Morrison et al., 1997). Stem cells could (Hack et al., 2004 ; Horiguchi et al., 2004 ; Parmar become an almost unlimited source for the genera- et al., 2002). tion of specific neurons. The cell preparations could be standardized and quality-controlled with Embryonic stem cells respect to viability and purity. Different types of stem cells could be used for neuronal transplanta- Embryonic stem cells are also an attractive cell tion. source for transplantation into the damaged brain. These cells are truly pluripotent and have an unlim- Neural stem cells ited capacity for in vitro expansion. The cells can easily be genetically manipulated. Several differen- Neural stem cells (NSCs) can be isolated from tiation protocols have already been developed for different regions of the embryonic central nervous differentiation of embryonic stem cells towards system (CNS) or from restricted areas in the adult neurons and neuronal-restricted precursors brain. Technical advances in recent years, includ- (Carpenter et al., 2001 ; Gokhan and Mehler, 2001 ; ing the use of bromodeoxyuridine (BrdU) and Kim et al., 2002 ; Li et al., 1998 ; Mujtaba et al., retroviral reporter mitotic labeling, have shown 1999 ; O’Shea, 2001 ; Okabe et al., 1996 ; Strubing that the hippocampal dentate gyrus and the fore- et al., 1995 ; Temple, 2001 ; Westmoreland et al., brain subventricular zone (SVZ), with a rostral 2001 ; Wichterle et al., 2002). ES cell-derived neur- migratory stream (RMS) of neuroblasts towards al precursors incorporate into the CNS and differ- the olfactory bulbs, are germinative regions in entiate into neurons and glia (Brustle et al., 1997 ; which neurogenesis is ongoing throughout life McDonald et al., 1999 ; Zhang et al., 2001). (Cameron et al., 1993 ; Lois et al., 1996 ; Lois and Electrophysiological studies have demonstrated Alvarez-Buylla, 1994 ; van Praag et al., 2002). It is that transplanted embryonic derived neurons presumed that this ongoing neurogenesis is an inte- (ESNs) display electrophysiological properties sim- gral part of ongoing plasticity in the adult mam- ilar to endogenous cells (Kim et al., 2002). malian brain. NSCs have been isolated from rodent Embryonic stem cell-derived glial precursors central nervous system (Galli et al., 2003 ; Galli et (ESGPs), have been used successfully for myelin al., 2003 ; Gobbel et al., 2003 ; Gritti et al., 1999 ; repair (Brustle et al., 1999 ;Liu et al., 2000) and dye Kim et al., 2003 ; Palmer et al., 1995 ; Palmer et coupling studies showed that the ESGP-derived al., 1997 ; Palmer et al., 1999 ; Reynolds and astrocytes formed gap junctions with each other but Weiss, 1996 ; Seaberg and van der Kooy, 2002 ; also with host astrocytes after transplantation in Shihabuddin et al., 1997 ; Shihabuddin et al., 2000 hippocampal slices (Scheffler et al., 2003). ; Temple and Alvarez-Buylla, 1999 ; Toda et al., Although embryonic stem cells seem to have an 2000 ; Vicario-Abejon et al., 2000 ; Weiss et al., unrestricted potential to differentiate towards neu- 1996 ; Weiss, 1999) and human brain (Akiyama et roectodermal phenotypes, embryonic stem cells al., 2001 ; Flax et al., 1998 ; Fricker et al., 1999 ; cannot be readily transplanted into the brain. Nunes et al., 2003 ; Svendsen et al., 1999 ; Because of the enormous random in vitro differenti- Svendsen and Caldwell, 2000 ;Vescovi et al., ation potential of embryonic stem cells, any remain- 1999). NSCs are defined by three main characteris- ing non-neural (Tabar and Studer, 2002) pluripotent tics : they can self-renew, give rise to all of the embryonic stem cell could give rise to teratomas major neural cells types, i.e. neurons, oligodendro- upon transplantation, resulting in significant con- cytes and astrocytes (Song et al., 2002b) and when cerns as to the clinical safety of this approach. When transplanted into the brain they are able to survive, ES cells are transplanted into the striatum of an ani- migrate and integrate in a functionally active way mal model for PD, they differentiate into a signifi- (Auerbach et al., 2000 ; Englund et al., 2002 ; Flax cant number of dopamine neurons but the incidence et al., 1998 ; Gage et al., 1995). When NSC are of ES-mediated tumor formation in this study was transplanted into the damaged brain, they migrate high (20%) (Bjorklund et al., 2002). preferentially towards the damaged areas, where they also seem to integrate and replace the lost cells Adult non-neuronal somatic stem cells (Barker and Dunnett, 1999 ; Bjorklund et al., 2002 ; Dziewczapolski et al., 2003 ; Pluchino et al., Several recent reports suggest that adult somatic 160 R. RAEDT AND P. BOON stem cells isolated from non-neuronal tissues may (Safford et al., 2002) and stem cells derived “transdifferentiate” across tissue lineage boundaries, from the dermis of mammalian skin (Toma et al., thus offering an accessible source for therapeutic 2001). applications even for neural tissue repair. Human and animal bone marrow (BM) transplantation BIO-ENGINEERED CELLS studies have shown that donor derived neurons and glial cells can be found in the brain of the host Cells can be genetically engineered to overcome (Brazelton et al., 2000 ; Eglitis and Mezey, 1997 ; problems such as senescence or to induce cells to Mezey et al., 2000 ; Mezey et al., 2003). However, release neurotrophic or neuromodulating factors. the number of these “transdifferentiated” cells is For example, neuroepithelial precursor cells, extremely low and recent works have demonstrat- derived from defined regions and prior to their ter- ed that donor BM cells contribute to adult Purkinje minal mitosis, have been infected with a retrovirus neurons through cell fusion (Alvarez-Dolado et al., encoding a temperature sensitive immortalizing 2003 ; Weimann et al., 2003). This is in contrast to oncogene. When transplanted into the intact brain, another study which demonstrated that human most of these cell lines will differentiate towards hematopoietic cells could contribute to long term neurons, astrocytes and oligodendrocytes. They adult human neuropoiesis without fusing (Cogle et even seem to respond to local microenvironmental al., 2004). It seems that fusion as well as transdif- cues, since the cells differentiate with morpholo- ferentiation can explain the presence of donor- gies indistinguishable from those of local endoge- derived cells in the brain of the recipient. Also puri- nous neurons (Martinez-Serrano and Bjorklund, fied mesenchymal stem cells, isolated from the 1997 ; Whittemore and Onifer, 2000). These bone marrow, seem to be capable of differentiating immortalized cell lines have been utilized in a vari- in vitro (Black and Woodbury, 2001 ; Deng et al., ety of ex vivo gene therapy experiments, in which 2001 ; Dezawa et al., 2004 ; Kohyama et al., 2001 ; they have been genetically modified in order to Rismanchi et al., 2003 ; Sanchez-Ramos et al., release different disease modifying molecules. As 2000 ; Sanchez-Ramos, 2002 ; Woodbury et al., an example NGF-secreting cells from the HiB5 cell 2000 ; Woodbury et al., 2002) and in vivo (Chopp line have been implanted into the adult rat striatum. and Li, 2002 ; Kopen et al., 1999) towards cells One week after transplantation a stroke was expressing neuronal and glial markers. Expression induced by middle cerebral artery occlusion. The of neuronal and glial markers, on the contrary, can- graft prevented striatal degeneration of both pro- not be seen as an absolute proof of neuronal differ- jection neurons and cholinergic interneurons entiation since it has been demonstrated that undif- (Andsberg et al., 1998). Different other growth fac- ferentiated mesenchymal stem cells also express tor-, neurotransmitter- or metabolite-releasing markers for neural lineage (Woodbury et al., 2002). immortalized cell lines have been created by genet- Moreover only one study has been able to demon- ic engineering. For example, cell lines releasing strate that MSC can differentiate towards neurons brain derived neurotrophic factor (BDNF) (Rubio displaying appropriate electrophysiological charac- et al., 1999) ; neurotrophin 3 (Liu et al., 1999) ; teristics (Kohyama et al., 2001). In addition to neurotransmitters, such as GABA (Eaton et al., hematopoietic and MSC stem cells, rare pluripotent 1999) ; or metabolites, such as b-glucuronidase stem cell subsets have been isolated from BM. A (Snyder et al., 1995) have been developed. Next to rare cell, called multipotent adult progenitor cell these immortalized cell lines other cell sources (MAPC), has been co-isolated with mesenchymal have been engineered to release disease-modifying stem cells and is able to differentiate towards cells substances. Commonly used cell types are fibrob- from the endodermal, mesodermal and ectodermal lasts (Blesch et al., 2001 ; Liu et al., 2002 ; Pizzo phenotypes (Jiang et al., 2002). This MAPC cell is et al., 2004 ; Tobias et al., 2003) and stem cells capable of differentiating toward cells with mor- (Arnhold et al., 2003 ; Behrstock and Svendsen, phological and electrophysiological properties of 2004 ; Zhao et al., 2004). midbrain neurons (Jiang et al., 2003). Recently a new pluripotent, CD45 negative population from STIMULATING ENDOGENOUS REPAIR human cord blood, termed unrestricted somatic stem cells (USSCs), has been described (Kogler et The finding that there is ongoing neurogenesis in al., 2004). It has been demonstrated that these cells dentate gyrus of the hippocampus and the forebrain can be differentiated towards neuronal cell types. SVZ, has led to the idea that stimulation of neuro- Implantation of these cells in rat brain revealed that genesis could enhance endogenous brain repair. human tau-positive neurons persisted in the rat There is some suggestion that neurogenesis also brain for up to 3 months. In this study, though, no can exist in other brain regions such as the neocor- electrophysiological experiments were done to tex (Gould et al., 2001 ; Magavi et al., 2000), the confirm that the cells were indeed functionally amygdala (Bernier et al., 2002) and the substantia active neurons. Other cells that display a presumed nigra (Zhao et al., 2003). These findings are con- neurogenic potential are adipose-derived stem cells troversial, however, (Koketsu et al., 2003 ; CELL THERAPY FOR NEURODEGENERATIVE DISEASES 161

Kornack and Rakic, 2001) and if neurogenesis more extensive cell death of different neuronal exists in these regions it is probably at much lower phenotypes throughout the brain. In the next chap- degree or may only be induced after insults ter three different disease models of an increasing (Mohapel and Brundin, 2004). Evidence from in complexity are presented and the possibilities for vivo studies suggests that specific growth and neu- developing cell therapy are evaluated. rotrophic factors influence neural precursor proliferation in the adult rodent dentate gyrus and SVZ, REPLACING SINGLE NEURONAL PHENOTYPES : and in some cases in other brain regions such as PARKINSON’S DISEASE (PD) striatum, thalamus, hypothalamus, septum and parenchymal regions lining the ventricles. These CNS diseases affecting specific neuronal cell factors include basic fibroblast growth factor populations are Parkinson’s disease (PD, loss of (bFGF), insulin growth factor-1 (IGF-1), epidermal striatal dopaminergic neurons), Huntington’s dis- growth factor (EGF), vascular endothelial groth ease (HD ; loss of GABAergic striatal spiny pro- factor (VEGF) and cilliary neurotrophic factor jection neurons) and amyotrophic lateral sclerosis (GDNF) (Aberg et al., 2000 ; Benraiss et al., 2001 (ALS, loss of cholinergic motorneurons). These ; Emsley and Hagg, 2003 ; Kuhn et al., 1997 ; neurodegenerative diseases are the most attractive Schanzer et al., 2004 ; Wagner et al., 1999b). ones to be treated with cell therapy and therefore a Several lines of evidence suggest that astrocytes considerable amount of research has been done to play important roles in the migration, differentia- investigate the possibilities of repair by cell trans- tion, integration and survival of neuroblasts plantation. The reader is referred to excellent derived from SVZ and dentate gyrus. (Doetsch et reviews of these studies (Bjorklund and Lindvall, al., 1999 ; Galli et al., 2003 ; Lim and Alvarez- 2000 ; Isacson, 2003 ; Lindvall et al., 2004). In this Buylla, 1999 ; Song et al., 2002a). Because astro- review only progress in cell therapy for PD will be cytes are activated by most brain insults, they are discussed. In PD there is specific loss of the major- most likely also involved in injury-induced neuro- ity of midbrain dopaminergic neurons projecting genesis. towards the striatum. Clinical trials for transplanta- A lot of work has been done on damaged tion of human embryonic mesencephalic tissue into induced neurogenesis in several models of stroke. the striatum of patients with severe Parkinson’s dis- Two recent reports indicate that forebrain SVZ ease have shown that neuronal replacement can neurogenesis increases ispilateral to the infarct work in the human brain. The grafted neurons sur- after adult rat transient middle cerebral artery vive and reïnnervate the striatum for as long as 10 occlusion (tMCAO) (Arvidsson et al., 2002 ; years despite an ongoing disease process Parent et al., 2002). The neuroblasts generated after (Kordower et al., 1995 ; Piccini et al., 1999). These stroke form chains closely apposed to astrocytes open trials have shown that after transplantation that extend from the SVZ to the injured striatum dopamine release is elevated and clinical benefit although it seems that only a small portion of the becomes evident (Piccini et al., 2000). A systemat- newly formed striatal neurons survive. When selec- ic review of 11 studies reporting 95 graft studies tive damage is induced to the hippocampal CA1 was made by Polgar et al., 2003. Two double blind region, by inducing transient four vessel ischemia sham surgery-controlled trials, however, showed in rats, and subsequently bFGF and EGF are no statistically significant improvement in behav- infused for three days in the first week after stroke, ioral score. It seems that the outcome of transplan- 40 % of the CA1 pyramidal neurons are regenerat- tation is dependent on the age of the donor, the ed. The source for the newly generated neurons is severity of the disease (Freed et al., 2001 ; Olanow demonstrated to be the SVZ in the posterior et al., 2003) and the variation in composition of the periventricular region (Nakatomi et al., 2002). graft. Several studies reported the occurrence of Transient global ischemia in young adult macaque dyskinesias as an important side effect of trans- monkeys also induces a significant postischemic plantation, which became troublesome in 7-15% of increase of the number of newly formed cells in the grafted patients (Freed et al., 2001 ; Hagell et al., hippocampal dentate gyrus, subventricular zone of 2002 ; Olanow et al., 2003). These rather disap- the temporal horn of the lateral ventricle and tem- pointing results and the occurrence of dyskinesias, poral neocortex (Tonchev et al., 2003). next to the limited tissue availability and the wide variation in functional outcome, impelled the search for alternative sources from which large Cell therapy for different neuronal disease numbers of dopaminergic neurons can be generat- mechanisms ed. Several recent publications provide a good review of the different studies in which dopaminer- It seems that the potential of cell therapy to gic differentiation of several types of stem cells restore neuronal damage mostly depends on the was investigated (Bjorklund and Lindvall, 2000 ; complexity of the disease. This ranges from focal Brundin and Hagell, 2001 ; Lindvall, 2003 ; cell death of only one neural or glial phenotype to Lindvall et al., 2004 ; Lindvall and Hagell, 2002 ; 162 R. RAEDT AND P. BOON

Lindvall and McKay, 2003). Functionally active caused by occlusion of a cerebral artery and leads dopaminergic neurons can be generated from to irreversible damage in a core region, which is mouse (Kim et al., 2002 ; Morizane et al., 2002) surrounded by a zone of partially reversible injury, and monkey embryonic stem cells (ECSs) the penumbra zone. The majority of cases with (Kawasaki et al., 2002) and from neural stem cells stroke in humans are caused by occlusion of the (NSCs) derived from the fetal rodent (Carvey et al., middle cerebral artery, which leads to infarction in 2001 ; Wagner et al., 1999a ; Yan et al., 2001) and the cerebral cortex, basal ganglia and internal cap- human brain (Storch et al., 2001), using different sule. In the only reported clinical trial, neurons neuronal differentiation protocols. However, up to generated from the human teratocarcinoma cell line now there is only one report describing differentia- NT-2 have been implanted in the infarcted area of tion of adult neural stem cells towards dopaminer- patients, who had experienced a stroke in the basal gic neurons (Daadi and Weiss, 1999). Also there is ganglia. Behavioral improvements were seen in little evidence that functional dopaminergic neu- some patients (Kondziolka et al., 2000) and autop- rons can be obtained from non-neural stem cells. sy in one patient revealed the presence of grafted One study described differentiation of mesenchy- cell expressing neuronal markers 2 years after mal stem cells towards functionally active grafting (Nelson et al., 2002). Next to this human dopaminergic neurons but when these cells where trial, cells from different origins (fetal cortical and transplanted into the diseased brain they did not striatal tissue, neural precursor cells, cell lines with differentiate towards neurons (Jiang et al., 2003 ; neurogenic potential, bone marrow stromal cells) Zhao et al., 2002). Dopaminergic neurons derived have been transplanted in different affected regions from stem cells have been transplanted into in the brain (cortex, striatum), in the ventricles or Parkinson’s models and in some cases clear behav- intravenously (Lindvall et al., 2004 ; Savitz et al., ioral recovery could be demonstrated (Lindvall, 2002). In most cases the transplanted cells survived 2003). and a partial behavioral recovery could be seen. However, in few studies there is evidence for a CELL THERAPY FOR DISEASES AFFECTING MULTIPLE functional integration of these cells into the dam- BRAIN REGIONS AND NEURONAL PHENOTYPES aged networks. It is possible that transplantation may enrich the local neural environment through Probably the most difficult to treat are diseases region-specific synaptic connections and trophic where transplanted cells should be able to generate factors. Alternatively, grafts may upregulate multiple phenotypes and reform long distance con- endogenous recovery mechanisms and induce sur- nections such as in the case of cerebral ischemic viving cells to establish new circuits. insults (Rossi and Cattaneo, 2002) and epilepsy (Grisolia, 2001). Epilepsy Epilepsy has many etiologies, all leading to an Cerebral ischemic insults imbalance between excitation and inhibition. There are two main types of ischemic insults that Unlike in the two other disease mechanisms pre- affect the brain in a specific way. First, cardiac sented so far, there is no identifiable defect to be arrest or coronary artery occlusion causes an abrupt restored by cell therapy. Nevertheless, in temporal and near-total interruption of total cerebral blood lobe epilepsy (TLE) there is a common lesion : hip- flow. This global ischemia causes selective neu- pocampal sclerosis (Blumcke et al., 1999 ; Liu et ronal death of certain vulnerable neuronal popula- al., 1995). Hippocampal sclerosis is characterized tions such as the pyramidal neurons of CA1 hip- by a selective loss of hippocampal neurons, axonal pocampal subregion. In the case of global sprouting and dense gliosis. However, it is still ischemia, fetal hippocampal CA1 tissue and condi- unproven whether seizures are a cause or an effect tionally immortalized neuroepithelial MHP36 cells of hippocampal sclerosis. Grafting of fetal hip- have been transplanted into the damaged CA1 pocampal tissue for repair of hippocampal net- region. In the case of transplantation of fetal CA1 works in the intrahippocampal kainic acid model tissue behavioral recovery is dependent on the for TLE led to the partial reversal of some of the establishment of some afferent and efferent con- characteristic anatomopathological changes of hip- nections. In the case of MHP36 cells there was also pocampal sclerosis, such as mossy fiber sprouting a behavioral improvement but only a small portion and loss of GABAergic interneurons. (Shetty et al., of the grafted cells displayed neuronal or glial 2000 ; Shetty and Turner, 1996 ; Shetty and Turner, markers. So it remains unclear whether behavioral 1997a ; Shetty and Turner, 1997b ; Shetty and recovery was caused by restoration of functional Turner, 2000 ; Zaman et al., 2000 ; Zaman and connectivity or by secretion of trophic substances Shetty, 2001 ; Zaman and Shetty, 2003). A major (Sinden et al., 1995 ; Sinden et al., 1997 ; Virley et caveat in these studies is that the authors have not al., 1999). investigated the influence of transplantation on the The second type of ischemic insult, stroke, is occurrence of epileptic seizures (personal commu- CELL THERAPY FOR NEURODEGENERATIVE DISEASES 163 nication, Ashok Shetty, 2002). Another transplanta- icant loss of the transplanted cells and the seizure tion strategy consists of grafting neurotransmitter suppressant effect. Embryonic stem cell derived releasing cells to modulate network excitability. glial cells have been engineered for adenosine When GABA-rich fetal striatal tissue is transplant- delivery (Fedele et al., 2004). These cells still have ed into the substantia nigra (SN) of fully amygdala to be transplanted into an epilepsy model but it is kindled rats this leads to a significant increase in expected that the survival of these glial cells will be the threshold to electrically evoke focal discharges greater compared to the kidney and fibroblast cells, (after discharge threshold [ADT]) and a significant which will probably lead to a more long term reduction of seizure severity (Loscher et al., 1998). seizure suppressant effect. However, this seizure-suppressing effect was only transient and disappeared over the weeks after Conclusion transplantation. Noradrenaline-rich locus coeruleus (LC) tissue has been transplanted in the damaged From the evaluation of different cell sources for hippocampus of status epilepticus models. Grafting transplantation it is evident that grafting of fetal led to a reduction of the number of spontaneous cells will not become the standard to treat neurode- seizures from (Bortolotto et al., 1990). But if the generative diseases because of ethical and practical transplanted rats were subjected to kindling stimu- problems and the high diversity in functional out- lations approximately eight months after transplan- come after transplantation. Embryonic and neural tation, no difference in afterdischarge threshold and stem cells are good alternatives for fetal tissue, kindling rate could be demonstrated (Holmes et al., given that we learn more about the mechanisms 1991). Next to neurotransmitter rich fetal brain tis- involved in control of cell proliferation and differ- sue, cells have been engineered to release agents entiation, neuronal integration and survival. for the inhibition of in vivo seizure activity. Genetic engineering provides a tool to modify the Thompson et al. engineered conditionally immor- cells in favor of their survival, integration and their talized mouse neurons to deliver GABA by driving capacity to modify underlying disease mecha-

GAD65 expression under the control of a tetracy- nisms. Other strategies for reconstruction of dam- cline regulatable promoter (Thompson et al., aged networks could be based on the stimulation of 2000). This cell line has been transplanted into the endogenous neurogenesis and repair by means of SNr (Thompson et al., 2000) or the pyriform cortex modulating neurotrophic mechanisms controlling (Gernert et al., 2002) of rats prior to kindling. In both. Another option could be to combine cell ther- both cases the transplantation had only weak apy with neurothrophic treatment in order to maxi- effects on ADT and kindling rate. These GABA mize the recruitment of newborn but also trans- releasing cells have also been transplanted in the planted cells. The cell therapy strategy for a given lithium pilocarpine status epilepticus model for disease highly depends on the complexity of the TLE, which displays spontaneous seizures. The disorder. In a disease such as PD, where there is animals were transplanted into the anterior SN 45- selective loss of dopaminergic neurons, the ulti- 65 days after SE (Thompson and Suchomelova, mate goal is to replace the lost cells, repair connec- 2004). Seven to 10 days after transplantation a tivity and normalize neurotransmitter release. That robust suppression of seizures and the reduction in is why lots of efforts are made to selectively gener- epileptiform spikes emerged in the group that was ate dopaminergic cells from different cells sources. transplanted with GABA releasing cells. The eval- In more complex disorders, such as stroke and uation of the seizure suppressant effect of GABA epilepsy, reconstructive therapy seems to be much releasing transplants was ended 13 days after trans- further away and therefore other strategies seem to plantation, while it would have been interesting to be appropriate in first instance. In stroke, partial investigate whether this anticonvulsant effect was recovery after transplantation sometimes occurs long lasting. without functional integration of transplanted cells. Adenosine and its analogues also have powerful Therefore neurotrophic responses of both donor antiseizure and neuroprotective activities and host cells, evoked by the transplantation itself, (Fredholm, 1997 ; Lee et al., 1984). Therefore may play an important role. Transplantation of baby hamster kidney cells have been engineered to cells, engineered to secrete neurotrophic factors, release adenosine in the environment by inactivat- could be a first option in the treatment of stroke. In ing of the adenosine metabolizing enzyme adeno- epilepsy most successes can be expected by trans- sine kinase (ADK). These adenosine-releasing planting cells, which secrete seizure suppressant cells have been encapsulated and transplanted into agents or neurotransmitters, in brain structures that the ventricles of the rat kindling model of epilepsy are presumed to play key roles in the generation or (Huber et al., 2001). After transplantation of the spread of epileptic seizures (Aberg et al., 2000 ; cells, behavioral seizure activity was almost com- Benraiss et al., 2001 ; Emsley and Hagg, 2003 ; pletely suppressed during four days after transplan- Kuhn et al., 1997 ; Schanzer et al., 2004 ; Wagner tation. This strong protection lasted for three weeks et al., 1999b). after transplantation after which there was a signif- 164 R. RAEDT AND P. BOON

Acknowledgements Neuroscience, 2000, 3 : 537-544. BJORKLUND L. M., SANCHEZ-PERNAUTE R., CHUNG S., Robrecht Raedt is supported by a grant from the ANDERSSON T., CHEN I. Y., MCNAUGHT K. S., Institute for Encouragement of Innovation through BROWNELL A. L., JENKINS B. G., WAHLESTEDT C., Science and Technology in Flanders (IWT). Paul Boon KIM K. S., ISACSON O. Embryonic stem cells is a Senior Clinical Investigator of the FWO-Flanders develop into functional dopaminergic neurons and is supported by grants 1.5236.99 and 6.0324.02 after transplantation in a Parkinson rat model. from the FWO-Flanders ; by grant 01105399 from BOF Proc. Natl. Acad. Sci. USA, 2002, 99 : 2344-9. and by the Clinical Epilepsy Grant, Ghent University BLACK I. B., WOODBURY D. Adult rat and human bone Hospital 2005. marrow stromal stem cells differentiate into neurons. Blood Cells Mol. Dis., 2001, 27 : 632-6. REFERENCES BLESCH A., CONNER J. M., TUSZYNSKI M. H. 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