Bilateral caudate and putamen grafts of embryonic mesencephalic tissue treated with lazaroids in Parkinson's disease

Cell therapy in Parkinson's disease

The clinical studies with intrastriatal transplants of fetal mesencephalic tissue in Parkinson's disease (PD) patients have provided proof-of-principle for the cell replacement strategy in this disorder. The grafted dopaminergic neurons can reinnervate the denervated striatum, restore regulated dopamine (DA) release and movement-related frontal cortical activation, and give rise to significant symptomatic relief. In the most successful cases, patients have been able to withdraw L-dopa treatment after transplantation and resume an independent life. However, there are currently several problems linked to the use of fetal tissue: 1) lack of sufficient amounts of tissue for transplantation in a large number of patients, 2) variability of functional outcome with some patients showing major improvement and others modest if any clinical benefit, and 3) occurrence of troublesome dyskinesias in a significant proportion of patients after transplantation. Thus, neural transplantation is still at an experimental stage in PD. For the development of a clinically useful cell therapy, we need to define better criteria for patient selection and how graft placement should be optimized in each patient. We also need to explore in more detail the importance for functional outcome of the dissection and cellular composition of the graft tissue as well as of immunological mechanisms. Strategies to prevent the development of dyskinesias after grafting have to be developed. Finally, we need to generate large numbers of viable DA neurons in preparations that are standardized and quality controlled. The stem cell technology may provide a virtually unlimited source of DA neurons, but several scientific issues need to be addressed before stem cell-based therapies can be tested in PD patients.

Positron emission tomography imaging of transplant function

In this article, the role of functional imaging for providing objective evidence that grafts of fetal tissue can survive and form connections in Parkinson's and Huntington's disease patients is reviewed. The dissociation between dopamine storage capacity, clinical improvement, and normalization of brain metabolism in PD is discussed, and possible mechanisms underlying the phenomenon of dyskinesias off medication are presented. It is concluded the positron emission tomography and single photon emission computed tomography can provide valuable ancillary information alongside clinical observations but are not currently appropriate modalities for use as surrogate endpoints.

Increased fiber outgrowth from xeno-transplanted human embryonic dopaminergic neurons with co-implants of polymer-encapsulated genetically modified cells releasing glial cell line-derived neurotrophic factor

We investigated whether a continuous supply of glial cell line-derived neurotrophic factor (GDNF) via encapsulated genetically modified cells can promote survival and fiber outgrowth from xenotransplanted human dopaminergic neurons. Cells genetically engineered to continuously secrete GDNF were confined in hollow fiber-based macrocapsules. Each hemiparkinsonian rat received either a single C2C12-hGDNF capsule (n=8) or a C2C12-control capsule (n=8) concomitantly with human embryonic ventral mesencephalic cell suspension transplants. Our results show that fiber outgrowth in the area between the capsule and the graft is more extensive in rats with GDNF-releasing capsules than in rats with control capsules. We suggest that continuous and safe delivery of GDNF to the brain could be a potential way to optimize neural transplantation as a therapy for Parkinson's disease.

Cell transplantation in Parkinson's disease: how can we make it work?

Previous open-label clinical trials have provided proof of principle that intrastriatal transplants of fetal dopaminergic neurons can induce substantial and long-lasting functional benefits in patients with Parkinson's disease. However, in two recent NIH-sponsored double-blind trials, functional improvements were only marginal and the primary endpoints were not met. Severe off-phase dyskinesias were observed in a significant proportion of the transplanted patients, raising doubts about the viability of the cell-transplantation approach. Here, we discuss the problems raised by the NIH-sponsored trials and point to several shortcomings that might explain the overall poor outcome, and we identify several crucial issues that remain to be resolved to develop cell replacement into an effective and safe therapy.

Dyskinesias and dopamine cell replacement in Parkinson's disease: a clinical perspective

Both increased and decreased dyskinesias have been reported from open label clinical trials of transplantation of human embryonic dopamine rich tissue in Parkinson's disease patients. In the first double-blind clinical transplantation trial, 15% of the grafted patients developed severe postoperative dyskinesias in the "off" phase. Since then, postoperative off-medication dyskinesias have been reported from two additional series of grafted patients. However, such dyskinesias are probably not a novel phenomenon. These dyskinesias have shown a different temporal development postoperatively compared to the antiparkinsonian graft effects, and no significant relationship with the magnitude of graft-derived dopaminergic reinnervation or symptomatic relief. However, positron emission tomography studies have indicated that an unbalanced putaminal dopaminergic function may contribute to this postoperative complication. While there is little doubt that intrastriatal grafts can induce dyskinesias, these appear to differ from common drug-induced dyskinesias. The term graft-induced dyskinesias (GID) is therefore suggested to more clearly identify this complication. While GID bear some phenomenological resemblance to biphasic drug induced dyskinesias, the mechanism(s) behind this complication remains obscure. Available data are scarce but allow for hypotheses to be generated that could (and should) be addressed in experimental animals.

Complement regulatory proteins are expressed at low levels in embryonic human, wild type and transgenic porcine neural tissue

Allotransplantation of human foetal neural tissue for neurodegenerative disorders has been shown to provide clinical benefit but is limited by a number of issues including donor supply. The use of porcine foetal tissue as an alternative source of cells is being investigated but xenotransplants survive poorly as a result of immunological rejection, which may involve complement. In this study we investigated the expression of the membrane-bound complement regulatory proteins--decay accelerating factor (DAF), membrane co-factor protein (MCP) and CD59 in embryonic neural tissue. Cells were derived from human foetuses, wild-type porcine foetuses and porcine foetuses transgenic for human complement regulatory proteins and analysed using flow cytometry and immunocytochemistry. Functional assessment of human complement regulatory protein expression in transgenic porcine tissue was assessed by C3b deposition and cell survival on exposure to human complement. Human and wild-type porcine foetal neural tissue expressed moderate levels of MCP and CD59 but low or no levels of DAF. Neural tissue from porcine foetuses transgenic for human MCP (E174) expressed the transgene but failed to significantly inhibit human C3b deposition compared with non-transgenic tissue. In contrast, foetal neural tissue from two different human DAF transgenic pig lines (A74 and E71) known to express high levels of human DAF on endothelial cells, failed to express significant levels of human DAF in foetal neural tissue. Complement regulatory proteins such as MCP and CD59 are expressed in the human and wild-type embryonic brain but in contrast, DAF is expressed at very low levels. Pigs transgenic for human DAF express very low levels of human DAF on embryonic neural tissue. In pigs transgenic for human MCP, the transgene is expressed at similar levels to that in human embryonic neural tissue but at an insufficient level to prevent activation of the complement cascade. Thus alternative approaches to reducing complement activation by xenografted neural foetal tissue will be required if this process proves to be important in the rejection process.

Stem cell therapy for Parkinson's disease

Transplantation of human fetal dopamine (DA) neurons to patients with Parkinson's disease (PD) has given proof of the principle that new neurons can survive for at least a decade, and then functionally integrate and provide significant symptomatic relief. Unfortunately, the ethical, technical, and practical limitations of using fetal DA neurons as the source for cell transplantation in PD, in combination with the development of unwanted grafting-related side effects, have put a halt to the spread of this treatment into clinical practice. Hopefully, recent advances in the fields of stem cell biology and adult neurogenesis research will lead totamen in new exciting ways to better understand and control the biological parameters necessary for achieving safe and successful neuronal replacement in PD patients.

The ability of grafted human sympathetic neurons to synthesize and store dopamine: a potential mechanism for the clinical effect of sympathetic neuron autografts in patients with Parkinson's disease

We have investigated the potential of autologous sympathetic neurons as a donor for cell therapy of Parkinson's disease (PD). Our recent study demonstrated that sympathetic neuron autografts increase the duration of levodopa-induced "on" periods with consequent reduction in the percent time spent in "off" phase. We also found that human sympathetic neurons grown in culture have the ability to convert exogenous levodopa to dopamine and to store the synthesized dopamine. This may explain the clinically observed prolongation in the duration of levodopa effects. To further analyze the mechanism for the graft-mediated effect, the present study investigated the metabolic function of human sympathetic ganglionic neurons xenografted into the dopamine (DA)-denervated striatum of rats by monitoring striatal levels of DA and its primary metabolite, 3,4-dihydroxyphenylacetic acid (DOPAC), after systemic administration of levodopa. We also explored whether the graft-mediated effect above may last in four PD patients who had been given the grafts and followed for 12-36 months postgrafting. Clinical evaluations showed that an increase in the duration of levodopa-induced "on" phase is detected during a follow-up period of 12-36 months postgrafting in all the four patients tested. Accordingly, the percent time spent in "off" phase exhibited a 30-40% reduction as compared to the pregrafting values. The animal experiment showed that a significant increase in striatal DA levels is noted after systemic levodopa treatment, and that the DA levels remain high for longer periods of time in the grafted rats than in control animals. When given reserpine pretreatment, the levodopa-induced rise of striatal DA levels was significantly attenuated with concomitant increase in DOPAC levels. Histological examinations demonstrated that the grafts contain some tyrosine hydroxylase (TH)-positive cells. These cells were also found to express aromatic-l-amino acid decarboxylase (AADC) and vesicular monoamine transporter-2 (VMAT), both of which are important molecules for the synthesis and the storage of DA, respectively. These results indicate that grafted sympathetic neurons can provide a site for both the conversion of exogenous levodopa to DA and the storage of the synthesized DA in the DA-denervated striatum, explaining a mechanism by which sympathetic neuron autografts can increase the duration of levodopa-induced "on" phase in PD patients.

Is there a future for neural transplantation?

Traditionally neural transplantation has had as its central tenet the replacement of missing neurons that have been lost because of neurodegenerative processes, as exemplified by diseases such as Parkinson disease (PD). However, the effectiveness and widespread application of this approach clinically has been limited, primarily because of the poor donor supply of human fetal neural tissue and the incomplete neurobiological understanding of the circuit reconstruction required to normalize function in these diseases. So, in PD the progress from promising neural transplantation in animal models to proof-of-principle, open-labeled clinical transplants, to randomized, placebo-controlled studies of neural transplantation has not been straightforward. The emergence of previously undescribed adverse effects and lack of significant functional advantage in recent clinical studies has been disappointing and has served to cast a new, and perhaps more realistic, perspective on this treatment approach. In fact, there have been calls by some involved in neural transplantation to return to the drawing board before pressing on with further clinical trials, and the return to basic experimentation. This therefore precipitates the question - is there a future for neural transplantation? It is important to remember that there are a number of possible explanations for the disappointing results from the recent clinical trials in PD, ranging from the mode of transplantation to patient selection. Nevertheless, almost irrespective of these reasons for the current trial results, there have always been significant practical and ethical problems with using human fetal tissue, and so a number of alternative cell sources have been investigated. These alternative sources include stem cells, which are attractive for cell-based therapies because of their potential ease of isolation, propagation and manipulation, and their ability in some cases to migrate to areas of pathology and differentiate into specific and appropriate cell types. Furthermore, the availability of stem cells derived from non-embryonic sources (e.g. adult stem cells derived from the sub-ventricular zone) has removed some of the ethical limitations associated with the use of embryonic human tissue. These potentially beneficial aspects of stem cells means that there is a future for neural transplantation as a means of treating patients with a range of neurological disorders, although whether this will ever translate into a truly effective, widely available therapy remains unknown.

In vivo monitoring of cellular transplants by magnetic resonance imaging and positron emission tomography

Cellular loss is a common pathological observation in many disease conditions. Recent evidence that these cells can be replaced has generated huge excitement over possible clinical applications. The use of stem or progenitor cells, which can differentiate into site-appropriate phenotypes required to "repair" the damaged tissue, has already demonstrated potential in animal models, but many aspects of this novel treatment strategy require further elucidation. Most importantly, the monitoring of the safety of cellular transplants in patients remains a challenge. Traditional histological methods do not address the dynamic nature of transplant-induced recovery and highlight the necessity of in vivo imaging to probe the survival, migration and functional consequences of transplanted cells. This paper reviews how non-invasive imaging technology can be used to serially assess intact living organisms in order to visualise and monitor cellular transplants.

Improving the survival of human CNS precursor-derived neurons after transplantation

We have examined the effects of predifferentiation and energy substrate deprivation on long-term expanded human neural precursor cells (HNPCs). The pre-differentiation of HNPC cultures produced large numbers of neurons (>60%) and mature glial cells capable of generating glycogen stores that protected the neuronal population from experimental metabolic stress. When predifferentiated HNPCs were transplanted into intact adult rat hippocampus, fewer cells survived compared to undifferentiated HNPC transplants. This cell death was completely attenuated, however, when predifferentiated HNPC cultures were pretreated to boost glial energy stores and resulted in greatly increased neuronal survival in vivo. The transplanted cells primarily engrafted within the granular layer of the dentate gyrus, where a large proportion of the predifferentiated HNPCs co-expressed neuronal markers whereas most HNPCs outside of the neuronal layer did not, indicating that the predifferentiated cells remained capable of responding to local cues in the adult brain. Undifferentiated HNPCs migrated more widely in the brain after grafting than did the predifferentiated cells, which generally remained within the hippocampus.

Neuroimaging in Parkinson's disease

In this review, the potential role of positron emission tomography and single photon emission computed tomography as biological markers for diagnosing and following the progression of Parkinson's disease (PD) is discussed. Their value for assessing the efficacy of putative neuroprotective agents in PD and for revealing the pharmacological changes underlying the symptomatology and complications of this disorder is also considered. It is concluded that in the future functional imaging will provide a valuable adjunct to clinical assessment when judging the efficacy of putative neuroprotective approaches to PD.

The cellular repair of the brain in Parkinson's disease—past, present and future

Damage to the central nervous system was once considered irreparable. However, there is now growing optimism that neural transplant therapies may one day enable complete circuit reconstruction and thus functional benefit for patients with neurodegenerative conditions such as Parkinson's disease (PD), and perhaps even those with more widespread damage such as stroke patients. Indeed, since the late 1980s hundreds of patients with Parkinson's disease have received allografts of dopamine-rich embryonic human neural tissue. The grafted tissue has been shown to survive and ameliorate many of the symptoms of the disease, both in the clinical setting and in animal models of the disease. However, practical problems associated with tissue procurement and storage, and ethical concerns over using aborted human fetal tissue have fuelled a search for alternative sources of suitable material for grafting. In particular, stem cells and xenogeneic embryonic dopamine-rich neural tissue are being explored, both of which bring their own practical and ethical dilemmas. Here we review the progress made in neural transplantation, both in the laboratory and in the clinic with particular attention to the development of stem cell and xenogeneic tissue based therapy.

Improvement of embryonic dopaminergic neurone survival in culture and after grafting into the striatum of hemiparkinsonian rats by CEP-1347

Transplantation of embryonic nigral tissue ameliorates functional deficiencies in Parkinson's disease (PD). A main constraint of neural grafting is the poor survival of dopaminergic neurones grafted into patients. Studies in rats indicated that many grafted neurones die by apoptosis. CEP-1347 is a mixed-lineage-kinase (MLK) inhibitor with neuroprotective action in several in vitro and in vivo models of neuronal apoptosis. We studied the effect of CEP-1347 on the survival of embryonic rat dopaminergic neurones in culture, and after transplantation in hemiparkinsonian rats. CEP-1347 and the alternative MLK inhibitor CEP-11004 significantly increased the survival of dopaminergic neurones in primary cultures from rat ventral mesencephalon and in Mn2+-exposed PC12 cells, a surrogate model of dopaminergic lethal stress. Moreover, combined treatment of the grafting cell suspension and the host animal with CEP-1347 significantly improved the long-term survival of rat dopaminergic neurones transplanted into the striatum of hemiparkinsonian rats. Also, the protective effect of CEP-1347 resulted in an increase in total graft size and in enhanced fibre outgrowth. Thus, treatment with CEP-1347 improved dopaminergic cell survival under severe stress and might be useful to improve the positive outcome of transplantation therapy in PD and reduce the amount of human tissue required.

Neural Stem Cell Therapy in the Aging Brain: Pitfalls and Possibilities

As aging progresses, there is a decline in the brain's capacity to produce new neurons in the two neurogenic regions, the subventricular zone surrounding the lateral ventricles and the subgranular layer of the hippocampal dentate gyrus. The underlying cause of the declining neurogenesis is unknown, but is presumably related to age-related changes that occur during normal aging of the brain. It is exacerbated by age-related neurodegenerative diseases such as Alzheimer's and Parkinson's diseases. Stem cell-based therapy to replace lost and/or damaged cells in the aging brain is currently the focus of intense research. The two most promising approaches involve transplantation of exogenous tissue and promoting proliferation of endogenous cells. However, age-related changes in the brain environment, including elevated oxidative stress and accumulation of protein and lipid by-products, present several unique challenges that must be addressed before cell-based therapy can be used as a viable option. Although progress has been made toward replacement of lost cells and recovery of lost function, there are fundamental issues that need to be addressed for stem cell therapy to be successful in the aging brain. In this review, we focus on recent progresses made toward understand the biology of neural stem cells in the aging brain, as well as progress toward using stem cells to replace cells lost during disease.

A double-blind controlled trial of bilateral fetal nigral transplantation in Parkinson's disease

Thirty-four patients with advanced Parkinson's disease participated in a prospective 24-month double-blind, placebo-controlled trial of fetal nigral transplantation. Patients were randomized to receive bilateral transplantation with one or four donors per side or a placebo procedure. The primary end point was change between baseline and final visits in motor component of the Unified Parkinson's Disease Rating Scale in the practically defined off state. There was no significant overall treatment effect (p = 0.244). Patients in the placebo and one-donor groups deteriorated by 9.4 +/- 4.25 and 3.5 +/- 4.23 points, respectively, whereas those in the four-donor group improved by 0.72 +/- 4.05 points. Pairwise comparisons were not significant, although the four-donor versus placebo groups yielded a p value of 0.096. Stratification based on disease severity showed a treatment effect in milder patients (p = 0.006). Striatal fluorodopa uptake was significantly increased after transplantation in both groups and robust survival of dopamine neurons was observed at postmortem examination. Fifty-six percent of transplanted patients developed dyskinesia that persisted after overnight withdrawal of dopaminergic medication ("off"-medication dyskinesia). Fetal nigral transplantation currently cannot be recommended as a therapy for PD based on these results.

Stem cells for neurodegenerative disorders: where can we go from here?

The use of stem cells in cell replacement therapy for neurodegenerative diseases has received a great deal of scientific and public interest in recent years. This is due to the remarkable pace at which paradigm-changing discoveries have been made regarding the neurogenic potential of embryonic, fetal, and adult cells. Over the last decade, clinical fetal tissue transplants have demonstrated that dopaminergic neurons can survive long term and provide functional clinical benefits for patients with Parkinson's disease. Pluripotent embryonic stem cells and multipotent neural stem cells may provide renewable sources that could replace these primary fetal grafts. Considerable advancement has been made in generating cultures with high numbers of neurons in general and of dopaminergic neurons using a varied array of techniques. However, much of this encouraging progress still remains to be tested on long-term expanded human cultures. Further problems include the low survival rate of these cells following transplantation and the tumorigenic tendencies of embryo-derived cells. However, pre-differentiation or genetic modification of stem cell cultures prior to transplantation may help lead to the generation of high numbers of cells of the desired phenotype following grafting. Boosting particular factors or substrates in the culture media may also protect grafted neurons from oxidative and metabolic stress, and provide epigenetic trophic support. Possible endogenous sources of cells for brain repair include the transdifferentiation of various types of adult cells into neurons. Despite the excitement generated by examples of this phenomenon, further work is needed in order to identify the precise instructive cues that generate neural cells from many other tissue types, and whether or not the new cells are functionally normal. Furthermore, issues such as cell homogeneity and fusion need to be addressed further before the true potential of transdifferentiation can be known. Endogenous stem cells also reside in the neurogenic zones of the adult brain (ventricle lining and hippocampus). Further elucidation of the mechanisms that stimulate cell division and migration are required in order to learn how to amplify the small amount of new cells generated by the adult brain and to direct these cells to areas of injury or degeneration. Finally, a more fundamental understanding of brain injury and disease is required in order to circumvent local brain environmental restrictions on endogenous cell differentiation and survival.

The Hedgehog signaling pathway--implications for drug targets in cancer and neurodegenerative disorders

The Hedgehog (Hh) pathway is a highly conserved signaling cascade involved in many developmental processes. Among others, these include patterning of the ventral neural tube and establishment of left-right asymmetry of the embryo. Additionally, the pathway regulates the development of numerous tissues and cell types. Mutations in elements of the pathway are associated with congenital diseases and defects, and ectopic Hh signaling activity is implicated in the development of a number of neoplasms. While little is known of Hh signaling function in the adult organism, a role of the pathway in maintenance of adult organs and cell types, including several neuronal subtypes in the central nervous system, is beginning to emerge. Elements of the Hh pathway are therefore potential drug targets for the treatment of cancers and degenerative diseases like Parkinson's disease, and the recent isolation of synthetic molecules capable of modulating the activity of the Hh cascade through a direct interaction with elements of the pathway is promising.

High efficacy of clonal growth and expansion of adult neural stem cells

Neural stem cells (NSCs) from the adult central nervous system are currently being investigated for their potential use in autologous cell replacement strategies. High expansion rates of NSCs in culture are crucial for the generation of a sufficient amount of cells needed for transplantation. Here, we describe efficient growth of adult NSCs in Neurobasal medium containing B27 supplement under clonal and low-density conditions in the absence of serum or conditioned medium. Expansion of up to 15-fold within 1 week was achieved on low-density NSC cultures derived from the lateral ventricle wall, the hippocampal formation, and the spinal cord of adult rats. A 27% single-cell cloning efficiency in Neurobasal/B27 combination further demonstrates its growth-promoting ability. Multipotency and nontumorgenicity of NSCs were retained despite the high rate of culture expansion. In addition, increased cell survival was obtained when Accutase, instead of trypsin, was used for enzymatic dissociation of NSC cultures. This work provides an important step toward the development of standardized protocols for highly efficient in vitro expansion of NSCs from the adult central nervous system to move more closely to the clinical use of NSCs.

Effects of antioxidant pretreatment on the survival of embryonic dopaminergic neurons in vitro and following grafting in an animal model of Parkinson's disease

The effect of pretreating cell suspensions of embryonic rat ventral mesencephala (VM) with antioxidant combinations on the survival of dopaminergic (DA) neurons was studied in vitro and following transplantation into the unilateral 6-hydroxydopamine (6-OHDA)-lesioned rat model of Parkinson's disease. The in vitro experiments examined the effects of two thiol antioxidants, N-acetyl-L-cysteine (NAC) and reduced glutathione (GSH), and a member of the lazaroid family of 21-aminosteroids, U-83836E, singly and in combination, on survival of DA neurons derived from dissociated E14 rat VM tissue. For in vivo studies, cell suspensions were pretreated with combinations of NAC, GSH, and U-83836E prior to transplanting into 6-OHDA-lesioned rats to investigate whether DA neuron survival could be further improved. NAC, GSH, and U-83836E individually increased DA neuron survival in vitro and a combination of all three resulted in the greatest survival. In vivo, pretreatment with U-83836E alone resulted in a significantly greater reduction in amphetamine-induced rotation 6 weeks postgrafting compared with a control group receiving nontreated graft tissue. This functional effect correlated with a significant improvement in DA neuron survival 6 weeks postgrafting. The thiol combination pretreatment of NAC and GSH, and the triple combination of NAC, GSH, and U-83836E, however, failed to improve both functional recovery and DA neuron survival when compared with the nontreated control grafts.

Reconstructive neurosurgery for Parkinson's disease: a systematic review and preliminary meta-analysis

This systematic review and meta-analysis aimed to identify the determinants for best practice and establish current benchmarks for recovery following reconstructive neurosurgery for people with Parkinson's disease. Eleven studies reporting results for 95 grafted patients were selected on the grounds of using optimal surgical techniques and the Core Assessment Program for Intracerebral Transplantation (CAPIT) protocol for data collection. Consistent trends demonstrating high levels of recovery were identified on most outcome measures. Determinants for best practice were identified as selecting younger patients; using low dose immunosuppression; bilateral grafting; and employing strategies to ensure the quantity and viability of the grafted cells. Secondary analysis of data demonstrated a correlation of rho=0.666 (P<0.05) between increases in striatal dopaminergic activity and UPDRS Motor (off) scores. Overall effect size 'd' was found to be 1.129 UPDRS Motor (off) condition and 0.719 for UPDRS Total (off) condition. The design of the studies and the variable standards for reporting the data precluded the use of more powerful and accurate meta-analyses. It was recommended that the creation of a collaborative database would improve the extraction of data and allow for more powerful statistical analyses for evaluating the overall harm and benefits associated with reconstructive neurosurgery.

Comparison between survival of lazaroid-treated embryonic nigral neurons in cell suspensions, cultures and transplants

Death of transplanted dopaminergic neurons is induced both during preparation of donor tissue and after intrastriatal grafting. Oxidative stress is thought to be partly responsible for this cell death. In the present study we compared the effects of three lipid peroxidation inhibitors, the lazaroids Tirilazad mesylate, U-83836E and U-101033, on survival of embryonic mesencephalic neurons in different paradigms. The lazaroids were equally potent in preventing serum deprivation-induced death of cultured dopaminergic neurons. In a second set of experiments, mesencephalic suspensions were pretreated with lazaroids and cell survival was analyzed immediately after dissociation, after 2 or 24 h in culture or after intrastriatal transplantation. Lazaroid pretreatment failed to protect mesencephalic neurons in the in vitro paradigms and U-101033E did not protect grafted dopaminergic neurons in contrast to the neuroprotective effects previously reported for U-83836E and Tirilazad. Pretreatment with the iron chelator deferoxamine mesylate did not protect cultured or grafted dopaminergic neurons, nor did it improve neuronal survival in the serum deprivation model. U-83836E and U-101033E, but not Tirilazad, prevented cell death induced by the pro-oxidant tert-butyl hydroperoxide in suspensions. In a final experiment, we found that systemic treatment of the graft recipient rat with Tirilazad mesylate (before and during the first 3 days after grafting) improved survival of transplanted dopaminergic neurons to 180% of control values. Our results show that systemic treatment with a lipid peroxidation inhibitor for 3 days can promote graft survival, but also highlights the poor correlation between neuroprotective effect of pharmacological compounds in vitro and in grafts.

Role of cell therapy in Parkinson disease

Clinical studies involving intrastriatal transplantation of embryonic mesencephalic tissue in patients with Parkinson disease (PD) have provided proof-of-principle for the cell replacement strategy in this disorder. The grafted dopaminergic neurons can reinnervate the denervated striatum, restore regulated dopamine release and movement-related frontal cortical activation, and produce significant symptomatic relief. In the most successful cases, patients have been able to withdraw from levodopa treatment after undergoing transplantation and resume an independent life. There are, however, several problems linked to the use of primary embryonic tissue: 1) lack of sufficient amounts of tissue for transplantation in a large number of patients; 2) variability of functional outcome (major improvement in some and modest if any clinical benefit in others); and 3) occurrence of troublesome dyskinesias in a significant proportion of patients after transplantation. Thus, neural transplantation is still at an experimental stage in the treatment of PD. For the development of a clinically useful cell therapy we need to define better criteria for patient selection and how graft placement should be optimized in each individual. Most importantly, we need to generate large numbers of viable dopamine neurons in preparations that are standardized and quality controlled. Stem cells could be useful as an unlimited source of dopamine neurons. Thus far, neurons with at least some dopaminergic characteristics have been generated from stem cells. In most cases, however, their survival after grafting in animals has been poor, and it is also unclear if they function as normal dopamine neurons. Several scientific issues need to be addressed before stem cell-based therapies can be tested in PD patients.

Functional imaging of the dopamine system: in vivo evaluation of dopamine deficiency and restoration

Dopamine deficiency causes a severe impairment in motor function in patients with Parkinson's disease (PD) and in experimental animal models. Recent developments in neuroimaging techniques provide a means to assess in vivo the state of the dopamine system. From a functional perspective, four levels need to be operative and integrated in the system: the dopamine cell (pre-synaptic), the striatal dopamine receptors (post-synaptic), adequate release of dopamine (intra-synaptic), and the cortico-subcortical motor projections. Neuroimaging functional methods can be used to estimate, at these four levels, dopamine cell degeneration, adaptive responses to injury and, importantly, the effect of therapeutic interventions. In this respect, data from functional imaging studies at clinical and pre-clinical stages, support the idea that cell replacement therapy might achieve a more physiological restoration of the dopamine motor system than other therapies (such as ablative surgery, administration of precursor, deep brain stimulation) that currently are equally or more effective in relieving motor symptoms.

Cellular replacement therapy for Parkinson's disease--where we are today?

The concept of replacing lost dopamine neurons in Parkinson's disease using mesencephalic brain cells from fetal cadavers has been supported by over 20 years of research in animals and over a decade of clinical studies. The ambitious goal of these studies was no less than a molecular and cellular "cure" for Parkinson's disease, other neurodegenerative diseases, and spinal cord injury. Much research has been done in rodents, and a few studies have been done in nonhuman primate models. Early uncontrolled clinical reports were enthusiastic, but the outcome of the first randomized, double blind, controlled study challenged the idea that dopamine replacement cells can cure Parkinson's disease, although there were some significant positive findings. Were the earlier animal studies and clinical reports wrong? Should we give up on the goal? Some aspects of the trial design and implantation methods may have led to lack of effects and to some side effects such as dyskinesias. But a detailed review of clinical neural transplants published to date still suggests that neural transplantation variably reverses some aspects of Parkinson's disease, although differing methods make exact comparisons difficult. While the randomized clinical studies have been in progress, new methods have shown promise for increasing transplant survival and distribution, reconstructing the circuits to provide dopamine to the appropriate targets and with normal regulation. Selected promising new strategies are reviewed that block apoptosis induced by tissue dissection, promote vascularization of grafts, reduce oxidant stress, provide key growth factors, and counteract adverse effects of increased age. New sources of replacement cells and stem cells may provide additional advantages for the future. Full recovery from parkinsonism appears not only to be possible, but a reliable cell replacement treatment may finally be near.

AAV2 vectors mediate efficient and sustained transduction of rat embryonic ventral mesencephalon

The success of transplantation of human embryonic mesencephalic tissue to treat parkinsonian patients is limited by the poor survival of the transplant. We show that an AAV2 vector mediates efficient expression of the egfp reporter gene in organotypic cultures of freshly explanted solid fragments of rat embryonic ventral mesencephalon (VM). We observed early and sustained transgene expression (4 days to > or = 6 weeks). Furthermore, rAAV-infected rat embryonic VM transplanted in the adult striatum continued to express EGFP for > or = 3 months. More than 95% of the transduced cells were neurons. Dopaminergic neurons were transduced at low frequency at earlier time points. This method of gene delivery could prove useful to achieve local, continuous secretion of neurotrophic factors at physiologically relevant doses to treat Parkinson's disease.

Antioxidant compounds protect dopamine neurons from death due to oxidative stress in vitro

Using tissue culture models of oxidative stress caused by serum deprivation or MPTP/MPP+ toxicity, the present study establishes that the antioxidants epigallocatechin gallate, lazaroids U74389G and U83836E, reservatrol, MnTBAP, MCI 186, trolox, and melatonin protect 68-100% of dopamine (DA) neurons from cell death. In contrast, the nitric oxide inhibitor LY83583, the caspase inhibitors Z-VAD-FMK, Ac-DQMD-CHO and Z-DEVD-FMK, and the CDK-5 inhibitor, roscovotine were not neuroprotective, although death was often delayed by 1 day in vitro. We conclude that antioxidants are more effective at preventing cell death in vitro than are inhibitors at later stages in the death cascade.

Alpha Gal is widely expressed in embryonic porcine stem cells and neural tissue

Fetal porcine neural xenografts are an alternative to human fetal tissue for cell based treatments of a number of neurodegenerative conditions but are currently limited by host immunological rejection. The expression of a major immunological epitope, Galalpha1-3Galbeta1-4GlcNAcbeta-R (alphaGal) was determined on stem cells and primary cells derived from E26 porcine fetal brains. alphaGal was detected on the majority of neural stem cells and cells from primary cell suspensions. The expression of this epitope paralleled the binding of human IgG and IgM to the cells, a binding that was significantly reduced with anti-alphaGal depleted human serum. This study demonstrates that alphaGal expression is extensive in embryonic porcine neural cells and will be of relevance to any clinical trials using this tissue.

Simultaneous intrastriatal and intranigral fetal dopaminergic grafts in patients with Parkinson disease: a pilot study. Report of three cases

The main neural transplantation strategy in Parkinson disease (PD) has been focused on reinnervating the striatum. The clinical results reported in patients who receive transplants have been limited and do not justify the use of neural transplantation as a routine therapeutic procedure for PD. Identifying the optimal target for transplantation may be one of the critical factors for optimizing clinical outcomes. Evidence from preclinical studies indicates that simultaneous intrastriatal and intranigral grafts (double grafts) may produce a more complete functional recovery. The authors report the clinical and positron emission tomography (PET) scanning results in three patients enrolled in a safety and feasibility pilot study who received double grafts and who have been followed for up to 13 months posttransplantation. Patients included in the study had idiopathic PD. All patients underwent detailed assessments before and after surgery, in accordance with the Core Assessment Program for Intracerebral Transplantation. The patients received implants of fetal mesencephalic cell suspensions in the putamen and substantia nigra (SN) bilaterally. There were no intraoperative or perioperative complications. Follow-up PET scans demonstrated an increase in the mean fluorodopa uptake constant values in the putamen and SN 12 months postsurgery. Improvements were also noted in the total Unified Parkinson's Disease Rating Scale, Hoehn and Yahr, Schwab and England, and pronation/supination scores after transplantation. The authors demonstrate the feasibility of reinnervating the SN and striatum by using a double transplant strategy in humans.

Sustained delivery of GDNF: towards a treatment for Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disease characterized by the progressive loss of nigral dopaminergic neurons. Although symptomatic therapies to substitute for the missing neurotransmitter dopamine are efficient at the early stages of the disease, the goal is to find alternative therapies which could protect dopaminergic neurons from the degenerative process. We have used two distinct gene therapy approaches to deliver the neuroprotective molecule glial cell line-derived neurotrophic factor (GDNF) in animal models of the disease: (i) an encapsulated genetically engineered cell line releasing GDNF (ex vivo gene therapy); and (ii) a lentiviral vector encoding the GDNF gene (in vivo gene therapy). Both approaches allowed protection of nigral dopaminergic neurons against lesion-induced cell death in rodent as well as monkey models of PD. Behavioral symptoms were also ameliorated in these animals. In addition, co-transplantation of embryonic dopaminergic neuronal grafts and a GDNF-releasing capsule allowed improvement of graft survival and differentiation, thereby accelerating behavioral recovery. These results should lead to clinical application in the near future.

Parkinson disease: etiology, pathogenesis and future of gene therapy

Parkinson disease (PD) is a progressive neurological disorder with a prevalence of 1-2% in people over the age of 50. It has a world-wide distribution and has no gender preference. The neurological hallmark of PD is the presence of Lewy bodies and is characterized by the degeneration of nigrostriatal dopaminergic neurons. The causes of PD are unknown but considerable evidence suggests a multifactorial etiology involving genetic and environmental factors. A molecular genetic approach identified three genes and at least two additional loci in rare familial forms of PD. Two of these genes are involved in the ubiquitin mediated pathway of protein degradation and the third one is a highly expressed protein in the synaptic terminal and is called alpha-synuclein. In animal models, it has been shown that use of the household pesticide which is known to contain rotenone, causes PD. Thus, a combined action of genetic and environmental factors is responsible for the pathogenesis of PD. Although use of levodopa or dopamine agonists can substantially reduce clinical symptoms, and transplantation of fetal nerve tissue still remains as an alternative therapy (although it has been recently shown to be having no overall benefit), directed delivery of glial cell derived neurotrophic factor (known to have trophic effects on dopaminergic neurons) may also be a beneficial therapeutic option for PD patients.

Cell survival and clinical outcome following intrastriatal transplantation in Parkinson disease

Intrastriatal transplantation of embryonic dopaminergic neurons is currently explored as a restorative cell therapy for Parkinson disease (PD). Clinical results have varied, probably due to differences in transplantation methodology and patient selection. In this review, we assess clinical trials and autopsy findings in grafted PD patients and suggest that a minimum number of surviving dopaminergic neurons is required for a favorable outcome. Restoration of [18F]-fluorodopa uptake in the putamen to about 50% of the normal mean seems necessary for moderate to marked clinical benefit to occur. Some studies indicate that this may require mesencephalic tissue from 3-5 human embryos implanted into each hemisphere. The volume, density and pattern of fiber outgrowth and reinnervation, as well as functional integration and dopamine release. are postulated as additional important factors for an optimal clinical outcome. For neural transplantation to become a feasible therapeutic alternative in PD, graft survival must be increased and the need for multiple donors of human embryonic tissue substantially decreased or alternate sources of donor tissue developed. Donor cells derived from alternative sources should demonstrate features comparable to those associated with successful implantation of human embryonic tissue before clinical trials are considered.

Anatomic and biochemical correlates of the dopamine transporter ligand 11C-PE2I in normal and parkinsonian primates: comparison with 6-[18F]fluoro-L-dopa

Positron emission tomography (PET) coupled to 6-[18F]Fluoro-L-Dopa (18F-Dopa) remains the gold standard for assessing dysfunctionality concerning the dopaminergic nigrostriatal pathway in Parkinson's disease and related disorders. The use of ligands of the dopamine transporters (DAT) is an attractive alternative target; consequently, the current aim was to validate one of them, 11C-PE2I, using a multiinjection modeling approach allowing accurate quantitation of DAT densities in the striatum. Experiments were performed in three controls, three MPTP-treated (parkinsonian) baboons, and one reserpine-treated baboon. 11C-PE2I B'max values obtained with this approach were compared with 18F-Dopa input rate constant values (Ki), in vitro Bmax binding of 125I-PE2I, and the number of dopaminergic neurons in the substantia nigra estimated postmortem by stereology. In the caudate nucleus and putamen, control values for 11C-PE2I B'max were 673 and 658 pmol/mL, respectively, whereas it was strongly reduced in the MPTP-treated (B'max = 26 and 36 pmol/mL) and reserpine-treated animals (B'max = 338 and 483 pmol/mL). In vivo 11C-PE2I B'max values correlated with 18F-Dopa Ki values and in vitro 125I-PE2I Bmax values in the striatum and with the number of nigral dopaminergic neurons. Altogether, these data support the use of 11C-PE2I for monitoring striatal dopaminergic disorders and the effect of potential neuroprotective strategies.

Parkinson's disease: surgical options

The introduction of levodopa revolutionized the treatment of Parkinson's disease. However, complications of therapy that diminish functional capacity eventually develop in the majority of patients. Studies in animal models have demonstrated that the parkinsonian state is associated with overactivity in the output nuclei of the basal ganglia. This provides a rationale for surgically targeting these nuclei to diminish this overactivity and reestablish a more balanced output (compensatory strategy). Lesioning and high-frequency stimulation of either the pallidum or the subthalamic nuclei are effective, but many questions remain regarding what surgery is best. Even more questions remain regarding the place of a restorative strategy, namely implantation of fetal midbrain tissue to replace the missing dopamine cells and "cure" the disease. Practical, ethical, and legal issues that complicate the use of human tissue have encouraged initial attempts at xenotransplantation using porcine fetal tissue.

Effects of immunosuppressive treatment on host responses against intracerebral porcine neural tissue xenografts in rats

BACKGROUND

Embryonic xenogeneic neural tissue is an alternative for transplantation in Parkinson's disease, but immune responses limit the application. The aims of this study were to enhance the in vitro viability rates by donor tissue pretreatment; to compare the efficacy of cyclosporine A (CsA) and tacrolimus (FK) in inhibiting xenograft rejection in rats; to evaluate additional inductive therapy with prednisolone (PRE) or mycophenolate mofetil (MMF).

METHODS

Tirilazad (a lipid peroxidase inhibitor) or FK and acYVAD-cmk (a caspase inhibitor), were added to embryonic porcine ventral mesencephalic tissue and viability was assessed in vitro. Tirilazad-treated tissue was grafted to the striatum of rats that were either left untreated or immunosuppressed with FK (1 mg/kg) or CsA (15 mg/kg) alone or in combination with a 2-week PRE (20 mg/kg) or MMF (40 mg/kg) induction course. Xenograft survival and host responses were determined using immunohistochemistry.

RESULTS

Pretreatment with tirilazad enhanced tissue survival in vitro. After transplantation into untreated controls, there was no graft survival at twelve weeks. Neural cell counts were significantly improved in immunosuppressed recipients, but there were no differences between the treatment groups. Additional inductive treatment reduced the infiltration with CD4+ and CD8+ cells, and macrophage infiltration was reduced compared with animals given CsA or FK alone.

CONCLUSION

Pretreatment of the donor tissue with free-radical scavengers reduces cell loss caused by tissue trauma. Porcine neural tissue xenografts survive significantly better in animals immunosuppressed with either FK or CsA. Additional inductive treatment with PRE or MMF reduced the infiltration of host cells into the xenografts.

Normalization of markers for dopamine innervation in striatum of MPTP-lesioned miniature pigs with intrastriatal grafts

As part of the DaNeX study, the uptake and binding of several positron emitting tracers was recorded in brain of healthy Göttingen minipigs, in minipigs with a syndrome of parkinsonism due to MPTP intoxication, and in parkinsonian minipigs which had received intrastriatal grafts of mesencephalic neurons from fetal pigs. The specific binding of [11C]NS 2214 to catecholamine uptake sites was reduced by two thirds in striatum of the intoxicated animals, while the rate constant for the decarboxylation of [18F]fluorodopa was reduced by 50% in the intoxicated animals. Several months after grafting, both pre-synaptic markers of dopamine fibres were normal in striatum. Dopamine depletion or grafting were without effect on the cerebral perfusion rate, measured with [15O]-water, did not alter the rate of oxygen metabolism (CMRO2) in brain, and did not alter the binding potential of tracers for dopamine D1 or D2 receptors in pig striatum. However, the grafting was associated with a local increase in the binding of [11C]PK 11195, a tracer for reactive gliosis, suggesting that an immunological reaction occurs at the site of graft, which might potentially have reduced the graft patency. However, this apparent immunological response did not preclude the re-establishment of normal [18F]fluorodopa and [11C]NS 2214 uptake in the allografted striatum.

Cell therapy in Parkinson's disease - stop or go?

The results of the first double-blind placebo-controlled trial using grafts of embryonic tissue to treat Parkinson's disease have aroused widespread interest and debate about the future of cell replacement therapies. What are the key issues that need to be resolved and the directions in which this technology is likely to develop?

Monitoring neuroprotection and restorative therapies in Parkinson's disease with PET

In this article the role of functional imaging (PET and SPECT) as a surrogate marker for following the progression of Parkinson's disease (PD) is discussed. The potential value of PET and SPECT for assessing the efficacy of putative neuroprotective agents in PD is considered and a review of 18F-dopa PET findings in transplantation trials involving implantation of human and procine fetal mesencephalic tissue is presented. It is concluded that functional imaging provides a valuable adjunct to clinical assessment when judging the efficacy of neuroprotective and restorative approaches to PD.

Cell therapy and transplantation in Parkinson's disease

Transplanted human fetal dopamine neurons can reinnervate the striatum in patients with Parkinson's disease (PD). Recent findings using positron emission tomography indicate that the grafts are functionally integrated and restore dopamine release in the patient's striatum. The grafts can exhibit long-term survival without immunological rejection and despite an ongoing disease process and continuous antiparkinsonian drug treatment. In the most successful cases, patients have been able to withdraw L-dopa treatment after transplantation and resume an independent life. About two-thirds of grafted patients have shown clinically useful, partial recovery of motor function. The major obstacle for the further development of this cell replacement strategy is that large amounts of human fetal mesencephalic tissue are needed for therapeutic effects. Stem cells hold promise as a virtually unlimited source of self-renewing progenitors for transplantation. The possibility to generate dopamine neurons from such cells is now being explored using different approaches. However, so far the generated neurons have survived poorly after transplantation in animals.