Short- and long-term survival and function of unilateral intrastriatal dopaminergic grafts in Parkinson's disease

Human ES and iPS cells as cell sources for the treatment of Parkinson's disease: current state and problems

Cell therapy using human embryonic stem cells (hESCs) is a promising therapeutic option for Parkinson's disease (PD), an incurable neurodegenerative disease. A prerequisite for clinical application of hESCs for PD is an efficient and strict differentiation of hESCs into midbrain dopamine (mDA) neuron-like cells, which would be directly translated into high effectiveness of the therapy with minimum risk of undesirable side effects. Due to fruitful efforts from many laboratories, a variety of strategies for improving efficiency of dopaminergic differentiation from hESCs have been developed, mostly by optimizing culture conditions, genetic modification, and modulating intracellular signaling pathways. The rapid advances in the fields of dopaminergic differentiation of hESCs, prevention of tumor formation, and establishment of safe human induced pluripotent stem cells (hiPSCs) would open the door to highly effective, tumor-free, and immune rejection-free cell therapy for PD in the near future.

Serotonergic neurons mediate dyskinesia side effects in Parkinson's patients with neural transplants

Troublesome involuntary movements in the absence of dopaminergic medication, so-called off-medication dyskinesias, are a serious adverse effect of fetal neural grafts that hinders the development of cell-based therapies for Parkinson's disease. The mechanisms underlying these dyskinesias are not well understood, and it is not known whether they are the same as in the dyskinesias induced by l-dopa treatment. Using in vivo brain imaging, we show excessive serotonergic innervation in the grafted striatum of two patients with Parkinson's disease, who had exhibited major motor recovery after transplantation with dopamine-rich fetal mesencephalic tissue but had later developed off-medication dyskinesias. The dyskinesias were markedly attenuated by systemic administration of a serotonin [5-hydroxytryptamine (5-HT)] receptor (5-HT(1A)) agonist, which dampens transmitter release from serotonergic neurons, indicating that the dyskinesias were caused by the serotonergic hyperinnervation. Our observations suggest strategies for avoiding and treating graft-induced dyskinesias that result from cell therapies for Parkinson's disease with fetal tissue or stem cells.

Lewy body pathology in fetal grafts

Although fetal nigral transplants have been shown to survive grafting into the striatum, increased [(18)F]6-fluroro-L-3,4-dihydroxyphenylalanine ((18)F-DOPA) uptake and improved motor function in open-label assessments have failed to establish any clinical benefits in double-blind, sham-controlled studies. To understand morphological and neurochemical alterations of grafted neurons, we performed postmortem analyses on six Parkinson's disease (PD) patients who had received fetal tissue transplantation 18-19 months, 4 years, and 14 years previously. These studies revealed robust neuronal survival with normal dopaminergic phenotypes in 18-month-old grafts and decreased dopamine transporter and increased cytoplasmic alpha-synuclein in 4-year-old grafts. We also found a decline of both dopamine transporter and tyrosine hydroxylase and the formation of Lewy body-like inclusions in 14-year-old grafts, which stained positive for alpha-synuclein and ubiquitin proteins. These pathological changes suggest that PD is an ongoing process that affects grafted cells in the striatum in a manner similar to how resident dopamine neurons are affected in the substantia nigra.

Open-labeled study of unilateral autologous bone-marrow-derived mesenchymal stem cell transplantation in Parkinson's disease

Parkinson's disease (PD) is a progressive neurodegenerative disease for which stem cell research has created hope in the last few years. Seven PD patients aged 22 to 62 years with a mean duration of disease 14.7+/-7.56 years were enrolled to participate in the prospective, uncontrolled, pilot study of single-dose, unilateral transplantation of autologous bone-marrow-derived mesenchymal stem cells (BM-MSCs). The BM-MSCs were transplanted into the sublateral ventricular zone by stereotaxic surgery. Patients were followed up for a period that ranged from 10 to 36 months. The mean baseline "off" score was 65+/-22.06, and the mean baseline "on" score was 50.6+/-15.85. Three of 7 patients have shown a steady improvement in their "off"/"on" Unified Parkinson's Disease Rating Scale (UPDRS). The mean "off" score at their last follow-up was 43.3 with an improvement of 22.9% from the baseline. The mean "on" score at their last follow-up was 31.7, with an improvement of 38%. Hoehn and Yahr (H&Y) and Schwab and England (S&E) scores showed similar improvements from 2.7 and 2.5 in H&Y and 14% improvement in S&E scores, respectively. A subjective improvement was found in symptoms like facial expression, gait, and freezing episodes; 2 patients have significantly reduced the dosages of PD medicine. These results indicate that our protocol seems to be safe, and no serious adverse events occurred after stem-cell transplantation in PD patients. The number of patients recruited and the uncontrolled nature of the trial did not permit demonstration of effectiveness of the treatment involved. However, the results encourage future trials with more patients to demonstrate efficacy.

How useful are stem cells in PD therapy?

Parkinson's disease (PD) is one of the most common neurodegenerative disorders. The onset of PD is usually after the age of 50. Clinical symptoms of PD are not manifested until 60-80% of dopaminergic neurons in the midbrain have been affected. Cell replacement has been a promising approach for the treatment of PD. Fetal mesencephalic dopaminergic neurons seemed to improve the motor disability in patients in some early studies. However, the clinical application of this approach may be limited by ethical and logistic concerns, as well as by side effects. On the other hand, embryonic stem (ES) cells are promising candidates because of their ability to provide an unlimited supply of specific cell types, their accessibility to genetic modifications, and their broad developmental potentials. Transplants of undifferentiated ES cells were able to proliferate and fully differentiate into dopaminergic neurons in a rodent PD model. One of the concerns though is the risk of tumor formation. The tumorigenic potential of ES cells seems to be greatly reduced when cells are predifferentiated into dopaminergic neurons in vitro before implantation. Recent developments in the induction of pluripotent stem cells from somatic adult cells provide a tremendous opportunity for this field. Initial success has been reported in a rodent PD model using iPS cells (induced pluripotent stem cells). However, whether this initial result can be successfully translated into human clinical studies still needs to be determined.

Cell Therapy From Bench to Bedside Translation in CNS Neurorestoratology Era

Recent advances in cell biology, neural injury and repair, and the progress towards development of neurorestorative interventions are the basis for increased optimism. Based on the complexity of the processes of demyelination and remyelination, degeneration and regeneration, damage and repair, functional loss and recovery, it would be expected that effective therapeutic approaches will require a combination of strategies encompassing neuroplasticity, immunomodulation, neuroprotection, neurorepair, neuroreplacement, and neuromodulation. Cell-based restorative treatment has become a new trend, and increasing data worldwide have strongly proven that it has a pivotal therapeutic value in CNS disease. Moreover, functional neurorestoration has been achieved to a certain extent in the CNS clinically. Up to now, the cells successfully used in preclinical experiments and/or clinical trial/treatment include fetal/embryonic brain and spinal cord tissue, stem cells (embryonic stem cells, neural stem/progenitor cells, hematopoietic stem cells, adipose-derived adult stem/precursor cells, skin-derived precursor, induced pluripotent stem cells), glial cells (Schwann cells, oligodendrocyte, olfactory ensheathing cells, astrocytes, microglia, tanycytes), neuronal cells (various phenotypic neurons and Purkinje cells), mesenchymal stromal cells originating from bone marrow, umbilical cord, and umbilical cord blood, epithelial cells derived from the layer of retina and amnion, menstrual blood-derived stem cells, Sertoli cells, and active macrophages, etc. Proof-of-concept indicates that we have now entered a new era in neurorestoratology.

Neural grafting in Parkinson's disease unraveling the mechanisms underlying graft-induced dyskinesia

The development of neural transplantation as a treatment for Parkinson's disease has been compromised by a lack of functional efficacy and the appearance of transplant-induced motor side-effects in some patients. Since the first reports of these graft-induced dyskinesias (GID), and the realization of their impact on the progress of the field, a great deal of experimental work has been performed to determine the underlying cause(s) of this problematic side-effect. In this review we describe the clinical phenomenon of GID, explore the different representations of GID in rodent models, and examine the various hypotheses that have been postulated to be the cause. Based on the available clinical and preclinical data we outline strategies to avoid GID in future clinical trials using fetal cell transplants or cell preparations derived from stem cells.

Dopamine cell implantation in Parkinson's disease: long-term clinical and (18)F-FDOPA PET outcomes

We have previously reported the results of a 1-y double-blind, placebo-controlled study of embryonic dopamine cell implantation for Parkinson's disease. At the end of the blinded phase, we found a significant increase in putamen uptake on (18)F-fluorodopa ((18)F-FDOPA) PET reflecting the viability of the grafts. Nonetheless, clinical improvement was significant only in younger (age < or = 60 y) transplant recipients, as indicated by a reduction in Unified Parkinson's Disease Rating Scale (UPDRS) motor scores.

METHODS

We now report long-term clinical and PET outcomes from 33 of the original trial participants who were followed for 2 y after transplantation and 15 of these subjects who were followed for 2 additional years. Longitudinal changes in UPDRS motor ratings and caudate and putamen (18)F-FDOPA uptake were assessed with repeated-measures ANOVA. Relationships between these changes over time were evaluated by the analysis of within-subject correlations.

RESULTS

We found that UPDRS motor ratings declined over time after transplantation (P < 0.001). Clinical improvement at 1 y was relatively better for the younger transplant recipients and for men, but these age and sex differences were not evident at longer-term follow-up. Significant increases in putamen (18)F-FDOPA uptake were evident at all posttransplantation time points (P < 0.001) and were not influenced by either age or sex. Posttransplantation changes in putamen PET signal and clinical outcome were significantly intercorrelated (P < 0.02) over the course of the study. Image analysis at the voxel level revealed significant bilateral increases in (18)F-FDOPA uptake at 1 y (P < 0.001) in the posterior putamen engraftment sites. PET signal in this region increased further at 2 and 4 y after engraftment. Concurrently, this analysis disclosed progressive declines in radiotracer uptake in the nonengrafted caudate and ventrorostral putamen. Clinical improvement after transplantation correlated with the retention of PET signal in this region at the preoperative baseline.

CONCLUSION

These results suggest that clinical benefit and graft viability are sustained up to 4 y after transplantation. Moreover, the dependence of clinical (but not imaging) outcomes on subject age and sex at 1 y may not persist over the long term. Last, the imaging changes reliably correlate with clinical outcome over the entire posttransplantation time course.

Neuronal cell replacement in Parkinson's disease

Transplantation of foetal dopamine neurons into the striatum of Parkinson's disease patients can provide restoration of the dopamine system and alleviate motor deficits. However, cellular replacement is associated with several problems. As with pharmacological treatments, cell therapy can lead to disabling abnormal involuntary movements (dyskinesias). The exclusion of serotonin and GABA neurons, and enrichment of substantia nigra (A9) dopamine neurons, may circumvent this problem. Furthermore, although grafted foetal dopamine neurons can survive in Parkinson's patients for more than a decade, the occurrence of Lewy bodies within such transplanted cells and reduced dopamine transporter and tyrosine hydroxylase expression levels indicate that grafted cells are associated with pathology. It will be important to understand if such abnormalities are host- or graft induced and to develop methods to ensure survival of functional dopamine neurons. Careful preparation of cellular suspensions to minimize graft-induced inflammatory responses might influence the longevity of transplanted cells. Finally, a number of practical and ethical issues are associated with the use of foetal tissue sources. Thus, future cell therapy is aiming towards the use of embryonic stem cell or induced pluripotent stem cell derived dopamine neurons.

Prospects of stem cell therapy for replacing dopamine neurons in Parkinson's disease

In Parkinson's disease (PD), the main pathology is a loss of nigrostriatal dopamine (DA) neurons. Clinical trials with intrastriatal transplantation of human embryonic mesencephalic tissue have shown that grafted DA neurons reinnervate the striatum, restore striatal DA release and, in some patients, induce major clinical benefit. Stem cells could provide an unlimited source of DA neurons for transplantation. Recent studies demonstrate that cells with properties of mesencephalic DA neurons can be produced from stem cells of different sources including reprogrammed somatic cells. However, as we discuss here, it remains to be shown that these cells can provide efficient functional reinnervation and behavioral recovery in animal PD models. Moreover, a clinically competitive cell therapy for PD will require better criteria for patient selection, improved functional efficacy of grafts by a tailor-made transplantation procedure providing optimum repair of the patient's DA system and strategies to prevent dyskinesias and tumor formation.

Implantation of c-mycER TAM immortalized human mesencephalic-derived clonal cell lines ameliorates behavior dysfunction in a rat model of Parkinson's disease

Human neural stem cells offer the hope that a cell therapy treatment for Parkinson's disease (PD) could be made widely available. In this study, we describe two clonal human neural cell lines, derived from two different 10-week-old fetal mesencephalic tissues and immortalized with the c-mycER(TAM) transgene. Under the growth control of 4-hydroxytamoxifen, both cell lines display stable long-term growth in culture with a normal karyotype. In vitro, these nestin-positive cells are able to differentiate into tyrosine hydroxylase (TH)-positive neurons and are multipotential. Implantation of the undifferentiated cells into the 6-OHDA substantia nigral lesioned rat model displayed sustained improvements in a number of behavioral tests compared with noncell-implanted, vehicle-injected controls over the course of 6 months. Histological analysis of the brains showed survival of the implanted cells but no evidence of differentiation into TH-positive neurons. An average increase of approximately 26% in host TH immunoreactivity in the lesioned dorsal striatum was observed in the cell-treated groups compared to controls, with no difference in loss of TH cell bodies in the lesioned substantia nigra. Further analysis of the cell lines identified a number of expressed trophic factors, providing a plausible explanation for the effects observed in vivo. The exact mechanisms by which the implanted human neural cell lines provide behavioral improvements in the PD model are not completely understood; however, these findings provide evidence that cell therapy can be a potent treatment for PD acting through a mechanism independent of dopaminergic neuronal cell replacement.

Manufacturing neurons from human embryonic stem cells: biological and regulatory aspects to develop a safe cellular product for stroke cell therapy

Demographic trends, particularly those related to longer life expectancy, suggest that the demand for tissue and organ transplants will further increase since many disorders result from degeneration, injury or organ failure. The most urgent problem in transplantation medicine is the shortage or lack of suitable donor organs and tissue, leading to ethical and societal problems such as organ trafficking. The discovery of stem cells in the inner cell mass of developing embryos and in adult tissue has revolutionized the medical field by introducing new therapeutic dimensions to consider for previously untreatable diseases and injuries. The unlimited self-renewal ability and pluripotent capacity to become any cell type of the organism make human embryonic stem cells (hESCs) a compelling source of cells to study tissue histogenesis and to apply in a wide array of tissue engineering, cell transplantation therapy and drug discovery applications. In this article, we will focus on hESCs and address the derivation of therapeutic neural stem cell lines from hESCs, as well as the biological and regulatory aspects to developing a safe cellular product for stroke cell therapy.

Neural stem cells: mechanisms of fate specification and nuclear reprogramming in regenerative medicine

Recently, intense interest in the potential use of neural stem cells (NSC) in the clinical therapy of brain disease and injury has resulted in rapid progress in research on the properties of NSC, their innate and directed differentiation potential and the induced reprogramming of differentiated somatic cells to revert to a pluripotent NSC-like state. The aim of this review is to give an overview of our current operational definitions of the NSC lineage, the growing understanding of extrinsic and intrinsic mechanisms, including heritable but reversible epigenetic chromatin modifications that regulate the maintenance and differentiation of NSC in vivo, and to emphasize ground-breaking efforts of cellular reprogramming with the view to generating patient-specific stem cells for cell replacement therapy. This is set against a summary of current practical procedures for the isolation, research and application of NSC, and of the state of the art in NSC-based regenerative medicine of the nervous system. Both provide the backdrop for the translation of recent findings into innovative clinical applications, with the hope of increasing the safety, efficiency and ethical acceptability of NSC-based therapies in the near future.

Gene and cell delivery to the degenerated striatum: status of preclinical efforts in primate models

Significant progress has been achieved in developing restorative neurosurgical strategies for movement disorders on the basis of preclinical gene and cell therapy experiments in primates. Because of the unique similarities between human and primate anatomy and physiology, experiments in primate models are the critical step in translating these innovative neurosurgical treatment concepts into successful human applications. To clarify progress toward this goal, we have examined recent preclinical data regarding the delivery of gene and cell therapy to the lesioned primate striatum. Improved behavioral outcomes after in vivo gene transduction, achieved by brain delivery of adeno-associated vectors, have resulted in the initiation of ongoing clinical trials. Cell transplantation experiments are transitioning from the grafting of fetal tissue, which has met with mixed clinical success, to the grafting of expanded neural stem cells, for which preliminary results in primates are encouraging. Careful attention to the surgical delivery parameters for these agents in primate studies, along with the ability to realistically model imaging and behavioral outcomes in these animals, is essential for optimizing the restoration of function for patients. The authors review data obtained from primate models that form the basis for ongoing clinical trials to consider how new preclinical models should be developed to answer questions that arise from experimental clinical data.

Emerging restorative treatments for Parkinson's disease

Several exciting approaches for restorative therapy in Parkinson's disease have emerged over the past two decades. This review initially describes experimental and clinical data regarding growth factor administration. We focus on glial cell line-derived neurotrophic factor (GDNF), particularly its role in neuroprotection and in regeneration in Parkinson's disease. Thereafter, we discuss the challenges currently facing cell transplantation in Parkinson's disease and briefly consider the possibility to continue testing intrastriatal transplantation of fetal dopaminergic progenitors clinically. We also give a more detailed overview of the developmental biology of dopaminergic neurons and the potential of certain stem cells, i.e. neural and embryonic stem cells, to differentiate into dopaminergic neurons. Finally, we discuss adult neurogenesis as a potential tool for restoring lost dopamine neurons in patients suffering from Parkinson's disease.

Protection of dopamine neurons by bone marrow stromal cells

Transplantation of bone marrow stromal cells (BMSC) has recently been demonstrated to provide neuroprotection in animal models of brain injuries such as ischemia and trauma. The present study was undertaken to explore whether BMSC can promote the survival of dopamine (DA) neurons in neuronal insult models in vitro. We also examined whether BMSC can increase the survival rate of embryonic DA neurons grafted into the striatum of a rat model of Parkinson's disease (PD). Treatment with conditioned media derived from BMSC cultures was found to significantly prevent the death of DA neurons in in vitro cell injury models such as serum deprivation and exposure to the neurotoxin 6-OHDA. In a transplantation study, we also found that the survival of grafted DA cells was significantly enhanced by treating donor cells with the conditioned media at the steps of both cell dissociation and implantation. The results suggest that BMSC may secrete diffusible factors able to protect DA neurons against neuronal injuries. Indeed, BMSC expressed mRNA encoding brain-derived neurotrophic factor, fibroblast growth factor-2 and glial cell line-derived neurotrophic factor, all of which have previously been shown to exhibit potent neurotrophic effects on DA cells. Enzyme-linked immunosorbent assay revealed that the cells release these growth factors into culture media. The present data indicate that BMSC may be a potential donor source of cell-based regenerative therapy for PD where the progressive loss of the midbrain DA neurons takes place.

Differential effects on forelimb grasping behavior induced by fetal dopaminergic grafts in hemiparkinsonian rats

Skilled forelimb movements depend on an intact dopaminergic (DA) neurotransmission and are substantially impaired in the unilateral rat model of Parkinson's disease. It has remained unclear, however, to what extent reaching and grasping movements can be influenced by intrastriatal transplantation of fetal DA neurons. Here an extensive behavioral assessment of skilled forelimb movement patterns in hemiparkinsonian and DA-grafted rats was carried out. Good DA graft survival was accompanied by a compensation of drug-induced rotational asymmetries. Interestingly, skilled forelimb use was significantly improved in transplanted animals as compared to lesion-only animals in the staircase test. Qualitative analysis of single forelimb reaching movement components revealed dissociable patterns of graft effects: while some movement components in grafted animals improved, others remained unchanged or even deteriorated. These findings provide novel insights into the complex interactions of graft-derived restoration of DA neurotransmission and skilled forelimb behavior.

Role of Endogenous Neural Stem Cells in Neurological Disease and Brain Repair

These examples show that stem-cell-based therapy of neuro-psychiatric disorders will not follow a single scheme, but rather include widely different approaches. This is in accordance with the notion that the impact of stem cell biology on neurology will be fundamental, providing a shift in perspective, rather than introducing just one novel therapeutic tool. Stem cell biology, much like genomics and proteomics, offers a "view from within" with an emphasis on a theoretical or real potential and thereby the inherent openness, which is central to the concept of stem cells. Thus, stem cell biology influences many other, more traditional therapeutic approaches, rather than introducing one distinct novel form of therapy. Substantial advances have been made i n neural stemcell research during the years. With the identification of stem and progenitor cells in the adult brain and the complex interaction of different stem cell compartments in the CNS--both, under physiological and pathological conditions--new questions arise: What is the lineage relationship between t he different progenitor cells in the CNS and how much lineage plasticity exists? What are the signals controlling proliferation and differentiation of neural stem cells and can these be utilized to allow repair of the CNS? Insights in these questions will help to better understand the role of stem cells during development and aging and the possible relation of impaired or disrupted stem cell function and their impact on both the development and treatment of neurological disease. A number o f studies have indicated a limited neuronal and glial regeneration certain pathological conditions. These fundamental observations have already changed our view on understanding neurological disease and the brain's capacity for endogenous repair. The following years will have to show how we can influence andmodulate endogenous repair nisms by increasing the cellular plasticity in the young and aged CNS.

Intracerebral co-transplantation of liposomal tacrolimus improves xenograft survival and reduces graft rejection in the hemiparkinsonian rat

Immunosuppression remains a key issue in neural transplantation. Systemic administration of cyclosporin-A is currently widely used but has many severe adverse side effects. Newer immunosuppressive agents, such as tacrolimus (TAC) and rapamycin (RAPA), have been investigated for their neuroprotective properties on dopaminergic neurons. These drugs have been formulated into liposomal preparations [liposomal tacrolimus (LTAC) and liposomal rapamycin (LRAPA)] which retain these neuroprotective properties. Due to the slower release of the drugs from the liposomes, we hypothesized that co-transplantation of either LTAC or LRAPA within a xenogeneic cell suspension would increase cell survival and decrease graft rejection in the hemiparkinsonian rat, and that a combination of the two drugs may have a synergistic effect. 6-hydroxydopamine-lesioned rats were divided to four groups which received intra-striatal transplants of the following: 1) a cell suspension containing 400,000 fetal mouse ventral mesencephalic cells; 2) the cell suspension containing 0.63 microM LRAPA; 3) the cell suspension containing a dose of 2.0 microM LTAC; 4) the cell suspension containing 2.0 microM LTAC and 0.63 microM LRAPA. Functional recovery was assessed by amphetamine-induced rotational behavior. Animals were killed at 4 days or 6 weeks post-transplantation, and immunohistochemistry was performed to look at the expression of tyrosine hydroxylase and major histocompatibility complex classes I and II. Only the group receiving LTAC had a decrease in rotational behavior. This observation correlated well with significantly more surviving tyrosine hydroxylase immunoreactive cells compared with the other groups and significantly lower levels of immunorejection as assessed by major histocompatibility complex class I and II staining. This study has shown the feasibility of using local immunosuppression in xenotransplantation. These findings may be useful in optimizing immunosuppression in experimental neural transplantation in the laboratory and its translation into the clinical setting.

Transplantation of autologous sympathetic neurons as a potential strategy to restore metabolic functions of the damaged nigrostriatal dopamine nerve terminals in Parkinson's disease

Grafting of catecholamine-producing cells can be a possible therapeutic strategy for attenuating motor symptoms in Parkinson's disease (PD). The potential of autologous sympathetic neurons has been investigated as a donor for cell therapy of PD. The clinical trials of autotransplantation of sympathetic ganglion cells in PD have revealed that the grafts increase the duration of L-DOPA (L-dihydroxy phenyl alanine)-induced beneficial effects, and that the graft-mediated effect is detectable during a follow-up period of at least 1 year postgrafting. In an in vitro analysis of the ability of human sympathetic neurons to release catecholamines, although DA was not detectable under basal conditions, DA levels were significantly increased upon exposure to exogenous L-DOPA. Furthermore, animal experiments with xenografting of human sympathetic ganglionic neurons in the DA-denervated striatum of rats demonstrated that a significant increase in striatal DA levels is noted after systemic L-DOPA treatment, and that the DA levels remain high for longer periods of time in the grafted rats than in control animals with sham surgery. The L-DOPA-induced rise of striatal DA levels was significantly attenuated when given reserpine pretreatment. This suggests that DA derived from exogenously administered L-DOPA is subjected to, at least in part, vesicular storage in grafted sympathetic neurons. Histological examinations indeed showed that the grafts express aromatic-L-amino acid decarboxylase and vesicular monoamine transporter-2, both of which are important molecules for the synthesis and the storage of DA, respectively. Taken together, grafted sympathetic neurons can provide a site for both the conversion of exogenous L-DOPA to DA and the storage of the synthesized DA in the DA-denervated striatum. This might be an explanation for a mechanism by which sympathetic neuron autografts can increase the duration of L-DOPA effects in PD patients. This review article summarizes the clinical effect of transplantation of autologous sympathetic neurons in PD and discusses the underlying mechanism for the effect based on experimental evidence previously obtained.

Imaging in cell-based therapy for neurodegenerative diseases

Fetal cell transplantation for the treatment of Parkinson's and Huntington's diseases has been developed over the past two decades and is now in early clinical testing phase. Direct assessment of the graft's survival, integration into the host brain and impact on neuronal functions requires advanced in vivo neuroimaging techniques. Owing to its high sensitivity, positron emission tomography is today the most widely used tool to evaluate the viability and function of the transplanted tissue in the brain. Nuclear magnetic resonance techniques are opening new possibilities for imaging neurochemical events in the brain. The ultimate goal will be to use the combination of multiple imaging modalities for complete functional monitoring of the repair processes in the central nervous system.

Human embryonic stem cell therapy for Parkinson's disease

Human embryonic stem cells (hESCs) may serve as the most enduring source of transplantable cells for Parkinson's disease patients. Accumulating experience in the transplantation of fetal midbrain tissue or cells into Parkinson's disease patients has set the stage for hESC therapy, but has also opened new controversies on the value and appropriate design of cell therapy. hESCs can be directed to differentiate into nigral dopaminergic neurons with high efficiency. The clinical use of hESCs will depend on their growth in controlled conditions, on whether safety can be proven, and on improving the survival of hESC-derived dopaminergic neurons in the host brain.

Neural transplantation in Huntington's disease: the NEST-UK donor tissue microbiological screening program and review of the literature

Neural transplantation of human fetal tissue for Huntington's disease (HD) is now entering the clinical arena. The safety of the procedure has now been demonstrated in a number of studies, although the efficacy of such an approach is still being investigated. Stringent but practicable screening of the donor tissue for potential pathogens is an essential prerequisite for successful implementation of any novel transplant program that uses human fetal tissue. In this article we summarize the UK-NEST protocol for the screening of human fetal tissue being grafted to patients with mild to moderate HD. We describe the results of microbiological screening of 87 potential tissue donors in a pilot study, and of the first four donor-recipient patients included in the UK-NEST series. The rationale for the adoption and interpretation of the various tests is described and our methodology is compared with those previously used by other centers. This article therefore presents a comprehensive, logical yet pragmatic screening program that could be employed in any clinical studies that use human fetal tissue for neurotransplantation.

Factors affecting the clinical outcome after neural transplantation in Parkinson's disease

Intrastriatal grafts of embryonic mesencephalic tissue can survive in the brains of patients with Parkinson's disease, but the degree of symptomatic relief is highly variable and some cases develop troublesome dyskinesias. Here we explored, using clinical assessment and 18F-dopa and 11C-raclopride PET, factors which may influence the functional outcome after transplantation. We observed increased 18F-dopa uptake in the grafted putamen, signifying continued survival of the transplanted dopaminergic neurons, in parallel with a progressive reduction of 18F-dopa uptake in non-grafted regions for the whole patient group. The patients with the best functional outcome after transplantation exhibited no dopaminergic denervation in areas outside the grafted areas either preoperatively or at 1 or 2 years post-operatively. In contrast, patients with no or modest clinical benefit showed reduction of 18F-dopa in ventral striatum prior to or following transplantation, which may have limited graft-induced improvement. We obtained no evidence that dyskinesias were caused by abnormal dopamine (DA) release from the grafts. As has been observed for intrinsic dopaminergic neurons, there was a significant correlation between 18F-dopa uptake and methamphetamine-induced change of 11C-raclopride binding (as a measure of DA release) in the putamen containing the graft. Furthermore, we observed no correlation between 11C-raclopride binding in anterior, posterior or entire putamen under basal conditions or after methamphetamine, and dyskinesia severity scores in the contralateral side of the body. Withdrawal of immunosuppression at 29 months after transplantation caused no reduction of 18F-dopa uptake or worsening of UPDRS motor score, indicating continued survival and function of the graft. However, patients showed increased dyskinesia scores, which might have been caused either by growth of the graft or worsening of a low-grade inflammation around the graft. These findings indicate that poor outcome after transplantation is associated with progressive dopaminergic denervation in areas outside the grafts, a process which may have started already before surgery. Also, that the development of dyskinesias after transplantation is not associated with excessive DA release from the grafts. Finally, our data provide evidence that long-term immunosuppression can be withdrawn without interfering with graft survival or the motor recovery induced by transplantation.

The controlled differentiation of human neural stem cells into TH-immunoreactive (ir) neurons in vitro

The expansion of human neural stem cells in vitro might overcome the poor donor supply of human fetal neural tissue in transplantation for Parkinson's disease. However, the differentiation of human neural stem cells into dopaminergic neurons has proven difficult. In the present study, we investigated the effects of cytokines, trophic factors of developmental striatum and Ginkgolide on differentiation of human neural stem cells (hNSCs) into TH-ir neurons. The immunoreactivity to tyrosine hydroxylase (TH), a distinctive marker for dopamine neurons was used to assess dopaminergic neuronal phenotype. We demonstrate that human neural stem cells expanded in vitro can efficiently differentiate into TH-ir neurons by induction. These stem cells might serve as a continuous, on-demand source of cells for therapeutic transplantation in patients with Parkinson's disease.

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.

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.

Dopaminergic reinnervation of the globus pallidus by fetal nigral grafts in the rodent model of Parkinson's disease

The current neural transplantation strategy for Parkinson's disease (PD) involves the dopaminergic reinnervation of the striatum (STR). Although up to 85% reinnervation of the STR has been attained by neural transplantation, functional recovery in animal models and transplanted patients is incomplete. This limitation may be due to an incomplete restoration of the dopaminergic input to other basal ganglia structures such as the external segment of the globus pallidus (GPe, homologue of the rodent GP), which normally receives dopaminergic input from the substantia nigra (SN). As part of our investigation into a multiple grafting strategy for PD, we have explored the effects of dopaminergic grafts in the GP of rodents with unilateral 6-hydroxydopamine (6-OHDA) lesions. In this experiment, lesioned rats received either 300,000 fetal ventral mesencephalic (FVM) cells or a sham injection into the GP. Functional assessment consisted of rotational behavior at 3 and 6 weeks posttransplantation. A fluorogold tracer study was conducted to rule out any behavioral improvement due to striatal outgrowth of the GP graft. Sections were stained for glial fibrillary acidic protein (GFAP) to assess the degree of trauma in the GP by the graft in comparison to the sham injection. Immunohistochemistry for tyrosine hydroxylase (TH) was performed after transplantation to assess graft survival. Animals with GP grafts demonstrated a significant improvement in rotational behavior at 3 and 6 weeks posttransplantation (p < 0.05) while sham control animals did not improve. All animals receiving FVM cells showed TH-immunoreactive grafts in the GP posttransplantation. TH-positive neurons in the GP showed no double labeling with an intrastriatal injection of fluorogold, indicating that behavioral improvement was not due to striatal innervation by the GP graft. These observations suggest that functional recovery was the result of dopaminergic reinnervation of the GP and that this nucleus may be a potential target for neural transplantation in clinical PD.

The homeodomain transcription factor Pitx3 facilitates differentiation of mouse embryonic stem cells into AHD2-expressing dopaminergic neurons

The A9 dopaminergic (DA) neuronal group projecting to the dorsal striatum is the most vulnerable in Parkinson's disease (PD). We genetically engineered mouse embryonic stem (ES) cells to express the transcription factors Nurr1 or Pitx3. After in vitro differentiation of Pitx3-expressing ES cells, the proportion of DA neurons expressing aldehyde dehydrogenase 2 (AHD2) increased, while the total number of DA neurons remained the same. The highest levels of AHD2 expression were observed in mouse A9 DA neurons projecting to the dorsal striatum. Furthermore, real-time PCR analyses of in vitro differentiated Pitx3-expressing ES cells revealed that genes highly expressed in A9 DA neurons were up-regulated. When transplanted into the mouse striatum, Pitx3-expressing cells generated an increased proportion of AHD2-expressing DA neurons. Contrastingly, in Nurr1-expressing ES cells, increases of all midbrain DA markers were observed, resulting in a higher total number of DA neurons in vitro and in vivo, whereas the proportion of AHD2-expressing DA neurons was not changed. Our data, using gain-of-function analysis of ES cells, suggest that Pitx3 may be important for specification and/or maintenance of A9-like neuronal properties, while Nurr1 influences overall midbrain DA specification. These findings may be important for modifying ES cells to generate an optimal cell source for transplantation therapy of PD.

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.

The clinical potential of stem cells

Stem cells are defined by their capacity for self-renewal and multilineage differentiation, making them uniquely situated to treat a broad spectrum of human diseases. For example, because hematopoietic stem cells can reconstitute the entire blood system, bone marrow transplantation has long been used in the clinic to treat various diseases. Similarly, the transplantation of other tissue-specific stem cells, such as stem cells isolated from epithelial and neural tissues, can treat mouse disease models and human patients in which epithelial and neural cells are damaged. An alternative to tissue-specific stem cell therapy takes advantage of embryonic stem cells, which are capable of differentiating into any tissue type. Furthermore, nuclear transfer, the transfer of a post-mitotic somatic cell nucleus into an enucleated oocyte, creates a limitless source of autologous cells that, when combined with gene therapy, can serve as a powerful therapeutic tool.

Dopaminergic neurons generated from monkey embryonic stem cells function in a Parkinson primate model

Parkinson disease (PD) is a neurodegenerative disorder characterized by loss of midbrain dopaminergic (DA) neurons. ES cells are currently the most promising donor cell source for cell-replacement therapy in PD. We previously described a strong neuralizing activity present on the surface of stromal cells, named stromal cell-derived inducing activity (SDIA). In this study, we generated neurospheres composed of neural progenitors from monkey ES cells, which are capable of producing large numbers of DA neurons. We demonstrated that FGF20, preferentially expressed in the substantia nigra, acts synergistically with FGF2 to increase the number of DA neurons in ES cell-derived neurospheres. We also analyzed the effect of transplantation of DA neurons generated from monkey ES cells into 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated (MPTP-treated) monkeys, a primate model for PD. Behavioral studies and functional imaging revealed that the transplanted cells functioned as DA neurons and attenuated MPTP-induced neurological symptoms.

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.

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.

Isolation and transplantation of dopaminergic neurons generated from mouse embryonic stem cells

Embryonic stem (ES) cells differentiate into dopamine (DA)-producing neurons when co-cultured with PA6 stromal cells, but the resulting cultures contain a variety of unidentified cells. In order to label live DA neurons in mixed populations, we introduced a GFP reporter under the control of the tyrosine hydroxylase (TH) gene promoter into ES cells. GFP expression was observed in TH-immunoreactive cells that differentiated from the ES cells that carried the TH-GFP reporter gene. DA neurons expressing GFP were sorted from the mixed cell population by fluorescence-activated cell sorting of cells exhibiting GFP fluorescence, and the sorted GFP(+) cells obtained were transplanted into a rat model of Parkinson's disease. Some of these cells survived and innervated the host striatum, resulting in a partial recovery from parkinsonian behavioral defects. This strategy of isolation and transplantation of ES-cell-derived DA neurons should be useful for cellular and molecular studies of DA neurons and for clinical application in the treatment of Parkinson's disease.

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.

Differentiation of idiopathic Parkinson's disease, multiple system atrophy, progressive supranuclear palsy, and healthy controls using magnetization transfer imaging

The differentiation of multiple system atrophy (MSA) and progressive supranuclear palsy (PSP) from idiopathic Parkinson's disease (IPD) is difficult. Magnetization transfer imaging (MTI), a measure that correlates with myelination and axonal density, was employed in this study in the attempt to distinguish between these disorders. Measurements were carried out in 15 patients with IPD, 12 patients with MSA, 10 patients with PSP, and in 20 aged-matched healthy control subjects. The main finding was a change in the magnetization transfer ratio in the globus pallidus, putamen, caudate nucleus, substantia nigra, and white matter in IPD, MSA, and PSP patients, matching the pathological features of the underlying disorder. Furthermore, stepwise linear discriminant analysis provided a good classification of the individual patients into the different disease groups. All IPD patients and control subjects were correctly separated from the MSA and PSP cohort, and all PSP patients and 11 of 12 MSA patients were correctly separated from the IPD and control cohort. There was also a fairly good discrimination of IPD patients from control subjects and of MSA from PSP patients. In conclusion, MTI revealed degenerative changes in patients with different parkinsonian syndromes matching the underlying pathological features of the different diseases, underlining the high potential of this method in distinguishing MSA and PSP from IPD.