Emerging restorative treatments for Parkinson's disease: manipulation and inducement of dopaminergic neurons from adult stem cells

Parkinson's disease (PD) is a common neurodegenerative disease, characterized by a selective loss of midbrain Dopaminergic (DA) neurons. To address this problem, various types of stem cells that have potential to differentiate into DA neurons are being investigated as cellular therapies for PD, including cells derived from embryonic or adult donor tissue, and embryonic stem cells. These cell sources, however, have raised certain questions with regard to ethical and rejection issues. Recent progress in adult stems has further proved that the cells derived from adult tissue could be expanded and differentiated into DA precursor cells in vitro, and cell therapy with adult stem cells could produce a clear improvement for PD models. Using adult stem cells for clinic application may not only overcome the ethical problem inherent in using human fetal tissue or embryonic stem cells, but also open the possibility for autologous transplantation. The patient-specific adult stem cell is therefore a potential and prospective candidate for PD treatment.     

Cell Therapeutics in Parkinson’s Disease

The main pathology underlying motor symptoms in Parkinson’s disease (PD) is a rather selective degeneration of nigrostriatal dopamine (DA) neurons. Intrastriatal transplantation of immature DA neurons, which replace those neurons that have died, leads to functional restoration in animal models of PD. Here we describe how far the clinical translation of the DA neuron replacement strategy has advanced. We briefly summarize the lessons learned from the early clinical trials with grafts of human fetal mesencephalic tissue, and discuss recent findings suggesting susceptibility of these grafts to the disease process long-term after implantation. Mechanisms underlying graft-induced dyskinesias, which constitute the only significant adverse event observed after neural transplantation, and how they should be prevented and treated are described. We summarize the attempts to generate DA neurons from stem cells of various sources and patient-specific DA neurons from fully differentiated somatic cells, with particular emphasis on the requirements of these cells to be useful in the clinical setting. The rationale for the new clinical trial with transplantation of fetal mesencephalic tissue is described. Finally, we discuss the scientific and clinical advancements that will be necessary to develop a competitive cell therapy for PD patients.     

Neuromodulation for Parkinson's disease]

Parkinson's disease (PD) is a neurodegenerative disorder characterized by a progressive loss of midbrain dopaminergic (DA) neurons and a subsequent reduction in striatal dopamine. As a treatment for advanced Parkinson's disease, deep brain stimulation (DBS) of the thalamus was introduced in 1987 to treat tremor, and was applied in 1993 to the subthalamic nucleus. Now high-frequency stimulation of the subthalamic nucleus has become a surgical therapy of choice. Another surgical treatment is a cell replacement therapy. Transplantation of fetal dopaminergic (DA) neurons can produce symptomatic relief, however, the technical and ethical difficulties in obtaining sufficient and appropriate donor fetal brain tissue have limited the application of this therapy. Then, neural precursor cells and embryonic stem (ES) cells are expected to be candidates of potential donor cells for transplantation. We induced DA neurons from monkey ES cells, and analyzed the effect of transplantation of the DA neurons into MPTP-treated monkeys as a primate model of Parkinson's disease. Behavioral studies and functional imaging revealed that the transplanted cells functioned as DA neurons, attenuating the MPTP-induced neurological symptoms. DA neurons have also been generated from several human ES cell lines. Furthermore, functional recovery of rat PD models after transplantation was observed. One of the major problems in ES cell transplantation is tumor formation, which is caused by a small fraction of undifferentiated ES cells in the graft. So, it is essential for undifferentiated ES cells to be eliminated from the graft in order for transplantation to be feasible. These efforts will lead to clinical application of ES cell transplantation to the patients with PD.     

A role for tumor necrosis factor alpha in death of dopaminergic neurons following neural transplantation

Poor survival of transplanted dopaminergic (DA) neurons remains a serious obstacle to the success of cell replacement therapy as an alternative to the current treatments for Parkinson's disease (PD). We have examined the temporal release profile of an inflammatory cytokine, tumor necrosis factor-alpha (TNFalpha), following transplantation of fetal mesencephalic tissue into the rat striatum. The amounts of TNFalpha released in vivo when added to cultures of embryonic DA neurons, significantly reduced the survival of DA neurons in vitro, and this cell death could be prevented by the inclusion of an antibody to the TNFalpha receptor type 1. Inclusion of this antibody in cell suspensions during transplantation also increased the survival of transplanted fetal DA neurons by approximately 250%. Use of this therapeutic antibody approach may offer significant improvements to neural transplantation as a treatment for PD.     

Cell implantation therapies for Parkinson's disease using neural stem, transgenic or xenogeneic donor cells

A new therapeutic neurological and neurosurgical methodology involves cell implantation into the living brain in order to replace intrinsic neuronal systems, that do not spontaneously regenerate after injury, such as the dopaminergic (DA) system affected in Parkinson's disease (PD) and aging. Current clinical data indicate proof of principle for this cell implantation therapy for PD. Furthermore, the disease process does not appear to negatively affect the transplanted cells, although the patient's endogenous DA system degeneration continues. However, the optimal cells for replacement, such as highly specialized human fetal dopaminergic cells capable of repairing an entire degenerated nigro-striatal system, cannot be reliably obtained or generated in sufficient numbers for a standardized medically effective intervention. Xenogeneic and transgenic cell sources of analogous DA cells have shown great utility in animal models and some promise in early pilot studies in PD patients. The cell implantation treatment discipline, using cell fate committed fetal allo- or xenogeneic dopamine neurons and glia, is currently complemented by research on potential stem cell derived DA neurons. Understanding the cell biological principles and developing methodology necessary to generate functional DA progenitors is currently our focus for obtaining DA cells in sufficient quantities for the unmet cell transplantation need for patients with PD and related disorders.     

Toward full restoration of synaptic and terminal function of the dopaminergic system in Parkinson's disease by stem cells

New therapeutic nonpharmacological methodology in Parkinson's disease (PD) involves cell and synaptic renewal or replacement to restore function of neuronal systems, including the dopaminergic (DA) system. Using fetal DA cell therapy in PD patients and laboratory models, it has been demonstrated that functional motor deficits associated with parkinsonism can be reduced. Similar results have been observed in animal models with stem cell-derived DA neurons. Evidence obtained from transplanted PD patients further shows that the underlying disease process does not destroy transplanted fetal DA cells, although degeneration of the host nigrostriatal system continues. The optimal DA cell regeneration system would reconstitute a normal neuronal network capable of restoring feedback-controlled release of DA in the nigrostriatal system. The success of cell therapy for PD is limited by access to preparation and development of highly specialized dopaminergic neurons found in the A9 and A10 region of the substantia nigra pars compacta as well as the technical and surgical steps associated with the transplantation procedure. Recent laboratory work has focused on using stem cells as a starting point for deriving the optimal DA cells to restore the nigrostriatal system. Ultimately, understanding the cell biological principles necessary for generating functional DA neurons can provide many new avenues for better treatment of patients with PD.     

Development of stem cell-based therapies for Parkinson's disease

Parkinson's disease (PD) is a common neurodegenerative disorder characterized by the progressive loss of dopaminergic neurons of the substantia nigra pars compacta in the brain with an unknown cause. Current pharmacological treatments for PD are only symptomatic and there is still no cure for this disease nowadays. In fact, transplantation of human fetal ventral midbrain cells into PD brains has provided a proof of concept that cell replacement therapy can be used for some PD patients, beneficial for improving their symptoms. However, the ethical and practical issues of human fetal tissue will inevitably limit its widespread clinical use. Therefore, it is essential to find alternative cell sources for the future cell transplantation for PD patients. With recent development in stem cell technology, here, we review the different types of stem cells and their main properties currently explored, which could be developed as a possible cell therapy for PD treatment.     

IL-1 beta is released from the host brain following transplantation but does not compromise embryonic dopaminergic neuron survival

Poor survival of transplanted dopaminergic (DA) neurons remains a serious obstacle to the success of cell replacement therapy as an alternative to the current treatments for Parkinson's disease. We have examined the temporal release profile of an inflammatory cytokine, interleukin-1 beta (IL-1 beta) following transplantation of fetal mesencephalic tissue into the rat striatum. The amounts of IL-1 beta released in vivo when added to cultures of embryonic DA neurons, did not significantly reduce the survival of DA neurons in vitro, and inclusion of the naturally-occurring IL-1 receptor antagonist, IL-1ra, did not appear to affect the numbers of surviving DA neurons present after 5 days in vitro. Neither did inclusion of IL-1ra in cell suspensions during transplantation increase the survival of transplanted fetal DA neurons. Thus, although IL-1 beta is released following implantation of a neural transplant, we suggest that this pro-inflammatory cytokine does not play an active role in reducing survival of transplanted DA neurons, unlike other cytokines such as tumor necrosis factor alpha. Modulation of IL-1 beta activity, therefore, will not offer significant improvements to neural transplantation as a treatment for PD.     

IL-1β is released from the host brain following transplantation but does not compromise embryonic dopaminergic neuron survival

Poor survival of transplanted dopaminergic (DA) neurons remains a serious obstacle to the success of cell replacement therapy as an alternative to the current treatments for Parkinson's disease. We have examined the temporal release profile of an inflammatory cytokine, interleukin-1 beta (IL-1 beta) following transplantation of fetal mesencephalic tissue into the rat striatum. The amounts of IL-1 beta released in vivo when added to cultures of embryonic DA neurons, did not significantly reduce the survival of DA neurons in vitro, and inclusion of the naturally-occurring IL-1 receptor antagonist, IL-1ra, did not appear to affect the numbers of surviving DA neurons present after 5 days in vitro. Neither did inclusion of IL-1ra in cell suspensions during transplantation increase the survival of transplanted fetal DA neurons. Thus, although IL-1 beta is released following implantation of a neural transplant, we suggest that this pro-inflammatory cytokine does not play an active role in reducing survival of transplanted DA neurons, unlike other cytokines such as tumor necrosis factor alpha. Modulation of IL-1 beta activity, therefore, will not offer significant improvements to neural transplantation as a treatment for PD.     

Intranigral transplants of GABA-rich striatal tissue induce behavioral recovery in the rat Parkinson model and promote the effects obtained by intrastriatal dopaminergic transplants

Intrastriatal transplantation of fetal ventral mesencephalon (VM) is currently explored as a potential clinical therapy in Parkinson's disease (PD). Although providing substantial benefit for the patient, behavioral recovery so far obtained with intrastriatal VM grafts is not complete. Using the 6-hydroxydopamine lesion model of PD, we show here that near-complete restoration of the striatal dopamine (DA) innervation can be achieved by multiple intrastriatal microtransplants of fetal DA cells; nevertheless, complete recovery in complex sensorimotor behaviors was not obtained in these animals. In line with the current model of basal ganglia function, this suggests that the lesion-induced overactivity of the basal ganglia output structures, i.e., the substantia nigra (SN) and the entopeduncular nucleus, may not be completely reversed by intrastriatal VM grafts. In the present study, we have transplanted fetal VM tissue or fetal striatal tissue, as a source of DA and GABA neurons, respectively, into the SN of DA-depleted rats. Intranigral VM grafts induced behavioral recovery in some sensorimotor behaviors (forelimb akinesia and balance tests), but the effect did not exceed the recovery observed after intrastriatal VM grafts. Intranigral grafts of striatal tissue induced a pattern of functional recovery which was distinctly different from that observed after intranigral VM grafts, and recovery in coordinated forelimb use in the paw-reaching test was even more pronounced than after intrastriatal transplantation of VM cells. Combined transplantation of DA neurons into the striatum and GABA-rich striatal neurons into the SN induced additive effects of behavioral recovery observed in the forelimb akinesia test. We propose that intranigral striatal transplants, by a GABA-mediated inhibitory action, can reduce the overactivity of the host SN projection neurons and can induce significant recovery in complex motor behavior in the rat PD model and that such grafts may be used to increase the overall functional efficacy of intrastriatal VM grafts.     

Cell Therapy in Parkinson's Disease

Summary: 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.     

Developmental perspectives on human midbrain-derived neural stem cells

Regeneration or restoration of lost or damaged neurons is very likely to profoundly alter the disability and needs of many patients. The replacement of dopaminergic (DA) neurons in patients with Parkinson's disease via implantation of embryonic midbrain tissue was taken from animal experiments to clinical applications. Ethical concerns related to the use of fetal tissue derived from abortions further argue for the search for alternative tissue sources. Today, it seems possible to generate functional DA neurons from a variety of stem cells, including embryonic and neural stem cells. Bone marrow stromal cells are another source for cell replacement. Neural stem cells derived from human fetal midbrain tissue maintain a considerable capacity to self-renew and to differentiate into DA neurons. Therefore, these cells may be a promising source to generate functional human DA neurons.     

Stem cells may reshape the prospect of Parkinson's disease therapy

The concept of cell replacement to compensate for cell loss and restore functionality has entered several disease entities including neurodegenerative disorders. Recent clinical studies have shown that transplantation of fetal dopaminergic (DA) cells into the brain of Parkinson's disease (PD) patients can reduce disease-associated motor deficits. However, the use of fetal tissue is associated with practical and ethical problems including low efficiency, variability in the clinical outcome and controversy regarding the use of fetuses as donor. An alternative cell resource could be embryonic stem (ES) cells, which can be cultivated in unlimited amounts and which have the potential to differentiate into mature DA cells. Several differentiation protocols have been developed, and some progress has been made in understanding the mechanisms underlying DA specification in ES cell development, but the "holy grail" in this paradigm, which is the production of sufficient amounts of the "right" therapeutic DA cell, has not yet been accomplished. To achieve this goal, several criteria on the transplanted DA cells need to be fulfilled, mainly addressing cell survival, accurate integration in the brain circuitry, normal function, no tumor formation, and no immunogenicity. Here, we summarize the current state of ES cell-derived DA neurogenesis and discuss the aspects involved in generating an optimal cell source for cell replacement in PD.     

Stem cell-based therapies in Parkinson’s disease: future hope or current treatment option?

Parkinson’s disease (PD) is one of the most frequent neurodegenerative diseases and represents a major therapeutic challenge because of the so far missing therapeutic means to influence the ongoing loss of dopaminergic innervation to the striatum. Cell replacement has raised hope to offer the first restorative treatment option. Clinical trials have provided “proof of principle” that transplantation of dopamine-producing neurons into the striatum of PD patients can achieve symptomatic relief given that the striatum is sufficiently re-innervated. Various cell sources have been tested, including fetal ventral midbrain tissue, embryonic stem cells, fetal and adult neural stem cells and, after a ground-breaking discovery, induced pluripotent stem cells. Although embryonic and induced pluripotent stem cells have emerged as the most promising candidates to overcome most of the obstacles to clinical successful cell replacement, each cell source has its unique drawbacks. This review does not only provide a comprehensive overview of the different cellular candidates, including their assets and drawbacks, but also of the various additional issues that need to be addressed in order to convert cellular replacement therapies from an experimental to a clinically relevant therapeutic alternative.

     

Cell therapy in Parkinson's disease]

An approach for symptomatic Parkinson's disease (PD) therapy is fetal dopamine neuron transplantation. This approach remains the technical and ethical difficulties in obtaining sufficient and appropriate donor fetal brain tissue. In developments of stem cell biology, neural stem cells exist in the adult brain as well as embryo and have the capacity to regenerate and to give rise to the three cell lineages in the nervous system. Embryonic stem cells (ES cells) and multipotent adult progenitor cells (MAPCs) are pluripotent cells, which give rise to all cells in the organism. Current findings suggest that stem cells but not fetal brain tissues may be suitable for cell replacement therapies in the treatment of neurodegenerative disorders. We will briefly review the current state of cell therapy, and will critically discuss the potential of stem cells for the treatment of PD.     

Zuckerkandl's organ improves long-term survival and function of neural stem cell derived dopaminergic neurons in Parkinsonian rats

Transplantation of neural stem cells (NSC) derived dopamine (DA) neurons has emerged as an alternative approach to fetal neural cell transplantation in Parkinson's disease (PD). However, similar to fetal neural cell, survival of these neurons following transplantation is also limited due to limited striatal reinnervation (graft with dense neuronal core), limited host-graft interaction, poor axonal outgrowth, lack of continuous neurotrophic factors supply and principally an absence of cell adhesion molecules mediated appropriate developmental cues. In the present study, an attempt has been made to increase survival and function of NSC derived DA neurons, by co-grafting with Zuckerkandl's organ (a paraneural organ that expresses neurotrophic factors as well as cell adhesion molecules); to provide continuous NTF support and developmental cues to transplanted DA neurons in the rat model of PD. 24 weeks post transplantation, a significant number of surviving functional NSC derived DA neurons were observed in the co-transplanted group as evident by an increase in the number of tyrosine hydroxylase immunoreactive (TH-IR) neurons, TH-IR fiber density, TH-mRNA expression and TH-protein level at the transplantation site (striatum). Significant behavioral recovery (amphetamine induced stereotypy and locomotor activity) and neurochemical recovery (DA-D2 receptor binding and DA and DOPAC levels at the transplant site) were also observed in the NSC+ZKO co-transplanted group as compared to the NSC or ZKO alone transplanted group. In vivo results were further substantiated by in vitro studies, which suggest that ZKO increases the NSC derived DA neuronal survival, differentiation, DA release and neurite outgrowth as well as protects against 6-OHDA toxicity in co-culture condition. The present study suggests that long-term and continuous NTF support provided by ZKO to the transplanted NSC derived DA neurons, helped in their better survival, axonal arborization and integration with host cells, leading to long-term functional restoration in the rat model of PD.     

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.     

Function of Wnts in dopaminergic neuron development

Dopaminergic (DA) neurons in the ventral midbrain (VM) are one of the major cell types lost in Parkinson's disease (PD). Proof of principle exists for cell replacement therapies for PD, but wider application is halted by the unavailability of abundant sources of DA neurons. Stem cells might constitute one of these sources. However, efficient protocols promoting their specific differentiation into a DA neuronal phenotype are required. In this review, we summarize the latest findings concerning the contribution of the Wnt family of glycolipoproteins in the development of VM DA neurons. Regulators of Wnt signaling are involved in several neurodevelopmental processes. Recent results indicate that Wnts are key regulators of proliferation and differentiation of DA precursors during VM neurogenesis and different Wnts have specific and unique activity profiles. Interestingly, chemical inhibitors of glycogen synthase kinase-3beta stabilize beta-catenin and increase DA differentiation in VM precursor cultures. We hereby propose that Wnts are likely to contribute in the future to improve stem/precursor cell replacement therapy approaches to PD.     

Apoptosis in neuronal development and transplantation: role of caspases and trophic factors

Fetal ventral mesencephalic (VM) transplants have been studied in the context of dopaminergic (DA) replacement therapy for Parkinson's disease (PD). DA neurons from VM transplants will grow axons and form functional synapses in the adult host central nervous system (CNS). Recently, studies have demonstrated that most of the transplanted DA neurons die in grafts within the first week after implantation. An important feature of neural development, also in transplanted developing fetal neural tissue, is cell death. However, while about 50% of cells born in the CNS will die naturally, up to 99% of fetal cells die after neural transplantation. It has been shown that VM grafts contain many apoptotic cells even at 14 days after transplantation. The interleukin-1beta converting enzyme (ICE) cysteine protease and 11 other ICE-like-related proteases have been identified, now named caspases. Activation of caspases is one of the final steps before a neuron is committed to die by apoptosis. Here we review this cell death process in detail: Since the growth of fetal neural grafts placed in the adult brain in many ways mimics normal development, it is likely that the caspases also play a functional role in transplants. Pharmacological inhibitors of caspases and genetically modified mice are now available for the study of neuronal death in fetal neuronal transplants. Understanding cell death mechanisms involved in acute cellular injury, necrosis, and programmed cell death (PCD) is useful in improving future neuronal transplantation methodology, as well as in neuroprotection, for patients with neurodegenerative diseases.     

Cell transplantation for Parkinson's disease

Parkinson's disease (PD) is a progressive neurodegenerative disorder characterized by the degeneration of the dopamine producing neurons projecting from the substantia nigra into the corpus striatum. Current medical therapy is limited and cannot stop or reverse the degeneration. Over the past 30 years, attempts were made to change the course of the disease by replacing the lost neurons with grafts from various sources. Recent controlled clinical trials of fetal cell transplantation for PD have had disappointing results. These events present an opportunity to examine the past developments and future direction of cell transplantation for PD.