From bench to bed: the potential of stem cells for the treatment of Parkinson's disease

Primate adult brain cell autotransplantation, a pilot study in asymptomatic MPTP-treated monkeys

Autologous brain cell transplantation might be useful for repairing lesions and restoring function of the central nervous system. We have demonstrated that adult monkey brain cells, obtained from cortical biopsy and kept in culture for a few weeks, exhibit neural progenitor characteristics that make them useful for brain repair. Following MPTP treatment, primates were dopamine depleted but asymptomatic. Autologous cultured cells were reimplanted into the right caudate nucleus of the donor monkey. Four months after reimplantation, histological analysis by stereology and TH immunolabeling showed that the reimplanted cells successfully survived, bilaterally migrated in the whole striatum, and seemed to have a neuroprotection effect over time. These results may add a new strategy to the field of brain neuroprotection or regeneration and could possibly lead to future clinical applications.

Pluripotent stem cells as new drugs? The example of Parkinson's disease

Cell replacement therapy is a widely discussed novel concept of medical treatment. The increased knowledge in the stem cell field, particularly pluripotent stem cells, potentially provides powerful tools for this therapeutic concept. A large number of disease characterized by the loss of functional cells are potential candidates for cell replacement therapy and, in this regards, Parkinson's disease is of particular interest. It is one of the most prevalent neurodegenerative diseases caused by the loss of dopaminergic neurons in the Substantia nigra pars compacta. Pharmacological therapies are valuable but suffer from the progressive decline of efficacy as the disease progresses. Cell therapy application has emerged about two decades ago as a valid therapeutic alternative and recent advances in stem cell research suggest that pluripotent stem cell transplantation may be a promising approach to replace degenerated neurons in Parkinson's disease. Various sources of pluripotent stem cells (PSC) currently tested in animal models of Parkinson's disease have proven their efficacy in relieving symptoms and restoring damaged brain function. This review summarizes and discusses the important challenges that actually must be solved before the first studies of PSC transplantation can be undertaken into humans.

Neural progenitors derived from human embryonic stem cells are targeted by allogeneic T and natural killer cells

Neural progenitor cells (NPC) of foetal origin or derived from human embryonic stem cells (HESC) have the potential to differentiate into mature neurons after transplantation into the central nervous system, opening the possibility of cell therapy for neurodegenerative disorders. In most cases, the transplanted NPC are genetically unrelated to the recipient, leading to potential rejection of the transplanted cells. Very few data provide reliable information as to the potential immune response of allogeneic neural progenitors derived from HESC. In this study, we analyzed in vitro the allogeneic immune response of T lymphocytes and natural killer (NK) cells to NPC derived from HESC or of foetal origin. We demonstrate that NPC induce T-cell stimulation and a strong NK cytotoxic response. NK-cell activity is unrelated to MHC-I expression but driven by the activating NKG2D receptor. Cyclosporine and dexamethasone previously used in clinical studies with foetal NPC did not only fail to prevent NK alloreactivity but strongly inhibited the terminal maturation from NPC into mature neurons. We conclude that allogenic transplantation of NPC in the central nervous system will most likely require an immunosuppressive regimen targeting allogenic T and NK cells, whereas possible interference with the differentiation of NPC needs to be carefully evaluated.

Deep brain stimulation of the subthalamic nucleus for the treatment of Parkinson's disease

High-frequency deep brain stimulation (DBS) of the subthalamic nucleus (STN-HFS) is the preferred surgical treatment for advanced Parkinson's disease. In the 15 years since its introduction into clinical practice, many studies have reported on its benefits, drawbacks, and insufficiencies. Despite limited evidence-based data, STN-HFS has been shown to be surgically safe, and improvements in dopaminergic drug-sensitive symptoms and reductions in subsequent drug dose and dyskinesias are well documented. However, the procedure is associated with adverse effects, mainly neurocognitive, and with side-effects created by spread of stimulation to surrounding structures, depending on the precise location of electrodes. Quality of life improves substantially, inducing sudden global changes in patients' lives, often requiring societal readaptation. STN-HFS is a powerful method that is currently unchallenged in the management of Parkinson's disease, but its long-term effects must be thoroughly assessed. Further improvements, through basic research and methodological innovations, should make it applicable to earlier stages of the disease and increase its availability to patients in developing countries.

alpha-Synuclein: a therapeutic target for Parkinson's disease?

Parkinson's disease is a progressive age-related neurodegenerative disease with invariant loss of substantia nigra dopamine neurons and striatal projections. This disorder is well known for the associated motoric symptoms including resting tremor and the inability to initiate movement. However, it is now apparent that Parkinson's disease is a multisystem disorder with patients exhibiting symptoms derived from peripheral nervous system and extra-nigral dysfunctions in addition to the prototypical nigrostriatal damage. Although the etiology for sporadic Parkinson's disease is unknown, information gleaned from both familial forms of the disease and animal models places misfolded alpha-synuclein at the forefront. The disease is currently without a cure and most therapies target the motoric symptoms relying on increasing dopamine tone. In this review, the role of alpha-synuclein in disease pathogenesis and as a potential therapeutic target focusing on toxic conformers of this protein is considered. The addition of protofibrillar/oligomer-directed neurotherapeutics to the existing armamentarium may extend the symptom-free stage of Parkinson's disease as well as alleviate pathogenesis.

Potential cellular and regenerative approaches for the treatment of Parkinson's disease

Parkinson's disease is most commonly treated with a range of pharmacotherapeutics, with the more recent introduction of surgical techniques including deep-brain stimulation. These have limited capabilities to improve symptoms of the disease in more advanced stages, thus new therapeutic strategies including the use of viral vectors and stem cells are in development. Providing a continuous supply of dopamine to the striatum in an attempt to improve the treatment of motor symptoms using enzymes in the dopamine synthesis and machinery is one approach. Alternatively, there are tools which may serve to both protect and encourage outgrowth of surviving neurons using growth factors or to directly replace lost innervation by transplantation of primary tissue or stem cell-derived dopaminergic neurons. We summarize some of the potential therapeutic approaches and also consider the recent EU directives on practical aspects of handling viral vectors, cells and tissues, and in the running of clinical trials in Europe which impact on their development.

Growth factors and feeder cells promote differentiation of human embryonic stem cells into dopaminergic neurons: a novel role for fibroblast growth factor-20

Human embryonic stem cells (hESCs) are a potential source of dopaminergic neurons for treatment of patients with Parkinson's disease (PD). Dopaminergic neurons can be derived from hESCs and display a characteristic midbrain phenotype. Once transplanted, they can induce partial behavioral recovery in animal models of PD. However, the potential research field faces several challenges that need to be overcome before clinical application of hESCs in a transplantation therapy in PD can be considered. These include low survival of the hESC-derived, grafted dopaminergic neurons after transplantation; unclear functional integration of the grafted neurons in the host brain; and, the risk of teratoma/tumor formation from the transplanted cells. This review is focused on our recent efforts to improve the survival of hESC-dervied dopaminergic neurons. In a recent study, we examined the effect of fibroblast growth factor (FGF)-20 in the differentiation of hESCs into dopaminergic neurons. We supplemented cultures of hESCs with FGF-20 during differentiation on PA6 mouse stromal cells for 3 weeks. When we added FGF-20 the yield of neurons expressing tyrosine hydroxylase increased. We demonstrated that at least part of the effect is contributed by enhanced cell differentiation towards the dopaminergic phenotype as well as reduced cell death. We compare our results with those obtained in other published protocols using different sets of growth factors. Taken together, our data indicate that FGF-20 has potent effects to generate large number of dopaminergic neurons derived from hESCs, which may be useful for hESC-based therapy in PD.

Embryonic stem cell biology

ES cell research represents an exploding field of exploration. Initially predicted to provide rapid cures for numerous human diseases, the clinical usefulness of ES cell-derived cells remains untested in humans. However, ES cells have rapidly expanded our knowledge of human development and the molecular details of differentiation. Our ability to generate relatively pure populations of specifically differentiated cells for transplantation has markedly improved. It is hoped that soon researchers will overcome the biologic impediments to successful treatment of human disease with ES cell-derived cells.