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

Lowered cAMP and cGMP signalling in the brain during levodopa-induced dyskinesias in hemiparkinsonian rats: new aspects in the pathogenetic mechanisms

Dysregulation of dopamine receptors is thought to underlie levodopa-induced dyskinesias in experimental models of Parkinson's disease. It is unknown whether an imbalance of the second messengers, cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP), is involved in the alterations of levodopa/dopamine signal transduction. We examined cAMP and cGMP signalling in the interconnected cortico-striatal-pallidal loop at the peak of levodopa-induced dyskinesias in rats with 6-hydroxydopamine lesions in the substantia nigra. In addition, we examined the role of phosphodiesterase (PDE) and the rate of cAMP and cGMP degradation on the severity of levodopa-induced dyskinesias in animals pretreated with PDE inhibitor, zaprinast. Unilateral lesion of substantia nigra led to an increase in cAMP but a decrease in cGMP levels in the ipsilateral basal ganglia. After chronic levodopa treatment, cAMP and cGMP were differentially regulated in eukinetic animals: the cAMP level increased in the cortex and striatum but decreased in the globus pallidus of both hemispheres, whereas the cGMP decreased below baseline levels in the contralateral cortico-striatal-pallidal regions. In dyskinetic animals chronic levodopa treatment led to an absolute decrease in cAMP and cGMP levels in cortico-striatal-pallidal regions of both hemispheres. Pretreatment with zaprinast reduced the severity of levodopa-induced dyskinesias, and partly prevented the decrease in cyclic nucleotides compared with pretreatment with saline-levodopa. In conclusion, using a rat model of hemiparkinsonism, we observed a significant reduction in the levels of cyclic nucleotides in both hemispheres at the peak of levodopa-induced dyskinesias. We propose that such a decrease in cyclic nucleotides may partly result from increased catabolism through PDE overactivity.

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.

Ascorbic Acid Increases the Number of Dopamine Neurons In Vitro and in Transplants to the 6-OHDA-Lesioned Rat Brain

The inadequate survival of dopamine neurons following intracerebral transplantation is in part attributed to the generation of reactive oxygen species and subsequent oxidative stress. To address this, we investigated whether the antioxidant ascorbic acid (vitamin C) had any effect on the yields of dopamine neurons derived from E14 rat ventral mesencephalic cells in vitro and in grafts. Following in vitro differentiation in medium containing ascorbic acid at concentrations ranging from 20 to 100 μM, significantly more neurons were immunopositive for the marker of mesencephalic dopamine neurons, tyrosine hydroxylase (TH), when compared to standard differentiation conditions containing no ascorbic acid. Mesencephalic cell suspensions supplemented with 100 μM ascorbic acid were also transplanted into unilateral 6-OHDA-lesioned rats and behavioral rotation was assessed at 2, 4, and 6 weeks posttransplantation. Grafts pretreated with ascorbic acid contained significantly more surviving dopamine neurons compared to nontreated grafts. However, no significant difference in rotation score was observed, with both groups showing a reversal and overcompensation of rotational bias. In addition, no evidence of neurogenesis of nigral dopamine neurons was observed in transplant groups. While the increased number of dopamine neurons observed in our study following ascorbic acid treatment may reflect a selective survival effect, our in vitro results suggest that ascorbic acid may act to increase the number dopamine neurons, both in culture and following transplantation, by stimulating dopaminergic differentiation of neural precursors from the fetal ventral mesencephalon.

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.

[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.

Neural stem cells in the mammalian brain

New fundamental results on stem cell biology have been obtained in the past 15 years. These results allow us to reinterpret the functioning of the cerebral tissue in health and disease. Proliferating stem cells have been found in the adult brain, which can be involved in postinjury repair and can replace dead cells under specific conditions. Numerous genomic mechanisms controlling stem cell proliferation and differentiation have been identified. The involvement of stem cells in the genesis of malignant tumors has been demonstrated. Neural stem cell tropism toward tumors has been shown. These findings suggest new lines of research on brain functioning and development. Stem cells can be used to develop radically new treatments of neurodegenerative and cancer diseases of the brain.

AAV2-mediated gene transfer of GDNF to the striatum of MPTP monkeys enhances the survival and outgrowth of co-implanted fetal dopamine neurons

Neural transplantation offers the potential of treating Parkinson's disease by grafting fetal dopamine neurons to depleted regions of the brain. However, clinical studies of neural grafting in Parkinson's disease have produced only modest improvements. One of the main reasons for this is the low survival rate of transplanted neurons. The inadequate supply of critical neurotrophic factors in the adult brain is likely to be a major cause of early cell death and restricted outgrowth of fetal grafts placed into the mature striatum. Glial derived neurotrophic factor (GDNF) is a potent neurotrophic factor that is crucial to the survival, outgrowth and maintenance of dopamine neurons, and so is a candidate for protecting grafted fetal dopamine neurons in the adult brain. We found that implantation of adeno-associated virus type 2 encoding GDNF (AAV2-GDNF) in the normal monkey caudate nucleus induced overexpression of GDNF that persisted for at least 6 months after injection. In a 6-month within-animal controlled study, AAV2-GDNF enhanced the survival of fetal dopamine neurons by 4-fold, and increased the outgrowth of grafted fetal dopamine neurons by almost 3-fold in the caudate nucleus of MPTP-treated monkeys, compared with control grafts in the other caudate nucleus. Thus, the addition of GDNF gene therapy to neural transplantation may be a useful strategy to improve treatment for Parkinson's disease.

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

Parkinson's disease (PD) is the most common movement disorder. The neuropathology is characterized by the loss of dopamine neurons in the substantia nigra pars compacta. Transplants of fetal/embryonic midbrain tissue have exhibited some beneficial clinical effects in open-label trials. Neural grafting has, however, not become a standard treatment for several reasons. First, the supply of donor cells is limited, and therefore, surgery is accompanied by difficult logistics. Second, the extent of beneficial effects has varied in a partly unpredictable manner. Third, some patients have exhibited graft-related side effects in the form of involuntary movements. Fourth, in two major double-blind placebo-controlled trials, there was no effect of the transplants on the primary endpoints. Nevertheless, neural transplantation continues to receive a great deal of interest, and now, attention is shifting to the idea of using stem cells as starting donor material. In the context of stem cell therapy for PD, stem cells can be divided into three categories: neural stem cells, embryonic stem cells, and other tissue-specific types of stem cells, e.g., bone marrow stem cells. Each type of stem cell is associated with advantages and disadvantages. In this article, we review recent advances of stem cell research of direct relevance to clinical application in PD and highlight the pros and cons of the different sources of cells. We draw special attention to some key problems that face the translation of stem cell technology into the clinical arena.

Surrogate outcomes in neurology, psychiatry, and psychopharmacology

A surrogate outcome can be defined as an outcome that can be observed sooner, at lower cost, or less invasively than the true outcome, and that enables valid inferences about the effect of intervention on the true outcome. There is increasing interest in the use of surrogate out comes of treatment efficacy measurement in investigational drug trials. However, the significance of surrogate markers of treatment outcome in neurology and psychiatry has not yet been sufficiently demonstrated. Few such markers have been adequately “validated, ” that is, shown to predict the effect of the treatment on the clinical outcome of interest. In this article, evidence that would support the validation of such markers is discussed, Biomarkers used during early clinical development programs of new psychotropic compounds are considered in the contexts of Parkinson's disease, affective disorder, and schizophrenia. The particular case of neuroprotective trials is exemplified by Parkinson's disease, where a biomarker substituting for a clinical measure of progression could be considered as a surrogate treatment outcome.

Induction of dopaminergic neurons from growth factor expanded neural stem/progenitor cell cultures derived from human first trimester forebrain

Multipotent stem/progenitor cells derived from human first trimester forebrain can be expanded as free-floating aggregates, so called neurospheres. These cells can differentiate into neurons, astrocytes and oligodendrocytes. In vitro differentiation protocols normally yield gamma-aminobutyric acid-immunoreactive neurons, whereas only few tyrosine hydroxylase (TH) expressing neurons are found. The present report describes conditions under which 4-10% of the cells in the culture become TH immunoreactive (ir) neurons within 24h. Factors including acidic fibroblast growth factor (aFGF) in combination with agents that increase intracellular cyclic AMP and activate protein kinase C, in addition to a substrate that promotes neuronal differentiation appear critical for efficient TH induction. The cells remain THir after trypsinization and replating, even when their subsequent culturing takes place in the absence of inducing factors. Consistent with a dopaminergic phenotype, mRNAs encoding aromatic acid decarboxylase, but not dopamine-beta-hydroxylase were detected by quantitative real time RT-PCR. Ten weeks after the cells had been grafted into the striatum of adult rats with unilateral nigrostriatal lesions, only very few of the surviving human neurons expressed TH. Our data suggest that a significant proportion of expandable human neural progenitors can differentiate into TH-expressing cells in vitro and that they could be useful for drug and gene discovery. Additional experiments, however, are required to improve the survival and phenotypic stability of these cells before they can be considered useful for cell replacement therapy in Parkinson's disease.