Clinical pattern and risk factors for dyskinesias following fetal nigral transplantation in Parkinson's disease: A double blind video-based analysis

Cell transplantation and gene therapy in Parkinson's disease

Parkinson's disease is a progressive neurodegenerative disorder affecting, in part, dopaminergic motor neurons of the ventral midbrain and their terminal projections that course to the striatum. Symptomatic strategies focused on dopamine replacement have proven effective at remediating some motor symptoms during the course of disease but ultimately fail to deliver long-term disease modification and lose effectiveness due to the emergence of side effects. Several strategies have been experimentally tested as alternatives for Parkinson's disease, including direct cell replacement and gene transfer through viral vectors. Cellular transplantation of dopamine-secreting cells was hypothesized as a substitute for pharmacotherapy to directly provide dopamine, whereas gene therapy has primarily focused on restoration of dopamine synthesis or neuroprotection and restoration of spared host dopaminergic circuitry through trophic factors as a means to enhance sustained controlled dopamine transmission. This seems now to have been verified in numerous studies in rodents and nonhuman primates, which have shown that grafts of fetal dopamine neurons or gene transfer through viral vector delivery can lead to improvements in biochemical and behavioral indices of dopamine deficiency. However, in clinical studies, the improvements in parkinsonism have been rather modest and variable and have been plagued by graft-induced dyskinesias. New developments in stem-cell transplantation and induced patient-derived cells have opened the doors for the advancement of cell-based therapeutics. In addition, viral-vector-derived therapies have been developed preclinically with excellent safety and efficacy profiles, showing promise in clinical trials thus far. Further progress and optimization of these therapies will be necessary to ensure safety and efficacy before widespread clinical use is deemed appropriate.

Graft-induced dyskinesias in Parkinson's disease: High striatal serotonin/dopamine transporter ratio

Graft-induced dyskinesias are a serious complication after neural transplantation in Parkinson's disease. One patient with Parkinson's disease, treated with fetal grafts 14 years ago and deep brain stimulation 6 years ago, showed marked improvement of motor symptoms but continued to suffer from OFF-medication graft-induced dyskinesias. The patient received a series of clinical and imaging assessments. Positron emission tomography and single-photon emission computed tomography 14 years posttransplantation revealed an elevated serotonin/dopamine transporter ratio in the grafted striatum compatible with serotonergic hyperinnervation. Inhibition of serotonin neuron activity by systemic administration of a 5-HT(1A) agonist suppressed graft-induced dyskinesias. Our data provide further evidence that serotonergic neurons mediate graft-induced dyskinesias in Parkinson's disease. Achieving a normal striatal serotonin/dopamine transporter ratio following transplantation of fetal tissue or stem cells should be necessary to avoid the development of graft-induced dyskinesias.

Optimizing functional imaging protocols for assessing the outcome of fetal cell transplantation in Parkinson's disease

Clinical trials aiming to assess the safety and efficacy of fetal cell transplantation in Parkinson's disease rely on the hypothesis that the grafted tissue will survive and grow, restore striatal dopaminergic neurotransmission, improve the connectivity between striatum, thalamus and cortex and, thereby, produce long-lasting clinical improvement while avoiding the development of adverse effects. Although transplantation of human fetal ventral mesencephalic tissue has been reported as one of the most effective reparative therapies in Parkinson's disease patients to date, different studies have shown inconsistent results causing a paucity of new trials over the last decade. However, during this period, functional imaging alongside other scientific developments from clinical observations and animal work has significantly aided in understanding the mechanisms responsible for the success or failure of grafting human fetal tissue. Recent advances in functional imaging including both positron emission tomography and functional magnetic resonance imaging could be proven useful in vivo tools for the development and assessment of new clinically competitive trials. In this commentary we discuss how an optimized functional imaging protocol could assist new clinical trials using fetal cell transplantation in Parkinson's disease.

The origins of oxidant stress in Parkinson's disease and therapeutic strategies

Parkinson's disease (PD) is a major world-wide health problem afflicting millions of the aged population. Factors that act on most or all cell types (pan-cellular factors), particularly genetic mutations and environmental toxins, have dominated public discussions of disease etiology. Although there is compelling evidence supporting an association between disease risk and these factors, the pattern of neuronal pathology and cell loss is difficult to explain without cell-specific factors. This article focuses on recent studies showing that the neurons at greatest risk in PD-substantia nigra pars compacta dopamine neurons-have a distinctive physiological phenotype that could contribute to their vulnerability. The opening of L-type calcium channels during autonomous pacemaking results in sustained calcium entry into the cytoplasm of substantia nigra pars compacta dopamine neurons, resulting in elevated mitochondrial oxidant stress and susceptibility to toxins used to create animal models of PD. This cell-specific stress could increase the negative consequences of pan-cellular factors that broadly challenge either mitochondrial or proteostatic competence. The availability of well-tolerated, orally deliverable antagonists for L-type calcium channels points to a novel neuroprotective strategy that could complement current attempts to boost mitochondrial function in the early stages of the disease.

Cell transplantation in Parkinson's disease: problems and perspectives

PURPOSE OF REVIEW

We review recent experiments conducted using embryonic tissue and stem cell transplants in experimental models of Parkinson's disease. We also highlight the challenges which remain to be met in order for cell therapy to become clinically effective and safe.

RECENT FINDINGS

The outcome of previous clinical transplantation trials was variable in terms of motor recovery. We discuss whether transplants can mitigate L-3,4-dihydroxyphenylalanine (L-DOPA)-induced dyskinesias and consider the risk factors which predispose to graft-induced dyskinesias. In addition, we introduce Transeuro, a new European Union-funded multicenter consortium which plans to perform transplantation trials.Stem cells have emerged as an alternative source for the generation of dopaminergic precursors. We briefly outline progress made in the use of human embryonic stem cells and focus predominantly on the emerging field of induced pluripotency. We conclude by introducing the exciting and novel method of direct reprogramming which involves the conversion of fibroblasts to neurons without inducing a pluripotent state.

SUMMARY

The area of cell transplantation has been revitalized by the identification of parameters which predispose patients to graft-induced dyskinesias and by the emergence of novel methods of generating dopaminergic neurons. Hopefully, the Transeuro clinical trials will give further impetus and act as a stepping stone to future trials employing stem-cell-derived neurons.

Understanding graft-induced dyskinesia

The transplantation of dopaminergic cells for the treatment of symptoms of Parkinson’s disease has several hurdles to overcome before it can be considered a successful therapeutic approach. One issue is the development of abnormal involuntary movements in the absence of L-3,4-dihydroxyphenylalanine following the transplantation of fetal ventral mesencephalon identified in three different clinical trials. Hypotheses as to the cause of these movements include: the composition of the graft, size of the graft, L-3,4-dihydroxyphenylalanine exposure and L-3,4-dihydroxyphenylalanine-induced dyskinesia prior to transplantation and inflammatory responses in and around the graft. We evaluate the clinical evidence supporting these hypotheses and the preclinical models upon which experiments are being based to resolve them.

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.

Established therapies and novel targets in the treatment of Parkinson's disease

Parkinson's disease affects more than 1% of individuals older than 60 years of age. The gold standard of its symptomatic treatment is levodopa therapy, which in time leads to motor fluctuations and dyskinesia due to noncontinuous receptor stimulation. Dopamine agonists and monoamine oxidase-B inhibitors are recommended as initial therapy, but they are less effective in the advanced stages of the disease. Treatment should be individualized for the patient, dependent on the stage, with attention to nonmotor symptoms. No effective neuroprotective therapy for Parkinson's disease is yet available, and there is currently substantial interest in the development of new nondopaminergic agents. Analogs of kynurenic acid and inhibitors of the enzymes involved in the synthesis of quinolinic acid may exert a neuroprotective effect.

Transgenic reporter mice as tools for studies of transplantability and connectivity of dopamine neuron precursors in fetal tissue grafts

Cell therapy for Parkinson's disease (PD) is based on the idea that new midbrain dopamine (mDA) neurons, implanted directly into the brain of the patient, can structurally and functionally replace those lost to the disease. Clinical trials have provided proof-of-principle that the grafted mDA neurons can survive and function after implantation in order to provide sustained improvement in motor function for some patients. Nonetheless, there are a number of issues limiting the application of this approach as mainstream therapy, including: the use of human fetal tissue as the only safe and reliable source of transplantable mDA neurons, and variability in the therapeutic outcome. Here we review recent progress in this area from investigations using rodent models of PD, paying particular attention to the use of transgenic reporter mice as tools for neural transplantation studies. Cell type-specific expression of reporter genes, such as green fluorescent protein, affords valuable technical advantages in transplantation experiments, such as the ability to selectively isolate specific cell fractions from mixed populations prior to grafting, and the unambiguous visualization of graft-derived dopamine neuron fiber patterns after transplantation. The results from these investigations have given new insights into the transplantability of mDA precursors as well as their connectivity after grafting and have interesting implications for the development of stem cell based approaches for the treatment of PD.