The BDNF Val66Met polymorphism (rs6265) enhances dopamine neuron graft efficacy and side-effect liability in rs6265 knock-in rats.

Prevalent in approximately 20% of the worldwide human population, the rs6265 (also called 'Val66Met') single nucleotide polymorphism (SNP) in the gene for brain-derived neurotrophic factor (BDNF) is a common genetic variant that can alter therapeutic responses in individuals with Parkinson's disease (PD). Possession of the variant Met allele results in decreased activity-dependent release of BDNF. Given the resurgent worldwide interest in neural transplantation for PD and the biological relevance of BDNF, the current studies examined the effects of the rs6265 SNP on therapeutic efficacy and side-effect development following primary dopamine (DA) neuron transplantation. Considering the significant reduction in BDNF release associated with rs6265, we hypothesized that rs6265-mediated dysfunctional BDNF signaling contributes to the limited clinical benefit observed in a subpopulation of PD patients despite robust survival of grafted DA neurons, and further, that this mutation contributes to the development of aberrant graft-induced dyskinesias (GID). To this end, we generated a CRISPR knock-in rat model of the rs6265 BDNF SNP to examine for the first time the influence of a common genetic polymorphism on graft survival, functional efficacy, and side-effect liability, comparing these parameters between wild-type (Val/Val) rats and those homozygous for the variant Met allele (Met/Met). Counter to our hypothesis, the current research indicates that Met/Met rats show enhanced graft-associated therapeutic efficacy and a paradoxical enhancement of graft-derived neurite outgrowth compared to wild-type rats. However, consistent with our hypothesis, we demonstrate that the rs6265 genotype in the host rat is strongly linked to development of GID, and that this behavioral phenotype is significantly correlated with neurochemical signatures of atypical glutamatergic neurotransmission by grafted DA neurons.

Combined cell-based therapy strategies for the treatment of Parkinson's disease: focus on mesenchymal stromal cells.

Parkinson's disease is a neurodegenerative condition characterized by motor impairments caused by the selective loss of dopaminergic neurons in the substantia nigra. Levodopa is an effective and well-tolerated dopamine replacement agent. However, levodopa provides only symptomatic improvements, without affecting the underlying pathology, and is associated with side effects after long-term use. Cell-based replacement is a promising strategy that offers the possibility to replace lost neurons in Parkinson's disease treatment. Clinical studies of transplantation of human fetal ventral mesencephalic tissue have provided evidence that the grafted dopaminergic neurons can reinnervate the striatum, release dopamine, integrate into the host neural circuits, and improve motor functions. One of the limiting factors for cell therapy in Parkinson's disease is the low survival rate of grafted dopaminergic cells. Different factors could cause cell death of dopaminergic neurons after grafting such as mechanical trauma, growth factor deprivation, hypoxia, and neuroinflammation. Neurotrophic factors play an essential role in the survival of grafted cells. However, direct, timely, and controllable delivery of neurotrophic factors into the brain faces important limitations. Different types of cells secrete neurotrophic factors constitutively and co-transplantation of these cells with dopaminergic neurons represents a feasible strategy to increase neuronal survival. In this review, we provide a general overview of the pioneering studies on cell transplantation developed in patients and animal models of Parkinson's disease, with a focus on neurotrophic factor-secreting cells, with a particular interest in mesenchymal stromal cells; that co-implanted with dopaminergic neurons would serve as a strategy to increase cell survival and improve graft outcomes.

Recent developments in the treatment of Parkinson's Disease.

Parkinson's disease (PD) is a common neurodegenerative disease typified by a movement disorder consisting of bradykinesia, rest tremor, rigidity, and postural instability. Treatment options for PD are limited, with most of the current approaches based on restoration of dopaminergic tone in the striatum. However, these do not alter disease course and do not treat the non-dopamine-dependent features of PD such as freezing of gait, cognitive impairment, and other non-motor features of the disorder, which often have the greatest impact on quality of life. As understanding of PD pathogenesis grows, novel therapeutic avenues are emerging. These include treatments that aim to control the symptoms of PD without the problematic side effects seen with currently available treatments and those that are aimed towards slowing pathology, reducing neuronal loss, and attenuating disease course. In this latter regard, there has been much interest in drug repurposing (the use of established drugs for a new indication), with many drugs being reported to affect PD-relevant intracellular processes. This approach offers an expedited route to the clinic, given that pharmacokinetic and safety data are potentially already available. In terms of better symptomatic therapies that are also regenerative, gene therapies and cell-based treatments are beginning to enter clinical trials, and developments in other neurosurgical strategies such as more nuanced deep brain stimulation approaches mean that the landscape of PD treatment is likely to evolve considerably over the coming years. In this review, we provide an overview of the novel therapeutic approaches that are close to, or are already in, clinical trials.

Neural grafting for Parkinson's disease: challenges and prospects.

Parkinson's disease (PD) is a neurodegenerative condition which causes a characteristic movement disorder secondary to loss of dopaminergic neurons in the substanitia nigra. The motor disorder responds well to dopamine-replacement therapies, though these result in significant adverse effects due to non-physiological release of dopamine in the striatum, and off-target effects. Cell-based regenerative treatments offer a potential means for targeted replacement of dopamine, in a physiological manner. Dopaminergic neurons for cell-based therapies can be obtained from several sources. Fetal ventral mesencephalon tissue contains dopaminergic neuron progenitors, and has been transplanted into the striatum of PD patients with good results in a number of cases. However, the ethical implications and logistical challenges of using fetal tissue mean that fetal ventral mesencephalon is unlikely to be used in a widespread clinical setting. Induced pluripotent stem cells can be used to generate dopaminergic neurons for transplantation, providing a source of autologous tissue for grafting. This approach means that challenges associated with allografts, such as the potential for immune rejection, can be circumvented. However, the associated cost and difficulty in producing a standardized product from different cell lines means that, at present, this approach is not commercially viable as a cell-based therapy. Dopaminergic neurons derived from embryonic stem cells offer the most promising basis for a cell-based therapy for Parkinson's disease, with trials due to commence in the next few years. Though there are ethical considerations to take into account when using embryonic tissue, the possibility of producing a standardized, optimized cell product means that this approach can be both effective, and commercially viable.

Cell therapies for Parkinson's disease: how far have we come?

Over the past three decades, significant progress has been made in the development of potential regenerative cell-based therapies for neurodegenerative disease, with most success being seen in Parkinson's disease. Cell-based therapies face many challenges including ethical considerations, potential for immune-mediated rejection with allogeneic and xenogeneic tissue, pathological spread of protein-related disease into the grafted tissue as well as the risk of graft overgrowth and tumorigenesis in stem cell-derived transplants. Preclinical trials have looked at many tissue types of which the most successful to date have been those using fetal ventral mesencephalon grafts, which led to clinical trials, which have shown that in some cases they can work very well. With important proof-of-concept derived from these studies, there is now much interest in how dopaminergic neurons derived from stem cell sources could be used to develop cell-based therapies suitable for clinical use, with clinical trials poised to enter the clinic in the next couple of years.

Charting new paths in neural regeneration: some prospects and possibilities after spinal cord injury.

Neural regeneration, once considered unlikely, is now a leading area of experimental research and its underpinning of future clinical approaches is a serious possibility. The background to current research into neural regeneration is reviewed and the major principles of neural grafting are explored. The tension between fundamental biological processes and therapeutic attempts to override these processes is assessed. Particular emphasis is placed on spinal cord transplantation studies. The roles of stem cells, foetal grafts, and gene therapy are investigated.

Scientific and ethical issues related to stem cell research and interventions in neurodegenerative disorders of the brain.

Should patients with Parkinson's disease participate in research involving stem cell treatments? Are induced pluripotent stem cells (iPSC) the ethical solution to the moral issues regarding embryonic stem cells? How can we adapt trial designs to best assess small numbers of patients in receipt of invasive experimental therapies? Over the last 20 years there has been a revolution in our ability to make stem cells from different sources and use them for therapeutic gain in disorders of the brain. These cells, which are defined by their capacity to proliferate indefinitely as well as differentiate into selective phenotypic cell types, are viewed as being especially attractive for studying disease processes and for grafting in patients with chronic incurable neurodegenerative disorders of the CNS such as Parkinson's disease (PD). In this review we briefly discuss and summarise where our understanding of stem cell biology has taken us relative to the clinic and patients, before dealing with some of the major ethical issues that work of this nature generates. This includes issues to do with the source of the cells, their ownership and exploitation along with questions about patient recruitment, consent and trial design when they translate to the clinic for therapeutic use.

Neural differentiation of transplanted neural stem cells in a rat model of striatal lacunar infarction: light and electron microscopic observations.

The increased risk and prevalence of lacunar stroke and Parkinson's disease (PD) makes the search for better experimental models an important requirement for translational research. In this study we assess ischemic damage of the nigrostriatal pathway in a model of lacunar stroke evoked by damaging the perforating arteries in the territory of the substantia nigra (SN) of the rat after stereotaxic administration of endothelin-1 (ET-1), a potent vasoconstrictor peptide. We hypothesized that transplantation of neural stem cells (NSCs) with the capacity of differentiating into diverse cell types such as neurons and glia, but with limited proliferation potential, would constitute an alternative and/or adjuvant therapy for lacunar stroke. These cells showed neuritogenic activity in vitro and a high potential for neural differentiation. Light and electron microscopy immunocytochemistry was used to characterize GFP-positive neurons derived from the transplants. 48 h after ET-1 injection, we characterized an area of selective degeneration of dopaminergic neurons within the nigrostriatal pathway characterized with tissue necrosis and glial scar formation, with subsequent behavioral signs of Parkinsonism. Light microscopy showed that grafted cells within the striatal infarction zone differentiated with a high yield into mature glial cells (GFAP-positive) and neuron types present in the normal striatum. Electron microscopy revealed that NSCs-derived neurons integrated into the host circuitry establishing synaptic contacts, mostly of the asymmetric type. Astrocytes were closely associated with normal small-sized blood vessels in the area of infarct, suggesting a possible role in the regulation of the blood brain barrier and angiogenesis. Our results encourage the use of NSCs as a cell-replacement therapy for the treatment of human vascular Parkinsonism.