Survival of Nigral Grafts within the Striatum of Marmosets with 6-Ohda Lesions Depends Critically on Donor Embryo Age

Repairing the Brain: Cell Replacement Using Stem Cell-Based Technologies

Current approaches to cell replacement therapy in Parkinson's disease are strongly focused on the dopamine system, with the view that restoring dopaminergic inputs in a localized and physiologic manner will provide superior benefits in terms of effect and longevity compared with oral medication. Experience using transplants of fetal tissue containing dopaminergic cell precursors has provided valuable proof that the approach is feasible, and that engrafted cells can survive and function over many years. However, multiple drawbacks and procedural complications are recognized in using fetal cells. Recent strides in stem cell technology now make it possible to overcome some of the barriers associated with fetal tissue. In particular the generation of high numbers of specific cell types, such as dopaminergic neurons, from stem cells means that quality, consistency, activity, and safety can be more thoroughly determined prior to transplantation, thus providing hope for more robust outcomes. These cells are also predicted to provide benefit without leading to the graft-induced dyskinesia that led to morbidity in a subset of individuals who underwent fetal mesencephalic cell and tissue grafting in the 1990s. In thinking about developing such novel therapeutics, the choice of starting material has also expanded, with the availability of multiple human embryonic stem cell lines, as well as the possibilities for producing induced pluripotent cells, or neuronal cells from a patient's own tissue. In this article, we speculate on how rapidly expanding knowledge and technical possibilities may impact on stem cell-based therapies for cell replacement in Parkinson's disease over the next two decades.

In vivo, in vitro and pharmacologic models of Parkinson's disease

Parkinson's disease (PD), which is the second most common neurodegenerative disorder after Alzheimer's disease, is firstly defined after James Parkinson's report. It carries motor symptoms such as resting tremor, bradykinesia and rigidity of skeletal muscle and freezing of gait. Furthermore, non-motor symptoms such as cognitive and behavioral problems, besides sensory impairments are seen in the patients. However, they may also suffer from sleep disorders or autonomic dysfunction. Although there are some medications in order to symptomatic management, but unfortunately, scientist could not have found exact approaches to cure this disease. Hence, producing a model which can express the most pathophysiologic and behavioral aspects of the disease is a desire. In this paper, we aimed to describe the different models of Parkinson's disease in brief.

Primary tissue for cellular brain repair in Parkinson's disease: Promise, problems and the potential of biomaterials

The dopamine precursor, levodopa, remains the "gold standard" treatment for Parkinson's disease, and, although it provides superlative efficacy in the early stages of the disease, its long-term use is limited by the development of severe motor side effects and a significant abating of therapeutic efficacy. Therefore, there remains a major unmet clinical need for the development of effective neuroprotective, neurorestorative or neuroreparatory therapies for this condition. The relatively selective loss of dopaminergic neurons from the nigrostriatal pathway makes Parkinson's disease an ideal candidate for reparative cell therapies, wherein the dopaminergic neurons that are lost in the condition are replaced through direct cell transplantation into the brain. To date, this approach has been developed, validated and clinically assessed using dopamine neuron-rich foetal ventral mesencephalon grafts which have been shown to survive and reinnervate the denervated brain after transplantation, and to restore motor function. However, despite long-term symptomatic relief in some patients, significant limitations, including poor graft survival and the impact this has on the number of foetal donors required, have prevented this therapy being more widely adopted as a restorative approach for Parkinson's disease. Injectable biomaterial scaffolds have the potential to improve the delivery, engraftment and survival of these grafts in the brain through provision of a supportive microenvironment for cell adhesion, growth and immune shielding. This article will briefly review the development of primary cell therapies for brain repair in Parkinson's disease and will consider the emerging literature which highlights the potential of using injectable biomaterial hydrogels in this context.

Transplantation in the nonhuman primate MPTP model of Parkinson's disease: update and perspectives

In order to calibrate stem cell exploitation for cellular therapy in neurodegenerative diseases, fundamental and preclinical research in NHP (nonhuman primate) models is crucial. Indeed, it is consensually recognized that it is not possible to directly extrapolate results obtained in rodent models to human patients. A large diversity of neurological pathologies should benefit from cellular therapy based on neural differentiation of stem cells. In the context of this special issue of Primate Biology on NHP stem cells, we describe past and recent advances on cell replacement in the NHP model of Parkinson's disease (PD). From the different grafting procedures to the various cell types transplanted, we review here diverse approaches for cell-replacement therapy and their related therapeutic potential on behavior and function in the NHP model of PD.

Neural Repair Strategies for Parkinson's Disease: Insights from Primate Models

Nonhuman primate models of Parkinson's disease (PD) have been invaluable to our understanding of the human disease and in the advancement of novel therapies for its treatment. In this review, we attempt to give a brief overview of the animal models of PD currently used, with a more comprehensive focus on the advantages and disadvantages presented by their use in the nonhuman primate. In particular, discussion addresses the 6-hydroxydopamine (6-OHDA), 1-methyl-1,2,3,6-tetrahydopyridine (MPTP), rotenone, paraquat, and maneb parkinsonian models. Additionally, the role of primate PD models in the development of novel therapies, such as trophic factor delivery, grafting, and deep brain stimulation, are described. Finally, the contribution of primate PD models to our understanding of the etiology and pathology of human PD is discussed.

La Enfermedad de Parkinson: Etiología, Tratamientos y Factores Preventivos

La enfermedad de Parkinson (EP) es la patología neurodegenerativa motora con mayor incidencia a nivel mundial. Esta afecta a aproximadamente 2-3% de la población mayor a 60 años de edad y sus causas aún no han sido bien determinadas. Actualmente no existe cura para esta patología; sin embargo, es posible contar con diferentes tratamientos que permiten aliviar algunos de sus síntomas y enlentecer su curso. Estos tratamientos tienen como premisa contrarrestar los efectos ocasionados por la pérdida de la función dopaminérgica de la sustancia nigra (SN) sobre estructuras como el núcleo subtálamico (NST) o globo pálido interno (GPi) ya sea por medio de tratamientos farmacológicos, estimulación cerebral profunda (ECP) o con el implante celular. Existen también investigaciones que están dirigiendo su interés al desarrollo de fármacos con potencial terapéutico, que presenten alta especificidad a receptores colinérgicos de nicotina (nAChRs) y antagonistas de receptores de adenosina, específicamente del subtipo A2A. Estos últimos, juegan un papel importante en el control de liberación dopaminérgica y en los procesos de neuroprotección. En esta revisión se pretende ofrecer una panorámica actual sobre algunos de los factores de riesgo asociados a EP, algunos de los tratamientos actuales más utilizados y acerca del rol de sustancias potencialmente útiles en la prevención de esta enfermedad.

Both creatine and its product phosphocreatine reduce oxidative stress and afford neuroprotection in an in vitro Parkinson's model

Creatine is the substrate for creatine kinase in the synthesis of phosphocreatine (PCr). This energetic system is endowed of antioxidant and neuroprotective properties and plays a pivotal role in brain energy homeostasis. The purpose of this study was to investigate the neuroprotective effect of creatine and PCr against 6-hydroxydopamine (6-OHDA)-induced mitochondrial dysfunction and cell death in rat striatal slices, used as an in vitro Parkinson's model. The possible involvement of the signaling pathway mediated by phosphatidylinositol-3 kinase (PI3K), protein kinase B (Akt), and glycogen synthase kinase-3β (GSK3β) was also evaluated. Exposure of striatal slices to 6-OHDA caused a significant disruption of the cellular homeostasis measured as 3-(4,5 dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide reduction, lactate dehydrogenase release, and tyrosine hydroxylase levels. 6-OHDA exposure increased the levels of reactive oxygen species and thiobarbituric acid reactive substances production and decreased mitochondrial membrane potential in rat striatal slices. Furthermore, 6-OHDA decreased the phosphorylation of Akt (Serine(473)) and GSK3β (Serine(9)). Coincubation with 6-OHDA and creatine or PCr reduced the effects of 6-OHDA toxicity. The protective effect afforded by creatine or PCr against 6-OHDA-induced toxicity was reversed by the PI3K inhibitor LY294002. In conclusion, creatine and PCr minimize oxidative stress in striatum to afford neuroprotection of dopaminergic neurons.

Long-term survival and development of fetal ventral spinal grafts into the motoneuron-depleted rat spinal cord: role of donor age

Fetal spinal cord (SC) tissue grafts can survive and develop into the lesioned SC, but no conclusive data are available concerning the long-term fate of transplanted material and the relation between the graft fate and the donor embryo age. Here, pre-labelled suspensions of ventral SC from E12 or E17 rat fetuses were grafted to the lumbar SC of adult rats with motoneuron depletion induced by perinatal injection of volkensin. E12 and E17 are presumably the stages when motoneuron development starts and terminates, respectively. Four or 10months post-grafting, SCs were analyzed to check the graft survival rate and to follow the differentiation and spatial distributions of grafted cells. Neurotoxic lesion produced a 61% motoneuronal loss in the lumbar SC. In transplanted animals, all E12 fetal grafts survived until the observed time-points and developed various mature cell phenotypes. Many motoneuron-like labelled cells were found within the graft area or adjacent to it. Conversely, none of the E17 fetal grafts survived, since no graft-derived elements with neuronal morphology were found either in the site of graft placement or adjacent to it. The present findings indicate that spinal neuroblasts can survive for a long time and develop within the motoneuron-depleted SC, and that the donor embryo age is crucial for successful engraftment.

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.

Human embryonic stem cells for brain repair?

Cell therapy has been perceived as the main or ultimate goal of human embryonic stem (ES) cell research. Where are we now and how are we going to get there? There has been rapid success in devising in vitro protocols for differentiating human ES cells to neuroepithelial cells. Progress has also been made to guide these neural precursors further to more specialized neural cells such as spinal motor neurons and dopamine-producing neurons. However, some of the in vitro produced neuronal types such as dopamine neurons do not possess all the phenotypes of their in vivo counterparts, which may contribute to the limited success of these cells in repairing injured or diseased brain and spinal cord in animal models. Hence, efficient generation of neural subtypes with correct phenotypes remains a challenge, although major hurdles still lie ahead in applying the human ES cell-derived neural cells clinically. We propose that careful studies on neural differentiation from human ES cells may provide more immediate answers to clinically relevant problems, such as drug discovery, mechanisms of disease and stimulation of endogenous stem cells.

Human embryonic stem cell-derived dopaminergic neurons reverse functional deficit in parkinsonian rats

We show that human embryonic stem cell-derived dopaminergic neurons survived transplantation to the neurotoxin 6-hydroxydopamine-lesioned rat striatum and, in combination with the cells newly differentiated from their progenitors, contributed to locomotive function recovery at 5 months. The animal behavioral improvement was correlated with the dopamine neurons present in the graft. Although the donor cells contained forebrain and midbrain dopamine neurons, the dopamine neurons present in the graft mainly exhibited a midbrain, or nigra, phenotype, suggesting the importance of midbrain dopamine neurons in functional repair. Furthermore, progenies of grafted cells were neurons and glia with greatly diminished mitotic activity by 5 months. Thus, the in vitro-produced human dopamine neurons can functionally engraft in the brain.

Transplanted dopamine neurons derived from primate ES cells preferentially innervate DARPP-32 striatal progenitors within the graft

The correct identity and functional capacity of transplanted dopamine (DA) neurons derived in vitro from embryonic stem (ES) cells is a critical factor for the development of an ES cell-based replacement therapy for Parkinson's disease. We transplanted primate Cyno-1 ES cells differentiated in vitro for 4 (progenitor ES cells) or 6 (differentiated ES cells) weeks, or control fetal primate cells into the striatum of hemi-parkinsonian rats. Partial behavioral recovery in amphetamine-induced rotation was correlated with the number of ES-derived tyrosine hydroxylase-positive (TH+) neurons in the grafts (r=0.5, P<0.05). Post mortem analysis of ES-derived grafts revealed TH+neurons with mature morphology, similar to fetal DA neurons, and expression of midbrain transcription factors, such as Engrailed (En) and Nurr-1. While the total number of TH+neurons was not different between the two groups, TH/En co-expression was significantly higher (>90%) in grafts from differentiated ES cells than in grafts derived from progenitor cells (<50%), reflecting a more heterogeneous cellular composition. Within the grafts there was an overlap between ES-derived TH+axonal arbors and clusters of primate ES-derived striatal neurons expressing brain factor 1 (Bf-1, Foxg1) and DA and cAMP-regulated phosphoprotein (DARPP-32). Such overlap was never observed for other regional transcription factors that define neighboring forebrain domains in the developing brain, such as Nkx2.1 (medial ganglionic eminence), Nkx2.2 (pallidal and diencephalic progenitors) or Pax6 (dorsal telencephalic progenitors). Despite the heterogeneity of ES-derived graft cell composition, these results demonstrate normal phenotypic specification, conserved natural axonal target selectivity and functionality of DA neurons derived from primate ES cells.

Neural tissue xenotransplantation: what is needed prior to clinical trials in Parkinson's disease? Neural Tissue Xenografting Project

Embryonic allografted human tissue in patients with Parkinson's disease has been shown to survive and ameliorate many of the symptoms of this disease. Despite this success, the practical problems of using this tissue coupled to the ethical restrictions of using aborted human fetal tissue have lead to an exploration for alternative sources of suitable material for grafting, including xenogeneic embryonic dopaminergic-rich neural tissue. Nevertheless, xenografted neural tissue itself generates a number of practical, ethical, safety, and immunological issues that have to be addressed prior to any clinical xenotransplant program. In this article we review these critical issues and set out the criteria that we consider need to be met in the development of our clinical xenotransplantation research programs. We advocate that these, or similar, criteria should be adopted and made explicit by other centers contemplating similar clinical trials.

Fetal porcine dopaminergic cell survival in vitro and its relationship to embryonic age

One of the critical factors in the survival of embryonic neural grafts is the age at which the population of donor neurons is harvested. This is especially the case for the developing dopaminergic neurons in the embryonic ventral mesencephalon, which are used for neural grafts in Parkinson's disease (PD). The donor age for optimal harvesting of these cells has been well characterized in the mouse, rat, and marmoset, and to a lesser extent in humans. However, the best donor age for porcine ventral mesencephalic tissue has not been ascertained, even though the use of this tissue for xenografts has been explored both experimentally and clinically. In this study the effect of donor age on dopaminergic cell survival was assessed in vitro, from a range of fetal pigs aged from E24 to E35. The number of tyrosine hydroxylase (TH)-positive cells per ventral mesencephalon was then calculated after 1 and 7 days in culture. E26-E27 embryos gave the highest yield of such cells at both survival time points, suggesting that this will be the optimal age for harvesting tissues whether for experimental or clinical nigral xenograft programs.