Dopamine cell implantation in Parkinson's disease: long-term clinical and (18)F-FDOPA PET outcomes

Treatment of Parkinson's disease using cell transplantation

The clinical trials with intrastriatal transplantation of human fetal mesencephalic tissue, rich in dopaminergic neurons, in Parkinson's disease (PD) patients show that cell replacement can work and in some cases induce major, long-lasting improvement. However, owing to poor tissue availability, this approach can only be applied in very few patients, and standardization is difficult, leading to wide variation in functional outcome. Stem cells and reprogrammed cells could potentially be used to produce dopaminergic neurons for transplantation. Importantly, dopaminergic neurons of the correct substantia nigra phenotype can now be generated from human embryonic stem cells in large numbers and standardized preparations, and will soon be ready for application in patients. Also, human induced pluripotent stem cell-derived dopaminergic neurons are being considered for clinical translation. Available data justify moving forward in a responsible way with these dopaminergic neurons, which should be tested, using optimal patient selection, cell preparation and transplantation procedures, in controlled clinical studies.

Cell Therapy for Parkinson's Disease: A Translational Approach to Assess the Role of Local and Systemic Immunosuppression

Neural transplantation is a promising therapeutic approach for neurodegenerative diseases; however, many patients receiving intracerebral fetal allografts exhibit signs of immunization to donor antigens that could compromise the graft. In this context, we intracerebrally transplanted mesencephalic pig xenografts into primates to identify a suitable strategy to enable long-term cell survival, maturation, and differentiation. Parkinsonian primates received WT or CTLA4-Ig transgenic porcine xenografts and different durations of peripheral immunosuppression to test whether systemic plus graft-mediated local immunosuppression might avoid rejection. A striking recovery of spontaneous locomotion was observed in primates receiving systemic plus local immunosuppression for 6 mo. Recovery was associated with restoration of dopaminergic activity detected both by positron emission tomography imaging and histological examination. Local infiltration by T cells and CD80/86+ microglial cells expressing indoleamine 2,3-dioxigenase were observed only in CTLA4-Ig recipients. Results suggest that in this primate neurotransplantation model, peripheral immunosuppression is indispensable to achieve the long-term survival of porcine neuronal xenografts that is required to study the beneficial immunomodulatory effect of local blockade of T cell costimulation.

Modulation of Abnormal Metabolic Brain Networks by Experimental Therapies in a Nonhuman Primate Model of Parkinson Disease: An Application to Human Retinal Pigment Epithelial Cell Implantation

Abnormal covariance pattern of regional metabolism associated with Parkinson disease (PD) is modulated by dopaminergic pharmacotherapy. Using high-resolution ¹⁸F-FDG PET and network analysis, we previously derived and validated a parkinsonism-related metabolic pattern (PRP) in nonhuman primate models of PD. It is currently not known whether this network is modulated by experimental therapeutics. In this study, we examined changes in network activity by striatal implantation of human levodopa-producing retinal pigment epithelial (hRPE) cells in parkinsonian macaques and evaluated the reproducibility of network activity in a small test-retest study.

METHODS

¹⁸F-FDG PET scans were acquired in 8 healthy macaques and 8 macaques with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced bilateral nigrostriatal dopaminergic lesions after unilateral putaminal implantation of hRPE cells or sham surgery. PRP activity was measured prospectively in all animals and in a subset of test-retest animals using a network quantification approach. Network activity and regional metabolic values were compared on a hemispheric basis between animal groups and treatment conditions.

RESULTS

All individual macaques showed clinical improvement after hRPE cell implantation compared with the sham surgery. PRP activity was elevated in the untreated MPTP hemispheres relative to those of the normal controls (P < 0.00005) but was reduced (P < 0.05) in the hRPE-implanted hemispheres. The modulation observed in network activity was supported by concurrent local and remote changes in regional glucose metabolism. PRP activity remained unchanged in the untreated MPTP hemispheres versus the sham-operated hemispheres. PRP activity was also stable (P ≥ 0.29) and correlated (R² ≥ 0.926; P < 0.00005) in the test-retest hemispheres. These findings were highly reproducible across several PRP topographies generated in multiple cohorts of parkinsonian and healthy macaques.

CONCLUSION

We have demonstrated long-term therapeutic effects of hRPE cell implantation in nonhuman primate models of PD. The implantation of such levodopa-producing cells can concurrently decrease the elevated metabolic network activity in parkinsonian brains on an individual basis. These results parallel the analogous findings reported in patients with PD undergoing levodopa therapy and other symptomatic interventions. With further validation in large samples, ¹⁸F-FDG PET imaging with network analysis may provide a viable biomarker for assessing treatment response in animal models of PD after experimental therapies.

Cell Therapy for Parkinson's Disease

In Parkinson’s disease (PD), dopamine neurons in the substantia nigra are degenerated and lost. Cell therapy for PD replaces the lost dopamine neurons by transplanting donor dopamine neural progenitor cells. Cell therapy for PD has been performed in the clinic since the 1980s and uses donor cells from the mesencephalon of aborted embryos. Regenerative medicine for PD using induced pluripotent stem (iPS) cell technology is drawing attention, because it offers a limitless and more advantageous source of donor cells than aborted embryos.

Embryonic stem cells derived neuron transplantation recovery in models of parkinsonism in relation to severity of the disorder in rats

6-Hydroxydopamine (6-OHDA)- and 1-methyl-4-phenylpyridinium (MPP(+))-induced hemi-parkinsonism was investigated in relation to the severity of the disorder in terms of behavioral disability and nigral neuronal loss and recovery regarding the number of stem cell-derived neurons transplanted in the striatum. Intra-median forebrain bundle infusion of the parkinsonian neurotoxins and intra-striatal transplantation of differentiated embryonic stem cells (ESCs) were carried out by rat brain stereotaxic surgery. The severity of the disease was determined using the number of amphetamine- or apomorphine-induced rotations, striatal dopamine levels as estimated by high-performance liquid chromatography (HPLC)-electrochemistry, and the number of surviving tyrosine hydroxylase immunoreactive dopaminergic neurons in the substantia nigra pars compacta. Rats that received unilateral infusion of 6-OHDA or MPP(+) responded with dose-dependent, unilateral bias in turning behavior when amphetamine or apomorphine was administered. Rotational asymmetry in both models correlated significantly well with the loss in the number of nigral dopaminergic neurons and striatal dopamine depletion. Transplantation of 2×10(5) differentiated murine ESCs revealed remarkably similar kinds of recovery in both animal models. The survival of the grafted dopaminergic cells in the striatum was better in animals with low-severity parkinsonism, but poor in the animals with severe parkinsonism. Amphetamine-induced rotational recovery correlated positively with an increasing number of cells transplanted in animals with uniform nigral neuronal lesion. These results suggest that disease severity is an important factor for determining the number of cells to be transplanted in parkinsonian rats for desirable recovery, which may be true in clinical conditions too.

Clinical translation of stem cell transplantation in Parkinson's disease

In Parkinson's disease (PD), the main pathology underlying the motor symptoms is a loss of nigrostriatal dopaminergic neurons. Clinical trials of intrastriatal transplantation of human foetal mesencephalic tissue have shown that the grafted dopaminergic neurons re-innervate the striatum, restore striatal dopamine release and, in some cases, induce major, long-lasting improvement of motor function. However, nonmotor symptoms originating from degeneration outside the striatum or in nondopaminergic systems are not alleviated by intrastriatal implantation of dopaminergic neurons. Stem cells and reprogrammed cells could potentially be used to produce dopaminergic neurons for transplantation in patients with PD. Recent studies demonstrate that standardized preparations of dopaminergic neurons of the correct substantia nigra phenotype can be generated from human embryonic stem cells in large numbers, and they will soon be available for patient application. In addition, dopaminergic neurons derived from human induced pluripotent stem cells are being considered for clinical translation. Important challenges include the demonstration of potency (growth capacity and functional efficacy) and safety of the generated dopaminergic neurons in preclinical animal models. The dopaminergic neurons should subsequently be tested, using optimal patient selection and cell preparation and transplantation procedures, in controlled clinical studies.

Imaging of genetic and degenerative disorders primarily causing Parkinsonism

In this chapter the structural and functional imaging changes associated with both genetic causes of Parkinson's disease and the sporadic condition are reviewed. The role of imaging for supporting diagnosis and detecting subclinical disease is discussed and the potential use and drawbacks of using imaging biomarkers for monitoring disease progression are debated. Additionally, the use of imaging for differentiating atypical parkinsonian syndromes from Parkinson's disease is presented.

Foetal Cell Transplantation for Parkinson's Disease: Focus on Graft-Induced Dyskinesia

Transplantation of dopamine- (DA-) rich foetal ventral mesencephalic cells emerged as a promising therapy for Parkinson's disease (PD), as it allowed significant improvement of motor symptoms in several PD patients in open-label studies. However, double-blind clinical trials have been largely disappointing. The general agreement in the field is that the lack of standardization of tissue collection and preparation, together with the absence of postsurgical immunosuppression, played a key role in the failure of these studies. Moreover, a further complication that emerged in previous studies is the appearance of the so-called graft-induced dyskinesia (GID), in a subset of grafted patients, which resembles dyskinesia induced by L-DOPA but in the absence of medication. Preclinical evidence pointed to the serotonin neurons as possible players in the appearance of GID. In agreement, clinical investigations have shown that grafted tissue may contain a large number of serotonin neurons, in the order of half of the DA cells; moreover, the serotonin 5-HT1A receptor agonist buspirone has been found to produce significant dampening of GID in grafted patients. In this paper, we will review the recent preclinical and clinical studies focusing on cell transplantation for PD and on the mechanisms underlying GID.

Neuroimaging of Parkinson's disease: Expanding views

Advances in molecular and structural and functional neuroimaging are rapidly expanding the complexity of neurobiological understanding of Parkinson's disease (PD). This review article begins with an introduction to PD neurobiology as a foundation for interpreting neuroimaging findings that may further lead to more integrated and comprehensive understanding of PD. Diverse areas of PD neuroimaging are then reviewed and summarized, including positron emission tomography, single photon emission computed tomography, magnetic resonance spectroscopy and imaging, transcranial sonography, magnetoencephalography, and multimodal imaging, with focus on human studies published over the last five years. These included studies on differential diagnosis, co-morbidity, genetic and prodromal PD, and treatments from L-DOPA to brain stimulation approaches, transplantation and gene therapies. Overall, neuroimaging has shown that PD is a neurodegenerative disorder involving many neurotransmitters, brain regions, structural and functional connections, and neurocognitive systems. A broad neurobiological understanding of PD will be essential for translational efforts to develop better treatments and preventive strategies. Many questions remain and we conclude with some suggestions for future directions of neuroimaging of PD.

Cell Therapy for Parkinson's Disease: New Hope from Reprogramming Technologies

Parkinson's disease (PD) is a neurodegenerative disease with the major pathology being the progressive loss of dopaminergic (DA) midbrain neurons in the substantia nigra. As early as in the 1980s, open-label clinical trials employing fetal ventral mesencephalon (fVM) tissues have demonstrated significant efficacy for PD treatment, which led to two NIH-sponsored double-blind placebo-controlled clinical trials. However, both trials showed only mild outcome. Retrospective analysis revealed several possible reasons that include patient selection, heterogeneity of grafts, immune recognition of grafts, lack of standardization of transplantation procedure and uneven distribution of grafts. Recent years have seen advances in reprogramming technologies which may provide solutions to the problems associated with fVM tissues. Induced pluripotent stem cells (iPSCs) and induced neural stem cells (iNSCs) hold promise for generating clinical grade DA neural cells that are safe, homogeneous, scalable and standardizable. These new technologies may bring back clinical trials using cell therapy for PD treatment in the future.

Potential of Neural Stem Cell-Based Therapy for Parkinson's Disease

Neural stem cell (NSC) transplantation is an emerging strategy for restoring neuronal function in neurological disorders, such as Parkinson's disease (PD), which is characterized by a profound and selective loss of nigrostriatal dopaminergic (DA) neurons. Adult neurogenesis generates newborn neurons that can be observed at specialized niches where endothelial cells (ECs) play a significant role in regulating the behavior of NSCs, including self-renewal and differentiating into all neural lineage cells. In this minireview, we highlight the importance of establishing an appropriate microenvironment at the target site of NSC transplantation, where grafted cells integrate into the surroundings in order to enhance DA neurotransmission. Using a novel model of NSC-EC coculture, it is possible to combine ECs with NSCs, to generate such a neurovascular microenvironment. With appropriate NSCs selected, the composition of the transplant can be investigated through paracrine and juxtacrine signaling within the neurovascular unit (NVU). With target site cellular and acellular compartments of the microenvironment recognized, guided DA differentiation of NSCs can be achieved. As differentiated DA neurons integrate into the existing nigrostriatal DA pathway, the symptoms of PD can potentially be alleviated by reversing characteristic neurodegeneration.

Stem cell transplantation therapy in Parkinson's disease

Ineffective therapeutic treatments and inadequate repair ability in the central nervous system are disturbing problems for several neurological diseases. Fortunately, the development of clinically applicable populations of stem cells has provided an avenue to overcome the failure of endogenous repair systems and substitute new cells into the damaged brain. However, there are still several existing obstacles to translating into clinical application. Here we review the stem-cell based therapies for Parkinson’s disease and discuss the potential advantages and drawbacks. We hope this review may provide suggestions for viable strategies to overcome the current technical and biological issues associated with the application of stem cells in Parkinson’s disease.

Development of stem cell-based therapy for Parkinson's disease

Parkinson’s disease (PD) is one of the most common neurodegenerative disorders of aging, characterized by the degeneration of dopamine neurons (DA neurons) in the substantial nigra, leading to the advent of both motor symptoms and non-motor symptoms. Current treatments include electrical stimulation of the affected brain areas and dopamine replacement therapy. Even though both categories are effective in treating PD patients, the disease progression cannot be stopped. The research advance into cell therapies provides exciting potential for the treatment of PD. Current cell sources include neural stem cells (NSCs) from fetal brain tissues, human embryonic stem cells (hESCs), induced pluripotent stem cells (iPSCs) and directly induced dopamine neurons (iDA neurons). Here, we evaluate the research progress in different cell sources with a focus on using iPSCs as a valuable source and propose key challenges for developing cells suitable for large-scale clinical applications in the treatment of PD.

Advances in Neurotrophic Factor and Cell-Based Therapies for Parkinson's Disease: A Mini-Review

Parkinson's disease (PD) affects an estimated 7-10 million people worldwide and remains without definitive or disease-modifying treatment. There have been many recent developments in cell-based therapy (CBT) to replace lost circuitry and provide chronic biological sources of therapeutic agents to the PD-affected brain. Early neural transplantation studies underscored the challenges of immune compatibility, graft integration and the need for renewable, autologous graft sources. Neurotrophic factors (NTFs) offer a potential class of cytoprotective pharmacotherapeutics that may complement dopamine (DA) replacement and CBT strategies in PD. Chronic NTF delivery may be an integral goal of CBT, with grafts consisting of autologous drug-producing (e.g., DA, NTF) cells that are capable of integration and function in the host brain. In this mini-review, we outline the past experience and recent advances in NTF technology and CBT as promising and integrated approaches for the treatment of PD.

Cell-based therapies for Parkinson disease—past insights and future potential

Parkinson disease (PD) is characterized by loss of the A9 nigral neurons that provide dopaminergic innervation to the striatum. This discovery led to the successful instigation of dopaminergic drug treatments in the 1960s, although these drugs were soon recognized to lose some of their efficacy and generate their own adverse effects over time. Despite the fact that PD is now known to have extensive non-nigral pathology with a wide range of clinical features, dopaminergic drug therapies are still the mainstay of therapy, and work well for many years. Given the success of pharmacological dopamine replacement, pursuit of cell-based dopamine replacement strategies seemed to be the next logical step, and studies were initiated over 30 years ago to explore the possibility of dopaminergic cell transplantation. In this Review, we outline the history of this therapeutic approach to PD and highlight the lessons that we have learned en route. We discuss how the best clinical outcomes have been obtained with fetal ventral mesencephalic allografts, while acknowledging inconsistencies in the results owing to problems in trial design, patient selection, tissue preparation, and immunotherapy used post-grafting. We conclude by discussing the challenges of bringing the new generation of stem cell-derived dopamine cells to the clinic.

A Meta-Analysis of Mesenchymal Stem Cells in Animal Models of Parkinson's Disease

Multiple studies have been performed to evaluate the effects of mesenchymal stem cells (MSCs) in animal models of Parkinson's disease (PD). We performed a meta-analysis to estimate the treatment effect of unmodified MSCs on behavioral outcomes in preclinical studies of PD. We performed a systematic literature search to identify studies that used behavioral testing to evaluate the treatment effect of unmodified MSCs in PD models. Meta-analysis was used to determine pooled effect size for rotational behavior and limb function, and meta-regression was performed to explore sources of heterogeneity. Twenty-five studies, including three delivery routes, a wide range of doses, and multiple PD models, were examined. Significant improvement was seen in the pooled standardized mean difference (SMD) for both rotational behavior [SMD: 1.24, 95% confidence interval (95% CI): 0.84, 1.64] and limb function (SMD: 0.84, 95% CI: 0.01, 1.66). Using meta-regression, intravenous administration and higher dose had a larger effect on limb function. Treatment with MSCs improves behavioral outcomes in PD models. Our analyses suggest that MSCs could be considered for early-stage clinical trials in the treatment of PD.

Moving stem cells to the clinic: potential and limitations for brain repair

Stem cell-based therapies hold considerable promise for many currently devastating neurological disorders. Substantial progress has been made in the derivation of disease-relevant human donor cell populations. Behavioral data in relevant animal models of disease have demonstrated therapeutic efficacy for several cell-based approaches. Consequently, cGMP grade cell products are currently being developed for first in human clinical trials in select disorders. Despite the therapeutic promise, the presumed mechanism of action of donor cell populations often remains insufficiently validated. It depends greatly on the properties of the transplanted cell type and the underlying host pathology. Several new technologies have become available to probe mechanisms of action in real time and to manipulate in vivo cell function and integration to enhance therapeutic efficacy. Results from such studies generate crucial insight into the nature of brain repair that can be achieved today and push the boundaries of what may be possible in the future.

Dopaminergic correlates of metabolic network activity in Parkinson's disease

Parkinson's disease (PD) is associated with distinct metabolic covariance patterns that relate to the motor and cognitive manifestations of the disorder. It is not known, however, how the expression of these patterns relates to measurements of nigrostriatal dopaminergic activity from the same individuals. To explore these associations, we studied 106 PD subjects who underwent cerebral PET with both (18) F-fluorodeoxyglucose (FDG) and (18) F-fluoro-L-dopa (FDOPA). Expression values for the PD motor- and cognition-related metabolic patterns (PDRP and PDCP, respectively) were computed for each subject; these measures were correlated with FDOPA uptake on a voxel-by-voxel basis. To explore the relationship between dopaminergic function and local metabolic activity, caudate and putamen FDOPA PET signal was correlated voxel-wise with FDG uptake over the entire brain. PDRP expression correlated with FDOPA uptake in caudate and putamen (P < 0.001), while PDCP expression correlated with uptake in the anterior striatum (P < 0.001). While statistically significant, the correlations were only of modest size, accounting for less than 20% of the overall variation in these measures. After controlling for PDCP expression, PDRP correlations were significant only in the posterior putamen. Of note, voxel-wise correlations between caudate/putamen FDOPA uptake and whole-brain FDG uptake were significant almost exclusively in PDRP regions. Overall, the data indicate that PDRP and PDCP expression correlates significantly with PET indices of presynaptic dopaminergic functioning obtained in the same individuals. Even so, the modest size of these correlations suggests that in PD patients, individual differences in network activity cannot be explained solely by nigrostriatal dopamine loss.

Phosphodiesterase 7 inhibition induces dopaminergic neurogenesis in hemiparkinsonian rats

Parkinson's disease is characterized by a loss of dopaminergic neurons in a specific brain region, the ventral midbrain. Parkinson's disease is diagnosed when approximately 50% of the dopaminergic neurons of the substantia nigra pars compacta (SNpc) have degenerated and the others are already affected by the disease. Thus, it is conceivable that all therapeutic strategies, aimed at neuroprotection, start too late. Therefore, an urgent medical need exists to discover new pharmacological targets and novel drugs with disease-modifying properties. In this regard, modulation of endogenous adult neurogenesis toward a dopaminergic phenotype might provide a new strategy to target Parkinson's disease by partially ameliorating the dopaminergic cell loss that occurs in this disorder. We have previously shown that a phosphodiesterase 7 (PDE7) inhibitor, S14, exerts potent neuroprotective and anti-inflammatory effects in different rodent models of Parkinson's disease, indicating that this compound could represent a novel therapeutic agent to stop the dopaminergic cell loss that occurs during the progression of the disease. In this report we show that, in addition to its neuroprotective effect, the PDE7 inhibitor S14 is also able to induce endogenous neuroregenerative processes toward a dopaminergic phenotype. We describe a population of actively dividing cells that give rise to new neurons in the SNpc of hemiparkinsonian rats after treatment with S14. In conclusion, our data identify S14 as a novel regulator of dopaminergic neuron generation.

SIGNIFICANCE

Parkinson's disease is a neurodegenerative disorder characterized by the loss of dopaminergic neurons in the ventral midbrain. Currently, no cure and no effective disease-modifying therapy are available for Parkinson's disease; therefore, an urgent medical need exists to discover new pharmacological targets and novel drugs for the treatment of this disorder. The present study reports that an inhibitor of the enzyme phosphodiesterase 7 (S14) induces proliferation in vitro and in vivo of neural stem cells, promoting its differentiation toward a dopaminergic phenotype and therefore enhancing dopaminergic neuron generation. Because this drug is also able to confer neuroprotection of these cells in animal models of Parkinson's disease, S14 holds great promise as a therapeutic new strategy for this disorder.

Parkinson's disease: fetal cell or stem cell-derived treatments

INTRODUCTION

The mean age of onset of Parkinson's disease is about 65 years, with a median time of 9 years between diagnosis and death.

METHODS AND OUTCOMES

We conducted a systematic review and aimed to answer the following clinical question: What are the effects of fetal cell or stem cell-derived therapy in people with Parkinson's disease? We searched: Medline, Embase, The Cochrane Library and other important databases up to September 2014 (Clinical Evidence reviews are updated periodically; please check our website for the most up-to-date version of this review). We included harms alerts from relevant organisations such as the US Food and Drug Administration (FDA) and the UK Medicines and Healthcare products Regulatory Agency (MHRA).

RESULTS

We found two studies that met our inclusion criteria. We performed a GRADE evaluation of the quality of evidence for interventions.

CONCLUSIONS

In this systematic review we present information relating to the effectiveness and safety of the following interventions: fetal cell therapy versus deep brain stimulation; fetal cell therapy versus sham surgery; stem cell-derived therapy versus deep brain stimulation; stem cell-derived therapy versus sham surgery.

The potential of alternate sources of cells for neural grafting in Parkinson's and Huntington's disease

Cell-based therapies for Parkinson's and Huntington's disease have provided mixed clinical outcomes and one of the reasons underlying this is the use of primary fetal tissue as the source of grafted cells. An alternate source of cells, such as stem cells, could overcome many of the issues associated with primary fetal tissue and would help bring forward cell replacement therapy as a reliable and effective treatment for these two neurodegenerative disorders. This review will discuss which stem cells are likely to go to clinic in the next generation of cells, based on trials for Parkinson's and Huntington's disease.

Fetal stem cell transplantation: Past, present, and future

Since 1928, human fetal tissues and stem cells have been used worldwide to treat various conditions. Although the transplantation of the fetal midbrain substantia nigra and dopaminergic neurons in patients suffering from Parkinson’s disease is particularly noteworthy, the history of other types of grafts, such as those of the fetal liver, thymus, and pancreas, should be addressed as there are many lessons to be learnt for future stem cell transplantation. This report describes previous practices and complications that led to current clinical trials of isolated fetal stem cells and embryonic stem (ES) cells. Moreover, strategies for transplantation are considered, with a particular focus on donor cells, cell processing, and the therapeutic cell niche, in addition to ethical issues associated with fetal origin. With the advent of autologous induced pluripotent stem cells and ES cells, clinical dependence on fetal transplantation is expected to gradually decline due to lasting ethical controversies, despite landmark achievements.

Molecular imaging using nanoparticle quenchers of Cerenkov luminescence

Cerenkov luminescence (CL) imaging is an emerging technique that collects the visible photons produced by radioisotopes. Here, molecular imaging strategies are investigated that switch the CL signal off. The noninvasive molecularly specific detection of cancer is demonstrated utilizing a combination of clinically approved agents, and their analogues. CL is modulated in vitro in a dose dependent manner using approved small molecules (Lymphazurin), as well as the clinically approved Feraheme and other preclinical superparamagnetic iron oxide nanoparticles (SPIO). To evaluate the quenching of CL in vivo, two strategies are pursued. [(18) F]-FDG is imaged by PET and CL in tumors prior to and following accumulation of nanoparticles. Initially, non-targeted particles are administered to mice bearing tumors in order to attenuate CL. For targeted imaging, a dual tumor model (expressing the human somatostatin receptor subtype-2 (hSSTr2) and a control negative cell line) is used. Targeting hSSTr2 with octreotate-conjugated SPIO, quenched CL enabling non-invasive distinction between tumors' molecular expression profiles is demonstrated. In this work, the quenching of Cerenkov emissions is demonstrated in several proof of principle models using a combination of approved agents and nanoparticle platforms to provide disease relevant information including tumor vascularity and specific antigen expression.

Proceedings: cell therapies for Parkinson's disease from discovery to clinic

In March 2013, the California Institute for Regenerative Medicine, in collaboration with the NIH Center for Regenerative Medicine, held a 2-day workshop on cell therapies for Parkinson's disease (PD), with the goals of reviewing the state of stem cell research for the treatment of PD and discussing and refining the approach and the appropriate patient populations in which to plan and conduct new clinical trials using stem cell-based therapies for PD. Workshop participants identified priorities for research, development, and funding; discussed existing resources and initiatives; and outlined a path to the clinic for a stem cell-based therapy for PD. A consensus emerged among participants that the development of cell replacement therapies for PD using stem cell-derived products could potentially offer substantial benefits to patients. As with all stem cell-based therapeutic approaches, however, there are many issues yet to be resolved regarding the safety, efficacy, and methodology of transplanting cell therapies into patients. Workshop participants agreed that designing an effective stem cell-based therapy for PD will require further research and development in several key areas. This paper summarizes the meeting.

Molecular imaging as a guide for the treatment of central nervous system disorders

Molecular imaging techniques have a number of advantages for research into the pathophysiology and treatment of central nervous system (CNS) disorders. Firstly, they provide a noninvasive means of characterizing physiological processes in the living brain, enabling molecular alterations to be linked to clinical changes. Secondly, the pathophysiological target in a given CNS disorder can be measured in animal models and in experimental human models in the same way, which enables translational research. Moreover, as molecular imaging facilitates the detection of functional change which precedes gross pathology, it is particularly useful for the early diagnosis and treatment of CNS disorders.

This review considers the application of molecular imaging to CNS disorders focusing on its potential to inform the development and evaluation of treatments. We focus on schizophrenia, Parkinson's disease, depression, and dementia as major CNS disorders. We also review the potential of molecular imaging to guide new drug development for CNS disorders.

Long-term health of dopaminergic neuron transplants in Parkinson's disease patients

To determine the long-term health and function of transplanted dopamine neurons in Parkinson's disease (PD) patients, the expression of dopamine transporters (DATs) and mitochondrial morphology were examined in human fetal midbrain cellular transplants. DAT was robustly expressed in transplanted dopamine neuron terminals in the reinnervated host putamen and caudate for at least 14 years after transplantation. The transplanted dopamine neurons showed a healthy and nonatrophied morphology at all time points. Labeling of the mitochondrial outer membrane protein Tom20 and α-synuclein showed a typical cellular pathology in the patients' own substantia nigra, which was not observed in transplanted dopamine neurons. These results show that the vast majority of transplanted neurons remain healthy for the long term in PD patients, consistent with clinical findings that fetal dopamine neuron transplants maintain function for up to 15-18 years in patients. These findings are critically important for the rational development of stem-cell-based dopamine neuronal replacement therapies for PD.

Potential for cell therapy in Parkinson's disease using genetically programmed human embryonic stem cell-derived neural progenitor cells

Neural transplantation is a promising strategy for restoring dopaminergic dysfunction and modifying disease progression in Parkinson's disease (PD). Human embryonic stem cells (hESCs) are a potential resource in this regard because of their ability to provide a virtually limitless supply of homogenous dopaminergic progenitors and neurons of appropriate lineage. The recent advances in developing robust cell culture protocols for directed differentiation of hESCs to near pure populations of ventral mesencephalic (A9-type) dopaminergic neurons has heightened the prospects for PD cell therapy. Here, we focus our review on current state-of-the-art techniques for harnessing hESC-based strategies toward development of a stem cell therapeutic for PD. Importantly, we also briefly describe a novel genetic-programming approach that may address many of the key challenges that remain in the field and that may hasten clinical translation.

Treating Parkinson's disease in the 21st century: can stem cell transplantation compete?

The characteristic and selective degeneration of a unique population of cells—the nigrostriatal dopamine (DA) neurons—that occurs in Parkinson’s disease (PD) has made the condition an iconic target for cell replacement therapies. Indeed, transplantation of fetal ventral mesencephalic cells into the DA-deficient striatum was first trialled nearly 30 years ago, at a time when other treatments for the disease were less well developed. Over recent decades standard treatments for PD have advanced, and newer biological therapies are now emerging. In the 21st century, stem cell technology will have to compete alongside other sophisticated treatments, including deep brain stimulation and gene therapies. In this review we examine how stem cell–based transplantation therapies compare with these novel and emerging treatments in the management of this common condition. J. Comp. Neurol. 522:2802–2816, 2014.

Region-specific restoration of striatal synaptic plasticity by dopamine grafts in experimental parkinsonism

Intrastriatal transplantation of dopaminergic neurons can restore striatal dopamine levels and improve parkinsonian deficits, but the mechanisms underlying these effects are poorly understood. Here, we show that transplants of dopamine neurons partially restore activity-dependent synaptic plasticity in the host striatal neurons. We evaluated synaptic plasticity in regions distal or proximal to the transplant (i.e., dorsolateral and ventrolateral striatum) and compared the effects of dopamine- and serotonin-enriched grafts using a rat model of Parkinson disease. Naïve rats showed comparable intrinsic membrane properties in the two subregions but distinct patterns of long-term synaptic plasticity. The ventrolateral striatum showed long-term potentiation using the same protocol that elicited long-term depression in the dorsolateral striatum. The long-term potentiation was linked to higher expression of postsynaptic AMPA and N2B NMDA subunits (GluN2B) and was dependent on the activation of GluN2A and GluN2B subunits and the D1 dopamine receptor. In both regions, the synaptic plasticity was abolished after a severe dopamine depletion and could not be restored by grafted serotonergic neurons. Solely, dopamine-enriched grafts could restore the long-term potentiation and partially restore motor deficits in the rats. The restoration could only be seen close to the graft, in the ventrolateral striatum where the graft-derived reinnervation was denser, compared with the distal dorsolateral region. These data provide proof of concept that dopamine-enriched transplants are able to functionally integrate into the host brain and restore deficits in striatal synaptic plasticity after experimental parkinsonism. The region-specific restoration might impose limitations in symptomatic improvement following neural transplantation.

Dopamine: PET Imaging and Parkinson Disease

This article discusses the current use of PET imaging in the evaluation of dopamine function in Parkinson disease (PD). The article reviews the major radioligands targeting dopaminergic systems in patients with parkinsonian disorders. The primary objective is to show the novel clinical applications of molecular imaging in the diagnosis and assessment of motor and nonmotor symptoms in PD.

Cell therapy for Parkinson's disease: what next?

The idea to use transplants of dopamine-producing cells to substitute for the lost midbrain dopamine neurons in Parkinson's disease (PD) goes back to the 1970s. In this review we give an overview of the history of cell transplantation in animal models of PD, and summarize the experience gained from the open-label and placebo-controlled clinical trials performed so far using intrastriatal transplants of human fetal dopamine neuroblasts. Further development of this therapeutic approach face numerous challenges, for example in the development of protocols that allow generation of fully functional and safe midbrain dopamine neurons from stem cells. Based on recent promising advancements, efforts are now being made to develop standardized and efficient protocols, and adapt these protocols to good laboratory practice (GLP)/good manufacturing practice (GMP) conditions, to move this technology closer to clinical translation.

Improved cell therapy protocols for Parkinson's disease based on differentiation efficiency and safety of hESC-, hiPSC-, and non-human primate iPSC-derived dopaminergic neurons

The main motor symptoms of Parkinson's disease are due to the loss of dopaminergic (DA) neurons in the ventral midbrain (VM). For the future treatment of Parkinson's disease with cell transplantation it is important to develop efficient differentiation methods for production of human iPSCs and hESCs-derived midbrain-type DA neurons. Here we describe an efficient differentiation and sorting strategy for DA neurons from both human ES/iPS cells and non-human primate iPSCs. The use of non-human primate iPSCs for neuronal differentiation and autologous transplantation is important for preclinical evaluation of safety and efficacy of stem cell-derived DA neurons. The aim of this study was to improve the safety of human- and non-human primate iPSC (PiPSC)-derived DA neurons. According to our results, NCAM(+) /CD29(low) sorting enriched VM DA neurons from pluripotent stem cell-derived neural cell populations. NCAM(+) /CD29(low) DA neurons were positive for FOXA2/TH and EN1/TH and this cell population had increased expression levels of FOXA2, LMX1A, TH, GIRK2, PITX3, EN1, NURR1 mRNA compared to unsorted neural cell populations. PiPSC-derived NCAM(+) /CD29(low) DA neurons were able to restore motor function of 6-hydroxydopamine (6-OHDA) lesioned rats 16 weeks after transplantation. The transplanted sorted cells also integrated in the rodent brain tissue, with robust TH+/hNCAM+ neuritic innervation of the host striatum. One year after autologous transplantation, the primate iPSC-derived neural cells survived in the striatum of one primate without any immunosuppression. These neural cell grafts contained FOXA2/TH-positive neurons in the graft site. This is an important proof of concept for the feasibility and safety of iPSC-derived cell transplantation therapies in the future.

Neurotrophic factor expression in expandable cell populations from brain samples in living patients with Parkinson's disease

Cell-based therapies offer promise for patients with Parkinson's disease (PD); however, durable and effective transplantation substrates need to be defined. This study characterized the feasibility and growth properties of primary cultures established from small-volume brain biopsies taken during deep brain stimulation (DBS) surgery in patients with PD. The lineage and expression of neurotrophic factors with known beneficial actions in PD-affected brain circuitry were also evaluated. Nineteen patients with PD undergoing DBS surgery consented to brain biopsies prior to electrode implantation. Cultures from these samples exhibited exponential and plateau phases of growth and were readily expanded throughout multiple passages. There was robust expression of progenitor markers and the unexpected colocalization of neural and mesenchymal proteins. The oligodendrocyte transcription factor, Olig1, and the myelin-specific sphingolipid, galactocerebroside, were coexpressed with each of glial-derived neurotrophic factor, brain-derived neurotrophic factor, and cerebral dopamine neurotrophic factor. Fluorescence-activated cell sorting demonstrated homogeneous expression of both nestin and Olig1 throughout the expanded cultures. Cells remained viable after a year in cryostorage. These findings confirm the feasibility of small brain biopsies as an expandable source of autologous cell substrate in living patients and demonstrate the complex phenotype of these cells, with implications for therapeutic application in PD and other neurological diseases.

Pluripotent stem cells for Parkinson's disease: progress and challenges

Parkinson's disease (PD) is a common debilitating neurodegenerative disease. The motor symptoms of PD are caused mainly by a progressive loss of dopaminergic neurons from the substania nigra, resulting in a loss of dopamine production. Current therapies are palliative and, in the long term, ineffective. In addition, some can result in significant clinical side effects. The relatively localized pathology of PD makes it an ideal candidate for cell replacement therapy. Initial efforts focused on fetal cell transplantation, and significant clinical benefit lasting more than 10 years has been reported in some cases. However, the approach is controversial and results have been inconsistent. Inherent limitations of this approach for widespread use are the limited availability and variability of transplant material. In contrast, the self-renewal and differentiation potential of human pluripotent stem cells (hPSCs) make them a promising alternative cell source for cell replacement therapy for PD. Efforts in the past decade have demonstrated that hPSCs can be induced to differentiate in culture to functional dopaminergic neurons. Studies in delivering these cells into PD animal models have demonstrated survival, engraftment, and behavioral deficit improvements. Several groups are developing these cells with clinical trials in mind. Here, we review the state of the technology and consider the suitability of current manufacturing processes, cell purity, and tumorgenicity for clinical testing.

Fetal dopaminergic transplantation trials and the future of neural grafting in Parkinson's disease

Clinical use of allografts of fetal ventral mesencephalic tissue as a treatment to replace dopaminergic neurons in patients with Parkinson's disease was first done more than 20 years ago. Since then, many patients have received transplants, with variable results. During this time, our knowledge of Parkinson's disease has changed and the nature and extent of problems associated with the disorder have been better defined. Our understanding on how best to implement this cell-replacement strategy for patients has grown, but gaining this insight has entailed critical reappraisal of data from transplant trials that have already been undertaken.

PET imaging of serotonin transporters with 4-[18F]-ADAM in a Parkinsonian rat model

This study was undertaken to address the effects of fetal mesencephalic tissue transplantation on the serotonin system in a rat model of Parkinson's disease (PD) while also investigating the usefulness of 4-[18F]-ADAM (a serotonin transporter imaging agent) coupled with micro-PET for imaging serotonin transporters (SERTs). A PD model was induced by unilateral injection of 6-hydroxydopamine (6-OHDA) into the right medial forebrain bundle of the nigrostriatal pathway, while cell transplantation was performed via intrastriatal injection of mesencephalic brain tissue dissected from embryonic (E14) rats. The 4-[18F]-ADAM/micro-PET scanning was performed following both 6-OHDA lesioning and transplantation. Immunohistochemistry (IHC) studies were also performed following the final PET scan, and the results were compared to show a 17-43% decrease in the specific uptake ratio (SUR) and a 23-52% decrease in serotonin transporter immunoreactivity (SERT-ir) within various brain regions on the lesioned side. The number of methamphetamine-induced rotations also decreased significantly at the 4th week postgraft. In addition, striatal SUR and the SERT-ir levels were restored to 77% and 83% 5 weeks postgraft. These results suggest that Parkinson's disease also affects the serotonergic system, while both the dopaminergic and serotonergic systems can be partially restored in a rat model of PD after E14 mesencephalic tissue transplantation. In addition, we have also determined that 4-[18F]-ADAM/micro-PET can be used to detect serotonergic neuron loss, monitor the progress of Parkinson's disease, and oversee the effectiveness of therapy.

The promise and perils of stem cell therapeutics

Stem cells are the seeds of tissue repair and regeneration and a promising source for novel therapies. However, apart from hematopoietic stem cell (HSC) transplantation, essentially all other stem cell treatments remain experimental. High hopes have inspired numerous clinical trials, but it has been difficult to obtain unequivocal evidence for robust clinical benefit. In recent years, unproven therapies have been widely practiced outside the standard clinical trial network, threatening the cause of legitimate clinical investigation. Numerous challenges and technical barriers must be overcome before novel stem cell therapies can achieve meaningful clinical impact.

Using stem cells and iPS cells to discover new treatments for Parkinson's disease

Fetal cell transplantation can improve the symptoms of Parkinson's disease (PD) patients for more than a decade. In some patients, alpha-synuclein aggregates and Lewy bodies have been observed in the transplanted neurons without functional significance. Recently stem cells have emerged as an ethically acceptable source of cells for transplantation but, importantly, the type of stem cell matters. While the lineage restriction of adult neural stem cells limits their clinical applicability for patients with PD, human pluripotent stem cells provide an opportunity to replace specific types of degenerating neurons. Now, cellular reprogramming technology can provide patient-specific neurons for neural transplantation and problems with cell fate specification and safety are resolving. Induced pluripotent stem (iPS) cell-derived neurons are also a unique tool for interpreting the genetic basis for an individual's risk of developing PD into clinically meaningful information. For example, clinical trials for neuroprotective molecules need to be tested in presymptomatic individuals when the neurons can still be protected. Patient-specific neural cells can also be used to identify an individual's responsiveness to drugs and to understand the mechanisms of the disease. Along these avenues of investigation, stem cells are enabling research for new treatments in PD.

Stem cells and regenerative therapies for Parkinson's disease

Currently the mainstay of Parkinson's disease (PD) therapy is the pharmacological replacement of the loss of the dopaminergic nigrostriatal pathway using drugs such as dopamine agonists and levodopa. Whilst these drugs effectively ameliorate some of the motor features of PD, they do not improve many of the nonmotor features that arise secondary to pathology outside of this system, nor do they slow the progressive neurodegeneration that is a characteristic of the disease. Regenerative therapies for PD seek to fill this therapeutic gap, with cell transplantation being the most explored approach to date. A number of different cell sources have been used in this therapeutic approach, but to date, the most successful has been the use of fetal ventral mesencephalic (VM) tissue that contains within it the developing nigral dopaminergic cells. Cell transplantation for PD was pioneered in the 1980-1990s, with several successful open-label trials of fetal VM transplantation in patients with relatively advanced PD. Whilst these findings were not replicated in two subsequent double-blind sham-surgery controlled trials, there were reasons to explain this outside of the one drawn at the time that these therapies are ineffective. Indeed all these studies have provided evidence that following the transplantation of fetal VM tissue, dopaminergic cells can survive long term, produce dopamine, and bring about clinical improvements in younger patients over many years. The use of fetal tissue, irrespective of its true efficacy, will never become a widely available therapy for PD for a host of practical and ethical reasons, and thus much work has been put in recently to exploring the utility of stem cells as a source of nigral dopaminergic neurons. In this respect, the advent of embryonic stem cell and induced pluripotent cells has heralded a new era in cell therapy for PD, and several groups have now demonstrated that these cells can form dopaminergic neurons which improve functional deficits in animal models of PD. Whilst encouraging, problems with respect to the immunogenicity and tumorigenicity of these cells means that they will need to be used in the clinic cautiously. Other regenerative therapies in PD have been tried over the years and include the use of trophic factors. This has primarily involved glial cell line-derived neurotrophic factor (GDNF) and again has produced mixed clinical effects, and in order to try and resolve this, a new trial of intraputamenal GDNF is now being planned. In addition, a new trial for platelet derived growth factor as a treatment for PD has just completed recruitment, and PYM50028 (Cogane) an oral agent shown in animal models to reduce the effects of MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) lesioning by the induction of growth factors is currently under investigation in a multicentre Phase II trial. Overall, there are a number of promising new regenerative therapies being developed and tested in PD, although the true long-term efficacy of any of these in large numbers of patients is still not known.

The development, past achievements, and future directions of brain PET

The early developments of brain positron emission tomography (PET), including the methodological advances that have driven progress, are outlined. The considerable past achievements of brain PET have been summarized in collaboration with contributing experts in specific clinical applications including cerebrovascular disease, movement disorders, dementia, epilepsy, schizophrenia, addiction, depression and anxiety, brain tumors, drug development, and the normal healthy brain. Despite a history of improving methodology and considerable achievements, brain PET research activity is not growing and appears to have diminished. Assessments of the reasons for decline are presented and strategies proposed for reinvigorating brain PET research. Central to this is widening the access to advanced PET procedures through the introduction of lower cost cyclotron and radiochemistry technologies. The support and expertize of the existing major PET centers, and the recruitment of new biologists, bio-mathematicians and chemists to the field would be important for such a revival. New future applications need to be identified, the scope of targets imaged broadened, and the developed expertize exploited in other areas of medical research. Such reinvigoration of the field would enable PET to continue making significant contributions to advance the understanding of the normal and diseased brain and support the development of advanced treatments.

Stem cells and stroke: are we further away than anyone is willing to admit?

Harnessing the potential of stem cells or other types of cell therapies to regenerate brain tissue lost from a stroke is a long way off, and a far more complicated process than is understood. There are critical safety issues regarding stem cells in stroke and many clinical trials have only just begun. We are at least 5-10 years away from knowing if cell therapies will improve clinical outcome in stroke patients. The use of stem cells in stroke therapy remains investigational only.

Recent advances in imaging of dopaminergic neurons for evaluation of neuropsychiatric disorders

Dopamine is the most intensely studied monoaminergic neurotransmitter. Dopaminergic neurotransmission plays an important role in regulating several aspects of basic brain function, including motor, behavior, motivation, and working memory. To date, there are numerous positron emission tomography (PET) and single photon emission computed tomography (SPECT) radiotracers available for targeting different steps in the process of dopaminergic neurotransmission, which permits us to quantify dopaminergic activity in the living human brain. Degeneration of the nigrostriatal dopamine system causes Parkinson's disease (PD) and related Parkinsonism. Dopamine is the neurotransmitter that has been classically associated with the reinforcing effects of drug abuse. Abnormalities within the dopamine system in the brain are involved in the pathophysiology of attention deficit hyperactivity disorder (ADHD). Dopamine receptors play an important role in schizophrenia and the effect of neuroleptics is through blockage of dopamine D₂ receptors. This review will concentrate on the radiotracers that have been developed for imaging dopaminergic neurons, describe the clinical aspects in the assessment of neuropsychiatric disorders, and suggest future directions in the diagnosis and management of such disorders.