PET imaging of implanted human retinal pigment epithelial cells in the MPTP-induced primate model of Parkinson's disease

A G-Protein Coupled Receptor and Macular Degeneration

Age-related macular degeneration (AMD) is a leading cause of irreversible blindness in the world. The risk of AMD increases with age and is most common among the white population. Here, we discuss the convergence of factors related to race, pigmentation, and susceptibility to AMD, where the primary defect occurs in retinal support cells, the retinal pigment epithelium (RPE). We explore whether the observed racial bias in AMD incidence is related to innate differences in the basal level of pigmentation between races, and whether the pigmentation pathway activity in the RPE might protect from retinal degeneration. More specifically, we explore whether the downstream signaling activity of GPR143, a G-protein coupled receptor in the pigmentation pathway, might underly the racial bias of AMD and be a target to prevent the disease. Lastly, we summarize the past findings of a large retrospective study that investigated the relationship between the stimulation of GPR143 with L-DOPA, the pigmentation pathway, and AMD, to potentially help develop new ways to prevent or treat AMD. The reader of this review will come to understand the racial bias of AMD, which is related to the function of the RPE.

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.

Differential behavioral outcomes following neonatal versus fetal human retinal pigment epithelial cell striatal implants in parkinsonian rats

Following the failure of a Phase II clinical study evaluating human retinal pigment epithelial (hRPE) cell implants as a potential treatment option for Parkinson's disease, speculation has centered on implant function and survival as possible contributors to the therapeutic outcomes. We recently reported that neonatal hRPE cells, similar to hRPE cells used in the Phase II clinical study, produced short-lived in vitro and limited in vivo trophic factors, which supports that assumption. We hypothesize that the switch from fetal to neonatal hRPE cells, between the Phase I and the Phase II clinical trial may be partly responsible for the later negative outcomes. To investigate this hypothesis, we used two neonatal hRPE cell lots, prepared in a similar manner to neonatal hRPE cells used in the Phase II clinical study, and compared them to previously evaluated fetal hRPE cells for behavioral changes following unilateral striatal implantation in 6-hydroxydopamine-lesioned rats. The results showed that only fetal, not neonatal, hRPE cell implants, were able to improve behavioral outcomes following striatal implantation in the lesioned rats. These data suggest that fetal hRPE cells may be preferential to neonatal hRPE cells in restoring behavioral deficits.

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.

Modulation of impulsivity and reward sensitivity in intertemporal choice by striatal and midbrain dopamine synthesis in healthy adults

Converging evidence links individual differences in mesolimbic and mesocortical dopamine (DA) to variation in the tendency to choose immediate rewards ("Now") over larger, delayed rewards ("Later"), or "Now bias." However, to date, no study of healthy young adults has evaluated the relationship between Now bias and DA with positron emission tomography (PET). Sixteen healthy adults (ages 24-34 yr; 50% women) completed a delay-discounting task that quantified aspects of intertemporal reward choice, including Now bias and reward magnitude sensitivity. Participants also underwent PET scanning with 6-[(18)F]fluoro-l-m-tyrosine (FMT), a radiotracer that measures DA synthesis capacity. Lower putamen FMT signal predicted elevated Now bias, a more rapidly declining discount rate with increasing delay time, and reduced willingness to accept low-interest-rate delayed rewards. In contrast, lower FMT signal in the midbrain predicted greater sensitivity to increasing magnitude of the Later reward. These data demonstrate that intertemporal reward choice in healthy humans varies with region-specific measures of DA processing, with regionally distinct associations with sensitivity to delay and to reward magnitude.

Neonatal human retinal pigment epithelial cells secrete limited trophic factors in vitro and in vivo following striatal implantation in parkinsonian rats

Human retinal pigment epithelial (hRPE) cell implants into the striatum have been investigated as a potential cell-based treatment for Parkinson's disease in a Phase II clinical trial that recently failed. We hypothesize that the trophic factor potential of the hRPE cells could potentially influence the function and/or survival of the implants and may be involved in an alternative mechanism of action. However, it is unclear if hRPE cells secreted trophic factors when handled in the manner used in the clinical Phase II trial. To address these questions, we investigated two neonatal hRPE cell lots, cultured in a similar manner to hRPE cells used in a Phase II clinical study, and longitudinally determined brain-derived neurotrophic factor (BDNF), fibroblast growth factor 2 (FGF2), and pigment epithelium-derived factor concentrations in vitro and following striatal implantation into 6-hydroxydopamine-lesioned rats. The results demonstrate short-lived BDNF and FGF2 concentrations in vitro from hRPE cells grown alone or attached to gelatin microcarriers (GM)s as well as limited trophic factor concentration differences in vivo following striatal implantation of hRPE-GM in 6-hydroxydopamine lesioned rats compared to sham (GM-only). The data suggest that trophic factors from neonatal hRPE cell implants likely did not participate in an alternative mechanism of action, which adds supports to a hypothesis that additional factors may have been necessary for the survival and/or function of hRPE implants and potentially the success of the Phase II clinical trial.

Bone Marrow-Derived Stem Cells and Strategies for Treatment of Nervous System Disorders: Many Protocols, and Many Results

Bone marrow stem cells are the best known stem cell type and have been employed for more than 50 years, especially in pathologies of the hematopoietic and immune systems. However, their therapeutic potential is much broader, and they can also be employed to palliate neural diseases. Apart from their plastic properties, these cells lack the legal or ethical constraints of other stem cell populations, that is, embryonic stem cells. Current research addressing the integration of bone marrow-derived cells into the neural circuits of the central nervous system, their features, and applications is a hotspot in neurobiology. Nevertheless, as in other leading research lines the efficacy and possibilities of their application depend on technical procedures, which are still far from being standardized. Accordingly, for efficient research this large range of variants should be taken into account as they could lead to unexpected results. Rather than focusing on clinical aspects, this review offers a compendium of the methodologies aimed at providing a guide for researchers who are working in the field of bone marrow transplantation in the central nervous system. It seeks to be useful for both introductory and trouble-shooting purposes, and in particular for dealing with the large array of bone marrow transplantation protocols available.

Carriers in cell-based therapies for neurological disorders

There is a pressing need for long-term neuroprotective and neuroregenerative therapies to promote full function recovery of injuries in the human nervous system resulting from trauma, stroke or degenerative diseases. Although cell-based therapies are promising in supporting repair and regeneration, direct introduction to the injury site is plagued by problems such as low transplanted cell survival rate, limited graft integration, immunorejection, and tumor formation. Neural tissue engineering offers an integrative and multifaceted approach to tackle these complex neurological disorders. Synergistic therapeutic effects can be obtained from combining customized biomaterial scaffolds with cell-based therapies. Current scaffold-facilitated cell transplantation strategies aim to achieve structural and functional rescue via offering a three-dimensional permissive and instructive environment for sustainable neuroactive factor production for prolonged periods and/or cell replacement at the target site. In this review, we intend to highlight important considerations in biomaterial selection and to review major biodegradable or non-biodegradable scaffolds used for cell transplantation to the central and peripheral nervous system in preclinical and clinical trials. Expanded knowledge in biomaterial properties and their prolonged interaction with transplanted and host cells have greatly expanded the possibilities for designing suitable carrier systems and the potential of cell therapies in the nervous system.

Intrastriatal transplantation of retinal pigment epithelial cells for the treatment of Parkinson disease: in vivo longitudinal molecular imaging with 18F-P3BZA PET/CT

PURPOSE

To evaluate the performance of N-[2-(diethylamino)ethyl]-(18)F-5-fluoropicolinamide ((18)F-P3BZA) for visualizing porcine retinal pigment epithelium (pRPE) cells transplanted in the striatum for the treatment of Parkinson disease and to monitor the long-term activity of implanted pRPE cells by means of (18)F-P3BZA positron emission tomography (PET)/computed tomography (CT) in vivo.

MATERIALS AND METHODS

Animal work was conducted in accordance with the administrative panel on laboratory animal care. In vitro cell uptake of (18)F-P3BZA was determined with incubation of melanotic pRPE or amelanotic ARPE-19 cells with (18)F-P3BZA. To visualize the implanted pRPE cells in vivo, normal rats (four per group) were injected with pRPE or ARPE-19 cells attached to gelatin microcarriers in the left striatum and with control gelatin microcarriers in the right striatum and followed up with small animal PET/CT. Longitudinal PET/CT scans were acquired in 12 rats up to 16 days after surgery. Postmortem analysis, which included autoradiography and hematoxylin-eosin, Fontana-Masson, and immunofluorescence staining, was performed. Data were compared with the Student t test, analysis of variance, and regression analysis.

RESULTS

(18)F-P3BZA accumulated in pRPE cells effectively (3.48% of the injected dose [ID] per gram of brain tissue ± 0.58 at 1 hour after injection of the probe at 2 days after surgery in vivo) but not in control ARPE-19 cells (P < .05). Longitudinal PET/CT scans revealed that the activity of implanted pRPE cells decreased over time, as evidenced by a reduction in (18)F-P3BZA uptake (3.39% ID/g ± 0.18, 2.49% ID/g ± 0.41, and 1.20% ID/g ± 0.13 at days 2, 9, and 16, respectively; P < .05). Postmortem analysis helped confirm the results of in vivo imaging.

CONCLUSION

(18)F-P3BZA PET/CT is a feasible technique for visualizing and detecting the activity of implanted RPE cells in vivo.

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.

Striatal dopamine influences the default mode network to affect shifting between object features

Cognitive flexibility or the ability to change behavior in response to external cues is conceptualized as two processes: one for shifting between perceptual features of objects and another for shifting between the abstract rules governing the selection of these objects. Object and rule shifts are believed to engage distinct anatomical structures and functional processes. Dopamine activity has been associated with cognitive flexibility, but patients with dopaminergic deficits are not impaired on all tasks assessing cognitive flexibility, suggesting that dopamine may have different roles in the shifting of objects and rules. The goals of this study were to identify brain regions supporting object and rule shifts and to examine the role of dopamine in modulating these two forms of cognitive flexibility. Sixteen young, healthy volunteers underwent fMRI while performing a set-shift task designed to differentiate shifting between object features from shifting between abstract task rules. Participants also underwent PET with 6-[¹⁸F]-fluoro-l-m-tyrosine (FMT), a radiotracer measuring dopamine synthesis capacity. Shifts of abstract rules were not associated with activation in any brain region, and FMT uptake did not correlate with rule shift performance. Shifting between object features deactivated the medial PFC and the posterior cingulate and activated the lateral PFC, posterior parietal areas, and the striatum. FMT signal in the striatum correlated negatively with object shift performance and deactivation in the medial PFC, a component of the default mode network, suggesting that dopamine influences object shifts via modulation of activity in the default mode network.

The immunological challenges of cell transplantation for the treatment of Parkinson's disease

Dopaminergic cell transplantation is an experimental therapy for Parkinson's disease (PD). It has many potential theoretical advantages over current treatment strategies such as providing continuous local dopaminergic replenishment, eliminating motor fluctuations and medication-induced dyskinesias, slowing down disease progression or even reversing disease pathology in the host. Recent studies also show that dopaminergic cell transplants provide long-term neuromodulation in the basal ganglia that simulates the combined effects of oral dopaminergic therapy and surgical therapies like deep brain stimulation, the contemporary therapeutic approach to advanced PD. However, dopaminergic cell transplantation in PD as not been optimized and current experimental techniques have many drawbacks. In published experiments to date of attempted dopaminergic grafting in PD, the major challenges are unacceptable graft-induced dyskinesias or failure of such grafts to exceed the benefits afforded by sham surgery. A deleterious host immune response to the transplant has been implicated as a major putative cause for these adverse outcomes. This article focuses on recent advances in understanding the immunology of the transplantation in PD and possible methods to overcome adverse events such that we could translate cell replacement strategies into viable clinical treatments in the future.

PEDF and VEGF-A output from human retinal pigment epithelial cells grown on novel microcarriers

Human retinal pigment epithelial (hRPE) cells have been tested as a cell-based therapy for Parkinson's disease but will require additional study before further clinical trials can be planned. We now show that the long-term survival and neurotrophic potential of hRPE cells can be enhanced by the use of FDA-approved plastic-based microcarriers compared to a gelatin-based microcarrier as used in failed clinical trials. The hRPE cells grown on these plastic-based microcarriers display several important characteristics of hRPE found in vivo: (1) characteristic morphological features, (2) accumulation of melanin pigment, and (3) high levels of production of the neurotrophic factors pigment epithelium-derived factor (PEDF) and vascular endothelial growth factor-A (VEGF-A). Growth of hRPE cells on plastic-based microcarriers led to sustained levels (>1 ng/ml) of PEDF and VEGF-A in conditioned media for two months. We also show that the expression of VEGF-A and PEDF is reciprocally regulated by activation of the GPR143 pathway. GPR143 is activated by L-DOPA (1 μM) which decreased VEGF-A secretion as opposed to the previously reported increase in PEDF secretion. The hRPE microcarriers are therefore novel candidate delivery systems for achieving long-term delivery of the neuroprotective factors PEDF and VEGF-A, which could have a value in neurodegenerative conditions such as Parkinson's disease.

Abnormal metabolic brain networks in a nonhuman primate model of parkinsonism

Parkinson's disease (PD) is associated with a characteristic regional metabolic covariance pattern that is modulated by treatment. To determine whether a homologous metabolic pattern is also present in nonhuman primate models of parkinsonism, 11 adult macaque monkeys with parkinsonism secondary to chronic systemic 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and 12 age-matched healthy animals were scanned with [(18)F]fluorodeoxyglucose (FDG) positron emission tomography (PET). A subgroup comprising five parkinsonian and six control animals was used to identify a parkinsonism-related pattern (PRP). For validation, analogous topographies were derived from other subsets of parkinsonian and control animals. The PRP topography was characterized by metabolic increases in putamen/pallidum, thalamus, pons, and sensorimotor cortex, as well as reductions in the posterior parietal-occipital region. Pattern expression was significantly elevated in parkinsonian relative to healthy animals (P<0.00001). Parkinsonism-related topographies identified in the other derivation sets were very similar, with significant pairwise correlations of region weights (r>0.88; P<0.0001) and subject scores (r>0.74; P<0.01). Moreover, pattern expression in parkinsonian animals correlated with motor ratings (r>0.71; P<0.05). Thus, homologous parkinsonism-related metabolic networks are demonstrable in PD patients and in monkeys with experimental parkinsonism. Network quantification may provide a useful biomarker for the evaluation of new therapeutic agents in preclinical models of PD.

Cell replacement therapy for Parkinson's disease: how close are we to the clinic?

Cell replacement therapy (CRT) offers great promise as the future of regenerative medicine in Parkinson´s disease (PD). Three decades of experiments have accumulated a wealth of knowledge regarding the replacement of dying neurons by new and healthy dopaminergic neurons transplanted into the brains of animal models and affected patients. The first clinical trials provided the proof of principle for CRT in PD. In these experiments, intrastriatal transplantation of human embryonic mesencephalic tissue reinnervated the striatum, restored dopamine levels and showed motor improvements. Sequential controlled studies highlighted several problems that should be addressed prior to the wide application of CRT for PD patients. Moreover, owing to ethical and practical problems, embryonic stem cells require replacement by better-suited stem cells. Several obstacles remain to be surpassed, including identifying the best source of stem cells for A9 dopaminergic neuron generation, eliminating the risk of tumor formation and the development of graft-induced dyskinesias, and standardizing dopaminergic cell production in order to enable clinical application. In this article, we present an update on CRT for PD, reviewing the research milestones, various stem cells used and tailored differentiation methods, and analyze the information gained from the clinical trials.

Image segmentation and activity estimation for microPET 11C-raclopride images using an expectation-maximum algorithm with a mixture of Poisson distributions

The objective of this study was to use a mixture of Poisson (MOP) model expectation maximum (EM) algorithm for segmenting microPET images. Simulated rat phantoms with partial volume effect and different noise levels were generated to evaluate the performance of the method. The partial volume correction was performed using an EM deblurring method before the segmentation. The EM-MOP outperforms the EM-MOP in terms of the estimated spatial accuracy, quantitative accuracy, robustness and computing efficiency. To conclude, the proposed EM-MOP method is a reliable and accurate approach for estimating uptake levels and spatial distributions across target tissues in microPET (11)C-raclopride imaging studies.

Dopamine cell transplantation in Parkinson's disease: challenge and perspective

BACKGROUND

Functional imaging provides a valuable adjunct to clinical evaluation for assessing the efficacy of cell-based restorative therapies in Parkinson's disease (PD).

SOURCES OF DATA

In this article, we review the latest advances on the use of positron emission tomography (PET) imaging in evaluating the surgical outcome of embryonic dopamine (DA) cell transplantation in PD patients.

AREAS OF AGREEMENT

These studies suggest long-term cell survival and clinical benefit following striatal transplantation of fetal nigral tissue in PD patients and in models of experimental parkinsonism.

AREAS OF CONTROVERSY

Adverse events subsequent to transplantation have also been noted and attributed to a variety of causes.

GROWING POINTS

Optimal outcomes of DA cell transplantation therapies are dependent on tissue composition and phenotype of DA neurons in the graft.

AREAS TIMELY FOR DEVELOPING RESEARCH

Given continued progress in DA neuron production from stem cells in recent years, transplantation of neural stem cells may be the next to enter clinical trials in patients.

CONCLUSION

The existing data from studies of embryonic DA transplantation for advanced PD have provided valuable insights for the design of new cell-based therapies for the treatment of this and related neurodegenerative disorders.

Cell-based therapies for Parkinson's disease: past, present, and future

Parkinson's disease (PD) researchers have pioneered the use of cell-based therapies (CBTs) in the central nervous system. CBTs for PD were originally envisioned as a way to replace the dopaminergic nigral neurons lost with the disease. Several sources of catecholaminergic cells, including autografts of adrenal medulla and allografts or xenografts of mesencephalic fetal tissue, were successfully assessed in animal models, but their clinical translation has yielded poor results and much controversy. Recent breakthroughs on cell biology are helping to develop novel cell lines that could be used for regenerative medicine. Their future successful clinical application depends on identifying and solving the problems encountered in previous CBTs trials. In this review, we critically analyze past CBTs' clinical translation, the impact of the host in graft survival, and the role of preclinical studies and emerging new cell lines. We propose that the prediction of clinical results from preclinical studies requires experimental designs that allow blind data acquisition and statistical analysis, assessment of the therapy in models that parallel clinical conditions, looking for sources of complications or side effects, and limiting optimism bias when reporting outcomes.

Retinal pigment epithelial cells secrete neurotrophic factors and synthesize dopamine: possible contribution to therapeutic effects of RPE cell transplantation in Parkinson's disease

BACKGROUND

New strategies for the treatment of Parkinson's disease (PD) are shifted from dopamine (DA) replacement to regeneration or restoration of the nigro-striatal system. A cell therapy using human retinal pigment epithelial (RPE) cells as substitution for degenerated dopaminergic (DAergic) neurons has been developed and showed promising prospect in clinical treatment of PD, but the exact mechanism underlying this therapy is not fully elucidated. In the present study, we investigated whether the beneficial effects of this therapy are related to the trophic properties of RPE cells and their ability to synthesize DA.

METHODS

We evaluated the protective effects of conditioned medium (CM) from cultured RPE cells on the DAergic cells against 6-hydroxydopamine (6-OHDA)- and rotenone-induced neurotoxicity and determined the levels of glial cell derived neurotrophic factor (GDNF) and brain derived neurotrophic factor (BDNF) released by RPE cells. We also measured the DA synthesis and release. Finally we transplanted microcarriers-RPE cells into 6-OHDA lesioned rats and observed the improvement in apomorphine-induced rotations (AIR).

RESULTS

We report here: (1) CM from RPE cells can secret trophic factors GDNF and BDNF, and protect DAergic neurons against the 6-OHDA- and rotenone-induced cell injury; (2) cultured RPE cells express L-dopa decarboxylase (DDC) and synthesize DA; (3) RPE cells attached to microcarriers can survive in the host striatum and improve the AIR in 6-OHDA-lesioned animal model of PD; (4) GDNF and BDNF levels are found significantly higher in the RPE cell-grafted tissues.

CONCLUSION

These findings indicate the RPE cells have the ability to secret GDNF and BDNF, and synthesize DA, which probably contribute to the therapeutic effects of RPE cell transplantation in PD.

New MRI, 18F-DOPA and 11C-(+)-alpha-dihydrotetrabenazine templates for Macaca fascicularis neuroimaging: advantages to improve PET quantification

Normalization of neuroimaging studies to a stereotaxic space allows the utilization of standard volumes of interest (VOIs) and voxel-based analysis (SPM). Such spatial normalization of PET and MRI studies requires a high quality template image. The aim of this study was to create new MRI and PET templates of (18)F-DOPA and (11)C-(+)-alpha-dihydrotetrabenazine ((11)C-DTBZ) of the Macaca fascicularis brain, an important animal model of Parkinson's disease. MRI template was constructed as a smoothed average of the scans of 15 healthy animals, previously transformed into the space of one representative MRI. In order to create the PET templates, (18)F-DOPA and (11)C-DTBZ PET of the same subjects were acquired in a dedicated small animal PET scanner and transformed to the created MRI template space. To validate these templates for PET quantification, parametric values obtained with a standard VOI-map applied after spatial normalization to each template were statistically compared to results computed using individual VOIs drawn for each animal. The high correlation between both procedures validated the utilization of all the templates, improving the reproducibility of PET analysis. To prove the utility of the templates for voxel-based quantification, dopamine striatal depletion in a representative monkey treated with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) was assessed by SPM analysis of (11)C-DTBZ PET. A symmetric reduction in striatal (11)C-DTBZ uptake was detected in accordance with the induced lesion. In conclusion, templates of M. fascicularis brain have been constructed and validated for reproducible and automated PET quantification. All templates are electronically available via the internet.

Pigment epithelium derived factor (PEDF) is neuroprotective in two in vitro models of Parkinson's disease

Transplantation of retinal pigment epithelial (RPE) cells in the basal ganglia has been proposed as a novel cell-based therapy for Parkinson's disease (PD), by providing a constant source of dopamine replacement via the melanin synthetic pathway enzyme tyrosinase. We have demonstrated previously that human RPE cells also produce a neurotrophic effect on primary cultures of rat striata mesencephalic (dopaminergic) neurons and showed that pigment epithelium derived factor (PEDF) accounted for a major portion of the neurotrophic effect. We now have also begun studies that demonstrate that the neurotrophic effect of PEDF corresponds to neuroprotection against toxins used to produce experimental PD. This was shown in (1) rotenone and (2) 6-hydroxydopamine (6-OHDA) in vitro models. The toxins were added at day 10 in culture, PEDF was added 1h prior. The cultures were fixed and analyzed after tyrosine hydroxylase (TH) immunocytochemical staining. Cell count of TH+ neurons clearly shows the neuroprotective potential of PEDF in both neurotoxin models. The neurotoxic effect of rotenone (25nM) on dopaminergic neurons is reversed by addition of PEDF. At a concentration of 1ng/ml PEDF the neurotoxic effect of rotenone is completely counteracted. PEDF (1ng/ml) has also a neuroprotective effect in the 6-OHDA midbrain in vitro model. The effect is most pronounced at concentrations of 25microM and 50microM 6-OHDA. We conclude that the neurotrophic factor PEDF, produced from RPE cells, can improve neuronal survival in models of PD, and plan to test if this effect can be observed using in vivo models of PD following RPE transplantation.

Abnormal regional brain function in Parkinson's disease: truth or fiction?

Normalization of regional measurements by the global mean is commonly employed to minimize inter-subject variability in functional imaging studies. This practice is based on the assumption that global values do not substantially differ between patient and control groups. In this issue of NeuroImage, Borghammer and colleagues challenge the validity of this assumption. They focus on Parkinson's disease (PD) and use computer simulations to show that lower global values can produce spurious increases in subcortical brain regions. The authors speculate that the increased signal observed in these areas in PD is artefactual and unrelated to localized changes in brain function. In this commentary, we summarize what is currently known of the relationship between regional and global metabolic activity in PD and experimental parkinsonism. We found that early stage PD patients exhibit global values that are virtually identical to those of age-matched healthy subjects. SPM analysis revealed increased normalized metabolic activity in a discrete set of biologically relevant subcortical brain regions. Because of their higher variability, the corresponding absolute regional measures did not differ across the two groups. Longitudinal imaging studies in this population showed that the subcortical elevations in normalized metabolism appeared earlier and progressed faster than did focal cortical or global metabolic reductions. The observed increases in subcortical activity, but not the global changes, correlated with independent clinical measures of disease progression. Multivariate analysis with SSM/PCA further confirmed that the abnormal spatial covariance structure of early PD is dominated by these subcortical increases as opposed to network-related reductions in cortical metabolic activity or global changes. Thus, increased subcortical activity in PD cannot be regarded as a simple artefact of global normalization. Moreover, stability of the normalized measurements, particularly at the network level, makes these metabolic indices suitable as imaging biomarkers of PD progression and the treatment response.

In-vivo PET imaging of implanted human retinal pigment epithelium cells in a Parkinson's disease rat model

BACKGROUND AND AIM

Researchers find that monitoring the differentiation of implanted cells in vivo is difficult. This study was designed to show that it is possible to track the efficacy of transplanted human retinal pigment epithelial cells (RPE cells) in a rat model of Parkinson's disease by using positron emission tomography (PET).

METHODS

RPE cells or normal saline were injected into striatum of the injured side of the rat model in treated and control groups, respectively. PET imaging of both groups was undertaken before transplantation and at intervals afterwards, using C-raclopride and C-beta-CFT as the markers. Observation of the rats' behaviour and immunofluorescence confocal microscopy were also used to prove the PET results.

RESULTS

PET studies showed increased accumulation of C-raclopride and decreased C-beta-CFT in the injured side of striatum in both groups. C-raclopride decreased along with a concomitant increase of C-beta-CFT after transplantation in the treated group. The changes shown by the PET studies paralleled the behavioural states and confocal microscopy observations in the treated animals.

CONCLUSION

These results suggest that even a clinical PET scanner could, to a certain extent, provide some information on the existence and in-vivo differentiation of RPE cells in a rat model of Parkinson's disease.

Human retinal pigment epithelial cell implants ameliorate motor deficits in two rat models of Parkinson disease

Intrastriatal transplantation of gelatin microcarrier-attached human retinal pigment epithelial cells (hRPE-GM) may represent an alternative source for cell therapy in Parkinson disease (PD). The use of human retinal pigment epithelial (hRPE) cells in PD relies on the capacity of these cells to produce l-dopa as an intermediate product in the eumelanin synthesis pathway. We investigated the behavioral effects of hRPE-GM implants on forelimb use asymmetries and hindlimb motor deficits in unilateral and bilateral 6-hydroxydopamine (6-OHDA) rat models of PD. We report that intrastriatal unilateral implantation of hRPE-GM in rats with 6-OHDA nigrostriatal lesions produce an amelioration of the contralateral forelimb disuse and the contralateral hindlimb deficits. These results further support the possibility that implantation of cultured hRPE cells may be a promising therapeutic option for patients with PD.

Characterization and survival of long-term implants of human retinal pigment epithelial cells attached to gelatin microcarriers in a model of Parkinson disease

Previous studies have demonstrated that the intrastriatal implantation of human retinal pigment epithelial cells attached to gelatin microcarriers (hRPE-GM) ameliorates behavioral deficits in animal models of Parkinson disease. However, there are only sparse data on cell survival in the host. In this study, we characterized a variety of retinal pigment epithelial (RPE)-specific markers in vitro and used these markers to investigate the long-term survival of hRPE-GM implants. Sprague-Dawley rats (n = 22) were unilaterally lesioned with 6-hydroxydopamine (6-OHDA) and implanted with hRPE-GM without immunosuppression. Rats were euthanized at 48 hours, 7 days, 4 weeks, and 5 months postimplant and immunohistochemically processed using the following antibodies: 1) human-specific nuclear mitotic apparatus protein (NuMA-Ab2), 2) epithelial-specific extracellular matrix metalloproteinase inducer (EMMPRIN), 3) RPE cell-specific RPE65, and the inflammation markers 4) glial fibrillary acidic protein and 5) ED1 (rat CD68). Our analysis revealed NuMA-, EMMPRIN-, and RPE65-immunoreactive cells at different times postimplant. The morphologic features of hRPE cell implants (at 48 hours and 5 months) were confirmed by electron microscopy. Furthermore, despite evidence of chronic inflammation at the later time point, there is an appreciable number of surviving hRPE cells. This study suggests that hRPE-GM implants can survive in the absence of immunosuppression and can be potentially used as an alternative for treating Parkinson disease.

Correlation between levels of pigment epithelium-derived factor and vascular endothelial growth factor in the striatum of patients with Parkinson's disease

Parkinson's disease (PD) is caused by progressive degeneration of nigrostriatal dopaminergic neurons and can potentially be treated by intrastriatal delivery of neurotrophic factors. Pigment epithelium-derived factor (PEDF), which exhibits protective effects on various neuronal populations, is up-/down-regulated in the cerebrospinal fluid in some neurodegenerative conditions. Here we investigated the level of PEDF protein in the striatum and immunoreactivity for PEDF in the substantia nigra (SN) of patients with PD to assess its role in the pathophysiology of PD. We also studied changes in PEDF expression in the striatum of the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of PD. We found a transient and rapid up-regulation of PEDF transcripts and a marked increase in immunoreactivity for PEDF protein in response to MPTP administration in mice. However, there were no significant changes in striatal levels of PEDF and immunoreactivity for PEDF in the SN of PD patients compared with age-matched non-PD patients. Intriguingly, the striatal levels of PEDF and vascular endothelial growth factor (VEGF), which has opposite functions to PEDF in terms of angiogenesis and vascular permeability, correlated positively in PD patients. Our results suggest up-regulation of PEDF in response to acute insult to the dopaminergic pathway, but such response might be disturbed in patients with advanced PD. The correlation between PEDF and VEGF striatal levels in PD patients suggests that concerted neurotrophic functions of these factors or structural changes in blood vessel walls play an important role in the pathophysiology of PD.

Animal Models of Neurodegenerative Disease: Insights from In vivo Imaging Studies

Animal models have been used extensively to understand the etiology and pathophysiology of human neurodegenerative diseases, and are an essential component in the development of therapeutic interventions for these disorders. In recent years, technical advances in imaging modalities such as positron emission tomography (PET) and magnetic resonance imaging (MRI) have allowed the use of these techniques for the evaluation of functional, neurochemical, and anatomical changes in the brains of animals. Combining animal models of neurodegenerative disorders with neuroimaging provides a powerful tool to follow the disease process, to examine compensatory mechanisms, and to investigate the effects of potential treatments preclinically to derive knowledge that will ultimately inform our clinical decisions. This article reviews the literature on the use of PET and MRI in animal models of Parkinson's disease, Huntington's disease, and Alzheimer's disease, and evaluates the strengths and limitations of brain imaging in animal models of neurodegenerative diseases.