Transplantation in the rat model of Parkinson's disease: ectopic versus homotopic graft placement

Impact of dietary vitamin A on striatal function in adult rats

The striatum is a brain structure involved in the control of voluntary movement. Striatum contains high amounts of retinoic acid, the active metabolite of vitamin A, as well as retinoid receptors, RARβ and RXRγ. Previous studies revealed that disruption of retinoid signaling initiated during development is deleterious for striatal physiology and related motor functions. However, the alteration of retinoid signaling, and the importance of vitamin A supply during adulthood on striatal physiology and function has never been established. In the present study, we investigated the impact of vitamin A supply on striatal function. Adult Sprague-Dawley rats were fed with three specific diets, either sub-deficient, sufficient, or enriched in vitamin A (0.4, 5, and 20 international units [IU] of retinol per g of diet, respectively) for 6 months. We first validated that vitamin A sub-deficient diet in adult rats constitutes a physiological model of retinoid signaling reduction in the striatum. We then revealed subtle alterations of fine motor skills in sub-deficient rats using a new behavioral apparatus specifically designed to test forepaw reach-and-grasp skills relying on striatal function. Finally, we showed using qPCR analysis and immunofluorescence that the striatal dopaminergic system per se was not affected by vitamin A sub-deficiency at adult age. Rather, cholinergic synthesis in the striatum and μ-opioid receptor expression in striosomes sub-territories were the most affected by vitamin A sub-deficiency starting at adulthood. Taken together these results revealed that retinoid signaling alteration at adulthood is associated with motor learning deficits together with discrete neurobiological alterations in the striatum.

Better Outcomes with Intranigral versus Intrastriatal Cell Transplantation: Relevance for Parkinson’s Disease

Intrastriatal embryonic ventral mesencephalon grafts have been shown to integrate, survive, and reinnervate the host striatum in clinical settings and in animal models of Parkinson’s disease. However, this ectopic location does not restore the physiological loops of the nigrostriatal pathway and promotes only moderate behavioral benefits. Here, we performed a direct comparison of the potential benefits of intranigral versus intrastriatal grafts in animal models of Parkinson’s disease. We report that intranigral grafts promoted better survival of dopaminergic neurons and that only intranigral grafts induced recovery of fine motor skills and normalized cortico-striatal responses. The increase in the number of toxic activated glial cells in host tissue surrounding the intrastriatal graft, as well as within the graft, may be one of the causes of the increased cell death observed in the intrastriatal graft. Homotopic localization of the graft and the subsequent physiological cell rewiring of the basal ganglia may be a key factor in successful and beneficial cell transplantation procedures.

A combined cell and gene therapy approach for homotopic reconstruction of midbrain dopamine pathways using human pluripotent stem cells

Midbrain dopamine (mDA) neurons can be replaced in patients with Parkinson's disease (PD) in order to provide long-term improvement in motor functions. The limited capacity for long-distance axonal growth in the adult brain means that cells are transplanted ectopically, into the striatal target. As a consequence, several mDA pathways are not re-instated, which may underlie the incomplete restoration of motor function in patients. Here, we show that viral delivery of GDNF to the striatum, in conjunction with homotopic transplantation of human pluripotent stem-cell-derived mDA neurons, recapitulates brain-wide mDA target innervation. The grafts provided re-instatement of striatal dopamine levels and correction of motor function and also connectivity with additional mDA target nuclei not well innervated by ectopic grafts. These results demonstrate the remarkable capacity for achieving functional and anatomically precise reconstruction of long-distance circuitry in the adult brain by matching appropriate growth-factor signaling to grafting of specific cell types.

Preventive Vitamin A Supplementation Improves Striatal Function in 6-Hydroxydopamine Hemiparkinsonian Rats

Background

The mechanisms leading to a loss of dopaminergic (DA) neurons from the substantia nigra pars compacta (SNc) in Parkinson's disease (PD) have multifactorial origins. In this context, nutrition is currently investigated as a modifiable environmental factor for the prevention of PD. In particular, initial studies revealed the deleterious consequences of vitamin A signaling failure on dopamine-related motor behaviors. However, the potential of vitamin A supplementation itself to prevent neurodegeneration has not been established yet.

Objective

The hypothesis tested in this study is that preventive vitamin A supplementation can protect DA neurons in a rat model of PD.

Methods

The impact of a 5-week preventive supplementation with vitamin A (20 IU/g of diet) was measured on motor and neurobiological alterations induced by 6-hydroxydopamine (6-OHDA) unilateral injections in the striatum of rats. Rotarod, step test and cylinder tests were performed up to 3 weeks after the lesion. Post-mortem analyses (retinol and monoamines dosages, western blots, immunofluorescence) were performed to investigate neurobiological processes.

Results

Vitamin A supplementation improved voluntary movements in the cylinder test. In 6-OHDA lesioned rats, a marked decrease of dopamine levels in striatum homogenates was measured. Tyrosine hydroxylase labeling in the SNc and in the striatum was significantly decreased by 6-OHDA injection, without effect of vitamin A. By contrast, vitamin A supplementation increased striatal expression of D2 and RXR receptors in the striatum of 6-OHDA lesioned rats.

Conclusions

Vitamin A supplementation partially alleviates motor alterations and improved striatal function, revealing a possible beneficial preventive approach for PD.

Restoring lost nigrostriatal fibers in Parkinson's disease based on clinically-inspired design criteria

Parkinson's disease is a neurodegenerative disease affecting around 10 million people worldwide. The death of dopaminergic neurons in the substantia nigra and the axonal fibers that constitute the nigrostriatal pathway leads to a loss of dopamine in the striatum that causes the motor symptoms of this disease. Traditional treatments have focused on reducing symptoms, while therapies with human fetal or stem cell-derived neurons have centered on implanting these cells in the striatum to restore its innervation. An alternative approach is pathway reconstruction, which aims to rebuild the entire structure of neurons and axonal fibers of the nigrostriatal pathway in a way that matches its anatomy and physiology. This type of repair could be more capable of reestablishing the signaling mechanisms that ensure proper dopamine release in the striatum and regulation of other motor circuit regions in the brain. In this manuscript, we conduct a review of the literature related to pathway reconstruction as a treatment for Parkinson's disease, delve into the limitations of these studies, and propose the requisite design criteria to achieve this goal at a human scale. We then present our tissue engineering-based platform to fabricate hydrogel-encased dopaminergic axon tracts in vitro for later implantation into the brain to replace and reconstruct the pathway. These tissue-engineered nigrostriatal pathways (TE-NSPs) can be characterized and optimized for cell number and phenotype, axon growth lengths and rates, and the capacity for synaptic connectivity and dopamine release. We then show original data of advances in creating these constructs matching clinical design criteria using human iPSC-derived dopaminergic neurons and a hyaluronic acid hydrogel. We conclude with a discussion of future steps that are needed to further optimize human-scale TE-NSPs and translate them into clinical products.

Neural stem cells derived from human midbrain organoids as a stable source for treating Parkinson's disease: Midbrain organoid-NSCs (Og-NSC) as a stable source for PD treatment

Successful clinical translation of stem cell-based therapy largely relies on the scalable and reproducible preparation of donor cells with potent therapeutic capacities. In this study, midbrain organoids were yielded from human pluripotent stem cells (hPSCs) to prepare cells for Parkinson's disease (PD) therapy. Neural stem/precursor cells (NSCs) isolated from midbrain organoids (Og-NSCs) expanded stably and differentiated into midbrain-type dopamine(mDA) neurons, and an unprecedentedly high proportion expressed midbrain-specific factors, with relatively low cell line and batch-to-batch variations. Single cell transcriptome analysis followed by in vitro assays indicated that the majority of cells in the Og-NSC cultures are ventral midbrain (VM)-patterned with low levels of cellular senescence/aging and mitochondrial stress, compared to those derived from 2D-culture environments. Notably, in contrast to current methods yielding mDA neurons without astrocyte differentiation, mDA neurons that differentiated from Og-NSCs were interspersed with astrocytes as in the physiologic brain environment. Thus, the Og-NSC-derived mDA neurons exhibited improved synaptic maturity, functionality, resistance to toxic insults, and faithful expressions of the midbrain-specific factors, in vitro and in vivo long after transplantation. Consequently, Og-NSC transplantation yielded potent therapeutic outcomes that are reproducible in PD model animals. Collectively, our observations demonstrate that the organoid-based method may satisfy the demands needed in the clinical setting of PD cell therapy.

hESC-Derived Dopaminergic Transplants Integrate into Basal Ganglia Circuitry in a Preclinical Model of Parkinson's Disease

Cell replacement is currently being explored as a therapeutic approach for neurodegenerative disease. Using stem cells as a source, transplantable progenitors can now be generated under conditions compliant with clinical application in patients. In this study, we elucidate factors controlling target-appropriate innervation and circuitry integration of human embryonic stem cell (hESC)-derived grafts after transplantation to the adult brain. We show that cell-intrinsic factors determine graft-derived axonal innervation, whereas synaptic inputs from host neurons primarily reflect the graft location. Furthermore, we provide evidence that hESC-derived dopaminergic grafts transplanted in a long-term preclinical rat model of Parkinson’s disease (PD) receive synaptic input from subtypes of host cortical, striatal, and pallidal neurons that are known to regulate the function of endogenous nigral dopamine neurons. This refined understanding of how graft neurons integrate with host circuitry will be important for the design of clinical stem-cell-based replacement therapies for PD, as well as for other neurodegenerative diseases.

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Pattern of graft-derived innervation is determined by phenotype of grafted cells

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Synaptic inputs from host-to-graft depend on location of graft

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Intrastriatal dopaminergic grafts receive correct excitatory and inhibitory host inputs

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Individual host neurons provide inputs to both dopaminergic grafts and the host nigra

Neuronal Replacement as a Tool for Basal Ganglia Circuitry Repair: 40 Years in Perspective

The ability of new neurons to promote repair of brain circuitry depends on their capacity to re-establish afferent and efferent connections with the host. In this review article, we give an overview of past and current efforts to restore damaged connectivity in the adult mammalian brain using implants of fetal neuroblasts or stem cell-derived neuronal precursors, with a focus on strategies aimed to repair damaged basal ganglia circuitry induced by lesions that mimic the pathology seen in humans affected by Parkinson’s or Huntington’s disease. Early work performed in rodents showed that neuroblasts obtained from striatal primordia or fetal ventral mesencephalon can become anatomically and functionally integrated into lesioned striatal and nigral circuitry, establish afferent and efferent connections with the lesioned host, and reverse the lesion-induced behavioral impairments. Recent progress in the generation of striatal and nigral progenitors from pluripotent stem cells have provided compelling evidence that they can survive and mature in the lesioned brain and re-establish afferent and efferent axonal connectivity with a remarkable degree of specificity. The studies of cell-based circuitry repair are now entering a new phase. The introduction of genetic and virus-based techniques for brain connectomics has opened entirely new possibilities for studies of graft-host integration and connectivity, and the access to more refined experimental techniques, such as chemo- and optogenetics, has provided new powerful tools to study the capacity of grafted neurons to impact the function of the host brain. Progress in this field will help to guide the efforts to develop therapeutic strategies for cell-based repair in Huntington’s and Parkinson’s disease and other neurodegenerative conditions involving damage to basal ganglia circuitry.

The Amphetamine Induced Rotation Test: A Re-Assessment of Its Use as a Tool to Monitor Motor Impairment and Functional Recovery in Rodent Models of Parkinson's Disease

Rats and mice with unilateral damage to the nigrostriatal dopamine system—induced by neurotoxins, such as 6-hydroxydopamine, overexpression of α-synuclein, or injections of toxic synuclein protofibrils—are widely used as experimental models to mimic the loss of dopamine neurons seen in Parkinson’s disease. The amphetamine rotation test is commonly used to monitor the extent of motor impairment induced by the lesion, and this test has also become the standard tool to demonstrate transplant-induced functional recovery or the efficacy of neuroprotective interventions aimed to preserve or restore DA neuron function. Although the amphetamine-induced rotation test is highly useful for this purpose it has some important pitfalls and the interpretation of the data may not always be straightforward. Unless the test is applied properly and the data are displayed and interpreted appropriately the conclusions may be misleading or simply totally wrong. The purpose of this review is to draw attention to the potential problems and pitfalls involved in the use of drug-induced rotation tests, and to provide recommendations and advice on how to avoid them.

Repairing the Brain: Gene Therapy

In vivo gene therapy for neurodegenerative disorders has turned out to be a formidable challenge. It is a field not much older than twenty years, but we were many who would have predicted a much easier path towards the clinic using this treatment modality. For Parkinson’s disease patients, this has meant a frustrating wait, seeing many promising therapies being forgotten after a few pre-clinical proof-of-concept studies. The reasons for this are both scientific and economical. However, this is slowly but surely changing and over the next two decades we will see a very exciting development in this field. In a foreseeable future, gene therapy will be a very natural component of many clinical therapies, not least in Parkinson’s disease.

Animal models of L-DOPA-induced dyskinesia: the 6-OHDA-lesioned rat and mouse

Appearance of L-DOPA-induced dyskinesia (LID) represents a major limitation in the pharmacological therapy with the dopamine precursor L-DOPA. Indeed, the vast majority of parkinsonian patients develop dyskinesia within 9-10 years of L-DOPA oral administration. This makes the discovery of new therapeutic strategies an important need. In the last decades, several animal models of Parkinson's disease (PD) have been developed, to both study mechanisms underlying PD pathology and treatment-induced side effects (i.e., LID) and to screen for new potential anti-parkinsonian and anti-dyskinetic treatments. Among all the models developed, the 6-OHDA-lesioned rodents represent the models of choice to mimic PD motor symptoms and LID, thanks to their reproducibility and translational value. Under L-DOPA treatment, rodents sustaining 6-OHDA lesions develop abnormal involuntary movements with dystonic and hyperkinetic features, resembling what seen in dyskinetic PD patients. These models have been extensively validated by the evidence that dyskinetic behaviors are alleviated by compounds reducing dyskinesia in patients and non-human primate models of PD. This article will focus on the translational value of the 6-OHDA rodent models of LID, highlighting their main features, advantages and disadvantages in preclinical research.

Early synaptic dysfunction induced by α-synuclein in a rat model of Parkinson's disease

Evidence suggests that synapses are affected first in Parkinson’s disease (PD). Here, we tested the claim that pathological accumulation of α-synuclein, and subsequent synaptic disruption, occur in absence of dopaminergic neuron loss in PD. We determined early synaptic changes in rats that overexpress human α-synuclein by local injection of viral-vectors in midbrain. We aimed to achieve α-synuclein levels sufficient to induce terminal pathology without significant loss of nigral neurons. We tested synaptic disruption in vivo by analyzing motor defects and binding of a positron emission tomography (PET) radioligand to the vesicular monoamine transporter 2, (VMAT2), [¹¹C]dihydrotetrabenazine (DTBZ). Animals overexpressing α-synuclein had progressive motor impairment and, 12 weeks post-surgery, showed asymmetric in vivo striatal DTBZ binding. The PET images matched ligand binding in post-mortem tissue, and histological markers of dopaminergic integrity. Histology confirmed the absence of nigral cell death with concomitant significant loss of striatal terminals. Progressive aggregation of proteinase-K resistant and Ser129-phosphorylated α-synuclein was observed in dopaminergic terminals, in dystrophic swellings that resembled axonal spheroids and contained mitochondria and vesicular proteins. In conclusion, pathological α-synuclein in nigro-striatal axonal terminals leads to early axonal pathology, synaptic disruption, dysfunction of dopaminergic neurotransmission, motor impairment, and measurable change of VMAT2 in the absence of cell loss.

DREADD Modulation of Transplanted DA Neurons Reveals a Novel Parkinsonian Dyskinesia Mechanism Mediated by the Serotonin 5-HT6 Receptor

Transplantation of DA neurons is actively pursued as a restorative therapy in Parkinson’s disease (PD). Pioneering clinical trials using transplants of fetal DA neuroblasts have given promising results, although a number of patients have developed graft-induced dyskinesias (GIDs), and the mechanism underlying this troublesome side effect is still unknown. Here we have used a new model where the activity of the transplanted DA neurons can be selectively modulated using a bimodal chemogenetic (DREADD) approach, allowing either enhancement or reduction of the therapeutic effect. We show that exclusive activation of a cAMP-linked (Gs-coupled) DREADD or serotonin 5-HT6 receptor, located on the grafted DA neurons, is sufficient to induce GIDs. These findings establish a mechanistic link between the 5-HT6 receptor, intracellular cAMP, and GIDs in transplanted PD patients. This effect is thought to be mediated through counteraction of the D2 autoreceptor feedback inhibition, resulting in a dysplastic DA release from the transplant.

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Bimodal DREADDs and transgenic rats enable remote control of transplant function

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Gs-coupled increase of cAMP using DREADDs leads to graft-induced dyskinesias (GIDs)

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Serotonin 5-HT6 receptor stimulation gives GIDs through dysplastic DA release

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DA neurons grafted to the human brain have high levels of the 5-HT6 receptor

The anti-dyskinetic effect of dopamine receptor blockade is enhanced in parkinsonian rats following dopamine neuron transplantation

Graft-induced dyskinesia (GID) is a serious complication induced by dopamine (DA) cell transplantation in parkinsonian patients. We have recently shown that DA D2 receptor blockade produces striking blockade of dyskinesia induced by amphetamine in grafted 6-OHDA-lesioned rats, a model of GID. This study was designed to investigate whether blockade of DA D1 receptors could produce similar outcome, and to see whether the effect of these treatments in grafted rats was specific for dyskinesia induced by amphetamine, or could also influence L-DOPA-induced dyskinesia (LID). L-DOPA-primed rats received transplants of fetal DA neurons into the DA-denervated striatum. Beginning at 20weeks after transplantation rats were subjected to pharmacological treatments with either L-DOPA (6mg/kg) or amphetamine (1.5mg/kg) alone, or in combination with the D1 receptor antagonist SCH23390, the D2 receptor antagonist eticlopride, and the 5-HT1A agonist/D2 receptor antagonist buspirone. Grafted rats developed severe GID, while LID was reduced. Both eticlopride and SCH23390 produced near-complete suppression of GID already at very low doses (0.015 and 0.1mg/kg, respectively). Buspirone induced similar suppression at a dose as low as 0.3mg/kg, which is far lower than the dose known to affect LID in non-grafted dyskinetic rats. In agreement with our previous results, the effect of buspirone was independent from 5-HT1A receptor activation, as it was not counteracted by the selective 5-HT1A antagonist WAY100635, but likely due to D2 receptor blockade. Most interestingly, the same doses of eticlopride, SCH23390 and buspirone were found to suppress LID in grafted but not in control dyskinetic rats. Taken together, these data demonstrate that the DA cell grafts strikingly exacerbate the effect of DA D1 and D2 receptor blockade against both GID and LID, and suggest that the anti-GID effect of buspirone seen in patients may also be due to blockade of DA D2 receptors.

Transplantation of mouse CGR8 embryonic stem cells producing GDNF and TH protects against 6-hydroxydopamine neurotoxicity in the rat

Embryonic stem cells (ESCs)-based therapies have been increasingly recognized as a potential tool to replace or support cells and their function damaged by the neurodegenerative process that underlies Parkinson's disease (PD). In this study, we implanted engineered mouse embryonic stem (ES) CGR8 cells, which stably co-express glial cell line-derived neurotrophic factor (GDNF) and tyrosine hydroxylase (TH), into striatum (Str) or both Str and substantia nigra (SN) of parkinsonian rats lesioned by 6-hydroxydopamine (6-OHDA). We found that cell transplantation into Str or both Str and SN rescued behavioral abnormalities and striatal DA depletion associated with 6-OHDA lesion. Our findings suggested that the profound functional impairment in nigrostriatal circuitry could be at least partially restored by ESCs-based expression of TH and GDNF, which may be developed into a useful tool for PD therapy.

Repair of the CNS using endogenous and transplanted neural stem cells

Restoration of the damaged central nervous system is a vast challenge. However, there is a great need for research into this topic, due to the prevalence of central nervous system disorders and the devastating impact they have on people's lives. A number of strategies are being examined to achieve this goal, including cell replacement therapy, enhancement of endogenous plasticity and the recruitment of endogenous neurogenesis. The current chapter reviews this topic within the context of Parkinson's disease, Huntington's disease and stroke. For each disease exogenous cell therapies are discussed including primary (foetal) cell transplants, neural stem cells, induced pluripotent stem cells and marrow stromal cells. This chapter highlights the different mechanistic approaches of cell replacement therapy versus cells that deliver neurotropic factors, or enhance the endogenous production of these factors. Evidence of exogenously transplanted cells functionally integrating into the host brain, replacing cells, and having a behavioural benefit are discussed, along with the ability of some cell sources to stimulate endogenous neuroprotective and restorative events. Alongside exogenous cell therapy, the role of endogenous neurogenesis in each of the three diseases is outlined and methods to enhance this phenomenon are discussed.

Neural tissue transplantation, repair, and rehabilitation

Transplants of cells and tissues to the central nervous system of adult mammals can, under appropriate conditions, survive, integrate, and function. In particular, the grafted cells can sustain functional recovery in animal models of a range of neurodegenerative conditions including genetic and idiopathic neurodegenerative diseases of adulthood and aging, ischemic stroke, and brain and spinal cord trauma. In a restricted subset of such conditions, cell transplantation has progressed to application in humans in early-stage clinical trials. At the present stage of play, there is clear evidence of clinical efficacy of fetal cell transplants in Parkinson disease (notwithstanding a range of technical difficulties still to be fully resolved), and preliminary claims of promising outcomes in several other severe neurodegenerative conditions, including Huntington disease and stroke. Moreover, the experimental literature is increasingly suggesting that the experience and training of the graft recipient materially affects the functional outcome. For example, environmental enrichment, behavioral activity, and specific training can enhance the recovery process to maximize functional recovery. There are even circumstances where the grafted cells have been demonstrated to restore the neural substrate for new learning. Consequently, it is not sufficient to replace lost cells anatomically; rather, for the grafts to be effective, they need to be integrated functionally into the host circuitry, and the host animal requires training and rehabilitation to maximize function of the reconstructed graft-host circuitry. Such observations require reconsideration of the design of the next generation of clinical trials and subsequent service delivery, to include physiotherapists, cognitive therapists, and rehabilitation experts as core members of the transplant team, along with the neurologists and neurosurgeons that have conventionally led the field.

Human mesenchymal stem cells: from immunophenotyping by flow cytometry to clinical applications

Modern medicine will unequivocally include regenerative medicine as a major breakthrough in the re-establishment of damaged or lost tissues due to degenerative diseases or injury. In this scenario, millions of patients worldwide can have their quality of life improved by stem cell implantation coupled with endogenous secretion or administration of survival and differentiation promoting factors. Large efforts, relying mostly on flow cytometry and imaging techniques, have been put into cell isolation, immunophenotyping, and studies of differentiation properties of stem cells of diverse origins. Mesenchymal stem cells (MSCs) are particularly relevant for therapy due to their simplicity of isolation. A minimal phenotypic pattern for the identification of MSCs cells requires them to be immunopositive for CD73, CD90, and CD105 expression, while being negative for CD34, CD45, and HLA-DR and other surface markers. MSCs identified by their cell surface marker expression pattern can be readily purified from patient's bone marrow and adipose tissues. Following expansion and/or predifferentiation into a desired tissue type, stem cells can be reimplanted for tissue repair in the same patient, virtually eliminating rejection problems. Transplantation of MSCs is subject of almost 200 clinical trials to cure and treat a very broad range of conditions, including bone, heart, and neurodegenerative diseases. Immediate or medium term improvements of clinical symptoms have been reported as results of many clinical studies.

Differential sensitivity of cranial and limb motor function to nigrostriatal dopamine depletion

The present study determined the differential effects of unilateral striatal dopamine depletion on cranial motor versus limb motor function. Forty male Long Evans rats were first trained on a comprehensive motor testing battery that dissociated cranial versus limb motor function and included: cylinder forepaw placement, single pellet reaching, vermicelli pasta handling; sunflower seed opening, pasta biting acoustics, and a licking task. Following baseline testing, animals were randomized to either a 6-hydroxydopamine (6-OHDA) (n=20) or control (n=20) group. Animals in the 6-OHDA group received unilateral intrastriatal 6-OHDA infusions to induce striatal dopamine depletion. Six-weeks following infusion, all animals were re-tested on the same battery of motor tests. Near infrared densitometry was performed on sections taken through the striatum that were immunohistochemically stained for tyrosine hydroxylase (TH). Animals in the 6-OHDA condition showed a mean reduction in TH staining of 88.27%. Although 6-OHDA animals were significantly impaired on all motor tasks, limb motor deficits were more severe than cranial motor impairments. Further, performance on limb motor tasks was correlated with degree of TH depletion while performance on cranial motor impairments showed no significant correlation. These results suggest that limb motor function may be more sensitive to striatal dopaminergic depletion than cranial motor function and is consistent with the clinical observation that therapies targeting the nigrostriatal dopaminergic system in Parkinson's disease are more effective for limb motor symptoms than cranial motor impairments.

Serotonergic and dopaminergic mechanisms in graft-induced dyskinesia in a rat model of Parkinson's disease

Dyskinesia seen in the off-state, referred as graft-induced dyskinesia (GID), has emerged as a serious complication induced by dopamine (DA) cell transplantation in parkinsonian patients. Although the mechanism underlying the appearance of GID is unknown, in a recent clinical study the partial 5-HT(1A) agonist buspirone was found to markedly reduce GID in three grafted patients, who showed significant serotonin (5-HT) hyperinnervation in the grafted striatum in positron emission tomography scanning (Politis et al., 2010, 2011). Prompted by these findings, this study was performed to investigate the involvement of serotonin neurons in the appearance of GID in the rat 6-hydroxydopamine model. L-DOPA-primed rats received transplants of DA neurons only, DA plus 5-HT neurons or 5-HT neurons only into the lesioned striatum. In DA cell-grafted rats, with or without 5-HT neurons, but not in 5-HT grafts, GID was observed consistently after administration of amphetamine (1.5mg/kg, i.p.) indicating that grafted DA neurons are required to induce GID. Strikingly, a low dose of buspirone produced a complete suppression of GID. In addition, activation of 5-HT(1A) and 5-HT(1B) receptors by 8-OH-DPAT and CP 94253, known to inhibit the activity of 5-HT neurons, significantly reduced GID, whereas induction of neurotransmitter release by fenfluramine administration significantly increased GID, indicating an involvement of the 5-HT system in the modulation of GID. To investigate the involvement of the host 5-HT system in GID, the endogenous 5-HT terminals were removed by intracerebral injection of 5,7-dihydroxytryptamine, but this treatment did not affect GID expression. However, 5-HT terminal destruction suppressed the anti-GID effect of 5-HT(1A) and 5-HT(1B) agonists, demonstrating that the 5-HT(1) agonist combination exerted its anti-GID effect through the activation of pre-synaptic host-derived receptors. By contrast, removal of the host 5-HT innervation or pre-treatment with a 5-HT(1A) antagonist did not abolish the anti-GID effect of buspirone, showing that its effect is independent from activation of either pre- or post-synaptic 5-HT(1A) receptors. Since buspirone is known to also act as a DA D(2) receptor antagonist, the selective D(2) receptor antagonist eticlopride was administered to test whether blockade of D(2) receptors could account for the anti-dyskinetic effect of buspirone. In fact, eticlopride produced complete suppression of GID in grafted animals already at very low dose. Together, these results point to a critical role of both 5-HT(1) and D(2) receptors in the modulation of GID, and suggest that 5-HT neurons exert a modulatory role in the development of this side effect of neuronal transplantation.

Comparison of the behavioural and histological characteristics of the 6-OHDA and α-synuclein rat models of Parkinson's disease

Development of relevant models of Parkinson's disease (PD) is essential for a better understanding of the pathological processes underlying the human disease and for the evaluation of promising targets for therapeutic intervention. To date, most pre-clinical studies have been performed in the well-established rodent and non-human primate models using injection of 6-hydroxydopamine (6-OHDA) or 1-methyl-4-phenyl-1,2,3,6-tetrahydroxypyridine (MPTP). Overexpression of the disease-causing protein α-synuclein (α-syn), using adeno-associated viral (AAV) vectors, has provided a novel model that recapitulates many features of the human disease. In the present study we compared the AAV-α-syn rat model with models where the nigro-striatal pathway is lesioned by injection of 6-OHDA in the striatum (partial lesion) or the medial forebrain bundle (full lesion). Examination of the behavioural changes over time revealed a different progression and magnitude of the motor impairment. Interestingly, dopamine (DA) neuron loss is prominent in both the toxin and the AAV-α-syn models. However, α-syn overexpressing animals were seen to exhibit less cell and terminal loss for an equivalent level of motor abnormalities. Prominent and persistent axonal pathology is only observed in the α-syn rat model. We suggest that, while neuronal and terminal loss mainly accounts for the behavioural impairment in the toxin-based model, similar motor deficits result from the combination of cell death and dysfunction of the remaining nigro-striatal neurons in the AAV-α-syn model. While the two models have been developed to mimic DA neuron deficiency, they differ in their temporal and neuropathological characteristics, and replicate different aspects of the pathophysiology of the human disease. This study suggests that the AAV-α-syn model replicates the human pathology more closely than either of the other two 6-OHDA lesion models.

Design of a single AAV vector for coexpression of TH and GCH1 to establish continuous DOPA synthesis in a rat model of Parkinson's disease

Preclinical efficacy of continuous delivery of 3,4-dihydroxyphenylalanine (DOPA) with adeno-associated viral (AAV) vectors has recently been documented in animal models of Parkinson's disease (PD). So far, all studies have utilized a mix of two monocistronic vectors expressing either of the two genes, tyrosine hydroxylase (TH) and GTP cyclohydrolase-1 (GCH1), needed for DOPA production. Here, we present a novel vector design that enables efficient DOPA production from a single AAV vector in rats with complete unilateral dopamine (DA) lesions. Functional efficacy was assessed with drug-induced and spontaneous motor behavioral tests where vector-treated animals showed near complete and stable recovery within 1 month. Recovery of motor function was associated with restoration of extracellular DA levels as assessed by online microdialysis. Histological analysis showed robust transgene expression not only in the striatum but also in overlying cortical areas. In globus pallidus, we noted loss of NeuN staining, which might be due to different sensitivity in neuronal populations to transgene expression. Taken together, we present a single AAV vector design that result in efficient DOPA production and wide-spread transduction. This is a favorable starting point for continued translation toward a therapeutic application, although future studies need to carefully review target region, vector spread and dilution with this approach.

Progressive neurodegenerative and behavioural changes induced by AAV-mediated overexpression of α-synuclein in midbrain dopamine neurons

Parkinson's disease (PD) is characterised by the progressive loss of nigral dopamine neurons and the presence of synucleinopathy. Overexpression of α-synuclein in vivo using viral vectors has opened interesting possibilities to model PD-like pathology in rodents. However, the attempts made so far have failed to show a consistent behavioural phenotype and pronounced dopamine neurodegeneration. Using a more efficient adeno-associated viral (AAV) vector construct, which includes a WPRE enhancer element and uses the neuron-specific synapsin-1 promoter to drive the expression of human wild-type α-synuclein, we have now been able to achieve increased levels of α-synuclein in the transduced midbrain dopamine neurons sufficient to induce profound deficits in motor function, accompanied by reduced expression of proteins involved in dopamine neurotransmission and a time-dependent loss of nigral dopamine neurons, that develop progressively over 2-4 months after vector injection. As in human PD, nigral cell loss was preceded by degenerative changes in striatal axons and terminals, and the appearance of α-synuclein positive inclusions in dystrophic axons and dendrites, supporting the idea that α-synuclein-induced pathology hits the axons and terminals first and later progresses to involve also the cell bodies. The time-course of changes seen in the AAV-α-synuclein treated animals defines distinct stages of disease progression that matches the pre-symptomatic, early symptomatic, and advanced stages seen in PD patients. This model provides new interesting possibilities for studies of stage-specific pathologic mechanisms and identification of targets for disease-modifying therapeutic interventions linked to early or late stages of the disease.

Cell therapeutics in Parkinson's disease

The main pathology underlying motor symptoms in Parkinson's disease (PD) is a rather selective degeneration of nigrostriatal dopamine (DA) neurons. Intrastriatal transplantation of immature DA neurons, which replace those neurons that have died, leads to functional restoration in animal models of PD. Here we describe how far the clinical translation of the DA neuron replacement strategy has advanced. We briefly summarize the lessons learned from the early clinical trials with grafts of human fetal mesencephalic tissue, and discuss recent findings suggesting susceptibility of these grafts to the disease process long-term after implantation. Mechanisms underlying graft-induced dyskinesias, which constitute the only significant adverse event observed after neural transplantation, and how they should be prevented and treated are described. We summarize the attempts to generate DA neurons from stem cells of various sources and patient-specific DA neurons from fully differentiated somatic cells, with particular emphasis on the requirements of these cells to be useful in the clinical setting. The rationale for the new clinical trial with transplantation of fetal mesencephalic tissue is described. Finally, we discuss the scientific and clinical advancements that will be necessary to develop a competitive cell therapy for PD patients.

Cell transplantation and gene therapy in Parkinson's disease

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

Directing dopaminergic fiber growth along a preformed molecular pathway from embryonic ventral mesencephalon transplants in the rat brain

To identify guidance molecules to promote long-distance growth of dopaminergic axons from transplanted embryonic ventral mesencephalon (VM) tissue, three pathways were created by expressing green fluorescent protein (GFP), glial cell line-derived neurotrophic factor (GDNF), or a combination of GDNF/GDNF receptor α1 (GFRα1) along the corpus callosum. To generate the guidance pathway, adenovirus encoding these transcripts was injected at four positions along the corpus callosum. In all groups, GDNF adenovirus was also injected on the right side 2.5 mm from the midline at the desired transplant site. Four days later, a piece of VM tissue from embryonic day 14 rats was injected at the transplant site. All rats also received daily subcutaneous injections of N-acetyl-L-cysteinamide (NACA; 100 μg per rat) as well as chondroitinase ABC at transplant site (10 U/ml, 2 μl). Two weeks after transplantation, the rats were perfused and the brains dissected out. Coronal sections were cut and immunostained with antibody to tyrosine hydroxylase (TH) to identify and count dopaminergic fibers in the corpus callosum. In GFP-expressing pathways, TH(+) fibers grew out of the transplants for a short distance in the corpus callosum. Very few TH(+) fibers grew across the midline. However, pathways expressing GDNF supported more TH(+) fiber growth across the midline into the contralateral hemisphere. Significantly greater numbers of TH(+) fibers grew across the midline in animals expressing a combination of GDNF and GFRα1 in the corpus callosum. These data suggest that expression of GDNF or a combination of GDNF and GFRα1 can support the long-distance dopaminergic fiber growth from a VM transplant, with the combination having a superior effect.

Characterization and differentiation potential of rat ventral mesencephalic neuronal progenitor cells immortalized with SV40 large T antigen

Neuronal progenitor cells (NPCs) possess high potential for use in regenerative medicine. To overcome their limited mitotic competence, various immortalization strategies have been applied that allow their prolonged maintenance and expansion in vitro. Such immortalized cells can be used for the design and discovery of new cell-based therapies for neurodegenerative diseases, such as Parkinson's disease. We immortalized rat ventral mesencephalic NPCs by using SV40 large T antigen (SV40Tag). All cell clones displayed a two- to three-fold higher proliferation rate compared with the primary cells. In order to induce dopaminergic differentiation of generated cell clones, both glial-derived neurotrophic factor and di-butyryl cyclic adenosine monophosphate were applied. Treated cells were then characterized regarding the expression of dopaminergic lineage markers, differentiation of various cell populations, calcium imaging in the presence of kainate, and immunohistochemistry after intrastriatal transplantation. Treated cells displayed morphological maturation, and calcium imaging revealed neuronal properties in the presence of kainate. These cells also expressed low mRNA levels of the dopamine transporter and tyrosine hydroxylase (TH), although no TH-immunopositive neurons were found. Intrastriatal transplantation into the neurotoxin-lesioned rats did not induce further differentiation. As an alternative approach, we silenced SV40Tag with short interfering RNA, but this was not sufficient to trigger differentiation into dopaminergic neurons. Nevertheless, neuronal and glial cells were detected as shown by beta-tubulin type III and glial fibrillary acidic protein staining, respectively. SV40Tag cells are suitable for carrying out controlled genetic modifications as shown by overexpression of enhanced green fluorescence protein after efficient non-viral transfection.

Neural grafting in Parkinson's disease unraveling the mechanisms underlying graft-induced dyskinesia

The development of neural transplantation as a treatment for Parkinson's disease has been compromised by a lack of functional efficacy and the appearance of transplant-induced motor side-effects in some patients. Since the first reports of these graft-induced dyskinesias (GID), and the realization of their impact on the progress of the field, a great deal of experimental work has been performed to determine the underlying cause(s) of this problematic side-effect. In this review we describe the clinical phenomenon of GID, explore the different representations of GID in rodent models, and examine the various hypotheses that have been postulated to be the cause. Based on the available clinical and preclinical data we outline strategies to avoid GID in future clinical trials using fetal cell transplants or cell preparations derived from stem cells.

Survival and early functional integration of dopaminergic progenitor cells following transplantation in a rat model of Parkinson's disease

Dopaminergic (DA) grafts in rat models of Parkinson's disease (PD) have previously been derived from embryonic day (E) 14 grafts. Because there is an increasing interest in the restorative capacity of DA stem and progenitor cells, in the present study we examined the survival and early and late functional behavioral effects of DA progenitor cells derived from E12, E13, E14, and E15 grafts transplanted into rats with unilateral 6-hydroxydopamin lesions. DA transplant-induced functional recovery was already observed in postural balancing reactions after 10 days and in stepping behavior after 13 days, that is, in spontaneous complex behaviors, and later, after 16 days, in the amphetamine-induced rotation test. Three distinct patterns of functional recovery could be observed at 6-9 weeks posttransplantation. First, behavioral improvements in drug-induced rotational asymmetry, stepping, and skilled forelimb behavior were directly related to DA neuron survival and TH-positive fiber reinnervation. Second, recovery in postural balancing reactions was closely related to a specific developmental time window of donor age, for example, only seen in E13 and E14 grafts. Finally, no functional graft effects were seen in the table lift test. Interestingly, DA neuron graft survival, TH-positive fiber outgrowth, and graft volume were significantly influenced by the developmental time window in which the DA progenitor cells were dissected from the ventral mesencephalon, that is, from E12, E13, E14, or E15 rat embryos. These data highlight the complexity of graft-host interactions and provide novel insights into the dynamics of DA progenitor graft-mediated functional recovery in animal models of Parkinson's disease.

Prospects of stem cell therapy for replacing dopamine neurons in Parkinson's disease

In Parkinson's disease (PD), the main pathology is a loss of nigrostriatal dopamine (DA) neurons. Clinical trials with intrastriatal transplantation of human embryonic mesencephalic tissue have shown that grafted DA neurons reinnervate the striatum, restore striatal DA release and, in some patients, induce major clinical benefit. Stem cells could provide an unlimited source of DA neurons for transplantation. Recent studies demonstrate that cells with properties of mesencephalic DA neurons can be produced from stem cells of different sources including reprogrammed somatic cells. However, as we discuss here, it remains to be shown that these cells can provide efficient functional reinnervation and behavioral recovery in animal PD models. Moreover, a clinically competitive cell therapy for PD will require better criteria for patient selection, improved functional efficacy of grafts by a tailor-made transplantation procedure providing optimum repair of the patient's DA system and strategies to prevent dyskinesias and tumor formation.

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

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

Potential cellular and regenerative approaches for the treatment of Parkinson's disease

Parkinson's disease is most commonly treated with a range of pharmacotherapeutics, with the more recent introduction of surgical techniques including deep-brain stimulation. These have limited capabilities to improve symptoms of the disease in more advanced stages, thus new therapeutic strategies including the use of viral vectors and stem cells are in development. Providing a continuous supply of dopamine to the striatum in an attempt to improve the treatment of motor symptoms using enzymes in the dopamine synthesis and machinery is one approach. Alternatively, there are tools which may serve to both protect and encourage outgrowth of surviving neurons using growth factors or to directly replace lost innervation by transplantation of primary tissue or stem cell-derived dopaminergic neurons. We summarize some of the potential therapeutic approaches and also consider the recent EU directives on practical aspects of handling viral vectors, cells and tissues, and in the running of clinical trials in Europe which impact on their development.

Emerging restorative treatments for Parkinson's disease

Several exciting approaches for restorative therapy in Parkinson's disease have emerged over the past two decades. This review initially describes experimental and clinical data regarding growth factor administration. We focus on glial cell line-derived neurotrophic factor (GDNF), particularly its role in neuroprotection and in regeneration in Parkinson's disease. Thereafter, we discuss the challenges currently facing cell transplantation in Parkinson's disease and briefly consider the possibility to continue testing intrastriatal transplantation of fetal dopaminergic progenitors clinically. We also give a more detailed overview of the developmental biology of dopaminergic neurons and the potential of certain stem cells, i.e. neural and embryonic stem cells, to differentiate into dopaminergic neurons. Finally, we discuss adult neurogenesis as a potential tool for restoring lost dopamine neurons in patients suffering from Parkinson's disease.

Human stem cells for CNS repair

Although most peripheral tissues have at least a limited ability for self-repair, the central nervous system (CNS) has long been known to be relatively resistant to regeneration. Small numbers of stem cells have been found in the adult brain but do not appear to be able to affect any significant recovery following disease or insult. In the last few decades, the idea of being able to repair the brain by introducing new cells to repair damaged areas has become an accepted potential treatment for neurodegenerative diseases. This review focuses on the suitability of various human stem cell sources for such treatments of both slowly progressing conditions, such as Parkinson's disease, Huntington's disease and multiple sclerosis, and acute insult, such as stroke and spinal cord injury. Despite stem cell transplantation having now moved a step closer to the clinic with the first trials of autologous mesenchymal stem cells, the effects shown are moderate and are not yet at the stage of development that can fulfil the hopes that have been placed on stem cells as a means to replace degenerating cells in the CNS. Success will depend on careful investigation in experimental models to enable us to understand not just the practicalities of stem cell use, but also the underlying biological principles.

Differential effects on forelimb grasping behavior induced by fetal dopaminergic grafts in hemiparkinsonian rats

Skilled forelimb movements depend on an intact dopaminergic (DA) neurotransmission and are substantially impaired in the unilateral rat model of Parkinson's disease. It has remained unclear, however, to what extent reaching and grasping movements can be influenced by intrastriatal transplantation of fetal DA neurons. Here an extensive behavioral assessment of skilled forelimb movement patterns in hemiparkinsonian and DA-grafted rats was carried out. Good DA graft survival was accompanied by a compensation of drug-induced rotational asymmetries. Interestingly, skilled forelimb use was significantly improved in transplanted animals as compared to lesion-only animals in the staircase test. Qualitative analysis of single forelimb reaching movement components revealed dissociable patterns of graft effects: while some movement components in grafted animals improved, others remained unchanged or even deteriorated. These findings provide novel insights into the complex interactions of graft-derived restoration of DA neurotransmission and skilled forelimb behavior.

The role of stem cells in Parkinson's disease

Parkinson's disease (PD) affects more than 1 million people in the United States, which makes it one of the most common age-related neurodegenerative disorders, second only to Alzheimer disease. In light of this significant health problem, this review places emphasis on the exciting prospect of using stem cells as an emerging therapeutic option in this neurologic field. To that goal, the authors first describe the clinical, genetic, and pathologic features of PD and proceed with discussing notions about disease mechanism as well as current medical and surgical treatments before focusing on the advantages, limitations, and feasibility of stem cell therapy.

Rat model of Parkinson's disease with bilateral motor abnormalities, reversible with levodopa, and dyskinesias

Parkinson's disease (PD) is characterized by the bilateral degeneration of the midbrain dopamine-containing neurons with the most severe lesion in the posterolateral part of the substantia nigra pars compacta (SNpc). In humans, such lesions lead to specific motor abnormalities (i.e., akinesia, rigidity, and tremor) that are greatly improved by levodopa treatment. After a few years, the beneficial effect of the treatment is frequently offset by the development of dyskinesias. To improve treatment strategies, an animal model showing most of the histological and clinical characteristics of the human disease is mandatory. Ten rats received a bilateral injection of small doses of 6-OHDA in the medial forebrain bundle (MFB) and were compared with five sham-lesioned rats. The 6-OHDA-lesioned rats progressively developed abnormal motor behavior (assessed by the stepping test) compared with the sham-lesioned rats. The lesioned rats greatly improved under levodopa treatment, but developed concomitant dyskinesias. All 6-OHDA-lesioned animals had bilateral partial lesions of the SNpc, with the most severe lesion being in its posterolateral part. There was a significant correlation between the severity of the dopaminergic cell loss and the severity of the levodopa-induced dyskinesias. These rats constitute an interesting model of PD, sharing some of the main characteristics of the human disease.

Cell repair in Parkinson’s disease

Parkinson’s disease (PD) is a progressive neurodegenerative movement disorder with the cardinal clinical features of muscular rigidity, resting tremor and bradykinesia. The prevalence of this disease is approximately 2% of those aged over 65 years, thus causing significant morbidity. The disease is characterized by degeneration of dopaminergic cells in the substantia nigra pars compacta, resulting in reduced dopaminergic input to the striatum. Significant clinical benefit can be achieved through the restoration of dopamine levels in this system with pharmacological interventions, although these therapies are only symptomatic and the disease progresses. Indeed, with disease progression other features often appear, including autonomic, affective and cognitive dysfunction, reflecting pathology at non-nigral sites. The occurrence of neural stem cells (NSCs) in the adult CNS, which, under certain conditions, are able to proliferate and renew neuronal numbers, has raised great expectations for alternative therapeutic applications in the treatment of PD. Indeed, it is potentially possible to harness this capacity either directly (increase of local proliferation, directed migration and differentiation) or indirectly (in vitro expansion before their transplantation), to facilitate the generation of specific cell types in order to replace missing neurons in neurodegenerative diseases. The manipulation of embryonic stem cells or their derivatives also offers a promising alternative as extensive proliferation may be achieved and, most importantly, directed differentiation to a dopaminergic phenotype is possible. Nevertheless, neuronal replacement will only be possible if proliferating or transplanted NSCs and their progeny can be harnessed at sites of pathology. It is the manipulation of stem cells both in vivo and in vitro, in the context of repairing the core pathological hallmark of PD, that is the main focus of this report.

Increased capacity for axonal outgrowth using xenogenic tissue in vitro and in a rodent model of Parkinson's disease

BACKGROUND

It has previously been proposed that grafted neurons may have the potential for more extensive axonal outgrowth in a xenogenic environment, and may thus possess a primary advantage over allografts in central nervous system repair and circuit reconstruction. In order to directly address this issue, fibre outgrowth from primary dopaminergic neurons was examined both in vitro and in vivo in an allogenic or xenogenic environment. A combination of species was used to circumvent problems relating to different gestational/developmental periods for such cells.

METHODS

In the in vitro experiments, axon length was measured over 2 to 14 days in cultures derived from either rat or mouse ventral mesencephalon (VM) tissue co-cultured onto either a monolayer of the rat Neu7 inhibitory cell line or fetal rodent cortical tissue. In the in vivo experiments, fetal rat or mouse VM tissue was transplanted into the striatum or substantia nigra of 6-hydroxydopamine-lesioned athymic rats. Amphetamine-induced rotations were observed for 3 months post-transplantation and the degree of graft-mediated neurite outgrowth was analyzed.

RESULTS

Embryonic VM manifested a greater capacity for neurite formation and outgrowth on xenogenic tissue, which was shown to be significant using co-cultures of cortical cells. The transplantation study showed that xenograft-derived fibres had a greater capacity for extensive fibre projection compared with those originating from allografts.

CONCLUSION

Results from the present study provide evidence for the hypothesis that xenografts are less responsive to inhibitory molecules present in the adult host environment and as such can project over great distances. Thus neural xenotransplantation may have the potential for more complete circuit reconstruction within the damaged host brain than equivalent allografted tissue.