Dopamine cells in nigral grafts differentiate prior to implantation

The Struggle to Make CNS Axons Regenerate: Why Has It Been so Difficult?

Axon regeneration in the CNS is inhibited by many extrinsic and intrinsic factors. Because these act in parallel, no single intervention has been sufficient to enable full regeneration of damaged axons in the adult mammalian CNS. In the external environment, NogoA and CSPGs are strongly inhibitory to the regeneration of adult axons. CNS neurons lose intrinsic regenerative ability as they mature: embryonic but not mature neurons can grow axons for long distances when transplanted into the adult CNS, and regeneration fails with maturity in in vitro axotomy models. The causes of this loss of regeneration include partitioning of neurons into axonal and dendritic fields with many growth-related molecules directed specifically to dendrites and excluded from axons, changes in axonal signalling due to changes in expression and localization of receptors and their ligands, changes in local translation of proteins in axons, and changes in cytoskeletal dynamics after injury. Also with neuronal maturation come epigenetic changes in neurons, with many of the transcription factor binding sites that drive axon growth-related genes becoming inaccessible. The overall aim for successful regeneration is to ensure that the right molecules are expressed after axotomy and to arrange for them to be transported to the right place in the neuron, including the damaged axon tip.

Encapsulation of young donor age dopaminergic grafts in a GDNF-loaded collagen hydrogel further increases their survival, reinnervation, and functional efficacy after intrastriatal transplantation in hemi-Parkinsonian rats

Biomaterials have been shown to significantly improve the outcome of cellular reparative approaches for Parkinson's disease in experimental studies because of their ability to provide transplanted cells with a supportive microenvironment and shielding from the host immune system. However, given that the margin for improvement in such reparative therapies is considerable, further studies are required to fully investigate and harness the potential of biomaterials in this context. Given that several recent studies have demonstrated improved brain repair in Parkinsonian models when using dopaminergic grafts derived from younger foetal donors, we hypothesized that encapsulating these cells in a supportive biomaterial would further improve their reparative efficacy. Thus, this study aimed to determine the impact of a GDNF-loaded collagen hydrogel on the survival, reinnervation, and functional efficacy of dopaminergic neurons derived from young donors. To do so, hemi-Parkinsonian (6-hydroxydopamine-lesioned) rats received intrastriatal transplants of embryonic day 12 cells extracted from the rat ventral mesencephalon either alone, in a collagen hydrogel, with GDNF, or in a GDNF-loaded collagen hydrogel. Methamphetamine-induced rotational behaviour was assessed at three weekly intervals for a total of 12 weeks, after which rats were sacrificed for postmortem assessment of graft survival. We found that, following intrastriatal transplantation to the lesioned striatum, the GDNF-loaded collagen hydrogel significantly increased the survival (4-fold), reinnervation (5.4-fold), and functional efficacy of the embryonic day 12 dopaminergic neurons. In conclusion, this study further demonstrates the significant potential of biomaterial hydrogel scaffolds for cellular brain repair approaches in neurodegenerative diseases such as Parkinson's disease.

Anatomically Inspired Three-dimensional Micro-tissue Engineered Neural Networks for Nervous System Reconstruction, Modulation, and Modeling

Functional recovery rarely occurs following injury or disease-induced degeneration within the central nervous system (CNS) due to the inhibitory environment and the limited capacity for neurogenesis. We are developing a strategy to simultaneously address neuronal and axonal pathway loss within the damaged CNS. This manuscript presents the fabrication protocol for micro-tissue engineered neural networks (micro-TENNs), implantable constructs consisting of neurons and aligned axonal tracts spanning the extracellular matrix (ECM) lumen of a preformed hydrogel cylinder hundreds of microns in diameter that may extend centimeters in length. Neuronal aggregates are delimited to the extremes of the three-dimensional encasement and are spanned by axonal projections. Micro-TENNs are uniquely poised as a strategy for CNS reconstruction, emulating aspects of brain connectome cytoarchitecture and potentially providing means for network replacement. The neuronal aggregates may synapse with host tissue to form new functional relays to restore and/or modulate missing or damaged circuitry. These constructs may also act as pro-regenerative "living scaffolds" capable of exploiting developmental mechanisms for cell migration and axonal pathfinding, providing synergistic structural and soluble cues based on the state of regeneration. Micro-TENNs are fabricated by pouring liquid hydrogel into a cylindrical mold containing a longitudinally centered needle. Once the hydrogel has gelled, the needle is removed, leaving a hollow micro-column. An ECM solution is added to the lumen to provide an environment suitable for neuronal adhesion and axonal outgrowth. Dissociated neurons are mechanically aggregated for precise seeding within one or both ends of the micro-column. This methodology reliably produces self-contained miniature constructs with long-projecting axonal tracts that may recapitulate features of brain neuroanatomy. Synaptic immunolabeling and genetically encoded calcium indicators suggest that micro-TENNs possess extensive synaptic distribution and intrinsic electrical activity. Consequently, micro-TENNs represent a promising strategy for targeted neurosurgical reconstruction of brain pathways and may also be applied as biofidelic models to study neurobiological phenomena in vitro.

Advanced biomaterial strategies to transplant preformed micro-tissue engineered neural networks into the brain

OBJECTIVE

Connectome disruption is a hallmark of many neurological diseases and trauma with no current strategies to restore lost long-distance axonal pathways in the brain. We are creating transplantable micro-tissue engineered neural networks (micro-TENNs), which are preformed constructs consisting of embedded neurons and long axonal tracts to integrate with the nervous system to physically reconstitute lost axonal pathways.

APPROACH

We advanced micro-tissue engineering techniques to generate micro-TENNs consisting of discrete populations of mature primary cerebral cortical neurons spanned by long axonal fascicles encased in miniature hydrogel micro-columns. Further, we improved the biomaterial encasement scheme by adding a thin layer of low viscosity carboxymethylcellulose (CMC) to enable needle-less insertion and rapid softening for mechanical similarity with brain tissue.

MAIN RESULTS

The engineered architecture of cortical micro-TENNs facilitated robust neuronal viability and axonal cytoarchitecture to at least 22 days in vitro. Micro-TENNs displayed discrete neuronal populations spanned by long axonal fasciculation throughout the core, thus mimicking the general systems-level anatomy of gray matter-white matter in the brain. Additionally, micro-columns with thin CMC-coating upon mild dehydration were able to withstand a force of 893 ± 457 mN before buckling, whereas a solid agarose cylinder of similar dimensions was predicted to withstand less than 150 μN of force. This thin CMC coating increased the stiffness by three orders of magnitude, enabling needle-less insertion into brain while significantly reducing the footprint of previous needle-based delivery methods to minimize insertion trauma.

SIGNIFICANCE

Our novel micro-TENNs are the first strategy designed for minimally invasive implantation to facilitate nervous system repair by simultaneously providing neuronal replacement and physical reconstruction of long-distance axon pathways in the brain. The micro-TENN approach may offer the ability to treat several disorders that disrupt the connectome, including Parkinson's disease, traumatic brain injury, stroke, and brain tumor excision.

Rebuilding Brain Circuitry with Living Micro-Tissue Engineered Neural Networks

Prominent neuropathology following trauma, stroke, and various neurodegenerative diseases includes neuronal degeneration as well as loss of long-distance axonal connections. While cell replacement and axonal pathfinding strategies are often explored independently, there is no strategy capable of simultaneously replacing lost neurons and re-establishing long-distance axonal connections in the central nervous system. Accordingly, we have created micro-tissue engineered neural networks (micro-TENNs), which are preformed constructs consisting of long integrated axonal tracts spanning discrete neuronal populations. These living micro-TENNs reconstitute the architecture of long-distance axonal tracts, and thus may serve as an effective substrate for targeted neurosurgical reconstruction of damaged pathways in the brain. Cerebral cortical neurons or dorsal root ganglia neurons were precisely delivered into the tubular constructs, and properties of the hydrogel exterior and extracellular matrix internal column (180-500 μm diameter) were optimized for robust neuronal survival and to promote axonal extensions across the 2.0 cm tube length. The very small diameter permits minimally invasive delivery into the brain. In this study, preformed micro-TENNs were stereotaxically injected into naive rats to bridge deep thalamic structures with the cerebral cortex to assess construct survival and integration. We found that micro-TENN neurons survived at least 1 month and maintained their long axonal architecture along the cortical-thalamic axis. Notably, we also found neurite penetration from micro-TENN neurons into the host cortex, with evidence of synapse formation. These micro-TENNs represent a new strategy to facilitate nervous system repair by recapitulating features of neural pathways to restore or modulate damaged brain circuitry.

Cell fate analysis of embryonic ventral mesencephalic grafts in the 6-OHDA model of Parkinson's disease

Evidence from carefully conducted open label clinical trials suggested that therapeutic benefit can be achieved by grafting fetal dopaminergic (DAergic) neurons derived from ventral mesencephalon (VM) into the denervated striatum of Parkinson's disease (PD) patients. However, two double-blind trials generated negative results reporting deleterious side effects such as prominent dyskinesias. Heterogeneous composition of VM grafts is likely to account for suboptimal clinical efficacy.We consider that gene expression patterns of the VM tissue needs to be better understood by comparing the genetic signature of the surviving and functioning grafts with the cell suspensions used for transplantation. In addition, it is crucial to assess whether the grafted cells exhibit the DAergic phenotype of adult substantia nigra pars compacta (SNpc). To investigate this further, we used a GFP reporter mouse as source of VM tissue that enabled the detection and dissection of the grafts 6 weeks post implantation. A comparative gene expression analysis of the VM cell suspension and grafts revealed that VM grafts continue to differentiate post-implantation. In addition, implanted grafts showed a mature SNpc-like molecular DAergic phenotype with similar expression levels of TH, Vmat2 and Dat. However, by comparing gene expression of the adult SNpc with dissected grafts we detected a higher expression of progenitor markers in the grafts. Finally, when compared to the VM cell suspension, post-grafting there was a higher expression of markers inherent to glia and other neuronal populations.In summary, our data highlight the dynamic development of distinctive DAergic and non-DAergic gene expression markers associated with the maturation of VM grafts in vivo. The molecular signature of VM grafts and its functional relevance should be further explored in future studies aimed at the optimization of DAergic cell therapy approaches in PD.

Cell intrinsic and extrinsic factors contribute to enhance neural circuit reconstruction following transplantation in Parkinsonian mice

Cell replacement therapy for Parkinson's disease has predominantly focused on ectopic transplantation of fetal dopamine (DA) neurons into the striatum as a means to restore neurotransmission, rather than homotopic grafts into the site of cell loss, which would require extensive axonal growth. However, ectopic grafts fail to restore important aspects of DA circuitry necessary for controlled basal ganglia output, and this may underlie the suboptimal and variable functional outcomes in patients. We recently showed that DA neurons in homotopic allografts of embryonic ventral mesencephalon (VM) can send long axonal projections along the nigrostriatal pathway in order to innervate forebrain targets, although the extent of striatal reinnervation remains substantially less than can be achieved with ectopic placement directly into the striatal target. Here, we examined the possible benefits of using younger VM donor tissue and over-expression of glial cell-derived neurotrophic factor (GDNF) in the striatal target to improve the degree of striatal innervation from homotopic grafts. Younger donor tissue, collected on embryonic day (E)10, generated 4-fold larger grafts with greater striatal targeting, compared to grafts generated from more conventional E12 donor VM. Over-expression of GDNF in the host brain also significantly increased DA axonal growth and striatal innervation. Furthermore, a notable increase in the number and proportion of A9 DA neurons, essential for functional recovery, was observed in younger donor grafts treated with GDNF. Behavioural testing confirmed functional integration of younger donor tissue and demonstrated that improved motor function could be attributed to both local midbrain and striatal innervation. Together, these findings suggest there is significant scope for further development of intra-nigral grafting as a restorative approach for Parkinson's disease.

Birth dating of midbrain dopamine neurons identifies A9 enriched tissue for transplantation into parkinsonian mice

Clinical trials have provided proof of principle that new dopamine neurons isolated from the developing ventral midbrain and transplanted into the denervated striatum can functionally integrate and alleviate symptoms in Parkinson's disease patients. However, extensive variability across patients has been observed, ranging from long-term motor improvement to the absence of symptomatic relief and development of dyskinesias. Heterogeneity of the donor tissue is likely to be a contributing factor in the variable outcomes. Dissections of ventral midbrain used for transplantation will variously contain progenitors for different dopamine neuron subtypes as well as different neurotransmitter phenotypes. The overall impact of the resulting graft will be determined by the functional contribution from these different cell types. The A9 substantia nigra pars compacta dopamine neurons, for example, are known to be particularly important for motor recovery in animal models. Serotonergic neurons, on the other hand, have been implicated in unwanted dyskinesias. Currently little knowledge exists on how variables such as donor age, which have not been controlled for in clinical trials, will impact on the final neuronal composition of fetal grafts. Here we performed a birth dating study to identify the time-course of neurogenesis within the various ventral midbrain dopamine subpopulations in an effort to identify A9-enriched donor tissue for transplantation. The results show that A9 neurons precede the birth of A10 ventral tegmental area dopamine neurons. Subsequent grafting of younger ventral midbrain donor tissue revealed significantly larger grafts containing more mitotic dopamine neuroblasts compared to older donor grafts. These grafts were enriched with A9 neurons and showed significantly greater innervation of the target dorso-lateral striatum and DA release. Younger donor grafts also contained significantly less serotonergic neurons. These findings demonstrate the importance of standardized methods to improve cell therapy for Parkinson's disease and have significant implications for the generation and selectivity of dopamine neurons from stem cell based sources.

Cultured subventricular zone progenitor cells transduced with neurogenin-2 become mature glutamatergic neurons and integrate into the dentate gyrus

We have previously shown that transplantation of immature DCX+/NeuN+/Prox1+ neurons (found in the neonatal DG), but not undifferentiated neuronal progenitor cells (NPCs) from ventral subventricular zone (SVZ), results in neuronal maturation in vivo within the dentate niche. Here we investigated whether we could enhance the integration of SVZ NPCs by forced expression of the proneural gene Neurogenin 2 (NEUROG2). NPCs cultured from neonatal GFP-transgenic rat SVZ for 7 days in a non-differentiating medium were transduced with a retrovirus encoding NEUROG2 and DsRed or the DsRed reporter gene alone (control). By 3 days post-transduction, the NEUROG2-transduced cells maintained in culture contained mostly immature neurons (91% DCX+; 76% NeuN+), whereas the control virus-transduced cells remained largely undifferentiated (30% DCX+; <1% NeuN+). At 6 weeks following transplantation into the DG of adult male rats, there were no neurons among the transplanted cells treated with the control virus but the majority of the NEUROG2-transduced DsRed+ SVZ cells became mature neurons (92% NeuN+; DCX-negative). Although the NEUROG2-transduced SVZ cells did not express the dentate granule neuron marker Prox1, most of the NEUROG2-transduced SVZ cells (78%) expressed the glutamatergic marker Tbr1, suggesting the acquisition of a glutamatergic phenotype. Moreover, some neurons extended dendrites into the molecular layer, grew axons containing Ankyrin G+ axonal initial segments, and projected into the CA3 region, thus resembling mature DG granule neurons. A proportion of NEUROG2 transduced cells also expressed c-Fos and P-CREB, two markers of neuronal activation. We conclude that NEUROG2-transduction is sufficient to promote neuronal maturation and integration of transplanted NPCs from SVZ into the DG.

Cell origin and culture history determine successful integration of neural precursor transplants into the dentate gyrus of the adult rat

The success of transplants of neural tissue into the adult dentate gyrus in generating mature neurons is highly variable. Here we address the roles of the origin of the tissue and its pre-implantation preparation, and show that both are critical. We transplanted neonatal cultured or primary rat cells from either the ventral subventricular zone (vSVZ) or the dentate gyrus (DG) into the adult rat DG. Only primary DG cells robustly generated DG neurons (80% NeuN and Prox1-positive cells at 6 weeks), substantially repaired the damaged DG, and formed glutamatergic projections to the target CA3 region. Cultured DG cells expanded for 7 days showed limited neuronal differentiation after transplantation (10% NeuN and Prox1-positive cells) whereas cultured or primary vSVZ cells failed to make any Prox1-positive DG granular neurons. We found that a specific population of postmitotic young neurons (triple doublecortin/NeuN/Prox1-positive) were particularly abundant in primary DG cells, but were markedly reduced in the cultured DG cells and were absent in the cultured and primary vSVZ cells. Labelling of primary DG cells with the mitotic marker BrdU suggested that postmitotic young neurons are the source of the transplanted mature neurons in-vivo. We conclude that both the origin and pre-transplantation history of donor cells are key factors that determine the outcome of transplantation. These findings may be of therapeutic interest for cell replacement therapy in treating the damaged hippocampus.

α-Synuclein propagates from mouse brain to grafted dopaminergic neurons and seeds aggregation in cultured human cells

Post-mortem analyses of brains from patients with Parkinson disease who received fetal mesencephalic transplants show that α-synuclein-containing (α-syn-containing) Lewy bodies gradually appear in grafted neurons. Here, we explored whether intercellular transfer of α-syn from host to graft, followed by seeding of α-syn aggregation in recipient neurons, can contribute to this phenomenon. We assessed α-syn cell-to-cell transfer using microscopy, flow cytometry, and high-content screening in several coculture model systems. Coculturing cells engineered to express either GFP- or DsRed-tagged α-syn resulted in a gradual increase in double-labeled cells. Importantly, α-syn-GFP derived from 1 neuroblastoma cell line localized to red fluorescent aggregates in other cells expressing DsRed-α-syn, suggesting a seeding effect of transmitted α-syn. Extracellular α-syn was taken up by cells through endocytosis and interacted with intracellular α-syn. Next, following intracortical injection of recombinant α-syn in rats, we found neuronal uptake was attenuated by coinjection of an endocytosis inhibitor. Finally, we demonstrated in vivo transfer of α-syn between host cells and grafted dopaminergic neurons in mice overexpressing human α-syn. In summary, intercellularly transferred α-syn interacts with cytoplasmic α-syn and can propagate α-syn pathology. These results suggest that α-syn propagation is a key element in the progression of Parkinson disease pathology.

Multitract microtransplantation increases the yield of DARPP-32-positive embryonic striatal cells in a rodent model of Huntington's disease

Embryonic striatal graft-mediated functional recovery in the rodent lesion model of Huntington's disease (HD) has been shown to correlate with the proportion of dopamine- and adenosine 3',5'-monophosphate-regulated phosphoprotein with a molecular weight of 32 kDa (DARPP-32)-positive neurons in the graft. The current study investigated the impact of graft distribution on the yield of DARPP-32-positive cells in the grafts following either single-tract or multitract cell delivery protocols using the microtransplantation approach. Cells derived from the whole ganglionic eminence of E15 rat embryos, ubiquitously expressing green fluorescent protein (GFP), were implanted into unilaterally QA-lesioned rat striatum either as 2 × 1.8 μl macrodeposits in a single tract, or as 18 × 0.2 μl microdeposits disseminated over six needle, multitract, penetrations. For both groups, an ultrathin glass capillary with an outer diameter of 50 μm was used. Histological assessment at 4 months after transplantation showed nearly twofold increase of DARRP-32-positive striatal-like neurons in the multitract compared to the single-tract group. However, the cellular make-up of the grafts did not translate into functional differences as tested in a basic spontaneous behavior test. Furthermore, the volumetric values for overall volume, DARPP-32-positive patches, and dopaminergic projection zones were similar between both groups. The results show that distribution of fetal striatal tissue in multiple submicroliter deposits provides for an increased yield of striatal-like neurons, potentially due to the enlargement of the graft-host border area intensifying the graft's exposure to host-derived factors. Furthermore, the use of embryonic tissue from GFP donors was validated in cell-based therapy studies in the HD model.

Slowly progressive dopamine cell loss--a model on which to test neuroprotective strategies for Parkinson's disease?

Making animal models of human disease is a very flawed process. Aspects of the disease can be imitated but models do not necessarily give reliable leads for treatment strategies. When Ungerstedt in Sweden first described the 6-hydroxydopamine (6-OHDA) treated rat model of Parkinson's disease /89/ we knew that the symptoms would not map readily to those of the human disease--rats have four legs after all. On the other hand, the neuropathology looked exactly like end-stage Parkinsonian pathology. That remained true even as we explored other types of neuropathology in the rats /24,43-46,80/. Many of today's treatments for Parkinsonism are developed from pharmacological studies on that model of rats with a chemically induced lesion. However, the 6-OHDA model does not address the important issue of a cure for the disease. The triggers and the time-course of dopamine (DA) cell death in rats are known for nearly every disease model - but for the human disease there is no equivalent knowledge. In the human, the neurons have been dying for a considerable time before the symptoms become obvious and they go on dying even with adequate symptomatic relief /94/, but after intracerebral administration of 6-OHDA to an animal the cells die quickly; all cells are destroyed in less than 5 days /42,88,89/. Thus, we were interested in developing an animal model of DA cell death with a slower time-course. After ibotenic acid injections into rat globus pallidus (GP), DA cells are lost from the ipsilateral substantia nigra over the slower time scale of about six weeks. This time scale has allowed us to test some interventions to prevent the cells from dying. Although some attempts have succeeded, cell death is prevented only for three weeks -beyond that treatments fail and DA cells die. At the moment, this model has at least opened a window into causes of neuronal death in a slower time scale /94/ than previous rodent models.

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.

Identification of transplantable dopamine neuron precursors at different stages of midbrain neurogenesis

Protocols used for generation of mesencephalic dopamine (mesDA) neurons from stem cells, or fetal brain tissue, invariably result in cell preparations that are highly mixed in composition, containing mesDA neuron precursors in various states of fate commitment and differentiation. For further optimisation and refinement of these procedures it is essential to determine the optimal stage of development and phenotypic characteristics of cells used for grafting. We have used fluorescence-activated cell sorting procedures to isolate mesDA precursors in defined stages of differentiation from mouse ventral mesencephalon (VM), at embryonic day 10.5 (E10.5), when the mesDA neuron domain consists of proliferative radial glia-like cells expressing the mesDA neuron determinant Lmx1a and the floorplate marker Corin, and at E12.5, when the VM has expanded to comprise a mixture of proliferative progenitors, neuroblasts and young neurons. The sorted cells were transplanted to the striatum of 6-hydroxydopamine-lesioned rats. Results show that the Lmx1a/Corin-expressing ventricular zone progenitors, which are the source of mesDA neurons in grafts from E10.5 VM, had lost this capacity at E12.5. At this later stage all transplantable mesDA precursors resided in the intermediate zone as postmitotic Nurr1-expressing neuroblasts. The more differentiated, TH-expressing cells survived sorting and transplantation poorly. We also provide evidence that, during early mesDA neurogenesis, the progenitors for nigral mesDA neurons segregate to lateral parts of the Lmx1a-expressing domain and can be selectively isolated based on their level of Corin expression. These results have implications for current efforts to develop well-characterized stem cell-derived mesDA progenitor cell preparations for cell therapy.

Molecular and cellular determinants for generating ES-cell derived dopamine neurons for cell therapy

Embryonic stem (ES) cells can generate midbrain dopaminergic (DA) neuronal phenotypes in vitro and have been successfully applied to restore function in animal models of Parkinson's disease (PD). How can we best integrate our growinginsight into the regulatory cascade of transcription factors guiding midbrain specification to further improve the in vitro differentiation of midbrain DA neurons for cell therapy of PD? To characterize the differentiation of authentic DA neurons in vitro, expression patterns of the numerous midbrain-characteristic markers need to be investigated. When using forced gene expression, such factors have to be closely monitored to avoid generation of nonphysiological cell types. Fluorescent markers such as Pitx3-GFP, TH-GFP, Sox1-GFP or surface antigens have proven useful for elimination of unwanted cell types by cell sorting, thereby averting tumors and increasing the DA fraction for transplantation studies. The importance of appropriate timing during application of extrinsic factors and the influence of cell-cell interactions in the dish has to be taken into account. This conceptual synopsis outlines current objectives, progress, but also challenges, in deriving midbrain DA neurons from pluripotent stem cells for clinical and scientific applications.

Ontogeny of substantia nigra dopamine neurons

Understanding the ontogeny of A9 dopamine (DA) neurons is critical not only to determining basic developmental events that facilitate the emergence of the substantia nigra pars compacta (SNc) but also to the extraction and de novo generation of DA neurons as a potential cell therapy for Parkinson's disease. Recent research has identified a precise window for DA cell birth (differentiation) in the ventral mesencephalon (VM) as well as a number of factors that may facilitate this process. However, application of these factors in vitro has had limited success in specifying a dopaminergic cell fate from undifferentiated cells, suggesting that other cell/molecular signals may as yet remain undiscovered. To resolve this, current work seeks to identify particularly potent and novel DA neuron differentiation factors within the developing VM specifically at the moment of ontogeny. Through such (past and present) studies, a catalog of proteins that play a pivotal role in the generation of nigral DA neurons during normal CNS development has begun to emerge. In the future, it will be crucial to continue to evaluate the critical developmental window where DA neuron ontogeny occurs, not only to facilitate our potential to protect these cells from degeneration in the adult brain but also to mimic the developmental environment in a way that enhances our ability to generate these cells anew either in vitro or in vivo. Here we review our present understanding of factors that are thought to be involved in the emergence of the A9 dopamine neuron group from the ventral mesencephalon.

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.

Recovery of functional deficits following early donor age ventral mesencephalic grafts in a rat model of Parkinson's disease

It has previously been reported that dopaminergic grafts derived from early donor age, embryonic age 12-day-old (E12) rat embryos produced a fivefold greater yield of dopamine neurons than those derived from conventional E14 donors. The present study addresses whether E12 grafts are able to ameliorate lesion-induced behavioral deficits to the same extent as E14 grafts. In a unilateral rat model of Parkinson's disease, animals received grafts derived from either E12 or E14 donor embryos, dispersed at four sites in the lesioned striatum. Both E12 and E14 grafts were able to induce recovery on both amphetamine and apomorphine rotation tests, and to ameliorate deficits in the cylinder, stepping test, and corridor tests, but were unable to restore function in the paw reaching task. E12 grafts were equivalent to E14 grafts in their effects on lesion-induced deficits. However, E12 grafts resulted in cell yields greater than previously reported for untreated primary tissue, with mean TH-positive cell counts in excess of 25,000 neurons, compared with E14 TH cell counts of 4000-5000 cells, representing survival rates of 75% and 12.5%, respectively, based on the expected adult complement. The equivalence of graft induced behavioral recovery between the two graft groups is attributed to a threshold number of cells, above which no further improvement is seen. Such high dopamine cell survival rates should mean that multiple, functioning grafts can be derived from a single embryonic donor, and if similar yields could be obtained from human tissues then the goal of one embryo per patient would be achieved.

Differential effects of glial cell line-derived neurotrophic factor on A9 and A10 dopamine neuron survival in vitro

Glial cell-line derived neurotrophic factor (GDNF) enhances dopamine (DA) cell survival and fiber outgrowth, and may be beneficial in enhancing cell restorative strategies for Parkinson's disease (PD). However, GDNF may have different roles for transplanted DA cell sub-types. The present in vitro study investigated the effect of GDNF on the survival of rat DA cells displaying a phenotype consistent with either the substantia nigra [A9 cells immunopositive for tyrosine hydroxylase (TH) and G-protein-gated inwardly rectifying potassium channel subunit 2 (GIRK2)] or with the ventral tegmental area [A10 cells immunopositive for TH and calbindin]. It was found that a single exposure of GDNF enhanced the number of DA cells of an A9 phenotype, without affecting DA cells of an A10 phenotype. Conversely, repeated GDNF exposure did not alter the survival of A9 phenotypic cells, but doubled the percentage of A10 cells. It was concluded that GDNF administration may affect dopaminergic cells differently depending on time and degree of GDNF exposure. For cell transplantation in PD, long-term GDNF administration may result in detrimental effects for transplanted A9 TH+ cells as this may introduce competition with A10 TH+ cells for survival and fiber outgrowth into the host striatum. These results may have important implications for clinical neural transplantation in PD.

Markers and methods for cell sorting of human embryonic stem cell-derived neural cell populations

Neural cells differentiated in vitro from human embryonic stem cells (hESC) exhibit broad cellular heterogeneity with respect to developmental stage and lineage specification. Here, we describe standard conditions for the use and discovery of markers for analysis and cell selection of hESC undergoing neuronal differentiation. To generate better-defined cell populations, we established a working protocol for sorting heterogeneous hESC-derived neural cell populations by fluorescence-activated cell sorting (FACS). Using genetically labeled synapsin-green fluorescent protein-positive hESC-derived neurons as a proof of principle, we enriched viable differentiated neurons by FACS. Cell sorting methodology using surface markers was developed, and a comprehensive profiling of surface antigens was obtained for immature embryonic stem cell types (such as stage-specific embryonic antigen [SSEA]-3, -4, TRA-1-81, TRA-1-60), neural stem and precursor cells (such as CD133, SSEA-1 [CD15], A2B5, forebrain surface embryonic antigen-1, CD29, CD146, p75 [CD271]), and differentiated neurons (such as CD24 or neural cell adhesion molecule [NCAM; CD56]). At later stages of neural differentiation, NCAM (CD56) was used to isolate hESC-derived neurons by FACS. Such FACS-sorted hESC-derived neurons survived in vivo after transplantation into rodent brain. These results and concepts provide (a) a feasible approach for experimental cell sorting of differentiated neurons, (b) an initial survey of surface antigens present during neural differentiation of hESC, and (c) a framework for developing cell selection strategies for neural cell-based therapies.

Improved survival of young donor age dopamine grafts in a rat model of Parkinson's disease

In an attempt to improve the survival of implanted dopamine cells, we have readdressed the optimal embryonic donor age for dopamine grafts. In a rat model of Parkinson's disease, animals with unilateral 6-hydroxydopamine lesions of the median forebrain bundle received dopamine-rich ventral mesencephalic grafts derived from embryos of crown to rump length 4, 6, 9, or 10.5 mm (estimated embryonic age (E) 11, E12, E13 and E14 days post-coitus, respectively). Grafts derived from 4 mm embryos survived poorly, with less than 1% of the implanted dopamine cells surviving. Grafts derived from 9 mm and 10.5 mm embryos were similar to those seen in previous experiments with survival rates of 8% and 7% respectively. The best survival was seen in the group that received 6 mm grafts, which were significantly larger than all other graft groups. Mean dopamine cell survival in the 6 mm group (E12) was 36%, an extremely high survival rate for primary, untreated ventral mesencephalic grafts applied as a single placement, and more than fivefold larger than the survival rate observed in the 10.5 mm (E14) group. As E12 ventral mesencephalic tissues contain few, if any, differentiated dopamine cells we conclude that the large numbers of dopamine cells seen in the 6 mm grafts must have differentiated post-implantation. We consider the in vivo conditions which allow this differentiation to occur, and the implications for the future of clinical trials based on dopamine cell replacement therapy.

Survival change of ventral mesencephalon-derived progenitor cells after grafting into unilateral intact adult rat striatum

Neural transplantation is one of the most promising treatments for neurodegenerative disorders. Survival rates of embryonic dopamine (DA) neurons following transplantation are low, between 2% and 20% in a number of animal models. To further establish survival changes of the transplanted gestational day 13.5 ventral mesencephalic (VM) cells into left intact adult rat striata so that design strategies of increasing survival of DA neurons, the tyrosine hydroxylase (TH) expression of VM-derived progenitor cells has been examined using immunohistochemistry and Western blot analysis. TH immunostaining revealed that the grafted VM cells developed to mature TH-positive neurons strongly at 3 weeks, peaked at 4 weeks, thereafter, gradually dropped following the degenerative expression of the grafted cells at both 5 and 6 weeks after transplantation. Western blot analysis also showed that the TH proteins were maximally expressed at 4 weeks post-grafting. Our finding suggested that the peak of surviving VM-derived TH positive cells occurred approximately 4 weeks after transplantation.

Stem cell transplantation for Huntington's disease

By way of commentary on a recent report that transplanted adult neural progenitor cells can alleviate functional deficits in a rat lesion model of Huntington's disease [Vazey, E.M., Chen, K., Hughes, S.M., Connor, B., 2006. Transplanted adult neural progenitor cells survive, differentiate and reduce motor function impairment in a rodent model of Huntington's disease. Exp. Neurol. 199, 384-396], we review the current status of the field exploring the use of stem cells, progenitor cells and immortalised cell lines to repair the lesioned striatum in animal models of the human disease. A remarkably rich range of alternative cell types have been used in various animal models, several of which exhibit cell survival and incorporation in the host brain, leading to subsequent functional recovery. In comparing the alternatives with the 'gold standard' currently offered by primary tissue grafts, key issues turn out to be: cell survival, differentiation prior to and following implantation into striatal-like phenotypes, integration and connectivity with the host brain, the nature of the electrophysiological, motor and cognitive tests used to assess functional repair, and the mechanisms by which the grafts exert their function. Although none of the alternatives yet has the capacity to match primary fetal tissues for functional repair, that standard is itself limited, and the long term goal must be not just to match but to surpass present capabilities in order to achieve fully functional reconstruction reliably, flexibly, and on demand.

Re-examining the ontogeny of substantia nigra dopamine neurons

Recently, the need to detail the precise ontogeny of nigrostriatal dopamine neurons has grown significantly. It is now thought that the gestational day on which the majority of these neurons are born is important not only for maximizing the yield of primary cells for transplantation but also for extracting suitable dopamine neural precursors (as stem cells) for expansion in vitro. Historically, peak ontogeny of substantia nigra pars compacta (SNc) dopamine neurons in the rat has been considered to occur around embryonic day (E)14. However, such a concept is at odds with recent studies that reveal not only that substantial numbers of tyrosine hydroxylase-immunopositive cells reside in the ventral mesencephalic region of rats at E14 but that many of these cells have matured extensive axonal projections to the ventral forebrain. Here, then, the ontogeny of SNc neurons in rats commonly used as a source of donor tissue for experimental cell transplantation in animal models of Parkinson's disease has been re-examined. Using a combination of bromodeoxyuridine (BrdU) administration at E11, E12, E13 or E14 with immunocytochemical stainings for both BrdU and tyrosine hydroxylase after 4 weeks of postnatal development, this characterization reveals that the vast majority (perhaps 80%) of SNc dopamine neurons are probably born on E12 in Sprague-Dawley rats. Such findings are important in refining the use of embryonic tissues for primary cell transplantation and may provide more precise timing for identifying the cellular and molecular events that drive neural stem cells toward a dopaminergic phenotype during development.

Dopaminergic properties and function after grafting of attached neural precursor cultures

Generation of dopaminergic (DA) neurons from multipotent embryonic progenitors represents a promising therapeutical strategy for Parkinson's disease (PD). Aim of the present study was the establishment of enhanced cell culture conditions, which optimize the use of midbrain progenitor cells in animal models of PD. In addition, the progenitor cells were characterized during expansion and differentiation according to morphological and electrophysiological criteria and compared to primary tissue. Here, we report that CNS precursors can be expanded in vitro up to 40-fold and afterwards be efficiently differentiated into DA neurons. After 4-5 days under differentiation conditions, more than 70% of the neurons were TH+, equivalent to 30% of the total cell population. Calcium imaging revealed the presence of calcium-permeable AMPA receptors in the differentiated precursors which are capable to contribute to many developmental processes. The overall survival rate, degree of reinnervation and the behavioral performance after transplantation of 4 days in-vitro-differentiated cells were similar to results after direct grafting of E14 ventral mesencephalic cells, whereas after shorter or longer differentiation periods, respectively, less effects were achieved. Compared to the amount of in-vitro-generated DA neurons, the survival rate was only 0.8%, indicating that these cells are very vulnerable. Our results suggest that expanded and differentiated DA precursors from attached cultures can survive microtransplantation and integrate within the striatum in terms of behavioral recovery. However, there is only a short time window during in vitro differentiation, in which enough cells are already differentiated towards a DA phenotype and simultaneously not too mature for implantation. However, additional factors and/or genetical manipulation of these expanded progenitors will be required to increase their in vivo survival in order to improve both the ethical and the technical outlook for the use of fetal tissue in clinical transplantation.

Neurogenin2 identifies a transplantable dopamine neuron precursor in the developing ventral mesencephalon

In neural transplantation studies, there is an interest in identifying and isolating mesencephalic dopamine (mesDA) neuron precursors that have the capacity to differentiate into fully mature mesDA neurons after transplantation. We report here that in the developing ventral mesencephalon (VM) the proneural gene Neurogenin2 (Ngn2) is expressed exclusively in the part of the ventricular zone that gives rise to the migrating mesDA neuroblasts, but not in the differentiated mesDA neurons. From other studies, we know that Ngn2 is involved in the generation of mesDA neurons and that the development of mesDA neurons is severely compromised in Ngn2-null mutant mice. We show here that cells isolated by FACS from the developing VM of Ngn2-GFP knock-in mice are capable of generating mesDA neurons, both in vitro and after transplantation to the striatum of neonatal rats. All mesDA neuron precursors, but not the serotonergic or GABAergic neuron precursors, are contained in the Ngn2-GFP-expressing population. Moreover, all glial cells were generated from cells contained in the GFP-negative cell fraction. The results show that surviving mesDA neurons in VM grafts are derived from early postmitotic, probably Nurr1-expressing precursors before they have acquired their fully differentiated neuronal phenotype. The Ngn2-GFP reporter construct used here thus provides a tool for the identification of mesDA neuron precursors in the VM and selective isolation of transplantable mesDA neuron precursors for transplantation.

Exogenous erythropoietin provides neuroprotection of grafted dopamine neurons in a rodent model of Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disease marked by severe loss of dopamine (DA) neurons in the nigrostriatal system, which results in depletion of striatal DA. Transplantation of embryonic ventral mesencephalic (VM) DA neurons into the striatum is a currently explored experimental treatment aimed at replacing lost DA in the nigrostriatal system, but is plagued with poor survival (5-20%) of implanted neurons. Here, we tested the ability of erythropoietin (Epo) to provide neuroprotection for embryonic day 14 (E14) VM DA neurons. Epo was tested in vitro for the ability to augment tyrosine hydroxylase-immunoreactive (TH-ir) neuron survival under normal cell culture conditions. In vitro, Epo did not increase the number of TH-ir neurons when administered at the time of plating the E14 VM cells in culture. We also tested the efficacy of Epo to enhance E14 VM transplants in vivo. Rats unilaterally lesioned with 6-hydroxydopamine received transplants that were incubated in Epo. Treatment with Epo produced significant increases in TH-ir neuron number, soma size, and staining intensity. Animals receiving Epo-treated grafts exhibited significantly accelerated functional improvements and significantly greater overall improvements from rotational asymmetry compared to control grafted rats. These data indicate that the survival of embryonic mesencephalic TH-ir neurons is increased when Epo is administered with grafted cells in a rodent model of PD. As direct neurotrophic effects of Epo were not observed in vitro, the mechanism of Epo neuroprotection remains to be elucidated.

Delivery of sonic hedgehog or glial derived neurotrophic factor to dopamine-rich grafts in a rat model of Parkinson's disease using adenoviral vectors Increased yield of dopamine cells is dependent on embryonic donor age

The poor survival of dopamine grafts in Parkinson's disease is one of the main obstacles to the widespread application of this therapy. One hypothesis is that implanted neurons, once removed from the embryonic environment, lack the differentiation factors needed to develop the dopaminergic phenotype. In an effort to improve the numbers of dopamine neurons surviving in the grafts, we have investigated the potential of adenoviral vectors to deliver the differentiation factor sonic hedgehog or the glial cell line-derived neurotrophic factor GDNF to dopamine-rich grafts in a rat model of Parkinson's disease. Adenoviral vectors containing sonic hedgehog, GDNF, or the marker gene LacZ were injected into the dopamine depleted striatum of hemiparkinsonian rats. Two weeks later, ventral mesencephalic cell suspensions were prepared from embryos of donor ages E12, E13, E14 or E15 and implanted into the vector-transduced striatum. Pre-treatment with the sonic hedgehog vector produced a three-fold increase in the numbers of tyrosine hydroxylase-positive (presumed dopaminergic) cells in grafts derived from E12 donors, but had no effect on E13-E15 grafts. By contrast, pre-treatment with the GDNF vector increased yields of dopamine cells in grafts derived from E14 and E15 donors but had no effect on grafts from younger donors. The results indicate that provision of both trophic and differentiation factors can enhance the yields of dopamine neurons in ventral mesencephalic grafts, but that the two factors differ in the age and stage of embryonic development at which they have maximal effects.

Selective rather than inductive mechanisms favour specific replacement of Purkinje cells by embryonic cerebellar cells transplanted to the cerebellum of adult Purkinje cell degeneration (pcd) mutant mice

Cell replacement after neuronal degeneration in the adult CNS depends on the availability of specific cues to direct specification, differentiation and integration of newly born neurons into mature circuits. Following recent reports indicating that neurogenic signals may be reactivated in the adult injured CNS, here we asked whether such signals are expressed in the cerebellum after Purkinje cell degeneration. Thus, we compared the fate of embryonic cerebellar cells transplanted to the cerebella of adult wild-type and Purkinje cell degeneration (pcd) mutant mice. Donor cells were dissected from beta-actin-enhanced green fluorescent protein (EGFP) transgenic mice and transplanted as a single cell suspension. In both hosts, grafted cells generated all major cerebellar phenotypes, with a precise localization in the recipient cortex or white matter. Nevertheless, the phenotypic distributions showed striking quantitative differences. Most notably, in the pcd cerebellum there was a higher amount of Purkinje cells, while other phenotypes were less frequent. Analysis of cell proliferation by 5-bromo-2'-deoxyuridine (BrDU) incorporation revealed that in both hosts mitotic activity was strongly reduced shortly after transplantation, and virtually all donor Purkinje cells were actually generated before grafting. Together, these results indicate that some compensatory mechanisms operate in the pcd environment. However, the very low mitotic rate of transplanted cells suggests that the adult cerebellum, either wild-type or mutant, does not provide instructive neurogenic cues to direct the specification of uncommitted progenitors. Rather, specific replacement in mutant hosts is achieved through selective mechanisms that favour the survival and integration of donor Purkinje cells at the expense of other phenotypes.

Combined toxicity of prenatal bacterial endotoxin exposure and postnatal 6-hydroxydopamine in the adult rat midbrain

We previously reported that injection of the Gram (-) bacteriotoxin, lipopolysaccharide (LPS), into gravid females at embryonic day 10.5 led to the birth of animals with fewer than normal dopamine (DA) neurons when assessed at postnatal days (P) 10 and 21. To determine if these changes continued into adulthood, we have now assessed animals at P120. As part of the previous studies, we also observed that the pro-inflammatory cytokine tumor necrosis factor alpha (TNFalpha) was elevated in the striatum, suggesting that these animals would be more susceptible to subsequent DA neurotoxin exposure. In order to test this hypothesis, we injected (at P99) 6-hydroxydopamine (6OHDA) or saline into animals exposed to LPS or saline prenatally. The results showed that animals exposed to prenatal LPS or postnatal 6OHDA alone had 33% and 46%, respectively, fewer DA neurons than controls, while the two toxins combined produced a less than additive 62% loss. Alterations in striatal DA were similar to, and significantly correlated with (r(2)=0.833) the DA cell losses. Prenatal LPS produced a 31% increase in striatal TNFalpha, and combined exposure with 6OHDA led to an 82% increase. We conclude that prenatal exposure to LPS produces a long-lived THir cell loss that is accompanied by an inflammatory state that leads to further DA neuron loss following subsequent neurotoxin exposure. The results suggest that individuals exposed to LPS prenatally, as might occur had their mother had bacterial vaginosis, would be at increased risk for Parkinson's disease.

The cellular repair of the brain in Parkinson's disease—past, present and future

Damage to the central nervous system was once considered irreparable. However, there is now growing optimism that neural transplant therapies may one day enable complete circuit reconstruction and thus functional benefit for patients with neurodegenerative conditions such as Parkinson's disease (PD), and perhaps even those with more widespread damage such as stroke patients. Indeed, since the late 1980s hundreds of patients with Parkinson's disease have received allografts of dopamine-rich embryonic human neural tissue. The grafted tissue has been shown to survive and ameliorate many of the symptoms of the disease, both in the clinical setting and in animal models of the disease. However, practical problems associated with tissue procurement and storage, and ethical concerns over using aborted human fetal tissue have fuelled a search for alternative sources of suitable material for grafting. In particular, stem cells and xenogeneic embryonic dopamine-rich neural tissue are being explored, both of which bring their own practical and ethical dilemmas. Here we review the progress made in neural transplantation, both in the laboratory and in the clinic with particular attention to the development of stem cell and xenogeneic tissue based therapy.

Striatal neurons in striatal grafts are derived from both post-mitotic cells and dividing progenitors

Transplants of embryonic striatal tissue are characteristically heterogeneous, containing patches (P-zones) of striatal medium spiny projection neurons. It is not yet known how this morphology develops, and whether the striatal neurons in the grafts are derived from post-mitotic neuroblasts in the embryonic brain or from striatal progenitors that continue to divide after transplantation. To address this question we labelled dividing cells in the transplants with bromodeoxyuridine (BrdU), either prior to or after transplantation into the adult lesioned rat striatum. Cells for transplantation were either pre-labelled in utero by intraperitoneal (i.p.) injections of BrdU, or post-labelled after transplantation by i.p. injections to the hosts. Either two or six months after transplantation the brains were processed using double immunohistochemical techniques to detect BrdU and calbindin-positive neurons in the transplants. In the transplants pre-labelled with BrdU, approximately 30% of calbindin-positive cells were heavily labelled with BrdU, suggesting these had undergone a final division prior to transplantation. In transplants where cells had been labelled post-transplantation, approximately 17% of calbindin cells were heavily BrdU labelled. These results suggest that whereas a proportion of striatal medium spiny neurons in the striatal grafts were post-mitotic at the time of transplantation, other striatal progenitor cells can continue to divide after transplantation, and then complete an appropriate neuronal maturation programme in the adult host brain environment.

Improvement of embryonic dopaminergic neurone survival in culture and after grafting into the striatum of hemiparkinsonian rats by CEP-1347

Transplantation of embryonic nigral tissue ameliorates functional deficiencies in Parkinson's disease (PD). A main constraint of neural grafting is the poor survival of dopaminergic neurones grafted into patients. Studies in rats indicated that many grafted neurones die by apoptosis. CEP-1347 is a mixed-lineage-kinase (MLK) inhibitor with neuroprotective action in several in vitro and in vivo models of neuronal apoptosis. We studied the effect of CEP-1347 on the survival of embryonic rat dopaminergic neurones in culture, and after transplantation in hemiparkinsonian rats. CEP-1347 and the alternative MLK inhibitor CEP-11004 significantly increased the survival of dopaminergic neurones in primary cultures from rat ventral mesencephalon and in Mn2+-exposed PC12 cells, a surrogate model of dopaminergic lethal stress. Moreover, combined treatment of the grafting cell suspension and the host animal with CEP-1347 significantly improved the long-term survival of rat dopaminergic neurones transplanted into the striatum of hemiparkinsonian rats. Also, the protective effect of CEP-1347 resulted in an increase in total graft size and in enhanced fibre outgrowth. Thus, treatment with CEP-1347 improved dopaminergic cell survival under severe stress and might be useful to improve the positive outcome of transplantation therapy in PD and reduce the amount of human tissue required.

Neuronal differentiation following transplantation of expanded mouse neurosphere cultures derived from different embryonic forebrain regions

In vitro, expanded neurospheres exhibit multipotent properties and can differentiate into neurons, astrocytes and oligodendrocytes. In vivo, cells from neurospheres derived from mouse fetal forebrain have previously been reported to predominantly differentiate into glial cells, and not into neurons. Here we isolated stem/progenitor cells from E13.5 lateral ganglionic eminence (LGE), medial ganglionic eminence (MGE) and cortical primordium, of a green fluorescent protein (GFP)-actin transgenic mouse. Free-floating neurospheres were expanded in the presence of epidermal growth factor (EGF) and basic fibroblast growth factor (bFGF) and implanted after five to six passages into the striatum, hippocampus and cortex of neonatal rats. Cell suspensions of primary LGE tissue were prepared and grafted in parallel. Grafted cells derived from the primary tissue displayed widespread incorporation into all regions, as visualized with the mouse-specific antibody M2, or mouse satellite DNA in situ hybridization, and differentiated into both neurons, astrocytes and oligodendrocytes. Grafts of neurosphere cells derived from the LGE, MGE and cortical primordium differentiated primarily into astrocytes, but contained low but significant numbers of GFP-immunoreactive neurons. Neurons derived from LGE neurospheres were of three types: cells with the morphology of medium-sized densely spiny projection neurons in the striatum; cells with interneuron-like morphologies in striatum, cortex and hippocampus; and cells integrating into SVZ and migrating along the RMS to the olfactory bulb. MGE- or cortical primordium-derived neurospheres differentiated into interneuron-like cells in both striatum and hippocampus. The results demonstrate the ability of in vitro expanded neural stem/progenitor cells to generate both neurons and glia after transplantation into neonatal recipients, and differentiate in a region-specific manner into mature neurons with morphological features characteristic for each target site.

The release of excitatory amino acids, dopamine, and potassium following transplantation of embryonic mesencephalic dopaminergic grafts to the rat striatum, and their effects on dopaminergic neuronal survival in vitro

A major limitation to the effectiveness of grafts of fetal ventral mesencephalic tissue for parkinsonism is that about 90-95% of grafted dopaminergic neurones die. In rats, many of the cells are dead within 1 day and most cell death is complete within 1 week. Our previous results suggest that a major cause of this cell death is the release of toxins from the injured CNS tissue surrounding the graft, and that many of these toxins have dissipated within 1 h of inserting the grafting cannula. In the present experiments we measured the change over time in the concentration of several potential toxins around an acutely implanted grafting cannula. We also measured the additional effect of injecting suspensions of embryonic mesencephalon, latex microspheres, or vehicle on these concentrations. Measurements of glutamate, aspartate, and dopamine by microdialysis showed elevated levels during the first 20-60 min, which then declined to baseline. In the first 20 min glutamate levels were 10.7 times, aspartate levels 5 times, and dopamine levels 24.3 times baseline. Potassium levels increased to a peak of 33 +/- 10.6 mM 4-5 min after cannula insertion, returning to baseline of <5 mM by 30 min. Injection of cell suspension, latex microspheres, or vehicle had no significant effect on these levels. We then assayed the effect of high concentrations of glutamate, aspartate, dopamine, and potassium on dopaminergic neuronal survival in E14 ventral mesencephalic cultures. In monolayer cultures only dopamine at 200 microM showed toxicity. In three-dimensional cultures only the combination of raised potassium, dopamine, glutamate, and aspartate together decreased dopaminergic neuronal survival. We conclude that toxins other than the ones measured are the main cause of dopaminergic cell death after transplantation, or the effects of the toxins measured are enhanced by anoxia and metabolic challenges affecting newly inserted grafts.

Pleiotrophin exhibits a trophic effect on survival of dopaminergic neurons in vitro

To understand what kind of trophic factors are up-regulated in dopamine (DA)-depleted striatum, we first analysed the up-regulation of mRNAs using a DNA microarray in DA-depleted striatum where DAergic inputs were denervated by 6-OHDA. We then investigated whether or not such trophic factors had an effect on cultured dopaminergic neurons. The microarray analysis revealed that pleiotrophin (PTN), glial-derived neurotopic factor (GDNF) and others were up-regulated in DA-depleted striatum. As PTN has been reported to have a wide range of trophic effects on neurons, we focused on the functional role of PTN in the present study. The increase in PTN mRNA was confirmed by Northern blotting at 1-3 weeks after the lesion, reaching a peak at 1 week. In embryonic day 15 mesencephalic neuron culture, PTN increased the number of tyrosine hydroxylase (TH) -positive neurons in a dose-dependent manner (125.2 +/- 2.0% of the control at 50 ng/mL), while a family protein, midkine (10 ng/mL) did not show any trophic effect (99.3 +/- 0.7%). In addition, the PTN effect on DAergic neurons was additive to the GDNF effect. As PTN did not increase the number of microtubule-associated protein-2 (MAP 2)-positive neurons or promote the proliferation of dopaminergic progenitors in a bromodeoxyuridine (BrdU) labelling study, the effect appeared to enhance the specific survival of dopaminergic neurons. Expression of PTN receptors (syndecan-3, PTP-zeta) was detected on the cultured mesencephalic neurons, and also up-regulated in DA-depleted striatum. The data indicate that PTN is up-regulated in DA-depleted striatum and exhibits a trophic effect specifically on the survival of cultured dopaminergic neurons.

The potential for circuit reconstruction by expanded neural precursor cells explored through porcine xenografts in a rat model of Parkinson's disease

Neural precursors with the properties of neural stem cells can be isolated from the developing brain, can be expanded in culture, and have been suggested as a potential source of cells for neuronal replacement therapies in degenerative disorders such as Parkinson's disease (PD). Under such conditions an improved spectrum of functional benefit may be obtained through homotypic reconstruction of degenerated neural circuitry, and to this end we have investigated the potential of expanded neural precursor cells (ENPs) to form long axonal projections following transplantation in the 6-hydroxydopamine-lesioned rat model of PD. ENPs have been isolated from the embryonic pig, since implantation in a xenograft environment is thought to favor axonal growth. These porcine ENPs possessed similar properties in vitro to those described in other species: they proliferated in response to epidermal and fibroblast growth factor-2, expressed the neuroepithelial marker nestin, and differentiated into neurons, astrocytes, and occasional oligodendrocytes on mitogen withdrawal. The use of pig-specific markers following xenotransplantion into cyclosporin A-immunosuppressed rats revealed that many cells differentiated into neurons and displayed extensive axogenesis, such that when placed in the region of the substantia nigra fibers projected throughout the striatal neuropil. These neurons were not restricted in the targets to which they could project since following intrastriatal grafting fibers were seen in the normal striatal targets of the pallidum and substantia nigra. Staining for a pig-specific synaptic marker suggested that synapses were formed in these distant sites. A small number of these cells differentiated spontaneously to express a catecholaminergic phenotype, but were insufficient to mediate behavioral recovery. Our results suggest that when the efficiency of neurochemical phenotype induction is increased, ENP-derived neurons have the potential to be a uniquely flexible source of cells for therapeutic cell replacement where anatomical reconstruction is advantageous.

Porcine neural xenografts in rats and mice: donor tissue development and characteristics of rejection

Embryonic ventral mesencephalic tissue from the pig is a potential alternative donor tissue for neural transplantation to Parkinson's disease patients. For stable graft survival, the host immune response has to be prevented. This study was performed in order to analyze the mechanisms and dynamics of neural xenograft rejection, as well as neurobiological properties of the donor tissue. Adult normal mice and rats, and cyclosporin A-treated rats, received intrastriatal transplants of dissociated embryonic ventral mesencephalic pig tissue that was 27 or 29 embryonic days of age (E27 and E29). The animals were perfused at 2, 4, 6, and 12 weeks after grafting and the brains were processed for immunohistochemistry of dopaminergic (tyrosine hydroxylase positive) neurons, CD4(+) and CD8(+) lymphocytes, natural killer cells, macrophages, microglia, and astrocytes. Thirty-five rats received daily injections of BrdU for 5 consecutive days at different time points after transplantation and were perfused at 6 weeks. These animals were analyzed for proliferation of cells in the donor tissue, both in healthy and in rejecting grafts. No tyrosine hydroxylase-positive cells proliferated after grafting. Our results demonstrated that E27 was superior to E29 donor tissue for neurobiological reasons. Cyclosporin A immunosuppression was protective only during the first weeks and failed to protect the grafts in a long-term perspective. Grafts in mice were invariably rejected between 2 and 4 weeks after transplantation, while occasional grafts in untreated rats survived up to 12 weeks without signs of an ongoing rejection process. CD8(+) lymphocytes and microglia cells are most likely important effector cells in the late, cyclosporin A-resistant rejection process.

In vitro generation and transplantation of precursor-derived human dopamine neurons

The use of in vitro expanded human CNS precursors has the potential to overcome some of the ethical, logistic and technical problems of fetal tissue transplantation in Parkinson disease. Cultured rat mesencephalic precursors proliferate in response to bFGF and upon mitogen withdrawal, differentiate into functional dopamine neurons that alleviate motor symptoms in Parkinsonian rats (Studer et al. [1998] Nat. Neurosci. 1:290-295). The successful clinical application of CNS precursor technology in Parkinson disease will depend on the efficient in vitro generation of human dopaminergic neurons. We demonstrate that human dopamine neurons can be generated from both midbrain and cortical precursors. Transplantation of midbrain precursor-derived dopamine neurons into Parkinsonian rats resulted in grafts rich in tyrosine hydroxylase positive neurons 6 weeks after transplantation. No surviving tyrosine hydroxylase positive neurons could be detected when dopamine neurons derived from cortical precursors were grafted. Our data demonstrate in vitro derivation of human dopamine neurons from expanded CNS precursors and encourage further studies that systematically address in vivo function and clinical potential.

Generation of dopaminergic neurons in the adult brain from mesencephalic precursor cells labeled with a nestin-GFP transgene

Mesencephalic precursor cells may one day provide dopaminergic neurons for the treatment of Parkinson's disease. However, the generation of dopaminergic neurons from mesencephalic precursors has been difficult to follow, partly because an appropriate means for recognizing mesencephalic ventricular zone precursors has not been available. To visualize and isolate mesencephalic precursor cells from a mixed population, we used transgenic mice and rats carrying green fluorescent protein (GFP) cDNA under the control of the nestin enhancer. nestin-driven GFP was detected in the mesencephalic ventricular zone, and it colocalized with specific markers for neural precursor cells. In addition, data from flow-cytometry indicated that Prominin/CD133, a cell-surface marker for ventricular zone cells, was expressed specifically in these GFP-positive (GFP(+)) cells. After sorting by fluorescence-activated cell sorting, the GFP(+) cells proliferated in vitro and expressed precursor cell markers but not neuronal markers. Using clonogenic sphere formation assays, we showed that this sorted population was enriched in multipotent precursor cells that could differentiate into both neurons and glia. Importantly, many neurons generated from nestin-GFP-sorted mesencephalic precursors developed a dopaminergic phenotype in vitro. Finally, nestin-GFP(+) cells were transplanted into the striatum of a rat model of Parkinson's disease. Bromodeoxyuridine-tyrosine hydroxylase double-labeling revealed that the transplanted cells generated new dopaminergic neurons within the host striatum. The implanted cells were able to restore dopaminergic function in the host striatum, as assessed by a behavioral measure: recovery from amphetamine-induced rotation. Together, these findings indicate that precursor cells harvested from the embryonic ventral mesencephalon can generate dopaminergic neurons able to restore function to the chemically denervated adult striatum.

Influence of environment on the efficacy of intrastriatal dopaminergic grafts

Functional recovery is influenced by experience. The aim of the present work was to examine the effects of "enriched" environment (EE) versus an "impoverished" environment on the anatomical and functional integration of intrastriatal dopaminergic grafts. These influences were studied using a paradigm where grafting was performed before the dopamine-depleting lesion. Dopaminergic grafts were implanted into the left neostriatum of adult male rats. In the enriched group, grafted rats were housed collectively and were trained on different behavioral tests following grafting. In contrast, impoverished grafted rats were housed individually and not further manipulated. Ten weeks after grafting, the mesotelencephalic dopaminergic pathway was destroyed unilaterally to the grafted side and different behaviors were followed for 7 months. Grafting prior to lesioning had no prophylactic effects on the performance as the graft did not prevent the onset of the lesion-induced impairments. However, under EE conditions, a graft effect was manifested in the reduction of drug-induced rotation and on the indices of bias as tested by a spatial alternation test. No positive graft effects were observed in the skilled paw reaching test. Grafted rats raised under impoverished conditions performed in a fashion indistinguishable from the control lesioned animals on most measures of behavior. A beneficial effect of EE conditions was observed on survival of TH-positive neurons within the grafts. The results suggest that survival of grafted neurons, and the reduction of the magnitude of particular behavioral impairments, can be optimized by increasing the complexity of the subject's environment.

Caspase inhibition increases embryonic striatal graft survival

In transplants of embryonic striatal cells placed into the excitotoxically lesioned rat striatum (a model of Huntington's disease), as many as 60 to 90% of the grafted cells are believed to die. Caspase activation is part of a cascade of events that can lead to apoptosis. We investigated the effect of the caspase inhibitor acetyl-tyrosinyl-valyl-alanyl-aspartyl-chloromethylketone (Ac-YVAD-cmk) on grafted embryonic striatal cells in the excitotoxically lesioned or intact rat striatum. Female Sprague-Dawley rats were subjected to unilateral intrastriatal injection of quinolinic acid. After 10 days, rats received bilateral intrastriatal grafts from embryonic day 14 rat lateral ganglionic eminence. Rats were divided into the following groups: Ac-YVAD-cmk, pretreatment of the graft tissue with the caspase inhibitor (500 microM); and control, untreated control grafts. Rats were perfused 10 days or 5 weeks postgrafting. Brain sections were processed immunohistochemically using an antibody against the striatal neuron marker dopamine- and adenosine 3',5'-monophosphate-regulated phosphoprotein with a molecular weight of 32 kDa (DARPP-32). Adjacent sections were stained for acetylcholinesterase/cresyl violet cytochemistry and Fluoro-Jade cytochemistry, a marker for degenerating neurons. Total graft volume, P-zone volume, total number of neuron-like cells, and number of DARPP-32-positive cells were increased, compared to control, in the group receiving Ac-YVAD-cmk-treated graft tissue. Moreover, transplants injected into the intact striatum were found to be significantly smaller compared to transplants placed into the excitotoxically lesioned striatum. The Fluoro-Jade staining revealed ongoing cell death in transplants 10 days after intrastriatal implantation and that cell death was significantly reduced 5 weeks after grafting.