Development of intrastriatal striatal grafts and their afferent innervation from the host

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.

A PITX3-EGFP Reporter Line Reveals Connectivity of Dopamine and Non-dopamine Neuronal Subtypes in Grafts Generated from Human Embryonic Stem Cells

Development of safe and effective stem cell-based therapies for brain repair requires an in-depth understanding of the in vivo properties of neural grafts generated from human stem cells. Replacing dopamine neurons in Parkinson's disease remains one of the most anticipated applications. Here, we have used a human PITX3-EGFP embryonic stem cell line to characterize the connectivity of stem cell-derived midbrain dopamine neurons in the dopamine-depleted host brain with an unprecedented level of specificity. The results show that the major A9 and A10 subclasses of implanted dopamine neurons innervate multiple, developmentally appropriate host targets but also that the majority of graft-derived connectivity is non-dopaminergic. These findings highlight the promise of stem cell-based procedures for anatomically correct reconstruction of specific neuronal pathways but also emphasize the scope for further refinement in order to limit the inclusion of uncharacterized and potentially unwanted cell types.

Fetal Neocortical Transplants Grafted into Neocortical Lesion Cavities Made in Newborn Rats: An Analysis of Transplant Integration with the Host Brain

Fetal neocortical transplants placed into frontal cortex aspiration lesion cavities in newborn rats have been shown to survive and exchange connections with the host brain. To further study the afferent innervation of such transplants, enzyme- and immunohistochemical techniques were employed to examine the distribution of cholinergic, cat-echolaminergic and serotonergic fibers within the transplants, and radiochemical enzyme assays and high performance liquid chromatography were used to determine the content of neurotransmitter markers for these same fiber systems. To examine functional integration of the transplanted neurons in terms of activation of molecular signaling systems, the graft recipient animals were exposed to a novel open field environment. This behavioral testing paradigm is known to induce c-fos mRNA and Fos protein within several areas of the normal brain, including the sensorimotor cortex. Subsequent detection of the induction of this particular immediate early gene (transcription as well as translation) in the grafts would accordingly indicate genomic activation and therefore functional integration at the level of molecular signaling systems. Our results showed that these global fiber systems are distributed evenly throughout the extent of three mo old neocortical grafts and that the content of transmitter-related markers for these systems do not differ significantly from control cortex. Open field exposure of the grafted animals resulted in c-fos mRNA and Fos protein expression of cells distributed throughout the transplants. We conclude that the “global” fiber system innervation of neocortical transplants placed into newborn rats is similar to the innervation of normal cortex and that grafted neurons respond to host brain activation at the level of molecular signaling systems.

Mechanisms and use of neural transplants for brain repair

Under appropriate conditions, neural tissues transplanted into the adult mammalian brain can survive, integrate, and function so as to influence the behavior of the host, opening the prospect of repairing neuronal damage, and alleviating symptoms associated with neuronal injury or neurodegenerative disease. Alternative mechanisms of action have been postulated: nonspecific effects of surgery; neurotrophic and neuroprotective influences on disease progression and host plasticity; diffuse or locally regulated pharmacological delivery of deficient neurochemicals, neurotransmitters, or neurohormones; restitution of the neuronal and glial environment necessary for proper host neuronal support and processing; promoting local and long-distance host and graft axon growth; formation of reciprocal connections and reconstruction of local circuits within the host brain; and up to full integration and reconstruction of fully functional host neuronal networks. Analysis of neural transplants in a broad range of anatomical systems and disease models, on simple and complex classes of behavioral function and information processing, have indicated that all of these alternative mechanisms are likely to contribute in different circumstances. Thus, there is not a single or typical mode of graft function; rather grafts can and do function in multiple ways, specific to each particular context. Consequently, to develop an effective cell-based therapy, multiple dimensions must be considered: the target disease pathogenesis; the neurodegenerative basis of each type of physiological dysfunction or behavioral symptom; the nature of the repair required to alleviate or remediate the functional impairments of particular clinical relevance; and identification of a suitable cell source or delivery system, along with the site and method of implantation, that can achieve the sought for repair and recovery.

ESC-Derived BDNF-Overexpressing Neural Progenitors Differentially Promote Recovery in Huntington's Disease Models by Enhanced Striatal Differentiation

Huntington's disease (HD) is characterized by fatal motoric failures induced by loss of striatal medium spiny neurons. Neuronal cell death has been linked to impaired expression and axonal transport of the neurotrophin BDNF (brain-derived neurotrophic factor). By transplanting embryonic stem cell-derived neural progenitors overexpressing BDNF, we combined cell replacement and BDNF supply as a potential HD therapy approach. Transplantation of purified neural progenitors was analyzed in a quinolinic acid (QA) chemical and two genetic HD mouse models (R6/2 and N171-82Q) on the basis of distinct behavioral parameters, including CatWalk gait analysis. Explicit rescue of motor function by BDNF neural progenitors was found in QA-lesioned mice, whereas genetic mouse models displayed only minor improvements. Tumor formation was absent, and regeneration was attributed to enhanced neuronal and striatal differentiation. In addition, adult neurogenesis was preserved in a BDNF-dependent manner. Our findings provide significant insight for establishing therapeutic strategies for HD to ameliorate neurodegenerative symptoms.

Direct Comparison of Rat- and Human-Derived Ganglionic Eminence Tissue Grafts on Motor Function

Huntington's disease (HD) is a debilitating, genetically inherited neurodegenerative disorder that results in early loss of medium spiny neurons from the striatum and subsequent degeneration of cortical and other subcortical brain regions. Behavioral changes manifest as a range of motor, cognitive, and neuropsychiatric impairments. It has been established that replacement of the degenerated medium spiny neurons with rat-derived fetal whole ganglionic eminence (rWGE) tissue can alleviate motor and cognitive deficits in preclinical rodent models of HD. However, clinical application of this cell replacement therapy requires the use of human-derived (hWGE), not rWGE, tissue. Despite this, little is currently known about the functional efficacy of hWGE. The aim of this study was to directly compare the ability of the gold standard rWGE grafts, against the clinically relevant hWGE grafts, on a range of behavioral tests of motor function. Lister hooded rats either remained as unoperated controls or received unilateral excitotoxic lesions of the lateral neostriatum. Subsets of lesioned rats then received transplants of either rWGE or hWGE primary fetal tissue into the lateral striatum. All rats were tested postlesion and postgraft on the following tests of motor function: staircase test, apomorphine-induced rotation, cylinder test, adjusting steps test, and vibrissae-evoked touch test. At 21 weeks postgraft, brain tissue was taken for histological analysis. The results revealed comparable improvements in apomorphine-induced rotational bias and the vibrissae test, despite larger graft volumes in the hWGE cohort. hWGE grafts, but not rWGE grafts, stabilized behavioral performance on the adjusting steps test. These results have implications for clinical application of cell replacement therapies, as well as providing a foundation for the development of stem cell-derived cell therapy products.

Differentiation of pluripotent stem cells into striatal projection neurons: a pure MSN fate may not be sufficient

Huntington's disease (HD) is an autosomal dominant inherited disorder leading to the loss inter alia of DARPP-32 positive medium spiny projection neurons ("MSNs") in the striatum. There is no known cure for HD but the relative specificity of cell loss early in the disease has made cell replacement by neural transplantation an attractive therapeutic possibility. Transplantation of human fetal striatal precursor cells has shown "proof-of-principle" in clinical trials; however, the practical and ethical difficulties associated with sourcing fetal tissues have stimulated the need to identify alternative source(s) of donor cells that are more readily available and more suitable for standardization. We now have available the first generation of protocols to generate DARPP-32 positive MSN-like neurons from pluripotent stem cells and these have been successfully grafted into animal models of HD. However, whether these grafts can provide stable functional recovery to the level that can regularly be achieved with primary fetal striatal grafts remains to be demonstrated. Of particular concern, primary fetal striatal grafts are not homogenous; they contain not only the MSN subpopulation of striatal projection neurons but also include all the different cell types that make up the mature striatum, such as the multiple populations of striatal interneurons and striatal glia, and which certainly contribute to normal striatal function. By contrast, present protocols for pluripotent stem cell differentiation are almost entirely targeted at specifying just neurons of an MSN lineage. So far, evidence for the functionality and integration of stem-cell derived grafts is correspondingly limited. Indeed, consideration of the features of full striatal reconstruction that is achieved with primary fetal striatal grafts suggests that optimal success of the next generations of stem cell-derived replacement therapy in HD will require that graft protocols be developed to allow inclusion of multiple striatal cell types, such as interneurons and/or glia. Almost certainly, therefore, more sophisticated differentiation protocols will be necessary, over and above replacement of a specific population of MSNs. A rational solution to this technical challenge requires that we re-address the underlying question-what constitutes a functional striatal graft?

Survival and differentiation of adenovirus-generated induced pluripotent stem cells transplanted into the rat striatum

Induced pluripotent stem cells (iPSCs) offer certain advantages over embryonic stem cells in cell replacement therapy for a variety of neurological disorders. However, reliable procedures, whereby transplanted iPSCs can survive and differentiate into functional neurons, without forming tumors, have yet to be devised. Currently, retroviral or lentiviral reprogramming methods are often used to reprogram somatic cells. Although the use of these viruses has proven to be effective, formation of tumors often results following in vivo transplantation, possibly due to the integration of the reprogramming genes. The goal of the current study was to develop a new approach, using an adenovirus for reprogramming cells, characterize the iPSCs in vitro, and test their safety, survivability, and ability to differentiate into region-appropriate neurons following transplantation into the rat brain. To this end, iPSCs were derived from bone marrow-derived mesenchymal stem cells and tail-tip fibroblasts using a single cassette lentivirus or a combination of adenoviruses. The reprogramming efficiency and levels of pluripotency were compared using immunocytochemistry, flow cytometry, and real-time polymerase chain reaction. Our data indicate that adenovirus-generated iPSCs from tail-tip fibroblasts are as efficient as the method we used for lentiviral reprogramming. All generated iPSCs were also capable of differentiating into neuronal-like cells in vitro. To test the in vivo survivability and the ability to differentiate into region-specific neurons in the absence of tumor formation, 400,000 of the iPSCs derived from tail-tip fibroblasts that were transfected with the adenovirus pair were transplanted into the striatum of adult, immune-competent rats. We observed that these iPSCs produced region-specific neuronal phenotypes, in the absence of tumor formation, at 90 days posttransplantation. These results suggest that adenovirus-generated iPSCs may provide a safe and viable means for neuronal replacement therapies.

Neurons derived from human embryonic stem cells extend long-distance axonal projections through growth along host white matter tracts after intra-cerebral transplantation

Human pluripotent stem cells have the capacity for directed differentiation into a wide variety of neuronal subtypes that may be useful for brain repair. While a substantial body of research has lead to a detailed understanding of the ability of neurons in fetal tissue grafts to structurally and functionally integrate after intra-cerebral transplantation, we are only just beginning to understand the in vivo properties of neurons derived from human pluripotent stem cells. Here we have utilized the human embryonic stem (ES) cell line Envy, which constitutively expresses green fluorescent protein (GFP), in order to study the in vivo properties of neurons derived from human ES cells. Rapid and efficient neural induction, followed by differentiation as neurospheres resulted in a GFP+ neural precursor population with traits of neuroepithelial and dorsal forebrain identity. Ten weeks after transplantation into neonatal rats, GFP+ fiber patterns revealed extensive axonal growth in the host brain, particularly along host white matter tracts, although innervation of adjacent nuclei was limited. The grafts were composed of a mix of neural cell types including differentiated neurons and glia, but also dividing neural progenitors and migrating neuroblasts, indicating an incomplete state of maturation at 10 weeks. This was reflected in patch-clamp recordings showing stereotypical properties appropriate for mature functional neurons, including the ability to generate action potentials, as well profiles consistent for more immature neurons. These findings illustrate the intrinsic capacity for neurons derived from human ES cells to integrate at a structural and functional level following transplantation.

Orthotopic transplantation of immortalized mesencephalic progenitors (CSM14.1 cells) into the substantia nigra of hemiparkinsonian rats induces neuronal differentiation and motoric improvement

Neural progenitor cell grafting is a promising therapeutic option in the treatment of Parkinson's disease. In previous experiments we grafted temperature-sensitive immortalized CSM14.1 cells, derived from the ventral mesencephalon of E14-rats, bilaterally in the caudate putamen of adult hemiparkinsonian rats. In these studies we were not able to demonstrate either a therapeutic improvement or neuronal differentiation of transplanted cells. Here we examined whether CSM14.1 cells grafted bilaterally orthotopically in the substantia nigra of hemiparkinsonian rats have the potential to differentiate into dopaminergic neurons. Adult male rats received 6-hydroxydopamine into the right medial forebrain bundle, and successful lesions were evaluated with apomorphine-induced rotations 12 days after surgery. Two weeks after a successful lesion the animals received bilateral intranigral grafts consisting of either about 50 000 PKH26-labelled undifferentiated CSM14.1 cells (n = 16) or a sham-graft (n = 9). Rotations were evaluated 3, 6, 9 and 12 weeks post-grafting. Animals were finally perfused with 4% paraformaldehyde. Cryoprotected brain slices were prepared for immunohistochemistry using the freeze-thaw technique to preserve PKH26-labelling. Slices were immunostained against neuronal epitopes (NeuN, tyrosine hydroxylase) or glial fibrillary acidic protein. The CSM14.1-cell grafts significantly reduced the apomorphine-induced rotations 12 weeks post-grafting compared to the sham-grafts (P < 0.05). There was an extensive mediolateral migration (400-700 microm) of the PKH26-labelled cells within the host substantia nigra. Colocalization with NeuN or glial fibrillary acidic protein in transplanted cells was confirmed with confocal microscopy. No tyrosine hydroxylase-immunoreactive grafted cells were detectable. The therapeutic effect of the CSM14.1 cells could be explained either by their glial cell-derived neurotrophic factor-expression or their neural differentiation with positive effects on the basal ganglia neuronal networks.

Glial overexpression of heme oxygenase-1: a histochemical marker for early stages of striatal damage

The level of heme oxygenase-1 (HO-1) in the normal striatum is below the limit of immunodetection. However, HO-1 is overexpressed in both neural and non-neural cells in response to a wide range of lesions. We induced different types of lesions affecting the striatal cells or the main striatal afferent systems in rats to investigate if overexpression of HO-1 could be a useful histochemical marker of striatal damage. Thirty-six hours after intrastriatal or intraventricular injection of excitotoxins that affect striatal neurons (ibotenic acid) or of neurotoxins that affect striatal dopaminergic (6-hydroxydopamine) or serotonergic (5,7-dihydroxytriptamine) afferent terminals, or after surgical lesioning of cortico-striatal projections, there was intense induction of striatal HO-1 immunoreactivity (HO-1-ir). Double immunolabeling revealed that the HO-1-ir was located in glial cells. After intrastriatal injection of ibotenic acid, a central zone of neuronal degeneration contained numerous round and pseudopodic HO-1-ir cells, and was surrounded by a ring of HO-1-ir cells, most of which were immunoreactive for astroglial markers. Intraventricular injection of neurotoxins induced astroglial HO-1-ir cells which were more evenly distributed throughout the lesioned or denervated areas. HO-1-ir microglial cells were also observed in areas subjected to mechanical damage. The HO-1-ir was markedly lower or absent 1 week after lesion, and even more so 3 weeks after, although some HO-1-ir cells were still observed after intrastriatal injection of ibotenic acid or surgical corticostriatal deafferentation. The results indicate that determination of glial HO-1-ir is a useful histochemical marker for early stages of striatal damage.

Expanded mesencephalic precursors develop into grafts of densely packed dopaminergic neurons that reinnervate the surrounding striatum and induce functional responses in the striatal neurons

The search for alternative sources of dopaminergic cells, other than primary fetal tissue for transplantation in Parkinson's disease has become a major focus of research. Different methodological approaches have led to generation in vitro of cells expressing DA-cell markers, although these cells are frequently unable to survive for a long time in vivo after transplantation and/or induce functional effects in the host brain. In the present study, we grafted cell aggregates treated with antibodies against fibroblast growth factor 4 into dopaminergic-denervated striata in rats. Furthermore, we grafted cell suspensions from primary mesencephalic fetal tissue. Grafts from expanded precursors were able to survive (at least 3 months postgrafting) and most decreased the lesion-induced ipsiversive rotation. In addition, immunolabeling for tyrosine hydroxylase and/or Fos showed that the grafts reinnervated the surrounding striatal tissue with dopaminergic terminals, and induced the expression of Fos in the striatal neurons of the reinnervated area after administration of amphetamine to the host rat. The number of dopaminergic cells in grafts from expanded precursors inducing rotational recovery was usually lower (1,226+/-314) than that in grafts from primary fetal tissue (1,671+/-122), but they were more densely packed in grafts that were of smaller volume and did not have the characteristic central nondopaminergic area observed in grafts from primary fetal tissue. The results suggest that long-term survival and functional integration into the DA-denervated striatum can be achieved with grafts of expanded mesencephalic precursors.

Cerebellar grafting in the oculomotor system as a model to study target influence on adult neurons

In the last decades, there have been many efforts directed to gain a better understanding on adult neuron-target cell relationships. Embryonic grafts have been used for the study of neural circuit rewiring. Thus, using several donor neuronal tissues, such as cerebellum or striatum, developing grafted cells have been shown to have the capability of substituting neural cell populations and establishing reciprocal connections with the host. In addition, different lesion paradigms have also led to a better understanding of target dependence in neuronal cells. Thus, for example, axotomy induces profound morphofunctional changes in adult neurons, including the loss of synaptic inputs and discharge alterations. These alterations are probably due to trophic factor loss in response to target disconnection. In this review, we summarize the different strategies performed to disconnect neurons from their targets, and the effects of target substitution, performed by tissue grafting, upon neural properties. Using the oculomotor system-and more precisely the abducens internuclear neurons-as a model, we describe herein the effects of disconnecting a population of central neurons from its natural target (i.e., the medial rectus motoneurons at the mesencephalic oculomotor nucleus). We also analyze target-derived influences in the structure and physiology of these neurons by using cerebellar embryonic grafts as a new target for the axotomized abducens internuclear neurons.

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.

Neural transplantation in patients with Huntington's disease

The gene for Huntington's disease was identified in 1993 as being a CAG repeat expansion in exon 1 of a gene now known as huntingtin on chromosome 4. Although many of the downstream effects of this mutant gene were identified in the subsequent years, a more detailed understanding of these events will be necessary in order to design specific interventions to interfere with the disease process and slow disease progression. In parallel, a number of groups have been investigating alternative approaches to treatment of Huntington's disease, including cell and tissue transplantation. As the brunt of cell dysfunction and loss is borne by the striatum, at least in the early to mid-stages of disease, the goal is to identify methods for replacing lost cells with fetal neuroblasts that can develop, integrate into the host circuitry and thereby restore lost function. Clinical studies in which primary fetal neuroblasts were transplanted into the brains of patients with advanced Parkinson's disease have demonstrated benefit when the transplant methodology closely follows the biological principles established in animal experiments. On the basis of demonstrated benefit following striatal cell transplantation in animal models of Huntington's disease, a small number of studies have now commenced in patients with Huntington's disease. To date, these clinical studies have demonstrated the feasibility and safety of transplantation in this condition, but it will require several more years yet before the efficacy of the procedure can be confidently established.

Axonal regrowth of layer II-III visual-projecting cortical neurons in rats fails beyond eye opening

Fetal neurons (embryonic age E16) of occipital origin grafted in the visual cortex of albino rats at increasing postnatal stages (P0, P7, P15, P30, P60, P120) can be activated by photic stimulation. Inputs originate from five major areas of the brain ipsilateral to the graft, namely, the claustrum, the periallocortex/proisocortex, the isocortex, the visual thalamus, and some unspecific subthalamic and hypothalamic nuclei. All inputs decrease in number with the age at which grafting was performed. Isocortical afferents exhibit furthermore a progressive laminar shaping. In neonates, layer II-III and layer V-VI neurons contribute equally to the graft input. In adults, grafts receive prominent input (approximately 70-80%) from layer VI neurons whereas layer II-III neurons account for less than 10%. Proportions of layer IV (approximately 2-4%) and layer V (approximately 15-20%) neurons innervating the graft remain stable, irrespective of the age of the recipient. The adult pattern of connectivity between the host brain and the graft establishes in frontal and temporal areas 1 week earlier than in occipital areas. It is nearly completed in postnatal day 15 (P15) grafted recipients. Supragranular neurons would be thus unable to innervate and to make stable synapses at the graft level beyond P15, i.e., when eyes open. Some infragranular neurons (supposedly remnants of the earliest generated cortical cell population) still have this capacity in adults.

Differences in host serotonin innervation of intrastriatal grafts are not determined by a glial scar or chondroitin sulfate proteoglycans

Serotoninergic (5-HT) neurons of adult recipients provide a much denser innervation of striatal than ventral mesencephalic grafts implanted into the neostriatum of the rat. Moreover, grafts from both brain regions are more innervated by host 5-HT axons after implantation in neonatal than adult hosts. To test the hypothesis that differences in glial scarring or expression of the growth inhibitory molecules, chondroitin sulfate proteoglycans (CSPG), be responsible for these differences in 5-HT innervation of neural grafts, we examined the 5-HT innervation, the astroglial reaction and the expression of CSPG in ventral mesencephalic grafts implanted into newborn (1-5 days old), juvenile (15 days old), or adult rats and in striatal grafts implanted in adult rats, using immunohistochemistry against 5-HT, glial fibrillary acidic protein (GFAP) and CSPG. Immunostaining for GFAP showed a stronger initial gliosis (1-10 days after grafting) in neonatal than adult recipients of mesencephalic grafts, but this gliosis subsided gradually at later time points. Nevertheless, a glial scar formed at the graft-host interface in both neonatal and adult recipients, 5-10 days after transplantation, although it decreased over a longer time course--up to 60 days--in adults. Immunostained astrocytes appeared first in the host brain tissue around the graft and then immunoreactive processes and perikarya gradually invaded the graft. Immunoreactivity for CSPG was similar in neonatal and adult hosts: it was strongly expressed inside the graft early after transplantation, and almost completely down-regulated at 60 days. The reaction of adult hosts to striatal and mesencephalic grafts was similar, although GFAP was more heterogeneously distributed and CSPG immunoreactivity remained in patches inside striatal grafts, even after 60 days. The 5-HT innervation of mesencephalic grafts was much denser after implantation in newborns than in adults. It was also stronger in striatal than in mesencephalic grafts implanted in adults. Thus, the presence of a glial scar or the expression of CSPG cannot totally account for the different degrees of 5-HT innervation in the various types of neural grafts.

Firing properties of axotomized central nervous system neurons recover after graft reinnervation

Axotomy produces changes in the electrical properties of neurons and in their synaptic inputs, leading to alterations in firing pattern. We have considered the possibility that these changes occur as a result of the target deprivation induced by the lesion. Thus, we have provided a novel target to axotomized central neurons by grafting embryonic tissue at the lesion site to study the target dependence of discharge characteristics. The extracellular single-unit electrical activity of abducens internuclear neurons was recorded in the alert behaving cat in control, after axotomy, and after axotomy plus the implantation of cerebellar primordium. As recently characterized (de la Cruz et al. [2000] J. Comp. Neurol. 427:391-404), firing alterations induced by axotomy included an overall decrease in firing rate and a loss of eye-related signals, i.e., eye position and velocity neuronal sensitivities, that do not resume to normality with time. The grafting of a novel target to the injured abducens internuclear neurons restored the normal firing and sensitivities as recorded in the majority of units. To study the reinnervation of the implant, we performed anterograde labeling with biocytin combined with electron microscopy visualization. Axons of abducens internuclear neurons grew into the transplant sprouting into granule cell and molecular layers, as characterized by the immunostaining for gamma-aminobutyric acid and calbindin D-28k. Ultrastructural examination of labeled axons and boutons revealed the establishment of synaptic contacts, mainly axodendritic, with different cell types of the grafted cerebellar cortex. Therefore, these data indicate that axotomized central neurons resume to normal firing after the reinnervation of a novel target.

Transplantation of human neural progenitor cells into the neonatal rat brain: extensive migration and differentiation with long-distance axonal projections

Here we examined the ability of human neural progenitors from the embryonic forebrain, expanded for up to a year in culture in the presence of growth factors, to respond to environmental signals provided by the developing rat brain. After survival times of up to more than a year after transplantation into the striatum, the hippocampus, and the subventricular zone, the cells were analyzed using human-specific antisera and the reporter gene green fluorescent protein (GFP). From grafts implanted in the striatum, the cells migrated extensively, especially within white matter structures. Neuronal differentiation was most pronounced at the striatal graft core, with axonal projections extending caudally along the internal capsule into mesencephalon. In the hippocampus, cells migrated throughout the entire hippocampal formation and into adjacent white matter tracts, with differentiation into neurons both in the dentate gyrus and in the CA1-3 regions. Directed migration along the rostral migratory stream to the olfactory bulb and differentiation into granule cells were observed after implantation into the subventricular zone. Glial differentiation occurred at all three graft sites, predominantly at the injection sites, but also among the migrating cells. A lentiviral vector was used to transduce the cells with the GFP gene prior to grafting. The reporter gene was expressed for at least 15 weeks and the distribution of the gene product throughout the entire cytoplasmic compartment of the expressing cells allowed for a detailed morphological analysis of a portion of the grafted cells. The extensive integration and differentiation of in vitro-expanded human neural progenitor cells indicate that multipotent progenitors are capable of responding in a regionally specific manner to cues present in the developing rat brain.

Astrocytes from cerebral cortex or striatum attract adult host serotoninergic axons into intrastriatal ventral mesencephalic co-grafts

The identification of axon growth inhibitory molecules offers new hopes for repair of the injured CNS. However, the navigational ability of adult CNS axons and the guidance cues they can recognize are still essentially unknown. Astrocytes may express guidance molecules and are known to have different regional phenotypes. To evaluate their influence on the affinity of adult serotoninergic (5-HT) axons for a projection target, we co-implanted astrocytes from the neonatal striatum, cortex, or ventral mesencephalon together with fetal ventral mesencephalic tissue into the striatum of adult rats. Two months after surgery, quantification after in vitro 5-[1,2-(3)H]serotonin ([(3)H]5-HT) uptake and autoradiography showed that ventral mesencephalic grafts with co-grafted cortical or striatal astrocytes were four times and three times, respectively, more densely innervated by host 5-HT axons than control ventral mesencephalic grafts with or without co-grafted ventral mesencephalic astrocytes. Immunohistochemistry for glial fibrillary acidic protein, vimentin, or chondroitin-sulfate proteoglycans revealed no qualitative or quantitative differences in host astroglial scar or production of inhibitory molecules that could explain these differences in 5-HT innervation. These results demonstrate that astrocytes grown in culture from different brain regions have the potential to influence the growth and maintenance of adult 5-HT axons in a graft of neural tissue from another brain region. It should now be feasible to identify the molecules expressed by cultured cortical or striatal, but not by ventral mesencephalic, astrocytes that have these tropic actions on 5-HT axons of the neostriatum.

Long-term cortical atrophy after excitotoxic striatal lesion: effects of intrastriatal fetal-striatum grafts and implications for Huntington disease

It is not currently clear whether the cortical atrophy observed in Huntington disease (HD) is entirely a direct consequence of the disease or at least partially a secondary consequence of striatal atrophy. This is of major importance for evaluating the possible therapeutic value of intrastriatal fetal-striatum grafts in HD. Cresyl violet-stained sections from rats that had received striatal excitotoxic lesions 1 wk or 4 wk previously showed small and statistically nonsignificant decreases in the thickness of cortical layers V and VI, while series from rats lesioned 12 months previously showed marked decreases in the thickness of the whole cortex (approximately 35% decrease), layer V (approximately 45%-50%) and layer VI (approximately 45%-50%), together with marked neuron loss in these layers. In deep layer V and layer VI, Fluoro-Jade staining showed labeled neurons in animals lesioned 1 wk previously, labeled neurons and astrocytes in animals lesioned 4 wk previously, and practically no labeling in animals lesioned 12 months previously. Intracortical injection of Phaseolus vulgaris leucoagglutinin revealed that corticostriatal fibers were practically absent from the lesioned area of striata lesioned 12 months previously. However, rats that received intrastriatal fetal-striatum grafts shortly after the lesion and were killed 12 months later showed a significant reduction in cortical atrophy, and a large number of labeled corticostriatal fibers surrounding and innervating the graft. In addition, a reduction in the number of Fluoro-Jade-labeled cells in the cortex was already apparent at 3 wk post-grafting. Regardless of whether HD has a primary effect on the cortex, the present results suggest that the striatal degeneration caused by HD contributes markedly to the cortical atrophy, and that intrastriatal grafts may ameliorate this secondary component of the cortical degeneration.

Transplanted fetal striatum in Huntington's disease: phenotypic development and lack of pathology

Neural and stem cell transplantation is emerging as a potential treatment for neurodegenerative diseases. Transplantation of specific committed neuroblasts (fetal neurons) to the adult brain provides such scientific exploration of these new potential therapies. Huntington's disease (HD) is a fatal, incurable autosomal dominant (CAG repeat expansion of huntingtin protein) neurodegenerative disorder with primary neuronal pathology within the caudate-putamen (striatum). In a clinical trial of human fetal striatal tissue transplantation, one patient died 18 months after transplantation from cardiovascular disease, and postmortem histological analysis demonstrated surviving transplanted cells with typical morphology of the developing striatum. Selective markers of both striatal projection and interneurons such as dopamine and c-AMP-related phosphoprotein, calretinin, acetylcholinesterase, choline acetyltransferase, tyrosine hydroxylase, calbindin, enkephalin, and substance P showed positive transplant regions clearly innervated by host tyrosine hydroxylase fibers. There was no histological evidence of immune rejection including microglia and macrophages. Notably, neuronal protein aggregates of mutated huntingtin, which is typical HD neuropathology, were not found within the transplanted fetal tissue. Thus, although there is a genetically predetermined process causing neuronal death within the HD striatum, implanted fetal neural cells lacking the mutant HD gene may be able to replace damaged host neurons and reconstitute damaged neuronal connections. This study demonstrates that grafts derived from human fetal striatal tissue can survive, develop, and are unaffected by the disease process, at least for 18 months, after transplantation into a patient with HD.

Morphology and neurochemistry of two striatal neuronal subtypes expressing the GABA(A) receptor alpha3-subunit in the rat

The morphological characteristics, distribution and neurochemical phenotype of striatal neuronal subtypes expressing the GABA(A) receptor alpha3-subunit were investigated using immunocytochemical and immunofluorescent techniques with an antibody specific for this subunit. alpha3-immunopositive neurons were infrequent in the rat striatum, but two morphologically different subtypes were observed: Cholinergic neurons, and a second cellular type that may correspond to neurogliaform neurons, although it may also be a novel subtype of striatal interneuron. To identify the second cellular subtype, co-localization of the GABA(A) receptor alpha3-subunit with markers of different classes of striatal interneurons was studied using specific antibodies. It was found that there was a lack of co-localization between all interneuronal markers used in this study and the alpha3-subunit. Although the alpha3-subunit immunopositive neurons represent a small percentage of the total of striatal neuronal populations, they may play an important role in the regulation of the microcircuitry of the striatum.

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.

Failure of neuroprotection by embryonic striatal grafts in a double lesion rat model of striatonigral degeneration (multiple system atrophy)

In the present experiment we studied the ability of embryonic striatal grafts to protect against striatal quinolinic acid (QA)-induced excitotoxicity in a previously established double lesion rat model of striatonigral degeneration (SND), the neuropathological substrate of parkinsonism associated with multiple system atrophy (MSA). Male Wistar rats received under halothane inhalation anesthesia a 6-hydroxydopamine 6-OHDA injection into the left medial forebrain bundle. Four to 5 weeks later apomorphine-induced rotation behavior was tested. Rats were divided into two treatment groups receiving either embryonic striatal cell suspensions or sham injections. Apomorphine-induced rotation behavior was retested 2 and 4 weeks after the grafting procedure. Following the rotation test animals of the striatal and sham graft group received a stereotaxic injection of 150 nmol QA. Again rotation behavior was assessed 2 and 4 weeks after lesioning. Brains were then processed to dopamine reuptake ([(3)H]mazindol), dopamine D1 ([(3)H]SCH23390), and D2 ([(3)H]spiperone) receptor autoradiography. Gliosis was detected using [(3)H]PK11195, a marker for peripheral benzodiazepine binding sites. Behavioral and autoradiographic analysis failed to show striatal protection in 6-OHDA prelesioned animals receiving embryonic striatal grafts. These findings indicate that beneficial protective effects of striatal grafts implanted into host striatum prior to excitotoxic insults are abolished in the presence of severe dopaminergic denervation. Our present results are relevant to future applications of neural grafting in MSA-SND.

Fetal tissue transplants in animal models of Huntington's disease: the effects on damaged neuronal circuitry and behavioral deficits

Accumulating evidence indicates that grafts of embryonic neurons achieve the anatomical and functional reconstruction of damaged neuronal circuitry. The restorative capacity of grafted embryonic neural tissue is most illustrated by studies with striatal tissue transplantation in animals with striatal lesions. Striatal neurons implanted into the lesioned striatum receive some of the major striatal afferents such as the nigrostriatal dopaminergic inputs and the gluatmatergic afferents from the neocortex and thalamus. The grafted neurons also send efferents to the primary striatal targets, including the globus pallidus (GP, the rodent homologue of the external segment of the globus pallidus) and the entopeduncular nucleus (EP, the rodent homologue of the internal segment of the globus pallidus). These anatomical connections provide the reversal of the lesion-induced alterations in neuronal activities of primary and secondary striatal targets. Furthermore, intrastriatal striatal grafts improve motor and cognitive deficits seen in animals with striatal lesions. Since the grafts affect motor and cognitive behaviors that are critically dependent on the integrity of neuronal circuits of the basal ganglia, the graft-mediated recovery in these behavioral deficits is most likely attributable to the functional reconstruction of the damaged neuronal circuits. The fact that the extent of the behavioral recovery is positively correlated to the amount of grafted neurons surviving in the striatum encourages this view. Based on the animal studies, embryonic striatal tissue grafting could be a viable strategy to alleviate motor and cognitive disorders seen in patients with Huntington's disease where massive degeneration of striatal neurons occurs.

Autoradiographic study of striatal dopamine re-uptake sites and dopamine D1 and D2 receptors in a 6-hydroxydopamine and quinolinic acid double-lesion rat model of striatonigral degeneration (multiple system atrophy) and effects of embryonic ventral mesencephalic, striatal or co-grafts

The influence of embryonic mesencephalic, striatal and mesencephalic/striatal co-grafts on amphetamine- and apomorphine-induced rotation behaviour was assessed in a rat model of multiple system atrophy/striatonigral degeneration type using dopamine D1 ([3H]SCH23390) and D2 ([3H]spiperone) receptor and dopamine re-uptake ([3H]mazindol) autoradiography. Male Wistar rats subjected to a sequential unilateral 6-hydroxydopamine lesion of the medial forebrain bundle followed by a quinolinic acid lesion of the ipsilateral striatum were divided into four treatment groups, receiving either mesencephalic, striatal, mesencephalic/striatal co-grafts or sham grafts. Amphetamine- and apomorphine-induced rotation behaviour was recorded prior to and up to 10 weeks following transplantation. 6-Hydroxydopamine-lesioned animals showed ipsiversive amphetamine-induced and contraversive apomorphine-induced rotation behaviour. Amphetamine-induced rotation rates persisted after the subsequent quinolinic acid lesion, whereas rotation induced by apomorphine was decreased. In 11 of 14 animals receiving mesencephalic or mesencephalic/striatal co-grafts, amphetamine-induced rotation scores were decreased by >50% at the 10-week post-grafting time-point. In contrast, only one of 12 animals receiving non-mesencephalic (striatal or sham) grafts exhibited diminished rotation rates at this time-point. Apomorphine-induced rotation rates were significantly increased following transplantation of mesencephalic, striatal or sham grafts. The largest increase of apomorphine-induced rotation rates approaching post-6-hydroxydopamine levels were observed in animals with striatal grafts. In contrast, in the co-graft group, there was no significant increase of apomorphine-induced rotation compared to the post-quinolinic acid time-point. Morphometric analysis revealed a 63-74% reduction of striatal surface areas across the treatment groups. Striatal [3H]mazindol binding on the lesioned side (excluding the demarcated graft area) revealed a marked loss of dopamine re-uptake sites across all treatment groups, indicating missing graft-induced dopaminergic re-innervation of the host. In eight (73%) of the 11 animals with mesencephalic grafts and reduced amphetamine-induced circling, discrete areas of [3H]mazindol binding ("hot spots") were observed, indicating graft survival. Dopamine D1 and D2 receptor binding was preserved in the remaining lesioned striatum irrespective of treatment assignment, except for a significant reduction of D2 receptor binding in animals receiving mesencephalic grafts. "Hot spots" of dopamine D1 and D2 receptor binding were observed in 10 (83%) and nine (75%) of 12 animals receiving striatal grafts or co-grafts, consistent with survival of embryonic primordial striatum grafted into a severely denervated and lesioned striatum. Our study confirms that functional improvement may be obtained from embryonic neuronal grafts in a double-lesion rat model of multiple system atrophy/striatonigral degeneration type. Co-grafts appear to be required for reversal of both amphetamine- and apomorphine-induced rotation behaviour in this model. We propose that the partial reversal of amphetamine-induced rotation asymmetry in double-lesioned rats receiving mesencephalic or mesencephalic/striatal co-grafts reflects non-synaptic graft-derived dopamine release. The changes of apomorphine-induced rotation following transplantation are likely to reflect a complex interaction of graft- and host-derived striatal projection pathways and basal ganglia output nuclei. Further studies in a larger number of animals are required to determine whether morphological parameters and behavioural improvement in the neurotransplantation multiple system atrophy rat model correlate.

Towards a protocol for the preparation and delivery of striatal tissue for clinical trials of transplantation in Huntington's disease

There is a growing body of scientific evidence contributing to the development of clinical transplantation programs in patients with Huntington's disease. Phase I clinical trials have already commenced in France and North America and are starting in the near future in Sweden and the UK. Protocols for patient selection, surgical implantation, and pre- and postoperative follow-up are well defined. However, considerable variability exists with respect to the harvesting, preparation, and timing of implantation of the donor material. In this article we review the scientific evidence on which a rational protocol for donor tissue preparation and delivery may be based. Strategies aimed at minimizing the variability of tissue preparation should reduce the variability of functional outcome of striatal transplantation observed in animal models of Huntington's disease.

Survival, neuronal differentiation, and fiber outgrowth of propagated human neural precursor grafts in an animal model of Huntington's disease

Expanded neural precursor cells provide an attractive alternative to primary fetal tissue for cell replacement therapies in neurodegenerative diseases. In this study we transplanted epigenetically propagated human neural precursor cells into a rat model of Huntington's disease. Neural precursors survived transplantation and large numbers differentiated to express neuronal antigens, including some that expressed DARPP-32, indicating a mature striatal phenotype had been adopted. Neuronal fibers from the grafts projected diffusely throughout the host brain, although there was no evidence that outgrowth was specifically target directed. This study supports the contention that propagated human neural precursors may ultimately be of use in therapeutic neural transplantation paradigms for diseases such as Huntington's disease.

Impact of a preceding striatal excitotoxic lesion and treatment with ciliary neurotrophic factor on striatal graft survival

The survival of grafted embryonic striatal tissue, dissected from the lateral ganglionic eminence, depends on the status of the host striatum. We found significantly larger volumes of surviving graft tissue and of striatal-like tissue (P-zone) within the graft, when the host striatum had been subjected to an excitotoxic lesion prior to transplantation surgery. Concomitantly the numbers of surviving grafted cells, assessed in both cresyl violet-stained sections and in sections stained with an immunohistochemical marker for striatal neurons, increased as compared to when graft tissue was placed in an intact unlesioned striatum. Finally, we examined the impact of treatment of the donor tissue with ciliary neurotrophic factor (CNTF) on graft survival. CNTF has previously been shown to protect striatal neurons against excitotoxic insults both in vitro and in vivo, but it did not improve striatal graft survival when added to the cell suspension prior to implantation.

Repair of the damaged brain. The Alfred Meyer Memorial Lecture 1998

Over the last decade, neural transplantation has progressed from being an experimental technique for studying regeneration and plasticity in the brain to clinical trials of reconstructive surgery in human neurodegenerative disease. Whereas clear evidence is only available at present for the viability of this technique in Parkinson's disease, applications to several other diseases, including Huntington's disease, multiple sclerosis, spinal cord injury, and chronic pain are currently under active consideration. It is clear that the techniques of transplantation can be functionally viable under certain well-defined biological circumstances, but significant problems remain in the availability of suitable donor tissues and defining the optimal conditions for reliable survival of the implanted cells. If we are to obtain improved reliability of the present techniques or identify suitable alternatives, we need a better understanding of the conditions for the survival and integration of grafts into the host brain, and the mechanisms by which they influence host function. In this review I consider the nature of the structural reconstruction required to achieve repair in animal models of Parkinson's and Huntington's diseases, contrasting the replacement of deficient neurochemicals within the striatum in the former case, and the need for reconstruction of input and output connections of the striatal circuitry in the latter.

Associative plasticity in striatal transplants

Striatal lesions disrupt both motor and cognitive performance in rats, many aspects of which can be restored by striatal transplants. Because the normal striatum is involved in the formation and maintenance of motor habits, it has been hypothesized that grafted animals may require explicit retraining to relearn previously established habits that have been disrupted by the lesions. We have used a lateralized-discrimination task to reproduce this "learning to use the transplant" effect, combined with a transfer-of-training paradigm to demonstrate that recovery requires relearning specific lateralized stimulus-response associations and cannot be explained simply by a generalized training-dependent improvement in motor skill. These results have clear implications for developing appropriate strategies for the rehabilitation of Huntington's disease patients participating in clinical transplantation programs.

In vivo magnetic resonance spectroscopy of human fetal neural transplants

To better define the survival and cellular composition of human fetal neurotransplants in vivo, we performed quantitative 1H MRS to determine the concentration of the neuronal amino acid [N-acetylaspartate] within MRI-visible grafts. In all, 71 grafts in 38 patients [24 Parkinson's disease (PD), 14 Huntington's disease (HD)] were examined, as well as 24 untreated PD and HD patients and 13 age-matched normal controls. MRI appearances of edema were present in three out of 71 grafts, the remainder being consistent with histologically identified viable neural transplant tissue. N-acetylaspartate (NAA), creatine, choline, myoinositol and glutamine plus glutamate (Glx) were identified in all post-transplant putamens, with abnormal metabolites, lactate and/or lipid detectable in only three patients. Of 71 grafts, 19 occupied more than 60% of the MRS-examined volume (VOI) (mean 84.2 +/- 3%; range 61-100%). In those, [NAA] was 8.50 +/- 0.99 mM in eight PD spectra and 6.59 +/- 0.81 mM in 11 HD spectra, and was not significantly different from controls. In contrast, transplanted fetal neurones contain less than 0.4 mM of the neuronal amino acid NAA. This suggests that established fetal neurotransplants in the human putamen of both PD and HD patients are populated by adult neurones, axons and dendrites.

Striatal dopaminergic afferents concentrate in GDNF-positive patches during development and in developing intrastriatal striatal grafts

Glial cell line-derived neurotrophic factor (GDNF) has potent trophic action on fetal dopaminergic neurons. We have used a double immunocytochemical approach with antibodies that recognize GDNF and tyroxine hydroxylase (TH) or the phosphoprotein DARPP-32, to study the developmental pattern of their interactions in the rat striatum and in intrastriatal striatal transplants. Postnatally, at one day and also at 1 week, GDNF showed a patchy distribution in the striatum, together with a high level of expression in the lateral striatal border, similar to that observed for the striatal marker DARPP-32 and also for TH. In the adult striatum, there was diffuse, weak immunopositivity for GDNF, together with widespread expression of DARPP-32-positive neurons and TH-immunoreactive (TH-ir) fibers. In 1-week-old intrastriatal striatal transplants, there were some GDNF immunopositive patches within the grafts and although there was not an abundance of TH-positive fibers, the ones that were seen were located in GDNF-positive areas. This was clearly evident in 2-week-old transplants, where TH-ir fibers appeared selectively concentrated in GDNF-positive patches. This pattern was repeated in 3-week-old grafts. In co-transplants of mesencephalic and striatal fetal tissue (in a proportion of 1:4), TH-ir somata were located mainly at the borders of areas that were more strongly immunostained for GDNF, and TH-ir fibers were also abundant in these areas and were found in smaller numbers in regions that were weakly positive for GDNF. These results demonstrate that GDNF-ir is coincident with that for TH and DARPP-32, and suggest that GDNF release by fetal striatal neurons both in normal development and in developing striatal grafts may have not only a trophic but also a tropic influence on TH-ir fibers and may be one of the factors that regulate dopaminergic innervation of the striatum.

Striatal grafts in a rat model of Huntington's disease: time course comparison of MRI and histology

Survival and integration into the host brain of grafted tissue are crucial factors in neurotransplantation approaches. The present study explored the feasibility of using a clinical MR scanner to study striatal graft development in a rat model of Huntington's disease. Rat fetal lateral ganglionic eminences grown as free-floating roller-tube cultures were grafted into the quinolinic acid-lesioned striatum, and T1- and T2-weighted sequences were acquired at 2, 7, 21, and 99 days posttransplantation. MR images were then compared with images of corresponding histological sections. The lesion-induced striatal degeneration caused a progressive ventricle enlargement, which was significantly different from controls at 21 days posttransplantation. Seven days posttransplantation, T1-weighted images revealed a defined liquid-isointense signal surrounded by a hyperintense rim at the site of graft placement, which was found unaltered for the first 21 days posttransplantation, whereas a hypointense graft signal was detected at 99 days posttransplantation. At 2 days posttransplantation, T2-weighted images showed the graft region as a hyperintense area surrounded by a rim of low signal intensity but at later time-points graft location could not be further verified. Measures for graft size and ventricle size obtained from MR images highly correlated with measures obtained from histologically processed sections (R = 0.8, P < 0.001). In conclusion, the present study shows that fetal rat lateral ganglionic eminences grown as free-floating roller-tube cultures can be successfully grafted in a rat Huntington model and that a clinical MR scanner offers a useful noninvasive tool for studying striatal graft development.

Striatal transplantation in a transgenic mouse model of Huntington's disease

Striatal grafts have been proposed as a potential strategy for striatal repair in Huntington's disease, but it is unknown whether the diseased brain will compromise graft survival. A transgenic mouse line has recently been described in which hemizygotes with an expanded CAG repeat in exon 1 of the HD gene exhibit a progressive neurological phenotype similar to the motor symptoms of Huntington's disease. We have therefore evaluated the effects of the transgenic brain environment on the survival, differentiation, and function of intrastriatal striatal grafts and undertaken a preliminary analysis of the effects of the grafts on the development of neurological deficits in the host mice. Hemizygote transgenic and wild-type littermate female mice received striatal grafts at 10 weeks of age and were allowed to survive 6 weeks. Normal healthy grafts were seen to survive and differentiate within the striatum of transgenic mice in a manner comparable to that seen in control mice. The transgenic mice exhibited a progressive decline in body weight from 9 weeks of age and a progressive hypoactivity in an open field test of general locomotor behavior. Although striatal grafts exerted a statistically significant influence on several indices of this impairment, all behavioral effects were small and did not exert any clinically relevant effect on the profound neurological deficiency of the transgenic mice.

Effects of severity of host striatal damage on the morphological development of intrastriatal transplants in a rodent model of Huntington's disease: implications for timing of surgical intervention

OBJECT

The goal of this study was to investigate the effect of the severity of host neural damage on the morphological development of intrastriatal transplants in a rodent model of Huntington's disease.

METHODS

Sprague-Dawley rats were subjected to unilateral striatal lesioning induced by administration of quinolinic acid (20 nM, 40 nM, or 90 nM). Seven days postlesioning, intrastriatal cell suspension grafts were placed in the right striatum in some of these animals. Grafts were also placed in the right striatum of additional animals that had not been subjected to lesioning. The rats were killed and processed for morphological analysis 8 weeks after grafting. The results indicate that striatal grafts survive and grow much better when implanted into a lesioned striatum rather than into an intact striatum, as measured both by the volume and the numbers of medium-sized spiny neurons within the graft. Only a small or modest lesion is necessary to produce this effect. By some measures (such as graft volume) grafts survive less well when the lesion is more extensive. The presence of a graft reduced the extent of striatal atrophy induced by the lesions, but this effect was not caused by differences in the numbers of surviving neurons per se.

CONCLUSIONS

These results have significant implications for the timing of surgical intervention and patient selection with respect to current and future clinical trials of striatal transplantation in the treatment of Huntington's disease.

Serotonin axons of the neostriatum show a higher affinity for striatal than for ventral mesencephalic transplants: a quantitative study in adult and immature recipient rats

We previously showed that grafts of fetal ventral mesencephalic tissue are practically not innervated by host serotonin (5-HT) axons after implantation into the striatum of rats aged more than 14 days, at variance with transplants of cortical or striatal tissue into the adult striatum, which are well innervated by these axons. Using 5-HT immunohistochemistry and in vitro [3H]5-HT uptake/autoradiography, we have examined and quantified the innervation of ventral mesencephalic versus striatal grafts several months after implantation into the striatum of neonatal (postnatal day 5 or P5), juvenile (P15), and adult rats. Ventral mesencephalic grafts implanted in P5 rats received a moderate 5-HT innervation, while similar grafts implanted in P15 or adult recipients were almost free of any 5-HT fibers (-80%, compared to P5). The density of 5-HT innervation showed a tendency toward higher values in striatal than in ventral mesencephalic grafts (1.6-2 times higher in P5 and adult recipients; 4 times higher in P15 recipients). The difference was more striking, and significant, when only the true striatal portions of the striatal grafts were considered, i.e., DARPP-32-immunopositive areas (4-5 times higher in P5 and adult recipients; 10 times higher in P15 recipients). Accordingly, these DARPP-32-positive areas were also more densely innervated than the DARPP-32-negative zones of the same grafts (3 times higher at any age). The 5-HT innervation density also decreased with increasing age of the recipients in DARPP-32-positive, as well as DARPP-32-negative compartments of the striatal grafts (-75% in adults), but this decrease appeared more gradual (-50% in juveniles) than with mesencephalic grafts. It is concluded that the 5-HT axons innervating the neostriatum have a better affinity for striatal grafts than for ventral mesencephalic grafts or the nonstriatal portions of striatal grafts. In adulthood, the relative affinity of these axons for the different types of grafts is maintained, even though their growth capacity decreases irrespective of the target tissue considered. This experimental model may prove useful for the identification of the receptors and ligands that are responsible for target recognition by 5-HT axons and to test the possibility that the progressive decrease of axonal growth capacity from neonatal age to adulthood be related to a downregulation of such molecules.

Mature intrastriatal striatal grafts revert the changes in the expression of pallidal and thalamic alpha 1, alpha 2 and beta 2/3 GABAA receptor subunit induced by ibotenic acid lesions in the rat striatum

A between-side comparison of GABAA receptor subunit expression levels in the globus pallidus and anterior-pole motor thalamic nuclei of rats with an ibotenate lesion of the striatum, and rats receiving a fetal striatal graft in the lesioned area was made by using immunocytochemistry with subunit-specific antibodies, at different times post-lesion or different times post-grafting. At 10 days post-lesion, there was already an increase in the labeling of the alpha 1- and beta 2/3-subunits in the globus pallidus, entopeduncular nucleus and ventrolateral nucleus ipsilateral to the lesion when compared with the contralateral side, while there were no significant changes at the level of the ventromedial nucleus. Labeling of the alpha 2-subunit showed a clear increase in the entopeduncular nucleus compared with the contralateral side at 10 days post-lesion. Similar changes were also observed for the different subunits studied at 30 and 120 days after lesioning. Rats with 20-day old transplants of fetal striatal neurons that were implanted in the ibotenate lesioned striatum at 10 days post-lesioning, continued to show changes in the expression of GABAA receptor subunits, albeit at a lower level than those of ibotenate lesioned rats at similar age post-lesion. However, when examining rats with 70-day old transplants, the ibotenate-lesion induced between-side changes were almost completely compensated. These findings suggest a correlation between the maturation of the grafts and their capability to function in reestablishing neuronal circuits as shown by the reduction of changes in GABAergic transmission induced by ibotenate lesions, as indicated by the reversal of changes in GABAA receptor subunit in several areas of the basal ganglia circuit.

GABA(A) receptor subunit expression in intrastriatal striatal grafts comparison between normal developing striatum and developing striatal grafts

Expression of the alpha1, alpha2 and beta2/3 GABA(A) receptor subunits in maturing cell-suspension striatal grafts and in normal developing striatum was studied by immunocytochemistry. During normal postnatal development, the alpha1 subunit was present in the striatum only at very low density, while the alpha2 and beta2/3 subunits were present with a patchy distribution, in some patches at high density. Double-staining techniques indicated that DARPP-32 (a marker of striatal projection neurons) was not colocalized with alpha1, but was present in some beta2/3-positive areas and all alpha2-positive areas. In striatal grafts, alpha1 immunoreactivity was first detected 2 weeks post-grafting (p.g.), and by 3-10 weeks p.g. the pattern was similar to that observed in mature grafts (1 year p.g.), in which alpha1-immunopositive patches surrounding DARPP-32-positive (i.e. striatum-like) areas are observed. Alpha2 and beta2/3 immunoreactivity was observed within the first week p.g., and by 3-10 weeks p.g. was similar to that observed in mature grafts (i.e. immunoreactivity throughout the graft but with patches of different intensity). During graft maturation there was a marked decline in alpha2 immunoreactivity in DARPP-32-negative areas, as is observed during normal development of the globus pallidus and ventral pallidum. Interestingly, alpha1- and beta2/3-positive fibers (perhaps mostly dendrites) entered DARPP-32-positive patches from DARPP-32-negative areas. This study indicates that the time course of expression of GABA(A) receptor subunits in grafted striatal neurons, closely matches that of morphological maturation of the transplant, that of the development of functional synaptic activity and that of GABA(A) receptor subunit immunoreactivity in normal developing striatum. Our results also suggest that there are significant interactions between DARPP-32-positive and DARPP-32-negative areas with respect to the expression of GABA(A) receptors, and support the suggestion that miniature 'striatopallidal systems' may develop within grafts; such interactions may be important for the functional integration of striatal grafts with the host brain.

The effects of donor stage on the survival and function of embryonic striatal grafts in the adult rat brain. I. Morphological characteristics

The effects of the stage of donor embryos on the survival of grafts from different neuronal cell types have been well documented. Indeed, this parameter has been shown to be highly important in the survival and function of transplants of various tissues of the CNS. However this question has not been addressed in grafts of embryonic striatal tissue transplanted into animal models of Huntington's disease. In this study, rats which had received a unilateral ibotenic acid lesion in the dorsal striatum received grafts from a standard dissection of embryonic striatal primordium taken from donors of embryonic stage either E14, E16, E17 or E19 days. Three months after transplantation six rats from each group were killed for analysis of graft survival and morphology. The remaining animals in each group were killed between 10 and 14 months after grafting. Graft morphology was detected using a range of markers including: acetylcholinesterase and Cresyl Violet, the 32,000 mol. wt dopamine- and cyclic AMP-regulated phosphoprotein (DARPP-32), tyrosine hydroxylase and striatally-enriched phosphatase. All the grafts from different donor stages survived well at both time-points and Cresyl Violet staining indicated neuronal cell types spread throughout the grafts. The transplants were seen to have a characteristic "patchy" appearance with areas of dense AChE activity and DARPP-32 immunopositivity interspersed with areas of much lighter expression. These areas also co-localized consistently with striatally-enriched phosphatase and tyrosine hydroxylase expression, indicating that they comprised the striatal-like compartment of the graft (the so called P zones, containing cells of the mature striatum), and receiving specific afferent input from the host dopaminergic system. There was no significant difference in total graft volume, when comparing individual groups at both time-points from grafting. However, when comparing the volume of the P zones, the striatal primordium from the youngest donor stages (E14 and E16) produced grafts with a significantly higher proportion of striatal-like tissue. Therefore, in order to increase the proportion of striatal tissue within these grafts, tissue from younger embryonic donors should be used. This has important implications in the application of this model towards clinical trials in Huntington's disease.

Extensive migration and target innervation by striatal precursors after grafting into the neonatal striatum

Embryonic striatal precursors grafted into the lesioned adult host striatum show limited integration with little migration and restricted efferent projections. In the present study, the influence of an immature striatal environment on the integrative capacity of grafted neuroblasts was examined after transplantation of striatal progenitors into the striatum at different stages of postnatal development. Mouse progenitors, derived from embryonic day 13.5-14 lateral or medial ganglionic eminence or the cerebellar primordium, were transplanted as a single cell suspension into the developing postnatal day 1, 7 and 21 rat striatum. The grafted cells and their axonal projections were visualized using antibodies raised against the mouse-specific neural markers, M6 and M2. Cells from the lateral (but not the medial) ganglionic eminence showed a remarkable capacity to innervate selectively the striatal target structures, globus pallidus, entopeduncular nucleus and substantia nigra, reminiscent of endogenous striatal neurons, which is not observed after grafting into adult hosts. M6 and M2-immunopositive cellular profiles from both the lateral and medial ganglionic eminences were observed to have migrated extensively away from the injection site, in contrast to the cerebellar precursors which remained clustered at the implantation site. Cells from the lateral ganglionic eminence were largely confined within the striatal complex where they developed striatal characteristics, displaying expression of DARPP-32, the 32,000 mol. wt dopamine- and cyclic AMP-regulated phosphoprotein, whereas cells from the medial ganglionic eminence had migrated caudally along the internal capsule and were observed predominantly in the globus pallidus and thalamus, in addition to the striatum. The cells located outside the striatum were all DARPP-32 negative. The improved integration and increased projection capacity of the lateral ganglionic eminence precursors grafted into postnatal day 1 hosts gradually declined as the host advanced into later stages of development (postnatal day 7), and in postnatal day 21 hosts the grafted striatal precursors behaved similarly to grafts implanted into adult recipients. These results demonstrate the specific capacity of embryonic striatal progenitors to integrate into the developing basal ganglia circuitry during early postnatal development, and that the extent of neuronal and glial integration and graft host connectivity declines when the host has developed beyond the first postnatal week.

Co-transplantation of fetal lateral ganglionic eminence and ventral mesencephalon can augment function and development of intrastriatal transplants

Methods to increase the development and sustained function of embryonic mesencephalic dopamine cells after transplantation into dopamine (DA)-depleted striatum are currently under investigation. Elements that are crucial for the maturation and connectivity of neurons during normal development of the brain may also play a role in the development and integration of grafted embryonic tissue. Based on in vitro and in vivo observations of the enhancing effects of striatal tissue on nigral dopaminergic cell development and survival, we demonstrate that inclusion of embryonic striatal cells, specifically from the lateral ganglionic eminence (LGE), produces dopaminergic transplants with augmented functional effects. Rats neonatally DA-depleted and co-transplanted with embryonic nigral and LGE cells developed improved functional outcome when compared with animals receiving only nigral cells, and they required the transplantation of fewer nigral cells to produce a strong behavioral effect. Anatomically, the inclusion of LGE cells produced increased DA cell survival, a higher density of reinnervation into the DA-depleted host striatum, and patches of DA fibers within the co-transplants. There were also an increased number of host striatal cells which induced the immediate-early gene c-fos in co-transplanted animals compared to animals receiving nigral cells alone, indicating a higher degree of host-cell activation. The ability to enhance function, cell survival, reinnervation, and host activation with nigral-striatal co-transplants in the presence of fewer nigral cells supports the hypothesis of a trophic influence of striatal cells on nigral DA cells.

Drug-free evaluation of rat models of parkinsonism and nigral grafts using a new automated rotarod test

A variety of tests are available for the evaluation of behavioural deficits in rat models of hemiparkinsonism; many, however, are of limited applicability or insufficiently objective. The drug-induced turning behaviour test is widely used. A disadvantage of this test is that the use of drugs may lead to misleading results. Here, we describe a drug-free rotarod test that was used to evaluate the effects of unilateral 6-hydroxydopamine lesions, nigral grafts, and subrotational doses of apomorphine. The rotarod unit was automated and interfaced to a personal computer allowing automatic recording of the time that each rat was able to stay on the rod at different rotational speeds (i.e., progressively increasing the difficulty of the task). A combination of lesion-induced deficits resembling those of Parkinson's disease appears to be involved in falling from the rod. The test shows high effectiveness for identifying rats with maximal dopaminergic lesions, but is also effective for identifying partial lesions. Rotarod performance profiles were useful for investigating the effects of intrastriatal nigral grafts, since low rotation speeds revealed differences from lesioned rats (i.e., improvements) while higher speeds revealed differences from normal rats (i.e., remaining deficits and partial lesions). The test was effective regardless of whether rats were trained on the rod before lesion, after lesion, or after grafting. Injections of apomorphine (0.0125 and 0.0250 mg/kg) did not induce consistent improvements. These results indicate that the rotarod test is a useful drug-free procedure for overall evaluation of basic motor abilities in rat models of parkinsonism and treatment-induced changes.

Dopaminergic innervation of striatal grafts placed into different sites of normal striatum: differences in the tyrosine hydroxylase immunoreactive growth pattern

When patients with Parkinson's disease initially show symptoms, approximately 80-85% of their dopaminergic nerve fibers in the striatum have degenerated. It is thus of importance to develop strategies to try to rescue the remaining dopaminergic neurons and to stimulate them to induce sprouting. In this study the goal was to examine whether the different subgroups of dopaminergic neurons in the ventral mesencephalon projecting to the basal ganglia have different sprouting capacities when stimulated by the trophic effect of a fetal striatal graft. Lateral ganglionic eminence was implanted into the lateral ventricle, the midportion of dorsal striatum, globus pallidus, or ventral striatum. Solid tissue pieces from 13- to 15-mm fetuses were stereotactically implanted into adult female Sprague-Dawley rats. At postgrafting week 4 the animals were perfused and processed for tyrosine hydroxylase (TH) immunohistochemistry. Transplants placed in the lateral ventricle were TH-negative, except for two cases with TH-positive fibers where the ependymal layer was disrupted, thereby allowing direct contact between the graft and the adjacent host striatum. The transplants placed into dorsal striatum were innervated by small patches of dopaminergic nerve fibers. Areas between the TH-positive patchy structures remained TH-negative. In grafts placed into globus pallidus, both patchy structures and a less dense TH-positive nerve fiber network was noted. The TH-positive growth pattern in transplants placed in ventral striatum was also divided into patchy and widespread growth. Grafts placed in globus pallidus and ventral striatum revealed significantly larger areas of TH-positive innervation compared with that measured in grafts placed in dorsal striatum and the lateral ventricle. In conclusion, it is possible to induce sprouting of TH-immunoreactive nerve fibers from all areas examined. The most potent areas to initiate dopaminergic growth were the globus pallidus and ventral striatum, where both a patchy dense and a widespread, less dense growth was induced. Thus, if using a trophic stimulus to induce sprouting from remaining dopaminergic nerve fibers in Parkinson's disease, the preferential target to induce sprouting would be ventromedial striatum and growth would be guided toward dorsal striatum owing to the enhanced dopaminergic growth properties in the ventromedial areas.