Connectivity of striatal grafts implanted into the ibotenic acid-lesioned striatum--I. Subcortical afferents

Neuronal threshold functions: Determining symptom onset in neurological disorders

Synaptic networks determine brain function. Highly complex interconnected brain synaptic networks provide output even under fluctuating or pathological conditions. Relevant to the treatment of brain disorders, understanding the limitations of such functional networks becomes paramount. Here we use the example of Parkinson's Disease (PD) as a system disorder, with PD symptomatology emerging only when the functional reserves of neurons, and their interconnected networks, are unable to facilitate effective compensatory mechanisms. We have denoted this the "threshold theory" to account for how PD symptoms develop in sequence. In this perspective, threshold functions are delineated in a quantitative, synaptic, and cellular network context. This provides a framework to discuss the development of specific symptoms. PD includes dysfunction and degeneration in many organ systems and both peripheral and central nervous system involvement. The threshold theory accounts for and explains the reasons why parallel gradually emerging pathologies in brain and peripheral systems generate specific symptoms only when functional thresholds are crossed, like tipping points. New and mounting evidence demonstrate that PD and related neurodegenerative diseases are multisystem disorders, which transcends the traditional brain-centric paradigm. We believe that representation of threshold functions will be helpful to develop new medicines and interventions that are specific for both pre- and post-symptomatic periods of neurodegenerative disorders.

Using gene or cell therapies to treat Huntington's disease

Huntington's disease is caused by a CAG repeat expansion in the first exon of the HTT gene, leading to the production of gain-of-toxic-function mutant huntingtin protein species and consequent transcriptional dysregulation and disrupted cell metabolism. The brunt of the disease process is borne by the striatum from the earliest disease stages, with striatal atrophy beginning approximately a decade prior to the onset of neurologic signs. Although the expanded CAG repeat in the HTT gene is necessary and sufficient to cause HD, other genes can influence the age at onset of symptoms and how they progress. Many of these modifier genes have roles in DNA repair and are likely to modulate the stability of the CAG repeat in somatic cells. Currently, there are no disease-modifying treatments for HD that can be prescribed to patients and few symptomatic treatments, but there is a lot of interest in therapeutics that can target the pathogenic pathways at the DNA and RNA levels, some of which have reached the stage of human studies. In contrast, cell therapies aim to replace key neural cells lost to the disease process and/or to support the host vulnerable striatum by direct delivery of cells to the brain. Ultimately it may be possible to combine gene and cell therapies to both slow disease processes and provide some level of neural repair. In this chapter we consider the current status of these therapeutic strategies along with their prospects and challenges.

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 Comparative Study of Preparation Techniques for Improving the Viability of Striatal Grafts Using Vital Stains, in Vitro Cultures, and in Vivo Grafts

Cell suspension grafts from embryonic striatal primordia placed into the adult rat striatum survive well and are able to alleviate a number of behavioral deficits caused by excitotoxic lesions to this structure. However, neither the anatomical connectivity between the graft and host nor the functional recovery elicited by the grafts is completely restored. One way in which the survival and function of embryonic striatal grafts may be enhanced is by the improvement of techniques for the preparation of the cell suspension prior to implantation, an issue that has been addressed only to a limited extent. We have evaluated a number of parameters during the preparation procedure, looking at the effects on cell survival over the first 24 h from preparation using vital dyes and the numbers of surviving neurons in vitro, after 4 days in culture, in addition to graft survival and function in vivo. Factors influencing cell survival include the type of trypsinization procedure and the age of donor tissues used for suspension preparation. The presence of DNase has no effect on cell viability but aids the dissociation of the tissue to form single cells. These results have important implications for the use of embryonic striatal grafts in animal models of Huntington's disease, and in any future clinical application of this research.

Behavioral Effects of Neural Transplantation

Considerable evidence suggests that transplantation of fetal neural tissue ameliorates the behavioral deficits observed in a variety of animal models of CNS disorders. However, it is also becoming increasingly clear that neural transplants do not necessarily produce behavioral recovery, and in some cases have either no beneficial effects, magnify existing behavioral abnormalities, or even produce a unique constellation of deficits. Regardless, studies demonstrating the successful use of neural transplants in reducing or eliminating behavioral deficits in these animal models has led directly to their clinical application in human neurodegenerative disorders such as Parkinson's disease. This review examines the beneficial and deleterious behavioral consequences of neural transplants in different animal models of human diseases, and discusses the possible mechanisms by which neural transplants might produce behavior recovery.

Establishment of a Stable Glutamate Decarboxylase (Gad) Expressing Cell-Line by Transfection

We have constructed a recombinant DNA clone containing the gene encoding glutamic acid decarboxylase (GAD), which catalyzes the synthesis of γ-amino-butyric acid (GABA). This recombinant DNA was then transfected into mouse NIH-3T3 fibroblast cells for transplantation into Swiss-Web mice. In order to construct a plasmid capable of transcribing the DNA insert in the eucaryotic cells, the GAD gene was removed from pSP65-13, and was ligated into the vector pSV2neo, which contains the SV40 early promoter, and the neomycin resistance gene. The pSV2GAD was then transfected into NIH-3T3 fibroblasts by calcium phosphate precipitation, or by electroporation. The transfected fibroblasts were then selected with antibiotic G418 for amplification. The transient expression of GAD in the transfected fibroblasts was detected by immunocytochemical staining using anti-GAD antibody. A small population of GAD immunoreactive cells were clearly stained, and were easily distinguished from the majority of unstained background cells. These GAD-immunoreactive cells were not seen in either mock-transfected, or pSV2neo-transfected cells (vector-alone control). The transfected fibroblasts were continuously selected with antibiotic G418. Six out of 35 subcultures that had GAD-positive immunostaining in the cell lines were selected. Granular GAD-positive staining was observed in the cytoplasm and fiber extensions of the transfected cell lines in varying densities. The GAD-mRNA was also detected in the subcultures by in situ hybridization using a 35S-labeled 369-nucleotide riboprobe in pBluescript. The GAD-transfected NIH-3T3 cells were then transplanted into Swiss-Web mice. Fifteen to 30 days later, transplanted animals were perfused for identification. These cells were first identified with anti-fibronectin antibody, and the adjacent sections with anti-GAD or anti-GABA antibodies. All the transplants are fibronectin-positive. Both GAD- and GABA-positive cells were observed in the transplant.

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.

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?

Transplantation of GABAergic cells derived from bioreactor-expanded human neural precursor cells restores motor and cognitive behavioral deficits in a rodent model of Huntington's disease

Huntington's disease (HD) is a neurodegenerative disorder that is characterized by progressive dementia, choreiform involuntary movements, and emotional deterioration. Neuropathological features include the progressive degeneration of striatal γ-aminobutyric acid (GABA) neurons. New therapeutic approaches, such as the transplantation of human neural precursor cells (hNPCs) to replace damaged or degenerated cells, are currently being investigated. The aim of this study was to investigate the potential for utilizing telencephalic hNPCs expanded in suspension bioreactors for cell restorative therapy in a rodent model of HD. hNPCs were expanded in a hydrodynamically controlled and homogeneous environment under serum-free conditions. In vitro analysis revealed that the bioreactor-expanded telencephalic (BET)-hNPCs could be differentiated into a highly enriched population of GABAergic neurons. Behavioral assessments of unilateral striatal quinolinic acid-lesioned rodents revealed a significant improvement in motor and memory deficits following transplantation with GABAergic cells differentiated from BET-hNPCs. Immunohistochemical analysis revealed that transplanted BET-hNPCs retained a GABAergic neuronal phenotype without aberrant transdifferentiation or tumor formation, indicating that BET-hNPCs are a safe source of cells for transplantation. This preclinical study has important implications as the transplantation of GABAergic cells derived from predifferentiated BET-hNPCs may be a safe and feasible cell replacement strategy to promote behavioral recovery in HD.

Clinical trials of neural transplantation in Huntington's disease

Clinical neural transplantation in Huntington's disease has moved forward as a series of small studies, which have provided some preliminary proof of principle that neural transplantation can provide benefit. However, to date, such benefits have not been robust, and there are a number of important issues that need to be addressed. These include defining the optimum donor tissue conditions and host characteristics in order to produce reliable benefit in transplant recipients, and whether, and for how long, immunosuppression is needed. Further clinical studies will be required to address these, and other issues, in order to better understand the processes leading to a properly functioning neural graft. Such studies will pave the way for future clinical trials of renewable donor sources, in particular, stem cell-derived neuronal progenitor grafts.

Validating the use of M4-BAC-GFP mice as tissue donors in cell replacement therapies in a rodent model of Huntington's disease

Huntington's disease (HD) is a neurodegenerative disease with currently only symptomatic treatment. Cell-based therapy, aiming at replacing the lost medium spiny neurons (MSN) with primary fetal striatal cells, has had some success at modifying the symptoms both in experimental studies and clinical trials. Additional pre-clinical studies are required to optimise transplantation protocols and conditions, learn about the limits of circuit reconstruction and functional recovery, and test alternative cell sources. Transgenic mice with integrated bacterial artificial chromosome (BAC) expressing the green fluorescent protein (GFP) can be used to study specific neuronal projections. The BAC transgenic line used in this study, with the GFP expression under the control of the muscarinic receptor M4 promoter, selectively expressed the reporter gene in the direct efferent pathway of the MSN projecting from the striatum to the substantia nigra pars reticulata and the entopeduncular nucleus, the rodent equivalent of the internal segment of the globus pallidus. The current work was designed to validate the use of M4-BAC-GFP mice as tissue donors in cell-based therapy in a rodent model of HD by examining the effect of the transplantation procedure on the GFP expression; the feasibility of identifying the GFP expression in vivo after different time points; and the survival and integration of the transgenic striatal tissue transplant up to 6 months in the host. The data confirm that embryonic striatal tissue from the M4-BAC-GFP mice survives, stably expresses GFP, and thus represents a powerful novel way to study graft-host interaction in this animal model neurodegeneration.

Cell transplantation for Huntington’s disease: practical and clinical considerations

Huntington’s disease is a dominantly inherited neurodegenerative disorder, usually starting in mid-life and leading to progressive disability and early death. There are currently no disease-modifying treatments available. Cell transplantation is being considered as a potential therapy, following proof of principle that cell transplantation can improve outcomes in another basal ganglia disorder, namely Parkinson’s disease. The principle aim is to replace the striatal medium spiny neurons lost in Huntington’s disease with new cells that are able to take over their function and reconnect the circuitry. This article reviews the experimental background and evidence from clinical studies that suggest that cell transplantation may improve function in Huntington’s disease, reviews the current status of the field and considers the current challenges to taking this experimental strategy forward to becoming a reliable therapeutic option.

Embryonic striatal grafts restore bi-directional synaptic plasticity in a rodent model of Huntington's disease

Embryonic striatal grafts integrate with the host striatal circuitry, forming anatomically appropriate connections capable of influencing host behaviour. In addition, striatal grafts can influence host behaviour via a variety of non-specific, trophic and pharmacological mechanisms; however, direct evidence that recovery is dependent on circuit reconstruction is lacking. Recent studies suggest that striatal grafts alleviate simple motor deficits, and also that learning of complex motor skills and habits can also be restored. However, although the data suggest that such 're-learning' requires integration of the graft into the host striatal circuitry, little evidence exists to demonstrate that such integration includes functional synaptic connections. Here we demonstrate that embryonic striatal grafts form functional connections with the host striatal circuitry, capable of restoring stable synaptic transmission, within an excitotoxic lesion model of Huntington's disease. Furthermore, such 'functional integration' of the striatal graft enables the expression of host-graft bi-directional synaptic plasticity, similar to the normal cortico-striatal circuit. These results indicate that striatal grafts express synaptic correlates of learning, and thereby provide direct evidence of functional neuronal circuit repair, an essential component of 'functional integration'.

Amygdalar GABAergic-rich neural grafts attenuate anxiety-like behavior in rats

Transplantation experiments have shown that neurologic deficits may be reversed by engrafting fresh tissue or engineered cells within dysfunctional neural circuitry. In experimental and clinical settings, this approach has provided insights into the pathology and treatment of neurologic diseases, primarily movement disorders. The present experiments were designed to investigate whether a similar strategy is feasible as a method to investigate, and perhaps repair, circuitry integral to emotional disorders. We focused on the amygdala, a macrostructure known to be involved in the expression of anxiety- and fear-related behaviors. GABAergic cell-rich suspensions were prepared from E17 rat lateral ganglionic eminence and engrafted bilaterally into the lateral and basolateral amygdaloid nuclei of young adult rats. After 6 weeks, increased numbers of GABAergic neurons were identified in the vicinity of the graft sites, and electron microscopy provided evidence for functional integration of transplanted cells. Rats with these grafts spent more time in the open arms of the elevated-plus maze, consistent with an anxioloytic-like phenotype. These rats were also less sensitive to the unconditioned anxiogenic effects of light on the acoustic startle response, although fear-potentiated startle was not affected, suggesting that the grafts produced an attenuation of unlearned fear but did not affect acquisition of conditioned fear. Our results raise the possibility that distinct components of emotion can be modulated by strategic neural engraftment.

Cell transplantation for Huntington's disease

Cell transplantation for Huntington's disease has developed over the last decade to clinical application in pilot trials in the USA, France and the UK. Although the procedures are feasible, and under appropriate conditions safe, evidence for efficacy is still limited, which has led to some calls that further development should be discontinued. We review the background of striatal cell transplantation in experimental animal models of Huntington's disease and the rationale for applying similar strategies in the human disease, and we survey the present status of the preliminary studies that have so far been undertaken in patients. When we consider the variety of parameters and principles that remain poorly defined -- such as the optimal source, age, dissection, preparation, implantation, immunoprotection and assessment protocols -- it is not surprising that clinical efficacy is still unreliable. However, since these protocols are all tractable to experimental refinement, we consider that the potential for cell transplantation in Huntington's disease is greater than has yet been realised, and remains a therapeutic strategy worthy of investigation and pursuit.

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.

Glutamatergic regulation of long-term grafts of fetal lateral ganglionic eminence in a rat model of Huntington's disease

Transplanting fetal striatal tissue is currently considered to be an important alternative strategy in the treatment of Huntington's disease. Although grafted striatal tissue differentiates and shows certain structural and neurochemical features of the normal striatum and receives host afferents, it is not clear whether host-derived afferent inputs can modulate the activity of neurotransmitter receptors and their signaling in the graft. An intricate interaction between dopaminergic and glutamatergic systems is pivotal for striatal function. In the present study, the modulation of D(2) receptors in the graft by host-derived glutamatergic afferents via NMDA receptors was investigated using haloperidol-induced c-Fos expression. The results indicate that haloperidol induces c-Fos in a large number of neurons in the P-zones of the graft and this induction is significantly suppressed by pretreatment with the NMDA receptor antagonist, MK-801. Therefore, the NMDA receptor-mediated modulation of D(2) receptor function seen in the normal striatum is established in the striatostriatal grafts.

Striatal graft-associated recovery of a lesion-induced performance deficit in the rat requires learning to use the transplant

Peformance in a prelearned choice reaction time task was studied 6 months after surgery in rats with ibotenate-induced lesions of the striatum either with or without striatal grafts, and in sham-operated controls. The long postoperative interval allowed full transplant maturation and the establishment of appropriate connections by the transplants. The animals were trained prior to surgery on a visual choice reaction time task which requires that a movement is made away from stimuli signalling reward. The use of multiple measures allowed a thorough analysis of several aspects of the animals' performance. Whilst sham-operated control animals recovered normal (preoperative) performance rapidly, the lesioned animals had a severe performance deficit. Although the transplanted animals were initially at least as deficient in performance as the lesioned group, repeated testing led to an amelioration of the lesion-induced deficit according to two distinct measures of spatial bias and reaction time. On a third measure, latency to complete the lateralized movement, the grafted group were initially worse than the lesioned group but repeated testing resulted in significant recovery. These results suggest that postoperative training may help to optimize the efficacy of graft-induced recovery, and that animals may need to learn to use a transplant in order for it to confer functional benefit in complex prelearned tasks.

Fetal striatal transplants restore electrophysiological sensitivity to dopamine in the lesioned striatum of rats with experimental Huntington's disease

Dopamine (DA), a major neurotransmitter used in the striatum, is involved in movement disorders such as Parkinson's disease and Huntington's chorea. With the loss of neurons in the striatum of patients with Huntington's disease (HD), there is an associated downregulation of DA receptors, which may alter DA-mediated responses. In the present study, DA-mediated electrophysiological depression was studied in animals with quinolinic acid (QA)-induced experimental HD. QA was directly applied to the right striatum of adult female Sprague-Dawley rats. Animals receiving QA developed ipsilateral rotation after the application of apomorphine. Fetal striatal tissue transplants grafted 1 month after lesioning attenuated apomorphine-induced rotation. Six months after lesioning, the animals were anesthetized with urethane for electrophysiological study. DA, applied directly to neurons by pressure microejection, inhibited spontaneous single-unit activity in the striatal neurons of nonlesioned, lesioned and lesioned/grafted rats. QA lesioning reduced responses to DA in the striatal neurons. The dose of DA required to inhibit striatal neuron activity in the lesioned rats was significantly increased compared to that in the nonlesioned rats. Transplantation of fetal striatal tissue restored the electrophysiological sensitivity to DA in the lesioned striatum. The dose of DA used to suppress striatal neuron activity was reduced after grafting. Immunohistostaining showed survival of gamma-aminobutyric acid neurons at the graft site. Tyrosine hydroxylase-positive terminals were found innervating the striatal grafts. In conclusion, our data demonstrate that fetal striatal transplants restore electrophysiological sensitivity to DA in the lesioned striatum of animals with experimental HD.

Fetal neural grafts for Huntington's disease: a prospective view

Intrastriatal transplantation of striatal neuroblasts from human fetuses is a promising approach for treatment of Huntington's disease, on the basis of many experimental animal studies and, most recently, pilot clinical trials. Technically, several issues remain to be resolved (e.g., the precise site of dissection of the fetal tissue; the number and location of the fetal striatal implants; or the use of immunosuppressive therapy), and await larger-scale trials and purposely designed protocols. Further clinical data must also be obtained, and preliminary promising results must be replicated in a patient group large enough to provide conclusive results. It is important to establish (1) the amount of clinical benefit provided to the patient by the grafted cells; (2) the anticipated duration of clinical benefits; and (3) the secondary rate of decline after the benefit of the graft has been overbalanced. Evaluation of these parameters will require very long-term follow-up of the patients involved, over several years after grafting, before the technique can eventually be proposed widely to patients.

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.

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.

Fetal striatal transplants reinstate the electrophysiological response of pallidal neurons to systemic apomorphine challenge in rats with excitotoxic striatal lesions

Previous studies with single-unit recording and 2-[14C]deoxyglucose quantitative autoradiography have shown that systemic administration of apomorphine increases the functional activity of pallidal neurons, and that the enhancement in the globus pallidus (GP) activity is abolished by striatal lesions. The present study employing electrophysiological techniques tested whether embryonic striatal tissue implanted in the excitotoxically damaged striatum of rats may affect the lesion-induced alteration in the neuronal response of GP to apomorphine. Systemically administered apomorphine significantly increased spontaneously firing rates of GP cells. The blockade of dopamine receptors with haloperidol reversed the increased rate to baseline levels. Quinolinate-induced striatal lesions attenuated the rate-increasing effect of apomorphine. Embryonic striatal grafts placed in the lesioned striatum restored the response of GP cells to systemic apomorphine. The graft-mediated restoration of the GP neuron response to apomorphine were accompanied by an improvement in the motor asymmetry induced by this drug. Considering previous anatomical data to demonstrate extensive innervation of the GP by embryonic striatal grafts, the present results suggest that the grafts reconstruct the functional striatopallidal pathway which is capable of transmitting apomorphine-induced changes in the neuronal activity.

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.

Embryonic striatal grafts restore neuronal activity of the globus pallidus in a rodent model of Huntington's disease

It has been demonstrated in rats that embryonic striatal grafts placed in the excitotoxically lesioned striatum establish neuronal connections with the host globus pallidus. In order to determine whether the morphologically verified connections between the grafts and host are functional, the present study investigated the effects of embryonic striatal grafts on changes in the neuronal activity of the globus pallidus in rats with quinolinic acid-induced striatal lesions. The activity of pallidal neurons was determined by use of quantitative cytochrome oxidase histochemistry and an electrophysiological technique. Striatal lesions induced an increase in both the cytochrome oxidase activity and the spontaneous firing rate of the globus pallidus ipsilateral to the lesions. Grafts derived from the lateral ganglionic eminence, but not the medial ganglionic eminence, reversed the lesion-induced increase in the cytochrome oxidase activity of the globus pallidus with concomitant reduction of apomorphine-induced rotational asymmetry. The lateral ganglionic eminence grafts also attenuate the increase in the firing rate of pallidal neurons in rats with striatal lesions. The present results provide evidence that striatal lesions lead to the loss of a tonic inhibitory input to the globus pallidus with consequent increase in the activity of pallidal neurons, and that intrastriatal striatal grafts reverse the altered activity of pallidal neurons. The findings strongly suggest that embryonic striatal grafts functionally repair the damaged striatopallidal pathway.

Restoration of circadian behavior by anterior hypothalamic grafts containing the suprachiasmatic nucleus: graft/host interconnections

Destruction of the hypothalamic suprachiasmatic nucleus (SCN) disrupts circadian behavior. Transplanting SCN tissue from fetal donors into SCN-lesioned recipients can restore circadian behavior to the arrhythmic hosts. In the transplantation model employing fetal hamster donors and SCN-lesioned hamsters as hosts, the period of the restored circadian behavior is hamster-typical. However, when fetal rat anterior hypothalamic tissue containing the SCN is implanted into SCN-lesioned rats, the period of the restored circadian rhythm is only rarely typical of that of the intact rat. The use of an anterior hypothalamic heterograft model provides new approaches to donor specificity of restored circadian behavior and with the aid of species-specific markers, provides a means for assessing connectivity between the graft and the host. Using an antibody that stains rat and mouse neuronal tissue but not hamster neurons, it has been demonstrated that rat and mouse anterior hypothalamic heterografts containing the SCN send numerous processes into the host (hamster) neuropil surrounding the graft, consistent with graft efferents reported in other hypothalamic transplantation models in which graft and host tissue can be differentiated (i.e., Brattleboro rat and hypogonadal mouse). Moreover, SCN neurons within anterior hypothalamic grafts send an appropriately restricted set of efferent projections to the host brain which may participate in the functional recovery of circadian locomotor activity.

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.

Safety of intrastriatal neurotransplantation for Huntington's disease patients

Fetal neural transplantation has been shown to be a feasible, safe, and according to a number of recent reports, effective treatment for Parkinson's disease (PD). Fetal striatal transplantation may be as feasible, safe, and effective a treatment for Huntington's disease (HD), a disorder for which there is currently no effective treatment. This report describes our experience with fetal striatal transplantation to adult striatum in three HD patients. Three moderately advanced, nondemented HD patients received transplantation of fetal striatal tissue. The striatal precursor was selectively obtained from the lateral ganglionic eminence. Each patient received bilateral grafts from five to eight donors, placed into the caudate nucleus (one graft on each side) and the putamen (four grafts on each side). All three patients had HD as documented by family history, DNA heterozygosity (17-20 and 48-51 repeats), magnetic resonance imaging (MRI) revealing striatal atrophy, and 2-deoxyglucose positron emission tomography revealing striatal hypometabolism. All patients had been evaluated using the Unified Huntington's Disease Rating Scale and appropriate neuropsychological tests for at least 3 months prior to transplantation. One year following transplantation, MRI of all three patients revealed that the grafts survived and grew within the striatum without displacing the surrounding tissue. No patients demonstrated adverse effects of the surgery or the associated cyclosporin immunosuppression, nor did any patient exhibit deterioration following the procedure. The limited experience provided by these three patients indicates that fetal tissue transplantation can be performed in HD patients without unexpected complications.

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. II. Correlation between positron emission tomography and reaching behaviour

Grafts of embryonic striatal primordia are able to elicit behavioural recovery in rats which have received an excitotoxic lesion to the striatum, and it is believed that the P zones or striatal-like tissue within the transplants play a crucial role in these functional effects. We performed this study to compare the effects of different donor stage of embryonic tissue on both the morphology (see accompanying paper) and function of striatal transplants. Both the medial and lateral ganglionic eminence was dissected from rat embryos of either 10 mm, 15 mm, 19 mm, or 23 mm crown-rump length, and implanted as a cell suspension into adult rats which had received an ibotenic acid lesion 10 days prior to transplantation. After four months the animals were tested on the "staircase task" of skilled forelimb use. At 10-14 months rats from the groups which had received grafts from 10 mm or 15 mm donor embryos were taken for positron emission tomography scanning in a small diameter positron emission tomography scanner, using ligands to the dopamine D1 and D2 receptors, [11C]SCH 23390 and [11C]raclopride, respectively. A lesion-alone group was also scanned with the same ligands for comparison. Animals which had received transplants from the 10 mm donors showed a significant recovery with their contralateral paw on the "staircase test". No other groups showed recovery on this task. Similarly, the animals with grafts from the youngest donors showed a significant increase in D1 and D2 receptor binding when compared to the lesion-alone group. No increase in signal was observed with either ligand in the group which had received grafts from 15 mm donors. Success in paw reaching showed a strong correlation to both the positron emission tomography signal obtained and the P zone volume of the grafts. These results suggest that striatal grafts from younger donors (10 mm CRL) give greater behavioural recovery than grafts prepared from older embryos. This recovery is due to both the increased proportion of striatal-like tissue within the grafts and an increase in functional D1 and D2 dopamine receptors measured by positron emission tomography, i.e. a more extensive integration of the graft 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.

Neuro-glial neurotrophic interaction in the S-100 beta retarded mutant mouse (Polydactyly Nagoya). III. Transplantation study

The hippocampus and caudo-dorsal cortex of the homozygote of polydactyly mutant mouse (Polydactyly Nagoya, Pdn/Pdn) were markedly reduced in S-100 beta positive astrocytes and serotonergic fibers as compared to the heterozygote (Pdn/+) and wild type (+/+) [39]. The Pdn/Pdn mice die within 2 days after birth, so it is impossible to examine postnatal changes. To demonstrate the developmental change of Pdn/Pdn hippocampal tissue, we transplanted hippocampal pieces of neonatal Pdn/Pdn and +/+ mice into the right and left hippocampus of the same adult +/+ mice, respectively, and immunocytochemically examined them. Two weeks after transplantation, +/+ hippocampal tissue contained a large number of glial fibrillary acidic protein (GFAP) and S-100 beta positive astrocytes and a number of serotonergic fibers. While Pdn/Pdn hippocampal tissue contained numerous GFAP positive astrocytes, S-100 beta positive astrocytes and serotonergic fibers were not observed. Two months after transplantation, GFAP and S-100 beta were expressed in the Pdn/Pdn hippocampal tissue similar to the +/+ tissue. Serotonergic fibers were distributed in the +/+ tissue, while no serotonergic fibers were observed in the Pdn/Pdn transplant tissue. In contrast, no difference was observed in the tyrosine hydroxylase positive fibers between Pdn/Pdn and +/+ grafts. The expression of 5-HT1A receptor-like immunoreactivity was higher in the +/+ tissue than that of Pdn/Pdn tissue. The present results suggest that the expression of S-100 beta in the astrocytes of early stage of transplantation is a critical for fiber ingrowth of serotonergic neurons and expressions of 5-HT1A receptor.

Influence of the postsynaptic target on the functional properties of neurons in the adult mammalian central nervous system

In this review we have attempted to summarize present knowledge concerning the regulatory role of target cells on the expression and maintenance of the neuronal phenotype during adulthood. It is well known that in early developmental stages the survival of neurons is maintained by specific neurotrophic factors derived from their target tissues. Neuronal survival is not the only phenotype that is regulated by target-derived neurotrophic factors since the expression of electrophysiological and cytochemical properties of neurons is also affected. However, a good deal of evidence indicates that the survival of neurons becomes less dependent on their targets in the adult stage. The question is to what extent are target cells still required for the maintenance of the pre-existing or programmed state of the neuron; i.e., what is the functional significance of target-derived factors during maturity? Studies addressing this question comprise a variety of neuronal systems and technical approaches and they indicate that trophic interactions, although less apparent, persist in maturity and are most easily revealed by experimental manipulation. In this respect, research has been directed to analyzing the consequences of disconnecting a group of neurons from their target-by either axotomy or selective target removal using different neurotoxins-and followed (or not) by the implant of a novel target, usually a piece of embryonic tissue. Numerous alterations have been described as taking place in neurons following axotomy, affecting their morphology, physiology and metabolism. All these neuronal properties return to normal values when regeneration is successful and reinnervation of the target is achieved. Nevertheless, most of the changes persist if reinnervation is prevented by any procedure. Although axotomy may represent, besides target disconnection, a cellular lesion, alternative approaches (e.g., blockade of either the axoplasmic transport or the conduction of action potentials) have been used yielding similar results. Moreover, in the adult mammalian central nervous system, neurotoxins have been used to eliminate a particular target selectively and to study the consequences on the intact but target-deprived presynaptic neurons. Target depletion performed by excitotoxic lesions is not followed by retrograde cell death, but targetless neurons exhibit several modifications such as reduction in soma size and in the staining intensity for neurotransmitter-synthesizing enzymes. Recently, the oculomotor system has been used as an experimental model for evaluating the functional effects of target removal on the premotor abducens internuclear neurons whose motoneuronal target is destroyed following the injection of toxic ricin into the extraocular medial rectus muscle. The functional characteristics of these abducens neurons recorded under alert conditions simultaneously with eye movements show noticeable changes after target loss, such as a general reduction in firing frequency and a loss of the discharge signals related to eye position and velocity. Nevertheless, the firing pattern of these targetless abducens internuclear neurons recovers in parallel with the establishment of synaptic contacts on a presumptive new target: the small oculomotor internuclear neurons located in proximity to the disappeared target motoneurons. The possibility that a new target may restore neuronal properties towards a normal state has been observed in other systems after axotomy and is also evident from experiments of transplantation of immature neurons into the lesioned central nervous system of adult mammals. It can be concluded that although target-derived factors may not control neuronal survival in the adult nervous system, they are required for the maintenance of the functional state of neurons, regulating numerous aspects of neuronal structure, chemistry and electro-physiology.(ABSTRUCT TRUNCATED)

Transplantation of human striatal tissue into a rodent model of Huntington's disease: phenotypic expression of transplanted neurons and host-to-graft innervation

The present study was undertaken to investigate the phenotypic expression and integration of human striatal neurons transplanted into an animal model of Huntington's disease. Sprague-Dawley rats were anesthetized and subjected to quinolinic acid lesions of the left striatum. Three human fetal cadavers were utilized for transplantation in this study (7, 8, and 10 weeks in gestation). The striatal primordia was dissected from each fetus and subsequently dissociated into cell suspensions. Following the initial lesion surgeries (3-4 months), the rats were reanesthetized and transplanted with human striatal cells (400,000 cells per rat). The animals were processed for histochemical analysis 9-17 weeks posttransplantation. Histochemistry was performed utilizing thionin (Nissl staining), acetylcholinesterase, NADPH-diaphorase, and antibodies against tyrosine hydroxylase and glial fibrillary acidic protein. Examination of stained brain sections demonstrate that human striatal transplants grow to fill a substantial portion of the remaining striatum, and contain clusters of immature and mature cells. Acetylcholinesterase activity is present in the transplant neuropil, varying in intensity, and distributed in a heterogeneous fashion. In addition, host afferent dopaminergic fibers penetrate into the transplant, and are occasionally found in patches. NADPH-diaphorase histochemistry revealed medium sized aspiny striatal neurons of donor origin in the transplants. The results of this study are similar to those obtained with rodent fetal striatal transplants, and suggest that human striatal tissue is capable of surviving, expressing normal striatal cell phenotypes, and receiving host dopaminergic innervation.

The pupillary light response: assessment of function mediated by intracranial retinal transplants

We have adapted a pupillometry measurement system to test the functional efficacy of retinae previously transplanted over the midbrain of neonatal rats in mediating a pupillary light reflex in the host eye. This has permitted us to examine factors influencing various parameters of the response, and to study transplant-mediated responses in comparison with responses mediated by way of the normal consensual pathway. Despite the unusual location of these transplanted retinae and the absence of supportive tissues normally associated with retinae in situ, it is clear that pupilloconstriction in the host eye can be elicited by transplant illumination. Under the optimal conditions here defined, response parameters for individual animals were stable with repeated testing over extended periods. When considered as individual cases, response amplitude, constriction rate and response latency were intensity dependent, although responses elicited by transplant illumination were less sensitive than normal, typically by 2-3 log units. Large-amplitude transplant-mediated pupillary responses could, however, be elicited repeatedly throughout long trains of stimuli, unlike normal responses, which rapidly failed to recover to baseline under similar test conditions. Finally, even though some cellular elements of the visual cycle are absent in transplanted retinae, pupilloconstriction in the host eye could be elicited repeatedly by transplant illumination as long as two years after transplantation took place. These observations indicate the applicability of this preparation as an assay for the effects of experimental manipulations on information processing and response plasticity in the visual system, and as a tool for examining, in general, the necessary conditions for optimal function of grafts that work by synthesizing and relaying neural signals.

Intrathalamic striatal grafts survive and affect circling behaviour in adult rats with excitotoxically lesioned striatum

Current models of basal ganglia disorders suggest that choreoathetosis is the end result of reduced GABAergic inhibition of the motor thalamus. Graft-derived release of GABA from intrastriatal striatal grafts has also been reported. In the present work, cell suspension grafts from embryonic day 14-15 rat striatal primordia were implanted close to the ventromedial thalamic nucleus to investigate whether they can develop and survive in this ectopic location, and whether they induce changes in the circling behaviour of the host. The grafts were implanted either in normal rats or in rats whose striatum had been lesioned with ibotenic acid. These grafts were implanted either ipsilateral or contralateral to the lesioned striatum. Additionally, some rats received intrastriatal grafts, and lesioned but non-grafted rats and lesioned rats that had received injections of saline or of cell suspensions from fetal spinal cord in the thalamus were used as control. Four to eight months after transplantation, circling behaviour after amphetamine or apomorphine injection was evaluated. Serial sections were stained with Cresyl Violet and studied immunohistochemically with antibodies against DARPP-32 (dopamine- and adenosine 3',5'-monophosphate-regulated phosphoprotein, as striatal marker), Fos protein, glutamate decarboxylase (67,000 mol. wt), glutamate decarboxylase (65,000 mol. wt) and GABA. Cresyl Violet sections showed that the intrathalamic striatal grafts developed into tissue masses resembling those observed in intrastriatal striatal grafts. DARPP-32 immunohistochemistry revealed that the grafts were composed of DARPP-32 immunoreactive (striatum-like) and DARPP-32-negative patches. The intrathalamic grafts of rats which had received a low dose of apomorphine (0.25 mg/kg) 2 h before perfusion showed clusters of intensely Fos-immunoreactive nuclei throughout the transplant, indicating that these cells had developed dopamine receptors and supersensitivity to dopamine agonists. Double Fos and DARPP-32 immunohistochemistry revealed that the Fos-positive nuclei were located in the striatum-like areas. Finally, the intrathalamic grafts also contained neurons immunoreactive to GABA and glutamate decarboxylase (65,000 and 67,000 mol. wt). Rats that had received intrathalamic grafts contralateral to the lesioned striatum (i.e. contralateral to the lesion-induced turning direction) showed a significant reduction of circling both after amphetamine (78% reduction) or apomorphine (77% reduction) injection. Rats that had received grafts ipsilateral to the lesioned striatum showed a 75% decrease in amphetamine-induced circling, but no significant change in apomorphine-induced circling. No significant drug-induced circling was observed in non-lesioned and grafted rats. Sham grafting (saline) or grafting of weakly GABAergic tissue (fetal spinal cord) had no significant effects on lesion-induced circling behaviour.(ABSTRACT TRUNCATED AT 400 WORDS)

Rationale for intrastriatal grafting of striatal neuroblasts in patients with Huntington's disease

Huntington's disease is a genetic disease, autosomal and dominant, that induces motor disorders, an inexorable deterioration of higher brain functions and psychiatric disturbances. At present, there are no known therapeutics against Huntington's disease. The Network of European CNS Transplantation and Restoration (NECTAR) has begun a program aimed at defining the conditions under which intrastriatal transplantation of fetal striatal cells could be attempted as an experimental treatment for Huntington's disease. This review presents the reasons why our group is considering participating in these trials. The validity of this therapeutic approach is supported by three main series of data: (i) neuropathological, clinical and imaging data indicate that Huntington's disease is, above all, a localized affection of a specific neuronal population ("medium-spiny" neurons) in the striatum; (ii) a large body of experimental results, obtained in rats and non-human primates, demonstrates that transplanted fetal striatal cells are able to integrate the host brain and to substitute for previously lesioned host striatal neurons; (iii) expertise in clinical neural transplantation has now been acquired from the treatment of patients with Parkinson's disease. These different sets of data are presented and discussed in this review. There are a number of problems which do not yet appear to be entirely resolved, nor are they likely to be using the experimental models currently available. These problems are identified and explicitly presented as working hypotheses. (1) Anatomo-functional results obtained in rodents and non-human primates with excitotoxic striatal lesions can serve as a basis for the extrapolation of what can be obtained from patients with Huntington's disease. (2). Huntington's disease can be efficiently fought by substituting degenerated striatal neurons alone. (3) Huntington's disease is due to a genetic defect which either hits the neurons that carry it directly or hits them indirectly only after several decades. Transplanted neurons, because they do not carry the gene or because they are of fetal origin, will not be rapidly affected by the ongoing disease process. Given the current state of knowledge, intracerebral transplantation appears to be the most serious opportunity (if not the only one that has been experimentally validated) for clinical improvement to be obtained in patients with Huntington's disease. The purpose of this review is to open a scientific discussion on its experimental bases before actual clinical trials start.

Functional repair of striatal systems by neural transplants: evidence for circuit reconstruction

Intrastriatal grafts of nigral and adrenal tissues have been found to be effective in alleviating many of the simple motor and sensorimotor deficits associated with lesions of the nigrostriatal dopamine system. However, the mechanisms by which such grafts exert their effects may be less specific than originally conceived, and both pharmacological and trophic actions play an essential role. Damage to intrinsic cortico-striatal circuits are unlikely to prove similarly amenable to such diffuse mechanisms of repair. Nevertheless, striatal grafts have been found to alleviate cognitive and motor deficits after excitotoxic lesions of the neostriatum. Accumulating evidence suggests that in this particular case many aspects of functional recovery may indeed be attributable to the striatal grafts providing an effective functional reconstruction of damaged neuronal circuits within the host brain.

Neurotransmitter-related gene expression in intrastriatal striatal transplants--II. Characterization of efferent projecting graft neurons

The phenotypic characteristics of identified graft neurons in intrastriatal striatal transplants which give rise to efferent projections innervating the host brain were examined using a combination of in situ hybridization histochemistry and fluorescent retrograde tracing. Cell suspension grafts of embryonic day 14-15 rat striatal primordia (including both the medial and lateral ganglionic eminences) were implanted into the previously excitotoxically lesioned striatum of adult rats, and after longer than one year the retrograde tracer Fluoro-Gold was injected bilaterally into either the globus pallidus or the substantia nigra. Injections into the globus pallidus resulted in significant retrograde labelling of graft neurons within most of the experimental animals, whereas very few graft cells were labelled after the nigral injections. The vast majority of the neurons retrogradely labelled from the globus pallidus occurred in clusters or patches in the caudal half of the transplants, which corresponded well with DARPP-32 messenger RNA expressing (i.e. striatal) regions of the grafts. Indeed, within these Fluoro-Gold-labelled graft patches, the proportion of retrogradely labelled cells found to contain DARPP-32 messenger RNA was identical to that observed in the intact striatum after similar pallidal injections (93%). In addition, some Fluoro-Gold-labelled cells were found scattered outside the DARPP-32-positive cell clusters; these cells were overall larger and rarely (c. 9%) DARPP-32 messenger RNA-positive. Messenger RNA encoding for glutamate decarboxylase (which was found in 95% of Fluoro-Gold-labelled neurons in the intact striatum) was detected in almost all retrogradely labelled graft neurons located in both the DARPP-32-positive patches of retrograde labelling (93%) and in the DARPP-32-negative regions (82%). In the intact striatum, neurons labelled after pallidal injections of Fluoro-Gold were observed to express preproenkephalin messenger RNA to a greater extent than preprotachykinin messenger RNA (81% vs 21%). Conversely, within the grafts, retrogradely labelled neurons in the patches of Fluoro-Gold-labelled cells were more often found to contain preprotachykinin messenger RNA (50%) than preproenkephalin messenger RNA (21%). The Fluoro-Gold-labelled cells scattered outside the patches of retrograde labelling rarely expressed either preproenkephalin or preprotachykinin messenger RNA. Fluoro-Gold injections into the host substantia nigra resulted in very few retrogradely labelled graft neurons; however, many (85%) of these cells were observed to express glutamate decarboxylase messenger RNA, while only rarely were they observed to contain either DARPP-32, preproenkephalin or preprotachykinin messenger RNAs (c. 10%).(ABSTRACT TRUNCATED AT 250 WORDS)

The lateral ganglionic eminence is the origin of cells committed to striatal phenotypes: neural transplantation and developmental evidence

In order to determine whether the lateral ganglionic eminence (LGE) of the fetal telencephalon is the primary source of striatal precursors in striatal transplants and tissue cultures, cells derived exclusively from the LGE of fetal rat brains were transplanted into the quinolinic-acid-lesioned striatum of adult rats. After 2-3 months they produced grafts that were almost entirely AChE-positive as well as DARPP-32-, TH-, and calbindin-immunoreactive. The grafts were integrated into the host striatum so that host corticofugal fiber tracts interdigitated with graft tissues similar to the way they penetrate the gray matter of the normal striatum. Fast Blue dye injected into the ipsilateral globus pallidus of LGE grafted produced retrogradely labeled neurons within the grafts, but Fluorogold dye injected into the ipsilateral substantia nigra did not. In a separate experiment using DARPP-32-immunohistochemstry as a striatal marker, fetal (E16) and neonatal (P2) rat brains showed DARPP-32 immunoreactivity in the LGE but not in the adjacent medial ganglionic eminence (MGE). In summary, both fetal LGE cells and LGE grafts express specific striatal markers, and LGE grafts integrate into the host striatum and innervate the major striatal efferent target within the host brain. These data suggest that the LGE is the origin of cells committed to striatal phenotypes in the developing brain.

A short term analysis of the behaviour of conditionally immortalized neuronal progenitors and primary neuroepithelial cells implanted into the fetal rat brain

Conditionally immortalized (temperature-sensitive) striatal-derived neuronal progenitor cell lines and primary neuroepithelial cells were transplanted into the CNS of gestational day 15-16 rat fetuses using an 'in utero' surgical procedure. Each fetus received 2.5-3 x 10(4) donor cells previously labelled in vitro by incubation with 5-bromo-2'-deoxyuridine (BrdU). At 5 days following transplantation, 69% of the fetuses were still alive. Engrafted cells were detected by BrdU immunohistochemistry, and the appearance of the engrafted cells and the time course of Nestin and PCNA expression were measured at 6, 24, 64 h and 5 days after transplantation. The evolution of Large T-Antigen immunoreactivity in engrafted temperature-sensitive (ts) cells was also evaluated at the above time intervals. The results indicate that the majority of the implanted cells were aggregated into clusters 24 h after transplantation. These clusters were not visible at 6 h, when most of the cells were isolated. The clusters were located in both the ventricles and parenchyma. These findings were common to both ts cells and striatal primary neuroepithelial cells. At 64 h and 5 days, isolated cells associated with the germinal layer and scattered throughout the parenchyma were also found. In the clusters, Nestin expression decreased proportionally with time following transplantation. Furthermore, Large T-Antigen immunoreactivity disappeared from ts cells between 6 and 24 h after transplantation. Finally, measurements of the temporal evolution of PCNA expression within the clusters indicate a progressive reduction in the mitotic activity of the transplanted cells. The results demonstrate that striatal primary neuroepithelial cells and conditionally immortalized neuronal progenitors can survive, migrate and/or compartimentalize into clusters whilst changing their antigenic properties and ability to proliferate.

Cortical stimulation induces fos expression in intrastriatal striatal grafts

Innervation of intrastriatal grafts of fetal striatal tissue by host corticostriatal projections has been shown in a number of previous studies in rats. In the work reported here, induction of Fos protein in grafted striatal neurons by electrical stimulation of the host frontoparietal cortex has been used as cell-level marker of corticostriatal postsynaptic responses within the striatal grafts. Unilateral cortical stimulation 30 min before sacrifice led to bilateral widespread and intense Fos induction throughout the normal striatum, although the response was somewhat more intense ipsilaterally and in the dorsolateral rostral striatum. In adult rats whose striatum had been lesioned with ibotenic acid 10-12 days prior to implantation of fetal striatal tissue, 3- and 18-month-old striatal grafts showed Fos immunoreactivity in a considerable number of cells after either bilateral, or ipsilateral (approximately 30-40% of the density of Fos-immunoreactive cells in the normal striatum) or contralateral cortical stimulation. Double-Fos and -DARPP-32 immunohistochemistry revealed that the Fos-immunoreactive nuclei were concentrated in the DARPP-32-positive (i.e. striatum-like) patches, which contained approximately 60% of the density of Fos-positive nuclei in the normal striatum after either ipsilateral or bilateral stimulation. However, Fos-immunoreactive nuclei were unevenly distributed within the DARPP-32-positive compartment of the graft, with some clusters of Fos-immunoreactive nuclei at 2-3 x the density observed in the normal striatum and other areas with Fos-immunoreactive nuclei present at lower density or absent. Fos induction was also observed in 4-week-old grafts, indicating that functional corticostriatal synaptic contacts develop rapidly. Striatal grafts implanted either in non-lesioned host striatum or in long-term (18 months) lesioned striatum, similarly showed Fos-positive nuclei after cortical stimulation, indicating that host corticostriatal fibers are equally capable of establishing functional synaptic contacts under these conditions. These results indicate that host corticostriatal fibres not only form an axonal network within the graft but also induce postsynaptic responses which may contribute to the observed graft-induced amelioration of lesion-derived behavioural deficits.

A primate model of Huntington's disease: functional neural transplantation and CT-guided stereotactic procedures

In this article, we show that 1) computed tomographic (CT)-guided stereotactic infusion of an excitotoxin into the striatum of a nonhuman primate provides a useful neuropathologic and behavioral model for Huntington's disease. 2) High-resolution positron emission tomography (PET) can be used to image the decreased glucose utilization and the preservation of dopaminergic terminals in the lesioned striatum by using 2-fluoro-deoxy-D-glucose (2FDG) and N-(C-11)-methyl-2-beta-carbomethoxy-3-beta-phenyl tropane (CPT) as tracers. 3) Transplantation of cross-species striatal fetal tissue into the lesioned caudate-putamen reduces many of the abnormal motor movements and behavioral changes seen in the Huntington's disease primate model. 4) Graft rejection results in the return of the abnormal signs of the pregrafted state. These results indicate that treatment of the neuronal deficit in Huntington's disease can involve intervention at the local neuronal circuit level. CT-guided stereotactic implantation of cells that might protect or replace this defective circuitry may eventually provide an effective treatment for Huntington's disease.