Intrinsic Organization and Connectivity of Intrastriatal Striatal Transplants in Rats as Revealed by DARPP-32 Immunohistochemistry: Specificity of Connections with the Lesioned Host Brain

hESC-derived striatal progenitors grafted into a Huntington's disease rat model support long-term functional motor recovery by differentiating, self-organizing and connecting into the lesioned striatum

BACKGROUND

Huntington's disease (HD) is a motor and cognitive neurodegenerative disorder due to prominent loss of striatal medium spiny neurons (MSNs). Cell replacement using human embryonic stem cells (hESCs) derivatives may offer new therapeutic opportunities to replace degenerated neurons and repair damaged circuits.

METHODS

With the aim to develop effective cell replacement for HD, we assessed the long-term therapeutic value of hESC-derived striatal progenitors by grafting the cells into the striatum of a preclinical model of HD [i.e., adult immunodeficient rats in which the striatum was lesioned by monolateral injection of quinolinic acid (QA)]. We examined the survival, maturation, self-organization and integration of the graft as well as its impact on lesion-dependent motor alterations up to 6 months post-graft. Moreover, we tested whether exposing a cohort of QA-lesioned animals to environmental enrichment (EE) could improve graft integration and function.

RESULTS

Human striatal progenitors survived up to 6 months after transplantation and showed morphological and neurochemical features typical of human MSNs. Donor-derived interneurons were also detected. Grafts wired in both local and long-range striatal circuits, formed domains suggestive of distinct ganglionic eminence territories and displayed emerging striosome features. Moreover, over time grafts improved complex motor performances affected by QA. EE selectively increased cell differentiation into MSN phenotype and promoted host-to-graft connectivity. However, when combined to the graft, the EE paradigm used in this study was insufficient to produce an additive effect on task execution.

CONCLUSIONS

The data support the long-term therapeutic potential of ESC-derived human striatal progenitor grafts for the replacement of degenerated striatal neurons in HD and suggest that EE can effectively accelerate the maturation and promote the integration of human striatal cells.

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.

Deconstructing Neurogenesis, Transplantation and Genome-Editing as Neural Repair Strategies in Brain Disease

Neural repair in injury and disease presents a pressing unmet need in regenerative medicine. Due to the intrinsically reduced ability of the brain to replace lost and damaged neurons, reversing long-term cognitive and functional impairments poses a unique problem. Over the years, advancements in cellular and molecular understanding of neurogenesis mechanisms coupled with sophistication of biotechnology tools have transformed neural repair into a cross-disciplinary field that integrates discoveries from developmental neurobiology, transplantation and tissue engineering to design disease- and patient-specific remedies aimed at boosting either native rehabilitation or delivering exogenous hypoimmunogenic interventions. Advances in deciphering the blueprint of neural ontogenesis and annotation of the human genome has led to the development of targeted therapeutic opportunities that have the potential of treating the most vulnerable patient populations and whose findings from benchside suggest looming clinical translation. This review discusses how findings from studies of adult neurogenesis have informed development of interventions that target endogenous neural regenerative machineries and how advances in biotechnology, including the use of new gene-editing tools, have made possible the development of promising, complex neural transplant-based strategies. Adopting a multi-pronged strategy that is tailored to underlying neural pathology and that encompasses facilitation of endogenous regeneration, correction of patient’s genomic mutations and delivery of transformed neural precursors and mature disease-relevant neuronal populations to replace injured or lost neural tissue remains no longer a fantasy.

Parvalbumin-producing striatal interneurons receive excitatory inputs onto proximal dendrites from the motor thalamus in male mice

In rodents, the dorsolateral striatum regulates voluntary movement by integrating excitatory inputs from the motor-related cerebral cortex and thalamus to produce contingent inhibitory output to other basal ganglia nuclei. Striatal parvalbumin (PV)-producing interneurons receiving this excitatory input then inhibit medium spiny neurons (MSNs) and modify their outputs. To understand basal ganglia function in motor control, it is important to reveal the precise synaptic organization of motor-related cortical and thalamic inputs to striatal PV interneurons. To examine which domains of the PV neurons receive these excitatory inputs, we used male bacterial artificial chromosome transgenic mice expressing somatodendritic membrane-targeted green fluorescent protein in PV neurons. An anterograde tracing study with the adeno-associated virus vector combined with immunodetection of pre- and postsynaptic markers visualized the distribution of the excitatory appositions on PV dendrites. Statistical analysis revealed that the density of thalamostriatal appositions along the dendrites was significantly higher on the proximal than distal dendrites. In contrast, there was no positional preference in the density of appositions from axons of the dorsofrontal cortex. Population observations of thalamostriatal and corticostriatal appositions by immunohistochemistry for pathway-specific vesicular glutamate transporters confirmed that thalamic inputs preferentially, and cortical ones less preferentially, made apposition on proximal dendrites of PV neurons. This axodendritic organization suggests that PV neurons produce fast and reliable inhibition of MSNs in response to thalamic inputs and process excitatory inputs from motor cortices locally and plastically, possibly together with other GABAergic and dopaminergic dendritic inputs, to modulate MSN inhibition.

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.

Effects of Hibernation or Cryopreservation on the Survival and Integration of Striatal Grafts Placed in the Ibotenate-Lesioned Rat Caudate-Putamen

Tissue storage prior to intracerebral transplantation would represent a major advantage when conducting clinical transplantation trials in that the procurement of the embryonic donor tissue and the timing of neurosurgery could be planned more efficiently. In the present study, the effects of storing rat embryonic striatal tissue at either +4°C or below freezing temperature prior to grafting to the adult striatum, were assessed with regard to transplant survival, morphology and integration. Eleven days following a unilateral injection of ibotenic acid into the head of the caudate-putamen, a control group of rats received grafts of striatal primordium prepared immediately after dissection from rat embryos (embryonic day 16). A second group of rat embryonic striatal tissue was stored at 4°C (hibernation) for 5 days and then transplanted. A third group of the striatal donor tissue was cryopreserved in liquid nitrogen for 5 days before implantation surgery. Six to seven weeks following transplantation surgery, the grafts were analysed in brain sections processed for acetylcholinesterase histochemistry, DARPP-32 (dopamine and cyclic AMP regulated phosphoprotein with a molecular weight of 32 kDa) and tyrosine hydroxylase (TH) immunocytochemistry. The mean total graft volume and the relative size of the AChE-positive regions were not significantly different between the three groups. Striatal-specific graft regions, positively stained for AChE and DARPP-32, generally exhibited TH immunoreactivity, suggesting that they had received dopaminergic afferents from the host brain. We conclude that embryonic rat striatal tissue can be cryopreserved or hibernated over 5 days without significant impairment in the yield of striatal neurons following intrastriatal implantation and without markedly affecting transplant morphology.

Neural Transplantation for Huntington's Disease: Experimental Rationale and Recommendations for Clinical Trials

Huntington's disease (HD) is a neurodegenerative disorder affecting motor function, personality, and cognition. This paper reviews the experimental data that demonstrate the potential for transplantation of fetal striatum and trophic factor secreting cells to serve as innovative treatment strategies for HD. Transplantation strategies have been effective in replacing lost neurons or preventing the degeneration of neurons destined to die in both rodent and nonhuman primate models of HD. In this regard, a logical series of investigations has proven that grafts of fetal striatum survive, reinnervate the host, and restore function impaired following excitotoxic lesions of the striatum. Furthermore, transplants of cells genetically modified to secrete trophic factors such as nerve growth factor protect striatal neurons from degeneration due to excitotoxicity or mitochondrial dysfunction. Given the disabling and progressive nature of HD, coupled with the absence of any meaningful medical therapy, it is reasonable to consider clinical trials of neural transplantation for this disease. Fetal striatal implants will most likely be the first transplant strategy attempted for HD. This paper describes the variable parameters we believe to be critical for consideration for the design of clinical trials using fetal striatal implants for the treatment of HD.

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.

Update on Huntington's disease: advances in care and emerging therapeutic options

INTRODUCTION

Huntington's disease (HD) is the most common hereditary neurodegenerative disorder. Despite the fact that both the gene and the mutation causing this monogenetic disorder were identified more than 20 years ago, disease-modifying therapies for HD have not yet been established.

REVIEW

While intense preclinical research and large cohort studies in HD have laid foundations for tangible improvements in understanding HD and caring for HD patients, identifying targets for therapeutic interventions and developing novel therapeutic modalities (new chemical entities and advanced therapies using DNA and RNA molecules as therapeutic agents) continues to be an ongoing process. The authors review recent achievements in HD research and focus on approaches towards disease-modifying therapies, ranging from huntingtin-lowering strategies to improving huntingtin clearance that may be promoted by posttranslational HTT modifications.

CONCLUSION

The nature and number of upcoming clinical studies/trials in HD is a reason for hope for HD patients and their families.

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?

Human induced pluripotent stem cells improve stroke outcome and reduce secondary degeneration in the recipient brain

Human induced pluripotent stem cells (hiPSCs) are a most appealing source for cell replacement therapy in acute brain lesions. We evaluated the potential of hiPSC therapy in stroke by transplanting hiPSC-derived neural progenitor cells (NPCs) into the postischemic striatum. Grafts received host tyrosine hydroxylase-positive afferents and contained developing interneurons and homotopic GABAergic medium spiny neurons that, with time, sent axons to the host substantia nigra. Grafting reversed stroke-induced somatosensory and motor deficits. Grafting also protected the host substantia nigra from the atrophy that follows disruption of reciprocal striatonigral connections. Graft innervation by tyrosine hydoxylase fibers, substantia nigra protection, and somatosensory functional recovery were early events, temporally dissociated from the slow maturation of GABAergic neurons in the grafts and innervation of substantia nigra. This suggests that grafted hiPSC-NPCs initially exert trophic effects on host brain structures, which precede integration and potential pathway reconstruction. We believe that transplantation of NPCs derived from hiPSCs can provide useful interventions to limit the functional consequences of stroke through both neuroprotective effects and reconstruction of impaired pathways.

Intracerebral transplantation for neurological disorders. Lessons from developmental, experimental, and clinical studies

The use of human pluripotent stem cells (PSCs) for cell therapy faces a number of challenges that are progressively answered by results from clinical trials and experimental research. Among these is the control of differentiation before transplantation and the prediction of cell fate after administration into the human brain, two aspects that condition both the safety and efficacy of the approach. For neurological disorders, this includes two steps: firstly, the identification of the optimal maturation stage for transplantation along the continuum that transforms PSCs into fully differentiated neural cell types, together with the derivation of robust protocols for large-scale production of biological products, and, secondly, the understanding of the effects of environmental cues and their possible interference with transplanted cells commitment. This review will firstly summarize our knowledge on developmental processes that have been applied to achieve robust in vitro differentiation of PSCs into neural progenitors. In a second part, we summarize results from experimental and clinical transplantation studies that help understanding the dialogue that establishes between transplanted cells and their host brain.

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.

Cellular therapy and induced neuronal replacement for Huntington's disease

Huntington's disease (HD) is an inherited, relentlessly progressive neurodegenerative disease with an invariably fatal outcome. HD is inherited in an autosomal dominant fashion, and is characterized pathologically by the loss of cortical and striatal neurons, and clinically by involuntary choreiform movements accompanied by progressive cognitive impairment and emotional lability. The disorder is caused by an expanded cystosine adenine guanine (CAG) tri-nucleotide repeat encoding polyglutamine (polyQ) in the first exon of the Huntingtin gene. There is a correlation between the number of CAG repeats and disease onset, such that in patients with CAG repeat lengths of 36 to 60, disease symptoms typically manifest after 35 years of age, whereas CAG repeat lengths >60 yield the more severe juvenile form of the disease. Even though mutant huntingtin is expressed throughout the brain, it is characterized by the selective degeneration of medium spiny neurons of the caudate and putamen, which heralds more widespread neuronal degeneration with disease progression. The mechanisms of cell dysfunction and death in HD have been the subjects of a number of studies, which have led to therapeutic strategies largely based on the amelioration of mutant huntingtin-related metabolic impairment and cellular toxicity. Each of these approaches has aimed to delay or stop the preferential degeneration of medium spiny neurons early in the disease course. Yet, in later stages of the disease, after cell death has become prominent, cell replacement therapy (whether by direct cell transplantation or by the mobilization of endogenous progenitors) may comprise a stronger potential avenue for therapy. In this review, we will consider recent progress in the transplantation of fetal striatal cells to the HD brain, as well as emerging alternative sources for human striatal progenitor cells. We will then consider the potential application of gene therapy toward the induction of striatal neurogenesis and neuronal recruitment, with an eye toward its potential therapeutic use in HD.

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.

The corridor task: Striatal lesion effects and graft-mediated recovery in a model of Huntington's disease

Experimental validation of cell replacement therapy as a treatment of neurodegenerative diseases requires the demonstration of graft-mediated behavioural recovery. The Corridor task proved to be simple and efficient to conduct with a robust ipsilateral retrieval bias in our rodent Huntington's disease model. The Corridor task is a viable behavioural option, particularly to non-specialised laboratories, for the evaluation of lateralised striatal damage and the probing of alternative therapeutic strategies, including transplantation.

Morphological and cellular changes within embryonic striatal grafts associated with enriched environment and involuntary exercise

Environmental enrichment (EE) and exercise have been implicated in influencing behaviour and altering neuronal processes associated with cellular morphology in both 'normal' and injured states of the CNS. Using a rodent model of Huntington's disease, we investigated whether prolonged EE or involuntary exercise can induce morphological and cellular changes within embryonic striatal transplants. Adult rats were trained on the Staircase test--requiring fine motor control to reach and collect reward pellets--prior to being lesioned unilaterally in the dorsal neostriatum with quinolinic acid. The lesioned animals received E15 whole ganglionic eminence cell suspension grafts followed by housing in EE or standard cages. Half of the animals in standard cages received daily forced exercise on a treadmill. The grafted animals showed significant functional recovery on both the Staircase test and in drug-induced rotation. Neither the housing conditions nor the training had an impact on the behaviour, with the exception of the treadmill reducing the ipsilateral drug-induced rotation observed amongst the lesioned animals. However, the animals housed in the EE had significantly increased striatal brain-derived neurotrophic factor (BDNF) levels, and graft neurons in these animals exhibited both greater spine densities and larger cell volumes. Animals on forced exercise regime had reduced BDNF levels and grafted cells with sparser spines. The study suggests that the context of the animal can affect the plasticity of transplanted cells. Appropriately exploiting the underlying, and yet unknown, mechanisms could lead the way to improved anatomical and potentially functional integration of the graft.

The effects of lateralized training on spontaneous forelimb preference, lesion deficits, and graft-mediated functional recovery after unilateral striatal lesions in rats

The ability of striatal embryonic grafts to promote functional recovery on complex behavioral tasks depends on various factors, including the amount of striatal-like tissue within the grafts and the duration of post-graft training. However, how the innate paw bias of animals is affected by experience, or influences recovery following injury, is less known. Here, we have examined the effects of intrinsic side bias and lateralized limb use training on spontaneous forelimb preference and graft-mediated functional recovery in a skilled reaching task in a rodent model of Huntington's disease. Naïve rats were assessed on their baseline paw preferences when reaching between the bars of their cage to retrieve sugar pellets from a tray attached outside. Next, rats were lesioned unilaterally in the lateral dorsal striatum with quinolinic acid, and 7-10 days later, half of the animals were given suspension grafts prepared from E15 whole ganglionic eminence implanted into the lesioned striatum. The animals then received extensive unilateral training, either ipsi- or contralateral to the side of the lesion and graft in separate subgroups, on the 'staircase' task until asymptotic performance was obtained. As reported previously, the grafts alleviated lesion-induced deficits in retrieving pellets from the contralateral staircase. Spontaneous biases were then reassessed in the cage-reaching task. Irrespective of whether the animal received ipsilateral or contralateral staircase training, the unilateral lesions induced a significant shift in spontaneous bias towards the ipsilateral paw. Grafted animals showed a similar shift in bias if staircase training was given to the ipsilateral paw but showed no change in spontaneous bias (similar to controls) if they had received contralateral training during the post-transplantation period. The results suggest that striatal grafts can alleviate lesion-induced changes in their spontaneous side preferences, but only if they receive extensive training in the use of the contralateral limb, compatible with the notion that recovery is use-dependent.

Treatment of neurodegenerative diseases with neural cell transplantation

Neural cell transplantation is an emerging therapy that may provide an effective treatment for neurodegenerative disorders. The most extensive work with neural transplants has been carried out for Parkinson's and Huntington's diseases. However, intensive efforts are also being made for the treatment of other neurological indications, such as spinal cord repair, stroke, epilepsy, multiple sclerosis (MS), Alzheimer's disease and amyotrophic lateral sclerosis (ALS or Lou Gehrig's disease), to name just a few. The major barrier for the successful application of cells as therapeutics is achieving long-term survival and function. The CNS has proven to be ideal for transplantation, in part because immune rejection is attenuated in the CNS compared to peripheral locations. However, some form of immunosuppression is desirable for optimal allograft survival and required for xenograft survival. This review will focus on the challenges of restoring function to something as intricate as the CNS and on the limitations imposed by this complexity on any cellular therapeutic.

Training specificity, graft development and graft-mediated functional recovery in a rodent model of Huntington’s disease

Neuronal function and morphology are affected by the environment and the behavioral experience. Here we report on the effects of differential training protocols on the development and the functional recovery mediated by intrastriatal striatal grafts. Rats were trained exclusively on the left or the right paw to perform on the skilled staircase task before being lesioned unilaterally in the dorsal striatum with quinolinic acid. E15 whole ganglionic eminence suspension grafts were implanted into the lesioned striatum. Subsequent testing probed unilateral performance of the affected contralateral paw, as well as bilateral performance. The grafted animals were initially as impaired as the lesioned, but partially recovered their performance with additional training. Grafted animals with appropriate previous experience initially performed better on the staircase test, but the advantage was transient. Furthermore, the grafted animals performed better with their affected paw under forced choice than under conditions when both paws were simultaneously probed. Improvements of the grafted animals were also observed on tests of forelimb akinesia and asymmetry. Morphological data suggest that the training conditions influenced the development specifically of striatal-like, but not of non-striatal like, neurones within the grafts. The grafts were smaller containing less striatal-like neurones in animals that were trained on the contralateral side prior to lesioning and grafting. The results support the hypothesis that unilateral training sensitizes the striatum that subserves the motor learning, leading to exacerbated excitotoxic lesions and to an environment less conducive for graft development.

Environmental enrichment affects striatal graft morphology and functional recovery

Environmental conditions and behavioural experience can affect neuronal function and morphology. It is less well known whether such factors also influence the growth, integration and functional recovery provided by neural grafts placed within the damaged brain. Here we report on the effects of differential housing conditions on striatal graft morphology and functional recovery after striatal lesions. Rats were pretrained on a skilled bilateral forelimb task, the staircase test, and lesioned unilaterally in the lateral dorsal striatum with quinolinic acid. One group of lesioned animals was given suspension grafts of E15 whole ganglionic eminence implanted into the lesioned striatum. Following transplantation, the animals were housed either in standard cages (four per cage) or in enriched environment housing conditions (10 per cage) with tunnels, ladders and increased living space available for exploration, social interaction and play. The differentially housed animals were retested on the skilled staircase test at two separate time points. Repeated testing, environmental enrichment and transplantation positively influenced behavioural recovery. Partial recovery was observed bilaterally amongst the grafted animals in both housing conditions. Nevertheless, the grafted animals housed in the enriched environment performed significantly better in the final test compared with all of the other experimental groups. The grafts survived equally well under both housing conditions but the grafts of animals housed in the enriched environment contained larger projection neurons and were somewhat better reinnervated by dopaminergic afferents. An increased level of striatal brain-derived neurotrophic factor was observed in the control animals housed under the enriched compared with the standard conditions. The results indicate that an enriched environment can affect both graft function and graft morphology through as yet unknown mechanisms.

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.

Regulation of neuropeptide mRNA expression in the basal ganglia by intrastriatal and intranigral transplants in the rat Parkinson model

Previous studies have shown that intrastriatal transplants of dopamine (DA)-rich fetal ventral mesencephalic (VM) tissue can correct denervation-induced changes in the cellular expression of neuropeptide and receptor mRNAs in the rat Parkinson model. However, with the standard transplantation approach normalization of all cellular parameters has not been obtained. This may be due either to the incomplete striatal reinnervation achieved by these transplants, or to the ectopic placement of the grafts. In the present study we have used a microtransplantation approach to obtain a more complete reinnervation of the denervated striatum (20 micrograft deposits spread over the entire structure). Neurons were also implanted directly into the substantia nigra. In rats with multiple intrastriatal VM transplants the lesion-induced upregulation of mRNAs encoding for preproenkephalin (PPE), the D(2)-type DA-receptor, and the GABA-synthesizing enzyme glutamic acid decarboxylase (GAD(67)) was normalized throughout the striatum, whereas the lesion-induced downregulation of preprotachykinin mRNA was unaffected. Intranigral grafts of either fetal DA-rich VM tissue or GABA-rich striatal tissue did not induce any changes in striatal neuropeptide and D(2)-receptor mRNA expression despite significant behavioral improvement. Comparison of the behavioral data with levels of neuropeptide expression showed that in rats with intrastriatal VM transplants a complete normalization of striatal PPE and GAD(67) mRNA expression did not translate into a complete recovery of spontaneous motor behaviors. The results show that extensive DA reinnervation of the host striatum by multiple VM microtransplants is insufficient to obtain full recovery of all lesion-induced changes at both the cellular and the behavioral level. A full reconstruction of the nigrostriatal pathway or, alternatively, modulation of basal ganglia function by grafting in non-striatal regions may be required to further improve the functional outcome in the DA-denervated brain.

Radial glial cell line C6-R integrates preferentially in adult white matter and facilitates migration of coimplanted neurons in vivo

C6-R is a cell line derived from C6 glioma cells that exhibits key properties of radial glia including the ability to support neuronal migration in culture. To explore its potential use in promoting neuronal migration in vivo, we analyzed the behavior of C6-R cells in the intact and injured adult rat CNS. At 6-11 days postimplantation at the splenium of the corpus callosum, green fluorescent protein-labeled C6-R cells were observed primarily in either the corpus callosum or the hippocampus in the brain, and in the spinal cord they migrated more extensively in the white matter than in the grey matter. To determine whether C6-R cells retain their ability to promote neuronal migration in vivo, they were coinjected with labeled neurons into adult brain. When rat embryonic neurons were coimplanted with C6-R cells, the neurons and C6-R cells comigrated through a much larger volume than neurons alone or neurons coimplanted with fibroblasts. In brains preinjured with ibotenic acid, C6-R cells as well as coimplanted neurons distributed widely within the lesion site and migrated into adjacent brain tissue, while transplants with neurons alone were restricted primarily to the lesion site. The results suggest that radial glial cell lines can serve as a scaffold for neuronal migration that may facilitate development of experimental models for neural transplantation and regeneration.

Retrograde tracing techniques influence reported death rates of adult rat nigrostriatal neurons

Injury often causes loss of neuronal markers and prior retrograde labeling can circumvent this problem of identification. We have previously used a time-consuming protocol for labeling all dopaminergic substantia nigra pars compacta neurons in adult rats by injecting the fluorescent tracer DiI into six sites throughout each neostriatum. Here, 2 weeks after injection of DiI into two central locations, only half of these nigrostriatal neurons were labeled. With six sites, more medial and lateral neurons were labeled, and also more in the midportion along the medial-lateral extent of the pars compacta. Less than 0.5% of the contralateral neurons were labeled. Two injections of Fluorogold also labeled fewer neurons, but their morphology was clearer. Two to 4 weeks after injection of the neurotoxin 6-OHDA into the two neostriatal sites, the total number of surviving neurons appeared greater with six sites of DiI than with two. However, within the middle region of the nigra, survival was lower with the six sites. This suggests that neurons that project outside the two central striatal tracer and 6-OHDA injection regions may be spared initially, but that those in the midportion that project to the central region are more vulnerable with the six-site protocol. Some reports suggest that Fluorogold prelabeling increases neuronal death. Here, survival after 6-OHDA or axotomy was similar with DiI or Fluorogold. These results suggest that because of a complex projection pattern of the nigrostriatal neurons, detailed quantification of neuronal survival should rely on extensive labeling. However, for drug screening purposes, faster labeling with Fluorogold using two sites is more suitable and should provide reliable data.

Striatal tissue transplantation in non-human primates

The caudate nucleus and putamen form part of a complex but topographically connected circuitry that links the cortex, the basal ganglia and the thalamus. Within this complex system lie a series of functionally and anatomically segregated loops that allow the concurrent processing of a wide range of cognitive and motor information (Alexander et al., 1986; Alexander and Crutcher, 1990). As a constituent of these loops it has been shown that the striatum is involved in movement initiation, response selection and attentional processes (Robbins and Brown, 1990; Alexander, 1994; Lawrence et al., 1998). Although it is the medium spiny GABAergic projection neurones that are primarily lost in HD, it is not sufficient merely to replace the GABA. Instead it is crucial for striatal tissue transplants to integrate with the host tissue in such a way that the cortico-striatal-thalamic circuitry is restored and is functional. Rodent studies have progressed a long way in establishing the principle that striatal grafts can, at least partially, restore function and integrate appropriately with the host (Dunnett and Svendsen, 1993; Björklund et al., 1994; Sanberg et al., 1998) but the limited behavioural repertoire and the undifferentiated striatum meant that it was inevitable that studies should progress into primate models. Anatomical tracing studies have demonstrated that motor, premotor and somatosensory cortical areas send corticostriatal projections primarily to the putamen region in primates, whereas the head and body of the caudate nucleus mostly receive efferent input from associative cortical areas (Kemp and Powell, 1970; Kunzle, 1975, 1977, 1978; Selemon and Goldman-Rakic, 1985). Based on such anatomical, and functional, studies Alexander and colleagues have proposed the existence of at least five cortico-striatal-thalamic loops including a motor, a dorsolateral-prefrontal and an orbito-frontal loop (Alexander et al., 1986). The concentration of motor inputs to the putamen region suggests a particular involvement of this structure in the motor loop. Indeed, unilateral lesions of the putamen disrupt motor performance in the marmoset and generate apomorphine-induced dyskinesias in larger primates (Burns et al., 1995; Kendall et al., 2000). The implantation of striatal grafts into marmosets that had previously received unilateral putamen lesions ameliorated some of the motor impairments, which suggested at least partial restoration of the motor loop. In support of this we found direct evidence of host-graft cortico-striatal connectivity using an anterograde tracer injected in the primary motor cortical region (Kendall et al., 1998a). In larger primates, with lesions of the caudate and putamen, striatal [figure: see text] allografts and xenografts have been shown to reduce apomorphine-induced dyskinesias (Isacson et al., 1989; Hantraye et al., 1992; Palfi et al., 1998). The mechanism by which dyskinesias are elicited is not fully understood but alterations in firing patterns within both segments of the globus pallidus have been identified during dyskinetic movements (Matsumura et al., 1995). It seems likely that it would actually require re-establishment of afferent connections between the implanted putamen and the globus pallidus as well as of functioning dopamine receptors within the graft for the reduction in the dyskinetic profile to be observed. Certainly there is evidence, from rodent studies and the marmoset study described here, that close proximity of the graft to the globus pallidus yields better functional recovery (Isacson et al., 1986). In addition, anatomical tracing studies in rats have demonstrated connections between the implanted tissue and the host globus pallidus (Wictorin et al., 1989b, 1990) However, the relationship between graft placement and functional recovery remains to be fully substantiated.

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.

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.

Simultaneous intrastriatal and intranigral grafting (double grafts) in the rat model of Parkinson's disease

Experimental and clinical studies of neural transplantation in Parkinson's disease have focused on the placement of fetal dopaminergic grafts not in their ontogenic site (substantia nigra) but in the main nigral target area (striatum). The reason for this is the apparent inability of intranigral nigral grafts to extend axons for long distances reinnervating the ipsilateral striatum. This review presents previous work by our laboratory [I. Mendez, M. Hong, Reconstruction of the striato-nigro-striatal circuitry by simultaneous double dopaminergic grafts: a tracer study using fluorogold and horseradish peroxidase, Brain Res. 778 (1997) 194-205; I. Mendez, D. Sadi, M. Hong., Reconstruction of the nigrostriatal pathway by simultaneous intrastriatal and intranigral dopaminergic transplants, J. Neurosci. 16 (1996) 7216-7227] using a new transplantation strategy aimed at restoring dopaminergic innervation of the nigra and striatum by simultaneous dopaminergic transplants placed in the substantia nigra and ipsilateral striatum (double grafts) in the 6-hydroxydopamine lesioned adult rat brain. These double grafts achieve not only greater striatal reinnervation than the standard intrastriatal grafts but also produce a faster and more complete behavioural recovery six weeks after transplantation. Injection of the retrograde tracer fluorogold into the striatum and nigra resulted in fluorescent labeled cells within the intranigral graft and the intrastriatal graft and surrounding striatum, respectively suggesting that these double grafts promote at least partial reconstruction of the nigrostriatal dopaminergic pathway. This double graft strategy may have potential implications in clinical neural transplantation for Parkinson's disease.

Expression of the striatal DARPP-32/ARPP-21 phenotype in GABAergic neurons requires neurotrophins in vivo and in vitro

The medium spiny neuron (MSN) is the major output neuron of the caudate nucleus and uses GABA as its primary neurotransmitter. A majority of MSNs coexpress DARPP-32 and ARPP-21, two dopamine and cyclic AMP-regulated phosphoproteins, and most of the matrix neurons express calbindin. DARPP-32 is the most commonly used MSN marker, but previous attempts to express this gene in vitro have failed. In this study we found that DARPP-32 is expressed in <12% of E13- or E17-derived striatal neurons when they are grown in defined media at high or low density in serum, dopamine, or Neurobasal/N2 (Life Technologies), and ARPP-21 is expressed in <1%. The percentage increases to 25% for DARPP-32 and 10% for ARPP-21 when the same cells are grown in Neurobasal/B27 (Life Technologies) for 7 d. After growth in Neurobasal/B27 plus brain-derived neurotrophic factor (BDNF) for 7 d, E13-derived MSNs are 53.7% DARPP-32-positive and 29. 0% ARPP-21-positive; E17-derived MSNs are 66.8% DARPP-32-positive and 51.5% ARPP-21-positive. The percentage of calbindin-positive neurons also is increased under these conditions. Finally, ARPP-21 expression is reduced in mice with a targeted deletion of the BDNF gene. We conclude that BDNF is required for the maturation of a large subset of patch and matrix MSNs in vivo and in vitro. In addition, we introduce a culture system in which highly differentiated MSNs may be generated, maintained, and studied.

Striatal reconstruction by striatal grafts

It is now well established that striatal lesions induce motor and cognitive deficits in rats, and that grafts of embryonic striatal tissue can survive, integrate into the lesioned host brain and alleviate the behavioural deficits in both motor and cognitive spheres. How? Since normal striatal function is dependent upon it's integration within a connected cortical-subcortical neuronal circuitry, and the deficits following striatal damage appear to reflect a "disconnexion" syndrome, the observation of recovery suggests that the grafts re-establish a connected circuitry within the host brain. Evidence to corroborate or refute this hypothesis, in comparison with a less-specific mechanism (or mechanisms) of recovery, is considered, including anatomical, electrophysiological and neurochemical demonstrations of functional circuit reconstruction in the host brain by striatal tissue transplants.

Protection and regeneration of nigral dopaminergic neurons by neurturin or GDNF in a partial lesion model of Parkinson's disease after administration into the striatum or the lateral ventricle

Both glial cell line-derived neurotrophic factor (GDNF) and its recently discovered congener, neurturin (NTN), have been shown to exert neuroprotective effects on lesioned nigral dopamine (DA) neurons when administered at the level of the substantia nigra. In the present study, we have explored the relative in vivo potency of these two neurotrophic factors using two alternative routes of administration, into the striatum or the lateral ventricle, which may be more relevant in a clinical setting. In rats subjected to an intrastriatal (IS) 6-hydroxydopamine (6-OHDA) lesion, GDNF and NTN were injected every third day for 3 weeks starting on the day after the 6-OHDA injection. GDNF provided almost complete (90-92%) protection of the lesioned nigral DA neurons after both IS and intracerebroventricular (ICV) administration. NTN, by contrast, was only partially effective after IS injection (72% sparing) and totally ineffective after ICV injection. Although the trophic factor injections protected the nigral neurons from lesion-induced cell death, the level of expression of the phenotypic marker, tyrosine hydroxylase (TH), was markedly reduced in the rescued cell bodies. The extent of 6-OHDA-induced DA denervation in the striatum was unaffected by both types of treatment; consistent with this observation, the high rate of amphetamine-induced turning seen in the lesioned control animals was unaltered by either GDNF or NTN treatment. In the GDNF-treated animals, and to a lesser extent also after IS NTN treatment, prominent axonal sprouting was observed within the globus pallidus, at the level where the lesioned nigrostriatal axons are known to end at the time of onset of the neurotrophic factor treatment. The results show that GDNF is highly effective as a neuroprotective and axon growth-stimulating agent in the IS 6-OHDA lesion model after both IS and ICV administration. The lower efficacy of NTN after IS, and particularly ICV, administration may be explained by the poor solubility and diffusion properties at neutral pH.

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.

Intranigral transplants of GABA-rich striatal tissue induce behavioral recovery in the rat Parkinson model and promote the effects obtained by intrastriatal dopaminergic transplants

Intrastriatal transplantation of fetal ventral mesencephalon (VM) is currently explored as a potential clinical therapy in Parkinson's disease (PD). Although providing substantial benefit for the patient, behavioral recovery so far obtained with intrastriatal VM grafts is not complete. Using the 6-hydroxydopamine lesion model of PD, we show here that near-complete restoration of the striatal dopamine (DA) innervation can be achieved by multiple intrastriatal microtransplants of fetal DA cells; nevertheless, complete recovery in complex sensorimotor behaviors was not obtained in these animals. In line with the current model of basal ganglia function, this suggests that the lesion-induced overactivity of the basal ganglia output structures, i.e., the substantia nigra (SN) and the entopeduncular nucleus, may not be completely reversed by intrastriatal VM grafts. In the present study, we have transplanted fetal VM tissue or fetal striatal tissue, as a source of DA and GABA neurons, respectively, into the SN of DA-depleted rats. Intranigral VM grafts induced behavioral recovery in some sensorimotor behaviors (forelimb akinesia and balance tests), but the effect did not exceed the recovery observed after intrastriatal VM grafts. Intranigral grafts of striatal tissue induced a pattern of functional recovery which was distinctly different from that observed after intranigral VM grafts, and recovery in coordinated forelimb use in the paw-reaching test was even more pronounced than after intrastriatal transplantation of VM cells. Combined transplantation of DA neurons into the striatum and GABA-rich striatal neurons into the SN induced additive effects of behavioral recovery observed in the forelimb akinesia test. We propose that intranigral striatal transplants, by a GABA-mediated inhibitory action, can reduce the overactivity of the host SN projection neurons and can induce significant recovery in complex motor behavior in the rat PD model and that such grafts may be used to increase the overall functional efficacy of intrastriatal VM grafts.

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.

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.

Early specification of striatal projection neurons and interneuronal subtypes in the lateral and medial ganglionic eminence

The striatum is thought to be generated from two transient swellings in the ventral telencephalon, the lateral and medial ganglionic eminences, present at mid-stages of embryonic rat development. We have studied the relative contribution of these structures to the specific generation of striatal neuronal subtypes such as projection neurons and cholinergic and somatostatin-containing interneurons at an early stage and a mid stage in striatal neurogenesis. Dissociated progenitors isolated from the embryonic day 12.5 and embryonic day 15.5 rat lateral ganglionic eminence grafted into the previously ibotenic acid lesioned adult striatum, produce grafts containing extensive numbers of neurons expressing messenger RNA for the striatal projection neuron marker, DARPP-32, whereas grafts of the embryonic day 12.5 and embryonic day 15.5 medial ganglionic eminences do not. While preprosomatostatin messenger RNA-expressing neurons were observed in grafts from each of the lateral ganglionic eminence and medial ganglionic eminence at both embryonic day 12.5 and embryonic day 15.5, choline acetyltransferase messenger RNA-expressing cholinergic neurons were largely found in grafts derived from the embryonic day 12.5 medial ganglionic eminence. These results suggest that the neuronal diversity of the adult striatum may derive both from the lateral ganglionic eminence, providing DARPP-32-expressing projection neurons as well as somatostatin-containing interneurons, and the early stage medial ganglionic eminence specifically contributing the cholinergic interneurons.

Seizure suppression in kindling epilepsy by grafts of fetal GABAergic neurons in rat substantia nigra

Compared with studies on models of neurodegenerative diseases, considerably less work has been performed with neural grafts in experimental epilepsy. The potential value of this approach, however, is already shown by evidence that noradrenergic grafts implanted bilaterally into the hippocampus or amygdala-piriform cortex can suppress seizure development in the kindling model of temporal lobe epilepsy. We previously showed that amygdala kindling results in a significant decrease of GABA and its synthesizing enzyme glutamate decarboxylase in substantia nigra (SN), i.e., a region thought to be critically involved in seizure propagation in various models of epilepsy. Thus, transplantation of fetal GABAergic neurons into SN might be an effective means of permanently blocking seizure generalization in kindling epilepsy and probably also other types of epilepsy. To test this hypothesis, three groups of female Wistar rats (n = 10 per group) were kindled by electrical stimulation via a bipolar electrode in the basolateral amygdala. After all rats were fully kindled, one group was implanted with GABA-rich cells prepared from the striatal eminence of Wistar rat fetuses at embryonic day 14. The striatal neurons were bilaterally microinjected at various sites over the anterior-posterior axis of the SN, aimed at the pars reticulata. The second group received microinjections of spinal cord cell preparations, whereas the third group received microinjections of cell-free medium only. In all rats, the threshold for focal discharges (afterdischarge threshold [ADT]) as well as afterdischarge duration and severity and duration of seizures occurring at ADT current were determined once weekly before and after transplantation. Eleven to 12 weeks following transplantation, the rats were killed, and location and integration of grafts were examined by immunohistological methods. Rats with GABAergic grafts in SN exhibited a significant increase in ADT and marked reduction in seizure severity compared with pretransplantation values, whereas no such alteration was seen in the other groups. However, the seizure-suppressing effect of GABAergic grafts was not permanent but slowly disappeared over the weeks after transplantation. Although the data indicate that intranigral transplantation of GABA-producing cells is no effective means of inducing long-lasting anticonvulsant effects in experimental epilepsy, this approach may be an initial step to develop more efficient strategies for seizure suppression.

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.

Implications of neurological rehabilitation for advancing intracerebral transplantation

Neurological rehabilitation involves the systematic presentation of environmental stimuli and challenges that enable the patient to learn strategies for minimizing their disabilities. Rehabilitation therapy of transplant recipients may be an important factor in enhancing the efficacy of the transplanted organ or tissue to promote functional recovery. Laboratory research and clinical trials on neural transplantation, as an experimental treatment for neurological disorders (e.g., Parkinson's disease, Huntington's disease, and cerebral ischemia), have focused primarily on devising effective surgical implantation strategies with little attention devoted to the interaction between environmental factors and restorative neurosurgery. Exercise training as part of neurological rehabilitation may be an important factor for neural transplantation therapy for Parkinson's disease. Rehabilitation providers are particularly well placed to provide the environment and the support to optimize the behavioral functioning of neural transplant patients in learning to use the new grafted tissue.

Fetal pig neural cells as a restorative therapy for neurodegenerative disease

With proper immunosuppression, interspecies transplantation of porcine as well as other species of neural cells survive, mature, and integrate into the host in a manner which reconstructs much of the appropriate neural circuitry. These transplants have been shown to alleviate many of the symptoms of various disorders of the central nervous system. In this study, we addressed immunological and maturation issues with regards to intracerebral transplantation of fetal porcine neural cells. First, we compared fetal neural xenograft survival in athymic nude rats versus rats immunosuppressed with cyclosporin A and found that there is little discernible difference between porcine grafts in the 2 recipients. We also found that ectopic transplantation of cells isolated from the porcine striatal primordium can survive and develop into grafts composed of both neuronal and glial phenotypes within the rat hippocampus. This fact raises the possibility that cells of a particular neurotransmitter type (e.g., GABAergic cells) developing from the striatal precursor cells can be transplanted outside the striatum of the adult brain and have physiological effects.

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.