Anatomy and connectivity of intrastriatal striatal transplants

Stem Cell-Derived Human Striatal Progenitors Innervate Striatal Targets and Alleviate Sensorimotor Deficit in a Rat Model of Huntington Disease

Huntington disease (HD) is an inherited late-onset neurological disorder characterized by progressive neuronal loss and disruption of cortical and basal ganglia circuits. Cell replacement using human embryonic stem cells may offer the opportunity to repair the damaged circuits and significantly ameliorate disease conditions. Here, we showed that in-vitro-differentiated human striatal progenitors undergo maturation and integrate into host circuits upon intra-striatal transplantation in a rat model of HD. By combining graft-specific immunohistochemistry, rabies virus-mediated synaptic tracing, and ex vivo electrophysiology, we showed that grafts can extend projections to the appropriate target structures, including the globus pallidus, the subthalamic nucleus, and the substantia nigra, and receive synaptic contact from both host and graft cells with 6.6 ± 1.6 inputs cell per transplanted neuron. We have also shown that transplants elicited a significant improvement in sensory-motor tasks up to 2 months post-transplant further supporting the therapeutic potential of this approach.

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hESC-derived striatal progenitors give rise to MSNs in a neurotoxin model of HD

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Donor transplants extend projections to appropriate striatal target regions

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Grafted cells establish synaptic contact with both donor and resident cells

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Transplanted animals show improvements in HD-related sensorimotor responses

The Effect of Tissue Preparation and Donor Age on Striatal Graft Morphology in the Mouse

Huntington's disease (HD) is a progressive neurodegenerative disease in which striatal medium spiny neurons (MSNs) are lost. Neuronal replacement therapies aim to replace MSNs through striatal transplantation of donor MSN progenitors, which successfully improve HD-like deficits in rat HD models and have provided functional improvement in patients. Transplants in mouse models of HD are more variable and have lower cell survival than equivalent rat grafts, yet mice constitute the majority of transgenic HD models. Improving the quality and consistency of mouse transplants would open up access to this wider range of rodent models and facilitate research to increase understanding of graft mechanisms, which is essential to progress transplantation as a therapy for HD. Here we determined how donor age, cell preparation, and donor/host strain choice influenced the quality of primary embryonic grafts in quinolinic acid lesion mouse models of HD. Both a within-strain (W-S) and a between-strain (B-S) donor/host paradigm were used to compare transplants of donor tissues derived from mice at embryonic day E12 and E14 prepared either as dissociated suspensions or as minimally manipulated tissue pieces (TP). Good graft survival was observed, although graft volume and cellular composition were highly variable. The effect of cell preparation on grafts differed significantly depending on donor age, with E14 cell suspensions yielding larger grafts compared to TP. Conversely, TP were more effective when derived from E12 donor tissue. A W-S model produced larger grafts with greater MSN content, and while high levels of activated microglia were observed across all groups, a greater number was found in B-S transplants. In summary, we show that the effect of tissue preparation on graft morphology is contingent on the age of donor tissue used. The presence of microglial activation in all groups highlights the host immune response as an important consideration in mouse transplantation.

Stem cell transplantation for Huntington's diseases

Therapeutic approaches based on stem cells have received considerable attention as potential treatments for Huntington's disease (HD), which is a fatal, inherited neurodegenerative disorder, caused by progressive loss of GABAergic medium spiny neurons (MSNs) in the striatum of the forebrain. Transplantation of stem cells or their derivatives in animal models of HD, efficiently improved functions by replacing the damaged or lost neurons. In particular, neural stem cells (NSCs) for HD treatments have been developed from various sources, such as the brain itself, the pluripotent stem cells (PSCs), and the somatic cells of the HD patients. However, the brain-derived NSCs are difficult to obtain, and the PSCs have to be differentiated into a population of the desired neuronal cells that may cause a risk of tumor formation after transplantation. In contrast, induced NSCs, derived from somatic cells as a new stem cell source for transplantation, are less likely to form tumors. Given that the stem cell transplantation strategy for treatment of HD, as a genetic disease, is to replace the dysfunctional or lost neurons, the correction of mutant genes containing the expanded CAG repeats is essential. In this review, we will describe the methods for obtaining the optimal NSCs for transplantation-based HD treatment and the differentiation conditions for the functional GABAergic MSNs as therapeutic cells. Also, we will discuss the valuable gene correction of the disease stem cells by the CRISPR/Cas9 system for HD treatment.

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.

Making Stem Cell Lines Suitable for Transplantation

Human stem cells, progenitor cells, and cell lines have been derived from embryonic, fetal, and adult sources in the search for graft tissue suitable for the treatment of CNS disorders. An increasing number of experimental studies have shown that grafts from several sources survive, differentiate into distinct cell types, and exert positive functional effects in experimental animal models, but little attention has been given to developing cells under conditions of good manufacturing practice (GMP) that can be scaled up for mass treatment. The capacity for continued division of stem cells in culture offers the opportunity to expand their production to meet the widespread clinical demands posed by neurodegenerative diseases. However, maintaining stem cell division in culture long term, while ensuring differentiation after transplantation, requires genetic and/or oncogenetic manipulations, which may affect the genetic stability and in vivo survival of cells. This review outlines the stages, selection criteria, problems, and ultimately the successes arising in the development of conditionally immortal clinical grade stem cell lines, which divide in vitro, differentiate in vivo, and exert positive functional effects. These processes are specifically exemplified by the murine MHP36 cell line, conditionally immortalized by a temperature-sensitive mutant of the SV40 large T antigen, and cell lines transfected with the c-myc protein fused with a mutated estrogen receptor (c-mycERTAM), regulated by a tamoxifen metabolite, but the issues raised are common to all routes for the development of effective clinical grade cells.

Xenogeneic Cell Therapy: Current Progress and Future Developments in Porcine Cell Transplantation

The multitude of distinct cell types present in mature and developing tissues display unique physiologic characteristics. Cellular therapy is a novel technology with the promise of utilizing this diversity to treat a wide range of human degenerative diseases. Intractable diseases, disorders, and injuries are characterized by cell death or aberrant cellular function. Cell transplantation can replace diseased or lost tissue to provide restorative therapy for these conditions. The limited use of cell transplants as a basis for current therapy can, in part, be attributed to the lack of available human cells suitable for transplantation. This has prevented further realization of the promise of cell transplantation as a platform technology. Accordingly, cell-based therapies such as blood transfusions, for which the cells are readily available, are a standard part of current medical practice. Despite numerous attempts to expand primary human cells in tissue culture, current technological limitations of this approach in regard to proliferative capacity and maintenance of the differentiated phenotype has prevented their use for transplantation. Further, use of human stem cells for the derivation of specific cell types for transplantation is an area of future application with great potential, but hurdles remain in regard to deriving and sufficiently expanding these multi-potential cells. Thus, it appears that primary cells are at present a superior source for transplantation. This review focuses on pigs as a source of a variety of primary cells to advance cell therapy to the clinic and implement achievement of its full potential. We outline the advantages and disadvantages of xenogeneic cell therapy while underscoring the utility of transplantable porcine cells for the treatment of human disease. © 1998 Elsevier Science Inc.

Transplanted hNT Cells (“LBS Neurons”) in a Rat Model of Huntington's Disease: Good Survival, Incomplete Differentiation, and Limited Functional Recovery

A variety of immortalized cell lines have been proposed to exhibit sufficient phenotypic plasticity to allow them to replace primary embryonic neurons for restorative cell transplantation. In the present experiments we evaluate the functional viability of one particular cell line, the hNT cells developed by Layton Bioscience, to replace lost neurons and alleviate asymmetrical motor deficits in a unilateral excitotoxic lesion model of Huntington's disease. Because the grafts involved implantation of human-derived cells into a rat host environment, all animals were immunosuppressed. Cyclosporin A and FK-506 were similar in providing effective immunoprotection of the hNT xenografts, and whereas the lesions induced a marked inflammatory response in the host brain, this was not exacerbated by the presence of xenograft cells. The presence of grafted cells was determined with the human-specific antigen HuNu, and good graft survival was demonstrated in almost all animals up to the longest survival examined, 16 weeks posttransplantation. Although the cells exhibited progressively greater maturation and differentiation at 10-day, 4- and 16-week time points, staining for the mature neuronal marker NeuN was at best very weak, and we were unable to detect unequivocal staining with any markers of mature striatal phenotype, including DARPP-32, calbindin, parvalbumin, choline acetyl transferase, or NADPH diaphorase (with in all cases positive control provided by good staining on the intact contralateral side of the brain). Nor were we able to detect any differences between rats with lesions alone and rats with grafts in the contralateral motor deficits exhibited in a test of skilled paw reaching or cylinder placing. These results suggest that further and more extensive studies should be undertaken to assess whether hNT neurons can show more extensive and appropriate maturation and be associated with recovery in appropriate behavioral models, before they may be considered a suitable replacement for primary embryonic cells for clinical application in Huntington's disease.

Is there a place for human fetal-derived stem cells for cell replacement therapy in Huntington's disease?

Huntington's disease (HD) is a neurodegenerative disease that offers an excellent paradigm for cell replacement therapy because of the associated relatively focal cell loss in the striatum. The predominant cells lost in this condition are striatal medium spiny neurons (MSNs). Transplantation of developing MSNs taken from the fetal brain has provided proof of concept that donor MSNs can survive, integrate and bring about a degree of functional recovery in both pre-clinical studies and in a limited number of clinical trials. The scarcity of human fetal tissue, and the logistics of coordinating collection and dissection of tissue with neurosurgical procedures makes the use of fetal tissue for this purpose both complex and limiting. Alternative donor cell sources which are expandable in culture prior to transplantation are currently being sought. Two potential donor cell sources which have received most attention recently are embryonic stem (ES) cells and adult induced pluripotent stem (iPS) cells, both of which can be directed to MSN-like fates, although achieving a genuine MSN fate has proven to be difficult. All potential donor sources have challenges in terms of their clinical application for regenerative medicine, and thus it is important to continue exploring a wide variety of expandable cells. In this review we discuss two less well-reported potential donor cell sources; embryonic germ (EG) cells and fetal neural precursors (FNPs), both are which are fetal-derived and have some properties that could make them useful for regenerative medicine applications.

Pluripotent stem cell-derived neurons for transplantation in Huntington's disease

Pluripotent stem cells present a potentially unlimited source of cells for regenerative medicine, providing that they can be efficiently and accurately differentiated to the target cell type. The principle target cell for Huntington's disease is the striatal medium spiny neuron. In this chapter, we review strategies for directing medium spiny neuron differentiation, based on known developmental principles, and we discuss the remaining hurdles on the road to engineering such cells for therapeutic application in Huntington's disease.

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.

Rehabilitation training in neural restitution

Over the last decade, neural transplantation has emerged as one of the more promising, albeit highly experimental, potential therapeutics in neurodegenerative disease. Preclinical studies in rat lesion models of Huntington's disease (HD) and Parkinson's disease (PD) have shown that transplanted precursor neuronal tissue from a fetus into the lesioned striatum can survive, integrate, and reconnect circuitry. Importantly, specific training on behavioral tasks that target striatal function is required to encourage functional integration of the graft to the host tissue. Indeed, "learning to use the graft" is a concept recently adopted in preclinical studies to account for unpredicted profiles of recovery posttransplantation and is an emerging strategy for improving graft functionality. Clinical transplant studies in HD and PD have resulted in mixed outcomes. Small sample sizes and nonstandardized experimental procedures from trial to trial may explain some of this variability. However, it is becoming increasingly apparent that simply replacing the lost neurons may not be sufficient to ensure the optimal graft effects. The knowledge gained from preclinical grafting and training studies suggests that lifestyle factors, including physical activity and specific cognitive and/or motor training, may be required to drive the functional integration of grafted cells and to facilitate the development of compensatory neural networks. The clear implications of preclinical studies are that physical activity and cognitive training strategies are likely to be crucial components of clinical cell replacement therapies in the future. In this chapter, we evaluate the role of general activity in mediating the physical ability of cells to survive, sprout, and extend processes following transplantation in the adult mammalian brain, and we consider the impact of general and specific activity at the behavioral level on functional integration at the cellular and physiological level. We then highlight specific research questions related to timing, intensity, and specificity of training in preclinical models and synthesize the current state of knowledge in clinical populations to inform the development of a strategy for neural transplantation rehabilitation training.

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

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

Repair of the CNS using endogenous and transplanted neural stem cells

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

Neural tissue transplantation, repair, and rehabilitation

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

Human embryonic stem cell-derived GABA neurons correct locomotion deficits in quinolinic acid-lesioned mice

Degeneration of medium spiny GABA neurons in the basal ganglia underlies motor dysfunction in Huntington's disease (HD), which presently lacks effective therapy. In this study, we have successfully directed human embryonic stem cells (hESCs) to enriched populations of DARPP32-expressing forebrain GABA neurons. Transplantation of these human forebrain GABA neurons and their progenitors, but not spinal GABA cells, into the striatum of quinolinic acid-lesioned mice results in generation of large populations of DARPP32(+) GABA neurons, which project to the substantia nigra as well as receiving glutamatergic and dopaminergic inputs, corresponding to correction of motor deficits. This finding raises hopes for cell therapy for HD.

The decrease of dopamine D₂/D₃ receptor densities in the putamen and nucleus caudatus goes parallel with maintained levels of CB₁ cannabinoid receptors in Parkinson's disease: a preliminary autoradiographic study with the selective dopamine D₂/D₃ antagonist [³H]raclopride and the novel CB₁ inverse agonist [¹²⁵I]SD7015

Cannabinoid type-1 receptors (CB₁Rs) modulate synaptic neurotransmission by participating in retrograde signaling in the adult brain. Increasing evidence suggests that cannabinoids through CB₁Rs play an important role in the regulation of motor activities in the striatum. In the present study, we used human brain samples to examine the relationship between CB₁R and dopamine receptor density in case of Parkinson's disease (PD). Post mortem putamen, nucleus caudatus and medial frontal gyrus samples obtained from PD patients were used for CB₁R and dopamine D₂/D₃ receptor autoradiography. [¹²⁵I]SD7015, a novel selective CB₁R inverse agonist, developed by a number of the present co-authors, and [³H]raclopride, a dopamine D₂/D₃ antagonist, were used as radioligands. Our results demonstrate unchanged CB₁R density in the putamen and nucleus caudatus of deceased PD patients, treated with levodopa (L-DOPA). At the same time dopamine D₂/D₃ receptors displayed significantly decreased density levels in case of PD putamen (control: 47.97 ± 10.00 fmol/g, PD: 3.73 ± 0.07 fmol/g (mean ± SEM), p<0.05) and nucleus caudatus (control: 30.26 ± 2.48 fmol/g, PD: 12.84 ± 5.49 fmol/g, p<0.0005) samples. In contrast to the putamen and the nucleus caudatus, in the medial frontal gyrus neither receptor densities were affected. Our data suggest the presence of an unaltered CB₁R population even in late stages of levodopa treated PD. This further supports the presence of an intact CB₁R population which, in line with the conclusion of earlier publications, may be utilized as a pharmacological target in the treatment of PD. Furthermore we found discrepancy between a maintained CB₁R population and a decreased dopamine D₂/D₃ receptor population in PD striatum. The precise explanation of this conundrum requires further studies with simultaneous examination of the central cannabinoid and dopaminergic systems in PD using higher sample size.

Role of experience, training, and plasticity in the functional efficacy of striatal transplants

Cell-based treatments of neurodegenerative diseases have been tested clinically with partial success. In the context of Huntington's disease (HD), experimental studies show that the grafted embryonic striatal cells survive, integrate within the host brain, and reverse some functional deficits. Importantly, once transplanted, the grafted striatal neurons retain a significant level of cellular, morphological, and functional plasticity which allows the experimental modification of their character through the manipulation of environmental cues or learning protocols. Using embryonic striatal grafts in the rodent model of HD as the principal example, this chapter summarizes seminal experiments that demonstrate that environmental factors, training, and activity can tap into mechanisms that influence the development of the grafted cells and can change the profile of graft-mediated behavioral recovery. Although currently there is limited understanding of the biological rationale behind the recovery, we put forward experimental data indicating that striatal grafts can express experience-dependent physiological plasticity at the synaptic as well as at the systemic functional level.

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.

The Current Status of Neural Grafting in the Treatment of Huntington's Disease. A Review

Huntington’s disease (HD) is a devastating, fatal, autosomal dominant condition in which the abnormal gene codes for a mutant form of huntingtin that causes widespread neuronal dysfunction and death. This leads to a clinical presentation, typically in midlife, with a combination of motor, psychiatric, cognitive, metabolic, and sleep abnormalities, for which there are some effective symptomatic therapies that can produce some transient benefits. The disease, though, runs a progressive course over a 20-year period ultimately leading to death, and there are currently no proven disease modifying therapies. However whilst the neuronal dysfunction and loss affects much of the central nervous system, the striatum is affected early on in the disease and is one of the areas most affected by the pathogenic process. As a result the prospect of treating HD using neural transplants of striatal tissue has been explored and to date the clinical data is inconclusive. In this review we discuss the rationale for treating HD using this approach, before discussing the clinical trial data and what we have learnt to date using this therapeutic strategy.

Human pluripotent stem cell therapy for Huntington's disease: technical, immunological, and safety challenges human pluripotent stem cell therapy for Huntington's disease: technical, immunological, and safety challenges

Intra-striatal transplantation of homotypic fetal tissue at the time of peak striatal neurogenesis can provide some functional benefit to patients suffering from Huntington's disease. Currently, the only approach shown to slow down the course of this condition is replacement of the neurons primarily targeted in this disorder, although it has been transient and has only worked with a limited number of patients. Otherwise, this dominantly inherited neurodegenerative disease inevitably results in the progressive decline of motricity, cognition, and behavior, and leads to death within 15 to 20 years of onset. However, fetal neural cell therapy of Huntington's disease, as with a similar approach in Parkinson's disease, is marred with both technical and biological hurdles related to the source of grafting material. This heavily restricts the number of patients who can be treated. A substitute cell source is therefore needed, but must perform at least as well as fetal neural graft in terms of brain recovery and reconstruction, while overcoming its major obstacles. Human pluripotent stem cells (embryonic in origin or induced from adult cells through genetic reprogramming) have the potential to meet those challenges. In this review, the therapeutic potential in view of 4 major issues is identified during fetal cell therapy clinical trials: 1) logistics of graft procurement, 2) quality control of the cell preparation, 3) immunogenicity of the graft, and 4) safety of the procedure.

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.

Cell-based treatments for huntington's disease

In experimental rats, mice, and monkeys, transplantation of embryonic striatal cells into the striatum can repair the damage and alleviate the functional deficits caused by striatal lesions. Such strategies have been translated to striatal repair by cell transplantation in small numbers of patients with progressive genetic striatal degeneration in Huntington's disease. In spite of some encouraging preliminary data, the clinical results are to date neither as reliable nor as compelling as the broad extend of recovery observed in the animal models across motor, cognitive, and skill and habit learning domains. Strategies to achieve immediate and long-term improvements in the clinical applications include identifying and limiting the causes of complications, standardization and quality control of preparation and delivery, appropriate patient selection to match the cellular repair to specific profiles of cell loss and degeneration in individual patients and different neurodegenerative diseases, and improving the availability of alternative sources of donor cells and tissues.

Thinking about repairing thinking

The work of Sinden et al. suggests that it may be possible to produce improvement in the “highest” areas of brain function by transplanting brain tissue. What appears to be the limiting factor is not the complexity of the mental process under consideration but the discreteness of the lesion which causes the impairment and the appropriateness and accuracy of placement of the grafted tissue.

Therapeutic uses for neural grafts: Progress slowed but not abandoned

In spite of Stein and Glasier's justifiable conclusion that initial optimism concerning the immediate clinical applicability of neural transplantation was premature, there exists much experimental evidence to support the potential for incorporating this procedure into a therapeutic arsenal in the future. To realize this potential will require continued evolution of our knowledge at multiple levels of the clinical and basic neurosciences.

The structure, operation, and functionality of intracerebral grafts

The concept of structure, operation, and functionality, as they may be understood by clinicians or researchers using neural transplantation techniques, are briefly defined. Following Stein & Glasier, we emphasize that the question of whether an intracerebral graft is really functional should be addressed not only in terms of what such a graft does in a given brain structure, but also in terms of what it does at the level of the organism.

The NGF superfamily of neurotrophins: Potential treatment for Alzheimer's and Parkinson's disease

Stein & Glasier suggest embryonic neural tissue grafts as a potential treatment strategy for Alzheimer's and Parkinson's disease. As an alternative, we suggest that the family of nerve growth factor-related neurotrophins and their trk (tyrosine kinase) receptors underlie cholinergic basal forebrain (CBF) and dopaminergic substantia nigra neuron degeneration in these diseases, respectively. Therefore, treatment approaches for these disorders could utilize neurotrophins.

Some practical and theoretical issues concerning fetal brain tissue grafts as therapy for brain dysfunctions

Grafts of embryonic neural tissue into the brains of adult patients are currently being used to treat Parkinson's disease and are under serious consideration as therapy for a variety of other degenerative and traumatic disorders. This target article evaluates the use of transplants to promote recovery from brain injury and highlights the kinds of questions and problems that must be addressed before this form of therapy is routinely applied. It has been argued that neural transplantation can promote functional recovery through the replacement of damaged nerve cells, the reestablishment of specific nerve pathways lost as a result of injury, the release of specific neurotransmitters, or the production of factors that promote neuronal growth. The latter two mechanisms, which need not rely on anatomical connections to the host brain, are open to examination for nonsurgical, less intrusive therapeutic use. Certain subjective judgments used to select patients who will receive grafts and in assessment of the outcome of graft therapy make it difficult to evaluate the procedure. In addition, little long-term assessment of transplant efficacy and effect has been done in nonhuman primates. Carefully controlled human studies, with multiple testing paradigms, are also needed to establish the efficacy of transplant therapy.

Models of neurological defects and defects in neurological models

The transition from research to patient following advances in transplantation research is likely to be disappointing unless it includes a better understanding of critically relevant characteristics of the neurological disorder and improvements in the animal models, particularly the behavioral features. The appropriateness of the model has less to do with the species than with how the species is used.

Neural transplants in patients with Huntington's disease undergo disease-like neuronal degeneration

The clinical evaluation of neural transplantation as a potential treatment for Huntington's disease (HD) was initiated in an attempt to replace lost neurons and improve patient outcomes. Two of 3 patients with HD reported here, who underwent neural transplantation containing striatal anlagen in the striatum a decade earlier, have demonstrated marginal and transient clinical benefits. Their brains were evaluated immunohistochemically and with electron microscopy for markers of projection neurons and interneurons, inflammatory cells, abnormal huntingtin protein, and host-derived connectivity. Surviving grafts were identified bilaterally in 2 of the subjects and displayed classic striatal projection neurons and interneurons. Genetic markers of HD were not expressed within the graft. Here we report in patients with HD that (i) graft survival is attenuated long-term; (ii) grafts undergo disease-like neuronal degeneration with a preferential loss of projection neurons in comparison to interneurons; (iii) immunologically unrelated cells degenerate more rapidly than the patient's neurons, particularly the projection neuron subtype; (iv) graft survival is attenuated in the caudate in comparison to the putamen in HD; (v) glutamatergic cortical neurons project to transplanted striatal neurons; and (vi) microglial inflammatory changes in the grafts specifically target the neuronal components of the grafts. These results, when combined, raise uncertainty about this potential therapeutic approach for the treatment of HD. However, these observations provide new opportunities to investigate the underlying mechanisms involved in HD, as well as to explore additional therapeutic paradigms.

Medium spiny neurons for transplantation in Huntington's disease

Cell-replacement therapy for Huntington's disease is one of very few therapies that has reported positive outcomes in clinical trials. However, for cell transplantation to be made more readily available, logistical, standardization and ethical issues associated with the current methodology need to be resolved. To achieve these goals, it is imperative that an alternative cell source be identified. One of the key requirements of the cells is that they are capable of acquiring an MSN (medium spiny neuron) morphology, express MSN markers such as DARPP-32 (dopamine- and cAMP-regulated phosphoprotein of 32 kDa), and function in vivo in a manner that replicates those that have been lost to the disease. Developmental biology has progressed in recent years to provide a vast array of information with regard to the key signalling events involved in the proliferation, specification and differentiation of striatal-specific neurons. In the present paper, we review the rationale for cell-replacement therapy in Huntington's disease, discuss some potential donor sources and consider the value of developmental markers in the identification of cells with the potential to develop an MSN phenotype.

Restoration of calbindin after fetal hippocampal CA3 cell grafting into the injured hippocampus in a rat model of temporal lobe epilepsy

Degeneration of the CA3 pyramidal and dentate hilar neurons in the adult rat hippocampus after an intracerebroventricular kainic acid (KA) administration, a model of temporal lobe epilepsy, leads to permanent loss of the calcium binding protein calbindin in major fractions of dentate granule cells and CA1 pyramidal neurons. We hypothesize that the enduring loss of calbindin in the dentate gyrus and the CA1 subfield after CA3-lesion is due to disruption of the hippocampal circuitry leading to hyperexcitability in these regions; therefore, specific cell grafts that are capable of both reconstructing the disrupted circuitry and suppressing hyperexcitability in the injured hippocampus can restore calbindin. We compared the effects of fetal CA3 or CA1 cell grafting into the injured CA3 region of adult rats at 45 days after KA-induced injury on the hippocampal calbindin. The calbindin immunoreactivity in the dentate granule cells and the CA1 pyramidal neurons of grafted animals was evaluated at 6 months after injury (i.e. at 4.5 months post-grafting). Compared with the intact hippocampus, the calbindin in "lesion-only" hippocampus was dramatically reduced at 6 months post-lesion. However, calbindin expression was restored in the lesioned hippocampus receiving CA3 cell grafts. In contrast, in the lesioned hippocampus receiving CA1 cell grafts, calbindin expression remained less than the intact hippocampus. Thus, specific cell grafting restores the injury-induced loss of calbindin in the adult hippocampus, likely via restitution of the disrupted circuitry. Since loss of calbindin after hippocampal injury is linked to hyperexcitability, re-expression of calbindin in both dentate gyrus and CA1 subfield following CA3 cell grafting may suggest that specific cell grafting is efficacious for ameliorating injury-induced hyperexcitability in the adult hippocampus. However, electrophysiological studies of KA-lesioned hippocampus receiving CA3 cell grafts are required in future to validate this possibility.

Stem cell-based cell therapy for Huntington disease: a review

Huntington disease (HD) is a devastating neurodegenerative disorder and no proven medical therapy is currently available to mitigate its clinical manifestations. Although fetal neural transplantation has been tried in both preclinical and clinical investigations, the efficacy is not satisfactory. With the recent explosive progress of stem cell biology, application of stem cell-based therapy in HD is an exciting prospect. Three kinds of stem cells, embryonic stem cells, bone marrow mesenchymal stem cells and neural stem cells, have previously been utilized in cell therapy in animal models of neurological disorders. However, neural stem cells were preferably used by investigators in experimental HD studies, since they have a clear capacity to become neurons or glial cells after intracerebral or intravenous transplantation, and they induce functional recovery. In this review, we summarize the current state of cell therapy utilizing stem cells in experimental HD animal models, and discuss the future considerations for developing new therapeutic strategies using neural stem cells.

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.

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.

Stem cell transplantation for Huntington's disease

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

Progressive Reparative Gliosis in Aged Hosts and Interferences with Neural Grafts in an Animal Model of Huntington's Disease

1. Neural transplantation in Huntington's diseased patients is currently the only approach in the treatment of this neurodegenerative disorder. The clinical trial, unfortunately, includes only a small number of patients until now, since many important questions have not been answered yet. One of them is only mild to moderate improvement of the state in most of grafted patients. 2. We examined the morphological correlates in the response to intrastriatal grafting of fragments of foetal rat ventral mesencephalic tissue 1 month after transplantation in male Wistar rats within varying durations (from 2 to 38 weeks) of experimentally induced neurodegenerative process of the striatum (used as a model of Huntington's disease). Our goal was to determine the impact of advanced striatal damage and gliosis on the graft viability and host-graft integration. 3. The findings can be summarized as follows: The progressive reactive gliosis, which is not able to compensate continual reduction of the grey matter leading to an extensive atrophy of the striatum in a long-term lesions, results in formation of the compact glial network. This tissue cannot be considered the suitable terrain for successful graft development and formation of host-graft interconnections. 4. The progression of irreversible morphological changes in long-lasting neurodegenerative process within the striatum can be supposed one of the important factors, which may decrease our prospect of distinct improvement after neural grafting in patients in advanced stage of Huntington's disease, who still remain the leading group in clinical trials.

Striatal grafts alleviate bilateral striatal lesion deficits in operant delayed alternation in the rat

In order to assess the capacity of striatal grafts to alleviate cognitive deficits of the frontal type that arise following bilateral striatal lesions, control, lesion and grafted rats were tested in an operant test of delayed alternation. Bilateral striatal lesions induced a marked impairment in choice accuracy, and signal detection analysis indicated that the lesion animals were reliably impaired on both parametric and non-parametric indices of discriminative sensitivity but not of response bias. The impairment was apparent at all intertrial interval delays, including the very shortest, suggesting the deficit is one of frontal-type executive function rather than of short-term memory. The grafted animals exhibited a significant alleviation of the deficit, again apparent at all delays. Histological analyses indicated good graft survival, and injections of a dextran amine anterograde tracer bilaterally into the host prefrontal cortex indicated reformation of extensive projections into the grafted tissues. Since performance of the operant delayed alternation task is dependent upon the integrity of corticostriatal connections, which is disrupted bilaterally by the lesions and restored to the grafts in the transplanted animals, the results corroborate the hypothesis that striatal grafts can alleviate complex cognitive functions of the frontal type by a mechanism that involves functional integration of the grafted neurons into the neural circuits of the host brain.

Transplanted adult neural progenitor cells survive, differentiate and reduce motor function impairment in a rodent model of Huntington's disease

The present study investigated the ability for adult rat neural progenitor cells to survive transplantation, structurally repopulate the striatum and improve motor function in the quinolinic acid (QA) lesion rat model of Huntington's disease. Neural progenitor cells were isolated from the subventricular zone of adult Wistar rats, propagated in culture and labeled with BrdU (50 microM). Fourteen days following QA lesioning, one group of rats (n = 12) received a unilateral injection of adult neural progenitor cells ( approximately 180,000 cells total) in the lesioned striatum, while a second group of rats (n = 10) received a unilateral injection of vehicle only (sham transplant). At the time of transplantation adult neural progenitor cells were phenotypically immature, as demonstrated by SOX2 immunocytochemistry. Eight weeks following transplantation, approximately 12% of BrdU-labeled cells had survived and migrated extensively throughout the lesioned striatum. Double-label immunocytochemical analysis demonstrated that transplanted BrdU-labeled progenitor cells differentiated into either astrocytes, as visualized by GFAP immunocytochemistry, or mature neurons, demonstrated with NeuN. A proportion of BrdU-labeled cells also expressed DARPP-32 and GAD67, specific markers for striatal medium spiny projection neurons and interneurons. Rats transplanted with adult neural progenitor cells also demonstrated a significant reduction in motor function impairment as determined by apomorphine-induced rotational asymmetry and spontaneous exploratory forelimb use when compared to sham transplanted animals. These results demonstrate that adult neural progenitor cells survive transplantation, undergo neuronal differentiation with a proportion of newly generated cells expressing markers characteristic of striatal neurons and reduce functional impairment in the QA lesion model of Huntington's disease.

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.

Neural progenitor implantation restores metabolic deficits in the brain following striatal quinolinic acid lesion

Neural progenitor transplantation is a potential treatment for neurodegenerative diseases, including Huntington's disease (HD). In the current study, we tested the potential of rat embryonic neural progenitors expanded in vitro as therapy in the rat quinolinic acid-lesioned striatum, a model that demonstrates some of the pathological features of HD. We used positron emission tomography (PET) to demonstrate that the intrastriatal injection of cultured rat neural progenitors results in improved metabolic function in the striatum and overlying cortex when compared to media-injected controls. Transplanted progenitors were capable of surviving, migrating long distances and differentiating into neurons and glia. The cortices of transplanted animals contained greater numbers of neurons in regions that had shown metabolic improvement. However, histological analysis revealed that only a small fraction of these increased neurons could be accounted for by engrafted cells, indicating that the metabolic sparing was likely the result of a trophic action of the transplanted cells on the host. Behavioral testing of the implanted animals did not reveal improvement in apomorphine-induced rotation. These data demonstrate that progenitor cell implantation results in enhanced metabolic function and sparing of neuron number, but that these functions do not necessarily result in the restoration of complex circuitry.

Neuronal replacement and integration in the rewiring of cerebellar circuits

Repair of CNS injury or degeneration by cell replacement may lead to significant functional recovery only through faithful reconstruction of the original anatomical architecture. This is particularly relevant for point-to-point systems, where precisely patterned connections have to be re-established to regain adaptive function. Despite the major interest recently drawn on cell therapies, little is known about the mechanisms and the potentialities for specific integration of new neurons in the mature CNS. Major findings and concepts about this issue will be reviewed here, with special focus on work dealing with the Purkinje cell transplantation in the rodent cerebellum. These studies show that the adult CNS may provide some efficient information to direct cell engraftment and process outgrowth. On their side, immature cells may be able to induce adaptive changes in their adult partners to facilitate their incorporation in the recipient network. Despite the rather high degree of specific integration achieved in several different CNS regions, these processes are usually defective and long-distance connections are not rewired. Thus, although some potentialities for cell replacement exist in the mature CNS, full incorporation of new neurons in adult circuits is rarely observed. Indeed, intrinsic mechanisms for growth control as well as injury-induced changes in the properties and architecture of the nervous tissue contribute to hamper repair processes. As a consequence, crucial to obtain successful cell replacement and integration in the mature CNS is a deep understanding of the basic biological mechanisms that regulate the interactions between newly added elements and the recipient environment.

Optimising plasticity: environmental and training associated factors in transplant-mediated brain repair

With progressively ageing populations, degeneration of nerve cells of the brain, due to accident or disease, represents one of the major problems for health and welfare in the developed world. The molecular environment in the adult brain promotes stability limiting its ability to regenerate or to repair itself following injury. Cell transplantation aims to repair the nervous system by introducing new cells that can replace the function of the compromised or lost cells. Alternatives to primary embryonic tissue are actively being sought but this is at present the only source that has been shown reliably to survive grafting into the adult brain and spinal cord, connect with the host nervous system, and influence behaviour. Based on animal studies, several clinical trials have now shown that embryonic tissue grafts can partially alleviate symptoms in Parkinson's disease, and related strategies are under evaluation for Huntington's disease, spinal cord injury, stroke and other CNS disorders. The adult brain is at its most plastic in the period following injury, offering a window of opportunity for therapeutic intervention. Enriched environment, behavioural experience and grafting can each separately influence neuronal plasticity and recovery of function after brain damage, but the extent to which these factors interact is at present unknown. To improve the outcome following brain damage, transplantation must make use of the endogenous potential for plasticity of both the host and the graft and optimise the external circumstances associated with graft-mediated recovery. Our understanding of mechanisms of brain plasticity subsequent to brain damage needs to be associated with what we know about enhancing intrinsic recovery processes in order to improve neurobiological and surgical strategies for repair at the clinical level. With the proof of principle beginning to emerge from clinical trials, a rich area for innovative research with profound therapeutic application, even broader than the specific context of transplantation, is now opening for investigation.