Review: Neurorehabilitation With Neural Transplantation

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2010 24: 692 originally published online 20 July 2010Neurorehabil Neural RepairMáté Döbrössy, Monica Busse, Tobias Piroth, Anne Rosser, Stephen Dunnett and Guido Nikkhah

Review: Neurorehabilitation With Neural Transplantation

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Review

Neurorehabilitation and Neural Repair24(8) 692 –701© The Author(s) 2010Reprints and permission: http://www. sagepub.com/journalsPermissions.navDOI: 10.1177/1545968310363586http://nnr.sagepub.com

Neurorehabilitation WithNeural Transplantation

Máté Döbrössy PhD1, Monica Busse PhD2, Tobias Piroth MD1,Anne Rosser MD2, Stephen Dunnett DSc2, and Guido Nikkhah MD1

Abstract

Cell replacement therapy has been tested clinically in Parkinson’s disease (PD) and Huntington’s disease (HD), epilepsy, spinal cord injury, and stroke. The clinical outcomes have been variable, perhaps partly because of the differing levels of preclinical, basic experimental evidence that was available prior to the trials. The most promising results have been seen in PD trials, with encouraging ones in HD. A common feature of most trials is that they have concentrated on the biological and technical aspects of transplantation without presupposing that the outcomes might be influenced by events after the surgery. The growing evidence of plasticity demonstrated by the brain and grafts in response to environmental and training stimuli such as rehabilitation interventions has been mostly neglected throughout the clinical application of cell therapy. This review suggests that a different approach may be required to maximize recovery: postoperative experiences, includ-ing rehabilitation with explicit behavioral retraining, could have marked direct as well as positive secondary effects on the integration and function of grafted cells in the host neural system. The knowledge gained about brain plasticity following brain damage needs to be linked with what we know about promoting intrinsic recovery processes and how this can boost neurobiological and surgical strategies for repair at the clinical level. With proof of principle now established, a rich area for innovative research with profound therapeutic application is open for investigation.

Keywords

neural transplantation, Parkinson’s disease, Huntington’s disease, spinal cord injury rehabilitation, stroke

Introduction

Cell replacement therapy is a brain repair strategy whereby the lost or dysfunctional cells of the central nervous system (CNS) are replaced by new cells, generally neurons. Fol-lowing many years of basic research, the approach has been offered to Parkinson’s disease (PD) and Huntington’s disease (HD) patients1 but has not become a routine treat-ment for 2 main reasons. First, the use of aborted fetal tissue as a source of cells raises ethical issues; second, the outcome of the therapy has been inconsistent.2,3

The emergence of the stem cell field has given a new lease of life to neural transplantation because it has the potential to resolve both the cell availability issue and the ethical questions.4 Generating cells of the desired phenotype and numbers under highly controlled and standardized Good Manufacturing Practice (GMP) conditions could open up transplantation to a larger number of patients.However, for the time being, primary fetal cells are used for transplantation into PD and HD patients because these are the only cells to have been fully validated preclinically. Fundamental research into alternative cell sources is an active area, but the translation into the clinic should only

occur if the newly identified cells achieve the gold stan-dards in safety and functional recovery set by primary fetal cells. The second issue, the one concerning the inconsis-tency of outcome within and across studies and, particularly, how transplant-mediated functional benefits in the clinic could be optimized, will be at the core of this review.

Dogma that the CNS loses its plasticity after develop-ment and becomes a rigid network of permanently fixed neurons has been surrendering credibility progressively over time. Donald Hebb in 1949 introduced the idea that behavioral adaptation throughout life might rely on activity-dependent modification of the configuration and the strength of connections between neurons.5 Further experiments demonstrated that the brain’s chemistry and anatomy are indeed sculpted by the environment and experiences,

1University Hospital Freiburg, Freiburg, Germany2Cardiff University, Cardiff, Wales, United Kingdom

Corresponding Author:Máté D. Döbrössy, Department of Stereotactic and Functional Neurosurgery, Laboratory of Molecular Neurosurgery, University Hospital Freiburg, Breisacher Str. 64, 79106 Freiburg, GermanyEmail: [email protected]



Döbrössy et al 693

suggesting that the adult brain retains some of the plasticity that it has widely been believed to have lost following the end of development and in the postmitotic state. In this review, the focus is on the degree to which brain repair can be achieved by intracerebral transplantation of cells into a diseased state; hence, we are particularly concerned with the plasticity of the transplant. Plasticity in this context refers to adaptive events affecting the morphology, ana-tomical connections, and the functions subserved by grafted neurons. Animal studies—some of which will be discussed in the next sections—have shown that the effects of enriched environment, training, and grafting can each separately influence neuronal plasticity and recovery of function after brain damage. However, the mechanisms by which these factors interact, so that the environment or training might modify the survival, integration, or function of grafted tissues, is unknown at the present time. To improve the outcome following brain damage, cell replacement therapy must both make use of the endogenous potential for recov-ery of the host and optimize the external circumstances associated with any intervention. In particular, the obser-vation that for some aspects of transplant-mediated recovery the grafted animals require specific training indicates that graft function and experience are not simply independent and additive but are fundamentally interactive.

The ProblemThe problem that this article addresses is the inconsistent and partial recovery often observed following transplanta-tion. In the past 10 years, there have been repeated calls—but little progress—relating to the need for developing activity-based rehabilitation processes using principles of activity-enhanced neuroplasticity and neurogenesis specifically relevant to individuals posttransplantation.6

The benefits of active rehabilitation in neurodegenera-tive diseases such as PD and HD include physical func-tioning, health-related quality of life, strength, balance, and gait.7-10 There is certainly clear evidence in support of task- and context-specific training for postural control and balance, gait and gait-related activities, and programs focusing on exercise and physical conditioning for people with mild to moderately impaired PD.11 There is also a small amount of evidence in support of physical therapy for people in the early to mid stages of HD. Recent results from an intensive inpatient rehabilitation program have indicated that a physical therapy program that includes gait, balance, and transfer training as well as strengthening, flexibility, and coordination activities resulted in improve-ment in motor performance following the 3-week interven-tion but little carryover effect of improvement.12

If clinical neurorehabilitation has positive effects on the patient, could neurorehabilitation, in conjunction with

cell replacement therapy, have an additive effect and improve overall functional recovery? To our knowledge, only 2 studies involving transplantation in humans have reported the application of rehabilitation in addition to transplantation. In the first study of neural transplantation of patients following subcortical motor stroke,13 participants either received transplantation plus rehabilitation or reha-bilitation only (nonsurgical control group). The specific content of the rehabilitation program was not described. Although safety was established, no significant changes in motor function as a result of combined transplantation and rehabilitation were identified in comparison to the nonsurgical control group.

A more recent study by Lima and colleagues,14 reported recently by this journal, investigated the safety and out-comes of implanting olfactory mucosal autografts in patients (n = 20) with chronic spinal cord injury (SCI). Extensive preoperative (31.8 ± 6.8 h/wk with a mean duration of 34.7 ± 30 weeks) and postoperative rehabilitation (32.7 ± 5.2 h/wk with a mean duration of 92 ± 37.6 weeks) was provided. Of interest in this study was that the focus of the rehabilitation protocol differed across study sites; the protocol that focused on freedom of movement and promo-tion of sensorimotor feedback to allow development of compensatory motor patterns resulted in better outcomes. The lack of a control group for rehabilitation alone (ie, surgical versus nonsurgical intervention) limits the conclu-sions that can be drawn from this study. Although it is unclear whether the transplantation process or the reha-bilitation resulted in the reported gains, it seems that rehabilitation is likely to play an important role in the success of surgical procedures.

Clinical Transplantation in Neurodegenerative Diseases and CNS DamagePD clinical trials. The late 1980s and early 1990s saw the first open-label fetal neural transplantation trials in PD patients.15 Open label studies applying different parameters helped identify variables that were critical for the patient outcome, and early studies showed a positive impact of the transplants in many cases.16 Key clinical features of PD sensitive to transplantation were bradykinesia, rigidity, time spent in off phase, off-phase dyskinesia, and the dose of antiparkinsonian drugs, whereas tremor was less affected. The typical course of postoperative clinical changes started between 3 to 12 months after grafting, allowing many patients to reduce their medication significantly.17

Independently from groups that had contributed to the animal research, 2 surgical teams sponsored by the National Institutes of Health conducted controlled trials on PD fetal tissue grafting.18,19 These studies used modified methods of immunosuppression, tissue processing, patient selection,



694 Neurorehabilitation and Neural Repair 24(8)

and implantation. For these first blinded transplantation trials, controls were deeply anesthetized and received a cranial borehole but did not receive intracerebral grafts. Using this alternative transplantation approach, the trials did not meet their primary outcome measures and failed to show constant improvement in transplanted patients. However, despite modified methods of tissue processing, a subgroup analysis of the Freed study revealed age to be a crucial factor because patients younger than 60 years of age improved.18 In the second published controlled trial, a clear improvement could be observed during the first 6 to 9 months after grafting when patients received immu-nosuppressants. Later, corresponding to the time of immu-nosuppression withdrawal, most patients deteriorated back to clinical baseline scores.19 Despite insufficient therapeutic effect in either trials, a significant portion of patients developed secondary dyskinetic movement disorders that persisted during the off states. This newly recognized side effect, referred to as graft-induced dyskinesia, is the subject of active preclinical research.20

During the past decade, a number of reports on the histological results of human trials have been published. The grafts contain surviving dopaminergic cells that express dopamine transporters and other proteins that characterize mature dopaminergic neurons and form extensive connec-tions into the surrounding host brain. In some reports, signs of inflammatory response of the host tissue were observed.17 This finding is not consistent in all reports and might there-fore hint at a possible influence of the method of tissue processing or the immunosuppressive regimen. Another surprising finding was that a minor subpopulation of grafted cells in some patients developed Lewy bodies that are typi-cal for PD pathology. This discovery might have an important impact on current PD pathophysiological models, but this is not believed to functionally compromise the graft.21

Although most clinical trials on fetal tissue transplantation for PD revealed encouraging results, the side effects and inconsistent efficacy, especially observed in the controlled trials, underscore the need for a new look at identifying crucial parameters that have an impact on the clinical out-come. Indeed, a clinical trial run by a consortium of centers with many years of experience has recently been awarded European Union funding; it addresses critical questions concerning patient selection, tissue preparation, delivery, immunosuppression, and off-state dyskinesia. A key objec-tive of this study will be to generate a protocol that can serve as a template for all future clinical trials in the cell therapy field, including stem cell–based therapies for PD.

HD clinical trials. The first clinical trial of cell transplan-tation in HD was reported in the early 1990s but provided no follow-up.22 In 1998, another report was released on intracerebral transplantation of fetal ganglionic eminence tissue in which tissue pieces were delivered within a

suspension by a bilateral stereotactical injection into the caudate nucleus and putamen.23 The investigators used the isolated lateral ganglionic eminence (LGE) of several donors per side. The immunosuppressive regimen was not specified, and only 1-year follow-up data were available. All 3 graft recipients displayed minor improvement in their Unified Huntington’s Disease Rating Scale (UHDRS) motor score, but details regarding which features of HD could be improved by cell therapy were not divulged.

The outcome of a more recent trial was published in 2002.24 The method of tissue processing (mechanical dis-sociation) and the choice of tissue (LGE) were comparable with previous trials. The surgeons stereotactically targeted the striatum, and oral immunosuppressants were adminis-trated for up to 6 months following grafting. The follow-up report gives rather sparse information about clinical outcome: at 12 months, a slight functional improvement in 5 but worsening in 2 of the patients was reported. The authors revealed increased mean putaminal D1-receptor binding and glucose uptake, but the UHDRS score did not signifi-cantly improve. However, after exclusion of 1 patient suf-fering from a severe subdural hemorrhage after transplantation, statistical analysis showed a significant improvement in the UHDRS motor score in the remaining group. A systematic long-term follow-up is lacking from this trial as well.

The first reports from trials using whole ganglionic eminence (WGE) transplantation into HD patients were published in 2000 and 2002, respectively. Bachoud-Lévi and colleagues25,26 grafted WGE tissue pieces into 5 patients with moderate to severe features of HD, with the first safety and efficacy reports published 2 years following the transplantation. Three patients had increasing striatal glucose uptake after grafting within the striatum, and these patients had improved or at least stabilized UHDRS motor and neuropsychological test scores. In 1 patient, decreasing metabolism was accompanied by cyst formation within that striatum. In another study, Rosser and colleagues27 made stereotaxic placements of enzymatically dissociated cell suspensions of WGE unilaterally into the striatum of 4 patients with early to moderate HD. In their 6-month follow-up report, the authors described general safety and efficiency of the procedure but no significant change of clinical features. A follow-up publication from Bachoud-Lévi’s group28 recently reported the outcome 6 years postgrafting: 2 patients continued to significantly decline, whereas 3 patients, who had initially improved, could maintain or even improve their initial clinical level. Simi-larly, another long-term report on 2 patients receiving enzymatically dissociated WGE tissue 5 years previously described 1 patient as improving and the other as deterio-rating.29 These publications suggest that cell replacement therapy in HD, in some of the cases, can alter the natural progression of the disease.




Postmortem findings have revealed additional informa-tion about the transplants. Graft maturation in WGE grafts continues for a longer period compared with ventral mes-encephalon (VM) grafts. The earliest histological findings after grafting of WGE are from a host brain 6 months after grafting (death not related to the transplantation) and shows developing precursor cells present within the graft.30 Fur-thermore, the report is in line with other postmortem data showing the expression of region-specific markers within grafts, only limited graft-to-host connections, and a lack of HD pathology within the graft tissue.31,32 There are very little histological data from long-term grafts. A recent publication describing postmortem analysis from 2 HD patients who had received striatal transplants 10 years prior to death described well-integrated grafts.33 These results need to be examined with caution because of several critical differences in the transplantation protocol used.

Overall, the clinical outcome to date implies that grafts can stabilize key features rather than reverse decline. The overall positive experiences from well-designed trials led to the initiation of a multicenter HD transplantation program led by Bachoud-Lévi and collaborators. A total of more than 50 patients have already received fetal WGE grafts, and a first report is expected within the next 2 years. It is our impression that providing optimal environmental conditions can amplify therapeutic effects to the HD patients. This might include targeted rehabilitation and physical therapy as well as efforts to support the patient’s independence.

Cellular interventions in stroke and SCI. Cell therapy interventions into cerebral ischemia–induced stroke or SCI have come to the forefront with the acceleration of research in stem cells. In contrast to PD/HD clinical trials of cell replacement therapy where the best practice in terms of preclinical animal models and clinical application have evolved progressively over many years and, what is impor-tant, within the context of primary fetal cells, cellular interventions for stroke and SCI are designed around engi-neered and cultured cells of various sources. In stroke, for example, preclinical work has been reported using immor-talized human neural stem cell lines, mesenchymal stem cells/stromal stem cells, hematopoietic stem cells, embryonic stem cells, and neuronal progenitors isolated from rodents and humans.34 Several different cell lines have demonstrated functional efficacy in animal models, including the NT2N immortalized line that was taken to phase I and II clinical trials, where they have been shown to be safe but not clearly effective when injected into the affected basal ganglion.35 However, a principle concern with carrying out large clini-cal trials is that any pluripotent cells might retain the potential to overproliferate and become cancerous over time, after transplantation.

A HD and PD cell intervention is based on direct, central, and stereotactic injection strategies because it intends to

replace lost neurons or transmitters. In the case of stroke and SCI, this strategy has been superseded in some instances by systemic injection strategies. This implies that the mechanism of action is not direct cellular replacement but more likely indirect actions of the transplanted cells boost-ing endogenous neurotrophic effects that enhance sprouting, angiogenesis, neuroprotection, remyelination, and immu-nomodulation, which could have consequences on axonal regeneration.36,37

A major shortcoming of cell intervention in stroke and SCI to date has been the lack of coordination within the community. As has been the case for HD and PD within the context of the Network of European CNS Transplanta-tion and Restoration (NECTAR) since the early 1990s, investigators developing cellular therapy for stroke are now coordinating their efforts and coming together to generate a standardized framework to guide future preclini-cal and clinical research.38 Similar group dynamics are needed for SCI.

The SolutionThere are many factors that influence the clinical outcome of cell replacement therapy, such as the disease stage when the intervention is made, the surgical center involved, and the cell preparation and storage conditions used. A potentially important aspect that has not been examined is whether the activities and the experience of the patient following trans-plantation could have an impact on the functional recovery mediated by the graft. This issue has been approached within the context of animal models of neurodegenerative diseases, principally HD and PD. What is the evidence from basic research that experience and the environment can influence the development of grafts posttransplantation?

Graft Plasticity: Emerging Concepts FromAnimal ModelsData supporting the rationale for translating experimental research of cell replacement therapy into the clinic have emerged exclusively from excitotoxic models of HD.39 There is overwhelming evidence that primary rat embryonic striatal tissue can survive, anatomically integrate with the host, and confer functional benefits on simple motor and sensorimotor and complex cognitive tasks.40,41 Experimental evidence strongly suggests that the basis for the graft-mediated functional and behavioral recovery is founded on 2 main factors.

1. Appropriate anatomical integration of the graft with the host: first, anterograde and retrograde tracing methods as well as xenografts have shown that the transplanted striatal tissue sends efferent




projections to the host globus pallidus and, under certain conditions, to the substantia nigra. It can receive afferent projections from the cortex, thalamus, substantia nigra, and the raphé nucleus.42 Second, electron microscopy data show that thalamic and nigral inputs make synaptic contacts with medium spiny neurons within the grafts.43 Third, activation of host nigral inputs to striatal grafts can stimulate GABA release from stria-topallidal terminals in the globus pallidus in the grafted animal with the same temporal resolution as in the intact striatopallidal neurons.44

2. Appropriate functional integration and plasticity of the graft within the host: first, grafted medial spiny neurons have similar electrophysiological characteristics, including long-term potentiation and depression profiles, as “endogenous” medial spiny neurons.45 Second, with well-designed training of the recipient animal, the graft can serve as the neural substrate for reacquiring a task—referred to as “learning to use the graft”—that has been lost following the lesion.40 Third, the grafted striatal neurons remain plastic: their morphological and ultrastructural properties can be “optimized” as they retain their sensitivity to environmental and activity-related factors to which the recipient is exposed to.46

Mechanisms of plasticity observed posttransplantation are not unique to striatal grafts, and examples from preclin-ical PD models will be described.

Environmental Enrichment: Graft Functionand PlasticityHousing animals in an enriched environment enhances their behavioral experiences by allowing greater opportunities for activity, exploration, play, and social interaction than standard, conventional housing provides. However, the life of most human beings is already far more enriched then any condition that scientists could recreate in a laboratory. The clinical relevance of environmental enrichment studies lies not principally in the observation that enrichment itself has a positive impact but in the knowledge gained regarding the substrates and mechanisms that are involved in the increased plasticity experienced by the host brain and the transplant. This section will give examples of the influence of the environment in the structural and functional plasticity of neural transplants in selected rodent models of brain damage and degeneration.

Cortical grafts in stroke and lesion models. The observation in human stroke victims of spontaneous functional recovery over several months following the incident offers strong

evidence for brain plasticity.47 In rodents, the functional integration of cortical tissue has been widely studied in a controlled model of ischemia induced by middle cerebral artery occlusion. Cortical grafts implanted into the infarcted area of the adult rat receive extensive afferent host con-nections from somatosensory and other pathways,48 which are functional, as demonstrated by an increased glucose uptake in the graft in response to stimulation of the con-tralateral vibrissae.49 Projections into the host brain are relatively sparse when the grafts are implanted into adult hosts, in contrast to the rich graft–host connections that are seen to form in the more permissive neonatal host brain.50 The absence of reciprocal anatomical integration partially explains why in this model the transplants may have little influence on host neuronal function.51 Neverthe-less, when grafted animals have been given the added benefit of housing in enriched environments, functional repair has been seen in particular on simple tests of motor asymmetry such as rotation, postural position, and balance on a rotating rod.52 Recovery was not seen on tests where functional effects of grafts appear to depend on the rees-tablishment of cortical and basal ganglia connections,53 suggesting that the effects are attributable to a secondary protection against thalamic atrophy rather than to a primary cortical reconstruction.54

Striatal grafts in the HD model. A recent study focused on how environmental manipulations can influence the morphological and cellular changes in transplanted embry-onic striatal neurons.55 Three aspects associated with striatal plasticity were examined: dendritic spine densities, cell volumes of the grafted neurons, and changes in the levels of brain-derived neurotrophic factor (BDNF). Dendritic spines are specialized structures located on dendrites interfacing with excitatory presynaptic terminals, and spine density reflects the level of synaptic activity of that par-ticular pathway.56 The study also looked at how the various experimental conditions affect the cell volume of the transplanted medial spiny neurons. Mammalian cells use a wide variety of cell volume regulatory mechanisms, such as the release or accumulation of ions through transport systems and ion channels, including Ca2+ permeable chan-nels,57 and correlations between cytoplasmic Ca2+ levels and cellular mechanisms associated with plasticity have been observed.58 Third, changes in the level of BDNF were examined on the grounds that it can modulate striatal neuronal survival,59 striatal differentiation,60 and synaptic function.61 The study showed functional recovery on behavior tests in the grafted animals of all groups inde-pendent of whether or not they were exposed to the enriched environment. However, at the level of morphological and cellular analysis, variations in graft development were apparent subsequent to the differential housing and exercise regimes. Animals housed in the enriched environment had




significantly more BDNF, a candidate substrate for influ-encing graft morphology and plasticity, and their grafts had both greater spine densities and larger cell volumes. The study gave a strong indication that environmental complexity is a factor that affects the plasticity of grafted cells in situ. These data, along with early evidence from 2 of the authors of this review (MD, SD) demonstrating that environmental enrichment can facilitate the expression of long-term potentiation—a proposed mechanism of synaptic plasticity—in the grafted striatum, suggest a high degree of physiological integration and interaction between the transplant and the host.

Influence of Experience, Training, andExercise on Graft FunctionWhereas enriched environment might have few direct clini-cal correlates, training and motor learning are the funda-mental pillars of rehabilitation and reeducation. Within the context of the laboratory, experience, training, and exercise refer to learned or spontaneous, voluntary or involuntary activities, with the common feature of increased motor output. The nature of the increased output is a question of debate, but accumulating data suggest that the more specific and more targeted the activity is with respect to the affected limb, as opposed to generic motor activity, the better the potential graft-mediated functional outcome can be.

Here, we examine, using examples from different models of transplantation into the experimentally damaged brain, how behavioral experience and training can effect graft function and repair.

Nigral grafts. The ability of dopaminergic transplants to restore complex sensorimotor behaviors in experimental PD is dependent on graft survival and reinnervation and is likely to be further modified by complex functional graft–host interactions, including the animal’s previous experi-ence. Within this context, a study examined the impact of hemispheric dominance and extensive testing regimes on the functional capabilities of dopaminergic transplants to restore skilled forelimb movements in rats with unilateral 6-hydroxydopamine lesions.62 Near complete recovery was observed in dopamine (DA) grafted animals that did not exhibit a strong hemispheric lateralization for paw use before lesion and implantation surgery, whereas animals with a clear lateralization of paw use and grafted into the contralateral hemisphere exhibited only moderate recovery.Finally, animals grafted ipsilateral to the preferred paw were most resistant to functional improvements in skilled forelimb use. However, the influence of hemispheric domi-nance on the degree of functional DA graft–induced res-toration was specific for skilled forelimb use, whereas no such differences were observed in other tests for motor and sensory functions related to the DA system.

How variable motor training schedules can affect func-tional recovery and graft development in the hemiparkin-sonian rat has also been examined recently (Dr Alex Klein, personal oral communication, November 2009). Animals with unilateral 6-OHDA medial forebrain bundle lesions, with or without intrastriatally grafted E14 ventral mesen-cephalon grafts, were subjected to training schedules with a varying amount of training in simple and complex motor tasks. Training sessions, including 4 different types of motor tests and lasting a total of 14 days, were organized before the lesion, at 2 and 3 months postlesion, and at 1 and 3 months posttransplantation: the 4 grafted groups and their controls were exposed to a different number of training ses-sions varying from a maximum of 5 to no training at all. The analysis of histological data revealed a strong correlation between the amount of training and graft integration: the more the training, the more the number of cells that survived, and the bigger the grafts, the denser the fiber outgrowth from the graft into the host. However, larger grafts did not neces-sarily translate into better functional recovery; what seems to be more important is the extent and the location of dopa-minergic reinnervation of the host. Additional data from the same animal model emphasize the importance of training protocol with better graft-mediated functional recovery observed under forced-choice conditions.63

Retinotectal grafts. Grafts of retina implanted into the midbrain of enucleated rats not only survive and connect anatomically with the host brain but can also transduce a light stimulus to drive a pupillary reflex.64 Does the percep-tion of the light confer a meaning, or if lost as a result of brain damage, is that meaning something that needs to be relearned? Coffey et al65 tested this in rats in 2 paradigms: a simple open-field avoidance chamber and an operant condi-tioned suppression test. The open field comprised a circular arena, divided into 3 equal segments with opaque, transpar-ent, and open ceilings. A bright light was positioned over-head. When allowed to explore, a normal rat spends most of the time under the shade of the opaque roof, whereas a sightless rat does not distinguish between the zones. The experimental rats had their normal vision obscured by eye patches and were prepared surgically with a Perspex window in the skull over the retinal grafts. When first exposed to the open field, they showed no differences in the time spent in each quadrant. However, it appeared that the failure was not in their ability to detect the light but in the fact that they did not attach meaning to this novel channel of sensory input. This was confirmed by providing specific training in which light was paired with a foot shock in the training phase of a conditioned suppression task. Effective learning was shown by a progressive reduction in lever pressing for food reward during light stimulation, indicating that the rats could detect the visual stimulus. More important, when the rats were placed back into the open arena, they avoided




the bright segments when viewing the world exclusively through the transplanted retina, indicating that they had learned to use the stimulus to control behavior.65 The con-cept that the grafted animals must be trained to benefit from their grafts is referred to as “learning to use the graft,” and it is similar to the long-established principles of visual development where animals (and people) have to be proac-tive in learning to interpret the visual world.

Striatal grafts. In a “9-hole box” operant chamber, ani-mals can be trained to poke their noses into a central hole and then to respond rapidly to brief light stimuli on the left or right sides. Unilateral lesions of the intrinsic striatal neurons disrupt the initiation of responses on the contra-lateral side, with lesions specifically affecting initiation without affecting the animal’s ability to detect or attend to the eliciting stimulus.66 In a seminal study by Brasted and colleagues40 in which unilateral striatal lesions and striatal grafts were given to rats that had been trained to respond on the opposite side to the stimulus, both lesioned and grafted animals exhibited profound deficits in responding on the contralateral side when returned to the test 4 months later. Whereas the lesioned rats could not relearn the task, the grafted animals did so with training. The relearning took place over a similar period to that required by naïve rats to learn the task de novo. Additional studies con-firmed that graft-associated relearning involves specific stimulus–response associations mediated by the transplanted striatum and is not achieved simply by general training in task performance.67 A mechanism for the relearning has been proposed by Mazzocchi-Jones and colleagues45 who demonstrated within grafts bidirectional synaptic plasticity, involving long-term depression or long-term potentiation dependent on the physiological environment, similar to the normal corticostriatal system. These results indicate that striatal grafts express physiological plasticity of a type, which at the synaptic level is required to represent new learning, and thereby provide direct evidence of functional neuronal circuit repair, an essential component of functional integration.

Recommendations for Rehabilitation in TransplantationAlthough the limited transferability of preclinical data to the clinical setting must be acknowledged,68 animal studies imply the need for specificity of retraining following graft-ing and provide support for active interventions in people with HD and PD posttransplantation.69 Existing evidence and knowledge of the biological bases of rehabilitation must be used and evaluated.70 The type of rehabilitation and, in particular, the dose and timing of interventions for optimizing graft function posttransplantation has yet to be established, and intervention effects have been reported to

decline on cessation. It would seem that to ensure sustained functional benefit, variations of shorter series of intense rehabilitation periods could be interspersed with less intense and less frequent training over a prolonged period of time.11 Furthermore, additional data are required to confirm that rehabilitation prior to transplantation—which is feasible as the dates for such interventions can be planned in advance—may be advantageous to achieve a stable baseline of function and familiarize patients with the posttransplant rehabilitation regimen.

The use of cueing strategies, repetitive task-specific training, and application of motor learning principles are hallmarks of rehabilitation interventions and have the potential to be specifically used to support targeted post-transplantation rehabilitation programs.71 Training should be meaningful to the individual and should be task and home based rather than generic and hospital based. This sugges-tion is clearly supported by the positive findings of the RESCUE study, where a 3-week period of home-based cueing training resulted in improvements in gait speed, step length, and a timed balance test.7 The concept of dual-task and cognitive load training may also be important in clinical conditions such as PD and HD, where cognitive training is relevant to improving functional gains. Recently, multiple-task training performed for 30 minutes once a week for 3 weeks showed beneficial effects on walking speed, stride length, and cadence under multiple-task conditions.72

Promotion of physical activity and facilitation of regular adherence to training routines for the long term is also likely to be important in supporting maintenance of functional gains. The exercise prescription, such as the timing and intensity, should be customized to the symptoms and functional capacity of the individual. Implementation of a graded exercise program emphasizing activities such as walking that initially place minimal demands on the car-diovascular system but promote restoration of mobility and fitness could be advantageous. Other forms of activity and exercise, including strength training, flexibility, neuromus-cular adaptation, and functional activity, should also be considered. Given that immunosuppression is an integral part of the posttransplantation protocol, the rehabilitation program should keep in mind that the immune system func-tion may be impaired by high-intensity exercise, whereas moderate exercise may reduce susceptibility to illness.71,73

In developing appropriate posttransplantation rehabilita-tion programs, it is important to consider ways in which not only task specificity of training can be encouraged but also ways in which clinical improvements can be measured. Outcome measures in rehabilitation trials need to be sensi-tive to the spectrum of impairments and functional loss that may be seen. Ordinal-type functional scales may lack the sensitivity and specificity required to identify clinically relevant change following rehabilitation interventions,68




so perhaps, quality-of-life measures should be considered as secondary outcome measures rather than as a primary outcome. The Unified Parkinson’s Disease Rating Scale (UPDRS) and the UHDRS are disease-specific rating systems that are used to quantify the severity of disease, assessing specific domains of clinical performance and capacity. Both scales appear to be appropriate for repeated administration during clinical studies but may not be suf-ficiently specific for the assessment of functional ability related to rehabilitation. Amelioration of any particular disease-specific impairment may not translate into functional improvement.68 Impairments in gait parameters are known to exist early on in the life cycle of both PD and HD, mak-ing clinical gait assessment one potential primary outcome measure in posttransplantation rehabilitation trials.74,75 Finally, structural and functional imaging may help define the biological basis for any serial changes in function fol-lowing transplantation and aid dose–response studies.68

ConclusionsNeural transplantation of embryonic cells can promote functional recovery in animal models of brain damage and disease. The transplanted cells retain many of the plastic features that are associated with embryonic neurons and can send and receive suitable efferent and afferent projec-tions that are necessary for anatomical, physiological, and functional integration with the host. The process of integra-tion at these various levels can be influenced by environ-mental conditions, exercise, and training. What is important is that functional recovery can be enhanced by targeted training of the affected deficit, whereby the recipient “learns to use the transplant.” Translating cell replacement therapy into clinical trials of PD and HD has shown it to be a safe and viable procedure with variable impact on behavioral symptoms. The knowledge gained about brain plasticity following brain damage needs to be linked with what we know about promoting intrinsic recovery processes, so that clinicians can integrate neurobiological and surgical strate-gies. Clinicians in the neurorehabilitation and cell trans-plantation fields should also cooperate to optimize cellular integration and maximize outcomes.

Declaration of Conflicting Interests

The authors declared no conflicts of interest with respect to the authorship and/or publication of this article.

Funding

The authors received no financial support for the research and/or authorship of this article.

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Documents

Review: Neurorehabilitation With Neural Transplantation