Pattern of long-distance projections from fetal hippocampal field CA3 and CA1 cell grafts in lesioned CA3 of adult hippocampus follows intrinsic character of respective donor cells

GABA-ergic cell therapy for epilepsy: Advances, limitations and challenges

Diminution in the number of gamma-amino butyric acid positive (GABA-ergic) interneurons and their axon terminals, and/or alterations in functional inhibition are conspicuous brain alterations believed to contribute to the persistence of seizures in acquired epilepsies such as temporal lobe epilepsy. This has steered a perception that replacement of lost GABA-ergic interneurons would improve inhibitory synaptic neurotransmission in the epileptic brain region and thereby reduce the occurrence of seizures. Indeed, studies using animal prototypes have reported that grafting of GABA-ergic progenitors derived from multiple sources into epileptic regions can reduce seizures. This review deliberates recent advances, limitations and challenges concerning the development of GABA-ergic cell therapy for epilepsy. The efficacy and limitations of grafts of primary GABA-ergic progenitors from the embryonic lateral ganglionic eminence and medial ganglionic eminence (MGE), neural stem/progenitor cells expanded from MGE, and MGE-like progenitors generated from human pluripotent stem cells for alleviating seizures and co-morbidities of epilepsy are conferred. Additional studies required for possible clinical application of GABA-ergic cell therapy for epilepsy are also summarized.

Colloids as mobile substrates for the implantation and integration of differentiated neurons into the mammalian brain

Neuronal degeneration and the deterioration of neuronal communication lie at the origin of many neuronal disorders, and there have been major efforts to develop cell replacement therapies for treating such diseases. One challenge, however, is that differentiated cells are challenging to transplant due to their sensitivity both to being uprooted from their cell culture growth support and to shear forces inherent in the implantation process. Here, we describe an approach to address these problems. We demonstrate that rat hippocampal neurons can be grown on colloidal particles or beads, matured and even transfected in vitro, and subsequently transplanted while adhered to the beads into the young adult rat hippocampus. The transplanted cells have a 76% cell survival rate one week post-surgery. At this time, most transplanted neurons have left their beads and elaborated long processes, similar to the host neurons. Additionally, the transplanted cells distribute uniformly across the host hippocampus. Expression of a fluorescent protein and the light-gated glutamate receptor in the transplanted neurons enabled them to be driven to fire by remote optical control. At 1-2 weeks after transplantation, calcium imaging of host brain slice shows that optical excitation of the transplanted neurons elicits activity in nearby host neurons, indicating the formation of functional transplant-host synaptic connections. After 6 months, the transplanted cell survival and overall cell distribution remained unchanged, suggesting that cells are functionally integrated. This approach, which could be extended to other cell classes such as neural stem cells and other regions of the brain, offers promising prospects for neuronal circuit repair via transplantation of in vitro differentiated, genetically engineered neurons.

Progress in cell grafting therapy for temporal lobe epilepsy

Temporal lobe epilepsy (TLE), exemplified by complex partial seizures, is recognized in ~30% of epileptic patients. Seizures in TLE are associated with cognitive dysfunction and are resistant to antiepileptic drug therapy in ~35% of patients. Although surgical resection of the hippocampus bestows improved seizure regulation in most cases of intractable TLE, this choice can cause lasting cognitive deficiency and reliance on antiepileptic drugs. Thus, alternative therapies that are proficient in both containing the spontaneous recurrent seizures and reversing the cognitive dysfunction are needed. The cell transplantation approach is promising in serving as an adept alternate therapy for TLE, because this strategy has shown the capability to curtail epileptogenesis when used soon after an initial precipitating brain injury, and to restrain spontaneous recurrent seizures and improve cognitive function when utilized after the occurrence of TLE. Nonetheless, this treatment needs further advancement and rigorous evaluation in animal prototypes of chronic TLE before the conceivable clinical use. It is especially vital to gauge the efficacy of distinct donor cell types, such as the hippocampal precursor cells, γ-aminobutyric acid-ergic progenitors, and neural stem cells derived from diverse human sources (including the embryonic stem cells and induced pluripotent stem cells) for longstanding seizure suppression using continuous electroencephalographic recordings for prolonged periods. Additionally, the identification of the mechanisms underlying the graft-mediated seizure suppression and improved cognitive function, and the development of apt grafting strategies that enhance the anti-seizure and pro-cognitive effects of grafts will be necessary. The goal of this review is to evaluate the progress made hitherto in this area and to discuss the prospect for cell-based therapy for TLE.

Hippocampal stem cell grafting-mediated recovery of injured hippocampus in the rat model of temporal lobe epilepsy

Hippocampal stem cells (HSCs) are considered promising donor cells to promote reorganization of degenerated regions of the injured hippocampus in the epileptic brain. However, the efficacy of HSC grafting for repairing injured hippocampus remains unclear. To address this issue, we transplanted neonatal rat HSCs into the right hippocampus in rats with kainite acid (KA)-induced epilepsy. The activity of the hippocampus and amygdala nucleus was monitored with electroencephalogram (EEG) throughout 24 weeks posttransplantation. Rats with grafted HSCs exhibited reduced frequency of epileptic wave discharge and a 50% decrease in the amplitude of discharge. At 1, 4, 8, and 24 weeks posttransplantation, the aberrant mossy fiber sprouting (MFS) was evaluated with Timm's stain and the number of CA3 pyramidal neurons was analyzed with Nissl staining. Aberrant MFS induced by KA-lesion was notably suppressed by HSC grafts beginning 4 weeks posttransplantation, and was most effective by 8 weeks. In addition, the loss of CA3 pyramidal neurons was partially restored and reached the most recovery at 8 weeks. Taken together, these results suggest that HSCs derived from the postnatal hippocampus offer a promising reparative effect on KA-induced epileptic brain.

Neuromodulation in Epilepsy

Neuromodulation strategies have been proposed to treat a variety of neurological disorders, including medication-resistant epilepsy. Electrical stimulation of both central and peripheral nervous systems has emerged as a possible alternative for patients who are not deemed to be good candidates for resective procedures. In addition to well-established treatments such as vagus nerve stimulation, epilepsy centers around the world are investigating the safety and efficacy of neurostimulation at different brain targets, including the hippocampus, thalamus, and subthalamic nucleus. Also promising are the preliminary results of responsive neuromodulation studies, which involve the delivery of stimulation to the brain in response to detected epileptiform or preepileptiform activity. In addition to electrical stimulation, novel therapeutic methods that may open new horizons in the management of epilepsy include transcranial magnetic stimulation, focal drug delivery, cellular transplantation, and gene therapy. We review the current strategies and future applications of neuromodulation in epilepsy.

GABAergic neuronal precursor grafting: implications in brain regeneration and plasticity

Numerous neurological disorders are caused by a dysfunction of the GABAergic system that impairs or either stimulates its inhibitory action over its neuronal targets. Pharmacological drugs have generally been proved very effective in restoring its normal function, but their lack of any sort of spatial or cell type specificity has created some limitations in their use. In the last decades, cell-based therapies using GABAergic neuronal grafts have emerged as a promising treatment, since they may restore the lost equilibrium by cellular replacement of the missing/altered inhibitory neurons or modulating the hyperactive excitatory system. In particular, the discovery that embryonic ganglionic eminence-derived GABAergic precursors are able to disperse and integrate in large areas of the host tissue after grafting has provided a strong rationale for exploiting their use for the treatment of diseased brains. GABAergic neuronal transplantation not only is efficacious to restore normal GABAergic activities but can also trigger or sustain high neuronal plasticity by promoting the general reorganization of local neuronal circuits adding new synaptic connections. These results cast new light on dynamics and plasticity of adult neuronal assemblies and their associated functions disclosing new therapeutic opportunities for the near future.

Medial ganglionic eminence-derived neural stem cell grafts ease spontaneous seizures and restore GDNF expression in a rat model of chronic temporal lobe epilepsy

Nearly 30% of patients with mesial temporal lobe epilepsy (TLE) are resistant to treatment with antiepileptic drugs. Neural stem cell (NSC) grafting into the hippocampus could offer an alternative therapy to hippocampal resection in these patients. As TLE is associated with reduced numbers of inhibitory gamma-amino butyric acid (GABA)-ergic interneurons and astrocytes expressing the anticonvulsant glial-derived neurotrophic factor (GDNF) in the hippocampus, we tested the hypothesis that grafting of NSCs that are capable of adding new GABA-ergic interneurons and GDNF-expressing astrocytes into the epileptic hippocampus restrains spontaneous recurrent motor seizures (SRMS) in chronic TLE. We grafted NSCs expanded in vitro from embryonic medial ganglionic eminence (MGE) into hippocampi of adult rats exhibiting chronic TLE with cognitive impairments. NSC grafting reduced frequencies of SRMS by 43% and stage V seizures by 90%. The duration of individual SRMS and the total time spent in seizures were reduced by 51 and 74%, respectively. Grafting did not improve the cognitive function however. Graft-derived cells (equivalent to approximately 28% of injected cells) were observed in various layers of the epileptic hippocampus where they differentiated into NeuN+ neurons (13%), S-100beta+ astrocytes (57%), and NG2+ oligodendrocyte-progenitors (3%). Furthermore, among graft-derived cells, 10% expressed GABA and 50% expressed GDNF. Additionally, NSC grafting restored GDNF in a vast majority of the hippocampal astrocytes but had no effect on neurogenesis. Thus, MGE-NSC therapy is efficacious for diminishing SRMS in chronic TLE. Addition of new GABA-ergic neurons and GDNF+ cells, and restoration of GDNF in the hippocampal astrocytes may underlie the therapeutic effect of MGE-NSC grafts.

Transplantation of GABA-producing cells for seizure control in models of temporal lobe epilepsy

A high percentage of patients with temporal lobe epilepsy (TLE) are refractory to conventional pharmacotherapy. The progressive neurodegenerative processes associated with a lifetime of uncontrolled seizures mandate the development of alternative approaches to treat this disease. Transplantation of inhibitory cells has been suggested as a potential therapeutic strategy to achieve seizure suppression in humans with intractable TLE. Preclinical investigations over 20 years have demonstrated that multiple cell types from several sources can produce anticonvulsant, and antiepileptogenic, effects in animal models of TLE. Transplanting GABA-producing cells, in particular, has been shown to reduce seizures in several well-established models. This review addresses experimentation using different sources of transplantable GABAergic cells, highlighting progress with fetal tissue, neural cell lines, and stem cells. Regardless of the source of the GABAergic cells used in seizure studies, common challenges have emerged. Several variables influence the anticonvulsant potential of GABA-producing cells. For example, tissue availability, graft survival, immunogenicity, tumorigenicity, and varying levels of cell migration, differentiation, and integration into functional circuits and the microenvironment provided by sclerotic tissue all contribute to the efficacy of transplanted cells. The challenge of understanding how all of these variables work in concert, in a disease process that has no well-established etiology, suggests that there is still much basic research to be done before rational cell-based therapies can be developed for TLE.

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.

Is exposure to enriched environment beneficial for functional post-lesional recovery in temporal lobe epilepsy?

Exposure to enriched environment has been shown to induce robust neuronal plasticity in both intact and injured adult central nervous system, including up-regulation of multiple neurotrophic factors, enhanced neurogenesis in the dentate gyrus of the hippocampus, and improved spatial learning and memory function. Neuronal plasticity, though mostly adaptive and abnormal, also occurs during certain neurodegenerative conditions such as the temporal lobe epilepsy (TLE). The TLE is characterized by hippocampal neurodegeneration, aberrant mossy fiber sprouting, spontaneous recurrent motor seizures, cognitive deficits, and abnormally enhanced neurogenesis during the early phase and dramatically declined neurogenesis during the chronic phase of the disease. As environmental enrichment has been found to be beneficial for treating animal models of Alzheimer's, Parkinson's, and Huntington's diseases, there is considerable interest in determining the efficacy of this strategy for preventing or treating chronic TLE after the initial precipitating brain injury. This review first discusses the proof of principle behind the potential application of the environmental enrichment strategy for preventing or treating TLE after brain injury. The subsequent chapters confer the portrayed beneficial effects of enrichment for functional post-lesional recovery in TLE and the possible complications which may arise from housing epilepsy-prone or epileptic rats in enriched environmental conditions. The final segment discusses studies that are essential for further understanding the efficacy of this approach for preventing or treating TLE.

Concise review: prospects of stem cell therapy for temporal lobe epilepsy

Certain regions of the adult brain have the ability for partial self-repair after injury through production of new neurons via activation of neural stem/progenitor cells (NSCs). Nonetheless, there is no evidence yet for pervasive spontaneous replacement of dead neurons by newly formed neurons leading to functional recovery in the injured brain. Consequently, there is enormous interest for stimulating endogenous NSCs in the brain to produce new neurons or for grafting of NSCs isolated and expanded from different brain regions or embryonic stem cells into the injured brain. Temporal lobe epilepsy (TLE), characterized by hyperexcitability in the hippocampus and spontaneous seizures, is a possible clinical target for stem cell-based therapies. This is because these approaches have the potential to curb epileptogenesis and prevent chronic epilepsy development and learning and memory dysfunction after hippocampal damage related to status epilepticus or head injury. Grafting of NSCs may also be useful for restraining seizures during chronic epilepsy. The aim of this review is to evaluate current knowledge and outlook pertaining to stem cell-based therapies for TLE. The first section discusses the behavior of endogenous hippocampal NSCs in human TLE and animal models of TLE and evaluates the role of hippocampal neurogenesis in the pathophysiology and treatment of TLE. The second segment considers the prospects for preventing or suppressing seizures in TLE using exogenously applied stem cells. The final part analyzes problems that remain to be resolved before initiating clinical application of stem cell-based therapies for TLE. Disclosure of potential conflicts of interest is found at the end of this article.

Cell therapy in models for temporal lobe epilepsy

For patients with refractory epilepsy it is important to search for alternative treatments. One of these potential treatments could be introducing new cells or modulating endogenous neurogenesis to reconstruct damaged epileptic circuits or to bring neurotransmitter function back into balance. In this review the scientific basis of these cell therapy strategies is discussed and the results are critically evaluated. Research on cell transplantation strategies has mainly been performed in animal models for temporal lobe epilepsy, in which seizure foci or seizure propagation pathways are targeted. Promising results have been obtained, although there remains a lot of debate about the relevance of the animal models, the appropriate target for transplantation, the suitable cell source and the proper time point for transplantation. From the presented studies it should be evident that transplanted cells can survive and sometimes even integrate in an epileptic brain and in a brain that is subjected to epileptogenic interventions. There is evidence that transplanted cells can partially restore damaged structures and/or release substances that modulate existent or induced hyperexcitability. Even though several studies show encouraging results, more studies need to be done in animal models with spontaneous seizures in order to have a better comparison to the human situation.

Strategies for promoting anti-seizure effects of hippocampal fetal cells grafted into the hippocampus of rats exhibiting chronic temporal lobe epilepsy

Efficacy of hippocampal fetal cell (HFC) grafting for restraining spontaneous recurrent motor seizures (SRMS) in chronic temporal lobe epilepsy (TLE) is unknown. We investigated both survival and anti-seizure effects of 5'-bromodeoxyuridine (BrdU) labeled embryonic day 19 (E19) HFC grafts pretreated with different neurotrophic factors and a caspase inhibitor. Grafts were placed bilaterally into the hippocampi of F344 rats exhibiting kainate (KA) induced chronic TLE, where the frequency of SRMS varied from 3.0 to 3.5 seizures/8-h duration. The first group received standard (untreated) HFC grafts, the second group received HFC grafts pretreated and transplanted with brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3) and caspase inhibitor Ac-YVAD-cmk (BNC-treated HFC grafts), the third group received HFC grafts pretreated and transplanted with fibroblast growth factor-2 (FGF-2) and caspase inhibitor Ac-YVAD-cmk (FC-treated HFC grafts), and the fourth group served as epilepsy-only controls. Epileptic rats receiving standard HFC grafts exhibited 119% increase in the frequency of SRMS at 2 months post-grafting consistent with 125% increase in seizure frequency observed in epilepsy-only controls during the same period. However, in epileptic rats receiving HFC grafts treated with BNC or FC, the frequency of SRMS was 33-39% less than their pre-transplant scores and 73-76% less than rats receiving standard HFC grafts or epilepsy-only rats. The yield of surviving neurons was equivalent to 30% of injected cells in standard HFC grafts, 57% in HFC grafts treated with BNC and 98% in HFC grafts treated with FC. Thus, standard HFC grafts survive poorly in the chronically epileptic hippocampus and fail to restrain the progression of chronic TLE. In contrast, HFCs treated and grafted with BNC or FC survive robustly in the chronically epileptic hippocampus, considerably reduce the frequency of SRMS and blunt the progression of chronic TLE.

Incorporation of embryonic CA3 cell grafts into the adult hippocampus at 4-months after injury: effects of combined neurotrophic supplementation and caspase inhibition

As receptivity of the injured hippocampus to cell grafts decreases with time after injury, strategies that improve graft integration are necessary for graft-mediated treatment of chronic neurodegenerative conditions such as temporal lobe epilepsy. We ascertained the efficacy of two distinct graft-augmentation strategies for improving the survival of embryonic day 19 hippocampal CA3 cell grafts placed into the adult hippocampus at 4-months after kainic acid induced injury. The donor cells were labeled with 5'-bromodeoxyuridine, and pre-treated and grafted with either brain-derived neurotrophic factor, neurotrophin-3 and a caspase inhibitor or fibroblast growth factor and caspase inhibitor. The yield of surviving grafted cells and neurons were quantified at 2-months post-grafting. The yield of surviving cells was substantially greater in grafts treated with brain-derived neurotrophic factor, neurotrophin-3 and caspase inhibitor (84%) or fibroblast growth factor and caspase inhibitor (99% of injected cells) than standard cell grafts (26%). Because approximately 85% of surviving grafted cells were neurons, increased yield in augmented groups reflects enhanced survival of grafted neurons. Evaluation of the mossy fiber synaptic re-organization in additional kainic acid-lesioned rats receiving grafts enriched with brain-derived neurotrophic factor, neurotrophin-3 and caspase inhibitor at 3-months post-grafting revealed reduced aberrant dentate mossy fiber sprouting in the dentate supragranular layer than "lesion-only" rats at 4 months post-kainic acid, suggesting that some of the aberrantly sprouted mossy fibers in the dentate supragranular layer withdraw when apt target cells (i.e. grafted neurons) become available in their vicinity. Thus, the yield of surviving neurons from CA3 cell grafts placed into the adult hippocampus at an extended time-point after injury could be enhanced through apt neurotrophic supplementation and caspase inhibition. Apt grafting is also efficacious for reversing some of the abnormal synaptic reorganization prevalent in the hippocampus at later time-points after injury.

Repair of the injured adult hippocampus through graft-mediated modulation of the plasticity of the dentate gyrus in a rat model of temporal lobe epilepsy

Intracerebroventricular kainate administration in rat, a model of temporal lobe epilepsy (TLE), causes degeneration of the hippocampal CA3 pyramidal and dentate hilar neurons. This leads to a robust but aberrant sprouting of the granule cell axons (mossy fibers) into the dentate supragranular layer and the CA3 stratum oriens. Because this plasticity is linked to an increased seizure susceptibility in TLE, strategies that restrain the aberrant mossy fiber sprouting (MFS) are perceived to be important for preventing the TLE development after the hippocampal injury. We ascertained the efficacy of fetal hippocampal CA3 or CA1 cell grafting into the kainate-lesioned CA3 region of the adult rat hippocampus at early post-kainic acid injury for providing a lasting inhibition of the aberrant MFS. Analyses at 12 months after grafting revealed that host mossy fibers project vigorously into CA3 cell grafts but avoid CA1 cell grafts. Consequently, in animals receiving CA3 cell grafts, the extent of aberrant MFS was minimal, in comparison with the robust MFS observed in both "lesion-only" animals and animals receiving CA1 cell grafts. Analyses of the graft axon growth revealed strong graft efferent projections into the dentate supragranular layer with CA3 cell grafting but not with CA1 cell grafting. Thus, the formation of reciprocal circuitry between the dentate granule cells and the grafted CA3 pyramidal neurons is likely the basis of inhibition of the aberrant MFS by CA3 cell grafts. The results also underscore that grafting of cells capable of differentiating into CA3 pyramidal neurons is highly efficacious for a lasting inhibition of the abnormal mossy fiber circuitry development in the injured hippocampus.

Fetal hippocampal CA3 cell grafts enriched with FGF-2 and BDNF exhibit robust long-term survival and integration and suppress aberrant mossy fiber sprouting in the injured middle-aged hippocampus

Cell transplants that successfully replace the lost neurons and facilitate the reconstruction of the disrupted circuitry in the injured aging hippocampus are invaluable for treating acute head injury, stroke and status epilepticus in the elderly. This is because apt graft integration has the potential to prevent the progression of the acute injury into chronic epilepsy in the elderly. However, neural transplants into the injured middle-aged or aged hippocampus exhibit poor cell survival, suggesting that apt graft augmentation strategies are critical for robust integration of grafted cells into the injured aging hippocampus. We examined the efficacy of pre-treatment and grafting of donor fetal CA3 cells with a blend of fibroblast growth factor-2 (FGF-2) and brain-derived neurotrophic factor (BDNF) for lasting survival and integration of grafted cells in the injured middle-aged (12 months old) hippocampus of F344 rats. Grafts were placed at 4 days after the kainic-acid-induced hippocampal injury and were analyzed at 6 months post-grafting. We demonstrate that 80% of grafted cells exhibit prolonged survival and 71% of grafted cells differentiate into CA3 pyramidal neurons. Grafts also receive a robust afferent input from the host mossy fibers and project efferent axons into the denervated zones of the dentate gyrus and the CA1 subfield. Consequently, the aberrant sprouting of the dentate mossy fibers, an epileptogenic change that typically ensues after the hippocampal injury, was suppressed. Thus, grafts of fetal CA3 cells enriched with FGF-2 and BDNF exhibit robust integration and dampen the abnormal mossy fiber sprouting in the injured middle-aged hippocampus. Because the aberrantly sprouted mossy fibers contribute to the generation of seizures, the results suggest that the grafting intervention using FGF-2 and BDNF is efficacious for suppressing epileptogenesis in the injured middle-aged hippocampus.

Focal treatment for refractory epilepsy: hope for the future?

Despite advances in anti-epileptic drug therapy and epilepsy surgery in recent years, intractable epilepsy remains a large clinical problem. Surgical resection, which can have an excellent outcome, is appropriate for only a minority of patients in whom an identifiable focus in non-eloquent brain can be identified. Systemic drug delivery is inevitably limited by the potential for unwanted side effects, due to actions both outside the CNS and in non-epileptic brain regions. Thus for a substantial number of patients novel treatment approaches are urgently needed. Both focal drug delivery and neuronal stem cell grafting have been evaluated in a variety of experimental epilepsy models in recent years, targeting either the seizure focus or key propagation pathways. The literature in this field is critically reviewed and considered in a clinical context. Studies in both areas are hampered by the limitations of available animal models, and by uncertainties in discerning which changes in the epileptic brain directly promote seizures, and which are compensatory. However, in many cases promising, though short-term, results have been obtained. Before such studies could be considered in humans further investigations that include long-term seizure and behavioural outcomes, in clinically relevant experimental models, are required. However, the current literature does provide proof in principle for a focal treatment approach, which may offer hope for many currently intractable patients for whom drug developments and surgical advances have proved disappointing.

Fetal hippocampal CA3 cell grafts enriched with fibroblast growth factor-2 exhibit enhanced neuronal integration into the lesioned aging rat hippocampus in a kainate model of temporal lobe epilepsy

Aging impairs the conduciveness of the lesioned hippocampus for robust survival of neurons derived from homotopic fetal cell grafts (Zaman and Shetty, Neuroscience 109:537-553, 2002), suggesting a need for graft augmentation in fetal graft-mediated therapeutic strategies for the lesioned aging hippocampus. We hypothesize that pretreatment and grafting of donor hippocampal CA3 cells with fibroblast growth factor-2 (FGF-2) considerably enhances graft neuronal integration into the lesioned CA3 region of the aging hippocampus. We employed the optical fractionator cell counting method and quantified the number of surviving cells and neurons derived from 5'-bromodoxyuridine-labeled embryonic day 19 CA3 cell grafts pre-treated and transplanted with FGF-2 into the lesioned CA3 region of the middle-aged and aged rat hippocampus at 4 days post-lesion. In both middle-aged and aged hippocampus, pre-treatment and transplantation of CA3 cell grafts with FGF-2 resulted in a robust yield of surviving cells (72-80% of injected cells) and neurons (62-69% of injected cells) from grafts. The overall yield was dramatically greater than the yield observed earlier from standard (untreated) fetal CA3 cell grafts into the lesioned aging hippocampus but was highly comparable to that observed for standard fetal CA3 cell grafts into the lesioned young hippocampus (Zaman and Shetty, Neuroscience 109:537-553, 2002). Thus, a robust neuronal integration from fetal CA3 cell grafts can be achieved into the lesioned CA3 region of the aging hippocampus with a simple pre-treatment and grafting of donor fetal CA3 cells with FGF-2. These results have implications toward the development of suitable cell grafting strategies for repair of the lesioned aging hippocampus in neurodegenerative diseases, particularly the temporal lobe epilepsy, stroke, and Alzheimer's disease.

Pretreatment of donor cells with FGF-2 enhances survival of fetal hippocampal CA3 cell transplants in the chronically lesioned young adult hippocampus

The lesioned CA3 region of the young adult hippocampus is very conducive for robust survival and integration of fetal hippocampal CA3 cell grafts when transplanted at an early postlesion delay of 4 days. However, similar CA3 cell grafts placed at 45 days postlesion display significantly diminished cell survival, implying that the receptivity of the lesioned young adult host hippocampus to grafts decreases considerably with a prolonged postlesion transplantation delay. We hypothesize that decreased cell survival in grafts placed into the chronically lesioned hippocampus is due to a reduced level of host neurotrophic factors that support fetal hippocampal cells; hence, pretreatment and grafting of donor fetal CA3 cells with fibroblast growth factor-2 (FGF-2) considerably enhances graft neuronal integration into the chronically lesioned young adult hippocampus. We employed the optical fractionator cell counting method and rigorously quantified the number of surviving cells and neurons derived from 5'-bromodeoxyuridine-labeled Embryonic Day 19 CA3 cell grafts pretreated and transplanted with FGF-2 into the lesioned CA3 region of the young adult rat hippocampus, at a delay of 60 days after a unilateral intracerebroventricular administration of the kainic acid. For comparison, we also analyzed the survival of standard fetal CA3 cell grafts (i.e., without FGF-2 treatment) after similar transplantation. Pre treatment and transplantation of CA3 cell grafts with FGF-2 resulted in a robust yield of surviving cells (115% of injected cells) and neurons (100% of injected cells) from grafts. In contrast, standard CA3 cell grafts exhibited a reduced yield of surviving cells (29%) and neurons (25%). Thus, the yield of neurons from fetal hippocampal CA3 cell grafts placed into the chronically lesioned young adult hippocampus can be greatly enhanced by a simple pretreatment and grafting of donor fetal CA3 cells with FGF-2. These results have significance toward advancement of clinically feasible cell grafting strategies for repair of the damaged young adult hippocampus, particularly at extended periods after the injury or the onset of neurodegenerative diseases.

Clinical prospects for neural grafting therapy for hippocampal lesions and epilepsy

OBJECTIVE

Hippocampal lesions and epilepsy may be potential clinical targets for neural grafting. We hypothesized that neural grafting could be a restorative therapy either acutely, adding unformed neural elements, or chronically, treating postlesioning epilepsy. The goal of this review was to assess the clinical reality of this hypothesis of neural grafting and to determine the problems that remain to be resolved before grafting can be applied clinically.

METHODS

We quantitatively defined graft integration within the host, on a cellular basis, by directly assessing survival of the transplanted neurons, graft cell dispersion and migration, neuronal differentiation and development, and establishment of appropriate local and long-distance synaptic connectivity.

RESULTS

Embryonic hippocampal suspension grafts demonstrate excellent survival rates (20-80%). Embryonic axons exhibit extensive, appropriate, local and long-distance connectivity, can facilitate reconstruction of excitatory and inhibitory cortical circuitry, and can prevent the formation of aberrant circuitry. Immature neural stem cells demonstrate lesser degrees of integration, likely because of a paucity of positional cues in the lesioned brain for the differentiation of stem cells into region-specific neuronal phenotypes. Labeled grafted cells may be selectively and noninvasively removed from the host with triggerable stealth toxins, for the late treatment of unanticipated graft problems.

CONCLUSION

Neural grafting with appropriate embryonic neurons may provide significant clinical benefits. However, embryonic cell availability is severely limited, and alternative sources of cells, such as stem cells, require significant additional research into the induction and maintenance of neuronal commitment and the ability of the cells to form functional synaptic connections in vivo.

Survival of fetal hippocampal CA3 cell grafts in the middle-aged and aged hippocampus: effect of host age and deafferentation

The potential application of neural transplantation to many neurodegenerative disorders at early stages of disease progression would involve middle-aged and aged persons. Hence, it is important to examine critically the extent of graft cell survival in both intact and partially deafferented middle-aged and aged brain. We investigated the degree of survival of 5'-bromodeoxyuridine (BrdU)-labeled fetal hippocampal CA3 cells after grafting into both intact hippocampus and partially deafferented hippocampus (i.e., hippocampus contralateral to intracerebroventricular administration of kainic acid) of middle-aged and aged Fischer 344 rats. Absolute cell survival within these grafts was rigorously analyzed using BrdU immunostaining of serial sections and the optical fractionator cell counting method. In the intact hippocampus, graft cell survival was 23% of injected cells for middle-aged rats and 18% for aged rats, which is consistent with the survival of fetal hippocampal cells in the intact young adult hippocampus reported earlier (Shetty and Turner [1995] Neuroscience 67:561-582). A partial deafferentation at the time of grafting significantly enhanced the degree of graft cell survival to 35% of injected cells in the middle-aged hippocampus and 27% in the aged hippocampus. However, the overall graft cell survival after deafferentation was significantly (30%) greater in the middle-aged hippocampus compared with the aged hippocampus. These results reveal that 1) the degree of survival of fetal neural cells in the intact mature brain remains constant with aging and 2) a partial deafferentation of the mature host brain at the time of grafting enhances survival of grafted fetal cells, regardless of the host age. However, the overall extent of graft cell survival after deafferentation depends on the age of the mature brain at the time of deafferentation.

Potential new methods for antiepileptic drug delivery

Use of novel drug delivery methods could enhance the efficacy and reduce the toxicity of antiepileptic drugs (AEDs). Slow-release oral forms of medication or depot drugs such as skin patches might improve compliance and therefore seizure control. In emergency situations, administration via rectal, nasal or buccal mucosa can deliver the drug more quickly than can oral administration. Slow-release oral forms and rectal forms of AEDs are already approved for use, nasal and buccal administration is currently off-label and skin patches for AEDs are an attractive but currently hypothetical option. Therapies under development may result in the delivery of AEDs directly to the regions of the brain involved in seizures. Experimental protocols are underway to allow continuous infusion of potent excitatory amino acid antagonists into the CSF. In experiments with animal models of epilepsy, AEDs have been delivered successfully to seizure foci in the brain by programmed infusion pumps, acting in response to computerised EEG seizure detection. Inactive prodrugs can be given systemically and activated at the site of the seizure focus by locally released compounds. One such drug under development is DP-VPA (or DP16), which is cleaved to valproic acid (sodium valproate) by phospholipases at the seizure focus. Liposomes and nanoparticles are engineered micro-reservoirs of a drug, with attached antibodies or receptor-specific binding agents designed to target the particles to a specific region of the body. Liposomes in theory could deliver a high concentration of an AED to a seizure focus. Penetration of the blood-brain barrier can be accomplished by linking large particles to iron transferrin or biological toxins that can cross the barrier. In the near future, it is likely that cell transplants that generate neurotransmitters and neuromodulators will accomplish renewable endogenous drug delivery. However, the survival and viability of transplanted cells have yet to be demonstrated in the clinical setting. Gene therapy also may play a role in local drug delivery with the use of adenovirus, adeno-associated virus, herpesvirus or other delivery vectors to induce brain cells to produce local modulatory substances. New delivery systems should significantly improve the therapeutic/toxic ratio of AEDs.

Combined neurotrophic supplementation and caspase inhibition enhances survival of fetal hippocampal CA3 cell grafts in lesioned CA3 region of the aging hippocampus

Fetal hippocampal CA3 cells show excellent survival when homotopically grafted into the kainic acid-lesioned CA3 region of the young adult hippocampus, a model of temporal lobe epilepsy. However, survival of these cells in the kainic acid-lesioned CA3 region of the aging hippocampus is unknown. We hypothesize that fetal CA3 grafts into the lesioned CA3 region of the middle-aged and aged hippocampus exhibit significantly diminished cell survival compared with similar grafts in the lesioned young adult hippocampus unless pre-treated and transplanted with factors that augment graft cell survival. We analyzed cell survival of 5'-bromodeoxyuridine-labeled embryonic day 19 CA3 grafts following their transplantation into the lesioned CA3 region of the middle-aged and aged rat hippocampus. Grafts were placed 4 days after an i.c.v. administration of kainic acid, and absolute cell survival of grafts was quantified 1 month after grafting using 5'-bromodeoxyuridine immunostaining of serial sections and the optical fractionator counting method. Grafts into both middle-aged and aged hippocampus exhibited analogous but significantly diminished cell survival (30% of injected cells) compared with similar grafts into the young adult hippocampus (72% cell survival). However, the extent of cell survival of CA3 grafts pre-treated and transplanted with a combination of neurotrophic factors brain-derived neurotrophic factor and neurotrophin-3 and the caspase inhibitor acetyl-tyrosinyl-valyl-alanyl-aspartyl-chloro-methylketone was significantly enhanced in both middle-aged and aged hippocampus (51-63% cell survival). These results underscore that aging impairs the conduciveness of the CA3 region for robust survival of homotopic fetal CA3 grafts after lesion. However, a combined neurotrophic supplementation and caspase inhibition significantly enhances survival of fetal CA3 cells in the lesioned aging hippocampus. Thus, pre-treatment and grafting of donor cells with a combination of factors that support growth of specific donor cells may considerably enhance survival and integration of fetal grafts into the lesioned aging CNS in clinical trials.

Preliminary study of the behavioral effects of LBS-neuron implantation on seizure susceptibility following middle cerebral artery occlusion in the rats

Neural transplantation is a promising treatment strategy that can restore the motor, sensory and cognitive functions in the rat middle cerebral artery occlusion (MCAO) model of stroke. In particular, neuronal cells derived from a human teratocarcinoma cell line, called hNT neurons or LBS neurons (clinical grade preparation), are effective in improving behavioral recovery after stroke. In the elderly, epilepsy is a common sequela of stroke, especially if the infarction involves cerebral cortex. However, the effect of implanting neural cells on seizure susceptibility in the MCAO model has not yet been determined. The purpose of this study was to determine the susceptibility to pentylenetetrazol (PTZ)-induced seizures in normal, MCAO-lesioned and MCAO-lesioned rats in which the LBS neurons were injected. Adult, male Sprague-Dawley rats were subjected to 60 min of MCAO using the intraluminal filament technique followed 3-4 weeks later by transplantation of 80,000 LBS-neurons into the ipsilateral cortex. Susceptibility to PTZ-induced seizures was tested 4-6 weeks post-transplant at doses of 35, 50 and 70 mg/kg, administered subcutaneously. Latency to the first lethal response, latency to first generalized seizure, duration of the first generalized seizure, and the number of generalized seizures in an hour post-PTZ treatment observation period was determined. Even thought there was a tendency for groups that underwent MCAO to be more susceptible to seizures, there were no statistically significant differences between the groups and no differences between MCAO alone and MCAO animals in which cells had been implanted. While grafted cells were identified in all but one injected animal, the results suggest that the grafts may not have been healthy either from immunological rejection or PTZ-induced injury. These results suggest that while placing cells within the cortex does not reduce seizure susceptibility, it also does not increase the incidence of seizures. Further investigations are warranted.

Fetal hippocampal CA3 cell grafts transplanted to lesioned CA3 region of the adult hippocampus exhibit long-term survival in a rat model of temporal lobe epilepsy

Intracerebroventricular administration of kainic acid in the adult rat, a widely used model for studying human temporal lobe epilepsy, results in widespread degeneration of CA3-pyramidal neurons. Transplantation of specific fetal hippocampal CA3 cell grafts into the lesioned CA3-region at a prolonged post lesion delay of 45-day leads to 31% graft cell survival at 1 month postgrafting and significantly facilitates appropriate recovery of the lesioned host hippocampus. However, the capability of hippocampal CA3 cell grafts for enduring survival in this model is unknown. We hypothesize that a significant fraction of fetal CA3 cells grafted into the lesioned CA3 region of the adult hippocampus at 45-days postlesion exhibit long-term survival. We measured the extent of cell survival within 5'-bromodeoxyuridine-labeled CA3 cell grafts at 1 year postgrafting, following their transplantation at 45 days postlesion into the lesioned CA3-region. Quantification of absolute graft cell survival using BrdU immunostaining and the optical fractionator counting method revealed survival of 36% of grafted cells at 1 year postgrafting. Thus, over a third of fetal hippocampal CA3 cells transplanted to the lesioned CA3-region at 45 days postlesion exhibit long-term survival. Further, the extent of cell survival in these grafts is highly analogous to the degree of cell survival in CA3 grafts analyzed earlier at 1 month postgrafting, suggesting that specific fetal cells that survive the first month of grafting into the lesioned CNS area are capable of exhibiting enduring survival.

Fetal hippocampal grafts containing CA3 cells restore host hippocampal glutamate decarboxylase-positive interneuron numbers in a rat model of temporal lobe epilepsy

Degeneration of CA3-pyramidal neurons in hippocampus after intracerebroventricular kainic acid (KA) administration, a model of temporal lobe epilepsy, results in hyperexcitability within both dentate gyrus and the CA1 subfield. It also leads to persistent reductions in hippocampal glutamate decarboxylase (GAD) interneuron numbers without diminution in Nissl-stained interneuron numbers, indicating loss of GAD expression in a majority of interneurons. We hypothesize that enduring loss of GAD expression in hippocampal interneurons after intracerebroventricular KA is attributable to degeneration of their CA3 afferent input; therefore, fetal CA3 grafts can restore GAD interneuron numbers through graft axon reinnervation of the host. We analyzed GAD interneuron density in the adult rat hippocampus at 6 months after KA administration after grafting of fetal mixed hippocampal, CA3 or CA1 cells into the CA3 region at 45 d after lesion, in comparison with "lesion-only" and intact hippocampus. In dentate and CA1 regions of the lesioned hippocampus receiving grafts of either mixed hippocampal or CA3 cells, GAD interneuron density was both significantly greater than lesion-only hippocampus and comparable with the intact hippocampus. In the CA3 region, GAD interneuron density was significantly greater than lesion-only hippocampus but less than the intact hippocampus. Collectively, the overall GAD interneuron density in the lesioned hippocampus receiving either mixed hippocampal or CA3 grafts was restored to that in the intact hippocampus. In contrast, GADinterneuron density in the lesioned hippocampus receiving CA1 grafts remained comparable with lesion-only hippocampus. Thus, grafts containing CA3 cells restore CA3 lesion-induced depletions in hippocampal GAD interneurons, likely by reinnervation of GAD-deficient interneurons. This specific graft-mediated effect is beneficial because reactivation of interneurons could ameliorate both loss of functional inhibition and hyperexcitability in CA3-lesioned hippocampus.