Development of fetal hippocampal grafts in intact and lesioned hippocampus

Toward Functional Restoration of the Central Nervous System: A Review of Translational Neuroscience Principles

Injury to the central nervous system (CNS) can leave patients with devastating neurological deficits that may permanently impair independence and diminish quality of life. Recent insights into how the CNS responds to injury and reacts to critically timed interventions are being translated into clinical applications that have the capacity to drastically improve outcomes for patients suffering from permanent neurological deficits due to spinal cord injury, stroke, or other CNS disorders. The translation of such knowledge into practical and impactful treatments involves the strategic collaboration between neurosurgeons, clinicians, therapists, scientists, and industry. Therefore, a common understanding of key neuroscientific principles is crucial. Conceptually, current approaches to CNS revitalization can be divided by scale into macroscopic (systems-circuitry) and microscopic (cellular-molecular). Here we review both emerging and well-established tenets that are being utilized to enhance CNS recovery on both levels, and we explore the role of neurosurgeons in developing therapies moving forward. Key principles include plasticity-driven functional recovery, cellular signaling mechanisms in axonal sprouting, critical timing for recovery after injury, and mechanisms of action underlying cellular replacement strategies. We then discuss integrative approaches aimed at synergizing interventions across scales, and we make recommendations for the basis of future clinical trial design. Ultimately, we argue that strategic modulation of microscopic cellular behavior within a macroscopic framework of functional circuitry re-establishment should provide the foundation for most neural restoration strategies, and the early involvement of neurosurgeons in the process will be crucial to successful clinical translation.

A Model of Chronic Temporal Lobe Epilepsy Presenting Constantly Rhythmic and Robust Spontaneous Seizures, Co-morbidities and Hippocampal Neuropathology

Many animal prototypes illustrating the various attributes of human temporal lobe epilepsy (TLE) are available. These models have been invaluable for comprehending multiple epileptogenic processes, modifications in electrophysiological properties, neuronal hyperexcitability, neurodegeneration, neural plasticity, and chronic neuroinflammation in TLE. Some models have also uncovered the efficacy of new antiepileptic drugs or biologics for alleviating epileptogenesis, cognitive impairments, or spontaneous recurrent seizures (SRS). Nonetheless, the suitability of these models for testing candidate therapeutics in conditions such as chronic TLE is debatable because of a lower frequency of SRS and an inconsistent pattern of SRS activity over days, weeks or months. An ideal prototype of chronic TLE for investigating novel therapeutics would need to display a large number of SRS with a dependable frequency and severity and related co-morbidities. This study presents a new kainic acid (KA) model of chronic TLE generated through induction of status epilepticus (SE) in 6-8 weeks old male F344 rats. A rigorous characterization in the chronic epilepsy period validated that the animal prototype mimicked the most salient features of robust chronic TLE. Animals displayed a constant frequency and intensity of SRS across weeks and months in the 5th and 6th month after SE, as well as cognitive and mood impairments. Moreover, SRS frequency displayed a rhythmic pattern with 24-hour periodicity and a consistently higher number of SRS in the daylight period. Besides, the model showed many neuropathological features of chronic TLE, which include a partial loss of inhibitory interneurons, reduced neurogenesis with persistent aberrant migration of newly born neurons, chronic neuroinflammation typified by hypertrophied astrocytes and rod-shaped microglia, and a significant aberrant mossy fiber sprouting in the hippocampus. This consistent chronic seizure model is ideal for investigating the efficacy of various antiepileptic drugs and biologics as well as understanding multiple pathophysiological mechanisms underlying chronic epilepsy.

Neurons Generated by Mouse ESCs with Hippocampal or Cortical Identity Display Distinct Projection Patterns When Co-transplanted in the Adult Brain

The capability of generating neural precursor cells with distinct types of regional identity in vitro has recently opened new opportunities for cell replacement in animal models of neurodegenerative diseases. By manipulating Wnt and BMP signaling, we steered the differentiation of mouse embryonic stem cells (ESCs) toward isocortical or hippocampal molecular identity. These two types of cells showed different degrees of axonal outgrowth and targeted different regions when co-transplanted in healthy or lesioned isocortex or in hippocampus. In hippocampus, only precursor cells with hippocampal molecular identity were able to extend projections, contacting CA3. Conversely, isocortical-like cells were capable of extending long-range axonal projections only when transplanted in motor cortex, sending fibers toward both intra- and extra-cortical targets. Ischemic damage induced by photothrombosis greatly enhanced the capability of isocortical-like cells to extend far-reaching projections. Our results indicate that neural precursors generated by ESCs carry intrinsic signals specifying axonal extension in different environments.

•

Wnt signaling induces hippocampal fate in neuralized mouse ESCs

•

Transplanted cortical and hippocampal neurons target distinct regions in adult brain

•

Photothrombotic lesion favors neurite elongation of cortical transplanted cells

•

Cortical cell transplantation improves the motor performance after ischemic damage

Grafted Neural Stem Cells Show Lesion-Specific Migration in Radiation-Injured Rat Brains

Neural stem cells (NSCs) exhibit preferential homing toward some types of brain lesion, but their migratory property during radiation brain injury (RBI) remains unexplored. Here, we use the superparamagnetic iron oxide (SPIO)-labeled magnetic resonance imaging (MRI) technology to determine the migration of transplanted NSCs in two partial RBI models in real time, created by administering 30–55 Gy of radiation to the right or posterior half of the adult rat brain. SPIO-labeled NSCs were stereotactically grafted into the uninjured side one week after RBI. The migration of SPIO-labeled NSCs in live radiation-injured brains was traced by MRI for up to 28 days after engraftment and quantified for their moving distances and speeds. A high labeling efficiency (>90%) was achieved by incubating NSCs with 100 μg ml⁻¹ of SPIO for 12–24 hours. Upon stereotactic transplantation into the healthy side of the brain, SPIO-labeled NSCs were distinctively detected as hypointense signals on T2-weighted images (T2WI), showed sustained survival for up to 4 weeks, and exhibited directional migration to the radiation-injured side of the brain with a speed of 86–127 μm per day. The moving kinetics of grafted NSCs displayed no difference in brains receiving a high (55 Gy) vs. moderate (45 Gy) dose of radiation, but was slower in the right RBI model than in the posterior RBI model. This study shows that NSCs can be effectively labeled by SPIO and traced in vivo by MRI, and that grafted NSCs exhibit directional migration toward RBI sites in a route-dependent but radiation dose-independent manner.

Neural stem cells (NSCs) exhibit preferential homing toward some types of brain lesion, but their migratory property during radiation brain injury (RBI) remains unexplored.

Neural Stem Cell or Human Induced Pluripotent Stem Cell-Derived GABA-ergic Progenitor Cell Grafting in an Animal Model of Chronic Temporal Lobe Epilepsy

Grafting of neural stem cells (NSCs) or GABA-ergic progenitor cells (GPCs) into the hippocampus could offer an alternative therapy to hippocampal resection in patients with drug-resistant chronic epilepsy, which afflicts >30% of temporal lobe epilepsy (TLE) cases. Multipotent, self-renewing NSCs could be expanded from multiple regions of the developing and adult brain, human embryonic stem cells (hESCs), and human induced pluripotent stem cells (hiPSCs). On the other hand, GPCs could be generated from the medial and lateral ganglionic eminences of the embryonic brain and from hESCs and hiPSCs. To provide comprehensive methodologies involved in testing the efficacy of transplantation of NSCs and GPCs in a rat model of chronic TLE, NSCs derived from the rat medial ganglionic eminence (MGE) and MGE-like GPCs derived from hiPSCs are taken as examples in this unit. The topics comprise description of the required materials, reagents and equipment, methods for obtaining rat MGE-NSCs and hiPSC-derived MGE-like GPCs in culture, generation of chronically epileptic rats, intrahippocampal grafting procedure, post-grafting evaluation of the effects of grafts on spontaneous recurrent seizures and cognitive and mood impairments, analyses of the yield and the fate of graft-derived cells, and the effects of grafts on the host hippocampus. © 2016 by John Wiley & Sons, Inc.

Neural stem cell grafting in an animal model of chronic temporal lobe epilepsy

Neural stem cell (NSC) transplantation into the hippocampus could offer an alternative therapy to hippocampal resection in patients with drug-resistant chronic epilepsy, which afflicts ∼30% of mesial temporal lobe epilepsy (TLE) cases. Multipotent, self-renewing NSCs could be expanded from multiple regions of the developing and adult brain, human embryonic stem cells (hESCs), and induced pluripotent stem cells (iPSCs). However, to provide a comprehensive methodology involved in testing the efficacy of transplantation of NSCs in a rat model of chronic TLE, NSCs derived from the embryonic medial ganglionic eminence (MGE) are taken as an example in this unit. The topics comprise description of the required materials, reagents and equipment, and protocols for expanding MGE-NSCs in culture, generating chronically epileptic rats, the intrahippocampal grafting, post-grafting evaluation of the effects of NSC grafts on spontaneous recurrent seizures and cognitive impairments, analyses of the yield and the fate of graft-derived cells, and the effects of NSC grafts on the host hippocampus.

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.

Transplantation of rat synapsin-EGFP-labeled embryonic neurons into the intact and ischemic CA1 hippocampal region: distribution, phenotype, and axodendritic sprouting

A major limitation of neural transplantation studies is assessing the degree of host-graft interaction. In the present study, rat hippocampal/cortical embryonic neurons (E18) were infected with a lentivirus encoding enhanced green fluorescent protein (GFP) under control of the neuron-specific synapsin promoter, thus permitting robust identification of labeled neurons after in vivo grafting. Two weeks after transient forebrain ischemia or sham-surgery, GFP-expressing neurons were transplanted into CA1 hippocampal regions in immunosuppressed adult Wistar rats. The survival, distribution, phenotype, and axonal projections of GFP-immunoreactive (IR) positive transplanted neurons were evaluated in the sham-operated and ischemia- damaged CA1 hippocampal regions 4, 8, and 12 weeks after transplantation. In both experimental groups, we observed that the main phenotype of host-derived afferents projecting towards grafted GFP-IR neurons as well as transplant-derived GFP-IR efferents were glutamatergic in both animal groups. GFP axonal projections were seen in the nucleus accumbens, septal nuclei, and subiculum-known target areas of CA1 pyramidal neurons. Compared to sham-operated animals, GFP-IR neurons grafted into the ischemia-damaged CA1 migrated more extensively throughout a larger volume of host tissue, particularly in the stratum radiatum. Moreover, enhanced axonal sprouting and neuronal plasticity of grafted cells were evident in the hippocampus, subiculum, septal nuclei, and nucleus accumbens of the ischemia-damaged rats. Our study suggests that the adult rat brain retains some capacity to direct newly sprouting axons of transplanted embryonic neurons to the correct targets and that this capacity is enhanced in previously ischemia-injured forebrain.

A novel animal model of hippocampal cognitive deficits, slow neurodegeneration, and neuroregeneration

Long-term adrenalectomy (ADX) results in an extensive and specific loss of dentate gyrus granule cells in the hippocampus of adult rats. This loss of granule cells extends over a period of weeks to months and ultimately results in cognitive deficits revealed in a number of tasks that depend on intact hippocampal function. The gradual nature of ADX-induced cell death and the ensuing deficits in cognition resemble in some important respects a variety of pathological conditions in humans. Here, we characterize behavioural and cellular processes, including adult neurogenesis, in the rat ADX model. We also provide experimental evidence for a neurogenic treatment strategy by which the lost hippocampal cells may be replaced, with the goal of functional recovery in mind.

Cell origin and culture history determine successful integration of neural precursor transplants into the dentate gyrus of the adult rat

The success of transplants of neural tissue into the adult dentate gyrus in generating mature neurons is highly variable. Here we address the roles of the origin of the tissue and its pre-implantation preparation, and show that both are critical. We transplanted neonatal cultured or primary rat cells from either the ventral subventricular zone (vSVZ) or the dentate gyrus (DG) into the adult rat DG. Only primary DG cells robustly generated DG neurons (80% NeuN and Prox1-positive cells at 6 weeks), substantially repaired the damaged DG, and formed glutamatergic projections to the target CA3 region. Cultured DG cells expanded for 7 days showed limited neuronal differentiation after transplantation (10% NeuN and Prox1-positive cells) whereas cultured or primary vSVZ cells failed to make any Prox1-positive DG granular neurons. We found that a specific population of postmitotic young neurons (triple doublecortin/NeuN/Prox1-positive) were particularly abundant in primary DG cells, but were markedly reduced in the cultured DG cells and were absent in the cultured and primary vSVZ cells. Labelling of primary DG cells with the mitotic marker BrdU suggested that postmitotic young neurons are the source of the transplanted mature neurons in-vivo. We conclude that both the origin and pre-transplantation history of donor cells are key factors that determine the outcome of transplantation. These findings may be of therapeutic interest for cell replacement therapy in treating the damaged hippocampus.

Recent advancements in stem cell and gene therapies for neurological disorders and intractable epilepsy

The potential applications of stem cell therapies for treating neurological disorders are enormous. Many laboratories are focusing on stem cell treatments for CNS diseases, including spinal cord injury, Amyotrophic lateral sclerosis, Parkinson's disease, Huntington's disease, multiple sclerosis, stroke, traumatic brain injury, and epilepsy. Among the many stem cell types under testing for neurological treatments, the most common are fetal and adult brain stem cells, embryonic stem cells, induced pluripotent stem cells, and mesenchymal stem cells. An expanding toolbox of molecular probes is now available to allow analyses of neural stem cell fates prior to and after transplantation. Concomitantly, protocols are being developed to direct the fates of stem cell-derived neural progenitors, and also to screen stem cells for tumorigenicity and aneuploidy. The rapid progress in the field suggests that novel stem cell and gene therapies for neurological disorders are in the pipeline.

Behavior of hippocampal stem/progenitor cells following grafting into the injured aged hippocampus

Multipotent neural stem/progenitor cells (NSCs) from the embryonic hippocampus are potentially useful as donor cells to repopulate the degenerated regions of the aged hippocampus after stroke, epilepsy, or Alzheimer's disease. However, the efficacy of the NSC grafting strategy for repairing the injured aged hippocampus is unknown. To address this issue, we expanded FGF-2-responsive NSCs from the hippocampus of embryonic day 14 green fluorescent protein-expressing transgenic mice as neurospheres in vitro and grafted them into the hippocampus of 24-month-old F344 rats 4 days after CA3 region injury. Engraftment, migration, and neuronal/glial differentiation of cells derived from NSCs were analyzed 1 month after grafting. Differentiation of neurospheres in culture dishes or after placement on organotypic hippocampal slice cultures demonstrated that these cells had the ability to generate considerable numbers of neurons, astrocytes, and oligodendrocytes. Following grafting into the injured aged hippocampus, cells derived from neurospheres survived and dispersed, but exhibited no directed migration into degenerated or intact hippocampal cell layers. Phenotypic analyses of graft-derived cells revealed neuronal differentiation in 3%-5% of cells, astrocytic differentiation in 28% of cells, and oligodendrocytic differentiation in 6%-10% cells. The results demonstrate for the first time that NSCs derived from the fetal hippocampus survive and give rise to all three CNS phenotypes following transplantation into the injured aged hippocampus. However, grafted NSCs do not exhibit directed migration into lesioned areas or widespread neuronal differentiation, suggesting that direct grafting of primitive NSCs is not adequate for repair of the injured aged brain without priming the microenvironment.

Implications of decreased hippocampal neurogenesis in chronic temporal lobe epilepsy

Temporal lobe epilepsy (TLE), characterized by spontaneous recurrent motor seizures (SRMS), learning and memory impairments, and depression, is associated with neurodegeneration, abnormal reorganization of the circuitry, and loss of functional inhibition in the hippocampal and extrahippocampal regions. Over the last decade, abnormal neurogenesis in the dentate gyrus (DG) has emerged as another hallmark of TLE. Increased DG neurogenesis and recruitment of newly born neurons into the epileptogenic hippocampal circuitry is a characteristic phenomenon occurring during the early phase after the initial precipitating injury such as status epilepticus. However, the chronic phase of the disease displays substantially declined DG neurogenesis, which is associated with SRMS, learning and memory impairments, and depression. This review focuses on DG neurogenesis in the chronic phase of TLE and first confers the extent and mechanisms of declined DG neurogenesis in chronic TLE. The available data on production, survival and neuronal fate choice decision of newly born cells, stability of hippocampal stem cell numbers, and changes in the hippocampal microenvironment in chronic TLE are considered. The next section discusses the possible contribution of declined DG neurogenesis to the pathophysiology of chronic TLE, which includes its potential effects on spontaneous recurrent seizures, cognitive dysfunction, and depression. The subsequent section considers strategies that may be useful for augmenting DG neurogenesis in chronic TLE, which encompass stem cell grafting, administration of distinct neurotrophic factors, physical exercise, exposure to enriched environment, and antidepressant therapy. The final section suggests possible ramifications of increasing the DG neurogenesis in chronic epilepsy.

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.

Lentiviral RNAi-induced downregulation of adenosine kinase in human mesenchymal stem cell grafts: a novel perspective for seizure control

Cell therapies based on focal delivery of the inhibitory neuromodulator adenosine were previously shown to provide potent seizure suppression in animal models of epilepsy. However, hitherto used therapeutic cells were derived from rodents and thus not suitable for clinical applications. Autologous patient-derived adenosine-releasing cell implants would constitute a major therapeutic advance to avoid both xenotransplantation and immunosuppression. Here we describe a novel approach based on lentiviral RNAi mediated downregulation of adenosine kinase (ADK), the major adenosine-removing enzyme, in human mesenchymal stem cells (hMSCs), which would be compatible with autologous cell grafting in patients. Following lentiviral transduction of hMSCs with anti-ADK miRNA expression cassettes we demonstrate up to 80% downregulation of ADK and a concentration of 8.5 ng adenosine per ml of medium after incubating 10(5) cells for 8 h. hMSCs with a knockdown of ADK or cells expressing a scrambled control sequence were transplanted into hippocampi of mice 1 week prior to the intraamygdaloid injection of kainic acid (KA). While mice with control implants expressing a scrambled miRNA sequence or sham treated control animals were characterized by KA-induced status epilepticus and subsequent CA3 neuronal cell loss, animals with therapeutic ADK knockdown implants displayed a 35% reduction in seizure duration and 65% reduction in CA3 neuronal cell loss, when analyzed 24 h after KA-injection. We conclude that lentiviral expression of anti-ADK miRNA constitutes a versatile tool to generate therapeutically effective adenosine releasing hMSCs, thus representing a model system to generate patient identical autologous adult stem cell grafts.

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.

Neuroprotection of adenoviral-vector-mediated GDNF expression against kainic-acid-induced excitotoxicity in the rat hippocampus

Glial-cell-line-derived neurotrophic factor (GDNF) is a potent survival factor for several types of neurons. In the present study, we examined the protective roles of adenoviral-vector-delivered GDNF (Ad-GDNF) in the hippocampus damaged by kainic-acid (KA)-induced excitotoxicity using GAD-67 immunoreactivity, immunoblot analysis, behavioral test, 5-bromo-2-deoxyuridine (BrdU) and TUNEL assay. Ad-GDNF was pre-inoculated into the KA-treated rat hippocampus 7 days before KA injection. Ad-GDNF resulted in the suppression of KA-induced tonic-clonic convulsions. In situ apoptosis assay demonstrated a significant reduction in apoptotic cells in the CA3 and dentate hilus regions of the Ad-GDNF-pre-inoculated rats (Ad-GDNF-KA), compared to the KA rats. Striking reductions in the density of GAD-67 neurons were also observed in the CA3 and dentate hilus regions of the KA rats. On the other hand, the number of GAD-67-positive cells was recovered to the control levels in the Ad-GDNF-KA rats. Immunoblot analysis further confirmed that GAD-67 and Bcl-2 expression increased in the Ad-GDNF-KA rats compared to KA rats. Taken together, these results suggest that Ad-GDNF may serve to control KA-induced hippocampal cell loss and behavioral seizure.

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.

Hippocampal neurotrophin levels after injury: Relationship to the age of the hippocampus at the time of injury

Aging impairs the competence of the hippocampus for synaptic reorganization after injury. This potentially is due to the inability of the aging hippocampus to up-regulate the critical neurotrophic factors for prolonged periods after injury to levels at which they can stimulate neurite outgrowth and facilitate synaptic reorganization. We hypothesize that the concentrations of neurotrophins in the hippocampus after injury depend on the age at the time of injury. We quantified the concentrations of brain-derived neurotrophic factor (BDNF), nerve growth factor (NGF), and neurotrophin-3 (NT-3) in the hippocampus of young, middle-aged, and aged Fischer 344 rats at 4 days after kainic acid (KA)-induced injury. In comparison with the age-matched intact hippocampus, the KA-lesioned hippocampus exhibited increased levels of BDNF and NGF in all three age groups. In contrast, the NT-3 concentration was unaltered after KA lesion. Notwithstanding similar percentage increases in BDNF after injury, the lesioned middle-aged and aged hippocampus contained 45-52% less BDNF than the lesioned young hippocampus. NGF and NT-3 levels after injury were comparable across the three age groups, however. Furthermore, lower BDNF concentration in the injured aging hippocampus was associated with normal astrocytic response but significantly diminished microglial reaction. Thus, in comparison with the injured young hippocampus, the injured aging hippocampus contains considerably less BDNF but similar levels of NGF and NT-3. Lower BDNF levels in the injured aging hippocampus might underlie the diminished spontaneous healing response observed in the aging hippocampus after injury, particularly in terms of synaptic reorganization and dentate neurogenesis.

Human neural stem cell transplantation reduces spontaneous recurrent seizures following pilocarpine-induced status epilepticus in adult rats

Transplantation of neural stem cells (NSCs) can replace lost neurons and improve the functional deficits. Cell transplantation strategies have been tried in the epileptic disorder, but the effect of exogenous NSCs is unknown. In this study, we attempted to test the anti-epileptogenic effect of NSCs in adult rats with status epilepticus. Experimental status epilepticus was induced by lithium-pilocarpine injection, and beta galactosidase-encoded human NSCs were transplanted intravenously on the next day of status epilepticus. Spontaneous recurrent seizures were monitored with Racine's seizure severity scale. Immunohistochemistry with anti-beta gal, Tuj-1, NeuN, GFAP, CNPase, GluR2, parvalbumin, and GABA were performed and extracellular field excitatory postsynaptic potentials (fEPSP) were recorded. Human NSCs suppressed spontaneous recurrent seizure formation and transplanted NSCs were differentiated into GABA-immunoreactive interneurons in the damaged hippocampus. Amplitude of fEPSP in the hippocampal CA1 was reduced, which was reversed by picrotoxin. These findings suggest that NSCs could be differentiated into inhibitory interneurons and decrease neuronal excitability, which could prevent spontaneous recurrent seizure formation in adult rats with pilocarpine-induced status epilepticus.

Neurogenesis of Rhesus adipose stromal cells

In this study, we isolated and characterized a population of non-human primate adipose tissue stromal cells (pATSCs) containing multipotent progenitor cells. We show that these pATSCs can differentiate into several mesodermal lineages, as well as neural lineage cells. For neural induction of pATSCs and non-human primate bone marrow stromal cells (pBMSCs), the cells were cultured in Neurobasal (NB) media supplemented with B27, basic fibroblast growth factor (bFGF), brain-derived neurotrophic factor (BDNF) and epidermal growth factor (EGF). After 4 days in culture, the pATSCs form compact, spheroid bodies that ultimately become neurospheres (NS). Free-floating neurospheres undergo extensive differentiation when cultured on PDL-laminin. Our data suggest that the neurogenic potential of pATSCs is markedly higher than that of pBMSCs. We have also performed microarray analysis and characterized the gene expression patterns in undifferentiated pATSCs. The direct comparison of gene expression profiles in undifferentiated pATSCs and pATSC-NS, and delineated specific members of important growth factor, signaling, cell adhesion and transcription factors families. Our data indicate that adipose tissue may be an alternative source of stem cells for therapy of central nervous system (CNS) defects.

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.

Progenitor cells from the CA3 region of the embryonic day 19 rat hippocampus generate region-specific neuronal phenotypes in vitro

Progenitor cells that endure in different regions of the CNS after the initial neurogenesis can be expanded in culture and used as a source of donor tissue for grafting in neurodegenerative diseases. However, the proliferation and differentiation characteristics of residual neural progenitor cells from distinct regions of the CNS are mostly unknown. This study elucidated the characteristics of progenitor cells that endure in the CA3 region of the hippocampus after neurogenesis, by in vitro analyses of cells that are responsive to epidermal growth factor (EGF) or fibroblast growth factor-2 (FGF-2) in the embryonic day 19 (E19) rat hippocampus. Isolated cells from the E19 CA3 region formed neurospheres in the presence of either EGF or FGF-2, but the yield of neurospheres was greater with FGF-2 exposure, Differentiation cultures revealed a greater yield of neurons from FGF-2 neurospheres (60%) than from EGF neurospheres (35%). Exposure to brain-derived neurotrophic factor (BDNF) enhanced the yield of neurons from EGF neurospheres but had no consequence on FGF-2 neurospheres. A large number of neurons from EGF/FGF-2 neurospheres demonstrated clearly palpable morphological features of CA3 pyramidal neurons and lacked gamma-aminobutyric acid (GABA) expression. However, a fraction of neurons (17-20%) from EGF/FGF-2 neurospheres expressed GABA, and exposure to BDNF increased the number of GABAergic neurons (30%) from EGF neurospheres. Neurons from EGF/FGF-2 neurospheres also contained smaller populations of calbindin- and calretinin-positive interneuron-like cells. Thus, progenitor cells responsive to FGF-2 are prevalent in the CA3 region of the E19 rat hippocampus and give rise to a greater number of neurons than progenitor cells responsive to EGF. However, both FGF-2- and EGF-responsive progenitor cells from E19 CA3 region are capable of giving rise to CA3 field-specific phenotypic neurons. These results imply that progenitor cells that persist in the hippocampus after neurogenesis remain regionally restricted and hence retain their ability to give rise to region-specific phenotypic neurons even after isolation and expansion in vitro.

Hippocampal neurotrophin levels in a kainate model of temporal lobe epilepsy: a lack of correlation between brain-derived neurotrophic factor content and progression of aberrant dentate mossy fiber sprouting

A significant upregulation of neurotrophins particularly brain-derived neurotrophic factor (BDNF) is believed to be involved in the initiation of epileptogenic changes such as the aberrant axonal sprouting and synaptic reorganization in the injured hippocampus. However, it is unknown which of the neurotrophins are upregulated during the peak period of aberrant mossy fiber sprouting in the chronically injured hippocampus. We measured chronic changes in the levels of BDNF, nerve growth factor (NGF) and neurotrophin-3 (NT-3) in the adult hippocampus using enzyme-linked immunosorbent assay (ELISA) after a unilateral intracerebroventricular administration of kainic acid (KA), a model of temporal lobe epilepsy. For comparison, neurotrophins were also measured from the control intact hippocampus. Further, to see the association between changes in neurotrophin levels and the progression of mossy fiber sprouting, chronic changes in the mossy fiber distribution within the dentate supragranular layer (DSGL) were quantified. In the KA-lesioned hippocampus, the neurotrophins BDNF and NGF were upregulated at 4 days post-lesion, in comparison to their levels in the intact hippocampus. However, the concentration of BDNF reached the baseline level at 45 days post-lesion and dramatically diminished at 120 days post-lesion. In contrast, the upregulation of NGF observed at 4 days post-lesion was sustained at both 45 days and 120 days post-lesion. The concentration of NT-3 was upregulated at 45 days post-lesion but remained comparable to baseline levels at 4 days and 120 days post-lesion. Interestingly, analysis of mossy fiber sprouting revealed that most of the aberrant sprouting in the lesioned hippocampus occurs between 45 days and 120 days post-lesion. Taken together, these results suggest that the period of robust mossy fiber sprouting does not correlate with the phase of post-lesion BDNF upregulation. Rather, it shows a relationship with the time of upregulation of neurotrophins NGF and NT-3.

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.

Entorhinal axons exhibit sprouting in CA1 subfield of the adult hippocampus in a rat model of temporal lobe epilepsy

Intracerebroventricular kainic acid administration in rat, a model of temporal lobe epilepsy, results in CA3 pyramidal neuron degeneration leading to deafferentation of CA1 pyramidal neurons. Denervation in CA1 shows a near-complete recovery of synaptic density over 2-3 months, but the source of axons participating in the reinnervation is not clear. This study investigated the contribution of the entorhinal cortex in this reinnervation by comparing the distribution of the entorhinal axons in the CA1 subfield between the intact hippocampus and the CA3-lesioned hippocampus at 3 months after administration of kainic acid. Entorhinal axons were visualized by anterograde tracing using injections of the biotinylated dextran amine into the entorhinal cortex. In the CA1 subfield of the intact hippocampus, entorhinal axons were conspicuous in the alveus and the stratum lacunosum moleculare. The distribution in the strata oriens, pyramidale, and radiatum was sparse and was characterized by isolated entorhinal fibers of the alvear pathway crossing these strata to the stratum lacunosum moleculare. However, after kainic acid-induced CA3 lesion, the density of entorhinal axons increased significantly in the CA1 stratum radiatum (375% of the intact hippocampus), as a large number of axons emanating from the entorhinal fiber plexus in the stratum lacunosum moleculare invaded the stratum radiatum. The stratum radiatum also exhibited wavy entorhinal axons filled with boutons and oriented parallel to the stratum pyramidale, suggesting collateral sprouting from entorhinal axons traversing the stratum radiatum. Thus, a significant aberrant sprouting of entorhinal axons occurs into the CA1 stratum radiatum after CA3 lesion. The sprouted fibers appear to come from both entorhinal fiber plexus in the stratum lacunosum moleculare (translaminar sprouting) and entorhinal axons traversing the stratum radiatum (intralaminar sprouting). However, the major contribution appears to be from the entorhinal plexus in the stratum lacunosum moleculare. This aberrant sprouting may lead to altered afferent excitatory connectivity in the CA1 subfield and contribute to the persistent CA1 hyperexcitability that occurs after the CA3 lesion.

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.

Fetal human hematopoietic stem cells can differentiate sequentially into neural stem cells and then astrocytes in vitro

In some rodent models, there is evidence that hematopoietic stem cells (HSC) can differentiate into neural cells. However, it is not known whether humans share this potential, and, if so, what conditions are sufficient for this transdifferentiation to occur. We addressed this question by assessing the ability of fetal human liver CD34(+)/CD133(+)/CD3(-) hematopoietic stem cells to generate neural cells and astrocytes in culture. We cultured fetal liver-derived hematopoietic stem cells in human astrocyte culture-conditioned medium or using a method wherein growing human astrocytes were separated from cultured, nonadherent hematopoietic stem cells by a semipermeable membrane in a double-chamber co-culture system. Hematopoietic stem cell cultures were probed for neural progenitor cell marker expression (nestin and bone morphogenic protein-2 [BMP-2]) during growth in both culture conditions. RT-PCR, western blotting, and immunocytochemistry assays showed that cells cultured in either condition expressed nestin mRNA and protein and BMP-2 mRNA. HSC similarly cultured in nonconditioned medium or in the absence of astrocytes did not express either marker. Cells expressing these neural markers were transferred and cultured on poly-D-lysine-coated dishes with nonconditioned growth medium for further study. Immunocytochemistry demonstrated that these cells differentiated into astrocytes after 8 days in culture as indicated by their morphology and expression of the astrocytic markers glial fibrillary acidic protein (GFAP) and S100, as well as by their rate of proliferation, which was identical to that of freshly isolated fetal brain astrocytes. These findings demonstrate that neural precursor gene expression can be induced when human hematopoietic stem cells are exposed to a suitable microenvironment. Furthermore, the neural stem cells generated in this environment can then differentiate into astrocytes. Therefore, human hematopoietic stem cells may be an alternative resource for generation of neural stem cells for therapy of central nervous system defects resulting from disease or trauma.

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.

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.

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.

Conditioned blocking is re-established by neurotransplantation in mature rats

Aging has been associated with a decay of hippocampal function that may begin well before senescence. Conditioned blocking is a complex learning phenomenon that requires an intact hippocampus in young-adult rats and is absent in middle-aged rats. The aim of the present study was to test the possibility of re-establishing conditioned blocking in 17-month-old Wistar rats by neurotransplantation. Solid embryonic hippocampal or nigral tissue was bilaterally transplanted in the proximity of the dorsal hippocampus (lateral ventricle and alveus). Conditioned blocking of an aversion to a cider vinegar (3%) solution presented in compound with a previously conditioned saccharin solution (0.1%) appeared 14 days after transplantation and persisted 3 months later only in the hippocampal grafted group, showing the possibility of restoring age-related cognitive deficits.

Transplantation of human fetal brain cells into ischemic lesions of adult gerbil hippocampus

OBJECT

The goal of this study was to establish whether transplanted cells derived from fetal human brain can survive in an ischemic lesion.

METHODS

Sixteen adult male Mongolian gerbils underwent transient bilateral common carotid artery occlusion. One week later, cell suspensions prepared from fetal human brain were injected using stereotactic guidance into the CA1 region of the hippocampus on one side. On the contralateral side injection of the cell suspension medium only was performed. One week after transplantation, the animals were perfusion fixed and their brains were processed for histological studies as well as expression of neuron and glia-specific antigens. Data from ischemic animals were compared with eight nonischemic gerbils that served as sham-operated controls. Last, the in vivo data were correlated with observations made from matching in vitro cultures of the fetal brain cell suspension. The in vivo data indicated that transplanted human fetus-derived brain cells survived in ischemic lesions of gerbil hippocampus after 1 week, provided that the host animal underwent adequate immunosuppression and the transplanted cells were not incorporated into the scar caused by the transplantation procedure. Unlike their in vivo counterparts, after 1 week, most cultured fetal brain cells expressed either neuron- or astrocyte-specific antigens.

CONCLUSIONS

This work demonstrates that xenotransplanted fetal human brain cells are able to survive in an ischemic lesion in a rodent model. These data might be useful for future neural transplantation studies of treatments for cerebrovascular ischemia in humans.

Glutamic acid decarboxylase-67-positive hippocampal interneurons undergo a permanent reduction in number following kainic acid-induced degeneration of ca3 pyramidal neurons

Kainic acid (KA)-induced degeneration of CA3 pyramidal neurons leads to synaptic reorganization and hyperexcitability in both dentate gyrus and CA1 region of the hippocampus. We hypothesize that the substrate for hippocampal inhibitory circuitry incurs significant and permanent alterations following degeneration of CA3 pyramidal neurons. We quantified changes in interneuron density (N(v)) in all strata of the dentate gyrus and the CA1 and CA3 subfields of adult rats at 1, 4, and 6 months following intracerebroventricular (icv) KA administration, using glutamic acid decarboxylase-67 (GAD-67) immunocytochemistry. At 1 month postlesion, GAD-67-positive interneuron density was significantly reduced in all strata of every hippocampal region except stratum pyramidale of CA1. The reduction in GAD-67-positive interneuron density either persisted or exacerbated at 4 and 6 months postlesion in every stratum of all hippocampal regions. Further, the soma of remaining GAD-67-positive interneurons in dentate gyrus and CA3 subfield showed significant hypertrophy. Thus, both permanent reductions in the density of GAD-67-positive interneurons in all hippocampal regions and somatic hypertrophy of remaining GAD-67-positive interneurons in dentate gyrus and CA3 subfield occur following icv KA. In contrast, the density of interneurons visualized with Nissl in CA1 and CA3 regions was nearly equivalent to that in the intact hippocampus at all postlesion time points. Collectively, these results suggest that persistent reductions in GAD-67-positive interneuron density observed throughout the hippocampus following CA3 lesion are largely due to a permanent loss of GAD-67 expression in a significant fraction of interneurons, rather than widespread degeneration of interneurons. Nevertheless, a persistent decrease in interneuron activity, as evidenced by permanent down-regulation of GAD-67 in a major fraction of interneurons, would likely enhance the degree of hyperexcitability in the CA3-lesioned hippocampus.

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

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

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.

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

Fetal hippocampal grafts transplanted to the lesioned CA3 of adult hippocampus can extend axonal projections to many regions of the host brain. However, the identity of grafted cells that project to specific host regions is unknown. We hypothesize that the pattern of long-distance axonal projections from distinct fetal hippocampal cells grafted to lesioned CA3 is specified by the intrinsic nature of respective donor cells rather than characteristics of the host graft region. We grafted fetal hippocampal CA3 or CA1 cells into kainic acid lesioned CA3 of adult hippocampus at four days post-lesion. Neurons projecting to either the contralateral hippocampus or the ipsilateral septum were then measured in these grafts at four months post-grafting using Fluoro-Gold and DiI tract tracing. CA3 grafts located close to the degenerated CA3 cell layer showed a high propensity for establishing projections into the contralateral hippocampus (commissural projections) compared to similarly located CA1 grafts, which exhibited negligible commissural projections. Similar distinction was observed between the two graft types even when they were located only partially in the lesioned CA3. Among CA3 grafts, those placed near the degenerated CA3 cell layer established significantly greater commissural projections than those placed only partially in the CA3 region. Septal projections, in contrast, were robust from both CA3 and CA1 grafts. This differential projection pattern between CA3 and CA1 grafts resembles projections of CA3 and CA1 cells in intact hippocampus.These results demonstrate that the intrinsic character of grafted fetal cells determines the type of efferent projections from fetal grafts into different targets in the lesioned adult host brain. However, the extent of efferent projections from specific grafts is also influenced by the location of grafted cells within the host region. Thus, graft-mediated appropriate reconstruction of damaged circuitry in the lesioned brain may require grafting of homotopic donor cells. Further, the robust and specific projections observed from CA3 grafts is likely beneficial for functional recovery of hippocampus following CA3 injury and hence of significance towards developing a graft-mediated therapy for human temporal lobe epilepsy.

Survival of grafted fetal neural cells in kainic acid lesioned CA3 region of adult hippocampus depends upon cell specificity

We hypothesize that the degree of graft cell survival within the damaged CNS correlates with the specificity of donor cells to the region of grafting. We investigated graft cell survival following transplantation of fetal micrografts into the CA3 region of the adult rat hippocampus at a time-point of 4 days after an intracerebroventricular administration of kainic acid (KA). Grafts consisted of 5'-bromodeoxyuridine (BrdU) labeled embryonic day (E) 19 cells from hippocampal fields CA3 and CA1 and E15 and E19 cells from the striatum. Absolute cell survival in these grafts was quantitatively analyzed at 1 month postgrafting, using BrdU immunostaining of serial sections and three-dimensional reconstruction of grafts. Absolute graft cell survival in lesioned CA3 was dramatically greater for cells having hippocampal origin (CA3 cells, 69% cell survival; CA1 cells, 42% cell survival) than those having nonhippocampal origin, such as striatal cells (E15 cells, 12% cell survival; E19 cells, 4% cell survival). This difference is in sharp contrast to survival of these cells in culture, where E19 cells from both hippocampal and nonhippocampal origins exhibited similar survival. Comparison of survival among hippocampal cell types indicated significantly greater survival for cells that are specific to the lesioned area (i.e., CA3 cells) than for those that are nonspecific to the lesioned area (i.e., CA1 cells). Graft cell survival in the intact CA3 region (contralateral to KA administration), however, did not differ either between cells having hippocampal and nonhippocampal origins or between CA3 and CA1 cells (CA3 cells, 26% cell survival; CA1 cells, 33% cell survival; and E15 striatal cells, 20% cell survival). These results underscore the finding that enhanced survival of fetal cell grafts in the lesioned CNS is critically dependent upon the specificity of donor fetal cells to the region of transplantation. Thus, grafting of cells that are specific to the lesioned area is a prerequisite for achieving maximal graft cell survival and integration in the lesioned host CNS.