Lineage-restricted neural precursors survive, migrate, and differentiate following transplantation into the injured adult spinal cord

Transplantation of neural stem cells clonally derived from embryonic stem cells promotes recovery after murine spinal cord injury

The pathology of spinal cord injury (SCI) makes it appropriate for cell-based therapies. Treatments using neural stem cells (NSCs) in animal models of SCI have shown positive outcomes, although uncertainty remains regarding the optimal cell source. Pluripotent cell sources such as embryonic stem cells (ESCs) provide a limitless supply of therapeutic cells. NSCs derived using embryoid bodies (EB) from ESCs have shown tumorigenic potential. Clonal neurosphere generation is an alternative method to generate safer and more clinically relevant NSCs without the use of an EB stage for use in cell-based therapies. We generated clonally derived definitive NSCs (dNSCs) from ESC. These cells were transplanted into a mouse thoracic SCI model. Embryonic stem cell-derived definitive neural stem cell (ES-dNSC)-transplanted mice were compared with controls using behavioral measures and histopathological analysis of tissue. In addition, the role of remyelination in injury recovery was investigated using transmission electron microscopy. The SCI group that received ES-dNSC transplantation showed significant improvements in locomotor function compared with controls in open field and gait analysis. The cell treatment group had a significant enhancement of spared neural tissue. Immunohistological assessments showed that dNSCs differentiated primarily to oligodendrocytes. These cells were shown to express myelin basic protein, associate with axons, and support nodal architecture as well as display proper compact, multilayer myelination in electron microscopic analysis. This study provides strong evidence that dNSCs clonally derived from pluripotent cells using the default pathway of neuralization improve motor function after SCI and enhance sparing of neural tissue, while remaining safe and clinically relevant.

Human iPS cell-derived astrocyte transplants preserve respiratory function after spinal cord injury

Transplantation-based replacement of lost and/or dysfunctional astrocytes is a promising therapy for spinal cord injury (SCI) that has not been extensively explored, despite the integral roles played by astrocytes in the central nervous system (CNS). Induced pluripotent stem (iPS) cells are a clinically-relevant source of pluripotent cells that both avoid ethical issues of embryonic stem cells and allow for homogeneous derivation of mature cell types in large quantities, potentially in an autologous fashion. Despite their promise, the iPS cell field is in its infancy with respect to evaluating in vivo graft integration and therapeutic efficacy in SCI models. Astrocytes express the major glutamate transporter, GLT1, which is responsible for the vast majority of glutamate uptake in spinal cord. Following SCI, compromised GLT1 expression/function can increase susceptibility to excitotoxicity. We therefore evaluated intraspinal transplantation of human iPS cell-derived astrocytes (hIPSAs) following cervical contusion SCI as a novel strategy for reconstituting GLT1 expression and for protecting diaphragmatic respiratory neural circuitry. Transplant-derived cells showed robust long-term survival post-injection and efficiently differentiated into astrocytes in injured spinal cord of both immunesuppressed mice and rats. However, the majority of transplant-derived astrocytes did not express high levels of GLT1, particularly at early times post-injection. To enhance their ability to modulate extracellular glutamate levels, we engineered hIPSAs with lentivirus to constitutively express GLT1. Overexpression significantly increased GLT1 protein and functional GLT1-mediated glutamate uptake levels in hIPSAs both in vitro and in vivo post-transplantation. Compared to human fibroblast control and unmodified hIPSA transplantation, GLT1-overexpressing hIPSAs reduced (1) lesion size within the injured cervical spinal cord, (2) morphological denervation by respiratory phrenic motor neurons at the diaphragm neuromuscular junction, and (3) functional diaphragm denervation as measured by recording of spontaneous EMGs and evoked compound muscle action potentials. Our findings demonstrate that hiPSA transplantation is a therapeutically-powerful approach for SCI.

Intermittent hypoxia and neurorehabilitation

In recent years, it has become clear that brief, repeated presentations of hypoxia [i.e., acute intermittent hypoxia (AIH)] can boost the efficacy of more traditional therapeutic strategies in certain cases of neurologic dysfunction. This hypothesis derives from a series of studies in animal models and human subjects performed over the past 35 yr. In 1980, Millhorn et al. (Millhorn DE, Eldridge FL, Waldrop TG. Respir Physiol 41: 87-103, 1980) showed that electrical stimulation of carotid chemoafferent neurons produced a persistent, serotonin-dependent increase in phrenic motor output that outlasts the stimulus for more than 90 min (i.e., a "respiratory memory"). AIH elicits similar phrenic "long-term facilitation" (LTF) by a mechanism that requires cervical spinal serotonin receptor activation and de novo protein synthesis. From 2003 to present, a series of studies demonstrated that AIH can induce neuroplasticity in the injured spinal cord, causing functional recovery of breathing capacity after cervical spinal injury. Subsequently, it was demonstrated that repeated AIH (rAIH) can induce recovery of limb function, and the functional benefits of rAIH are greatest when paired with task-specific training. Since uncontrolled and/or prolonged intermittent hypoxia can elicit pathophysiology, a challenge of intermittent hypoxia research is to ensure that therapeutic protocols are well below the threshold for pathogenesis. This is possible since many low dose rAIH protocols have induced functional benefits without evidence of pathology. We propose that carefully controlled rAIH is a safe and noninvasive modality that can be paired with other neurorehabilitative strategies including traditional activity-based physical therapy or cell-based therapies such as intraspinal transplantation of neural progenitors.

Transplantation of Induced Pluripotent Stem Cell-Derived Neural Stem Cells Mediate Functional Recovery Following Thoracic Spinal Cord Injury Through Remyelination of Axons

: Neural stem cells (NSCs) from embryonic or fetal/adult tissue sources have shown considerable promise in regenerative strategies for traumatic spinal cord injury (SCI). However, there are limitations with their use related to the availability, immunogenicity, and uncertainty of the mechanisms involved. To address these issues, definitive NSCs derived from induced pluripotent stem (iPS) cells generated using a nonviral, piggyBac transposon approach, were investigated. Committed NSCs were generated from iPS cells using a free-floating neurosphere methodology previously described by our laboratory. To delineate the mechanism of action, specifically the role of exogenous myelination, NSCs derived from wildtype (wt) and nonmyelinating Shiverer (shi) iPS cell lines were used following thoracic SCI with subacute intraspinal transplantation. Behavioral, histological, and electrophysiological outcomes were analyzed to assess the effectiveness of this treatment. The wt- and shi-iPS-NSCs were validated and shown to be equivalent except in myelination capacity. Both iPS-NSC lines successfully integrated into the injured spinal cord and predominantly differentiated to oligodendrocytes, but only the wt-iPS-NSC treatment resulted in a functional benefit. The wt-iPS-dNSCs, which exhibited the capacity for remyelination, significantly improved neurobehavioral function (Basso Mouse Scale and CatWalk), histological outcomes, and electrophysiological measures of axonal function (sucrose gap analysis) compared with the nonmyelinating iPS-dNSCs and cell-free controls. In summary, we demonstrated that iPS cells can generate translationally relevant NSCs for applications in SCI. Although NSCs have a diverse range of functions in the injured spinal cord, remyelination is the predominant mechanism of recovery following thoracic SCI.

SIGNIFICANCE

Gain-of-function/loss-of-function techniques were used to examine the mechanistic importance of graft-derived remyelination following thoracic spinal cord injury (SCI). The novel findings of this study include the first use of neural stem cells (NSCs) from induced pluripotent stem cells (iPSCs) derived using the clonal neurosphere expansion conditions, for the treatment of SCI, the first characterization and in vivo application of iPSCs from Shiverer mouse fibroblasts, and the first evidence of the importance of remyelination by pluripotent-sourced NSCs for SCI repair and regeneration.

Polarized Macrophages Have Distinct Roles in the Differentiation and Migration of Embryonic Spinal-cord-derived Neural Stem Cells After Grafting to Injured Sites of Spinal Cord

Spinal cord injury (SCI) frequently provokes serious detrimental outcomes because neuronal regeneration is limited in the central nervous system (CNS). Thus, the creation of a permissive environment for transplantation therapy with neural stem/progenitor cells (NS/PCs) is a promising strategy to replace lost neuronal cells, promote repair, and stimulate functional plasticity after SCI. Macrophages are important SCI-associated inflammatory cells and a major source of secreted factors that modify the lesion milieu. Here, we used conditional medium (CM) from bone marrow-derived M1 or M2 polarized macrophages to culture murine NS/PCs. The NS/PCs showed enhanced astrocytic versus neuronal/oligodendrocytic differentiation in the presence of M1- versus M2-CM. Similarly, cotransplantation of NS/PCs with M1 and M2 macrophages into intact or injured murine spinal cord increased the number of engrafted NS/PC-derived astrocytes and neurons/oligodendrocytes, respectively. Furthermore, when cotransplantated with M2 macrophages, the NS/PC-derived neurons integrated into the local circuitry and enhanced locomotor recovery following SCI. Interesting, engrafted M1 macrophages promoted long-distance rostral migration of NS/PC-derived cells in a chemokine (C-X-C motif) receptor 4 (CXCR4)-dependent manner, while engrafted M2 macrophages resulted in limited cell migration of NS/PC-derived cells. Altogether, these findings suggest that the cotransplantation of NS/PCs together with polarized macrophages could constitute a promising therapeutic approach for SCI repair.

The Neural Stem Cell Microenvironment: Focusing on Axon Guidance Molecules and Myelin-Associated Factors

Neural stem cells (NSCs) could produce various cell phenotypes in the subventricular zone (SVZ) and dentate gyrus of the hippocampus in the central nervous system (CNS), where neurogenesis has been determined to occur. The extracellular microenvironment also influences the behaviors of NSCs during development and at CNS injury sites. Our previous study indicates that myelin, a component of the CNS, could regulate the differentiation of NSCs in vitro. Recent reports have implicated three myelin-derived inhibitors, NogoA, myelin-associated glycoprotein (MAG), and oligodendrocyte-myelin glycoprotein (OMgp), as well as several axon guidance molecules as regulators of NSC survival, proliferation, migration, and differentiation. However, the molecular mechanisms underlying the behavior of NSCs are not fully understood. In this study, we summarize the current literature on the effects of different extrinsic factors on NSCs and discuss possible mechanisms, as well as future possible clinical applications.

Characterization of ectopic colonies that form in widespread areas of the nervous system with neural stem cell transplants into the site of a severe spinal cord injury

We reported previously the formation of ectopic colonies in widespread areas of the nervous system after transplantation of fetal neural stem cells (NSCs) into spinal cord transection sites. Here, we characterize the incidence, distribution, and cellular composition of the colonies. NSCs harvested from E14 spinal cords from rats that express GFP were treated with a growth factor cocktail and grafted into the site of a complete spinal cord transection. Two months after transplant, spinal cord and brain tissue were analyzed histologically. Ectopic colonies were found at long distances from the transplant in the central canal of the spinal cord, the surface of the brainstem and spinal cord, and in the fourth ventricle. Colonies were present in 50% of the rats, and most rats had multiple colonies. Axons extended from the colonies into the host CNS. Colonies were strongly positive for nestin, a marker for neural precursors, and contained NeuN-positive cells with processes resembling dendrites, GFAP-positive astrocytes, APC/CC1-positive oligodendrocytes, and Ki-67-positive cells, indicating ongoing proliferation. Stereological analyses revealed an estimated 21,818 cells in a colony in the fourth ventricle, of which 1005 (5%) were Ki-67 positive. Immunostaining for synaptic markers (synaptophysin and VGluT-1) revealed large numbers of synaptophysin-positive puncta within the colonies but fewer VGluT-1 puncta. Continuing expansion of NSC-derived cell masses in confined spaces in the spinal cord and brain could produce symptoms attributable to compression of nearby tissue. It remains to be determined whether other cell types with self-renewing potential can also form colonies.

Repair of spinal cord injury with neuronal relays: From fetal grafts to neural stem cells

Spinal cord injury (SCI) disrupts the long axonal tracts of the spinal cord leading to devastating loss of function. Cell transplantation in the injured spinal cord has the potential to lead to recovery after SCI via a variety of mechanisms. One such strategy is the formation of neuronal relays between injured long tract axons and denervated neurons. The idea of creating a neuronal relay was first proposed over 25 years ago when fetal tissue was first successfully transplanted into the injured rodent spinal cord. Advances in labeling of grafted cells and the development of neural stem cell culturing techniques have improved the ability to create and refine such relays. Several recent studies have examined the ability to create a novel neuronal circuit between injured axons and denervated targets. This approach is an alternative to long-distance regeneration of damaged axons that may provide a meaningful degree of recovery without direct recreation of lost pathways. This brief review will examine the contribution of fetal grafting to current advances in neuronal grafting. Of particular interest will be the ability of transplanted neurons derived from fetal grafts, neural precursor cells and neural stem cells to reconnect long distance motor and sensory pathways of the injured spinal cord. This article is part of a Special Issue entitled SI: Spinal cord injury.

Transplantation of glial progenitors that overexpress glutamate transporter GLT1 preserves diaphragm function following cervical SCI

Approximately half of traumatic spinal cord injury (SCI) cases affect cervical regions, resulting in chronic respiratory compromise. The majority of these injuries affect midcervical levels, the location of phrenic motor neurons (PMNs) that innervate the diaphragm. A valuable opportunity exists following SCI for preventing PMN loss that occurs during secondary degeneration. One of the primary causes of secondary injury is excitotoxicity due to dysregulation of extracellular glutamate homeostasis. Astrocytes express glutamate transporter 1 (GLT1), which is responsible for the majority of CNS glutamate clearance. Given our observations of GLT1 dysfunction post-SCI, we evaluated intraspinal transplantation of Glial-Restricted Precursors (GRPs)--a class of lineage-restricted astrocyte progenitors--into ventral horn following cervical hemicontusion as a novel strategy for reconstituting GLT1 function, preventing excitotoxicity and protecting PMNs in the acutely injured spinal cord. We find that unmodified transplants express low levels of GLT1 in the injured spinal cord. To enhance their therapeutic properties, we engineered GRPs with AAV8 to overexpress GLT1 only in astrocytes using the GFA2 promoter, resulting in significantly increased GLT1 protein expression and functional glutamate uptake following astrocyte differentiation in vitro and after transplantation into C4 hemicontusion. Compared to medium-only control and unmodified GRPs, GLT1-overexpressing transplants reduced lesion size, diaphragm denervation and diaphragm dysfunction. Our findings demonstrate transplantation-based replacement of astrocyte GLT1 is a promising approach for SCI.

Time course of spinal doublecortin expression in developing rat and porcine spinal cord: implication in in vivo neural precursor grafting studies

Expression of doublecortin (DCX), a 43-53 kDa microtubule binding protein, is frequently used as (i) an early neuronal marker to identify the stage of neuronal maturation of in vivo grafted neuronal precursors (NSCs), and (ii) a neuronal fate marker transiently expressed by immature neurons during development. Reliable identification of the origin of DCX-immunoreactive cells (i.e., host vs. graft) requires detailed spatial and temporal mapping of endogenous DCX expression at graft-targeted brain or spinal cord regions. Accordingly, in the present study, we analyzed (i) the time course of DCX expression in pre- and postnatal rat and porcine spinal cord, and (ii) the DCX expression in spinally grafted porcine-induced pluripotent stem cells (iPS)-derived NSCs and human embryonic stem cell (ES)-derived NSCs. In addition, complementary temporospatial GFAP expression study in porcine spinal cord was also performed. In 21-day-old rat fetuses, an intense DCX immunoreactivity distributed between the dorsal horn (DH) and ventral horn was seen and was still present in the DH neurons on postnatal day 20. In animals older than 8 weeks, no DCX immunoreactivity was seen at any spinal cord laminae. In contrast to rat, in porcine spinal cord (gestational period 113-114 days), DCX was only expressed during the pre-natal period (up to 100 days) but was no longer present in newborn piglets or in adult animals. Immunohistochemical analysis was confirmed with a comparable expression profile by western blot analysis. Contrary, the expression of porcine GFAP started within 70-80 days of the pre-natal period. Spinally grafted porcine iPS-NSCs and human ES-NSCs showed clear DCX expression at 3-4 weeks postgrafting. These data indicate that in spinal grafting studies which employ postnatal or adult porcine models, the expression of DCX can be used as a reliable marker of grafted neurons. In contrast, if grafted neurons are to be analyzed during the first 4 postnatal weeks in the rat spinal cord, additional markers or grafted cell-specific labeling techniques need to be employed to reliably identify grafted early postmitotic neurons and to differentiate the DCX expression from the neurons of the host.

Increasing label-free stem cell sorting capacity to reach transplantation-scale throughput

Dielectrophoresis (DEP) has proven an invaluable tool for the enrichment of populations of stem and progenitor cells owing to its ability to sort cells in a label-free manner and its biological safety. However, DEP separation devices have suffered from a low throughput preventing researchers from undertaking studies requiring large numbers of cells, such as needed for cell transplantation. We developed a microfluidic device designed for the enrichment of stem and progenitor cell populations that sorts cells at a rate of 150,000 cells/h, corresponding to an improvement in the throughput achieved with our previous device designs by over an order of magnitude. This advancement, coupled with data showing the DEP-sorted cells retain their enrichment and differentiation capacity when expanded in culture for periods of up to 2 weeks, provides sufficient throughput and cell numbers to enable a wider variety of experiments with enriched stem and progenitor cell populations. Furthermore, the sorting devices presented here provide ease of setup and operation, a simple fabrication process, and a low associated cost to use that makes them more amenable for use in common biological research laboratories. To our knowledge, this work represents the first to enrich stem cells and expand them in culture to generate transplantation-scale numbers of differentiation-competent cells using DEP.

Neural stem cell-conditioned medium protects neurons and promotes propriospinal neurons relay neural circuit reconnection after spinal cord injury

Human fetal neural stem cells (hNSCs) are used to treat a variety of neurological disorders involving spinal cord injury (SCI). Although their mechanism of action has been attributed to cell substitution, we examined the possibility that NSCs may have neuroprotective activities. The present article studied the action of hNSCs on protecting neurons and promoting corticospinal tract (CST) axon regeneration after SCI. hNSCs were isolated from the cortical tissue of spontaneously aborted human fetuses. The cells were removed from the NSC culture medium to acquire NSCM, thus excluding the effect of cell substitution. Continuous administration of the NSCM after the SCI resulted in extensive growth of the CST in the cervical region and more than tripled the formation of synaptic contacts between CST collaterals and propriospinal interneurons that project from the cervical level of the spinal cord to the lumbar level. NSCM reduced the number of caspase 3-positive apoptotic profiles at 7 days and protected against loss of the neurons 6 weeks after injury. NSCM promoted locomotor recovery with a five-point improvement on the BBB scale in adult rats. Thus, hNSCs help to set up a contour neural circuit via secretory factors, which may be the mechanism for their action in SCI rats. This manuscript is published as part of the International Association of Neurorestoratology (IANR) special issue of Cell Transplantation.

Persistent at-level thermal hyperalgesia and tactile allodynia accompany chronic neuronal and astrocyte activation in superficial dorsal horn following mouse cervical contusion spinal cord injury

In humans, sensory abnormalities, including neuropathic pain, often result from traumatic spinal cord injury (SCI). SCI can induce cellular changes in the CNS, termed central sensitization, that alter excitability of spinal cord neurons, including those in the dorsal horn involved in pain transmission. Persistently elevated levels of neuronal activity, glial activation, and glutamatergic transmission are thought to contribute to the hyperexcitability of these dorsal horn neurons, which can lead to maladaptive circuitry, aberrant pain processing and, ultimately, chronic neuropathic pain. Here we present a mouse model of SCI-induced neuropathic pain that exhibits a persistent pain phenotype accompanied by chronic neuronal hyperexcitability and glial activation in the spinal cord dorsal horn. We generated a unilateral cervical contusion injury at the C5 or C6 level of the adult mouse spinal cord. Following injury, an increase in the number of neurons expressing ΔFosB (a marker of chronic neuronal activation), persistent astrocyte activation and proliferation (as measured by GFAP and Ki67 expression), and a decrease in the expression of the astrocyte glutamate transporter GLT1 are observed in the ipsilateral superficial dorsal horn of cervical spinal cord. These changes have previously been associated with neuronal hyperexcitability and may contribute to altered pain transmission and chronic neuropathic pain. In our model, they are accompanied by robust at-level hyperaglesia in the ipsilateral forepaw and allodynia in both forepaws that are evident within two weeks following injury and persist for at least six weeks. Furthermore, the pain phenotype occurs in the absence of alterations in forelimb grip strength, suggesting that it represents sensory and not motor abnormalities. Given the importance of transgenic mouse technology, this clinically-relevant model provides a resource that can be used to study the molecular mechanisms contributing to neuropathic pain following SCI and to identify potential therapeutic targets for the treatment of chronic pathological pain.

Effects of resveratrol on hydrogen peroxide-induced oxidative stress in embryonic neural stem cells

Resveratrol, a natural phenolic compound, has been shown to prevent cardiovascular diseases and cancer and exhibit neuroprotective effects. In this study, we examined the neuroprotective and antioxidant effects of resveratrol against hydrogen peroxide in embryonic neural stem cells. Hydrogen peroxide treatment alone increased catalase and glutathione peroxidase activities but did not change superoxide dismutase levels compared with hydrogen peroxide + resveratrol treatment. Nitric oxide synthase activity and concomitant nitric oxide levels increased in response to hydrogen peroxide treatment. Conversely, resveratrol treatment decreased nitric oxide synthase activity and nitric oxide levels. Resveratrol also attenuated hydrogen peroxide-induced nuclear or mitochondrial DNA damage. We propose that resveratrol may be a promising agent for protecting embryonic neural stem cells because of its potential to decrease oxidative stress by inducing higher activity of antioxidant enzymes, decreasing nitric oxide production and nitric oxide synthase activity, and alleviating both nuclear and mitochondrial DNA damage.

Overexpression of the astrocyte glutamate transporter GLT1 exacerbates phrenic motor neuron degeneration, diaphragm compromise, and forelimb motor dysfunction following cervical contusion spinal cord injury

A major portion of spinal cord injury (SCI) cases affect midcervical levels, the location of the phrenic motor neuron (PhMN) pool that innervates the diaphragm. While initial trauma is uncontrollable, a valuable opportunity exists in the hours to days following SCI for preventing PhMN loss and consequent respiratory dysfunction that occurs during secondary degeneration. One of the primary causes of secondary injury is excitotoxic cell death due to dysregulation of extracellular glutamate homeostasis. GLT1, mainly expressed by astrocytes, is responsible for the vast majority of functional uptake of extracellular glutamate in the CNS, particularly in spinal cord. We found that, in bacterial artificial chromosome-GLT1-enhanced green fluorescent protein reporter mice following unilateral midcervical (C4) contusion SCI, numbers of GLT1-expressing astrocytes in ventral horn and total intraspinal GLT1 protein expression were reduced soon after injury and the decrease persisted for ≥6 weeks. We used intraspinal delivery of adeno-associated virus type 8 (AAV8)-Gfa2 vector to rat cervical spinal cord ventral horn for targeting focal astrocyte GLT1 overexpression in areas of PhMN loss. Intraspinal delivery of AAV8-Gfa2-GLT1 resulted in transduction primarily of GFAP(+) astrocytes that persisted for ≥6 weeks postinjury, as well as increased intraspinal GLT1 protein expression. Surprisingly, we found that astrocyte-targeted GLT1 overexpression increased lesion size, PhMN loss, phrenic nerve axonal degeneration, and diaphragm neuromuscular junction denervation, and resulted in reduced functional diaphragm innervation as assessed by phrenic nerve-diaphragm compound muscle action potential recordings. These results demonstrate that GLT1 overexpression via intraspinal AAV-Gfa2-GLT1 delivery exacerbates neuronal damage and increases respiratory impairment following cervical SCI.

Promotion of survival and differentiation of neural stem cells with fibrin and growth factor cocktails after severe spinal cord injury

Neural stem cells (NSCs) can self-renew and differentiate into neurons and glia. Transplanted NSCs can replace lost neurons and glia after spinal cord injury (SCI), and can form functional relays to re-connect spinal cord segments above and below a lesion. Previous studies grafting neural stem cells have been limited by incomplete graft survival within the spinal cord lesion cavity. Further, tracking of graft cell survival, differentiation, and process extension had not been optimized. Finally, in previous studies, cultured rat NSCs were typically reported to differentiate into glia when grafted to the injured spinal cord, rather than neurons, unless fate was driven to a specific cell type. To address these issues, we developed new methods to improve the survival, integration and differentiation of NSCs to sites of even severe SCI. NSCs were freshly isolated from embryonic day 14 spinal cord (E14) from a stable transgenic Fischer 344 rat line expressing green fluorescent protein (GFP) and were embedded into a fibrin matrix containing growth factors; this formulation aimed to retain grafted cells in the lesion cavity and support cell survival. NSCs in the fibrin/growth factor cocktail were implanted two weeks after thoracic level-3 (T3) complete spinal cord transections, thereby avoiding peak periods of inflammation. Resulting grafts completely filled the lesion cavity and differentiated into both neurons, which extended axons into the host spinal cord over remarkably long distances, and glia. Grafts of cultured human NSCs expressing GFP resulted in similar findings. Thus, methods are defined for improving neural stem cell grafting, survival and analysis of in vivo findings.

Chronic at-level thermal hyperalgesia following rat cervical contusion spinal cord injury is accompanied by neuronal and astrocyte activation and loss of the astrocyte glutamate transporter, GLT1, in superficial dorsal horn

Neuropathic pain is a form of pathological nociception that occurs in a significant portion of traumatic spinal cord injury (SCI) patients, resulting in debilitating and often long-term physical and psychological burdens. While many peripheral and central mechanisms have been implicated in neuropathic pain, central sensitization of dorsal horn spinothalamic tract (STT) neurons is a major underlying substrate. Furthermore, dysregulation of extracellular glutamate homeostasis and chronic astrocyte activation play important underlying roles in persistent hyperexcitability of these superficial dorsal horn neurons. To date, central sensitization and astrocyte changes have not been characterized in cervical SCI-induced neuropathic pain models, despite the fact that a major portion of SCI patients suffer contusion trauma to cervical spinal cord. In this study, we have characterized 2 rat models of unilateral cervical contusion SCI that behaviorally result in chronic persistence of thermal hyperalgesia in the ipsilateral forepaw. In addition, we find that STT neurons are chronically activated in both models when compared to laminectomy-only uninjured rats. Finally, persistent astrocyte activation and significantly reduced expression of the major CNS glutamate transporter, GLT1, in superficial dorsal horn astrocytes are associated with both excitability changes in STT neurons and the neuropathic pain behavioral phenotype. In conclusion, we have characterized clinically-relevant rodent models of cervical contusion-induced neuropathic pain that result in chronic activation of both STT neurons and astrocytes, as well as compromise in astrocyte glutamate transporter expression. These models can be used as important tools to further study mechanisms underlying neuropathic pain post-SCI and to test potential therapeutic interventions.

A re-assessment of long distance growth and connectivity of neural stem cells after severe spinal cord injury

As part of the NIH "Facilities of Research Excellence-Spinal Cord Injury" project to support independent replication, we repeated key parts of a study reporting robust engraftment of neural stem cells (NSCs) treated with growth factors after complete spinal cord transection in rats. Rats (n=20) received complete transections at thoracic level 3 (T3) and 2weeks later received NSC transplants in a fibrin matrix with a growth factor cocktail using 2 different transplantation methods (with and without removal of scar tissue). Control rats (n=9) received transections only. Hindlimb locomotor function was assessed with the BBB scale. Nine weeks post injury, reticulospinal tract axons were traced in 6 rats by injecting BDA into the reticular formation. Transplants grew to fill the lesion cavity in most rats although grafts made with scar tissue removal had large central cavities. Grafts blended extensively with host tissue obliterating the astroglial boundary at the cut ends, but in most cases there was a well-defined partition within the graft that separated rostral and caudal parts of the graft. In some cases, the partition contained non-neuronal scar tissue. There was extensive outgrowth of GFP labeled axons from the graft, but there was minimal ingrowth of host axons into the graft revealed by tract tracing and immunocytochemistry for 5HT. There were no statistically significant differences between transplant and control groups in the degree of locomotor recovery. Our results confirm the previous report that NSC transplants can fill lesion cavities and robustly extend axons, but reveal that most grafts do not create a continuous bridge of neural tissue between rostral and caudal segments.

Co-grafting of neural stem cells with olfactory en sheathing cells promotes neuronal restoration in traumatic brain injury with an anti-inflammatory mechanism

BACKGROUND

We sought to investigate the effects of co-grafting neural stem cells (NSCs) with olfactory ensheathing cells (OECs) on neurological behavior in rats subjected to traumatic brain injury (TBI) and explore underlying molecular mechanisms.

METHODS

TBI was established by percussion device made through a weight drop (50 g) from a 30 cm height. Cultured NSCs and OECs isolated from rats were labeled by Hoechst 33342 (blue) and chloromethyl-benzamidodialkyl carbocyanine (CM-Dil) (red), respectively. Then, NSCs and/or OECs, separately or combined, were transplanted into the area surrounding the injury site. Fourteen days after transplantation, neurological severity score (NSS) were recorded. The brain tissue was harvested and processed for immunocytochemistry, terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL), and reverse transcription-polymerase chain reaction (RT-PCR).

RESULTS

Significant neurological function improvement was observed in the three transplant groups, compared to the TBI group, and co-transplantation gave rise to the best improvement. Morphological evaluation showed that the number of neurons in cortex from combination implantation was more than for other groups (P <0.05); conversely, the number of apoptotic cells showed a significant decrease by TUNEL staining. Transplanted NSCs and OECs could survive and migrate in the brain, and the number of neurons differentiating from NSCs in the co-transplantation group was significantly greater than in the NSCs group. At the molecular level, the expressions of IL-6 and BAD in the co-graft group were found to be down regulated significantly, when compared to either the NSC or OEC alone groups.

CONCLUSION

The present study demonstrates for the first time the optimal effects of co-grafting NSCs and OECs as a new strategy for the treatment of TBI via an anti-inflammation mechanism.

Transplanting neural progenitors into a complete transection model of spinal cord injury

Neural progenitor cell (NPC) transplantation is a promising therapeutic strategy for spinal cord injury (SCI) because of the potential for cell replacement and restoration of connectivity. Our previous studies have shown that transplants of NPC, composed of neuron- and glia-restricted progenitors derived from the embryonic spinal cord, survived well in partial lesion models and generated graft-derived neurons, which could be used to form a functional relay. We have now examined the properties of a similar NPC transplant using a complete transection model in juvenile and adult rats. We found poor survival of grafted cells despite using a variety of lesion methods, matrices, and delays of transplantation. If, instead of cultured progenitor cells, the transplants were composed of segmental or dissociated segments of fetal spinal cord (FSC) derived from similar-staged embryos, grafted cells survived and integrated well with host tissue in juvenile and adult rats. FSC transplants differentiated into neurons and glial cells, including astrocytes and oligodendrocytes. Graft-derived neurons expressed glutaminergic and GABAergic markers. Grafted cells also migrated and extended processes into host tissue. Analysis of axon growth from the host spinal cord showed serotonin-positive fibers and biotinylated dextran amine-traced propriospinal axons growing into the transplants. These results suggest that in treating severe SCI, such as complete transection, NPC grafting faces major challenges related to cell survival and formation of a functional relay. Lessons learned from the efficacy of FSC transplants could be used to develop a therapeutic strategy based on neural progenitor cells for severe SCI.

Bridging defects in chronic spinal cord injury using peripheral nerve grafts combined with a chitosan-laminin scaffold and enhancing regeneration through them by co-transplantation with bone-marrow-derived mesenchymal stem cells: case series of 14 patients

OBJECTIVE

To investigate the effect of bridging defects in chronic spinal cord injury using peripheral nerve grafts combined with a chitosan-laminin scaffold and enhancing regeneration through them by co-transplantation with bone-marrow-derived mesenchymal stem cells.

METHODS

In 14 patients with chronic paraplegia caused by spinal cord injury, cord defects were grafted and stem cells injected into the whole construct and contained using a chitosan-laminin paste. Patients were evaluated using the International Standards for Classification of Spinal Cord Injuries.

RESULTS

Chitosan disintegration leading to post-operative seroma formation was a complication. Motor level improved four levels in 2 cases and two levels in 12 cases. Sensory-level improved six levels in two cases, five levels in five cases, four levels in three cases, and three levels in four cases. A four-level neurological improvement was recorded in 2 cases and a two-level neurological improvement occurred in 12 cases. The American Spinal Impairment Association (ASIA) impairment scale improved from A to C in 12 cases and from A to B in 2 cases. Although motor power improvement was recorded in the abdominal muscles (2 grades), hip flexors (3 grades), hip adductors (3 grades), knee extensors (2-3 grades), ankle dorsiflexors (1-2 grades), long toe extensors (1-2 grades), and plantar flexors (0-2 grades), this improvement was too low to enable them to stand erect and hold their knees extended while walking unaided.

CONCLUSION

Mesenchymal stem cell-derived neural stem cell-like cell transplantation enhances recovery in chronic spinal cord injuries with defects bridged by sural nerve grafts combined with a chitosan-laminin scaffold.

From basics to clinical: a comprehensive review on spinal cord injury

Spinal cord injury (SCI) is a devastating neurological disorder that affects thousands of individuals each year. Over the past decades an enormous progress has been made in our understanding of the molecular and cellular events generated by SCI, providing insights into crucial mechanisms that contribute to tissue damage and regenerative failure of injured neurons. Current treatment options for SCI include the use of high dose methylprednisolone, surgical interventions to stabilize and decompress the spinal cord, and rehabilitative care. Nonetheless, SCI is still a harmful condition for which there is yet no cure. Cellular, molecular, rehabilitative training and combinatorial therapies have shown promising results in animal models. Nevertheless, work remains to be done to ascertain whether any of these therapies can safely improve patient's condition after human SCI. This review provides an extensive overview of SCI research, as well as its clinical component. It starts covering areas from physiology and anatomy of the spinal cord, neuropathology of the SCI, current clinical options, neuronal plasticity after SCI, animal models and techniques to assess recovery, focusing the subsequent discussion on a variety of promising neuroprotective, cell-based and combinatorial therapeutic approaches that have recently moved, or are close, to clinical testing.

Intraspinal transplantation and modulation of donor neuron electrophysiological activity

Rat fetal spinal cord (FSC) tissue, naturally enriched with interneuronal progenitors, was introduced into high cervical, hemi-resection (Hx) lesions. Electrophysiological analyses were conducted to determine if such grafts exhibit physiologically-patterned neuronal activity and if stimuli which increase respiratory motor output also alter donor neuron bursting. Three months following transplantation, the bursting activity of FSC neurons and the contralateral phrenic nerve were recorded in anesthetized rats during a normoxic baseline period and brief respiratory challenges. Spontaneous neuronal activity was detected in 80% of the FSC transplants, and autocorrelation of action potential spikes revealed distinct correlogram peaks in 87% of neurons. At baseline, the average discharge frequency of graft neurons was 13.0 ± 1.7 Hz, and discharge frequency increased during a hypoxic respiratory challenge (p<0.001). Parallel studies in unanesthetized rats showed that FSC tissue recipients had larger inspiratory tidal volumes during brief hypoxic exposures (p<0.05 vs. C2Hx rats). Anatomical connectivity was explored in additional graft recipients by injecting a transsynaptic retrograde viral tracer (pseudorabies virus, PRV) directly into matured transplants. Neuronal labeling occurred throughout graft tissues and also in the host spinal cord and brainstem nuclei, including those associated with respiratory control. These results underscore the neuroplastic potential of host-graft interactions and training approaches to enhance functional integration within targeted spinal circuitry.

The survival of engrafted neural stem cells within hyaluronic acid hydrogels

Successful cell-based therapy of neurological disorders is highly dependent on the survival of transplanted stem cells, with the overall graft survival of naked, unprotected cells in general remaining poor. We investigated the use of an injectable hyaluronic acid (HA) hydrogel for enhancement of survival of transplanted mouse C17.2 cells, human neural progenitor cells (ReNcells), and human glial-restricted precursors (GRPs). The gelation properties of the HA hydrogel were first characterized and optimized for intracerebral injection, resulting in a 25 min delayed-injection after mixing of the hydrogel components. Using bioluminescence imaging (BLI) as a non-invasive readout of cell survival, we found that the hydrogel can protect xenografted cells as evidenced by the prolonged survival of C17.2 cells implanted in immunocompetent rats (p < 0.01 at day 12). The survival of human ReNcells and human GRPs implanted in the brain of immunocompetent or immunodeficient mice was also significantly improved after hydrogel scaffolding (ReNcells, p < 0.05 at day 5; GRPs, p < 0.05 at day 7). However, an inflammatory response could be noted two weeks after injection of hydrogel into immunocompetent mice brains. We conclude that hydrogel scaffolding increases the survival of engrafted neural stem cells, justifying further optimization of hydrogel compositions.

Neuro-immune interactions of neural stem cell transplants: from animal disease models to human trials

Stem cell technology is a promising branch of regenerative medicine that is aimed at developing new approaches for the treatment of severely debilitating human diseases, including those affecting the central nervous system (CNS). Despite the increasing understanding of the mechanisms governing their biology, the application of stem cell therapeutics remains challenging. The initial idea that stem cell transplants work in vivo via the replacement of endogenous cells lost or damaged owing to disease has been challenged by accumulating evidence of their therapeutic plasticity. This new concept covers the remarkable immune regulatory and tissue trophic effects that transplanted stem cells exert at the level of the neural microenvironment to promote tissue healing via combination of immune modulatory and tissue protective actions, while retaining predominantly undifferentiated features. Among a number of promising candidate stem cell sources, neural stem/precursor cells (NPCs) are under extensive investigation with regard to their therapeutic plasticity after transplantation. The significant impact in vivo of experimental NPC therapies in animal models of inflammatory CNS diseases has raised great expectations that these stem cells, or the manipulation of the mechanisms behind their therapeutic impact, could soon be translated to human studies. This review aims to provide an update on the most recent evidence of therapeutically-relevant neuro-immune interactions following NPC transplants in animal models of multiple sclerosis, cerebral stroke and traumas of the spinal cord, and consideration of the forthcoming challenges related to the early translation of some of these exciting experimental outcomes into clinical medicines.

Modeling to optimize terminal stem cell differentiation

Embryonic stem cell (ESC), iPCs, and adult stem cells (ASCs) all are among the most promising potential treatments for heart failure, spinal cord injury, neurodegenerative diseases, and diabetes. However, considerable uncertainty in the production of ESC-derived terminally differentiated cell types has limited the efficiency of their development. To address this uncertainty, we and other investigators have begun to employ a comprehensive statistical model of ESC differentiation for determining the role of intracellular pathways (e.g., STAT3) in ESC differentiation and determination of germ layer fate. The approach discussed here applies the Baysian statistical model to cell/developmental biology combining traditional flow cytometry methodology and specific morphological observations with advanced statistical and probabilistic modeling and experimental design. The final result of this study is a unique tool and model that enhances the understanding of how and when specific cell fates are determined during differentiation. This model provides a guideline for increasing the production efficiency of therapeutically viable ESCs/iPSCs/ASC derived neurons or any other cell type and will eventually lead to advances in stem cell therapy.

Endogenous proliferation after spinal cord injury in animal models

Spinal cord injury (SCI) results in motor and sensory deficits, the severity of which depends on the level and extent of the injury. Animal models for SCI research include transection, contusion, and compression mouse models. In this paper we will discuss the endogenous stem cell response to SCI in animal models. All SCI animal models experience a similar peak of cell proliferation three days after injury; however, each specific type of injury promotes a specific and distinct stem cell response. For example, the transection model results in a strong and localized initial increase of proliferation, while in contusion and compression models, the initial level of proliferation is lower but encompasses the entire rostrocaudal extent of the spinal cord. All injury types result in an increased ependymal proliferation, but only in contusion and compression models is there a significant level of proliferation in the lateral regions of the spinal cord. Finally, the fate of newly generated cells varies from a mainly oligodendrocyte fate in contusion and compression to a mostly astrocyte fate in the transection model. Here we will discuss the potential of endogenous stem/progenitor cell manipulation as a therapeutic tool to treat SCI.

Long-distance growth and connectivity of neural stem cells after severe spinal cord injury

Neural stem cells (NSCs) expressing GFP were embedded into fibrin matrices containing growth factor cocktails and grafted to sites of severe spinal cord injury. Grafted cells differentiated into multiple cellular phenotypes, including neurons, which extended large numbers of axons over remarkable distances. Extending axons formed abundant synapses with host cells. Axonal growth was partially dependent on mammalian target of rapamycin (mTOR), but not Nogo signaling. Grafted neurons supported formation of electrophysiological relays across sites of complete spinal transection, resulting in functional recovery. Two human stem cell lines (566RSC and HUES7) embedded in growth-factor-containing fibrin exhibited similar growth, and 566RSC cells supported functional recovery. Thus, properties intrinsic to early-stage neurons can overcome the inhibitory milieu of the injured adult spinal cord to mount remarkable axonal growth, resulting in formation of new relay circuits that significantly improve function. These therapeutic properties extend across stem cell sources and species.

Functional assessment of the acute local and distal transplantation of human neural stem cells after spinal cord injury

BACKGROUND CONTEXT

Spinal cord injury can lead to severe functional impairments secondary to axonal damage, neuronal loss, and demyelination. The injured spinal cord has limited regrowth of damaged axons. Treatment remains controversial, given inconsistent functional improvement. Previous studies demonstrated functional recovery of rats with spinal cord contusion after transplantation of rat fetal neural stem cells.

PURPOSE

We hypothesized that acute transplantation of human fetal neural stem cells (hNSCs) both locally at the injury site as well as distally via intrathecal injection would lead to improved functional recovery compared with controls.

STUDY DESIGN/SETTING

Twenty-four adult female Long-Evans hooded rats were randomized into four groups with six animals in each group: two experimental and two control. Functional assessment was measured after injury and then weekly for 6 weeks using the Basso, Beattie, and Bresnahan Locomotor Rating Score. Data were analyzed using two-sample t test and linear mixed-effects model analysis.

METHODS

Posterior exposure and laminectomy at T10 level was used. Moderate spinal cord contusion was induced by the Multicenter Animal Spinal Cord Injury Study Impactor with 10-g weight dropped from a height of 25 mm. Experimental subjects received either a subdural injection of hNSCs locally at the injury site or intrathecal injection of hNSCs through a separate distal laminotomy. Controls received control media injection either locally or distally.

RESULTS

Statistically significant functional improvement was observed in local or distal hNSCs subjects versus controls (p=.034 and 0.016, respectively). No significant difference was seen between local or distal hNSC subjects (p=.66).

CONCLUSIONS

Acute local and distal transplantation of hNSCs into the contused spinal cord led to significant functional recovery in the rat model. No statistical difference was found between the two techniques.

Differential effects of distinct central nervous system regions on cell migration and axonal extension of neural precursor transplants

Transplantation of neural precursor cells (NPCs) is a promising therapeutic strategy in CNS injury. However, the adult CNS lacks instructive signals present during development and, depending on the region and type of transplant, may be inhibitory for neuron generation and axonal growth. We examined the effects of the white matter in different regions of the adult CNS on the properties of NPC transplants with respect to cell survival, differentiation, migration, and axonal growth. NPCs were prepared from day 13.5 embryonic spinal cord of transgenic rats that express the human placental alkaline phosphatase (AP) reporter. These NPCs were injected unilaterally into the cervical spinal cord white matter and into the corpus callosum of adult rats and were analyzed immunohistochemically 2 weeks later. NPCs survived in both regions and differentiated into astrocytes, oligodendrocytes, and neurons, with no apparent differences in survival or phenotypic composition. However, in the spinal cord white matter, graft-derived cells, identified as precursors and glial cells, migrated from the injection site rostrally and caudally, whereas, in the corpus callosum, graft-derived cells did not migrate and remained at the injection site. Importantly, graft-derived neurons extended axons from the grafting site along the corpus callosum past the midline, entering into the contralateral side of the corpus callosum. These results demonstrate dramatic differences between white matter regions in the spinal cord and brain with respect to cell migration and axonal growth and underscore the importance of considering the effects of the local CNS environment in the design of effective transplantation strategies.

Recent therapeutic strategies for spinal cord injury treatment: possible role of stem cells

Spinal cord injury (SCI) often results in significant dysfunction and disability. A series of treatments have been proposed to prevent and overcome the formation of the glial scar and inhibitory factors to axon regrowth. In the last decade, cell therapy has emerged as a new tool for several diseases of the nervous system. Stem cells act as minipumps providing trophic and immunomodulatory factors to enhance axonal growth, to modulate the environment, and to reduce neuroinflammation. This capability can be boosted by genetical manipulation to deliver trophic molecules. Different types of stem cells have been tested, according to their properties and the therapeutic aims. They differ from each other for origin, developmental stage, stage of differentiation, and fate lineage. Related to this, stem cells differentiating into neurons could be used for cell replacement, even though the feasibility that stem cells after transplantation in the adult lesioned spinal cord can differentiate into neurons, integrate within neural circuits, and emit axons reaching the muscle is quite remote. The timing of cell therapy has been variable, and may be summarized in the acute and chronic phases of disease, when stem cells interact with a completely different environment. Even though further experimental studies are needed to elucidate the mechanisms of action, the therapeutic, and the side effects of cell therapy, several clinical protocols have been tested or are under trial. Here, we report the state-of-the-art of cell therapy in SCI, in terms of feasibility, outcome, and side effects.

Neurotrophic factors in combinatorial approaches for spinal cord regeneration

Axonal regeneration is inhibited by a plethora of different mechanisms in the adult central nervous system (CNS). While neurotrophic factors have been shown to stimulate axonal growth in numerous animal models of nervous system injury, a lack of suitable growth substrates, an insufficient activation of neuron-intrinsic regenerative programs, and extracellular inhibitors of regeneration limit the efficacy of neurotrophic factor delivery for anatomical and functional recovery after spinal cord injury. Thus, growth-stimulating factors will likely have to be combined with other treatment approaches to tap into the full potential of growth factor therapy for axonal regeneration. In addition, the temporal and spatial distribution of growth factors have to be tightly controlled to achieve biologically active concentrations, to allow for the chemotropic guidance of axons, and to prevent adverse effects related to the widespread distribution of neurotrophic factors. Here, we will review the rationale for combinatorial treatments in axonal regeneration and summarize some recent progress in promoting axonal regeneration in the injured CNS using such approaches.

The roles of neuronal and glial precursors in overcoming chondroitin sulfate proteoglycan inhibition

The extension of axons through the major inhibitory component of the glial scar, chondroitin sulfate proteoglycans (CSPGs), remains a key obstacle for regeneration following spinal cord injury (SCI). We have previously shown that transplants composed of neuronal and glial restricted precursors (NRP and GRP respectively) promote regeneration and connectivity in the injured spinal cord (Bonner et al., 2010, 2011), however, little is known about the properties of these precursors at a cellular level. We now report that NRP-derived neurons, in contrast to dorsal root ganglion (DRG) neurons, have the ability to extend axons and cross over from a permissive substratum (laminin) onto inhibitory CSPG in vitro. Growth cones of neurons derived from NRP, compared to DRG, exhibit significantly lower levels of the CSPG receptors protein tyrosine phosphatase sigma (PTPσ) and leukocyte common antigen-related phosphatase (LAR). GRP-conditioned medium prepared from the same cell densities did not affect the response of primary sensory neurons to CSPG confirming that the ability of NRP-derived neurons to cross onto CSPG is determined intrinsically. However, GRP-conditioned medium collected from high density cultures increased the probability of DRG axons to cross from LN onto CSPG and increased the length of DRG axons extending on CSPG. Collectively, these results suggest that (1) neurons derived from NRPs are intrinsically insensitive to CSPGs due to low levels of receptor expression, and (2) high levels of factors secreted by GRP can reduce the inhibitory effects of CSPG and promote axonal growth. These observations provide mechanistic insights into the specific roles of NRPs and GRPs in promoting regeneration and repair following SCI.

Neurotrophin-3 gene transduction of mouse neural stem cells promotes proliferation and neuronal differentiation in organotypic hippocampal slice cultures

BACKGROUND

The transplantation of neural stem cells (NSCs) has been accepted as a promising therapeutic strategy for central nervous system disorders. However, the beneficial effect of NSC transplantation upon functional recovery is limited due to the unfavorable microenvironment (niche) at the site of trauma or degenerative disease in the brain. Combination of transplantation of NSCs with neurotrophins may overcome the hurdles of impaired cell survival and neuronal differentiation.

MATERIAL/METHODS

In the current study, the neurotrophin-3 (NT-3) gene was transduced into cultured mouse embryonic cortical NSCs via an AAV vector (NSC-NT-3). The effect of NT-3 over-expression on cell proliferation and differentiation in NSCs was observed by immunohistochemistry, cell culture and organotypic hippocampal slice cultures.<br />

RESULTS

The characteristics of self-renewal and multiple differentiation of NSCs were well-preserved. Cells in the NSC-NT-3 group proliferated faster and differentiated into more β-tubulin III-positive neurons compared to the control group in vitro. Furthermore, cells in the NSC-NT-3 group survived in a significantly higher percentage and undertook neuronal differentiation preferably in organotypic hippocampal slice cultures.

CONCLUSIONS

Our results suggest that the transduction of NT-3 into NSCs could effectively promote NSCs survival, proliferation, and neuronal differentiation in vitro without change of the stemness of NSCs. This work also offers evidence to better understand the safety and efficiency of combined treatment with NT-3 and NSCs for the central nervous system disorders.

Phenotypic analysis of astrocytes derived from glial restricted precursors and their impact on axon regeneration

Although astrocytes are involved in the production of an inhibitory glial scar following injury, they are also capable of providing neuroprotection and supporting axonal growth. There is growing appreciation for a diverse and dynamic population of astrocytes, specified by a variety of glial precursors, whose function is regulated regionally and temporally. Consequently, the therapeutic application of glial precursors and astrocytes by effective transplantation protocols requires a better understanding of their phenotypic and functional properties and effective protocols for their preparation. We present a systematic analysis of astrocyte differentiation using multiple preparations of glial-restricted precursors (GRP), evaluating their morphological and phenotypic properties following treatment with fetal bovine serum (FBS), bone morphogenetic protein 4 (BMP-4), or ciliary neurotrophic factor (CNTF) in comparison to controls treated with basic fibroblast growth factor (bFGF), which maintains undifferentiated GRP. We found that treatments with FBS or BMP-4 generated similar profiles of highly differentiated astrocytes that were A2B5-/GFAP+. Treatment with FBS generated the most mature astrocytes, with a distinct and near-homogeneous morphology of fibroblast-like flat cells, whereas BMP-4 derived astrocytes had a stellate, but heterogeneous morphology. Treatment with CNTF induced differentiation of GRP to an intermediate state of GFAP+cells that maintained immature markers and had relatively long processes. Furthermore, astrocytes generated by BMP-4 or CNTF showed considerable experimental plasticity, and their morphology and phenotypes could be reversed with complementary treatments along a wide range of mature-immature states. Importantly, when GRP or GRP treated with BMP-4 or CNTF were transplanted acutely into a dorsal column lesion of the spinal cord, cells from all 3 groups survived and generated permissive astrocytes that supported axon growth and regeneration of host sensory axons into, but not out of the lesion. Our study underscores the dynamic nature of astrocytes prepared from GRP and their permissive properties, and suggest that future therapeutic applications in restoring connectivity following CNS injury are likely to require a combination of treatments.

Cell transplantation: stem cells and precursor cells

Stem cells have been used to approach four different therapeutic repair strategies in spinal cord injury (SCI): (1) replacement of lost neurons, (2) replacement of oligodendrocytes to promote remyelination of demyelinated and/or regenerated axons, (3) providing a permissive substrate for axonal regeneration to overcome the intrinsic inhibition of surface molecules, and (4) engendering host repair. The first two strategies involve cell-specific differentiation of engrafted neural cells and the latter two may involve grafted neural or non-neural cells. The preclinical data for all of these approaches is at times contradictory and there is no consensus as to what type of stem cell is optimal to facilitate repair in specific injuries. Remyelination has been the most successful stem cell replacement strategy. Partial lineage restriction and pharmacological and/or genetic manipulation to express additional trophic support or restrict responses to host signals appears necessary for optimal neuronal and oligodendrocytic differentiation. However, these modifications will make their clinical application exceedingly difficult. Effects of grafted stem cells on abrogating host immune responses and engendering intrinsic repair is also a mechanism through which stem cells are likely therapeutically beneficial. While clinical trials with stem cell grafting into the injured spinal cord are ongoing, preclinical studies have yet to define mechanisms of action that can be definitively translated to those clinical approaches.

Gene therapy, neurotrophic factors and spinal cord regeneration

Significant advances have been made in understanding the mechanisms that limit axon regeneration in the adult mammalian central nervous system and in addressing some of the obstacles for axon growth. Despite this progress numerous challenges remain to achieve regeneration of a large number of axons sufficient to mediate functional improvement. Given the complexity of injury-induced changes in axon, cell body, and parenchyma surrounding a spinal cord lesion, it seems likely that multiple factors both intrinsic and extrinsic to injured neurons have to be addressed to augment axon regeneration and useful reorganization of spared circuitry. Neurotrophic factors have been shown to be one potent means to increase the number and range of regenerating axons, to guide regenerating axons across a lesion site, and to augment regenerative cell body responses to injury. In this chapter we will review the potential and current limitations of neurotrophic factors and gene therapy, in combination with cellular transplants, for axon regeneration and sprouting in the injured spinal cord.

Cell therapies for the central nervous system

PURPOSE OF REVIEW

Central to the obstacles to be overcome in moving promising cell-based therapies from the laboratory to the clinic is that of determining which of the many cell types being examined are optimal for repairing particular lesions.

RECENT FINDINGS

Our studies on astrocyte replacement therapies demonstrate clearly that some cells are far better than others at promoting recovery in spinal cord injury and that, at least in some cases, transplanting undifferentiated precursor cells is far less useful than transplanting specific astrocytes derived from those precursor cells. But further comparison between different approaches is hindered by the difficulties in replicating results between laboratories, even for well defined pharmacological agents and bioactive proteins. These difficulties in replication appear most likely to be due to unrecognized nuances in lesion characteristics and in the details of delivery of therapies.

SUMMARY

We propose that the challenge of reproducibility provides a critical opportunity for refining cell-based therapies. If the utility of a particular approach is so restricted that even small changes in lesions or treatment protocols eliminate benefit, then the variability inherent in clinical injuries will frustrate translation. In contrast, rising to this challenge may enable discovery of refinements needed to confer the robustness needed for successful clinical trials.

Human glial-restricted progenitor transplantation into cervical spinal cord of the SOD1 mouse model of ALS

Cellular abnormalities are not limited to motor neurons in amyotrophic lateral sclerosis (ALS). There are numerous observations of astrocyte dysfunction in both humans with ALS and in SOD1(G93A) rodents, a widely studied ALS model. The present study therapeutically targeted astrocyte replacement in this model via transplantation of human Glial-Restricted Progenitors (hGRPs), lineage-restricted progenitors derived from human fetal neural tissue. Our previous findings demonstrated that transplantation of rodent-derived GRPs into cervical spinal cord ventral gray matter (in order to target therapy to diaphragmatic function) resulted in therapeutic efficacy in the SOD1(G93A) rat. Those findings demonstrated the feasibility and efficacy of transplantation-based astrocyte replacement for ALS, and also show that targeted multi-segmental cell delivery to cervical spinal cord is a promising therapeutic strategy, particularly because of its relevance to addressing respiratory compromise associated with ALS. The present study investigated the safety and in vivo survival, distribution, differentiation, and potential efficacy of hGRPs in the SOD1(G93A) mouse. hGRP transplants robustly survived and migrated in both gray and white matter and differentiated into astrocytes in SOD1(G93A) mice spinal cord, despite ongoing disease progression. However, cervical spinal cord transplants did not result in motor neuron protection or any therapeutic benefits on functional outcome measures. This study provides an in vivo characterization of this glial progenitor cell and provides a foundation for understanding their capacity for survival, integration within host tissues, differentiation into glial subtypes, migration, and lack of toxicity or tumor formation.

Precursor cell biology and the development of astrocyte transplantation therapies: lessons from spinal cord injury

This review summarizes current progress on development of astrocyte transplantation therapies for repair of the damaged central nervous system. Replacement of neurons in the injured or diseased central nervous system is currently one of the most popular therapeutic goals, but if neuronal replacement is attempted in the absence of appropriate supporting cells (astrocytes and oligodendrocytes), then the chances of restoring neurological functional are greatly reduced. Although the past 20 years have offered great progress on oligodendrocyte replacement therapies, astrocyte transplantation therapies have been both less explored and comparatively less successful. We have now developed successful astrocyte transplantation therapies by pre-differentiating glial restricted precursor (GRP) cells into a specific population of GRP cell-derived astrocytes (GDAs) by exposing the GRP cells to bone morphogenetic protein-4 (BMP) prior to transplantation. When transplanted into transected rat spinal cord, rat and human GDAs(BMP) promote extensive axonal regeneration, rescue neuronal cell survival, realign tissue structure, and restore behavior to pre-injury levels on a grid-walk analysis of volitional foot placement. Such benefits are not provided by GRP cells themselves, demonstrating that the lesion environment does not direct differentiation in a manner optimally beneficial for the restoration of function. Such benefits also are not provided by transplantation of a different population of astrocytes generated from GRP cells exposed to ciliary neurotrophic factor (GDAs(CNTF)), thus providing the first transplantation-based evidence of functional heterogeneity in astrocyte populations. Moreover, lessons learned from the study of rat cells are strongly predictive of outcomes using human cells. Thus, these studies provide successful strategies for the use of astrocyte transplantation therapies for restoration of function following spinal cord injury.

A pilot study of poly(N-isopropylacrylamide)-g-polyethylene glycol and poly(N-isopropylacrylamide)-g-methylcellulose branched copolymers as injectable scaffolds for local delivery of neurotrophins and cellular transplants into the injured spinal cord

OBJECT

The authors investigated the feasibility of using injectable hydrogels, based on poly(N-isopropylacrylamide) (PNIPAAm), lightly cross-linked with polyethylene glycol (PEG) or methylcellulose (MC), to serve as injectable scaffolds for local delivery of neurotrophins and cellular transplants into the injured spinal cord. The primary aims of this work were to assess the biocompatibility of the scaffolds by evaluating graft cell survival and the host tissue immune response. The scaffolds were also evaluated for their ability to promote axonal growth through the action of released brain-derived neurotrophic factor (BDNF).

METHODS

The in vivo performance of PNIPAAm-g-PEG and PNIPAAm-g-MC was evaluated using a rodent model of spinal cord injury (SCI). The hydrogels were injected as viscous liquids into the injury site and formed space-filling hydrogels. The host immune response and biocompatibility of the scaffolds were evaluated at 2 weeks by histological and fluorescent immunohistochemical analysis. Commercially available matrices were used as a control and examined for comparison.

RESULTS

Experiments showed that the scaffolds did not contribute to an injury-related inflammatory response. PNIPAAm-g-PEG was also shown to be an effective vehicle for delivery of cellular transplants and supported graft survival. Additionally, PNIPAAm-g-PEG and PNIPAAm-g-MC are permissive to axonal growth and can serve as injectable scaffolds for local delivery of BDNF.

CONCLUSIONS

Based on the results, the authors suggest that these copolymers are feasible injectable scaffolds for cell grafting into the injured spinal cord and for delivery of therapeutic factors.

Grafted neural progenitors integrate and restore synaptic connectivity across the injured spinal cord

Transplantation of neural progenitor cells (NPC) is a promising therapeutic strategy for replacing neurons lost after spinal cord injury, but significant challenges remain regarding neuronal integration and functional connectivity. Here we tested the ability of graft-derived neurons to reestablish connectivity by forming neuronal relays between injured dorsal column (DC) sensory axons and the denervated dorsal column nuclei (DCN). A mixed population of neuronal and glial restricted precursors (NRP/GRP) derived from the embryonic spinal cord of alkaline phosphatase (AP) transgenic rats were grafted acutely into a DC lesion at C1. One week later, BDNF-expressing lentivirus was injected into the DCN to guide graft axons to the intended target. Six weeks later, we observed anterogradely traced sensory axons regenerating into the graft and robust growth of graft-derived AP-positive axons along the neurotrophin gradient into the DCN. Immunoelectron microscopy revealed excitatory synaptic connections between regenerating host axons and graft-derived neurons at C1 as well as between graft axons and DCN neurons in the brainstem. Functional analysis by stimulus-evoked c-Fos expression and electrophysiological recording showed that host axons formed active synapses with graft neurons at the injury site with the signal propagating by graft axons to the DCN. We observed reproducible electrophysiological activity at the DCN with a temporal delay predicted by our relay model. These findings provide the first evidence for the ability of NPC to form a neuronal relay by extending active axons across the injured spinal cord to the intended target establishing a critical step for neural repair with stem cells.

Spatial and temporal changes in promoter activity of the astrocyte glutamate transporter GLT1 following traumatic spinal cord injury

After traumatic spinal cord injury (SCI), there is an opportunity for preserving function by attenuating secondary cell loss. Astrocytes play crucial roles in the adult CNS and are responsible for the vast majority of glutamate buffering, potentially preventing excitotoxic loss of neurons and oligodendrocytes. We examined spatial and temporal changes in gene expression of the major astrocyte glutamate transporter GLT1 following moderate thoracic contusion SCI using transgenic BAC-GLT1-eGFP promoter reporter mice. In dorsal column white matter, total intensity of GLT1-eGFP expression per region was significantly reduced following SCI at both lesion epicenter and at rostral and caudal areas where no tissue loss occurred. This regional decrease in GLT1 expression was due to significant loss of GLT1-eGFP(+) cells, partially accounted for by apoptosis of eGFP(+) /GFAP(+) astrocytes in both white and gray matter. There were also decreased numbers of GLT1-eGFP-expressing cells in multiple gray matter regions following injury; nevertheless, there was sustained or even increased regional GLT1-eGFP expression in gray matter as a result of up-regulation in astrocytes that continued to express GLT1-eGFP. Although there were increased numbers of GFAP(+) cells both at the lesion site and in surrounding intact spinal cord following SCI, the majority of proliferating Ki67(+) /GFAP(+) astrocytes did not express GLT1-eGFP. These findings demonstrate that spatial and temporal alterations in GLT1 expression observed after SCI result from both astrocyte death and gene expression changes in surviving astrocytes. Results also suggest that following SCI a significant portion of astrocytes lacks GLT1 expression, possibly compromising the important role of astrocytes in glutamate homeostasis.