Oligodendrocyte precursors survive poorly and do not migrate following transplantation into the normal adult central nervous system

Trophic factors are essential for the survival of grafted oligodendrocyte progenitors and for neuroprotection after perinatal excitotoxicity

The consequences of neonatal white matter injury are devastating and represent a major societal problem as currently there is no cure. Prematurity, low weight birth and maternal pre-natal infection are the most frequent causes of acquired myelin deficiency in the human neonate leading to cerebral palsy and cognitive impairment. In the developing brain, oligodendrocyte (OL) maturation occurs perinatally, and immature OLs are particularly vulnerable. Cell replacement therapy is often considered a viable option to replace progenitors that die due to glutamate excitotoxicity. We previously reported directed specification and mobilization of endogenous committed and uncommitted neural progenitors by the combination of transferrin and insulin growth factor 1 (TSC1). Here, considering cell replacement and integration as therapeutic goals, we examined if OL progenitors (OLPs) grafted into the brain parenchyma of mice that were subjected to an excitotoxic insult could rescue white matter injury. For that purpose, we used a well-established model of glutamate excitotoxic injury. Four-day-old mice received a single intraparenchymal injection of the glutamate receptor agonist N-methyl-D-aspartate alone or in conjunction with TSC1 in the presence or absence of OLPs grafted into the brain parenchyma. Energetics and expression of stress proteins and OL developmental specific markers were examined. A comparison of the proteomic profile per treatment was also ascertained. We found that OLPs did not survive in the excitotoxic environment when grafted alone. In contrast, when combined with TSC1, survival and integration of grafted OLPs was observed. Further, energy metabolism in OLPs was significantly increased by N-methyl-D-aspartate and modulated by TSC1. The proteomic profile after the various treatments showed elevated ubiquitination and stress/heat shock protein 90 in response to N-methyl-D-aspartate. These changes were reversed in the presence of TSC1 and ubiquitination was decreased. The results obtained in this pre-clinical study indicate that the use of a combinatorial intervention including both trophic support and healthy OLPs constitutes a promising approach for long-term survival and successful graft integration. We established optimal conditioning of the host brain environment to promote long-term survival and integration of grafted OLPs into an inflamed neonate host brain. Experimental procedures were performed under the United States Public Health Service Guide for the Care and Use of Laboratory Animals and were approved by the Institutional Animal Care Committee at (UCLA) (ARC #1992-034-61) on July 1, 2010.

Neuroprotective effects of oligodendrocyte progenitor cell transplantation in premature rat brain following hypoxic-ischemic injury

Periventricular leukomalacia (PVL) is a common ischemic brain injury in premature infants for which there is no effective treatment. The objective of this study was to determine whether transplanted mouse oligodendrocyte progenitor cells (OPCs) have neuroprotective effects in a rat model of PVL. Hypoxia-ischemia (HI) was induced in 3-day-old rat pups by left carotid artery ligation, followed by exposure to 6% oxygen for 2.5 h. Animals were assigned to OPC transplantation or sham control groups and injected with OPCs or PBS, respectively, and sacrificed up to 6 weeks later for immunohistochemical analysis to investigate the survival and differentiation of transplanted OPCs. Apoptosis was evaluated by double immunolabeling of brain sections for caspase-3 and neuronal nuclei (NeuN), while proliferation was assessed using a combination of anti-Nestin and -bromodeoxyuridine antibodies. The expression of brain-derived neurotrophic factor (BDNF) and Bcl-2 was examined 7 days after OPC transplantation. The Morris water maze was used to test spatial learning and memory. The results showed that transplanted OPCs survived and formed a myelin sheath, and stimulated BDNF and Bcl-2 expression and the proliferation of neural stem cells (NSC), while inhibiting HI-induced neuronal apoptosis relative to control animals. Moreover, deficits in spatial learning and memory resulting from HI were improved by OPC transplantation. These results demonstrate an important neuroprotective role for OPCs that can potentially be exploited in cell-based therapeutic approaches to minimize HI-induced brain injury.

Intranasal delivery of stem cells as therapy for central nervous system disease

Stem cells, upon entering the CNS, can preferentially migrate into disease foci, where they exert therapeutic effects that compensate for lost tissue, reconstructing damaged neuronal circuitry and establishing in the brain a new microenvironment suitable for cell survival. However, the route of stem cell delivery into the CNS remains a challenge: with systemic administration (e.g., intravenous injection), a fraction of cells may be trapped in other organs than the CNS, while direct CNS injections, e.g., intracerebroventricular or transcranial, are invasive. Intranasal (i.n.) delivery of stem cells, in contrast, can effectively bypass the blood-brain barrier, rapidly enter the CNS, and minimize systemic distribution. I.n. delivery of stem cells may therefore be a safe and non-invasive way of targeting the CNS and would thus be a promising therapeutic option for CNS disease. In this review we discuss the i.n. route for stem cell delivery into the CNS, and the perspectives of i.n. stem cell-based therapy in CNS disease.

Oligodendrocytes engineered with migratory proteins as effective graft source for cell transplantation in multiple sclerosis

Multiple sclerosis (MS) is characterized by widespread immunomodulatory demyelination of the CNS resulting in nerve cell dysfunction. Accordingly, treatment strategies have been centered on immunodulation and remyelination, with the former primarily focused on reducing the pathology rather than enhancing myelin repair which the latter targets. While conceding to the emerging view of heterogeneity in the pathology of MS, which precludes variations in degree of immune response (i.e., inflammation) and demyelination, the concept of enhancing myelin repair is appealing since it is likely to provide both disease-reducing and disease-inhibiting therapeutic approach to MS. In this regard, we and several others, have proposed that cell replacement therapy is an effective strategy to repair the myelin in MS. Here, we hypothesize that transplantation of mouse bone marrow-derived oligodendrocytes (BMDOs) and BMDOs transfected with Ephrin proteins (BMDO+Ephrin), which are known to enhance cell and axonal migratory capacity, may produce therapeutic benefits in animal models of MS.

Cell therapy for multiple sclerosis

The spontaneous recovery observed in the early stages of multiple sclerosis (MS) is substituted with a later progressive course and failure of endogenous processes of repair and remyelination. Although this is the basic rationale for cell therapy, it is not clear yet to what degree the MS brain is amenable for repair and whether cell therapy has an advantage in comparison to other strategies to enhance endogenous remyelination. Central to the promise of stem cell therapy is the therapeutic plasticity, by which neural precursors can replace damaged oligodendrocytes and myelin, and also effectively attenuate the autoimmune process in a local, nonsystemic manner to protect brain cells from further injury, as well as facilitate the intrinsic capacity of the brain for recovery. These fundamental immunomodulatory and neurotrophic properties are shared by stem cells of different sources. By using different routes of delivery, cells may target both affected white matter tracts and the perivascular niche where the trafficking of immune cells occur. It is unclear yet whether the therapeutic properties of transplanted cells are maintained with the duration of time. The application of neural stem cell therapy (derived from fetal brain or from human embryonic stem cells) will be realized once their purification, mass generation, and safety are guaranteed. However, previous clinical experience with bone marrow stromal (mesenchymal) stem cells and the relative easy expansion of autologous cells have opened the way to their experimental application in MS. An initial clinical trial has established the probable safety of their intravenous and intrathecal delivery. Short-term follow-up observed immunomodulatory effects and clinical benefit justifying further clinical trials.

Migration and remyelination by oligodendrocyte progenitor cells transplanted adjacent to focal areas of spinal cord inflammation

Multiple sclerosis (MS) is an inflammatory demyelinating disease of the central nervous system. Exogenous cell replacement in MS lesions has been proposed as a means of achieving remyelination when endogenous remyelination has failed. However, the ability of exogenous cells to remyelinate axons in the presence of inflammation remains uncertain. We have explored the remyelinating capacity of an oligodendrocyte progenitor cell line CG-4 transduced with the GFP gene and transplanted adjacent to a zymosan-induced focal demyelination model in the rat spinal cord. The resulting zymosan-induced lesions were characterized by persistent macrophage/microglia activation, focal demyelination, degeneration of axons, and reactive astrogliosis. GFP(+) CG-4 cells were found to migrate preferentially toward the inflammatory lesion and survive inside the lesion. A proportion of GFP(+) CG-4 cells differentiated into mature oligodendrocytes and remyelinated axons within the lesion. These findings suggest that grafted oligodendrocyte progenitors may migrate toward areas of inflammation in the adult rat spinal cord, where they can survive and differentiate into myelinating oligodendrocytes.

Importance of oligodendrocyte protection, BBB breakdown and inflammation for remyelination

Multiple sclerosis (MS) is a chronic inflammatory demyelinating disease of the CNS. A better understanding of why remyelination fails in MS is necessary to improve remyelination strategies. Remyelination is mediated by oligodendrocyte precursor cells (OPCs), which are widely distributed throughout the adult CNS. However, it is still unclear whether OPCs detectable in MS lesions survive the inflammatory response but are unable to myelinate or whether OPC and oligodendrocyte death is primarily responsible for remyelination failure and detectable OPCs enter demyelinated areas from adjacent tissue as the lesion evolves. Remyelination strategies should, therefore, focus on stimulation of differentiation or prevention of apoptosis, as well as establishment of a supportive environment for OPC-mediated remyelination, which may be especially important in chronically demyelinated lesions.

Unique in vivo properties of olfactory ensheathing cells that may contribute to neural repair and protection following spinal cord injury

Olfactory ensheathing cells (OECs) are specialized glial cells that guide olfactory receptor axons from the nasal mucosa into the brain where they make synaptic contacts in the olfactory bulb. While a number of studies have demonstrated that in vivo transplantation of OECs into injured spinal cord results in improved functional outcome, precise cellular mechanisms underlying this improvement are not fully understood. Current thinking is that OECs can encourage axonal regeneration, provide trophic support for injured neurons and for angiogenesis, and remyelinate axons. However, Schwann cell (SC) transplantation also results in significant functional improvement in animal models of spinal cord injury. In culture SCs and OECs share a number of phenotypic properties such as expression of the low affinity NGF receptor (p75). An important area of research has been to distinguish potential differences in the in vivo behavior of OECs and SCs to determine if one cell type may offer greater advantage as a cellular therapeutic candidate. In this review we focus on several unique features of OECs when they are transplanted into the spinal cord.

Derivation of high-purity oligodendroglial progenitors

Oligodendrocytes are a type of glial cells that play a critical role in supporting the central nervous system (CNS), in particular insulating axons within the CNS by wrapping them with a myelin sheath, thereby enabling saltatory conduction. They are lost, and myelin damaged - demyelination - in a wide variety of neurological disorders. Replacing depleted cell types within demyelinated areas, however, has been shown experimentally to achieve remyelination and so help restore function. One method to produce oligodendrocytes for cellular replacement therapies is through the use of progenitor or stem cells. The ability to differentiate progenitor or stem cells into high-purity fates not only permits the generation of specific cells for transplantation therapies, but also provides powerful tools for studying cellular mechanisms of development. This chapter outlines methods of generating high-purity OPCs from multipotent neonatal progenitor or human embryonic stem cells.

Prospects of cell therapy for disorders of myelin

Recent advances in stem cell biology have raised expectations that both diseases of, and injuries to, the central nervous system may be ameliorated by cell transplantation. In particular, cell therapy has been studied for inducing efficient remyelination in disorders of myelin, including both the largely pediatric disorders of myelin formation and maintenance and the acquired demyelinations of both children and adults. Potential cell-based treatments of two major groups of disorders include both delivery of myelinogenic replacements and mobilization of residual oligodendrocyte progenitor cells as a means of stimulating endogenous repair; the choice of modality is then predicated upon the disease target. In this review we consider the potential application of cell-based therapeutic strategies to disorders of myelin, highlighting the promises as well as the problems and potential perils of this treatment approach.

Glial precursor cell transplantation therapy for neurotrauma and multiple sclerosis

Traumatic injury to the brain or spinal cord and multiple sclerosis (MS) share a common pathophysiology with regard to axonal demyelination. Despite advances in central nervous system (CNS) repair in experimental animal models, adequate functional recovery has yet to be achieved in patients in response to any of the current strategies. Functional recovery is dependent, in large part, upon remyelination of spared or regenerating axons. The mammalian CNS maintains an endogenous reservoir of glial precursor cells (GPCs), capable of generating new oligodendrocytes and astrocytes. These GPCs are upregulated following traumatic or demyelinating lesions, followed by their differentiation into oligodendrocytes. However, this innate response does not adequately promote remyelination. As a result, researchers have been focusing their efforts on harvesting, culturing, characterizing, and transplanting GPCs into injured regions of the adult mammalian CNS in a variety of animal models of CNS trauma or demyelinating disease. The technical and logistic considerations for transplanting GPCs are extensive and crucial for optimizing and maintaining cell survival before and after transplantation, promoting myelination, and tracking the fate of transplanted cells. This is especially true in trials of GPC transplantation in combination with other strategies such as neutralization of inhibitors to axonal regeneration or remyelination. Overall, such studies improve our understanding and approach to developing clinically relevant therapies for axonal remyelination following traumatic brain injury (TBI) or spinal cord injury (SCI) and demyelinating diseases such as MS.

Dual-modality monitoring of targeted intraarterial delivery of mesenchymal stem cells after transient ischemia

BACKGROUND AND PURPOSE

In animal models of stroke, functional improvement has been obtained after stem cell transplantation. Successful therapy depends largely on achieving a robust and targeted cell engraftment, with intraarterial (IA) injection being a potentially attractive route of administration. We assessed the suitability of laser Doppler flow (LDF) signal measurements and magnetic resonance (MR) imaging for noninvasive dual monitoring of targeted IA cell delivery.

METHODS

Transient cerebral ischemia was induced in adult Wistar rats (n=25) followed by IA or intravenous (IV) injection of mesenchymal stem cells (MSCs) labeled with superparamagnetic iron oxide. Cell infusion was monitored in real time with transcranial laser Doppler flowmetry while cellular delivery was assessed with MRI in vivo (4.7 T) and ex vivo (9.4 T).

RESULTS

Successful delivery of magnetically labeled MSCs could be readily visualized with MRI after IA but not IV injection. IA stem cell injection during acute stroke resulted in a high variability of cerebral engraftment. The amount of LDF reduction during cell infusion (up to 80%) was found to correlate well with the degree of intracerebral engraftment, with low LDF values being associated with significant morbidity.

CONCLUSIONS

High cerebral engraftment rates are associated with impeded cerebral blood flow. Noninvasive dual-modality imaging enables monitoring of targeted cell delivery, and through interactive adjustment may improve the safety and efficacy of stem cell therapy.

Applicability and limitations of MR tracking of neural stem cells with asymmetric cell division and rapid turnover: the case of the shiverer dysmyelinated mouse brain

LacZ-transfected C17.2 neural stem cells (NSCs) were labeled with the superparamagnetic iron oxide formulation Feridex prior to ICV injection in shi/shi neonates. Feridex labeling did not alter cell differentiation in vitro and in vivo. Initially, MR images obtained at 11.7T correlated closely to NSC distribution as assessed with anti-dextran and anti-beta-galactosidase double-fluorescent immunostaining. However, at 6 days postgrafting there was already a pronounced mismatch between the hypointense MR signal and the histologically determined cell distribution, with a surprisingly sharp cutoff rather than a gradual decrease of signal. Positive in vivo BrdU labeling of NSCs showed that significant cell replication occurred post-transplantation, causing rapid dilution of Feridex particles between mother and daughter cells toward undetectable levels. Neural differentiation experiments demonstrated asymmetric cell division, explaining the observed sharp cutoff. At later time points (2 weeks), the mismatch further increased by the presence of non-cell-associated Feridex particles resulting from active excretion or cell death. These results are a first demonstration of the inability of MRI to track rapidly dividing and self-renewing, asymmetrically dividing SCs. Therefore, MR cell tracking should only be applied for nonproliferating cells or short-term monitoring of highly-proliferative cells, with mitotic symmetry or asymmetry being important for determining its applicability.

Endogenous or exogenous oligodendrocytes for remyelination

The relative merits of endogenous and exogenous oligodendrocyte progenitor cells (OPCs) for remyelination are compared in terms of their ability to repopulate OPC-depleted tissue and generate remyelinating oligodendrocytes. Exogenous neonatal OPCs can repopulate OPC-depleted tissue 5-10 times faster than endogenous cells and as a result are capable of more extensive remyelination. Both endogenous and exogenous cells will only repopulate normal tissue if there is extensive depletion of the local OPC population and both show reduced ability to generate remyelinating cells in the absence of acute inflammation. When endogenous OPCs are depleted by X-irradiation during cuprizone intoxication, where there is a combination of astrocytosis and acute demyelination, endogenous but not exogenous embryo-derived OPCs fail to repopulate the OPC-depleted cortex.

Neural progenitor cell transplantation and imaging in a large animal model

To evaluate neural stem/progenitor cell (NPC) transplantation therapy in cat models of neurodegenerative diseases, we have isolated, expanded and characterized feline NPCs (fNPCs) from normal fetal cat brain. Feline NPCs responsive to both human epidermal growth factor (hEGF) and human fibroblast growth factor 2 (hFGF2) proliferated as neurospheres, which were able to differentiate to neurons and glial cells. The analysis of growth factors indicated that both hEGF and hFGF2 were required for proliferation of fNPCs. In contrast to the effect on human NPCs, human leukemia inhibitory factor (hLIF) enhanced differentiation of fNPCs. Expanded fNPCs were injected into the brains of normal adult cats. Immunohistochemical analysis showed that the majority of transplanted cells were located adjacent to the injection site and some fNPCs differentiated into neurons. The survival of transplanted fNPCs over time was monitored using non-invasive bioluminescent imaging technology. This study provided the first evidence of allotransplantation of fNPCs into feline CNS. Cats have heterogeneous genetic backgrounds and possess neurological diseases that closely resemble analogous human diseases. The characterization of fNPCs and exploration of non-invasive bioluminescent imaging to track transplanted cells in this study will allow evaluation of NPC transplantation therapy using feline models of human neurological diseases.

Regeneration and repair in multiple sclerosis: the view of experimental pathology

In order to devise a strategy to enhance remyelination in multiple sclerosis (MS) it is necessary to understand the cause of remyelination failure in MS. A case is made that areas of chronic demyelination arise because of concurrent loss of oligodendrocyte progenitor cells (OPCs) and oligodendrocytes and that because of the slow rate of repopulation that occurs in old individuals the recruited OPCs are not exposed to the acute inflammatory environment required to generate remyelinating oligodendrocytes. Based on this analysis the case is made that only areas of acute demyelination will be amenable to transplant-mediated remyelination. An analysis of the many cells that could be used to provide a source of remyelinating cells would indicate that structural repair of the CNS in MS would likely only be possible if neural precursors were used and the most promising route for their introduction would appear to be by intraventricular injection. Both neural precursors and mesenchymal stromal cells can be immunomodulatory and neuroprotective following intravenous injection; however, only neural precursors are likely to be able to contribute to structural repair of the damaged nervous system.

Neutralization of ciliary neurotrophic factor reduces astrocyte production from transplanted neural stem cells and promotes regeneration of corticospinal tract fibers in spinal cord injury

Transplantation of neural stem cells (NSC) into lesioned spinal cord offers the potential to increase regeneration by replacing lost neurons or oligodendrocytes. The majority of transplanted NSC, however, typically differentiate into astrocytes that may exacerbate glial scar formation. Here we show that blocking of ciliary neurotrophic factor (CNTF) with anti-CNTF antibodies after NSC transplant into spinal cord injury (SCI) resulted in a reduction of glial scar formation by 8 weeks. Treated animals had a wider distribution of transplanted NSC compared with the control animals. The NSC around the lesion coexpressed either nestin or markers for neurons, oligodendrocytes, or astrocytes. Approximately 20% fewer glial fibrillary acidic protein-positive/bromodeoxyuridine (BrdU)-positive cells were seen at 2, 4, and 8 weeks postgrafting, compared with the control animals. Furthermore, more CNPase(+)/BrdU(+) cells were detected in the treated group at 4 and 8 weeks. These CNPase(+) or Rip(+) mature oligodendrocytes were seen in close proximity to host corticospinal tract (CST) and 5HT(+) serotonergic axon. We also demonstrate that the number of regenerated CST fibers both at the lesion and at caudal sites in treated animals was significantly greater than that in the control animals at 8 weeks. We suggest that the blocking of CNTF at the beginning of SCI provides a more favorable environment for the differentiation of transplanted NSC and the regeneration of host axons.

Role of Endogenous Neural Stem Cells in Neurological Disease and Brain Repair

These examples show that stem-cell-based therapy of neuro-psychiatric disorders will not follow a single scheme, but rather include widely different approaches. This is in accordance with the notion that the impact of stem cell biology on neurology will be fundamental, providing a shift in perspective, rather than introducing just one novel therapeutic tool. Stem cell biology, much like genomics and proteomics, offers a "view from within" with an emphasis on a theoretical or real potential and thereby the inherent openness, which is central to the concept of stem cells. Thus, stem cell biology influences many other, more traditional therapeutic approaches, rather than introducing one distinct novel form of therapy. Substantial advances have been made i n neural stemcell research during the years. With the identification of stem and progenitor cells in the adult brain and the complex interaction of different stem cell compartments in the CNS--both, under physiological and pathological conditions--new questions arise: What is the lineage relationship between t he different progenitor cells in the CNS and how much lineage plasticity exists? What are the signals controlling proliferation and differentiation of neural stem cells and can these be utilized to allow repair of the CNS? Insights in these questions will help to better understand the role of stem cells during development and aging and the possible relation of impaired or disrupted stem cell function and their impact on both the development and treatment of neurological disease. A number o f studies have indicated a limited neuronal and glial regeneration certain pathological conditions. These fundamental observations have already changed our view on understanding neurological disease and the brain's capacity for endogenous repair. The following years will have to show how we can influence andmodulate endogenous repair nisms by increasing the cellular plasticity in the young and aged CNS.

A different regional response by mouse oligodendrocyte progenitor cells (OPCs) to high-dose X-irradiation has consequences for repopulating OPC-depleted normal tissue

This study was designed to investigate whether the residual, dysfunctional oligodendrocyte progenitor cells (OPCs) observed following X-irradiation of the mouse spinal cord [D. M. Chari et al. (2003) Exp. Neurol., 198, 145-153], the presence of which prevented the endogenous repopulation of these areas from normal tissue, reflects a general response of OPCs in the mouse central nervous system (CNS) to X-irradiation. The brains of adult mice were exposed to 40 Gy of X-irradiation and the effect of X-irradiation on the OPCs was assessed up to 4 weeks post-irradiation using anti-NG2 antibodies. X-irradiation resulted in almost complete depletion of OPCs within the telencephalon (cortex, corpus callosum and hippocampus) by 7 days post-irradiation, which was followed by progressive repopulation of OPCs from non-irradiated areas of the cortex. By contrast, within the lower brain centres (the diencephalon and mesencephalon) OPC loss occurred much more slowly so that 26% of the OPCs still remained 4 weeks after X-irradiation. The consequence of this heterogeneous response to X-irradiation was that whereas transplanted and endogenous OPCs rapidly established themselves in the OPC-depleted telencephalon this did not occur in the areas where there was incomplete depletion of endogenous OPCs. Our findings confirm not only the requirement for almost complete OPC depletion in order to establish transplanted OPCs in normal tissue but also highlight a heterogeneity of progenitor populations in different areas of the mouse CNS.

Remyelination in multiple sclerosis

Remyelination is the phenomenon by which new myelin sheaths are generated around axons in the adult central nervous system (CNS). This follows the pathological loss of myelin in diseases like multiple sclerosis (MS). Remyelination can restore conduction properties to axons (thereby restoring neurological function) and is increasingly believed to exert a neuroprotective role on axons. Remyelination occurs in many MS lesions but becomes increasingly incomplete/inadequate and eventually fails in the majority of lesions and patients. Efforts to understand the causes for this failure of regeneration have fueled research into the biology of remyelination and the complex, interdependent cellular and molecular factors that regulate this process. Examination of the mechanisms of repair of experimental lesions has demonstrated that remyelination occurs in two major phases. The first consists of colonization of lesions by oligodendrocyte progenitor cells (OPCs), the second the differentiation of OPCs into myelinating oligodendrocytes that contact demyelinated axons to generate functional myelin sheaths. Several intracellular and extracellular molecules have been identified that mediate these two phases of repair. Theoretically, the repair of demyelinating lesions can be promoted by enhancing the intrinsic repair process (by providing one or more remyelination-enhancing factors or via immunoglobulin therapy). Alternatively, endogenous repair can be bypassed by introducing myelinogenic cells into demyelinated areas; several cellular candidates have been identified that can mediate repair of experimental demyelinating lesions. Future challenges confronting therapeutic strategies to enhance remyelination will involve the translation of findings from basic science to clinical demyelinating disease.

Transplantation of human embryonic stem cell-derived oligodendrocyte progenitors into rat spinal cord injuries does not cause harm

Demyelination contributes to loss of function following spinal cord injury. We have shown previously that transplantation of human embryonic stem cell-derived oligodendrocyte progenitors into adult rat 200 kD contusive spinal cord injury sites enhances remyelination and promotes recovery of motor function. Previous studies using oligodendrocyte lineage cells have noted a correlation between the presence of demyelinating pathology and the survival and migration rate of the transplanted cells. The present study compared the survival and migration of human embryonic stem cell-derived oligodendrocyte progenitors injected 7 days after a 200 or 50 kD contusive spinal cord injury, as well as the locomotor outcome of transplantation. Our findings indicate that a 200 kD spinal cord injury induces extensive demyelination, whereas a 50 kD spinal cord injury induces no detectable demyelination. Cells transplanted into the 200 kD injury group survived, migrated, and resulted in robust remyelination, replicating our previous studies. In contrast, cells transplanted into the 50 kD injury group survived, exhibited limited migration, and failed to induce remyelination as demyelination in this injury group was absent. Animals that received a 50 kD injury displayed only a transient decline in locomotor function as a result of the injury. Importantly, human embryonic stem cell-derived oligodendrocyte progenitor transplants into the 50 kD injury group did not cause a further decline in locomotion. Our studies highlight the importance of a demyelinating pathology as a prerequisite for the function of transplanted myelinogenic cells. In addition, our results indicate that transplantation of human embryonic stem cell-derived oligodendrocyte progenitor cells into the injured spinal cord is not associated with a decline in locomotor function.

Oligodendrocyte progenitor cell (OPC) transplantation is unlikely to offer a means of preventing X-irradiation induced damage in the CNS

Oligodendrocyte lineage cells [oligodendrocytes and their parent cells, the oligodendrocyte progenitor cells (OPCs)] are depleted by X-irradiation and progenitor cell transplantation has been proposed as a therapeutic strategy to counteract radiation induced myelopathy. Previous studies have demonstrated that oligodendrocyte progenitor cell (OPC) depletion is a prerequisite for establishing transplanted OPCs in normal tissue. One can therefore predict that the extent and timing of OPC depletion and regeneration following X-irradiation will be crucial factors in determining the feasibility of this therapeutic approach. To address this issue, we have examined the time course of OPC depletion and regeneration following a range of X-irradiation doses (5 to 40 Gy), and its relationship to establishing transplanted OPCs in X-irradiated tissue. Doses above 10 Gy resulted in rapid death of OPCs. With doses up to 20 Gy, surviving X-irradiated OPCs were capable of robust regeneration, restoring normal densities within 6 weeks. Transplanted OPCs could only be established in tissue that had been exposed to > or =20 Gy. Since 20 Gy is close to the ED50 for radiation necrosis, our findings demonstrate the limitation of OPC replacement strategies.

Survival of neural precursor cells in growth factor-poor environment: Implications for transplantation in chronic disease

A key issue for therapeutic neural stem cell transplantation in chronic diseases is the long-term survival of transplanted cells in the brain. The normal adult central nervous system does not support the survival of transplanted cells. Presumably, the limited availability of trophic factors maintains the survival of resident cells but is insufficient for supporting the survival of transplanted cells. Specifically, in multiple sclerosis, a chronic relapsing disease, it would be necessary to maintain long-term survival of transplanted cells through phases of relapses and remissions. It may be beneficial to transplant cells as early as possible, in a form that will keep their survival independent of tissue support and ready for immediate mobilization upon tissue demand during disease relapse. In the present study, we examined whether, in the form of neurospheres, multipotential neural precursor cells (NPCs) survive in a growth factor-poor environment while maintaining their potential to respond to environmental cues. We found that after removal of growth factors from the culture medium of neurospheres in vitro, NPC proliferation decreased significantly, but most cells survived for a prolonged time and maintained their stem cell characteristics. After re-exposure to growth factors, neurosphere cells resumed proliferation and could differentiate along neural lineages. Furthermore, neurospheres, but not single NPCs, that were transplanted into the brain ventricles of intact animals survived within the ventricles for at least a month and responded to induction of experimental autoimmune encephalomyelitis and brain inflammation by extensive migration into the brain white matter and differentiated into glial lineage cells.

Transplantation of glial-committed progenitor cells into a viral model of multiple sclerosis induces remyelination in the absence of an attenuated inflammatory response

Transplantation of remyelination-competent cells represents a promising strategy for the treatment of demyelinating diseases. As the environment dictates the success or failure of remyelination, it is critical to understand the role that the immune system plays in transplant-mediated remyelination. In this study, we evaluated the severity of neuroinflammation following transplantation of glial-committed progenitor cells into the spinal cords of mice chronically infected with mouse hepatitis virus (MHV), a model in which T cells and macrophages are critical in amplifying the severity of demyelination. Transplantation was performed following viral persistence in which inflammation and demyelination are established and clinical disease is evident. Mice were sacrificed 10 and 21 days following progenitor cell transplantation and the effect on neuroinflammation evaluated. Treatment did not alter accumulation of T cells or macrophages within the CNS as compared to control mice. Moreover, progenitor cell implantation did not affect local cytokine/chemokine gene expression in the CNS. Finally, remyelination associated with transplantation did not result in an imbalance of T_H1-associated cytokine production by virus-specific T cells. These studies demonstrate that progenitor cell-mediated remyelination is not the result of modulating the composition of the cellular infiltrate nor cytokine expression by virus-specific T cells and suggest that remyelination may not depend on amelioration of the inflammatory response or alteration of cytokine secretion by virus-specific T cells.

Human embryonic stem cell-derived oligodendrocyte progenitors for the treatment of spinal cord injury

Stem cells are self-renewing, pluripotent cells that can be manipulated in vitro to differentiate into virtually any cell type. Stem cells are highly proliferative and have the potential to expand into very large numbers of a desired cell lineage. As such, they represent an excellent source of cells for cellular replacement strategies in disease states that are typified by a loss of a particular cell population. Recent studies have indicated that spinal cord injury is accompanied by chronic progressive demyelination, and have thus identified oligodendrocytes as a desirable transplant population for remyelination strategies. To address this need, we developed a method to differentiate hESCs into high purity human oligodendrocyte progenitor cells (OPCs). Transplantation into spinal cord injury sites in adult rats resulted in remyelination and functional repair. Here, we summarize these findings and present new data concerning the effects of hESC-derived OPC transplantation on the host environment.

Apoptosis and proliferation of oligodendrocyte progenitor cells in the irradiated rodent spinal cord

PURPOSE

Oligodendrocytes undergo early apoptosis after irradiation. The aim of this study was to determine the relationship between oligodendroglial apoptosis and proliferation of oligodendrocyte progenitor cells (OPC) in the irradiated central nervous system.

METHODS AND MATERIALS

Adult rats and p53 transgenic mice were given single doses of 2 Gy, 8 Gy, or 22 Gy to the cervical spinal cord. Apoptosis was assessed using TUNEL (Tdt-mediated dUTP terminal nick-end labeling) staining or by examining nuclear morphology. Oligodendrocyte progenitor cells were identified with an NG2 antibody or by in situ hybridization for platelet-derived growth factor receptor alpha. Proliferation of OPC was assessed by in vivo bromodeoxyuridine (BrdU) labeling and subsequent immunohistochemistry. Because radiation-induced apoptosis of oligodendroglial cells is p53 dependent, p53 transgenic mice were used to study the relationship between apoptosis and cell proliferation.

RESULTS

Oligodendrocyte progenitor cells underwent apoptosis within 24 h of irradiation in the rat. That did not result in a change in OPC density at 24 h. Oligodendrocyte progenitor cell density was significantly reduced by 2-4 weeks, but showed recovery by 6 weeks after irradiation. An increase in BrdU-labeled cells was observed at 2 weeks after 8 Gy or 22 Gy, and proliferating cells in the rat spinal cord were immunoreactive for NG2. The mouse spinal cord showed a similar early cell proliferation after irradiation. No difference was observed in the proliferation response in the spinal cord of p53 -/- mice compared with wild type animals.

CONCLUSIONS

Oligodendroglial cells undergo early apoptosis and OPC undergo early proliferation after ionizing radiation. However, apoptosis is not likely to be the trigger for early proliferation of OPC in the irradiated central nervous system.

Y chromosome detection of three-dimensional tissue-engineered skeletal muscle constructs in a syngeneic rat animal model

Surgical reconstruction of muscle tissue lost by trauma or tumor ablation is limited by the lack of availability of functional native tissue substitution. Moreover, so far most inherited or acquired muscle diseases are lacking sufficient treatment, because only few alternatives exist to provide functional restoration of lost muscle tissues. Engineering those tissues and transplantation into sites of dysfunction may be an alternative approach and may allow replacement of such damaged or failing skeletal muscle tissues. Techniques attempting reconstruction of some human tissues and organs (tissue engineering) have been introduced into clinical practice recently. One major problem that previous transplantation studies were facing is the ability of detection of transplanted cells after integration. Using the Y chromosome in situ hybridization technique in a syngeneic rat model allows transplantation of cell constructs orthotopically, without manipulation of the cells, with no rejection or immunosuppression being implied, but providing a nondilutable genetic marker to identify transplanted cells. The purpose of our study was to create functional skeletal muscle tissue in vivo using the transplantation of primary myoblasts precultivated within a three-dimensional (3D) fibrin matrix and to determine the fate of the transplanted cells using the Y chromosome detection technique. 3D myoblast cultures were established derived from male donor rats and after 7 days of cultivation we performed an orthotopic transplantation of 3D cell constructs into a created muscle defect within the gracilis muscle of syngeneic female rats. Anti-desmin immunostaining and Y chromosome in situ hybridization indicated the survival and integration of transplanted male myoblasts into the female recipient animal, thus demonstrating the feasibility of this approach in tissue engineering and the research of cell transplantation in general.

Is there a future for neural transplantation?

Traditionally neural transplantation has had as its central tenet the replacement of missing neurons that have been lost because of neurodegenerative processes, as exemplified by diseases such as Parkinson disease (PD). However, the effectiveness and widespread application of this approach clinically has been limited, primarily because of the poor donor supply of human fetal neural tissue and the incomplete neurobiological understanding of the circuit reconstruction required to normalize function in these diseases. So, in PD the progress from promising neural transplantation in animal models to proof-of-principle, open-labeled clinical transplants, to randomized, placebo-controlled studies of neural transplantation has not been straightforward. The emergence of previously undescribed adverse effects and lack of significant functional advantage in recent clinical studies has been disappointing and has served to cast a new, and perhaps more realistic, perspective on this treatment approach. In fact, there have been calls by some involved in neural transplantation to return to the drawing board before pressing on with further clinical trials, and the return to basic experimentation. This therefore precipitates the question - is there a future for neural transplantation? It is important to remember that there are a number of possible explanations for the disappointing results from the recent clinical trials in PD, ranging from the mode of transplantation to patient selection. Nevertheless, almost irrespective of these reasons for the current trial results, there have always been significant practical and ethical problems with using human fetal tissue, and so a number of alternative cell sources have been investigated. These alternative sources include stem cells, which are attractive for cell-based therapies because of their potential ease of isolation, propagation and manipulation, and their ability in some cases to migrate to areas of pathology and differentiate into specific and appropriate cell types. Furthermore, the availability of stem cells derived from non-embryonic sources (e.g. adult stem cells derived from the sub-ventricular zone) has removed some of the ethical limitations associated with the use of embryonic human tissue. These potentially beneficial aspects of stem cells means that there is a future for neural transplantation as a means of treating patients with a range of neurological disorders, although whether this will ever translate into a truly effective, widely available therapy remains unknown.

Transplantation of glial-restricted precursor cells into the adult spinal cord: survival, glial-specific differentiation, and preferential migration in white matter

Glial-restricted precursor (GRP) cells are among a number of candidate cells for transplantation repair of CNS injury. The isolation and characterization of these cells in vitro have been described previously, but their in vivo properties are not well understood. We examined the fate and migration of grafted fetal GRP cells harvested from alkaline phosphatase-expressing transgenic rats into intact and injured spinal cord. Transplanted GRP cells survived for at least 6 weeks and differentiated along astrocytic and oligodendrocytic but not neuronal lineages. Cells grafted into the intact spinal cord exhibited robust migration along longitudinal white matter tracts and by 6 weeks migrated more than 15 mm. In contrast, migration of GRP cells in the gray matter was very limited. We then examined the phenotypic properties of proliferating endogenous precursors in response to injury by BrdU labeling. The predominant proliferating population seen after injury consisted of GRP-like cells with Nkx2.2/olig2 phenotype. Incorporation of BrdU by endogenous cells suggests that the environment provides proliferation signals and is permissive to glial precursor survival. To test if exogenous GRP cells would respond similarly, we transplanted GRP cells into a lateral funiculus injury. GRP cells survived and differentiated along glial lineages and migrated along white matter tracts in the injured spinal cord. Directed homing toward the lesion was not seen and there was no significant bias in differentiation between cells transplanted into injured and uninjured spinal cord. GRP cell transplants may therefore provide a cellular transplant that can respond to appropriate endogenous cues to produce therapeutic molecules and new glial cells after injury.

Olfactory ensheathing cells induce less host astrocyte response and chondroitin sulphate proteoglycan expression than Schwann cells following transplantation into adult CNS white matter

Both Schwann cells and olfactory ensheathing cells (OECs) create an environment favorable to axon regeneration when transplanted into the damaged CNS. However, transplanted cells can also exert an effect on the host tissue that will influence the extent to which regenerating axons can grow beyond the transplanted area and reenter the host environment. In this study equivalent numbers of Lac-Z-labeled Schwann cells and OECs have been separately transplanted into normal white matter of adult rat spinal cord and the host astrocyte response to each compared. Schwann cell transplantation resulted in a greater area of increased glial fibrillary acidic protein (GFAP) expression compared to that associated with OEC transplantation. This was accompanied by a greater increase in the expression of axon growth inhibitory chrondroitin sulfate proteoglycans (CSPGs) following Schwann cell transplantation compared to OEC transplantation. However, no differences were detected in the increased expression of the specific CSPG neurocan following transplantation of the two cell types. These results mirror differences in the interactions between astrocytes and either Schwann cells or OECs observed in tissue culture models and reveal one aspect of the complex biology of creating regeneration-promoting environments by cell transplantation where transplanted OECs have favorable properties compared to transplanted Schwann cells.

Remyelination in multiple sclerosis: a new role for neurotrophins?

Multiple sclerosis (MS) is a common neurological disease, which affects young adults. Its course is unpredictable and runs over decades. It is considered as an autoimmune disease, and is neuropathologically characterized by demyelination, variable loss of oligodendroglial cells, and axonal degeneration. Demyelination provides a permitting condition for axonal degeneration, which seems to be causative of permanent neurological deficits. Hence, the current treatment, which works preferentially immunmodulatory, should be complemented by therapeutics, which improves remyelination not only for restoring conduction velocity but also for preventing an irreversible axonal damage. One strategy to achieve this aim would be to promote remyelination by stimulating oligodendroglial cells remaining in MS lesions. While central nervous system neurons were already known to respond to neurotrophins (NT), interactions with glial cells became apparent more recently. In vitro and in vivo studies have shown that NT influence proliferation, differentiation, survival, and regeneration of mature oligodendrocytes and oligodendroglial precursors in favor of a myelin repair. Two in vivo models provided direct evidence that NT can improve remyelination. In addition, their neuroprotective and anti-inflammatory role would support a repair. Hence, a wealth of data point to NT as promising therapeutical candidates.

Fetal and adult human oligodendrocyte progenitor cell isolates myelinate the congenitally dysmyelinated brain

Both late-gestation and adult human forebrain contain large numbers of oligodendrocyte progenitor cells (OPCs). These cells may be identified by their A2B5(+)PSA-NCAM(-) phenotype (positive for the early oligodendrocyte marker A2B5 and negative for the polysialylated neural cell adhesion molecule). We used dual-color fluorescence-activated cell sorting (FACS) to extract OPCs from 21- to 23-week-old fetal human forebrain, and A2B5 selection to extract these cells from adult white matter. When xenografted to the forebrains of newborn shiverer mice, fetal OPCs dispersed throughout the white matter and developed into oligodendrocytes and astrocytes. By 12 weeks, the host brains showed extensive myelin production, compaction and axonal myelination. Isolates of OPCs derived from adult human white matter also myelinated shiverer mouse brain, but much more rapidly than their fetal counterparts, achieving widespread and dense myelin basic protein (MBP) expression by 4 weeks after grafting. Adult OPCs generated oligodendrocytes more efficiently than fetal OPCs, and ensheathed more host axons per donor cell than fetal cells. Both fetal and adult OPC phenotypes mediated the extensive and robust myelination of congenitally dysmyelinated host brain, although their differences suggested their use for different disease targets.

Dysfunctional oligodendrocyte progenitor cell (OPC) populations may inhibit repopulation of OPC depleted tissue

We have attempted to extend a previously described rat model of focal oligodendrocyte progenitor cell (OPC) depletion, using 40 Gy X-irradiation (Chari and Blakemore [2002] Glia 37:307-313), to the adult mouse spinal cord, to examine the ability of OPCs present in adjacent normal areas to colonise areas of progenitor depletion. In contrast to rat, OPCs in the mouse spinal cord appeared to be a comparatively radiation-resistant population, as 30-35% of OPCs survived in X-irradiated tissue (whereas <1% of OPCs survive in X-irradiated rat spinal cord). The numbers of surviving OPCs remained constant with time indicating that this population was incapable of regenerating itself in response to OPC loss. Additionally, these OPCs did not contribute to remyelination of axons when demyelinating lesions were placed in X-irradiated tissue, suggesting that the surviving cells are functionally impaired. Importantly, the length of the OPC-depleted area did not diminish with time, as would be expected if progressive repopulation of OPC-depleted areas by OPCs from normal areas was occurring. Our findings therefore raise the possibility that the presence of a residual dysfunctional OPC population may inhibit colonisation of such areas by normal OPCs.

Migration and differentiation of neural progenitor cells from two different regions of embryonic central nervous system after transplantation into the intact spinal cord

Transplantation of in vitro-expanded neural stem cells (NSCs) is a potentially powerful tool to repair functions of the injured spinal cord. A prerequisite for the successful transplantation therapy is identification of optimized experimental parameters that can promote maximal survival, extensive migration and selective differentiation of the transplanted NSC population in the spinal cord. To this end, we evaluated the basic characteristics of NSC-like cells from two different donor sources, the embryonic hippocampus and spinal cord, after transplantation into the neonatal spinal cord. Proliferation and differentiation phenotypes of both NSC-like cells can be controlled by the concentration of fibroblast growth factor-2 (FGF-2) in vitro. Both NSC-like cells can survive within the environment of the intact neonatal spinal cord and showed extensive migratory behaviour shortly after transplantation. However, quantitative analysis revealed preferential migration of hippocampus-derived cells in the dorsal white matter. Both NSC-like cells showed restricted phenotype toward the oligodendroglial lineage after transplantation. Transplantation of the mixture of two cell types revealed selective survival of hippocampus-derived NSC-like cells. This study indicates the possibility of transplanting hippocampus-derived NSCs to supply the cell source for immature oligodendrocytes, which are thought to be essential for both the myelination and trophic support of regenerating axons in the dorsal white matter of the spinal cord.

Olfactory ensheathing glia transplantation: a therapy to promote repair in the mammalian central nervous system

A therapy to treat injuries to the central nervous system (CNS) is, to date, a major clinical challenge. The devastating functional consequences they cause in human patients have encouraged many scientists to search, in animal models, for a repair strategy that could, in the future, be applied to humans. However, although several experimental approaches have obtained some degree of success, very few have been translated into clinical trials. Traumatic and demyelinating lesions of the spinal cord have attracted several groups with the same aim: to find a way to promote axonal regeneration, remyelination, and functional recovery, by using a simple, safe, effective, and viable procedure. During the past decade, olfactory ensheathing glia (OEG) transplantation has emerged as a very promising experimental therapy to promote repair of spinal cords, after different types of injuries. Transplants of these cells promoted axonal regeneration and functional recovery after partial and complete spinal cord lesions. Moreover, olfactory ensheathing glia were able to form myelin sheaths around demyelinated axons. In this article, we review these recent advances and discuss to what extent olfactory ensheathing glia transplantation might have a future as a therapy for different spinal cord affections in humans.

Remyelination-promoting antibodies activate distinct Ca2+ influx pathways in astrocytes and oligodendrocytes: relationship to the mechanism of myelin repair

Our laboratory has identified mouse and human monoclonal antibodies that promote myelin repair in multiple models of demyelinating disease. We have proposed that these antibodies promote remyelination by directly activating central nervous system glia. Intracellular calcium concentration was monitored using a Fura2 ratiometric assay. Repair-promoting antibodies induced distinct Ca2+ signals in both astrocytes and oligodendrocytes. Astrocyte Ca2+ signaling is mediated by a phospholipase C-dependent pathway while oligodendrocyte Ca2+ signaling is mediated via AMPA-sensitive glutamate receptors. An antibody's ability to induce Ca2+ signals is statistically correlated with promotion of myelin repair. These findings support the hypothesis that remyelination-promoting antibodies are acting directly at the surface of glial cells to induce calcium-dependent physiologic reparative function.

Transplanted multipotential neural precursor cells migrate into the inflamed white matter in response to experimental autoimmune encephalomyelitis

Transplanted neural precursor cells remyelinate efficiently acutely demyelinated focal lesions. However, the clinical value of cell transplantation in a chronic, multifocal disease like multiple sclerosis will depend on the ability of transplanted cells to migrate to the multiple disease foci in the brain. Here, we expanded newborn rat neural precursor cells in spheres and transplanted them intracerebroventricularly or intrathecally in rats. The cells were labeled by the nuclear fluorescent dye Hoechst or by incubation with BrdU to enable their identification at 2 days and 2 weeks after transplantation, respectively. Spheres consisted of PSA-NCAM(+), nestin(+), NG2(-) undifferentiated precursor cells that differentiated in vitro into astrocytes, oligodendrocytes, and neurons. Spheres that were transplanted into intact rats remained mostly in the ventricles or in the spinal subarachnoid space. Following transplantation at peak of experimental autoimmune encephalomyelitis, cells migrated into the brain or spinal cord parenchyma, exclusively into inflamed white matter but not into adjacent gray matter regions. After 2 weeks, many transplanted cells had migrated into distant white matter tracts and acquired specific markers of the astroglial and oligodendroglial lineages. Thus, the inflammatory process may attract targeted migration of transplanted precursor cells into the brain parenchyma.

Heterogeneity of pathogenesis in multiple sclerosis: implications for promotion of remyelination

Enhancing myelin repair remains an important therapeutic goal in primary demyelinating diseases of the central nervous system (CNS) such as multiple sclerosis (MS). The emerging heterogeneity of pathology within MS lesions, and differential oligodendrocyte survival in particular, suggests that therapeutic strategies may need to be tailored to an individual patient's requirements. A number of therapeutic strategies have been proposed to enhance myelin repair in the CNS: cell transplantation, growth factor therapy, and antibody therapy, but each proposed therapy has different implications with respect to pathogenetic mechanisms of demyelination. Of these, antibody therapy is the most amenable to immediate application in patients-but a combination of therapeutic approaches may be required in practice.

Remyelination of the demyelinated CNS: the case for and against transplantation of central, peripheral and olfactory glia

Although originally developed as a research tool for studying glial-glial and glial-axonal interactions, the technique of transplanting glial cell into the central nervous system has more recently been employed as a potential means for repairing persistent demyelination in clinical disease. It has now been clearly established using various experimental models that oligodendrocyte lineage cells, Schwann cells and olfactory ensheathing cells can all produce new myelin sheaths around demyelinated or amyelinated axons following transplantation. However, this property alone does not necessarily mean that transplantation of these cells into demyelinated lesions in clinical disease will be successful. This article considers some of the properties that would be required of a transplanted myelinogenic cell and assesses the advantages and disadvantages of the currently available cell types.

Application of bone marrow-derived stem cells in experimental nephrology

Recent advancement in developmental biology has led to the discovery of immature mesenchymal stem cells in bone marrow and several established organs. The therapeutic potentials of such stem cells for treating serious diseases constitute a major rationale for every research effort, and clinical trials for replacing some damaged tissues such as cartilage are currently under way. Although the feasibility of such stem cell therapies for renal diseases remains unknown, it is worthy of pursuing possibilities. As an experimental tool, we have made chimeric rats that carry the bone marrow of transgenic rats expressing green fluorescent protein (GFP) throughout the body. All marrow lineages can be traced with bright green fluorescence regardless of their terminal phenotypes in chimeric rats. When compared with other conventional strategies for tracking bone marrow-derived cells, our system is highly sensitive and allows us to take advantage of the rat nephrology. Using this system, we have reported that bone marrow provides mesangial cells during repair process. In this manuscript, we briefly summarize our experiences.

New tools to trace populations of inflammatory cells in the CNS

Cells of the central nervous system (CNS) and immune system communicate regularly. There is a constant surveillance of the intact, healthy CNS by activated T-cells, and massive infiltration of the CNS by immune cells under pathological conditions such as neurodegeneration or neuroinflammation. Labeling CNS-infiltrating T-cells is an essential tool to identify the signals and mechanisms, which mediate the interaction between immune cells and cells of the CNS. In this article, we will present an overview describing currently used cellular markers and demonstrate how these markers have contributed to our current knowledge of CNS inflammation and immune surveillance.

Embryonic-derived glial-restricted precursor cells (GRP cells) can differentiate into astrocytes and oligodendrocytes in vivo

We have isolated and characterized a unique glial-restricted precursor cell (GRP) from the embryonic spinal cord. Clonal analysis demonstrated that these cells are able to generate oligodendrocytes and two distinct type of astrocytes (type 1 and type 2) when exposed to appropriate signals in vitro. We now show that many aspects of these cells are retained in vivo. GRP cells are restricted to the glial lineage in vivo as they seem to be unable to generate neuronal phenotypes in an in vivo neurogenic environment. GRP cells survive and migrate in the neonatal and adult brain. Transplanted GRP cells differentiate into myelin-forming oligodendrocytes in a myelin-deficient background and also generate immature oligodendrocytes in the normal neonatal brain. In addition, GRP cells also consistently generated glial fibrillary protein-expressing cells in the neonatal and adult brain, a property not consistently expressed by other glial precursor cells like the O-2A/OPC cells. We suggest that the lineage restriction of GRP cells and their ability to generate both oligodendrocytes and astrocytes in vivo together with their embryonic character that allows for extensive in vitro expansion of the population makes the cell useful for clinical application.

Depletion of endogenous oligodendrocyte progenitors rather than increased availability of survival factors is a likely explanation for enhanced survival of transplanted oligodendrocyte progenitors in X-irradiated compared to normal CNS

Oligodendrocyte progenitors (OPs) survive and migrate following transplantation into adult rat central nervous system (CNS) exposed to high levels of X-irradiation but fail to do so if they are transplanted into normal adult rat CNS. In the context of developing OP transplantation as a potential therapy for repairing demyelinating diseases it is clearly of some importance to understand what changes have occurred in X-irradiated CNS that permit OP survival. This study addressed two alternative hypotheses. Firstly, X-irradiation causes an increase in the availability of OP survival factors, allowing the CNS to support a greater number of progenitors. Secondly, X-irradiation depletes the endogenous OP population thereby providing vacant niches that can be occupied by transplanted OPs. In situ hybridization was used to examine whether X-irradiation causes an increase in mRNA expression of five known OP survival factors, CNTF, IGF-I, PDGF-A, NT-3 and GGF-2. The levels of expression of these factors at 4 and 10 days following exposure of the adult rat spinal cord to X-irradiation remain the same as the expression levels in normal tissue. Using intravenous injection of horseradish peroxidase, no evidence was found of X-irradiation-induced change in blood-brain barrier permeability that might have exposed X-irradiated tissue to serum-derived survival factors. However, in support of the second hypothesis, a profound X-irradiation-induced decrease in the number of OPs was noted. These data suggest that the increased survival of transplanted OPs in X-irradiated CNS is not a result of the increases in the availability of the OP survival factors examined in this study but rather the depletion of endogenous OPs creating 'space' for transplanted OPs to integrate into the host tissue.

Areas of demyelination do not attract significant numbers of schwann cells transplanted into normal white matter

If Schwann cell transplantation is to be used as a therapy for demyelinating disease, it is important to know if the number of transplanted cells and their transplantation site affects the extent of remyelination. Primary Schwann cell cultures were obtained from neonatal rat sciatic nerve, purified, and expanded using bovine pituitary extract and forskolin. Areas of persistent demyelination were created in the dorsal funiculus of the thoracolumbar spinal cord of rats by injecting ethidium bromide into white matter exposed to 40 Gy of X-irradiation, and a high and low number of Schwann cells were transplanted, into either the area of demyelination or the dorsal funiculus cranial to the area of demyelination. Animals were perfused 4 weeks after transplantation. After injection of 4 x 10(4) cells into the area of demyelination, the area of Schwann cell remyelination was 0.88 +/- 0.16 mm(2), while following the injection of 3 x 10(3) cells it was significantly smaller, 0.29 +/- 0.09 mm(2). After implantation of Schwann cells 1-3 mm (mean 2.5 mm) cranial to the area of demyelination, only one of the eight animals (a high-dose animal) showed extensive Schwann cell remyelination. In this animal, the cells were transplanted within 1 mm of the area of demyelination, well within the length of tissue over which cells are passively spread by the injection procedure (1-3 mm). Our results show that significant numbers of transplanted Schwann cells are not attracted through normal tissue to areas of demyelination and when transplanted into areas of demyelination the extent of myelination is related to the number of Schwann cells transplanted.