Building brains: neural chimeras in the study of nervous system development and repair

Fucoidan improving spinal cord injury recovery: Modulating microenvironment and promoting remyelination

INTRODUCTION

Excessive neuroinflammation, apoptosis, glial scar, and demyelination triggered by spinal cord injury (SCI) are major obstacles to SCI repair. Fucoidan, a natural marine plant extract, possesses broad-spectrum anti-inflammatory and immunomodulatory effects and is regarded as a potential therapeutic for various diseases, including neurological disorders. However, its role in SCI has not been investigated.

METHODS

In this study, we established an SCI model in mice and intervened in injury repair by daily intraperitoneal injections of different doses of fucoidan (10 and 20 mg/kg). Concurrently, primary oligodendrocyte precursor cells (OPCs) were treated in vitro to validate the differentiation-promoting effect of fucoidan on OPCs. Basso Mouse Scale (BMS), Louisville Swim Scale (LSS), and Rotarod test were carried out to measure the functional recovery. Immunofluorescence staining, and transmission electron microscopy (TEM) were performed to assess the neuroinflammation, apoptosis, glial scar, and remyelination. Western blot analysis was conducted to clarify the underlying mechanism of remyelination.

RESULTS

Our results indicate that in the SCI model, fucoidan exhibits significant anti-inflammatory effects and promotes the transformation of pro-inflammatory M1-type microglia/macrophages into anti-inflammatory M2-type ones. Fucoidan enhances the survival of neurons and axons in the injury area and improves remyelination. Additionally, fucoidan promotes OPCs differentiation into mature oligodendrocytes by activating the PI3K/AKT/mTOR pathway.

CONCLUSION

Fucoidan improves SCI repair by modulating the microenvironment and promoting remyelination.

Engineered Macrophage Membrane-Coated Nanoparticles with Enhanced CCR2 Expression Promote Spinal Cord Injury Repair by Suppressing Neuroinflammation and Neuronal death

Spinal cord injury (SCI) is a severe neurological disorder characterized by significant disability and limited treatment options. Mitigating the secondary inflammatory response following the initial injury is the primary focus of current research in the treatment of SCI. CCL2 (C─C motif chemokine ligand 2) serves as the primary regulator responsible for inflammatory chemotaxis of the majority of peripheral immune cells, blocking the CCL2-CCR2 (C─C chemokine receptor type 2) axis has shown considerable therapeutic potential for inflammatory diseases, including SCI. In this study, it presents a multifunctional biomimetic nanoplatform (CCR2-MM@PLGA/Cur) specifically designed to target the CCL2-CCR2 axis, which consisted of an engineered macrophage membrane (MM) coating with enhanced CCR2 expression and a PLGA (poly (lactic-co-glycolic acid)) nanoparticle that encapsulated therapeutic drugs. CCR2 overexpression on MM not only enhanced drug-targeted delivery to the injury site, but also attenuated macrophage infiltration, microglia pro-inflammatory polarization, and neuronal apoptosis by trapping CCL2. Consequently, it facilitated neural regeneration and motor function recovery in SCI mice, enabling a comprehensive treatment approach for SCI. The feasibility and efficacy of this platform are confirmed through a series of in vitro and in vivo assays, offering new insights and potential avenues for further exploration in the treatment of SCI.

Hierarchically clustering to 1,033 genes differentially expressed in mouse superior colliculus in the courses of optic nerve development and injury

Tempo spatially specific expression of many development-related genes is the molecular basis for the formation of the central nervous system (CNS), especially those genes regulating the proliferation, differentiation, migration, axon growth, and orientation of nerve cells. The development-related genes are usually prominent during the embryonic and newborn stages, but rarely express during the adulthood. These genes are believed to be suitable target genes for promoting CNS regeneration, despite majority of which remains unknown. Hence, the aim of this study was to screen development-related genes which might contribute to CNS regeneration. In this study, 1,033 differentially-expressed genes of superior colliculus in the courses of mouse optic nerve development and injury, as previously identified by cDNA microarrays, were hierarchically clustered to display expression pattern of each gene and reveal the relationships among these genes, and infer the functions of some unknown genes based on function-identified genes with the similar expression patterns. Consequently, the expression patterns of 1,033 candidate genes were revealed at eight time points during optic nerve development or injury. According to the similarity among gene expression patterns, 1,033 genes were divided into seven groups. The potential function of genes in each group was inferred on the basis of the dynamic trend for mean gene expression values. Moreover, the expression patterns of six function-unidentified genes were extremely similar to that of the ptn gene which could promote and guide axonal extension. Therefore, these six genes are temporally regarded as candidate genes related to axon growth and guidance. The results may help to better understand the roles of function-identified genes in the stages of CNS development and injury, and offer useful clues to evaluate the functions of hundreds of unidentified genes.

Screening genes related to development and injury of the mouse optic nerve by cDNA microarrays

The aim of this study was to screen genes related to the development and injury of the mouse optic nerve so as to provide possible target genes for gene-engineering therapy of central nervous system (CNS) injury. Gene expression was profiled by cDNA microarrays in the mouse superior colliculus at 8-time points during the development or following injury of the optic nerve; consequently, 1,095 highly expressed genes (ratio > or =2) were identified. Then, these genes were categorized functionally; there were 561 genes (51.19%) with unidentified functions and 534 genes (48.81%) with identified or partially identified functions. After discounting the overlapping genes, 486 genes with identified or partially identified functions were categorized into 17 functional groups. The 17 functional groups were as follows: I transcription regulation, II signal transduction, III protein synthesis, IV materials transporting, V RNA processing, VI metabolism-related genes, VII cell cycle or apoptosis-related genes, VIII extracellular matrix, IX protein folding and degradation, X cytoskeleton, XI histone metabolism, XII nervous system specific functional genes, XIII tumor related genes, XIV DNA replication and repair, XV axon growth and guidance, XVI immune response, and XVII cell adhesion. These genes may play key roles in the development, injury, and repairment of the optic nerve.

Metaphysical and ethical perspectives on creating animal-human chimeras

This paper addresses several questions related to the nature, production, and use of animal-human (a-h) chimeras. At the heart of the issue is whether certain types of a-h chimeras should be brought into existence, and, if they are, how we should treat such creatures. In our current research environment, we recognize a dichotomy between research involving nonhuman animal subjects and research involving human subjects, and the classification of a research protocol into one of these categories will trigger different ethical standards as to the moral permissibility of the research in question. Are a-h chimeras entitled to the more restrictive and protective ethical standards applied to human research subjects? We elucidate an Aristotelian-Thomistic metaphysical framework in which to argue how such chimeras ought to be defined ontologically. We then examine when the creation of, and experimentation upon, certain types of a-h chimeras may be morally permissible.

Stem cell-based therapies for spinal cord injury

Spinal cord injury (SCI) results in loss of nervous tissue and consequently loss of motor and sensory function. There is no treatment available that restores the injury-induced loss of function to a degree that an independent life can be guaranteed. Transplantation of stem cells or progenitors may support spinal cord repair. Stem cells are characterized by self-renewal and their ability to become any cell in an organism. Promising results have been obtained in experimental models of SCI. Stem cells can be directed to differentiate into neurons or glia in vitro, which can be used for replacement of neural cells lost after SCI. Neuroprotective and axon regeneration-promoting effects have also been credited to transplanted stem cells. There are still issues related to stem cell transplantation that need to be resolved, including ethical concerns. This paper reviews the current status of stem cell application for spinal cord repair.

Stem cell-based therapies for spinal cord injury

Spinal cord injury (SCI) results in loss of nervous tissue and consequently loss of motor and sensory function. There is no treatment available that restores the injury-induced loss of function to a degree that an independent life can be guaranteed. Transplantation of stem cells or progenitors may support spinal cord repair. Stem cells are characterized by self-renewal and their ability to become any cell in an organism. Promising results have been obtained in experimental models of SCI. Stem cells can be directed to differentiate into neurons or glia in vitro, which can be used for replacement of neural cells lost after SCI. Neuroprotective and axon regeneration-promoting effects have also been credited to transplanted stem cells. There are still issues related to stem cell transplantation that need to be resolved, including ethical concerns. This paper reviews the current status of stem cell application for spinal cord repair.

Electrophysiological evaluation of engrafted stem cell-derived neurons

Recent advances in the neural stem cell field have provided a wealth of methods for generating large amounts of purified neuronal precursor cells. It has become a question of paramount importance to determine whether these cells integrate and interact with established neural circuitry after engraftment. In principle, neurons have to fulfill three basic functions: receive incoming signals via synapses, compute and forward processed information to other neurons or effector cells. It is anticipated that functionally integrating stem cell-derived donor neurons perform accordingly. Here we provide protocols for the efficient electrophysiological evaluation of engrafted cells and highlight current limitations thereof.

Human embryonic stem cell-derived neural precursors develop into neurons and integrate into the host brain

Whether and how in-vitro-produced human neural precursors mature and integrate into the brain are crucial to the utility of human embryonic stem (hES) cells in treating neurological disorders. After transplantation into the ventricles of neonatal immune-deficient mice, hES-cell-derived neural precursors stopped expressing the cell division marker Ki67, except in neurogenic areas, and differentiated into neurons and then glia in a temporal course intrinsic to that of human cells regardless of location. The human cells located in the gray matter became neurons in the olfactory bulb and striatum, whereas those in the white matter produced exclusively glia. Importantly, the grafted human cells formed synapses. Thus, the in-vitro-produced human neural precursors follow their intrinsic temporal program to produce neurons and glia and, in response to environmental signals, generate cells appropriate to their target regions and integrate into the brain.

Neurotransmitter receptor expression and activity during neuronal differentiation of embryonal carcinoma and stem cells: from basic research towards clinical applications

Embryonal carcinoma and embryonic stem cells have served as models to understand basic aspects of neuronal differentiation and are promising candidates for regenerative medicine. Besides being well characterized regarding the capability of embryonal carcinoma and embryonic stem cells to be precursors of different tissues, the molecular mechanisms controlling neuronal differentiation are hardly understood. Neuropeptide and neurotransmitter receptors are expressed at early stages of differentiation prior to synaptogenesis, triggering transient changes in calcium concentration and inducing neurone-specific gene expression. In vitro neuronal differentiation of embryonal carcinoma and embryonic stem cells closely resembles early neuronal development in vivo. Murine P19 EC cells are a well-characterized model for in vitro differentiation, which upon treatment with retinoic acid differentiate into neurones. Expression and activity of various receptor proteins is regulated during their differentiation. Stimulation of kinin-B2, endothelin-B, muscarinic acetylcholine, and N-methyl-D-aspartate receptors results in transient increases of intracellular free calcium concentration [Ca(2+)](i) in P19 cells undergoing neuronal differentiation, whereas embryonal cells do not respond or show a smaller change in [Ca(2+)](i) than differentiating cells. Receptor inhibition, as studied with the example of the kinin-B2 receptor, aborts neuronal maturation of P19 cells, demonstrating the crucial importance of B2 receptors during the differentiation process. Future success in obtaining desired neuronal phenotypes from pluripotent cells in vitro may offer new therapeutic perspectives for curing genetic and acquired dysfunctions of the developing and adult nervous system.

Magnetic resonance tracking of implanted adult and embryonic stem cells in injured brain and spinal cord

Stem cells are a promising tool for treating brain and spinal cord injury. Magnetic resonance imaging (MRI) provides a noninvasive method to study the fate of transplanted cells in vivo. We studied implanted rat bone marrow stromal cells (MSCs) and mouse embryonic stem cells (ESCs) labeled with iron-oxide nanoparticles (Endorem) and human CD34+ cells labeled with magnetic MicroBeads (Miltenyi) in rats with a cortical or spinal cord lesion. Cells were grafted intracerebrally, contralaterally to a cortical photochemical lesion, or injected intravenously. During the first week post transplantation, transplanted cells migrated to the lesion. About 3% of MSCs and ESCs differentiated into neurons, while no MSCs, but 75% of ESCs differentiated into astrocytes. Labeled MSCs, ESCs, and CD34+ cells were visible in the lesion on MR images as a hypointensive signal, persisting for more than 50 days. In rats with a balloon-induced spinal cord compression lesion, intravenously injected MSCs migrated to the lesion, leading to a hypointensive MRI signal. In plantar and Basso-Beattie-Bresnehan (BBB) tests, grafted animals scored better than lesioned animals injected with saline solution. Histologic studies confirmed a decrease in lesion size. We also used 3-D polymer constructs seeded with MSCs to bridge a spinal cord lesion. Our studies demonstrate that grafted adult as well as embryonic stem cells labeled with iron-oxide nanoparticles migrate into a lesion site in brain as well as in spinal cord.

Neural stem cell systems: diversities and properties after transplantation in animal models of diseases

Currently available effective treatments of the diseased or damaged central nervous system (CNS) are restricted to a limited pharmacological relief of symptoms or those given to avoid further damage. Therefore the search is on for treatments that can restore function in the CNS. During recent years replacement of damaged neurons by cell transplantation is being enthusiastically explored as a potential treatment for many neurodegenerative diseases, stroke and traumatic brain injury. Several references in both scientific journals and popular newspapers concerning different types of cultured stem cells, potentially exploitable to treat pathological conditions of the brain, raise important questions pertinent to the fundamental and realistic differences between grafts of primary neural cells and the transplantation of in vitro expanded neural stem cells (NSCs). Our aim is to review the available information on the grafting of different NSC types into the adult rodent brain, focusing on critical aspects for the development of clinical therapies to replace damaged neurons.

Centriolar Mechanisms of Differentiation and Replicative Aging of Higher Animal Cells

The centrosome (centriole) and the cytoskeleton produced by it are structures, which probably determine differentiation, morphogenesis, and switching on the mechanism of replicative aging in all somatic cells of multicellular animals. The mechanism of such programming of the events seems to include cytoskeleton influences and small RNAs related to the centrosome. 1) If these functions are really related with centrioles, the multicellular organism's cells which: a) initially lack centrioles (e.g., higher plant cells and also zygote and early blastomeres of some animals) or cytoskeleton (e.g., embryonic stem cells); or b) generate centrioles de novo (e.g., zygote and early blastomeres of some animals), will be totipotent and lack replicative aging. Consequently, the absence (constant or temporary) of the structure determining the counting of divisions also means the absence of counting of differentiation processes. 2) Although a particular damage to centrioles or cytoskeleton (e.g., in tumor cells) fails to make the cells totipotent (because the morphogenetic status of these cells, as differentiated from that of totipotent ones, is not zero), but such a transformation can suppress the initiation of the aging mechanism induced by these structures and, thus, make such cells replicatively "immortal".

Generating chimeric spinal cord: a novel model for transplantable oligodendrocyte progenitors derived from embryonic stem cells

Object

To identify and evaluate stem cell–derived oligodendrocytes obtained for cell transplantation therapies, the authors developed a novel model to examine single, adult oligodendrocytes in situ.

Methods

Green fluorescent protein–expressing, mouse embryonic stem cells (ESCs) were neural induced and additionally staged in an oligosphere preparatory step for high-yield derivation of oligodendrocyte progenitors. These transplantable, induced progenitors were injected into postnatal Day 2 rat pups, in which spinal cord sections were then examined at 3 and 9 weeks posttransplantation.

Conclusions

Transplanted oligosphere ESCs survived and integrated anatomically into postnatal and adult white matter, generating targeted regions of chimeric spinal cord. A simple model for identifying adult oligodendrocytes in situ is presented, which is suitable for use in further studies examining functional myelination and derivation of oligodendrocytes from genetically engineered ESC lines, including human ESCs. Results from the model presented here demonstrate a unique method for examining transplantable oligodendrocyte progenitors derived from ESCs for repair of white matter disease.

Stem cell and progenitor cell transplantation in multiple sclerosis: The discrepancy between neurobiological attraction and clinical feasibility

Recent developments in our understanding of stem- and progenitor cell differentiation raises hopes that brain damage in chronic neurological diseases may become repaired by systemic or focal transplantation of such cells. In this review the potential of such an approach is discussed, but it is also highlighted that many aspects regarding its feasibility or safety are currently unresolved. Furthermore, recent findings on the pathogenesis of multiple sclerosis lesions indicate that major problems in this disease rather are related to axonal pathology and neurodegeneration rather than to the absence of oligodendrocyte progenitor cells within the lesions. In light of this complex situation, it is concluded that clinical trials of stem- or progenitor cell transplantation in multiple sclerosis are currently premature.

Perinatal applications of neural stem cells

The brain, unlike many tissues, has a limited capacity for self-repair and so there has been great interest in the possibility of transplanting neural cells to replace those lost through injury or disease. Encouraging research in humans is already underway examining the possibility of neural cell replacement in adult neurodegenerative conditions such as Parkinson's disease and Huntington disease. In addition, experiments exploring neural stem cell replacement in rodent models of acute stroke, demyelination and spinal cord injury have demonstrated functional improvements in treated animals. When considering perinatal neural stem cell therapy, it should not be overlooked that the immature, developing brain might provide a more favourable environment for stem cell integration. However, considerable advances need to be made both in understanding the basic biology of neural stem cells, including the instructive signals that determine their proliferation and differentiation, and in characterising their responses when transplanted in a damaged or diseased area of the brain.

Integration and differentiation of neural stem cells after transplantation into the dysmyelinated central nervous system of adult mice

Mutant mice deficient in the myelin-associated glycoprotein (MAG) and the nonreceptor-type tyrosine kinase Fyn are characterized by a severely hypomyelinated central nervous system (CNS) and morphologically abnormal myelin sheaths. Despite this pronounced phenotype, MAG/Fyn-deficient mice have a normal longevity. In the present study, we took advantage of the normal life expectancy of this myelin mutant and grafted neural stem cells (NSCs) into the CNS of MAG/Fyn-deficient mice to study in short- and long-term experiments the fate of NSCs in adult dysmyelinated brains. Neural stem cells were isolated from spinal cords of transgenic mouse embryos ubiquitously expressing enhanced green fluorescent protein. Cells were expanded in vitro in the presence of mitogens for up to 5 weeks before they were grafted into the lateral ventricles or injected into white matter tracts. Analysis of mutant brains 3-15 weeks after intracerebroventricular transplantation of NSCs revealed only limited integration of donor cells into the host brains. However, injection of NSCs directly into white matter tracts resulted in widespread distribution of donor cells within the host tissue. Donor cells survived for at least 15 weeks in adult host brains. The majority of grafted cells populated white matter tracts and differentiated into oligodendrocytes that myelinated host axons. Results suggest that intraparenchymal transplantation of NSCs might be a strategy to reconstruct myelin in dysmyelinated adult brains.

Differentiation and tropic/trophic effects of exogenous neural precursors in the adult spinal cord

The fate of exogenous neural stem cells (NSCs) in the environment of the adult nervous system continues to be a matter of debate. In the present study, we report that cells of the murine NSC clone C17.2, when grafted into the lumbar segments of the spinal cord of adult rats, survive and undergo partial differentiation. C17.2 cells migrate avidly toward axonal tracts and nerve roots and differentiate into nonmyelinating ensheathing cells. Notably, C17.2 cells induce the de novo formation of host axon tracts aiming at graft innervation. Differentiation and inductive properties of C17.2 cells are independent of the presence of lesions in the spinal cord. The tropic/trophic interactions of C17.2 NSCs with host axons, the avid C17.2 cell-host axon contacts, and the ensheathing properties of these cells are related to their complex molecular profile, which includes the expression of trophic cytokines and neurotrophins such as glial cell line-derived neurotrophic factor and brain-derived neurotrophic factor, glial growth factor receptors such as ErbB-2; and PASK, the mammalian homologue of the fray gene that is involved in axon ensheathment. These results show that NSCs might not only play a critical supportive role in repairing axonal injury in the adult spinal cord but also can be used as probes for exploring the molecular underpinnings of the regenerative potential of the mature nervous system after injury.

Regenerative biology: the emerging field of tissue repair and restoration

Regenerative biology has now been recognized as a new field with certain aims and goals. One direction of this new field is to understand the basic mechanisms by which tissues can be repaired and restored. The other direction examines the possibility of using this basic knowledge to apply it to medicine with the goal to clinically repair damaged tissues. Regeneration of tissues can occur by the differentiation of stem cells (local or non-local) or by the transdifferentiation of local terminally differentiated cells. While the transdifferentiation aspects are old, during the past few years many data have accumulated regarding the existence of stem cells and their participation in tissue renewal. This review will present an overview of the potential of all vertebrate organs to regenerate and of the basic mechanisms involved.

Neural stem cells

Neural stem cells (NSCs) have the ability to self-renew, and are capable of differentiating into neurones, astrocytes and oligodendrocytes. Such cells have been isolated from the developing brain and more recently from the adult central nervous system. This review aims to provide an overview of the current research in this evolving area. There is now increasing knowledge of the factors controlling the division and differentiation of NSCs during normal brain development. In addition, the cues for differentiation in vitro, and the possibility of transdifferentiation are reviewed. The discovery of these cells in the adult brain has encouraged research into their role during neurogenesis in the normal mature brain and after injury. Lastly other sources of neural precursors are discussed, and the potential for stem cells to be used in cell replacement therapy for brain injury or degenerative brain diseases with a particular emphasis on cerebral ischaemia and Parkinson's disease.

Of layers and spheres: the reaggregate approach in tissue engineering

The reaggregate approach involves the regeneration of histotypical three-dimensional spheres from dispersed cells of a given tissue in suspension culture. Reaggregated spheres are used as tumour, genetic, toxicological, biohybrid and neurosphere models, and often replace animal experimentation. A particularly instructive example is the use of reaggregation to regenerate complete laminar tissue from avian embryonic retina. By revealing constraints of layered tissue formation, such retinal spheres could be instrumental for regenerative medicine, including stem cell-based tissue engineering.

Basic and clinical neuroscience applications of embryonic stem cells

There have been recent dramatic advances in our understanding of the molecular mechanisms governing the elaboration of mature tissue-specific cellular subpopulations from embryonic stem (ES) cells. These investigations have generated a range of new biological and potential therapeutic reagents to allow us to dissect specific stages of mammalian development that were previously experimentally inaccessible. Ultimately, we will be able to reconstitute seminal signaling pathways to promote regeneration of the nervous system. Totipotent ES cells possess an unlimited proliferative capacity that make them attractive candidates for use in a series of innovative transplantation paradigms. Elucidation of the molecular and physiologic properties of ES cells also has important implications for our understanding of the integrative cellular processes underlying neural induction, patterning of the neural tube, neural lineage restriction and commitment, neuronal differentiation, regional neuronal subtype specification, and the specific pathological consequences of alterations in discrete components of these fundamental neurodevelopmental pathways. In addition, recent experimental observations suggest that neurodegenerative disease pathology may involve alterations in a range of progressive neural inductive and neurodevelopmental events through novel biological mechanisms that result in sublethal impairments in cellular homeostasis within evolving regional neuronal precursor populations containing the mutant proteins, culminating in increased vulnerability of their differentiated neuronal progeny to late-onset apoptosis. Future discoveries in ES cell research will offer unique conceptual and therapeutic perspectives that representing an alternative to neural stem cell therapeutic strategies for ameliorating the pathologic consequences of a broad range of genetic and acquired insults to the developing, adult, and aging brain. Evolving regenerative strategies for both neurodevelopmental and neurodegenerative diseases will likely involve the targeting of vulnerable regional neural precursor populations during "presymptomatic" clinicopathological stages prior to the occurrence of irrevocable neural cell injury and cell death.

Migration and multipotentiality of PSA-NCAM+ neural precursors transplanted in the developing brain

By optimizing the previously described strategy for obtention of spheres enriched in PSA-NCAM+ precursors, we prepared PSA-NCAM-immunoselected cell populations from cerebral hemispheres of neonatal MBP-LacZ transgenic mice. These cells expressed Nestin, exhibited clonal expansion potential and formed spheres, which were initially enriched in PSA-NCAM+ cells but became enriched in GD3+ oligodendrocyte progenitors after 1 week in B104 contionned medium. One month after their periventricular transplantation into the brain of wild-type and/or shiverer newborn mice, cells from PSA-NCAM+ spheres exhibited a higher rostral migration potential than cells from GD3+ spheres, and clearly contributed to myelination in the olfactory bulb. In shiverer hosts, both sphere populations generated oligodendrocytes with similar myelination potential. In addition PSA-NCAM+ sphere cells generated GFAP+ astrocytes and NeuN+ neurons, depending on their site of insertion. These results evidence the high plasticity of newborn PSA-NCAM+ neural precursors and suggest that they are promising tools for cell therapy of CNS diseases, including myelin disorders.

From stem cells towards neural layers: a lesson from re-aggregated embryonic retinal cells

Cells from dissociated embryonic avian retinae have the capacity to re-aggregate in rotation culture and form cellular spheres reconstituting a complete arrangement of all retinal layers. This exquisite phenomenon is based upon in vitro proliferation of multipotent precursor stem cells and spatial organization of their differentiating descendants. The addition of soluble factors from cultured retinal pigmented epithelial (RPE) or radial glial cells is essential to revert inside-out spheres (rosetted retinal spheres) into correctly laminated outside-out spheres (stratified spheres). Such complete restoration of a laminated brain tissue by cell re-aggregation has been achieved only for the embryonic avian retina, but not the mammalian retina, nor for other brain parts. This review summarises the history of the re-aggregation approach, presents avian retinal re-aggregate models, and analyses roles of the RPE and Müller cells for successful retinal tissue regeneration. It is predicted that these results will become biomedically relevant, as stem cell biology will soon open ways to produce large amounts of human retinal precursors.

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

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

Immature human NT2 cells grafted into mouse brain differentiate into neuronal and glial cell types

NT2 cells are a transfectable human embryonal carcinoma cell line, that can be differentiated into postmitotic neuron-like cells (NT2N cells), and transplanted into rodent brains. Differentiation requires a 5-week-long treatment with retinoic acid prior to transplantation. Here, we show that this step can be omitted, and that undifferentiated NT2 cells migrate over long distances and differentiate into both neuron- and oligodendrocyte-like cell types upon grafting into brains of immunocompetent newborn mice. Grafted cells can be traced by fluorogold, with no evidence for tumor formation. Our approach provides an experimental model system which allows the immunohistological and biochemical study of neuronal and glial differentiation of human cells in vivo, and which may be suitable as an in vivo model for pharmacological studies.

Cytomegalovirus infection of the central nervous system stem cells from mouse embryo: a model for developmental brain disorders induced by cytomegalovirus

Cytomegalovirus (CMV) is the most frequent infectious cause of developmental disorders of the central nervous system (CNS) in humans. Infection of the CNS stem cells seems to be primarily responsible for the generation of the brain abnormalities. In this study, we evaluated the infectivity of murine CMV (MCMV) in epidermal growth factor (EGF)-responsive CNS stem cells prepared from fetal mouse brains, and studied the effect of infection on growth and differentiation of the stem cells. The CNS stem cells were permissive for MCMV infection, although MCMV replication was slower than in mouse embryonic fibroblasts. MCMV infection inhibited the growth and DNA replication of the stem cells. A clonogenic assay revealed that MCMV infection suppressed generation of colonies from single stem cells. When uninfected stem cells were induced to differentiate, a decrease in expression of the primitive neuroepidermal marker nestin was observed by immunocytochemistry and flow cytometry, whereas expression of neurofilament and glial fibrillary acidic protein (GFAP) were induced. In virus-infected CNS stem cells, nestin expression was retained, whereas the expression of neurofilament was more severely inhibited than that of GFAP in these cells. Two-color flow cytometry showed that differentiated glial precursor cells were preferentially susceptible to MCMV infection. MCMV-infected and uninfected CNS stem cells were transplanted into the neonatal rat brains. The reduced number of infected stem cells were engulfed into the subventricular zone and expressed GFAP, but did not migrate further, in contrast to the uninfected stem cells. These results suggest that suppression of the growth of the CNS stem cells and inhibition of the neuronal differentiation by CMV infection may be primary causes of disorders of brain development in congenital CMV infection.

Differentiation and histological analysis of embryonic stem cell-derived neural transplants in mice

We report here that neural transplantation of in vitro-differentiated embryonic stem (ES) cells provides a versatile strategy for gene transfer into the central nervous system. ES cells were subjected to an optimized in vitro differentiation protocol to obtain embryoid bodies. These aggregates were stereotaxically transplanted into the brain of recipient adult mice, where they followed a strictly controlled differentiation pattern and eventually formed mature neural grafts. A marker gene, introduced into the ROSA26 locus allowed for precise determination of the fate of the descendants of the transplanted embryoid bodies and revealed that not only neurons but also astrocytes, oligodendrocytes and even microglial cells were graft-derived. Evaluation of long-term experiments showed viable grafts with a stable transgene expression and proved that this approach provides a tool for reliable gene expression within a spatially delimited area of neural tissue.

Human neural precursor cells express low levels of telomerase in vitro and show diminishing cell proliferation with extensive axonal outgrowth following transplantation

Worldwideattention is presently focused on proliferating populations of neural precursor cells as an in vitro source of tissue for neural transplantation and brain repair. However, successful neuroreconstruction is contingent upon their capacity to integrate within the host CNS and the absence of tumorigenesis. Here we show that human neural precursor cells express very low levels of telomerase at early passages (less than 20 population doublings), but that this decreases to undetectable levels at later passages. In contrast, rodent neural precursors express high levels of telomerase at both early and late passages. The human neural precursors also have telomeres (approximately 12 kbp) significantly shorter than those of their rodent counterparts (approximately 40 kbp). Human neural precursors were then expanded 100-fold prior to intrastriatal transplantation in a rodent model of Parkinson's disease. To establish the effects of implanted cell number on survival and integration, precursors were transplanted at either 200,000, 1 million, or 2 million cells per animal. Interestingly, the smaller transplants were more likely to extend neuronal fibers and less likely to provoke immune rejection than the largest transplants in this xenograft model. Cellular proliferation continued immediately post-transplantation, but by 20 weeks there were virtually no dividing cells within any of the grafts. In contrast, fiber outgrowth increased gradually over time and often occupied the entire striatum at 20 weeks postgrafting. Transient expression of tyrosine hydroxylase-positive cells within the grafts was found in some animals, but this was not sustained at 20 weeks and had no functional effects. For Parkinson's disease, the principal aim now is to induce the dopaminergic phenotype in these cells prior to transplantation. However, given the relative safety profile for these human cells and their capacity to extend fibers into the adult rodent brain, they may provide the ideal basis for the repair of other lesions of the CNS where extensive axonal outgrowth is required.

Gliogenesis in the central nervous system

Multipotential neuroepithelial stem cells are thought to give rise to all the differentiated cells of the central nervous system (CNS). The developmental potential of these multipotent stem cells becomes more restricted as they differentiate into progressively more committed cells and ultimately into mature neurons and glia. In studying gliogenesis, the optic nerve and spinal cord have become invaluable models and the progressive stages of differentiation are being clarified. Multiple classes of glial precursors termed glial restricted precursors (GRP), oligospheres, oligodendrocyte-type2 astrocyte (O-2A) and astrocyte precursor cells (APC) have been identified. Similar classes of precursor cells can be isolated from human neural stem cell cultures and from embryonic stem (ES) cell cultures providing a non-fetal source of such cells. In this review, we discuss gliogenesis, glial stem cells, putative relationships of these cells to each other, factors implicated in gliogenesis, and therapeutic applications of glial precursors.