Immunotyping of radial glia and their glial derivatives during development of the rat spinal cord

Biomaterial strategies for creating in vitro astrocyte cultures resembling in vivo astrocyte morphologies and phenotypes

Astrocytes are dynamic cells residing in the central nervous system exhibiting many diverse functions. Astrocytes quickly change and present unique phenotypes in response to injury or disease. Here, we briefly summarize recent information regarding astrocyte morphology and function and provide brief insight into their phenotypic changes following injury or disease. We also present the utility of in vitro astrocyte cultures and present recent advances in biomaterial development that enable better recapitulation of their in vivo behavior and morphology.

Differences in the Cellular Response to Acute Spinal Cord Injury between Developing and Mature Rats Highlights the Potential Significance of the Inflammatory Response

There exists a trend for a better functional recovery from spinal cord injury (SCI) in younger patients compared to adults, which is also reported for animal studies; however, the reasons for this are yet to be elucidated. The post injury tissue microenvironment is a complex milieu of cells and signals that interact on multiple levels. Inflammation has been shown to play a significant role in this post injury microenvironment. Endogenous neural progenitor cells (NPC), in the ependymal layer of the central canal, have also been shown to respond and migrate to the lesion site. This study used a mild contusion injury model to compare adult (9 week), juvenile (5 week) and infant (P7) Sprague-Dawley rats at 24 h, 1, 2, and 6 weeks post-injury (n = 108). The innate cells of the inflammatory response were examined using counts of ED1/IBA1 labeled cells. This found a decreased inflammatory response in the infants, compared to the adult and juvenile animals, demonstrated by a decreased neutrophil infiltration and macrophage and microglial activation at all 4 time points. Two other prominent cellular contributors to the post-injury microenvironment, the reactive astrocytes, which eventually form the glial scar, and the NPC were quantitated using GFAP and Nestin immunohistochemistry. After SCI in all 3 ages there was an obvious increase in Nestin staining in the ependymal layer, with long basal processes extending into the parenchyma. This was consistent between age groups early post injury then deviated at 2 weeks. The GFAP results also showed stark differences between the mature and infant animals. These results point to significant differences in the inflammatory response between infants and adults that may contribute to the better recovery indicated by other researchers, as well as differences in the overall injury progression and cellular responses. This may have important consequences if we are able to mirror and manipulate this response in patients of all ages; however much greater exploration in this area is required.

Intraspinal transplantation of subventricular zone-derived neural progenitor cells improves phrenic motor output after high cervical spinal cord injury

Following spinal cord injury (SCI), intraspinal transplantation of neural progenitor cells (NPCs) harvested from the forebrain sub-ventricular zone (SVZ) can improve locomotor outcomes. Cervical SCI often results in respiratory-related impairments, and here we used an established model cervical SCI (C2 hemisection, C2Hx) to confirm the feasibility of mid-cervical transplantation of SVZ-derived NPCs and the hypothesis that that this procedure would improve spontaneous respiratory motor recovery. NPCs were isolated from the SVZ of enhanced green fluorescent protein (GFP) expressing neonatal rats, and then intraspinally delivered immediately caudal to an acute C2Hx lesion in adult non-GFP rats. Whole body plethysmography conducted at 4 and 8wks post-transplant demonstrated increased inspiratory tidal volume in SVZ vs. sham transplants during hypoxic (P=0.003) or hypercapnic respiratory challenge (P=0.019). Phrenic nerve output was assessed at 8wks post-transplant; burst amplitude recorded ipsilateral to C2Hx was greater in SVZ vs. sham rats across a wide range of conditions (e.g., quiet breathing through maximal chemoreceptor stimulation; P<0.001). Stereological analyses at 8wks post-injury indicated survival of ~50% of transplanted NPCs with ~90% of cells distributed in ipsilateral white matter at or near the injection site. Peak inspiratory phrenic bursting after NPC transplant was positively correlated with the total number of surviving cells (P<0.001). Immunohistochemistry confirmed an astrocytic phenotype in a subset of the transplanted cells with no evidence for neuronal differentiation. We conclude that intraspinal transplantation of SVZ-derived NPCs can improve respiratory recovery following high cervical SCI.

"Targeting astrocytes in CNS injury and disease: A translational research approach"

Astrocytes are a major constituent of the central nervous system. These glia play a major role in regulating blood-brain barrier function, the formation and maintenance of synapses, glutamate uptake, and trophic support for surrounding neurons and glia. Therefore, maintaining the proper functioning of these cells is crucial to survival. Astrocyte defects are associated with a wide variety of neuropathological insults, ranging from neurodegenerative diseases to gliomas. Additionally, injury to the CNS causes drastic changes to astrocytes, often leading to a phenomenon known as reactive astrogliosis. This process is important for protecting the surrounding healthy tissue from the spread of injury, while it also inhibits axonal regeneration and plasticity. Here, we discuss the important roles of astrocytes after injury and in disease, as well as potential therapeutic approaches to restore proper astrocyte functioning.

Fibroblast growth factor-2 up-regulates the expression of nestin through the Ras-Raf-ERK-Sp1 signaling axis in C6 glioma cells

Nestin is a 240-kDa intermediate filament protein expressed mainly in neural and myogenic stem cells. Although a substantial number of studies have focused on the expression of nestin during development of the central nervous system, little is known about the factors that induce and regulate its expression. Fibroblast growth factor-2 (FGF-2) is an effective mitogen and stimulates the proliferation and differentiation of a subset of nestin-expressing cells, including neural progenitor cells, glial precursor cells, and smooth muscle cells. To assess whether FGF-2 is a potent factor that induces the expression of nestin, C6 glioma cells were used. The results showed that nestin expression was up-regulated by FGF-2 via de novo RNA and protein synthesis. Our RT-PCR results showed that C6 glioma cells express FGFR1/3, and FGFRs is required for FGF-2-induced nestin expression. Further signaling analysis also revealed that FGF-2-induced nestin expression is mediated through FGFR-MAPK-ERK signaling axis and the transcriptional factor Sp1. These findings provide new insight into the regulation of nestin in glial system and enable the further studies on the function of nestin in glial cells.

Identification and cytoprotective function of a novel nestin isoform, Nes-S, in dorsal root ganglia neurons

In this study, the first nestin isoform, Nes-S, was identified in neurons of dorsal root ganglia (DRG) of adult rats. Nes-S cannot form filaments by itself in cytoplasmic intermediate filament-free SW13 cells. Instead, it co-assembles into filaments with vimentin when transfected into vimentin(+) SW13 cells, and with peripherin and neurofilament proteins when transfected into N2a cells. In primary DRG neurons, endogenous Nes-S co-assembles with peripherin and neurofilament proteins. The expression of Nes-S first appears in DRG at postnatal day 5 and persists to adulthood. Among the adult tissues we examined, the expression of Nes-S is restricted to the sensory and motor neurons. Finally, exogenous Nes-S enhances viability when transfected into N2a cells, and knockdown of endogenous Nes-S impairs the survival of DRG neurons in primary cultures. Taken together, Nes-S is a new neuronal intermediate filament protein that exerts a cytoprotective function in mature sensory and motor neurons.

ErbB2 activation contributes to de-differentiation of astrocytes into radial glial cells following induction of scratch-insulted astrocyte conditioned medium

Radial glial cells play a significant role in the repair of spinal cord injuries as they exert critical role in the neurogenesis and act as a scaffold for neuronal migration. Our previous study showed that mature astrocytes of spinal cord can undergo a de-differentiation process and further transform into pluripotential neural precursors; the occurrence of these complex events arise directly from the induction of diffusible factors released from scratch-insulted astrocytes. However, it is unclear whether astrocytes can also undergo rejuvenation to revert to a radial glial progenitor phenotype after the induction of scratch-insulted astrocytes conditioned medium (ACM). Furthermore, the mechanism of astrocyte de-differentiation to the progenitor cells is still unclear. Here we demonstrate that upon treating mature astrocytes with ACM for 10 days, the astrocytes exhibit progressive morphological and functional conversion to radial glial cells. These changes include the appearance of radial glial progenitor cells, changes in the immunophenotypical profiles, characterized by the co-expression of nestin, paired homeobox protein (Pax6) and RC2 as well as enhanced capability of multipotential differentiation. Concomitantly, ErbB2 protein level was progressively up-regulated. Thereby these results provide a potential mechanism by which ACM could induce mature astrocytes to regain the profile of radial glial progenitors due to activating the ErbB2 signaling pathways.

The expression of nestin delineates skeletal muscle differentiation in the developing rat esophagus

The muscularis externa of the developing rodent esophagus is initially composed of smooth muscle, and later replaced by skeletal muscle in a craniocaudal progression. There is growing evidence of distinct developmental origins for esophageal smooth and skeletal muscles. However, the identification of skeletal muscle progenitor cells is controversial, and the detailed cell lineage of their descendants remains elusive. In the current study, we carried out multiple labeling immunofluorescence microscopy of nestin and muscle type-specific markers to characterize the dynamic process of rat esophageal myogenesis. The results showed that nestin was transiently expressed in immature esophageal smooth muscle cells in early developing stages. After nestin was downregulated in smooth muscle cells, a distinct population of nestin-positive cells emerged as skeletal muscle precursors. They were mitotically active, and subsequently co-expressed MyoD, followed by the embryonic and later the fast type of skeletal muscle myosin heavy chain. Thus, the cell lineage of esophageal skeletal muscle differentiation was established by an immunotyping approach, which revealed that skeletal myocytes arise from a distinct lineage rather than through transdifferentiation of smooth muscle cells during rat esophageal myogenesis.

Nervous-tissue-specific elimination of microtubule-actin crosslinking factor 1a results in multiple developmental defects in the mouse brain

The microtubule-actin crosslinking factor 1 (MACF1) is a ubiquitous cytoskeletal linker protein with multiple spliced isoforms expressed in different tissues. The MACF1a isoform contains microtubule and actin-binding regions and is expressed at high levels in the nervous system. Macf1-/- mice are early embryonic lethal and hence the role of MACF1 in the nervous system could not be determined. We have specifically knocked out MACF1a in the developing mouse nervous system using Cre/loxP technology. Mutant mice died within 24-36h after birth of apparent respiratory distress. Their brains displayed a disorganized cerebral cortex with a mixed layer structure, heterotopia in the pyramidal layer of the hippocampus, disorganized thalamocortical and corticofugal fibers, and aplastic anterior and hippocampal commissures. Embryonic neurons showed a defect in traversing the cortical plate. Our data suggest a critical role for MACF1 in neuronal migration that is dependent on its ability to interact with both microfilaments and microtubules.

The glial nature of embryonic and adult neural stem cells

Glial cells were long considered end products of neural differentiation, specialized supportive cells with an origin very different from that of neurons. New studies have shown that some glial cells--radial glia (RG) in development and specific subpopulations of astrocytes in adult mammals--function as primary progenitors or neural stem cells (NSCs). This is a fundamental departure from classical views separating neuronal and glial lineages early in development. Direct visualization of the behavior of NSCs and lineage-tracing studies reveal how neuronal lineages emerge. In development and in the adult brain, many neurons and glial cells are not the direct progeny of NSCs, but instead originate from transit amplifying, or intermediate, progenitor cells (IPCs). Within NSCs and IPCs, genetic programs unfold for generating the extraordinary diversity of cell types in the central nervous system. The timing in development and location of NSCs, a property tightly linked to their neuroepithelial origin, appear to be the key determinants of the types of neurons generated. Identification of NSCs and IPCs is critical to understand brain development and adult neurogenesis and to develop new strategies for brain repair.

Circumventricular organs: a novel site of neural stem cells in the adult brain

Neurogenesis in the adult mammalian nervous system is now well established in the subventricular zone of the anterolateral ventricle and subgranular zone of the hippocampus. In these regions, neurons are thought to arise from neural stem cells, identified by their expression of specific intermediate filament proteins (nestin, vimentin, GFAP) and transcription factors (Sox2). In the present study, we show that in adult rat and mouse, the circumventricular organs (CVOs) are rich in nestin+, GFAP+, vimentin+ cells which express Sox2 and the cell cycle-regulating protein Ki67. In culture, these cells proliferate as neurospheres and express neuronal (doublecortin+, beta-tubulin III+) and glial (S100beta+, GFAP+, RIP+) phenotypic traits. Further, our in vivo studies using bromodeoxyuridine show that CVO cells proliferate and undergo constitutive neurogenesis and gliogenesis. These findings suggest that CVOs may constitute a heretofore unknown source of stem/progenitor cells, capable of giving rise to new neurons and/or glia in the adult brain.

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.

Oligodendrocytes and radial glia derived from adult rat spinal cord progenitors: morphological and immunocytochemical characterization

Self-renewing, multipotent neural progenitor cells (NPCs) reside in the adult mammalian spinal cord ependymal region. The current study characterized, in vitro, the native differentiation potential of spinal cord NPCs isolated from adult enhanced green fluorescence protein rats. Neurospheres were differentiated, immunocytochemistry (ICC) was performed, and the positive cells were counted as a percentage of Hoescht+ nuclei in 10 random fields. Oligodendrocytes constituted most of the NPC progeny (58.0% of differentiated cells; 23.4% in undifferentiated spheres). ICC and electron microscopy (EM) showed intense myelin production by neurospheres and progeny. The number of differentiated astrocytes was 18.0%, but only 2.8% in undifferentiated spheres. The number of differentiated neurons was 7.4%, but only 0.85% in undifferentiated spheres. The number of differentiated radial glia (RG) was 73.0% and in undifferentiated spheres 80.9%. EM showed an in vitro phagocytic capability of NPCs. The number of undifferentiated NPCs was 32.8% under differentiation conditions and 78.9% in undifferentiated spheres. Compared with ependymal region spheres, the spheres derived from the peripheral white matter of the spinal cord produced glial-restricted precursors. These findings indicate that adult rat spinal cord ependymal NPCs differentiate preferentially into oligodendrocytes and RG, which may support axonal regeneration in future trials of transplant therapy for spinal cord injury.

Role of radial glia in cytogenesis, patterning and boundary formation in the developing spinal cord

Radial glial fibres provide a transient scaffold and impose constraints in the developing central nervous system (CNS) that facilitate cell migration and axon growth. Recent reports have raised doubts about the distinction between radial glia and precursor cells by demonstrating that radial glia are themselves neuronal progenitor cells in the developing cortex, indicating a dual role for radial glia in both neurogenesis and migration guidance. Radial glia shift toward exclusive generation of astrocytes after neurogenesis has ceased. Radial progenitor cell differentiation and lineage relationships in CNS development are complex processes depending on genetic programming, cell-cell interaction and microenvironmental factors. In the spinal cord, radial cells that arise directly from the neuroepithelium have been identified. At least in the spinal cord, these radial cells appear to be the precursors to radial glia. It remains unknown whether radial glial cells or their precursors, the radial cells, or both can give rise to neurons in the spinal cord. Radial glial cells are also important in regulating the axon out-growth and pathfinding processes that occur during white matter patterning of the developing spinal cord.

Radial glial cells derived from the neonatal rat spinal cord: morphological and immunocytochemical characterization

Radial glial cells are transiently bipolar cells in the developing central nervous system, best known for their role in guiding migrating neurons. The aim of the present study was to investigate phenotypic characteristics of these bipolar precursor cells in a mixed glial cell culture system derived from the rat neonatal spinal cord. Morphological characterization was assessed by cell-specific immunocytochemical markers (nestin, vimentin, 3CB2) and transmission electron microscopy. Our study yielded substantial evidence showing that the bipolar cells exhibit immunocytochemical and ultrastructural features of radial glial cells. Immunohistochemistry of the neonatal rat spinal cord using the same cell-specific markers suggested these cells are likely derived from the subependymal zone, ventral commissure, and dorsomedial septum. We believe our data recommend this mixed glial culture system to be a valuable tool in studying radial glial cells in vitro.

Gliogenic radial glial cells show heterogeneity in the developing mouse spinal cord

The central nervous system of the mammalian embryo is organized according to the expression of region-specific transcription factors along the anteroposterior and/or the dorsoventral axis. For example, the dorsal ventricular zone (VZ) of the embryonic spinal cord expresses Pax3 and Pax7, the ventral VZ expresses Pax6, and the more ventral VZ expresses Nkx2.2. Properties of neuronal precursors located in the VZ are determined by the characteristic expression patterns of these transcription factors, leading to the generation of distinct classes of neurons. Recent studies demonstrated that radial glial cells produce neurons in addition to glia during central nervous system development. Thus, neuronal precursor diversity may be dependent upon the diversity of radial glial cells. To investigate this hypothesis, we analyzed the expression of radial glial cell markers and transcription factors in the mouse embryonic spinal cord. We show that radial glial cells indeed express domain-specific transcription factor. Moreover, they varied in expression of the astrocyte-specific glutamate transporter. The region where the astrocyte-specific glutamate transporter is strongly expressed in the ventral radial glial cells is closely related to the Pax6-expressing domain, and the weakly expressing region corresponding to the Nkx2.2-expressing domain. Furthermore, dorsal radial fibers expressed ephrin-B1. Thus, different types of radial glial cells exist in different domains defined by the transcription factor expression at E12.5. We also show that this diversity continues to the gliogenic stage of radial glial cells. This raises the idea that astrocytes generated from different domains along the dorsoventral axis in the mouse spinal cord have distinct characteristics.

Differentiation of radial glia from radial precursor cells and transformation into astrocytes in the developing rat spinal cord

Radial glial cell origins and functions have been studied extensively in the brain; however, questions remain relating to their origin and fate in the spinal cord. In the present study, radial glia are investigated in vivo using the neuroepithelial markers nestin and vimentin and the gliogenic markers GLAST, BLBP, 3CB2, and glial fibrillary acidic protein (GFAP). This has revealed heterogeneity among nestin/vimentin-positive precursor cells and suggests a lineage progression from neuroepithelial cell through to astrocyte in the developing spinal cord. A population of self-renewing radial cells, distinct from an earlier pseudo-stratified neuroepithelium, that resemble radial glial cells in morphology but do not express GLAST, BLBP, or 3CB2, is revealed. These radial cells arise directly from the spinal cord neuroepithelium and are probably the progenitors of neurons and the earliest appearing radial glial cells. GLAST/BLBP-positive radial glia first appear in the ventral cord at E14, and these cells gradually transform through one or more intermediate stages into differentiated astrocytes. Few if any neurons appear to be derived from radial glial cells, which are instead the major sources of astrocytes in the spinal cord. Evidence for the nonradial glial cell origins of some white matter astrocytes is also presented.

Spontaneous regeneration of the corticospinal tract after transection in young rats: a key role of reactive astrocytes in making favorable and unfavorable conditions for regeneration

We demonstrated the occurrence of marked regeneration of the corticospinal tract (CST) after a single transection and failure of regeneration after a repeated transection in young rats. To provide convincing evidence for the complete transection and regeneration we used retrograde neuronal double labeling. Double-labeled neurons that took up the first tracer from the transection site and the second tracer from the injection site caudal to the transection site were observed in the sensorimotor cortex. The anterograde tracing method revealed various patterns of regeneration. In the most successful cases the vast majority of regenerated fibers descended in the normal tract and terminated normally whereas a trace amount of fibers coursed aberrantly. In the less successful cases fibers descended partly normally and partly aberrantly or totally aberrantly. To clarify the role of astrocytes in determining the success or failure of regeneration we compared expression of glial fibrillary acidic protein (GFAP), vimentin and neurofilament (NF) immunoreactivity (IR) in the lesion between single and repeated transections. In either transection, astrocytes disappeared from the CST near the lesion site as early as 3 h after lesioning. However, by 24 h after a single transection, immature astrocytes coexpressing GFAP- and vimentin-IR appeared in the former astrocyte-free area and NF-positive axons crossed the lesion. By contrast, after a repeated transection the astrocyte-free area spread and NF-positive axons never crossed the lesion. It appears likely that the major sign, and possibly cause of failure of regeneration is the prolonged disappearance of astrocytes in the lesioned tract area.

Spontaneous regeneration of the corticospinal tract after transection in young rats: collagen type IV deposition and astrocytic scar in the lesion site are not the cause but the effect of failure of regeneration

In young rats the corticospinal tract regenerated after a single transection of the spinal cord with a sharp blade, but regeneration failed if the transection was repeated to make a more traumatic injury. To identify cells and associated molecules that promote or impede regeneration, we compared expression of collagen type IV, glial fibrillary acidic protein (GFAP), and vimentin immunoreactivity (IR) at the lesion sites in combination with anterograde axonal tracing between animals with two types of transection. Axonal regeneration occurred as early as 18 hours after transection; regenerating axons penetrated vessel-like structures with collagen type IV-IR at the lesion site, while reactive astrocytes coexpressing GFAP- and vimentin-IR appeared in the lesioned white matter. In contrast, when regeneration failed astrocytes were absent near the lesion. By 7 days sheet-like structures with collagen type IV-IR and astrocytic scar appeared in the lesioned white matter and persisted until the end of the observation period (31 days). On the basis of their spatiotemporal appearance, collagen type IV-IR sheet-like structures and the astrocytic scar follow, rather than cause, the failure of regeneration. The major sign, and perhaps cause, of failure of axonal regeneration is likely the prolonged disappearance of astrocytes around the lesion site in the early postinjury period.

Signaling to and from radial glia

Radial glia represent the major glial cell type in the developing CNS and perform many essential functions, which range from acting as neural precursors to providing physical substrates for newborn neurons to migrate on. Previous work has shown that cell-cell signaling is important for the development of the radial glial phenotype. In particular, signals from newborn neurons appear to contribute significantly to the formation of this cell type. In addition, radial glia may be involved in reciprocal signaling roles that contribute to regional patterning and differentiation in the developing CNS.

Morphology and differentiation of radial glia in the developing rat spinal cord

Important events underlying the proper functioning of the central nervous system (CNS) include the production, assembly, and differentiation of appropriate types and numbers of cells during development. The mechanisms that control these events are difficult to unravel because of displacement of cells from their sites of origin to their permanent locations and because of the diverse cellular composition of the CNS. As in other regions of the mammalian CNS, the two major classes of neuroglial cells in the rat spinal cord are oligodendrocytes and astrocytes. In the developing spinal cord, radial glia are prominent. In this study, radial glia in the cervical region of the spinal cord were analysed. 1,1'Dioctadecyl-3,3,3'-tetramethylindocarbocyanine perchlorate (DiI) was used to determine the morphology and distribution of radial glia during spinal cord development. The DiI labelling technique enabled locating glial precursor cells during spinal cord development. Radial fibres that extended from the central canal to the pial surface were present at embryonic days 14, 16, and 18 in the developing spinal cord. Their distribution was restricted with increasing development, and by embryonic day 20 the only remaining evidence of radial glia were short radial processes in the white matter.

The effect of type 1 astrocytes on neuronal complexity: a fractal analysis

Embryonic, ventral spinal cord neurons were grown on poly(d-lysine) (PDL) or on a monolayer of type 1 astrocytes. At various times from 6 h to 2 weeks postplating, cells were fluorescently labeled and fixed with 4% paraformaldehyde. The cell surface immunoreaction allowed visualization of neurons in their entirety, namely, cell bodies and various membranous extensions that included lamellipodia, growth cones, axons, and dendrites. Outlines were drawn for individual neurons and their fractal dimension (D) was calculated. Neurons on poly(d-lysine) reached a peak D at 3 days in vitro, 1 day later than neurons on astrocytes (2 days in vitro). The maximum D was greater for cells on poly(d-lysine) when compared with neurons on astrocytes. In a second experiment the maximum D was similar for neurons on both surfaces but neurons on PDL maintained a higher D for a much longer period than neurons on astrocytes. An examination of fluorescent images revealed that neurons on poly(d-lysine) exhibited lamellipodia and large growth cones for several days and these structures were likely responsible for the high D seen in these cells. These structures were rarely observed in neurons plated on astrocytes. Interestingly, D on both surfaces decreased to a similar value at between 1 and 2 weeks in vitro. The trend for D in these cultures, an initial increase to a peak value followed by a decrease to a stable value, is discussed in light of the chemical nature of the two surfaces and synapse formation and stabilization.

Neuroligin 3 is a vertebrate gliotactin expressed in the olfactory ensheathing glia, a growth-promoting class of macroglia

The molecular mechanisms that drive glia-glial interactions and glia-neuronal interactions during the development of the nervous system are poorly understood. A number of membrane-bound cell adhesion molecules have been shown to play a role, although the precise nature of their involvement is unknown. One class of molecules with cell adhesive properties used in the nervous system is the serine-esterase-like family of transmembrane proteins. A member of this class, a glia-specific protein called gliotactin, has been shown to be necessary for the development of the glial sheath in the peripheral nervous system of Drosophila melanogaster. Gliotactin is essential for the development of septate junctions in the glial sheath of individual and neighboring glia. Mutations that remove this protein result in paralysis and eventually death due to a breakdown in the glial-based blood-nerve barrier. To study the role of gliotactin during vertebrate nervous system development, we have isolated a potential vertebrate gliotactin homologue from mice and rat and found that it corresponds to neuroligin 3. Using a combination of RT-PCR and immunohistochemistry, we have found that neuroligin 3 is expressed during the development of the nervous system in many classes of glia. In particular neuroligin 3 is expressed in the olfactory ensheathing glia, retinal astrocytes, Schwann cells, and spinal cord astrocytes in the developing embryo. This expression is developmentally controlled such that in postnatal and adult stages, neuroligin 3 continues to be expressed at high levels in the olfactory ensheathing glia, a highly plastic class of glia that retain many of their developmental characteristics throughout life.

A 295-kDA intermediate filament-associated protein in radial glia and developing muscle cells in vivo and in vitro

The RC2 antibody is frequently used to label mouse radial glial cells in all parts of the nervous system where neuronal migration occurs during embryonic and early postnatal life. The antigen recognized by this antibody still needs to be identified. We have characterized further its localization in vivo, its expression and subcellular localization in vitro, as well as its molecular nature. Histologic investigations of whole mouse embryos reveal an equally intense expression of RC2 immunostaining in radial glial cells in brain and spinal cord and in skeletal muscle. In glial cells cultures, the RC2 antibody recognizes an epitope located on the glial cytoskeleton and identified as an intermediate filament associated protein (IFAP) at the ultrastructural level. RC2 immunostaining in those cells is strongly dependent on the presence of a serum-derived activity. Serum-removal causes a decrease of the staining while adding serum back to the cells induces reexpression of RC2 immunoreactivity. By Western blotting, we find that in intermediate filament (IF) preparations obtained from cultured cerebellar glia, the RC2 antibody recognizes a 295-kDa protein whose expression is also dependent on the presence of serum in culture medium. In developing muscle cells, RC2 immunostaining is observed from the myoblast stage and disappears after complete myotube fusion. Both in vivo and in vitro, staining is first seen as a loose capping around myoblasts nuclei and progressively concentrates into Z-disks in association with the muscle IF protein desmin. The RC2 antibody also recognizes a 295-kDa protein band in muscle tissue protein extracts. Thus, the RC2 antibody recognizes a developmentally regulated cytoskeletal protein that is expressed, like other previously identified IFAPs, by cells of the glial and myogenic lineages and whose expression in vitro seems to be controlled by a signaling mechanism known to modulate astroglial morphology.

Astrocytes regulate the developmental appearance of GABAergic and glutamatergic postsynaptic currents in cultured embryonic rat spinal neurons

The effects of astrocytes on the emergence of synaptic transients and excitable membrane properties in cultured, embryonic, rat ventral spinal neurons were studied with electrical and optical recording techniques. Neurons on astrocytes had significantly longer neurites and an accelerated rate of growth in surface membrane during the initial 24 h in culture compared to neurons on poly-D-lysine (PDL). GABAergic (GABA, gamma-aminobutyric acid) and glutamatergic transients appeared spontaneously in co-cultured neurons by 24 h. GABAergic quanta did not appear in neurons on PDL until 4 days in culture, and glutamatergic transients did not emerge until 7 days in culture. Astrocyte-conditioned medium (ACM) partially mimicked the effects of direct astrocytic contact. GABAergic transients appeared by 2 days, and glutamatergic signals by 4 days in neurons on PDL exposed to ACM. All of the spontaneous, synaptic-like transients were eliminated by tetrodotoxin or Ca2+o-free saline, implicating voltage-dependent cation channels in their generation. Astrocytes immediately and significantly increased the density of voltage-dependent Na+ currents compared to neurons on PDL, but by the end of 24 h, Na+ current densities were identical. Electrophysiological and optical recording revealed comparable densities of high-voltage-activated (HVA) Ca2+ currents on both co-cultured neurons and neurons on PDL throughout the first week. However, neurons on astrocytes had significantly greater contributions of P/Q-type currents and lesser contributions of L-type currents beginning at 24 h and continuing for 7 days. The contribution of N-type current was significantly more in co-cultured neurons only at 24 h. Thus, in vitro, astrocytes help to differentiate specific excitable membrane properties in spinal neurons, along with GABAergic and glutamatergic forms of synaptic transmission.

A rod end deletion in the intermediate filament protein nestin alters its subcellular localization in neuroepithelial cells of transgenic mice

Neuroepithelial and radial glial cells span between the ventricular and the pial surfaces of the neural tube and express two intermediate filaments (IFs), nestin and vimentin, which form a filamentous network throughout the length of the cells. In this report we study the polymerization characteristics of nestin and examine how mutations affect the assembly and localization of the nestin protein in cultured cells and in the developing CNS of transgenic mice. A wild-type rat nestin gene transfected into the IF-free SW13 cell line failed to assemble into a filamentous network but was incorporated into the existing IF network of a subclone expressing vimentin, demonstrating that nestin requires vimentin for proper assembly. In transgenic mice, rat nestin formed a network indistinguishable from that formed by endogenous nestin and vimentin, but a mutant form lacking five amino acids at the carboxy terminus of the rod domain was largely restricted to the pial endfeet. Since nestin mRNA is localized to the pial endfoot region we propose that both transgenes are translated there, but that the wild-type protein is preferentially incorporated into the IF network. These observations provide evidence for hierarchical assembly and a complex organization of the IF network along the ventricular-pial axis in the early CNS.

Astrocytes regulate developmental changes in the chloride ion gradient of embryonic rat ventral spinal cord neurons in culture

1. Embryonic rat ventral spinal cord neurons were dissociated at day 15 and grown on: (i) poly-D-lysine (PDL); (ii) a confluent monolayer of type I astrocytes; or (iii) PDL in astrocyte-conditioned medium (ACM) to examine the influence of astroglia on the regulation of GABAA receptor/Cl- channel properties. 2. Potentiometric oxonol dye recordings of intact cells indicated that embryonic neurons were uniformly depolarized by muscimol. The depolarizing effects disappeared in cells dissociated during the early postnatal period and recovered in culture for 24 h. Similar recordings using the calcium-imaging dye fura-2 AM revealed that GABA or muscimol triggered a sustained rise in cytosolic Ca2+ (Ca2+c ) in embryonic neurons that was dependent on extracellular Ca2+, blocked by bicuculline and nifedipine and sensitive to changes in extracellular chloride. The incidence and amplitude of the Ca2+ response decreased with time in vitro and was accelerated in neurons cultured on astrocytes compared with those on PDL. 3. Perforated patch-clamp recordings revealed that GABA depolarized neurons in a Cl--dependent and bicuculline-sensitive manner. Both the resting membrane potential and the GABA equilibrium potential became more hyperpolarized with time in vitro. 4. Astrocytes and ACM accelerated the transformation of GABAergic potential responses from depolarizing to hyperpolarizing. The change occurred over the first 4 days in co-culture or in ACM but took more than 2 weeks in neurons cultured on PDL alone. 5. The intrinsic, elementary properties of GABAA receptor/Cl- channels including open time and unitary conductance changed independently of the presence of astrocytes or ACM. Mean open time of the dominant kinetic component decreased and conductance increased with time in vitro. 6. In sum, astrocytes accelerate the developmental change in the Cl- ion gradient extrinsic to GABAA receptor/Cl- channels, which is critical for triggering Ca2+ entry, without influencing parallel changes in the intrinsic properties of the channels.

Border controls at the mammalian spinal cord: late-surviving neural crest boundary cap cells at dorsal root entry sites may regulate sensory afferent ingrowth and entry zone morphogenesis

Boundary caps (BCs) form when neural crest cells, migrating ventrally alongside the neural tube, arrest at sites where axons will enter and exit. However, nothing is known of their subsequent fate and functions. We have found late-surviving neural crest BC cell clusters at proximal dorsal root entry sites throughout rat spinal cord development. Sensory afferents cross BCs to enter the spinal cord, while exiting astrocyte processes, destined to form the dorsal root entry zone (DREZ) after birth, are temporarily stalled in their vicinity. To test whether contact with BC cells influences neurite outgrowth from dorsal root ganglia, neurons were cultured on embryonic dorsal root/spinal cord cryosections. Neurites that entered CNS territory preferentially extended over BC cells. Thus, BC cells could be instrumental in regulating afferent ingrowth and DREZ morphogenesis in mammalian spinal cord development.

Maturation of the mammalian dorsal root entry zone--from entry to no entry

Interfaces between glial cell precursors of the PNS and CNS are established early in development and form the sites where sensory axons enter and motor axons exit the developing CNS. The molecular and cellular interactions that lead to the formation of these glial interfaces are only now becoming apparent. New in-vitro techniques are providing clues as to how the maturation of PNS-CNS glial interfaces generates barriers to regenerating axons.

Transient increase of tenascin-C in immature hippocampus: astroglial and neuronal expression

In the present report we describe the anatomical localization of cells expressing tenascin-C, an extracellular matrix glycoprotein, in the hippocampal complex of developing rats. We report a development-dependent down regulation of both tenascin-C protein and mRNA. The highest levels of expression of tenascin-C was observed in rat pups from embryonic day 18 to postnatal day 7. Double labelling experiments performed with a tenascin-C antibody or tenascin-C probes combined with specific markers of astrocytes (GFAP) or neurons (MAP2 and Tau) allowed us to demonstrate that tenascin-C is expressed by both immature astrocytes and neurons in immature hippocampus. The temporal and topographic distribution of cells expressing tenascin-C (in the hilus and the stratum oriens of CA3) correlate with the localization and period of migration and maturation of post-mitotic cells. In view of these data we discuss the hypothesis that tenascin-C, as a mediator of neuron-glia interactions, may contribute to the development of hippocampal cells.

Cytoskeleton gradients in three dimensions during neurulation in the rabbit

Morphogenetic movements leading to the formation of the neural tube and cellular differentiation leading to neuronal and glial cell lineages are both part of early development of the vertebrate nervous system. In order to analyze the degree of overlap between these processes, cellular differentiation during the shaping of the neural plate is investigated immunohistochemically by using monoclonal intermediate filament protein antibodies and the 7.5-8.0-day-old rabbit embryo as a model. Western blotting is used to confirm the specificity of the antibodies, which include a new monoclonal vimentin antibody suitable for double-labeling in combination with monoclonal cytokeratin (and fibronectin) antibodies. Starting in the early somite embryo and concomitant with neural plate folding, a gradual loss of cytokeratin 8 (and 18) expression in the neuroepithelium is mirrored by a gain in vimentin expression with partial coexpression of both proteins. At the prospective rhombencephalic and spino-caudal levels, vimentin expression, in particular, changes (i.e., increases) along gradients in three dimensions: along the longitudinal axis of each neuroepithelial cell from basal to apical, in the transverse plane of the embryo from dorsolateral to ventromedial and along the craniocaudal axis from prospective rhombencephalic toward spino-caudal levels of the neural plate. At the prospective mes- and prosencephalic levels, the expression change also proceeds from basal to apical within each neuroepithelial cell, but along the other axes described here, the progress in expression change is more complex. Although the functional meaning of these highly ordered expression changes is at present unclear, the gradients suggest a novel pattern of neuroepithelial differentiation which may be functionally related to the process of interkinetic nuclear migration (Sauer [1935] J. Comp. Neurol. 62:377-402) and which partially coincides with the morphogenetic movements involved in the shaping of the neural plate.

Neuropeptide receptors in developing and adult rat spinal cord: an in vitro quantitative autoradiography study of calcitonin gene-related peptide, neurokinins, mu-opioid, galanin, somatostatin, neurotensin and vasoactive intestinal polypeptide receptors

A number of neuroactive peptides including calcitonin gene-related peptide (CGRP), substance P, neurokinin B, opioids, somatostatin (SRIF), galanin, neurotensin and vasoactive intestinal polypeptide (VIP) have been localized in adult rat spinal cord and are considered to participate either directly and/or indirectly in the processing of sensory, motor and autonomic functions. Most of these peptides appear early during development, leading to the suggestion that peptides, in addition to their neurotransmitter/neuromodulator roles, may possibly be involved in the normal growth and maturation of the spinal cord. To provide an anatomical substrate for a better understanding of the possible roles of peptides in the ontogenic development of the cord, we investigated the topographical profile as well as variation in densities of [125I]hCGRP alpha, [125I]substance P/neurokinin-1 (NK-1), [125I]eledoisin/neurokinin-3 (NK-3), [125I]FK 33-824 ([D-Ala2, Me-Phe4, Met(O)ol5]enkephalin)/mu-opioid, [125I]galanin, [125I]T0D8-SRIF14 (an analog of somatostatin); [125I]neurotensin and [125I]VIP binding sites in postnatal and adult rat spinal cord using in vitro quantitative receptor autoradiography. Receptor binding sites recognized by each radioligand are found to be distributed widely during early stages of postnatal development and then to undergo selective modification to attain their adult profile of distribution during the third week of postnatal development. The apparent density of various receptor sites, however, are differently regulated depending on the lamina and the stage of development studied. For example, the density of mu-opioid binding sites, following a peak at postnatal day 4 (P4), declines gradually in almost all regions of the spinal cord with the increasing age of the animal. [125I]substance P/NK-1 binding sites, on the other hand, show very little variation until P14 and then subsequently decrease as the development proceeds. In the adult rat, most of these peptide receptor binding sites are localized in relatively high amounts in the superficial laminae of the dorsal horn. To varying extents, moderate to low density of various peptide receptor binding sites are also found to be present in the ventral horn, intermediolateral cell column and around the central canal. Taken together, these results suggest that each receptor-ligand system is regulated differently during development and may each uniquely be involved in cellular growth, differentiation and in maturation of the normal neural circuits of the spinal cord. Furthermore, the selective localization of various receptor binding sites in adult rat spinal cord over a wide variety of functionally distinct regions reinforces the neurotransmitter/modulator roles of these peptides in sensory, motor and autonomic functions associated with the spinal cord.

Glial domains and axonal reordering in the chiasmatic region of the developing ferret

This study has examined the developing glial architecture of the optic pathway and has related this to the changing organization of the constituent axons. Immunocytochemistry was used to reveal the distribution of glial profiles, and DiI was used to label either radial glial profiles or optic axons. Electron microscopy was used to determine the distribution of glial profiles, axons, growth cones, and wrists at different locations along the pathway. Three different glial boundaries were defined: Two of these are revealed as changes in the distribution of vimentin-immunoreactive profiles occurring in the prechiasmatic optic nerve and at the threshold of the optic tract, respectively, and one by the presence of glial fibrillary acidic protein (GFAP)-immunoreactive profiles at the chiasmatic midline. The latter, midline boundary may be related to the segregation of nasal from temporal optic axons. The boundary at the threshold of the optic tract coincides with the segregation of dorsal from ventral optic axons that emerges at this location in the pathway. The segregation of old from young optic axons is shown to occur only gradually along the pathway. Glial profiles are most frequent in the deeper parts of the tract, coursing parallel to the optic axons and orthogonal to their usual radial axis. These are suggested to arise from later-growing radial glial fibers that are diverted to grow amongst the older optic axons. Those glial profiles may subsequently impede axonal invasion, thus creating the chronotopic reordering by forcing the later-arriving axons to accumulate superficially.

Proteins of the intermediate filament cytoskeleton as markers for astrocytes and human astrocytomas

There is a pressing need for a more accurate system of classifying human astrocytomas, one that is based on morphologic characteristics and that could also make use of distinctive biochemical markers. However, little is known about the phenotypic characteristics of astrocytomas. Recent studies have shown that the expression of proteins comprising the intermediate filament (IF) cytoskeleton of astrocytic cells is developmentally regulated. It is our hypothesis that this changing protein profile can be used as the basis of a system for clearly and objectively classifying astrocytomas. A spectrum of human astrocytomas has been examined by immunofluorescence microscopy employing antibodies to several IF structural subunit proteins (GFAP, vimentin, and keratins) and an IF-associated protein, IFAP-300kDa. These proteins occupy unique temporal niches in the cytogenesis of the astrocytic cells: keratins in cells of the neuroectoderm; vimentin and IFAP-300kDa in radial glia and immature glia; GFAP in mature astrocytes; and vimentin in some mature astrocytes. In agreement with previous reports, our immunofluorescence studies have revealed both GFAP and vimentin in all astrocytoma specimens. Two new observations, however, are of particular interest: IFAP-300kDa is detectable in all astrocytic tumors, and the proportion of keratin-containing cells present in the astrocytomas is in direct relationship to the degree of the malignancy. Because IFAP-300kDa is not present in either normal mature or reactive astrocytes, this protein appears to represent a specific marker of transformed (malignant) astrocytes. If it is presumed that higher malignancy grades represent the most dedifferentiated cellular state of the astrocytes, the presence of keratin-containing cells is not totally unexpected, given the ectodermal (epithelial) origin of the CNS. Specific developmentally regulated proteins of the IF cytoskeleton thus appear to hold great potential as diagnostic markers of astrocytomas and as tools for investigating the biology of these tumors.