Astrocyte cell lineage. IV. Changes in the organization of microfilaments and adhesion patterns during astrocyte differentiation in culture

Crown of Microfilaments in the Extending Cytoplasmic Processes of Medulloblastoma Glial Progenitors

Microfilaments and microtubules play a part in the extension of neuronal processes but their roles in the formation of glial processes have not yet been determined. The objectives of this study were to determine the organization of microfilaments in differentiating glial progenitors (RB2 cells) and to study the effects of microfilament or microtubule disruption on process extension. Intense F-actin staining (crown of microfilaments) was observed at the leading edge of a small extending conical tip in differentiating RB2 cells, but was absent in process-bearing TE671 rhabdomyosarcoma cells. No significant difference was noted in the mean number of TE671 cells with processes treated with a microfilament disrupter from that of similarly treated controls. In contrast, a significant difference was noted in the mean number of RB2 cells with processes after microfilament disruption treatment from that of similarly treated controls. Microtubule disruption arrested extension and caused process retraction in both cell types. The results of this study demonstrate that microtubules play an equally important part in the extension and stabilization of the RB2 and TE671 processes. Moreover, the crown of microfilaments concentrated in the glial RB2 process (and not in the TE671 process) may be critical to its extension during differentiation.

Eph receptor tyrosine kinases regulate astrocyte cytoskeletal rearrangement and focal adhesion formation

EphA4 null mice have impaired astrocytic gliosis following spinal cord injury. This may be because of altered cytoskeletal regulation and is examined herein using cultured astrocytes from wildtype and EphA4 null mice. Under basal conditions EphA4 null astrocytes appeared relatively normal but following stimuli resulting in cytoskeletal rearrangement, EphA4 null cells responded more slowly. When F-actin stress fibers were collapsed using the Rho kinase inhibitor HA1077, fewer EphA4 null cells showed stress fiber collapse in response to HA1077 and recovered stress fibers more slowly following HA1077 removal. EphA4 null astrocytes were less adherent and had smaller focal adhesions, while activation of Eph receptors with ephrin-A5-Fc increased the numbers of focal adhesions in both wildtype and knockout astrocytes following serum starvation. Using scratch wound assays, EphA4 null astrocytes invading the scratch showed impaired glial fibrillary acidic protein expression, particularly in proliferative cells. Astrocytes did not express Ephexin, a major Eph-interacting Rho guanine exchange factor, but they expressed Vav proteins, with lower levels of phospho-Vav in EphA4 null compared to wildtype astrocytes. This may contribute to the slower cytoskeletal responses generally observed in the EphA4 null astrocytes. Eph receptor signaling therefore regulates astrocyte reactivity through modulation of cytoskeletal responses.

The glial design of a teleost optic nerve head supporting continuous growth

This study demonstrates the peculiarities of the glial organization of the optic nerve head (ONH) of a fish, the tench (Tinca tinca), by using immunohistochemistry and electron microscopy. We employed antibodies specific for the macroglial cells: glutamine synthetase (GS), glial fibrillary acidic protein (GFAP), and S100. We also used the N518 antibody to label the new ganglion cells' axons, which are continuously added to the fish retina, and the anti-proliferating cell nuclear antigen (PCNA) antibody to specifically locate dividing cells. We demonstrate a specific regional adaptation of the GS-S100-positive Müller cells' vitreal processes around the optic disc, strongly labeled with the anti-GFAP antibody. In direct contact with these Müller cells' vitreal processes, there are S100-positive astrocytes and S100-negative cells ultrastructurally identified as microglial cells. Moreover, a population of PCNA-positive cells, characterized as glioblasts, forms the limit between the retina and the optic nerve in a region homologous to the Kuhnt intermediary tissue of mammals. Finally, in the intraocular portion of the optic nerve there are differentiating oligodendrocytes arranged in rows. Both the glioblasts and the rows of developing cells could serve as a pool of glial elements for the continuous growth of the visual system.

The effect of dibutyryl cyclic AMP on the expression of actin isoforms in astroglia

Mammalian cells contain at least 8 actin isoforms. The functional significance and the mechanisms that regulate the expression of each actin isoform are not yet known. Using immunofluorescence staining, it was found that all astroglia in tissue culture express beta-actin isoform and about 86% of astroglia express alpha-smooth muscle actin isoform. When astroglia were treated with dibutyryl cyclic AMP for 4, 7, 14 and 21 days, it was found that the number of the cells expressing alpha-smooth muscle actin isoform progressively decreased, whereas, the number of the cells expressing beta-actin isoform remained constant. The western blot experiment showed that the amount of total alpha-smooth muscle actin isoform (soluble and insoluble) and of the insoluble isoform expressed by astroglia treated with dibutyryl cAMP decreased whereas, the amount of total and insoluble beta-actin isoform expressed by the same cells did not show any significant changes. The cells treated with the cAMP failed to migrate and to close the area created by the scratch wound in monolayer culture. However, the non-treated cells migrated and closed the area created by the scratch after 3 days. This study shows that the astroglia have different mechanisms in regulating the expression of different actin isoforms and that the alpha-sm actin isoform is important in migration of astroglia.

Upregulation of F-actin and alpha-actinin in reactive astrocytes

We have shown previously that in tissue culture stellate astrocytes downregulate F-actin and actin binding proteins (ABPs) (Abd-El-Basset et al.: J Neurosci Res 30:1-17, 1991), whereas the reactive-like astrocytes upregulate their F-actin (Fedoroff et al.: Neuroscience 22:255-266, 1987). In the present study we report that in normal brain, as in tissue culture, neither F-actin nor alpha-actinin (an ABP) could be detected in stellate astrocytes. When a stab wound was made in brain, F-actin and alpha-actinin were upregulated in reactive astrocytes. We also demonstrated that reactive-like astrocytes in tissue culture express alpha-actinin, which has a "dotted" appearance when immunostained, and is colocalized with F-actin in a specific arrangement.

Reexpression of glial fibrillary acidic protein rescues the ability of astrocytoma cells to form processes in response to neurons

Astroglial cells play an important role in orchestrating the migration and positioning of neurons during central nervous system development. Primary astroglia, as well as astrocytoma cells will extend long stable processes when co-cultured with granule neurons. In order to determine the function of the glial fibrillary acidic protein (GFAP), the major intermediate filament protein in astroglia and astrocytoma cells, we suppressed the expression of GFAP by stable transfection of an anti-sense GFAP construct in human astrocytoma U251MG cells. The resulting AS2-U251 cells can no longer extend stable processes in the presence of granule neurons. To show that this effect is due specifically to the absence of GFAP, we reintroduced a fully encoding rat brain GFAP cDNA into these AS2-U251 cells. The resulting rat GFAP appeared as a filamentous network and the reexpression of GFAP rescued the ability of these astrocytoma cells to form stable processes when co-cultured with neurons. From these results, it is clear that the glial specific intermediate filament protein, GFAP, is required for process extension of these astrocytoma cells in response to granule neurons.

Contact-spacing among astrocytes is independent of neighbouring structures: in vivo and in vitro evidence

We have examined the morphology of astrocytes and the arrays they form in two situations, in retinas from which ganglion cells and blood vessels have been caused to degenerate, and in vitro. These observations were made to test whether the regularity of the spacing of astrocytes within normal central nervous tissue results from interaction among astrocytes, or from interaction between astrocytes and other elements of that tissue. Both in the partially degenerated cat retina, and in cultures of astrocytes from neonatal rat cortex, astrocytes make and maintain contact with neighbouring astrocytes, yet space their somas apart, giving regularity to the arrays. These results support the hypothesis that the regularity observed in arrays of astrocytes in intact tissue results from an interaction among astrocytes, independent of neighbouring structures, and lead us to suggest that the cell-cell interactions involved in contact spacing serve to distribute astrocytes through the central nervous system, and may, in other tissues, underlie the formation of epithelia.

Immunocytochemical demonstration of early appearing astroglial structures that form boundaries and pathways along axon tracts in the fetal brain

During normal development of the mammalian forebrain, the paired cerebral hemispheres are initially separated midsagittally by the connective tissue-filled longitudinal fissure. During subsequent stages, the hemispheres fuse as basal lamina is remodeled and fibroblasts are eliminated from the fissure to create new central nervous system (CNS) territory in the midline. Two axon pathways, the corpus callosum and dorsal callosal stria, eventually use this region as part of their pathway. In order to assess the possible role of glial cells in the fusion process and in the guidance of axons in this and several other areas of the forebrain, we have analyzed the developing brain in timed cat and mouse embryos with immunohistochemical and morphological techniques. With the use of astroglial-specific antibodies and electron microscopy, we have visualized two distinct, primitive astroglial structures associated with the cerebral midline, and seven more associated with other specific brain regions. The way in which one of these structures moves as a column along the hemispheric midline in synchrony with seam formation suggests the possibility that during morphogenesis of the telencephalon, astrocytes may aid in the fusion process. In addition, the compact assemblage, early appearance and location of this and the other glial structures in relation to well defined neuroanatomical landmarks or axon pathways suggest that they may transiently compartmentalize relatively large regions of the CNS and organize certain developing fiber systems by acting as guides or barriers at critical stages of ontogeny.

The neuronal cytoskeleton

In this paper we have described the organization of F-actin and actin-binding proteins (ABP): alpha-actinin, myosin, tropomyosin, caldesmon, vinculin, talin, and spectrin, in differentiating astroglia in colony cultures. We observed that the microfilament (MF) network arrangements differ at various stages of astroglia development, but the composition of MF bundles and stress fibers is the same at all developmental stages. F-actin is closely colocalized with myosin, tropomyosin, caldesmon, and alpha-actinin. The striated pattern of myosin, tropomyosin, and caldesmon are superimposable. Tropomyosin and caldesmon extend along F-actin but are interrupted for short periods, whereas myosin is interrupted for longer periods. alpha-actinin colocalizes with tropomyosin and caldesmon but not with myosin. In astroglia at different stages of development spectrin is arranged in the form of fine networks spreading through the cell and does not follow the arrangement of MF bundles. Only F-actin, alpha-actinin, and vinculin can be detected at cell-cell junctions. In the areas of the focal contacts, F-actin, alpha-actinin, vinculin, and talin are present. They overlap each other, although talin and vinculin extend toward the cell membrane beyond F-actin and alpha-actinin. Astroglia undergo well-defined states of nonmotility, motility, and nonmotility again during differentiation. The changes in motility are paralleled by changes in the organization of F-actin and ABP: as GFAP-containing intermediate filaments increase in differentiating astroglia, the F-actin and ABP are down-regulated, leading to non motility.

Immunolocalization of the alpha isoform of smooth muscle actin in mouse astroglia in cultures

In mouse astroglia colony cultures, cells display dramatic changes in their 'social behavior' during differentiation. The non-motile glioblasts progress through highly motile pro-astroblast and astroblast stages before becoming non-motile fibrous astrocytes. These changes in the behavior of astroglia are paralleled by changes in the organization of their microfilaments. Immunofluorescence staining of the astroglia colony cultures with rhodamine phalloidin, which binds to actin filaments (F-actin) formed by any of the 6 actin isoforms and with monoclonal antibody to alpha-smooth muscle (alpha-sm) actin (an actin isoform typical of smooth muscle cells), shows that the majority of the cells are positive for both alpha-sm actin and F-actin. Approximately 15% of the cells, however, were negative for alpha-sm actin but stained positively for F-actin, indicating that the actin of these cells belongs to isoforms other than the alpha-sm isoform.

Simple method for the culture of glial cells from embryonic rat brain: implications for regional heterogeneity and the radial glial lineage

A simple method is described for the production of glial cell cultures from specific regions of the day 13-15 embryonic rat brainstem and midbrain based on differential cell attachment to a relatively nonadhesive substrate, which inhibits the growth of neurons. Regional differences in the ability of specific populations of brainstem and midbrain cells to attach and spread on the substrate suggest that embryonic glial populations may differ in their cell surface properties even when they derive from the same general area of the developing brain. Based on observations of the spatiotemporal distribution of radial-like glial cells and astrocytes with time in vitro, we suggest that this culture system may prove useful for investigation of the radial glial lineage.

Radioautographic evidence for slow astrocyte turnover and modest oligodendrocyte production in the corpus callosum of adult mice infused with 3H-thymidine

To find out whether glial cells proliferate in the corpus callosum of adult mice, two series of experiments were carried out. The first one made use of 9-month-old "aged" male mice. Some of them were given 3H-thymidine as a 2-hour pulse to examine which cells became labeled and, therefore, had the ability to divide. Others were sacrificed after a continuous infusion of 3H-thymidine for 30 days to examine whether the label would then appear in different cells. In other aged animals, the 30-day infusion was followed by 60 or 180 days without 3H-thymidine to determine whether cells retained or lost their label with time. A second series of experiments was carried out in 4-month old "young adult" male mice to seek confirmation of the main conclusions. Following the 3H-thymidine pulse given to aged mice, only immature glial cells were labeled. After a 30-day infusion, 12.1% astrocytes and 1.1% oligodendrocytes were labeled, so that the net daily addition rate of astrocytes averaged 0.4% and of oligodendrocytes, 0.04%. In young adult mice, the rate after a 7-day infusion averaged 0.9% for astrocytes and 0.08% for oligodendrocytes. However, when the 30-day infusion into aged mice was followed by 60 and 180 days without 3H-thymidine, the labeled astrocytes decreased to 5.3% and 0%, respectively, whereas the number of labeled oligodendrocytes did not change significantly. The interpretation of the results is that the immature cells present in the corpus callosum of mice continue dividing throughout life and their progeny give rise to astrocytes and oligodendrocytes. In the case of astrocytes, the production of new cells occurs in parallel with a loss, so that the astrocyte population turns over. In the case of oligodendrocytes, there is a small production of new, apparently stable cells.

48-Kilodalton intermediate-filament-associated protein in astrocytes

We provide evidence that a protein of 48 kilodaltons (KD), recognized by a normal rabbit serum (F2N), is associated with intermediate filaments (IF) of astrocytes both in cell cultures and in situ. Immunofluorescence staining shows that the F2N serum gives a fibrous staining pattern similar to that seen with anti-serum to glial filament protein (GFP), a protein specific for IF of astrocytes, and that both proteins are present in the perinuclear fibrous aggregates of IF produced by treating the cells with colchicine. At the ultrastructural level the gold particles decorating the 48-KD protein are localized in clusters along the IF, whereas the gold particles decorating the GFP are localized on the IF in a linear pattern. This difference in distribution and the fact that the two proteins have different electrophoretic mobilities on SDS gels indicates that the 48-KD protein although associated with IF is different from GFP. The 48-KD protein appears to be a distinct, developmentally regulated intermediate-filament-associated protein (IFAP), different from other IFAPs reported to date and the first IFAP described in astrocytes. Its appearance in late developmental stages when motile astroblasts are changing into nonmotile stellate cells suggests that the 48-KD protein may be involved in cross-linking the GFP-containing IF to provide more tensile strength to the cytoplasm at the expense of flexibility.

Expression of glial fibrillary acidic protein in rat C6 glioma relates to vimentin and is independent of cell-cell contact

Glial fibrillary acidic protein (GFAP) was induced in rat C6 glioma cells grown in M199 and HAM F10 media by addition of 1 mM dibutyryl cyclic AMP. The amount of GFAP per cell increased 7- and 33-fold in M199 and HAM F10 media, respectively. GFAP could be induced in each phase of the cell culture except for the lag phase, where GFAP synthesis was delayed until the onset of the logarithmic growth. The induction took place under conditions where the total protein content of the cell decreased. Measurement of the amount of vimentin indicated that GFAP was induced under conditions of low vimentin concentration. Our results do not support the hypothesis that GFAP induction depends on cell-cell contact or cell proliferation. They indicate a shift from vimentin to GFAP synthesis by an as yet unknown mechanism.

Organization of microfilaments in astrocytes that form in the presence of dibutyryl cyclic AMP in cultures, and which are similar to reactive astrocytes in vivo

This paper deals with changes in the arrangement of microfilaments at various stages during the transformation of astroblasts into reactive astrocytes in the presence of dibutyryl 3',5'-cyclic adenosine monophosphate in vitro. When cultures of two-week-old mouse astroblasts are treated with dibutyryl cyclic AMP, drastic changes occur in cell shape and in the organization of microfilaments, resulting in cells that closely resemble reactive astrocytes in vivo. A thick, prominent ring of microfilaments in such cells which stains strongly with 7-nitrobenz-2 oxa-1,3-diazole-phallacidin, delineates the perikaryon. Electron microscope examination showed that the ring is composed of many smaller bundles of microfilaments running parallel to each other. Prominent bundles of microfilaments radiate from the cell body into the cell processes. Based on the observation of intermediate forms, we propose that the microfilament ring may be important in the development of cell processes in reactive astrocytes.

Gradient isolation of glial cells: evidence that flat epithelial cells are astroglial cell precursors

Discontinuous gradients of metrizamide were used to separate the cell components of monolayers of primary cultures of embryonic rat brains. These primary cell cultures were of two types: long-term cultures (more than a year) of embryonic rat brain, which contained several glial cell types, and monolayers of cell cultures (several weeks old), which contained a complex population of cells, including neuronal elements. The gradient separation produces fractions of pure flat epithelial cells that are able to survive and proliferate. After a few days, all flat epithelial cells become confluent and show a positive reaction to glial fibrillary acidic protein (GFAP); this indicates that these cells astroglial precursor cells. Following their maintenance in vitro for several months, all cultures give rise to a pure population of astrocytes identified not only by their characteristic morphology, but also by their content of GFAP. It is proposed that the differentiation controls are dependent on cell interactions that are influenced by the composition of the cell population and/or the molecular growth and differentiation factors released by these cells into the medium.

The organization and solubility properties of intermediate filaments and microtubules of cortical astrocytes in culture

The organization of intermediate filaments (IF) and microtubules (MT) and the solubility of intermediate filament proteins and tubulin in astrocytes which develop from cerebral hemispheres of neonatal rats in culture were examined using immunocytochemical and immunochemical approaches. Results of immunocytochemical studies demonstrated that in flat astrocytes which develop after 3 weeks of culturing in serum-supplemented medium, the IF containing vimentin and glial fibrillary acidic protein (GFAP) are concentrated around the nucleus and dispersed in an irregular fashion throughout the cytoplasm. Astrocytes which develop in serum-free hormonally-defined medium irrespective of whether they are bipolar, multipolar or flattened, have IF organized as a fibrous network of filaments distributed from the nuclear regions to the cell periphery. Under both culture conditions, vimentin and GFAP are resistant to extraction with low salt buffer containing nonionic detergent, indicating that the different cytoplasmic distribution of IF is unrelated to the solubility properties of vimentin and GFAP. Double immunolabelling experiments with polyclonal antibody to GFAP and monoclonal antibody to each alpha-tubulin or beta-tubulin reveal an extensive codistribution and parallel organization of IF and MT in all morphological types of astrocytes studied. Stabilization of MT with taxol, or depolymerization of MT with colchicine, cause dramatic changes in the distribution of IF and inhibit the extension of astrocyte processes in response to dibutyryl cyclic AMP (dBcAMP). In early stages of treatment with dBcAMP, renewal of culture medium without dBcAMP produces a rapid and permanent retraction of astrocyte processes, whereas in later stages the processes only retract partially and are then restored and maintained for several days in the absence of dBcAMP. The retraction of processes is accompanied by changes of immunocytochemical staining of IF with antibody to GFAP, which appears more intense and diffuse. However, electrophoretic and immunoblot analyses of detergent-extracted proteins from parallel cultures demonstrate that neither the amount nor the solubility of GFAP and vimentin are changed. Detergent extraction in MT stabilizing conditions shows that a substantial proportion of tubulin in astrocytes cultured in serum-containing and serum-free media is assembled into MT, most of which depolymerize on treatment with low temperature and Ca2+. Following long exposure to dBcAMP the proportion of cold/Ca2+-stable MT increases. The results suggest that the IF of astrocytes in culture are dependent on MT with respect to their cytoplasmic distribution.(ABSTRACT TRUNCATED AT 400 WORDS)