Rapid and gentle extraction, reconstitution and characterization of microfilament and glia filament from rat astrocytes

Sialosylcholesterol induces reorganization of astrocyte filament network

Sialosylcholesterol induces the differentiation of astrocytes with respect to their morphological appearance (Kato et al., Brain Res. 438 (1988) 277-285; Ito et al., 481 (1989) 335-343), while in a cell-free condition it depolymerizes the astrocyte cellular filaments, the glia filaments and microfilaments (Ito et al., J. Neurochem. 61 (1993) 80-84). To solve this paradox, we examined hetero-interaction between the glia filaments and microfilaments in the presence of sialosylcholesterol. Each filament was prepared in a depolymerized form in low ionic strength, and was adjusted to physiological ionic strength to prevent from repolymerization by sialosylcholesterol. When the two filament preparations in this form were mixed, repolymerization took place in spite of the presence of sialosylcholesterol. The filament formed in the mixture was found almost exclusively composed of vimentin and actin, the major component of the glia filaments and microfilaments preparation, respectively. An excess amount of vimentin over actin in the precipitate implicated that the main mechanism for the hetero-polymerization was the enhancement of vimentin polymerization by actin. To support this view, pre-polymerization of the microfilaments before mixing with the depolymerized glia filaments resulted in a marked decrease in polymerization of the glia filaments. A similar hetero-interaction was found between the purified vimentin and actin. When polymerized vimentin and actin were directly depolymerized by sialosylcholesterol and mixed, polymer formation was demonstrated between these two proteins. Electronmicroscopy indicated direct interaction of the actin filament with the vimentin filament. The results indicate that sialosylcholesterol induces reorganization of the cellular filament network, such as disorganization of vimentin and actin filaments, and provokes their hetero-interaction to form the hetero-filament. Hence, this may be one of the key mechanisms for the induction of cellular differentiation by sialocylcholesterol.

Effects of epidermal growth factor and dibutyryl cyclic adenosine monophosphate on the migration pattern of astrocytes grafted into adult rat brain

OBJECTIVE

Neonatal rat astrocytes transplanted into host brains migrate in specific patterns, which are determined by the developmental stage of the host brain and the region of implantation. We hypothesized that the differentiation state of the implanted astrocytes could also affect astrocyte migration.

METHODS

Astrocytes derived from neonatal rats (1-4 d) were placed in culture and exposed to growth- or differentiation-promoting agents (e.g., epidermal growth factor or dibutyryl cyclic adenosine monophosphate). Treated cells were then injected into different regions of the adult rat brain. At 3, 6, and 9 days after implantation, the extent and pattern of astrocyte migration after injection into the cortex, hippocampus, and corpus callosum were assessed.

RESULTS

Astrocytes pretreated with either factor did not migrate during the first 3 days after implantation into the host cortex and hippocampus, whereas untreated cells migrated extensively by Day 3. After 9 days, implanted cells that had been pretreated with dibutyryl cyclic adenosine monophosphate began to demonstrate migratory activity, whereas those exposed to epidermal growth factor remained at the site of implantation. These findings corresponded to the effects of these agents in culture. On the other hand, cells implanted into the corpus callosum migrated in spite of pretreatment.

CONCLUSION

Epidermal growth factor and dibutyryl cyclic adenosine monophosphate each altered the cells in culture such that they were inhibited from migrating after transplantation into the host cortex and hippocampus. This finding suggests that the activation of either growth or differentiation cascades partially inhibits the migratory ability in these cells either through effects on their internal migratory potentials or their responsiveness to external migratory signals. In contrast, cells implanted into the corpus callosum migrated in spite of pretreatment, suggesting that this structure may present migratory cues sufficient to override the effects of treatment.

Role of glial fibrillary acidic protein expression in the biology of human glioblastoma U-373MG cells

OBJECT

The relationship between glial fibrillary acidic protein (GFAP) expression and glial tumor cell behavior has not been well defined. The goal of this study was to examine this relationship further.

METHODS

To investigate the relationship between GFAP expression and glial tumor cell behavior, the authors isolated clones from the human glioblastoma cell line, U-373MG, according to their level of GFAP expression. Immunochemical analysis demonstrated that one clone had consistently low GFAP expression (approximately 93% of cells were GFAP negative), whereas a second clone had consistently high GFAP expression (approximately 80% of the cells were GFAP positive). The structure, population doubling time, saturation density, anchorage-independent growth, migratory rate, and invasive potential of these two clones were determined in relation to their level of GFAP expression. Morphologically, both clones were composed of ameboid as well as stellate components. Although the population doubling times of the two clones were equally rapid, the clone with low GFAP expression demonstrated a slightly higher saturation density compared with the clone with high GFAP expression. In an anchorage-independent environment (soft agar), a greater difference in growth characteristics was noted between the two clones: the high-expression clone formed more colonies and these colonies were compact, well defined, and spherical, whereas the low-expression clone formed predominantly smaller, two-dimensional colonies with vague boundaries and isolated cells or groups of cells at the periphery. In contrast to these minor differences between the clones, the low-expression clone showed a markedly increased migratory rate and invasive potential compared with the high-expression clone. Therefore, the clone with reduced GFAP expression appeared more aggressive, demonstrating decreased contact inhibition, increased migratory rate, and increased invasive potential.

CONCLUSIONS

These results suggest a direct correlation between GFAP expression and some measures of aggressive tumor growth and transformation properties.

Suppression by gangliosides of polymerization of glial cytoskeletons prepared from rat astrocytes: a role of sialic acid moiety

This study investigated in vitro the effects of gangliosides on polymerization of either the depolymerized microfilament preparation (MF) or depolymerized glia filament preparation (GF) extracted separately from the crude cytoskeletal fraction of rat astrocytes. Gangliosides GM1, GM2 and GM3 markedly suppressed polymerization of both MF and GF. The concentration of GM1, GM2 or GM3 required to induce 50% inhibition of the polymerization of 7.5 micrograms MF protein/200 microliters (IC50 of GM1, GM2, or GM3) was 3.2, 2.8 or 5.6 micrograms/200 microliters, respectively. The IC50 of each ganglioside for the polymerization of 7.5 micrograms/200 microliters of GF, furthermore, was 3.3, 3.5 or 7.4 micrograms/200 microliters, respectively, suggesting that the inhibitory activities of GM1 and GM2 on polymerization of both MF and GF were greater than those of GM3. GM1, GM2 and GM3 also suppressed dose-dependently the polymerization of both actin and vimentin. The inhibitory activities of GM1 and GM2 on the polymerization of actin or vimentin were greater than GM3, as in the case of polymerization of MF or GF. The IC50S of GD1a and GT1b for MF polymerization at the same concentration were 2.2 and 1.2 micrograms/200 microliters, respectively, and those for GF polymerization were 2.7 and 1.7 micrograms/200 microliters, respectively. The IC50 of GD3 for MF polymerization was 3.9 micrograms/200 microliters, and that for GF polymerization 4.0 micrograms/200 microliters, implying that the inhibitory activities of GD3 on polymerization of both MF and GF were greater than those of GM3. The findings suggested that the inhibitory activities of gangliosides on MF or GF polymerization became greater with increasing number of sialic acid residues. AsialoGM1 suppressed neither MF nor GF polymerization, and inhibited dose-dependently the ability of GM1 to suppress MF polymerization.