Identification of a collagen potentiated neurite promoting factor isolated from C6 glioma cells

A quantitative bioassay for nerve growth factor, using PC12 clones expressing different levels of trkA receptors

Nerve growth factor (NGF) is a neurotrophin required for differentiation, development, and survival of the sympathetic nervous system, with many of its biological effects being mediated via trkA receptors. There is a need for a standard quantitative bioassay for NGF, to be used in basic research and in pharmaceutical studies. The objective of the present research was to develop a selective, quantitative, and reliable bioassay for NGF, using a morphological criterion: neurite cell outgrowth. In addition, we aimed to apply the aforementioned bioassay to measure NGF administered to mice. Pheochromocytoma PC12 cell variants including wild-type cultures, and a trkA-overexpressing stable transfectant PC12-6.24-I, PC12nnr5, and PC12EN lacking trkA receptors, were used. Dose-response curves were generated with NGF beta-subunit (2.5S) purified from mouse submaxillary glands. Our results demonstrated that the bioassay was sensitive to 0.3-20 ng/mL, and selective, as neurite outgrowth was not seen by any other growth factor other than NGF. In addition, variant clones PC12nnr5 and PC12EN, lacking trkA receptors, did not respond to NGF. The bioassay detected NGF in serum of mice injected with NGF. This novel developed bioassay can serve as a model system for various neuroscience purposes.

Dexamethasone enhances serum deprivation-induced necrotic death of rat C6 glioma cells through activation of glucocorticoid receptors

Glucocorticoids have been shown to be neurotoxic and appear to play a role in neuronal cell loss during aging and following neuropathological insults. However, very little is known about the effects of these steroid hormones on glial cells. The effect of the synthetic glucocorticoid dexamethasone (DEX) on glial cell viability was therefore examined by measuring neutral red uptake into rat C6 glioma cells. Serum deprivation markedly reduced cell viability, and this effect was significantly enhanced by DEX. Electrophoretic analysis showed that the cell damage induced by either serum deprivation alone or in combination with DEX was not accompanied by the degradation of DNA into nucleosomic fragments. Electron microscopic studies confirmed that serum deprivation and glucocorticoid treatment caused necrotic cell death. Furthermore, the effect of DEX on cell viability could be mimicked by the glucocorticoid receptor agonist RU28362, and completely prevented by the glucocorticoid receptor antagonist RU38486. These results indicate that dexamethasone can enhance the necrotic death of glioma cells induced by serum deprivation, suggesting that glucocorticoids may be involved in the chronic alteration of brain function arising from neuropathological damage to glial cells.

Adaptation of C6 glioma cells to serum-free conditions leads to the expression of a mixed astrocyte-oligodendrocyte phenotype and increased production of neurite-promoting activity

C6 cells were adapted to proliferate in defined culture medium to allow the study of long-term effects of serum-free growth conditions on their phenotypic antigen expression and production of neurite promoting factors (NPFs). Cultures were grown in either Ham's F-12 or supplemented Opti-MEM-I containing 15% heat-inactivated horse serum and 2.5% fetal calf serum (serum-containing) or in supplemented Opti-MEM-I alone (serum-free). Immunocytochemical and immunofluorescence techniques were used to determine the antigenic expression of A2B5, galactocerebroside (GalC), and glial fibrillary acidic protein (GFAP) in passage matched and sister cultures of serum and serum-free grown C6 cells. When C6 cells were grown under serum-containing conditions, two populations of cells were seen: young oligodendrocytes (A2B5+, GFAP-, GalC+), and mixed astrocyte-oligodendrocyte phenotype (A2B5+, GFAP+, GalC+). After adaptation of the C6 cells to serum-free conditions over 2-3 passages, only one population of cells was observed, the mixed astrocyte-oligodendrocyte phenotype. The serum-free conditions also resulted in greater staining of the C6 cells. Conditioned media from the two growth conditions were fractionated by ultrafiltration into two fractions: components > 50 kDa and components of 10-50 kDa. The amount of neurite promoting activity seen between the two culture conditions resulted in a 3-fold increase in NPF activity under serum-free conditions in the > 50 kDa fraction. The 10-50 kDa fraction only expressed NPF activity if obtained from the serum-grown C6 cells. This alteration in NPF activity appears to be the result of the phenotypical alteration of the C6 cells, and may suggest that the NPF activities from the two culture conditions may not be identical.

Intracellular increases of cAMP induce opposite effects in glutamic acid decarboxylase (GAD67) and glial fibrillary acidic protein immunoreactivities in C6 cells

C6 is a cell line that expresses glial and neuronal markers. Treatments that increase intracellular cAMP levels induce the differentiation of these cells. We had previously demonstrated that forskolin, an agent that activates adenylate cyclase, produced changes in gene expression in C6 cells. As a consequence of this treatment, glutamic acid decarboxylase (GAD) activity and the mRNA for GAD67, one of the isoforms of the enzyme, decreased. In contrast, this treatment increased the transcription of the glial fibrillary acidic protein (GFAP) gene. We now show, by immunocytochemistry, that the changes in gene expression are phenotypically reflected by corresponding changes in the levels of the proteins encoded by the GAD67 and GFAP genes. Computer-assisted image analysis demonstrated that both the increase in GFAP immunofluorescence, and the decrease in GAD67 immunofluorescence are statistically significant. The changes in gene expression and in protein immunoreactivity are part of the differentiation process of the C6 cells towards a more mature glial phenotype.