Maturation-related gene expression of rat astroblasts in vitro studied by two-dimensional polyacrylamide gel electrophoresis

Regulation of kappa-opioid receptor mRNA level by cyclic AMP and growth factors in cultured rat glial cells

The mRNA of the kappa-opioid receptor (KOR) has been found recently in cultured astrocytes and in microglia. By using RT-PCR and Southern hybridization, we confirmed these observations and, in addition, we observed that KOR mRNA was expressed in oligodendrocytes and in the precursors of astrocytes and oligodendrocytes. KOR mRNA level was the highest in the immature astrocytes and decreased with their maturation. Very few data are available on the regulation of KOR level by extracellular signals. Therefore, we examined the effect of three growth factors known to be present in the adult brain, basic fibroblast growth factor (bFGF), platelet-derived growth factor (PDGF-BB) and leukemia inhibitory factor (LIF) and of two cyclic AMP (cAMP) generating systems, the cAMP analog, 8-(4-chlorophenylthio)-cAMP and forskolin, on this level. It was found that in astrocytes, KOR mRNA level decreased dramatically under the effect of cAMP and less under the effect of bFGF while it did not change significantly after LIF treatment. In oligodendrocytes, it also decreased with cAMP, but increased under the effect of bFGF and PDGF-BB. In microglia, a decrease was observed with cAMP and lipopolysaccharides (LPS), the most used activators of macrophages. These results shed new evidence on the expression of opioid receptor mRNA in the glial cells of the rat CNS. The regulation of KOR mRNA level under the effect of extracellular signals suggests that opioids take part in dynamic processes in glial cells, possibly related to glial-neuron communication.

Calpain inhibitors, but not caspase inhibitors, prevent actin proteolysis and DNA fragmentation during apoptosis

Apoptosis, or programmed cell death, involves a cascade of regulatory events leading to the activation of specific proteases. However, the key substrates for these proteases remain to be identified. We previously demonstrated that levels of five unidentified polypeptides were specifically increased in neurons from embryonic chicken ciliary ganglia undergoing apoptosis by trophic deprivation. Here we show by microsequencing of two of these polypeptides that they are fragments of actin. One of them represents cleavage of actin at the site of interaction with DNase I. The same actin fragments are also found at early stages of apoptosis in chicken and rat dorsal root ganglion neurons, chicken spinal motoneurons and rat thymocytes. Actin fragmentation may play a role in the apoptotic process, since calpain inhibitors I and II both inhibit neuronal death and suppress actin fragmentation. In contrast, caspase (ICE family) inhibitors, though effective in delaying neuronal death, do not prevent actin cleavage or DNA fragmentation. These results indicate a key role for calpain-like proteases in neuronal programmed cell death and suggest that actin fragmentation in the cell is correlated with subsequent DNA fragmentation.

Glucocorticoids enhance the potency of Schwann cell mitogens

Previous studies have documented that cultured Schwann cells require serum-containing medium to respond maximally to mitogens. We now report that Schwann cells are able to proliferate to a mitogenic response in a serum-free defined medium termed oligodendrocyte defined media (ODM). Glucocorticoids are the essential component of ODM which allow Schwann cell proliferation in the serum-free medium. Charcoal treatment of the fetal calf serum decreases the mitogenic potency of the axolemma-enriched fraction (AEF) by 50%. The addition of 2 microM hydrocortisone to charcoal-treated fetal calf serum restores 75% of the lost mitogenicity. These observations are consistent with the view that glucocorticoids present in fetal calf serum are potent co-mitogens essential for AEF-induced Schwann cell proliferation. The synthetic glucocorticoid, dexamethasone, is a more potent co-mitogen than hydrocortisone, with a maximal effect at concentrations less than 10 nM. In contrast, other steroids including aldosterone, progesterone, testosterone, and 17 beta-estradiol have no effect on enhancing the mitogenic response of Schwann cells to the AEF. The glucocorticoid antagonists RU 486 and dehydroepiandrosterone (DHEA), but not the antiestrogenic compound tamoxifen, block AEF-induced Schwann cell proliferation. These results suggest that glucocorticoid-induced Schwann cell proliferation is mediated through a glucocorticoid receptor (GR) mechanism. We detected immunoreactivity to the GR in the cytoplasm, but not in the nuclei of Schwann cells grown in ODM lacking dexamethasone. The addition of 100 nM dexamethasone to these cultures resulted in immunoreactivity in the nucleus. This data suggests that glucocorticoids working through the GR are potent co-mitogens for Schwann cell proliferation.

FGFs stimulate IGF binding protein synthesis without affecting IGF synthesis in rat astroblasts in primary culture

The production of insulin-like growth factor (IGF)-I and -II and their binding proteins (BPs) has been studied in new-born rat astroblasts at confluency in primary culture. Under the influence of fibroblast growth factors (FGFs) (acidic and basic), the morphology of the astroblasts was altered, 125I-deoxyuridine incorporation was increased, and glutamine synthetase activity was stimulated. IGF production and IGF mRNA expression remained unchanged. Production of the 32 kDa BP (IGFBP-2), the sole or predominant form under base-line conditions, was enhanced and the 43-39 kDa forms (IGFBP-3) appeared or were increased. Epidermal growth factor (EGF) also stimulated production of these BPs, whereas thrombin and db-cAMP had no effect. Our data suggest that a relationship exists between FGF-induced maturation of astroblasts and the forms of BP they produce. The data also indicate that some factors may act specifically on BP synthesis, without affecting IGF synthesis, and in this way play a role in regulating the bioavailability of the IGFs.

Acidic and basic fibroblast growth factors similarly regulate the rate of biosynthesis of rat astroblast proteins

Quiescent rat astroblasts in culture have been treated for various periods of time with acidic and basic fibroblast growth factors. Both factors elicited similar effects on the cell proliferation and glutamine synthetase activity. The rate of biosynthesis of the proteins analyzed on autoradiograms of polyacrylamide gels after two-dimensional electrophoresis was also similarly modulated by the two growth factors. These results suggest that the two fibroblast growth factors act through the same membrane receptors on rat astroblasts in culture.