Increased 5'-nucleotidase activity induced by dibutyryl cyclic AMP treatment of cultured glial cells

Inactivation of membrane surface ecto-5'-nucleotidase by sodium nitroprusside in C6 glioma cells

Ecto-5'-nucleotidase (NT5E), a predominant enzyme that produces extracellular adenosine from AMP, plays an important role in a variety of physiological and pathophysiological processes. This study was performed to identify agents that affect NT5E activity using C6 glioma cells. When cells were incubated with sodium nitroprusside (SNP), phorbol 12-myristate 13-acetate, forskolin, lipopolysaccharide, or interferon-γ, only SNP inhibited NT5E activity in a time- and concentration-dependent manner (IC(50) = 1.2 µM). The inhibitory effect of SNP was long-lasting even after SNP washout; and its action was not mimicked by nitric oxide generating agents, 8-bromo cyclic GMP, ferricyanide, ferrocyanide, or sodium cyanide. SNP did not change NT5E mRNA level or membrane surface protein expression. Similar to SNP, Fe(2+) inhibited NT5E activity, but to a lesser extent. Although Fe(2+) is known to increase oxidative stress, Fe(2+)-mediated oxidative stress was not involved in SNP inhibition of NT5E because the inhibition of NT5E by SNP was not affected by superoxide dismutase and catalase. In contrast, addition of Zn(2+), an essential metal co-factor of NT5E activity, prevented SNP from inhibiting NT5E. These results suggest that SNP disrupts a critical Zn(2+)-dependent enzyme activity and might be useful as a pharmacological tool for inhibiting NT5E.

5’-Nucleotidase Activity and Galactolipid Accumulation in Triethyllead-exposed Glial Cell Cultures

The developing nervous system is vulnerable to heavy metal exposure, which can cause alterations in neuronal and glial cells in the brain. Thus, heavy metals such as mercury and lead reduce myelin galactocerebroside (GalC) synthesis, and increase the ratio of non-hydroxylated fatty acids to hydroxylated fatty acids (GalC-N:GalC-OH) in the GalC molecules in newborn rats. This study investigated the effect of lead on the expression of myelin components by the myelin-forming oligodendroglial cells (OG) in vitro. Primary cultures of mixed glial cells from brains of newborn rats were continuously exposed to triethyllead (TEL; 1nM, 10nM, 50nM and 100nM) for 3 weeks, one week after seeding. The first morphological alteration observed was an increased proliferation of OG in cultures exposed to 10nM TEL. Biochemical analyses showed up-regulation of the enzymes, 2'3’-cyclic nucleotide 3’-phosphodiesterase and 5’-nucleotidase. GalC synthesis was also stimulated, and the ratio GalC-N:GalC-OH was reduced. The results indicate that TEL stimulates the differentiation and maturation of OG in cultures, which suggests that the alterations induced by heavy metals in newborn rats are not due to interference with the OG maturation.

Two distinct inwardly rectifying conductances are expressed in long term dibutyryl-cyclic-AMP treated rat cultured cortical astrocytes

Long term incubation (1-3 weeks) with 250 microM dibutyryl-cyclic-AMP (dBcAMP) of pure cultured cortical astrocytes from newborn rats leads to the expression of voltage-dependent, inward-rectifying potassium (K+) and chloride (Cl-) currents which are lacking in shortly treated (4-24 h) and in control cultured astrocytes. Both conductances are already activated at the holding potential of -60 mV and are distinguishable for their gating kinetics and pharmacological sensitivity. K+ currents have a fast activation kinetic and show a time- and voltage-dependent inactivation at potentials negative to -120 mV. The conductive property of the K+ currents increases upon elevation of the extracellular K+ concentration ([K+]o) and they are reversibly blocked by extracellular 0.1 mM barium ions (Ba2+). Cl- currents are activated only at negative membrane potentials; they display a slow activation kinetic, no time-dependent inactivation and are not affected by 0.1 mM Ba2+. In individual astrocyte the K+ and Cl- conductances can be expressed singularly or in combination. The results indicate that the expression of these two conductances is controlled by a cAMP-dependent molecular signalling, presumably by regulating a late gene activation. Thus, the strengthening of this signalling would contribute to promote the maturation of less differentiated astrocytes in culture, implicating the expression of K+ and Cl- membrane conductances which may operate together in the regulation of [K+]o homeostasis via the mechanism of the local accumulation.

Proliferative effects of humoral factors derived from neuroblastoma cells on cultured astrocytes

The proliferative effects of humoral factors released from N18-RE105 neuroblastoma (NRE) cells on cultured astrocytes were assessed in separate co-culture and conditioned medium studies. In both experimental conditions, the humoral factors derived from neuroblastoma cells had growth-promoting effects on C6 glioma cells of astroglial lineage, but not on primary cultured astrocytes from new-born rat cerebral cortex. It is assumed that neuron-derived humoral factors include astroglial growth factors and that differences in responsiveness between two kinds of cells are probably related to the stages of astroglial maturation processes.

Humoral factors derived from glial cells protect cultured neuroblastoma cells against glutamate toxicity

The protective effects of glial cells against glutamate cytotoxicity on neuronal cells were studied using clonal neuroblastoma cells and two types of glial cells. Neuronal cells treated with glia-conditioned medium became able to tolerate glutamate toxicity. It is suggested that the preparatory processes against glutamate toxicity might be developed in neurons by the humoral factor(s) released from glial cells.

Effects of Polyunsaturated Fatty Acids on Glial Cell Activation. A Study Using Primary Cultures

The aim of this study was to examine the effects of essential fatty acids (gammalinolenic acid [18:3 n-6; GLA] and alpha-linolenic acid [18:3 n-3; Lin]) on the activation of glial cells, using lipopolysaccharides as the activating agent. Primary cultures of mixed glial cells from rat brain were used as the model. The morphological activation of microglia was the most significant response to the exposure. This activation was followed by an increase in 5’-nucleotidase (5’-NT) activity. The 5’-NT activity was increased by GLA or Lin alone to 250–350% of the control value and further increased by co-incubation with lipoteichoic acid (a lipopolysaccharide) to 500–600% of the control value.

The lipopolysaccharide-induced activation of glial cells was also followed by an augmented release of prostaglandin E2. GLA increased the release of prostaglandin E2 in a dose-dependent manner, whereas Lin had no effect on its release.

The results show that this model system is useful for studies on factors affecting the activation of glial cells. GLA and Lin did not reverse glial activation induced by lipopolysaccharides under these experimental conditions.

Hypoxic effects on glutamate uptake in cultured glial cells

Hypoxic effects on glutamate uptake and ATP content in glial cells were investigated by using cultured C6 glioma cells. Mild regressive changes were found depending on the duration of the hypoxic insult, but necrosis or detachment of the cells from the substratum was rarely observed. Glutamate uptake was relatively well preserved after a short hypoxic insult, while a marked decrease in glutamate uptake was observed after hypoxia of long duration. The uptake of sucrose was reduced in a similar pattern to glutamate uptake. Hypoxic insult resulted in a significant reduction of the ATP content in glial cells. Therefore, the decrease in glutamate uptake by glial cells under hypoxia is likely to be due to ATP dependency, and not to the failure of a specific glutamate uptake system, but the failure of a general uptake of the glial cells owing to the energy-dependent membrane dysfunction by ATP depletion. These findings suggest that there are phased changes of astrocytic functions in a hypoxic condition, a preservative phase in the initial stages and then a dysfunctional phase in the later stages of hypoxia.

Cytochemical redistribution of 5'-nucleotidase in the developing cat visual cortex

The adenosine-producing ectoenzyme 5'-nucleotidase has recently been shown to undergo a marked redistribution during development of the cat visual cortex and to be involved in the remodelling of ocular dominance columns (Schoen et al., J. Comp. Neurol., 296, 379-392, 1990). Using an enzyme-cytochemical technique, we now investigate the developmental redistribution of 5'-nucleotidase activity in area 17 of kittens at the ultrastructural level. Between postnatal days 35 and 42, when 5'-nucleotidase is concentrated in layer IV, enzyme reaction product occupies the clefts of asymmetrical synapses within the neuropil. During later development (9th and 13th postnatal weeks), when 5'-nucleotidase spreads over all cortical laminae, the enzyme disappears from its synaptic localization and becomes increasingly associated with astrocytic membranes. The transient appearance of 5'-nucleotidase at synapses parallels the time-course and laminar profile of the synaptic remodelling which takes place during the critical period of visual cortex development. This suggests that synapse-bound 5'-nucleotidase activity plays a role in synaptic malleability, whereas its later association with glial profiles is likely to reflect other functions of the enzyme.

Dibutyryl cyclic AMP-induced morphological differentiation of rat brain astrocytes increases alpha 1-adrenoceptor induced phosphoinositide breakdown by a mechanism involving protein synthesis

Elevation of intracellular cAMP levels by treatment of cultured astrocytes with dibutyryl cyclic AMP (dBcAMP) resulted in a dose-dependent morphological transformation from a flat, polygonal phenotype into a stellate-like cell shape. This morphological differentiation was accompanied by an increase in maximal inositolphosphate (InsPn)-accumulation after stimulation of phosphoinositide (PI)-breakdown by norepinephrine (NE). Maximal enhancement of NE-induced PI-breakdown was observed after treatment of the cells with 0.15 mM dBcAMP for 7 days. While there was a clear effect of dBcAMP-induced differentiation on the maximal NE-induced PI-response, no effect on the dose-response relationship was detectable, resulting in similar EC50-values for astrocytes cultured either in the absence or presence of dBcAMP. The enhancement of NE-stimulated InsPn-formation was dependent on the duration of dBcAMP-treatment. More than a 6 h incubation time was needed to observe an increase in NE-induced PI-breakdown. Furthermore, the enhancing effect of dBcAMP could be prevented by inclusion of the protein-synthesis inhibitor cycloheximide and the blocker of mRNA-transcription actinomycin D. Both the alpha 1-adrenoceptor antagonists prazosin and WB 4101 potently inhibited NE-mediated PI-breakdown. Pretreatment of astrocytes with 100 microM CEC, an alpha 1B-adrenoceptor-specific, irreversible antagonist increased the EC50 values for NE-induced InsPn-accumulation in non-treated as well as in dBcAMP-treated cultures, indicating that both the alpha 1A- and alpha 1B-adrenoceptor subtypes were expressed under both culturing conditions. Reduction of extracellular Ca2+ or pretreatment of the cells with either 12-O-tetradecanoyl-phorbol-13-acetate (TPA), or pertussis toxin (PTX) resulted in a significant reduction of NE-stimulated InsPn formation. The effects of the tested effectors were similar under both culturing conditions indicating that the susceptibility of components of the signalling pathway via alpha 1-adrenoceptors to these modulators was not influenced by morphological differentiation. Different mechanistic aspects of dBcAMP-action on NE-mediated signal-transduction are discussed.