Modulation of beta-adrenergic response in rat brain astrocytes by serum and hormones

TLR3 ligation protects human astrocytes against oxidative stress

Astrocytic Toll-like receptor 3 (TLR3) plays an important role not only in antiviral response but also in regeneration/healing of the CNS. The present study was undertaken to determine whether the neuroprotective effects of TLR3 signaling also include antioxidative protection. TLR3 ligation in human astrocytes induced protracted resistance of the cells to H(2)O(2) toxicity. Similar resistance was induced by conditioned medium from TLR3-ligated astrocytes indicating the involvement of paracrine signaling mechanisms. Out of 13 major antioxidative genes only the gene encoding superoxide dismutase 2 (SOD2) was postligationally upregulated suggesting that SOD2 is the major enzyme responsible for this protection.

Cyclic AMP regulation of messenger RNA level of the stimulatory GTP-binding protein Gs alpha. Isoproterenol, forskolin and 8-bromoadenosine 3':5'-cyclic monophosphate increase the level of Gs alpha mRNA in cultured astroglial cells

The existence of a cAMP-dependent regulation of the expression of the alpha-subunit of the stimulatory guanine nucleotide-binding protein (Gs) in a well characterized astroglial cells culture was established. The culture of astroglial cells for 3-6 h with isoproterenol (10 microM) or forskolin (10 microM) (a cAMP-inducing agent) increased (200-400%) the response of adenylylcyclase to agents which bypass the receptor; GTP, GTP[S] or forskolin. For prolonged exposure times (15 h or more) to isoproterenol or forskolin, the adenylycyclase activity decreased to the value observed in control cells. The same biphasic response of adenylylcyclase to isoproterenol (10 microM) plus GTP (10 microM) occurred in membrane fractions from cells cultured with forskolin, whereas a diminished response to isoproterenol was observed in isoproterenol-treated cells, indicating that the beta-adrenergic receptor was desensitized. To understand the molecular mechanism of these phenomena, we measured the levels of the alpha subunits of the guanine-nucleotide binding protein (Gs and Gi) by Western-blot analysis. The culture of astroglial cells with isoproterenol or forskolin (3-24 h) resulted in a transient increase of both the Gs alpha and the Gi alpha protein levels, while the level of G beta subunits was unaffected. We also identified Gs alpha protein (about 40% of the total cellular protein) in the supernatant fraction of astroglial cells but its level was not modified by the stimulation of cells by forskolin. The level of Gs alpha mRNA measured by Northern-blot analysis was transiently increased (200%) after stimulation of astroglial cells with isoproterenol or forskolin for an incubation period of 6-9 h, then returned to that of control cells for longer period of time. In addition, the Gs alpha mRNA level was threefold increased when cells were cultured for 2-6 h with 8-bromoadenosine 3':5'-cyclic monophosphate (10 microM), a permeant analogue of cAMP. These results indicate that cAMP induces a time-dependent increase of Gs alpha mRNA. The half-life of Gs alpha protein and Gs alpha mRNA were determined. Pulse-chase studies revealed that the decay of Gs alpha protein was clearly biphasic with an early phase (5-6 h) and a slower second phase (20-25 h) but the treatment of cells with forskolin did not accelerate or slow down the turnover of Gs alpha protein.(ABSTRACT TRUNCATED AT 400 WORDS)

Catecholamine effects on dissociated tiger salamander Muller (glial) cells

Dissociated Muller (glial) cells from the neotenous tiger salamander retina respond electrogenically and rheogenically to three putative catecholamine (CA) neurotransmitters, epinephrine, norepinephrine and dopamine. All 3 CAs stimulate a net inward current and an increase in input resistance (Rn). The Muller cell response to the CAs is concentration dependent. At high concentrations ascorbate, an antioxidant used to protect the CAs from oxidation, stimulated a net outward current and a decrease in Rn. When the effects of ascorbate were considered, the CA response at 1 mM was no larger than the response at 100 microM, indicating that 100 microM CA maximally stimulated the Muller cell response.

Interleukin-1 beta regulates proenkephalin gene expression in astrocytes cultured from rat cortex

Glial cells execute essential functions in central nervous system (CNS) development and are also believed to play important roles during gliosis in response to trauma or disease. These developmental and pathological states have also been associated with elevated expression of opioid genes. Because levels of the cytokine interleukin-1 beta (IL-1 beta) increase following CNS lesions, we examined the possible influence of IL-1 beta on the expression of opioid genes in astrocytes cultured from rat cortex. Proenkephalin mRNA expression was stimulated by IL-1 beta in a time- and concentration-dependent manner, being maximal with 5 U/ml IL-1 beta at 4 h. Although the beta-adrenergic agonist isoproterenol was also active, interferon, glutamate, and carbachol were not. Unlike isoproterenol, the actions of IL-1 beta were not associated with a cyclic adenosine monophosphate (AMP)-dependent pathway. Interleukin-1 beta also regulated a proenkephalin-chloramphenicol acetyltransferase fusion gene transiently transfected into astrocytes, with a dose-response similar to that active in proenkephalin mRNA. These effects of IL-1 beta were region-specific, not being observed with either cerebellar or hippocampal astrocytes; however, isoproterenol was active in the latter cell populations. Proenkephalin mRNA in cortical astrocytes was stimulated following a temperature stress. These results suggest that enhanced proenkephalin gene expression in astrocytes by IL-1 beta may be important in neuroimmune interactions and in trauma-induced CNS injury or stress.

Chapter 15: Early response gene induction in astrocytes as a mechanism for encoding and integrating neuronal signals

Astrocytes in vitro readily respond to a wide variety of neuronal and systemic signals by inducing a complex pattern of early response genes (ERGs). The level of complexity is evident in both the ligand-associated expression kinetics and levels of message accumulation as well as in the heterogeneity of response within a population of astrocytes. Ligand-restricted expression of ERG mRNAs suggest that all astrocytes in culture are not alike. Although the ability of glial cells to express ERGs appears to be highly restricted in vivo, one important exception is the category of glial response to injury. Long-term expression of multiple ERG proteins in glial cells stimulated by neuropathological conditions may play an important role in the outcome of brain injury and neurodegenerative disease. The extensive and staggered expression of ERG proteins acting as transcription factors may represent a mechanism for temporally coordinating the genomic program of large sets of genes associated with glial cell response to neuronal dysfunction. As part of the neuronal-glial interdependency, glial ERG expression may encode and integrate the environmental signals associated with neuronal damage and promote the proper restoration of neuronal function. For example, ligand-induced ERG expression regulates the transcription of the nerve growth factor (NGF) gene in glia which could have important functional consequences on neuronal survival and process outgrowth. Future studies will clarify the mechanisms that lead to the expression of ERGs and the subsequent complex, temporally ordered combinatorial consequence of numerous ERG proteins acting as transcription factors impinging upon target gene promoters. Such studies will enrich our understanding of astrocyte-neuronal interaction, clarify how distinct sets of genes in glial cells contribute to the problem and/or solution of neuropathological situations and guide our efforts to diagnose and treat neuropathological conditions.

Serum factor induces selective increase in Na-channel expression in cultured skeletal muscle

We have examined effects of horse serum (HS) and various fractions (1 million-1M, 300K, 100K, and 30K nominal molecular weight limit) obtained by ultrafiltration on expression of TTX-sensitive Na-channels and on activities of the Na-K pump and glucose transport systems in cultured myotubes obtained from 1-2-day-old neonatal rat pups. Five-day-old cells were transferred to serum-free medium with no hormone or growth factor supplements (DMEM) for 24 hr and then treated with the various serum fractions for 48 hr. Measurements were made of specific [3H]-saxitoxin (STX) binding, action potential properties, 86Rb-uptake and 2-deoxyglucose (2-DG) uptake. HS significantly increased all parameters compared to DMEM (increases in STX-binding, 69%; Rb-uptake, 65%; 2-DG uptake, 93%). Results of treatment with the separate fractions showed that the 300K fraction caused a significantly greater increase in STX-binding than either HS or the other fractions. In contrast, the increases in Rb and 2-DG uptakes induced by the different fractions were not different from that obtained with HS. We conclude that serum contains a factor that selectively increases expression of TTX-sensitive Na-channels in skeletal muscle.

Ca2+ waves in astrocytes

The glial cell is the most numerous cell type in the central nervous system and is believed to play an important role in guiding brain development and in supporting adult brain function. One type of glial cell, the astrocyte also may be an integral computational element in the brain since it undergoes neurotransmitter-triggered signalling. Here we review the role of the astrocyte in the central nervous system, emphasizing receptor-mediated Ca2+ physiology. One focus is the recent discovery that the neurotransmitter glutamate induces a variety of intracellular Ca2+ changes in astrocytes. Simple Ca2+ spikes or intracellular Ca2+ oscillations often appear spatially uniform. However, in many instances, the Ca2+ rise has a significant spatial dimension, beginning in one part of the cell it spreads through the rest of the cell in the form of a wave. With high enough agonist concentration an astrocyte syncitium supports intercellular waves which propagate from cell to cell over relatively long distances. We present results of experiments using more specific pharmacological glutamate receptor agonists. In addition to describing the intercellular Ca2+ wave we present evidence for another form of intercellular signalling. Some possible functions of a long-range glial signalling system are also discussed.

Neuronal modulation of calcium channel activity in cultured rat astrocytes

The patch-clamp technique was used to study whether cocultivation of neurons and astrocytes modulates the expression of calcium channel activity in astrocytes. Whole-cell patch-clamp recordings from rat brain astrocytes cocultured with rat embryonic neurons revealed two types of voltage-dependent inward currents carried by Ca2+ and blocked by either Cd2+ or Co2+ that otherwise were not detected in purified astrocytes. This expression of calcium channel activity in astrocytes was neuron dependent and was not observed when astrocytes were cocultured with purified oligodendrocytes.

Growth, differentiation and the beta-adrenergic signal system of HL-60 cells. Characterization in a medium with insulin as the only added protein

The purpose of the present investigation was to define experimental conditions for studies on growth, differentiation and the beta-adrenergic signal system of HL-60 cells. The cell medium was completely devoid of added proteins and hormones other than insulin. The HL-60 cell was able to grow and differentiate in this medium. The spontaneous differentiation along the granulocytic pathway after 72 hr, as assessed by the Nitro Blue tetrazolium test, increased by 400% compared to the serum supplemented medium, but the response to 1 microM retinoic acid was equal in the two media. Induction of monocytic differentiation by 0.16 microM phorbol-13-acetate-12-myristate, as determined by surface adherence after 24 hr, was lower in the absence than in the presence of serum. cAMP levels were elevated in response to (-)-isoproterenol. The EC50 was 0.36 +/- 0.01 microM (mean +/- SE, N = 3). The beta-adrenergic ligand 3H-CGP 12177 was specifically bound to 1 single class of binding sites (Kd: 0.15 +/- 0.04 nM, Bmax: 2220 +/- 150, mean +/- SE, N = 3). These data are comparable to our previously reported findings in serum supplemented medium. The present data show that HL-60 cells are able to grow and differentiate in the absence of serum proteins and hormones other than insulin. Under the present experimental conditions, these cells possessed functional beta-adrenergic receptors.

Expression of beta-adrenergic receptors by astrocytes isolated from adult rat cortex

Astrocytes are the most numerous cell type in the central nervous system. When cultured, these cells express a wide variety of receptors for neurotransmitters. Nonetheless, it has yet to be firmly established that adult astrocytes in situ express receptors for neurotransmitters. In this report the technique of combined receptor autoradiography and immunocytochemistry has been applied to isolated astrocytes from adult animals to examine beta-adrenergic receptor (beta-AR) expression by these cells. It is shown that fibrous and protoplasmic astrocytes isolated from adult rat cerebral cortices specifically bind the beta-AR ligand iodocyanopindolol and that this binding is inhibited by the beta-AR ligands propranolol and isoproterenol. These results indicate that at least two of the major morphological subtypes of adult astrocytes express beta-ARs.

Pharmacological identification of the alpha-adrenergic receptor type which inhibits the beta-adrenergic activated adenylate cyclase system in cultured astrocytes

The adrenergic agonist norepinephrine can exert its influence on cell function by activating both alpha- and beta-adrenergic receptors. In astrocytes, the alpha-adrenergic receptor activity of norepinephrine is known to inhibit the cyclic AMP response elicited by its action at beta-adrenergic receptors. Pharmacological studies were conducted to identify the subtype of alpha-adrenergic receptor which mediates this inhibitory action. The alpha 2-adrenergic antagonist yohimbine potentiated the cyclic AMP response elicited by norepinephrine, whereas the alpha 1-adrenergic antagonist prazosin did not affect the response. The alpha 2-adrenergic agonist clonidine inhibited the cyclic AMP response elicited by the beta-adrenergic agonist isoproterenol and this inhibition could be blocked by yohimbine but not by prazosin. In contrast, the alpha 1-adrenergic agonist phenylephrine did not inhibit the cyclic AMP response to isoproterenol. These studies indicate that the inhibitory action of norepinephrine is mediated by its action at alpha 2-adrenergic receptors.

Fibroblast (heparin-binding) growing factors in neuronal development and repair

Nearly thirty growth and trophic factors that have been purified from mammalian tissues in the last 15 yr have been found to share chemical identity. The results of their chemical purification and molecular cloning show that they are two distinct polypeptides (Mr 17,400 and 18,400), each of which gives rise to families of smaller size peptides. These peptides share a common affinity for heparin. In view of this property, a common nomenclature for the two principle peptide growth factors (heparin-binding growth factor classes 1 and 2; HBGF-1 and -2) has been proposed. However, the names acidic and basic Fibroblast Growth Factors (aFGF,bFGF), which were applied to them originally to describe their mitogenic activity, are more commonly in use and will therefore be adopted in this review. Brain tissue is one of the richest sources of FGFs. It has been used as a starting point for their chemical purification and to prepare genomic libraries for molecular cloning of the aFGF and bFGF genes. There is increasing evidence that these growth factors, expressed in neurons and glia throughout the mammalian nervous system, are implicated in neuronal cell proliferation, differentiation, and histogenesis. FGFs have a strong affinity not only for heparin, but also for the related heparan sulphate proteoglycans that are abundant in neural tissues. This fact provides a clue to the importance of tissue-associated proteoglycans in mediating the release, sequestration, and activation of FGFs and the modulation of their receptor binding and bioactivity. The relevance of FGFs to neural development and their mechanisms of action in neurons will be considered in light of the existing literature describing their biological properties and activity in mesodermal cell types. Evidence is reviewed showing that FGFs have in vivo biological activity, ameliorating the degeneration of central and peripheral neurons after axotomy. The presence and implications of high levels of FGFs in adult mammalian brain provides a direction for future research into neural regeneration. The bioactivity of FGFs in neural tissue may not depend on the regulation of their expression per se, but on the subregional modification of their interaction with proteoglycans.

Inositol phospholipid hydrolysis in cultured astrocytes and oligodendrocytes

Cultures of astrocytes and oligodendrocytes were prelabeled with 3H-inositol and the accumulation of 3H-inositol phosphates was determined following stimulation with a number of neuroactive substances. In astrocytes, norepinephrine (NE) produced the greatest stimulation with significant increase also observed with bradykinin. In oligodendrocytes, the greatest stimulation was produced by carbachol with significant increase also produced by bradykinin, histamine and NE. Carbachol was found to be ineffective in producing stimulation in astrocytes. The accumulation of 3H-inositol phosphates in astrocytes in response to NE was found to be dependent on the presence of Li+. The NE stimulation in astrocytes was dose-dependent and had an EC50 of 1.2 microM. This stimulation was blocked by the low concentration of the alpha 1-adrenergic antagonist prazosin but not by the alpha 2-adrenergic antagonist yohimbine. The NE-stimulated accumulation of 3H-inositol phosphates in astrocytes was inhibited by the cyclic nucleotide phosphodiesterase inhibitor isobutylmethylxanthine as well as by the cAMP analog dibutyryl cAMP.