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.

Glycerophosphorylcholine phosphocholine phosphodiesterase activity in cultured oligodendrocytes, astrocytes, and central nervous tissue of dysmyelinating rodent mutants

The levels of GPC phosphocholine phosphodiesterase, pNP phosphocholine phosphodiesterase, CNPase, and UDP galactose: ceramide galactosyltransferase activities were estimated with pure cultures of oligodendrocytes and astrocytes; mixed primary glial cells cultures; C-6 cells; and CNS tissue of the dysmyelinating md rat, the jimpy mouse, and the quaking mouse. The highest activity of GPC and pNP phosphocholine phosphodiesterases as with CNPase and C gal T was found in the pure cultured oligodendrocytes. C-6 cells had very low or undetectable activities for these two phosphodiesterases but possessed very high CNPase activity. The activity of GPC phosphocholine phosphodiesterase was significantly decreased in the CNS tissue of the md rat and the jimpy and the quaking mouse. Similar reductions were observed for the pNP phosphocholine phosphodiesterase, CNPase, and C gal T activities. The selective cellular enrichment in oligodendrocytes of the GPC phosphocholine phosphodiesterase activity and decreases of its activity in three dysmyelinating mutants in the same ratio as for CNPase and C gal T suggest that GPC phosphocholine phosphodiesterase is a myelin marker enzyme and it may reflect the quantity of myelin and oligodendrocyte present.

Regulation of heat shock protein synthesis in rat astrocytes

Rat forebrain astrocytes synthesize heat shock proteins with molecular weights 97, 89, 70, 68, and 30-34 kilodaltons. The stress inducible 68-kDa heat shock protein (HSP-68) was vigorously expressed by astrocytes in culture after a 45 degrees C, 20 min heat shock. HSP-68 synthesis was poorly inducible by a second heat shock given 16 hr after the initial heat shock. Decreased [35S]methionine incorporation into HSP-68 correlated with low levels of HSP-68 mRNA present after the second heat shock. The data suggest that control of HSP-68 mRNA levels by transcriptional/posttranscriptional mechanisms is a major site for regulation of HSP-68 synthesis.

Comparison of the heat shock response in cultured cortical neurons and astrocytes

Cultured cortical neurons and astrocytes were compared for synthesis of the major inducible 68 kDa heat shock protein. By one- and two-dimensional electrophoresis the inducible 68 kDa protein appeared similar, but astrocytes produced greater amounts of the protein by 3 h than did neurons. Antibodies raised against HeLa cell inducible 72 and constitutive 73 kDa heat shock proteins were used to characterizes the inducible heat shock proteins in neurons and astrocytes. Unlike the gels, major differences were noted of the major inducible heat shock protein in astrocytes compared with neurons when analyzed by Western immunoblots. Heat shock protein 68 kDa mRNA induction in neurons was less than astrocytes suggesting an attenuated inducible 68 kDa heat shock protein response in neurons. The neuronal protein may be a different isoform of the 70 kDa family of heat shock proteins.

Laminin and s-laminin are produced and released by astrocytes, Schwann cells, and schwannomas in culture

Components of the extracellular matrix (ECM) have been implicated in the regulation of neuronal migration, axonal growth, and synaptogenesis. We have examined cultures of glial cells, Schwann cells, and schwannomas for the expression of two components of the ECM, laminin and s-laminin, using immunohistochemical and Western blot techniques. Laminin is a potent promotor of neurite outgrowth in cultures of both central and peripheral neurons, and is present in all ECMs. In contrast, s-laminin (for synaptic laminin), a recently described homolog of laminin, is highly localized at the neuromuscular synaptic cleft (Sanes and Chiu, Cold Spring Harbor Symp. Quant. Biol. 1983;48:667-678; Chiu and Sanes, Dev. Biol. 1984;103:456-467) and shows selective adhesivity for motor neurons (Hunter et al. Cell 1989;59:905-913). While the distribution of these ECM components have been well documented in situ, the sources of these extracellular molecules are unclear. We report that astrocytes cultured in serum-free medium maintain an organized ECM that only bears laminin immunoreactivity; s-laminin appears to be sequestered intracellularly. However, both molecules are found in the astrocyte conditioned medium. Thus, under these growth conditions, astrocytes produce and release laminin and s-laminin, but only incorporate the former into an ECM. In contrast, neither molecule is present in comparable cultures of oligodendrocytes. Although no established ECM is seen in cultures of Schwann cells or schwannomas, laminin and s-laminin immunoreactivity are present within cells and in the conditioned media. These results indicate that certain populations of non-neuronal support cells and cell lines can produce and release both synaptic and extrasynaptic components of the ECM. The assembly of these different molecules into an organized basal lamina may require the presence of additional factors or interaction with neurons.

Fetal alcohol delays the developmental expression of myelin basic protein and transferrin in rat primary oligodendrocyte cultures

This study has examined the development of immunoreactive myelin basic protein and transferrin in primary glial cell cultures. Cultures were initiated from control and experimental Sprague-Dawley rats 1-2 days postnatally. Experimental treatment involved exposure to 5% (w/v) ethanol in a liquid diet during the last two weeks of gestation. Prenatal alcohol administration delayed the expression of myelin basic protein and transferrin during the first three weeks postnatally. Other oligodendroglial and astroglial markers were little affected, if at all, by fetal alcohol exposure.

Characterization of functional nerve growth factor-receptors in a CNS glial cell line: monoclonal antibody 217c recognizes the nerve growth factor-receptor on C6 glioma cells

The biological effects of nerve growth factor (NGF) have been shown to be mediated by the high-affinity form of the nerve growth factor receptor (NGF-R) in sympathetic and sensory neurons, and in PC12 cells. We report here that the central nervous system C6 rat glioma cell line likewise expresses functional high-affinity NGF-Rs. The expression of NGF-R mRNA in C6 cells can be up-regulated by cycloheximide and its own ligand, NGF; and it can be rapidly down-regulated by epidermal growth factor (EGF). Furthermore, C6 cells display NGF responsiveness by expressing c-fos mRNA within 30 minutes of treatment with NGF; and after 4-5 days of NGF exposure, C6 cells cease dividing as measured by [3H]-thymidine uptake, change shape, and reveal neurite-like processes. Scatchard analysis of [125I]-labelled NGF bound to solubilized C6 cells confirms the presence of both high- and low-affinity receptor protein. Crosslinking radiolabeled NGF to its receptor in the presence or absence of excess unlabeled NGF, followed by immunoprecipitation with monoclonal antibody (mAb) 192-IgG (a known anti-NGF-R antibody) and SDS-PAGE reveals a 100 kD band corresponding to the NGF/NGF-R complex. An identical band is observed when the immunoprecipitation is carried out with mAb 217c, suggesting that the 217c epitope is related to NGF-R. The 217c antibody was generated against C6 cells and shown to be a cell surface antibody (Peng et al., Science 215:1102-4, 1982); several investigators have used it subsequently as an immunocytochemical marker for Schwann cells. The significance of NGF-Rs in a CNS glial cell line is unclear, but association of NGF with the control of proliferation and/or differentiation of primitive glial cells is suggested.

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.

Transferrin gene expression and secretion by rat brain cells in vitro

We have previously shown by immunocytochemistry in rat primary glial cultures that transferrin (Tf) is an early developmental marker for oligodendrocytes. The present work addresses the issue of Tf gene expression and synthesis by neural cells in vitro. For this purpose, we used rat embryonic neuronal cultures and newborn glial cultures of astrocytes and oligodendrocytes. Cultured fibroblasts and C6 glioma cells were used as negative controls. We found that Tf mRNA is present in oligodendrocytes, astrocytes, and neurons. However, oligodendrocytes and astrocytes, but not neurons, were shown to synthesize and secrete Tf. Neither fibroblasts nor C6 glioma cells expressed detectable amounts of Tf mRNA. Tf mRNA levels in astrocyte cultures appeared to be under hormonal control since hydrocortisone markedly reduced message levels. These results show that both astrocytes and oligodendrocytes can synthesize and secrete Tf under cell culture conditions. However, epigenetic factors, such as hydrocortisone, may repress the expression of Tf in astrocytes in vivo.

The myelin-deficient rat mutant: partial recovery of oligodendrocyte maturation in vitro

The morphological and immunocytochemical identification and characterization of the myelin-forming cell, the oligodendrocyte, have defined a model system for developmental studies. The myelin-deficient (md) rat mutant lacks myelin in the central nervous system and fails to express the normal developmental increase in oligodendroglial and myelin markers, apparently as a consequence of a point mutation in the proteolipid protein gene. In the present work, we compared the developmental pattern of primary glial cultures derived from newborn md rat brains to those derived from wild-type animals. Brain cell suspensions were prepared from each rat pup and cultured separately. We found by immunocytochemical and enzymatic analyses for the various markers that the developmental cascade of oligodendroglial marker expression is delayed, oligodendrocytes failing to mature compared to normal cultures. However, a partial recovery of marker expression was observed in md-derived cultures as compared to development previously reported in the intact md animals. We suggest that the partial recovery of the sequential expression of oligodendroglial markers may be due to a supportive environment provided to the oligodendrocyte progenitor cell (1) by tissue culture conditions or (2) by the absence of the blood-brain barrier in contrast to its presence in the intact animal.

Transcriptional regulation studies of myelin-associated genes in myelin-deficient mutant rats

To identify and assess the consequences of the mutation in myelin-deficient (md) rats, the myelin proteolipid protein (PLP) gene and its expression were studied in md rats. Southern blots of the PLP gene demonstrated that no major deletions or insertions have occurred in this gene. In addition, the mutation in this gene does not result in a splicing defect in the RNAs, since all exons are represented in md PLP RNAs. These data are consistent with results in another laboratory indicating that a point mutation in the PLP gene in md rats results in a single amino acid alteration in the protein. To elucidate the molecular mechanisms producing reduced levels of PLP, myelin basic protein (MBP) and glycerol phosphate dehydrogenase (GPDH) mRNAs, and their corresponding proteins in md rats, in vitro transcription assays were performed. Transcription of the PLP gene in nuclei isolated from 23-day-old md rat brains was dramatically reduced relative to normal tissue. Thus, the single amino acid alteration in this protein alters the regulation of transcription of this gene. In contrast, the transcriptional activities of the MBP and GPDH genes in md rats were indistinguishable from normal animals. Thus, the lower level of MBP and GPDH mRNA and protein in md rats relative to normal results from a posttranscriptional event.

Developmental regulation of myelin-associated genes in the normal and the myelin deficient mutant rat

Oligodendrocyte development and myelinogenesis, both in vivo and in vitro, are characterized by the sequential and coordinate expression of markers which participate in the differentiation of oligodendrocytes as a prerequisite for myelination. The myelin deficient (md) rat shows greatly reduced mRNA expression for several oligodendrocyte markers: glycerol phosphate dehydrogenase (GPDH), myelin basic protein (MBP) and proteolipid protein (PLP). Brain GPDH mRNA levels are initially equivalent in md and unaffected littermates, but the mutant rats fail to display the normal developmental increase in gene expression. Immunostaining of brain tissue sections also reveals decreased expression of these oligodendrocyte markers. The number of oligodendrocytes containing GPDH-like immunoreactivity is reduced in mutant rats, and in general these cells appear morphologically less complex with shorter processes. However, the intensity of staining in many oligodendrocytes appears equivalent to that observed in unaffected rats. Expression of the neuronal marker, glutamic acid decarboxylase, and the astrocyte markers, glutamine synthetase and glial fibrillary acidic protein, are largely unaffected at either the mRNA or protein level. Mixed glial cultures prepared from the brains of neonatal male md rats possess fewer oligodendrocytes compared to cultures derived from unaffected littermates, and the temporal sequence of marker development is delayed. Although an abnormality in the PLP gene is suspected in the md rat, these findings document profound deficits in many oligodendrocyte gene products.

Retinoids increase perinatal spinal cord neuronal survival and astroglial differentiation

In this report we demonstrate that retinol and retinoic acid (RA) increase the survival and morphological differentiation of rat spinal cord neurons in vitro. Micromolar amounts of retinol and RA increased the number of surviving neurons by 2- to 3-fold and affected neuritic density resulting in increased secondary and tertiary processes compared to untreated sister cultures. A marked morphological differentiation of the astrocyte population in conjunction with an antiproliferative effect in the presence of retinoids were apparent. These trophic effects occurred mainly after 5 days in vitro, a time that corresponds to the time of birth in vivo. Retinoic acid exerted a direct trophic effect on spinal cord neurons in the absence of glial cells while retinol lost its effectiveness. Metabolic labeling suggested that retinol is converted to the biologically active RA within astrocytes but not in neurons. Taken together, our results have demonstrated direct trophic effects of RA on spinal cord neurons and have suggested another role for astrocytes in the maintenance of normal neural physiology by regulating RA concentrations through the oxidation of retinol.

Interferon inhibits the accumulation of glycerol phosphate dehydrogenase mRNA in oligodendrocytes and C6 cells

We investigated the effect of rat interferon-alpha/beta (IFN) on the expression of glycerol phosphate dehydrogenase (E.C.1.1.1.8; GPDH), in both C6 cells and pure cultures of oligodendrocytes. IFNs are naturally produced inhibitors of cell growth that can also affect differentiated cell functions. GPDH is a biochemical marker for oligodendrocytes and is known to be developmentally regulated and steroid inducible. GPDH activity is induced by hydrocortisone (HC) 3.5 fold in C6 cells and 5 fold in oligodendrocytes compared to untreated cultures. A pretreatment of these cells with 75 U/ml of rat IFN-alpha/beta resulted in an inhibition of the HC induction of GPDH enzymatic activity by 50% and 40% in C6 cells and oligodendrocytes respectively. We also found that IFN impaired the accumulation of GPDH mRNA in both cell types. These results demonstrate that IFNs are capable of modifying the cellular response to hormones in cells of neuroepithelial origin, and suggest the possibility that IFNs may be able to influence the development and function of the brain.

Induction of c-fos and TIS genes in cultured rat astrocytes by neurotransmitters

The interaction of neurotransmitters with their specific receptors initiates a cascade of intracellular biochemical events which lead to induction of specific genes. Included in this cascade is the rapid and transient induction of a family of primary early response genes we term TIS genes (Lim et al.: Oncogene 1: 263-270, 1987). Expression of six TIS gene, including c-fos, was examined in secondary cultures of rat neocortical astrocytes exposed to muscarinic and adrenergic agonists and antagonists to study the early genomic responses which accompany neurotransmitter-induced alteration of glial morphology and physiology. Carbachol induced accumulation of mRNA for c-fos and the other TIS genes. Carbachol-mediated induction of TIS mRNA expression was sensitive to atropine blockade and was potentiated by lithium. Norepinephrine (NE), isoproterenol, or phenylephrine also induced TIS mRNA accumulation. In order to determine which second-messenger pathways mediate NE induction of TIS gene expression, the influences of the beta(B) antagonist propranolol (PR), the alpha I(AI) antagonist prazosin (PZ), and the alpha 2(A2) antagonist yohimbine (YB) were examined. The induction of TIS1 mRNA by NE was partially blocked by PR or PZ alone, and completely abolished by both antagonists in combination. YB had no effect on TIS1 mRNA expression. These results suggest that NE induces TIS1 mRNA through both B- and A 1-adrenergic, but not A2, pathways. The lack of effect of inhibitors of phospholipase A2 and cyclooxygenase suggests that the A1 component is mediated through a protein kinase C pathway. The induction of transient gene expression by neurotransmitters may mediate the secondary genomic responses and phenotypic changes occurring in astrocytes in response to alterations in neuronal neurotransmitter release.

Differential regulation of oligodendrocyte markers by glucocorticoids: post-transcriptional regulation of both proteolipid protein and myelin basic protein and transcriptional regulation of glycerol phosphate dehydrogenase

During neonatal development glucocorticoids potentiate oligodendrocyte differentiation and myelinogenesis by regulating the expression of myelin basic protein, proteolipid protein, and glycerol phosphate dehydrogenase (sn-glycerol-3-phosphate: NAD+ 2-oxidoreductase, EC 1.1.1.8). The actual locus at which hydrocortisone exerts its developmental influence on glial physiology is, however, not well understood. Glycerol phosphate dehydrogenase is glucocorticoid-inducible in oligodendrocytes at all stages of development both in vivo and in vitro. In newborn rat cerebral cultures, between 9 and 15 days in vitro, a 2- to 3-fold increase in myelin basic protein and proteolipid protein mRNA levels occurs in oligodendrocytes within 12 hr of hydrocortisone treatment. Immunostaining demonstrates that this increase in mRNAs is followed by a 2- to 3-fold increase in the protein levels within 24 hr. In vitro transcription assays performed with oligodendrocyte nuclei show an 11-fold increase in the transcriptional activity of glycerol phosphate dehydrogenase in response to hydrocortisone but no increase in transcription of myelin basic protein or proteolipid protein. These results indicate that during early myelinogenesis, glucocorticoids influence the expression of key oligodendroglial markers by different processes: The expression of glycerol phosphate dehydrogenase is regulated at the transcriptional level, whereas the expression of myelin basic protein and proteolipid protein is modulated via a different, yet uncharacterized, mechanism involving post-transcriptional regulation.

TIS gene expression in cultured rat astrocytes: induction by mitogens and stellation agents

The expression of a number of TIS genes (Lim et al.: Oncogene 1:263-270, 1987) was examined in secondary cultures of rat neocortical astrocytes treated with mitogens and stellation agents, to study the early nuclear events which accompany the induction of glial proliferation and/or differentiation. Tetradecanoyl phorbol acetate (TPA), epidermal growth factor, and fibroblast growth factor, three mitogens for astrocytes, stimulated marked, rapid, and transient increases in TIS gene mRNAS. TIS10, which is not expressed in rat PC12 pheochromocytoma cells, could be induced by these mitogens in rat astrocytes. Dibutyryl cyclic adenosine monophosphate and forskolin, which induce rapid stellation in astrocytes, and ganglioside GM1, a potent mitogen as well as an antagonist of the induction and maintenance of stellation, all induced TIS gene expression. Thus, a broad range of agents which elicit both proliferative and differentiation responses in astrocytes are capable of inducing a family of genes that may play a role in the early events of signal transduction.

TIS gene expression in cultured rat astrocytes: multiple pathways of induction by mitogens

Accumulation of TIS1 and TIS11 (Lim et al.: Oncogene 1:263-270, 1987) mRNAs in secondary cultures of rat neocortical astrocytes was much greater in response to tetradecanoyl phorbol acetate (TPA) than in response to either epidermal growth factor (EGF) or fibroblast growth factor (FGF). In contrast, EGF, FGF, and TPA were equally effective in inducing accumulation of TIS8 and TIS28/c-fos mRNAs. These data suggested that TPA and the polypeptide mitogens might induce TIS gene expression by distinct pathways. When maximally inducing concentrations of EGF and FGF were co-administered to astrocyte cultures, TIS mRNA accumulations were no greater than those observed for the individual growth factors, suggesting that EGF and FGF saturate a common, limiting step in their induction pathways. In contrast, when either EGF or FGF was presented to astrocytes in combination with maximally inducing levels of TPA, the resulting levels of accumulation of TIS mRNAs were at least as great as the sum of the levels induced by the individual mitogens. Stimulation of [3H]-thymidine incorporation demonstrated an identical pattern of interaction; EGF and FGF co-administration was no more effective than either polypeptide mitogen alone, but, when presented to astrocyte cultures along with maximally inducing concentrations of TPA, either EGF or FGF was able to increase incorporation of [3H]-thymidine. Superinduction of all the TIS genes occurred if cycloheximide (CHX) was present during TPA exposure. Once again, two distinct classes of responses of the various TIS genes occurred; superinduction of TIS1, TIS7, TIS11, and TIS28/c-fos mRNA accumulation ranged from 10- to 20-fold, while CHX superinduction of TIS8 and TIS10 was far more modest, ranging from 2- to 3-fold.(ABSTRACT TRUNCATED AT 250 WORDS)

Induction of the major inducible 68-kDa heat-shock protein after rapid changes of extracellular pH in cultured rat astrocytes

Cultured rat astrocytes were exposed for 1 or 3 h to acidic medium (pH adjusted to 5.0, 5.5, or 6.0). Radioactive labeling for 3 h after exposure to acidic medium revealed increased synthesis of many proteins, including an inducible 68-kDa protein. Optimal extracellular (medium) pH for the induction of this 68-kDa protein was 5.5. Immunoblotting demonstrated that this 68-kDa protein induced by acidosis was the 68-kDa heat-shock protein previously described in cultured astrocytes.

Corticosterone-induced responses in rat brain RNA are also evoked in hippocampus by acute vibratory stress

Corticosterone (CORT) induces responses in brain cells that are mediated by glucocorticoid receptors through regulation of gene activity. We previously found rapid increases in select poly(A)-containing RNAs in rat hippocampus following treatment with CORT that are mediated by low-affinity glucocorticoid receptors. To determine if these responses are hippocampal specific, we examined RNA responses to glucocorticoids in several brain regions, myocardium, and cultured astrocytes by two-dimensional gel electrophoretic resolution of 35S-methionine labelled, in vitro translation products. RNAs coding for similar 35-, 33-, and 20-kdalton polypeptides are induced after 3 days of CORT treatment (40 mg/kg/day) in hippocampus, hypothalamus, cortex, striatum, cerebellum, and myocardium. Primary astrocyte cultures (neonatal rat), however, showed increases after hydrocortisone (1 microgram/ml) in only the 20- and 33-kdalton translation products, while the 35-kdalton polypeptide was not detected. The hippocampal responses were maintained for up to 3 months during chronic daily CORT treatment. To determine if an increase in endogenous CORT levels would also evoke the RNA responses, we subjected rats to 2 h vibratory stress and analyzed the in vitro translation products. RNAs coding for the 35- and 20-kdalton polypeptides were increased 3- to 5-fold in the hippocampus after acute stress in intact rats, but not in stressed adrenalectomized rats. These results suggest a new class of molecular stress responses in brain cells that is glucocorticoid dependent under physiological conditions.