Differences between reactive astrocytes and cultured astrocytes treated with di-butyryl-cyclic AMP

Dynamic expression of homeostatic ion channels in differentiated cortical astrocytes in vitro

The capacity of astrocytes to adapt their biochemical and functional features upon physiological and pathological stimuli is a fundamental property at the basis of their ability to regulate the homeostasis of the central nervous system (CNS). It is well known that in primary cultured astrocytes, the expression of plasma membrane ion channels and transporters involved in homeostatic tasks does not closely reflect the pattern observed in vivo. The individuation of culture conditions that promote the expression of the ion channel array found in vivo is crucial when aiming at investigating the mechanisms underlying their dynamics upon various physiological and pathological stimuli. A chemically defined medium containing growth factors and hormones (G5) was previously shown to induce the growth, differentiation, and maturation of primary cultured astrocytes. Here we report that under these culture conditions, rat cortical astrocytes undergo robust morphological changes acquiring a multi-branched phenotype, which develops gradually during the 2-week period of culturing. The shape changes were paralleled by variations in passive membrane properties and background conductance owing to the differential temporal development of inwardly rectifying chloride (Cl⁻) and potassium (K⁺) currents. Confocal and immunoblot analyses showed that morphologically differentiated astrocytes displayed a large increase in the expression of the inward rectifier Cl⁻ and K⁺ channels ClC-2 and Kir4.1, respectively, which are relevant ion channels in vivo. Finally, they exhibited a large diminution of the intermediate filaments glial fibrillary acidic protein (GFAP) and vimentin which are upregulated in reactive astrocytes in vivo. Taken together the data indicate that long-term culturing of cortical astrocytes in this chemical-defined medium promotes a quiescent functional phenotype. This culture model could aid to address the regulation of ion channel expression involved in CNS homeostasis in response to physiological and pathological challenges.

Graphene Oxide Upregulates the Homeostatic Functions of Primary Astrocytes and Modulates Astrocyte-to-Neuron Communication

Graphene-based materials are the focus of intense research efforts to devise novel theranostic strategies for targeting the central nervous system. In this work, we have investigated the consequences of long-term exposure of primary rat astrocytes to pristine graphene (GR) and graphene oxide (GO) flakes. We demonstrate that GR/GO interfere with a variety of intracellular processes as a result of their internalization through the endolysosomal pathway. Graphene-exposed astrocytes acquire a more differentiated morphological phenotype associated with extensive cytoskeletal rearrangements. Profound functional alterations are induced by GO internalization, including the upregulation of inward-rectifying K⁺ channels and of Na⁺-dependent glutamate uptake, which are linked to the astrocyte capacity to control the extracellular homeostasis. Interestingly, GO-pretreated astrocytes promote the functional maturation of cocultured primary neurons by inducing an increase in intrinsic excitability and in the density of GABAergic synapses. The results indicate that graphene nanomaterials profoundly affect astrocyte physiology in vitro with consequences for neuronal network activity. This work supports the view that GO-based materials could be of great interest to address pathologies of the central nervous system associated with astrocyte dysfunctions.

Cyclic AMP signaling restricts activation and promotes maturation and antioxidant defenses in astrocytes

Background

cAMP signaling produces dramatic changes in astrocyte morphology and physiology. However, its involvement in phenotype acquisition and the transcriptionally mediated mechanisms of action are largely unknown.

Results

Here we analyzed the global transcriptome of cultured astroglial cells incubated with activators of cAMP pathways. A bulk of astroglial transcripts, 6221 annotated genes, were differentially regulated by cAMP signaling. cAMP analogs strongly upregulated genes involved in typical functions of mature astrocytes, such as homeostatic control, metabolic and structural support to neurons, antioxidant defense and communication, whereas they downregulated a considerable number of proliferating and immaturity-related transcripts. Moreover, numerous genes typically activated in reactive cells, such as scar components and immunological mediators, were repressed by cAMP. GSEA analysis contrasting gene expression profiles with transcriptome signatures of acutely isolated astrocytes and in situ evaluation of protein levels in these cells showed that cAMP signaling conferred mature and in vivo–like transcriptional features to cultured astrocytes.

Conclusions

These results indicate that cAMP signaling is a key pathway promoting astrocyte maturation and restricting their developmental and activation features. Therefore, a positive modulation of cAMP signaling may promote the normal state of differentiated astrocytes and favor the protection and function of neuronal networks.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2623-4) contains supplementary material, which is available to authorized users.

Astrocyte Cultures Mimicking Brain Astrocytes in Gene Expression, Signaling, Metabolism and K+ Uptake and Showing Astrocytic Gene Expression Overlooked by Immunohistochemistry and In Situ Hybridization

Based on differences in gene expression between cultured astrocytes and freshly isolated brain astrocytes it has been claimed that cultured astrocytes poorly reflect the characteristics of their in vivo counterparts. This paper shows that this is not the case with the cultures of mouse astrocytes we have used since 1978. The culture is prepared following guidelines provided by Drs. Monique Sensenbrenner and John Booher, with the difference that dibutyryl cyclic AMP is added to the culture medium from the beginning of the third week. This addition has only minor effects on glucose and glutamate metabolism, but it is crucial for effects by elevated K⁺ concentrations and for Ca²⁺ homeostasis, important aspects of astrocyte function. Work by Liang Peng and her colleagues has shown identity between not only gene expression but also drug-induced gene upregulations and editings in astrocytes cultured by this method and astrocytes freshly isolated from brains of drug-treated animals. Dr. Norenberg's laboratory has demonstrated identical upregulation of the cotransporter NKCC1 in ammonia-exposed astrocytes and rats with liver failure. Similarity between cultured and freshly isolated astrocytes has also been shown in metabolism, K⁺ uptake and several aspects of signaling. However, others have shown that the gene for the glutamate transporter GLT1 is not expressed, and rat cultures show some abnormalities in K⁺ effects. Nevertheless, the overall reliability of the cultured cells is important because immunohistochemistry and in situ hybridization poorly demonstrate many astrocytic genes, e.g., those of nucleoside transporters, and even microarray analysis of isolated cells can be misleading.

Ammonium increases Ca(2+) signalling and up-regulates expression of TRPC1 gene in astrocytes in primary cultures and in the in vivo brain

Rapid rise in ammonium concentration in the brain is the major pathogenic factor in hepatic encephalopathy that is manifested by state of confusion, forgetfulness and irritability, psychotic symptoms, delusions, lethargy, somnolence and, in the terminal stages, coma. Primary cultures of mouse astrocytes were used to investigate effects of chronic treatment (3 days) with ammonium chloride (ammonium) at 3 mM, this being a relevant concentration for hepatic encephalopathy condition, on metabotropic receptor agonist-induced increases in free cytosolic Ca(2+) concentration [(Ca(2+))i], measured with fura-2 based microfluorimetry and on store-operated Ca(2+) entry (SOCE) activated following treatment with the SERCA inhibitor thapsigargin. The agonists used were the β-adrenergic agonist isoproterenol, the α2-adrenergic agonist dexmedetomidine, the InsP3 receptor (InsP3R) agonist adenophostin A and ryanodine receptor agonist 4-Chloro-m-cresol (4-CMC). Agonist-induced [Ca(2+)]i responses were significantly increased in astrocytes chronically exposed to ammonium. Similarly, the SOCE, meditated by the transient receptor potential channel 1 (TRPC1), was significantly augmented. The ammonium-induced increase in SOCE was a result of an up-regulation of mRNA and protein expression of TRPC1 in astrocytes. Increase in TRPC1 expression and in SOCE were both prevented by ouabain antagonist canrenone. Similar up-regulation of TRPC1 gene expression was found in the brain of adult mice subjected to intraperitoneal injection of urease for 3 days. In transgenic mice tagged with an astrocyte-specific or a neurone-specific markers and treated with intraperitoneal injections of urease for 3 days, the fluorescence-activated cell sorting of neurones and astrocytes demonstrated that TRPC1 mRNA expression was up-regulated in astrocytes, but not in neurones.

Aralar mRNA and protein levels in neurons and astrocytes freshly isolated from young and adult mouse brain and in maturing cultured astrocytes

Intense glucose-based energy metabolism and glutamate synthesis by astrocytes require malate-aspartate-shuttle (MAS) activity to regenerate NAD⁺ from NADH formed during glycolysis, since brain lacks significant glycerophosphate shuttle activity. Aralar is a necessary aspartate/glutamate exchanger for MAS function in brain. Based on cytochemical immunoassays the absence of aralar in adult astrocytes was repeatedly reported. This would mean that adult astrocytes must regenerate NAD⁺ by producing lactate from pyruvate, eliminating its use by oxidative and biosynthetic pathways. We alternatively used astrocytes and neurons from adult brain, freshly isolated by fluorescence-activated cell sorting, to determine aralar protein by a specific antibody and its mRNA by real-time PCR. Both protein and mRNA expressions were identical in adult neurons and astrocytes and similar to whole brain levels. The same level of aralar expression was reached in well-differentiated astrocyte cultures, but not until late development, coinciding with the late-maturing brain capability for glutamate formation and degradation.

An in vitro trauma model to study rodent and human astrocyte reactivity

Protocols are presented describing a unique in vitro injury model and how to culture and mature mouse, rat, and human astrocytes for its use. This injury model produces widespread injury and astrocyte reactivity that enable quantitative measurements of morphological, biochemical, and functional changes in rodent and human reactive astrocytes. To investigate structural and molecular mechanisms of reactivity in vitro, cultured astrocytes need to be purified and then in vitro "matured" to reach a highly differentiated state. This is achieved by culturing astrocytes on deformable collagen-coated membranes in the presence of adult-derived horse serum (HS), followed by its stepwise withdrawal. These in vitro matured, process-bearing, quiescent astrocytes are then subjected to mechanical stretch injury by an abrupt pressure pulse from a pressure control device that briefly deforms the culture well bottom. This inflicts a measured reproducible, widespread strain that induces reactivity and injury in rodent and human astrocytes. Cross-species comparisons are possible because mouse, rat, and human astrocytes are grown using essentially the same in vitro treatment regimen. Human astrocytes from fetal cerebral cortex are compared to those derived from cortical biopsies of epilepsy patients (ages 1-12 years old), with regard to growth, purity, and differentiation. This opens a unique opportunity for future studies on glial biology, maturation, and pathology of human astrocytes. Prototypical astrocyte proteins including GFAP, S100, aquaporin4, glutamate transporters, and tenascin are expressed in mouse, rat, and human in vitro matured astrocyte. Upon pressure-stretching, rodent and human astrocytes undergo dynamic morphological, gene expression, and protein changes that are characteristic for trauma-induced reactive astrogliosis.

Adrenoceptors in brain: cellular gene expression and effects on astrocytic metabolism and [Ca(2+)]i

Recent in vivo studies have established astrocytes as a major target for locus coeruleus activation (Bekar et al., 2008), renewing interest in cell culture studies on noradrenergic effects on astrocytes in primary cultures and calling for additional information about the expression of adrenoceptor subtypes on different types of brain cells. In the present communication, mRNA expression of alpha(1)-, alpha(2)- and beta-adrenergic receptors and their subtypes was determined in freshly isolated, cell marker-defined populations of astrocytes, NG2-positive cells, microglia, endothelial cells, and Thy1-positive neurons (mainly glutamatergic projection neurons) in murine cerebral cortex. Immediately after dissection of frontal, parietal and occipital cortex of 10-12-week-old transgenic mice, which combined each cell-type marker with a specific fluorescent signal, the tissue was digested, triturated and centrifuged, yielding a solution of dissociated cells of all types, which were separated by fluorescence-activated cell sorting (FACS). mRNA expression in each cell fraction was determined by microarray analysis. alpha(1A)-Receptors were unequivocally expressed in astrocytes and NG2-positive cells, but absent in other cell types, and alpha(1B)-receptors were not expressed in any cell population. Among alpha(2)-receptors only alpha(2A)-receptors were expressed, unequivocally in astrocytes and NG-positive cells, tentatively in microglia and questionably in Thy1-positive neurons and endothelial cells. beta(1)-Receptors were unequivocally expressed in astrocytes, tentatively in microglia, and questionably in neurons and endothelial cells, whereas beta(2)-adrenergic receptors showed tentative expression in neurons and astrocytes and unequivocal expression in other cell types. This distribution was supported by immunochemical data and its relevance established by previous studies in well-differentiated primary cultures of mouse astrocytes, showing that stimulation of alpha(2)-adrenoceptors increases glycogen formation and oxidative metabolism, the latter by a mechanism depending on intramitochondrial Ca(2+), whereas alpha(1)-adrenoceptor stimulation enhances glutamate uptake, and beta-adrenoceptor activation causes glycogenolysis and increased Na(+), K(+)-ATPase activity. The Ca(2+)- and cAMP-mediated association between energy-consuming and energy-yielding processes is emphasized.

CCR7 is expressed in astrocytes and upregulated after an inflammatory injury

Neurodegenerative or autoimmune diseases are frequently regulated by chemokines and their receptors, controlling both glial activation and immune cell infiltration. CCL19 and CCL21 have been described to mediate crucial functions during CNS pathological states, regulating both immune cell traffic to the CNS and communication between glia and neurons. Here, we describe the expression pattern and cellular sources of CCR7, receptor of CCL19 and CCL21, in the normal mouse brain. Moreover, we found that CCR7 is upregulated in reactive astrocytes upon intracerebral LPS, regulating early glial reactivity through its ligands CCL19 and CCL21. Our results indicate that CCR7 is playing an important role for the intercellular communication during the inflammatory activation in the CNS.

Ouabain binding kinetics and FXYD7 expression in astrocytes and neurons in primary cultures: implications for cellular contributions to extracellular K+ homeostasis?

Although Na+,K+-ATPase-mediated K+ uptake into astrocytes plays a major role in re-establishing resting extracellular K+ following neuronal excitation little information is available about astrocytic Na+,K+-ATPase function, let alone mechanisms returning K+ to neurons. The catalytic units of the Na+,K+-ATPase are the astrocyte-specific α2, the neuron-specific α3 and the ubiquitously expressed α1. In the present work, Bmax and KD values for α1, α2 and α3 subunits were computed in cultured cerebro-cortical mouse astrocytes and cerebellar granule neurons by non-linear regression as high-affinity (α2, α3) and low-affinity (α1) [3H]ouabain binding sites, which stoichiometrically equal transporter sites. Cellular expression was also determined of the brain- and α1-β1 isoform-specific FDYX7, regulating Na+,K+-ATPase efficiency and K+-sensitivity. From ouabain-sensitive K+ uptake rates published by ourselves (Walz and Hertz, 1982) or others (Atterwill et al., 1985), Na+,K+-ATPase turnover was determined. Subunits α2 and α3 showed Bmax of 15-30 pmol/mg protein, with maximum turnover rates of 70-80/s. Bmax of the α1 subunit was low in neurons but very high in astrocytes (645 pmol/mg protein), where turnover rate was slow, reflecting expression of selectively expressed FXYD7, and binding was increased by K+. The role of these characteristics for K+ homeostasis are discussed.

Nucleoside transporter expression and function in cultured mouse astrocytes

Uptake of purine and pyrimidine nucleosides in astrocytes is important for several reasons: (1) uptake of nucleosides contributes to nucleic acid synthesis; (2) astrocytes synthesize AMP, ADP, and ATP from adenosine and GTP from guanosine; and (3) adenosine and guanosine function as neuromodulators, whose effects are partly terminated by cellular uptake. It has previously been shown that adenosine is rapidly accumulated by active uptake in astrocytes (Hertz and Matz, Neurochem Res 14:755-760, 1989), but the ratio between active uptake and metabolism-driven uptake of adenosine is unknown, as are uptake characteristics for guanosine. The present study therefore aims at providing detailed information of nucleoside transport and transporters in primary cultures of mouse astrocytes. Reverse transcription-polymerase chain reaction identified the two equilibrative nucleoside transporters, ENT1 and ENT2, together with the concentrative nucleoside transporter CNT2, whereas CNT3 was absent, and CNT1 expression could not be investigated. Uptake studies of tritiated thymidine, formycin B, guanosine, and adenosine (3-s uptakes at 1-4 degrees C to study diffusional uptake and 1-60-min uptakes at 37 degrees C to study concentrative uptake) demonstrated a fast diffusional uptake of all four nucleosides, a small, Na(+)-independent and probably metabolism-driven uptake of thymidine (consistent with DNA synthesis), larger metabolism-driven uptakes of guanosine (consistent with synthesis of DNA, RNA, and GTP) and especially of adenosine (consistent with rapid nucleotide synthesis), and Na(+)-dependent uptakes of adenosine (consistent with its concentrative uptake) and guanosine, rendering neuromodulator uptake independent of nucleoside metabolism. Astrocytes are accordingly well suited for both intense nucleoside metabolism and metabolism-independent uptake to terminate neuromodulator effects of adenosine and guanosine.

Central nervous system lesions that can and those that cannot be repaired with the help of olfactory bulb ensheathing cell transplants

Growth-promoting macroglia (aldynoglia) with growth properties and immunological markers similar to Schwann cells, are found in loci of the mammalian CNS where axon regeneration occurs throughout life, like the olfactory sytem, hypothalamus-hypophysis and the pineal gland. Contrary to Schwann cells, aldynoglia mingle freely with astrocytes and can migrate in brain and spinal cord. Transplantation of cultured and immunopurified olfactory ensheathing cells (OECs) in the spinal cord after multiple central rhizotomy, promoted sensory and central axon growth and partial functional restoration, judging by anatomical, electrophysiological and behavioural criteria. OEC transplants suppressed astrocyte reactivity, thus generally favouring axon growth after a lesion. However, the functional repair promoted by OEC transplants was partial in the best cases, depending on lesion type and location. Cyst formation after photochemical cord lesion was partially prevented but neither the corticospinal tract, interrupted by a mild contusion, nor the sectioned medial longitudinal fascicle, did regrow after OEC transplantation in the injured area.

Developmental changes of parameters for astrogliosis during cultivation of purified cerebral astrocytes from newborn rats

Astrogliosis is a common phenomenon seen in most neuropathological changes of the central nervous system. Several in vitro models have been used to study the mechanisms and conditions for the induction of astrogliosis, however many do not take into account that the metabolic and structural characteristics of astrocytes change with time in culture. Thus, it appears difficult to attribute changes of, e.g., GFAP to the normal change in vitro as opposed to additional changes due to an astrogliotic reaction. The present study was therefore undertaken to characterize these developmental changes in purified astroglial secondary cultures during cultivation to provide a basis for further investigations of astrogliosis in vitro. During 6 weeks of cultivation (3-43 days) GFAP (ELISA) increased much more (22-fold) than the cell number (2.5-fold) and the total protein (3.5-fold). The GFAP/protein ratio increased during the first 4 weeks of cultivation and reached a plateau thereafter, which was accompanied by a significant increase of GFAP mRNA (Northern blot). At the ultrastructural level (transmission electron microscopy) gliofilaments in the perinuclear region as well as in the cell processes of 4-day-old astrocytes showed a dispersed pattern, whereas an accumulation of gliofilaments was found in 39-day-old cells, which formed large aggregated bundles localized mostly in the cell processes. Our results show that in vitro astrocytes undergo developmental changes in their accumulation of GFAP and intermediate filaments which reach a stable steady state after 4 weeks in culture. These 'normal' developmental changes will have to be taken into account, when experiments with variations of the level of GFAP are performed. Stable culture conditions for experimentation appear to be present after 4 weeks in culture.

Cataplerotic TCA cycle flux determined as glutamate-sustained oxygen consumption in primary cultures of astrocytes

Utilization of glucose by adult brain as its metabolic substrate does not mean that glutamate cannot be synthesized from glucose and subsequently oxidatively degraded. Between 10 and 20% of total pyruvate metabolism in brain occurs as formation of oxaloacetate (OAA), a tricarboxylic acid (TCA) cycle intermediate, from pyruvate plus CO(2). This anaplerotic ('pool-filling') process occurs in astrocytes, which in contrast to neurons express pyruvate carboxylase (PC) activity. Equivalent amounts of pyruvate are converted to acetylcoenzyme A and condensed with oxaloacetate to form citrate (Cit), which is metabolized to alpha-ketoglutarate (generating oxidatively-derived energy), glutamate and glutamine and transferred to neurons in the glutamate-glutamine cycle and used as precursor for transmitter glutamate. Since the blood-brain barrier is poorly permeable to glutamate and its metabolites, net synthesis of glutamate must be followed by degradation of equivalent amounts of glutamate, a cataplerotic ('pool-emptying') process, in which glutamate is converted in the TCA cycle to malate or oxaloacetate (generating additional energy), which exit the cycle to form one molecule pyruvate. To obtain an estimate of the rate of astrocytic oxidation of glutamate the rate of oxygen consumption was measured in primary cultures of mouse astrocytes metabolizing glutamate in the absence of other metabolic substrates. The observed rate is compatible with complete oxidative degradation of glutamate.

Cytosine arabinofuranoside-induced activation of astrocytes increases the susceptibility of neurons to glutamate due to the release of soluble factors

Activation of astrocytes occurs during many forms of CNS injury, but its importance for neuronal survival is poorly understood. When hippocampal cultures of neurons and astrocytes were treated from day 2-4 in vitro (DIV 2-4) with 1 microM cytosine arabinofuranoside (AraC), we observed a stellation of astrocytes, an increase in glial fibrillary acidic protein (GFAP) level as well as a higher susceptibility of the neurons to glutamate compared with cultures treated from DIV 2-4 with vehicle. To find out whether factors released into the culture medium were responsible for the observed differences in glutamate neurotoxicity, conditioned medium of AraC-treated cultures (MCMAraC) was added to vehicle-treated cultures and conditioned medium of vehicle-treated cultures (MCMvh) was added to AraC-treated cultures 2 h before and up to 18 h after the exposure to 1mM glutamate for 1 h. MCMAraC increased glutamate neurotoxicity in vehicle-treated cultures and MCMvh reduced glutamate neurotoxicity in AraC-treated cultures. Heat-inactivation of MCMvh increased, whereas heat-inactivation of MCMAraC did not affect glutamate toxicity suggesting that heat-inactivation changed the proportion of factors in MCMvh inhibiting and exacerbating the excitotoxic injury. Similar findings were obtained using conditioned medium of pure astrocyte cultures of DIV 12 treated from DIV 2-4 with vehicle or 1 microM AraC suggesting that heat-sensitive factors in MCMvh were mainly derived from astrocytes. Treatment of hippocampal cultures with 1mM dibutyryl-cAMP for 3 days induced an activation of the astrocytes similar to AraC and increased neuronal susceptibility to glutamate. Our findings provide evidence that activation of astrocytes impairs their ability to protect neurons after excitotoxic injury due to changes in the release of soluble and heat-sensitive factors.

Expression and regulation of voltage-gated sodium channel beta1 subunit protein in human gliosis-associated pathologies

Auxiliary beta1 subunits of voltage-gated sodium channels (NaChs) critically regulate channel activity and may also act as cell adhesion molecules (CAMs). In a recent study we have shown that the expression of beta1 NaCh protein is increased in reactive astrocytes in a rat epilepsy model of mesial temporal lobe epilepsy. The present study was undertaken to examine whether changes of NaCh beta1 subunit protein expression are also associated with structural changes occurring in human reactive astrocytes under different pathological conditions in vivo, as well as in response to changing environmental conditions in vitro. Strong beta1 astroglial immunoreactivity was present in human brain tissue from patients with astrogliosis. The over-expression of beta1 protein in reactive glia was observed in both epilepsy-associated brain pathologies (temporal lobe epilepsy, cortical dysplasia), as well as non-epileptic (cerebral infarction, multiple sclerosis, amyotrophic lateral sclerosis, meningo-encephalitis) disorders. The up-regulation of beta1 subunit protein in astrocytes can be reproduced in vitro. beta1 protein is highly expressed in human astrocytes cultured in the presence of trophic factors, under conditions in which they show morphology similar to the morphology of cells undergoing reactive gliosis. The growth factor-induced overexpression of beta1 protein was abrogated by PD98059, which inhibits the mitogen-activated protein kinase pathway. These findings demonstrate that the expression of NaCh beta1 subunit protein in astrocytes is plastic, and indicate a novel mechanism for modulation of glial function in gliosis-associated pathologies.

Astrocytic amino acid metabolism under control conditions and during oxygen and/or glucose deprivation

Amino acid contents were measured in 1- and 3-week-old primary cultures of astrocytes and in their incubation media, an amino acid-free salt solution with or without glucose, during 3-h incubation under normoxic or anoxic conditions. Most essential amino acids were rapidly released to the medium during the beginning of the incubation. A subsequent slow medium increase reflected proteolysis. Glutamate and aspartate were absent from the media during all conditions, indicating fueling of their uptake by either glycolytically or oxidatively derived energy. The total content of glutamine increased, except during incubation in glucose-deprived media, when it declined or remained constant. Changes in aspartate were negligible, suggesting oxidative degradation of aspartate-derived oxaloacetate during normoxia and its reduction to succinate during anoxia, driving regeneration of NAD+ from NADH. An increase of alanine was reduced in glucose-free media, whereas serine showed especially large increase during isolated glucose deprivation, suggesting its production from glutamine via 3-phosphoglycerate.

Up-regulation of 5-HT2B receptor density and receptor-mediated glycogenolysis in mouse astrocytes by long-term fluoxetine administration

The effects were studied of short-term (1 week) versus long-term (2-3 weeks) fluoxetine treatment of primary cultures of mouse astrocytes, differentiated by treatment with dibutyryl cyclic AMP. From previous experiments it is known that acute treatment with fluoxetine stimulates glycogenolysis and increases free cytosolic Ca2+ concentration ([Ca2+]i]) in these cultures, whereas short-term (one week) treatment with 10 microM down-regulates the effects on glycogen and [Ca2+]i, when fluoxetine administration is renewed (or when serotonin is administered). Moreover, antagonist studies have shown that these responses are evoked by activation of a 5-HT2, receptor that is different from the 5-HT2A receptor and therefore at that time tentatively were interpreted as being exerted on 5-HT2C receptors. In the present study the cultures were found by RT-PCR to express mRNA for 5-HT2A and 5-HT2B receptors, but not for the 5-HT2C receptor, identifying the 5-HT2 receptor activated by fluoxetine as the 5-HT2B receptor, the most recently cloned 5-Ht2 receptor and a 5-HT receptor known to be more abundant in human, than in rodent, brain. Both short-term and long-term treatment with fluoxetine increased the specific binding of [3H]mesulergine, a ligand for alL three 5-HT2 receptors. Long-term treatment with fluoxetine caused an agonist-induced up-regulation of the glycogenolytic response to renewed administration of fluoxetine, whereas short-term treatment abolished the fluoxetine-induced hydrolysis of glycogen. Thus, during a treatment period similar to that required for fluoxetine's clinical response to occur, 5-HT2B-mediated effects are initially down-regulated and subsequently up-regulated.

Guanosine-induced increase in free cytosolic calcium concentration in mouse astrocytes in primary cultures: does it act on an A3 adenosine receptor?

Purinergic receptors play an important role in the regulation of free cytosolic calcium concentration ([Ca(2+)](i)) in astrocytes. In the present study, 10 microM adenosine caused an increase in [Ca(2+)](i) in 85% of the cultures studied, i.e., primary cultures of mouse astrocytes, differentiated by culturing in the presence of dibutyryl cyclic AMP. Antagonist sensitivity and rapid desensitization suggested that it did so by acting on A3 receptors. Another biologically important purine, guanosine, also caused an increase in astrocytic [Ca(2+)](i) (at concentrations of 0.1-100 microM). Although this response did not show the same rapid desensitization as the response to adenosine, it may also have been exerted on an A3 receptor. It supports this idea that inosine also caused an increase in [Ca(2+)](i), because inosine is known to activate A3 receptors in mast cells and structurally is even more closely related to guanosine than is adenosine.

Increasing neurite outgrowth capacity of beta-amyloid precursor protein proteoglycan in Alzheimer's disease

Progressive cerebral deposition of beta-amyloid peptide either in blood vessels or around neurites is one of the most important features of Alzheimer's disease (AD). The beta-peptide, known as Abeta or A4, is produced by proteolytic cleavage of the amyloid precursor protein (APP). Two APP processing pathways have been proposed as physiological alternatives; only one of which leads to the production of Abeta or amyloidogenic peptides. However, we have little information regarding these processing pathways in the brain, or on whether posttranslational modifications such as glycosylation affect APP processing in vivo. Furthermore, the physiological function(s) of this protein in nervous tissue remains unclear, although modulatory roles in cell adhesion and neuritic extension have been suggested. It has been reported that APP may be glycosylated as a proteoglycan. We purified this APP population from human brain, and our data indicate that PG-APP supports neurite extension of hippocampal neurons. Neurons grown on this substratum showed an increased capacity to elongate neurites and increased neuritic "branching" compared to culture on laminin. These effects were enhanced with PG-APP samples obtained from AD brains. Our results suggest that this APP population may act as a neurite outgrowth and branching promoter and may thus play a role in some pathological conditions. These findings may have significant implications in understanding normal brain development and pathological situations (such as AD).

Neurite outgrowth inhibitors in gliotic tissue

Gliotic tissue is the major obstacle to axon regeneration after CNS injury. We designed tissue culture assays to search for molecules responsible for neurite outgrowth inhibition in gliotic tissue. All the inhibitory activity in injured brain tissue was located in a plasma membrane heparan-sulphate and condroitin-sulphate type-proteoglycan of apparent molecular weight 200 kDalton. The proteoglycan core protein (apparent MW 48,000 kD) was biologically inactive, whereas the glycosamine-glycan (GAG) chains accounted for the inhibitory activity. Because of its cell location and mode of induction, the inhibitor was called injured membrane proteoglycan, IMP. IMP prevented neurite outgrowth initiation when attached to the culture substrate and caused growth cone collapse when added in solution to neurons with already growing neurites. We concluded that IMP was responsible for preventing injured CNS fibre regeneration. Double-staining immunohistochemistry of normal and gliotic tissue with anti-IMP monoclonal antibodies together with glial and neuronal markers, permitted the unequivocal definition of inhibitor presenting cells by confocal microscopy. IMP-immunostaining in normal CNS was observed exclusively on neurons. However, after a lesion, immunostaining occurred primarily on intensely GFAP-positive reactive astrocytes, but not on OX-42 positive microglia. The availability of antibodies permitted rapid affinity-purification of the neurite inhibitor and comparison with similar molecules possibly expressed during development. IMP itself or a highly related form, was expressed in embryonic brain, reaching maximal expression around postnatal day 3 and decreasing strongly in normal adult tissue. Perinatal rat brain proteoglycans inhibited neurite outgrowth similarly, though not identically, to IMP. Our data suggest that perinatal membrane and injured membrane proteoglycans may differ in GAG composition. IMP-like immunoreactivity was also found in developing brain, predominantly in neurons in normal brain, associating after a lesion with reactive astrocytes. Thes results suggest that injury evokes re-expression of IMP previously expressed during CNS development. One of the monoclonal antibodies to IMP blocked inhibitory activity, restoring neurite outgrowth in vitro. We are currently preparing Fab fragments to test the possibility that the antibody may block inhibition of central sprout growth in vivo. The combined use of blocking antibody fragments to neurite outgrowth inhibitors and transplants of growth-promoting glia, may help in the repair brain and spinal cord lesions.

Cytodifferentiation potentiates aFGF-induced p21(ras)/Erk signaling pathway in rat cultured astrocytes

MBP kinase detection assay revealed that acidic FGF (aFGF) augmented MBP kinase activity in a dose-dependent manner in astrocytes (AC). The molar potency of this action of aFGF in dibutyryl cyclic AMP (DBcAMP)-treated AC was significantly higher than that in quiescent AC. Consistently, the molar potency of accumulation of p21(ras)-GTP by aFGF was significantly higher in DBcAMP-treated AC than in quiescent AC. However, binding study showed that B(max) and K(D) for [(125)I]aFGF in DBcAMP-treated AC were quite similar to those in quiescent AC. Furthermore, the expression levels of Grb2, SOS, and p21(ras) were not changed by treatment of AC with DBcAMP. These results suggest that cytodifferentiation potentiates the p21(ras)/Erk signaling pathway in AC in response to aFGF without changing the expression levels of signaling molecules mediating from the FGF receptor to p21(ras).

Developmental regulation of glutamate transporters and glutamine synthetase activity in astrocyte cultures differentiated in vitro

Glutamate plays an important role in brain development, physiological function, and neurodegeneration. Astrocytes control synaptic concentration of glutamate via the high affinity glutamate transporters, GLT-1 and GLAST, and the glutamate catabolizing enzyme, glutamine synthetase. In this study we show that astrocytes cultured from rat brain in various stages of development including embryonic (E18), postnatal (P1-P21) and mature (P50), show distinct patterns of GLT-1 and GLAST expression, glutamine synthetase activity, and phenotypic changes induced by dibutyryl-cyclic adenosine monophosphate. The transcripts for GLT-1 message were detectable in embryonic astrocytes only, whereas the GLAST message was highly expressed in E18 and P1-P4 astrocyte cultures, declined in P10-P21, and was undetectable in P50 astrocytes. Uptake of 3H-glutamate correlated well with GLAST expression in astrocyte cultures of all developmental stages. Glutamine synthetase activity significantly declined from high embryonic levels in P4 astrocytes and remained low throughout postnatal maturation. Exposure of astrocyte cultures to the differentiating agent, db-cAMP (250-500 microM; 6 days), resulted in a pronounced stellation, up-regulation of GLT-1 and GLAST in E18, and GLAST in P4 cultures, while it was ineffective in P10 astrocytes. By contrast, db-cAMP induced a more pronounced stimulation of glutamine synthetase activity (up to 10-fold above basal) in P10 than in E18 cultures (up to 2 times above basal). The differences in expression/inducibility of glutamate transporters and glutamine synthetase observed in astrocyte cultures derived from various stages of fetal and postnatal development suggest that astrocytes in vivo might also respond differently to environmental or injurious stimuli during development and maturation.

Amyloid precursor protein proteoglycan is increased after brain damage

The beta-amyloid peptide (Abeta or A4) is produced by proteolytic cleavage from amyloid precursor protein (APP). The progressive cerebral deposition of this peptide is one of the most important features of Alzheimer's disease. From the study of normal and transfected cells, two APP processing pathways have been proposed as physiological alternatives. One of these can produce Abeta or amyloidogenic peptides, whereas the second does not. However, it is not completely clear how APPs are post-translationally modified, proteolytically processed and metabolized in the brain. We report here that APPs also exist as proteoglycan, chondroitin-sulfate (ChS). We have identified in normal rat brain a complex pool of 8 to 130 kDa ChS-core proteins. The main portion of these proteoglycan (PGs) APPs contains complete amyloidogenic sequence, suggesting a novel proteolytic processing of APP from the amino-terminal to the transmembrane region. This population appears augmented after brain damage. These findings may have significant implications in understanding the initial deposition and kinetics of amyloid aggregation in a pathological situation like Alzheimer's disease.

Reactive astrocyte formation in vivo is regulated by noradrenergic axons

Beta adrenergic receptor antagonists greatly reduce reactive astrocyte formation induced by neuronal degeneration. To test the hypothesis that the density of noradrenergic innervation is a factor in the regulation of astrocytosis, we measured glial fibrillary acidic protein (GFAP) optical density after neuronal injury in central nervous system (CNS) regions with permanent noradrenergic sprouting or norepinephrine (NE) depletion. The injury model employs the injection of Ricinus communis lectin into a cranial or peripheral nerve to destroy CNS neurons without the blood-brain barrier disruption and lymphocyte infiltration associated with contusive or surgical lesions. We took advantage of the lack of an NE transporter in the terminals of certain classes of noradrenergic axons to produce noradrenergic sprouting in the trigeminal motor nucleus (MoV) with neonatal 6-hydroxydopamine (6-OHDA) treatment and to produce depletion of NE in the spinal cord dorsal horn with N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine hydrochloride (DSP-4) administration. In each of these regions, GFAP optical density in the region of reactive astrocytes on the Ricin lectin-treated side was compared with the untreated contralateral (control) side in animals with NE hyperinnervation or NE depletion. GFAP density was increased about 55% in the injured NE-hyperinnervated MoV and was decreased about 35% in the injured NE-depleted dorsal horn. The degree of reactive astrocyte formation to injury is known to vary in different regions of the CNS, and our results suggest that differences in noradrenergic innervation may contribute to this variation. Along with earlier findings that beta-adrenergic receptor blockade reduces reactive astrocyte formation, these data indicate that the noradrenergic innervation is a factor in the degree of astrocyte reactivity following injury.

Differential cellular response after glutamate analog hippocampal damage

The tissue response after brain damage implicates the cellular "activation" of astrocytes and microglia. This glial response is referred as reactive gliosis. Using immunohistochemical markers, we have analyzed the neuronal and glial response to some neurotoxic-induced lesions. We have compared the effects of two glutamate analogs, AMPA and kainic acid, with those of traumatic injury. Our data showed that the time-course of appearance, the relative contribution of and the behavior of reactive astrocytes and microglial cells were clearly different after AMPA or kainic acid administration. The immunoreactivity associated with microglia response, with respect to the immunoreactivity associated with reactive astrocytes, was higher after AMPA damage than after kainic acid treatment. In both cases, however, glial cells were more abundant than after traumatic lesions. Interestingly, the CA1 pyramidal neurons affected by AMPA and some cortical neurons affected by traumatic injury responded with an overexpression of amyloid precursor protein, whereas no neuronal response was detected after the kainic acid treatment. Our data suggest that the gliotic response is highly specific to the type of insult and heterogeneous depending on the brain area affected.

Effect of ascorbic acid on hyperoxic rat astrocytes

The effect of ascorbic acid on cell size and ascorbic acid transport was studied in hyperoxic astrocytes. Subcultured rat astrocytes plated on poly-L-lysine-coated coverslips or on plastic dishes were exposed to serum-free culture medium and 20% or 42% ambient oxygen for 48 h. Vehicle (homocysteine) or L-ascorbic acid was added to the medium at 0 and 24 h. Cell size and relative optical density of glial fibrillary acidic protein-positive astrocytes were measured by a computerized imaging system. Cells on the dishes were used for ascorbic acid transport studies. Hyperoxia significantly increased the cell size of astrocytes, and this effect was inhibited by ascorbic acid. The rate of L-[14C]ascorbic acid Na(+)-dependent uptake was also inhibited by hyperoxia in vehicle-treated cultures but not in ascorbic acid-supplemented cultures. These results indicate that the presence of ascorbic acid during the hyperoxic episode can prevent astrocytic cell swelling and preserve membrane transport function.

Verapamil treatment attenuates immunoreactive GFAP at cerebral cortical lesion site

Cerebral cortical lesions were produced using a stereotactic injection system in Sprague-Dawley rats randomly assigned to three groups: (1) needle lesioned and uninjected (Lesioned), (2) needle lesion and simultaneous local injection of 50 or 100 microliters 0.9% saline (L/Saline), and (3) needle lesion and simultaneous local injection of 50 or 100 microliters Verapamil-HCl (VHCL) (2.5 mg/ml (5 mM) Abbott Labs, Chicago, IL), a passive, L-type calcium channel blocker (L/VHCL). The lesioning induced expression of glial fibrillary acidic protein (GFAP), a type of intermediate filament protein expressed in reactive astrocytes, at the lesion site. There was a reduction in GFAP-like immunoreactivity (GFAP-IR) in the L/VHCL group versus the Lesioned and the L/Saline groups. There was a five-fold increase of GFAP-IR at 24 h post lesion in the L/Saline group, but no statistically significant increase seen in the Lesioned or L/VHCL groups at either volume. Pretreatment of the anti-GFAP with VHCL did not impair the antigen labeling. To determine whether differences in pHs, or volume could account for these findings, a second experiment was performed using pH-matched saline or VHCL in 10 microliters volume injected into contralateral hemispheres at the time of lesioning. There was an 80% reduction in GFAP-IR in the L/VHCL group at 72 h compared with the L/Saline group. These data suggest that VHCL may suppress the early increase of GFAP-IR in response to cortical lesion and that reducing transmembrane calcium flux through L-type calcium channels may be the mechanism involved.

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.

Brain injury in a dish: a model for reactive gliosis

Reactive gliosis is a powerful response to brain injury and subsequent neuronal damage in vivo. Neuronal cell cultures are now well established as assays to study this process in vitro. However, equivalent studies of purified glial cell populations have only recently been achieved, following the realization that glial cells produce many of the neuropeptides, transmitters and growth factors that are produced also by neurons. There is now scope for studies in vitro that use mixed, identified populations of glial and neuronal cells to dissect the interactions between the two. Such cultures also lend themselves to assays for potential therapeutic strategies for brain injury that take account of all the different cell types found in the brain.

Characterization of a neurite outgrowth inhibitor expressed after CNS injury

Reactive gliosis, a general response to injury in the central system grey and white matter, represents a serious obstacle to axonal regeneration in mammals. In culture, myelin-free plasma membranes from normal rat brain tissue promoted neurite outgrowth, whereas myelin-free membranes purified from injured tissue were inhibitory. The inhibitory activity could be solubilized by detergent, was sensible to glycosaminoglycan lyase digestion and eluted with an apparent molecular weight of 160-220 kDa in gel filtration chromatography. When presented as a surface-bound molecule, the inhibitor prevented neurite initiation; when added in a soluble form to growing neurites, it induced their retraction. These results provide cellular and molecular evidence supporting the classical view that, in the mammalian central nervous system, damage-evoked gliosis correlates with the expression of molecules capable of preventing neurite outgrowth.