Molecular profile of reactive astrocytes--implications for their role in neurologic disease

Fulminant jejuno-ileitis following ablation of enteric glia in adult transgenic mice

To investigate the roles of astroglial cells, we targeted their ablation genetically. Transgenic mice were generated expressing herpes simplex virus thymidine kinase from the mouse glial fibrillary acidic protein (GFAP) promoter. In adult transgenic mice, 2 weeks of subcutaneous treatment with the antiviral agent ganciclovir preferentially ablated transgene-expressing, GFAP-positive glia from the jejunum and ileum, causing a fulminating and fatal jejuno-ileitis. This pathology was independent of bacterial overgrowth and was characterized by increased myeloperoxidase activity, moderate degeneration of myenteric neurons, and intraluminal hemorrhage. These findings demonstrate that enteric glia play an essential role in maintaining the integrity of the bowel and suggest that their loss or dysfunction may contribute to the cellular mechanisms of inflammatory bowel disease.

Immune regulation within the central nervous system

The brain constitutes an environment that is specifically designed to accommodate, regulate and shape immune responses. On one hand, the central nervous system (CNS) has traditionally been regarded as an immunologically privileged organ, owing to local tissue barrier and immunosuppressive microenvironment. On the other hand, activated microglia and astrocytes express MHC and adhesion/costimulatory molecules, release reactive oxygen intermediates and cytokines, and participate in local immune regulation. Bidirectional interactions between immune and neuroglial components occur in response to infectious and traumatic lesions. Glial cells may facilitate and amplify immune effector mechanisms within the CNS. Cytokines and chemokines within the CNS constitute a specialized CNS-cytokine network, and regulate the development and recovery from autoimmune diseases within the CNS. The interactions between glial cells and lymphoid cells are constituents of a complex immune regulatory system within the CNS. New data on the cross-talk between the CNS and the immune systems are envisaged, and followed by an attempt to create a synthesis of current knowledge.

Fibrillar beta-amyloid induces microglial phagocytosis, expression of inducible nitric oxide synthase, and loss of a select population of neurons in the rat CNS in vivo

To determine the stability of beta-amyloid peptide (Abeta) and the glial and neuronal changes induced by Abeta in the CNS in vivo, we made single injections of fibrillar Abeta (fAbeta), soluble Abeta (sAbeta), or vehicle into the rat striatum. Injected fAbeta is stable in vivo for at least 30 d after injection, whereas sAbeta is primarily cleared within 1 d. After injection of fAbeta, microglia phagocytize fAbeta aggregates, whereas nearby astrocytes form a virtual wall between fAbeta-containing microglia and the surrounding neuropil. Similar glial changes are not observed after sAbeta injection. Microglia and astrocytes near the injected fAbeta show a significant increase in inducible nitric oxide synthase (iNOS) expression compared with that seen with sAbeta or vehicle injection. Injection of fAbeta but not sAbeta or vehicle induces a significant loss of parvalbumin- and neuronal nitric oxide synthase-immunoreactive neurons, whereas the number of calbindin-immunoreactive neurons remains unchanged. These data demonstrate that fAbeta is remarkably stable in the CNS in vivo and suggest that fAbeta neurotoxicity is mediated in large part by factors released from activated microglia and astrocytes, as opposed to direct interaction between Abeta fibrils and neurons.

Fibrillar beta-amyloid induces microglial phagocytosis, expression of inducible nitric oxide synthase, and loss of a select population of neurons in the rat CNS in vivo

To determine the stability of beta-amyloid peptide (Abeta) and the glial and neuronal changes induced by Abeta in the CNS in vivo, we made single injections of fibrillar Abeta (fAbeta), soluble Abeta (sAbeta), or vehicle into the rat striatum. Injected fAbeta is stable in vivo for at least 30 d after injection, whereas sAbeta is primarily cleared within 1 d. After injection of fAbeta, microglia phagocytize fAbeta aggregates, whereas nearby astrocytes form a virtual wall between fAbeta-containing microglia and the surrounding neuropil. Similar glial changes are not observed after sAbeta injection. Microglia and astrocytes near the injected fAbeta show a significant increase in inducible nitric oxide synthase (iNOS) expression compared with that seen with sAbeta or vehicle injection. Injection of fAbeta but not sAbeta or vehicle induces a significant loss of parvalbumin- and neuronal nitric oxide synthase-immunoreactive neurons, whereas the number of calbindin-immunoreactive neurons remains unchanged. These data demonstrate that fAbeta is remarkably stable in the CNS in vivo and suggest that fAbeta neurotoxicity is mediated in large part by factors released from activated microglia and astrocytes, as opposed to direct interaction between Abeta fibrils and neurons.

Effects of feline immunodeficiency virus on astrocyte glutamate uptake: implications for lentivirus-induced central nervous system diseases

Feline immunodeficiency virus (FIV) is a lentivirus of domestic cats that causes a spectrum of diseases remarkably similar to AIDS in HIV-infected humans. As part of this spectrum, both HIV-1 and FIV induce neurologic disorders. Because astrocytes are essential in maintaining the homeostasis of the central nervous system, we analyzed FIV for the ability to infect feline astrocytes. Through immunocytochemistry and reverse transcriptase activity, it was demonstrated that two molecular clones of FIV (FIV-34TF10 and FIV-PPR) produce a chronic low level productive infection of feline astrocyte cultures. To investigate the consequences of this infection, selected astrocyte functions were examined. Infection with FIV-34TF10 significantly decreased the ability of astrocytes to scavenge extracellular glutamate (with a peak inhibition of 74%). The effects of the infection did not appear to be a result of toxicity but rather were more selective in nature because the glucose uptake function of the infected astrocyte cultures was not altered. Our data demonstrate that FIV productively infected, at a low level, feline astrocyte cultures, and as a consequence of this infection, an important astroglial function was altered. These findings suggest that a chronic low grade infection of astrocytes may impair the ability of these cells to maintain homeostasis of the central nervous system that, in turn, may contribute to a neurodegenerative disease process that is often associated with lentivirus infections.

Amyloid-beta peptide activates cultured astrocytes: morphological alterations, cytokine induction and nitric oxide release

A common feature of many neurodegenerative disorders is an abundance of activated glial cells (astrocytes and microglia). In Alzheimer's disease (AD), activated astrocytes are in close apposition to and surrounding the amyloid plaques. The mechanisms by which the astrocytes become activated in AD and the consequences of reactive astrocytosis to disease progression are not known. We examined the possibility that the amyloid-beta (Abeta) peptide, a major constituent of the amyloid plaque, could act as a stimulus leading to activation. We found that treatment of rat cortical astrocyte cultures with aggregated Abeta 1-42 peptide induces activation, as assessed by reactive morphological changes and upregulation of selective glial mRNA and proteins, such as the inflammatory cytokine interleukin-1beta. Abeta also stimulates inducible nitric oxide synthase (iNOS) mRNA levels and nitric oxide (NO) release. Abeta 1-42, a major form of amyloid associated with neurotoxicity, activated astrocytes in a time- and dose-dependent manner, whereas a scrambled Abeta 1-42 sequence or Abeta 17-42 had little or no effect. We also determined that the Abeta activity can be found in a supernatant fraction containing soluble Abeta oligomers. Our data suggest that Abeta plays a role in the reactive astrocytosis of AD and that the inflammatory response induced upon glial activation is a critical component of the neurodegenerative process.

Cytokines, signal transduction, and inflammatory demyelination: review and hypothesis

The mechanism of focal demyelination in multiple sclerosis has been a long-standing enigma of this disorder. Cytokines, a diverse family of signalling molecules, are viewed as potential mediators of the process based on clinical observations and studies with animal models and tissue/cell culture systems. Myelin and oligodendrocyte (OL) destruction occur in cultured preparations subjected to cytokines such as tumor necrosis factor-alpha (TNF alpha) and lymphotoxin (LT). Many studies have shown these and other cytokines to be elevated at lesion sites and in the CSF of multiple sclerosis (MS) patients, with similar findings in animal models. Some variability in the nature of MS lesion formation has been reported, both OLs and myelin being primary targets. To account for myelin destruction in the presence of apparently functional OLs we hypothesize that cytokines such as TNF alpha and LT alpha contribute to myelin damage through triggering of specific reactions within the myelin sheath. We further propose that neutral sphingomyelinase (SMase) is one such enzyme, two forms of which have been detected in purified myelin. An additional event is accumulation of cholesterol ester, apparently a downstream consequence of cytokine-induced SMase. The resulting lipid changes are viewed as potentially destabilizing to myelin, which may render it more vulnerable to attack by invading and resident phagocytes.

Inflammation in traumatic brain injury: role of cytokines and chemokines

A traumatic injury to the adult mammalian central nervous system (CNS), such as a stab wound lesion, results in reactive astrogliosis and the migration of hematogenous cells into the damaged neural tissue. The roles of cytokines and growth factors released locally by the damaged endogenous cells are recognized in controlling the cellular changes that occur following CNS injury. However, the role of chemokines, a novel class of chemoattractant cytokines, is only recently being studied in regulating inflammatory cell invasion in the injured/diseased CNS (1). The mRNAs for several chemokines have been shown to be upregulated in experimental allergic encephalomyelitis (EAE), an inflammatory demyelinating disease of the CNS, but chemokine expression in traumatic brain injury has not been studied in detail. Astrocytes have been demonstrated to participate in numerous processes that occur following injury to the CNS. In particular, astrocytic expression of cytokines and growth factors in the injured CNS has been well reviewed (2). Recently a few studies have detected the presence of chemokines in astrocytes following traumatic brain injury (3,4). These studies have suggested that chemokines may represent a promising target for future therapy of inflammatory conditions. This review summarizes the events that occur in traumatic brain injury and discusses the roles of resident and non-resident cells in the expression of growth factors, cytokines and chemokines in the injured CNS.

Effects of chronic placental insufficiency on brain development in fetal sheep

Clinical evidence has linked intrauterine compromise such as fetal hypoxemia to poor neurologic outcome in the newborn. In this study we examined the effects of inducing chronic fetal hypoxemia by impairment of placental function on brain development in fetal sheep. Placental insufficiency was induced from 120 to 140 d of gestation (term = 145-148 d) by injection of microspheres into the umbilical circulation in five fetal sheep. Fetal partial pressure of oxygen, PaO2, was reduced from 24.1 +/- 0.5 mm Hg before embolization to 14.8 +/- 0.4 mm Hg after embolization (p < 0.05). In another three fetuses a similar level of hypoxemia (PaO2, 13.8 +/- 0.4 mm Hg) occurred spontaneously. At 140 d of gestation the fetal brains were perfused with fixatives and compared with five control fetuses for the assessment of structural and immunohistochemical alterations. Hypoxemic fetuses demonstrated severe gliosis in the cerebral cortex and reduced myelination of subcortical white matter as visualized by glial fibrillary acidic protein and myelin basic protein staining, respectively (p < 0.05). White matter lesions were observed in two fetuses. The diameter of cerebral capillaries was increased in hypoxemic fetuses (p < 0.05), but there was no change in the number of nitric oxide synthase immunoreactive cells. Growth of neuronal processes was affected in the cerebellum, where there was also a reduction in the number of Purkinje neurons (p < 0.05). These results show that a prolonged period of placental insufficiency, resulting in moderate fetal hypoxemia during the last third of gestation, can affect neurodevelopmental processes that occur late in gestation such as myelination and growth of the cerebellum. This prenatal damage could affect neural connectivity and have functional consequences after birth.

Fatal encephalopathy with astrocyte inclusions in GFAP transgenic mice

Increased expression of glial fibrillary acidic protein (GFAP) is a hallmark of gliosis, the astrocytic hypertrophy that occurs during a wide variety of diseases of the central nervous system. To determine whether this increase in GFAP expression per se alters astrocyte function, we generated transgenic mice that carry copies of the human GFAP gene driven by its own promoter. Astrocytes of these mice are hypertrophic, up-regulate small heat-shock proteins, and contain inclusion bodies identical histologically and antigenically to the Rosenthal fibers of Alexander's disease. Mice in the highest expressing lines die by the second postnatal week. The results support the notion that Alexander's disease is a disorder of astrocytes, and provide an animal model for studying the causes and consequences of inclusion body disease.

Chronic neuroinflammation in rats reproduces components of the neurobiology of Alzheimer's disease

Inflammatory processes may play a critical role in the pathogenesis of the degenerative changes and cognitive impairments associated with Alzheimer's disease (AD). In the present study, lipopolysaccharide (LPS) from the cell wall of gram-negative bacteria was used to produce chronic, global inflammation within the brain of young rats. Chronic infusion of LPS (0.25 microgram/h) into the 4th ventricle for four weeks produced (1) an increase in the number of glial fibrillary acidic protein-positive activated astrocytes and OX-6-positive reactive microglia distributed throughout the brain, with the greatest increase occurring within the temporal lobe, particularly the hippocampus, (2) an induction in interleukin-1 beta, tumor necrosis factor-alpha and beta-amyloid precursor protein mRNA levels within the basal forebrain region and hippocampus, (3) the degeneration of hippocampal CA3 pyramidal neurons, and (4) a significant impairment in spatial memory as determined by decreased spontaneous alternation behavior on a T-maze.

Peptide receptors on astrocytes

In recent years, it has become apparent that astrocytes (at least in vitro) harbor functional receptors to almost all possible neurotransmitters (with the potential noticeable exception of acetylcholine nicotinic receptors). Peptides are no exception, since receptors to all neuropeptides known to be produced in the CNS have been found on cultured astrocytes, and the presence of many of these has been confirmed on astrocytes in vivo. A variety of methodologies have been used to detect peptide receptors on astrocytes, as summarized in the current review. Special emphasis is also put on the possible roles that peptides may play in the regulation of astrocyte functions. These include proliferation, morphology, release of eicosanoids and arachidonic acid, induction of calcium transients and calcium waves, and control of internal pH, glucose uptake, glycogen metabolism, and gap junctional conductance. Recent data concerning the effects of natriuretic peptides on astrocytes are reviewed, and why these peptides may constitute priviledged tools to test the effects of peptides on astrocyte-neuron interactions is also discussed.

Differential regulation of astrocytic mRNAs in the rat striatum after lesions of the cortex or substantia nigra

This study evaluates the time course of expression of three astrocytic mRNAs, glial fibrillary acidic protein (GFAP), apolipoprotein E (ApoE), and clusterin, in the rat striatum (ST) following a unilateral lesion of either the cortex (CX) or the substantia nigra (SN), using Northern blot and in situ hybridization analyses. We found that while there was a time-dependent increase in astrocytic GFAP mRNA in the deafferented ST following both the CX and the SN lesions, the time course of the response was different between the two lesion paradigms. Specifically, the increase in GFAP mRNA in striatal astrocytes after the SN lesion was rapid and transient returning to control levels by 10 days postlesion, while the response was long lasting and remained increased until at least 27 days after the CX lesion. In addition, the mRNA response for both ApoE and clusterin was differentially regulated in response to the two lesions. Specifically, both clusterin and ApoE mRNAs were rapidly increased in the ST following the CX lesion while both mRNAs remained unchanged following the SN lesion. Data from this study extend information derived from previous investigations on the multifunctional role of astrocytes in the response to brain injury. Specifically, our data support the notion that while the time course of the GFAP response in striatal astrocytes may vary between lesion paradigms, the upregulation of GFAP is part of a generalized response of reactive astrocytes to diverse brain injuries. By comparison, upregulation of the mRNAs for the lipoproteins clusterin and ApoE are lesion specific and may play a role in the transport of recycled myelin lipids from dying axons to actively growing axons and dendrites in reactive synaptogenesis.

Manganese augments nitric oxide synthesis in murine astrocytes: a new pathogenetic mechanism in manganism?

Since manganese (Mn2+) is known to be sequestered in glial cells, we investigated possible neurotoxic mechanisms involving astrocytes in vitro. Low concentrations of Mn2+ were toxic only in astrocyte-neuronal cocultures but not in pure astrocyte or neuronal cultures. As a possible mediator of manganese-derived neurotoxicity, we measured the production of nitric oxide in astrocytes. Manganese, but not other transition metals, dose dependently increased iNOS mRNA and protein levels and the release of nitric oxide in activated astrocytes. This effect was specific for astrocytes, since we observed no stimulation in microglial cells. The observations suggest that besides the known inhibition of mitochondrial function the neurotoxic effect of manganese in low concentrations might be mediated by the increased production of nitric oxide in astrocytes.

Functional IL-4 receptors on mouse astrocytes: IL-4 inhibits astrocyte activation and induces NGF secretion

IL-4 is a Th2-derived cytokine which plays an important role in the function of various immunocompetent cells as well as in the pathophysiology of various CNS disorders. In this study we characterized the expression of IL-4R in cultured astrocytes and explored the effects of IL-4 on the function of these cells. We found that astrocytes express the mRNA of both the membrane-bound and the soluble forms of the IL-4R, whereas they do not secrete IL-4. IL-4 inhibited both NO production and iNOS expression induced by LPS stimulation and decreased the secretion of TNF-alpha and the expression of ICAM-1. In contrast, IL-4 induced the secretion of NGF by astrocytes and synergized with LPS and TNF-alpha in this effect. These results suggest an important role for IL-4 as an immunosuppressive and a neurotrophic factor in the CNS.

Reactive astrogliosis in neonatal rat spinal cord after exposure to cerebrospinal fluid from patients with amyotrophic lateral sclerosis

Previous studies have proposed the presence of circulating toxic factor(s) in the cerebrospinal fluid (CSF) of patients with amyotrophic lateral sclerosis (ALS). In the present study we show that there is an increased number of astrocytes intensely immunoreactive for glial fibrillary acidic protein (GFAP) in the gray matter of the spinal cords of neonatal rats exposed to ALS CSF. There is also increased expression of GFAP in the astrocytes of the white matter of neonatal rat spinal cords exposed to ALS CSF. Western blot analysis also confirmed the increased expression of GFAP. Accordingly, our study provides for the first time a clear evidence for the pathological response of glia to the circulating toxic factor(s) in the CSF of ALS patients.

Substance P induces secretion of immunomodulatory cytokines by human astrocytoma cells

In human astrocytoma cell lines, substance P (SP) stimulated interleukin (IL)-8, IL-6, granulocyte macrophage colony-stimulating factor and leukemia inhibitory factor protein secretion. These SP effects were blocked by a specific NK1 tachykinin receptor antagonist. Further, SP stimulation increased the half-life of IL-6 and IL-8 messenger RNAs, suggesting that the synthesis of these cytokines is also regulated post-transcriptionally. SP-induced cytokine release was inhibited by staurosporine and phorbol 12-myristate 13-acetate desensitization suggesting protein kinase C involvement. The demonstration that SP affects cytokine production in glioma cells might be of relevance for the biology of such tumors.

Glutamate-modulated production of GABA in immortalized astrocytes transduced by a glutamic acid decarboxylase-expressing retrovirus

Replication-defective Moloney murine leukemia virus expressing the GAD67 gene under the control of the GFAP promoter was produced using selected clones of a fibroblast-packaging cell line. A spontaneously immortalized astrocyte cell line was infected with this virus and cellular clones expressing GAD67 selected. Astrocyte and fibroblast clones expressed functional GAD (detected by glutamic acid decarboxylation), but only fibroblasts were able to also produce GABA in the extracellular medium. When exposed to 200 microM glutamate, despite an observed difference in the rates of glutamate accumulation in control and GAD67-expressing astrocytes, similar proportions of glutamate taken up were detected. In GAD67-expressing astrocytes, the glutamate was mainly converted into GABA, suggesting GAD transgene activity to be dominant over other glutamate metabolic pathways, such as glutamine synthetase and glutamate dehydrogenase. Moreover, rapid GABA release into the cell medium was also observed, suggesting the involvement of reverse GABA transporters. The use of the GFAP promoter might be able to take advantage of its activation in response to factors inducing reactive gliosis observed in pathological insults. GAD67-expressing astrocytes might therefore be used for future grafting in pathological situations in which an excess of glutamate results in neuronal dysfunction or cell death.

Expression of nitric oxide synthase-2 in glia associated with CNS pathology

This chapter discusses the expression of nitric oxide synthase-2 (NOS-2) in glia associated with central nervous system (CNS) pathology. The production of nitric oxide (NO) in the nervous system is catalyzed by three, highly homologous isoforms of NO synthase (NOS). NOS-2, the dimeric, heme-containing, soluble protein whose activity is independent of a rise in intracellular calcium, is variously termed ‘inducible,’ ‘immunologic,’ and ‘macrophage NOS (macNOS).’ Nitric oxide inhibits not only NOS-2 activity but also regulates the level of NOS-2 messenger RNA (mRNA) expression through a mechanism involving NF-^K B. There is specific evidence for the glial expression of NOS-2 associated with neuronal injury and infection of the CNS and in neurodegenerative and demyelinating diseases. Direct injury in the CNS results in a reactive gliosis, characterized by the induction of the glial fibrillary acidic protein gene and changes in astrocyte morphology.

The A3 adenosine receptor mediates cell spreading, reorganization of actin cytoskeleton, and distribution of Bcl-XL: studies in human astroglioma cells

The pathophysiological role of the adenosine A3 receptor in the central nervous system is largely unknown. We have investigated the effects of the selective A3 receptor agonist 2-chloro-N6-(3-iodobenzyl)-adenosine, Cl-IB-MECA, in cells of the astroglial lineage (human astrocytoma ADF cells). A marked reorganization of the cytoskeleton, with appearance of stress fibers and numerous cell protrusions, was found following exposure of cells to low (nM) concentrations of Cl-IB-MECA. These "trophic" effects were accompanied by induction of the expression of Rho, a small GTP-binding protein, which was virtually absent in control cells, and by changes of the intracellular distribution of the antiapoptotic protein Bcl-XL, that, in agonist-exposed cells, became specifically associated to cell protrusions. This is the first demonstration that the intracellular organization of Bcl-XL can be modulated by the activation of a G-protein-coupled membrane receptor, such as the A3 adenosine receptor. Moreover, modulation of the astrocytic cytoskeleton by adenosine may have intriguing implications in both nervous system development and in the response of the brain to trauma and ischemia.

Physiological and pathological roles of interleukin-6 in the central nervous system

The cytokine interleukin-6 (IL-6) is an important mediator of inflammatory and immune responses in the periphery. IL-6 is produced in the periphery and acts systemically to induce growth and differentiation of cells in the immune and hematopoietic systems and to induce and coordinate the different elements of the acute-phase response. In addition to these peripheral actions, recent studies indicate that IL-6 is also produced within the central nervous system (CNS) and may play an important role in a variety of CNS functions such as cell-to-cell signaling, coordination of neuroimmune responses, protection of neurons from insult, as well as neuronal differentiation, growth and survival. IL-6 may also contribute to the etiology of neuropathological disorders. Elevated levels of IL-6 in the CNS are found in several neurological disorders including AIDS dementia complex, Alzheimer's disease, multiple sclerosis, systemic lupus erythematosus, CNS trauma, and viral and bacterial meningitis. Moreover, several studies have shown that chronic overexpression of IL-6 in transgenic mice can lead to significant neuroanatomical and neurophysiological changes in the CNS similar to that commonly observed in various neurological diseases. Thus, it appears that IL-6 may play a role in both physiological and pathophysiological processes in the CNS.

Effect of global system for mobile communication microwave exposure on the genomic response of the rat brain

The acute effect of global system for mobile communication (GSM) microwave exposure on the genomic response of the central nervous system was studied in rats by measuring changes in the messenger RNAs of hsp70, the transcription factor genes c-fos and c-jun and the glial structural gene GFAP using in situ hybridization histochemistry. Protein products of transcription factors, stress proteins and marker proteins of astroglial and microglial activation were assessed by immunocytochemistry. Cell proliferation was evaluated by bromodeoxyuridine incorporation. A special GSM radiofrequency test set, connected to a commercial cellular phone operating in the discontinuous transmission mode, was used to simulate GSM exposure. The study was conducted at time averaged and brain averaged specific absorption rates of 0.3 W/kg (GSM exposure), 1.5 W/kg (GSM exposure) and 7.5 W/kg (continuous wave exposure), respectively. Immediately after exposure, in situ hybridization revealed slight induction of hsp70 messenger RNA in the cerebellum and hippocampus after 7.5 W/kg exposure, but not at lower intensities. A slightly increased expression of c-fos messenger RNA was observed in the cerebellum, neocortex and piriform cortex of all groups subjected to immobilization, but no differences were found amongst different exposure conditions. C-jun and GFAP messenger RNAs did not increase in any of the experimental groups. 24 h after exposure, immunocytochemical analysis of FOS and JUN proteins (c-FOS, FOS B, c-JUN JUN B, JUN D), of HSP70 or of KROX-20 and -24 did not reveal any alterations. Seven days after exposure, neither increased cell proliferation nor altered expression of astroglial and microglial marker proteins were observed. In conclusion, acute high intensity microwave exposure of immobilized rats may induce some minor stress response but does not result in lasting adaptive or reactive changes of the brain.

Activated astrocytes induce nitric oxide synthase-2 in cerebral endothelium via tumor necrosis factor alpha

Astrocytes under pathological conditions become activated and produce a variety of cytokines and low molecular weight signal molecules. Previously we demonstrated that activated astrocytes release nitric oxide which can downregulate the expression of nitric oxide synthase (NOS)-2 in co-cultured cerebral endothelium, and also release a transcriptionally regulated factor that can induce NOS-2 expression in endothelium (Borgerding and Murphy: J Neurochem 65:1342, 1995). The activity of this NOS-2-inducing factor was impeded by inhibitors of tyrosine kinases and NF-kappaB activation. Tumor necrosis factor (TNF alpha) alone, or in combination with IL-6, induced NOS-2 expression in endothelial cells. A neutralizing antibody against TNF alpha attenuated the NOS-2 expression in endothelial cells exposed to activated astrocytes. These results imply that cytokine-activated astrocytes release TNF alpha which can induce NOS-2 expression in endothelium and suggest that activated astrocytes within the CNS may induce expression of NOS-2 in cells of the adjacent microvasculature. The ensuing alterations in blood-brain barrier properties may be either beneficial or detrimental.

Reactive astrocytes: cellular and molecular cues to biological function

For several decades, the reactive gliosis that occurs after an injury to the CNS has been considered one of the major impediments to axonal regeneration. Nevertheless, recent studies have suggested that in certain conditions, reactive astrocytes may provide a permissive substratum to support axonal regrowth. The important criteria, allowing for the distinction between permissive and non-permissive gliosis, are the ultrastructural 3D organization of the scar and more importantly the recognition molecules expressed by reactive astrocytes. Reactive astrocytes express surface molecules and produce various neurotrophic factors and cytokines. The latter in turn might modulate the production of recognition molecules by reactive astrocytes, allowing them to support post-lesional axonal regrowth. Although numerous recent articles have focused on cytokines and cell adhesion molecules, scant attention has been paid to reactive astrocytes. Reactive astrocytes should be considered a key element, like neurons, of a dynamic environment, thus forming with neurons a functional unit involved in homeostasis, plasticity and neurotransmission. Attempts are in progress to identify molecular markers for reactive astrocytes.

Optic disc and optic nerve of the blind cape mole-rat (Georychus capensis): a proposed model for naturally occurring reactive gliosis

Recent studies of the visual system of animal species that live in a subterranean environment show not only regressive but also progressive morphological features. In this regard the aim of the present investigation is to describe the structural organisation of the eye and optic nerve of the adult Cape mole-rat, with special emphasis on both glial cell population and myelination. The main results are: (a) astrocytes show identical features to those occurring in reactive gliosis; (b) optic fibers vary greatly in diameter; (c) very small axons are myelinated and are often surrounded by a thicker sheath than larger optic fibers; (d) a large onion bulb-like structure composed of optic fibers, glia, and ganglion cells is found within the choriocapillary layer. These results suggest that the Cape mole-rat and probably other subterranean rodents may serve as a model to study spontaneous gliosis as well as mechanisms involved in myelination and degenerative processes.

Regulation of astroglial-derived dopaminergic neurotrophic factors by interleukin-1 beta in the striatum of young and middle-aged mice

Interleukin-1 beta (IL-1 beta) can induce dopaminergic axonal sprouting in the denervated striatum of parkinsonian animals. In order to determine whether IL-1 beta effects on dopaminergic axonal sprouting are mediated by the induction of astroglial-derived dopaminergic neurotrophic factors, effects of IL-1 beta treatment on acidic and basic fibroblast growth factor (aFGF and bFGF) and glial cell line-derived growth factor (GDNF) gene expression were examined in primary striatal astrocyte cultures and after in vivo administration. We found a selective induction of bFGF mRNA synthesis but not aFGF or GDNF mRNA after IL-1 beta treatment both in vitro and in vivo. This suggests that bFGF may be the putative endogenous dopaminergic neurotrophic factor mediating lesion-induced plasticity of dopamine neurons. In addition, to determine why recovery from injury becomes reduced with age, we examined whether there was an aging-associated decline in the ability of IL-1 beta to induce the synthesis of neurotrophic factors in middle-aged animals compared to young mice. Interestingly, IL-1 beta stimulated a greater induction in bFGF mRNA levels in the middle-aged mice compared to young mice. These results suggest that the regulation of bFGF and possibly its receptor signaling efficacy may vary as the brain ages.

Function in neurotoxicity: index of effect and also determinant of vulnerability

1. In neurotoxicity, functional indices may be the only available measures of effect, as many potent neurotoxic agents produce no morphological change. Examples of these are strychnine, dieldrin and pyrethroids, which produce excitation but no pathology, and barbiturates, xylene and lithium, which produce depression but no pathology. 2. In other cases where both functional and morphological effects are seen, functional measures often produce the most convenient, if not always the most specific, indices of toxicity. Appropriate functional measures can be highly sensitive, both in humans and in experimental animals, and can also give vital mechanistic information. However, it is essential that functional measures are reproducible and interpretable (some behavioural measures are not) and also provide a reasonably exacting test of function (passive observation of resting behaviour can miss many effects). 3. In addition to their use as an index of toxicity, changes in function, even within the normal range, can themselves influence susceptibility to toxins. Tissue perfusion can determine delivered dose and is influenced by function, while metabolic transformation is modified by nutritional state. Nutritional state can also influence absorption, with anaemia enhancing manganese toxicity and calcium deficiency enhancing lead toxicity. Functional activity can influence target susceptibility directly: thus, noise exposure enhances the ototoxicity of carbon monoxide, toluene or aminoglycoside antibiotics; noise, motor activity or anaesthesia all influence the central neurotoxicity of dinitrobenzene or metronidazole; motor activity enhances the peripheral nerve toxicity of lead or thallium; and nerve regeneration enhances the toxicity of hexane. These functional factors can be very important in determining individual susceptibility.

Regulation of GDNF expression in cultured astrocytes by inflammatory stimuli

Astrocytes express increased levels of neurotrophic factors in response to pathological conditions in the CNS such as injury and inflammation. We have examined the effects of lipopolysaccharide (LPS) and inflammatory cytokines on the expression of GDNF by mouse astrocytes and by C6 glial cells. LPS and tumor necrosis factor-alpha (TNF-alpha) induced an increase in level of glial-derived neurotrophic factor (GDNF) mRNA in both cell types. Similarly, the synthesis of GDNF protein was increased by both treatments. Interleukin-1beta (IL-1beta) and interferon-gamma (IFN-gamma) induced similar effects on GDNF production, whereas IL-2 and IL-6 had no significant effects. These results indicate that the expression of GDNF in astrocytes is regulated by inflammatory stimuli and therefore may provide neurotrophic support to injured neurons in inflammatory conditions in the CNS.

Astrocyte-mediated enhancement of neuronal survival is abolished by glutathione deficiency

Astrocytes promote the survival of neurons. Conditions characterized by loss of neurons, such as aging and aging-related neurodegenerative disorders, are accompanied by both disturbances in astrocyte-neuron interactions and signs of oxidative damage. Neuronal glutathione, a major antioxidant in the brain, is maintained by astrocytes and brain levels of glutathione are reduced in named conditions. Therefore, we focused on a possible link between glutathione deficiency and loss of astrocyte-derived neuronal support. For this purpose, we used a coculture system consisting of rat striatal astrocytes and mesencephalic, dopaminergic (DAergic) neurons. Using tyrosine hydroxylase immunocytochemistry and radiolabeled dopamine uptake as parameters, an increase in the number and outgrowth of DAergic neurons was noted in cocultures as compared to cultures of mesencephalic neurons alone. This enhanced survival of DAergic neurons in cocultures was abolished following depletion of glutathione with buthionine sulfoximine. As demonstrated by glial fibrillary acidic protein immunocytochemistry and a microtiter tetrazolium assay, under these conditions no change in astrocyte survival occurred. However, glutathione depletion in cocultures was accompanied by loss of astrocyte-mediated neuroprotection against hydrogen peroxide toxicity. Thus, our results indicate that glutathione is important for the maintenance of the neuronal support function of astrocytes and that glutathione deficiency in the brain may lead to enhanced vulnerability of neurons to (oxidative) damage.

GFAP and S100beta expression in the cortex and hippocampus in response to mild cortical contusion

We studied the acute response of glial fibrillary acidic protein (GFAP) and S100beta gene expression in the cerebral cortex and hippocampus to mild unilateral cortical contusion. Our goal was to evaluate and compare the expression patterns of each gene in the early stages of the astrocytic response to brain injury. RNA was extracted from the cerebral cortex and hippocampus of male rats at 0, 3, 12, 24, or 96 h after lesion or sham-operation, then quantified using an RNase protection assay. Contusion produced a robust elevation in GFAP mRNA by 12 h in both brain regions on the ipsilateral side to the contusion. In the cortex, but not the hippocampus, this elevation was sustained at 96 h. S100beta mRNA levels were elevated bilaterally in lesioned animals at 24 h in both brain regions. However, these data are difficult to interpret because sham mRNA levels decreased with time, making it unclear whether contusion stimulates S100beta gene expression or whether it mitigates the inhibitory effect of sham. We further analyzed the effect of contusion on GFAP and S100beta immunoreactive astrocyte density at 96 h postlesion or postsham by double-label immunocytochemistry. All detectable astrocytes under all conditions were S100beta immunoreactive in both brain regions. Furthermore, all S100beta immunoreactive astrocytes in the lesioned ipsilateral cortex were also GFAP immunoreactive, whereas only about 11% of S100beta positive cells were also GFAP labeled in the contralateral lesioned or the ipsilateral sham cortex. In the hippocampus, all S100beta immunoreactive cells were also GFAP immunoreactive under all conditions. These data correlate with the gene expression data at 96 h, and suggest that, at least in the cortex, resident S100beta-expressing astrocytes produce GFAP at levels that are undetectable by immunocytochemistry until they are activated in response to injury.

Functional characterization of substance P receptors on cultured human spinal cord astrocytes: synergism of substance P with cytokines in inducing interleukin-6 and prostaglandin E2 production

Following brain injury, astrocytes express receptors for cytokines and neuropeptides and secrete several regulatory mediators that have a well established role in inflammation, immunity, and tissue development or repair. To elucidate the role of substance P (SP), a neurotransmitter peptide of the tachykinin family, in inducing astrocyte secretory activities, we have examined the expression of SP receptors and the functional consequences of their activation in cultured astrocytes from the human embryonic brain or spinal cord. Radioligand binding studies revealed that only one type of SP receptors, the high affinity NK-1 receptor, was present on human astrocytes and that spinal cord astrocytes expressed about 6 times as many SP binding sites as brain astrocytes. Following SP treatment, a substantial inositol phosphate formation was observed in spinal cord astrocytes only. Stimulation of spinal cord astrocytes with SP alone did not induce secretion of cytokines [interleukin-6 (IL-6), granulocyte-macrophage-CSF, macrophage chemoattractant protein-1 or leukemia inhibitory factor] or prostaglandin E2 (PGE2). Interestingly, however, SP selectively potentiated the inducing effect of IL-1beta on IL-6 and PGE2 secretion by spinal cord astrocytes without affecting the IL-1-beta-evoked secretion of other cytokines. SP also enhanced the small inducing effect of tumor necrosis factor-alpha (TNF-alpha) on IL-6 and PGE2 secretion and that of transforming growth factor-beta on PGE2 secretion. These results suggest that SP can enhance immunoregulatory and neurotrophic astroglial functions mediated by IL-6 and PGE2 by acting in concert with a set of cytokines whose cerebral expression has been reported during development and in a variety of diseases.

A time-dependent increase in glial fibrillary acidic protein expression and glutamine synthetase activity in long-term subculture of the GL15 glioma cell line

1. Astrocytes are the most numerous cellular elements in the central nervous tissue, where they play a critical role in physiological and pathological events. The biological signals regulating astrocyte growth and differentiation are relevant for both physiology and pathology, but they are still little understood. 2. Using a poorly differentiated glioma cell line, GL15, we investigated whether, in long-term subculture, this could upregulate the expression of glial fibrillary acidic protein (GFAP), as described in some rodent astrocyte cell lines. Under the same culture conditions, we investigated glutamine synthetase (GS) activity, growth-associated protein (GAP)-43 expression, and expression of several neutrotrophic factors. 3. A dramatic increase in GFAP expression was evidenced by Western blotting during progressive in vitro growth of GL15 cells. GS specific activity was also upregulated in long-term culture. The time spent in vitro by GL15 cells did not affect GAP-43 and neutrophic factor BDNF and NT3 expression as revealed by RT-PCR analysis. 4. Our results suggest that, in GL15, GFAP and GS genes may have common or integrated regulatory mechanisms elicited at the cell confluency which could be relevant for both astrocyte physiology and astrocyte pathology. These mechanisms are not involved in GAP-43 and neutrophic factor BDNF and NT3 expression.

Obtention and characterization of primary astrocyte and microglial cultures from adult monkey brains

Simple methods for obtention of primary cultures of isolated astrocytes and microglia from adult simian brain have been developed. Characterization of these two glial cell populations were performed by morphological observations and by immunocytochemistry. The astroglial cultures were obtained by an indirect method. After L-leucine methyl-ester treatment and trypsinizations, more than 99% of cells expressed glial fibrillary acidic protein (GFAP), whereas no macrophages or microglia could be detected. Likely, the 1% remaining cells were immature astrocytes or cells that lost their GFAP expression. Cultured simian astrocytes expressed vimentin, laminin, and fibronectin. We also found a constitutively low expression of major histocompatibility complex (MHC) class II by cultured astrocytes which was significantly enhanced by lipopolysaccharide (LPS), interferon gamma (IFN-gamma), or tumor necrosis factor alpha (TNF-alpha) treatments. Microglial cultures were obtained by a direct method of isolation using Percoll gradient separations and compared to simian monocyte-derived macrophages or alveolar macrophages. Microglial cells differed from macrophages by their proliferation upon granulocyte-macrophage colony stimulating factor (GM-CSF) treatment and by their typical morphology when observed by scanning electron microscopy. As macrophages, they expressed in vitro CD68, CD64, CD14, CD11b, MHC class II, and fibronectin. However, contrary to macrophages, simian cultured microglia expressed laminin. This observation suggests that microglia represent a new potential source of this extracellular matrix protein in the brain.

Glial and endothelial cell response to a fetal transplant of purified neurons

Astrocytes, microglia and endothelial cells display very specific phenotypic characteristics in the intact adult CNS, which appear quite versatile when grown in culture without neurons. Indirect evidence from in vitro co-culture studies and analysis of the effects of specific neuronal removal in vivo, does accordingly favour a role of neurons for the phenotypic repression of these cells in the intact brain. In order to provide more direct evidence for such neuronal influence, we attempted to induce, in the rat brain, a reversal of the post-lesional activation of astrocytes, microglia and endothelial cells by transplantation of fetal neurons purified by immunopanning. Host microglial cells which have been activated by the lesion process, penetrated the neuronal graft during the few days after the transplantation. Reactive astrocytes began to appear in the lesioned parenchyma and gathered around the transplant. Thereafter they first sent their processes in the direction of the neuronal graft, before they migrated into the graft a few days later. At this time, which was at the end of the first week post-transplantation, the host endothelial cells sprouted "streamers" of basal lamina within the graft forming small capillaries. During the second week post-transplantation, numerous astrocytes and microglial cells, both displaying a reactive hypertrophied morphology, were observed throughout the grafts. Finally, by the end of the first month, the activated cells differentiated towards a quiescent, resting morphology. At this time the grafts contained a vascular network with morphological characteristics comparable to those observed in the intact brain parenchyma. The results indicate that the interaction of activated astroglia and microglia and endothelial cells with neurons causes the cells to re-differentiate and regain phenotypic features characteristic of intact brain parenchyma, strongly suggesting that neurons play an essential role in the phenotypic restriction of glial and endothelial cells in the adult central nervous system.

Neurotoxic consequences of central long-term administration of interleukin-2 in rats

Interleukin-2 is an immunoregulatory cytokine with several recently established CNS activities. Central effects of interleukin-2 include growth promotion for neuronal and glial cells as well as modulatory influences on neurotransmission and hormone release. However, little is known about the consequences in the CNS of chronically elevated levels of interleukin-2. Alterations in the interleukin-2/interleukin-2 receptor system are not only associated with CNS trauma, inflammation and certain neuropathologies; elevated interleukin-2 concentrations are especially induced during the therapeutic use of interleukin-2 in cancer treatments. In the present study, intracerebroventricular (i.c.v.) interleukin-2 infusions (5 15 U/h) were performed in Sprague Dawley rats for up to 14 days. Interleukin-2-treated animals showed significantly increased plasma levels of corticosterone indicating an hyperfunctioning of the hypothalamic-pituitary-adrenocortical axis that lasted over the 14 day infusion period. Moreover, the performance of interleukin-2-treated animals in the Morris swim maze task was transiently impaired. Quantitative receptor autoradiographic analyses revealed changes in the binding levels of cholinergic M1 and M2 as well as dopaminergic D1 and D2 receptors in selected brain areas in which interleukin-2 was shown to modulate neurotransmission and which are enriched with interleukin-2 receptor expression. Decreased receptor binding levels were observed in the frontoparietal cortex (M2, D1, D2), hippocampal CA1 region (M1, M2) and the nucleus accumbens (D2). Histological and immunohistochemical examination of the brains of interleukin-2-treated animals revealed multiple alterations. Interleukin-2 treatment resulted in an intracranial accumulation of non-neural, MHC class II-positive cells as well as T and B lymphocytes within the infused brain hemisphere. Cellular infiltrates were associated with angiogenesis and the deposition of extracellular matrix material, such as fibronectin. Adjacent brain regions that were partly invaded and dislodged by the cellular masses were characterized by reactive astrogliosis, microglial activation, endothelial upregulation of adhesion molecules, myelin damage and neuronal loss. Together the data suggest that persistently elevated central levels of interleukin-2 can interfere with several CNS functions and may lead to nervous tissue injury. These findings could be relevant to CNS pathologies characterized by abnormal interleukin-2 production and to central responses to interleukin-2 treatments.

Bone morphogenetic proteins: neurotrophic roles for midbrain dopaminergic neurons and implications of astroglial cells

Bone morphogenetic proteins (BMPs) are members of the transforming growth factor beta (TGF-beta) superfamily that have been implicated in tissue growth and remodelling. Recent evidence suggests that several BMPs are expressed in the developing and adult brain. Specifically, we show that BMP 2 and BMP 6 are expressed in the developing midbrain floor of the rat. We studied potential neurotrophic effects of BMPs on the in vitro survival, transmitter uptake and protection against MPP+ toxicity of mesencephalic dopaminergic neurons cultured from the embryonic midbrain floor at embryonic day (E) 14. At 10 ng/ml and under serum-free conditions, most BMPs promoted the survival of dopaminergic neurons visualized by tyrosine hydroxylase immunocytochemistry during an 8-day culture period, but to varying extents (relative potencies: BMP 6 = 12 > 2, 4, 7). BMPs 6 and 12 were as effective as fibroblast growth factor-2 (FGF-2) and glial cell line-derived neurotrophic factor, promoting survival 1.7-fold compared with controls. BMPs 9 and 11 were not effective. Dose-response curves revealed an EC50 for BMPs 2, 6 and 12 of 2 ng/ml. BMPs 2, 4, 6, 7, 9 and 12 also promoted DNA synthesis and astroglial cell differentiation, visualized by 5-bromodeoxyuridine (BrdU) incorporation and glial fibrillary acidic protein (GFAP) immunocytochemistry respectively. Suppression of cell proliferation and subsequent maturation of GFAP-positive cells by 5-fluorodeoxyuridine or aminoadipic acid abolished the neuron survival-promoting effect of BMP 2. This suggests that BMPs, like other non-TGF-beta factors affecting dopaminergic neuron survival, act indirectly, probably by stimulating the synthesis and/or release of glial-derived trophic factors. BMP 6 and BMP 7 also increased the uptake of [3H]dopamine without affecting the uptake of [3H]5-hydroxytryptamine and [3H]GABA, underscoring the specificity of the trophic effect. We conclude that several BMPs share a neurotrophic capacity for dopaminergic midbrain neurons with other members of the TGF-beta superfamily, but act indirectly, possibly through glial cells.

The isolation and characterization of a novel G protein-coupled receptor regulated by immunologic challenge

Using a degenerate PCR based approach, a fragment of the novel G protein-coupled receptor, VTR 15-20, was identified from the rat ventral tegmentum. Hybridization screening and RACE PCR were employed to isolate the full length clone. The cDNA encodes a protein of 305 amino acids which shares homology to several orphan as well as known G protein-coupled receptors. Amino acid analysis demonstrates the VTR 15-20 contains specific regions conserved among the G protein-coupled receptor superfamily. Messenger RNA encoding VTR 15-20 is expressed throughout the mammalian nervous system. Using primary rat culture systems we have demonstrated the expression of VTR 15-20 mRNA in both microglia and astrocytes. The highest levels of VTR 15-20 mRNA expression are detected in peripheral tissues including the spleen. Moreover, we have found that the expression of VTR 15-20 mRNA in brain and spleen is regulated by immunologic challenge. Based on the cellular distribution and regulation by immune challenge and neuronal insult, we hypothesize that VTR 15-20 plays a role in neuroimmune function.

Temporal profiles of the in vitro phosphorylation rate and immunocontent of glial fibrillary acidic protein (GFAP) after kainic acid-induced lesions in area CA1 of the rat hippocampus: demonstration of a novel phosphoprotein associated with gliosis

The in vitro phosphorylation rate and immunocontent of glial fibrillary acidic protein was studied in slices of area CA1 of the rat hippocampus after stereotaxic injection of 1 nmol of kainic acid. For controls the contralateral hippocampus was injected with saline. Hippocampal tissue was incubated with [32P]phosphate and analysed by two-dimensional electrophoresis for phosphorylation rate and by immunoblotting for immunocontent. Both these parameters decreased during the first 4 days after injection and then started to increase at 10 days and continued to increase until at least 84 days. Except for a small excess of phosphorylation rate at 28 days, the relationship between immunocontent and in vitro phosphorylation rate of glial fibrillary acidic protein remained constant, indicating that the reactive gliosis was not associated with hypo- or a major hyperphosphorylation of this protein. Histology showed a pronounced loss of CA1 pyramidal cells 1 day after injection. At 28 days after injection the pyramidal cells had disappeared and only a few abnormal neurones were present. In contrast, immunocytochemistry after 28 days showed a marked increase in astrocytes reacting positive to the antibody in the strata radiatum and lacunosum moleculare. Besides glial fibrillary acidic protein the expression of several other proteins was upregulated as a result of the injection of kainic acid. These included phosphovimentin and an unknown phosphoprotein designated pp25 which co-migrated on 2-D gels with a prominent phosphoprotein expressed in primary cultures of astrocytes. Pp25 was expressed in lesioned tissue more frequently than phosphovimentin and with a time course that started earlier. Of particular interest was the expression of pp25 in the contralateral saline-injected hippocampus 1 day after injection of kainic acid. It is possible that pp25 will prove to be a sensitive marker of gliosis.

Characterization of the signalling pathways involved in ATP and basic fibroblast growth factor-induced astrogliosis

1. A brief challenge of rat astrocytes with either alpha, beta-methyleneATP (alpha, beta-meATP) or basic fibroblast growth factor (bFGF) resulted, three days later, in morphological differentiation of cells, as shown by marked elongation of astrocytic processes. The P2 receptor antagonist suramin prevented alpha, beta-meATP- but not bFGF-induced astrocytic elongation. Similar effects on astrocytic elongation were also observed with ATP and other P2 receptor agonists (beta, gamma meATP, ADP beta S, 2meSATP and, to a lesser extent, UTP). 2. Pertussis toxin completely abolished alpha, beta-meATP- but not bFGF-induced effects. No effects were exerted by alpha, beta-meATP on cyclic AMP production; similarly, neomycin had no effects on elogation of processes induced by the purine analogue, suggesting that adenylyl cyclase and phospholipase C are probably not involved in alpha, beta-meATP-induced effects (see also the accompanying paper by Centemeri et al., 1997). The tyrosine-kinase inhibitor genistein greatly reduced bFGF- but not alpha, beta-meATP-induced astrocytic elongation. 3. Challenge of cultures with alpha, beta-meATP rapidly and concentration-dependently increased [3H]-arachidonic acid (AA) release from cells, suggesting that activation of phospholipase A2 (PLA2) may be involved in the long-term functional effects evoked by purine analogues. Consistently, exogenously added AA markedly elongated astrocytic processes. Moreover, various PLA2 inhibitors (e.g. mepacrine and dexamethasone) prevented both the early alpha, beta-meATP-induced [3H]-AA release and/or the associated long-term morphological changes, without affecting the astrocytic elongation induced by bFGF. Finally, the protein kinase C (PKC) inhibitor H7 fully abolished alpha, beta-meATP- but not bFGF-induced effects. 4. Both alpha, beta-meATP and bFGF rapidly and transiently induced the nuclear accumulation of Fos and Jun. Both c-fos and c-jun induction by the purine analogue could be fully prevented by pretreatment with suramin. In contrast, the effects of bFGF were unaffected by this P2 receptor antagonist. 5. It was concluded that alpha, beta-meATP- and bFGF-morphological differentiation of astrocytes occurs via independent transductional pathways. For the purine analogue, signalling involves a Gi/G(o) protein-coupled P2Y-receptor which may be linked to activation of PLA2 (involvement of an arachidonate-sensitive PKC is speculated); for bFGF, a tyrosine kinase receptor is involved. Both pathways merge on some common intracellular target, as suggested by induction of primary response genes, which in turn may regulate late response genes mediating long-term phenotypic changes of astroglial cells. 6. These findings implicate P2 receptors as novel targets for the pharmacological regulation of reactive astrogliosis, which has intriguing implications in nervous system diseases characterized by degenerative events.

Effects of pro-inflammatory cytokines in experimental spinal cord injury

Following injury to the spinal cord, secondary tissue damage leading to massive additional tissue loss and inflammatory reactions as well as scar formation takes place. The precise functions and effects of the inflammatory cells and their secreted factors are largely unclear. The present study investigates whether the exogenous local administration of pro-inflammatory cytokines to mice after spinal cord injury can influence these intrinsic processes. A mixture of murine recombinant interleukin-1beta (IL-1beta), interleukin-6 (IL-6) and tumour necrosis factor alpha (TNF alpha) was administered to the lesioned spinal cord of adult mice. These cytokines provoked an increased recruitment and activation of macrophages and microglial cells in the lesion area when administered 1 day post lesion. In contrast, when administered 4 days after the lesion, recruitment of macrophages was slightly increased while activation of microglia was decreased as compared to controls. The amount of tissue loss 7 days after trauma was smaller in the animals receiving the cytokine mixture than in the mice receiving Ringer control solution on day 4 after lesion. Thus the role of the inflammatory response in spinal cord injury seems to be complex and well regulated. Anti-inflammatory cytokines and factors probably also contribute to the outcome of the damage following injury to the spinal cord.

Interrelationships between astrocyte function, oxidative stress and antioxidant status within the central nervous system

Astrocytes have, until recently, been thought of as the passive supporting elements of the central nervous system. However, recent developments suggest that these cells actually play a crucial and vital role in the overall physiology of the brain. Astrocytes selectively express a host of cell membrane and nuclear receptors that are responsive to various neuroactive compounds. In addition, the cell membrane has a number of important transporters for these compounds. Direct evidence for the selective co-expression of neurotransmitters, transporters on both neurons and astrocytes, provides additional evidence for metabolic compartmentation within the central nervous system. Oxidative stress as defined by the excessive production of free radicals can alter dramatically the function of the cell. The free radical nitric oxide has attracted a considerable amount of attention recently, due to its role as a physiological second messenger but also because of its neurotoxic potential when produced in excess. We provide, therefore, an in-depth discussion on how this free radical and its metabolites affect the intra and intercellular physiology of the astrocyte(s) and surrounding neurons. Finally, we look at the ways in which astrocytes can counteract the production of free radicals in general by using their antioxidant pathways. The glutathione antioxidant system will be the focus of attention, since astrocytes have an enormous capacity for, and efficiency built into this particular system.

Expression of pro-inflammatory cytokine and chemokine mRNA upon experimental spinal cord injury in mouse: an in situ hybridization study

Injury to the spinal cord induces a complex cascade of cellular reactions at the local lesion area: secondary cell death and inflammatory reactions as well as scar and cavity formation take place. In order to investigate the molecular features underlying this local wounding response and to determine their pathophysiological implications, we studied the expression pattern of pro-inflammatory and chemoattractant cytokines in an experimental spinal cord injury model in mouse. We show by in situ hybridization that transcripts for the pro-inflammatory cytokines TNF alpha and IL-1 as well as the chemokines MIP-1alpha and MIP-1beta are upregulated within the first hour following injury. In this early phase, the expression of the pro-inflammatory cytokines is restricted to cells in the surroundings of the lesion area probably resident CNS cells. While TNF alpha is expressed in a very narrow time window, IL-1 can be detected in a second phase in a subset of polymorphonuclear granulocytes which immigrate into the spinal cord around 6 h. Message for the chemokines MIP-1alpha and beta is expressed in a generalized way in the grey matter of the entire spinal cord around 24 h and gets again restricted to the cellular infiltrate at the lesion site at 4 days following injury. Interestingly, our data suggest that resident CNS cells, most probably microglial cells, and not peripheral inflammatory cells, are the main source for cytokine and chemokine mRNAs. The defined cytokine pattern observed indicates that the inflammatory events upon lesioning the CNS are tightly controlled. The very early expression of pro-inflammatory cytokine and chemokine messages may represent an important element of the recruitment of inflammatory cells. Additional pathophysiological consequences of the specific cytokine pattern observed remain to be determined.

Toxicity of ricin and volkensin, two ribosome-inactivating proteins, to microglia, astrocyte, and neuron cultures

Ricin and volkensin, two potent toxins belonging to the family of ribosome-inactivating proteins (RIPs), have been largely exploited in recent years in in vivo experiments of neuronal degeneration consequent to suicide transport or immunolesioning. We have determined both the toxicity of, and the inhibition of, protein synthesis by ricin and volkensin in in vitro cultures enriched in microglial cells, astrocytes, or neurons. In microglial cultures, 50% of toxicity (estimated by LDH released from dead cells) after 24 h exposure to RIPs was obtained with volkensin at 2.2x10(-12) M concentration and 50% of protein synthesis inhibition at 2x10(-14) M concentration. Both values were higher by about one order of magnitude in astrocyte-enriched cultures. Toxicity of, and inhibition of, protein synthesis by, ricin were lower for both cell types by about 1 order of magnitude as compared to volkensin. Cerebellar granule neurons in culture survived remarkably well to 24 h exposure to ricin or volkensin, although their protein synthesis was effectively inhibited by the two toxins with a potency similar to that found for astrocytes. These results demonstrate that glial cells, in particular microglia, are very sensitive to RIPs toxicity and should, therefore, be a primary target of these toxins when injected in vivo. Thus, the damage observed after in vivo experiments could be partly related to diffusion of toxic substances from early-affected glial cells.

Tenascin-R is an intrinsic autocrine factor for oligodendrocyte differentiation and promotes cell adhesion by a sulfatide-mediated mechanism

O4(+) oligodendrocyte (OL) progenitors in the mammalian CNS are committed fully to terminal differentiation into myelin-forming cells. In the absence of other cell types in vitro, OL differentiation reproduces the in vivo development with a correct timing, suggesting the existence of an intrinsic regulatory mechanism that presently is unknown. We have examined the effect of two isoforms of the extracellular matrix (ECM) molecule tenascin-R (TN-R), which is expressed by OLs during the process of myelination, on the adhesion and maturation of OLs in vitro. Here we show that the substrate-bound molecules supported the adhesion of O4(+) OLs independently of the CNS region or age from which they were derived. At the molecular level this process was mediated by protein binding to membrane surface sulfatides (Sulf), as indicated by the interference of O4 antibody and Sulf with the attachment of OLs or other Sulf+ cells, erythrocytes, to TN-R substrates and by direct protein-glycolipid binding studies. In the absence of platelet-derived growth factor (PDGF), exogenous TN-R induced myelin gene expression and the upregulation of its own synthesis by cultured cells, resulting in a rapid terminal differentiation of O4(+) progenitors. Our findings strongly suggest that TN-R represents an intrinsic regulatory molecule that controls the timed OL differentiation by an autocrine mechanism and imply the relevance of TN-R for CNS myelination and remyelination.

Tenascin-R is an intrinsic autocrine factor for oligodendrocyte differentiation and promotes cell adhesion by a sulfatide-mediated mechanism

O4(+) oligodendrocyte (OL) progenitors in the mammalian CNS are committed fully to terminal differentiation into myelin-forming cells. In the absence of other cell types in vitro, OL differentiation reproduces the in vivo development with a correct timing, suggesting the existence of an intrinsic regulatory mechanism that presently is unknown. We have examined the effect of two isoforms of the extracellular matrix (ECM) molecule tenascin-R (TN-R), which is expressed by OLs during the process of myelination, on the adhesion and maturation of OLs in vitro. Here we show that the substrate-bound molecules supported the adhesion of O4(+) OLs independently of the CNS region or age from which they were derived. At the molecular level this process was mediated by protein binding to membrane surface sulfatides (Sulf), as indicated by the interference of O4 antibody and Sulf with the attachment of OLs or other Sulf+ cells, erythrocytes, to TN-R substrates and by direct protein-glycolipid binding studies. In the absence of platelet-derived growth factor (PDGF), exogenous TN-R induced myelin gene expression and the upregulation of its own synthesis by cultured cells, resulting in a rapid terminal differentiation of O4(+) progenitors. Our findings strongly suggest that TN-R represents an intrinsic regulatory molecule that controls the timed OL differentiation by an autocrine mechanism and imply the relevance of TN-R for CNS myelination and remyelination.