The p44/42 mitogen-activated protein kinase cascade is involved in the induction and maintenance of astrocyte stellation mediated by protein kinase C

Docosahexaenoic Acid (DHA) Induced Morphological Differentiation of Astrocytes Is Associated with Transcriptional Upregulation and Endocytosis of β2-AR

Docosahexaenoic acid (DHA), an important ω-3 fatty acid, is abundantly present in the central nervous system and is important in every step of brain development. Much of this knowledge has been based on studies of the role of DHA in the function of the neurons, and reports on its effect on the glial cells are few and far between. We have previously reported that DHA facilitates astrocyte differentiation in primary culture. We have further explored the signaling mechanism associated with this event. It was observed that a sustained activation of the extracellular signal-regulated kinase (ERK) appeared to be critical for DHA-induced differentiation of the cultured astrocytes. Prior exposure to different endocytic inhibitors blocked both ERK activation and differentiation of the astrocytes during DHA treatment suggesting that the observed induction of ERK-2 was purely endosomal. Unlike the β₁-adrenergic receptor (β₁-AR) antagonist, atenolol, pre-treatment of the cells with the β₂-adrenergic receptor (β₂-AR) antagonist, ICI-118,551 inhibited the DHA-induced differentiation process, indicating a downstream involvement of β₂-AR in the differentiation process. qRT-PCR and western blot analysis demonstrated a significant induction in the mRNA and protein expression of β₂-AR at 18-24 h of DHA treatment, suggesting that the induction of β₂-AR may be due to transcriptional upregulation. Moreover, DHA caused activation of PKA at 6 h, followed by activation of downstream cAMP response element-binding protein, a known transcription factor for β₂-AR. Altogether, the observations suggest that DHA upregulates β₂-AR in astrocytes, which undergo endocytosis and signals for sustained endosomal ERK activation to drive the differentiation process.

Electroconductive nanoscale topography for enhanced neuronal differentiation and electrophysiological maturation of human neural stem cells

Biophysical cues, such as topography, and electrical cues can provide external stimulation for the promotion of stem cell neurogenesis. Here, we demonstrate an electroconductive surface nanotopography for enhancing neuronal differentiation and the functional maturation of human neural stem cells (hNSCs). The electroconductive nanopatterned substrates were prepared by depositing a thin layer of titanium (Ti) with nanograting topographies (150 to 300 nm groove/ridge, the thickness of the groove - 150 μm) onto polymer surfaces. The Ti-coated nanopatterned substrate (TNS) induced cellular alignment along the groove pattern via contact guidance and promoted focal adhesion and cytoskeletal reorganization, which ultimately led to enhanced neuronal differentiation and maturation of hNSCs as indicated by significantly elevated neurite extension and the upregulated expression of the neuronal markers Tuj1 and NeuN compared with the Ti-coated flat substrate (TFS) and the nanopatterned substrate (NS) without Ti coating. Mechanosensitive cellular events, such as β1-integrin binding/clustering and myosin-actin interaction, and the Rho-associated protein kinase (ROCK) and mitogen-activated protein kinase/extracellular signal regulated kinase (MEK-ERK) pathways, were found to be associated with enhanced focal adhesion and neuronal differentiation of hNSCs by the TNS. Among the neuronal subtypes, differentiation into dopaminergic and glutamatergic neurons was promoted on the TNS. Importantly, the TNS increased the induction rate of neuron-like cells exhibiting electrophysiological properties from hNSCs. Finally, the application of pulsed electrical stimulation to the TNS further enhanced neuronal differentiation of hNSCs due probably to calcium channel activation, indicating a combined effect of topographical and electrical cues on stem cell neurogenesis, which postulates the novelty of our current study. The present work suggests that an electroconductive nanopatterned substrate can serve as an effective culture platform for deriving highly mature, functional neuronal lineage cells from stem cells.

Thyroid Hormone and Astrocyte Differentiation

Thyroid hormones (THs) have important contributions to the development of the mammalian brain, targeting its actions on both neurons and glial cells. Astrocytes, which constitute about half of the glial cells, characteristically undergo dramatic changes in their morphology during development and such changes become necessary for the proper development of the brain. Interestingly, a large number of studies have suggested that THs play a profound role in such morphological maturation of the astrocytes. This review discusses the present knowledge on the mechanisms by which THs elicit progressive differentiation and maturation of the astrocytes. As a prelude, information on astrocyte morphology during development and its regulations, the role of THs in the various functions of astrocyte shall be dealt with for a thorough understanding of the subject of this review.

NF-κB transcriptional activation by TNFα requires phospholipase C, extracellular signal-regulated kinase 2 and poly(ADP-ribose) polymerase-1

BACKGROUND

The nuclear enzyme poly(ADP-ribose) polymerase-1 (PARP-1) is required for pro-inflammatory effects of TNFα. Our previous studies demonstrated that PARP-1 mediates TNFα-induced NF-κB activation in glia. Here, we evaluated the mechanisms by which TNFα activates PARP-1 and PARP-1 mediates NF-κB activation.

METHODS

Primary cultures of mouse cortical astrocytes and microglia were treated with TNFα and suitable signaling pathway modulators (pharmacological and molecular). Outcome measures included calcium imaging, PARP-1 activation status, NF-κB transcriptional activity, DNA damage assesment and cytokine relesease profiling.

RESULTS

TNFα induces PARP-1 activation in the absence of detectable DNA strand breaks, as measured by the PANT assay. TNFα-induced transcriptional activation of NF-κB requires PARP-1 enzymatic activity. Enzymatic activation of PARP-1 by TNFα was blocked in Ca(2+)-free medium, by Ca(2+) chelation with BAPTA-AM, and by D609, an inhibitor of phoshatidyl choline-specific phospholipase C (PC-PLC), but not by thapsigargin or by U73112, an inhibitor of phosphatidyl inisitol-specific PLC (PI -PLC). A TNFR1 blocking antibody reduced Ca(2+) influx and PARP-1 activation. TNFα-induced PARP-1 activation was also blocked by siRNA downregulation of ERK2 and by PD98059, an inhibitor of the MEK / ERK protein kinase cascade. Moreover, TNFα-induced NF-κB (p65) transcriptional activation was absent in cells expressing PARP-1 that lacked ERK2 phosphorylation sites, while basal NF-κB transcriptional activation increased in cells expressing PARP-1 with a phosphomimetic substitution at an ERK2 phophorylation site.

CONCLUSIONS

These results suggest that TNFα induces PARP-1 activation through a signaling pathway involving TNFR1, Ca(2+) influx, activation of PC-PLC, and activation of the MEK1 / ERK2 protein kinase cascade. TNFα-induced PARP-1 activation is not associated with DNA damage, but ERK2 mediated phosphorylation of PARP-1.

Pentoxifylline and propentofylline prevent proliferation and activation of the mammalian target of rapamycin and mitogen activated protein kinase in cultured spinal astrocytes

Astrocyte activation is an important feature in many disorders of the central nervous system, including chronic pain conditions. Activation of astrocytes is characterized by a change in morphology, including hypertrophy and increased size of processes, proliferation, and an increased production of proinflammatory mediators. The xanthine derivatives pentoxifylline and propentofylline are commonly used experimentally as glial inhibitors. These compounds are generally believed to attenuate glial activity by raising cyclic AMP (cAMP) levels and inhibiting glial tumor necrosis factor (TNF) production. In the present study, we show that these substances inhibit TNF and serum-induced astrocyte proliferation and signaling through the mammalian target of rapamycin (mTOR) pathway, demonstrated by decreased levels of phosphorylated S6 kinase (S6K), commonly used as a marker of mTOR complex (mTORC) activation. Furthermore, we show that pentoxifylline and propentofylline also inhibit JNK and p38, but not ERK, activation induced by TNF. In addition, the JNK antagonist SP600125, but not the p38 inhibitor SB203580, prevents TNF-induced activation of S6 kinase, suggesting that pentoxifylline and propentofylline may regulate mTORC activity in spinal astrocytes partially through inhibition of the JNK pathway. Our results suggest that pentoxifylline and propentofylline inhibit astrocyte activity in a broad fashion by attenuating flux through specific pathways.

Kallikrein 6 is a novel molecular trigger of reactive astrogliosis

Kallikrein-related peptidase 6 (KLK6) is a trypsin-like serine protease upregulated at sites of central nervous system (CNS) injury, including de novo expression by reactive astrocytes, yet its physiological actions are largely undefined. Taken with recent evidence that KLK6 activates G-protein-coupled protease-activated receptors (PARs), we hypothesized that injury-induced elevations in KLK6 contribute to the development of astrogliosis and that this occurs in a PAR-dependent fashion. Using primary murine astrocytes and the Neu7 astrocyte cell line, we show that KLK6 causes astrocytes to transform from an epitheliod to a stellate morphology and to secrete interleukin 6 (IL-6). By contrast, KLK6 reduced expression of glial fibrillary acidic protein (GFAP). The stellation-promoting activities of KLK6 were shown to be dependent on activation of the thrombin receptor, PAR1, as a PAR1-specific inhibitor, SCH79797, blocked KLK6-induced morphological changes. The ability of KLK6 to promote astrocyte stellation was also shown to be linked to activation of protein kinase C (PKC). These studies indicate that KLK6 is positioned to serve as a molecular trigger of select physiological processes involved in the development of astrogliosis and that this is likely to occur at least in part by activation of the G-protein-coupled receptor, PAR1.

Caspase activation contributes to astrogliosis

Caspases, a family of cysteine proteases, are widely activated in neurons and glia in the injured brain, a response thought to induce apoptosis. However, caspase activation in astrocytes following injury is not strongly associated with apoptosis. The present study investigates the potential role of caspase activation in astrocytes with another characteristic response to neural injury, astrogliosis. Caspase activity and morphological and biochemical indices of astrogliosis and apoptosis were assessed in (i) cultured neonatal rat astrocytes treated with astrogliosis-inducing stimuli (dibutryl cAMP, β-amyloid peptide), and (ii) cultures of adult rat hippocampal astrocytes generated from control and kainate-lesioned rats. The effects of broad spectrum and specific pharmacological caspase inhibitors were assessed on indicators of astrogliosis, including stellate morphology and expression of glutamine synthetase and fibroblast growth factor-2. Reactive neonatal and adult astrocytes demonstrated an increase in total caspase activity with a corresponding increase in the expression of active caspase-3 in the absence of cell death. Broad spectrum caspase inhibition with zVAD significantly attenuated increases in glutamine synthetase and fibroblast growth factor-2 in the reactive astrocytes. In the reactive neonatal astrocyte cultures, specific inhibition of caspases-3 and -11 also attenuated glutamine synthetase and fibroblast growth factor-2 expression, but did not reverse the morphological reactive phenotype. Astrogliosis is observed in all forms of brain injury and despite extensive study, its molecular triggers remain largely unknown. While previous studies have demonstrated active caspases in astrocytes following acute brain injury, here we present evidence functionally implicating the caspases in astrogliosis.

Experimental investigation of HGF inhibiting glial scar in vitro

To study the inhibitory effect of Hepatocyte growth factor (HGF) on the responsive hyperplasia of damaged astrocytes in vitro. We prepared damaged model of astrocytes to simulate the responsive hyperplasia of damaged astrocytes in vivo by culturing astrocytes in vitro; After the first day of Ad-HGF transfection, astrocytes were scratched, then after the first, the third, and the fifth day of scratch, we detect the expression amount of astrocytes specific glial fibrillary acidic protein (GFAP) and the ratio of S-phase cells with flow cytometry, both of which can reflect the proliferation status of damaged astrocytes; After HGF was added in scratched astrocytes, the activity of SPK and MAPK (P42/44) were detected by autoradiography and immunoblotting test; After adding different concentrations of HGF protein in astrocytes cultured in different serum concentrations and adding diverse concentrations of HGF protein, SPK and SPK inhibitor DMS in scratched astrocytes, we detect cell proliferation with 3H-TDR incorporation. The first day after Ad-HGF transfected astrocytes were scratched, the amount of GFAP secreted by astrocytes were decreased significantly (P < 0.05), and the cells in S phase were declined obviously. HGF has bidirectional regulation on SPK of scratched astrocytes: increases the SPK activity when HGF in low dose, while inhibits when in high dose. In addition, DMS can block the signal passage; HGF had no effects on MAPK (P42/44) of damaged astrocytes cells. In conclusion, after the transfection of Ad-HGF, it can inhibit the responsive hyperplasia of damaged astrocytes by the means of blocking SPK passage.

Serofendic acid promotes stellation induced by cAMP and cGMP analogs in cultured cortical astrocytes

We investigated the effect of serofendic acid, a neuroprotective substance derived from fetal calf serum, on the morphological changes in cultured cortical astrocytes. Cultured astrocytes developed a stellate morphology with several processes following exposure to dibutylyl cAMP (dbcAMP), a membrane-permeable cAMP analog; 8-Br-cGMP, a membrane-permeable cGMP analog; or phorbol-12-myristate-13-acetate (PMA), a protein kinase C activator. Serofendic acid significantly accelerated the stellation induced by dbcAMP- and 8-Br-cGMP. In contrast, the PMA-induced stellation was not affected by serofendic acid. Next, we attempted to elucidate the mechanism underlying the dbcAMP-induced stellation and explore the site of action of serofendic acid. Both the stellation induced by dbcAMP and the promotional effect of serofendic acid were partially inhibited by KT5720, a specific protein kinase A (PKA) inhibitor. Furthermore, serofendic acid failed to facilitate the stellation induced by Y-27632, an inhibitor of Rho-associated kinase (ROCK). These results indicate that serofendic acid promotes dbcAMP- and 8-Br-cGMP-induced stellation and the promotional effect on dbcAMP-induced stellation is mediated at least partly by the regulation of PKA activity and not by controlling ROCK activity.

Developmental regulation of SSeCKS expression in rat brain

SSeCKS (src suppressed C kinase substrate) was identified as a PKC substrate/PKC-binding protein, which plays a role in mitogenic regulatory activity and has a function in the control of cell signaling and cytoskeletal arrangement. Previous studies showed that expression of SSeCKS mRNA and protein levels were developmentally regulated in rat testis and the molecular might have some effects on the process of spermiogenesis. Here we carried out experiments to investigate the expression of SSeCKS in rat brain. Western blot analysis indicated that SSeCKS could be detected in the whole brain of developing rat embryos and reached its peak at 1 week after birth, while during mature period, its level was decreasing. Regional-distribution analysis showed that the expression pattern of SSeCKS in telencephalon, hippocampus and diencephalons was in accordance with the result from whole brain both in mRNA and protein level. However, in cerebellum, SSeCKS was almost in the same level, and in brainstem, the expression level was higher in 4-week-old rat brain than in 1-week-old one. Immunohistochemistry results showed SSeCKS was in diffused and granule-like distribution. Double immunofluorescence staining showed that it was expressed by some GFAP positive cells. All the results suggested that SSeCKS might affect brain development and further research is needed to have a good understanding of its function and mechanism.

The high affinity peripheral benzodiazepine receptor ligand DAA1106 binds to activated and infected brain macrophages in areas of synaptic degeneration: implications for PET imaging of neuroinflammation in lentiviral encephalitis

HIV encephalitis (HIVE) is characterized by neurodegeneration mediated by toxins derived from infected and activated brain macrophages. Since the peripheral benzodiazepine receptor (PBR) is abundant on brain macrophages, we hypothesized that [(3)H]DAA1106, a new PBR ligand, can label infected and activated brain macrophages in HIVE. Using cell culture and postmortem brain tissues from HIVE and a macaque model of HIVE, we show that [(3)H]DAA1106 binds with high affinity to activated and infected macrophages in regions of synaptic damage. Further, binding affinity reflected by lower K(D) (dissociation constant) values and the B(max) (total number of binding sites) to K(D) ratios reflective of ligand-binding potential was significantly higher with [(3)H]DAA1106 compared to the extensively characterized PBR ligand [(3)H](R)-PK11195. These data suggest that DAA1106 binds with high affinity to activated and infected brain macrophages and possesses binding characteristics beneficial for in vivo use in the detection and clinical monitoring of HIVE using positron emission tomography.

Changes of Src-suppressed C kinase substrate expression in cytokine induced reactive C6 glioma cells

OBJECTIVE

To investigate effect of tumor necrosis factor-alpha (TNF-alpha) on the Src-suppressed C kinase substrate (SSeCKS) in C6 glioma cells.

METHODS

Cultured C6 glioma cells were randomly divided into two groups. In time-dependent group, cells were cultured with TNF-alpha (2 ng/mL) for 0 h, 1 h, 3 h, 6 h, 12 or 24 h, respectively; in dose-dependent group, cells were cultured with TNF-alpha (0 ng/mL, 0.02 ng/mL, 0.2 ng/mL, or 2 ng/mL) for 6 h. The expression of SSeCKS was detected by Realtime PCR and Western blot analysis, and immunocytochemistry was used to investigate SSeCKS's subcellular localization.

RESULTS

TNF-alpha induced rapid phosphorylations of protein kinase C (PKC) substrates in C6 glioma cells, and upregulated SSeCKS expression in a time and concentration dependent manner. Immunocytochemistry suggested that SSeCKS was localized in the cyroplasm and the leading end of podosomal extensions in control groups, while TNF-alpha induced translocation of SSeCKS perinuclear. This effect could be partly reversed by PKC inhibitor Ro-31-8220.

CONCLUSION

TNF-alpha activates PKC and upregulates SSeCKS expression in C6 glioma cells. These effects are associated with PKC activity, suggesting that SSeCKS plays a role in response to glia activation in PKC mediated pathway.

A pharmacological activator of AMP-activated protein kinase (AMPK) induces astrocyte stellation

AMP-activated protein kinase (AMPK) represents a key energy-sensing molecule in many cell types. Because astrocytes are key mediators of metabolic signaling in the brain, we have initiated studies on the expression and activation of AMPK in these cells. Treatment of cultured rat cortical astrocytes with a pharmacological AMPK activator, AICA-riboside (AICAR) resulted in a time- and concentration-dependent increase in phosphorylation of AMPK and acetyl-CoA carboxylase (ACC), a direct substrate. AICAR treatment also induced a transition from epithelioid to stellate morphology in a time- and concentration-dependent manner. As stellation is indicative of actin cytoskeletal reorganization, the formation of stress fibers and focal adhesions in response to AICAR was assessed. AICAR-induced stellation correlated with F-actin disassembly and focal adhesion dispersal. Furthermore, transient transfection of an activated RhoA construct prevented AICAR-induced stellation, indicating a mechanism upstream of RhoA. Use of pharmacological inhibitor compound C prevented AICAR-induced stellation demonstrating necessity of AMPK activity for the response. Our findings suggest that AMPK mediates morphological alterations of astrocytes in response to energy depletion.

Src suppressed C kinase substrate regulates the lipopolysaccharide-induced TNF-α biosynthesis in rat astrocytes

The protein kinase C (PKC) is known to be a critical component in the signaling cascades that lead to astrocyte-activation. To further understand the mechanism of PKC signaling in astrocyte-activation, we investigated the effect of SSeCKS, a PKC substrate, on LPS-induced cytokine expression in astrocytes by RT-PCR and enzyme-linked immunosorbent assay. Exposure of the cells to LPS induced rapid translocation of SSeCKS to the perinuclear sides, ERK activation and pronounced TNF-alpha production, which can be inhibited by the PKC inhibitor Gö6983. By using siRNA knockdown of SSeCKS expression, LPS-induced signaling events were partly inhibited, including ERK activation, inducible TNF-alpha biosynthesis and secretion. These results suggest that SSeCKS is involved in the LPS-induced TNF-alpha expression in astrocytes mediated by PKC.

Cell type specific signalling by hematopoietic growth factors in neural cells

Correct timing and spatial location of growth factor expression is critical for undisturbed brain development and functioning. In terminally differentiated cells distinct biological responses to growth factors may depend on cell type specific activation of signalling cascades. We show that the hematopoietic growth factors thrombopoietin (TPO) and granulocyte colony-stimulating factor (GCSF) exert cell type specific effects on survival, proliferation and the degree of phosphorylation of Akt1, ERK1/2 and STAT3 in rat hippocampal neurons and cortical astrocytes. In neurons, TPO induced cell death and selectively activated ERK1/2. GCSF protected neurons from TPO- and hypoxia-induced cell death via selective activation of Akt1. In astrocytes, neither TPO nor GCSF had any effect on cell viability but inhibited proliferation. This effect was accompanied by activation of ERK1/2 and inhibition of STAT3 activity. A balance between growth factors, their receptors and signalling proteins may play an important role in regulation of neural cell survival.

Thyroid hormone-induced morphological differentiation and maturation of astrocytes involves activation of protein kinase A and ERK signalling pathway

Thyroid hormone (TH) has a profound effect on astrocyte differentiation and maturation. Astrocytes cultured under TH-deficient conditions fail to transform from flat polygonal morphology to mature, process-bearing, stellate cells. Supplementation of physiological concentrations of TH initiate gradual transformation of the cells and the process takes approximately 48 h to complete. The signal transduction pathways associated with TH-mediated maturation of astrocytes have been investigated. TH treatment caused an initial activation of protein kinase A (PKA), with a peak activity at 2 h which fell back to basal level there after. Although there was no visible change in morphology of the cells during the observed activation of PKA, it was sufficient to drive the process of transformation to completion, suggesting the involvement of downstream regulators of PKA. PKA inhibitors as well as the MEK inhibitor PD098059 attenuated the TH-induced morphological transformation. Further studies showed that TH treatment resulted in a biphasic response on the cellular phospho-MAP kinase (p-MAPK or p-ERK) level: an initial decline in the p-ERK level followed by an induction at 18-24 h, both of which could be blocked by a PKA inhibitor. Such sustained activation of p-ERK levels by TH at this later stage coincided with initiation of morphological differentiation of the astrocytes and appeared to be critical for the transformation of astrocytes. The nitric oxide synthase (NOS) inhibitor 7-NI inhibited this induction of p-ERK activity. Moreover, the induction was accompanied by a parallel increase in phospho-CREB activity which, however, persisted at the end of the transformation of the astroglial cells.

Opposing roles of ERK and p38 MAP kinases in FGF2-induced astroglial process extension

The stellate processes of astroglial cells undergo extensive remodeling in response to neural injury. Little is known about intracellular signaling mechanisms controlling process extension. We tested roles for the ERK and p38 MAP kinase pathways in a simplified culture model. FGF2-induced process extension was preceded by a strong and transient phosphorylation of ERK, and a modest activation of p38 MAP kinase, which exhibited significant basal activity. Phosphorylated ERK was found predominantly in the cytoplasm, whereas activated p38 MAP kinase was nuclear. Process extension was completely blocked by the specific MEK inhibitor U0126. Conversely, inhibition of the p38 MAP kinase pathway with SB202190 stimulated spontaneous process growth and greatly potentiated FGF2-induced process extension. The p38 inhibitor effect was reproduced with an adenovirus expressing dominant-negative p38 MAP kinase. Selective pharmacological blockade of MAP kinase pathways may enable modulation of the astroglial response to injury so as to promote neural regeneration.

Strain-induced vascular endothelial cell proliferation requires PI3K-dependent mTOR-4E-BP1 signal pathway

The aim of this study was to determine whether the phosphatidylinositol 3-kinase (PI3K)-dependent mammalian target of rapamycin (mTOR)-eukaryotic initiation factor 4E binding protein 1 (4E-BP1) signal pathway and S6 kinase (S6K), the major element of the mTOR pathway, play a role in the enhanced vascular endothelial cell (EC) proliferation induced by cyclic strain. Bovine aortic ECs were subjected to an average of 10% strain at a rate of 60 cycles/min for ≤24 h. Cyclic strain-induced EC proliferation was reduced by pretreatment with rapamycin but not the MEK1 inhibitor PD-98059. The PI3K inhibitors wortmannin and LY-294002 also attenuated strain-induced EC proliferation and strain-induced activation of S6K. Rapamycin but not PD-98059 prevented strain-induced S6K activation, and PD-98059 but not rapamycin prevented strain-induced activation of extracellular signal-regulated kinases 1 and 2. Cyclic strain also activated 4E-BP1, which could be inhibited by PI3K inhibitors. These data suggest that the PI3K-dependent S6K-mTOR-4E-BP1 signal pathway may be critically involved in strain-induced bovine aortic EC proliferation.

System L-amino acid transporters are differently expressed in rat astrocyte and C6 glioma cells

The system L-amino acid transporter is a major nutrient transport system that is responsible for Na+-independent transport of neutral amino acids including several essential amino acids. We have compared and examined the expressions and functions of the system L-amino acid transporters in both rat astrocyte cultures and C6 glioma cells. The rat astrocyte cultures expressed the l-type amino acid transporter 2 (LAT2) with its subunit 4F2hc, whereas the l-type amino acid transporter 1 (LAT1) was not expressed in these cells. The C6 glioma cells expressed LAT1 but not LAT2 with 4F2hc. The [14C]l-leucine uptakes by the rat astrocyte cultures and C6 glioma cells were Na+-independent and were completely inhibited by the system l selective inhibitor, BCH. These results suggest that the transport of neutral amino acids including several essential amino acids into rat astrocyte cultures and C6 glioma cells are for the most part mediated by LAT2 and LAT1, respectively. Therefore, the rat astrocyte cultures and C6 glioma cells are excellent tools for examining the properties of LAT2 and LAT1, respectively. Moreover, the specific inhibition of LAT1 in cancer cells might be a new rationale for anti-cancer therapy.

Deficiency of PAR-2 gene increases acute focal ischemic brain injury

The expression profile of the protease-activated receptor-2 (PAR-2) and effects of PAR-2 gene knockout (PAR-2 KO) on the infarct size were investigated after 60 minutes of transient middle cerebral artery occlusion (tMCAO) in mice in relation to phosphorylated extracellular signal-regulated kinase (p-ERK) and astrocyte activation. PAR-2 was normally distributed mainly in neurons of the central nervous system (CNS), and strongly upregulated at 8-24 hours after tMCAO. Deficiency of PAR-2 gene significantly increased the infarct volume and the number of TUNEL-positive cells at 24 hours of reperfusion. The strong neuronal expression of p-ERK was induced at 5 minutes as a peak after reperfusion in wild-type mice, but the signal change was significantly reduced in PAR-2 KO mice. Astroglial activation was also greatly inhibited at 24 hours after tMCAO in PAR-2 KO mice. These results show that the deficiency of PAR-2 gene increases the acute ischemic cerebral injury associating with suppression of neuronal ERK activation and reactive astroglial activation.

Cytoplasmic vacuolation in cultured rat astrocytes induced by an organophosphorus agent requires extracellular signal-regulated kinase activation

There are various toxic chemicals that cause cell death. However, in certain cases deleterious agents elicit various cellular responses prior to cell death. To determine the cellular mechanisms by which such cellular responses are induced is important, but sufficient attention has not been paid to this issue to date. In this study, we showed the characteristic effects of an organophosphorus (OP) agent, bis(pinacolyl methyl)phosphonate (BPMP), which we synthesized for the study of OP nerve agents, on cultured rat astrocytes. Morphologically, BPMP induced cytoplasmic vacuolation and stellation in the rat astrocytes. Cytoplasmic vacuolation is a cell pathological change observed, for example, in vacuolar degeneration, and stellation has been reported in astrocytic reactions against various stimuli. By pretreatment with cycloheximide, a protein synthesis inhibitor, stellation was inhibited, although vacuolation was not. Cell staining with a mitochondrion-selective dye indicated that the vacuolation probably occurs in the mitochondria that are swollen and vacuolatred in the center. Interestingly, the extracellular signal-regulated kinase (ERK) cascade inhibitor inhibited vacuolation and, to some extent, stellation. These results suggest that the ERK signaling cascade is important for the induction of mitochondrial vacuolation. We expect that a detailed study of these astrocytic reactions will provide us new perspectives regarding the variation and pathological significance of cell morphological changes, such as vacuolar degeneration, and also the mechanisms underlying various neurological disorders.

Delayed but sustained induction of mitogen-activated protein kinase activity is associated with β-adrenergic receptor-mediated morphological differentiation of astrocytes

Astroglial beta-adrenergic receptors (beta-ARs) are functionally linked to regulate cellular morphology. In primary cultures, the beta-AR agonist isoproterenol (ISP) can transform flat polygonal astrocytes into process-bearing, mature stellate cells by 48 h, an effect that can be blocked by the beta-AR antagonist, propranolol. ISP induced immediate activation of protein kinase A (PKA) which persisted up to 2 h, with no visible change in cell morphology. However, activation of PKA was sufficient to drive the process of transformation to completion, suggesting the involvement of downstream regulators of PKA. In addition to PKA inhibitors, the mitogen-activated protein kinase (MAPK) kinase inhibitor PD098059 also blocked ISP-induced morphological transformation. ISP treatment resulted in a biphasic response of cellular phosphorylated MAPK (phosphorylated extracellular signal-regulated kinase; p-ERK) level: an initial decline in p-ERK level followed by a sustained induction at 12-24 h, both of which were blocked by PKA inhibitor. The induction in pERK level coincided with initiation of morphological differentiation of the astrocytes and nuclear translocation of p-ERK. A long-lasting activation of p-ERK activity by ISP, at a later stage, appears to be critical for the transformation of astrocytes.

Glia-conditioned medium induces de novo synthesis of tyrosine hydroxylase and increases dopamine cell survival by differential signaling pathways

The mesencephalic astroglia-conditioned medium (GCM) greatly increases dopamine (DA) phenotype expression, and it also protects from spontaneous and toxin-induced cell death in midbrain cultures. In this study, we have investigated the signaling pathways implicated in those effects. Genistein at 5 microM, an inhibitor of tyrosine kinase receptors, and KT-5720, a protein kinase A inhibitor, blocked the GCM-induced effects on DA phenotype expression and DA cell survival but did not abolish the increased astrocytic (glial fibrillary acidic protein-positive; GFAP+) processes. We analyzed the role of phosphatidylinositol-3 kinase (PI-3K) on TH induction and cell survival, with the PI-3K inhibitors LY-294002 and wortmannin, and the role of the phosphorylation of mitogen-activated protein kinase (MAPK) with PD-98059, a p-ERK1/2 MAPK inhibitor. LY-294002 at 20-30 microM blocked the GCM-induced effects on TH expression and DA cell survival but did not abolish the increased astrocytic processes. PD-98059 at 20 and 40 microM blocked the GCM-induced effects on DA phenotype, cell survival, and GFAP expression. However, staurosporine at 10 nM, a protein kinase C inhibitor, only blocked the protective effects induced by GCM on midbrain cell apoptosis. The data presented herein show that tyrosine kinase receptors, cAMP-dependent protein kinase, PI-3K, and MAPK signaling pathways are implicated in de novo synthesis of TH+ cells induced by GCM as well as in DA cell apoptosis and that these effects are unrelated to increased GFAP expression. PKC inhibitors only abolished the GCM-induced effects on midbrain neuronal survival, suggesting that signaling pathways for DA phenotype expression and survival may be independent.

Phosphorylation of extracellular signal-regulated kinases 1/2 is predominantly enhanced in the microglia of the rat spinal cord following dorsal root transection

The present study was initiated to investigate the role of extracellular signal-regulated kinases (ERK) 1/2 signaling pathway in the early response of spinal cord and associated dorsal root ganglion (DRG) to rhizotomy by using Western blotting and immunohistochemical techniques in a rat model of L3 and L4 dorsal root transection. The results showed that there were a considerable amount of total and phosphorylated ERK 1/2 protein in both spinal cord and DRG in normal animals killed under pentobarbital anesthesia. The total ERK 1/2 distributed in both glia and neurons, while phosphorylated ERK 1/2 dominantly existed in the latter in the gray matter of spinal cord, as demonstrated with double immunofluorescent staining. Twenty-four and forty-eight hours after axotomy, the phosphorylation level of ERK 1/2 in the operation side of dorsal spinal cord was much higher than that in the contralateral side, while the total ERK 1/2 level seemed unchanged. The increased expression of Fos protein was also seen in the dorsal spinal cord at lesion side twelve and twenty-four hours after axotomy. Double fluorescent staining proved that the phosphorylated ERK 1/2 positive cells in the ipsilateral dorsal spinal cord after axotomy predominantly were microglia and small portion was oligodendrocytes, whereas the Fos expression was mainly in neurons. In normal DRG, most neurons, especially the medium and small-sized ones, and the satellite cells contained total ERK 1/2-like immunoreactivity, whereas only a small portion of neurons and satellite cells contained phosphorylated ERK 1/2. After unilateral dorsal rhizotomy, there were no detectable changes for the phosphorylation of ERK 1/2 in either neurons or satellite cells in DRG.Collectively, the present results suggest that both ERK and Fos signal pathways involve the cellular activation in the spinal cord following dorsal rhizotomy, with ERK mainly in microglia and Fos in neurons. The increase of phosphorylation of ERK 1/2 in microglia of spinal cord after rhizotomy implicates that ERK signaling pathway involves intracellular activity of microglia responding to the experimental injury.

Vasopressin-induced cytoplasmic and nuclear calcium signaling in embryonic cortical astrocytes: dynamics of calcium and calcium-dependent kinase translocation

The present study sought to determine the downstream consequences of V1a vasopressin receptor (V1aR) activation of Ca2+ signaling in cortical astrocytes. Results of these analyses demonstrated that V1aR activation led to a marked increase in both cytoplasmic and nuclear Ca2+. We also investigated V1aR activation of Ca2+-activated signaling kinases, protein kinase C (PKC), Ca2+/calmodulin-dependent protein kinase II (CaMKII), and the mitogen-activated protein (MAP) kinases [MAPK and extracellular signal-regulated kinases 1 and 2 (ERK1/2)], their localization within cytoplasmic and nuclear compartments, and activation of their downstream nuclear target, the transcription factor cAMP response element-binding protein (CREB). Results of these analyses demonstrated that V1aR activation led to a significant rise in PKC, CaMKII, and ERK1/2 activation, with CaMKII and ERK1/2 demonstrating dynamic transport between cytoplasmic and nuclear compartments. Although no evidence of PKC translocation was apparent, PKC and CaMKs were required for activation and nuclear translocation of ERK1/2. Subsequent to CaMKII and ERK1/2 translocation to the nucleus, CREB activation occurred and was found to be dependent on upstream activation of ERK1/2 and CaMKs. These data provide the first systematic analysis of the V1aR-induced Ca2+ signaling cascade in cortical astrocytes. In addition, results of this study introduce a heretofore unknown effect of vasopressin, dynamic Ca2+ signaling between the cytoplasm and nucleus that leads to comparable dynamics of kinase activation and shuttling between cytoplasmic and nuclear compartments. Implications for development and regeneration induced by V1aR activation of CREB-regulated gene expression in cortical astrocytes are discussed.

Astrocyte stellation induced by Rho kinase inhibitors in culture

To understand the role of Rho kinases in regulation of astrocyte morphology, we investigated the effects of Rho kinase inhibitors on the morphology of cultured rat cortical astrocytes. Cultured astrocytes exhibited flattened, polygonal morphology in the absence of stimulation, but changed into process-bearing stellate cells following treatment with the selective Rho kinase inhibitor Y-27632 (1-10 microM). The Y-27632-induced astrocyte stellation was abolished by treatment with colchicine, indicating that the response requires reorganization of cytoskeletal elements. The effect of Y-27632 was mimicked by another Rho kinase inhibitor HA1077, but not by the protein kinase C inhibitor GF-109203X or the protein kinase A inhibitor KT5720. These results suggest that Rho kinases are in an activated state in the absence of stimuli and contribute to the maintenance of polygonal morphology of cultured astrocytes.

Cytokine-stimulated inducible nitric oxide synthase expression in astroglia: role of Erk mitogen-activated protein kinase and NF-kappaB

Expression of inducible nitric oxide synthase (iNOS), which leads to the production of nitric oxide (NO), is stimulated by proinflammatory cytokines such as interleukin-1beta (IL-1beta) and tumor necrosis factor-alpha (TNF-alpha). Here we report on the roles of nuclear factor-kappaB (NF-kappaB) and mitogen-activated protein (MAP) kinases in IL-1beta/TNF-alpha-induced iNOS expression in adult rat astroglia. Cytokine-induced increases in nitrite accumulation (an index of NO production) and iNOS expression were attenuated by inhibition of NF-kappaB with pyrrolidine dithiocarbamate (PDTC). Similar attenuation of these cytokine-induced responses was produced by inhibition of MAP kinase (MEK), the immediate upstream activator of Erk, using PD098,059. Combined treatment of astroglia with PDTC and PD098,059 completely abolished the cytokine-induced increases in iNOS expression and nitrite accumulation. By contrast, the selective p38 kinase inhibitor SB203,580 amplified the effects of IL-1beta/TNF-alpha on nitrite accumulation. In accordance with these findings, IL-1beta- and TNF-alpha-induced a time-dependent increase in Erk1/Erk2 activation. This cytokine action was completely abolished by PD098,059 but was not altered by PDTC. Finally, IL-1beta and TNF-alpha induced degradation of NF-kappaB's bound inhibitory protein, IkappaB-alpha, leading to translocation of NF-kappaB into the nucleus. IkappaB-alpha expression was not restored to control levels by inhibition of MEK. Furthermore, inhibition of MEK with PD098,059 did not alter IL-1beta- and TNF-alpha-induced expression of active NF-kappaB. The results demonstrate that autonomous Erk and NF-kappaB pathways mediate cytokine-induced increases in iNOS expression in astroglia.

1alpha,25(OH)2-vitamin D3 inhibits HGF synthesis and secretion from MG-63 human osteosarcoma cells

Several mesenchymally derived cells, including osteoblasts, secrete hepatocyte growth factor (HGF). 1alpha,25(OH)(2)-vitamin D(3) [1,25(OH)(2)D(3)] inhibits proliferation and induces differentiation of MG-63 osteoblastic cells. Here we show that MG-63 cells secrete copious amounts of HGF and that 1,25(OH)(2)D(3) inhibits HGF production. MG-63 cells also express HGF receptor (c-Met) mRNA, suggesting an autocrine action of HGF. Indeed, although exogenous HGF failed to stimulate cellular proliferation, neutralizing endogenous HGF with a neutralizing antibody inhibited MG-63 cell proliferation; moreover, inhibiting HGF synthesis with 1,25(OH)(2)D(3) followed by addition of HGF rescued hormone-induced inhibition of proliferation. Nonneutralized cells displayed constitutive phosphorylation of c-Met and the mitogen-activated protein kinases mitogen/extracellular signal-regulated kinase (MEK) 1 and extracellular signal-regulated kinase (Erk) 1/2, which were inhibited by anti-HGF antibody. Constitutive phosphorylation of Erk1/2 was also abolished by 1,25(OH)(2)D(3). Addition of HGF to MG-63 cells treated with neutralizing HGF antibody induced rapid phosphorylation of c-Met, MEK1, and Erk1/2. Thus endogenous HGF induces a constitutively active, autocrine mitogenic loop in MG-63 cells. The known antiproliferative effect of 1,25(OH)(2)D(3) on MG-63 cells can be accounted for by the concomitant 1,25(OH)(2)D(3)-induced inhibition of HGF production.

Increased expression and secretion of r-Gsp protein, rat counterpart of complement C1s precursor, during cyclic AMP-induced differentiation in rat C6 glioma cells

The gene, termed r-gsp, was originally isolated during identification of differentiation-associated molecules in rat C6 glial cells. Its mRNA expression was markedly increased during cAMP-induced glial cell differentiation. The deduced amino acid sequence of r-gsp was homologous to those of complement C1s precursors of hamsters and humans. In the present study, we raised anti-peptide antibody against r-Gsp protein and analyzed its change during cAMP-induced differentiation. The 90-kDa r-Gsp protein increased time-dependently and reached the maximal level ( approximately 7.6-fold increase) at 24 h in response to dibutyryl cyclic AMP (dbcAMP) and theophylline. Moreover, it was secreted into the medium and then was cleaved to form disulfide-linked fragments, one of which was 30 kDa, similar to C1s, suggesting its processing in the extracellular space. In fact, the partially purified r-Gsp from culture medium was cleaved by active human C1r to form a 30-kDa polypeptide. Moreover, secreted r-Gsp protein cleaved human C4alpha to yield C4alpha' and associated with human serum C1-esterase inhibitor, strongly suggesting that r-Gsp protein is rat C1s. However, in C6 cells overexpressing r-Gsp, their morphology and proliferation rate were similar to those in parent C6 cells. These results suggest that r-Gsp protein could not induce glial differentiation alone, and suggest that r-Gsp protein was secreted as a proenzyme and processed in culture medium. Its possible role in glial cell differentiation will be discussed.

Oxidative stress involves in astrocytic alterations induced by manganese

It is hypothesized that manganese neurotoxicity could be secondary to a diminution of cellular protective and scavenger mechanisms. Since manganese is known to be sequestered in glial cells, we investigated possible neurotoxic mechanisms involving astrocytes in vitro. Astrocytes differentiated into process-bearing stellate cells in response to manganese treatment. Manganese concentration dependently decreased cellular DNA synthesis, glial fibrillary acidic protein expression, energy production, antioxidant capacity, and glutamate transporter activity. In contrast, manganese increased glutamine synthetase protein expression and cytokine-stimulated interleukin 6 mRNA expression. Under the concentration of 0.1 mM, manganese chloride caused no significant astrocyte death even up to 48 h after treatment. That is, these astrocytic alterations proceeded before the onset of cell demise. As a possible mediator of manganese-derived alterations, we determined intracellular redox state in astrocytes. Manganese time-dependently changed intracellular redox potential into oxidized state. The influx of manganese and its resultant oxidative stress was essential to most of the alterations, except for the action on stellation. Astrocytes are central component of the brain's antioxidant defense. Therefore, the observations suggest that dysfunction of astrocytes possibly involved in neurotoxic action of manganese.

Sphingosine-1-phosphate stimulates human Caco-2 intestinal epithelial proliferation via p38 activation and activates ERK by an independent mechanism

Sphingosine-1-phosphate (S-1-P) has been identified as an extracellular mediator and an intracellular second messenger that may modulate cell motility, adhesion, proliferation, and differentiation and cancer cell invasion. Widely distributed, S-1-P is most abundant in the intestine. Although S-1-P is likely to modulate various intracellular pathways, activation of the mitogen-activated protein kinases (MAPKs) such as extracellular signal-regulated kinase 1 (ERK1), ERK2, and p38 is among the best-characterized S-1-P effects. Because the MAPKs regulate proliferation, we hypothesized that S-1-P might stimulate intestinal epithelial cell proliferation by MAPK activation. Human Caco-2 intestinal epithelial cells were cultured on a fibronectin matrix because fibronectin is an important constituent of the gut mucosal basement membrane. We assessed ERK1, ERK2, and p38 activation by Western blotting with antibodies specific for their active forms and proliferation by Coulter counting at 24 h. Specific MAP kinase kinase (MEK) and p38 inhibitors PD98059 (20 microM) and SB202190 and SB203580 (10 and 20 microM) were used to probe the role of ERK and p38 in S-1-P-mediated proliferation. Three or more similar studies were pooled for the analysis. S-1-P stimulated Caco-2 proliferation and dose-responsively activated ERK1, ERK2, and p38. Proliferation peaked at 5 microM, yielding a cell number 166.3 +/- 2.7% of the vehicle control (n = 6, P < 0.05). S-1-P also maximally stimulated ERK1, ERK2, and p38 at 5 microM, to 164.4 +/- 19.9%, 232.2 +/- 38.5%, and 169.2 +/- 20.5% of the control, respectively. Although MEK inhibition prevented S-1-P activation of ERK1 and ERK2 and slightly but significantly inhibited basal Caco-2 proliferation, MEK inhibition did not block the S-1-P mitogenic effect. However, pretreatment with 10 microM SB202190 or SB203580 (putative p38 inhibitors) attenuated the stimulation of proliferation by S-1-P. Twenty micromolars of SB202190 or SB203580 completely blocked the mitogenic effect of S-1-P. Ten to twenty micromolars of SB202190 and SB203580 also dose-dependently ablated the effects of 5 microM S-1-P on heat shock protein 27 accumulation, a downstream consequence of p38 MAPK activation. Consistent with the reports in some other cell types, S-1-P appears to activate ERK1, ERK2, and p38 and to stimulate proliferation. However, in contrast to the mediation of the S-1-P effects in some other cell types, S-1-P appears to stimulate human intestinal epithelial proliferation by activating p38. ERK activation by S-1-P is not required for its mitogenic effect.

Regulation of human beta-defensin-2 in gingival epithelial cells: the involvement of mitogen-activated protein kinase pathways, but not the NF-kappaB transcription factor family

Stratified epithelia of the oral cavity are continually exposed to bacterial challenge that is initially resisted by neutrophils and epithelial factors, including antimicrobial peptides of the beta-defensin family. Previous work has shown that multiple signaling pathways are involved in human beta-defensin (hBD)-2 mRNA regulation in human gingival epithelial cells stimulated with a periodontal bacterium, Fusobacterium nucleatum, and other stimulants. The goal of this study was to further characterize these pathways. The role of NF-kappaB in hBD-2 regulation was investigated initially due to its importance in inflammation and infection. Nuclear translocation of p65 and NF-kappaB activation was seen in human gingival epithelial cells stimulated with F. nucleatum cell wall extract, indicating possible involvement of NF-kappaB in hBD-2 regulation. However, hBD-2 induction by F. nucleatum was not blocked by pretreatment with two NF-kappaB inhibitors, pyrrolidine dithiocarbamate and the proteasome inhibitor, MG132. To investigate alternative modes of hBD-2 regulation, we explored involvement of mitogen-activated protein kinase pathways. F. nucleatum activated p38 and c-Jun NH(2)-terminal kinase (JNK) pathways, whereas it had little effect on p44/42. Furthermore, inhibition of p38 and JNK partially blocked hBD-2 mRNA induction by F. nucleatum, and the combination of two inhibitors completely blocked expression. Our results suggest that NF-kappaB is neither essential nor sufficient for hBD-2 induction, and that hBD-2 regulation by F. nucleatum is via p38 and JNK, while phorbol ester induces hBD-2 via the p44/42 extracellular signal-regulated kinase pathway. Studies of hBD-2 regulation provide insight into how its expression may be enhanced to control infection locally within the mucosa and thereby reduce microbial invasion into the underlying tissue.

L-glutamate suppresses astrocyte stellation induced by actin breakdown in culture

We have recently found that L-glutamate suppresses morphological changes of astrocytes induced by amyloid beta protein, adenosine 3',5'-cyclic monophosphate or phorbol ester in culture. To test the possibility that L-glutamate affects organization of the cytoskeleton, we investigated its effect on morphological changes induced by disruption of actin filaments with cytochalasin B. Cultured rat cortical astrocytes exhibited flat, polygonal morphology in the absence of stimulation, and changed into process-bearing stellate cells following treatment with cytochalasin B (50 microM). L-Glutamate strongly suppressed the stellation induced by cytochalasin B. The effect of L-glutamate was mimicked by D- and L-aspartate and transportable glutamate uptake inhibitors. These results suggest that glutamate transporter activity leads to cytoskeletal actin organization in astrocytes.

Sustained phosphorylation of mitogen-activated protein kinase is required for basic fibroblast growth factor-mediated axonal branch formation in cultured rat hippocampal neurons

Basic fibroblast growth factor (bFGF) has been reported to promote the formation of axonal branches in cultured brain neurons. In the present study, we investigated whether the mitogen-activated protein kinase (MAPK) cascade was involved in this action of bFGF in cultured rat hippocampal neurons. Recombinant human bFGF (0.1-10 ng/ml) induced phosphorylation of p44/42 MAPK in a concentration and time-dependent manner. The phosphorylation of p44/42 MAPK occurred rapidly within 5 min after addition of bFGF, and lasted for 48 h. The bFGF-induced phosphorylation of p44/42 MAPK and axonal branch formation were both blocked by simultaneous addition of U0126 and PD98059, specific inhibitors of MAPK kinases. Furthermore, when U0126 and PD98059 were added 24 h after bFGF, phosphorylation of p44/42 m MAPK was decreased, and axonal branch formation was stopped. These results suggest that sustained activation of the MAPK cascade is required for bFGF-mediated axonal branch formation.

Possible linkage between glutamate transporter and mitogen-activated protein kinase cascade in cultured rat cortical astrocytes

The mitogen-activated protein kinases (MAPKs) play a pivotal role in the mediation of cellular responses to a variety of signalling molecules. In the present study, we investigated possible linkage between glutamate signalling and the MAPK cascade in cultured rat cortical astrocytes. Exposure of the cells to L-glutamate (100-1000 microM) resulted in an increase in phosphorylated p44/42 MAPK (ERK1/2) in a concentration- and time-dependent manner. The glutamate-induced ERK1/2 phosphorylation was blocked by U0126 and PD98059, specific inhibitors of the MAPK-activating enzyme MEK. Furthermore, L-glutamate-induced ERK1/2 phosphorylation was not mimicked by glutamate receptor agonists and was not blocked by glutamate receptor antagonists. In contrast, the effect of L-glutamate was mimicked by D- and L-aspartate and transportable glutamate uptake inhibitors. These results suggest that the MEK/ERK cascade is activated by a mechanism related to glutamate transporters. We propose that the glutamate transporter functions as a receptor transmitting extracellular glutamate signal to intracellular messengers.

Na+ and K+ dependence of L-glutamate-induced suppression of astrocyte stellation in culture

We have recently found that L-glutamate suppresses astrocyte stellation induced by various stimuli, and that this effect of L-glutamate is mimicked by transportable glutamate uptake inhibitors. To test the possible role of the glutamate transporter in the regulation of astrocyte morphology, we investigated the Na+ and K+ dependence of this effect of L-glutamate. In astrocyte cultures obtained from the cerebral cortex of neonatal rats, the L-glutamate-induced suppression of astrocyte stellation was significantly attenuated in a low- Na+/high- K+ medium and by the Na+ -K+ pump inhibitor ouabain. These results support that astrocyte morphology is affected by the activity of the Na+ -dependent glutamate transporter.

Neurotrophic effect of basic fibroblast growth factor is mediated by the p42/p44 mitogen-activated protein kinase cascade in cultured rat cortical neurons

Basic fibroblast growth factor (bFGF) has been reported to support the survival of brain neurons. In the present study, we investigated whether the neurotrophic effect of bFGF is mediated by the mitogen-activated protein kinase (MAPK) cascade in cultured rat cerebral cortical neurons. Recombinant human bFGF (0.1-10 ng/ml) induced phosphorylation of p44/42 MAPK (ERK1/2) in a concentration- and time-dependent manner. bFGF-induced ERK1/2 phosphorylation and promotion of neuronal survival were both blocked by U0126 and PD98059, inhibitors of the MAPK-activating enzyme MEK. These results suggest that the MEK/ERK signal transduction cascade is involved in the neurotrophic effect of bFGF.

The mitogen-activated protein kinase cascade mediates neurotrophic effect of epidermal growth factor in cultured rat hippocampal neurons

Epidermal growth factor (EGF) has been reported to support the survival of cultured brain neurons. In the present study, we investigated whether the neurotrophic effect of EGF is mediated by the mitogen-activated protein kinase (MAPK) cascade in cultured rat hippocampal neurons. Recombinant human EGF (0.1-10 ng/ml) induced phosphorylation of p44/42 MAPK (ERK1/2) in a concentration-and time-dependent manner. EGF-induced ERK1/2 phosphorylation and promotion of neuronal survival were both blocked by U0126 and PD98059, inhibitors of the MAPK-activating enzyme MEK. These results suggest that the MEK/ERK signal transduction cascade is involved in the neurotrophic effect of EGF.