Inhibition of vanadate-induced astrocytic stress fiber formation by C3 ADP-ribosyltransferase

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.

Role of protein-tyrosine phosphatases on β-adrenergic receptor mediated morphological differentiation of astrocytes

A role of protein-tyrosine phosphatases in isoproterenol induced differentiation of cultured astrocytes was investigated. Unlike serine/threonine phosphatase inhibitors, the tyrosine phosphatase inhibitor, sodium orthovanadate effectively blocked transformation of the polygonal astrocytes to process bearing stellate cells on exposure to isoproterenol for 2 days. Isoproterenol caused a stimulation of c-AMP dependent protein kinase activity in the cells only at the initial stages (45 min) and at 12 and 24 h, there was a decline in the level of phospho-tyrosinated proteins which could be antagonised by the protein kinase A inhibitor, H89. Genestein, a protein-tyrosine kinase inhibitor, had no effect on the alteration in the morphology of the astroglial cells induced by isoproterenol but by itself, decreased the dephosphorylation of the phospho-tyrosinated proteins, the decline being less than that observed in isoproterenol treated cells. Moreover, unlike H89, genestein had no effect on isoproterenol-induced dephosphorylation of phospho-tyrosinated proteins. Taken together it appears that the dephosphorylation of tyrosine residues during isoproterenol-induced astrocyte differentiation is a downstream event of protein kinase A stimulation and needs to attain a critical level in order for the cells to differentiate.

Focal adhesion kinase is required for endothelin-induced cell cycle progression of cultured astrocytes

When the brain is damaged, astrocytes often cause hyperplasia resulting in glial scar formation at the injured sites. Endothelins (ETs) have been shown to be involved in the pathophysiologic responses of astrocytes, including proliferation. In this study, we examined the mechanisms underlying the ET-induced astrocytic G1/S-phase cell cycle transition by focusing on focal adhesion kinase (FAK). A transient transfection with wild-type FAK was followed by an increase in bromodeoxyuridine (BrdU) incorporation into cultured rat astrocytes. The increases in BrdU incorporation induced by 100 nM ET-1 were not found in astrocytes transfected with dominant-negative FAK mutants (FRNK and dC14-FAK). The increases in BrdU incorporation induced by 10 nM phorbol 12-myristate 13-acetate (PMA) were not affected by the FAK mutants. Wild-type FAK did not induce stress fiber formation in cultured astrocytes. The dominant negative FAK mutant dC14-FAK did not prevent ET-induced astrocytic stress fiber formation. These results suggest that FAK mediated the astrocytic G1/S cell cycle transition induced by ET-1 downstream of the cytoskeletal actin reorganization.

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.

Astrocyte stellation in saline media lacking bicarbonate: possible relation to intracellular pH and tyrosine phosphorylation

Primary cultures of astrocytes exhibit a polygonal morphology, but on treatment with agents that increase cAMP they change to stellate cells. We found that astrocyte stellation also occurred on replacing the culture medium with saline buffered with HEPES. However, stellation did not occur when the medium was replaced with saline buffered with bicarbonate/CO(2) provided Ca(2+) was present. Since exposure of astrocytes to media lacking bicarbonate results in a decrease in intracellular pH (pH(i)) we sought evidence for an association between pH(i) and morphology. Astrocytic pH(i) was monitored for 60 min after transferring the cells to HEPES or bicarbonate-buffered saline. HEPES-induced stellation was associated with transient acidification which coincided with the morphological changes. Acidification was not observed in cells transferred to bicarbonate-saline. However when cytoplasmic acidification of cells in bicarbonate-saline was induced pharmacologically, rapid stellation occurred. Stellation induced by cAMP is reversed by activation of the RhoA pathway with lysophosphatidic acid (LPA). Here we found that LPA inhibited HEPES-induced stellation, but only with Ca(2+) present. Inhibition of stellation by LPA+Ca(2+) was associated with transient acidification followed by modest alkanization. A close association of tyrosine phosphorylation with stellation and pH(i) was observed. Thus incubation of astrocytes in HEPES-saline with orthovanadate to inhibit dephosphorylation abolished stellation and acidification; conversely incubation of cells in bicarbonate-saline with genistein to inhibit tyrosine kinases caused stellation and major acidification. Acidification may be one of several factors resulting in stellation, but it is not a necessary factor since stellation without acidification was observed in bicarbonate-saline lacking Ca(2+).

The Rho exchange factor Net1 is regulated by nuclear sequestration

Net1 is a guanine nucleotide exchange factor specific for the small GTPase Rho. Oncogenic activation of Net1 occurs by truncation of the N-terminal part of the protein, which functions as a negative regulatory domain. Here, we have investigated the mechanism of Net1 regulation via its N terminus. We find that Net1 localizes to the nucleus, whereas oncogenic Net1 is found in the cytoplasm. Nuclear import of Net1 is mediated by two nuclear localization signals present in the N terminus of the protein, and forced cytoplasmic localization of Net1 is sufficient to activate Rho. In addition, the pleckstrin homology (PH) domain of Net1 acts as a nuclear export signal. Because an amino acid substitution in the PH domain that inhibits guanine nucleotide exchange factor activity does not inhibit nuclear export, we conclude that this PH domain has at least two functions. Together, our results suggest that Net1 can shuttle in and out of the nucleus, and that activation of Rho by Net1 is controlled by changes in its subcellular localization.

Desensitization of the PDGFbeta receptor by modulation of the cytoskeleton: the role of p21(Ras) and Rho family GTPases

Ligand-induced PDGF-type beta receptor (PDGFbeta-R) autophosphorylation is profoundly suppressed in cells transformed by activated p21(Ras). We report here that the integrity of the actin cytoskeleton is a critical regulator of PDGFbeta-R function in the presence of p21(Ras). Morphological reversion of Balb cells expressing a constitutively activated p21(Ras), with re-formation of actin stress fibers and cytoskeletal architecture, rendering them phenotypically similar to untransformed fibroblasts, allowed recovery of ligand-dependent PDGFbeta-R autophosphorylation. Conversely, disruption of the actin cytoskeleton in Balb/c-3T3 cells obliterated the normal ligand-induced phosphorylation of the PDGFbeta-R. The Rho family GTPases Rac and Rho are activated by p21(Ras) and are critical mediators of cell motility and morphology via their influence on the actin cytoskeleton. Transient expression of wild-type or constitutively active mutant forms of RhoA suppressed ligand-dependent PDGFbeta-R autophosphorylation and downstream signal transduction. These studies demonstrate the necessary role of Rho in the inhibition of PDGFbeta-R autophosphorylation in cells containing activated p21(Ras) and also demonstrate the importance of cell context and the integrity of the actin cytoskeleton in the regulation of PDGFbeta-R ligand-induced autophosphorylation.

[Functional alterations of astroglia on brain pathologies and their intracellular mechanisms]

A phenotypic alteration of astroglia, "astroglial activation", is a common phenomenon observed on brain pathologies. The hypertrophy/hyperplasia of activated astroglia causes a glial scar, which prevents synaptic re-generation. In contrast, many neurotrophic substances are produced by the activated astroglia. Thus, the functional alteration of astroglia is important in tissue repair processes of the damaged CNS. Endothelins (ETs) are involved in the pathophysiological responses of the CNS. We found that injection of ETs into rat brain induced activated astroglia. A selective ETB-receptor antagonist attenuated the induction of activated astroglia. In cultured astroglia, ETs reproduce the functional alterations characterizing activated astroglia; i.e., increases in proliferation, morphological changes and stimulation of several gene transcriptions. ETs re-organized astroglial cytoskeletal actin through a small GTP-binding protein, rho, which may underlie the astroglial hypertrophy. Analysis of gene expression showed that transcriptions of neurotrophic factors (GDNF and BDNF) were stimulated by ETs. ETs stimulated astroglial proliferation by both adhesion-dependent and -independent mechanisms, where FAK and ERK plays key roles, respectively. These findings suggest important roles of ETs in the regulation of astroglial functions.

Endothelins increase tyrosine phosphorylation of astrocytic focal adhesion kinase and paxillin accompanied by their association with cytoskeletal components

Astrocytic endothelin receptors are involved in the appearance of activated astrocytes upon injury of the brain [Ishikawa N. et al. (1997) Eur. J. Neurosci. 9, 895-901; Koyama Y. et al. (1999) Glia 26, 268-271]. To clarify signal transduction triggered by endothelin receptors, we examined the effects of endothelins on protein tyrosine phosphorylation in cultured rat astrocytes. Endothelin-1 (1 nM) increased tyrosine phosphorylation of focal adhesion kinase and paxillin. The tyrosine phosphorylation was also induced by endothelin-1 (1 nM) and Ala(1,3,11,15)-endothelin-1 (10nM), an endothelin-B receptor agonist. BQ788 (100 nM), an endothelin-B receptor antagonist, inhibited the effects of endothelin-3. Orthovanadate (VO(4)(3-)), a tyrosine phosphatase inhibitor, but not bradykinin (1 microM), angiotensin II (100 nM), A23187 (5 microM) and phorbol 12-myristate 13-acetate (100 nM), increased tyrosine phosphorylation of focal adhesion kinase and paxillin. The tyrosine phosphorylation by endothelin-3 was not prevented by pertussis toxin, Ca(2+) chelation, protein kinase C inhibitors (calphostin C and staurosporine) or wortmannin. Immunocytochemical staining showed that endothelin-3 and VO(4)(3-) induced redistribution of focal adhesion kinase and paxillin to focal adhesions concomitant with stress fiber formation in dibutyryl cyclic-AMP-treated astrocytes. Treatment with endothelin-3 and VO(4)(3-) increased focal adhesion kinase and paxillin associated with astrocytic cytoskeletal fraction. In the presence of cytochalasin B, an actin disrupting agent, endothelin-3 and VO(4)(3-) did not phosphorylate focal adhesion kinase and paxillin. Application of cytochalasin B after treatment with endothelin-3 and VO(4)(3-) stimulated dephosphorylation of focal adhesion kinase and paxillin. These results suggest that the associations of focal adhesion kinase and paxillin with cytoskeletal components are required in the endothelin-induced tyrosine phosphorylation of the astrocytic proteins.

Distinct roles for the N- and C-terminal regions in the cytotoxicity of pierisin-1, a putative ADP-ribosylating toxin from cabbage butterfly, against mammalian cells

Pierisin-1 is an 850-aa cytotoxic protein found in the cabbage butterfly, Pieris rapae, and has been suggested to consist of an N-terminal region with ADP-ribosyltransferase domain and of a C-terminal region that might have a receptor-binding domain. To elucidate the role of each region, we investigated the functions of various fragments of pierisin-1. In vitro expressed polypeptide consisting of amino acid residues 1-233 or 234-850 of pierisin-1 alone did not show cytotoxicity against human cervical carcinoma HeLa cells. However, the presence of both polypeptides in the culture medium showed some of the original cytotoxic activity. Introduction of the N-terminal polypeptide alone by electroporation also induced cell death in HeLa cells, and even in the mouse melanoma MEB4 cells insensitive to pierisin-1. Thus, the N-terminal region has a principal role in the cytotoxicity of pierisin-1 inside mammalian cells. Analyses of incorporated pierisin-1 indicated that the entire protein, regardless of whether it consisted of a single polypeptide or two separate N- and C-terminal polypeptides, was incorporated into HeLa cells. However, neither of the terminal polypeptides was incorporated when each polypeptide was present separately. These findings indicate that the C-terminal region is important for the incorporation of pierisin-1. Moreover, presence of receptor for pierisin-1 in the lipid fraction of cell membrane was suggested. The cytotoxic effects of pierisin-1 were enhanced by previous treatment with trypsin, producing "nicked" pierisin-1. Generation of the N-terminal fragment in HeLa cells was detected after application of intact entire molecule of pierisin-1. From the above observations, it is suggested that after incorporation of pierisin-1 into the cell by interaction of its C-terminal region with the receptor in the cell membrane, the entire protein is cleaved into the N- and C-terminal fragments with intracellular protease, and the N-terminal fragment then exhibits cytotoxicity.

Cortical astrocytes exposed to tributyltin undergo morphological changes in vitro

We investigated the effect of tributyltin (TBT), an endocrine-disrupting chemical, on the morphology and viability of cultured rat cortical astrocytes. Cultured astrocytes exhibited smooth and planiform morphology under normal conditions. Following exposure to TBT, however, they showed rapid morphological changes that are characterized by asteriated cell bodies and process formation in a time- and concentration-dependent manner. Higher concentrations of TBT produced progressive cell death of the astrocytes. In serum-free medium, TBT at a concentration as low as 200 nM induced the stellation. Pharmacological studies revealed that the morphological changes were alleviated by application of diverse free radical scavengers or antioxidants such as catalase, superoxide dismutase, Trolox, ascorbic acid and N-acetyl-L-cysteine, suggesting that TBT-induced stellation is caused by oxidative stress involving free radicals, particularly reactive oxygen species. Furthermore, we found that the astrocyte stellation was abolished by treatment with inhibitors of phospholipase C, mitogen-activated protein kinase kinase or tyrosine phosphatase. The data suggest that TBT causes the stellation through intracellular signaling cascades rather than its non-specific toxicity. These findings provide an important insight for reconciling the problems in assumed aversive actions of this environmental pollutant for mammals.

Astrocyte stellation induced by tyrosine kinase inhibitors in culture

To understand the role of tyrosine kinases in regulation of astrocyte morphology, we investigated the effects of tyrosine kinase inhibitors on morphology of cultured rat cortical astrocytes. Cultured astrocytes exhibited flattened, polygonal morphology in the absence of stimulation, but changed into process-bearing stellate cells in the presence of the tyrosine kinase inhibitor genistein (3-100 microM). Genistein-induced astrocyte stellation was abolished by treatment with colchicine or paclitaxel, indicating the involvement of cytoskeletal elements. The effect of genistein was mimicked by another tyrosine kinase inhibitor herbimycin A, but not by daidzein, an inactive analog of genistein. These results suggest that tyrosine kinases are in an activated state in the absence of stimuli and contribute to the maintenance of polygonal morphology of cultured astrocytes.

Evidence for a calpeptin-sensitive protein-tyrosine phosphatase upstream of the small GTPase Rho. A novel role for the calpain inhibitor calpeptin in the inhibition of protein-tyrosine phosphatases

Activation of the thiol protease calpain results in proteolysis of focal adhesion-associated proteins and severing of cytoskeletal-integrin links. We employed a commonly used inhibitor of calpain, calpeptin, to examine a role for this protease in the reorganization of the cytoskeleton under a variety of conditions. Calpeptin induced stress fiber formation in both forskolin-treated REF-52 fibroblasts and serum-starved Swiss 3T3 fibroblasts. Surprisingly, calpeptin was the only calpain inhibitor of several tested with the ability to induce these effects, suggesting that calpeptin may act on targets besides calpain. Here we show that calpeptin inhibits tyrosine phosphatases, enhancing tyrosine phosphorylation particularly of paxillin. Calpeptin preferentially inhibits membrane-associated phosphatase activity. Consistent with this observation, in vitro phosphatase assays using purified glutathione S-transferase fusion proteins demonstrated a preference for the transmembrane protein-tyrosine phosphatase-alpha over the cytosolic protein-tyrosine phosphatase-1B. Furthermore, unlike wide spectrum inhibitors of tyrosine phosphatases such as pervanadate, calpeptin appeared to inhibit a subset of phosphatases. Calpeptin-induced assembly of stress fibers was inhibited by botulinum toxin C3, indicating that calpeptin is acting on a phosphatase upstream of the small GTPase Rho, a protein that controls stress fiber and focal adhesion assembly. Not only does this work reveal that calpeptin is an inhibitor of protein-tyrosine phosphatases, but it suggests that calpeptin will be a valuable tool to identify the phosphatase activity upstream of Rho.

Not just scaffolding: plectin regulates actin dynamics in cultured cells

Plectin, a major linker and scaffolding protein of the cytoskeleton, has been shown to be essential for the mechanical integrity of skin, skeletal muscle, and heart. Studying fibroblast and astroglial cell cultures derived from plectin (-/-) mice, we found that their actin cytoskeleton, including focal adhesion contacts, was developed more extensively than in wild-type cells. Also it failed to show characteristic short-term rearrangments in response to extracellular stimuli activating the Rho/Rac/Cdc42 signaling cascades. As a consequence, cell motility, adherence, and shear stress resistance were altered, and morphogenic processes were delayed. Furthermore, we show that plectin interacts with G-actin in vitro in a phosphatidylinositol-4,5-biphosphate-dependent manner and associates with actin stress fibers in living cells. The actin stress fiber phenotype of plectin-deficient fibroblasts could be reversed to a large degree by transient transfection of full-length plectin or plectin fragments containing the amino-terminal actin-binding domain (ABD). These results reveal a novel role of plectin as regulator of cellular processes involving actin filament dynamics that goes beyond its proposed role in scaffolding and mechanical stabilization of cells.

Adenosine stimulates stellation of cultured rat cortical astrocytes

We investigated the effect of adenosine on astrocyte morphology by using cell cultures prepared from the cerebral cortices of neonatal rats. Cultured rat cortical astrocytes exhibited flattened, polygonal morphology in the absence of stimulation, but differentiated into process-bearing stellate cells in response to adenosine (1-1000 microM). Adenosine-induced astrocyte stellation was abolished by treatment with microtubule inhibitors, colchicine and paclitaxel, indicating the involvement of cytoskeletal elements. The effect of adenosine was mimicked by other adenosine receptor agonists, and blocked by adenosine receptor antagonists and guanosine 5'-O-(2-thiodiphosphate), indicating that the effect of adenosine is mediated by G protein-coupled adenosine receptors. Although adenosine receptors are known to be linked to adenylate cyclase or phospholipase C, adenosine did not change intracellular cyclic AMP level nor intracellular Ca2+ concentration in astrocytes. Alternatively, adenosine-induced stellation was abolished by tyrosine phosphatase inhibitors, orthovanadate and phenylarsine oxide, suggesting that adenosine causes astrocyte stellation through tyrosine dephosphorylation. Adenosine may function as a factor regulating astrocyte differentiation.

Role of endothelin B receptor signals in reactive astrocytes

We investigated the involvement of endothelinB receptor signals in the activation of astrocytes in vitro and in vivo. Endothelin reversed the stellate morphology of astrocytes induced by several agents. Endothelin stimulates astrocytic stress fiber formation. This effect of endothelin is Ca2+-independent and mediated by rho protein signal cascades linked to tyrosine phosphorylation. Injection of endothelin into striatum caused reactive astrocytosis which is prevented by a systemic injection of endothelinB receptor antagonist. We propose the role of endothelinB receptor signal transduction in reactive astrocytes.

Guanine nucleotide-dependent translocation of RhoA from cytosol to high affinity membrane binding sites in human erythrocytes

The translocation of the small GTP-binding protein Rho from the cytosolic to membrane-bound form is an early step in many cellular signal-transduction events, but little is known regarding the mechanism of Rho association with the plasma membrane. We have used membranes from human erythrocytes to uncover a novel class of integral membrane components involved in the Rho-membrane association. Membranes of human erythrocytes contain several proteins of the Ras superfamily. Using specific antibodies and C3 exoenzyme of Clostridium botulinum we have identified one of them as RhoA. This protein was detected in both cytosol and membrane fractions of hypotonically lysed erythrocytes. We found that cytosolic Rho bound specifically to the cytoplasmic surface of the erythrocyte membrane and that the translocation of Rho to the membrane was absolutely dependent on the prior incubation of the cytosol with guanosine 5'--gamma-thio-triphosphate (1-50 microM) at low Mg2+ concentration. Rho binding sites could not be extracted from the membrane using conditions that extracted all other peripheral proteins and were unaffected by heat treatment and protease digestion. Rho binding was saturable, with a Kd in the range 1-5.0 nM, and the number of binding sites was estimated to be approx. (1-2) x 10(3) sites per cell. This is the first report of Rho binding to integral membrane components. The identity of these components may reveal novel aspects of the mechanism by which Rho exerts its multiple biochemical effects.

Bovine serum albumin and lysophosphatidic acid stimulate calcium mobilization and reversal of cAMP-induced stellation in rat spinal cord astrocytes

We report that lysophosphatidic acid (LPA) stimulates dynamic calcium (Ca2+) fluctuations and morphological rearrangements in astrocytes derived from neonatal rat spinal cord. Addition of 10 microM LPA elicited single Ca2+ transients, or biphasic oscillations and sustained increases in intracellular Ca2+ ([Ca2+]i). The biphasic Ca2+ response consisted of an initial release from intracellular stores, often followed by a sustained elevation or continued oscillations that required Ca2+ flux across the cell membrane. The type of Ca2+ response, but not the overall magnitude, was dependent on LPA concentration. Higher concentrations (> 10 microM) often elicited sustained increases in [Ca2+]i, while lower concentrations stimulated oscillations or single Ca2+ transients. It has previously been established that agents that elevate cyclic adenosine monophosphate (cAMP) induce flat astrocytes to adopt a more stellate morphology. LPA can completely reverse this morphological change at a half-maximal concentration of 215 nM. Inhibiting LPA-induced [Ca2+]i fluctuations using BAPTA-AM to buffer [Ca2+]i and EGTA in the bath to prevent transmembrane flux had little effect on the ability of LPA to reverse stellation. LPA is found bound to serum albumin, in which crude preparations have been shown to induce various physiological responses in a number of cell types. Many of the activities have been attributed to albumin-associated lipid factors including LPA. We show that lipid factors associated with BSA can mimic the effect of LPA in both Ca2+ mobilization and reversal of cAMP-induced stellation.