Thrombin reverts the beta-adrenergic agonist-induced morphological response in rat glioma C6 cells

β adrenergic receptor modulated signaling in glioma models: promoting β adrenergic receptor-β arrestin scaffold-mediated activation of extracellular-regulated kinase 1/2 may prove to be a panacea in the treatment of intracranial and spinal malignancy and extra-neuraxial carcinoma

Neoplastically transformed astrocytes express functionally active cell surface β adrenergic receptors (βARs). Treatment of glioma models in vitro and in vivo with β adrenergic agonists variably amplifies or attenuates cellular proliferation. In the majority of in vivo models, β adrenergic agonists generally reduce cellular proliferation. However, treatment with β adrenergic agonists consistently reduces tumor cell invasive potential, angiogenesis, and metastasis. β adrenergic agonists induced decreases of invasive potential are chiefly mediated through reductions in the expression of matrix metalloproteinases types 2 and 9. Treatment with β adrenergic agonists also clearly reduce tumoral neoangiogenesis, which may represent a putatively useful mechanism to adjuvantly amplify the effects of bevacizumab. Bevacizumab is a monoclonal antibody targeting the vascular endothelial growth factor receptor. We may accordingly designate βagonists to represent an enhancer of bevacizumab. The antiangiogenic effects of β adrenergic agonists may thus effectively render an otherwise borderline effective therapy to generate significant enhancement in clinical outcomes. β adrenergic agonists upregulate expression of the major histocompatibility class II DR alpha gene, effectively potentiating the immunogenicity of tumor cells to tumor surveillance mechanisms. Authors have also demonstrated crossmodal modulation of signaling events downstream from the β adrenergic cell surface receptor and microtubular polymerization and depolymerization. Complex effects and desensitization mechanisms of the β adrenergic signaling may putatively represent promising therapeutic targets. Constant stimulation of the β adrenergic receptor induces its phosphorylation by β adrenergic receptor kinase (βARK), rendering it a suitable substrate for alternate binding by β arrestins 1 or 2. The binding of a β arrestin to βARK phosphorylated βAR promotes receptor mediated internalization and downregulation of cell surface receptor and contemporaneously generates a cell surface scaffold at the βAR. The scaffold mediated activation of extracellular regulated kinase 1/2, compared with protein kinase A mediated activation, preferentially favors cytosolic retention of ERK1/2 and blunting of nuclear translocation and ensuant pro-transcriptional activity. Thus, βAR desensitization and consequent scaffold assembly effectively retains the cytosolic homeostatic functions of ERK1/2 while inhibiting its pro-proliferative effects. We suggest these mechanisms specifically will prove quite promising in developing primary and adjuvant therapies mitigating glioma growth, angiogenesis, invasive potential, and angiogenesis. We suggest generating compounds and targeted mutations of the β adrenergic receptor favoring β arrestin binding and scaffold facilitated activation of ERK1/2 may hold potential promise and therapeutic benefit in adjuvantly treating most or all cancers. We hope our discussion will generate fruitful research endeavors seeking to exploit these mechanisms.

Dioleoyl phosphatidic acid induces morphological changes through an endogenous LPA receptor in C6 glioma cells

Previously, we suggested that dioleoyl phosphatidic acid (PA) and lysophosphatidic acid (LPA) increased [Ca(2+)](i) through endogenous LPA receptors coupled to pertussis toxin-sensitive G proteins in rat C6 glioma cells. In the present report, we investigated morphological changes and cytotoxicity induced by PA and LPA in C6 glioma cells. Isoproterenol treatment led to changes in the cell morphology of rat C6 glioma cells, which were reverted by the addition of PA and LPA. PA-and LPA-induced morphological reversions were inhibited by treatment with Ki16425, an LPA(1)/LPA(3) receptor antagonist. VPC32183, another LPA(1)/LPA(3) receptor antagonist with a different structure, only inhibited PA-induced morphological reversion but not LPA-induced reversion. However, the reversions were not inhibited by treatment with pertussis toxin, a specific inhibitor of G(i/o) proteins. In addition, cytotoxicity was only induced by LPA but not by PA in C6 glioma cells. Our results suggest that PA may act as a partial agonist at endogenous LPA receptors, which are sensitive to Ki16425 and coupled to PTX-insensitive G proteins, to evoke morphological changes in C6 glioma cells.

Volatile anesthetics affect the morphology of rat glioma C6 cells via RhoA, ERK, and Akt activation

Treatment of rat glioma C6 cells with the beta-receptor agonist isoproterenol induces a massive increase in cAMP. Concomitantly the cells change their morphology from a fibroblast-type to an astrocyte-like (stellated) cell shape. The stellated morphology can be completely reverted by thrombin and sphingosine-1-phosphate (S-1-P) but also to a certain extent by clinical concentrations of volatile anesthetics. The anesthetic-induced reversion of the stellated cell shape seems to be mediated by a number of cellular alterations. Central to the effect is most likely a RhoA/Rho-kinase activation, but also the MAPKK/MEK and the Akt/protein kinase B pathway are activated by the anesthetics. With the use of specific inhibitors we were able to show that activation of the MAPKK/MEK pathway inhibits, whereas activation of the Akt/protein kinase B pathway stimulates the reversal of the stellated cell shape by the anesthetics. In summary, volatile anesthetics affect the morphology of rat glioma C6 cells by activation of the RhoA/Rho kinase, the MAPKK/MEK, and the Akt/protein kinase B signaling pathways.

Sphingosine 1-phosphate (S1P) induces shape change in rat C6 glioma cells through the S1P2 receptor: development of an agonist for S1P receptors

Treatment with isoprenaline led to a change in the cell morphology of rat C6 glioma cells. This morphological change was reverted by the addition of sphingosine 1-phosphate (S1P). Using this morphological change as a response marker we determined that DS-SG-44 ((2S,3R)-2-amino-3-hydroxy-4-(4-octylphenyl)butyl phosphoric acid) was an agonist of S1P receptors. The DS-SG-44-induced morphological reversion was not observed with such structurally related molecules as DS-SG-45 ((2S,3R)-2-amino-3-hydroxy-4-(3-octylphenyl)butyl phosphoric acid) and DS-SG-12 ((2S,3R)-2-amino-4-(4-octylphenyl)butane-1,3-diol). The S1P- and DS-SG-44-induced shape changes were neither reproduced with the S1P1/S1P3 receptor agonist VPC24191 nor inhibited by the S1P1/S1P3 receptor antagonist, VPC23019. Transfection with small interfering RNA (siRNA) for the S1P2 receptor greatly inhibited the DS-SG-44-induced shape change, and in part an S1P-induced response. In the presence of VPC23019, siRNA transfection for the S1P2 receptor almost completely blocked the S1P- and DS-SG-44-induced shape changes. Our results suggested that DS-SG-44, a newly-synthesized S1P analogue, acted as an S1P receptor agonist and that the S1P-induced shape change in rat C6 glioma cells was mediated mainly through the S1P2 receptor, and cooperatively through the S1P1/S1P3 receptors.

Possible involvement of 5α-reduced neurosteroids in adrenergic and serotonergic stimulation of GFAP gene expression in rat C6 glioma cells

Influence of adrenergic and serotonergic stimulation on glial fibrillary acidic protein (GFAP) gene expression in rat C6 glioma cells was first examined as an in vitro model experiment for investigating the neuronal regulation of glial cell differentiation. Stimulation of these cells with isoproterenol and serotonin elevated GFAP mRNA levels followed by an increase in its protein contents, thus suggesting that both adrenergic and serotonergic stimulation might induce the differentiation of the glioma cells. In addition, progesterone and its 5alpha-reduced metabolite dihydroprogesterone also elevated GFAP mRNA levels in rat C6 glioma cells, consistent with their stimulatory actions on GFAP gene expression observed in rat astrocytes. Further studies showed that the elevation of GFAP mRNA levels induced by isoproterenol and serotonin as well as progesterone was abolished by pretreatment of the glioma cells with finasteride, an inhibitor of 5alpha-reduced steroid production. Moreover, the stimulatory actions of isoproterenol and serotonin on GFAP gene expression were inhibited by pretreatment with a GABA(A) receptor antagonist bicuculline and a progesterone receptor antagonist RU486. These findings suggest that both adrenergic and serotonergic stimulation may indirectly activate GFAP gene expression probably through the production of 5alpha-reduced steroid metabolites in rat C6 glioma cells, proposing the possibility that 5alpha-reduced neurosteroids may play a potential role in the neuronal regulation of glial cell differentiation.

Calcium-independent, tyrosine phosphorylation-dependent effects of serum on the morphology of cultured neurohypophysial astrocytes

Activation of adenylate cyclase induces cultured neurohypophysial astrocytes (pituicytes) to change from a protoplasmic, nonstellate form to a stellate form. Stellation is inhibited and reversed (destellation) by serum. The objective of the present studies was to examine the roles of Ca2+ and tyrosine phosphorylation in mediating these morphological changes. The effects of forskolin (to induce stellation) and serum (to inhibit and reverse stellation) were not affected by replacement of Ca2+ with Co2+ in the medium or by treatment of cultures with thapsigargin. However, genistein, a specific inhibitor of tyrosine kinase(s), significantly reduced the effect of serum on forskolin-induced stellation. Also, dephostatin, a specific inhibitor of tyrosine phosphatase, inhibited forskolin-induced stellation. In contrast, genistein did not have a dramatic effect on serum-induced destellation. The data demonstrate that morphological changes exhibited by cultured pituicytes are independent of Ca2+ but may be modulated by the activity of tyrosine kinase(s) and phosphatase(s).

Identification of thrombin receptors in rat brain capillary endothelial cells

Both thrombin and plasmin induce contraction of brain endothelial cells, which may increase capillary permeability thereby leading to disruption of the blood-brain barrier. Identification of thrombin receptors, as well as the influence of plasmin on their activation, in capillary endothelial cells and astrocytes are therefore essential for understanding injury-related actions of thrombin in the brain. Using the reverse transcriptase-polymerase chain reaction method, the present study shows that primary cultures of rat brain capillary endothelial (RBCE) cells and astrocytes derived from rat brain express two different thrombin receptors. The first is proteolytically activated receptor (PAR)-1, the receptor responsible for the vast majority of the thrombin's cellular activation functions; the second is PAR-3, a receptor described to be essential for normal responsiveness to thrombin in mouse platelets. In addition to these thrombin receptors, the mRNA (messenger RNA) for PAR-2, a possible trypsin receptor, was also identified. Functional significance of thrombin receptors was indicated by changes in [Ca2+]i in response to thrombin, as measured by FURA-2 fluorescence in RBCE cells. Thrombin as low as 4 nmol/L induced an abrupt increase in [Ca2+]i whereas, upon addition of active site-blocked thrombin or plasmin, [Ca2+]i remained unchanged. The [Ca2+]i signal attributable to thrombin was smaller in a low Ca2+-containing medium, indicating that an influx of Ca2+ from the extracellular medium makes a contribution to the overall [Ca2+]i rise. The amplitude of the transient [Ca2+]i signal was dependent on the concentration of thrombin, and repeated application of the enzyme caused an essentially complete and long-term desensitization of the receptor. The PAR-1 agonist peptide SFLLRN also elicited a transient increase in [Ca2+]i. After activation by SFLLRN, cells showed a diminished response to thrombin, but the response was not absent, indicating that PAR-3 might contribute to the generation of the [Ca2+]i signal. Pretreatment of RBCE cells with 100 nmol/L plasmin completely prevented [Ca2+]i rise attributable to thrombin. These data show that RBCE cells and astrocytes express at least two receptors for thrombin, PAR-1 and PAR-3, and probably both receptors are involved in thrombin-induced [Ca2+]i signals. Plasmin itself does not elevate [Ca2+]i but prevents the activation of receptors by thrombin.

Lithium chloride inhibits thrombin-induced intracellular calcium mobilization in C6 rat glioma cells

In this study, the authors have demonstrated the effect of lithium, a typical mood stabilizer, on thrombin-evoked Ca2+ mobilization in C6 cells to elucidate the action mechanisms of the drug. Thrombin-induced Ca2 mobilization was reduced 24 hr after 1 or 10 mM lithium chloride (LiCl) pretreatment. The Ca2+ rise was reduced in a time-dependent manner, and the significant inhibition was observed 9 hr pretreatment with 10 mM LiCl. On the other hand, pretreatment of the cells with 10 mM LiCl for 24 hr did not alter the amount of Galphaq/11 significantly. Pretreatment with 10 mM LiCl for 24 hr failed to reduce the 5-HT-induced Ca2+ mobilization or to affect the desensitization of the 5-HT signal. Finally, thrombin-elicited Ca2+ rise was markedly inhibited in the presence of 0.05 U/ml plasmin, however, the Ca2+ rise was not further attenuated in the presence of plasmin in C6 cells pretreated with LiCl for 24 hr. These results indicate that pretreatment with LiCl attenuated thrombin-evoked intracellular Ca2+ mobilization in plasmin sensitive manner in C6 rat glioma cells. Thus, it is important to investigate the effect of lithium on thrombin-induced cellular responses to clarify the action mechanism of lithium in relation to some abnormality in thrombin-evoked Ca2+ rise observed in bipolar disorders.

Influence of serum-free culture conditions on steroid 5alpha-reductase mRNA expression in rat C6 glioma cells

Immunocytochemical studies previously showed that serum deprivation resulted in the appearance of steroid 5alpha-reductase (5alpha-R) in the cytoplasm of rat C6 glioma cells. To determine whether this increase in cytoplasmic 5alpha-R was due to changes in 5alpha-R gene expression, the effect of serum deprivation on 5alpha-R mRNA expression was examined. No significant change in the mRNA levels was observed in cells grown in serum-free culture medium. Therefore, the appearance of 5alpha-R immunoreactivity in the cell cytoplasm observed under serum-free conditions is probably not due to changes in 5alpha-R gene expression.

Influence of serum-free culture conditions on subcellular localization of steroid 5alpha-reductase in rat C6 glioma cells

Rat C6 glioma cells are considered to be well characterized, and therefore commonly used as a model system to investigate the function of glial cells. However, recent study has shown that an alteration in the expression of their phenotypic antigens is observed when the cells are maintained under the serum-free conditions, proposing the possibility that various properties of glioma cells can be altered by the growth conditions. To test this possibility, the effects of serum-free culture conditions on the expression of steroid 5alpha-reductase (5alpha-R) type 1 isozyme in glioma cells were examined using immunocytochemical technique. Immunoreactivity of 5alpha-R type 1 was confined to the perinuclear region of glioma cells cultured in serum-containing medium, and observed in the cytoplasmic space as well as the perinuclear region of the cells cultured in serum-free medium. In contrast, serum deprivation failed to affect the expression of phenotypic antigens, glial fibrillary acidic protein (GFAP) and 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNPase). Further studies showed that the expression of cytoplasmic 5alpha-R immunoreactivity induced by serum deprivation was reversible, and might be attributed to removal of serum proteins rather than biologically active small molecules from culture medium. This alteration in the expression of 5alpha-R immunoreactivity is therefore considered to reflect the translocation of the enzyme from the perinuclear region to the cell cytoplasm rather than the induction of cytoplasmic enzyme, and suggest that the culture conditions cause an alteration in the subcellular localization of 5alpha-R type 1 isozyme without phenotypic change of the glioma cell.

Changes in the expression of protease-activated receptor 1 and protease nexin-1 mRNA during rat nervous system development and after nerve lesion

HDs racI Thrombin causes profound metabolic and morphological changes in cultured neural cells via activation of the thrombin receptor, also called protease-activated receptor 1 (PAR1). PAR1 mRNA is present in the rat brain, but the role of this receptor in the nervous system remains elusive. The expression of PAR1 and the potent thrombin inhibitor protease nexin-1 (PN-1) was investigated in the developing rat brain and spinal cord and after peripheral nerve lesion. As seen by in situ hybridization, the PAR1 mRNA signal in the late embryonic and early postnatal nervous system was widespread, but generally of low intensity whereas in the adult it was more pronounced and confined to particular neuronal cells. These include the mesencephalic dopaminergic neurons, several thalamic and brainstem nuclei, the mitral cells in the olfactory bulb and the Purkinje cells in the cerebellum. In the spinal cord, PAR1 mRNA was abundant in motoneurons and a particularly high expression was detected in the preganglionic neurons of the autonomic nervous system. High PAR1 mRNA expression was also found in the dorsal root ganglia. Interestingly, strong immunoreactivity for the protease inhibitor PN-1 was present in spinal motoneuron cell bodies, although its transcript was undetectable there. In response to sciatic nerve transection, the signal intensity of PAR1 mRNA as seen by Northern analysis increased in the proximal and the distal part of the lesioned nerve and in the denervated muscle, whereas the PN-1 mRNA signal strongly increased only in the distal part of the nerve but remained unchanged in the proximal part and in the muscle. After facial nerve transection, PAR1 mRNA expression substantially decreased in facial motoneurons. No PAR1 transcript was detected in reactive astrocytes. Similar to PAR1, PN-1 mRNA which was expressed in interneurons within the facial nucleus was also decreased following facial nerve transection.

Astrocyte spreading in response to thrombin and lysophosphatidic acid is dependent on the Rho GTPase

Astrocytes are typically star shaped cells playing diverse roles in the function of the nervous system. In astrocyte cultures established from the cerebral hemispheres of newborn rats, the cells have generally a polygonal fibroblast-like morphology, but acquire a stellate shape upon serum removal. When the serine protease thrombin or the bioactive lipid lysophosphatidic acid is added, the stellate cells revert to the flat morphology. Here we show that the effect of these agents is mediated via activation of the small GTP-binding protein Rho. Neither thrombin nor lysophosphatidic acid induced spreading of astrocytes microinjected with C3 transferase, an exoenzyme which ADP-ribosylates and thereby inactivates Rho. In contrast, the response of cells injected with a dominant negative form of Rac was unaffected. In addition, the injection of active Rho into stellate astrocytes mimicked the effect of thrombin and lysophosphatidic acid and an injection of C3 into flat cells grown in serum induced stellation. The conversion from a stellate to a spread morphology upon activation of Rho resulted in the formation of stress fibers and focal adhesions which most probably are key events in establishing and stabilizing the altered cytoarchitecture. These results suggest that Rho plays a crucial role in determining the shape of astrocytes and thereby may modulate their interaction with neurons in vivo.

Hirudin suppresses the invasion of inflammatory cells and the appearance of vimentin-positive astrocytes in the rat cerebral ablation model

Hirudin is a specific and direct-acting thrombin inhibitor superior to heparin as an anticoagulant. Thrombin is a multifunctional molecule that acts as a serine protease locally generated from prothrombin during blood coagulation related to injury and/or inflammation. We previously reported that thrombin might be involved in the inflammatory response, glial reaction, and scar formation that occurred in central nervous system (CNS). Here we studied the suppressive effects of hirudin on the inflammation, vimentin-positive astrocytes, and glial fibrillary acidic protein (GFAP)-positive astrocytes using rat cerebral ablation models. Hirudin and vehicle solution soaked in Gelform were administered to the cavity of the traumatic brain defect. Brains were examined by conventional histologic and immunohistologic technique. Antibodies for monocytes/macrophages, GFAP, and vimentin were used to assess the infiltration of inflammatory cells and reaction of astrocytes. The number of the inflammatory cells, vimentin-positive astrocytes, and GFAP-positive astrocytes were quantitatively analyzed. Hirudin suppressed the infiltration of inflammatory cells and the increase in vimentin-positive astrocytes, but had no effects on the increase in GFAP-positive astrocytes. These data suggest that thrombin may play an important role in inflammatory and glial responses to CNS injury, and that hirudin can be a candidate for the therapeutic agent that minimizes the secondary brain damage following the inflammation, and the glial reaction mediated by vimentin-positive astrocytes near the lesion site.

Serum uncouples elevation of cyclic adenosine monophosphate concentration from cyclic adenosine monophosphate dependent morphological changes exhibited by cultured pituicytes

Cultured pituicytes (neurohypophysial astrocytes) are normally flat amorphous cells when incubated (90 min) in a HEPES balanced salt solution (HBSS) but become stellate when incubated in HBSS supplemented with forskolin. This stellation process is attenuated by serum (0.5% vol/vol). The experiments described here were designed to determine whether serum attenuates stellation by modulation of the intracellular cyclic adenosine monophosphate (cAMP) concentration or some other mechanism. It was observed that the effect of serum on forskolin-induced stellation was not affected by pertussis toxin (100 ng/ml) and that serum also inhibited stellation induced by the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX; 100 microM). Further, serum inhibited stellation induced by the membrane permeable cAMP analog 8-bromo cAMP (150 microM). These results indicate that although an increase of intracellular cAMP concentration is necessary for pituicyte stellation, an increase of intracellular cAMP concentration may be decoupled from stellation.

Thrombin has a bimodal effect on glioma cell growth

Using rat glioma C6 cells as a model, we have found a bimodal effect of alpha-thrombin on cell growth. In C6 cells treated with alpha-thrombin at concentrations from 0.02 nM to 1.0 nM, inhibition of cell proliferation was noted. Because the thrombin receptor agonist peptide TRAP-6 also induced inhibition of cell proliferation and the thrombin receptor antagonist peptide T1 prevented the inhibitory effect of alpha-thrombin on C6 glioma cell growth, thrombin receptor involvement in antiproliferative action of alpha-thrombin in C6 glioma cells is highly likely. However, stimulation of cell proliferation observed when C6 cells were treated with alpha-thrombin at higher doses (> 1.0 nM) seems to be mediated by as yet undefined thrombin receptor-independent biochemical mechanisms.

Lysophosphatidic acid depletes intracellular calcium stores different from those mediating capacitative calcium entry in C6 rat glioma cells

Lysophosphatidic acid (LPA) functions as an extracellular lipid mediator stimulating phospholipase C and affecting the structure of the cytoskeleton in several cell types. In rat glioma C6 cells, LPA mobilizes calcium from intracellular calcium stores and reverts morphological changes induced by elevated cytosolic cAMP-concentrations. Here we show that LPA-stimulation of C6 cells loaded with the calcium-sensitive fluorescent dye indo-1 results in calcium release from a subset of intracellular calcium stores that are not sensitive to the tumor promoter thapsigargin and do not overlap with calcium stores depleted during purinergic receptor stimulation with ATP. Furthermore, depletion of LPA-sensitive calcium stores does not induce capacitative calcium entry from the extracellular space into the cytosol to the same extent as ATP. These results indicate that inositol phosphate signaling induced by LPA or ATP may differ in kinetics or in spatial organisation within the cell. This may represent a possible explanation for the previous observation that only LPA, but not other calcium-mobilizing agonists, reverts cAMP-induced changes in the cytoskeletal organization in C6 cells.

Signaling effects of alpha-thrombin and SFLLRN in rat glioma C6 cells

Effects of thrombin on brain cells, including change of neurite outgrowth and astrocyte shape, are described, but the molecular mechanisms are unclear. We investigated the effects of human alpha-thrombin and a six amino acid thrombin receptor activating peptide (TRAP-6, SFLLRN) on [Ca2+]i, phosphoinositide hydrolysis, and protein kinase C in rat glioma C6 cells. Stimulation of C6 cells with both alpha-thrombin and TRAP-6 resulted in [Ca2+]i mobilization, [3H]Inositol phosphate response, and enhanced immunoreactivity of the protein kinase C (PKC) isoenzymes alpha, beta, gamma, delta, and epsilon. Results suggest that alpha-thrombin and TRAP-6 activate at least partially the same intracellular signaling pathways in rat glioma C6 cells, which is evidence for involvement of "tethered ligand" receptor in thrombin induced signaling in glioma C6 cells.

Serum modulates cyclic AMP-dependent morphological changes in cultured neurohypophysial astrocytes

The effects of serum on the morphological plasticity exhibited by pituicytes in explant cultures of the neurohypophysis of adult rats have been examined. Cultured pituicytes are normally nonstellate, protoplasmic, amorphous cells (< 25% are stellate with a distinct cell body and phase bright processes). After incubation (90 min) of pituicyte cultures in a HEPES buffered salt solution (HBSS) supplemented with isoproterenol or forskolin, the fraction of stellate pituicytes significantly increased. The increase in the fraction of stellate cells induced by isoproterenol was not reversed by subsequent incubation in isoproterenol-free HBSS for 90 min. In contrast, after stellation was induced in cultures by exposure to forskolin (90 min), the fraction of stellate cells was significantly reduced if these cultures were incubated in forskolin-free, serum (0.5%) supplemented HBSS for the same duration. Serum also blocked the increase in the fraction of stellate pituicytes induced by forskolin. These experiments suggest that serum components may have a significant role in controlling the plasticity of neuroglial relations in the neurohypophysis previously demonstrated in vivo.

Granzyme A released upon stimulation of cytotoxic T lymphocytes activates the thrombin receptor on neuronal cells and astrocytes

Granzymes are a family of serine proteases that are harbored in cytoplasmic granules of activated T lymphocytes and are released upon target cell interaction. Immediate and complete neurite retraction was induced in a mouse neuronal cell line when total extracts of granule proteins were added. This activity was isolated and identified as granzyme A. This protease not only induced neurite retraction at nanomolar concentrations but also reversed the stellation of astrocytes. Both effects were critically dependent on the esterolytic activity of granzyme A. As neurite retraction is known to be induced by thrombin, possible cleavage and activation of the thrombin receptor were investigated. A synthetic peptide spanning the N-terminal thrombin receptor activation sequence was cleaved by granzyme A at the authentic thrombin cleavage site Leu-Asp-Pro-Arg-Ser. Antibodies to the thrombin receptor inhibited both thrombin and granzyme A-mediated neurite retraction. Thus, T-cell-released granzyme A induces cellular responses by activation of the thrombin receptor. As brain-infiltrating CD4+ lymphocytes are the effector cells in experimental allergic encephalomyelitis, granzyme A released in the brain may contribute to the etiology of autoimmune disorders in the nervous system.

Plasmin modulates the thrombin-evoked calcium response in C6 glioma cells

Extracellular proteinases may be selectively targeted to cell surfaces by specific receptors or binding sites. In previous studies, we have characterized cellular binding sites for plasminogen and plasmin on rat C6 glioma cells. In this investigation, we studied the response of C6 cells to alpha-thrombin and plasmin by measuring the rapid kinetics of free intracellular Ca2+ concentrations ([Ca2+]i). Thrombin produced a strong, concentration-dependent rise in [Ca2+]i with an onset within 3 s and peak levels achieved in less than 10 s. A similar response was also evoked by an SFLLRN-containing thrombin-agonist peptide. C6 cells did not respond to plasmin (25 nM-1.5 microM). By contrast, pretreatment of C6 cells with 100 nM plasmin significantly inhibited the [Ca2+]i response to thrombin and the thrombin-agonist peptide. The peak [Ca2+]i response to thrombin, in cells pretreated with plasmin, was reduced by approx. 50%. The effect of plasmin on the cellular response to thrombin was selective, as pretreatment of the cells with plasmin did not affect the [Ca2+]i response to platelet-activating factor. Di-isopropylphosphorylplasmin and plasminogen did not inhibit the cellular response to thrombin, indicating that plasmin activity is required and that occupancy of cellular plasmin(ogen)-binding sites alone is insufficient. These studies demonstrate that plasmin does not directly induce a response in C6 cells, but may affect cellular function by specifically modulating the thrombin response.

Glial calcium

This review summarizes current knowledge relating intracellular calcium and glial function. During steady state, glia maintain a low cytosolic calcium level by pumping calcium into intracellular stores and by extruding calcium across the plasma membrane. Glial Ca2+ increases in response to a variety of physiological stimuli. Some stimuli open membrane calcium channels, others release calcium from intracellular stores, and some do both. The temporal and spatial complexity of glial cytosolic calcium changes suggest that these responses may form the basis of an intracellular or intercellular signaling system. Cytosolic calcium rises effect changes in glial structure and function through protein kinases, phospholipases, and direct interaction with lipid and protein constituents. Ultimately, calcium signaling influence glial gene expression, development, metabolism, and regulation of the extracellular milieu. Disturbances in glial calcium homeostasis may have a role in certain pathological conditions. The discovery of complex calcium-based glial signaling systems, capable of sensing and influencing neural activity, suggest a more integrated neuro-glial model of information processing in the central nervous system.

Homologous desensitization of serotonin 5-HT2 receptor-stimulated intracellular calcium mobilization in C6BU-1 glioma cells via a mechanism involving a calmodulin pathway

Serotonin 5-HT2 receptor-mediated intracellular Ca2+ mobilization was investigated in rat glioma C6BU-1 cells. The receptors became desensitized after previous exposure to 5-HT in a time- and concentration-dependent manner. The desensitization of 5-HT2 receptor-mediated intracellular signaling appeared to be homologous because previous exposure to 5-HT did not alter the response to other transmitters such as thrombin or isoproterenol and because previous exposure to thrombin or isoproterenol did not diminish the response to 5-HT. The desensitization induced by pretreatment with 5-HT was potently prevented by the naphthalenesulfonamide derivative W-7, a calmodulin antagonist, when it was cosupplied with 5-HT. Furthermore, the preventive effect of W-7 was greater than that of W-5, a weak analogue of W-7, and than that of H-7, a nonselective inhibitor of protein kinases. These results suggest that 5-HT2 receptor-mediated Ca2+ mobilization can be desensitized homologously after prolonged exposure to 5-HT in a calmodulin-dependent manner in rat glioma C6BU-1 cells.

Thrombin is a regulator of astrocytic endothelin-1

Endothelin-1, a potent vasoconstrictor of cerebral vessels, is produced by rat primary astrocytes and is subject to autostimulatory regulation in these cells. In this study we examined the effect of thrombin on astrocytic endothelins and report that endothelin-1 is released into the culture fluid in response to thrombin treatment. However, increased production of endothelin-1 is not accompanied by a concomitant increase in steady-state levels of endothelin-1 mRNA as assessed by reverse transcriptase-polymerase chain reaction, even though thrombin stimulation leads to increased inositolphospholipid turnover and activation of the nuclear factor AP1. Thus, astrocytic production of endothelin-1 may be mainly post-transcriptionally regulated in response to thrombin stimulation. In addition, two endothelin receptor genes (ET(A) and ETB) were found to be transcribed simultaneously in primary astrocyte cultures, and both thrombin and endothelin-1 stimulation result in a distinct temporary decrease in ET(A) mRNA. These studies suggest a role for thrombin in the regulation of brain perfusion through astrocytic endothelin-1 expression.

Thrombin may contribute to the pathophysiology of central nervous system injury

Thrombin has multiple functions, including its function as a key enzyme during blood coagulation and other physiologic activities. We studied brain tissue reactions to thrombin that might be present in the central nervous system (CNS) following injury. Thrombin and three different types of controls--buffer, albumin, and plasmin--were individually infused into the rat caudate nucleus by a continuous osmotic mini-pump. Brains were examined by conventional histologic and immunohistologic techniques. Antibodies for bromodeoxyuridine (BrdU), glial fibrillary acidic protein (GFAP), vimentin, and laminin were employed to assess the infiltration of inflammatory cells, proliferation activity of cells, and reaction of astrocytes and mesenchymal cells, respectively. The number of inflammatory cells, number of BrdU-positive cells, area and number of vimentin-positive astrocytes, and the area of GFAP-positive astrocytes were quantitatively analyzed. Thrombin caused infiltration of inflammatory cells, proliferation of mesenchymal cells, induction of angiogenesis, and an increase in vimentin-positive reactive astrocytes. These histologic changes caused by thrombin infusion resembled the inflammation, scar formation, and reactive gliosis in the CNS following injury. These results suggest that thrombin may play an important role in inflammatory responses to CNS injury since thrombin is one of the blood borne factors that may interact with brain tissue after CNS injury. The data further suggest that the therapeutic application of antithrombin agents for CNS injury suppresses inflammation and the excessive gliosis and scar formation, which are barriers to neuronal regeneration.

Synthesis of glial fibrillary acidic protein in rat C6 glioma in chemically defined medium: cyclic AMP-dependent transcriptional and translational regulation

Glial fibrillary acidic protein (GFA) expression was induced in rat C6 glioma in chemically defined medium by the addition of N6, O2'-dibutyryl cyclic AMP (dbcAMP). Induction was dependent on the increase in intracellular cyclic AMP (cAMP), which was linearly correlated with added dbcAMP. Contrary to GFA mRNA synthesis, which can be obtained by cAMP-dependent and -independent pathways, translation of mRNA into GFA was observed only above a cellular cAMP concentration of approximately 0.2 fmol/cell. dbcAMP stimulation did not affect the vimentin concentration, which remained at a low level, but changed the cellular morphology from a bipolar to a stellate shape. A similar morphological change was observed after stimulation of C6 with lipopolysaccharide (LPS). However, LPS did not significantly increase the intracellular concentration of cAMP and the LPS-induced mRNA was not translated into GFA. Our results indicate that GFA synthesis is regulated at the mRNA level and at the translational level and that a cAMP-dependent mechanism determines the ultimate synthesis of GFA by a yet unknown mechanism.

Thrombin causes neurite retraction in neuronal cells through activation of cell surface receptors

The mechanism by which thrombin induces neurite retraction was studied in NB2a mouse neuroblastoma cells. The rapid effect of thrombin (completed within minutes) appears to involve an interaction between its anion-binding exosite and the thrombin receptor. Structural alterations of this site increase the EC50 for thrombin-mediated retraction, and a hirudin C-terminal peptide that blocks this site inhibits the response. The thrombin effect was mimicked by a 14 amino acid peptide starting with Ser-42, at the proposed cleavage site of the human thrombin receptor. The protein kinase inhibitors staurosporine and H-7 blocked thrombin-induced retraction. It is therefore proposed that thrombin-mediated neurite retraction is caused by cleavage-induced activation of the thrombin receptor and involves stimulation of a protein kinase(s).