Protein phosphorylation in astrocytes mediated by protein kinase C: comparison with phosphorylation by cyclic AMP-dependent protein kinase

Control of the phosphorylation of the astrocyte marker glial fibrillary acidic protein (GFAP) in the immature rat hippocampus by glutamate and calcium ions: possible key factor in astrocytic plasticity

The present review describes recent research on the regulation by glutamate and Ca2+ of the phosphorylation state of the intermediate filament protein of the astrocytic cytoskeleton, glial fibrillary acidic protein (GFAP), in immature hippocampal slices. The results of this research are discussed against a background of modern knowledge of the functional importance of astrocytes in the brain and of the structure and dynamic properties of intermediate filament proteins. Astrocytes are now recognized as partners with neurons in many aspects of brain function with important roles in neural plasticity. Site-specific phosphorylation of intermediate filament proteins, including GFAP, has been shown to regulate the dynamic equilibrium between the polymerized and depolymerized state of the filaments and to play a fundamental role in mitosis. Glutamate was found to increase the phosphorylation state of GFAP in hippocampal slices from rats in the post-natal age range of 12-16 days in a reaction that was dependent on external Ca2+. The lack of external Ca2+ in the absence of glutamate also increased GFAP phosphorylation to the same extent. These effects of glutamate and Ca2+ were absent in adult hippocampal slices, where the phosphorylation of GFAP was completely Ca(2+)-dependent. Studies using specific agonists of glutamate receptors showed that the glutamate response was mediated by a G protein-linked group II metabotropic glutamate receptor (mGluR). Since group II mGluRs do not act by liberating Ca2+ from internal stores, it is proposed that activation of the receptor by glutamate inhibits Ca2+ entry into the astrocytes and consequently down-regulates a Ca(2+)-dependent dephosphorylation cascade regulating the phosphorylation state of GFAP. The functional significance of these results may be related to the narrow developmental window when the glutamate response is present. In the rat brain this window corresponds to the period of massive synaptogenesis during which astrocytes are known to proliferate. Possibly, glutamate liberated from developing synapses during this period may signal an increase in the phosphorylation state of GFAP and a consequent increase in the number of mitotic astrocytes.

Selective enhancement by serum factors of cyclic AMP accumulation in rat microglial cultures

Using purified microglial cultures obtained from the neonatal rat brain we found that media containing fetal calf serum (as well as human, horse and goat sera) enhanced by about 3-fold the accumulation of cyclic AMP induced by the beta-adrenergic agonist isoproterenol and did not affect in a significant way that induced by the direct adenylyl cyclase stimulator forskolin. The effect of fetal calf serum was (i) dose dependent, and statistically significant also at serum concentrations below 1%; (ii) rapidly lost (half life of about 15 min) when the serum-containing medium was exposed to microglia, astrocytes or neuroblastoma cells; (iii) present also when cyclic AMP accumulation was enhanced by prostaglandin E2 or by cholera toxin; (iv) absent on basal cyclic AMP levels. When media containing fetal calf serum or the other mammalian sera mentioned above were tested on astrocyte cultures, an inhibitory, rather than enhancing activity on cyclic AMP levels was observed, indicating that the facilitatory factor(s) present in serum acts specifically on microglial cells. Moreover, in astrocytes the effect of serum was identical when tested on basal and on isoproterenol or forskolin-stimulated cyclic AMP levels. Thus, the mechanism of cyclic AMP inhibition in astrocytes is unrelated to the mechanism of activation in microglia. Our observations suggest that serum contains factor(s), promptly cleared by different cell types. Such factors may interact with so far unidentified microglial receptors responsible for a facilitation of G protein-mediated activation of adenylyl cyclase. Regulation of the cyclic AMP cascade at this step has not been described previously, and may be important for the modulation of microglial functions controlled by the cyclic nucleotide.

Age-dependent changes in the regulation by external calcium ions of the phosphorylation of glial fibrillary acidic protein in slices of rat hippocampus

We studied the effect of external Ca2+ on the incorporation of [32P]phosphate into the astrocytic marker protein, glial fibrillary acidic protein (GFAP), in hippocampal slices from rats in the postnatal age range 12-16 days to +60 days (P12-P16 to +P60). At age P12-P16 the presence of Ca2+ in the incubation medium inhibited the incorporation of 32P into GFAP; this inhibition declined to near zero by P21 and subsequently 32P-incorporation became progressively more dependent on Ca2+ until by P60 no GFAP phosphorylation was observed in the absence of Ca2+. With tissue from immature rats inhibition of 32P-incorporation into GFAP started at a medium concentration of 7.5 microM Ca2+, reached 50% at 100 microM and then remained constant up to 1 mM; with adults maximal phosphorylation required 1 mM Ca2+ in the medium. The inorganic Ca(2+)-channel blockers, Co2+ and Ni2+, and a high concentration of the L-type blocker, nifedipine, reversed the effects of external Ca2+ on GFAP phosphorylation. The results suggest a late developmental change in the compartmental disposition of Ca2+ in astrocytes.

Human immunodeficiency virus protein gp120 interferes with beta-adrenergic receptor-mediated protein phosphorylation in cultured rat cortical astrocytes

1. We have previously shown that acute exposure to the HIV coat protein gp120 interferes with the beta-adrenergic regulation of astroglial and microglial cells (Levi et al., 1993). In particular, exposure to 100 pM gp120 for 30 min depressed the phosphorylation of vimentin and glial fibrillary acidic protein (GFAP) induced by isoproterenol in rat cortical astrocyte cultures. In the present study we have extended our analysis on the effects of gp120 on astroglial protein phosphorylation. 2. We found that chronic (3-day) treatment of the cells with 100 pM gp120 before exposure to isoproterenol was substantially more effective than acute treatment in depressing the stimulatory effect of the beta-adrenergic agonist on vimentin and GFAP phosphorylation. 3. Even after chronic treatment with gp120, no differences were found in the levels and solubility of these proteins. 4. Besides stimulating the phosphorylation of intermediate filament proteins, isoproterenol inhibited the incorporation of 32P into a soluble acidic protein of 80,000 M(r), which was only minimally present in Triton X-100-insoluble extracts. 5. Treatment of astrocytes with a phorbol ester or exposure to 3H-myristic acid indicated that the acidic 80,000 M(r) protein is a substrate for protein kinase C (PKC) and is myristoylated, thus suggesting that it is related to the MARCKS family of PKC substrates. 6. Acute (30-min) treatment with 100 pM gp120 totally prevented the inhibitory effect of isoproterenol on the phosphorylation of the 80,000 M(r) MARCKS-like protein. 7. Our studies corroborate the hypothesis that viral components may contribute to the neuropathological changes observed in AIDS through the alteration of signal transduction systems in glial cells.

Transition metal chelator TPEN counteracts phorbol ester-induced actin cytoskeletal disruption in C6 rat glioma cells without inhibiting activation or translocation of protein kinase C

Phorbol ester-induced reorganization of the actin cytoskeleton was investigated in C6 rat glioma cells. Observations by fluorescence microscopy and photoelectron microscopy indicated that pretreatment with the transition metal chelator N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN) for 1-2 h at 50 microM reduced the sensitivity of the actin cytoskeleton to disruption by the subsequent addition of 200 nM phorbol myristate acetate (PMA). The protective effect of TPEN was eliminated by adding back Zn2+ prior to PMA addition, implicating chelation of metal ions as the mechanism of action of TPEN. C6 cells exposed to PMA experience potent activation of protein kinase C (PKC) and substantial redistribution of the kinase from a soluble to a particulate cellular fraction (translocation). TPEN pretreatment did not block PKC translocation in PMA-exposed cells. By two-dimensional gel analysis, TPEN also did not reduce, but rather slightly increased, the PMA-stimulated phosphorylation of the acidic 80 kDa endogenous PKC substrate, as well as two other proteins at 18 kDa and 50 kDa. In contrast, TPEN significantly suppressed phosphorylation of a 20 kDa protein, both in cells treated with TPEN only and in TPEN-pretreated PMA-exposed cells. The results indicate that the ability of TPEN to protect against PKC-mediated actin cytoskeletal disruption is not due to either a block of PKC translocation or to general inhibition of PKC activity. Rather, the action of TPEN is more selective and probably involves chelation of Zn2+ at a critical Zn(2+)-dependent phosphorylation step downstream from the initial tumor promoter-induced effects on PKC.

Ethanol induced alterations in plasma membrane protein phosphorylation of neurons and astrocytes

The endogenous protein phosphorylation patterns in plasma membranes of bulk isolated neurons and astroglia from control and chronic ethanol treated rats have been investigated. Chronic ethanol treatment resulted in increased phosphorylation of specific proteins with molecular weights 116, 63 and 60 kDa in both neurons and astrocytes. These proteins were further resolved by 2-DE and the analysis suggested an increased phosphorylation of congruent to 10-15 proteins, of which 116 kDa protein is phosphorylated to a higher extent by ethanol. Further, decreased phosphorylation was noticed in D-95 and D-63 proteins in neurons and D-78 and D-54 proteins in astrocytes. Alkali stability experiments for identifying the phosphoamino acid involved in phosphorylation of 116, 63 and 60 kDa proteins suggested that tyrosine and threonine are not involved and probably serine is the likely site for phosphorylation during chronic ethanol treatment. The phosphorylation of specific membrane proteins during chronic ethanol treatment might contribute to ethanol evoked cellular dysfunction.

Cloning and characteristics of fish glial fibrillary acidic protein: implications for optic nerve regeneration

Mammalian central nervous system neurons do not regenerate after axonal injury, unlike their counterparts in fish and amphibians. After axonal injury, glial cells in mammals do not support regrowth of axons, while in fish they support the regeneration process. Controversy exists as to whether or not the intact fish optic nerve expresses glial fibrillary acidic protein, a well-known marker for mature astrocytes, and thus whether its astrocytes differ in this respect from those of the brain and spinal cord, as well as from optic nerve astrocytes of other species. In an attempt to resolve this question we cloned fish glial fibrillary acidic protein. Two different complementary DNA clones were isolated from a carp brain complementary DNA library, each encoding a different form of glial fibrillary acidic protein apparently originating from different genes. Monospecific polyclonal antibodies were raised against a peptide synthesized according to the predicted amino acid sequence, and used to identify and localize the fish glial fibrillary acidic protein. Two glial fibrillary acidic proteins (of 49 kDa and 51 kDa) were identified by the antibodies in all tested fish central nervous system tissues. The antibodies were then used to examine glial fibrillary acidic protein immunoreactivity in sections taken from uninjured and injured optic nerves of goldfish. Injury was followed by an elevation in glial fibrillary acidic protein immunoreactivity along the whole length of the nerve, except at the site of the injury, where--as in the case of vimentin--no immunoreactivity was detectable. However, in contrast to vimentin-positive glial cells, which repopulate the site of the injury soon after the optic nerve is injured, glial fibrillary acidic protein-positive glial cells remained outside the injury site for as long as 6 weeks after the injury. Despite the injury-induced changes in glial fibrillary acidic protein immunoreactivity, no change was observed in the level of transcript encoding glial fibrillary acidic protein after injury, while there was an increase in the amount of glial fibrillary acidic protein associated with the cytoskeleton and a reduction in the soluble form. These results suggest that the injury-induced changes in immunoreactivity on sections involve changes not in transcription or translation of glial fibrillary acidic protein, but in glial fibrillary acidic protein compartmentalization.

Differential effects of a phorbol ester on carboxypeptidase E in cultured astrocytes and AtT-20 cells, a neuroendocrine cell line

Cultured astrocytes have been shown to secrete various neuropeptides and the neuropeptide processing enzyme, carboxypeptidase E (CPE). The secretion of CPE enzymatic activity from astrocytes has been shown previously to be increased approximately twofold by treatment with tetradecanoylphorbol 13-acetate (TPA), a phorbol ester. In this study, metabolic labeling with [35S]Met was utilized to examine the effect of TPA on the biosynthesis of CPE protein in cultured astrocytes and in AtT-20 cells, a pituitary-derived cell line. Treatment of astrocytes with 0.1 micrograms/ml TPA for 24 h caused an 80% increase in the level of radiolabeled CPE in both the media and the cells, indicating that the synthesis of CPE was stimulated by the TPA. AtT-20 cells also secreted more radiolabeled CPE in response to TPA, but this increase was offset by a proportional decrease in the cellular level of radiolabeled CPE, and synthesis of CPE was not stimulated in this cell line. Northern blot analysis demonstrated that 0.1 micrograms/ml TPA elevated CPE mRNA by approximately 50% in cultured astrocytes but not in AtT-20 cells. Quantitative in situ hybridization studies demonstrated that the TPA-induced increase in CPE mRNA expression was largely due to increases in the number of cells expressing CPE mRNA, although for astrocytes from some brain regions the average level of CPE mRNA per cell was also elevated by TPA. These results suggest that astrocytes can be induced to express CPE, which is consistent with a role for astrocytes in intercellular signaling.

Dissociation of phosphorylation and translocation of a myristoylated protein kinase C substrate (MARCKS protein) in C6 glioma and N1E-115 neuroblastoma cells

An 80-kDa protein labeled with [3H]myristic acid in C6 glioma and N1E-115 neuroblastoma cells has been identified as the myristoylated alanine-rich C kinase substrate (MARCKS protein) on the basis of its calmodulin-binding, acidic nature, heat stability, and immunochemical properties. When C6 cells preincubated with [3H]myristate were treated with 200 nM 4 beta-12-O-tetradecanoylphorbol 13-acetate (beta-TPA), labeled MARCKS was rapidly increased in the soluble digitonin fraction (maximal, fivefold at 10 min) with a concomitant decrease in the Triton X-100-soluble membrane fraction. However, phosphorylation of this protein was increased in the presence of beta-TPA to a similar extent in both fractions (maximal, fourfold at 30 min). In contrast, beta-TPA-stimulated phosphorylation of MARCKS in N1E-115 cells was confined to the membrane fraction only and no change in the distribution of the myristoylated protein was noted relative to alpha-TPA controls. These results indicate that although phosphorylation of MARCKS by protein kinase C occurs in both cell lines, it is not directly associated with translocation from membrane to cytosol, which occurs in C6 cells only. The cell-specific translocation of MARCKS appears to correlate with previously demonstrated differential effects of phorbol esters on stimulation of phosphatidylcholine turnover in these two cell lines.

Norepinephrine-mediated protein phosphorylation in astrocytes

Studies were conducted to determine if norepinephrine activates both protein kinase C and the cyclic AMP-dependent protein kinase in cultured rat astrocytes using phosphoproteins as markers. Norepinephrine was found to decrease 32P incorporation into an acidic 80,000 M(R) protein. A similar response was observed with isoproterenol and cyclic AMP analogs. In contrast, phorbol myristate acetate (PMA) increased 32P incorporation into this protein. Further studies looked at phosphorylation sites on glial fibrillary acidic protein and vimentin using two-dimensional tryptic phosphopeptide maps. The pattern of phosphorylation of these two proteins by norepinephrine resembles that of 8-bromo cyclic AMP and isoproterenol, and not that of PMA. Additionally, the effect of norepinephrine on the phosphorylation of GFAP and vimentin was blocked by alprenolol. One difference noted between norepinephrine and isoproterenol was the phosphorylation of an 18,000 M(R) protein. Norepinephrine increased, and isoproterenol decreased, 32P incorporation into this protein; however, the mechanism which mediates the norepinephrine effect remains to be determined. Overall, these studies indicate that the most prominent phosphorylation events mediated by norepinephrine are the consequence of the activation of cyclic AMP-dependent protein kinase.

Role of calmodulin and protein kinase C in astrocytic cell volume regulation

We investigated the role of Ca(2+)-dependent protein kinases in the regulation of astrocytic cell volume. Calmodulin (CaM) antagonists were used to inhibit CaM and thus Ca2+/CaM-dependent protein kinase. The effect of these inhibitors as well as activators and inhibitors of protein kinase C (PKC) on astrocytic volume was measured in response to hypoosmotic stress and under isoosmotic conditions. In conditions of hypoosmolarity, CaM antagonists had no effect on swelling, but inhibited the regulatory volume decrease. PKC activation facilitated the swelling induced by hypoosmotic stress. PKC inhibitors induced cell shrinkage and inhibited the initial phase of regulatory volume decrease, whereas PKC down-regulation caused pronounced swelling and partial inhibition of regulatory volume decrease. In isoosmotic conditions, CaM antagonists and PKC activation did not affect astrocytic volume, but PKC inhibitors caused shrinking and PKC down-regulation led to swelling of these cells. These studies indicate the importance of Ca(2+)-dependent protein kinases in the regulation of astrocytic cell volume.

Isolation of cDNA clones encoding rat glial fibrillary acidic protein: expression in astrocytes and in Schwann cells

Glial fibrillary acidic protein (GFAP) expressed by astrocytes in the central nervous system (CNS) has been extensively characterized but the molecular identity of related molecules in the peripheral nervous system (PNS) remains unclear. To examine possible structural differences between CNS and PNS GFAP, we have isolated cDNA clones for rat GFAP from both cultured astrocyte and Schwann cell libraries. Nucleotide sequence analysis indicated that the PNS and CNS GFAP clones contained identical coding regions, with a predicted protein product of 430 amino acids. However, the 5'-untranslated region of clone rGFA15, isolated from the Schwann cell library, was longer than that predicted for brain-derived GFAP mRNA. Primer extension analysis of RNA isolated from the RT4-D6 Schwann cell line indicated that the start site for PNS GFAP mRNA lies 169 bases upstream from that used in the CNS. In addition, tryptic peptide mapping of GFAP prepared from cultured astrocytes and Schwann cells revealed one major peptide fragment present in CNS GFAP but absent from PNS GFAP. These results suggest structural differences between GFAP in these two cell types, at both the nucleic acid and protein level, and are consistent with previous observations of immunochemical differences existing between CNS and PNS GFAP.

Stimulation of the P2Y purinergic receptor on type 1 astroglia results in inositol phosphate formation and calcium mobilization

Cultured astroglia express purinergic receptors that initiate phosphoinositide metabolism and calcium mobilization. Experiments were conducted to characterize the purinergic receptor subtype on type 1 astroglia responsible for stimulation these second-messenger systems. Inositol phosphate (IP) accumulation and calcium mobilization were measured after stimulation with ATP or purinergic receptor subtype-selective ATP analogues. ATP (10(-5) M) increased IP accumulation severalfold. Dose-effect assays monitoring astroglial IP accumulation revealed the order of potency that defines the P2Y receptor: 2-methylthioadenosine 5'-triphosphate greater than ATP greater than alpha beta-methyleneadenosine 5'-triphosphate greater than beta gamma-methyleneadenosine 5'-triphosphate. The influence of ATP on intracellular calcium levels in individual type 1 astroglia was examined using the calcium indicator dye, fura-2. Dose-effect experiments indicated that ATP was equally potent for generating inositol phosphates and increasing cellular calcium. The most prevalent response (87% of total responses) to ATP consisted of a rapid increase in calcium to a peak level that was approximately five times greater than the prestimulation level. This peak was followed by a decline to a plateau level that was significantly above baseline. This plateau phase of the calcium increase was maintained for at least 5 min in the presence of ATP and was dependent on external calcium. Many (23%) astroglia exhibited spontaneous calcium oscillations whose frequency and magnitude increased after the addition of 10(-5) M ATP. Immunocytochemical staining indicated that the responses occurred in glial fibrillary acidic protein positive cells. We conclude that type 1 astroglia express the P2Y purinergic receptor which regulates IP production and calcium mobilization.

Phosphorylation of glial fibrillary acidic protein and vimentin by cytoskeletal-associated intermediate filament protein kinase activity in astrocytes

These studies describe a cytoskeletal-associated protein kinase activity in astrocytes that phosphorylated the intermediate filament proteins glial fibrillary acidic protein (GFAP) and vimentin and that appeared to be distinct from protein kinase C (PK-C) and the cyclic AMP-dependent protein kinase (PK-A). The cytoskeletal-associated kinase activity phosphorylated intermediate filament proteins in the presence of 10 mM MgCl2 and produced an even greater increase in 32P incorporation into these proteins in the presence of calcium/calmodulin. Tryptic peptide mapping of phosphorylated intermediate filament proteins showed that the intermediate filament protein kinase activity produced unique phosphopeptide maps, in both the presence and the absence of calcium/calmodulin, as compared to that of PK-C and PK-A, although there were some common sites of phosphorylation among the kinases. In addition, it was determined that the intermediate filament protein kinase activity phosphorylated both serine and threonine residues of the intermediate filament proteins, vimentin and GFAP. However, the relative proportion of serine and threonine residues phosphorylated varied depending on the presence or absence of calcium/calmodulin. The magnesium-dependent activity produced the highest proportion of threonine phosphorylation, suggesting that the calcium/calmodulin-dependent kinase activity acts mainly at serine residues. PK-A and PK-C phosphorylated mainly serine residues. Also, the intermediate filament protein kinase activity phosphorylated both the N-and the C-terminal domains of vimentin and the N-terminal domain of GFAP. In contrast, both PK-C and PK-A are known to phosphorylate the N-terminal domains of both proteins.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of plasminogen activators and type-1 plasminogen activator inhibitor by cyclic AMP and phorbol ester in rat astrocytes

Two plasminogen activators (PAs): tissue-type plasminogen activator (t-PA) and urokinase-type plasminogen activator (u-PA), as well as the type-1 plasminogen activator inhibitor (PAI-1) are synthesized and secreted by rat astrocytes. Preliminary studies suggest that PA activity plays a role in astrocyte development and differentiation. We have examined the regulation of the PA system by the cAMP-dependent protein kinase (PKA) and protein kinase C (PKC) in purified rat astrocyte cultures. PKA activity was increased by exposing cultured astrocytes to forskolin or dibutyryl cyclic AMP, whereas PKC activity was stimulated with phorbol-12-myristate 13-acetate (PMA). Activation of both second-messenger pathways produced a time- and dose-dependent increase in the total PA activity. However, based on SDS-PAGE/zymography we found that forskolin increased t-PA activity and reduced u-PA activity, whereas PMA treatment caused a significant increase in u-PA activity without altering t-PA activity. Reverse zymography analysis revealed that astrocyte PAI-1 activity is decreased by forskolin and increased by PMA. Together, these results demonstrate that the components of the PA system in rat astrocytes are independently and reciprocally regulated by PKA and PKC. Our findings raise the possibility that the plasminogen activator system could be involved in some of the actions of growth factors and/or neuromodulators that modulate PKC or PKA in astrocytes.

Protein kinase C-induced redistribution of the cytoskeleton and phosphorylation of vimentin in cultured brain macrophages

The phorbol ester 12-O-tetradecanoyl-acetate (TPA) induced prominent and transient changes in the organization of the cytoskeleton in cultured amoeboid microglial cells including redistribution of actin toward the center of the cells and in the subplasmalemmal region, appearance of fine actin filaments, retraction of microtubules (MT), and rearrangement of intermediate filaments (IF) containing vimentin. The possible implication of protein kinase C (PKC) in mediating the effects of TPA was suggested by a parallel shift of PKC activity from the soluble to membrane fractions and phosphorylation of several microglial proteins. The rearrangement of IF closely correlated with increased vimentin phosphorylation, detected by pulse labeling of intact cells. Two monoclonal antivimentin antibodies, B3 and V9, showed different staining patterns. Immunoreactivity with the antibody B3 was more restricted and could be abolished by treatment of fixed, permeabilized cells with alkaline phosphatase, thus suggesting that the antibody reacts with a phosphorylated epitope. Using this antibody, rearrangement of IF involving vimentin phosphorylation was detected within 15 to 60 min of treatment with 50 nM TPA and consisted in the appearance of intense perinuclear fluorescent label. This perinuclear fluorescence persisted up to 24 hr after TPA removal and gradually diminished during the following 2 to 3 days. Immunochemical analysis of nonionic detergent-soluble and -insoluble extracts from untreated and TPA-treated cells revealed no differences in vimentin solubility suggesting that TPA induced vimentin phosphorylation does not result in notable vimentin filament disassembly. However the extent of vimentin degradation was more prominent in TPA-treated cultures indicating a higher sensitivity of vimentin to proteolytic degradation. The data show that PKC-mediated phosphorylation of vimentin results in precise spatial and temporal rearrangement of IF which are not associated with altered vimentin solubility, but possibly changes the mechanical properties and interactions of vimentin filaments.

Distribution of protein kinase C isozymes in rat optic nerves

Light (LM) and electron (EM) microscopic immunocytochemical methods were used to study the distribution of protein kinase C (PKC) isozymes in adult rat optic nerves. In cryostat and vibratome sections examined by LM, type II (beta) isozyme was localized almost exclusively in the axons. In the EM, immunoreaction products were found to associate with microtubules and neurofilaments. The inner surface of axonal membranes were occasionally stained. Analysis of PKC isozyme composition of the optic nerves by using immunoblot techniques revealed that type II (beta) isozyme accounted for approximately 80% of the total immunoreactivity. By contrast, type III (alpha) isozyme, which accounted for the remaining 20% of PKC, was found mainly in the astrocytes. Astrocytic processes next to blood vessels and between myelinated axons were stained. In the EM, immunoreaction products were found in the cytoplasm and along astroglial filaments. Segments of plasma membranes also were stained; but nuclei were unstained. Adult glial cells were not stained by an antibody to type II (beta) isozyme except for the occurrence of a few punctate cytoplasmic densities in occasional astrocytes. Very faint or no immunostaining was observed in sections treated with a monoclonal antibody to type I (gamma) isozyme. Immunoblot analyses also did not reveal this subspecies. The absence of type I (gamma) isozyme in optic nerves is not due to a down-regulation of the enzyme during development. In developing (5 and 11 day) rats, immunoreactivity of protein kinase C was very faint or absent. After 15 days, reaction products of both type III (alpha) and type II (beta) isozymes were found throughout the nerve. These findings suggest that type II (beta) isozyme may be involved in axonal transport whereas type III (alpha) isozyme may play a role in some astrocyte functions in mature optic nerves.

Effects of sphingosine on phorbol ester-mediated changes in astrocyte morphology and protein phosphorylation

Previous studies indicate that phorbol myristate acetate (PMA) can induce morphological changes in astrocytes cultured from the rat neocortex. PMA also increased 32P incorporation into several proteins, including glial fibrillary acidic protein (GFAP), vimentin, and proteins with molecular weights of 80,000 (pI 4.5), 50,000 (pI 4.9), and 30,000 (pI 5.5). The present studies were conducted to determine if the morphological effect and the phosphorylation effect of PMA could be blocked by treatment with sphingosine, a protein kinase C inhibitor. Treatment with 15 microM sphingosine inhibited the effect of PMA on astrocyte morphology. This agent also inhibited the increase in phosphorylation mediated by PMA. The percent inhibition ranged from approximately 20% for the 30,000-Mr protein to 70% for GFAP. Analysis of phosphorylation sites on GFAP and vimentin using two-dimensional tryptic mapping techniques indicate that the partial inhibition of phosphorylation is likely the consequence of partial inhibition of protein kinase C rather than a selective inhibition at some phosphorylation sites and not others. In addition to increasing 32P incorporation into various proteins, PMA also decreased 32P incorporation in several 20,000-Mr proteins (pI values of 6.7, 6.4, 6.2, 4.9). However, this effect was not blocked by treatment with sphingosine. This suggests that the actions of PMA to increase and decrease 32P incorporation are mediated by different mechanisms.

Phorbol myristate acetate and 8-bromo-cyclic AMP-induced phosphorylation of glial fibrillary acidic protein and vimentin in astrocytes: comparison of phosphorylation sites

Both the protein kinase C (PK-C) activator, phorbol 12-myristate 13-acetate (PMA), and the cyclic AMP-dependent protein kinase (PK-A) activator, 8-bromo-cyclic AMP (8-BR), have been shown to increase 32P incorporation into glial fibrillary acidic protein (GFAP) and vimentin in cultured astrocytes. Also, treatment of astrocytes with PMA or 8-BR results in the morphological transformation of flat, polygonal-shaped cells into stellate, process-bearing cells, suggesting the possibility that signals mediated by these two kinase systems converge at the level of protein phosphorylation to elicit similar changes in cell morphology. Therefore, studies were conducted to determine whether treatment with PMA and 8-BR results in the phosphorylation of the same tryptic peptide fragments on GFAP and vimentin in astrocytes. Treatment with PMA increased 32P incorporation into all the peptide fragments that were phosphorylated by 8-BR on both vimentin and GFAP; however, PMA also stimulated phosphorylation of additional fragments of both proteins. The phosphorylation of vimentin and GFAP resulting from PMA or 8-BR treatment was restricted to serine residues in the N-terminal domain of these proteins. Studies were also conducted to compare the two-dimensional tryptic phosphopeptide maps of GFAP and vimentin from intact cells treated with PMA and 8-BR with those produced when the proteins were phosphorylated with purified PK-C or PK-A. PK-C phosphorylated the same fragments of GFAP and vimentin that were phosphorylated by PMA treatment. Additionally, PK-C phosphorylated some tryptic peptide fragments of these proteins that were not observed with PMA treatment in intact cells.(ABSTRACT TRUNCATED AT 250 WORDS)

The use of microwave tissue fixation to demonstrate the in vivo phosphorylation of an acidic 80,000 molecular weight protein in the rat neocortex following treatment with soman

Studies were conducted to determine if soman, a cholinesterase inhibitor, could activate the protein kinase C system in the rat neocortex. Using microwave radiation for rapid tissue fixation, it was demonstrated that treatment with soman increased 32P incorporation into an acidic 80,000 molecular weight, heat-stable protein in vivo. Based on relative molecular weight and isoelectric point this protein appears to be identical to a protein identified as a substrate for protein kinase C. Additionally, a protein of the same molecular weight and isoelectric point could be phosphorylated in tissue slices prepared from the neocortex by cholinergic dependent mechanisms. Also, treatment with soman decreased protein kinase C in the soluble fraction of this brain region; however, no corresponding increase was observed in the particulate fraction. These results suggest that soman can activate protein kinase C in vivo, and demonstrate the utility of using microwave tissue fixation to study protein phosphorylation events in vivo.

Phosphorylation of the glial fibrillary acidic protein

The glial fibrillary acidic protein (GFAP) was found to be phosphorylated in vivo after intracerebral injection of [32P]-orthophosphate, in brain slices, and in a cell free system. The phosphorylated proteins were separated by two-dimensional gel electrophoresis and then transferred to nitrocellulose sheets. Two isoelectric variants of GFAP were immunochemically identified by monoclonal antibodies. Autoradiography demonstrated that only the more acidic isoelectric variant of GFAP was phosphorylated. Phosphoamino acid analysis revealed that under all conditions GFAP was phosphorylated at serine and threonine residues. Incubation of brain slices with [32P]-orthophosphate and the protein kinase C activator phorbol 12-myristate 13-acetate or forskolin, an activator of cyclic AMP-dependent protein kinase, stimulated phosphorylation of GFAP. Likewise phosphorylation of GFAP was also accentuated by calcium/phosphatidylserine/diolein and by exogenous cyclic AMP-dependent kinase in a cell free system. These findings announce that protein kinase C and cyclic-AMP dependent kinase may play physiologic roles in the in situ phosphorylation of GFAP. When isolated cytoskeletal preparations were incubated with [gamma-32P] ATP, GFAP was phosphorylated in vitro by two additional protein kinases, a Ca2++/calmodulin-dependent kinase and an effector-independent kinase. The results of these investigations strongly suggest that phosphorylation of GFAP appears to be regulated by multiple second messenger pathways.

Synthesis and phosphorylation of the glial fibrillary acidic protein during brain development: a tissue slice study

Brain slices were incubated with either [3H] amino acids or [32P] orthophosphate in order to characterize the synthesis and phosphorylation of the glial fibrillary acidic protein (GFAP) in the rat nervous system. The incorporation of [3H] amino acids into GFAP was found to increase significantly during early postnatal development, reaching a peak of activity on day 5 of life and then declining over the next 2 weeks. Concomitant with this peak of synthetic activity the content of GFAP in rat brain was also observed to increase dramatically. GFAP continued to accumulate in brain through postnatal day 30 despite a decrease in the synthesis of the protein. These results indicate that the increase in GFAP during the first month of life cannot be ascribed solely to the rate of GFAP synthesis. The findings are consistent with the hypothesis that during later stages of astrocytic development the accumulation of GFAP may be primarily dependent upon a low rate of protein degradation. The pattern of GFAP phosphorylation in the developing rat brain differed from that observed for the incorporation of [3H] amino acids. The peak incorporation of 32P into GFAP occurred on postnatal day 10 at a time when synthesis of the protein had declined by 43%. These findings suggest that during development phosphorylation of GFAP is mediated by factors different from those directing its synthesis. In addition, phosphorylation of GFAP did not alter its solubility in cytoskeletal preparations indicating that GFAP phosphorylation is probably not a major regulatory mechanism in disassembly of the astroglial filaments.

Phorbol ester-induced change in astrocyte morphology: correlation with protein kinase C activation and protein phosphorylation

Treatment with 300 nM phorbol 12-myristate 13-acetate (PMA) transforms polygonal-shaped cultured astrocytes into process-bearing cells and produces a shift in protein kinase C (PK-C) from the cytosol to the membrane. Exposure to PMA also produces increases in the phosphorylation of several proteins including vimentin, glial fibrillary acidic protein (GFAP), an acidic 80,000 molecular weight protein, and two 30,000 molecular weight proteins (pI 5.5 and 5.7). The effects of PMA on the translocation of PK-C and on protein phosphorylation precede the PMA-induced changes in astrocyte morphology, and a close correlation exists between the concentration of PMA necessary to elicit half-maximal and maximal effects on the shift of PK-C to the membrane and on protein phosphorylation. In addition, the PMA-induced alterations in cell morphology are not permanent, and within 24 hr after PMA treatment the cells have reverted almost to their original morphology. A second exposure to PMA at this time fails to elicit further change in cell shape and is also incapable of producing increases in the phosphorylation of proteins. It was determined that there is little, if any, PK-C present in these PMA-pretreated cells. The morphological responsiveness to PMA gradually returns in 5 to 8 days after the initial treatment with PMA, and this is accompanied by the recovery of PK-C activity and the phosphorylation response. Therefore, these studies suggest that the effect of PMA on astrocyte morphology is mediated by the activation of PK-C and subsequent protein phosphorylation.