Phosphorylation of the glial fibrillary acidic protein

Aging-related tau astrogliopathy (ARTAG): not only tau phosphorylation in astrocytes

Aging-related tau astrogliopathy (ARTAG) is defined by the presence of two types of tau-bearing astrocytes: thorn-shaped astrocytes (TSAs) and granular/fuzzy astrocytes in the brain of old-aged individuals. The present study is focused on TSAs in rare forms of ARTAG with no neuronal tau pathology or restricted to entorhinal and transentorhinal cortices, to avoid bias from associated tauopathies. TSAs show 4Rtau phosphorylation at several specific sites and abnormal tau conformation, but they lack ubiquitin and they are not immunostained with tau-C3 antibodies which recognize truncated tau at Asp421. Astrocytes in ARTAG have atrophic processes, reduced glial fibrillary acidic protein (GFAP) and increased superoxide dismutase 2 (SOD2) immunoreactivity. Gel electrophoresis and western blotting of sarkosyl-insoluble fractions reveal a pattern of phospho-tau in ARTAG characterized by two bands of 68 and 64 kDa, and several middle bands between 35 and 50 kDa which differ from what is seen in AD. Phosphoproteomics of dissected vulnerable regions identifies an increase of phosphorylation marks in a large number of proteins in ARTAG compared with controls. GFAP, aquaporin 4, several serine-threonine kinases, microtubule associated proteins and other neuronal proteins are among the differentially phosphorylated proteins in ARTAG thus suggesting a hyper-phosphorylation background that affects several molecules, including many kinases and proteins from several cell compartments and various cell types. Finally, present results show for the first time that tau seeding is produced in neurons of the hippocampal complex, astrocytes, oligodendroglia and along fibers of the corpus callosum, fimbria and fornix following inoculation into the hippocampus of wild type mice of sarkosyl-insoluble fractions enriched in hyper-phosphorylated tau from selected ARTAG cases. These findings show astrocytes as crucial players of tau seeding in tauopathies.

Identification of regulators of chaperone-mediated autophagy

Chaperone-mediated autophagy (CMA) is a selective mechanism for the degradation of cytosolic proteins in lysosomes that contributes to cellular quality control and becomes an additional source of amino acids when nutrients are scarce. A chaperone complex delivers CMA substrates to a receptor protein at the lysosomal membrane that assembles into multimeric translocation complexes. However, the mechanisms regulating this process remain, for the most part, unknown. In this work, we have identified two regulatory proteins, GFAP and EF1alpha, that mediate a previously unknown inhibitory effect of GTP on CMA. GFAP stabilizes the multimeric translocation complex against chaperone-mediated disassembly, whereas GTP-mediated release of EF1alpha from the lysosomal membrane promotes self-association of GFAP, disassembly of the CMA translocation complex, and the consequent decrease in CMA. The dynamic interactions of these two proteins at the lysosomal membrane unveil now a role for GTP as a negative regulator of CMA.

N-Methyl-D-aspartate receptor antagonist MK-801 attenuates morphine tolerance and associated glial fibrillary acid protein up-regulation: a proteomic approach

BACKGROUND

It is well known that long-term morphine administration results in tolerance, which limits the clinical use of this drug in pain management.

METHODS

Male Wistar rats were randomly assigned to receive one of four different infusions: morphine [15 microg/h, intrathecal (i.t.)], saline, MK-801 (5 microg/h, i.t.) plus morphine (15 microg/h, i.t.), or MK-801 (5 microg/h, i.t.) alone.

RESULTS

Morphine infusion induced a maximal antinociceptive effect on day 1 and tolerance on day 3, and the maximal anti-receptive tolerance was observed on day 5. Co-infusing MK-801 with morphine attenuated morphine's anti-receptive tolerance. Two-dimensional gel electrophoretic analysis of spinal proteins revealed that eight protein spots were up-regulated in morphine-tolerant rats, and that they were significantly inhibited by MK-801 co-infusion. Among the up-regulated proteins, glial fibrillary acid protein (GFAP), a glial-specific maker, was identified by mass spectrometry. This finding was also confirmed by Western blot analysis.

CONCLUSION

Using proteomic analysis, we identified eight GFAP protein spots that were up-regulated in the dorsal horn of morphine-tolerant rat spinal cords. This up-regulation was partly inhibited by N-methyl-D-aspartate receptor antagonist MK-801 co-infusion, which suggests that GFAP protein can be considered to be a pathogenesis marker of morphine tolerance.

Effect of fatty acids isolated from edible oils like mustard, linseed or coconut on astrocytes maturation

The omega-3 polyunsaturated fatty acid, docosahexaenoic acid (DHA, 22:6n-3) has been previously shown to facilitate some of the vital functions of astrocytes. Since some dietary oils contain alpha-linolenic acid (ALA, 18:3n-3), which is a precursor of DHA, we examined their effect on astrocyte development. Fatty acids (FAs) were isolated from commonly used oils and their compositions were determined by GLC. FAs from three oils, viz. coconut, mustard and linseed were studied for their effect on astrocyte morphology. Parallel studies were conducted with FAs from the same oils after heating for 72 h. Unlike coconut oil, FAs from mustard and linseed, both heated and raw, caused significant morphogenesis of astrocytes in culture. ss-AR binding was also substantially increased in astrocytes treated with FAs from raw mustard and linseed oils as compared to astrocytes grown in normal medium. The expression profile of the isoforms of GFAP showed that astrocyte maturation by FAs of mustard and linseed oil was associated with appearance of acidic variants of GFAP and disappearance of some neutral isoforms similar to that observed in cultures grown in serum containing medium or in the presence of DHA. Taken together, the study highlights the contribution of specific dietary oils in facilitating astrocyte development that can have potential impact on human health.

Retinal function and PKC alpha expression after focal laser photocoagulation

PURPOSE

To examine the effects of focal laser photocoagulation on general and local retinal function and to relate electrophysiological findings with changes in protein kinase C (PKC) alpha expression.

METHODS

Twelve rabbits were treated with 70 spots of laser photocoagulation in the central cone-rich retina. The operated eyes were investigated with electroretinography (full-field ERG and multifocal electroretinography, mfERG) preoperatively and at 1, 3, and 5 weeks after surgery. The expression of PKC alpha was examined at all three time points using immunohistochemistry, and PKC alpha mRNA levels were quantified using real-time polymerase chain reaction (PCR). Immunohistochemistry for glial fibrillary acidic protein (GFAP) and hematoxylin and eosin staining was employed to monitor the extent and dynamics of the morphological response.

RESULTS

The full-field ERG revealed a significant increase in b-wave amplitudes derived from the isolated rod response (blue light) at all three time points after surgery (p < 0.05). Supernormal b-wave amplitudes were also found for the combined rod-cone response at 3 weeks (white light), and for the isolated cone response (light-adapted 30-Hz flicker) at 5 weeks after treatment. In the mfERG, amplitudes derived from the central retina did not change postoperatively, while the implicit time was significantly increased at all time points. Immunohistochemistry for PKC alpha revealed a reduced expression of the enzyme in rod bipolar cells 1 and 3 weeks after laser treatment compared with untreated controls. Five weeks postoperatively, no PKC alpha labeling in rod bipolar cells was found in any part of the retina. Real-time PCR 1 and 3 weeks after treatment displayed a decreased level of PKC alpha mRNA compared to the controls. Immunolabeled tissue sections from laser-treated eyes displayed GFAP expression in Müller cells in the treated as well as untreated retina 1 week postoperatively. At 3 and 5 weeks, GFAP labeling was less pronounced and was concentrated around the laser-treated spots.

CONCLUSIONS

Focal laser treatment in the rabbit eye induces local and wide-spread alterations in both rod- and cone-mediated retinal function in the form of supernormal b-wave amplitudes in the full-field ERG and increased latency in the mfERG. The electrophysiological abnormalities are accompanied by a progressive down-regulation of the PKC alpha isoenzyme in rod bipolar cells, reaching far beyond the treated area. PKC alpha is down-regulated directly by impaired protein synthesis, and also possibly indirectly by protein consumption related to GFAP up-regulation. The results indicate that focal laser photocoagulation interferes with PKC-alpha-mediated inhibitory regulation of inner retinal signal transmission.

Proteomic analysis of glial fibrillary acidic protein in Alzheimer's disease and aging brain

Chronic inflammation is known to play an important role in the heterogeneous pathogenesis of Alzheimer's disease (AD). Activated astrocytes expressing glial fibrillary acidic protein (GFAP) are closely associated with AD pathology, such as tangles, neuritic plaques and amyloid depositions. Altogether, 46 soluble isoforms of GFAP were separated and most of them quantified by two-dimensional immunoblotting in frontal cortices of AD patients and age-matched controls. A 60% increase in the amount of more acidic isoforms of GFAP was observed in AD and these isoforms were both phosphorylated and N-glycosylated, while more basic isoforms were O-glycosylated and exhibited no quantitative differences between post-mortem AD and control brains. These data highlight the importance of exploring isoform-specific levels of proteins in pathophysiological conditions since modifications of proteins determine their activity state, localization, turnover and interaction with other molecules. Mechanisms, structures and functional consequences of modification of GFAP isoforms remain to be clarified.

Estradiol modulation of astrocytes and the establishment of sex differences in the brain

The role of steroid hormones as a conduit for reciprocal glial-neuronal communication is an emerging but relatively unexplored concept. Research in our laboratory has discovered that the relationship between astrocytic and neuronal morphology during development is distinct for different brain regions and provides a fundamental basis for region-specific sexual differentiation. The functional significance of estradiol-induced differentiation of astrocytes and the cross-talk of these cells with neurons includes permanent changes in synaptic patterning and control of adult reproductive behaviors. The cellular mechanisms as currently understood for each region are discussed and unanswered questions as well as other areas for future research are reviewed.

Calcium-sensing receptor in the brain

Following its cloning through an homology-based method from a rat striatal library, the calcium-sensing receptor (CaR) has been localized in the brains of adult and developing rats by immunocytochemistry and in situ hybridization with CaR-specific antibodies and cDNA probes, respectively. The receptor resides in numerous regions of the brain at widely varying levels. The highest levels are present within the subfornical organ (SFO) and the olfactory bulbs. Substantial levels of expression are also evident within the hippocampus, striatum, cingulate cortex, cerebellum, ependymal zones of the cerebral ventricles, and perivascular nerves around cerebral arteries. There are abundant levels of CaR expression within the SFO, an important hypothalamic thirst center, suggesting that it participates in the central control of systemic fluid and electrolyte balance. Therefore, while mineral ion homeostasis is not often considered to have central regulatory elements (i.e. in the brain), there are perhaps more complex relationships than recognized previously among the system governing mineral ion homeostasis and other homeostatic systems known to exhibit prominent neuroendocrine elements (i.e. water homeostasis). Furthermore, the expression of the CaR in all three types of glial cells indicates potential roles in the maintenance of local ionic homeostasis as well as in disease processes such as glioma.

Protein carbonyl formation and tyrosine nitration as markers of oxidative damage during ischaemia-reperfusion injury to rat sciatic nerve

We have investigated the role of oxidative damage in peripheral nerve ischaemia-reperfusion injury using a rat sciatic nerve model. After 5 h ischaemia blood flow to the sciatic nerve was restarted and markers of oxidative damage measured after various times of reperfusion. As a marker of protein oxidative damage, protein carbonyl formation was measured using a sensitive enzyme-linked immunosorbent assay. Protein carbonyl content was unaffected by ischaemia alone, but increased by 55% after 12-18 h reperfusion, correlating with the onset of nerve pathology. Pretreatment with the xanthine oxidase inhibitor allopurinol prevented these abnormalities, suggesting that xanthine oxidase activity is proximal to oxidative damage during reperfusion injury. To determine whether formation of the potent oxidant peroxynitrite from nitric oxide and superoxide contributed to ischaemia-reperfusion injury, we measured the accumulation of 3-nitrotyrosine residues in proteins. Only one protein of 49,000 mol. wt contained significant amounts of 3-nitrotyrosine residues which was shown to be glial fibrillary acidic protein, an abundant cytoskeletal protein in Schwann cells. However glial fibrillary acidic protein contained 3-nitrotyrosine residues prior to ischaemia-reperfusion, and the amount of nitrated tyrosine residues in total glial fibrillary acidic protein did not increase significantly during reperfusion, therefore it was not possible to draw conclusions about the role of peroxynitrite in nerve reperfusion injury.

Glial fibrillary acidic protein and its fragments discriminate astrocytoma from oligodendroglioma

In the last few years it has been shown that anaplastic oligodendrogliomas, in contrast to anaplastic astrocytomas, are responsive to a three drug regimen chemotherapy. The histologic criteria for the discrimination between oligodendrogliomas and astrocytomas are subject to substantial interobserver variability, particularly in anaplastic and mixed gliomas. In the present study a two-dimensional electrophoresis technique (2-DE) has been applied to glioma samples in an attempt to discriminate the glioma subtypes. It was found that the presence of glial fibrillary acidic protein (GFAP) fragments distinguishes oligodendroglioma from astrocytoma. One-dimensional (1-DE) immunoblots were compared with immunohistologically stained tissue sections in which various GFAP-positive cell types were seen. It is concluded that 2-DE and 1-DE GFAP immunoblotting provide accurate information for the reliable discrimination of anaplastic astrocytomas and oligodendrogliomas.

Adenovirus-mediated suicide gene therapy in an in vitro model of reactive gliosis

Adenovirus-mediated herpes simplex thymidine kinase/ganciclovir (HSV-tk/GCV) system has been demonstrated to be efficient for the treatment of experimental brain tumors. However, no study has been directed to the elimination of proliferating cellular populations in other pathological conditions. In this study we used this suicide gene approach in a primary culture of astrocytes, as a model of reactive gliosis, in order to evaluate its efficiency as a therapeutic strategy for post-traumatic astrogliosis in vivo. First, we evaluated the peak of astrocytic proliferation to characterize our model. Second, the efficiency of adenovirus-mediated lacZ gene transfer is shown to be dependent on vector multiplicity of infection (MOI). As expected, the cells transfected with the HSV-tk gene showed an increase in sensibility to GCV compared with cells transfected with lacZ gene. Finally, an unexpected interaction between the adenoviral vector and bromodeoxyuridine (BrdU) or [3H]-Thymidine ([3H]-Thy) was evidenced in transfected cultures, whose interpretation is discussed. The present study demonstrates that a recombinant adenoviral vector carrying the tk gene confers to in vitro cultured astrocytes a cytotoxic sensibility to GCV, and that this system constitutes a potentially efficient tool to eliminate the hyperplasia of astrocytes following injury to the central nervous system in vivo.

Calcium-dependent phosphorylation of glial fibrillary acidic protein (GFAP) in the rat hippocampus: a comparison of the kinase/phosphatase balance in immature and mature slices using tryptic phosphopeptide mapping

In previous work we showed that phosphorylation of the astrocyte marker glial fibrillary acidic protein (GFAP) in hippocampal slices from adult rats is dependent on external Ca2+, whereas in slices from immature rats aged 12-16 days postnatal 32P incorporation into GFAP is inhibited by external Ca2+. The nature of this late developmental change in Ca2+ sensitivity for GFAP phosphorylation was investigated in the present work by comparing in immature and adult animals phosphorylation of GFAP by endogenous protein kinase activity in cytoskeletal fractions and tryptic phosphopeptide maps prepared from cytoskeletal fractions labelled with [gamma-32P]ATP and from slices labelled with [32P]phosphate. Cytoskeletal fractions prepared from immature and adult hippocampus both contained endogenous protein kinase activity towards GFAP and other proteins stimulated by Ca2+/calmodulin and by cyclic AMP. The maps of GFAP isolated from the cytoskeletal fractions labelled in the presence of Ca2+/calmodulin were very similar and exhibited two major and several minor phosphopeptides. Comparison with maps derived from these fractions labelled in the presence of cyclic AMP showed that one of the major phosphopeptides was either directly or indirectly phosphorylated by Ca2+/calmodulin-stimulated kinase activity. Maps derived from GFAP isolated from adult slices labelled in the presence of Ca2+ and immature slices labelled in the absence of Ca2+ were qualitatively identical, with minor differences from the cytoskeletal maps. At both ages the slice maps displayed the phosphopeptide phosphorylated through the activity of a Ca2+/calmodulin kinase in the cytoskeletal fractions. By its migration properties this peptide appears to correspond to a sequence containing a site shown by other workers to be phosphorylated in vitro by CaM kinase II, suggesting that even in the absence of external Ca2+, kinase activity directly or indirectly dependent on Ca2+ was occurring in the immature slices. The near identity of the phosphorylation sites at the two ages suggest that the change in Ca2+ sensitivity of GFAP phosphorylation during development is not due to a change in the balance of kinase and phosphatase activities, but rather to a change in the mechanism(s) whereby Ca2+ controls the relative activity of these enzymes.

Glutamate metabolism is down-regulated in astrocytes during experimental allergic encephalomyelitis

Experimental allergic encephalomyelitis (EAE) was induced in SJL/J mice by adoptive transfer of MBP-reactive T cells in order to investigate the role of astrocytes in pathology. GFAP protein and mRNA expression (analyzed using semiquantitative Western blot and RT-PCR techniques) were upregulated in the spinal cord of mice, which had developed a complete paralysis of hind- and fore-limbs and tail (grade 4 EAE), thus establishing that reactive gliosis occurred under these experimental conditions. Within the same samples and using similar techniques, we found that glutamine synthetase (GS) and glutamate dehydrogenase (GDH) expression were dramatically reduced. These two astrocytic enzymes are responsible for degradation of glutamate, the most abundant excitatory neurotransmitter in the brain. Since elevated levels of glutamate may be neurotoxic, we propose that the decreased capacity of astrocytes to metabolize glutamate may contribute to EAE pathology.

Verapamil treatment attenuates immunoreactive GFAP at cerebral cortical lesion site

Cerebral cortical lesions were produced using a stereotactic injection system in Sprague-Dawley rats randomly assigned to three groups: (1) needle lesioned and uninjected (Lesioned), (2) needle lesion and simultaneous local injection of 50 or 100 microliters 0.9% saline (L/Saline), and (3) needle lesion and simultaneous local injection of 50 or 100 microliters Verapamil-HCl (VHCL) (2.5 mg/ml (5 mM) Abbott Labs, Chicago, IL), a passive, L-type calcium channel blocker (L/VHCL). The lesioning induced expression of glial fibrillary acidic protein (GFAP), a type of intermediate filament protein expressed in reactive astrocytes, at the lesion site. There was a reduction in GFAP-like immunoreactivity (GFAP-IR) in the L/VHCL group versus the Lesioned and the L/Saline groups. There was a five-fold increase of GFAP-IR at 24 h post lesion in the L/Saline group, but no statistically significant increase seen in the Lesioned or L/VHCL groups at either volume. Pretreatment of the anti-GFAP with VHCL did not impair the antigen labeling. To determine whether differences in pHs, or volume could account for these findings, a second experiment was performed using pH-matched saline or VHCL in 10 microliters volume injected into contralateral hemispheres at the time of lesioning. There was an 80% reduction in GFAP-IR in the L/VHCL group at 72 h compared with the L/Saline group. These data suggest that VHCL may suppress the early increase of GFAP-IR in response to cortical lesion and that reducing transmembrane calcium flux through L-type calcium channels may be the mechanism involved.

Oxysterol (7 beta-hydroxycholesteryl-3-oleate) promotes serotonergic reinnervation in the lesioned rat spinal cord by reducing glial reaction

In the present study, following previous experience with electrolytic lesion of the rat brain, and subsequent reduction of reactive gliosis with 7 beta-hydroxycholesterol derivatives (Bochelen et al.: Neuroscience 51:827-834, 1992), we have performed a hemisection of the spinal cord in adult rats and investigated the influence of 7 beta-hydroxycholesteryl-3-oleate (oxysterol) on the intensity of the astrocytic reaction and the axonal regeneration. We have shown here that local administration of liposomes containing this oxysterol reduced the intensity of the astroglial reaction on the sectioned side, as seen with immunocytochemical detection of glial fibrillary acidic protein (GFAP) and by in situ hybridization with a specific RNA probe. Moreover, radioautographic evaluation of astrocyte proliferation with tritiated thymidine evidenced a reduction of the astrocyte labelling index. In addition, double immunocytochemical detection of GFAP and polysialylated neural cell adhesion molecule (E-NCAM) revealed a decrease of the expression of this molecule in reactive astrocytes of the treated animals. Finally, immunocytochemical detection of serotonin (5HT) was determined in the raphespinal projections, which constitute a major descending system. In treated animals, serotonergic axons originating from the intact side reinnervated the dorsal horn of the sectioned side, below the hemisection. These results demonstrate that 7 beta-hydroxycholesteryl-3-oleate can reduce the astrocytic reaction following spinal cord injury, promoting the serotonergic reinnervation of a denervated territory.

Synaptic regulation of glial protein expression in vivo

We investigated signaling between individual nerve terminals and perisynaptic Schwann cells, the teloglial cells that cover neuromuscular junctions. When deprived of neuronal activity in vivo, either by motor nerve transection or tetrodotoxin injection, perisynaptic Schwann cells rapidly up-regulated glial fibrillary acidic protein. Addition of transcription or translation inhibitors to excised muscles prevented this increase. Stimulation of cut nerves prevented glial fibrillary acidic protein increases even when postsynaptic nicotinic receptors were blocked, but not when neurotransmitter release was blocked with omega-conotoxin GVIA. We conclude that there is a nerve terminal to glial signal, requiring presynaptic neurotransmitter release, which regulates perisynaptic Schwann cell genes. This may be a general principle since many types of glial are sensitive to transmitters applied in vitro or released in situ.

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.

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.