PKC-dependent stimulation of EAAT3 glutamate transporter does not require the integrity of actin cytoskeleton

Rapid Regulation of Glutamate Transport: Where Do We Go from Here?

Glutamate is the predominant excitatory neurotransmitter in the mammalian central nervous system (CNS). A family of five Na⁺-dependent transporters maintain low levels of extracellular glutamate and shape excitatory signaling. Shortly after the research group of the person being honored in this special issue (Dr. Baruch Kanner) cloned one of these transporters, his group and several others showed that their activity can be acutely (within minutes to hours) regulated. Since this time, several different signals and post-translational modifications have been implicated in the regulation of these transporters. In this review, we will provide a brief introduction to the distribution and function of this family of glutamate transporters. This will be followed by a discussion of the signals that rapidly control the activity and/or localization of these transporters, including protein kinase C, ubiquitination, glutamate transporter substrates, nitrosylation, and palmitoylation. We also include the results of our attempts to define the role of palmitoylation in the regulation of GLT-1 in crude synaptosomes. In some cases, the mechanisms have been fairly well-defined, but in others, the mechanisms are not understood. In several cases, contradictory phenomena have been observed by more than one group; we describe these studies with the goal of identifying the opportunities for advancing the field. Abnormal glutamatergic signaling has been implicated in a wide variety of psychiatric and neurologic disorders. Although recent studies have begun to link regulation of glutamate transporters to the pathogenesis of these disorders, it will be difficult to determine how regulation influences signaling or pathophysiology of glutamate without a better understanding of the mechanisms involved.

Methylglyoxal Induces Changes in the Glyoxalase System and Impairs Glutamate Uptake Activity in Primary Astrocytes

The impairment of astrocyte functions is associated with diabetes mellitus and other neurodegenerative diseases. Astrocytes have been proposed to be essential cells for neuroprotection against elevated levels of methylglyoxal (MG), a highly reactive aldehyde derived from the glycolytic pathway. MG exposure impairs primary astrocyte viability, as evaluated by different assays, and these cells respond to MG elevation by increasing glyoxalase 1 activity and glutathione levels, which improve cell viability and survival. However, C6 glioma cells have shown strong signs of resistance against MG, without significant changes in the glyoxalase system. Results for aminoguanidine coincubation support the idea that MG toxicity is mediated by glycation. We found a significant decrease in glutamate uptake by astrocytes, without changes in the expression of the major transporters. Carbenoxolone, a nonspecific inhibitor of gap junctions, prevented the cytotoxicity induced by MG in astrocyte cultures. Thus, our data reinforce the idea that astrocyte viability depends on gap junctions and that the impairment induced by MG involves glutamate excitotoxicity. The astrocyte susceptibility to MG emphasizes the importance of this compound in neurodegenerative diseases, where the neuronal damage induced by MG may be aggravated by the commitment of the cells charged with MG clearance.

Glutamate Transport System as a Novel Therapeutic Target in Chronic Pain: Molecular Mechanisms and Pharmacology

The vast majority of peripheral neurons sensing noxious stimuli and conducting pain signals to the dorsal horn of the spinal cord utilize glutamate as a chemical transmitter of excitation. High-affinity glutamate transporter subtypes GLAST/EAAT1, GLT1/EAAT2, EAAC1/EAAT3, and EAAT4, differentially expressed on sensory neurons, postsynaptic spinal interneurons, and neighboring glia, ensure fine modulation of glutamate neurotransmission in the spinal cord. The glutamate transport system seems to play important roles in molecular mechanisms underlying chronic pain and analgesia. Downregulation of glutamate transporters (GluTs) often precedes or occurs simultaneously with development of hypersensitivity to thermal or tactile stimuli in various models of chronic pain. Moreover, antisense knockdown or pharmacological inhibition of these membrane proteins can induce or aggravate pain. In contrast, upregulation of GluTs by positive pharmacological modulators or by viral gene transfer to the spinal cord can reverse the development of such pathological hypersensitivity. Furthermore, some multi-target drugs displaying analgesic properties (e.g., tricyclic antidepressant amitriptyline, riluzole, anticonvulsant valproate, tetracycline antibiotic minocycline, β-lactam antibiotic ceftriaxone and its structural analog devoid of antibacterial activity, clavulanic acid) can significantly increase the spinal glutamate uptake. Thus, mounting evidence points at GluTs as prospective therapeutic target for chronic pain treatment. However, design and development of new analgesics based on the modulation of glutamate uptake will require more precise knowledge of molecular mechanisms underlying physiological or aberrant functioning of this transport system in the spinal cord.

Gliopreventive effects of guanosine against glucose deprivation in vitro

Guanosine, a guanine-based purine, is recognized as an extracellular signaling molecule that is released from astrocytes and confers neuroprotective effects in several in vivo and in vitro studies. Astrocytes regulate glucose metabolism, glutamate transport, and defense mechanism against oxidative stress. C6 astroglial cells are widely used as an astrocyte-like cell line to study the astrocytic function and signaling pathways. Our previous studies showed that guanosine modulates the glutamate uptake activity, thus avoiding glutamatergic excitotoxicity and protecting neural cells. The goal of this study was to determine the gliopreventive effects of guanosine against glucose deprivation in vitro in cultured C6 cells. Glucose deprivation induced cytotoxicity, an increase in reactive oxygen and nitrogen species (ROS/RNS) levels and lipid peroxidation as well as affected the metabolism of glutamate, which may impair important astrocytic functions. Guanosine prevented glucose deprivation-induced toxicity in C6 cells by modulating oxidative and nitrosative stress and glial responses, such as the glutamate uptake, the glutamine synthetase activity, and the glutathione levels. Glucose deprivation decreased the level of EAAC1, the main glutamate transporter present in C6 cells. Guanosine also prevented this effect, most likely through PKC, PI3K, p38 MAPK, and ERK signaling pathways. Taken together, these results show that guanosine may represent an important mechanism for protection of glial cells against glucose deprivation. Additionally, this study contributes to a more thorough understanding of the glial- and redox-related protective properties of guanosine in astroglial cells.

The glutamate transporter excitatory amino acid carrier 1 associates with the actin-binding protein alpha-adducin

Excitatory amino acid carrier 1 (EAAC1) belongs to the family of the Na(+)-dependent glutamate carriers. Although the association between defective EAAC1 function and neurologic disease has been repeatedly studied, EAAC1 regulation is not yet fully understood. We have reported that in C6 glioma cells both the activity and membrane targeting of EAAC1 require the integrity of actin cytoskeleton. Here we show that, in the same model, EAAC1 partially co-localizes with actin filaments at the level of cell processes. Moreover, perinuclear spots in which EAAC1 co-localizes with the actin binding protein alpha-adducin are observed in some cells and, consistently, faint co-immunoprecipitation bands between EAAC1 and alpha-adducin are detected. Co-localization and partial co-immunoprecipitation of EAAC1 and adducin are still detectable after cell treatment with phorbol esters, a condition that leads to a protein kinase C (PKC)-dependent increase of EAAC1 expression on the membrane and to the phosphorylation of adducin. A co-immunoprecipitation band was also detected in protein extracts of rat hippocampus. The amount of adducin co-immunoprecipitated with EAAC1 increases after the treatment of C6 cells with retinoic acid, a differentiating agent that induces EAAC1 overexpression in this cell model. Moreover, in clones of C6 cells transfected with a hemagglutinin (HA)-tagged adducin, the bands of EAAC1 immunoprecipitated by an anti-HA antiserum were proportional to EAAC1 expression. These results suggest the existence of a pool of EAAC1 transporters associated with the actin binding protein alpha-adducin in a PKC-insensitive manner.

Muscarinic receptor stimulation of D-aspartate uptake into human SH-SY5Y neuroblastoma cells is attenuated by hypoosmolarity

In addition to its function as an excitatory neurotransmitter, glutamate plays a major role as an osmolyte within the central nervous system (CNS). Accordingly, mechanisms that regulate glutamate release and uptake are of physiological importance not only during conditions in which cell volume remains constant but also when cells are subjected to hypoosmotic stress. In the present study, the ability of muscarinic cholinergic receptors (mAChRs) to regulate the uptake of glutamate (monitored as D-aspartate) into human SH-SY5Y neuroblastoma cells under isotonic or hypotonic conditions has been examined. In isotonic media, agonist activation of mAChRs resulted in a significant increase (250-300% of control) in the uptake of D-aspartate and, concurrently, a cellular redistribution of the excitatory amino acid transporter 3 (EAAT3) to the plasma membrane. mAChR-mediated increases in d-aspartate uptake were potently blocked by the EAAT3 inhibitor l-beta-threo-benzyl-aspartate. In hypotonic media, the ability of mAChR activation to facilitate D-aspartate uptake was significantly attenuated (40-50%), and the cellular distribution of EAAT3 was disrupted. Reduction of mAChR-stimulated D-aspartate uptake under hypoosmotic conditions could be fully reversed upon re-exposure of the cells to isotonic media. Under both isotonic and hypotonic conditions, mAChR-mediated increases in D-aspartate uptake depended on cytoskeletal integrity, protein kinase C and phosphatidylinositol 3-kinase activities, and the availability of intracellular Ca2+. In contrast, dependence on extracellular Ca2+ was observed only under isotonic conditions. The results suggest that, although the uptake of D-aspartate into SH-SY5Y cells is enhanced after mAChR activation, this process is markedly attenuated by hypoosmolarity.

Effect of Cytoskeletal Disruption on Mechanotransduction of Hydrostatic Pressure by C3H10T1/2 Murine Fibroblasts

Cyclic hydrostatic pressure of physiological magnitude (< 10 MPa) stimulates chondrogenic differentiation of mesenchymal stem cells, but mechanotransduction mechanisms are not well understood. It was hypothesized that an intact cytoskeleton would be required for uninhibited mechanotransduction of hydrostatic pressure. Therefore we examined the effects of drugs which selectively interfere with actin and tubulin polymerization on pressure-induced upregulation of aggrecan and col2a1 (type II collagen) mRNA expression. C3H10T1/2 cells were cultured as pellets in either 4µM cytochalasin D or 4µM nocodazole and subjected to 3 days of cyclic hydrostatic compression (1 Hz, 5 MPa, 2 h per day). Phalloidin staining and indirect immunostaining with anti α-tubulin antibody confirmed disruption of microfilament and microtubule assemblies, respectively. Real time RT-PCR revealed that both drugs substantially lowered the basal level of aggrecan and col2a1 mRNA, but that neither drug prevented a pressure-stimulated increase in gene expression relative to the altered basal state. Thus upregulation of macromolecular gene expression by cyclic hydrostatic pressure did not require a completely intact cytoskeleton.

Stimulation of EAAC1 in C6 glioma cells by store-operated calcium influx

This study investigated how modulation of intracellular calcium alters the functional activity of the EAAC1 glutamate transporter in C6 glioma cells. Pre-incubation of C6 glioma cells with the endoplasmic reticulum Ca2+ ATP pump inhibitor, thapsigargin (10 microM) produced a time-dependent increase in the Vmax for D-[3H]aspartate transport that reached a maximum at 15 min (143% of control; P<0.001) that was accompanied by increased plasma membrane expression of EAAC1 and was blocked by inhibition of protein kinase C. Pre-incubation of C6 glioma cells with phorbol myristate-3-acetate (100 nM for 20 min) also caused a significant increase in the Vmax of sodium-dependent D-[3H]aspartate transport (190% of control; P<0.01). In contrast, in the absence of extracellular calcium, thapsigargin caused a significant inhibition in D-[3H]aspartate transport that was not mediated by protein kinase C. Blockade of store-operated calcium channels with 2-aminoethoxydiphenyl borate (50 microM) or SKF 96365 (10 microM) caused a net inhibition of D-[3H]aspartate uptake. Co-incubation of C6 glioma cells with both thapsigargin and 2-aminoethoxydiphenyl borate (but not SKF 96365) prevented the increase in D-[3H]aspartate transport that was observed in the presence of thapsigargin alone. Furthermore, 2-aminoethoxydiphenyl borate, but not SKF 96365, reduced the increase in intracellular calcium that occurred following pre-incubation of the cells with thapsigargin. It is concluded that, in C6 glioma cells, stimulation of EAAC1-mediated glutamate transport by thapsigargin is dependent on entry of calcium via the NSCC-1 subtype of store operated calcium channel and is mediated by protein kinase C. In contrast, in the absence of store operated calcium entry, thapsigargin inhibits transport.

In human endothelial cells rapamycin causes mTORC2 inhibition and impairs cell viability and function

AIM

Drug-eluting stents are widely used to prevent restenosis but are associated with late endothelial damage. To understand the basis for this effect, we have studied the consequences of a prolonged incubation with rapamycin on the viability and functions of endothelial cells.

METHODS AND RESULTS

Human umbilical vein or aorta endothelial cells were exposed to rapamycin in the absence or in the presence of tumour necrosis factor alpha (TNFalpha). After a 24 h-incubation, rapamycin (100 nM) caused a significant cell loss associated with the increase of both apoptosis and necrosis, as quantified by propidium iodide staining, caspase 3 activity, and lactate dehydrogenase release. Rapamycin also impaired cell mobility, as assessed by a wound test, and promoted the formation of actin stress fibres, as determined with confocal microscopy. Moreover, the inhibitor prolonged TNFalpha-dependent E-selectin induction, inhibited endothelial nitric oxide synthase expression at both mRNA (quantitative real-time polymerase chain reaction) and protein level (enzyme-linked immunosorbent assay and western blot), and lowered bioactive nitric oxide output (RFL-6 reporter cell assay). Under the conditions adopted, rapamycin inhibited both mammalian target-of-rapamycin complexes (mTORC1 and mTORC2), as indicated by the reduced amount of raptor and rictor bound to mTOR in immunoprecipitates and by the marked hypophosphorylation of protein S6 kinase I (p70S6K) and Akt, determined by western blotting. The selective inhibition of mTORC1 by AICAR did not affect endothelial viability.

CONCLUSION

A prolonged treatment with rapamycin impairs endothelial function and hinders cell viability. Endothelial damage seems dependent on mTORC2 inhibition.

Effect of eslicarbazepine acetate (BIA 2-093) on latrunculin A-induced seizures and extracellular amino acid concentrations in the rat hippocampus

PURPOSE

Eslicarbazepine acetate (ESL, BIA 2-093) is a novel antiepileptic drug endowed with an anticonvulsant potency similar to that of carbamazepine, and shares with carbamazepine and oxcarbazepine the capability to inhibit voltage-gated sodium channels. ESL is efficacious against maximal electroshock seizure-induced seizures, protects against picrotoxin-induced seizures in mice and rats, and prevents development of kindling in rats. In vivo, latrunculin A microperfusion in the rat hippocampus induces acute epileptic seizures and long-term biochemical changes leading to decreased picrotoxin seizure threshold and spontaneous seizures. We have tested the effect of ESL on latrunculin A-induced seizures, and its effect on the changes in extracellular amino acid levels induced by latrunculin A.

METHODS

Rat hippocampus was continuously perfused with a latrunculin A solution (4 microM) through CMA/12 microdialysis probes at a flow rate of 2 microl/min during 8 h with continuous EEG and videotape recording for 3 consecutive days. The same protocol was repeated after oral administration of ESL (3, 10 and 30 mg/kg). Samples from the microdialysate were collected and analyzed by HPLC using pre-column derivatization with 6 aminoquinolyl-N-hydroxysuccinimidyl carbamate (AQC) and fluorescence detection.

RESULTS

After the administration of 3 mg/kg of ESL, seizures were completely suppressed in the 66.7% of the rats. 10 and 30 mg/kg of ESL did completely suppressed seizures in the 100% of the animals studied. Hippocampal extracellular levels of glutamate, glycine and aspartate were significantly increased during latrunculin A microperfusion, while GABA levels remained unchanged. At the doses studied, ESL reversed the increases in extracellular glutamate and aspartate concentrations to basal levels and significantly reduced glycine levels.

CONCLUSIONS

ESL, at oral doses of 3, 10 and 30 mg/kg, shows an excellent anticonvulsant effect against seizures induced by latrunculin A microperfusion in the rat, and prevents the increases in glutamate and aspartate induced by latrunculin A.

Magnetoreception system in honeybees (Apis mellifera)

Honeybees (Apis mellifera) undergo iron biomineralization, providing the basis for magnetoreception. We showed earlier the presence of superparamagnetic magnetite in iron granules formed in honeybees, and subscribed to the notion that external magnetic fields may cause expansion or contraction of the superparamagnetic particles in an orientation-specific manner, relaying the signal via cytoskeleton (Hsu and Li 1994). In this study, we established a size-density purification procedure, with which quantitative amount of iron granules was obtained from honey bee trophocytes and characterized; the density of iron granules was determined to be 1.25 g/cm³. While we confirmed the presence of superparamagnetic magnetite in the iron granules, we observed changes in the size of the magnetic granules in the trophycytes upon applying additional magnetic field to the cells. A concomitant release of calcium ion was observed by confocal microscope. This size fluctuation triggered the increase of intracellular Ca⁺² , which was inhibited by colchicines and latrunculin B, known to be blockers for microtubule and microfilament syntheses, respectively. The associated cytoskeleton may thus relay the magnetosignal, initiating a neural response. A model for the mechanism of magnetoreception in honeybees is proposed, which may be applicable to most, if not all, magnetotactic organisms.

Changes in extracellular amino acid concentrations in the rat hippocampus after in vivo actin depolymerization with latrunculin A

The effect of latrunculin A microperfusion on hippocampal extracellular concentrations of glutamate, aspartate, glycine and GABA, as measured by in vivo microdialysis, was investigated. Latrunculin A (4 microg/ml) was perfused for three consecutive days (8h a day) to promote in vivo F-actin depolymerization. Intrahippocampal latrunculin A microdialysis induced seizures during the second and third day of perfusion, and the animals started showing spontaneous seizures 1 month after lartrunculin A administration. Hippocampal glutamate levels were significantly increased during the first day of latrunculin A microperfusion without significant changes during the second and third day of perfusion. Aspartate levels were significantly increased during the first and second days of treatment. The rise on glutamate and asparate levels was partially reversed by perfusion of NMDA antagonist MK-801. Glycine concentrations were significantly increased during the 3 days of latrunculin A microdialyis, but no significant effect was observed on baseline GABA levels. One month after latrunculin A microperfusion, no significant differences in glutamate and aspartate extracellular concentrations were detected as compared to controls, however, significant increases in glycine and GABA extracellular concentrations were observed. The immediate increases in glutamate, aspartate and glycine levels indicate a modulatory effect of the F-actin cytoskeleton on extracellular concentrations of glutamate, aspartate and glycine. The chronic elevations in GABA and glycine levels are more likely to be related with long-term epileptogenesis processes. Our results suggest that the in vivo biochemical study of actin-dependent processes seems to be a promising approach to the neuropathology and neuropharmacology of epileptic seizures.