Glutamate in the human brain: possible roles in synaptic transmission and ischemia

CORM-A1 prevents blood-brain barrier dysfunction caused by ionotropic glutamate receptor-mediated endothelial oxidative stress and apoptosis

In cerebral microvascular endothelial cells (CMVEC) of newborn pigs, glutamate at excitotoxic concentrations (mM) causes apoptosis mediated by reactive oxygen species (ROS). Carbon monoxide (CO) produced by CMVEC or delivered by a CO-releasing molecule, CORM-A1, has antioxidant properties. We tested the hypothesis that CORM-A1 prevents cerebrovascular endothelial barrier dysfunction caused by glutamate excitotoxicity. First, we identified the glutamate receptors (GluRs) and enzymatic sources of ROS involved in the mechanism of endothelial apoptosis. In glutamate-exposed CMVEC, ROS formation and apoptosis were blocked by rotenone, 2-thenoyltrifluoroacetone (TTFA), and antimycin, indicating that mitochondrial complexes I, II, and III are the major sources of oxidative stress. Agonists of ionotropic GluRs (iGluRs) N-methyl-D-aspartate (NMDA), cis-ACPD, AMPA, and kainate increased ROS production and apoptosis, whereas iGluR antagonists exhibited antiapoptotic properties, suggesting that iGluRs mediate glutamate-induced endothelial apoptosis. The functional consequences of endothelial injury were tested in the model of blood-brain barrier (BBB) composed of CMVEC monolayer on semipermeable membranes. Glutamate and iGluR agonists reduced transendothelial electrical resistance and increased endothelial paracellular permeability to 3-kDa dextran. CORM-A1 exhibited potent antioxidant and antiapoptotic properties in CMVEC and completely prevented BBB dysfunction caused by glutamate and iGluR agonists. Overall, the endothelial component of the BBB is a cellular target for excitotoxic glutamate that, via a mechanism involving a iGluR-mediated activation of mitochondrial ROS production and apoptosis, leads to BBB opening that may be prevented by the antioxidant and antiapoptotic actions of CORMs. Antioxidant CORMs therapy may help preserve BBB functional integrity in neonatal cerebrovascular disease.

Cerebroprotective functions of HO-2

The constitutive isoform of heme oxygenase, HO-2, is highly expressed in the brain and in cerebral vessels. HO-2 functions in the brain have been evaluated using pharmacological inhibitors of the enzyme and HO-2 gene deletion in in vivo animal models and in cultured cells (neurons, astrocytes, cerebral vascular endothelial cells). Rapid activation of HO-2 via post-translational modifications without upregulation of HO-2 expression or HO-1 induction coincides with the increase in cerebral blood flow aimed at maintaining brain homeostasis and neuronal survival during seizures, hypoxia, and hypotension. Pharmacological inhibition or gene deletion of brain HO-2 exacerbates oxidative stress induced by seizures, glutamate, and inflammatory cytokines, and causes cerebral vascular injury. Carbon monoxide (CO) and bilirubin, the end products of HO-catalyzed heme degradation, have distinct cytoprotective functions. CO, by binding to a heme prosthetic group, regulates the key components of cell signaling, including BK(Ca) channels, guanylyl cyclase, NADPH oxidase, and the mitochondria respiratory chain. Cerebral vasodilator effects of CO are mediated via activation of BK(Ca) channels and guanylyl cyclase. CO, by inhibiting the major components of endogenous oxidant-generating machinery, NADPH oxidase and the cytochrome C oxidase of the mitochondrial respiratory chain, blocks formation of reactive oxygen species. Bilirubin, via redox cycling with biliverdin, is a potent oxidant scavenger that removes preformed oxidants. Overall, HO-2 has dual housekeeping cerebroprotective functions by maintaining autoregulation of cerebral blood flow aimed at improving neuronal survival in a changing environment, and by providing an effective defense mechanism that blocks oxidant formation and prevents cell death caused by oxidative stress.

Critical roles of Src family tyrosine kinases in excitatory neuronal differentiation of cultured embryonic stem cells

Embryonic stem (ES) cells have been tested for potential cell transplantation therapy for CNS disorders. Understanding their differentiation mechanism and identifying factors involved in driving excitatory and inhibitory neuron lineages should enhance the efficacy and efficiency of the cell transplantation therapy. We tested the hypothesis that selective expression of Src family tyrosine kinases is required for phenotype-specific differentiation and functional maturation of ES cell derived neurons. Cultured mouse pluripotent ES cells were treated with retinoic acid (RA) to induce neural differentiation. After RA induction, neurons derived from ES cells showed significant neurite growth, increased expression of Src, Fyn and Lck and an extension of Src kinase expression from cell body to neurite processes. ES cell derived neuron-like cells expressed neurofilament, synaptophysin, glutamate receptors, NMDA and kainate currents, became vulnerable to excitotoxicity and formed functional excitatory synapses. These developmental events were blocked or attenuated when cells were grown in the presence of Src family kinase inhibitor PP2. However, there was no change in the expression of GABAergic-specific protein GAD67 during PP2 treatment. Our data suggest that Src tyrosine kinases are involved in the terminal differentiation of excitatory neuronal phenotype during ES cell neural differentiation after RA induction.

Development of neuronal networks from single stem cells harvested from the adult human brain

OBJECTIVE

It was long held as an axiom that new neurons are not produced in the adult human brain. More recent studies, however, have identified multipotent cells whose progeny express glial or neuronal markers. This discovery may lead to new therapeutic strategies against central nervous system disorders by transplanting stem cells that have been propagated in vitro. Still, it is not known whether stem cells from the adult human brain retain the potential to mature into neurons that integrate and communicate in a network.

METHODS

We cultured cells from the ventricular wall of the adult human brain as monoclonal neurospheres. After two passages, the neurospheres were dissociated and the cells were allowed to differentiate. After 4 weeks of maturation, the cells were studied by immunocytochemistry, confocal microscopy, and whole-cell patch-clamp.

RESULTS

We show that monoclonal stem cells harvested from the ventricular wall of the adult human brain develop into mature neurons with functional glutamate receptors and glutamatergic nerve terminals. By patching pairs of cells simultaneously, we also present direct evidence for synaptic communication between neurons developed from the same monoclonal cell.

CONCLUSION

Neural stem cells harvested from the adult human brain retain the potential to mature into fully differentiated neurons that integrate and communicate by synapses. This opens a possible future scenario of autotransplantation, in which stem cells are harvested from small biopsies of the ventricular wall and propagated in vitro before transplantation.

Change in plasma glutamate concentration during cardiac surgery is a poor predictor of cognitive outcome

OBJECTIVE

To develop a simple and reliable method for quantitating plasma glutamate concentration and apply this method to monitor systemic glutamate levels during coronary artery bypass graft (CABG) surgery, a procedure associated with neurologic deficits.

DESIGN

Prospective serial investigation of cardiac surgery patients.

SETTING

Tertiary-care university teaching hospital.

PARTICIPANTS

Patients undergoing CABG surgery (n = 33).

INTERVENTIONS

Preoperative and postoperative neurologic and neurocognitive testing were done. Intraoperative blood samples for glutamate quantitation were obtained from jugular bulb and pulmonary artery catheters before, during, and after cardiopulmonary bypass.

MEASUREMENTS AND MAIN RESULTS

Glutamate concentrations were determined using a reverse-phase high-pressure liquid chromatography method coupled to precolumn derivatization of the analyte with o-phthalaldehyde. The mean prebypass plasma glutamate concentration was 79.4 +/- 41.8 micromol/L. Plasma glutamate levels fluctuated during surgery with considerable degrees of temporal and quantitative interpatient variability. Neurologic and neurocognitive deficits were observed after CABG surgery. However, neither the occurrence nor the severity of cognitive decline could be predicted by the magnitude of increase in plasma glutamate concentration.

CONCLUSION

Fluctuations in intraoperative systemic glutamate levels do not predict post-CABG surgery neurologic outcome.

Endothelin-1 decreases glutamate uptake in primary cultured rat astrocytes

Endothelin-1 (ET-1) is a potent vasoconstrictor peptide that is also known to induce a wide spectrum of biological responses in nonvascular tissue. In this study, we found that ET-1 (100 nM) inhibited the glutamate uptake in cultured astrocytes expressing the glutamate/aspartate transporter (GLAST); astrocytes did not express the glutamate transporter-1 (GLT-1). The V(max) and the K(m) of the glutamate uptake were reduced by 57% and 47%, respectively. Application of the ET(A) and ET(B) receptor antagonists BQ-123 and BQ-788 partly inhibited the ET-1-evoked decrease in the glutamate uptake, whereas the nonspecific ET receptor antagonist bosentan completely inhibited this decrease. Incubation of the cultures with pertussis toxin abolished the effect of ET-1 on the uptake. The ET-1-induced decrease in the glutamate uptake was independent of extracellular free Ca(2+) concentration, whereas the intracellular Ca(2+) antagonists thapsigargin and 3,4,5-trimethoxybenzoic acid 8-(diethylamino)octyl ester abolished the effect of ET-1 on the glutamate uptake. Incubation with the protein kinase C (PKC) antagonist staurosporine, but not with the fatty acid-binding protein bovine serum albumin, prevented the ET-1-induced decrease in the glutamate uptake. These results suggest that ET-1 impairs the high-affinity glutamate uptake in cultured astrocytes through a G protein-coupled mechanism, involving PKC and changes in intracellular Ca(2+).

Baclofen prevents hypoxia-induced consolidation impairment for passive avoidance in rats

We investigated the effects of baclofen, a selective GABA-B receptor agonist, on certain behaviours in rats after short-term hypoxia, as a model of experimentally induced amnesia. Baclofen given intraperitoneally (i.p.) in a dose of 0.25 mg kg(-1) increased the number of crossings and bar approaches in the open field, but was ineffective in the passive avoidance tests; it also shortened the time spent in open arms and reduced the number of open arms entries in an elevated 'plus' maze, being a measure of anxiety. Hypoxia (2% O2, 98% N2) within 4 min profoundly impaired locomotor activity, consolidation and retrieval of conditioned responses, and exhibited a proaxiogenic effect in the elevated 'plus' maze in rats--it reduced the time spent in open arms and the number of entries to closed and open arms. Baclofen's effect on locomotor and exploratory activity was substantially impaired after hypoxia, i.e. rats exhibited a significant reduction in those activities. This agonist of GABA-B receptor used before hypoxia significantly improved consolidation, but had no effect on retrieval. In the elevated 'plus' maze rats pre-treated with baclofen and then subjected to hypoxia prolonged the time spent in open arms, reduced the time spent in closed arms, and increased the number of entries to the arms, i.e. exhibited anxiolytic effect. We conclude, therefore, that baclofen improved consolidation of passive avoidance in rats undergoing hypoxia.