Development of brain damage after neonatal hypoxia-ischemia: excitatory amino acids and cysteine

Mitochondrial dynamics, mitophagy and biogenesis in neonatal hypoxic-ischaemic brain injury

Hypoxic-ischaemic encephalopathy, resulting from asphyxia during birth, affects 2-3 in every 1000 term infants and depending on severity, brings about life-changing neurological consequences or death. This hypoxic-ischaemia (HI) results in a delayed neural energy failure during which the majority of brain injury occurs. Currently, there are limited treatment options and additional therapies are urgently required. Mitochondrial dysfunction acts as a focal point in injury development in the immature brain. Not only do mitochondria become permeabilised, but recent findings implicate perturbations in mitochondrial dynamics (fission, fusion), mitophagy and biogenesis. Mitoprotective therapies may therefore offer a new avenue of intervention for babies who suffer lifelong disabilities due to birth asphyxia.

Increased concentrations of both NMDA receptor co-agonists D-serine and glycine in global ischemia: a potential novel treatment target for perinatal asphyxia

Worldwide, perinatal asphyxia is an important cause of morbidity and mortality among term-born children. Overactivation of the N-methyl-d-aspartate receptor (NMDAr) plays a central role in the pathogenesis of cerebral hypoxia–ischemia, but the role of both endogenous NMDAr co-agonists d-serine and glycine remains largely elusive. We investigated d-serine and glycine concentration changes in rat glioma cells, subjected to oxygen and glucose deprivation (OGD) and CSF from piglets exposed to hypoxia–ischemia by occlusion of both carotid arteries and hypoxia. We illustrated these findings with analyses of cerebrospinal fluid (CSF) from human newborns affected by perinatal asphyxia. Extracellular concentrations of glycine and d-serine were markedly increased in rat glioma cells exposed to OGD, presumably through increased synthesis from l-serine. Upon reperfusion glycine concentrations normalized and d-serine concentrations were significantly lowered. The in vivo studies corroborated the finding of initially elevated and then normalizing concentrations of glycine and decreased d-serine concentrations upon reperfusion These significant increases of both endogenous NMDAr co-agonists in combination with elevated glutamate concentrations, as induced by global cerebral ischemia, are bound to lead to massive NMDAr activation, excitotoxicity and neuronal damage. Influencing these NMDAr co-agonist concentrations provides an interesting treatment target for this common, devastating and currently poorly treatable condition.

Excitatory amino acids and magnesium sulfate in neonatal asphyxia

The excitatory amino acids (EAA); glutamate and aspartate are released into the cerebrospinal fluids (CSF) of asphyxiated newborns. The objectives of this study were: (a) to examine the relation of the concentration of EAA in the CSF with the degree of brain injury, (b) To determine the time of the release of these EAA into the CSF, and (c) to detect the effect of magnesium sulfate (MgSO(4)) on their levels. DESIGNS AND METHODS. A randomized controlled trial was conducted on 47 full term asphyxiated newborns. Twenty three infants received an intravenous 10% solution of MgSO(4) at a dose of 250 mg/kg within the first 24h of life while the other 24 newborns received isotonic saline (0.9%) of an equal volume. Levels of glutamate and aspartate were measured before and 72 h after giving the trial solution. Results. In the study population (n=47) both glutamate and aspartate were significantly elevated in infants with higher grades of HIE compared to those with lower grades (P=0.013 and 0.031, respectively). Compared to baseline level, glutamate decreased significantly over time in placebo group (-8.28+/-14.26, P=0.025) and in MgSO(4) group (-14.39+/-18.72, P=0.005). Glutamate concentration did not differ between groups when measured at baseline (29.26+/-16.31 vs. 31.27+/-22.62, P=0.82) and at 72 h (19.28+/-15.63 vs. 19.6+/-16.54, P=0.87). The change in aspartate concentration over time was not significant in placebo group (-0.45+/-1.96, P=0.34) or in MgSO(4) group (-0.7+/-3.19, P=0.37). Aspartate did not differ between groups when measured at baseline (3.52+/-2.4 vs. 3.92+/-2.59, P=0.49) or at 72 h (2.79+/-1.24 vs. 3.05+/-2.48, P=0.92). Conclusions. The EAA; glutamate and aspartate are released in the CSF of asphyxiated newborns immediately after birth and declined by 72 h. Their initial concentrations correlated with the severity of HIE. Postnatal administration of MgSO(4) did not alter the levels of these 2 EAA.

Hypoxia/ischemia modulates G protein-coupled receptor kinase 2 and beta-arrestin-1 levels in the neonatal rat brain

BACKGROUND AND PURPOSE

Neurotransmitters, neuropeptides, chemokines, and many other molecules signal through G protein-coupled receptors (GPCRs). GPCR kinases (GRKs) and beta-arrestins play a crucial role in regulating the responsiveness of multiple GPCRs. Reduced expression of GRK and beta-arrestins leads to supersensitization of GPCRs and will thereby increase the response to neuropeptides and neurotransmitters. We analyzed GRK and beta-arrestin expression after cerebral hypoxia/ischemia (HI).

MATERIALS AND METHODS

Twelve-day-old rat pups were exposed to 90 minutes of hypoxia (fraction of inspired oxygen [FiO2] 0.08) after ligation of the right carotid artery, a procedure that induces unilateral damage in the right hemisphere. At 6, 12, 24, and 48 hours after HI, the left (hypoxic) and right (hypoxic/ischemic) hemispheres were analyzed for GRK and beta-arrestin protein and mRNA expression by Western blotting and real-time polymerase chain reaction, respectively. In addition, we analyzed GRK2 expression in the hippocampus by immunohistochemistry.

RESULTS

HI downregulated GRK2 protein expression in both hemispheres at 24 to 48 hours after HI, and the effect was more pronounced in the ipsilateral hemisphere. HI induced no global change in GRK6 protein expression. However, GRK2 was markedly decreased in the hippocampal region of the ipsilateral hemisphere that will be severely damaged after HI. No changes in global mRNA levels for GRK2 were detected. In contrast, HI increased beta-arrestin-1 protein expression as well as mRNA levels at 6 to 12 hours after HI.

CONCLUSIONS

Neonatal HI-induced brain damage is associated with specific changes in the GPCR desensitization machinery. We hypothesize that these changes result in supersensitization of multiple GPCRs and might therefore contribute to HI-induced brain damage.

Extracellular glutamate levels and neuropathology in cerebral white matter following repeated umbilical cord occlusion in the near term fetal sheep

Umbilical cord occlusion causes fetal hypoxemia which can result in brain injury including damage to cerebral white matter. Excessive glutamate release may be involved in the damage process. This study examined the relation between extracellular glutamate levels in the cerebral white matter of the ovine fetus during and after intermittent umbilical cord occlusion and the degree of resultant fetal brain injury. Fetal sheep underwent surgery for chronic catheterisation and implantation of an intra-cerebral microdialysis probe at 130 days of gestation (term approximately 147 days). Four days after surgery (day 1), seven fetuses were subjected to 5x2 min umbilical cord occlusions, and on the following day (day 2) they were subjected to either 4 or 5x4 min umbilical cord occlusions; seven fetuses served as controls. Microdialysis samples were collected before, during and after the umbilical cord occlusions to determine extracellular glutamate levels in the cerebral white matter. Fetal blood gas status was measured and the fetal electrocorticogram was recorded continuously. During the periods of umbilical cord occlusions on both days 1 and 2, fetal arterial oxygen saturation, arterial partial pressure of oxygen and arterial pH decreased (P<0.05) while arterial partial pressure of carbon dioxide increased (P<0.05). All fetuses showed episodes of isoelectric electrocortical activity during umbilical cord occlusions on both days 1 and 2. In fetuses with patent microdialysis probes there were marked increases of glutamate efflux in the cerebral white matter following umbilical cord occlusion. Fetal brains were removed at autopsy on day 5 and subjected to histological assessment. Brain damage was observed in all fetuses exposed to cord occlusion, particularly in the periventricular white matter, with the most extensive damage occurring in the fetuses with the greatest increases in glutamate levels. We conclude that, in the unanesthetised fetus in utero, glutamatergic processes are associated with umbilical cord occlusion-induced brain damage in the cerebral white matter.

Compromised glutamate transport in human glioma cells: reduction-mislocalization of sodium-dependent glutamate transporters and enhanced activity of cystine-glutamate exchange

Elevated levels of extracellular glutamate ([Glu](o)) can induce seizures and cause excitotoxic neuronal cell death. This is normally prevented by astrocytic glutamate uptake. Neoplastic transformation of human astrocytes causes malignant gliomas, which are often associated with seizures and neuronal necrosis. Here, we show that Na(+)-dependent glutamate uptake in glioma cell lines derived from human tumors (STTG-1, D-54MG, D-65MG, U-373MG, U-251MG, U-138MG, and CH-235MG) is up to 100-fold lower than in astrocytes. Immunohistochemistry and subcellular fractionation show very low expression levels of the astrocytic glutamate transporter GLT-1 but normal expression levels of another glial glutamate transporter, GLAST. However, in glioma cells, essentially all GLAST protein was found in cell nuclei rather than the plasma membrane. Similarly, brain tissues from glioblastoma patients also display reduction of GLT-1 and mislocalization of GLAST. In glioma cell lines, over 50% of glutamate transport was Na(+)-independent and mediated by a cystine-glutamate exchanger (system x(c)(-)). Extracellular L-cystine dose-dependently induced glutamate release from glioma cells. Glutamate release was enhanced by extracellular glutamine and inhibited by (S)-4-carboxyphenylglycine, which blocked cystine-glutamate exchange. These data suggest that the unusual release of glutamate from glioma cells is caused by reduction-mislocalization of Na(+)-dependent glutamate transporters in conjunction with upregulation of cystine-glutamate exchange. The resulting glutamate release from glioma cells may contribute to tumor-associated necrosis and possibly to seizures in peritumoral brain tissue.

Brain injury after neonatal hypoxia-ischemia in rats: a role of cysteine?

The aim of this study was to investigate the role of cysteine in development of brain damage after hypoxia-ischemia (HI) in neonatal rats. Rat pups were subjected to unilateral carotid ligation and exposure to hypoxia (7.7% oxygen) for 60 or 90 min. A subtoxic dose of cysteine were administered before or after HI and the unilateral brain injury was evaluated 14 days after the insult and expressed as ipsilateral weight deficit as % of the contralateral hemisphere. In some experiments the changes of extracellular (e.c.) cysteine in the cerebral cortex were sampled with microdialysis and analyzed with HPLC. Cysteine in a dose of 0.2 mg/g s.c. given before 60 min of HI increased the extent of brain injury by 59%. The effect of posttreatment was limited and dependent on the duration of HI: 0.2 mg/g of cysteine given after 90 min of HI increased the degree of brain injury by 25%, whereas the same dose administered after 60 min of HI was ineffective in spite of that this combination of cysteine and HI resulted in e.c. cysteine concentrations 3-4 times higher than those observed in non-treated HI controls. These data show that subtoxic doses of cysteine administered before or after HI enhances brain injury. However, e.c. cysteine levels exceeding those induced by HI are required which makes a substantial contribution of cysteine in the pathophysiology of HI brain injury in the neonatal rat unlikely.

Mitochondrial function and energy metabolism after hypoxia-ischemia in the immature rat brain: involvement of NMDA-receptors

Treatment after hypoxia-ischemia (HI) in immature rats with the N-methyl-D-aspartate receptor (NMDAR) antagonist dizocilpine maleate (MK-801) reduces areas with high glucose utilization and reduces brain damage. The object was to study the metabolic effects of MK-801 treatment after HI. Seven-day-old rats were randomized to the following groups: non-HI, HI, or HI plus MK-801 (0.5 mg/kg immediately after HI). In the parietal cortex, the mitochondrial respiration was measured in homogenates 1 to 4 hours, and the energy metabolites at 3 and 8 hours after HI. The energy use was calculated from changes in energy metabolites after decapitation at 3 hours after HI. State 3 respiration was reduced by 46%, 32%, and 25% after HI compared with non-HI with pyruvate plus malate, glutamate plus malate, or glutamate plus succinate as substrates, respectively. Uncoupler-stimulated but not state 4 respiration was similarly reduced. The MK-801 augmented pyruvate plus malate-supported state 3 respiration after HI by 42%. The energy utilization was not affected by HI but was reduced by MK-801 treatment in the ipsilateral cortex from 4.6 +/- 2.3 to 2.6 +/- 1.8 micromol high-energy phosphate bond/min/g. The levels of ATP and phosphocreatine did not differ between the HI and HI plus MK-801 groups at 3 hours, but were lower in the HI than in the HI plus MK-801 group at 8 hours after HI. In conclusion, treatment with MK-801 reduced energy utilization and improved mitochondrial function and energy status after HI, suggesting a linkage between NMDAR activation and impaired energy metabolism during reperfusion.

Alterations in K+ evoked profiles of neurotransmitter and neuromodulator amino acids after focal ischemia-reperfusion

Secondary elevations in extracellular amino acids occur during reperfusion after transient cerebral ischemia. The delayed accumulation of excitatory amino acids may contribute to the progressive development of neuronal injury. In this study, we explored the mechanisms that may be involved in this phenomenon. Microdialysis samples from probes located in rabbit cortex were analysed with a chiral amino acid procedure. Concentrations of neurotransmitters (L-Glu, GABA), N-methyl-D-aspartate receptor modulators (D-Ser, Gly), an inhibitory neuromodulator (Tau), the lipid component phosphoethanolamine, and L-Gln, L-Ser and L-Ala were measured. Depolarization via perfusion with potassium was used to assess the status of release/reuptake systems at 2 and 4 h reperfusion after 2 h transient focal ischemia. Background experiments classified potassium evoked responses as calcium dependent or calcium-independent by inclusion of 30 microM omega-conopeptide MVIIC or by inclusion of 20 mM magnesium and ommision of calcium. During ischemia, large elevations of almost all amino acids occurred. During reperfusion, secondary elevations in transmitter amino acids (L-Glu, GABA) and N-methyl-D-aspartate receptor modulators (D-Ser, Gly) occurred. Tau remained slightly elevated whereas the lipid component phosphoethanolamine remained high and stable during reperfusion. Reperfusion significantly potentiated the potassium response for amino acids with calcium-dependent responses (L-Glu and GABA). In contrast, calcium-independent responses (Tau, phosphoethanolamine, L-Gln) were significantly attenuated. Intermediate behavior was observed with Gly, while no potassium responses were observed for D-Ser, L-Ser or L-Ala. These data demonstrate that perturbations in evoked amino acid profiles after ischemia-reperfusion are selective. Reduction of calcium-independent responses implicate a general decline in efficacy of transporter mechanisms that restore transmembrane gradients of ions and transmitters. Decreased efficacy of transporter systems may reduce transmitter reuptake and account for the amplified release of L-Glu and GABA, thus contributing to progressive neural dysfunction after cerebral ischemia.

Acute and long-lasting effects of neonatal hypoxia on (+)-3-[125I]MK-801 binding to NMDA brain receptors

The NMDA receptor subtype is the major excitatory mediator for glutamate neurotoxicity. To assess its participation in the noxious effects of postnatal hypoxia, we have characterized the binding of the ionophoric marker of NMDA receptor, dizocilpine (MK-801). Binding of (+)-3-[125I]MK-801 to NMDA brain receptors under nonequilibrium conditions was quantified by in vitro autoradiography in rats exposed to hypoxia induced by 93% N2/6.5% O2 exposure for 70 min on Postnatal Day 4. Acute and long-lasting effects were investigated at 4 h after injury and on Postnatal Day 40. At the acute stage, a transient decrease in binding was found in several specific brain areas, hypothalamus, amygdaloid nuclei, entorhinal cortex, perirhinal cortex, and hippocampus, and no differences were found in temporal cortex, thalamus, and geniculate nucleus, when compared to sham-treated animals. At this early age, there was no increase of binding when slices from both groups were incubated in the presence of glutamate and glycine (Glu/Gly), positive allosteric modulators of MK-801 binding. In the 40-day-old brains, the binding to the NMDA receptors of hypoxiatreated animals was not different with respect to controls in most of the areas studied, but the Glu/Gly stimulation of binding in hypoxic rats showed a reduced, or absent, response to the allosteric modulators. In contrast, control rats showed a remarkable increase of the specific binding induced by the presence of the modulators in the incubation buffer. Binding of (+)-3-[125I]MK-801 was also performed at a higher concentration to clarify whether the altered response to Glu/Gly may be due to differences in the number of channels; however, the density of NMDA receptors at this concentration was similar in both control and hypoxia-treated rats. We conclude that the effect of exposure of newborn rats to hypoxia can generate acute and long-lasting effects on the NMDA receptor. The deleterious action of this kind of noxa on the CNS could be exerted by interference with normal glutamatergic transmission and hence over normal growth and development.