TrkB-mediated neuroprotection in female hippocampal neurons is autonomous, estrogen receptor alpha-dependent, and eliminated by testosterone: a proposed model for sex differences in neonatal hippocampal neuronal injury

BACKGROUND

Neonatal hypoxia ischemia (HI) related brain injury is one of the major causes of learning disabilities and memory deficits in children. In both human and animal studies, female neonate brains are less susceptible to HI than male brains. Phosphorylation of the nerve growth factor receptor TrkB has been shown to provide sex-specific neuroprotection following in vivo HI in female mice in an estrogen receptor alpha (ERα)-dependent manner. However, the molecular and cellular mechanisms conferring sex-specific neonatal neuroprotection remain incompletely understood. Here, we test whether female neonatal hippocampal neurons express autonomous neuroprotective properties and assess the ability of testosterone (T) to alter this phenotype.

METHODS

We cultured sexed hippocampal neurons from ERα+/+ and ERα-/- mice and subjected them to 4 h oxygen glucose deprivation and 24 h reoxygenation (4-OGD/24-REOX). Sexed hippocampal neurons were treated either with vehicle control (VC) or the TrkB agonist 7,8-dihydroxyflavone (7,8-DHF) following in vitro ischemia. End points at 24 h REOX were TrkB phosphorylation (p-TrkB) and neuronal survival assessed by immunohistochemistry. In addition, in vitro ischemia-mediated ERα gene expression in hippocampal neurons were investigated following testosterone (T) pre-treatment and TrkB antagonist therapy via q-RTPCR. Multifactorial analysis of variance was conducted to test for significant differences between experimental conditions.

RESULTS

Under normoxic conditions, administration of 3 µM 7,8-DHF resulted an ERα-dependent increase in p-TrkB immunoexpression that was higher in female, as compared to male neurons. Following 4-OGD/24-REOX, p-TrkB expression increased 20% in both male and female ERα+/+ neurons. However, with 3 µM 7,8-DHF treatment p-TrkB expression increased further in female neurons by 2.81 ± 0.79-fold and was ERα dependent. 4-OGD/24-REOX resulted in a 56% increase in cell death, but only female cells were rescued with 3 µM 7,8-DHF, again in an ERα dependent manner. Following 4-OGD/3-REOX, ERα mRNA increased ~ 3 fold in female neurons. This increase was blocked with either the TrkB antagonist ANA-12 or pre-treatment with T. Pre-treatment with T also blocked the 7,8-DHF- dependent sex-specific neuronal survival in female neurons following 4-OGD/24-REOX.

CONCLUSIONS

OGD/REOX results in sex-dependent TrkB phosphorylation in female neurons that increases further with 7,8-DHF treatment. TrkB phosphorylation by 7,8-DHF increased ERα mRNA expression and promoted cell survival preferentially in female hippocampal neurons. The sex-dependent neuroprotective actions of 7,8-DHF were blocked by either ANA-12 or by T pre-treatment. These results are consistent with a model for a female-specific neuroprotective pathway in hippocampal neurons in response to hypoxia. The pathway is activated by 7,8-DHF, mediated by TrkB phosphorylation, dependent on ERα and blocked by pre-exposure to T.

TrkB-mediated sustained neuroprotection is sex-specific and Erα-dependent in adult mice following neonatal hypoxia ischemia

BACKGROUND

Neonatal hypoxia ischemia (HI) related brain injury is one of the major causes of life-long neurological morbidities that result in learning and memory impairments. Evidence suggests that male neonates are more susceptible to the detrimental effects of HI, yet the mechanisms mediating these sex-specific responses to neural injury in neonates remain poorly understood. We previously tested the effects of treatment with a small molecule agonist of the tyrosine kinase B receptor (TrkB), 7,8-dihydroxyflavone (DHF) following neonatal HI and determined that females, but not males exhibit increased phosphorylation of TrkB and reduced apoptosis in their hippocampi. Moreover, these female-specific effects of the TrkB agonist were found to be dependent upon the expression of Erα. These findings demonstrated that TrkB activation in the presence of Erα comprises one pathway by which neuroprotection may be conferred in a female-specific manner. The goal of this study was to determine the role of Erα-dependent TrkB-mediated neuroprotection in memory and anxiety in young adult mice exposed to HI during the neonatal period.

METHODS

In this study, we used a unilateral hypoxic ischemic (HI) mouse model. Erα+/+ or Erα-/- mice were subjected to HI on postnatal day (P) 9 and mice were treated with either vehicle control or the TrkB agonist, DHF, for 7 days following HI. When mice reached young adulthood, we used the novel object recognition, novel object location and open field tests to assess long-term memory and anxiety-like behavior. The brains were then assessed for tissue damage using immunohistochemistry.

RESULTS

Neonatal DHF treatment prevented HI-induced decrements in recognition and location memory in adulthood in females, but not in males. This protective effect was absent in female mice lacking Erα. The female-specific improved recognition and location memory outcomes in adulthood conferred by DHF therapy after neonatal HI tended to be or were Erα-dependent, respectively. Interestingly, DHF triggered anxiety-like behavior in both sexes only in the mice that lacked Erα. When we assessed the severity of injury, we found that DHF therapy did not decrease the percent tissue loss in proportion to functional recovery. We additionally observed that the presence of Erα significantly reduced overall HI-associated mortality in both sexes.

CONCLUSIONS

These observations provide evidence for a therapeutic role for DHF in which TrkB-mediated sustained recovery of recognition and location memories in females are Erα-associated and dependent, respectively. However, the beneficial effects of DHF therapy did not include reduction of gross tissue loss but may be derived from the enhanced functioning of residual tissues in a cell-specific manner.

TrkB-mediated sustained neuroprotection is sex-specific and ERα dependent in adult mice following neonatal hypoxia ischemia

Background

Neonatal hypoxia ischemia (HI) related brain injury is one of the major causes of life-long neurological morbidities that result in learning and memory impairments. Evidence suggests that male neonates are more susceptible to the detrimental effects of HI, yet the mechanisms mediating these sex-specific responses to neural injury in neonates remain poorly understood. We previously tested the effects of treatment with a small molecule agonist of the tyrosine kinase B receptor (TrkB), 7,8-dihydroxyflavone (DHF) following neonatal HI and determined that females, but not males exhibit increased phosphorylation of TrkB and reduced apoptosis in their hippocampi. Moreover, these female-specific effects of the TrkB agonist were found to be dependent upon the expression of ERα. These findings demonstrated that TrkB activation in the presence of ERα comprises one pathway by which neuroprotection may be conferred in a female-specific manner. The goal of this study was to determine the role of ERα-dependent TrkB-mediated neuroprotection in memory and anxiety in young adult mice exposed to HI during the neonatal period.

Methods

In this study we used a unilateral hypoxic ischemic (HI) mouse model. ERα+/+ or ERα-/- mice were subjected to HI on postnatal day (P) 9 and mice were treated with either vehicle control or the TrkB agonist, DHF, for seven days following HI. When mice reached young adulthood, we used the novel object recognition, novel object location and open field tests to assess long-term memory and anxiety like behavior. The brains were then assessed for tissue damage using immunohistochemistry.

Results

Neonatal DHF treatment prevented HI-induced decrements in recognition and location memory in adulthood in females, but not in males. This protective effect was absent in female mice lacking ERα. Thus, the female-specific and ERα-dependent neuroprotection conferred by DHF therapy after neonatal HI was associated with improved learning and memory outcomes in adulthood. Interestingly, DHF triggered anxiety like behavior in both sexes only in the mice that lacked ERα. When we assessed the severity of injury, we found that DHF therapy did not decrease the percent tissue loss in proportion to functional recovery. We additionally observed that the presence of ERα significantly reduced overall HI-associated mortality in both sexes.

Conclusions

These observations provide evidence for a therapeutic role for DHF in which sustained recovery of memory in females is TrkB-mediated and ERα-dependent. However, the beneficial effects of DHF therapy did not include reduction of gross tissue loss but may be derived from the enhanced functioning of residual tissues in a cell-specific manner.

Developmental differences in microglia morphology and gene expression during normal brain development and in response to hypoxia-ischemia

BACKGROUND

Neuroinflammation plays an important role in ischemic brain injury and recovery, however the interplay between brain development and the neuroinflammatory response is poorly understood. We previously described age-dependent differences in the microglial response and the effect of microglial inhibition. Here we investigate whether age-dependent microglial responses may be related to pre-injury developmental differences in microglial phenotype.

METHODS

Measures of microglia morphology were quantified using semi-automated software analysis of immunostained sections from postnatal day 2 (P2), P9, P30 and P60 mice using IMARIS. Microglia were isolated from P2, P9, P30 and P60 mice, and expression of markers of classical and alternative microglial activation was assessed, as well as transforming growth factor beta (TGF-β) receptor, Serpine1, Mer Tyrosine Kinase (MerTK), and the suppressor of cytokine signaling (SOCS3). Hypoxia-ischemia (HI) was induced in P9 and P30 mice using unilateral carotid artery ligation and exposure to 10% oxygen for 50 min. Microglia morphology and microglial expression of genes in the TGF-β and MerTK pathways were determined in ipsilateral and contralateral hippocampus.

RESULTS

A progressive and significant increase in microglia branching morphology was seen in all brain regions from P2 to P30. No consistent classical or alternative activation profile was seen in isolated microglia. A clear transition to increased expression of TGF-β and its downstream effector serpine1 was seen between P9 and P30. A similar increase in expression was seen in MerTK and its downstream effector SOCS3. HI resulted in a significant decrease in branching morphology only in the P9 mice, and expression of TGF-β receptor, Serpine1, MerTK, and SOCS3 were elevated in P30 mice compared to P9 post-HI.

CONCLUSION

Microglia maturation is associated with changes in morphology and gene expression, and microglial responses to ischemia in the developing brain differ based on the age at which injury occurs.

Hippocampal increase of 5-hmC in the glucocorticoid receptor gene following acute stress

5-Hydroxymethylcytosine (5-hmC) is a novel environmentally sensitive DNA modification that is highly enriched in post-mitotic neurons and is associated with active transcription of neuronal genes. Recently, 5-hmC was functionally linked to learning and cognition and these studies revealed an accumulation of 5-hmC in the prefrontal cortex of mice undergoing fear extinction. These studies led us to hypothesize a role for 5-hmC in response to stress. To test this hypothesis, we combined immunohistochemistry, tandem mass spectrometry, and tet-assisted sodium bisulfite sequencing (TAB-seq) analyses on tissue and DNA from the hippocampus of 7-week old male mice exposed to a single 30-min restraint stress. After first identifying that the broad neuronal distribution of 5-hmC is not disrupted by acute stress, we used TAB-seq to find a stress-induced increase of 5-hmC in the 3'UTR of the glucocorticoid receptor gene (Nr3c1). Nr3c1 has a well-defined role in the stress pathway and these data suggest that 5-hmC contributes to these processes. Together, these data indicate that a deeper investigation of stress-related 5-hmC levels may reveal an environmental impact on this newly discovered epigenetic mark in the brain.

Age-dependent microglial activation in immature brains after hypoxia- ischemia

In the present study, we tested whether the ongoing differentiation of microglia in the immature brain results in more robust microglial activation and pro-inflammatory responses than juvenile brains following hypoxia-ischemia (HI). Under normoxic conditions, microglial activation profiles were assessed in postnatal day 9 and postnatal day 30 mice (P9 and P30) by analyzing relative expression levels of CD45 in CD11b+/CD45+ microglia/macrophages. Flow cytometry analysis revealed that the hippocampi of P9 and P30 brains exhibited higher levels of CD45 expression in CD11b+/CD45+ cells than in the cortex and striatum. In response to HI, there was an early increase in number of CD11b+/CD45+ microglia/macrophages in the ipsilateral hippocampus of P9 mice. These cells transformed from a "ramified" to an "amoeboid" morphology in the CA1 region, which was accompanied by a loss of microtubule-associated protein 2 immunostaining in this brain region. The peak response of microglial activation in the ipsilateral hippocampus of P9 mice occurred on day 2 post-HI, which was in contrast to a delayed and persistent microglial activation in the cortex and striatum (peak on day 9 post-HI). P9 brains demonstrated a 2-3 fold greater increase in microglia counts than P30 brains in each region (hippocampus, cortex, and striatum) during day 1-17 post-HI. P9 brains also showed more robust expression of pro-inflammatory cytokines (tumor necrosis factor-alpha, interleukin-1β) than P30 brains. Taken together, compared to P30 mice, P9 mice demonstrated differences in microglial activation and pro-inflammatory responses after HI, which may be important in brain damage and tissue repair.

Inhibition of Na(+)-K(+)-2Cl(-) cotransporter isoform 1 accelerates temozolomide-mediated apoptosis in glioblastoma cancer cells

The hallmark of apoptosis is a significant reduction in cell volume (AVD) resulting from loss of K(+)(i) and Cl(-)(i). Loss of cell volume and lowering of ionic strength of intracellular K(+) and Cl(-) occur before any other detectable characteristics of apoptosis. In the present study, temozolomide (TMZ) triggered loss of K(+)(i) and Cl(-)(i) and AVD in primary glioblastoma multiforme (GBM) cancer cells (GC) and GC cancer stem cells (GSC). We hypothesize that Na(+)-K(+)-2Cl(-) cotransporter isoform 1 (NKCC1) counteracts AVD during apoptosis in GBM cancer cells by regulating cell volume and Cl(-) homeostasis. NKCC1 protein was expressed in both GC and GSC and played an essential role in regulatory volume increase (RVI) in response to hypertonic cell shrinkage and isotonic cell shrinkage. Blocking NKCC1 activity with its potent inhibitor bumetanide abolished RVI. These cells maintained a basal [Cl(-)](i) (~ 68 mM) above the electrochemical equilibrium for Cl(-)(i). NKCC1 also functioned to replenish Cl(-)(i) levels following the loss of Cl(-)(i). TMZ-treated cells exhibited increased phosphorylation of NKCC1 and its up-stream novel Cl(-)/volume-sensitive regulatory kinase WNK1. Inhibition of NKCC1 activity with bumetanide accelerated AVD, early apoptosis, as well as activation of caspase-3 and caspase-8. Taken together, this study strongly suggests that NKCC1 is an essential mechanism in GBM cells to maintain K(+), Cl(-), and volume homeostasis to counteract TMZ-induced loss of K(+), Cl(-) and AVD. Therefore, blocking NKCC1 function augments TMZ-induced apoptosis in glioma cells.

Excessive Na+/H+ exchange in disruption of dendritic Na+ and Ca2+ homeostasis and mitochondrial dysfunction following in vitro ischemia

Neuronal dendrites are vulnerable to injury under diverse pathological conditions. However, the underlying mechanisms for dendritic Na(+) overload and the selective dendritic injury remain poorly understood. Our current study demonstrates that activation of NHE-1 (Na(+)/H(+) exchanger isoform 1) in dendrites presents a major pathway for Na(+) overload. Neuronal dendrites exhibited higher pH(i) regulation rates than soma as a result of a larger surface area/volume ratio. Following a 2-h oxygen glucose deprivation and a 1-h reoxygenation, NHE-1 activity was increased by ∼70-200% in dendrites. This elevation depended on activation of p90 ribosomal S6 kinase. Moreover, stimulation of NHE-1 caused dendritic Na(+)(i) accumulation, swelling, and a concurrent loss of Ca(2+)(i) homeostasis. The Ca(2+)(i) overload in dendrites preceded the changes in soma. Inhibition of NHE-1 or the reverse mode of Na(+)/Ca(2+) exchange prevented these changes. Mitochondrial membrane potential in dendrites depolarized 40 min earlier than soma following oxygen glucose deprivation/reoxygenation. Blocking NHE-1 activity not only attenuated loss of dendritic mitochondrial membrane potential and mitochondrial Ca(2+) homeostasis but also preserved dendritic membrane integrity. Taken together, our study demonstrates that NHE-1-mediated Na(+) entry and subsequent Na(+)/Ca(2+) exchange activation contribute to the selective dendritic vulnerability to in vitro ischemia.

Endoplasmic reticulum Ca2+ signaling and mitochondrial Cyt c release in astrocytes following oxygen and glucose deprivation

In the present study, we investigated changes of cytosolic Ca2+([Ca2+](cyt)), endoplasmic reticulum Ca2+([Ca2+](ER)) and mitochondrial Ca2+(Ca2+(m)) in astrocytes following oxygen/glucose deprivation and reoxygenation (OGD/REOX). Two hours OGD did not cause changes in [Ca2+](cyt), but led to a significant increase in [Ca2+](ER). The elevation in [Ca2+](ER) continued and reached a peak level (130 +/- 2 microM) by 90 min REOX. An abrupt release of Ca2+(ER) occurred during 1.5-2.5 h REOX, which was accompanied with a delayed and sustained rise in [Ca2+](cyt). Moreover, Ca2+(m) content was increased significantly within 15 min REOX followed by a secondary rise (approximately 4.5-fold) and a release of mitochondrial cytochrome c (Cyt c). Astrocytes exhibited translocation of Cyt c from mitochondria to endoplasmic reticulum (ER) and up regulation of ER stress protein p-eIF2alpha. Blocking Na+-K+-Cl(-) cotransporter isoform 1 activity, either by its potent inhibitor bumetanide or genetic ablation, abolished release of ER Ca2+, delayed rise in [Ca2+](cyt) and Ca2+(m). Inhibition of the reverse mode operation of the Na+/Ca2+ exchanger significantly attenuated OGD/REOX-mediated Cyt c release. In summary, this study illustrates that OGD/REOX triggers a time-dependent loss of Ca2+ homeostasis in cytosol and organelles (ER and mitochondria) in astrocytes. Collective stimulation of Na+-K+-Cl(-) cotransporter isoform 1 and reverse mode function of Na+/Ca2+ exchanger contributes to these changes.

Study of Na+/H+ exchange-mediated pHi regulations in neuronal soma and neurites in compartmentalized microfluidic devices

Regulation of intracellular pH (pH(i)) in neurons is crucial to maintain their physiological function. In the current study, newly-developed polydimethylsiloxane (PDMS) microfluidic devices were used to independently investigate pH(i) regulation in neuronal soma and neurites. Embryonic cortical neurons were cultured in PDMS microfluidic devices with soma growing in one chamber (seeded) and neurites extending through a set of perpendicular microchannels into the opposite parallel chamber (non-seeded). Neurons in the microchambers were characterized by the vital dye calcein-red, polarized mitochondria, and expression of neuronal specific beta-tubulin (type-III), axonal Tau-1 protein, dendritic microtubule associated protein (MAP-2), and Na(+)/H(+) exchanger isoform 1 (NHE-1). Neurites exhibited higher resting pH(i) than soma (7.16 +/- 0.09 vs. 6.90 +/- 0.15). The neurites had a proton extrusion rate 3.7-fold faster than in soma following NH(4)Cl prepulse-mediated acidification (p < 0.05). The difference in the pH(i) regulation rates between neurites and soma can be accounted for by the larger surface area to volume ratio in the neurites. Interestingly, pharmacological inhibition of NHE-1 activity blocked the pH(i) regulation in soma and in neurites by approximately 70% (p < 0.05). Taken together, our study demonstrated that the microfluidic devices provide a useful tool to study neuronal pH(i) regulation in soma and their neurites. We conclude that NHE-1 plays an important role in regulation of pH(i) in both compartments.

Protein aggregation in neurons following OGD: a role for Na+ and Ca2+ ionic dysregulation

In this study, we investigated whether disruption of Na(+) and Ca(2+) homeostasis via activation of Na(+)-K(+)-Cl(-) cotransporter isoform 1 (NKCC1) and reversal of Na(+)/Ca(2+) exchange (NCX(rev)) affects protein aggregation and degradation following oxygen-glucose deprivation (OGD). Cultured cortical neurons were subjected to 2 h OGD and 1-24 h reoxygenation (REOX). Redistribution of ubiquitin and formation of ubiquitin-conjugated protein aggregates occurred in neurons as early as 2 h REOX. The protein aggregation progressed further by 8 h REOX. There was no significant recovery at 24 h REOX. Moreover, the proteasome activity in neurons was inhibited by 80-90% during 2-8 h REOX and recovered partially at 24 h REOX. Interestingly, pharmacological inhibition or genetic ablation of NKCC1 activity significantly decreased accumulation of ubiquitin-conjugated protein aggregates and improved proteasome activity. A similar protective effect was obtained by blocking NCX(rev) activity. Inhibition of NKCC1 activity also preserved intracellular ATP and Na(+) homeostasis during 0-24 h REOX. In a positive control study, disruption of endoplasmic reticulum Ca(2+) with thapsigargin triggered redistribution of free ubiquitin and protein aggregation. We conclude that overstimulation of NKCC1 and NCX(rev) following OGD/REOX partially contributes to protein aggregation and proteasome dysfunction as a result of ionic dysregulation.

Calcium dysregulation induces apoptosis-inducing factor release: cross-talk between PARP-1- and calpain-signaling pathways

Recent discoveries show that caspase-independent cell death pathways are a pervasive mechanism in neurodegenerative diseases, and apoptosis-inducing factor (AIF) is an important effector of this mode of neuronal death. There are currently two known mechanisms underlying AIF release following excitotoxic stress, PARP-1 and calpain. To test whether there is an interaction between PARP-1 and calpain in triggering AIF release, we used the NMDA toxicity model in rat primary cortical neurons. Exposure to NMDA resulted in AIF truncation and nuclear translocation, and shRNA-mediated knockdown of AIF resulted in neuroprotection. Both calpain and PARP-1 are involved with AIF processing as AIF truncation, nuclear translocation and neuronal death were attenuated by calpain inhibition using adeno-associated virus-mediated overexpression of the endogenous calpain inhibitor, calpastatin, or treatment with the PARP-1 inhibitor 3-ABA. Activation of PARP-1 is necessary for calpain activation as PARP-1 inhibition blocked mitochondrial calpain activation. Finally, NMDA toxicity induces mitochondrial Ca(2+) dysregulation in a PARP-1 dependent manner. Thus, PARP-1 and mitochondrial calpain activation are linked via PARP-1-induced alterations in mitochondrial Ca(2+) homeostasis. Collectively, these findings link the two seemingly independent mechanisms triggering AIF-induced neuronal death.

Endoplasmic reticulum Ca2+ dysregulation and endoplasmic reticulum stress following in vitro neuronal ischemia: role of Na+-K+-Cl- cotransporter

We investigated the role of Na(+)-K(+)-Cl(-) cotransporter (NKCC1) in conjunction with Na(+)/Ca(2+) exchanger (NCX) in disruption of endoplasmic reticulum (ER) Ca(2+) homeostasis and ER stress development in primary cortical neurons following in vitro ischemia. Oxygen-glucose deprivation (OGD) and reoxygenation (REOX) caused a rise in [Na(+)](cyt) which was accompanied by an elevation in [Ca(2+)](cyt). Inhibition of NKCC1 with its potent inhibitor bumetanide abolished the OGD/REOX-induced rise in [Na(+)](cyt) and [Ca(2+)](cyt). Moreover, OGD significantly increased Ca(2+)(ER) accumulation. Following REOX, a biphasic change in Ca(2+)(ER) occurred with an initial release of Ca(2+)(ER) which was sensitive to inositol 1,4,5-trisphosphate receptor (IP(3)R) inhibition and a subsequent refilling of Ca(2+)(ER) stores. Inhibition of NKCC1 activity with its inhibitor or genetic ablation prevented the release of Ca(2+)(ER). A similar result was obtained with inhibition of reversed mode operation of NCX (NCX(rev)). OGD/REOX also triggered a transient increase of glucose regulated protein 78 (GRP78), phospho-form of the alpha subunit of eukaryotic initiation factor 2 (p-eIF2alpha), and cleaved caspase 12 proteins. Pre-treatment of neurons with NKCC1 inhibitor bumetanide inhibited upregulation of GRP78 and attenuated the level of cleaved caspase 12 and p-eIF2alpha. Inhibition of NKCC1 reduced cytochrome C release and neuronal death. Taken together, these results suggest that NKCC1 and NCX(rev) may be involved in ischemic cell damage in part via disrupting ER Ca(2+) homeostasis and ER function.

A concerted role of Na+ -K+ -Cl- cotransporter and Na+/Ca2+ exchanger in ischemic damage

Na+-K+-Cl(-) cotransporter isoform 1 (NKCC1) and Na+/Ca2+ exchanger isoform 1 (NCX1) were expressed in cortical neurons. Three hours of oxygen and glucose deprivation (OGD) significantly increased expression of full-length NCX1 protein ( approximately 116 kDa), which remained elevated during 1 to 21 h reoxygenation (REOX) and was accompanied with concurrent cleavage of NCX1. Na+/Ca2+ exchanger isoform 1 heterozygous (NCX1+/-) neurons with approximately 50% less of NCX1 protein exhibited approximately 64% reduction in NCX-mediated Ca2+ influx. Expression of NCX1 and NKCC1 proteins was reduced in double heterozygous (NCX1+/-/NKCC1+/-) neurons. NCX-mediated Ca2+ influx was nearly abolished in these neurons. Three-hour OGD and 21-h REOX caused approximately 80% mortality rate in NCX1+/+ neurons and in NCX1+/- neurons. In contrast, NKCC1+/- neurons exhibited approximately 45% less cell death. The lowest mortality rate was found in NCX1+/-/NKCC1+/- neurons ( approximately 65% less neuronal death). The increased tolerance to ischemic damage was also observed in NCX1+/-/NKCC1+/- brains after transient cerebral ischemia. NCX1+/-/NKCC1+/- mice had a significantly reduced infarct volume at 24 and 72 h reperfusion. In conclusion, these data suggest that NKCC1 in conjunction with NCX1 plays a role in reperfusion-induced brain injury after ischemia.

ERK1/2-p90RSK-mediated phosphorylation of Na+/H+ exchanger isoform 1. A role in ischemic neuronal death

The function and regulation of Na(+)/H(+) exchanger isoform 1 (NHE1) following cerebral ischemia are not well understood. In this study, we demonstrate that extracellular signal-related kinases (ERK1/2) play a role in stimulation of neuronal NHE1 following in vitro ischemia. NHE1 activity was significantly increased during 10-60 min reoxygenation (REOX) after 2-h oxygen and glucose deprivation (OGD). OGD/REOX not only increased the V(max) for NHE1 but also shifted the K(m) toward decreased [H(+)](i). These changes in NHE1 kinetics were absent when MAPK/ERK kinase (MEK) was inhibited by the MEK inhibitor U0126. There were no changes in the levels of phosphorylated ERK1/2 (p-ERK1/2) after 2 h OGD. The p-ERK1/2 level was significantly increased during 10-60 min REOX, which was accompanied by nuclear translocation. U0126 abolished REOX-induced elevation and translocation of p-ERK1/2. We further examined the ERK/90-kDa ribosomal S6 kinase (p90(RSK)) signaling pathways. At 10 min REOX, phosphorylated NHE1 was increased with a concurrent elevation of phosphorylation of p90(RSK), a known NHE1 kinase. Inhibition of MEK activity with U0126 abolished phosphorylation of both NHE1 and p90(RSK). Moreover, neuroprotection was observed with U0126 or genetic ablation or pharmacological inhibition of NHE1 following OGD/REOX. Taken together, these results suggest that activation of ERK1/2-p90(RSK) pathways following in vitro ischemia phosphorylates NHE1 and increases its activity, which subsequently contributes to neuronal damage.

AMPA-mediated excitotoxicity in oligodendrocytes: role for Na(+)-K(+)-Cl(-) co-transport and reversal of Na(+)/Ca(2+) exchanger

We investigated the role of Na(+)-K(+)-Cl(-) co-transporter isoform 1 (NKCC1) and reversal of Na(+)/Ca(2+) exchanger (NCX(rev)) in glutamate-mediated excitotoxicity in oligodendrocytes obtained from rat spinal cords (postnatal day 6-8). An immunocytochemical characterization showed that these cultures express NKCC1 and Na(+)/Ca(2+) exchanger isoforms 1, 2, and 3 (NCX1, NCX2, NCX3). Exposing the cultures to alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) plus cyclothiazide (CTZ) led to a transient rise in intracellular (), which was followed by a sustained overload, NKCC1 phosphorylation, and a NKCC1-mediated Na(+) influx. In the presence of a specific AMPA receptor inhibitor 6-cyano-7-nitroquinoxaline-2, 3-dione (CNQX), the AMPA/CTZ failed to elicit any changes in . The AMPA/CTZ-induced sustained rise led to mitochondrial Ca(2+) accumulation, release of cytochrome c from mitochondria, and cell death. The AMPA/CTZ-elicited increase, mitochondrial damage, and cell death were significantly reduced by inhibiting NKCC1 or NCX(rev). These data suggest that in cultured oligodendrocytes, activation of AMPA receptors leads to NKCC1 phosphorylation that enhances NKCC1-mediated Na(+) influx. The latter triggers NCX(rev) and NCX(rev)-mediated overload and compromises mitochondrial function and cellular viability.

Role of Na+-K+-Cl−cotransport and Na+/Ca2+exchange in mitochondrial dysfunction in astrocytes following in vitro ischemia

Na+-K+-Cl−cotransporter isoform 1 (NKCC1) and reverse mode operation of the Na+/Ca2+exchanger (NCX) contribute to intracellular Na+and Ca2+overload in astrocytes following oxygen-glucose deprivation (OGD) and reoxygenation (REOX). Here, we further investigated whether NKCC1 and NCX play a role in mitochondrial Ca2+(Cam2+) overload and dysfunction. OGD/REOX caused a doubling of mitochondrial-releasable Ca2+( P < 0.05). When NKCC1 was inhibited with bumetanide, the mitochondrial-releasable Ca2+was reduced by ∼42% ( P < 0.05). Genetic ablation of NKCC1 also reduced Cam2+accumulation. Moreover, OGD/REOX in NKCC1+/+astrocytes caused dissipation of the mitochondrial membrane potential (Ψm) to 42 ± 3% of controls. In contrast, when NKCC1 was inhibited with bumetanide, depolarization of Ψmwas attenuated significantly (66 ± 10% of controls, P < 0.05). Cells were also subjected to severe in vitro hypoxia by superfusion with a hypoxic, acidic, ion-shifted Ringer buffer (HAIR). HAIR/REOX triggered a secondary, sustained rise in intracellular Ca2+that was attenuated by reversal NCX inhibitor KB-R7943. The hypoxia-mediated increase in Cam2+was accompanied by loss of Ψmand cytochrome c release in NKCC1+/+astrocytes. Bumetanide or genetic ablation of NKCC1 attenuated mitochondrial dysfunction and astrocyte death following ischemia. Our study suggests that NKCC1 acting in concert with NCX causes a perturbation of Cam2+homeostasis and mitochondrial dysfunction and cell death following in vitro ischemia.

Increased tolerance to ischemic neuronal damage by knockdown of Na+-Ca2+ exchanger isoform 1

We hypothesize that stimulation of Na+-K+-Cl+ cotransporter (NKCC1) causes Na+ overload that may lead to reversal of Na+-Ca2+ exchanger isoform 1 (NCX1) and ischemic neuronal damage. NCX1 protein expression and Ca2+ influx via reversal of NCX were decreased by approximately 70% in NCX1+/- neurons. Compared to NCX1+/+ neurons, NCX1+/- neurons exhibited significantly less cell death (approximately 30%) after 3 h oxygen and glucose deprivation (OGD) and 21 h reoxygenation. Additional neuroprotection was found in NCX1+/- neurons treated with NCX inhibitor KB-R7943. Moreover, expression of NCX1 protein was approximately 40% lower in NCX1+/- brains than in NCX1+/+ brains. However, there was no significant reduction in cerebral infarction in NCX1+/- mice following middle cerebral artery occlusion (MCAO). These data suggest that moderate reduction of NCX1 protein may be not enough to exert protection. We used small RNA-interference (siRNA) approach to further elucidate the role of NCX1 in ischemic cell damage. Efficacy of anti-NCX1 siRNA was tested in astrocytes and approximately 50% knockdown of NCX1 protein expression was achieved after 24-72 h transfection. Reduction in NCX1 protein expression was also found in brains of NCX1+/- mice after the siRNA injection. NCX1+/- mice treated with siRNA showed approximately 20% less MCAO-induced infarction, compared to NCX1+/- mice. Approximately 50% neuroprotection was detected in NKCC1+/-/NCX1+/- mice following MCAO. In conclusion, these data suggest that NCX1 plays an important role in ischemia/reperfusion-induced neuronal injury.

Role of membrane ion transport proteins in cerebral ischemic damage

Loss of ion homeostasis plays a central role in pathogenesis of ischemic cell damage. Ischemia-induced perturbation of ion homeostasis leads to intracellular accumulation of Ca2+ and Na+ and subsequent activation of proteases, phospholipases, and formation of oxygen and nitrogen free radicals. This signal transduction cascade results in long-term functional and structural changes in membrane and cytoskeletal integrity and eventual cell death. Both ion conductances and ion transporters could affect ion homeostasis. Considerable research effort has been centered on roles of passive fluxes via cation and anion conductances in cerebral ischemic damage. This review will instead focus on the recent studies into the role of secondary active transport proteins in ischemia-induced dissipation of ion homeostasis. Secondary active ion transport proteins are a membrane protein-mediated solute transport mechanism that derives its energy from the combined chemical gradients of the transported ions. They are important in maintaining steady-state intracellular ion concentrations. These include Na+-dependent chloride transport (NKCC), Na+/H+ exchange (NHE), and Na+/Ca2+ exchange (NCX). Results from both in vitro and in vivo experimental studies suggest that these ion transport proteins are potential targets to reduce or prevent ischemia-mediated loss of ion homeostasis.

Decreased neuronal death in Na+/H+ exchanger isoform 1-null mice after in vitro and in vivo ischemia

Na+/H+ exchanger isoform 1 (NHE1) is a major acid extrusion mechanism after intracellular acidosis. We hypothesized that stimulation of NHE1 after cerebral ischemia contributes to the disruption of Na+ homeostasis and neuronal death. In the present study, expression of NHE1 was detected in cultured mouse cortical neurons. Three hours of oxygen and glucose deprivation (OGD) followed by 21 h of reoxygenation (REOX) led to 68 +/- 10% cell death. Inhibition of NHE1 with the potent inhibitor cariporide (HOE 642) or genetic ablation of NHE1 reduced OGD-induced cell death by approximately 40-50% (p < 0.05). In NHE1(+/+) neurons, OGD caused a twofold increase in [Na+]i, and 60 min REOX triggered a sevenfold increase. Genetic ablation of NHE1 or HOE 642 treatment had no effects on the OGD-mediated initial Na+(i) rise but reduced the second phase of Na+(i) rise by approximately 40-50%. In addition, 60 min REOX evoked a 1.5-fold increase in [Ca2+]i in NHE1(+/+) neurons, which was abolished by inhibition of either NHE1 or reverse-mode operation of Na+/Ca2+ exchange. OGD/REOX-mediated mitochondrial Ca2+ accumulation and cytochrome c release were attenuated by inhibition of NHE1 activity. In an in vivo focal ischemic model, 2 h of left middle cerebral artery occlusion followed by 24 h of reperfusion induced 84.8 +/- 8.0 mm3 infarction in NHE1(+/+) mice. NHE1(+/+) mice treated with HOE 642 or NHE1 heterozygous mice exhibited a approximately 33% decrease in infarct size (p < 0.05). These results imply that NHE1 activity disrupts Na+ and Ca2+ homeostasis and contributes to ischemic neuronal damage.

Inhibition of Na+/H+ exchanger isoform 1 attenuates mitochondrial cytochrome C release in cortical neurons following in vitro ischemia

Na+/H+ exchanger isoform 1 (NHE1) is a major acid extrusion mechanism following intracellular acidosis. We hypothesized that stimulation of NHE1 after cerebral ischemia contributes to disruption of Na+ homeostasis and neuronal death. In the present study, expression of NHE1 was detected in cultured mouse cortical neurons. Oxygen and glucose deprivation (OGD) for 3 hours followed by 21 hours of reoxygenation (REOX) led to 68 +/- 10% cell death. Inhibition of NHE1 with the potent inhibitor HOE 642 or genetic ablation of NHE1 reduced OGD-induced cell death by approximately 40% to 50% (p < 0.05). In NHE1 +/+ neurons, OGD/REOX triggered significant increases in Na+ and Ca(i)2+. Genetic ablation of NHE1 and HOE 642 treatment reduced the rise of Na(i)+ by approximately 40% to 50% and abolished the OGD/REOX-mediated Ca2+ accumulation. Moreover, mitochondrial cytochrome C release was significantly attenuated by inhibition of NHE1 activity. These results imply that NHE1 activity disrupts Na+ and Ca2+ homeostasis and contributes to ischemic neuronal damage.

Na-K-Cl cotransporter-mediated intracellular Na+ accumulation affects Ca2+ signaling in astrocytes in an in vitro ischemic model

Na-K-Cl cotransporter isoform 1 (NKCC1) plays an important role in maintenance of intracellular Na+, K+, and Cl- levels in astrocytes. We propose that NKCC1 may contribute to perturbations of ionic homeostasis in astrocytes under ischemic conditions. After 3-8 hr of oxygen and glucose deprivation (OGD), NKCC1-mediated 86Rb influx was significantly increased in astrocytes from NKCC1 wild-type (NKCC1+/+) and heterozygous mutant (NKCC1+/-) mice. Phosphorylated NKCC1 protein was increased in NKCC1+/+ astrocytes at 2 hr of OGD. Two hours of OGD and 1 hr of reoxygenation (OGD/REOX) triggered an 3.6-fold increase in intracellular Na+ concentration ([Na+]i) in NKCC1+/+ astrocytes. Inhibition of NKCC1 activity by bumetanide or ablation of the NKCC1 gene significantly attenuated the rise in [Na+]i. Moreover, NKCC1+/+ astrocytes swelled by 10-30% during 20-60 min of OGD. Either genetic ablation of NKCC1 or inhibition of NKCC1 by bumetanide-attenuated OGD-mediated swelling. An NKCC1-mediated increase in [Na+]i may subsequently affect Ca2+ signaling through the Na+/Ca2+ exchanger (NCX). A rise in [Ca2+]i was detected after OGD/REOX in the presence of a sarcoplasmic-endoplasmic reticulum (ER) Ca2+-ATPase inhibitor thapsigargin. Moreover, OGD/REOX led to a significant increase in Ca2+ release from ER Ca2+ stores. Furthermore, KB-R7943 (2-[2-[4(4-nitrobenzyloxy)phenyl]ethyl]isothiourea mesylate), an inhibitor of reverse-mode operation of NCX, abolished the OGD/REOX-induced enhancement in filling of ER Ca2+ stores. OGD/REOX-mediated Ca2+ accumulation in ER Ca2+ stores was absent when NKCC1 activity was ablated or pharmacologically inhibited. These findings imply that stimulation of NKCC1 activity leads to Na+ accumulation after OGD/REOX and that subsequent reverse-mode operation of NCX contributes to increased Ca2+ accumulation by intracellular Ca2+ stores.

Stimulation of astrocyte Na+/H+ exchange activity in response to in vitro ischemia depends in part on activation of ERK1/2

We recently reported that Na+/H+ exchanger isoform 1 (NHE1) activity in astrocytes is stimulated and leads to intracellular Na+ loading after oxygen and glucose deprivation (OGD). However, the underlying mechanisms for this stimulation of NHE1 activity and its impact on astrocyte function are unknown. In the present study, we investigated the role of the ERK1/2 pathway in NHE1 activation. NHE1 activity was elevated by approximately 75% in NHE1+/+ astrocytes after 2-h OGD and 1-h reoxygenation (REOX). The OGD/REOX-mediated stimulation of NHE1 was partially blocked by 30 microM PD-98059. Increased expression of phosphorylated ERK1/2 was detected in NHE1+/+ astrocytes after OGD/REOX. Moreover, stimulation of NHE1 activity disrupted not only Na+ but also Ca2+ homeostasis via reverse-mode operation of Na+/Ca2+ exchange. OGD/REOX led to a 103% increase in intracellular Ca2+ concentration ([Ca2+]i) in NHE1+/+ astrocytes in the presence of thapsigargin. Inhibition of NHE1 activity with the NHE1 inhibitor HOE-642 decreased OGD/REOX-induced elevation of [Ca2+]i by 73%. To further investigate changes of Ca2+ signaling, bradykinin-mediated Ca2+ release was evaluated. Bradykinin-mediated intracellular Ca2+ transient in NHE1+/+ astrocytes was increased by approximately 84% after OGD/REOX. However, in NHE1-/- astrocytes or NHE1+/+ astrocytes treated with HOE-642, the bradykinin-induced Ca2+ release was increased by only approximately 34%. Inhibition of the reverse mode of Na+/Ca2+ exchange abolished OGD/REOX-mediated Ca2+ rise. Together, our data suggest that ERK1/2 is involved in activation of NHE1 in astrocytes after in vitro ischemia. NHE1-mediated Na+ accumulation subsequently alters Ca2+ homeostasis via Na+/Ca2+ exchange.

Na(+)-dependent chloride transporter (NKCC1)-null mice exhibit less gray and white matter damage after focal cerebral ischemia

We previously demonstrated that pharmacological inhibition of Na(+)-K(+)-Cl- cotransporter isoform 1 (NKCC1) is neuroprotective in in vivo and in vitro ischemic models. In this study, we investigated whether genetic ablation of NKCC1 provides neuroprotection after ischemia. Focal ischemia was induced by 2 hours occlusion of the left middle cerebral artery (MCAO) followed by 10 or 24 hours reperfusion. Two hours MCAO and ten or twenty-four hours reperfusion caused infarction (approximately 85 mm3) in NKCC1 wild-type (NKCC1(+/+)) mice. Infarction volume in NKCC1(-/-) mice was reduced by approximately 30% to 46%. Heterozygous mutant (NKCC1(+/-)) mice showed approximately 28% reduction in infarction (P>0.05). Two hours MCAO and twenty-four hours reperfusion led to a significant increase in brain edema in NKCC1(+/+) mice. In contrast, NKCC1(+/-) and NKCC1(-/-) mice exhibited approximately 50% less edema (P<0.05). Moreover, white matter damage was assessed by immunostaining of amyloid precursor protein (APP). An increase in APP was detected in NKCC1(+/+) mice after 2 hours MCAO and 10 hours reperfusion. However, NKCC1(-/-) mice exhibited significantly less APP accumulation (P<0.05). Oxygen-glucose deprivation (OGD) induced approximately 67% cell death and a fourfold increase in Na+ accumulation in cultured NKCC1(+/+) cortical neurons. OGD-mediated cell death and Na+ influx were significantly reduced in NKCC1(-/-) neurons (P<0.05). In addition, inhibition of NKCC1 by bumetanide resulted in similar protection in NKCC1(+/+) neurons and astrocytes (P<0.05). These results imply that stimulation of NKCC1 activity is important in ischemic neuronal damage.

Increased tolerance to oxygen and glucose deprivation in astrocytes from Na+/H+ exchanger isoform 1 null mice

The ubiquitously expressed Na+/H+ exchanger isoform 1 (NHE1) functions as a major intracellular pH (pHi) regulatory mechanism in many cell types, and in some tissues its activity may contribute to ischemic injury. In the present study, cortical astrocyte cultures from wild-type (NHE1+/+) and NHE1-deficient (NHE1−/−) mice were used to investigate the role of NHE1 in pHi recovery and ischemic injury in astrocytes. In the absence of HCO3−, the mean resting pHi levels were 6.86 ± 0.03 in NHE1+/+ astrocytes and 6.53 ± 0.04 in NHE1−/− astrocytes. Removal of extracellular Na+ or blocking of NHE1 activity by the potent NHE1 inhibitor HOE-642 significantly reduced the resting level of pHi in NHE1+/+ astrocytes. NHE1+/+ astrocytes exhibited a rapid pHi recovery (0.33 ± 0.08 pH unit/min) after NH4Cl prepulse acid load. The pHi recovery in NHE1+/+ astrocytes was reversibly inhibited by HOE-642 or removal of extracellular Na+. In NHE1−/− astrocytes, the pHi recovery after acidification was impaired and not affected by either Na+-free conditions or HOE-642. Furthermore, 2 h of oxygen and glucose deprivation (OGD) led to an ∼80% increase in pHi recovery rate in NHE1+/+ astrocytes. OGD induced a 5-fold rise in intracellular [Na+] and 26% swelling in NHE1+/+ astrocytes. HOE-642 or genetic ablation of NHE1 significantly reduced the Na+ rise and swelling after OGD. These results suggest that NHE1 is the major pHi regulatory mechanism in cortical astrocytes and that ablation of NHE1 in astrocytes attenuates ischemia-induced disruption of ionic regulation and swelling.

GABA-mediated trophic effect on oligodendrocytes requires Na-K-2Cl cotransport activity

The Na-K-2Cl cotransporter isoform1 (NKCC1) is present in many animal cells where it plays prominent roles in regulating cell volume and maintaining intracellular Cl(-) concentration ([Cl(-)]i) above electrochemical equilibrium. We show here that NKCC1 is present and active in cultured oligodendrocytes. Expression of NKCC1 in the rat spinal cord increased during development from postnatal day 6 through 21 in parallel with that of myelin basic protein. In cultured oligodendrocytes, 39% of the total K+ (86Rb+) influx represented NKCC1 activity. Activation of GABA(A) receptors with muscimol produced a reduction in intracellular Cl(-) content, cell shrinkage, and a stimulation of NKCC1 activity. Muscimol also triggered an increase in intracellular Ca2+, which depended on NKCC1 activity. Survival of oligodendrocytes following withdrawal of growth factors was enhanced by muscimol and this effect also required NKCC1 activity. Our results suggest that NKCC1 functions in oligodendrocytes to maintain [Cl(-)]i above electrochemical equilibrium and that NKCC1 is required for GABAergic trophic effects.

Na-K-Cl cotransporter contributes to glutamate-mediated excitotoxicity

We hypothesized that cation-dependent Cl- transport protein Na-K-Cl cotransporter isoform 1 (NKCC1) plays a role in the disruption of ion homeostasis in cerebral ischemia. In the current study, a role for NKCC1 in neuronal death was elucidated in neurotoxicity induced by glutamate and oxygen and glucose deprivation (OGD). Incubation of cortical neurons cultured for 14-15 d in vitro (DIV) with 100 microm glutamate for 24 hr resulted in 50% cell death. Three hours of OGD followed by 21 hr of reoxygenation led to 70% cell death. Inhibition of NMDA receptors with dizocilpine hydrogen maleate (1 microm) prevented both OGD- and glutamate-mediated cell death. Moreover, blocking of NKCC1 activity with bumetanide (5-10 microm) abolished glutamate- or OGD-induced neurotoxicity. Bumetanide was ineffective if added after 10-120 min of glutamate incubation or 3-6 hr of OGD treatment. Accumulation of intracellular Na+ and 36Cl content after NMDA receptor activation was inhibited by bumetanide. Blockage of NKCC1 significantly attenuated cell swelling after OGD or NMDA receptor activation. This neuroprotection was age dependent. Inhibition of NKCC1 did not protect DIV 7-8 neurons against OGD-mediated cell death. In contrast, cell death in DIV 7-8 neurons was prevented by the protein-synthesis inhibitor, cycloheximide. Taken together, the results suggest that NKCC1 activity is involved in the acute excitotoxicity as a result of excessive Na+ and Cl- entry and disruption of ion homeostasis.

Cross talk between the GABA(A) receptor and the Na-K-Cl cotransporter is mediated by intracellular Cl-

It has been suggested that the GABA(A) receptor-mediated depolarization in immature neurons depends on a high intracellular Cl(-) concentration maintained by Na-K-Cl cotransporter isoform1 (NKCC1). We previously found that activation of the GABA(A) receptor led to stimulation of NKCC1. This stimulation could be a result of GABA(A) receptor-mediated Cl(-) efflux. However, a loss of intracellular Cl(-) is associated with cell shrinkage, membrane depolarization, as well as a rise of intracellular Ca(2+) concentration ([Ca(2+)](i)). To determine which cellular mechanism is underlying NKCC1 stimulation, we investigated changes of intracellular Cl(-) content, [Ca(2+)](i), cell volume, and NKCC1 activity following GABA(A) receptor activation. The basal levels of intracellular (36)Cl were 0.70 +/- 0.04 micromol/mg protein. The intracellular (36)Cl content decreased to 0.53 +/- 0.03 micromol/mg protein in response to 30 microM muscimol (P < 0.05). The loss of intracellular (36)Cl was blocked by 10 microM bicuculline. Muscimol triggered a rise in [Ca(2+)](i), but did not cause cell shrinkage. In contrast, 10-50 mM [Cl(-)](o) or hypertonic HEPES-MEM resulted in reversible cell shrinkage (P < 0.05). Moreover, the GABA-mediated stimulation of NKCC1 activity was not abolished either by removal of extracellular Ca(2+) or BAPTA-AM. An increase in phosphorylation of NKCC1 was detected under both 10 mM [Cl(-)](o) and muscimol conditions. These results suggest that a GABA-mediated loss of intracellular Cl(-), but not a subsequent rise in [Ca(2+)](i) or shrinkage, leads to stimulation of NKCC1. This stimulation may be an important positive feedback mechanism to maintain intracellular Cl(-) level and GABA function in immature neurons.

Astrocytes from Na(+)-K(+)-Cl(-) cotransporter-null mice exhibit absence of swelling and decrease in EAA release

We reported previously that inhibition of Na(+)-K(+)-Cl(-) cotransporter isoform 1 (NKCC1) by bumetanide abolishes high extracellular K(+) concentration ([K(+)](o))-induced swelling and intracellular Cl(-) accumulation in rat cortical astrocytes. In this report, we extended our study by using cortical astrocytes from NKCC1-deficient (NKCC1(-/-)) mice. NKCC1 protein and activity were absent in NKCC1(-/-) astrocytes. [K(+)](o) of 75 mM increased NKCC1 activity approximately fourfold in NKCC1(+/+) cells (P < 0.05) but had no effect in NKCC1(-/-) astrocytes. Intracellular Cl(-) was increased by 70% in NKCC1(+/+) astrocytes under 75 mM [K(+)](o) (P < 0.05) but remained unchanged in NKCC1(-/-) astrocytes. Baseline intracellular Na(+) concentration ([Na(+)](i)) in NKCC1(+/+) astrocytes was 19.0 +/- 0.5 mM, compared with 16.9 +/- 0.3 mM [Na(+)](i) in NKCC1(-/-) astrocytes (P < 0.05). Relative cell volume of NKCC1(+/+) astrocytes increased by 13 +/- 2% in 75 mM [K(+)](o), compared with a value of 1.0 +/- 0.5% in NKCC1(-/-) astrocytes (P < 0.05). Regulatory volume increase after hypertonic shrinkage was completely impaired in NKCC1(-/-) astrocytes. High-[K(+)](o)-induced (14)C-labeled D-aspartate release was reduced by approximately 30% in NKCC1(-/-) astrocytes. Our study suggests that stimulation of NKCC1 is required for high-[K(+)](o)-induced swelling, which contributes to glutamate release from astrocytes under high [K(+)](o).

Contribution of Na(+)-K(+)-Cl(-) cotransporter to high-[K(+)](o)- induced swelling and EAA release in astrocytes

We hypothesized that high extracellular K(+) concentration ([K(+)](o))-mediated stimulation of Na(+)-K(+)-Cl(-) cotransporter isoform 1 (NKCC1) may result in a net gain of K(+) and Cl(-) and thus lead to high-[K(+)](o)-induced swelling and glutamate release. In the current study, relative cell volume changes were determined in astrocytes. Under 75 mM [K(+)](o,) astrocytes swelled by 20.2 +/- 4.9%. This high-[K(+)](o)-mediated swelling was abolished by the NKCC1 inhibitor bumetanide (10 microM, 1.0 +/- 3.1%; P < 0.05). Intracellular (36)Cl(-) accumulation was increased from a control value of 0.39 +/- 0.06 to 0.68 +/- 0.05 micromol/mg protein in response to 75 mM [K(+)](o). This increase was significantly reduced by bumetanide (P < 0.05). Basal intracellular Na(+) concentration ([Na(+)](i)) was reduced from 19.1 +/- 0.8 to 16.8 +/- 1.9 mM by bumetanide (P < 0.05). [Na(+)](i) decreased to 8.4 +/- 1.0 mM under 75 mM [K(+)](o) and was further reduced to 5.2 +/- 1.7 mM by bumetanide. In addition, the recovery rate of [Na(+)](i) on return to 5.8 mM [K(+)](o) was decreased by 40% in the presence of bumetanide (P < 0.05). Bumetanide inhibited high-[K(+)](o)-induced (14)C-labeled D-aspartate release by ~50% (P < 0.05). These results suggest that NKCC1 contributes to high-[K(+)](o)-induced astrocyte swelling and glutamate release.

31P-MRS-based determination of brain intracellular and interstitial pH: its application to in vivo H+ compartmentation and cellular regulation during hypoxic/ischemic conditions

In the last decade, significant progress has been made in the characterization of pH regulation in nervous tissue in vitro. However, little work has been directed at understanding how pH regulatory mechanisms function in vivo. We are interested in how ischemic acidosis can effect pH regulation and modulate the extent of post-ischemic brain damage. We used 31P-MRS to determine normal in vivo pH(i) and pH(e) simultaneously in both the isolated canine brain and the intact rat brain. We observed that the 31P(i) peak in the 31P-MRS spectrum is heterogeneous and can be deconvoluted into a number of discrete constituent peaks. In a series of experiments, we identified these peaks as arising from either extracellular or intracellular sources. In particular, we identified the peak representing the neurons and astrocytes and showed that they maintain different basal pH (6.95 and 7.05, respectively) and behave differently during hypoxic/ischemic episodes.

In vivo microdialysis of 2-deoxyglucose 6-phosphate into brain: a novel method for the measurement of interstitial pH using 31P-NMR

A unique method for simultaneously measuring interstitial (pHe) as well as intracellular (pHi) pH in the brains of lightly anesthetized rats is described. A 4-mm microdialysis probe was inserted acutely into the right frontal lobe in the center of the area sampled by a surface coil tuned for the collection of 31P-NMR spectra. 2-Deoxyglucose 6-phosphate (2-DG-6-P) was microdialyzed into the rat until a single NMR peak was detected in the phosphomonoester region of the 31P spectrum. pHe and pHi values were calculated from the chemical shift of 2-DG-6-P and inorganic phosphate, respectively, relative to the phosphocreatine peak. The average in vivo pHe was 7.24+/-0.01, whereas the average pHi was 7.05+/-0.01 (n = 7). The average pHe value and the average CSF bicarbonate value (23.5+/-0.1 mEq/L) were used to calculate an interstitial Pco2 of 55 mm Hg. Rats were then subjected to a 15-min period of either hypercapnia, by addition of CO2 (2.5, 5, or 10%) to the ventilator gases, or hypocapnia (PCO2 < 30 mm Hg), by increasing the ventilation rate and volume. pHe responded inversely to arterial Pco2 and was well described (r2 = 0.91) by the Henderson-Hasselbalch equation, assuming a pKa for the bicarbonate buffer system of 6.1 and a solubility coefficient for CO2 of 0.031. This confirms the view that the bicarbonate buffer system is dominant in the interstitial space. pHi responded inversely and linearly to arterial PCO2. The intracellular effect was muted as compared with pHe (slope = -0.0025, r2 = 0.60). pHe and pHi values were also monitored during the first 12 min of ischemia produced by cardiac arrest. pHe decreases more rapidly than pHi during the first 5 min of ischemia. After 12 min of ischemia, pHe and pHi values were not significantly different (6.44+/-0.02 and 6.44+/-0.03, respectively). The limitations, advantages, and future uses of the combined microdialysis/31P-NMR method for measurement of pHe and pHi are discussed.

NMR-based identification of intra- and extracellular compartments of the brain Pi peak

The Pi peak in a 31P NMR spectrum of the brain can be deconvoluted into six separate Lorentzian peaks with the same linewidth as that of the phosphocreatine peak in the spectrum. In an earlier communication we showed that the six Pi peaks in normal brain represent two extracellular and four intracellular compartments. In that report we have identified the first of the extracellular peaks by marking plasma with infused Pi, thereby substantially increasing the amplitude of the single peak at pH 7.35. 2-Deoxyglucose-6-phosphate (2-DG-6-P) was placed in the brain interstitial space by microdialysis. The resulting 2-DG-6-P peak was deconvoluted into three separate peaks. The chemical shift of the principle 2-DG-6-P peak gave a calculated pH of 7.24 +/- 0.02 for interstitial fluid pH, a value that agreed well with the pH of the second extracellular Pi peak at pH 7.25 +/- 0.01. We identified the intracellular compartments by selectively stressing cellular energy metabolism in three of the four intracellular spaces. A seizure-producing chemical, flurothyl, was used to activate the neuron, thereby causing a demand for energy that could not be completely met by oxidative phosphorylation alone. The resulting loss of high-energy phosphate reserves caused a significant increase in intracellular Pi only in those cells associated with the Pi peak at pH 6.95 +/- 0.01. This suggests that this compartment represents the neuron. Ammonia is detoxified in the astrocyte (glutamine synthetase) by incorporating it into glutamine, a process that requires large amounts of glucose and ATP. The intraarterial infusion of ammonium acetate into the brain stressed astrocyte energy metabolism resulting in an increase in the Pi of the cells at pH of 7.05 +/- 0.01 and 7.15 +/- 0.02. This finding, coupled with our observation that these same cells take up infused Pi probably via the astrocyte end-foot processes, lead us to conclude that these two compartments represent two different types of astrocytes, probably protoplasmic and fibrous, respectively. As a result of this study, we now believe the brain contains four extracellular and four intracellular compartments.

Hyperglycemic damage to mitochondrial membranes during cerebral ischemia: amelioration by platelet-activating factor antagonist BN 50739

The Pulsinelli-Brierley four-vessel occlusion model was used to study the consequences of hyperglycemic ischemia and reperfusion. Rats were subjected to either 30 min of normo- or hyperglycemic ischemia or 30 min of normo- or hyperglycemic ischemia followed by 60 min of reperfusion. In some animals, 2 mg/kg BN 50739, a platelet-activating factor receptor antagonist, was administered intraarterially either before or after the ischemic insult. The changes in mitochondrial membrane free fatty acid levels, phosphatidylcholine fatty acyl composition, and thiobarbituric acid-reactive material (TBAR) content plus the mitochondrial respiratory control ratio (RCR) were monitored. When the platelet-activating factor antagonist was present during normoglycemia, (a) the mitochondrial free fatty acid release both during and after ischemia was slowed, (b) reacylation of phosphatidylcholine following ischemia was promoted, and (c) TBAR accumulation during and following ischemia was decreased. The detrimental effects of hyperglycemia were muted when BN 50739 was present during ischemia. The RCR was preserved and phosphatidylcholine hydrolysis during ischemia was decreased. TBAR levels were consistently higher in hyperglycemic brain mitochondria both during and after ischemia. The RCR correlated directly with mitochondrial phosphatidylcholine polyunsaturated fatty acid content during ischemia and reperfusion. BN 50739 protection of mitochondrial membranes in brain may be influenced by tissue pH.

Evaluation of artificial plasma for maintaining the isolated canine brain

The use of canine erythrocytes suspended in artificial plasma to maintain the isolated brain was investigated in 18 preparations. Two plasmas were studied: One (AP1) contained electrolytes, amino acids, and albumin; the other (AP2) was similar to CSF and contained a mixture of 37 organic nutrients plus electrolytes and albumin. The CMRO2, CMRglu, and cerebral vascular resistance (CVR) were measured during 2 h of perfusion, and tissue high-energy phosphates were measured at the end of perfusion. The AP1 and AP2 groups were compared with control preparations perfused with canine red blood cells suspended in buffy coat-poor canine plasma. Both CMRO2 and ATP decreased to 60% of the control value; CVR increased to 187% of the control value in both groups following 2 h of perfusion. After 2 h of perfusion, the calculated value of intracellular pH (pHi)--based on creatine kinase equilibrium--remained normal (6.96) for the control brains, but decreased to 6.49 and 6.63, respectively, for the AP1- and AP2-perfused brains. Thus, there appears to be an eventual disruption of normal oxidative metabolism resulting in energy failure, possibly caused by the absence of an essential nutrient from the artificial plasma. For studies of intermediary metabolism in isolated normothermic brain, diluted whole blood appears to be the perfusate of choice.

Cerebral vascular resistance following platelet and leukocyte removal from perfusate

The rapid increases in cerebral vascular resistance (CVR) frequently observed during extracorporeal perfusion of isolated organs with whole or diluted blood have been investigated using the isolated canine brain preparation. Our data strongly suggest that these increases are caused by a vasoconstrictor that is present in the buffy coat. Plasma serotonin levels were measured and found to be insignificant after conditioning and storage. Conditioning the blood reduced the platelets and the potential for thromboxane A2 production to approximately 40% of normal. However, there was no correlation between thromboxane B2 levels and increases in CVR. Removal of the buffy coat effectively removed all of the leukocytes. Thus, one of the leukotrienes may have been responsible for the buffy coat-induced vasoconstriction.