Failure and function of intracellular pH regulation in acute hypoxic-ischemic injury of astrocytes

Potential for in vivo visualization of intracellular pH gradient in the brain using PET imaging

Intracellular pH is a valuable index for predicting neuronal damage and injury. However, no PET probe is currently available for monitoring intracellular pH in vivo. In this study, we developed a new approach for visualizing the hydrolysis rate of monoacylglycerol lipase, which is widely distributed in neurons and astrocytes throughout the brain. This approach uses PET with the new radioprobe [11C]QST-0837 (1,1,1,3,3,3-hexafluoropropan-2-yl-3-(1-phenyl-1H-pyrazol-3-yl)azetidine-1-[11C]carboxylate), a covalent inhibitor containing an azetidine carbamate skeleton for monoacylglycerol lipase. The uptake and residence of this new radioprobe depends on the intracellular pH gradient, and we evaluated this with in silico, in vitro and in vivo assessments. Molecular dynamics simulations predicted that because the azetidine carbamate moiety is close to that of water molecules, the compound containing azetidine carbamate would be more easily hydrolyzed following binding to monoacylglycerol lipase than would its analogue containing a piperidine carbamate skeleton. Interestingly, it was difficult for monoacylglycerol lipase to hydrolyze the azetidine carbamate compound under weakly acidic (pH 6) conditions because of a change in the interactions with water molecules on the carbamate moiety of their complex. Subsequently, an in vitro assessment using rat brain homogenate to confirm the molecular dynamics simulation-predicted behaviour of the azetidine carbamate compound showed that [11C]QST-0837 reacted with monoacylglycerol lipase to yield an [11C]complex, which was hydrolyzed to liberate 11CO2 as a final product. Additionally, the 11CO2 liberation rate was slower at lower pH. Finally, to indicate the feasibility of estimating how the hydrolysis rate depends on intracellular pH in vivo, we performed a PET study with [11C]QST-0837 using ischaemic rats. In our proposed in vivo compartment model, the clearance rate of radioactivity from the brain reflected the rate of [11C]QST-0837 hydrolysis (clearance through the production of 11CO2) in the brain, which was lower in a remarkably hypoxic area than in the contralateral region. In conclusion, we indicated the potential for visualization of the intracellular pH gradient in the brain using PET imaging, although some limitations remain. This approach should permit further elucidation of the pathological mechanisms involved under acidic conditions in multiple CNS disorders.

Inward Operation of Sodium-Bicarbonate Cotransporter 1 Promotes Astrocytic Na+ Loading and Loss of ATP in Mouse Neocortex during Brief Chemical Ischemia

Ischemic conditions cause an increase in the sodium concentration of astrocytes, driving the breakdown of ionic homeostasis and exacerbating cellular damage. Astrocytes express high levels of the electrogenic sodium-bicarbonate cotransporter1 (NBCe1), which couples intracellular Na+ homeostasis to regulation of pH and operates close to its reversal potential under physiological conditions. Here, we analyzed its mode of operation during transient energy deprivation via imaging astrocytic pH, Na+, and ATP in organotypic slice cultures of the mouse neocortex, complemented with patch-clamp and ion-selective microelectrode recordings and computational modeling. We found that a 2 min period of metabolic failure resulted in a transient acidosis accompanied by a Na+ increase in astrocytes. Inhibition of NBCe1 increased the acidosis while decreasing the Na+ load. Similar results were obtained when comparing ion changes in wild-type and Nbce1-deficient mice. Mathematical modeling replicated these findings and further predicted that NBCe1 activation contributes to the loss of cellular ATP under ischemic conditions, a result confirmed experimentally using FRET-based imaging of ATP. Altogether, our data demonstrate that transient energy failure stimulates the inward operation of NBCe1 in astrocytes. This causes a significant amelioration of ischemia-induced astrocytic acidification, albeit at the expense of increased Na+ influx and a decline in cellular ATP.

Dysregulation of Astrocyte Ion Homeostasis and Its Relevance for Stroke-Induced Brain Damage

Ischemic stroke is a leading cause of mortality and chronic disability. Either recovery or progression towards irreversible failure of neurons and astrocytes occurs within minutes to days, depending on remaining perfusion levels. Initial damage arises from energy depletion resulting in a failure to maintain homeostasis and ion gradients between extra- and intracellular spaces. Astrocytes play a key role in these processes and are thus central players in the dynamics towards recovery or progression of stroke-induced brain damage. Here, we present a synopsis of the pivotal functions of astrocytes at the tripartite synapse, which form the basis of physiological brain functioning. We summarize the evidence of astrocytic failure and its consequences under ischemic conditions. Special emphasis is put on the homeostasis and stroke-induced dysregulation of the major monovalent ions, namely Na+, K+, H+, and Cl-, and their involvement in maintenance of cellular volume and generation of cerebral edema.

Tissue Acidosis Associated with Ischemic Stroke to Guide Neuroprotective Drug Delivery

Simple Summary

Ischemic stroke is caused by the blockade of a blood vessel in the brain. Consequently, the brain region supplied by the blocked vessel suffers brain damage and becomes acidic. Here we provide a summary of the causes and consequences of acid accumulation in the brain tissue. Ischemic stroke requires immediate medical attention to minimize the damage of brain tissue, and to save function. It would be desirable for the medical treatment to target the site of injury selectively, to enrich the site of ongoing injury with the protective agent, and to avoid undesirable side effects at the same time. We propose that acid accumulation at the sight of brain tissue injury can be used to delineate the region that would benefit most from medical treatment. Tiny drug carriers known as nanoparticles may be loaded with drugs that protect the brain tissue. These nanoparticles may be designed to release their drug cargo in response to an acidic environment. This would ensure that the therapeutic agent is directed selectively to the site where it is needed. Ultimately, this approach may offer a new way to treat stroke patients with the hope of more effective therapy, and better stroke outcome.

Abstract

Ischemic stroke is a leading cause of death and disability worldwide. Yet, the effective therapy of focal cerebral ischemia has been an unresolved challenge. We propose here that ischemic tissue acidosis, a sensitive metabolic indicator of injury progression in cerebral ischemia, can be harnessed for the targeted delivery of neuroprotective agents. Ischemic tissue acidosis, which represents the accumulation of lactic acid in malperfused brain tissue is significantly exacerbated by the recurrence of spreading depolarizations. Deepening acidosis itself activates specific ion channels to cause neurotoxic cellular Ca²⁺ accumulation and cytotoxic edema. These processes are thought to contribute to the loss of the ischemic penumbra. The unique metabolic status of the ischemic penumbra has been exploited to identify the penumbra zone with imaging tools. Importantly, acidosis in the ischemic penumbra may also be used to guide therapeutic intervention. Agents with neuroprotective promise are suggested here to be delivered selectively to the ischemic penumbra with pH-responsive smart nanosystems. The administered nanoparticels release their cargo in acidic tissue environment, which reliably delineates sites at risk of injury. Therefore, tissue pH-targeted drug delivery is expected to enrich sites of ongoing injury with the therapeutical agent, without the risk of unfavorable off-target effects.

Roles of glial ion transporters in brain diseases

Glial ion transporters are important in regulation of ionic homeostasis, cell volume, and cellular signal transduction under physiological conditions of the central nervous system (CNS). In response to acute or chronic brain injuries, these ion transporters can be activated and differentially regulate glial functions, which has subsequent impact on brain injury or tissue repair and functional recovery. In this review, we summarized the current knowledge about major glial ion transporters, including Na⁺ /H⁺ exchangers (NHE), Na⁺ /Ca²⁺ exchangers (NCX), Na⁺ -K⁺ -Cl^- cotransporters (NKCC), and Na⁺ -HCO₃ ^- cotransporters (NBC). In acute neurological diseases, such as ischemic stroke and traumatic brain injury (TBI), these ion transporters are rapidly activated and play significant roles in regulation of the intra- and extracellular pH, Na⁺ , K⁺ , and Ca²⁺ homeostasis, synaptic plasticity, and myelin formation. However, overstimulation of these ion transporters can contribute to glial apoptosis, demyelination, inflammation, and excitotoxicity. In chronic brain diseases, such as glioma, Alzheimer's disease (AD), Parkinson's disease (PD), and multiple sclerosis (MS), glial ion transporters are involved in the glioma Warburg effect, glial activation, neuroinflammation, and neuronal damages. These findings suggest that glial ion transporters are involved in tissue structural and functional restoration, or brain injury and neurological disease development and progression. A better understanding of these ion transporters in acute and chronic neurological diseases will provide insights for their potential as therapeutic targets.

Acute sensitivity of astrocytes in the Substantia Nigra to oxygen and glucose deprivation (OGD) compared with hippocampal astrocytes in brain slices

The Substantia Nigra is a brainstem nucleus critical for movement control. Although its dopamine-producing neurons degenerate in Parkinsons disease, little is known of the acute effects of ischemia in this region. We recently reported that oxygen and glucose deprivation (OGD) in brain slices, an in vitro ischemia model, evokes a profound depolarization and swelling of GABAergic neurons in the Substantia Nigra pars reticulata (SNr), but not dopaminergic neurons in the Substantia Nigra pars compacta (SNc). The current study characterised the effects of OGD on nigral astrocytes, and compared these with the established responses of astrocytes in the CA1 hippocampal region. Intracellular recordings were made from astrocytes at the border between SNc and SNr subregions, in midbrain slices from postnatal day 21-23 rats. Immunoreactivity for astrocyte-specific proteins was also assessed. OGD evoked a slow, then fast depolarization of nigral astrocytes. The fast phase developed during the anoxic depolarization (indicated by a fast negative shift of extracellular DC potential and increase in light transmittance) and rapid increase in extracellular K⁺ concentration in the SNr. This biphasic response resembled the OGD-evoked depolarization of hippocampal astrocytes. However, unlike the partial repolarization seen in hippocampal cells after reperfusion with O₂ and glucose, nigral astrocytes remained depolarized near 0 mV. In addition, immunoreactivity for glial fibrillary acidic protein-positive astrocytes markedly decreased in the Substantia Nigra after OGD, while in the hippocampus remained unchanged. These data indicate an acute post-ischemic withdrawal of astrocytic support in the Substantia Nigra, but not in the hippocampus.

Intracellular pH Regulation in iPSCs-derived Astrocytes from Subjects with Chronic Mountain Sickness

Chronic Mountain Sickness (CMS) occurs in high-altitude residents with major neurological symptoms such as migraine headaches, dizziness and cognitive deficits. Recent work demonstrated that highlanders have increased intracellular pH (pH_i) in their brain cells, perhaps for the sake of adaptation to hypoxemia and help to facilitate glycolysis, DNA synthesis, and cell cycle progression. Since there are well adapted (non-CMS) and maladapted (CMS) high-altitude dwellers, it is not clear whether pH_i is differently regulated in these two high-altitude populations. In this work, we obtained induced pluripotent stem cell (iPSC)-derived astrocytes from both CMS and non-CMS highlanders who live in the Peruvian Andes (>14,000 ft) and studied pH_i regulation in these astrocytes using pH-sensitive dye BCECF. Our results show that the steady-state pH_i (ss pH_i) is lower in CMS astrocytes compared with non-CMS astrocytes. In addition, the acid extrusion following an acid loading is faster and the pH_i dependence of H⁺ flux rate becomes steeper in CMS astrocytes. Furthermore, the Na⁺ dependency of ss pH_i is stronger in CMS astrocytes and the Na⁺/H⁺ exchanger (NHE) inhibitors blunted the acid extrusion in both CMS and non-CMS astrocytes. We conclude that (a) NHE contributes to the ss pH_i stabilization and mediates active acid extrusion during the cytosolic acidosis in highlanders; (b) acid extrusion becomes less pH_i sensitive in non-CMS (versus CMS) astrocytes which may prevent NHE from over-activated in the hypoxia-induced intracellular acidosis and render the non-CMS astrocytes more resistant to hypoxemia challenges.

Knock-out of a mitochondrial sirtuin protects neurons from degeneration in Caenorhabditis elegans

Sirtuins are NAD⁺-dependent deacetylases, lipoamidases, and ADP-ribosyltransferases that link cellular metabolism to multiple intracellular pathways that influence processes as diverse as cell survival, longevity, and cancer growth. Sirtuins influence the extent of neuronal death in stroke. However, different sirtuins appear to have opposite roles in neuronal protection. In Caenorhabditis elegans, we found that knock-out of mitochondrial sirtuin sir-2.3, homologous to mammalian SIRT4, is protective in both chemical ischemia and hyperactive channel induced necrosis. Furthermore, the protective effect of sir-2.3 knock-out is enhanced by block of glycolysis and eliminated by a null mutation in daf-16/FOXO transcription factor, supporting the involvement of the insulin/IGF pathway. However, data in Caenorhabditis elegans cell culture suggest that the effects of sir-2.3 knock-out act downstream of the DAF-2/IGF-1 receptor. Analysis of ROS in sir-2.3 knock-out reveals that ROS become elevated in this mutant under ischemic conditions in dietary deprivation (DD), but to a lesser extent than in wild type, suggesting more robust activation of a ROS scavenging system in this mutant in the absence of food. This work suggests a deleterious role of SIRT4 during ischemic processes in mammals that must be further investigated and reveals a novel pathway that can be targeted for the design of therapies aimed at protecting neurons from death in ischemic conditions.

Glia-neuron interactions in neurological diseases: Testing non-cell autonomy in a dish

For the past century, research on neurological disorders has largely focused on the most prominently affected cell types - the neurons. However, with increasing knowledge of the diverse physiological functions of glial cells, their impact on these diseases has become more evident. Thus, many conditions appear to have more complex origins than initially thought. Since neurological pathologies are often sporadic with unknown etiology, animal models are difficult to create and might only reflect a small portion of patients in which a mutation in a gene has been identified. Therefore, reliable in vitro systems to studying these disorders are urgently needed. They might be a pre-requisite for improving our understanding of the disease mechanisms as well as for the development of potential new therapies. In this review, we will briefly summarize the function of different glial cell types in the healthy central nervous system (CNS) and outline their implication in the development or progression of neurological conditions. We will then describe different types of culture systems to model non-cell autonomous interactions in vitro and evaluate advantages and disadvantages. This article is part of a Special Issue entitled SI: Exploiting human neurons.

Acute neuroinflammation provokes intracellular acidification in mouse hippocampus

Background

Maintaining pH levels within the physiological norm is an important component of brain homeostasis. However, in some pathological or physiological conditions, the capacity of the pH regulatory system could be overpowered by various factors resulting in a transient or permanent alteration in pH levels. Such changes are often observed in pathological conditions associated with neuroinflammation. We hypothesized that neuroinflammation itself is a factor affecting pH levels in neural tissue. To assess this hypothesis, we examined the effects of acute LPS-induced neuroinflammation on intra- and extracellular pH (pHi and pHo) levels in the CA1 region of mouse hippocampus.

Methods

Acute neuroinflammation was induced using two approaches: (1) in vivo by i.p. injections of LPS (5 mg/kg) and (2) in vitro by incubating hippocampal slices of naïve animals in the LPS-containing media (1 μg/mL, 1 h at 35 °C). Standard techniques were used to prepare hippocampal slices. pHi was measured using ratiometric pH-sensitive fluorescent dye BCECF-AM. pHo was assessed using calibrated pH-sensitive micropipettes. The presence of neuroinflammation was verified with immunohistochemistry (IL-1β and Iba1) and ELISA (IL-1β and TNF-α).

Results

A significant reduction of pHi was observed in the slices of the LPS-injected 3-month-old (LPS 7.13 ± 0.03; Sal 7.22 ± 0.03; p = 0.043, r = 0.43) and 19-month-old (LPS 6.78 ± 0.08; Sal 7.13 ± 0.03; p = 0.0001, r = 0.32) mice. In contrast, the levels of pHo within the slice, measured in 19-month-old animals, were not affected (LPS 7.27 ± 0.02; Sal 7.26 ± 0.02; p = 0.6, r = 0.13). A reduction of pHi was also observed in the LPS-treated slices during the interval 3.5–7 h after the LPS exposure (LPS 6.92 ± 0.07; Veh 7.28 ± 0.05; p = 0.0001, r = 0.46).

Conclusions

Acute LPS-induced neuroinflammation results in a significant intracellular acidification of the CA1 neurons in mouse hippocampus, while the pHo remains largely unchanged. Such changes may represent a specific protective reaction of neural tissue in unfavorable external conditions or be a part of the pathological process.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-016-0747-8) contains supplementary material, which is available to authorized users.

The Na+/HCO3- co-transporter is protective during ischemia in astrocytes

The sodium bicarbonate co-transporter (NBC) is the major bicarbonate-dependent acid-base transporter in mammalian astrocytes and has been implicated in ischemic brain injury. A malfunction of astrocytes could have great impact on the outcome of stroke due to their participation in the formation of blood-brain barrier, synaptic transmission, and electrolyte balance in the human brain. Nevertheless, the role of NBC in the ischemic astrocyte death has not been well understood. In this work, we obtained skin biopsies from healthy human subjects and had their fibroblasts grown in culture and reprogrammed into human-induced pluripotent stem cells (hiPSCs). These hiPSCs were further differentiated into neuroprogenitor cells (NPCs) and then into human astrocytes. These astrocytes express GFAP and S100β and readily propagate calcium waves upon mechanical stimulation. Using pH-sensitive dye BCECF [2',7'-bis-(carboxyethyl)-5-(and-6)-carboxyfluorescein] and qPCR technique, we have confirmed that these astrocytes express functional NBC including electrogenic NBC (NBCe). In addition, astrocytes exposed to an ischemic solution (IS) that mimics the ischemic penumbral environment enhanced both mRNA and protein expression level of NBCe1 in astrocytes. Using IS and a generic NBC blocker S0859, we have studied the involvement of NBC in IS-induced human astrocytes death. Our results show that a 30μM S0859 induced a 97.5±1.6% (n=10) cell death in IS-treated astrocytes, which is significantly higher than 43.6±4.5%, (n=10) in the control group treated with IS alone. In summary, a NBC blocker exaggerates IS-induced cell death, suggesting that NBC activity is essential for astrocyte survival when exposed to ischemic penumbral environment.

Global brain blood-oxygen level responses to autonomic challenges in obstructive sleep apnea

Obstructive sleep apnea (OSA) is accompanied by brain injury, perhaps resulting from apnea-related hypoxia or periods of impaired cerebral perfusion. Perfusion changes can be determined indirectly by evaluation of cerebral blood volume and oxygenation alterations, which can be measured rapidly and non-invasively with the global blood oxygen level dependent (BOLD) signal, a magnetic resonance imaging procedure. We assessed acute BOLD responses in OSA subjects to pressor challenges that elicit cerebral blood flow changes, using a two-group comparative design with healthy subjects as a reference. We separately assessed female and male patterns, since OSA characteristics and brain injury differ between sexes. We studied 94 subjects, 37 with newly-diagnosed, untreated OSA (6 female (age mean ± std: 52.1±8.1 yrs; apnea/hypopnea index [AHI]: 27.7±15.6 events/hr and 31 male 54.3±8.4 yrs; AHI: 37.4±19.6 events/hr), and 20 female (age 50.5±8.1 yrs) and 37 male (age 45.6±9.2 yrs) healthy control subjects. We measured brain BOLD responses every 2 s while subjects underwent cold pressor, hand grip, and Valsalva maneuver challenges. The global BOLD signal rapidly changed after the first 2 s of each challenge, and differed in magnitude between groups to two challenges (cold pressor, hand grip), but not to the Valsalva maneuver (repeated measures ANOVA, p<0.05). OSA females showed greater differences from males in response magnitude and pattern, relative to healthy counterparts. Cold pressor BOLD signal increases (mean ± adjusted standard error) at the 8 s peak were: OSA 0.14±0.08% vs. Control 0.31±0.06%, and hand grip at 6 s were: OSA 0.08±0.03% vs. Control at 0.30±0.02%. These findings, indicative of reduced cerebral blood flow changes to autonomic challenges in OSA, complement earlier reports of altered resting blood flow and reduced cerebral artery responsiveness. Females are more affected than males, an outcome which may contribute to the sex-specific brain injury in the syndrome.

Heart rate responses to autonomic challenges in obstructive sleep apnea

Obstructive sleep apnea (OSA) is accompanied by structural alterations and dysfunction in central autonomic regulatory regions, which may impair dynamic and static cardiovascular regulation, and contribute to other syndrome pathologies. Characterizing cardiovascular responses to autonomic challenges may provide insights into central nervous system impairments, including contributions by sex, since structural alterations are enhanced in OSA females over males. The objective was to assess heart rate responses in OSA versus healthy control subjects to autonomic challenges, and, separately, characterize female and male patterns. We studied 94 subjects, including 37 newly-diagnosed, untreated OSA patients (6 female, age mean ± std: 52.1 ± 8.1 years; 31 male aged 54.3 ± 8.4 years), and 57 healthy control subjects (20 female, 50.5 ± 8.1 years; 37 male, 45.6 ± 9.2 years). We measured instantaneous heart rate with pulse oximetry during cold pressor, hand grip, and Valsalva maneuver challenges. All challenges elicited significant heart rate differences between OSA and control groups during and after challenges (repeated measures ANOVA, p<0.05). In post-hoc analyses, OSA females showed greater impairments than OSA males, which included: for cold pressor, lower initial increase (OSA vs. control: 9.5 vs. 7.3 bpm in females, 7.6 vs. 3.7 bpm in males), OSA delay to initial peak (2.5 s females/0.9 s males), slower mid-challenge rate-of-increase (OSA vs. control: -0.11 vs. 0.09 bpm/s in females, 0.03 vs. 0.06 bpm/s in males); for hand grip, lower initial peak (OSA vs. control: 2.6 vs. 4.6 bpm in females, 5.3 vs. 6.0 bpm in males); for Valsalva maneuver, lower Valsalva ratio (OSA vs. control: 1.14 vs. 1.30 in females, 1.29 vs. 1.34 in males), and OSA delay during phase II (0.68 s females/1.31 s males). Heart rate responses showed lower amplitude, delayed onset, and slower rate changes in OSA patients over healthy controls, and impairments may be more pronounced in females. The dysfunctions may reflect central injury in the syndrome, and suggest autonomic deficiencies that may contribute to further tissue and functional pathologies.

Hypoxia promotes tumor cell survival in acidic conditions by preserving ATP levels

The efficacy of targeting pH disruption to induce cell death in the acidic and hypoxic tumor microenvironment continues to be assessed. Here we analyzed the impact of varying levels of hypoxia in acidic conditions on fibroblast and tumor cell survival. Across all cell lines tested, hypoxia (1% O(2)) provided protection against acidosis induced cell death compared to normoxia. Meanwhile severe hypoxia (0.1% O(2)) removed this protection and in some cases exacerbated acidosis-induced cell death. Differential survival between cell types during external acidosis correlated with their respective intracellular pH regulating capabilities. Cellular ATP measurements were conducted to determine their contribution to cell survival under these combined stresses. In general, hypoxia (1% O(2)) maintained elevated ATP levels in acidic conditions while severe hypoxia did not. To further explore this interaction we combined acidosis with ATP depletion using 2-deoxyglucose and observed an enhanced rate of cell mortality. Striking results were also observed with hypoxia providing protection against cell death in spite of a severe metabolic stress induced by a combination of acidosis and oligomycin. Finally, we demonstrated that both HIF1α and HIF2α expression were drastically reduced in hypoxic and acidic conditions indicating a sensitivity of this protein to cellular pH conditions. This knockdown of HIF expression by acidosis has implications for the development of therapies targeting the disruption of cellular pH regulation. Our results reinforce the proof of concept that acidosis and metabolic disruption affecting ATP levels could be exploited as a tumor cell killing strategy.

Two sides of the same coin: sodium homeostasis and signaling in astrocytes under physiological and pathophysiological conditions

The intracellular sodium concentration of astrocytes is classically viewed as being kept under tight homeostatic control and at a relatively stable level under physiological conditions. Indeed, the steep inwardly directed electrochemical gradient for sodium, generated by the Na⁺/K⁺-ATPase, contributes to maintain the electrochemical gradient of K⁺ and the highly K⁺-based negative membrane potential, and is a central element in energizing membrane transport. As such it is tightly coupled to the homeostasis of extra- and intracellular potassium, calcium or pH and to the reuptake of transmitters such as glutamate. Recent studies, however, have demonstrated that this picture is far too simplistic. It is now firmly established that transmitters, most notably glutamate, and excitatory neuronal activity evoke long-lasting sodium transients in astrocytes, the properties of which are distinctly different from those of activity-related glial calcium signals. From these studies, it emerges that sodium homeostasis and signaling are two sides of the same coin: sodium-dependent transporters, primarily known for their role in ion regulation and homeostasis, also generate relevant ion signals during neuronal activity. The functional consequences of activity-related sodium transients are manifold and are just coming into view, enabling surprising and important new insights into astrocyte function and neuron-glia interaction in the brain. The present review will highlight current knowledge about the mechanisms that contribute to sodium homeostasis in astrocytes, present recent data on the spatial and temporal properties of activity-related glial sodium signals and discuss their functional consequences with a special emphasis on pathophysiological conditions.

Persistent astroglial swelling accompanies rapid reversible dendritic injury during stroke-induced spreading depolarizations

Spreading depolarizations are a key event in the pathophysiology of stroke, resulting in rapid dendritic beading, which represents acute damage to synaptic circuitry. The impact of spreading depolarizations on the real-time injury of astrocytes during ischemia is less clear. We used simultaneous in vivo 2-photon imaging and electrophysiological recordings in adult mouse somatosensory cortex to examine spreading depolarization-induced astroglial structural changes concurrently with signs of neuronal injury in the early periods of focal and global ischemia. Astrocytes in the metabolically compromised ischemic penumbra-like area showed a long lasting swelling response to spontaneous spreading depolarizations despite rapid dendritic recovery in a photothrombotic occlusion model of focal stroke. Astroglial swelling was often facilitated by recurrent depolarizations and the magnitude of swelling strongly correlated with the total duration of depolarization. In contrast, spreading depolarization-induced astroglial swelling was transient in normoxic healthy tissue. In a model of transient global ischemia, the occurrence of a single spreading depolarization elicited by a bilateral common carotid artery occlusion coincided with astroglial swelling alongside dendritic beading. With immediate reperfusion, dendritic beading subsides. Astroglial swelling was either transient during short ischemic periods distinguished by a short-lasting spreading depolarization, or persistent during severe ischemia characterized by a long-lasting depolarization with the ultraslow negative voltage component. We propose that persistent astroglial swelling is initiated and exacerbated during spreading depolarization in brain tissue with moderate to severe energy deficits, disrupting astroglial maintenance of normal homeostatic function thus contributing to the negative outcome of ischemic stroke as astrocytes fail to provide neuronal support.

Neurological diseases as primary gliopathies: a reassessment of neurocentrism

Diseases of the human brain are almost universally attributed to malfunction or loss of nerve cells. However, a considerable amount of work has, during the last decade, expanded our view on the role of astrocytes in CNS (central nervous system), and this analysis suggests that astrocytes contribute to both initiation and propagation of many (if not all) neurological diseases. Astrocytes provide metabolic and trophic support to neurons and oligodendrocytes. Here, we shall endeavour a broad overviewing of the progress in the field and forward the idea that loss of homoeostatic astroglial function leads to an acute loss of neurons in the setting of acute insults such as ischaemia, whereas more subtle dysfunction of astrocytes over periods of months to years contributes to epilepsy and to progressive loss of neurons in neurodegenerative diseases. The majority of therapeutic drugs currently in clinical use target neuronal receptors, channels or transporters. Future therapeutic efforts may benefit by a stronger focus on the supportive homoeostatic functions of astrocytes.

Cultured astrocytes do not release adenosine during hypoxic conditions

Recent reports based on a chemiluminescent enzymatic assay for detection of adenosine conclude that cultured astrocytes release adenosine during mildly hypoxic conditions. If so, astrocytes may suppress neural activity in early stages of hypoxia. The aim of this study was to reevaluate the observation using high-performance liquid chromatography (HPLC). The HPLC analysis showed that exposure to 20 or 120 minutes of mild hypoxia failed to increase release of adenosine triphosphate (ATP), adenosine diphosphate (ADP), adenosine monophosphate (AMP), and adenosine from cultured astrocytes. Similar results were obtained using a chemiluminescent enzymatic assay. Moreover, since the chemiluminescent enzymatic assay relies on hydrogen peroxide generation, release of free-radical scavengers from hypoxic cells can interfere with the assay. Accordingly, adenosine added to samples collected from hypoxic cultures could not be detected using the chemiluminescent enzymatic assay. Furthermore, addition of free-radical scavengers sharply reduced the sensitivity of adenosine detection. Conversely, use of a single-step assay inflated measured values due to the inability of the assay to distinguish adenosine and its metabolite inosine. These results show that cultured astrocytes do not release adenosine during mild hypoxia, an observation consistent with their high resistance to hypoxia.

Effects of intracellular acidosis on endothelial function: an overview

The endothelium represents the largest functional organ in the human body playing an active role in vasoregulation, coagulation, inflammation, and microvascular permeability. Endothelium contributes to maintain vascular integrity, intravascular volume, and tissue oxygenation promoting inflammatory network response for local defense and repair. Acid-basis homeostasis is an important physiologic parameter that controls cell function, and changes in pH can influence vascular tone by regulating endothelium and vascular smooth muscle cells. This review presents a current perspective of the effects of intracellular acidosis on the function and the basic regulatory mechanisms of endothelial cells.

A combined a-EEG and MR spectroscopy study in term newborns with hypoxic-ischemic encephalopathy

OBJECTIVES

Brain damage following a perinatal hypoxic-ischemic (HI) insult has been documented by different diagnostic techniques. The aim of the present study was to relate a-EEG time course during the first 24h of life to brain metabolic changes detected by proton MR spectroscopy ((1)H-MRS) at 7-10days of life and to evaluate their correlation with outcome.

METHODS

Thirty-two patients with any grade HI encephalopathy were studied. Thirty-one out of 32 patients survived and underwent (1)H-MRS examination at 7-10days of life; a-EEG was recorded during the first 24h of life in 27/32 newborns; 26 patients underwent both examinations. Griffiths test, evaluation of motor skills, visual and hearing function were performed at regular intervals until the age of 2years.

RESULTS

a-EEG at 6, 12 and 24h of life showed a significant correlation with outcome. N-acetyl-aspartate/creatine (Cr), Lactate/Cr and myo-inositol differed significantly between patients with normal or poor outcome. a-EEG time course during the first 24h of life showed improvement in newborns with normal (1)H-MRS and good outcome and a deterioration in those with abnormal (1)H-MRS and poor outcome.

CONCLUSIONS

a-EEG time course may be able to document the severity and the evolution of the cerebral damage following an HI event. a-EEG is related to the severity of cerebral injury as defined by (1)H-MRS and both examinations showed a good correlation with outcome. These data, obtained in non-cooled infants, may represent reference data for future investigations in cooled infants.

Ischemic post-conditioning to counteract intestinal ischemia/reperfusion injury

Intestinal ischemia is a severe disorder with a variety of causes. Reperfusion is a common occurrence during treatment of acute intestinal ischemia but the injury resulting from ischemia/reperfusion (IR) may lead to even more serious complications from intestinal atrophy to multiple organ failure and death. The susceptibility of the intestine to IR-induced injury (IRI) appears from various experimental studies and clinical settings such as cardiac and major vascular surgery and organ transplantation. Whereas oxygen free radicals, activation of leukocytes, failure of microvascular perfusion, cellular acidosis and disturbance of intracellular homeostasis have been implicated as important factors in the pathogenesis of intestinal IRI, the mechanisms underlying this disorder are not well known. To date, increasing attention is being paid in animal studies to potential pre- and post-ischemia treatments that protect against intestinal IRI such as drug interference with IR-induced apoptosis and inflammation processes and ischemic pre-conditioning. However, better insight is needed into the molecular and cellular events associated with reperfusion-induced damage to develop effective clinical protection protocols to combat this disorder. In this respect, the use of ischemic post-conditioning in combination with experimentally prolonged acidosis blocking deleterious reperfusion actions may turn out to have particular clinical relevance.

Targeting astrocytes for stroke therapy

Stroke remains a major health problem and is a leading cause of death and disability. Past research and neurotherapeutic clinical trials have targeted the molecular mechanisms of neuronal cell death during stroke, but this approach has uniformly failed to reduce stroke-induced damage or to improve functional recovery. Beyond the intrinsic molecular mechanisms inducing neuronal death during ischemia, survival and function of astrocytes is absolutely required for neuronal survival and for functional recovery after stroke. Many functions of astrocytes likely improve neuronal viability during stroke. For example, uptake of glutamate and release of neurotrophins enhances neuronal viability during ischemia. Under certain conditions, however, astrocyte function may compromise neuronal viability. For example, astrocytes may produce inflammatory cytokines or toxic mediators, or may release glutamate. The only clinical neurotherapeutic trial for stroke that specifically targeted astrocyte function focused on reducing release of S-100β from astrocytes, which becomes a neurotoxin when present at high levels. Recent work also suggests that astrocytes, beyond their influence on cell survival, also contribute to angiogenesis, neuronal plasticity, and functional recovery in the several days to weeks after stroke. If these delayed functions of astrocytes could be targeted for enhancing stroke recovery, it could contribute importantly to improving stroke recovery. This review focuses on both the positive and the negative influences of astrocytes during stroke, especially as they may be targeted for translation to human trials.

Electrical coupling of astrocytes in rat hippocampal slices under physiological and simulated ischemic conditions

Mammalian protoplasmic astrocytes are extensively coupled through gap junction channels but the biophysical properties of these channels under physiological and ischemic conditions in situ are not well defined. Using confocal morphometric analysis of biocytin-filled astrocytic syncytia in rat hippocampal CA1 stratum radiatum we found that each astrocyte directly couples, on average, to 11 other astrocytes with a mean interastrocytic distance of 45 microm. Voltage-independent and bidirectional transjunctional currents were always measured between directly coupled astrocyte pairs in dual voltage-clamp recordings, but never from astrocyte-NG2 glia or astrocyte-interneuron pairs. The electrical coupling ratio varied considerably among astrocytes in developing postnatal day 14 rats (P14, 0.5-12.4%, mean = 3.6%), but became more constant in young adult P21 rats (0.18-3.9%, mean = 1.6%), and the coupling ratio declined exponentially with increasing pair distance. Electrical coupling was not affected by short-term oxygen-glucose deprivation (OGD) treatment, but showed delayed inhibition in an acidic extracellular pH of 6.4. Combination of acidic pH (6.4) and OGD, a condition that better represents cerebral ischemia in vivo, accelerated the inhibition of electrical coupling. Our results show that, under physiological conditions, 20.7-24.2% of K(+) induced currents can travel from any astrocytic soma in CA1 stratum radiatum to the gap junctions of the nearest neighbor astrocytes, but this should be severely inhibited as a consequence of the OGD and acidosis seen in the ischemic brain.

Unusual stability of human neuroglobin at low pH--molecular mechanisms and biological significance

Neuroglobin (Ngb) is a recently discovered globin that is predominantly expressed in the brain, retina and other nerve tissues of human and other vertebrates. Ngb has been shown to act as a neuroprotective factor, promoting neuronal survival in conditions of hypoxic-ischemic insult, such as those occurring during stroke. In this work, the conformational and functional stability of Ngb at acidic pH was analyzed, and the results were compared to those obtained with Mb. It was shown by spectroscopic and biochemical (limited proteolysis) techniques that, at pH 2.0, apoNgb is a folded and rigid protein, retaining most of the structural features that the protein displays at neutral pH. Conversely, apoMb, under the same experimental conditions of acidic pH, is essentially a random coil polypeptide. Urea-mediated denaturation studies revealed that the stability displayed by apoNgb at pH 2.0 is very similar to that of Mb at pH 7.0. Ngb also shows enhanced functional stability as compared with Mb, being capable of heme binding over a more acidic pH range than Mb. Furthermore, Ngb reversibly binds oxygen at acidic pH, with an affinity that increases as the pH is decreased. It is proposed that the acid-stable fold of Ngb depends on the particular amino acid composition of the protein polypeptide chain. The functional stability at low pH displayed by Ngb was instead shown to be related to hexacoordination of the heme group. The biological implications of the unusual acid resistance of the folding and function of Ngb are discussed.

The annexins: spatial and temporal coordination of signaling events during cellular stress

Annexins are a family of structurally related, Ca2+-sensitive proteins that bind to negatively charged phospholipids and establish specific interactions with other lipids and lipid microdomains. They are present in all eukaryotic cells and share a common folding motif, the "annexin core", which incorporates Ca2+- and membrane-binding sites. Annexins participate in a variety of intracellular processes, ranging from the regulation of membrane dynamics to cell migration, proliferation, and apoptosis. Here we focus on the role of annexins in cellular signaling during stress. A chronic stress response triggers the activation of different intracellular pathways, resulting in profound changes in Ca2+ and pH homeostasis and the production of lipid second messengers. We review the latest data on how these changes are sensed by the annexins, which have the ability to simultaneously interact with specific lipid and protein moieties at the plasma membrane, contributing to stress adaptation via regulation of various signaling pathways.

Intestinal ischemia-reperfusion injury: reversible and irreversible damage imaged in vivo

The early events in an intestinal ischemic episode have been difficult to evaluate. Using in vivo microscopy we have analyzed in real-time the effects of short (15 min) and long (40-50 min) ischemia with subsequent reperfusion (IR), evaluating structure, integrity, and functioning of the mouse jejunal mucosa while monitoring blood flow by confocal microscopy. IR was imposed by inflation/deflation of a vascular occluder, and blood flow was monitored and confirmed with scanning confocal imaging. After short ischemia, villus tip cells revealed a rapid increase (23%) in the intracellular NAD(P)H concentration (confocal autofluorescence microscopy), and the pH-sensitive probe BCECF showed a biphasic response of the intracellular pH (pH(i)), quickly alkalinizing from the resting value of 6.8 +/- 0.1 to 7.1 +/- 0.1 but then strongly acidifying to 6.3 +/- 0.1. Upon reperfusion, values returned toward control. In contrast, results were heterogeneous after long IR. During long ischemia, one-third of the epithelial cells remained viable with reversible changes upon reperfusion, but remaining cells lost membrane integrity (Lucifer Yellow uptake, LY) and had membrane blebs during ischemia. These effects became more pronounced as the reperfusion interval progressed when cells exhibited more severely affected NAD(P)H and pH(i) values, larger blebs, and more LY uptake and eventually were shed from the villus. Results from stereo microscopy suggest that these irreversible effects of IR may have occurred as a result of incomplete restorations of local blood flow, especially at the antimesenteric side of the intestine. We conclude that the adverse effects of short ischemia on the jejunum epithelium are fully reversible during the reperfusion interval. However, after long ischemia, reperfusion cannot restore normal structure and functioning of a majority of cells, which deteriorate further. Our results provide a basis for defining the cellular events that cause tissue to transit from reversible to irreversible damage during IR.

Modulation of brain hemichannels and gap junction channels by pro-inflammatory agents and their possible role in neurodegeneration

In normal brain, neurons, astrocytes, and oligodendrocytes, the most abundant and active cells express pannexins and connexins, protein subunits of two families forming membrane channels. Most available evidence indicates that in mammals endogenously expressed pannexins form only hemichannels and connexins form both gap junction channels and hemichannels. Whereas gap junction channels connect the cytoplasm of contacting cells and coordinate electric and metabolic activity, hemichannels communicate the intra- and extracellular compartments and serve as a diffusional pathway for ions and small molecules. A subthreshold stimulation by acute pathological threatening conditions (e.g., global ischemia subthreshold for cell death) enhances neuronal Cx36 and glial Cx43 hemichannel activity, favoring ATP release and generation of preconditioning. If the stimulus is sufficiently deleterious, microglia become overactivated and release bioactive molecules that increase the activity of hemichannels and reduce gap junctional communication in astroglial networks, depriving neurons of astrocytic protective functions, and further reducing neuronal viability. Continuous glial activation triggered by low levels of anomalous proteins expressed in several neurodegenerative diseases induce glial hemichannel and gap junction channel disorders similar to those of acute inflammatory responses triggered by ischemia or infectious diseases. These changes are likely to occur in diverse cell types of the CNS and contribute to neurodegeneration during inflammatory process.

Traumatic brain injury-induced expression and phosphorylation of pyruvate dehydrogenase: a mechanism of dysregulated glucose metabolism

Dysregulated brain glucose metabolism and lactate accumulation are seen following traumatic brain injury (TBI). The underlying molecular mechanism is poorly understood. Pyruvate dehydrogenase (PDH), the rate-limiting enzyme coupling cytosolic glycolysis to mitochondrial citric acid cycle, plays a critical role in maintaining homeostasis of brain glucose metabolism. PDH activity is maintained by the expression of its E1alpha1 subunit 1 (PDHE1alpha1) and is inhibited by the phosphorylation of PDHE1alpha1 (p-PDHE1alpha1). We hypothesized that PDHE1alpha1 expression and phosphorylation was altered in rat brain following controlled cortical impact (CCI)-induced TBI. Compared to naïve controls (=100%), PDHE1alpha1 protein decreased significantly ipsilateral to CCI (62%, P<0.05; 75%, P<0.05; 57%, P<0.05; and 39%, P<0.01) and contralateral to CCI (77%, 78%, 78% and 36% P<0.01) at 4h, 24h, 3- and 7-day post-CCI, respectively. PDHE1alpha1 protein phosphorylation level also decreased significantly ipsilateral to CCI (31%, P<0.01; 102%, P>0.05; 64%, P<0.05; and 14%, P<0.01) and to contralateral CCI (35%, 74%, P<0.05; 60%, P<0.05; 20%, P<0.01) at 4h, 24h, 3- and 7-day post-CCI, respectively. Similar reduction in PDHE1alpha1 and p-PDHE1alpha1 protein was found in the craniotomy (sham CCI) group. TBI-induced change in PDHE1alpha1 expression and phosphorylation could alter brain PDH activity and glucose metabolism.

Differential influence of arterial blood glucose on cerebral metabolism following severe traumatic brain injury

INTRODUCTION

Maintaining arterial blood glucose within tight limits is beneficial in critically ill patients. Upper and lower limits of detrimental blood glucose levels must be determined.

METHODS

In 69 patients with severe traumatic brain injury (TBI), cerebral metabolism was monitored by assessing changes in arterial and jugular venous blood at normocarbia (partial arterial pressure of carbon dioxide (paCO2) 4.4 to 5.6 kPa), normoxia (partial arterial pressure of oxygen (paO2) 9 to 20 kPa), stable haematocrit (27 to 36%), brain temperature 35 to 38 degrees C, and cerebral perfusion pressure (CPP) 70 to 90 mmHg. This resulted in a total of 43,896 values for glucose uptake, lactate release, oxygen extraction ratio (OER), carbon dioxide (CO2) and bicarbonate (HCO3) production, jugular venous oxygen saturation (SjvO2), oxygen-glucose index (OGI), lactate-glucose index (LGI) and lactate-oxygen index (LOI). Arterial blood glucose concentration-dependent influence was determined retrospectively by assessing changes in these parameters within pre-defined blood glucose clusters, ranging from less than 4 to more than 9 mmol/l.

RESULTS

Arterial blood glucose significantly influenced signs of cerebral metabolism reflected by increased cerebral glucose uptake, decreased cerebral lactate production, reduced oxygen consumption, negative LGI and decreased cerebral CO2/HCO3 production at arterial blood glucose levels above 6 to 7 mmol/l compared with lower arterial blood glucose concentrations. At blood glucose levels more than 8 mmol/l signs of increased anaerobic glycolysis (OGI less than 6) supervened.

CONCLUSIONS

Maintaining arterial blood glucose levels between 6 and 8 mmol/l appears superior compared with lower and higher blood glucose concentrations in terms of stabilised cerebral metabolism. It appears that arterial blood glucose values below 6 and above 8 mmol/l should be avoided. Prospective analysis is required to determine the optimal arterial blood glucose target in patients suffering from severe TBI.

Ouabain-induced stimulation of sodium-hydrogen exchange in rat optic nerve astrocytes

Sodium-dependent transporters are inhibited indirectly by the Na-K-ATPase inhibitor ouabain. Here we report stimulation of sodium-hydrogen exchange (NHE) in ouabain-treated cells. BCECF was used to measure cytoplasmic pH in cultured rat optic nerve astrocytes. Ammonium chloride was applied to acid load the cells. On removal of ammonium chloride, cytoplasmic pH fell abruptly, then gradually recovered toward baseline. Ouabain (1 microM) did not change cell sodium content, but the rate of pH recovery increased by 68%. Ouabain speeded pH recovery both in the presence and absence of bicarbonate. In bicarbonate-free medium, dimethylamiloride, an NHE inhibitor, eliminated the effect of 1 microM ouabain on pH recovery. Western blot analysis showed an NHE1 immunoreactive band but not NHE2, NHE3, or NHE4. Immunoprecipitation studies showed phosphorylation of NHE1 in cells treated with 1 microM ouabain. Ouabain evoked an increase of cAMP, and the effect of 1 microM ouabain on pH recovery was abolished by H-89, a protein kinase A inhibitor. 8-Bromoadenosine-cAMP increased the pH recovery rate, and this recovery was not further increased by ouabain. Although 1 microM ouabain did not alter cytoplasmic calcium concentration, it stimulated calcium entry after store depletion, a response abolished by 2-APB. Ouabain-induced stimulation of pH recovery was suppressed by inhibitors of capacitative calcium entry, SKF-96365, and 2-APB, as well as the cytoplasmic calcium chelator BAPTA. The cAMP increase in ouabain-treated cells was abolished by BAPTA and 2-APB. Taken together, the results are consistent with increased capacitative calcium entry and subsequent cAMP-PKA-dependent stimulation of NHE1 in ouabain-treated cells.

Late expression of Na+/H+ exchanger 1 (NHE1) and neuroprotective effects of NHE inhibitor in the gerbil hippocampal CA1 region induced by transient ischemia

Although acidosis may be involved in neuronal death, the participation of Na(+)/H(+) exchanger (NHE) in delayed neuronal death in the hippocampal CA1 region induced by transient forebrain ischemia has not been well established. In the present study, we investigated the chronological alterations of NHE1 in the hippocampal CA1 region using a gerbil model after ischemia/reperfusion. In the sham-operated group, NHE1 immunoreactivity was weakly detected in the CA1 region. Two and 3 days after ischemia/reperfusion, NHE1 immunoreactivity was observed in glial components, not in neurons, in the CA1 region. Four days after ischemia/reperfusion, NHE1 immunoreactivity was markedly increased in CA1 pyramidal neurons as well as glial cells. These glial cells were identified as astrocytes based on double immunofluorescence staining. Western blot analysis also showed that NHE protein level in the CA1 region began to increase 2 days after ischemia/reperfusion. The treatment of 10 mg/kg 5-(N-ethyl-N-isopropyl) amiloride, a NHE inhibitor, significantly reduced the ischemia-induced hyperactivity 1 day after ischemia/reperfusion. In addition, NHE inhibitor potently protected CA1 pyramidal neurons from ischemic damage, and NHE inhibitor attenuated the activation of astrocytes and microglia in the ischemic CA1 region. In addition, NHE inhibitor treatment blocked Na(+)/Ca(2+) exchanger 1 immunoreactivity in the CA1 region after transient forebrain ischemia. These results suggest that NHE1 may play a role in the delayed death, and the treatment with NHE inhibitor protects neurons from ischemic damage.

Neuroprotection for ischemic stroke: past, present and future

Neuroprotection for ischemic stroke refers to strategies, applied singly or in combination, that antagonize the injurious biochemical and molecular events that eventuate in irreversible ischemic injury. There has been a recent explosion of interest in this field, with over 1000 experimental papers and over 400 clinical articles appearing within the past 6 years. These studies, in turn, are the outgrowth of three decades of investigative work to define the multiple mechanisms and mediators of ischemic brain injury, which constitute potential targets of neuroprotection. Rigorously conducted experimental studies in animal models of brain ischemia provide incontrovertible proof-of-principle that high-grade protection of the ischemic brain is an achievable goal. Nonetheless, many agents have been brought to clinical trial without a sufficiently compelling evidence-based pre-clinical foundation. At this writing, around 160 clinical trials of neuroprotection for ischemic stroke have been initiated. Of the approximately 120 completed trials, two-thirds were smaller early-phase safety-feasibility studies. The remaining one-third were typically larger (>200 subjects) phase II or III trials, but, disappointingly, only fewer than one-half of these administered neuroprotective therapy within the 4-6h therapeutic window within which efficacious neuroprotection is considered to be achievable. This fact alone helps to account for the abundance of "failed" trials. This review presents a close survey of the most extensively evaluated neuroprotective agents and classes and considers both the strengths and weakness of the pre-clinical evidence as well as the results and shortcomings of the clinical trials themselves. Among the agent-classes considered are calcium channel blockers; glutamate antagonists; GABA agonists; antioxidants/radical scavengers; phospholipid precursor; nitric oxide signal-transduction down-regulator; leukocyte inhibitors; hemodilution; and a miscellany of other agents. Among promising ongoing efforts, therapeutic hypothermia, high-dose human albumin therapy, and hyperacute magnesium therapy are considered in detail. The potential of combination therapies is highlighted. Issues of clinical-trial funding, the need for improved translational strategies and clinical-trial design, and "thinking outside the box" are emphasized.

Annexins sense changes in intracellular pH during hypoxia

The pH(i) (intracellular pH) is an important physiological parameter which is altered during hypoxia and ischaemia, pathological conditions accompanied by a dramatic decrease in pH(i). Sensors of pH(i) include ion transport systems which control intracellular Ca2+ gradients and link changes in pH(i) to functions as diverse as proliferation and apoptosis. The annexins are a protein family characterized by Ca2+-dependent interactions with cellular membranes. Additionally, in vitro evidence points to the existence of pH-dependent, Ca(2+)-independent membrane association of several annexins. We show that hypoxia promotes the interaction of the recombinant annexin A2-S100A10 (p11) and annexin A6 with the plasma membrane. We have investigated in vivo the influence of the pH(i) on the membrane association of human annexins A1, A2, A4, A5 and A6 tagged with fluorescent proteins, and characterized this interaction for endogenous annexins present in smooth muscle and HEK (human embryonic kidney)-293 cells biochemically and by immunofluorescence microscopy. Our results show that annexin A6 and the heterotetramer A2-S100A10 (but not annexins A1, A4 and A5) interact independently of Ca2+ with the plasma membrane at pH 6.2 and 6.6. The dimerization of annexin A2 within the annexin A2-S100A10 complex is essential for the pH-dependent membrane interaction at this pH range. The pH-induced membrane binding of annexins A6 and A2-S100A10 might have consequences for their functions as membrane organizers and channel modulators.

Regulation of initiation factors controlling protein synthesis on cultured astrocytes in lactic acid-induced stress

The goals of this work were first to assess whether the lactic acidosis observed in vivo in ischemia may by itself explain the inhibition of protein synthesis described in the literature and second to study the factors controlling the initiation of protein synthesis under lactic acid stress. Primary rat astrocyte cultures exposed to pH 5.25 underwent cell death and a strong inhibition of protein synthesis assessed by [3H]methionine incorporation, which was solely due to acidity of the extracellular medium and was not related to lactate concentrations. This result was associated with a weak phosphorylation of eukaryotic initiation factor (eIF)4E and a rapid phosphorylation of eIF2alpha via the kinases PKR and PKR-like endoplasmic reticulum kinase. The inhibition of PKR by PRI led first to a significant but not complete dephosphorylation of eIF2alpha that probably contributed to maintain the inhibition of the protein synthesis and second to surprising phosphorylations of extracellular signal-regulated protein kinase, p70S6K and eIF4E, suggesting a possible cross-link between the two pathways. Conversely, cell death was weak at pH 5.5. Protein synthesis was decreased to a lesser extent, the phosphorylation of eIF2alpha was limited, extracellular signal-regulated protein kinase 1/2 was activated and its downstream targets, p70S6K and eIF4E, were phosphorylated. However, the strong phosphorylation of eIF4E was not associated with an activation of the eIF4F complex. This last result may explain why protein synthesis was not stimulated at pH 5.5. However, when astrocytes were exposed at pH 6.2, corresponding to the lower pH observed in hyperglycemic ischemia, no modification in protein synthesis was observed. Consequently, lactic acidosis cannot, by itself, provide an explanation for the decrease in protein synthesis previously reported in vivo in ischemia.

Chronic continuous hypoxia decreases the expression of SLC4A7 (NBCn1) and SLC4A10 (NCBE) in mouse brain

In the mammalian CNS, hypoxia causes a wide range of physiological effects, and these effects often depend on the stage of development. Among the effects are alterations in pH homeostasis. Na+-coupled HCO3(-) transporters can play critical roles in intracellular pH regulation and several, such as NCBE and NBCn1, are expressed abundantly in the central nervous system. In the present study, we examined the effect of chronic continuous hypoxia on the expression of two electroneutral Na-coupled HCO3(-) transporters, SLC4a7 (NBCn1) and SLC4a10 (NCBE), in mouse brain, the first such study on any acid-base transporter. We placed the mice in normobaric chambers and either maintained normoxia (21% inspired O2) or imposed continuous chronic hypoxia (11% O2) for a duration of either 14 days or 28 days, starting from ages of either postnatal age 2 days (P2) or P90. We assessed protein abundance by Western blot analysis, loading equal amounts of total protein for each condition. In most cases, hypoxia reduced NBCn1 levels by 20-50%, and NCBE levels by 15-40% in cerebral cortex, subcortex, cerebellum, and hippocampus, both after 14 and 28 days, and in both pups and adults. We hypothesize that these decreases, which are out of proportion to the expected overall decreases in brain protein levels, may especially be important for reducing energy consumption.

Cell death in an ischemic infarct rim model

Using an in vitro model that simulates the microenvironment in the ischemic infarct rim, we have examined the temporal profile and possible mechanisms of cell death in the neuropil (an astrocyte-rich area or ARA) of organotypic hippocampal slice cultures. Two-photon confocal microscopy, propidium iodide, and GFAP-GFP transgenic mice were used to confirm cell death in astrocytes. An 'ischemic solution' (IS) induced major cell death throughout the hippocampus over 24 h, with the earliest injury starting in ARA. Our studies using IS or ion replacements in IS revealed that cell death in ARA was modest when K(+) was increased or pH lowered. High K(+) is most effective in reducing cell death when HCO(3)(-) is normal or high. When Cl(-) or HCO(3)(-) was reduced, cell injury was worsened. 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) protected cells from IS-induced death in a dose-dependent manner (1-4000 micromol/L). We conclude that (i) various areas of the hippocampal formation respond differently to ionic replacements; (ii) K(+) interacts with other ions to protect cells in ARA; and (iii) DIDS has a substantial protective effect in ARA by blocking DIDS-sensitive membrane exchangers or by interfering with intracellular signaling pathways.

Prostaglandin transporter expression in mouse brain during development and in response to hypoxia

Prostaglandins (PGs) are bioactive lipid mediators released following brain hypoxic-ischemic injury. Clearance and re-uptake of these prostaglandins occur via a transmembrane prostaglandin transporter (PGT), which exchanges PG for lactate. We used Western blot analyses to examine the PGT developmental profile and its regional distribution as well as changes in transporter expression during chronic hypoxia in the neonatal mouse brain. Microsomal preparations from four brain regions (cortex, hippocampus, cerebellum and brainstem/diencephalon) showed gradual increases in prostaglandin transporter expression in all brain regions examined from postnatal day 1 till day 30. There was a significant regional heterogeneity in the prostaglandin transporter expression with highest expression in the cortex, followed by cerebellum and hippocampus, and least expressed in the brainstem/diencephalon. To further delineate the pattern of prostaglandin transporter expression, separate astrocytic and neuronal microsomal preparations were also examined. In contrast to neurons, which had a robust expression of prostaglandin transporters, astrocytes had very little PGT expression under basal conditions. In response to chronic hypoxia, there was a significant decline in PGT expression in vivo and in neurons in vitro, whereas cultured astrocytes increased their PGT expression. This is the first report on PGT expression in the CNS and our studies suggest that PGTs have 1) a widespread distribution in the CNS; 2) a gradual increase and a differential expression in various regions during brain development; and 3) striking contrast in expression between glia and neurons, especially in response to hypoxia. Since PGTs play a role as prostaglandin-lactate exchangers, we hypothesize that PGTs are important in the CNS during stress such as hypoxia.

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.

Factors influencing cell fate in the infarct rim

In focal ischemia, the fate of penumbral cells is closely linked to the infarcted tissue. Because of the release of cytosolic material from damaged cells, the biochemical and ionic alterations within the core are dramatic. Hence, adjacent cells ( infarct rim) are generally exposed to these changes and may be deleteriously affected. To mimic such conditions in vitro, we have employed a slice culture system and used an ischemic solution (IS) that resembles the milieu in the territory of infarct rim. In contrast to normal artificial cerebral spinal fluid, IS is characterized by low O(2), glucose, pH; excitotoxic levels of glutamate; and ionic alterations. In organotypic hippocampal slice cultures, we examined cell injury/death using propidium iodide following exposure to IS. Our data show significant cell injury starting at approximately 8 h following IS exposure with cell injury spreading as a function of exposure duration. We further studied the effect of each component in the IS separately, i.e., acidosis, hypoxia, ionic shifts or glutamate exitotoxicity and were able to isolate the contribution of each of these effectors to the IS-induced cell death. Our results suggest that in IS, acidosis exacerbates the potential for injury while ionic shifts, especially those of K(+) and Na(+), alleviate the potential for cell death.

Gene expression profiling reveals the profound upregulation of hypoxia-responsive genes in primary human astrocytes

Oxygen is vital for the development and survival of mammals. In response to hypoxia, the brain initiates numerous adaptive responses at the organ level as well as at the molecular and cellular levels, including the alteration of gene expression. Astrocytes play critical roles in the proper functioning of the brain; thus the manner in which astrocytes respond to hypoxia is likely important in determining the outcome of brain hypoxia. Here, we used microarray gene expression profiling and data-analysis algorithms to identify and analyze hypoxia-responsive genes in primary human astrocytes. We also compared gene expression patterns in astrocytes with those in human HeLa cells and pulmonary artery endothelial cells (ECs). Remarkably, in astrocytes, five times as many genes were induced as suppressed, whereas in HeLa and pulmonary ECs, as many as or more genes were suppressed than induced. More genes encoding hypoxia-inducible functions, such as glycolytic enzymes and angiogenic growth factors, were strongly induced in astrocytes compared with HeLa cells. Furthermore, gene ontology and computational algorithms revealed that many target genes of the EGF and insulin signaling pathways and the transcriptional regulators Myc, Jun, and p53 were selectively altered by hypoxia in astrocytes. Indeed, Western blot analysis confirmed that two major signal transducers mediating insulin and EGF action, Akt and MEK1/2, were activated by hypoxia in astrocytes. These results provide a global view of the signaling and regulatory network mediating oxygen regulation in human astrocytes.

Expression of recombinant goldfish glutamic acid decarboxylase 65 and evidence for differential pH and PLP responsiveness compared to the human enzyme

Glutamic acid decarboxylase (GAD) catalyzes the conversion of glutamate to gamma-aminobutyric acid (GABA) that acts as an important inhibitory neurotransmitter in the vertebrate brain, as well as in the regulation of neuroendocrine function. GAD65 and GAD67 are the two main isoforms that exist in vertebrates. The biochemical properties of recombinant forms of goldfish and human GAD65 were examined. The recombinant goldfish GAD65 (gfGAD65) was expressed at high levels using a maltose binding protein fusion system for biochemical characterization. The human GAD65 (hGAD65) was expressed as a GST fusion and was also purified. The recombinant goldfish GAD65 protein has properties that are different from the human counterpart. In particular, the gfGAD65 is less active at acidic pH compared to hGAD65, which is moderately active over a wider range of acidic and basic pH. Interestingly, however, gfGAD65 is less dependent on a cofactor pyridoxal-5'-L-phosphate (PLP) for activity. In the absence of added PLP, cleaved recombinant gfGAD65 showed approximately 20% of maximal activity whereas hGAD65 showed no detectable activity. The physiological and evolutionary significance of these findings is discussed in light of the conserved function of GAD in two vertebrate species that are separated in evolutionary time by more than 200 million years.

Role of glial cells in cerebral ischemia

Despite intense efforts at the bench and at the bedside, few therapeutic strategies exist to combat the consequences of cerebral ischemia. Traditionally, a "neurocentric" view has dominated research in this field. Evidence is now accumulating that glial cells, in particular astrocytes, play an active and important role in the pathophysiology of cerebral ischemia. Brain energetics, water and ion homeostasis, inflammation, trophic factor production, vascular regulation, neuroneogenesis, and vasculogenesis, among others, are all under the control of glial cells. As a consequence, glial cells have been identified as promising targets for novel therapeutic approaches in brain protection. This review aims at dissecting possible protective as well as destructive roles of astrocytes (and other glial cells) in cerebral ischemia. By emphasizing open issues in this field, we hope to stimulate further research into this relatively unexplored aspect of brain pathophysiology.