Plasticity of Na,K-ATPase isoform expression in cultures of flat astrocytes: species differences in gene expression

Astrocytic Na+, K+ ATPases in physiology and pathophysiology

The Na+, K+ ATPases play a fundamental role in the homeostatic functions of astrocytes. After a brief historic prologue and discussion of the subunit composition and localization of the astrocytic Na+, K+ ATPases, the review focuses on the role of the astrocytic Na+, K+ pumps in extracellular K+ and glutamate homeostasis, intracellular Na+ and Ca2+ homeostasis and signaling, regulation of synaptic transmission and neurometabolic coupling between astrocytes and neurons. Loss-of-function mutations in the gene encoding the astrocytic α2 Na+, K+ ATPase cause a rare monogenic form of migraine with aura (familial hemiplegic migraine type 2). On the other hand, the α2 Na+, K+ ATPase is upregulated in spinal cord and brain samples from amyotrophic lateral sclerosis and Alzheimer disease patients, respectively. In the last part, the review focuses on i) the migraine relevant phenotypes shown by familial hemiplegic migraine type 2 knock-in mice with 50 % reduced expression of the astrocytic α2 Na+, K+ ATPase and the insights into the pathophysiology of migraine obtained from these genetic mouse models, and ii) the evidence that upregulation of the astrocytic α2 Na+, K+ ATPase in mouse models of amyotrophic lateral sclerosis and Alzheimer disease promotes neuroinflammation and contributes to progressive neurodegeneration.

Metabolic Recruitment in Brain Tissue

Information processing imposes urgent metabolic demands on neurons, which have negligible energy stores and restricted access to fuel. Here, we discuss metabolic recruitment, the tissue-level phenomenon whereby active neurons harvest resources from their surroundings. The primary event is the neuronal release of K⁺ that mirrors workload. Astrocytes sense K⁺ in exquisite fashion thanks to their unique coexpression of NBCe1 and α2β2 Na⁺/K⁺ ATPase, and within seconds switch to Crabtree metabolism, involving GLUT1, aerobic glycolysis, transient suppression of mitochondrial respiration, and lactate export. The lactate surge serves as a secondary recruiter by inhibiting glucose consumption in distant cells. Additional recruiters are glutamate, nitric oxide, and ammonium, which signal over different spatiotemporal domains. The net outcome of these events is that more glucose, lactate, and oxygen are made available. Metabolic recruitment works alongside neurovascular coupling and various averaging strategies to support the inordinate dynamic range of individual neurons.

Cell biology and dynamics of Neuronal Na+/K+-ATPase in health and diseases

Neuronal Na⁺/K⁺-ATPase is responsible for the maintenance of ionic gradient across plasma membrane. In doing so, in a healthy brain, Na⁺/K⁺-ATPase activity accounts for nearly half of total brain energy consumption. The α3-subunit containing Na⁺/K⁺-ATPase expression is restricted to neurons. Heterozygous mutations within α3-subunit leads to Rapid-onset Dystonia Parkinsonism, Alternating Hemiplegia of Childhood and other neurological and neuropsychiatric disorders. Additionally, proteins such as α-synuclein, amyloid-β, tau and SOD1 whose aggregation is associated to neurodegenerative diseases directly bind and impair α3-Na⁺/K⁺-ATPase activity. The review will provide a summary of neuronal α3-Na⁺/K⁺-ATPase functional properties, expression pattern, protein-protein interactions at the plasma membrane, biophysical properties (distribution and lateral diffusion). Lastly, the role of α3-Na⁺/K⁺-ATPase in neurological and neurodegenerative disorders will be discussed. This article is part of the special issue entitled 'Mobility and trafficking of neuronal membrane proteins'.

α2 isoform of Na+,K+-ATPase via Na+,Ca2+ exchanger modulates myelin basic protein synthesis in oligodendrocyte lineage cells in vitro

Oligodendrocytes in the CNS myelinate neuronal axons, facilitating rapid propagation of action potentials. Myelin basic protein (MBP) is an essential component of myelin and its absence results in severe hypomyelination. In oligodendrocyte lineage cell (OLC) monocultures MBP synthesis starts at DIV4. Ouabain (10 nM), a Na⁺,K⁺-ATPase (NKA) blocker, stimulates MBP synthesis. As OLCs express the α2 isoform of NKA (α2-NKA) that has a high affinity for ouabain, we hypothesized that α2-NKA mediates this effect. Knockdown of α2-NKA with small interfering (si)RNA (α2-siRNA) significantly potentiated MBP synthesis at DIV4 and 5. This effect was completely blocked by KB-R7943 (1 μM), a Na⁺,Ca²⁺ exchanger (NCX) antagonist. α2-NKA ablation increased the frequency of NCX-mediated spontaneous Ca²⁺ transients ([Ca²⁺]t) at DIV4, whereas in control OLC cultures comparable frequency of [Ca²⁺]t was observed at DIV5. At DIV6 almost no [Ca²⁺]t were observed either in control or in α2-siRNA-treated cultures. Immunocytochemical analyses showed that α2-NKA co-localizes with MBP in proximal processes of immature OLCs but is only weakly present in MBP-enriched membrane sheets. Knockdown of α2-NKA in cortical slice cultures did not change MBP levels but reduced co-localization of neurofilament- and MBP-positive compartments. We conclude that α2-NKA activity in OLCs affects NCX-mediated [Ca²⁺]t and the onset of MBP synthesis. We suggest therefore that neuronal activity, presumably in form of local extracellular [K⁺] changes, might locally influence NCX-mediated [Ca²⁺]t in OLC processes thus triggering local MBP synthesis in the vicinity of an active axon.

The Na, K-ATPase β-Subunit Isoforms Expression in Glioblastoma Multiforme: Moonlighting Roles

Glioblastoma multiforme (GBM) is the most common form of malignant glioma. Recent studies point out that gliomas exploit ion channels and transporters, including Na, K-ATPase, to sustain their singular growth and invasion as they invade the brain parenchyma. Moreover, the different isoforms of the β-subunit of Na, K-ATPase have been implicated in regulating cellular dynamics, particularly during cancer progression. The aim of this study was to determine the Na, K-ATPase β subunit isoform subcellular expression patterns in all cell types responsible for microenvironment heterogeneity of GBM using immunohistochemical analysis. All three isoforms, β1, β2/AMOG (Adhesion Molecule On Glia) and β3, were found to be expressed in GBM samples. Generally, β1 isoform was not expressed by astrocytes, in both primary and secondary GBM, although other cell types (endothelial cells, pericytes, telocytes, macrophages) did express this isoform. β2/AMOG and β3 positive expression was observed in the cytoplasm, membrane and nuclear envelope of astrocytes and GFAP (Glial Fibrillary Acidic Protein) negative cells. Interestingly, differences in isoforms expression have been observed between primary and secondary GBM: in secondary GBM, β2 isoform expression in astrocytes was lower than that observed in primary GBM, while the expression of the β3 subunit was more intense. These changes in β subunit isoforms expression in GBM could be related to a different ionic handling, to a different relationship between astrocyte and neuron (β2/AMOG) and to changes in the moonlighting roles of Na, K-ATPase β subunits as adaptor proteins and transcription factors.

The α2β2 isoform combination dominates the astrocytic Na+ /K+ -ATPase activity and is rendered nonfunctional by the α2.G301R familial hemiplegic migraine type 2-associated mutation

Synaptic activity results in transient elevations in extracellular K+ , clearance of which is critical for sustained function of the nervous system. The K+ clearance is, in part, accomplished by the neighboring astrocytes by mechanisms involving the Na+ /K+ -ATPase. The Na+ /K+ -ATPase consists of an α and a β subunit, each with several isoforms present in the central nervous system, of which the α2β2 and α2β1 isoform combinations are kinetically geared for astrocytic K+ clearance. While transcript analysis data designate α2β2 as predominantly astrocytic, the relative quantitative protein distribution and isoform pairing remain unknown. As cultured astrocytes altered their isoform expression in vitro, we isolated a pure astrocytic fraction from rat brain by a novel immunomagnetic separation approach in order to determine the expression levels of α and β isoforms by immunoblotting. In order to compare the abundance of isoforms in astrocytic samples, semi-quantification was carried out with polyhistidine-tagged Na+ /K+ -ATPase subunit isoforms expressed in Xenopus laevis oocytes as standards to obtain an efficiency factor for each antibody. Proximity ligation assay illustrated that α2 paired efficiently with both β1 and β2 and the semi-quantification of the astrocytic fraction indicated that the astrocytic Na+ /K+ -ATPase is dominated by α2, paired with β1 or β2 (in a 1:9 ratio). We demonstrate that while the familial hemiplegic migraine-associated α2.G301R mutant was not functionally expressed at the plasma membrane in a heterologous expression system, α2+/G301R mice displayed normal protein levels of α2 and glutamate transporters and that the one functional allele suffices to manage the general K+ dynamics.

The Structure and Function of the Na,K-ATPase Isoforms in Health and Disease

The sodium and potassium gradients across the plasma membrane are used by animal cells for numerous processes, and the range of demands requires that the responsible ion pump, the Na,K-ATPase, can be fine-tuned to the different cellular needs. Therefore, several isoforms are expressed of each of the three subunits that make a Na,K-ATPase, the alpha, beta and FXYD subunits. This review summarizes the various roles and expression patterns of the Na,K-ATPase subunit isoforms and maps the sequence variations to compare the differences structurally. Mutations in the Na,K-ATPase genes encoding alpha subunit isoforms have severe physiological consequences, causing very distinct, often neurological diseases. The differences in the pathophysiological effects of mutations further underline how the kinetic parameters, regulation and proteomic interactions of the Na,K-ATPase isoforms are optimized for the individual cellular needs.

Astrocytic transporters in Alzheimer's disease

Astrocytes play a fundamental role in maintaining the health and function of the central nervous system. Increasing evidence indicates that astrocytes undergo both cellular and molecular changes at an early stage in neurological diseases, including Alzheimer's disease (AD). These changes may reflect a change from a neuroprotective to a neurotoxic phenotype. Given the lack of current disease-modifying therapies for AD, astrocytes have become an interesting and viable target for therapeutic intervention. The astrocyte transport system covers a diverse array of proteins involved in metabolic support, neurotransmission and synaptic architecture. Therefore, specific targeting of individual transporter families has the potential to suppress neurodegeneration, a characteristic hallmark of AD. A small number of the 400 transporter superfamilies are expressed in astrocytes, with evidence highlighting a fraction of these are implicated in AD. Here, we review the current evidence for six astrocytic transporter subfamilies involved in AD, as reported in both animal and human studies. This review confirms that astrocytes are indeed a viable target, highlights the complexities of studying astrocytes and provides future directives to exploit the potential of astrocytes in tackling AD.

Glutamate transporter activity promotes enhanced Na+ /K+ -ATPase-mediated extracellular K+ management during neuronal activity

KEY POINTS

Management of glutamate and K⁺ in brain extracellular space is of critical importance to neuronal function. The astrocytic α2β2 Na⁺ /K⁺ -ATPase isoform combination is activated by the K⁺ transients occurring during neuronal activity. In the present study, we report that glutamate transporter-mediated astrocytic Na⁺ transients stimulate the Na⁺ /K⁺ -ATPase and thus the clearance of extracellular K⁺ . Specifically, the astrocytic α2β1 Na⁺ /K⁺ -ATPase subunit combination displays an apparent Na⁺ affinity primed to react to physiological changes in intracellular Na⁺ . Accordingly, we demonstrate a distinct physiological role in K⁺ management for each of the two astrocytic Na⁺ /K⁺ -ATPase β-subunits.

ABSTRACT

Neuronal activity is associated with transient [K⁺ ]_o increases. The excess K⁺ is cleared by surrounding astrocytes, partly by the Na⁺ /K⁺ -ATPase of which several subunit isoform combinations exist. The astrocytic Na⁺ /K⁺ -ATPase α2β2 isoform constellation responds directly to increased [K⁺ ]_o but, in addition, Na⁺ /K⁺ -ATPase-mediated K⁺ clearance could be governed by astrocytic [Na⁺ ]_i . During most neuronal activity, glutamate is released in the synaptic cleft and is re-absorbed by astrocytic Na⁺ -coupled glutamate transporters, thereby elevating [Na⁺ ]_i . It thus remains unresolved whether the different Na⁺ /K⁺ -ATPase isoforms are controlled by [K⁺ ]_o or [Na⁺ ]_i during neuronal activity. Hippocampal slice recordings of stimulus-induced [K⁺ ]_o transients with ion-sensitive microelectrodes revealed reduced Na⁺ /K⁺ -ATPase-mediated K⁺ management upon parallel inhibition of the glutamate transporter. The apparent intracellular Na⁺ affinity of isoform constellations involving the astrocytic β2 has remained elusive as a result of inherent expression of β1 in most cell systems, as well as technical challenges involved in measuring intracellular affinity in intact cells. We therefore expressed the different astrocytic isoform constellations in Xenopus oocytes and determined their apparent Na⁺ affinity in intact oocytes and isolated membranes. The Na⁺ /K⁺ -ATPase was not fully saturated at basal astrocytic [Na⁺ ]_i , irrespective of isoform constellation, although the β1 subunit conferred lower apparent Na⁺ affinity to the α1 and α2 isoforms than the β2 isoform. In summary, enhanced astrocytic Na⁺ /K⁺ -ATPase-dependent K⁺ clearance was obtained with parallel glutamate transport activity. The astrocytic Na⁺ /K⁺ -ATPase isoform constellation α2β1 appeared to be specifically geared to respond to the [Na⁺ ]_i transients associated with activity-induced glutamate transporter activity.

Signaling function of Na,K-ATPase induced by ouabain against LPS as an inflammation model in hippocampus

BACKGROUND

Ouabain (OUA) is a newly recognized hormone that is synthesized in the adrenal cortex and hypothalamus. Low doses of OUA can activate a signaling pathway by interaction with Na,K-ATPase, which is protective against a number of insults. OUA has central and peripheral anti-inflammatory effects. Lipopolysaccharide (LPS), via toll-like receptor 4 activation, is a widely used model to induce systemic inflammation. This study used a low OUA dose to evaluate its effects on inflammation induced by LPS injection in rats.

METHODS

Adult male Wistar rats received acute intraperitoneal (ip) OUA (1.8 μg/kg) or saline 20 minutes before LPS (200 μg/kg, ip) or saline injection. Some of the animals had their femoral artery catheterized in order to assess arterial blood pressure values before and after OUA administration. Na,K-ATPase activity, cytokine mRNA levels, apoptosis-related proteins, NF-κB activation brain-derived neurotrophic factor BDNF, corticosterone and TNF-α levels were measured.

RESULTS

OUA pretreatment decreased mRNA levels of the pro-inflammatory cytokines, inducible nitric oxide synthase (iNOS) and IL-1β, which are activated by LPS in the hippocampus, but with no effect on serum measures of these factors. None of these OUA effects were linked to Na,K-ATPase activity. The involvement of the inflammatory transcription factor NF-κB in the OUA effect was indicated by its prevention of LPS-induced nuclear translocation of the NF-κB subunit, RELA (p65), as well as the decreased cytosol levels of the NF-κB inhibitor, IKB, in the hippocampus. OUA pretreatment reversed the LPS-induced glial fibrillary acidic protein (GFAP) activation and associated inflammation in the dentate gyrus. OUA also prevented LPS-induced increases in the hippocampal Bax/Bcl2 ratio suggesting an anti-apoptotic action in the brain.

CONCLUSION

Our results suggest that a low dose of OUA has an important anti-inflammatory effect in the rat hippocampus. This effect was associated with decreased GFAP induction by LPS in the dentate gyrus, a brain area linked to adult neurogenesis.

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.

Evaluation of aggregating brain cell cultures for the detection of acute organ-specific toxicity

As part of the ACuteTox project aimed at the development of non-animal testing strategies for predicting human acute oral toxicity, aggregating brain cell cultures (AGGR) were examined for their capability to detect organ-specific toxicity. Previous multicenter evaluations of in vitro cytotoxicity showed that some 20% of the tested chemicals exhibited significantly lower in vitro toxicity as expected from in vivo toxicity data. This was supposed to be due to toxicity at supracellular (organ or system) levels. To examine the capability of AGGR to alert for potential organ-specific toxicants, concentration-response studies were carried out in AGGR for 86 chemicals, taking as endpoints the mRNA expression levels of four selected genes. The lowest observed effect concentration (LOEC) determined for each chemical was compared with the IC20 reported for the 3T3/NRU cytotoxicity assay. A LOEC lower than IC20 by at least a factor of 5 was taken to alert for organ-specific toxicity. The results showed that the frequency of alerts increased with the level of toxicity observed in AGGR. Among the chemicals identified as alert were many compounds known for their organ-specific toxicity. These findings suggest that AGGR are suitable for the detection of organ-specific toxicity and that they could, in conjunction with the 3T3/NRU cytotoxicity assay, improve the predictive capacity of in vitro toxicity testing.

Regional heterogeneity and diversity in cytokine and chemokine production by astroglia: differential responses to HIV-1 Tat, gp120, and morphine revealed by multiplex analysis

HIV-infected individuals who abuse opiates show a faster progression to AIDS and higher incidence of encephalitis. The HIV-1 proteins Tat and gp120 have been shown to cause neurodegenerative changes either in vitro or when injected or expressed in the CNS, and we have shown that opiate drugs can exacerbate neurotoxic effects in the striatum through direct actions on pharmacologically discrete subpopulations of mu-opioid receptor-expressing astroglia. Opiate coexposure also significantly enhances release of specific inflammatory mediators by astroglia from the striatum, and we theorize that astroglial reactivity may underlie aspects of HIV neuropathology. To determine whether astroglia from different regions of the central nervous system have distinct, intrinsic responses to HIV-1 proteins and opiates, we used multiplex suspension array analyses to define and compare the inflammatory signature of cytokines released by murine astrocytes grown from cerebral cortex, cerebellum, and spinal cord. Results demonstrate significant regional differences in baseline secretion patterns, and in responses to viral proteins. Of importance for the disease process, astrocytes from all regions have very limited inflammatory response to gp120 protein, as compared to Tat protein, either in the presence or absence of morphine. Overall, the chemokine/cytokine release is higher from spinal cord and cortical astroglia than from cerebellar astroglia, paralleling the relatively low incidence of HIV-related neuropathology in the cerebellum.

Increased Na, K-ATPase alpha2 isoform gene expression by ammonia in astrocytes and in brain in vivo

In mouse astrocyte cultures identical to those used in the present study ammonia increases the production of ouabain-like compounds and Na, K-ATPase activity (Kala et al., 2000). Increased activity of Na, K-ATPase could be the result of enhanced production of ouabain-like compounds, since cultured rat astrocytes react to prolonged exposure to a high concentration of ouabain with an upregulation of the Na, K-ATPase alpha(1) isoform (Hosoi et al., 1997). However, unlike astrocytes in brain in vivo and mouse primary cultures, cultured rat astrocytes do not express the astrocyte-specific alpha(2) isoform, which shows a higher affinity for ouabain (EC(50) approximately 0.1 microM) than the alpha(1) isoform (EC(50) approximately 10 microM). In the present study we have investigated (i) effects of ammonia on mRNA and protein expression of alpha(1) and alpha(2) isoforms in primary cultures of mouse astrocytes; (ii) effects of hyperammonia obtained by urease injection on mRNA and protein expression of alpha(1) and alpha(2) isoforms in the brain in vivo; and (iii) effect on observed upregulation of gene expression of AG1478, an inhibitor of the EGF receptor-tyrosine kinase, PP1, an inhibitor of Src, and GM6001, an inhibitor of Zn(2+)-dependent metalloproteinases in the cultured cells. It was established that alpha(2) mRNA and protein expression, but not alpha(1) expression, was upregulated in cultured astrocytes by 1-4 days of exposure to 3 or 5 mM ammonia and that similar upregulation, contrasted by a downregulation of the neuronal alpha(3) subunit occurred in the hyperammonemic brain. Based on the effects of the inhibitors and literature data it is concluded that ammonia activates formation of an endogenous ouabain-like compound, which binds to the Na, K-ATPase, activating Src, which in turn stimulates the receptor-tyrosine kinase of the EGF receptor, leading to activation of the Ras, Raf, MEK pathway and phosphorylation of ERK(1/2), which eventually causes upregulation of alpha(2) gene expression.

Ouabain binding kinetics and FXYD7 expression in astrocytes and neurons in primary cultures: implications for cellular contributions to extracellular K+ homeostasis?

Although Na+,K+-ATPase-mediated K+ uptake into astrocytes plays a major role in re-establishing resting extracellular K+ following neuronal excitation little information is available about astrocytic Na+,K+-ATPase function, let alone mechanisms returning K+ to neurons. The catalytic units of the Na+,K+-ATPase are the astrocyte-specific α2, the neuron-specific α3 and the ubiquitously expressed α1. In the present work, Bmax and KD values for α1, α2 and α3 subunits were computed in cultured cerebro-cortical mouse astrocytes and cerebellar granule neurons by non-linear regression as high-affinity (α2, α3) and low-affinity (α1) [3H]ouabain binding sites, which stoichiometrically equal transporter sites. Cellular expression was also determined of the brain- and α1-β1 isoform-specific FDYX7, regulating Na+,K+-ATPase efficiency and K+-sensitivity. From ouabain-sensitive K+ uptake rates published by ourselves (Walz and Hertz, 1982) or others (Atterwill et al., 1985), Na+,K+-ATPase turnover was determined. Subunits α2 and α3 showed Bmax of 15-30 pmol/mg protein, with maximum turnover rates of 70-80/s. Bmax of the α1 subunit was low in neurons but very high in astrocytes (645 pmol/mg protein), where turnover rate was slow, reflecting expression of selectively expressed FXYD7, and binding was increased by K+. The role of these characteristics for K+ homeostasis are discussed.

When the brain goes diving: glial oxidative metabolism may confer hypoxia tolerance to the seal brain

Deep diving mammals have developed strategies to cope with limited oxygen availability when submerged. These adaptations are associated with an increased neuronal hypoxia tolerance. Brain neurons of the hooded seal Cystophora cristata remain much longer active in hypoxic conditions than those of mice. To understand the cellular basis of neuronal hypoxia tolerance, we studied neuroglobin and cytochrome c in C. cristata brain. Neuroglobin, a respiratory protein typically found in vertebrate neurons, displays three unique amino acid substitutions in hooded seal. However, these substitutions unlikely contribute to a modulation of O(2) affinity. Moreover, there is no significant difference in total neuroglobin protein levels in mouse, rat and seal brains. However, in terrestrial mammals neuroglobin resided exclusively in neurons, whereas in seals neuroglobin is mainly located in astrocytes. This unusual localization of neuroglobin is accompanied by a shift in the distribution of cytochrome c. In seals, this marker for oxidative metabolism is mainly localized in astrocytes, whereas in terrestrial mammals it is essentially found in neurons. Our results indicate that in seals aerobic ATP production depends significantly on astrocytes, while neurons rely less on aerobic energy metabolism. This adaptation may imbue seal neurons with an increased tolerance to hypoxia and potentially also to reactive oxygen species, and may explain in part the ability of deep diving mammals to sustain neuronal activity during prolonged dives.

Energy metabolism in astrocytes: high rate of oxidative metabolism and spatiotemporal dependence on glycolysis/glycogenolysis

Astrocytic energy demand is stimulated by K(+) and glutamate uptake, signaling processes, responses to neurotransmitters, Ca(2+) fluxes, and filopodial motility. Astrocytes derive energy from glycolytic and oxidative pathways, but respiration, with its high-energy yield, provides most adenosine 5' triphosphate (ATP). The proportion of cortical oxidative metabolism attributed to astrocytes ( approximately 30%) in in vivo nuclear magnetic resonance (NMR) spectroscopic and autoradiographic studies corresponds to their volume fraction, indicating similar oxidation rates in astrocytes and neurons. Astrocyte-selective expression of pyruvate carboxylase (PC) enables synthesis of glutamate from glucose, accounting for two-thirds of astrocytic glucose degradation via combined pyruvate carboxylation and dehydrogenation. Together, glutamate synthesis and oxidation, including neurotransmitter turnover, generate almost as much energy as direct glucose oxidation. Glycolysis and glycogenolysis are essential for astrocytic responses to increasing energy demand because astrocytic filopodial and lamellipodial extensions, which account for 80% of their surface area, are too narrow to accommodate mitochondria; these processes depend on glycolysis, glycogenolysis, and probably diffusion of ATP and phosphocreatine formed via mitochondrial metabolism to satisfy their energy demands. High glycogen turnover in astrocytic processes may stimulate glucose demand and lactate production because less ATP is generated when glucose is metabolized via glycogen, thereby contributing to the decreased oxygen to glucose utilization ratio during brain activation. Generated lactate can spread from activated astrocytes via low-affinity monocarboxylate transporters and gap junctions, but its subsequent fate is unknown. Astrocytic metabolic compartmentation arises from their complex ultrastructure; astrocytes have high oxidative rates plus dependence on glycolysis and glycogenolysis, and their energetics is underestimated if based solely on glutamate cycling.

Ethanol acutely decreases astroglial gap junction permeability in primary cultures from defined brain regions

The acute effect of hyperosmotic ethanol on gap junction permeability was examined in astroglial cells in primary culture from five different brain regions. Gap junction permeability was analyzed by measuring dye spreading from cell to cell with the low molecular weight dye Lucifer Yellow. Ethanol concentrations 25-300 mM significantly decreased dye spreading in cultures from the cerebral cortex in a dose-dependent but time-independent manner for up to 60 min. Besides cerebral cortex, exposure to 150 mM ethanol decreased dye spreading in astroglial cultures from the hippocampus and from the brain stem, while cultures from the olfactory bulb and from the hypothalamus were not significantly affected. The ethanol-induced decrease in dye spreading in cultures from the cerebral cortex was not mediated through changes in cell volume, osmolarity, protein kinase C (PKC) phosphorylation, intracellular pH, or intracellular calcium concentration ([Ca(2+)](i)). The decrease in dye spreading was abolished upon incubation in sodium-reduced buffer, and after blockage of the Na(+)/K(+)/2Cl(-) cotransporter with furosemide. The results presented here indicate that ethanol-mediated decrease in dye spreading is directly or indirectly dependent on sodium.

Encéphalopathie myoclonique : rôle des inhibiteurs de la pompe à protons

Two men (66 and 73 Years) with a cardiovascular history were hospitalized for rapid onset encephalopathy associated with myoclonia and an extrapyramidal syndrome. On the basis of the French Pharmacovigilance system, this symptomatology has been attributed to the coadministration of a proton pump inhibitor, lansoprazole (15mg/day) with levodopa. Lansoprazole withdrawal led to a normalisation of the situation.

The neurobiology of glia in the context of water and ion homeostasis

Astrocytes are highly complex cells that respond to a variety of external stimulations. One of the chief functions of astrocytes is to optimize the interstitial space for synaptic transmission by tight control of water and ionic homeostasis. Several lines of work have, over the past decade, expanded the role of astrocytes and it is now clear that astrocytes are active participants in the tri-partite synapse and modulate synaptic activity in hippocampus, cortex, and hypothalamus. Thus, the emerging concept of astrocytes includes both supportive functions as well as active modulation of neuronal output. Glutamate plays a central role in astrocytic-neuronal interactions. This excitatory amino acid is cleared from the neuronal synapses by astrocytes via glutamate transporters, and is converted into glutamine, which is released and in turn taken up by neurons. Furthermore, metabotropic glutamate receptor activation on astrocytes triggers via increases in cytosolic Ca(2+) a variety of responses. For example, calcium-dependent glutamate release from the astrocytes modulates the activity of both excitatory and inhibitory synapses. In vivo studies have identified the astrocytic end-foot processes enveloping the vessel walls as the center for astrocytic Ca(2+) signaling and it is possible that Ca(2+) signaling events in the cellular component of the blood-brain barrier are instrumental in modulation of local blood flow as well as substrate transport. The hormonal regulation of water and ionic homeostasis is achieved by the opposing effects of vasopressin and atrial natriuretic peptide on astroglial water and chloride uptake. In conjuncture, the brain appears to have a distinct astrocytic perivascular system, involving several potassium channels as well as aquaporin 4, a membrane water channel, which has been localized to astrocytic endfeet and mediate water fluxes within the brain. The multitask functions of astrocytes are essential for higher brain function. One of the major challenges for future studies is to link receptor-mediated signaling events in astrocytes to their roles in metabolism, ion, and water homeostasis.

AMOG/beta2 and glioma invasion: does loss of AMOG make tumour cells run amok?

The beta2 subunit of Na,K-ATPase, initially described as adhesion molecule on glia (AMOG), has been shown to mediate neurone-astrocyte adhesion as well as neural cell migration in vitro. We have investigated the expression of AMOG/beta2 in human gliomas and its effect on glioma cell adhesion and migration. Compared to normal astrocytes of human brain, AMOG/beta2 expression levels of neoplastic astrocytes were down-regulated in biopsy specimens and inversely related to the grade of malignancy. One rat and four human glioma cell lines showed complete loss of AMOG. To investigate the function of AMOG/beta2, its expression was re-established by transfecting an expression plasmid into AMOG/beta2-negative C6 rat glioma cells. In vitro assays revealed increased adhesion and decreased migration on matrigel of AMOG/beta2-positive cells as compared to their AMOG/beta2-negative counterparts. We conclude that increasing loss of AMOG/beta2 during malignant progression parallels and may underlie the extensive invasion pattern of malignant gliomas.

Cell-specific expression pattern of monocarboxylate transporters in astrocytes and neurons observed in different mouse brain cortical cell cultures

Evidence suggests that lactate could be a preferential energy substrate transferred from astrocytes to neurons. Such a process implies the presence of specific monocarboxylate transporters on both cell types. Expression of MCT1 and MCT2, two isoforms of the monocarboxylate transporter (MCT) family, was studied in enriched cultures of mouse cortical astrocytes or neurons. It was observed that, at both the mRNA and the protein levels, astrocytes strongly expressed MCT1 but had very little if any MCT2. By contrast, neurons had high amounts of MCT2 mRNA, although MCT1 mRNA was also detected. Double immunofluorescent labelings with appropriate markers confirmed the cell-specific preference in the expression of MCT1 and MCT2, but they revealed that a subset of neurons expresses low to moderate levels of MCT1. Parallel immunocytochemical stainings of cultured neurons with the presynaptic marker synaptophysin showed that MCT2 expression is correlated with synaptic development. Although MCT2 and synaptophysin were not colocalized, their distribution was similar, and they were often closely apposed, suggesting that MCT2 could be associated with postsynaptic terminals. Interaction between astrocytes and neurons, as occurring in layered cultures, did not modify the levels of MCT1 and MCT2 expression or their distribution and cell-specific preference under the conditions used. However, a close apposition between neurites and MCT1-expressing astrocytic processes was apparent and developed as cultures evolved. In addition to providing an extensive description of MCT distribution in cultured cells, our data underscore the potential of such preparations for future studies on the regulation of MCT expression.

Degeneration of the amygdala/piriform cortex and enhanced fear/anxiety behaviors in sodium pump alpha2 subunit (Atp1a2)-deficient mice

The sodium pump is the enzyme responsible for the maintenance of Na+ and K+ gradients across the cell membrane. Four isoforms of the catalytic alpha subunit have been identified, but their individual roles remain essentially unknown. To investigate the necessary functions of the alpha2 subunit in vivo, we generated and analyzed mice defective in the alpha2 subunit gene. Mice homozygous for the alpha2 mutation died just after birth and displayed selective neuronal apoptosis in the amygdala and piriform cortex. In these regions, high expression of c-Fos before apoptosis indicated neural hyperactivity, and re-uptake of glutamic acid and GABA into P2 fraction containing crude synaptosome was impaired. These results indicate that the alpha2 subunit plays a critical role regulating neural activity in the developing amygdala and piriform cortex. Further supporting a role of the alpha2 subunit in the function of the amygdala, heterozygous adult mice showed augmented fear/anxiety behaviors and enhanced neuronal activity in the amygdala and piriform cortex after conditioned fear stimuli.

The Na,K-ATPase alpha 2 isoform is expressed in neurons, and its absence disrupts neuronal activity in newborn mice

Na,K-ATPase is an ion transporter that impacts neural and glial physiology by direct electrogenic activity and the modulation of ion gradients. Its three isoforms in brain have cell-type and development-specific expression patterns. Interestingly, our studies demonstrate that in late gestation, the alpha2 isoform is widely expressed in neurons, unlike in the adult brain, in which alpha2 has been shown to be expressed primarily in astrocytes. This unexpected distribution of alpha2 isoform expression in neurons is interesting in light of our examination of mice lacking the alpha2 isoform which fail to survive after birth. These animals showed no movement; however, defects in gross brain development, muscle contractility, neuromuscular transmission, and lung development were ruled out. Akinesia suggests a primary neuronal defect and electrophysiological recordings in the pre-Bötzinger complex, the brainstem breathing center, showed reduction of respiratory rhythm activity, with less regular and smaller population bursts. These data demonstrate that the Na,K-ATPase alpha2 isoform could be important in the modulation of neuronal activity in the neonate.

Thyroidal regulation of different isoforms of NaKATPase in the primary cultures of neurons derived from fetal rat brain

The developmental profile of the different isoforms of NaKATPase have been investigated using primary cultures of isolated neurons initiated from 17 day old fetal rat brain. Northern blot analysis showed that the expression of three alpha isoforms (alpha(1), alpha(2) and alpha(3)) and two beta isoforms (beta(1) and beta(2)) increased progressively and reached a peak between 12 to 16 days of culture. Comparison of the mRNA levels of these isoforms in the cells maintained in thyroid hormone deficient (TH def) and thyroid hormone supplemented (TH sup) media for 6-12 days, revealed for the first time that in the neurons three alpha and two beta isoforms of NaKATPase are sensitive to TH. Furthermore immunocytochemical staining of these cells with isoform specific NaKATPase antibodies showed that the uniform distribution of alpha(2), alpha(3) and beta(2) isoforms in the neuronal processes require the presence of TH. These results establish neurons as the target cells for the regulation of NaKATPase by TH in the developing brain.

Differential gene expression in astrocytes from human normal and glaucomatous optic nerve head analyzed by cDNA microarray

Recent advances in cDNA microarray technology have made it possible to analyze expression of several thousand genes at the same time. Using this technique, gene expression in human astrocytes cultured from glaucomatous and normal optic nerve heads (ONH) was compared. One hundred-fifty genes were differentially expressed more than 5-fold in glaucomatous cell cultures compared with normal. These genes are involved in a number of biological processes, including signal transduction, cell adhesion and proliferation, ECM synthesis, and degradation. Confirmation of differential gene expression was performed by quantitative RT-PCR. Western blots and immunohistochemistry demonstrated gene products in cell cultures or in human ONH tissues. Proliferation, adhesion and migration assays tested physiological responses suggested by differential gene expression. Our study suggests that cultured glaucomatous ONH astrocytes retain in culture many phenotypic characteristics that may be relevant to their role in the pathogenesis of glaucoma and, in general to reactive astrocytes in the CNS. Potential applications of these data include the identification and characterization of signaling pathways involved in astrocyte function, studies of the role of steroid-metabolizing enzymes in the glaucomatous ONH, and further exploration of the role of selected identified genes in experimental animal and in vitro models of glaucoma.

Expression, activity and distribution of Na,K-ATPase subunits during in vitro neuronal induction

The expression pattern of the alpha and beta isoforms and the gamma subunit of the Na,K-ATPase was investigated during in vitro induction of pluripotent murine embryonic stem (ES) cells into neuronal cells. alpha1 protein was expressed in undifferentiated ES (UES) cells and throughout all stages studied. In contrast, alpha3 protein was prominent only when neuronal cells have reached full differentiation. In this model, neuron-depleted cultures did not express the alpha3 isoform, indicating its specificity for mature neuronal cells. UES possessed Na,K-ATPase activity consistent with a single isoform (alpha1), whereas in fully mature neuronal cells a ouabain-sensitive isoform (alpha3) accounted for 27+/-4% of the activity, and a ouabain-resistant isoform (alpha1) 66+/-3%. Immunocytochemistry of mature neuronal cells for alpha1 and alpha3 proteins showed a similar distribution, including cell soma and processes, without evidence of polarization. beta1 protein was expressed in uninduced ES, embryonic bodies (EB) and neuronal cells. While proteins of the beta2 and beta3 isoforms were not detected by immunoblots (except for beta2 in UES), their mRNAs were detected in UES and EB (beta2 and beta3), and in immature and fully differentiated neuronal cells (beta3). Message for the beta2 isoform, however, was not present in neuronal cells. gamma subunit mRNA and protein were undetectable at any stage. These results provide further characterization of neuron-like cells obtained by induction of ES cells in vitro, and establish a model for the expression of isoforms of the Na,K-ATPase during neuronal differentiation. The relation to other aspects of neuronal cell development and relevance to a specialised function for the alpha3 subunit in neurons are discussed.

Oligodendrocytes in brain and optic nerve express the beta3 subunit isoform of Na,K-ATPase

The Na,K-ATPase, which catalyzes the active transport of Na(+) and K(+), has two principal subunits (alpha and beta) that have several genetically distinct isoforms. Most of these isoforms are expressed in the nervous system, but certain ones are preferentially expressed in glia and others in neurons. Of the beta isoforms, beta1 predominates in neurons and beta2 in astrocytes, although there are some exceptions. Here we demonstrate that beta3 is expressed in rat and mouse white matter oligodendrocytes. Immunofluorescence microscopy identified beta3 in oligodendrocytes of rat brain white matter in typical linear arrays of cell bodies between fascicles of axons. The intensity of stain peaked at 20 postnatal days. beta3 was identified in cortical oligodendrocytes grown in culture, where it was expressed in processes and colocalized with antibody to galactocerebroside. In the mouse and rat optic nerve, beta3 stain was seen in oligodendrocytes, where it colocalized with carbonic anhydrase II. For comparison, optic nerve was stained for the beta1 and beta2 subunits, showing distinct patterns of labelling of axons (beta1) and astrocytes (beta2). The C6 glioma cell line was also found to express the beta3 isoform preferentially. Since beta3 was not found at detectable levels in astrocytes, this suggests that C6 is closer to oligodendrocytes than astrocytes in the glial cell lineage.

Separate neuronal and glial Na+,K+-ATPase isoforms regulate glucose utilization in response to membrane depolarization and elevated extracellular potassium

The role of cell type-specific Na+,K+-ATPase isozymes in function-related glucose metabolism was studied using differentiated rat brain cell aggregate cultures. In mixed neuron-glia cultures, glucose utilization, determined by measuring the rate of radiolabeled 2-deoxyglucose accumulation, was markedly stimulated by the voltage-dependent sodium channel agonist veratridine (0.75 micromol/L), as well as by glutamate (100 micromol/L) and the ionotropic glutamate receptor agonist N-methyl-D-aspartate (NMDA) (10 micromol/L). Significant stimulation also was elicited by elevated extracellular potassium (12 mmol/L KCl), which was even more pronounced at 30 mmol/L KCl. In neuron-enriched cultures, a similar stimulation of glucose utilization was obtained with veratridine, specific ionotropic glutamate receptor agonists, and 30 mmol/L but not 12 mmol/L KCl. The effects of veratridine, glutamate, and NMDA were blocked by specific antagonists (tetrodotoxin, CNQX, or MK801, respectively). Low concentrations of ouabain (10(-6) mol/L) prevented stimulation by the depolarizing agents but reduced only partially the response to 12 mmol/L KCl. Together with previous data showing cell type-specific expression of Na+,K+-ATPase subunit isoforms in these cultures, the current results support the view that distinct isoforms of Na+,K+-ATPase regulate glucose utilization in neurons in response to membrane depolarization, and in glial cells in response to elevated extracellular potassium.