A putative fatty acid-binding domain of the NMDA receptor

Autism as a disorder of deficiency of brain-derived neurotrophic factor and altered metabolism of polyunsaturated fatty acids

Autism has a strong genetic and environmental basis in which inflammatory markers and factors concerned with synapse formation, nerve transmission, and information processing such as brain-derived neurotrophic factor (BDNF), polyunsaturated fatty acids (PUFAs): arachidonic (AA), eicosapentaenoic (EPA), and docosahexaenoic acids (DHA) and their products and neurotransmitters: dopamine, serotonin, acetylcholine, γ-aminobutyric acid, and catecholamines and cytokines are altered. Antioxidants, vitamins, minerals, and trace elements are needed for the normal metabolism of neurotrophic factors, eicosanoids, and neurotransmitters, supporting reports of their alterations in autism. But, the exact relationship among these factors and their interaction with genes and proteins concerned with brain development and growth is not clear. It is suggested that maternal infections and inflammation and adverse events during intrauterine growth of the fetus could lead to alterations in the gene expression profile and proteomics that results in dysfunction of the neuronal function and neurotransmitters, alteration(s) in the metabolism of PUFAs and their metabolites resulting in excess production of proinflammatory eicosanoids and cytokines and a deficiency of anti-inflammatory cytokines and bioactive lipids that ultimately results in the development of autism. Based on these evidences, it is proposed that selective delivery of BDNF and methods designed to augment the production of anti-inflammatory cytokines and eicosanoids and PUFAs may prevent, arrest, or reverse the autism disease process.

Baifuzi reduces transient ischemic brain damage through an interaction with the STREX domain of BKCa channels

Stroke is a long-term disability and one of the leading causes of death. However, no successful therapeutic intervention is available for the majority of stroke patients. In this study, we explored a traditional Chinese medicine Baifuzi (Typhonium giganteum Engl.). We show, at first, that the ethanol extract of Baifuzi exerts neuroprotective effects against brain damage induced by transient global or focal cerebral ischemia in rats and mice. Second, the extract activated large-conductance Ca(2+)-activated K(+) channel (BK(Ca)) channels, and BK(Ca) channel blockade suppressed the neuroprotection of the extract, suggesting that the BK(Ca) is the molecular target of Baifuzi. Third, Baifuzi cerebroside (Baifuzi-CB), purified from its ethanol extract, activated BK(Ca) channels in a manner similar to that of the extract. Fourth, the stress axis hormone-regulated exon (STREX) domain of the BK(Ca) channel directly interacted with Baifuzi-CB, and its deletion suppressed channel activation by Baifuzi-CB. These results indicate that Baifuzi-CB activated the BK(Ca) channel through its direct interaction with the STREX domain of the channel and suggests that Baifuzi-CB merits exploration as a potential therapeutic agent for treating brain ischemia.

A neuroscientist's guide to lipidomics

Nerve cells mould the lipid fabric of their membranes to ease vesicle fusion, regulate ion fluxes and create specialized microenvironments that contribute to cellular communication. The chemical diversity of membrane lipids controls protein traffic, facilitates recognition between cells and leads to the production of hundreds of molecules that carry information both within and across cells. With so many roles, it is no wonder that lipids make up half of the human brain in dry weight. The objective of neural lipidomics is to understand how these molecules work together; this difficult task will greatly benefit from technical advances that might enable the testing of emerging hypotheses.

Lipid emulsions reduce NMDA-evoked currents

Membrane currents conducted by the NMDA receptor channels were investigated in cultured cortical neurons and TsA cells transfected with NR1-1a/NR2A subunits of the NMDA receptor. The whole-cell recording technique was used. Current transients evoked by bath application of NMDA for 5 s were characterized by a fast peak and a slow decay to 46.1 +/-15.5% of the peak level at the end. When NMDA was applied in combination with various lipid emulsions (Intralipid, ClinOleic, Lipofundin or Abbolipid, the NMDA-induced currents were reduced, although this reduction did not affect the fast peak, it did affect the decay phase. The amount of reduction depended on the concentration of the lipids (in the case of Abbolipid diluted at 1:40, the current at the end of the 5-s drug application was approximately 2/3 of control). When Abbolipid was applied 40 s before NMDA, peak and late current were reduced to approximately 2/3. The effect of current reduction was the same at either of the two chosen membrane potentials (-80 and +40 mV) which indicates that the effect was not mediated by contamination of the emulsions with Mg(2+). The current reduction produced by Abbolipid was about the same in native neuronal cells and in TsA cells expressing the NR1-1a/NR2A subunits. The current-reducing effect of the lipid emulsions may add to the anesthetic, analgesic and neuroprotective effects seen with hypnotics administered by way of lipid carriers.

Heart type fatty acid binding protein (H-FABP) is decreased in brains of patients with Down syndrome and Alzheimer’s disease

Fatty acid binding proteins (FABPs) are thought to play a role in the binding, targeting and transport of long-chain fatty acids, and at least three types of FABPs are found in human brain; heart type (H)-FABP, brain type (B)-FABP and epidermal type (E)-FABP. Although all three FABPs could be involved in normal brain function in prenatal and postnatal life, a neurobiological role of FABPs in neurodegenerative diseases has not been reported yet. These made us evaluate the protein levels of FABPs in brains from patients with Down syndrome (DS) and Alzheimer's disease (AD) and fetal cerebral cortex with DS using two-dimensional (2-D) gel electrophoresis with subsequent matrix-assisted laser desorption ionization mass spectroscopy (MALDI-MS) identification and specific software for quantification of proteins. In adult brain, B-FABP was significantly increased in occipital cortex of DS, and H-FABP was significantly decreased in DS (frontal, occipital and parietal cortices) and AD (frontal, temporal, occipital and parietal cortices). In fetal brain, B-FABP and epidermal E-FABP levels were comparable in controls and DS. We conclude that aberrant expression of FABPs, especially H-FABP may alter membrane fluidity and signal transduction, and consequently could be involved in cellular dysfunction in neurodegenerative disorders.

Role of three isoforms of phospholipase A2 in capacitative calcium influx in human T-cells

The present study was conducted on human Jurkat T-cell lines in order to elucidate the role of phospholipase A2 in capacitative calcium entry. We have employed thapsigargin (TG) that induces increases in [Ca2+]i by emptying the calcium pool of endoplasmic reticulum, followed by capacitative calcium entry. We designed a Ca2+ free/Ca2+ reintroduction (CFCR) protocol for the experiments, conducted in Ca2+-free medium. By employing CFCR protocol, we observed that addition of exogenous arachidonic acid (AA) stimulated TG-induced capacitative calcium influx. The liberation of endogenous AA and its autocrine action seems to be implicated during TG-induced capacitative calcium influx: TG potentiates the induction of constitutively expressed mRNA of four PLA2 isoforms (type 1B, IV, V, VI), the inhibitors of the three PLA2 isotypes (type 1B, V, VI) inhibit TG-induced release of [3H]AA into the extracellular medium, and finally, these PLA2 inhibitors do curtail TG-stimulated capacitative calcium entry in these cells. These results suggest that stimulation of three isoforms of PLA2 by thapsigargin liberates free AA that, in turn, induces capacitative calcium influx in human T-cells.

Activation of neuronal N-methyl-D-aspartate receptor channels by lipid emulsions

Lipid emulsions are widely used as carriers for hypnotics such as propofol, etomidate, and diazepam. It is assumed that the emulsions alone exert no effect on cellular functions nor influence the pharmacokinetics, pharmacodynamics, or anesthetic and analgetic potency of the hypnotics they carry. To elucidate possible interactions between lipid emulsions and cell membranes, in particular membrane-bound proteins, we investigated the effects of commercially available lipid emulsions on the cell membranes of cultured cortical neurons from the mouse by using the whole-cell configuration of the patch-clamp technique. Of nine lipid emulsions tested, three, i.e., Intralipid, Structolipid, and, to a much lesser extent, Abbolipid, activated membrane currents in the neuronal cells in a dilution-dependent manner. The emulsion-induced currents were not affected by picrotoxin or bicuculline but were inhibited by DL-AP5 and ketamine. The voltage dependence of the currents was influenced by the presence of Mg(2+) in a way that is typical for currents conducted by N-methyl-D-aspartate receptor channels. We conclude that Intralipid, Structolipid, and Abbolipid activate N-methyl-D-aspartate receptor channels in cortical neurons.

IMPLICATIONS

Lipid emulsions are widely used as carriers for hypnotics such as propofol, etomidate, or diazepam. We tested nine commercially available lipid emulsions and demonstrate that three of them--Intralipid, Structolipid, and Abbolipid--activate NMDA receptor channels in the membranes of cortical neuronal cells.

Direct activation of an inwardly rectifying potassium channel by arachidonic acid

Arachidonic acid (AA) is an important constituent of membrane phospholipids and can be liberated by activation of cellular phospholipases. AA modulates a variety of ion channels via diverse mechanisms, including both direct effects by AA itself and indirect actions through AA metabolites. Here, we report excitatory effects of AA on a cloned human inwardly rectifying K(+) channel, Kir2.3, which is highly expressed in the brain and heart and is critical in regulating cell excitability. AA potently and reversibly increased Kir2.3 current amplitudes in whole-cell and excised macro-patch recordings (maximal whole-cell response to AA was 258 +/- 21% of control, with an EC(50) value of 447 nM at -97 mV). This effect was apparently caused by an action of AA at an extracellular site and was not prevented by inhibitors of protein kinase C, free oxygen radicals, or AA metabolic pathways. Fatty acids that are not substrates for metabolism also potentiated Kir2.3 current. AA had no effect on the currents flowing through Kir2.1, Kir2.2, or Kir2.4 channels. Experiments with Kir2.1/2.3 chimeras suggested that, although AA may bind to both Kir2.1 and Kir2.3, the transmembrane and/or intracellular domains of Kir2.3 were essential for channel potentiation. These results argue for a direct mechanism of AA modulation of Kir2.3.

Effects of fatty acids on BK channels in GH(3) cells

Ca(2+)-activated K(+) (BK) channels in GH(3) cells are activated by arachidonic acid (AA). Because cytosolic phospholipase A(2) can produce other unsaturated free fatty acids (FFA), we examined the effects of FFA on BK channels in excised patches. Control recordings were made at several holding potentials. The desired FFA was added to the bath solution, and the voltage paradigm was repeated. AA increased the activity of BK channels by 3.6 +/- 1.6-fold. The cis FFA, palmitoleic, oleic, linoleic, linolenic, eicosapentaenoic, and the triple bond analog of AA, eicosatetraynoic acid, all increased BK channel activity, whereas stearic (saturated) or the trans isomers elaidic, linolelaidic, and linolenelaidic had no effect. The cis unsaturated FFA shifted the open probability vs. voltage relationships to the left without a change in slope, suggesting no change in the sensitivity of the voltage sensor. Measurements of membrane fluidity showed no correlation between the change of membrane fluidity and the change in BK channel activation. In addition, AA effects on BK channels were unaffected in the presence of N-acetylcysteine. Arachidonyl-CoA, a membrane impermeable analog of AA, activates channels when applied to the cytosolic surface of excised patches, suggesting an effect of FFAs from the cytosolic surface of BK channels. Our data imply a direct interaction between cis FFA and the BK channel protein.

Influence of cannabinoids on the delayed rectifier in freshly dissociated smooth muscle cells of the rat aorta

The influence of the cannabinoids anandamide, methanandamide and WIN 55212-2 on the delayed rectifier K(+) current (I(K(V))) in rat arterial myocytes was investigated. Anandamide caused a concentration-dependent reduction of total peak and late K(+) current (I(K)). The maximal effect (about 50% inhibition of I(K)) was reached with 3 microM, and half-maximal current block was observed at 0.6 microM. Blockade was voltage-independent. Inhibition of I(K) by the cannabinoid was associated with a characteristic increase in the rate of current relaxation. Methanandamide (10 microM), a metabolically more stable analogue of anandamide, decreased I(K) with a similar time course. Current traces in the presence of the drug also showed an acceleration of inactivation. The presence of TEA did not impair the inhibition by anandamide or methanandamide, but inhibition was prevented by pre-exposure to 4-AP, showing that both cannabinoids inhibited I(K(V)) while having no influence on Ca(2+)-dependent K(+) current (I(K(Ca))). The CB(1) receptor antagonist SR141716A (10 microM) did not influence the action of anandamide or methanandamide. Arachidonic acid (1 microM) increased I(K) considerably. However, in the presence of TEA it caused a decrease of I(K(V)) with a characteristic increase in the rate of current relaxation. WIN 55212-2 (20 microM) caused similar inhibition of I(K). Internally applied anandamide (10 microM) or methanandamide (10 microM) was ineffective at influencing I(K). In the dialyzed cells, the additional external application of a cannabinoid promptly initiated inhibition. The results show that anandamide, methanandamide and WIN 55212-2 affect I(K(V)) in a cannabinoid receptor-independent way similar to that of arachidonic acid, which, unlike the cannabinoids, additionally increases a Ca(2+)-activated K(+) current. It is suggested that cannabinoids might bind to an external site on or near the K(v) channel of the vascular smooth muscle cells.

Cytosolic phospholipase A2 and the distinct transcriptional programs of astrocytoma cells

Astrocytes constitute the most abundant cell type in the nervous system. Under physiological conditions, they respond to the stimuli to which neurons are also responsive. The use of astrocytoma cell lines with well-defined morphological and functional markers has been helpful for addressing the mechanisms of signal transduction that operate in the nervous system. On the basis of the effects produced by agonists of different types of receptor (muscarinic ACh receptors, thrombin receptors, phospholipases A2 receptors and tumor necrosis factor alpha receptors), several different transcriptional programs that involve the MAP kinase-cytosolic phospholipase A2 system and the transcription factor NF-kappaB have been described.

Arachidonic acid stimulates a novel cocaine-sensitive cation conductance associated with the human dopamine transporter

The dopamine transporter (DAT) exhibits several ionic currents that are either coupled to or uncoupled from the transport of substrate. Second messenger systems have been shown to modulate dopamine (DA) transport, however, the modulation of DAT-associated currents has not been studied in depth. Using the two-electrode voltage-clamp method to record from Xenopus oocytes expressing the human DAT, we examined the effects of arachidonic acid (AA) on membrane currents. AA (10-100 microM) stimulates a novel nonselective cation conductance seen only in oocytes expressing human DA transporter (hDAT). The AA-stimulated conductance is up to 50-fold greater than the current normally elicited by DA, but does not appear to arise from the modulation of previously described hDAT conductances, including the leak current and the current associated with electrogenic transport. In addition, DA dramatically potentiates and cocaine blocks the AA-stimulated DAT current. DA potentiates the AA-induced currents in the absence of sodium and chloride, indicating that these currents arise from processes distinct from those associated with substrate transport. The effects of AA were mimicked by other fatty acids with a rank order of potency correlated with their degree of unsaturation, suggesting that AA directly stimulates the novel cation current. Therefore, AA stimulation of this DAT-associated conductance may provide a novel mechanism for modulation of neuronal signaling.

Subunit- and site-specific pharmacology of the NMDA receptor channel

N-Methyl-D-aspartate (NMDA) receptor channels play important roles in various physiological functions such as synaptic plasticity and synapse formation underlying memory, learning and formation of neural networks during development. They are also important for a variety of pathological states including acute and chronic neurological disorders, psychiatric disorders, and neuropathic pain syndromes. cDNA cloning has revealed the molecular diversity of NMDA receptor channels. The identification of multiple subunits with distinct distributions, properties and regulation, implies that NMDA receptor channels are heterogeneous in their pharmacological properties, depending on the brain region and the developmental stage. Furthermore, mutation studies have revealed a critical role for specific amino acid residues in certain subunits in determining the pharmacological properties of NMDA receptor channels. The molecular heterogeneity of NMDA receptor channels as well as their dual role in physiological and pathological functions makes it necessary to develop subunit- and site-specific drugs for precise and selective therapeutic intervention. This review summarizes from a molecular perspective the recent advances in our understanding of the pharmacological properties of NMDA receptor channels with specific references to agonists binding sites, channel pore regions, allosteric modulation sites for protons, polyamines, redox agents, Zn2+ and protein kinases, phosphatases.

Oxidative downmodulation of the transient K-current IA by intracellular arachidonic acid in rat hippocampal neurons

Membrane-permeable arachidonic acid (AA) is liberated in a Ca2+-dependent way inside cells. By using whole cell patch clamp we show that intracellular AA (1 pM) selectively reduces IA in rat hippocampal neurons, whereas extracellular application requires a 10(6)-fold concentration. The nonmetabolized AA analogue ETYA mimics the effect of AA that is blocked by ascorbic acid or intracellular glutathione, suggesting an intracellular oxidative mechanism. We conclude that intracellular AA is extremely potent in reducing IA by an oxidative mechanism, particularly during oxidative stress.

Opposite modulation of NMDA receptors by lysophospholipids and arachidonic acid: common features with mechanosensitivity

1. Two classes of amphiphilic compounds, lysophospholipids and arachidonic acid, have been suggested to produce opposite deformations of the lipid bilayer. We have found that their effects on N-methyl-D-aspartate (NMDA) responses are opposite, and resemble those of mechanical deformations of the plasma membrane. 2. Lysophospholipids inhibited NMDA responses both in nucleated patches taken from cultured neurons and in cells expressing recombinant NMDA receptors. This inhibition was reversible, voltage independent and stronger at non-saturating doses of agonist. It was not linked to the charge of the polar head, and was not mimicked by lysophosphatidic acid or phosphatidylcholine. In outside-out patches, lysophospholipids reduced the open probability of NMDA-activated channels without changing their single-channel conductance. 3. The inhibition produced by lysophospholipids occluded that produced by a mechanical compression induced by changes in osmotic or hydrostatic pressure. 4. The potentiation of NMDA responses by arachidonic acid was observed both in native and recombinant receptors, including those in which the putative 'fatty acid binding domain' had been deleted. This suggests that, like lysophospholipids, arachidonic acid alters the NMDA receptor by insertion into the lipid bilayer. 5. Recombinant receptors in which the cytoplasmic tails had been modified or deleted were still sensitive to mechanical deformation. A linkage to the cytoskeleton is therefore not required for NMDA receptor mechanosensitivity. 6. The fact that the NMDA responses are depressed similarly by compression and lysophospholipids, and potentiated similarly by stretch and arachidonic acid supports the notion that the modulation of NMDA receptor activity by asymmetrical amphiphilic compounds involves pressure changes transmitted through the lipid bilayer. Compounds with a large hydrophilic head mimic the effects of a compression, and compounds with a small hydrophilic head mimic the effects of stretch.

Inhibition of rat neuronal kainate receptors by cis-unsaturated fatty acids

1. Whole-cell recordings from cultured rat hippocampal neurons, from freshly dissociated dorsal root ganglion (DRG) neurons and from human embryonic kidney (HEK) 293 cells expressing the glutamate receptor GluR6 subunit were used to study the modulation of kainate receptor channels by long chain fatty acids. 2. In all three cell types, application of cis-unsaturated fatty acids caused a dose-dependent reduction in whole-cell currents evoked by kainate. Docosahexaenoic acid (DHA), arachidonic acid (AA), linolenic acid and linoleic acid all produced substantial inhibition at a concentration of 50 microM, whereas inhibition by linolenelaidic acid and linolelaidic acid was significantly weaker. Fully saturated fatty acids were essentially inactive. 3. With continuous exposure to active fatty acids, the peak current elicited by kainate declined over a time course of several minutes to reach a steady-state level less than 50 % of the initial amplitude. Recovery was slow in control solution, but was speeded up by exposure to bovine serum albumin (0.5 mg ml-1), a protein that binds fatty acids with submicromolar affinity. The inhibition in neurons was half-maximal with 5-15 microM AA or DHA, but potency was at least 10-fold greater at GluR6 in HEK 293 cells. 4. Inhibition by AA or DHA was unaffected by extracellular nordihydroguaiaretic acid (10 microM), indomethacin (10 microM), 17-octadecynoic acid (30 microM) or 1-(5-isoquinolinylsulphonyl)-2-methylpiperazine dihydrochloride (H-7; 10 microM). Furthermore, inclusion of H-7 (100 microM), BAPTA (10 mM), AA (50 microM), antioxidants, or the protein kinase C inhibitor PKC19-36 (20 microM) in the internal solution had little effect on whole-cell currents and did not prevent inhibition of currents by extracellular application of AA or DHA. 5. We conclude that the inhibition produced by cis-unsaturated fatty acids does not require conversion to oxidized metabolites or activation of PKC. Instead, active compounds may interact directly with an extracellular, or intramembraneous, site on kainate receptors.

Alterations of potassium channel activity in retinal Müller glial cells induced by arachidonic acid

Arachidonic acid, which is thought to be involved in pathogenetic mechanisms of the central nervous system, has been shown previously to modulate neuronal ion channels and the glutamate uptake carrier of retinal glial (Müller) cells. We have used various configurations of the patch-clamp technique to determine the effects of arachidonic acid on the K+ currents of freshly isolated Müller glial cells from rabbit and human. Arachidonic acid reduced the peak amplitude of the transient (A-type) outward K+ currents in a dose-dependent and reversible manner, with a 50% reduction achieved by 4.1 microM arachidonic acid. The inward rectifier-mediated currents remained unchanged after arachidonic acid application. The amplitude of the Ca(2+)-activated K+ outward currents (KCa), which were blocked by 1 mM tetraethylammonium chloride and 40 nM iberiotoxin, respectively, was dose-dependently elevated by bath application of arachidonic acid. The activation curve of the KCa currents shifted towards more negative membrane potentials. Furthermore, arachidonic acid was found to suppress inwardly directed Na+ currents. In cell-attached recordings with 3 mM K+ in the bath and 130 mM K+ in the pipette, the KCa channels of rabbit Müller cells displayed a linear current-voltage relation, with a mean slope conductance of 102 pS. In excised patches, the slope conductance was 220 pS (150 mM K+i/130 mM K+o). The opening probability of the KCa channels increased during membrane depolarization and during elevation of the free Ca2+ concentration at the intracellular face of the membrane patches. Bath application of arachidonic acid caused a reversible increase of the single-channel opening probability, as well as an increase of the number of open channels. Arachidonic acid did not affect the single-channel conductance. Since arachidonic acid also stimulates the KCa channel activity in excised patches, the action of arachidonic acid is assumed to be independent of changes of the intracellular calcium concentration. Our results demonstrate that arachidonic acid exerts specific effects on distinct types of K+ channels in retinal glial, cells. In pathological cases, elevated arachidonic acid levels may contribute to prolonged Müller cell depolarizations, and to the initiation of reactive glial cell proliferation.

Mass spectrometric evidence that agents that cause loss of Ca2+ from intracellular compartments induce hydrolysis of arachidonic acid from pancreatic islet membrane phospholipids by a mechanism that does not require a rise in cytosolic Ca2+ concentration

Stimulation of pancreatic islets with glucose induces phospholipid hydrolysis and accumulation of nonesterified arachidonic acid, which may amplify the glucose-induced Ca2+ entry into islet beta-cells that triggers insulin secretion. Ca2+ loss from beta-cell intracellular compartments has been proposed to induce both Ca2+ entry and events dependent on arachidonate metabolism. We examine here effects of inducing Ca2+ loss from intracellular sequestration sites with ionophore A23187 and thapsigargin on arachidonate hydrolysis from islet phospholipids. A23187 induces a decline in islet arachidonate-containing phospholipids and release of nonesterified arachidonate. A23187-induced arachidonate release is of similar magnitude when islets are stimulated in Ca2+-replete or in Ca2+-free media or when islets loaded with the intracellular Ca2+ chelator BAPTA are stimulated in Ca2+-free medium, a condition in which A23187 induces no rise in beta-cell cytosolic [Ca2+]. Thapsigargin also induces islet arachidonate release under these conditions. A23187- or thapsigargin-induced arachidonate release is prevented by a bromoenol lactone (BEL) inhibitor of a beta-cell phospholipase A2 (iPLA2), which does not require Ca2+ for catalytic activity and which is negatively modulated by and physically interacts with calmodulin by Ca2+-dependent mechanisms. Agents that cause Ca2+ loss from islet intracellular compartments thus induce arachidonate hydrolysis from phospholipids by a BEL-sensitive mechanism that does not require a rise in cytosolic [Ca2+], and a BEL-sensitive enzyme-like iPLA2 or a related membranous activity may participate in sensing Ca2+ compartment content.

Arachidonic acid elicits a substrate-gated proton current associated with the glutamate transporter EAAT4

Arachidonic acid modulates both electrical and biochemical properties of membrane proteins involved in cellular signaling. In Xenopus laevis oocytes expressing the excitatory amino acid transporter EAAT4, physiologically relevant concentrations of arachidonic acid increase the amplitude of the substrate-activated current by roughly twofold at -60 mV. This stimulation is not attributable to the modulation of either substrate/ion cotransport or the ligand-gated chloride current, the major conductance associated with this carrier. Ion-substitution experiments reveal that arachidonic acid stimulates a proton-selective conductance. The effect does not require metabolism of arachidonic acid and is not blocked by inhibitors of endogenous oocyte ion-exchangers. This proton conductance expands the complex repertoire of the ligand-gated channel properties associated with EAAT4.

Modulation of ACh receptor currents by arachidonic acid

The present study investigated the effects of arachidonic acid on Torpedo (alpha beta gamma delta) and neuronal nicotinic acetylcholine (ACh) receptors (chick alpha7; rat alpha7, alpha3 beta2, alpha3 beta4, alpha4 beta2, and alpha4 beta4). Arachidonic acid (10 microM) depressed currents through normal Torpedo ACh receptors during treatment and afterward, persistently (>/=30 min) potentiated the currents. The potentiation was blocked by the selective protein kinase C (PKC) inhibitor, GF109203X or PKC inhibitor peptide (PKCI). The depression was not inhibited by any protein kinase inhibitor examined here, but greater in Ca2+-free extracellular solution. Arachidonic acid also potentiated currents through mutant Torpedo ACh receptors lacking PKC phosphorylation sites at Ser333 on the alpha subunit and Ser377 on the delta subunit without depression, but otherwise, it depressed currents through mutant receptors replacing of each Ser by negatively charged amino acid residue, possibly that mimics PKC phosphorylation of the receptors. These results suggest that the depression was due to the direct blocking effect on Ca2+-modulatory sites, which was accelerated under conditions of the receptors phosphorylated by PKC, and that the potentiation was caused by PKC activation, independently of PKC phosphorylation of the receptors. Arachidonic acid reduced currents through chick alpha7 receptors by a mechanism independent of protein kinase activation. In contrast, arachidonic acid potentiated currents through rat alpha7, alpha3 beta2, alpha4 beta2, and alpha4 beta4 receptors, perhaps by the same mechanism as the potentiation observed in Torpedo ACh receptors, although it had no effect on rat alpha3 beta4 receptors. The results of the present study thus demonstrate that arachidonic acid exerts diverse actions on nicotinic ACh receptors by different mechanisms.

Functional modulation of human recombinant gamma-aminobutyric acid type A receptor by docosahexaenoic acid

Human gamma-aminobutyric acid type A (GABAA) receptors were expressed in the baculovirus/Sf-9 insect cell expression system using recombinant cDNA of alpha1beta2gamma2s subunits. The effect of unsaturated fatty acids on GABAA receptor complexes was investigated electrophysiologically using conventional whole cell recording under voltage clamp. Three distinct effects of docosahexaenoic acid (DHA) on the GABA responses were observed. First, DHA, at a concentration of 10(-7) M or greater, accelerated the desensitization after the peak of the GABA-induced current. Second, DHA (10(-6) M) potentiated the peak amplitude of GABA response. This potentiation by DHA was inhibited in the presence of Zn2+ (10(-5) M); Cu2+ and Ni2+ mimicked the action of Zn2+. Zn2+ (10(-5) M) did not block the GABA response on alpha1beta2gamma2s receptor complexes. Third, DHA, at a concentration of 3 x 10(-6) M or higher, gradually suppressed the peak amplitude of GABA response. A protein kinase A inhibitor, a protein kinase C inhibitor, and a Ca2+ chelator did not modify the effects of DHA on GABA-induced chloride ion current. Six unsaturated fatty acids other than DHA were examined. Arachidonic acid mimicked the effect of DHA while e.g. oleic acid had no effect. The inhibition of the GABA response in the presence of DHA was also observed in cells expressing GABAA receptors of alpha1 and beta2 subunit combinations. The data show that the gamma subunit is essential for DHA and arachidonic acid to potentiate the GABA-induced Cl- channel activity and to affect the desensitization kinetics of the GABAA receptor.

Electrospray ionization mass spectrometric analyses of phospholipids from rat and human pancreatic islets and subcellular membranes: comparison to other tissues and implications for membrane fusion in insulin exocytosis

Glucose-induced insulin secretion from pancreatic islets involves hydrolysis of arachidonic acid from phospholipids as an intermediary event. Accumulation of nonesterified arachidonate in islet membranes may influence both ion fluxes that trigger insulin secretion and fusion of secretory granule and plasma membranes. Recent findings indicate that plasmenylethanolamine species may also participate in fusion of such membranes, but high-performance liquid chromatographic (HPLC) and gas chromatographic/mass spectrometric (GC/MS) analyses of islet secretory granule phospholipids suggested that they contain little plasmenylethanolamine. Here, electrospray ionization mass spectrometry (ESI/MS) of intact phospholipid molecules is used to demonstrate that the most prominent components of all major glycerophospholipid headgroup classes in islets are arachidonate-containing species. Such species contribute the majority of the ESI/MS negative ion current from rat and human islet glycerophosphoethanolamine (GPE), and the fraction of GPE negative ion current contributed by plasmenylethanolamine species in rat islets is higher than that for rat liver or heart and similar to that for brain. The most prominent sn-2 substituent of plasmenylethanolamine species in brain is docosahexaenoate and in islets is arachidonate. Arachidonate-containing plasmenylethanolamine species are also prominent components of GPE from islet secretory granules and plasma membranes. Fusion of islet secretory granule and plasma membranes is demonstrated to be catalyzed by cytosolic components from insulinoma cells and rat brain with chromatographic similarities to a rabbit brain factor that specifically catalyzes fusion of plasmenylethanolamine-containing membranes.

Two distinct types of fatty acid-binding protein are expressed in heart ventricle of Antarctic teleost fishes

This report provides the first evidence for the existence of two distinct types of fatty acid-binding protein (FABP) in cardiac tissue of vertebrates. Four species of Antarctic teleost fish (Chaenocephalus aceratus, Cryodraco antarcticus, Gobionotothen gibberifrons and Notothenia coriiceps) exhibited two FABP mRNAs of 1. 0 kb and 0.8 kb, which we have termed Hh-FABP and Had-FABP (isolated from Heart tissue, with similarity to mammalian heart-type FABP or mammalian adipose-type FABP respectively). These FABP types appear to be products of distinct genes. Both FABP transcripts were abundant in cardiac and aerobic pectoral muscle. However, relative abundance of the two types varied distinctly among other tissues such as kidney, brain, spleen and white muscle. Neither FABP type was expressed in liver or intestine. The coding regions of Hh-FABP and Had-FABP cDNAs from the same species are only approximately 60% identical with one another. However, homologues of each FABP species, which exhibit >98% identity to their respective types, were isolated from three other Antarctic teleosts. Phylogenetic analysis of aligned amino-acid sequences places Hh-FABP with other vertebrate heart-type FABPs, and Had with adipose/cutaneous FABPs. Expression of two distinct FABPs in cardiac tissue of Antarctic teleosts may be related to their ability to both utilize fatty acid as the primary metabolic fuel and to store lipid intracellularly.

Modulation of K+ channels by arachidonic acid in T84 cells. II. Activation of a Ca(2+)-independent K+ channel

We used single-channel recording techniques to identify and characterize a large-conductance, Ca(2+)-independent K+ channel in the colonic secretory cell line T84. In symmetric potassium gluconate, this channel had a linear current-voltage relationship with a single-channel conductance of 161 pS. Channel open probability (Po) was increased at depolarizing potentials. Partial substitution of bath K+ with Na+ indicated a permeability ratio of K+ to Na+ of 25:1. Channel Po was reduced by extracellular Ba2+. Event-duration analysis suggested a linear kinetic model for channel gating having a single open state and three closed states: C3<-->C2<-->C1<-->O. Arachidonic acid (AA) increased the Po of the channel, with an apparent stimulatory constant (Ks) of 1.39 microM. Neither channel open time (O) nor the fast closed time (C1) was affected by AA. In contrast, AA dramatically reduced mean closed time by decreasing both C3 and C2. The cis-unsaturated fatty acid linoleate increased Po also, whereas the saturated fatty acid myristate and the trans-unsaturated fatty acid elaidate did not affect Po. This channel is activated also by negative pressure applied to the pipette during inside-out recording. Thus we determined the effect of the stretch-activated channel blockers amiloride and Gd3+ on the K+ channel after activation by AA. Amiloride (2 mM) on the extracellular side reduced single-channel amplitude in a voltage-dependent manner, whereas Gd3+ (100 microM) had no effect on channel activity. Activation of this K+ channel may be important during stimulation of Cl- secretion by agonists that use AA as a second messenger (e.g., vasoactive intestinal polypeptide, adenosine) or during the volume regulatory response to cell swelling.

Distinct sites for inverse modulation of N-methyl-D-aspartate receptors by sulfated steroids

Steroid sulfation occurs in nervous tissue and endogenous sulfated steroids can act as positive or negative modulators of N-methyl-D-aspartate (NMDA) receptor function. In the current study, structure-activity relationships for sulfated steroids were examined in voltage-clamped chick spinal cord and rat hippocampal neurons in culture and in Xenopus laevis oocytes expressing NR1(100) and NR2A subunits. The ability of pregnenolone sulfate (a positive modulator) and epipregnanolone sulfate (a negative modulator) to compete with each another, as well as with other known classes of NMDA receptor modulators, was examined. The results show that steroid positive and negative modulators act at specific, extracellularly directed sites that are distinct from one another and from the spermine, redox, glycine, Mg2+, MK-801, and arachidonic acid sites. Sulfated steroids are effective as modulators of ongoing glutamate-mediated synaptic transmission, which is consistent with their possible role as endogenous neuromodulators in the CNS.

Effects of arachidonyltrifluoromethyl ketone on cytosolic [Ca2+] in HIT insulinoma cells

Arachidonyltrifluoromethyl ketone (ATFMK), an analogue of arachidonic acid (AA), inhibits an 85 kDa cytosolic phospholipase A2 enzyme. Exposure of HIT insulinoma cells to ATFMK induced a delayed, sustained, and irreversible increase in cytosolic [Ca2+] that required extracellular Ca2+ and a concentration-dependent inhibition of depolarization-induced increases in cytosolic [Ca2+] prior to onset of the delayed response to AFTMK. These results suggest a disruptive effect of ATFMK on calcium mobilization which may contribute to its effects on insulin secretion from beta-cells.

Inhibition of store-dependent capacitative Ca2+ influx by unsaturated fatty acids

The effects of the unsaturated fatty acids, arachidonic and oleic acid, on the influx of Ca2+ activated by depletion of intracellular stores with thapsigargin were investigated in various cell types. By using a Ca2+ free/Ca2+ reintroduction protocol, we observed that arachidonic acid (2 to 5 microM) inhibited thapsigargin-induced rises in cytosolic free Ca2+ ([Ca2+]i) in Ehrlich tumor cells, Jurkat T lymphocytes, rat thymocytes, and Friend erythroleukemia and PC12 rat pheochromocytoma cells. This effect was attributed to the inhibition of Ca2+ entry, since arachidonate also inhibited thapsigargin-stimulated unidirectional entry of the Ca2+ surrogates Ba2+ and Mn2+. In Ehrlich cells, the IC50 for arachidonic and oleic acid was 1.2 and 1.8 microM, respectively. The inhibition appeared to depend on the ratio [fatty acid]/[cells] rather than on the absolute fatty acid concentration. Experiments with [3H]-oleic acid revealed that the inhibitory activity was not correlated with cell internalisation and metabolism of the fatty acid. The inhibition was reverted by removal of the fatty acid bound to cell membrane by fatty acid-free albumin treatment. The unsaturated fatty acids had no effect on ATP/ADP cell levels and plasma membrane potential. Pharmacological evidence indicated that cell phosphorylation/dephosphorylation events, and pertussis toxin-sensitive G proteins were not involved. Other amphipathic lipophilic compounds, i.e. 2-bromopalmitic acid, retinoic acid, sphingosine, and dihydrosphingosine, mimicked arachidonic/oleic acid as they inhibited thapsigargin-stimulated Ca2+ influx in an albumin-reversible fashion. These results suggest that physiologically relevant (unsaturated) fatty acids can inhibit capacitative Ca2+ influx possibly because they intercalate into the plasma membrane and directly affect the activity of the channels involved.

Arachidonic acid activation of potassium channels in rat visual cortex neurons

We investigated the effects of arachidonic acid on K+ channels in freshly dissociated neurons of 10- to 20-day-old rat visual cortex, using a perforated and conventional whole-cell patch-clamp and inside-out excised patch configurations. Arachidonic acid at 5-30 microM induced an outward current in 88.1% of the neurons in whole-cell mode, and evoked channel opening with a conductance of 170-238 pS in 90.5% of neurons under inside-out patch recording. Arachidonic acid-activated K+ channels were partially blocked by extracellular administration of 1 mM tetraethylammonium and 100 nM charybdotoxin. However, Ba2+ completely blocked the channel in all cases. None of the other K+ channel blockers, including 4-aminopyridine, quinidine, apamin and glibenclamide, inhibited the arachidonic acid-activated channels. Intracellular perfusion with Ca2+-free and 5 mM BAPTA in Ca2+-free extracellular perfusate containing 2 mM EGTA in conventional whole-cell recording did not inhibit the K+ channel, implying that the channel is not Ca2+ dependent. Neither guanosine 5'-O-(2-thiodiphosphate) nor staurosporine applied in inside-out mode affected the arachidonic acid-activated channels, indicating that G-protein and protein kinase C are not involved in this phenomenon. In addition, neither indomethacin nor nordihydroguaiaretic acid blocked the channel currents, demonstrating that it is arachidonic acid itself but not its metabolites that induced the effect. Among the fatty acids tested, only cis-unsaturated fatty acids, having more than two double bonds, such as arachidonic acid, docosahexaenoic acid and linolenic acid, activated the K+ channels. These findings suggest that there exists a novel type of K+ channel activated by arachidonic acid which may play a critical role in modulating neuronal excitability in cortical neurons.

Short-term depression and long-term enhancement of ACh-gated channel currents induced by linoleic and linolenic acid

The effects of cis-unsaturated free fatty acids such as linoleic and linolenic acid on ACh-evoked currents were examined using normal and mutant nicotinic acetylcholine (ACh) receptors lacking protein kinase C (PKC) phosphorylation sites on the alpha and delta subunits expressed in Xenopus oocytes. These free fatty acids reduced ACh-gated channel currents during treatment and to a greater extent in Ca2+-free extracellular solution. After treatment, the currents were enhanced as the drug was washed out, but this effect was not observed in the absence of extracellular Ca2+. Linolenic acid was more potent of the current enhancement (300% of the control) than linoleic acid (190% of the control). The current enhancement induced by these free fatty acids was inhibited by the selective PKC inhibitor, GF109203X, while the current depression was not affected. Furthermore, these lipids decreased ACh-evoked currents in mutant ACh receptors to the same extent as in normal ACh receptors, but never enhanced the currents. These results indicate that linoleic and linolenic acid have biphasic actions on ACh receptor currents; a short-term depression and a long-term enhancement. The short-term depression may be due to an interaction with the ACh receptor channels, presumably at Ca2+ binding sites. The long-lasting enhancement appears to result from Ca2+-dependent PKC activation followed by PKC phosphorylation of the ACh receptors.

Effects of arachidonic acid on A-type potassium currents in smooth muscle cells of the guinea pig

Effects of arachidonic acid (AA) and related fatty acids on Ca2+ -independent transient (A-type) K+ current (I(A)) were examined in single myocytes of guinea pig vas deferens, ureter, and proximal colon as well as in rabbit vas deferens. The peak amplitude of I(A) was reduced by external application of AA (half-maximal inhibitory concentration = approximately 1 microM). The blocking effect was not changed significantly by indomethacin, nordihydroguaiaretic acid, guanosine 5'-O-(2-thiodiphosphate), or guanosine 5'-O-(3-thiotriphosphate). Pharmacological studies suggested that the effect of AA was not mediated by activation of protein kinases A or C or tyrosine kinase. AA (20:4) was the most potent of the four types of cis-eicosanoic acids with two to five double bonds (20:2 to 20:5) that were tested. I(A)-like current in cardiac atrial myocytes of the rabbit was not affected significantly by 30 microM AA. These results indicate that AA itself directly blocks A-type K+ channels. A relationship between stereospecific chemical structure of fatty acids and their blockade of A-type K+ channels is suggested. A-type K+ channels in smooth muscle cells can be clearly resolved from those in atrial myocytes by the responses to AA.

Oleic acid enhances ACh receptor currents by activation of Ca2+/calmodulin-dependent protein kinase II

Oleic acid, a cis-unsaturated free fatty acid, is proposed to be involved in the protein kinase C (PKC) activation pathway. Its biological actions, however, have not been well-characterized. We examined the effects of oleic acid on acetylcholine (ACh)-gated channel currents using Torpedo nicotinic ACh receptors expressed in Xenopus oocytes. Oleic acid (10 microM) enhanced the currents, reaching a maximum (140%) 20 min after treatment, while no enhancement was observed in Ca(2+)-free extracellular solution. The current potentiation by oleic acid was not inhibited by PKC inhibitors such as PKCI or GF109203X. Furthermore, oleic acid potentiated the currents in mutant ACh receptors lacking potential PKC phosphorylation sites. In contrast, the potentiation was fully inhibited by a CaMKII inhibitor, KN-62. These results strongly suggest that oleic acid potentiates ACh receptor currents by activation of calmodulin-dependent protein kinase II (CaMKII), independent of the PKC pathway.

LP2, a differentiation-associated lipid-binding protein expressed in bovine lens

A 13 kDa protein from bovine lens was identified and characterized by protein microsequencing and by rapid amplification of cDNA ends (RACE) PCR. Its complete sequence shows that this protein belongs to a family of fatty acid-binding proteins (FABPs), including myelin and adipocyte P2, that are associated with cellular differentiation. The bovine lens protein, designated LP2, shows very close similarity to human epidermal FABP (eFABP) and human eFABP was detected in human lens, suggesting that the two proteins might be orthologous. Reverse transcriptase-PCR (RT-PCR) was used to compare expression patterns of LP2 with those for actin and for the differentiation markers gamma B-crystallin and gamma s-crystallin in lens. Actin was most abundant in the relatively undifferentiated epithelial cells and decreased with lens cell differentiation. In contrast gamma B-crystallin and gamma s-crystallin were detected only in fibres (nuclear and cortical respectively). LP2 transcripts were detected most abundantly in fibre cells and apparently increased with cellular differentiation. Molecular modelling confirms that the sequence of LP2 fits the tertiary template of adipocyte P2 but reveals the presence of two close pairs of cysteine residues that might be susceptible to intramolecular disulphide bond formation under appropriate oxidizing conditions. LP2 is thus another potential target for oxidative stress during cataract formation in lens.

Vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP) potentiate the glutamate-evoked release of arachidonic acid from mouse cortical neurons. Evidence for a cAMP-independent mechanism

Glutamatergic neurotransmission is associated with release of arachidonic acid (AA) from membrane phospholipids of both neurons and astrocytes. Since free AA has been shown to enhance glutamate-mediated synaptic transmission, it can be postulated that glutamate release and AA formation constitute a positive feed-back mechanism for sustained excitatory neurotransmission. In the present study, we examined whether the glutamate-evoked release of AA could be modulated by peptides. Using mouse cortical neurons in primary cultures, we show that the release of AA evoked by glutamate is potentiated by vasoactive intestinal peptide and pituitary adenylate cyclase-activating polypeptide (PACAP). This effect is mediated through the activation of PACAP I receptors. However, several arguments show that this potentiating mechanism does not involve the cAMP/PKA pathway. 1) Increasing intracellular cAMP by either cholera toxin, forskolin, or 8-Br-cAMP treatments does not affect the glutamate-evoked release of AA; 2) potentiation of the glutamate response by PACAP is not prevented by the PKA inhibitor 8-Br-Rp-cAMPS. Also, an involvement of the phospholipase C protein kinase C pathways is unlikely since inhibitors of both phospholipase C (i.e. U-73122) and protein kinase C (i.e. Ro 31-8220) do not affect the potentiation of the glutamate response by PACAP. These observations indicate an effect mediated by PACAP I receptors, which does not involve the second messenger pathways classically associated with activation of this type of receptors. Furthermore, results indicate that this potentiating mechanism mediated by PACAP I receptor acts at a level downstream of the glutamate receptor-mediated calcium influx.

Altered sodium current response to intracellular fatty acids in halothane-hypersensitive skeletal muscle

Biopsies of human skeletal muscle were analyzed by an in vitro contracture test (IVCT) for responsiveness to a halothane challenge: noncontracting (nonresponsive; IVCT-) and contracting (IVCT+). A muscle biopsy that is IVCT+ indicates potential malignant hyperthermia (MH) susceptibility. Primary cultures were grown from portions of the skeletal muscle biopsies, and voltage-activated currents were measured by whole cell recording in the presence or absence of 2-5 microM intracellular arachidonic or oleic acids. In untreated IVCT- cells, Na+ currents were predominantly tetrodotoxin (TTX) insensitive, indicating that most of the current was carried through the embryonic SkM2 isoform of the Na+ channel. Inclusion of fatty acids in the recording pipette of IVCT- cells produced an increase in voltage-activated Na+ currents during 20 min of recording. Approximately 70% of currents in fatty acid-treated cells were TTX sensitive, indicating activation of the adult SkM1 isoform of the Na+ channel. In contrast to IVCT- cells, IVCT+ cells expressed Na+ currents that were predominantly TTX sensitive even in the absence of added fatty acid, thus showing a relatively large baseline functional expression of SkM1 channels. Addition of fatty acids to the recording pipette produced little further change in the magnitude or TTX sensitivity of the whole cell currents in IVCT+ cells, suggesting altered functional regulation of Na+ channels in MH muscle.

Effects of arachidonic acid upon the volume-sensitive chloride current in rat osteoblast-like (ROS 17/2.8) cells

1. The effects of arachidonic acid upon the volume-sensitive Cl- current present in cultured osteoblastic cells (ROS 17/2.8) was studied using the whole-cell patch-clamp technique. 2. Arachidonate produced two distinct phases of inhibition, a rapid phase occurring within 10-15 s of application, preceding a slower phase that occurred 2 min after onset of arachidonate superfusion. Accompanying the slower inhibitory phase was an acceleration of the time-dependent inactivation exhibited by the current at strongly depolarized potentials (> + 50 mV). The half-maximal inhibitory concentrations (IC50) were 177 +/- 31 and 10 +/- 4 microM for the two phases, respectively. 3. Arachidonate was still effective in the presence of inhibitors of cyclo-oxygenase (indomethacin, 10 microM), lipoxygenase (nordihydroguaretic acid, 10-100 microM) and cytochrome P450 (SKF525A, 100 microM; ethoxyresorufin, 10 microM; metyrapone, 500 microM; piperonyl butoxide, 500 microM; cimetidine, 1 mM). The effects of arachidonate could not be produced by another cis unsaturated fatty acid, oleic acid. 4. Measurements of cell volume showed that arachidonate effectively inhibited the regulatory volume decrease elicited by ROS 17/2.8 cells in response to a reduction in extracellular osmolarity. 5. It is concluded that the volume-sensitive Cl- conductance in ROS 17/2.8 cells is directly modulated by arachidonate and may represent a physiological mechanism by which volume regulation can be controlled in these cells.

ATP-regulated K+ channel and cytosolic Ca2+ mobilization in cultured rat spinal neurons

ATP activated the K+ channel responsible for outwardly rectifying currents via a P2Y purinoceptor linked to a pertussis toxin-insensitive G-protein in cultured rat spinal neurons. The evoked currents were inhibited by a selective protein kinase C inhibitor, GF109203X, whereas a phospholipase C inhibitor, neomycin had no effect. These indicate that the currents are regulated by phospholipase C-independent protein kinase C activation. In addition, ATP enhanced intracellular free Ca2+ concentration. The increase in intracellular free Ca2+ concentration was inhibited by a broad G-protein inhibitor, GDP beta S, but not affected by neomycin or an inositol 1,4,5-triphosphate receptor antagonist, heparin, suggesting that the cytosolic Ca2+ mobilization is regulated by a mechanism independent of a phospholipase C-mediated phosphatidylinositol signaling. These results, thus, demonstrate that ATP has dual actions on the coupled K+ channel and cytosolic Ca2+ release.

Evidence that free polyunsaturated fatty acids modify Na+ channels by directly binding to the channel proteins

The effects of free polyunsaturated fatty acids (PUFA) on the binding of ligands to receptors on voltage-sensitive Na+ channels of neonatal rat cardiac myocytes were assessed. The radioligand was [benzoyl-2,5-(3)H] batrachotoxinin A 20alpha-benzoate ([(3)H]BTXB), a toxin that binds to the Na+ channel. The PUFA that have been shown to be antiarrhythmic, including eicosapentaenoic acid (EPA; C20:5n-3), docosahexaenoic acid (DHA; C22:6n-3), eicosatetraynoic acid (ETYA), linolenic acid (C18:3n-3), and linoleic acid (C18:2n-6), inhibited [(3)H]BTXB binding in a dose-dependent fashion with IC50 values of 28-35 microM, whereas those fatty acids that have no antiarrhythmic effects including saturated fatty acid (stearic acid, C18:0), monounsaturated fatty acid (oleic acid; C18:1n-9), and EPA methyl ester did not have a significant effect on [(3)H]BTXB binding. Enrichment of the myocyte membrane with cholesterol neither affected [(3)H]BTXB binding when compared with control cells nor altered the inhibitory effects of PUFA on [(3)H]BTXB binding. Scatchard analysis of [(3)H]BTXB binding showed that EPA reduced the maximal binding without altering the Kd for [(3)H]BTXB binding, indicating allosteric inhibition. The inhibition by EPA of [(3)H]BTXB binding was reversible (within 30 min) when delipidated bovine serum albumin was added. The binding of the PUFA to this site on the Na+ channel is reversible and structure-specific and occurs at concentrations close to those required for apparent antiarrhythmic effects and a blocking effect on the Na+ current, suggesting that binding of the PUFA at this site relates to their antiarrhythmic action.

Direct binding of ethanol to bovine serum albumin: a fluorescent and 13C NMR multiplet relaxation study

Molecular mechanisms of ethanol interaction with proteins are not well-understood. In the present study, direct interaction of ethanol with hydrophobic binding sites on fatty acid free bovine serum albumin (BSA) was determined using the fluorescent probe 1-anilinonaphthalene-8-sulfonic acid (1,8-ANS), cis-parinaric acid, and 13C NMR. The affinity of ethanol for BSA (Kd) was (5.21 +/- 0.31) x 10(-2) mol. Ethanol (25-200 mmol) competitively inhibited 1,8-ANS binding to BSA in a concentration-dependent manner with a Ki (concentration of ethanol that decreased 1,8-ANS binding by 50%) of 658 mmol. Preincubation of BSA with ethanol significantly decreased cis-parinaric acid binding to BSA, indicating interaction of ethanol with hydrophobic fatty acid-binding site(s) on BSA. Furthermore, ethanol was found to act on three of the five fatty acid-binding sites on BSA. These data indicated selectivity in the interaction of ethanol with hydrophobic sites on BSA. 13C NMR multiplet relaxation was used to characterize the interaction of ethanol with binding sites on BSA. Detailed analysis of [13C]ethanol relaxation data obtained in the presence of increasing BSA concentrations (25-200 mg/mL) led to the conclusion that the ethanol methyl group, as opposed to its hydroxyl group, binds in a hydrophobic pocket(s) on the protein. Ethanol-induced changes in activity of certain proteins may result from direct binding of ethanol to specific hydrophobic binding sites and/or displacement of endogenous ligands from those sites.

Alternatively spliced isoforms of the NMDARI receptor subunit

Molecularly diverse forms of the NMDA-receptor subunit NRI are formed by alternative RNA splicing. Differential splicing of three exons generates as many as eight NRI splice variants, seven of which have been identified in cDNA libraries. The alternatively spliced exons encode a 21 amino acid sequence in the N-terminus domain (termed NI), and adjacent sequences of 37 and 38 amino acids in the C-terminus domain (termed C1 and C2, respectively). Splicing out the exon segment that encodes the C2 cassette removes the first stop codon, resulting in a new open reading frame that encodes an unrelated sequence of 22 amino acids (C2') before a second stop codon is reached. Differential RNA splicing alters the structural, physiological and pharmacological properties of receptors that comprise NRI subunits. Diversity of NMDA receptors is also caused by differential association with members of the NR2 gene family. The finding of cell-specific expression and developmental regulation of NRI splice variants, and of the NR2 subunits, provides an explanation for the diversity of properties of NMDA receptors in different neuronal populations.

PXA1, a possible Saccharomyces cerevisiae ortholog of the human adrenoleukodystrophy gene

The adrenoleukodystrophy protein (ALDp) is an ATP-binding cassette (ABC) transporter in the human peroxisome membrane. It is defective in X chromosome-linked adrenoleukodystrophy (ALD), a neurodegenerative disorder with impaired peroxisomal oxidation of very long chain fatty acids. We report cloning and characterization of PXA1, a yeast gene encoding a protein (Pxa1p) exhibiting high similarity to ALDp. Disruption of PXA1 results in impaired growth on oleic acid and reduced ability to oxidize oleate. Pxa1p is peroxisome associated; however, in the PXA1 mutant yeast, as in ALD cells, peroxisomes are morphologically intact. Disruption of a second yeast gene, YKL741, which encodes a more distantly related ALDp homolog (Yk174p), in either wild-type or PXA1 mutant yeast, results in a growth phenotype identical to that of the PXA1 mutant. This result suggests that Yk1741p and Pxa1p may be subunits of the same transporter. Sequence analysis of Pxa1p, ALDp, and related ABC transporters reveals a possible fatty acid binding domain and a 14-amino acid EAA-like motif, previously described only in prokaryotes. Because of the similarities in sequence and function, we propose that Pxa1p is the Saccharomyces cerevisiae ortholog of ALDp.

Modulation of neuronal calcium channels by arachidonic acid and related substances

Low-voltage-activated (l-v-a) and high-voltage-activated (h-v-a) Ca2+ currents (ICa) were recorded in whole-cell voltage clamped NG108-15 neuroblastoma x glioma hybrid cells. We studied the effects of arachidonic acid (AA), oleic acid, myristic acid and of the positively charged compounds tetradecyltrimethylammonium (C14TMA) and sphingosine. At pulse potentials > -20 mV, AA (25-100 microM) decreased l-v-a and h-v-a ICa equally. The decrease developed slowly and became continually stronger with increasing time of application. It was accompanied by a small negative shift and a slight flattening of the activation and inactivation curves of the l-v-a ICa. The shift of the activation curve manifested itself in a small increase of l-v-a ICa at pulse potentials < -30 mV. The effects were only partly reversible. The AA effect was not prevented by 50 microM 5, 8, 11, 14-eicosatetraynoic acid, an inhibitor of the AA metabolism, and not mimicked by 0.1-1 microM phorbol 12, 13-dibutyrate, an activator of protein kinase C. Probably, AA directly affects the channel protein or its lipid environment. Oleic and myristic acid acted similarly to AA but were much less effective. The positively charged compounds C14TMA and sphingosine had a different effect: They shifted the activation curve of l-v-a ICa in the positive direction and suppressed l-v-a more than h-v-a ICa; their effect reached a steady-state within 5-10 min and was readily reversible. C14TMA blocked l-v-a ICa with an IC50 of 4.2 microM while sphingosine was less potent.

Arachidonic acid activation of a new family of K+ channels in cultured rat neuronal cells

1. The presence and properties of K+ channels activated by arachidonic acid were studied in neuronal cells cultured from the mesencephalic and hypothalamic areas of rat brain. 2. Arachidonic acid produced a concentration-dependent (5-50 microM) and reversible activation of whole-cell currents. 3. In excised membrane patches, arachidonic acid applied to the cytoplasmic or extracellular side of the membrane caused opening of three types of channels whose current-voltage relationships were slightly outwardly rectifying, inwardly rectifying and linear, and whose single channel slope conductances at +60 mV were 143, 45 and 52 pS, respectively. 4. All three currents were K+ selective and blocked by 2 mM Ba2+ but not by other K+ channel blockers such as tetraethylammonium chloride, 4-aminopyridine and quinidine. The outwardly and inwardly rectifying currents were slightly voltage dependent with higher channel activity at more depolarized potentials. 5. Arachidonic acid activated the K+ channels in cells treated with cyclo-oxygenase and lipoxygenase inhibitors (indomethacin and nordihydroguaiaretic acid), indicating that arachidonic acid itself can directly activate the channels. Alcohol and methyl ester derivatives of arachidonic acid failed to activate the K+ channels, indicating that the charged carboxyl group is important for activation. 6. Certain unsaturated fatty acids (linoleic, linolenic and docosahexaenoic acids), but not saturated fatty acids (myristic, palmitic, stearic acids), also reversibly activated all three types of K+ channel. 7. All three K+ channels were activated by pressure applied to the membrane (i.e. channels were stretch sensitive) with a half-maximal pressure of approximately 18 mmHg. The K+ channels were not blocked by 100 microM GdCl3. 8. A decrease in intracellular pH (over the range 5.6-7.2) caused a reversible, pH-dependent increase in channel activity whether the channel was initially activated by arachidonic acid or stretch. 9. Glutamate, a neurotransmitter reported to generate arachidonic acid in striatal neurons, did not cause activation of the K+ channels when applied extracellularly in cell-attached patches. 10. It is suggested that the K+ channels described here belong to a distinct family of ion channels that are activated by either fatty acids or membrane stretch. Although the physiological roles of these K+ channels are not yet known, they may be involved in cellular processes such as cell volume regulation and ischaemia-induced elevation of K+ loss.

Stretch activation of a toad smooth muscle K+ channel may be mediated by fatty acids

1. using standard single channel patch clamp techniques we studied the stretch sensitivity of a 20 pS K(+)-selective channel which is activated by fatty acids and found in freshly dissociated smooth muscle cells from the stomach of the toad Bufo marinus. 2. A pulse of suction applied to the back of the patch pipette in order to stretch the membrane resulted in activation of this K+ channel. A train of suction pulses resulted in a gradually increased level of channel activity during each successive pulse, as well as an increase in baseline activity between pulses. This pattern contrasts markedly with many other stretch-activated channels whose activation is limited to the duration of the suction pulse. 3. Application of fatty acids augmented the response to stretch. In contrast, application of 10 microM defatted albumin, which removes fatty acids from membranes, rapidly and reversibly decreased the response to stretch. 4. These results are consistent with the hypothesis that fatty acids which are generated by mechanical stimuli, perhaps by mechanically activated phospholipases, are the intermediaries in activation of certain mechanically sensitive ion channels.

Intracellular acidification is not a prerequisite for glutamate-triggered death of cultured hippocampal neurons

Glutamate decreased intracellular pH (pHi) in cultured rat hippocampal neurons. The protonophore, FCCP (1 microM), produced an acidification comparable to that produced by glutamate. Application of glutamate to FCCP-treated cells, returned pHi to resting levels. This alkaline shift resulted from a glutamate-induced membrane depolarization that removed the driving force across the plasmalemma for H+ entry via FCCP. The endogenous protonophore, arachidonic acid (10 microM), produced pHi changes similar to those elicited by FCCP. Because application of glutamate and FCCP in combination did not change pHi, this treatment was used to determine the role of glutamate-induced acidification in neurotoxicity. FCCP (1 microM, 5 min) did not affect neuronal viability, either alone or in combination with various concentrations of glutamate, as indicated by the release of lactate dehydrogenase into the bathing medium. Thus, acidification was not the cause of glutamate-induced cell death although, it may be symptomatic of neurotoxic processes.

Direct effects of fatty acids and other charged lipids on ion channel activity in smooth muscle cells

A variety of fatty acids increase the activity of certain types of K+ channels. This effect is not dependent on the three enzymatic pathways that convert arachidonic acid to various bioactive oxygenated metabolites. One type of K+ channel in toad stomach smooth muscle cell membranes in activated by fatty acids and other single chain lipids which possess both a negatively charged head group and a sufficiently hydrophobic acyl chain. Neutral lipids have no effect on K+ channel activity, while positively charged lipids with a sufficiently hydrophobic acyl chain suppress channel activity. Acyl Coenzyme A's, which do not flip across the bilayer, act only from the cytosolic surface of the membrane, suggesting that the binding site for channel activation is also located there. This fatty acid-activated channel is also activated by membrane stretch. Moreover, this mechanical response is either mediated or modulated by fatty acids. Thus, fatty acids and other charged single chain lipids may comprise another class of first or second messenger molecules that target ion channels.