Ca2+ sensitivity of BK channels in GH3 cells involves cytosolic phospholipase A2

Actions and Mechanisms of Polyunsaturated Fatty Acids on Voltage-Gated Ion Channels

Polyunsaturated fatty acids (PUFAs) act on most ion channels, thereby having significant physiological and pharmacological effects. In this review we summarize data from numerous PUFAs on voltage-gated ion channels containing one or several voltage-sensor domains, such as voltage-gated sodium (Na_V), potassium (K_V), calcium (Ca_V), and proton (H_V) channels, as well as calcium-activated potassium (K_Ca), and transient receptor potential (TRP) channels. Some effects of fatty acids appear to be channel specific, whereas others seem to be more general. Common features for the fatty acids to act on the ion channels are at least two double bonds in cis geometry and a charged carboxyl group. In total we identify and label five different sites for the PUFAs. PUFA site 1: The intracellular cavity. Binding of PUFA reduces the current, sometimes as a time-dependent block, inducing an apparent inactivation. PUFA site 2: The extracellular entrance to the pore. Binding leads to a block of the channel. PUFA site 3: The intracellular gate. Binding to this site can bend the gate open and increase the current. PUFA site 4: The interface between the extracellular leaflet of the lipid bilayer and the voltage-sensor domain. Binding to this site leads to an opening of the channel via an electrostatic attraction between the negatively charged PUFA and the positively charged voltage sensor. PUFA site 5: The interface between the extracellular leaflet of the lipid bilayer and the pore domain. Binding to this site affects slow inactivation. This mapping of functional PUFA sites can form the basis for physiological and pharmacological modifications of voltage-gated ion channels.

Unoprostone activation of BK (KCa1.1) channel splice variants

This investigation was conducted to study the relationship between intracellular Ca(2+) and activation of large conductance Ca(2+)-activated K(+) (BK) currents by unoprostone, the first synthetic docosanoid. We used HEK293 cells stably transfected with two BK channel splice variants, one sensitive to unoprostone and the other insensitive. We examined the effects of unoprostone on channel activity in excised inside-out patches and cell-attached patches. The half-maximal stimulation of the sensitive BK channels by Ca(2+) was shifted from 3.4±0.017 nM to 0.81±.0058 nM in the presence of 10 nM unoprostone. There was no effect on insensitive channels even at unoprostone concentrations as high as 1000 nM. There was no effect of unoprostone on the voltage dependence of the BK channels. Changes in open probability and effects of Ca(2+) and unoprostone were best described by a synergistic binding model. These data would suggest that Ca(2+) and unoprostone were binding to sites close to one another on the channel protein and that unoprostone binding causes the affinity of the calcium binding site to increase. This idea is consistent with three dimensional models of the Ca(2+) binding site and a putative unoprostone binding domain. Our results have important implications for the clinical use of unoprostone to activate BK channels. Channel activation will be limited if intracellular Ca(2+) is not elevated.

Oxidative-stress induced increase in circulating fatty acids does not contribute to phospholipase A2-dependent appetitive long-term memory failure in the pond snail Lymnaea stagnalis

Background

Reactive oxygen species (ROS) are essential for normal physiological functioning of the brain. However, uncompensated increase in ROS levels may results in oxidative stress. Phospholipase A₂ (PLA₂) is one of the key players activated by elevated ROS levels resulting in the hydrolysis of various products from the plasmamembrane such as peroxidized fatty acids. Free fatty acids (FFAs) and fatty acid metabolites are often implicated to the genesis of cognitive impairment. Previously we have shown that age-, and experimentally induced oxidative stress causes PLA₂-dependent long-term memory (LTM) failure in an aversive operant conditioning model in Lymnaea stagnalis. In the present study, we investigate the effects of experimentally induced oxidative stress and the role of elevated levels of circulating FFAs on LTM function using a non-aversive appetitive classical conditioning paradigm.

Results

We show that intracoelomic injection of exogenous PLA₂ or pro-oxidant induced PLA₂ activation negatively affects LTM performance in our learning paradigm. In addition, we show that experimental induction of oxidative stress causes significant temporal changes in circulating FFA levels. Importantly, the time of training coincides with the peak of this change in lipid metabolism. However, intracoelomic injection with exogenous arachidonic acid, one of the main FFAs released by PLA₂, does not affect LTM function. Moreover, sequestrating circulating FFAs with the aid of bovine serum albumin does not rescue pro-oxidant induced appetitive LTM failure.

Conclusions

Our data substantiates previous evidence linking lipid peroxidation and PLA₂ activation to age- and oxidative stress-related cognitive impairment, neuronal dysfunction and disease. In addition however, our data indicate that lipid peroxidation induced increased levels of circulating (per)oxidized FFAs are not a factor in oxidative stress induced LTM impairment.

Protein-protein interaction between cPLA2 and splice variants of alpha-subunit of BK channels

Altering the splice variant composition of large-conductance Ca(2+)-activated potassium (BK) channels can alter their activity and apparent sensitivity to Ca(2+) and other regulators of activity. We hypothesized that differences in the responsiveness to arachidonic acid of GH3 and GH4 cells was due to a difference in two splice variants, one present in GH3 cells and the other in GH4 cells. The sequences of the two splice variants differ from one another in several ways, but the largest difference is the presence or absence of 27 amino acids in the COOH terminus of the BK alpha-subunit. Open probability of the variant containing the 27 amino acids is significantly increased by arachidonic acid, while the variant lacking the 27 amino acids is insensitive to arachidonic acid. In addition, sensitivity of BK channels to arachidonic acid depends on cytosolic phospholipase A(2) (cPLA(2)). Here we used the Mammalian Matchmaker two-hybrid assay and two BK alpha-subunit constructs with [rSlo(27)] and without [rSlo(0)] the 27-amino acid motif to determine whether cPLA(2) associates with one construct [rSlo(27)] and not the other. We hypothesized that differential association of cPLA(2) might explain the differing responsiveness of the two constructs and GH3 and GH4 cells to arachidonic acid. We found that cPLA(2) is strongly associated with the COOH terminus of rSlo(27) and only very weakly associated with rSlo(0). We also found that arachidonic acid has a lower affinity for rSlo(0) than for rSlo(27). We conclude that the lack of response of BK channels in GH4 cells to arachidonic acid can be explained, in part, by the poor binding of cPLA(2) to the COOH terminus of the rSlo(0) alpha-subunit, which is very similar to the splice variant found in the arachidonic acid-insensitive GH4 cells.

Co-localization of the alpha-subunit of BK-channels and c-PLA2 in GH3 cells

Large conductance, calcium-activated potassium channels (maxi K- or BK-channels) can be regulated by arachidonic acid produced by c-Phospholipase A2 (c-PLA2). Since in every excised patch from GH3 cells where there was BK-channel activity, treatment with either a stimulator or inhibitor of c-PLA2 resulted in a corresponding increase or decrease in BK-channel activity, we hypothesized that there must be a close association between BK-channel proteins and c-PLA2 in the cell membrane. To test this hypothesis, we first determined whether the two proteins would co-immunoprecipitate. We then used confocal imaging of fluorescently tagged proteins to determine where in the cells BK-channel proteins and c-PLA2 co-localize. The alpha-subunit of the BK-channel was strongly co-immunoprecipitated by c-PLA2 antibodies, suggesting that most of the BK channel alpha-subunits are associated with c-PLA2. This interaction was not affected by pharmacologically inhibiting c-PLA2 suggesting that the association does not require functionally active c-PLA2. Following dual immunohistochemical labeling and confocal microscopy, image analysis revealed that in the cytosol there was some co-localization, but most of the c-PLA2 was separate from BK-channel proteins. On the other hand, the c-PLA2 and BK-channel proteins at the plasma membrane were strongly co-localized. Immunoprecipitation experiments conducted with plasma membrane proteins support these findings. We conclude that c-PLA2 is likely physically associated with BK-channel proteins at the cell surface.

Activation of BK channels in GH3 cells by a c-PLA2-dependent G-protein signaling pathway

BK-channels in GH3 cells are activated by arachidonic acid produced by c-PLA2. beta-adrenergic agonists also activate BK channels and were presumed to do so via production of cAMP. We, however, show for the first time in GH3 cells that a beta-adrenergic agonist activates a pertussis-toxin-sensitive G protein that activates c-PLA2. The arachidonic acid produced by c-PLA2 then activates BK channels. We examined BK channels in cell-attached patches and in excised patches from untreated GH3 cells and from GH3 cells exposed to c-PLA2 antisense oligonucleotides. For the cell-attached patch experiments, physiologic pipette and bath solutions were used. For the excised patches, 150 mM KCl was used in both the pipette and bath solutions, and the cytosolic surface contained 1 microM free Ca2+ (buffered with 5 mM K2EGTA). Treatment of GH3 cells with the G protein activator, fluoroaluminate, (AlF4-) produced an increase in the Po of BK channels of 177 +/- 41% (mean +/- SD) in cell-attached patches. Because G proteins are membrane associated, we also added an activator of G proteins, 100 microM GTP-gamma-S, to the cytosolic surface of excised patches. This treatment leads to an increase in Po of 50 +/- 9%. Similar treatment of excised patches with GDP-beta-S had no effect on Po. Isoproterenol (1 microM), an activator of beta-adrenergic receptors and, consequently, some G proteins, increased BK channel activity 229 +/- 37% in cell-attached patches from cultured GH3 cells. Western blot analysis showed that GH3 cells have beta-adrenergic receptor protein and that isoproterenol acts through these receptors because the beta-adrenergic receptor antagonist, propanolol, blocks the action of isoproterenol. To test whether G protein activation of BK channels involves c-PLA2, we studied the effects of GTP-gamma-S on excised patches and isoproterenol on cell attached patches from GH3 cells previously treated with c-PLA2 antisense oligonucleotides or pharmacological inhibitors of c-PLA2. Neither isoproterenol nor GTP-gamma-S had any effect on Po in these patches. Similarly, neither isoproterenol nor GTP-gamma-S had any effect on Po in cultured GH3 cells pretreated with pertussis toxin. Isoproterenol also significantly increased the rate of arachidonic production in GH3 cells. These results show that some receptor-linked, pertussis-toxin-sensitive G protein in GH3 cells can activate c-PLA2 to increase the amount of arachidonic acid present and ultimately increase BK-channel activity.

Identification and localization of an arachidonic acid-sensitive potassium channel in the cochlea

Receptor cells of the auditory and vestibular end organs of vertebrates acquire various types of potassium channels during development. Their expression and kinetics can differ along the tonotopic axis as well as in different cell types of the sensory epithelium. These variations can play a crucial role in modulating sensory transduction and cochlear tuning. Whole-cell tight-seal recordings of isolated hair cells revealed the presence of an arachidonic acid-sensitive A-type channel in the short (outer) hair cells of the chicken cochlea. This polyunsaturated fatty acid blocked the A-current, thereby increasing the amplitude and duration of the voltage response in these cells. We identified the gene encoding this channel as belonging to a member of the Shal subfamily, Kv4.2. Expression of the recombinant channel shows half-activation and inactivation potentials shifted to more positive values relative to native channels, suggesting that the native channel is coexpressed with an accessory subunit. RT-PCR revealed that transcription begins early in development, whereas in situ hybridization showed mRNA expression limited to the intermediate and short hair cells located in specific regions of the adult cochlea. Additional localization, using immunofluorescent staining, revealed clustering in apical-lateral regions of the receptor cell as well as in the cochlear ganglion. These experiments provide evidence that in addition to membrane proteins modulating excitation in these receptor cells, fatty acids contribute to the coding of auditory stimuli via these channels.

Downregulation of nitric oxide formation by cytosolic phospholipase A2-released arachidonic acid

Exposure of PC12 cells to A23187 or thapsigargin caused a concentration-dependent release of arachidonic acid (AA) mediated by cytosolic phospholipase A2 (PLA2). Under the same conditions, however, analysis of nitric oxide (NO) formation revealed that activation of NO synthase (NOS) is best described by a bell-shaped curve. Reduced detection of NO observed at increasing A23187 or thapsigargin concentrations was not due to formation of peroxynitrite or to activation of NO-consuming processes, but rather to AA-dependent inhibition of NOS activity. Furthermore, NO formation observed under optimal conditions for NOS activity was suppressed by AA as well as by the PLA2 activator melittin. Finally, the effects of AA were not the consequence of direct enzyme inhibition, because this lipid messenger failed to inhibit formation of NO by purified neuronal NOS, but were mediated by an AA-dependent signaling and not by downstream products of the cyclooxygenase and lipoxygenase pathways. In conclusion, the present study underscores a novel mechanism whereby endogenous, or exogenous, AA promotes inhibition of NOS activity. Because AA is generated in response to various agonists acting on membrane receptors and extensively released in inflammatory conditions, these findings have important physiopathological implications.

Cytosolic phospholipase A2 is an effector of Jak/STAT signaling and is involved in platelet-derived growth factor BB-induced growth in vascular smooth muscle cells

Platelet-derived growth factor-BB (PDGF-BB) is a potent mitogen and chemoattractant for vascular smooth muscle cells (VSMC). To understand its mitogenic and chemotactic signaling events, we studied the role of cytosolic phospholipase A(2) (cPLA(2)) and the Jak/STAT pathway. PDGF-BB induced the expression and activity of cPLA(2) in a time-dependent manner in VSMC. Arachidonyl trifluoromethyl ketone, a potent and specific inhibitor of cPLA(2), significantly reduced PDGF-BB-induced arachidonic acid release and DNA synthesis. PDGF-BB stimulated tyrosine phosphorylation of Jak-2 in a time-dependent manner. In addition, PDGF-BB activated STAT-3 as determined by its tyrosine phosphorylation, DNA-binding activity, and reporter gene expression, and these responses were suppressed by AG490, a selective inhibitor of Jak-2. AG490 and a dominant-negative mutant of STAT-3 also attenuated PDGF-BB-induced expression of cPLA(2,) arachidonic acid release, and DNA synthesis in VSMC. Together, these results suggest that induction of expression of cPLA(2) and arachidonic acid release are involved in VSMC growth in response to PDGF-BB and that these events are mediated by Jak-2-dependent STAT-3 activation.

Site of action of fatty acids and other charged lipids on BKCa channels from arterial smooth muscle cells

Fatty acids and other negatively charged single-chain lipids increase large-conductance Ca(2+)-activated K(+) (BK(Ca)) channel activity, whereas sphingosine and other positively charged single-chain lipids suppress activity. Because these molecules are effective on both inside-out and outside-out patches and because they can flip across the bilayer, the location of their site of action is unclear. To identify the site of action of charged lipids on this channel, we used two compounds that are unlikely to flip across the lipid bilayer. Palmitoyl coenzyme A (PCoA) was used to identify the site of action of negatively charged lipids, and a positively charged myristoylated pentapeptide (myr-KPRPK) was used to investigate the site of action of positively charged lipids. The effect of these compounds on channel activity was studied in excised patches using patch-clamp techniques. In "normal" ionic strength solutions and in experiments where high-ionic strength solutions were used to shield membrane surface charge, PCoA increased channel activity only when applied to outside-out patches, suggesting that the site of action of negatively charged lipids is located on the outer surface of the membrane. A decrease in activity, similar to that of other positively charged lipids, was observed only when myr-KPRPK was applied to outside-out patches, suggesting that positively charged lipids suppress activity by also acting on the outer membrane surface. Some channel blockade effects of myr-KPRPK and KPRPK are also described. The sidedness of action suggests that modulation of channel activity by single-chain lipids can occur by their interaction with the channel protein.

Modulation of BK(Ca) channel activity by fatty acids: structural requirements and mechanism of action

To determine the mechanism of fatty acid modulation of rabbit pulmonary artery large-conductance Ca2+ -activated K+ (BK(Ca)) channel activity, we studied effects of fatty acids and other lipids on channel activity in excised patches with patch-clamp techniques. The structural features of the fatty acid required to increase BK(Ca) channel activity (or average number of open channels, NP(o)) were identified to be the negatively charged head group and a sufficiently long (C > 8) carbon chain. Positively charged lipids like sphingosine, which have a sufficiently long alkyl chain (C >or= 8), produced a decrease in NP(o). Neutral and short-chain lipids did not alter NP(o). Screening of membrane surface charge with high-ionic-strength bathing solutions (330 mM K+ or 130 mM K+, 300 mM Na+) did not alter the modulation of the BK(Ca) channel NP(o) by fatty acids and other charged lipids, indicating that channel modulation is unlikely to be due to an alteration of the membrane electric field or the attraction of local counterions to the channel. Fatty acids and other negatively charged lipids were able to modulate BK(Ca) channel activity in bathing solutions containing 0 mM Ca2+, 20 mM EGTA, suggesting that calcium is not required for this modulation. Together, these results indicate that modulation of BK(Ca) channels by fatty acids and other charged lipids most likely occurs by their direct interaction with the channel protein itself or with some other channel-associated component.

Beta-adrenergic regulation of amiloride-sensitive lung sodium channels

We investigated the mechanism by which cAMP increases sodium transport in lung epithelial cells. Alveolar type II (ATII) cells have two types of amiloride-sensitive, cation channels: a nonselective cation channel (NSC) and a highly selective channel (HSC). Exposure of ATII cells to cAMP, beta-adrenergic agonists, or other agents that increase adenylyl cyclase activity increased activity of both channel types, albeit by different mechanisms. NSC open probability (P(o)) increased severalfold when exposed to terbutaline, isoproterenol, forskolin, or cAMP analogs without any change in NSC number. In contrast, terbutaline increased HSC number with no significant change in HSC P(o). For both channels, the effect of terbutaline was blocked by propranolol and H-89, suggesting a protein kinase A (PKA) requirement for beta-adrenergic-induced changes in channel activity. Terbutaline increased cAMP levels in ATII cells, but intracellular calcium also increased. Calcium sequestration with BAPTA blocked beta-adrenergic-induced increases in NSC P(o) but did not alter HSC activity. These observations suggest that beta-adrenergic stimulation increases intracellular cAMP and activates PKA. PKA increases HSC number and increases intracellular calcium. The increase in calcium increases NSC P(o). Thus increased cAMP levels are likely to increase lung sodium transport regardless of which channel type is present.

S-adenosyl-L-homocysteine hydrolase is necessary for aldosterone-induced activity of epithelial Na(+) channels

The A6 cell line was used to study the role of S-adenosyl-L-homocysteine hydrolase (SAHHase) in the aldosterone-induced activation of the epithelial Na(+) channel (ENaC). Because aldosterone increases methylation of several different molecules, and because this methylation is associated with increased Na(+) reabsorption, we tested the hypothesis that aldosterone increases the expression and activity of SAHHase protein. The rationale for this work is that general methylation may be promoted by activation of SAHHase, the only enzyme known to metabolize SAH, a potent end-product inhibitor of methylation. Although aldosterone increased SAHHase activity, steroid did not affect SAHHase expression. Antisense SAHHase oligonucleotide decreased SAHHase expression and activity. Moreover, this oligonucleotide, as well as a pharmacological inhibitor of SAHHase, decreased aldosterone-induced activity of ENaC via a decrease in ENaC open probability. The kinetics of ENaC in cells treated with antisense plus aldosterone were similar to those reported previously for the channel in the absence of steroid. This is the first report showing that active SAHHase, in part, increases ENaC open probability by reducing the transition rate from open states in response to aldosterone. Thus aldosterone-induced SAHHase activity plays a critical role in shifting ENaC from a gating mode with short open and closed times to one with longer open and closed times.

Contrasting effects of cPLA2 on epithelial Na+ transport

Activity of the epithelial Na+ channel (ENaC) is the limiting step for discretionary Na+ reabsorption in the cortical collecting duct. Xenopus laevis kidney A6 cells were used to investigate the effects of cytosolic phospholipase A2 (cPLA2) activity on Na+ transport. Application of aristolochic acid, a cPLA2 inhibitor, to the apical membrane of monolayers produced a decrease in apical [3H]arachidonic acid (AA) release and led to an approximate twofold increase in transepithelial Na+ current. Increased current was abolished by the nonmetabolized AA analog 5,8,11,14-eicosatetraynoic acid (ETYA), suggesting that AA, rather than one of its metabolic products, affected current. In single channel studies, ETYA produced a decrease in ENaC open probability. This suggests that cPLA2 is tonically active in A6 cells and that the end effect of liberated AA at the apical membrane is to reduce Na+ transport via actions on ENaC. In contrast, aristolochic acid applied basolaterally inhibited current, and the effect was not reversed by ETYA. Basolateral application of the cyclooxygenase inhibitor ibuprofen also inhibited current. Both effects were reversed by prostaglandin E2 (PGE2). This suggests that cPLA2 activity and free AA, which is metabolized to PGE2, are necessary to support transport. This study supports the fine-tuning of Na+ transport and reabsorption through the regulation of free AA and AA metabolism.

Cytosolic phospholipase A2 is required for optimal ATP activation of BK channels in GH(3) cells

To test the hypothesis that ATP activation of BK channels in GH(3) cells involves cytosolic phospholipase A(2) (cPLA(2)) as a potential protein target for phosphorylation, we first inhibited the activity of cPLA(2) by both pharmacologic and molecular biologic approaches. Both approaches resulted in a decrease rather than an increase in BK channel activity by ATP, suggesting that in the absence of cPLA(2), phosphorylation of other regulatory elements, possibly the BK channel protein itself, results in inactivation rather than activation of the channel. The absence of changes in activity in the presence of the non-substrate ATP analog 5'-adenylyl-beta,gamma-imidodiphosphate verified that ATP hydrolysis was required for channel activation by ATP. Experiments with an activator and inhibitor of protein kinase C (PKC) support the hypothesis that PKC can be involved in the activation of BK channels by ATP; and in the absence of PKC, other kinases appear to phosphorylate additional elements in the regulatory pathway that reduce channel activity. Our data point to cPLA(2)-alpha (but not cPLA(2)-gamma) as one target protein for phosphorylation that is intimately associated with the BK channel protein.

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.