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

Role of Airway Smooth Muscle in Inflammation Related to Asthma and COPD

Airway smooth muscle contributes to both contractility and inflammation in the pathophysiology of asthma and COPD. Airway smooth muscle cells can change the degree of a variety of functions, including contraction, proliferation, migration, and the secretion of inflammatory mediators (phenotype plasticity). Airflow limitation, airway hyperresponsiveness, β₂-adrenergic desensitization, and airway remodeling, which are fundamental characteristic features of these diseases, are caused by phenotype changes in airway smooth muscle cells. Alterations between contractile and hyper-contractile, synthetic/proliferative phenotypes result from Ca²⁺ dynamics and Ca²⁺ sensitization. Modulation of Ca²⁺ dynamics through the large-conductance Ca²⁺-activated K⁺ channel/L-type voltage-dependent Ca²⁺ channel linkage and of Ca²⁺ sensitization through the RhoA/Rho-kinase pathway contributes not only to alterations in the contractile phenotype involved in airflow limitation, airway hyperresponsiveness, and β₂-adrenergic desensitization but also to alteration of the synthetic/proliferative phenotype involved in airway remodeling. These Ca²⁺ signal pathways are also associated with synergistic effects due to allosteric modulation between β₂-adrenergic agonists and muscarinic antagonists. Therefore, airway smooth muscle may be a target tissue in the therapy for these diseases. Moreover, the phenotype changing in airway smooth muscle cells with focuses on Ca²⁺ signaling may provide novel strategies for research and development of effective remedies against both bronchoconstriction and inflammation.

Permissive Modulation of Sphingosine-1-Phosphate-Enhanced Intracellular Calcium on BKCa Channel of Chromaffin Cells

Sphingosine-1-phosphate (S1P), is a signaling sphingolipid which acts as a bioactive lipid mediator. We assessed whether S1P had multiplex effects in regulating the large-conductance Ca²⁺-activated K⁺ channel (BK_Ca) in catecholamine-secreting chromaffin cells. Using multiple patch-clamp modes, Ca²⁺ imaging, and computational modeling, we evaluated the effects of S1P on the Ca²⁺-activated K⁺ currents (I_K(Ca)) in bovine adrenal chromaffin cells and in a pheochromocytoma cell line (PC12). In outside-out patches, the open probability of BK_Ca channel was reduced with a mean-closed time increment, but without a conductance change in response to a low-concentration S1P (1 µM). The intracellular Ca²⁺ concentration (Ca_i) was elevated in response to a high-dose (10 µM) but not low-dose of S1P. The single-channel activity of BK_Ca was also enhanced by S1P (10 µM) in the cell-attached recording of chromaffin cells. In the whole-cell voltage-clamp, a low-dose S1P (1 µM) suppressed I_K(Ca), whereas a high-dose S1P (10 µM) produced a biphasic response in the amplitude of I_K(Ca), i.e., an initial decrease followed by a sustained increase. The S1P-induced I_K(Ca) enhancement was abolished by BAPTA. Current-clamp studies showed that S1P (1 µM) increased the action potential (AP) firing. Simulation data revealed that the decreased BK_Ca conductance leads to increased AP firings in a modeling chromaffin cell. Over a similar dosage range, S1P (1 µM) inhibited I_K(Ca) and the permissive role of S1P on the BK_Ca activity was also effectively observed in the PC12 cell system. The S1P-mediated I_K(Ca) stimulation may result from the elevated Ca_i, whereas the inhibition of BK_Ca activity by S1P appears to be direct. By the differentiated tailoring BK_Ca channel function, S1P can modulate stimulus-secretion coupling in chromaffin cells.

Arachidonic acid effect on the allosteric gating mechanism of BK (Slo1) channels associated with the β1 subunit

Arachidonic acid (AA) is a fatty acid involved in the modulation of several ion channels. Previously, we reported that AA activates the high conductance Ca²⁺- and voltage-dependent K⁺ channel (BK) in vascular smooth muscle depending on the expression of the auxiliary β1 subunit. Here, using the patch-clamp technique on BK channel co-expressed with β1 subunit in a heterologous cell expression system, we analyzed whether AA modifies the three functional modules involved in the channel gating: the voltage sensor domain (VSD), the pore domain (PD), and the intracellular calcium sensor domain (CSD). We present evidence that AA activates BK channel in a direct way, inducing VSD stabilization on its active configuration observed as a significant left shift in the Q-V curve obtained from gating currents recordings. Moreover, AA facilitates the channel opening transitions when VSD are at rest, and the CSD are unoccupied. Furthermore, the activation was independent of the intracellular Ca²⁺ concentration and reduced when the BK channel was co-expressed with the Y74A mutant of the β1 subunit. These results allow us to present new insigths in the mechanism by which AA modulates BK channels co-expressed with its auxiliary β1 subunit.

KCNMA1-linked channelopathy

Bailey et al. review a new neurological channelopathy associated with KCNMA1, encoding the BK voltage- and Ca²⁺-activated K⁺ channel.

KCNMA1 encodes the pore-forming α subunit of the “Big K⁺” (BK) large conductance calcium and voltage-activated K⁺ channel. BK channels are widely distributed across tissues, including both excitable and nonexcitable cells. Expression levels are highest in brain and muscle, where BK channels are critical regulators of neuronal excitability and muscle contractility. A global deletion in mouse (KCNMA1^−/−) is viable but exhibits pathophysiology in many organ systems. Yet despite the important roles in animal models, the consequences of dysfunctional BK channels in humans are not well characterized. Here, we summarize 16 rare KCNMA1 mutations identified in 37 patients dating back to 2005, with an array of clinically defined pathological phenotypes collectively referred to as “KCNMA1-linked channelopathy.” These mutations encompass gain-of-function (GOF) and loss-of-function (LOF) alterations in BK channel activity, as well as several variants of unknown significance (VUS). Human KCNMA1 mutations are primarily associated with neurological conditions, including seizures, movement disorders, developmental delay, and intellectual disability. Due to the recent identification of additional patients, the spectrum of symptoms associated with KCNMA1 mutations has expanded but remains primarily defined by brain and muscle dysfunction. Emerging evidence suggests the functional BK channel alterations produced by different KCNMA1 alleles may associate with semi-distinct patient symptoms, such as paroxysmal nonkinesigenic dyskinesia (PNKD) with GOF and ataxia with LOF. However, due to the de novo origins for the majority of KCNMA1 mutations identified to date and the phenotypic variability exhibited by patients, additional evidence is required to establish causality in most cases. The symptomatic picture developing from patients with KCNMA1-linked channelopathy highlights the importance of better understanding the roles BK channels play in regulating cell excitability. Establishing causality between KCNMA1-linked BK channel dysfunction and specific patient symptoms may reveal new treatment approaches with the potential to increase therapeutic efficacy over current standard regimens.

Phospholipid effects on SGLT1-mediated glucose transport in rabbit ileum brush border membrane vesicles

In small intestine, sodium-glucose cotransporter SGLT1 provides the main mechanism for sugar uptake. We investigated the effect of membrane phospholipids (PL) on this transport in rabbit ileal brush border membrane vesicles (BBMV). For this, PL of different charge, length, and saturation were incorporated into BBMV. Transport was measured related to (i) membrane surface charge (membrane-bound MC540 fluorescence), (ii) membrane thickness (PL incorporation of different acyl chain length), and (iii) membrane fluidity (r_12AS, fluorescence anisotropy of 12-AS). Compared to phosphatidylcholine (PC) carrying a neutral head group, inhibition of SGLT1 increased considerably with the acidic phosphatidic acid (PA) and phosphatidylinositol (PI) that increase membrane negative surface charge. The order of PL potency was PI>PA > PE = PS > PC. Inhibition by acidic PA-oleate was 5-times more effective than with neutral PE (phosphatidylethanolamine)-oleate. Lineweaver-Burk plot indicated uncompetitive inhibition of SGLT1 by PA. When membrane thickness was increased by neutral PC of varying acyl chain length, transport was increasingly inhibited by 16:1 PC to 22:1 PC. Even more pronounced inhibition was observed with mono-unsaturated instead of saturated acyl chains which increased membrane fluidity (indicated by decreased r_12AS). In conclusion, sodium-dependent glucose transport of rabbit ileal BBMV is modulated by (i) altered membrane surface charge, (ii) length of acyl chains via membrane thickness, and (iii) saturation of PL acyl chains altering membrane fluidity. Transport was attenuated by charged PL with longer and unsaturated acyl residues. Alterations of PL may provide a principle for attenuating dietary glucose uptake.

Regulation of Coronary Arterial Large Conductance Ca2+-Activated K+ Channel Protein Expression and Function by n-3 Polyunsaturated Fatty Acids in Diabetic Rats

AIM

The objective of this study was to examine the effects of n-3 polyunsaturated fatty acids (n-3 PUFAs) on coronary arterial large conductance Ca2+-activated K+ (BK) channel function in coronary smooth muscle cells (SMCs) of streptozotocin-induced diabetic rats.

METHODS

The effects of docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) on coronary BK channel open probabilities were determined using the patch clamp technique. The mRNA and protein expressions of BK channel subunits were measured using qRT-PCR and Western blots. The coronary artery tension and coronary SMC Ca2+ concentrations were measured using a myograph system and fluorescence Ca2+ indicator.

RESULTS

Compared to nondiabetic control rats, the BK channel function was impaired with a reduced response to EPA and DHA in freshly isolated SMCs of diabetic rats. Oral administration of n-3 PUFAs had no effects on protein expressions of BK channel subunits in nondiabetic rats, but significantly enhanced those of BK-β1 in diabetic rats without altering BK-α protein levels. Moreover, coronary ring tension induced by iberiotoxin (a specific BK channel blocker) was increased and cytosolic Ca2+ concentrations in coronary SMCs were decreased in diabetic rats, but no changes were found in nondiabetic rats.

CONCLUSIONS

n-3 PUFAs protect the coronary BK channel function and coronary vasoreactivity in diabetic rats as a result of not only increasing BK-β1 protein expressions, but also decreasing coronary artery tension and coronary smooth muscle cytosolic Ca2+ concentrations.

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.

Fatty Acid Regulation of Voltage- and Ligand-Gated Ion Channel Function

Free fatty acids (FFA) are essential components of the cell, where they play a key role in lipid and carbohydrate metabolism, and most particularly in cell membranes, where they are central actors in shaping the physicochemical properties of the lipid bilayer and the cellular adaptation to the environment. FFA are continuously being produced and degraded, and a feedback regulatory function has been attributed to their turnover. The massive increase observed under some pathological conditions, especially in brain, has been interpreted as a protective mechanism possibly operative on ion channels, which in some cases is of stimulatory nature and in other cases inhibitory. Here we discuss the correlation between the structure of FFA and their ability to modulate protein function, evaluating the influence of saturation/unsaturation, number of double bonds, and cis vs. trans isomerism. We further focus on the mechanisms of FFA modulation operating on voltage-gated and ligand-gated ion channel function, contrasting the still conflicting evidence on direct vs. indirect mechanisms of action.

Atomic determinants of BK channel activation by polyunsaturated fatty acids

Docosahexaenoic acid (DHA), a polyunsaturated ω-3 fatty acid enriched in oily fish, contributes to better health by affecting multiple targets. Large-conductance Ca²⁺- and voltage-gated Slo1 BK channels are directly activated by nanomolar levels of DHA. We investigated DHA-channel interaction by manipulating both the fatty acid structure and the channel composition through the site-directed incorporation of unnatural amino acids. Electrophysiological measurements show that the para-group of a Tyr residue near the ion conduction pathway has a critical role. To robustly activate the channel, ionization must occur readily by a fatty acid for a good efficacy, and a long nonpolar acyl tail with a Z double bond present at the halfway position for a high affinity. The results suggest that DHA and the channel form an ion-dipole bond to promote opening and demonstrate the channel druggability. DHA, a marine-derived nutraceutical, represents a promising lead compound for rational drug design and discovery.

Inhibition of TRPV1 channels by a naturally occurring omega-9 fatty acid reduces pain and itch

The transient receptor potential vanilloid 1 (TRPV1) ion channel is mainly found in primary nociceptive afferents whose activity has been linked to pathophysiological conditions including pain, itch and inflammation. Consequently, it is important to identify naturally occurring antagonists of this channel. Here we show that a naturally occurring monounsaturated fatty acid, oleic acid, inhibits TRPV1 activity, and also pain and itch responses in mice by interacting with the vanilloid (capsaicin)-binding pocket and promoting the stabilization of a closed state conformation. Moreover, we report an itch-inducing molecule, cyclic phosphatidic acid, that activates TRPV1 and whose pruritic activity, as well as that of histamine, occurs through the activation of this ion channel. These findings provide insights into the molecular basis of oleic acid inhibition of TRPV1 and also into a way of reducing the pathophysiological effects resulting from its activation.

TRPV1 channels are known to mediate pathological pain and itch. Here, the authors find a naturally occurring fatty acid, oleic acid, acts as a TRPV1 antagonist and can modulate capsaicin and histamine-mediated pain and itch response in mouse models.

Modulation of BK Channels by Small Endogenous Molecules and Pharmaceutical Channel Openers

Voltage- and Ca(2+)-activated K(+) channels of big conductance (BK channels) are abundantly found in various organs and their relevance for smooth muscle tone and neuronal signaling is well documented. Dysfunction of BK channels is implicated in an array of human diseases involving many organs including the nervous, pulmonary, cardiovascular, renal, and urinary systems. In humans a single gene (KCNMA1) encodes the pore-forming α subunit (Slo1) of BK channels, but the channel properties are variable because of alternative splicing, tissue- and subcellular-specific auxiliary subunits (β, γ), posttranslational modifications, and a multitude of endogenous signaling molecules directly affecting the channel function. Initiatives to develop drugs capable of activating BK channels (channel openers) therefore need to consider the tissue-specific variability of BK channel structure and the potential interference with endogenously produced regulatory factors. The atomic structural basis of BK channel function is only beginning to be revealed. However, building on detailed knowledge of BK channel function, including its single-channel characteristics, voltage- and Ca(2+) dependence of channel gating, and modulation by diffusible messengers, a multi-tier allosteric model of BK channel gating (Horrigan and Aldrich (HA) model) has become a valuable tool in studying modulation of the channel. Using the conceptual framework of the HA model, we here review the functional impact of endogenous modulatory factors and select small synthetic compounds that regulate BK channel activity. Furthermore, we devise experimental approaches for studying BK channel-drug interactions with the aim to classify BK-modulating substances according to their molecular mode of action.

BK Channels in the Vascular System

Autoregulation of blood flow is essential for the preservation of organ function to ensure continuous supply of oxygen and essential nutrients and removal of metabolic waste. This is achieved by controlling the diameter of muscular arteries and arterioles that exhibit a myogenic response to changes in arterial blood pressure, nerve activity and tissue metabolism. Large-conductance voltage and Ca(2+)-dependent K(+) channels (BK channels), expressed exclusively in smooth muscle cells (SMCs) in the vascular wall of healthy arteries, play a critical role in regulating the myogenic response. Activation of BK channels by intracellular, local, and transient ryanodine receptor-mediated "Ca(2+) sparks," provides a hyperpolarizing influence on the SMC membrane potential thereby decreasing the activity of voltage-dependent Ca(2+) channels and limiting Ca(2+) influx to promote SMC relaxation and vasodilation. The BK channel α subunit, a large tetrameric protein with each monomer consisting of seven-transmembrane domains, a long intracellular C-terminal tail and an extracellular N-terminus, associates with the β1 and γ subunits in vascular SMCs. The BK channel is regulated by factors originating within the SMC or from the endothelium, perivascular nerves and circulating blood, that significantly alter channel gating properties, Ca(2+) sensitivity and expression of the α and/or β1 subunit. The BK channel thus serves as a central receiving dock that relays the effects of the changes in several such concomitant autocrine and paracrine factors and influences cardiovascular health. This chapter describes the primary mechanism of regulation of myogenic response by BK channels and the alterations to this mechanism wrought by different vasoactive mediators.

Activation of calcium- and voltage-gated potassium channels of large conductance by leukotriene B4

Calcium/voltage-gated, large conductance potassium (BK) channels control numerous physiological processes, including myogenic tone. BK channel regulation by direct interaction between lipid and channel protein sites has received increasing attention. Leukotrienes (LTA4, LTB4, LTC4, LTD4, and LTE4) are inflammatory lipid mediators. We performed patch clamp studies in Xenopus oocytes that co-expressed BK channel-forming (cbv1) and accessory β1 subunits cloned from rat cerebral artery myocytes. Leukotrienes were applied at 0.1 nm-10 μm to either leaflet of cell-free membranes at a wide range of [Ca(2+)]i and voltages. Only LTB4 reversibly increased BK steady-state activity (EC50 = 1 nm; Emax reached at 10 nm), with physiological [Ca(2+)]i and voltages favoring this activation. Homomeric cbv1 or cbv1-β2 channels were LTB4-resistant. Computational modeling predicted that LTB4 docked onto the cholane steroid-sensing site in the BK β1 transmembrane domain 2 (TM2). Co-application of LTB4 and cholane steroid did not further increase LTB4-induced activation. LTB4 failed to activate β1 subunit-containing channels when β1 carried T169A, A176S, or K179I within the docking site. Co-application of LTB4 with LTA4, LTC4, LTD4, or LTE4 suppressed LTB4-induced activation. Inactive leukotrienes docked onto a portion of the site, probably preventing tight docking of LTB4. In summary, we document the ability of two endogenous lipids from different chemical families to share their site of action on a channel accessory subunit. Thus, cross-talk between leukotrienes and cholane steroids might converge on regulation of smooth muscle contractility via BK β1. Moreover, the identification of LTB4 as a highly potent ligand for BK channels is critical for the future development of β1-specific BK channel activators.

Pharmacological consequences of the coexpression of BK channel α and auxiliary β subunits

Coded by a single gene (Slo1, KCM) and activated by depolarizing potentials and by a rise in intracellular Ca(2+) concentration, the large conductance voltage- and Ca(2+)-activated K(+) channel (BK) is unique among the superfamily of K(+) channels. BK channels are tetramers characterized by a pore-forming α subunit containing seven transmembrane segments (instead of the six found in voltage-dependent K(+) channels) and a large C terminus composed of two regulators of K(+) conductance domains (RCK domains), where the Ca(2+)-binding sites reside. BK channels can be associated with accessory β subunits and, although different BK modulatory mechanisms have been described, greater interest has recently been placed on the role that the β subunits may play in the modulation of BK channel gating due to its physiological importance. Four β subunits have currently been identified (i.e., β1, β2, β3, and β4) and despite the fact that they all share the same topology, it has been shown that every β subunit has a specific tissue distribution and that they modify channel kinetics as well as their pharmacological properties and the apparent Ca(2+) sensitivity of the α subunit in different ways. Additionally, different studies have shown that natural, endogenous, and synthetic compounds can modulate BK channels through β subunits. Considering the importance of these channels in different pathological conditions, such as hypertension and neurological disorders, this review focuses on the mechanisms by which these compounds modulate the biophysical properties of BK channels through the regulation of β subunits, as well as their potential therapeutic uses for diseases such as those mentioned above.

Lipid regulation of BK channel function

This mini-review focuses on lipid modulation of BK (MaxiK, BK_Ca) current by a direct interaction between lipid and the BK subunits and/or their immediate lipid environment. Direct lipid-BK protein interactions have been proposed for fatty and epoxyeicosatrienoic acids, phosphoinositides and cholesterol, evidence for such action being less clear for other lipids. BK α (slo1) subunits are sufficient to support current perturbation by fatty and epoxyeicosatrienoic acids, glycerophospholipids and cholesterol, while distinct BK β subunits seem necessary for current modulation by most steroids. Subunit domains or amino acids that participate in lipid action have been identified in a few cases: hslo1 Y318, cerebral artery smooth muscle (cbv1) R334,K335,K336, cbv1 seven cytosolic CRAC domains, slo1 STREX and β1 T169,L172,L173 for docosahexaenoic acid, PIP₂, cholesterol, sulfatides, and cholane steroids, respectively. Whether these protein motifs directly bind lipids or rather transmit the energy of lipid binding to other areas and trigger protein conformation change remains unresolved. The impact of direct lipid-BK interaction on physiology is briefly discussed.

Modulation of the mitochondrial large-conductance calcium-regulated potassium channel by polyunsaturated fatty acids

Polyunsaturated fatty acids (PUFAs) and their metabolites can modulate several biochemical processes in the cell and thus prevent various diseases. PUFAs have a number of cellular targets, including membrane proteins. They can interact with plasma membrane and intracellular potassium channels. The goal of this work was to verify the interaction between PUFAs and the most common and intensively studied mitochondrial large conductance Ca(2+)-regulated potassium channel (mitoBKCa). For this purpose human astrocytoma U87 MG cell lines were investigated using a patch-clamp technique. We analyzed the effects of arachidonic acid (AA); eicosatetraynoic acid (ETYA), which is a non-metabolizable analog of AA; docosahexaenoic acid (DHA); and eicosapentaenoic acid (EPA). The open probability (Po) of the channel did not change significantly after application of 10μM ETYA. Po increased, however, after adding 10μM AA. The application of 30μM DHA or 10μM EPA also increased the Po of the channel. Additionally, the number of open channels in the patch increased in the presence of 30μM EPA. Collectively, our results indicate that PUFAs regulate the BKCa channel from the inner mitochondrial membrane.

A point mutation in the human Slo1 channel that impairs its sensitivity to omega-3 docosahexaenoic acid

Long-chain polyunsaturated omega-3 fatty acids such as docosahexaenoic acid (DHA) at nanomolar concentrations reversibly activate human large-conductance Ca²⁺- and voltage-gated K⁺ (Slo1 BK) channels containing auxiliary β1 or β4 subunits in cell-free patches. Here we examined the action of DHA on the Slo1 channel without any auxiliary subunit and sought to elucidate the biophysical mechanism and the molecular determinants of the DHA sensitivity. Measurements of ionic currents through human Slo1 (hSlo1) channels reveal that the stimulatory effect of DHA does not require activation of the voltage or Ca²⁺ sensors. Unlike gating of the hSlo1 channel, that of the Drosophila melanogaster Slo1 (dSlo1) channel is unaltered by DHA. Our mutagenesis study based on the differential responses of human and dSlo1 channels to DHA pinpoints that Y318 near the cytoplasmic end of S6 in the hSlo1 channel is a critical determinant of the stimulatory action of DHA. The mutation Y318S in hSlo1, which replaces Y with S as found in dSlo1, greatly diminishes the channel’s response to DHA with a 22-carbon chain whether β1 or β4 is absent or present. However, the responses to α-linolenic acid, an omegea-3 fatty acid with an 18-carbon chain, and to arachidonic acid, an omega-6 fatty acid with a 20-carbon chain, remain unaffected by the mutation. Y318 in the S6 segment of hSlo1 is thus an important determinant of the electrophysiological response of the channel to DHA. Furthermore, the mutation Y318S may prove to be useful in dissecting out the complex lipid-mediated modulation of Slo1 BK channels.

Omega-3 fatty acids lower blood pressure by directly activating large-conductance Ca²⁺-dependent K⁺ channels

Long-chain polyunsaturated omega-3 fatty acids such as docosahexaenoic acid (DHA), found abundantly in oily fish, may have diverse health-promoting effects, potentially protecting the immune, nervous, and cardiovascular systems. However, the mechanisms underlying the purported health-promoting effects of DHA remain largely unclear, in part because molecular signaling pathways and effectors of DHA are only beginning to be revealed. In vascular smooth muscle cells, large-conductance Ca(2+)- and voltage-activated K(+) (BK) channels provide a critical vasodilatory influence. We report here that DHA with an EC50 of ∼500 nM rapidly and reversibly activates BK channels composed of the pore-forming Slo1 subunit and the auxiliary subunit β1, increasing currents by up to ∼20-fold. The DHA action is observed in cell-free patches and does not require voltage-sensor activation or Ca(2+) binding but involves destabilization of the closed conformation of the ion conduction gate. DHA lowers blood pressure in anesthetized wild-type but not in Slo1 knockout mice. DHA ethyl ester, contained in dietary supplements, fails to activate BK channels and antagonizes the stimulatory effect of DHA. Slo1 BK channels are thus receptors for long-chain omega-3 fatty acids, and these fatty acids--unlike their ethyl ester derivatives--activate the channels and lower blood pressure. This finding has practical implications for the use of omega-3 fatty acids as nutraceuticals for the general public and also for the critically ill receiving omega-3-enriched formulas.

Function and regulation of large conductance Ca(2+)-activated K+ channel in vascular smooth muscle cells

Large conductance Ca(2+)-activated K(+) (BK(Ca)) channels are abundantly expressed in vascular smooth muscle cells. Activation of BK(Ca) channels leads to hyperpolarization of cell membrane, which in turn counteracts vasoconstriction. Therefore, BK(Ca) channels have an important role in regulation of vascular tone and blood pressure. The activity of BK(Ca) channels is subject to modulation by various factors. Furthermore, the function of BK(Ca) channels are altered in both physiological and pathophysiological conditions, such as pregnancy, hypertension and diabetes, which has dramatic impacts on vascular tone and hemodynamics. Consequently, compounds and genetic manipulation that alter activity and expression of the channel might be of therapeutic interest.

Dietary intake of unsaturated fatty acids modulates physiological properties of entorhinal cortex neurons in mice

Dietary lipids modify brain fatty acid profile, but evidence of their direct effect on neuronal function is sparse. The enthorinal cortex (EC) neurons connecting to the hippocampus play a critical role in learning and memory. Here, we have exposed mice to diets based on canola:soybean oils (40 : 10, g/kg) or safflower : corn oils (25 : 25, g/kg) to investigate the relationship between the lipid profile of brain fatty acids and the intrinsic properties of EC neurons. Consumption of canola : soybean oil-enriched diet led to the increase of the monounsaturated fatty acid oleic acid and to a decrease of arachidonic acid in ethanolamine glycerophospholipids of the white matter. We also found an important rise in docosahexaenoic acid (DHA) within ethanolamine glycerophospholipids and phosphatidylserine of gray matter. The canola:soybean oil treatment led to a shorter duration of action potential (-21%), a reduction in the duration of postsynaptic response (-21%) and increased firing activity (+43%). Data from additional experiments with animals fed DHA alone or DHA with canola oil suggested that dietary monounsaturated fatty acid may have contributed to these effects on EC neuron physiology. Since neuronal function within the enthorhinal-hippocampal loop is critical to learning and memory processes, the present data may provide a functional basis for the beneficial cognitive effects of canola oil-based diets.

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.

Biphasic effect of linoleic acid on connexin 46 hemichannels

Connexins form hemichannels at undocked plasma membranes and gap-junction channels (GJCs) at intercellular contacting zones. Under physiological conditions, hemichannels have low open probabilities, but their activation under pathological conditions, such as ischemia, induces and/or accelerates cell death. Connexin 46 (Cx46) is a major connexin of the lens, and mutations of this connexin induce cataracts. Here, we report the effects of linoleic acid (LA) on the electrical properties of Cx46 GJCs and hemichannels expressed in Xenopus laevis oocytes. LA has a biphasic effect, increasing hemichannel current at 0.1 μM and decreasing it at concentrations of 100 μM or higher. The effects of extracellular and microinjected LA conjugated to coenzyme A (LA-CoA) suggest that the current activation site is accessible from the intracellular but not extracellular compartment, whereas the current inhibitory site is either located in a region of the hemichannel pore inaccessible to intracellular LA-CoA, or requires crossing of LA through an organelle membrane. Experiments with other fatty acids demonstrated that the block of hemichannels depends on the presence of a hydrogenated double bond at position 9 and is directly proportional to the number of double bonds. Experiments in paired oocytes expressing Cx46 showed that LA does not affect GJCs. The block by unsaturated fatty acids reported here opens the possibility that increases in the concentration of these lipids in the lens induce cataract formation by blocking Cx46 hemichannels.

The effect of single cerebroside compounds on activation of BKCa channels

We have previously shown that a mixture of cerebrosides obtained from dried tubers of herb Typhonium giganteum Engl. plays a neuroprotective role in the ischemic brain through its effect on activation of BK(Ca) channels. It is very curious to know whether a single pure cerebroside compound could activate the BK(Ca) channel as well. This study explored the possible effects of pure cerebroside compounds, termitomycesphins A and B, on the BK(Ca) channel activation. Both termitomycesphins A and B activated the BK(Ca) channels at micromole concentration without significant difference. Termitomycesphin A increased the single channel open probability of the BK(Ca) channels in a dose-dependent manner without modifying the single channel conductance. Termitomycesphin A activated BK(Ca) channel more efficiently when it was applied to the cytoplasmic face of the membrane, suggesting that binding site for termitomycesphin A is located at the cytoplasmic side. Termitomycesphin A shifted the voltage-dependent activation curve to less positive membrane potentials and the Ca(2+)-dependent activation curve of the channel upwards, suggesting that termitomycesphin A could activate the channels even without intracellular free Ca(2+). Furthermore, STREX-deleted BK(Ca) channels were completely insensitive to termitomycesphin A, indicating that STREX domain is required for the activation of the BK(Ca) channel. These data provide evidence that termitomycesphins are potent in stimulating the activity of the BK(Ca) channels. As BK(Ca) channels are associated with pathology of many diseases, termitomycesphins might be used as therapeutic agents for treating these diseases through its regulatory effect on the BK(Ca) channels.

Large conductance, Ca2+-activated K+ channels (BKCa) and arteriolar myogenic signaling

Myogenic, or pressure-induced, vasoconstriction is critical for local blood flow autoregulation. Underlying this vascular smooth muscle (VSM) response are events including membrane depolarization, Ca(2+) entry and mobilization, and activation of contractile proteins. Large conductance, Ca(2+)-activated K(+) channel (BK(Ca)) has been implicated in several of these steps including, (1) channel closure causing membrane depolarization, and (2) channel opening causing hyperpolarization to oppose excessive pressure-induced vasoconstriction. As multiple mechanisms regulate BK(Ca) activity (subunit composition, membrane potential (Em) and Ca(2+) levels, post-translational modification) tissue level diversity is predicted. Importantly, heterogeneity in BK(Ca) channel activity may contribute to tissue-specific differences in regulation of myogenic vasoconstriction, allowing local hemodynamics to be matched to metabolic requirements. Knowledge of such variability will be important to exploiting the BK(Ca) channel as a therapeutic target and understanding systemic effects of its pharmacological manipulation.

Arachidonic acid inhibition of L-type calcium (CaV1.3b) channels varies with accessory CaVbeta subunits

Arachidonic acid (AA) inhibits the activity of several different voltage-gated Ca²⁺ channels by an unknown mechanism at an unknown site. The Ca²⁺ channel pore-forming subunit (Ca_Vα₁) is a candidate for the site of AA inhibition because T-type Ca²⁺ channels, which do not require accessory subunits for expression, are inhibited by AA. Here, we report the unanticipated role of accessory Ca_Vβ subunits on the inhibition of Ca_V1.3b L-type (L-) current by AA. Whole cell Ba²⁺ currents were measured from recombinant channels expressed in human embryonic kidney 293 cells at a test potential of −10 mV from a holding potential of −90 mV. A one-minute exposure to 10 µM AA inhibited currents with β_1b, β₃, or β₄ 58, 51, or 44%, respectively, but with β_2a only 31%. At a more depolarized holding potential of −60 mV, currents were inhibited to a lesser degree. These data are best explained by a simple model where AA stabilizes Ca_V1.3b in a deep closed-channel conformation, resulting in current inhibition. Consistent with this hypothesis, inhibition by AA occurred in the absence of test pulses, indicating that channels do not need to open to become inhibited. AA had no effect on the voltage dependence of holding potential–dependent inactivation or on recovery from inactivation regardless of Ca_Vβ subunit. Unexpectedly, kinetic analysis revealed evidence for two populations of L-channels that exhibit willing and reluctant gating previously described for Ca_V2 channels. AA preferentially inhibited reluctant gating channels, revealing the accelerated kinetics of willing channels. Additionally, we discovered that the palmitoyl groups of β_2a interfere with inhibition by AA. Our novel findings that the Ca_Vβ subunit alters kinetic changes and magnitude of inhibition by AA suggest that Ca_Vβ expression may regulate how AA modulates Ca²⁺-dependent processes that rely on L-channels, such as gene expression, enzyme activation, secretion, and membrane excitability.

Cytochrome P450 Products and Arachidonic Acid–Induced, Non–Store-Operated, Ca2+Entry in Cultured Bovine Endothelial Cells

Endothelial cells possess multiple mechanisms for the control of Ca2+ influx during agonist and mechanical stimulation. Increased intracellular Ca2+ during such events is important in the production of vasoactive substances including NO, prostacyclin, and, possibly, endothelium-derived hyperpolarizing factor(s). The present studies examined the effect of arachidonic acid on cellular Ca2+ entry and the underlying mechanisms by which this fatty acid regulates entry. Studies were conducted in cultured bovine aortic endothelial cells (passages 3 to 6) with changes in intracellular Ca2+ determined using the fluorescent Ca2+-sensitive indicator fura 2. Arachidonic acid (1 to 50 microM) stimulated Ca2+ entry from the superfusate without affecting Ca2+ release from intracellular stores. 2-aminoethoxydiphenyl borate (2APB) (100 microM) added at the peak of Ca2+ entry did not inhibit arachidonic acid-induced Ca2+ entry but, in contrast, significantly inhibited entry stimulated by ATP (1 microM). Arachidonic acid-induced Ca2+ entry was inhibited by econazole (1 microM), but not indomethacin (10 microM) or nordihydroguairetic acid (10 microM), suggesting the involvement of cytochrome P450 monooxygenase metabolite of arachidonic acid. Oleic acid (10 microM) was ineffective in inducing Ca2+ entry, whereas linoleic acid (10 microM) stimulated Ca2+ entry but by a mechanism insensitive to econazole. Collectively the data demonstrate that primary cultured aortic endothelial cells possess a Ca2+ entry mechanism modulated by arachidonic acid. This mode of Ca2+ entry appears to operate independently of store depletion-mediated mechanisms.

Modulation of BKCa channel gating by endogenous signaling molecules

Large-conductance Ca(2+)- and voltage-activated K(+) (BK(Ca), MaxiK, or Slo1) channels are expressed in almost every tissue in our body and participate in many critical functions such as neuronal excitability, vascular tone regulation, and neurotransmitter release. The functional versatility of BK(Ca) channels owes in part to the availability of a spectacularly wide array of biological modulators of the channel function. In this review, we focus on modulation of BK(Ca) channels by small endogenous molecules, emphasizing their molecular mechanisms. The mechanistic information available from studies on the small naturally occurring modulators is expected to contribute to our understanding of the physiological and pathophysiological roles of BK(Ca) channels.

Polyunsaturated fatty acid modulation of voltage-gated ion channels

Arachidonic acid (AA) was found to inhibit the function of whole-cell voltage-gated (VG) calcium currents nearly 16 years ago. There are now numerous examples demonstrating that AA and other polyunsaturated fatty acids (PUFAs) modulate the function of VG ion channels, primarily in neurons and muscle cells. We will review and extract some common features about the modulation by PUFAs of VG calcium, sodium, and potassium channels and discuss the impact of this modulation on the excitability of neurons and cardiac myocytes. We will describe the fatty acid nature of the membrane, how fatty acids become available to function as modulators of VG channels, and the physiologic importance of this type of modulation. We will review the evidence for molecular mechanisms and assess our current understanding of the structural basis for modulation. With guidance from research on the structure of fatty acid binding proteins, the role of lipids in gating mechanosensitive (MS) channels, and the impact of membrane lipid composition on membrane-embedded proteins, we will highlight some avenues for future investigations.

Urine stimulation activates BK channels in mouse vomeronasal neurons

Most odor responses in mouse vomeronasal neurons are mediated by the phospholipase C (PLC) pathway, activation of which elevates diacylglycerol (DAG). Lucas et al. showed that DAG activates transient receptor potential channels, subfamily C, member 2 (TRPC2), resulting in a depolarizing Ca2+ influx. DAG can be subsequently converted to arachidonic acid (AA) by a DAG lipase, the role of which remains largely unknown. In this study, we found that urine stimulation of vomeronasal neurons activated large-conductance Ca2+-activated K+ (BK) channels via AA production. Using isolated neurons, we demonstrated that repetitive applications of AA potentiated a K+ current that required a Ca2+ influx and was sensitive to specific BK blockers. Using immunocytochemistry, we found that BK channels are present in vomeronasal neurons with labeling on the soma and heavy labeling on the dendrite with a BK channel antibody. We examined the role of these BK channels in regulating neuronal firing when the neuron was activated by membrane depolarization or urine. Contrary to a recent report, our data suggest that BK channels contribute to adaptation of urine/odor responses because the inhibition of BK channels during urine stimulation promoted repetitive firing. These data strongly support the hypothesis that AA mediates an inhibitory pathway through BK channels, a possible mechanism for odor adaptation in vomeronasal neurons.

Structural determinants of monohydroxylated bile acids to activate beta 1 subunit-containing BK channels

Lithocholate (LC) (10-300 microM) in physiological solution is sensed by vascular myocyte large conductance, calcium- and voltage-gated potassium (BK) channel beta(1) accessory subunits, leading to channel activation and arterial dilation. However, the structural features in steroid and target that determine LC action are unknown. We tested LC and close analogs on BK channel (pore-forming cbv1+beta(1) subunits) activity using the product of the number of functional ion channels in the membrane patch (N) and the open channel probability (Po). LC (5beta-cholanic acid-3alpha-ol), 5alpha-cholanic acid-3alpha-ol, and 5beta-cholanic acid-3beta-ol increased NPo (EC(50) approximately 45 microM). At maximal increase in NPo, LC increased NPo by 180%, whereas 5alpha-cholanic acid-3alpha-ol and 5beta-cholanic acid-3beta-ol raised NPo by 40%. Thus, the alpha-hydroxyl and the cis A-B ring junction are both required for robust channel potentiation. Lacking both features, 5alpha-cholanic acid-3beta-ol and 5-cholenic acid-3beta-ol were inactive. Three-dimensional structures show that only LC displays a bean shape with clear-cut convex and concave hemispheres; 5alpha-cholanic acid-3alpha-ol and 5beta-cholanic acid-3beta-ol partially matched LC shape, and 5alpha-cholanic acid-3beta-ol and 5-cholenic acid-3beta-ol did not. Increasing polarity in steroid rings (5beta-cholanic acid-3alpha-sulfate) or reducing polarity in lateral chain (5beta-cholanic acid 3alpha-ol methyl ester) rendered poorly active compounds, consistent with steroid insertion between beta(1) and bilayer lipids, with the steroid-charged tail near the aqueous phase. Molecular dynamics identified two regions in beta(1) transmembrane domain 2 that meet unique requirements for bonding with the LC concave hemisphere, where the steroid functional groups are located.

Direct regulation of BK channels by phosphatidylinositol 4,5-bisphosphate as a novel signaling pathway

Large conductance, calcium- and voltage-gated potassium (BK) channels are ubiquitous and critical for neuronal function, immunity, and smooth muscle contractility. BK channels are thought to be regulated by phosphatidylinositol 4,5-bisphosphate (PIP(2)) only through phospholipase C (PLC)-generated PIP(2) metabolites that target Ca(2+) stores and protein kinase C and, eventually, the BK channel. Here, we report that PIP(2) activates BK channels independently of PIP(2) metabolites. PIP(2) enhances Ca(2+)-driven gating and alters both open and closed channel distributions without affecting voltage gating and unitary conductance. Recovery from activation was strongly dependent on PIP(2) acyl chain length, with channels exposed to water-soluble diC4 and diC8 showing much faster recovery than those exposed to PIP(2) (diC16). The PIP(2)-channel interaction requires negative charge and the inositol moiety in the phospholipid headgroup, and the sequence RKK in the S6-S7 cytosolic linker of the BK channel-forming (cbv1) subunit. PIP(2)-induced activation is drastically potentiated by accessory beta(1) (but not beta(4)) channel subunits. Moreover, PIP(2) robustly activates BK channels in vascular myocytes, where beta(1) subunits are abundantly expressed, but not in skeletal myocytes, where these subunits are barely detectable. These data demonstrate that the final PIP(2) effect is determined by channel accessory subunits, and such mechanism is subunit specific. In HEK293 cells, cotransfection of cbv1+beta(1) and PI4-kinaseIIalpha robustly activates BK channels, suggesting a role for endogenous PIP(2) in modulating channel activity. Indeed, in membrane patches excised from vascular myocytes, BK channel activity runs down and Mg-ATP recovers it, this recovery being abolished by PIP(2) antibodies applied to the cytosolic membrane surface. Moreover, in intact arterial myocytes under physiological conditions, PLC inhibition on top of blockade of downstream signaling leads to drastic BK channel activation. Finally, pharmacological treatment that raises PIP(2) levels and activates BK channels dilates de-endothelized arteries that regulate cerebral blood flow. These data indicate that endogenous PIP(2) directly activates vascular myocyte BK channels to control vascular tone.

Arachidonic acid and ion channels: an update

Arachidonic acid (AA), a polyunsaturated fatty acid with four double bonds, has multiple actions on living cells. Many of these effects are mediated by an action of AA or its metabolites on ion channels. During the last 10 years, new types of ion channels, transient receptor potential (TRP) channels, store-operated calcium entry (SOCE) channels and non-SOCE channels have been studied. This review summarizes our current knowledge about the effects of AA on TRP and non-SOCE channels as well as classical ion channels. It aims to distinguish between effects of AA itself and effects of AA metabolites. Lipid mediators are of clinical interest because some of them (for example, leukotrienes) play a role in various diseases, others (such as prostaglandins) are targets for pharmacological therapeutic intervention.

Modulation of hSlo BK current inactivation by fatty acid esters of CoA

Lipid metabolism influences membrane proteins, including ion channels, in health and disease. Fatty acid esters of CoA are important intermediates in fatty acid metabolism and lipid biosynthesis. In the present study, we examined the effect of acyl-CoAs on hSlo BK currents. Arachidonoyl-CoA (C(20)-CoA) induced beta2-dependent inhibition of hSlo-alpha current when applied intracellularly but not extracellularly. This action was also mimicked by other long-chain acyl-CoAs such as oleoyl-CoA (C(18)-CoA) and palmitoyl-CoA (C(16)-CoA), but not acetyl-CoA (C(2)-CoA, shorter chain), suggesting that the length of acyl chains, rather than CoA headgroups, is critical. When hSlo-alpha inactivation was induced by a free synthetic cationic beta2 NH2-terminus inactivation ball peptide, long-chain acyl-CoAs inhibited hSlo-alpha current and facilitated inactivation. The precursor fatty acids also facilitated the ball peptide-induced inactivation in a chain length-dependent manner, whereas sphingosine (positively charged) slowed this inactivation. When the beta2-induced inactivation was compared with that of the ball peptide, there was a negative shift in the steady state inactivation, slower recovery, and a reduced voltage-dependence of inactivation onset. These data suggest that electrostatic interactions with the cytosolic inactivation domain of beta2 mediate acyl-CoA modulation of BK currents. BK channel inactivation may be a specific target for lipid modulation in physiological and pathophysiological conditions.

A model for modulation of neuronal synchronization by D4 dopamine receptor-mediated phospholipid methylation

We describe a new molecular mechanism of dopamine-induced membrane protein modulation that can tune neuronal oscillation frequency to attention-related gamma rhythm. This mechanism is based on the unique ability of D4 dopamine receptors (D4R) to carry out phospholipid methylation (PLM) that may affect the kinetics of ion channels. We show that by deceasing the inertia of the delayed rectifier potassium channel, a transition to 40 Hz oscillations can be achieved. Decreased potassium channel inertia shortens spike duration and decreases the interspike interval via its influence on the calcium-dependent potassium current. This mechanism leads to a transition to attention-related gamma oscillations in a pyramidal cell-interneuron network. The higher frequency and better synchronization is observed with PLM affecting pyramidal neurons only, and recurrent excitation between pyramidal neurons is important for synchronization. Thus dopamine-stimulated methylation of membrane phospholipids may be an important mechanism for modulating firing activity, while impaired methylation can contribute to disorders of attention.

β-Subunit–Dependent Modulation ofhSloBK Current by Arachidonic Acid

In this study, we examined the effect of arachidonic acid (AA) on the BK α-subunit with or without β-subunits expressed in Xenopus oocytes. In excised patches, AA potentiated the hSlo-α current and slowed inactivation only when β2/3 subunit was co-expressed. The β2-subunit–dependent modulation by AA persisted in the presence of either superoxide dismutase or inhibitors of AA metabolism such as nordihydroguaiaretic acid and eicosatetraynoic acid, suggesting that AA acts directly rather than through its metabolites. Other cis unsaturated fatty acids (docosahexaenoic and oleic acid) also enhanced hSlo-α + β2 currents and slowed inactivation, whereas saturated fatty acids (palmitic, stearic, and caprylic acid) were without effect. Pretreatment with trypsin to remove the cytosolic inactivation domain largely occluded AA action. Intracellularly applied free synthetic β2-ball peptide induced inactivation of the hSlo-α current, and AA failed to enhance this current and slow the inactivation. These results suggest that AA removes inactivation by interacting, possibly through conformational changes, with β2 to prevent the inactivation ball from reaching its receptor. Our data reveal a novel mechanism of β-subunit–dependent modulation of BK channels by AA. In freshly dissociated mouse neocortical neurons, AA eliminated a transient component of whole cell K+currents. BK channel inactivation may be a specific mechanism by which AA and other unsaturated fatty acids influence neuronal death/survival in neuropathological conditions.

Regulatory effect of sulphatides on BKCa channels

BACKGROUND AND PURPOSE

Sulphatides are sulphated glycosphingolipids expressed on the surface of many cell types, particularly neurones. Changes in sulphatide species or content have been associated with epilepsy and Alzheimer's disease. As the large conductance, calcium sensitive K(+) channel (BK(Ca)) are modulated by membrane lipids, the aim of the study was to explore possible effects of sulphatides on BK(Ca) channels.

EXPERIMENTAL APPROACH

Using patch-clamp techniques, we studied effects of exogenous sulphatides on BK(Ca) channels expressed in Chinese hamster ovary cells.

KEY RESULTS

Sulphatides reversibly increased the whole-cell current and the single channel open probability of BK(Ca) channels dose-dependently. The EC(50) value on the channel at +10 mV was 1.6 microM and the Hill coefficient was 2.5. In inside-out patches, sulphatides increased the single channel open probability from both intra- and extra-cellular faces of the membrane, but more effectively with external application. Furthermore, activation of the channels by sulphatides was independent of intracellular Ca(2+) concentration. Sulphatides also shifted the activation curve of the channels to less positive membrane potentials. Mutant BK(Ca) channels lacking a 59 aminoacid region important for amphipath activation (STREX) were less activated by the sulphatides.

CONCLUSIONS AND IMPLICATIONS

Sulphatides are novel activators of BK(Ca) channels, independent of intracellular Ca(2+) or other signalling molecules but partly dependent on the STREX sequence of the channel protein. As changes of sulphatide content are associated with neuronal dysfunction, as in epilepsy and Alzheimer's disease, our results imply that these effects of sulphatides may play important pathophysiological roles in regulation of BK(Ca) channels.

Dendritic calcium spikes are tunable triggers of cannabinoid release and short-term synaptic plasticity in cerebellar Purkinje neurons

Understanding the relationship between dendritic excitability and synaptic plasticity is vital for determining how dendrites regulate the input-output function of the neuron. Dendritic calcium spikes have been associated with the induction of long-term changes in synaptic efficacy. Here we use direct recordings from cerebellar Purkinje cell dendrites to show that synaptically activated local dendritic calcium spikes are potent triggers of cannabinoid release, producing a profound and short-term reduction in synaptic efficacy at parallel fiber synapses. Enhancing dendritic excitability by modulating dendritic large-conductance calcium-activated potassium (BK) channels improves the spread of dendritic calcium spikes and enhances cannabinoid release at the expense of spatial specificity. Our findings reveal that dendritic calcium spikes provide a local and tunable coincidence detection mechanism that readjusts synaptic gain when synchronous activity reaches a threshold, and they reveal a tight link between the regulation of dendritic excitability and the induction of synaptic plasticity.

Voltage-dependent K+ channel acts as sex steroid sensor in endocrine cells of the human ovary

Molecular targets of rapid non-genomic steroid actions are not well known compared to those of the classical transcription pathway, but ion channels have recently been identified to be steroid-sensitive. Especially, in the ovary, the very organ producing high amounts of sex steroids, their rapid actions are not well examined. We now identified a yet unknown target for sex steroids, a voltage-dependent K+ channel (Kv4.2) that contributes to a transient outward K+ current (I(A)) in human granulosa cells (GCs). Sex steroid hormones at concentrations typical for the ovary (1 microM) blocked Kv4.2 thereby attenuating I(A) by about 25% within seconds. We also found both Kv4.2 (KCND2) mRNA and protein in endocrine cells of the human and rhesus macaque ovary, emphasizing the physiological relevance of this channel. Therefore, we propose a role as fast-responding steroid sensor for the Kv4.2 channel. The direct regulation of K+ channel activity by sex steroids might represent a yet unknown mechanism of rapid steroid action in close proximity to the site of steroid production in the primate ovary. Our data might also be important for Kv4 channels in the brain and the cardiovascular system where rapid steroid effects are discussed in the context of prevention of cell death.

Beta2 and beta4 subunits of BK channels confer differential sensitivity to acute modulation by steroid hormones

Membrane-associated receptors for rapid, steroidal neuromodulation remain elusive. Estradiol has been reported to facilitate activation of voltage- and Ca(2+)-dependent BK potassium channels encoded by Slo, if associated with beta1 subunits. We show here that 1) multiple members of the beta family confer sensitivity to multiple steroids on BK channels, 2) that beta subunits differentiate between steroids, and 3) that different betas have distinct relative preferences for particular steroids. Expressed in HEK 293 cells, inside-out patches with channels composed of Slo-alpha alone showed no steroid sensitivity. Cells expressing alphabeta4 exhibited potent, rapid, reversible, and dose-dependent potentiation by corticosterone (CORT; a glucocorticoid), and were potentiated to a lesser degree by other sex and stress steroids. In contrast, alphabeta2 channels were potentiated more strongly by dehydroepiandrosterone (DHEA; an enigmatic, stress-related adrenal androgen), and to a lesser extent by CORT, estradiol, testosterone, and DHEA-S. Cholesterol had no effect on any BK channel compositions tested. Conductance-voltage plots of channels composed of alpha plus beta2 or beta4 subunits were shifted in the negative direction by steroids, indicating greater activation at negative voltages. Thus our results argue that the variety of Slo-beta subunit coexpression patterns occurring in vivo expands the repertoire of Slo channel gating in yet another dimension not fully appreciated, rendering BK gating responsive to dynamic fluctuations in a multiple of steroid hormones.

Activation of large-conductance, Ca2+-activated K+channels by cannabinoids

We have examined the effects of the cannabinoid anandamide (AEA) and its stable analog, methanandamide (methAEA), on large-conductance, Ca2+-activated K+(BK) channels using human embryonic kidney (HEK)-293 cells, in which the α-subunit of the BK channel (BK-α), both α- and β1-subunits (BK-αβ1), or both α- and β4-subunits (BK-αβ4) were heterologously expressed. In a whole cell voltage-clamp configuration, each cannabinoid activated BK-αβ1within a similar concentration range. Because methAEA could potentiate BK-α, BK-αβ1, and BK-αβ4with similar efficacy, the β-subunits may not be involved at the site of action for cannabinoids. Under cell-attached patch-clamp conditions, application of methAEA to the bathing solution increased BK channel activity; however, methAEA did not alter channel activity in the excised inside-out patch mode even when ATP was present on the cytoplasmic side of the membrane. Application of methAEA to HEK-BK-α and HEK-BK-αβ1did not change intracellular Ca2+concentration. Moreover, methAEA-induced potentiation of BK channel currents was not affected by pretreatment with a CB1antagonist (AM251), modulators of G proteins (cholera and pertussis toxins) or by application of a selective CB2agonist (JWH133). Inhibitors of CaM, PKG, and MAPKs (W7, KT5823, and PD-98059) did not affect the potentiation. Application of methAEA to mouse aortic myocytes significantly increased BK channel currents. This study provides the first direct evidence that unknown factors in the cytoplasm mediate the ability of endogenous cannabinoids to activate BK channel currents. Cannabinoids may be hyperpolarizing factors in cells, such as arterial myocytes, in which BK channels are highly expressed.

Sphingosine-1-phosphate activates BKCa channels independently of G protein-coupled receptor in human endothelial cells

The effect of sphingosine-1-phosphate (S1P) on large-conductance Ca(2+)-activated K(+) (BK(Ca)) channels was examined in primary cultured human umbilical vein endothelial cells by measuring intracellular Ca(2+) concentration ([Ca(2+)](i)), whole cell membrane currents, and single-channel activity. In nystatin-perforated current-clamped cells, S1P hyperpolarized the membrane and simultaneously increased [Ca(2+)](i). [Ca(2+)](i) and membrane potentials were strongly correlated. In whole cell clamped cells, BK(Ca) currents were activated by increasing [Ca(2+)](i) via cell dialysis with pipette solution, and the activated BK(Ca) currents were further enhanced by S1P. When [Ca(2+)](i) was buffered at 1 microM, the S1P concentration required to evoke half-maximal activation was 403 +/- 13 nM. In inside-out patches, when S1P was included in the bath solution, S1P enhanced BK(Ca) channel activity in a reversible manner and shifted the relationship between Ca(2+) concentration in the bath solution and the mean open probability to the left. In whole cell clamped cells or inside-out patches loaded with guanosine 5'-O-(2-thiodiphosphate) (GDPbetaS; 1 mM) using a patch pipette, GDPbetaS application or pretreatment of cells with pertussis toxin (100 ng/ml) for 15 h did not affect S1P-induced BK(Ca) current and channel activation. These results suggest that S1P enhances BK(Ca) channel activity by increasing Ca(2+) sensitivity. This channel activation hyperpolarizes the membrane and thereby increases Ca(2+) influx through Ca(2+) entry channels. Inasmuch as S1P activates BK(Ca) channels via a mechanism independent of G protein-coupled receptors, S1P may be a component of the intracellular second messenger that is involved in Ca(2+) mobilization in human endothelial cells.

Impaired arachidonic acid-mediated activation of large-conductance Ca2+-activated K+ channels in coronary arterial smooth muscle cells in Zucker Diabetic Fatty rats

We studied the arachidonic acid (AA)-mediated modulation of large-conductance Ca2+-activated K+ (BK) channels in coronary arterial smooth myocytes from lean control and Zucker Diabetic Fatty (ZDF) rats. A total of 1 micromol/l AA enhanced BK current by 274% in lean and by 98% in ZDF rats. After incubation with 10 micromol/l indomethacin, 1 micromol/l AA increased BK currents by 80% in lean and by 70% in ZDF rats. Vasoreactivity studies showed that the dilation of small coronary arteries produced by 1 micromol/l AA was reduced by 44% in ZDF rats. [3H]6-keto-prostagladin F1alpha ([3H]6-keto-PGF1alpha,), the stable metabolite of prostacyclin (PGI2), was the major [3H]AA metabolite produced by coronary arteries of lean vessels, but ZDF vessels produced only 15% as much [3H]6-keto-PGF1alpha. BK channel activation and vasorelaxation by iloprost were similar in lean and ZDF rats. Immunoblots showed a 73% reduction in PGI2 synthase (PGIS) expression in ZDF vessels compared with lean vessels, and there was no change in cyclooxygenase (COX) and BK channel expressions. Real-time PCR studies showed that mRNA levels of PGIS, COX-1, and COX-2 were similar between lean and ZDF vessels. We conclude that PGI2 is the major AA metabolite in lean coronaries, and AA-mediated BK channel activation is impaired in ZDF coronaries due to reduced PGIS activity.

Arachidonic acid inhibits the store-operated Ca2+ current in rat liver cells

Vasopressin and other phospholipase-C-coupled hormones induce oscillations (waves) of [Ca2+]cyt (cytoplasmic Ca2+ concentration) in liver cells. Maintenance of these oscillations requires replenishment of Ca2+ in intracellular stores through Ca2+ inflow across the plasma membrane. While this may be achieved by SOCs (store-operated Ca2+ channels), some studies in other cell types indicate that it is dependent on AA (arachidonic acid)-activated Ca2+ channels. We studied the effects of AA on membrane conductance of rat liver cells using whole-cell patch clamping. We found no evidence that concentrations of AA in the physiological range could activate Ca2+-permeable channels in either H4IIE liver cells or rat hepatocytes. However, AA (1-10 microM) did inhibit (IC50=2.4+/-0.1 microM) Ca2+ inflow through SOCs (ISOC) initiated by intracellular application of Ins(1,4,5)P3 in H4IIE cells. Pre-incubation with AA did not inhibit ISOC development, but decreased maximal amplitude of the current. Iso-tetrandrine, widely used to inhibit receptor-activation of phospholipase A2, and therefore AA release, inhibited ISOC directly in H4IIE cells. It is concluded that (i) in rat liver cells, AA does not activate an AA-regulated Ca2+-permeable channel, but does inhibit SOCs, and (ii) iso-tetrandrine and tetrandrine are effective blockers of CRAC (Ca2+-release-activated Ca2+) channel-like SOCs. These results indicate that AA-activated Ca2+-permeable channels do not contribute to hormone-induced increases or oscillations in [Ca2+]cyt in liver cells. However, AA may be a physiological modulator of Ca2+ inflow in these cells.

Somatic localization of a specific large-conductance calcium-activated potassium channel subtype controls compartmentalized ethanol sensitivity in the nucleus accumbens

Alcohol is an addictive drug that targets a variety of ion channels and receptors. To address whether the effects of alcohol are compartment specific (soma vs dendrite), we examined the effects of ethanol (EtOH) on large-conductance calcium-activated potassium channels (BK) in cell bodies and dendrites of freshly isolated neurons from the rat nucleus accumbens (NAcc), a region known to be critical for the development of addiction. Compartment-specific drug action was indeed observed. Clinically relevant concentrations of EtOH increased somatic but not dendritic BK channel open probability. Electrophysiological single-channel recordings and pharmacological analysis of the BK channel in excised patches from each region indicated a number of differences, suggestive of a compartment-specific expression of the beta4 subunit of the BK channel, that might explain the differential alcohol sensitivity. These parameters included activation kinetics, calcium dependency, and toxin blockade. Reverse transcription-PCR showed that both BK channel beta1 and beta4 subunit mRNAs are found in the NAcc, although the signal for beta1 is significantly weaker. Immunohistochemistry revealed that beta1 subunits were found in both soma and dendrites, whereas beta4 appeared restricted to the soma. These findings suggest that the beta4 subunit may confer EtOH sensitivity to somatic BK channels, whereas the absence of beta4 in the dendrite results in insensitivity to the drug. Consistent with this idea, acute EtOH potentiated alphabeta4 BK currents in transfected human embryonic kidney cells, whereas it failed to alter alphabeta1 BK channel-mediated currents. Finally, an EtOH concentration (50 mm) that increased BK channel open probability strongly decreased the duration of somatic-generated action potential in NAcc neurons.