The steroid interaction site in transmembrane domain 2 of the large conductance, voltage- and calcium-gated potassium (BK) channel accessory β1 subunit

The sperm-specific K+ channel Slo3 is inhibited by albumin and steroids contained in reproductive fluids

To locate and fertilize the egg, sperm probe the varying microenvironment prevailing at different stages during their journey across the female genital tract. To this end, they are equipped with a unique repertoire of mostly sperm-specific proteins. In particular, the flagellar Ca2+ channel CatSper has come into focus as a polymodal sensor used by human sperm to register ligands released into the female genital tract. Here, we provide the first comprehensive study on the pharmacology of the sperm-specific human Slo3 channel, shedding light on its modulation by reproductive fluids and their constituents. We show that seminal fluid and contained prostaglandins and Zn2+ do not affect the channel, whereas human Slo3 is inhibited in a non-genomic fashion by diverse steroids as well as by albumin, which are released into the oviduct along with the egg. This indicates that not only CatSper but also Slo3 harbours promiscuous ligand-binding sites that can accommodate structurally diverse molecules, suggesting that Slo3 is involved in chemosensory signalling in human sperm.

Cholesterol regulation of mechanosensitive ion channels

The purpose of this review is to evaluate the role of cholesterol in regulating mechanosensitive ion channels. Ion channels discussed in this review are sensitive to two types of mechanical signals, fluid shear stress and/or membrane stretch. Cholesterol regulates the channels primarily in two ways: 1) indirectly through localizing the channels into cholesterol-rich membrane domains where they interact with accessory proteins and/or 2) direct binding of cholesterol to the channel at specified putative binding sites. Cholesterol may also regulate channel function via changes of the biophysical properties of the membrane bilayer. Changes in cholesterol affect both mechanosensitivity and basal channel function. We focus on four mechanosensitive ion channels in this review Piezo, Kir2, TRPV4, and VRAC channels. Piezo channels were shown to be regulated by auxiliary proteins that enhance channel function in high cholesterol domains. The direct binding mechanism was shown in Kir2.1 and TRPV4 where cholesterol inhibits channel function. Finally, cholesterol regulation of VRAC was attributed to changes in the physical properties of lipid bilayer. Additional studies should be performed to determine the physiological implications of these sterol effects in complex cellular environments.

A Novel Role of Uricosuric Agent Benzbromarone in BK Channel Activation and Reduction of Airway Smooth Muscle Contraction

The uricosuric drug benzbromarone, widely used for treatment of gout, hyperpolarizes the membrane potential of airway smooth muscle cells, but how it works remains unknown. Here we show a novel role of benzbromarone in activation of large conductance calcium-activated K⁺ channels. Benzbromarone results in dose-dependent activation of macroscopic big potassium (BK) currents about 1.7- to 14.5-fold with an EC₅₀ of 111 μM and shifts the voltage-dependent channel activation to a more hyperpolarizing direction about 10 to 54 mV in whole-cell patch clamp recordings. In single-channel recordings, benzbromarone decreases single BKα channel closed dwell time and increases the channel open probability. Coexpressing β1 subunit also enhances BK activation by benzbromarone with an EC₅₀ of 67 μM and a leftward shift of conductance-voltage (G-V) curve about 44 to 138 mV. Site-directed mutagenesis reveals that a motif of three amino acids ³²⁹RKK³³¹ in the cytoplasmic linker between S6 and C-terminal regulator of potassium conductance (RCK) gating ring mediates the pharmacological activation of BK channels by benzbromarone. Further ex vivo assay shows that benzbromarone causes reduction of tracheal strip contraction. Taken together, our findings demonstrate that uricosuric benzbromarone activates BK channels through molecular mechanism of action involving the channel RKK motif of S6-RCK linker. Pharmacological activation of BK channel by benzbromarone causes reduction of tracheal strip contraction, holding a repurposing potential for asthma and pulmonary arterial hypertension or BK channelopathies. SIGNIFICANCE STATEMENT: We describe a novel role of uricosuric agent benzbromarone in big potassium (BK) channel activation and relaxation of airway smooth muscle contraction. In this study, we find that benzbromarone is an activator of the large-conductance Ca²⁺- and voltage-activated K⁺ channel (BK channel), which serves numerous cellular functions, including control of smooth muscle contraction. Pharmacological activation of BK channel by the FDA-approved drug benzbromarone may hold repurposing potential for treatment of asthma and pulmonary arterial hypertension or BK channelopathies.

H2S donating corticosteroids: Design, synthesis and biological evaluation in a murine model of asthma

Introduction

Hydrogen sulfide (H2S) is a fundamental biological endogenous gas-mediator in the respiratory system. It regulates pivotal patho-physiological processes such as oxidative stress, pulmonary circulation, airway tone and inflammation.

Objectives

We herein describe the design and synthesis of molecular hybrids obtained by the condensation of several corticosteroids with different hydrogen sulfide releasing moieties.

Methods

All the molecules are characterized for their ability to release H2S both via amperometric approach and using a fluorescent probe. The chemical stability of the newly synthesized hybrid molecules has been investigated at differing pH values and in human serum.

Results

Prednisone-TBZ hybrid (compound 7) was selected for further evaluations. The obtained results from the in vitro and in vivo studies clearly show evidence in favor of the anti-inflammatory properties of the released H2S.

Conclusions

The protective effect on airway remodeling makes the hybrid Prednisone-TBZ (compound 7) as a promising therapeutic option in reducing allergic asthma symptoms and exacerbations.

The Membrane Cholesterol Modulates the Interaction Between 17-βEstradiol and the BK Channel

BK channels are composed by the pore forming α subunit and, in some tissues, is associated with different accessory β subunits. These proteins modify the biophysical properties of the channel, amplifying the range of BK channel activation according to the physiological context. In the vascular cells, the pore forming BKα subunit is expressed with the β1 subunit, where they play an essential role in the modulation of arterial tone and blood pressure. In eukaryotes, cholesterol is a structural lipid of the cellular membrane. Changes in the ratio of cholesterol content in the plasma membrane (PM) regulates the BK channel activation altering its open probability, and hence, vascular contraction. It has been shown that the estrogen 17β-Estradiol (E2) causes a vasodilator effect in vascular cells, inducing a leftward shift in the V_0.5 of the GV curve. Here, we evaluate whether changes in the membrane cholesterol concentration modify the effect that E2 induces on the BKα/β1 channel activity. Using binding and electrophysiology assays after cholesterol depletion or enrichment, we show that the cholesterol enrichment significantly decreases the expression of the α subunit, while cholesterol depletion increased the expression of that α subunit. Additionally, we demonstrated that changes in the membrane cholesterol cause the loss of the modulatory effect of E2 on the BKα/β1 channel activity, without affecting the E2 binding to the complex. Our data suggest that changes in membrane cholesterol content could affect channel properties related to the E2 effect on BKα/β1 channel activity. Finally, the results suggest that an optimal membrane cholesterol content is essential for the activation of BK channels through the β1 subunit.

Cortisol modulates calcium release-activated calcium channel gating in fish hepatocytes

Glucocorticoids (GCs) are rapidly released in response to stress and play an important role in the physiological adjustments to re-establish homeostasis. The mode of action of GCs for stress coping is mediated largely by the steroid binding to the glucocorticoid receptor (GR), a ligand-bound transcription factor, and modulating the expression of target genes. However, GCs also exert rapid actions that are independent of transcriptional regulation by modulating second messenger signaling. However, a membrane-specific protein that transduces rapid GCs signal is yet to be characterized. Here, using freshly isolated hepatocytes from rainbow trout (Oncorhynchus mykiss) and fura2 fluorescence microscopy, we report that stressed levels of cortisol rapidly stimulate the rise in cytosolic free calcium ([Ca²⁺]i). Pharmacological manipulations using specific extra- and intra-cellular calcium chelators, plasma membrane and endoplasmic reticulum channel blockers and receptors, indicated extracellular Ca²⁺ entry is required for the cortisol-mediated rise in ([Ca²⁺]i). Particularly, the calcium release-activated calcium (CRAC) channel gating appears to be a key target for the rapid action of cortisol in the ([Ca²⁺]i) rise in trout hepatocytes. To test this further, we carried out in silico molecular docking studies using the Drosophila CRAC channel modulator 1 (ORAI1) protein, the pore forming subunit of CRAC channel that is highly conserved. The result predicts a putative binding site on CRAC for cortisol to modulate channel gating, suggesting a direct, as well as an indirect regulation (by other membrane receptors) of CRAC channel gating by cortisol. Altogether, CRAC channel may be a novel cortisol-gated Ca²⁺ channel transducing rapid nongenomic signalling in hepatocytes during acute stress.

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.

Celastrol Dilates and Counteracts Ethanol-Induced Constriction of Cerebral Arteries

The increasing recognition of the role played by cerebral artery dysfunction in brain disorders has fueled the search for new cerebrovascular dilators. Celastrol, a natural triterpene undergoing clinical trials for treating obesity, exerts neuroprotection, which was linked to its antioxidant/anti-inflammatory activities. We previously showed that celastrol fit pharmacophore criteria for activating calcium- and voltage-gated potassium channels of large conductance (BK channels) made of subunits cloned from cerebrovascular smooth muscle (SM). These recombinant BK channels expressed in a heterologous system were activated by celastrol. Activation of native SM BK channels is well known to evoke cerebral artery dilation. Current data demonstrate that celastrol (1-100 µM) dilates de-endothelialized, ex vivo pressurized middle cerebral arteries (MCAs) from rats, with EC₅₀ = 45 µM and maximal effective concentration (E_max)= 100 µM and with MCA diameter reaching a 10% increase over vehicle-containing, time-matched values (P < 0.05). A similar vasodilatory efficacy is achieved when celastrol is probed on MCA segments with intact endothelium. Selective BK blocking with 1 μM paxilline blunts celastrol vasodilation. Similar blunting is achieved with 0.8 mM 4-aminopirydine, which blocks voltage-gated K⁺ channels other than BK. Using an in vivo rat cranial window, we further demonstrate that intracarotid injections of 45 μM celastrol into pial arteries branching from MCA mimics celastrol ex vivo action. MCA constriction by ethanol concentrations reached in blood during moderate-heavy alcohol drinking (50 mM), which involves SM BK inhibition, is both prevented and reverted by celastrol. We conclude that celastrol could be an effective cerebrovascular dilator and antagonist of alcohol-induced cerebrovascular constriction, with its efficacy being uncompromised by conditions that disrupt endothelial and/or BK function. SIGNIFICANCE STATEMENT: Our study demonstrates for the first time that celastrol significantly dilates rat cerebral arteries both ex vivo and in vivo and both prevents and reverses ethanol-induced cerebral artery constriction. Celastrol actions are endothelium-independent but mediated through voltage-gated (K_V) and calcium- and voltage-gated potassium channel of large conductance (BK) K⁺ channels. This makes celastrol an appealing new agent to evoke cerebrovascular dilation under conditions in which endothelial and/or BK channel function are impaired.

Interfacial Binding Sites for Cholesterol on Kir, Kv, K2P, and Related Potassium Channels

Inwardly rectifying, voltage-gated, two-pore domain, and related K+ channels are located in eukaryotic membranes rich in cholesterol. Here, molecular docking is used to detect specific binding sites ("hot spots") for cholesterol on K+ channels with characteristics that match those of known cholesterol binding sites. The transmembrane surfaces of all available high-resolution structures for K+ channels were swept for potential binding sites. Cholesterol poses were found to be located largely in hollows between protein ridges. A comparison between cholesterol poses and resolved phospholipids suggests that not all cholesterol molecules binding to the transmembrane surface of a K+ channel will result in displacement of a phospholipid molecule from the surface. Competition between cholesterol binding and binding of anionic phospholipids essential for activity could explain some of the effects of cholesterol on channel function.

Comparative docking analysis of cholesterol analogs to ion channels to discriminate between stereospecific binding vs. stereospecific response

Cholesterol is a major component of the membrane and a key regulator of many ion channels. Multiple studies showed that cholesterol regulates ion channels in a stereospecific manner, with cholesterol but not its chiral isomers having a functional effect. This stereospecificity has been universally attributed to the specificity of cholesterol binding, with the assumption that only native cholesterol binds to the channels whereas its isomers do not. In this study, we challenge this paradigm by docking analyses of cholesterol and its chiral isomers to five ion channels whose response to cholesterol was shown to be stereospecific, Kir2.2, KirBac1.1, TRPV1, GABA_A and BK. The analysis is performed using AutoDock Vina to predict the binding poses and energies of the sterols to the channels and identify amino acids interacting with the sterol molecules. We found that for every ion channel tested herein all three sterols showed similar binding poses and significant overlap in the set of the amino acids that comprise the predicted binding sites, along with similar energetic favorability to these overlapping sites. We also found, however, that specific orientations of the three sterols within the binding sites of the channels are distinct, so that a subset of the interacting amino acids is unique to each sterol. We propose therefore, that contrary to previous thought, stereospecific effects of cholesterol should be attributed not to the lack of binding of the stereoisomers but to specific, unique interactions between the cholesterol molecule and the residues within the binding sites of the channels.

The molecular nature of the 17β-Estradiol binding site in the voltage- and Ca2+-activated K+ (BK) channel β1 subunit

The accessory β1 subunit modulates the Ca2+- and voltage-activated K+ (BK) channel gating properties mainly by increasing its apparent Ca2+ sensitivity. β1 plays an important role in the modulation of arterial tone and blood pressure by vascular smooth muscle cells (SMCs). 17β-estradiol (E2) increases the BK channel open probability (Po) in SMCs, through a β1 subunit-dependent modulatory effect. Here, using molecular modeling, bioinformatics, mutagenesis, and electrophysiology, we identify a cluster of hydrophobic residues in the second transmembrane domain of the β1 subunit, including the residues W163 and F166, as the binding site for E2. We further show that the increase in Po induced by E2 is associated with a stabilization of the voltage sensor in its active configuration and an increase in the coupling between the voltage sensor activation and pore opening. Since β1 is a key molecular player in vasoregulation, the findings reported here are of importance in the design of novel drugs able to modulate BK channels.

The Biosynthesis, Signaling, and Neurological Functions of Bile Acids

Bile acids (BA) are amphipathic steroid acids synthesized from cholesterol in the liver. They act as detergents to expedite the digestion and absorption of dietary lipids and lipophilic vitamins. BA are also considered to be signaling molecules, being ligands of nuclear and cell-surface receptors, including farnesoid X receptor and Takeda G-protein receptor 5. Moreover, BA also activate ion channels, including the bile acid-sensitive ion channel and epithelial Na⁺ channel. BA regulate glucose and lipid metabolism by activating these receptors in peripheral tissues, such as the liver and brown and white adipose tissue. Recently, 20 different BA have been identified in the central nervous system. Furthermore, BA affect the function of neurotransmitter receptors, such as the muscarinic acetylcholine receptor and γ-aminobutyric acid receptor. BA are also known to be protective against neurodegeneration. Here, we review recent findings regarding the biosynthesis, signaling, and neurological functions of BA.

Molecular Dynamics Simulations of Kir2.2 Interactions with an Ensemble of Cholesterol Molecules

Cholesterol is a major regulator of multiple types of ion channels, but the specific mechanisms and the dynamics of its interactions with the channels are not well understood. Kir2 channels were shown to be sensitive to cholesterol through direct interactions with "cholesterol-sensitive" regions on the channel protein. In this work, we used Martini coarse-grained simulations to analyze the long (μs) timescale dynamics of cholesterol with Kir2.2 channels embedded into a model membrane containing POPC phospholipid with 30 mol% cholesterol. This approach allows us to simulate the dynamic, unbiased migration of cholesterol molecules from the lipid membrane environment to the protein surface of Kir2.2 and explore the favorability of cholesterol interactions at both surface sites and recessed pockets of the channel. We found that the cholesterol environment surrounding Kir channels forms a complex milieu of different short- and long-term interactions, with multiple cholesterol molecules concurrently interacting with the channel. Furthermore, utilizing principles from network theory, we identified four discrete cholesterol-binding sites within the previously identified cholesterol-sensitive region that exist depending on the conformational state of the channel-open or closed. We also discovered that a twofold decrease in the cholesterol level of the membrane, which we found earlier to increase Kir2 activity, results in a site-specific decrease of cholesterol occupancy at these sites in both the open and closed states: cholesterol molecules at the deepest of these discrete sites shows no change in occupancy at different cholesterol levels, whereas the remaining sites showed a marked decrease in occupancy.

How lipids contribute to ion channel function, a fat perspective on direct and indirect interactions

Membrane lipid composition and remodeling influence the function of ion channels. Polyunsaturated fatty acids (PUFAs) and their derivatives modulate ion channel function; whether this effect occurs directly by binding to the protein or indirectly through alteration of membranes' mechanical properties has been difficult to distinguish. There are a large number of studies addressing the effect of fatty acids; recent structural and functional analyses have identified binding sites and provided further evidence for the role of the plasma membrane in ion channel function. Here, we review cation channels that do not share a common topology or lipid-binding signature sequence, but for which there are recent compelling data that support both direct and indirect modulation by PUFAs or their derivatives.

Calcium- and voltage-gated BK channels in vascular smooth muscle

Ion channels in vascular smooth muscle regulate myogenic tone and vessel contractility. In particular, activation of calcium- and voltage-gated potassium channels of large conductance (BK channels) results in outward current that shifts the membrane potential toward more negative values, triggering a negative feed-back loop on depolarization-induced calcium influx and SM contraction. In this short review, we first present the molecular basis of vascular smooth muscle BK channels and the role of subunit composition and trafficking in the regulation of myogenic tone and vascular contractility. BK channel modulation by endogenous signaling molecules, and paracrine and endocrine mediators follows. Lastly, we describe the functional changes in smooth muscle BK channels that contribute to, or are triggered by, common physiological conditions and pathologies, including obesity, diabetes, and systemic hypertension.

BK β1 subunit-dependent facilitation of ethanol inhibition of BK current and cerebral artery constriction is mediated by the β1 transmembrane domain 2

BACKGROUND AND PURPOSE

Ethanol at concentrations obtained in the circulation during moderate-heavy episodic drinking (30-60 mM) causes cerebral artery constriction in several species, including humans. In rodents, ethanol-induced cerebral artery constriction results from ethanol inhibition of large conductance voltage/Ca²⁺_i -gated K⁺ (BK) channels in cerebral artery myocytes. Moreover, the smooth muscle-abundant BK β1 accessory subunit is required for ethanol to inhibit cerebral artery myocyte BK channels under physiological Ca²⁺_i and voltages and thus constrict cerebral arteries. The molecular bases of these ethanol actions remain unknown. Here, we set to identify the BK β1 region(s) that mediates ethanol-induced inhibition of cerebral artery myocyte BK channels and eventual arterial constriction.

EXPERIMENTAL APPROACH

We used protein biochemistry, patch-clamp on engineered channel subunits, reversible cDNA permeabilization of KCNMB1 K/O mouse arteries and artery in vitro pressurization.

KEY RESULTS

Ethanol inhibition of BK current was facilitated by β1 but not β4 subunits. Furthermore, only BK complexes containing β chimeras with β1 transmembrane (TM) domains on a β4 background or with a β1 TM2 domain on a β4 background displayed ethanol responses identical to those of BK complexes including wild-type β1. Moreover, β1 TM2 itself but not other β regions were necessary for ethanol-induced cerebral artery constriction.

CONCLUSIONS AND IMPLICATIONS

BK β1 TM2 is necessary for this subunit to enable ethanol-induced inhibition of myocyte BK channels and cerebral artery constriction at physiological Ca²⁺ and voltages. Thus, novel agents that target β1 TM2 may be considered to counteract ethanol-induced cerebral artery constriction and associated cerebrovascular conditions.

Regulation of Ca2+-Sensitive K+ Channels by Cholesterol and Bile Acids via Distinct Channel Subunits and Sites

Cholesterol (CLR) conversion into bile acids (BAs) in the liver constitutes the major pathway for CLR elimination from the body. Moreover, these steroids regulate each other's metabolism. While the roles of CLR and BAs in regulating metabolism and tissue function are well known, research of the last two decades revealed the existence of specific protein receptors for CLR or BAs in tissues with minor contribution to lipid metabolism, raising the possibility that these lipids serve as signaling molecules throughout the body. Among other lipids, CLR and BAs regulate ionic current mediated by the activity of voltage- and Ca²⁺-gated, K⁺ channels of large conductance (BK channels) and, thus, modulate cell physiology and participate in tissue pathophysiology. Initial work attributed modification of BK channel function by CLR or BAs to the capability of these steroids to directly interact with bilayer lipids and thus alter the physicochemical properties of the bilayer with eventual modification of BK channel function. Based on our own work and that of others, we now review evidence that supports direct interactions between CLR or BA and specific BK protein subunits, and the consequence of such interactions on channel activity and organ function, with a particular emphasis on arterial smooth muscle. For each steroid type, we will also briefly discuss several mechanisms that may underlie modification of channel steady-state activity. Finally, we will present novel computational data that provide a chemical basis for differential recognition of CLR vs lithocholic acid by distinct BK channel subunits and recognition sites.

Molecular Determinants of BK Channel Functional Diversity and Functioning

Large-conductance Ca2+- and voltage-activated K+ (BK) channels play many physiological roles ranging from the maintenance of smooth muscle tone to hearing and neurosecretion. BK channels are tetramers in which the pore-forming α subunit is coded by a single gene ( Slowpoke, KCNMA1). In this review, we first highlight the physiological importance of this ubiquitous channel, emphasizing the role that BK channels play in different channelopathies. We next discuss the modular nature of BK channel-forming protein, in which the different modules (the voltage sensor and the Ca2+ binding sites) communicate with the pore gates allosterically. In this regard, we review in detail the allosteric models proposed to explain channel activation and how the models are related to channel structure. Considering their extremely large conductance and unique selectivity to K+, we also offer an account of how these two apparently paradoxical characteristics can be understood consistently in unison, and what we have learned about the conduction system and the activation gates using ions, blockers, and toxins. Attention is paid here to the molecular nature of the voltage sensor and the Ca2+ binding sites that are located in a gating ring of known crystal structure and constituted by four COOH termini. Despite the fact that BK channels are coded by a single gene, diversity is obtained by means of alternative splicing and modulatory β and γ subunits. We finish this review by describing how the association of the α subunit with β or with γ subunits can change the BK channel phenotype and pharmacology.

Differential distribution and functional impact of BK channel beta1 subunits across mesenteric, coronary, and different cerebral arteries of the rat

Large conductance, Ca²⁺_i- and voltage-gated K⁺ (BK) channels regulate myogenic tone and, thus, arterial diameter. In smooth muscle (SM), BK channels include channel-forming α and auxiliary β1 subunits. BK β1 increases the channel's Ca²⁺ sensitivity, allowing BK channels to negatively feedback on depolarization-induced Ca²⁺ entry, oppose SM contraction and favor vasodilation. Thus, endothelial-independent vasodilation can be evoked though targeting of SM BK β1 by endogenous ligands, including lithocholate (LCA). Here, we investigated the expression of BK β1 across arteries of the cerebral and peripheral circulations, and the contribution of such expression to channel function and BK β1-mediated vasodilation. Data demonstrate that endothelium-independent, BK β1-mediated vasodilation by LCA is larger in coronary (CA) and basilar (BA) arteries than in anterior cerebral (ACA), middle cerebral (MCA), posterior cerebral (PCA), and mesenteric (MA) arteries, all arterial segments having a similar diameter. Thus, differential dilation occurs in extracranial arteries which are subjected to similar vascular pressure (CA vs. MA) and in arteries that irrigate different brain regions (BA vs. ACA, MCA, and PCA). SM BK channels from BA and CA displayed increased basal activity and LCA responses, indicating increased BK β1 functional presence. Indeed, in the absence of detectable changes in BK α, BA and CA myocytes showed an increased location of BK β1 in the plasmalemma/subplasmalemma. Moreover, these myocytes distinctly showed increased BK β1 messenger RNA (mRNA) levels. Supporting a major role of enhanced BK β1 transcripts in artery dilation, LCA-induced dilation of MCA transfected with BK β1 complementary DNA (cDNA) was as high as LCA-induced dilation of untransfected BA or CA.

Decoding the vasoregulatory activities of bile acid-activated receptors in systemic and portal circulation: role of gaseous mediators

Bile acids are end products of cholesterol metabolism generated in the liver and released in the intestine. Primary and secondary bile acids are the result of the symbiotic relation between the host and intestinal microbiota. In addition to their role in nutrient absorption, bile acids are increasingly recognized as regulatory signals that exert their function beyond the intestine by activating a network of membrane and nuclear receptors. The best characterized of these bile acid-activated receptors, GPBAR1 (also known as TGR5) and the farnesosid-X-receptor (FXR), have also been detected in the vascular system and their activation mediates the vasodilatory effects of bile acids in the systemic and splanchnic circulation. GPBAR1, is a G protein-coupled receptor, that is preferentially activated by lithocholic acid (LCA) a secondary bile acid. GPBAR1 is expressed in endothelial cells and liver sinusoidal cells (LSECs) and responds to LCA by regulating the expression of both endothelial nitric oxide synthase (eNOS) and cystathionine-γ-lyase (CSE), an enzyme involved in generation of hydrogen sulfide (H₂S). Activation of CSE by GPBAR1 ligands in LSECs is due to genomic and nongenomic effects, involves protein phosphorylation, and leads to release of H₂S. Despite that species-specific effects have been described, vasodilation caused by GPBAR1 ligands in the liver microcirculation and aortic rings is abrogated by inhibition of CSE but not by eNOS inhibitor. Vasodilation caused by GPBAR1 (and FXR) ligands also involves large conductance calcium-activated potassium channels likely acting downstream to H₂S. The identification of GPBAR1 as a vasodilatory receptor is of relevance in the treatment of complex disorders including metabolic syndrome-associated diseases, liver steatohepatitis, and portal hypertension.

Modulation of BK Channels by Ethanol

In alcohol-naïve systems, ethanol (<100mM) exposure of calcium-gated BK channels perturbs physiology and behavior. Brief (several minutes) ethanol exposure usually leads to increased BK current, which results from ethanol interaction with a pocket mapped to the BK channel-forming slo1 protein cytosolic tail domain. The importance of this region in ethanol-induced intoxication has been independently supported by an unbiased screen of Caenorhabditis elegans slo1 mutants. However, ethanol-induced BK activation is not universal as refractoriness and inhibition have been reported. The final effect depends on many factors, including intracellular calcium levels, slo1 isoform, BK beta subunit composition, posttranslational modification of BK proteins, channel lipid microenvironment, and type of ethanol administration. Studies in Drosophila melanogaster, C. elegans, and rodents show that protracted/repeated ethanol administration leads to tolerance to ethanol-induced modification of BK-driven physiology and behavior. Unveiling the mechanisms underlying tolerance is of major importance, as tolerance to ethanol has been proposed as predictor of risk for alcoholism.

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.

Modulation of BK Channel Function by Auxiliary Beta and Gamma Subunits

The large-conductance, Ca(2+)- and voltage-activated K(+) (BK) channel is ubiquitously expressed in mammalian tissues and displays diverse biophysical or pharmacological characteristics. This diversity is in part conferred by channel modulation with different regulatory auxiliary subunits. To date, two distinct classes of BK channel auxiliary subunits have been identified: β subunits and γ subunits. Modulation of BK channels by the four auxiliary β (β1-β4) subunits has been well established and intensively investigated over the past two decades. The auxiliary γ subunits, however, were identified only very recently, which adds a new dimension to BK channel regulation and improves our understanding of the physiological functions of BK channels in various tissues and cell types. This chapter will review the current understanding of BK channel modulation by auxiliary β and γ subunits, especially the latest findings.

Bisphenol A activates BK channels through effects on α and β1 subunits

We demonstrated previously that BK (K(Ca)1.1) channel activity (NP(o)) increases in response to bisphenol A (BPA). Moreover, BK channels containing regulatory β1 subunits were more sensitive to the stimulatory effect of BPA. How BPA increases BK channel NPo remains mostly unknown. Estradiol activates BK channels by binding to an extracellular site, but neither the existence nor location of a BPA binding site has been demonstrated. We tested the hypothesis that an extracellular binding site is responsible for activation of BK channels by BPA. We synthesized membrane-impermeant BPA-monosulfate (BPA-MS) and used patch clamp electrophysiology to study channels composed of α or α + β1 subunits in cell-attached (C-A), whole-cell (W-C), and inside-out (I-O) patches. In C-A patches, bath application of BPA-MS (100 μM) had no effect on the NP(o) of BK channels, regardless of their subunit composition. Importantly, however, subsequent addition of membrane-permeant BPA (100 μM) increased the NP(o) of both α and α + β1 channels in C-A patches. The C-A data indicate that in order to alter BK channel NP(o), BPA must interact with the channel itself (or some closely associated partner) and diffusible messengers are not involved. In W-C patches, 100 μM BPA-MS activated current in cells expressingα subunits, whereas cells expressing α + β1 subunits responded similarly to a log-order lower concentration (10 μM). The W-C data suggest that an extracellular activation site exists, but do not eliminate the possibility that an intracellular site may also be present. In I-O patches, where the cytoplasmic face was exposed to the bath, BPA-MS had no effect on the NP(o) of BK α subunits, but BPA increased it. BPA-MS increased the NP(o) of α + β1 channels in I-O patches, but not as much as BPA. We conclude that BPA activates BK α via an extracellular site and that BPA-sensitivity is increased by the β1 subunit, which may also constitute part of an intracellular binding site.

BK channels: multiple sensors, one activation gate

Ion transport across cell membranes is essential to cell communication and signaling. Passive ion transport is mediated by ion channels, membrane proteins that create ion conducting pores across cell membrane to allow ion flux down electrochemical gradient. Under physiological conditions, majority of ion channel pores are not constitutively open. Instead, structural region(s) within these pores breaks the continuity of the aqueous ion pathway, thereby serves as activation gate(s) to control ions flow in and out. To achieve spatially and temporally regulated ion flux in cells, many ion channels have evolved sensors to detect various environmental stimuli or the metabolic states of the cell and trigger global conformational changes, thereby dynamically operate the opening and closing of their activation gate. The sensors of ion channels can be broadly categorized as chemical sensors and physical sensors to respond to chemical (such as neural transmitters, nucleotides and ions) and physical (such as voltage, mechanical force and temperature) signals, respectively. With the rapidly growing structural and functional information of different types of ion channels, it is now critical to understand how ion channel sensors dynamically control their gates at molecular and atomic level. The voltage and Ca²⁺ activated BK channels, a K⁺ channel with an electrical sensor and multiple chemical sensors, provide a unique model system for us to understand how physical and chemical energy synergistically operate its activation gate.

Dehydroepiandrosterone, its metabolites and ion channels

This review is focused on the physiological and pathophysiological relevance of steroids influencing the activities of the central and peripheral nervous systems with regard to their concentrations in body fluids and tissues in various stages of human life like the fetal development or pregnancy. The data summarized in this review shows that DHEA and its unconjugated and sulfated metabolites are physiologically and pathophysiologically relevant in modulating numerous ion channels and participate in vital functions of the human organism. DHEA and its unconjugated and sulfated metabolites including 5α/β-reduced androstane steroids participate in various physiological and pathophysiological processes like the management of GnRH cyclic release, regulation of glandular and neurotransmitter secretions, maintenance of glucose homeostasis on one hand and insulin insensitivity on the other hand, control of skeletal muscle and smooth muscle activities including vasoregulation, promotion of tolerance to ischemia and other neuroprotective effects. In respect of prevalence of steroid sulfates over unconjugated steroids in the periphery and the opposite situation in the CNS, the sulfated androgens and androgen metabolites reach relevance in peripheral organs. The unconjugated androgens and estrogens are relevant in periphery and so much the more in the CNS due to higher concentrations of most unconjugated steroids in the CNS tissues than in circulation and peripheral organs. This article is part of a Special Issue entitled "Essential role of DHEA".

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.

Multi-generational pharmacophore modeling for ligands to the cholane steroid-recognition site in the β₁ modulatory subunit of the BKCa channel

Large conductance, voltage- and Ca(2+)-gated K(+) (BKCa) channels play a critical role in smooth muscle contractility and thus represent an emerging therapeutic target for drug development to treat vascular disease, gastrointestinal, bladder and uterine disorders. Several compounds are known to target the ubiquitously expressed BKCa channel-forming α subunit. In contrast, just a few are known to target the BKCa modulatory β1 subunit, which is highly expressed in smooth muscle and scarce in most other tissues. Lack of available high-resolution structural data makes structure-based pharmacophore modeling of β1 subunit-dependent BKCa channel activators a major challenge. Following recent discoveries of novel BKCa channel activators that act via β1 subunit recognition, we performed ligand-based pharmacophore modeling that led to the successful creation and fine-tuning of a pharmacophore over several generations. Initial models were developed using physiologically active cholane steroids (bile acids) as template. However, as more compounds that act on BKCa β1 have been discovered, our model has been refined to improve accuracy. Database searching with our best-performing model has uncovered several novel compounds as candidate BKCa β1 subunit ligands. Eight of the identified compounds were experimentally screened and two proved to be activators of recombinant BKCa β1 complexes. One of these activators, sobetirome, differs substantially in structure from any previously reported activator.

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.

Both transmembrane domains of BK β1 subunits are essential to confer the normal phenotype of β1-containing BK channels

Voltage/Ca²⁺_i-gated, large conductance K⁺ (BK) channels result from tetrameric association of α (slo1) subunits. In most tissues, BK protein complexes include regulatory β subunits that contain two transmembrane domains (TM1, TM2), an extracellular loop, and two short intracellular termini. Four BK β types have been identified, each presenting a rather selective tissue-specific expression profile. Thus, BK β modifies current phenotype to suit physiology in a tissue-specific manner. The smooth muscle-abundant BK β1 drastically increases the channel's apparent Ca²⁺_i sensitivity. The resulting phenotype is critical for BK channel activity to increase in response to Ca²⁺ levels reached near the channel during depolarization-induced Ca²⁺ influx and myocyte contraction. The eventual BK channel activation generates outward K⁺ currents that drive the membrane potential in the negative direction and eventually counteract depolarization-induced Ca²⁺ influx. The BK β1 regions responsible for the characteristic phenotype of β1-containing BK channels remain to be identified. We used patch-clamp electrophysiology on channels resulting from the combination of smooth muscle slo1 (cbv1) subunits with smooth muscle-abundant β1, neuron-abundant β4, or chimeras constructed by swapping β1 and β4 regions, and determined the contribution of specific β1 regions to the BK phenotype. At Ca²⁺ levels found near the channel during myocyte contraction (10 µM), channel complexes that included chimeras having both TMs from β1 and the remaining regions (“background”) from β4 showed a phenotype (V_half, τ_act, τ_deact) identical to that of complexes containing wt β1. This phenotype could not be evoked by complexes that included chimeras combining either β1 TM1 or β1 TM2 with a β4 background. Likewise, β “halves” (each including β1 TM1 or β1 TM2) resulting from interrupting the continuity of the EC loop failed to render the normal phenotype, indicating that physical connection between β1 TMs via the EC loop is also necessary for proper channel function.

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.

Type 2 ryanodine receptors are highly sensitive to alcohol

Exposure to ethanol levels reached in circulation during alcohol intoxication (>10mM) constricts cerebral arteries in rats and humans. Remarkably, targets and mechanisms underlying this action remain largely unidentified. Artery diameter is regulated by myocyte Ca(2+) sparks, a vasodilatory signal contributed to by type 2 ryanodine receptors (RyR2). Using laser confocal microscopy in rat cerebral arteries and bilayer electrophysiology we unveil that ethanol inhibits both Ca(2+) spark and RyR2 activity with IC50<20 mM, placing RyR2 among the ion channels that are most sensitive to ethanol. Alcohol directly targets RyR2 and its lipid microenvironment, leading to stabilization of RyR2 closed states.

Cholesterol down-regulates BK channels stably expressed in HEK 293 cells

Cholesterol is one of the major lipid components of the plasma membrane in mammalian cells and is involved in the regulation of a number of ion channels. The present study investigates how large conductance Ca²⁺-activated K⁺ (BK) channels are regulated by membrane cholesterol in BK-HEK 293 cells expressing both the α-subunit hKCa1.1 and the auxiliary β1-subunit or in hKCa1.1-HEK 293 cells expressing only the α-subunit hKCa1.1 using approaches of electrophysiology, molecular biology, and immunocytochemistry. Membrane cholesterol was depleted in these cells with methyl-β-cyclodextrin (MβCD), and enriched with cholesterol-saturated MβCD (MβCD-cholesterol) or low-density lipoprotein (LDL). We found that BK current density was decreased by cholesterol enrichment in BK-HEK 293 cells, with a reduced expression of KCa1.1 protein, but not the β1-subunit protein. This effect was fully countered by the proteasome inhibitor lactacystin or the lysosome function inhibitor bafilomycin A1. Interestingly, in hKCa1.1-HEK 293 cells, the current density was not affected by cholesterol enrichment, but directly decreased by MβCD, suggesting that the down-regulation of BK channels by cholesterol depends on the auxiliary β1-subunit. The reduced KCa1.1 channel protein expression was also observed in cultured human coronary artery smooth muscle cells with cholesterol enrichment using MβCD-cholesterol or LDL. These results demonstrate the novel information that cholesterol down-regulates BK channels by reducing KCa1.1 protein expression via increasing the channel protein degradation, and the effect is dependent on the auxiliary β1-subunit.

Cerebrovascular dilation via selective targeting of the cholane steroid-recognition site in the BK channel β1-subunit by a novel nonsteroidal agent

The Ca(2+)/voltage-gated K(+) large conductance (BK) channel β1 subunit is particularly abundant in vascular smooth muscle. By determining their phenotype, BK β1 allows the BK channels to reduce myogenic tone, facilitating vasodilation. The endogenous steroid lithocholic acid (LCA) dilates cerebral arteries via BK channel activation, which requires recognition by a BK β1 site that includes Thr169. Whether exogenous nonsteroidal agents can access this site to selectively activate β1-containing BK channels and evoke vasodilation remain unknown. We performed a chemical structure database similarity search using LCA as a template, along with a two-step reaction to generate sodium 3-hydroxyolean-12-en-30-oate (HENA). HENA activated the BK (cbv1 + β1) channels cloned from rat cerebral artery myocytes with a potency (EC₅₀ = 53 μM) similar to and an efficacy (×2.5 potentiation) significantly greater than that of LCA. This HENA action was replicated on native channels in rat cerebral artery myocytes. HENA failed to activate the channels made of cbv1 + β2, β3, β4, or β1T169A, indicating that this drug selectively targets β1-containing BK channels via the BK β1 steroid-sensing site. HENA (3-45 μM) dilated the rat and C57BL/6 mouse pressurized cerebral arteries. Consistent with the electrophysiologic results, this effect was larger than that of LCA. HENA failed to dilate the arteries from the KCNMB1 knockout mouse, underscoring BK β1's role in HENA action. Finally, carotid artery-infusion of HENA (45 μM) dilated the pial cerebral arterioles via selective BK-channel targeting. In conclusion, we have identified for the first time a nonsteroidal agent that selectively activates β1-containing BK channels by targeting the steroid-sensing site in BK β1, rendering vasodilation.

Calcium- and voltage-gated potassium (BK) channel activators in the 5β-cholanic acid-3α-ol analogue series with modifications in the lateral chain

Large conductance, calcium- and voltage-gated potassium (BK) channels regulate various physiological processes and represent an attractive target for drug discovery. Numerous BK channel activators are available. However, these agents usually interact with the ubiquitously distributed channel-forming subunit and thus cannot selectively target a particular tissue. We performed a structure-activity relationship study of lithocholic acid (LCA), a cholane that activates BK channels via the accessory BK β1 subunit. The latter protein is highly abundant in smooth muscle but scarce in most other tissues. Modifications to the LCA lateral chain length and functional group yielded two novel smooth muscle BK channel activators in which the substituent at C24 has a small volume and a net negative charge. Our data provide detailed structural information that will be useful to advance a pharmacophore in search of β1 subunit-selective BK channel activators. These compounds are expected to evoke smooth muscle relaxation, which would be beneficial in the pharmacotherapy of prevalent human disorders associated with increased smooth muscle contraction, such as systemic hypertension, cerebral or coronary vasospasm, bronchial asthma, bladder hyperactivity, and erectile dysfunction.

BASIC--a bile acid-sensitive ion channel highly expressed in bile ducts

Brain liver intestine Na+ channel (BLINaC) is an ion channel of the DEG/ENaC gene family of unknown function. BLINaC from rats (rBLINaC) and humans (INaC) is inactive at rest, and its mode of activation has remained unclear. Here, we show that the BLINaC protein localizes to cholangiocytes, epithelial cells that line bile ducts. Moreover, we provide evidence that rBLINaC and INaC are robustly activated by bile acids, in particular chenodeoxycholic acid and hyodeoxycholic acid (EC50=2.1±0.05 mM). Thus, BLINaC appears to be an epithelial cation channel of bile ducts sensitive to physiological concentrations of bile acids. BLINaC is related to acid-sensing ion channels (ASICs) and to the epithelial Na+ channel (ENaC) and shares ligand activation with ASICs and epithelial localization with ENaC. Therefore, based on the close homology of BLINaC to ASICs and its activation by bile acids, we propose to rename BLINaC bile acid-sensitive ion channel (BASIC).

Large conductance, calcium- and voltage-gated potassium (BK) channels: regulation by cholesterol

Cholesterol (CLR) is an essential component of eukaryotic plasma membranes. CLR regulates the membrane physical state, microdomain formation and the activity of membrane-spanning proteins, including ion channels. Large conductance, voltage- and Ca²⁺-gated K⁺ (BK) channels link membrane potential to cell Ca²⁺ homeostasis. Thus, they control many physiological processes and participate in pathophysiological mechanisms leading to human disease. Because plasmalemma BK channels cluster in CLR-rich membrane microdomains, a major driving force for studying BK channel-CLR interactions is determining how membrane CLR controls the BK current phenotype, including its pharmacology, channel sorting, distribution, and role in cell physiology. Since both BK channels and CLR tissue levels play a pathophysiological role in human disease, identifying functional and structural aspects of the CLR-BK channel interaction may open new avenues for therapeutic intervention. Here, we review the studies documenting membrane CLR-BK channel interactions, dissecting out the many factors that determine the final BK current response to changes in membrane CLR content. We also summarize work in reductionist systems where recombinant BK protein is studied in artificial lipid bilayers, which documents a direct inhibition of BK channel activity by CLR and builds a strong case for a direct interaction between CLR and the BK channel-forming protein. Bilayer lipid-mediated mechanisms in CLR action are also discussed. Finally, we review studies of BK channel function during hypercholesterolemia, and underscore the many consequences that the CLR-BK channel interaction brings to cell physiology and human disease.