Multiple regulatory sites in large-conductance calcium-activated potassium channels

The contribution of RCK domains to human BK channel allosteric activation

Large conductance voltage- and Ca(2+)-activated K(+) (BK) channels are potent regulators of cellular processes including neuronal firing, synaptic transmission, cochlear hair cell tuning, insulin release, and smooth muscle tone. Their unique activation pathway relies on structurally distinct regulatory domains including one transmembrane voltage-sensing domain (VSD) and two intracellular high affinity Ca(2+)-sensing sites per subunit (located in the RCK1 and RCK2 domains). Four pairs of RCK1 and RCK2 domains form a Ca(2+)-sensing apparatus known as the "gating ring." The allosteric interplay between voltage- and Ca(2+)-sensing apparati is a fundamental mechanism of BK channel function. Using voltage-clamp fluorometry and UV photolysis of intracellular caged Ca(2+), we optically resolved VSD activation prompted by Ca(2+) binding to the gating ring. The sudden increase of intracellular Ca(2+) concentration ([Ca(2+)](i)) induced a hyperpolarizing shift in the voltage dependence of both channel opening and VSD activation, reported by a fluorophore labeling position 202, located in the upper side of the S4 transmembrane segment. The neutralization of the Ca(2+) sensor located in the RCK2 domain abolished the effect of [Ca(2+)](i) increase on the VSD rearrangements. On the other hand, the mutation of RCK1 residues involved in Ca(2+) sensing did not prevent the effect of Ca(2+) release on the VSD, revealing a functionally distinct interaction between RCK1 and RCK2 and the VSD. A statistical-mechanical model quantifies the complex thermodynamics interplay between Ca(2+) association in two distinct sites, voltage sensor activation, and BK channel opening.

Mg(2+) binding to open and closed states can activate BK channels provided that the voltage sensors are elevated

BK channels are activated by intracellular Ca²⁺ and Mg²⁺ as well as by depolarization. Such activation is possible because each of the four subunits has two high-affinity Ca²⁺ sites, one low-affinity Mg²⁺ site, and a voltage sensor. This study further investigates the mechanism of Mg²⁺ activation by using single-channel recording to determine separately the action of Mg²⁺ on the open and closed states of the channel. To limit Mg²⁺ action to the Mg²⁺ sites, the two high-affinity Ca²⁺ sites are disabled by mutation. When the voltage is stepped from negative holding potentials to +100 mV, we find that 10 mM Mg²⁺ decreases the mean closed latency to the first channel opening 2.1-fold, decreases the mean closed interval duration 8.7-fold, increases mean burst duration 10.1-fold, increases the number of openings per burst 4.4-fold, and increases mean open interval duration 2.3-fold. Hence, Mg²⁺ can bind to closed BK channels, increasing their opening rates, and to open BK channels, decreasing their closing rates. To explore the relationship between Mg²⁺ action and voltage sensor activation, we record single-channel activity in macropatches containing hundreds of channels. Open probability (P_o) is dramatically increased by 10 mM Mg²⁺ when voltage sensors are activated with either depolarization or the mutation R210C. The increased P_o arises from large decreases in mean closed interval durations and moderate increases in mean open interval durations. In contrast, 10 mM Mg²⁺ has no detectable effects on P_o or interval durations when voltage sensors are deactivated with very negative potentials or the mutation R167E. These observations are consistent with a model in which Mg²⁺ can bind to and alter the gating of both closed and open states to increase P_o, provided that one or more voltage sensors are activated.

On the simple random-walk models of ion-channel gate dynamics reflecting long-term memory

Several approaches to ion-channel gating modelling have been proposed. Although many models describe the dwell-time distributions correctly, they are incapable of predicting and explaining the long-term correlations between the lengths of adjacent openings and closings of a channel. In this paper we propose two simple random-walk models of the gating dynamics of voltage and Ca(2+)-activated potassium channels which qualitatively reproduce the dwell-time distributions, and describe the experimentally observed long-term memory quite well. Biological interpretation of both models is presented. In particular, the origin of the correlations is associated with fluctuations of channel mass density. The long-term memory effect, as measured by Hurst R/S analysis of experimental single-channel patch-clamp recordings, is close to the behaviour predicted by our models. The flexibility of the models enables their use as templates for other types of ion channel.

Tungstate activates BK channels in a β subunit- and Mg2+-dependent manner: relevance for arterial vasodilatation

AIMS

Tungstate reduces blood pressure in experimental animal models of both hypertension and metabolic syndrome, although the underlying mechanisms are not fully understood. Given that the large-conductance voltage- and Ca(2+)-dependent K(+) (BK) channel is a key element in the control of arterial tone, our aim was to evaluate whether BK channel modulation by tungstate can contribute to its antihypertensive effect.

METHODS AND RESULTS

Patch-clamp studies of heterologously expressed human BK channels (α + β(1-4) subunits) revealed that cytosolic tungstate (1 mM) induced a significant left shift (∼20 mV) in the voltage-dependent activation curve only in BK channels containing αβ(1) or αβ(4) subunits, but reduced the amplitude of K(+) currents through all BK channels tested. The β(1)-dependent activation of BK channels by tungstate was enhanced at cytosolic Ca(2+) levels reached during myocyte contraction, and prevented either by removal of cytosolic Mg(2+) or by mutations rendering the channel insensitive to Mg(2+). A lower concentration of tungstate (0.1 mM) induced voltage-dependent activation of the vascular BKαβ(1) channel without reducing current amplitude, and consistently exerted a vasodilatory action on wild-type but not on β(1)-knockout mouse arteries pre-contracted with endothelin-1.

CONCLUSION

Tungstate activates BK channels in a β subunit- and Mg(2+)-dependent manner and induces vasodilatation only in mouse arteries that express the BK β(1) subunit.

Ca(2+)-regulated ion channels

Due to its high external and low internal concentration the Ca(2+) ion is used ubiquitously as an intracellular signaling molecule, and a great many Ca(2+)-sensing proteins have evolved to receive and propagate Ca(2+) signals. Among them are ion channel proteins, whose Ca(2+) sensitivity allows internal Ca(2+) to influence the electrical activity of cell membranes and to feedback-inhibit further Ca(2+) entry into the cytoplasm. In this review I will describe what is understood about the Ca(2+) sensing mechanisms of the three best studied classes of Ca(2+)-sensitive ion channels: Large-conductance Ca(2+)-activated K(+) channels, small-conductance Ca(2+)-activated K(+) channels, and voltage- gated Ca(2+) channels. Great strides in mechanistic understanding have be made for each of these channel types in just the past few years.

Molecular mechanism of pharmacological activation of BK channels

Large-conductance voltage- and Ca(2+)-activated K(+) (Slo1 BK) channels serve numerous cellular functions, and their dysregulation is implicated in various diseases. Drugs activating BK channels therefore bear substantial therapeutic potential, but their deployment has been hindered in part because the mode of action remains obscure. Here we provide mechanistic insight into how the dehydroabietic acid derivative Cym04 activates BK channels. As a representative of NS1619-like BK openers, Cym04 reversibly left-shifts the half-activation voltage of Slo1 BK channels. Using an established allosteric BK gating model, the Cym04 effect can be simulated by a shift of the voltage sensor and the ion conduction gate equilibria toward the activated and open state, respectively. BK activation by Cym04 occurs in a splice variant-specific manner; it does not occur in such Slo1 BK channels using an alternative neuronal exon 9, which codes for the linker connecting the transmembrane segment S6 and the cytosolic RCK1 domain--the S6/RCK linker. In addition, Cym04 does not affect Slo1 BK channels with a two-residue deletion within this linker. Mutagenesis and model-based gating analysis revealed that BK openers, such as Cym04 and NS1619 but not mallotoxin, activate BK channels by functionally interacting with the S6/RCK linker, mimicking site-specific shortening of this purported passive spring, which transmits force from the cytosolic gating ring structure to open the channel's gate.

Hydrogen sulfide preconditioning or neutrophil depletion attenuates ischemia-reperfusion-induced mitochondrial dysfunction in rat small intestine

The objectives of this study were to determine whether neutrophil depletion with anti-neutrophil serum (ANS) or preconditioning with the hydrogen sulfide (H(2)S) donor NaHS (NaHS-PC) 24 h prior to ischemia-reperfusion (I/R) would prevent postischemic mitochondrial dysfunction in rat intestinal mucosa and, if so, whether calcium-activated, large conductance potassium (BK(Ca)) channels were involved in this protective effect. I/R was induced by 45-min occlusion of the superior mesenteric artery followed by 60-min reperfusion in rats preconditioned with NaHS (NaHS-PC) or a BK(Ca) channel activator (NS-1619-PC) 24 h earlier or treated with ANS. Mitochondrial function was assessed by measuring mitochondrial membrane potential, mitochondrial dehydrogenase function, and cytochrome c release. Mucosal myeloperoxidase (MPO) and TNF-α levels were also determined, as measures of postischemic inflammation. BK(Ca) expression in intestinal mucosa was detected by immunohistochemistry and Western blotting. I/R induced mitochondrial dysfunction and increased tissue MPO and TNF-α levels. Although mitochondrial dysfunction was attenuated by NaHS-PC or NS-1619-PC, the postischemic increases in mucosal MPO and TNF-α levels were not. The protective effect of NaHS-PC or NS-1619-PC on postischemic mitochondrial function was abolished by coincident treatment with BK(Ca) channel inhibitors. ANS prevented the I/R-induced increase in tissue MPO levels and reversed mitochondrial dysfunction. These data indicate that neutrophils play an essential role in I/R-induced mucosal mitochondrial dysfunction. In addition, NaHS-PC prevents postischemic mitochondrial dysfunction (but not inflammation) by a BK(Ca) channel-dependent mechanism.

Alternatively spliced domains interact to regulate BK potassium channel gating

Most human genes contain multiple alternative splice sites believed to extend the complexity and diversity of the proteome. However, little is known about how interactions among alternative exons regulate protein function. We used the Caenorhabditis elegans slo-1 large-conductance calcium and voltage-activated potassium (BK) channel gene, which contains three alternative splice sites (A, B, and C) and encodes at least 12 splice variants, to investigate the functional consequences of alternative splicing. These splice sites enable the insertion of exons encoding part of the regulator of K(+) conductance (RCK)1 Ca(2+) coordination domain (exons A1 and A2) and portions of the RCK1-RCK2 linker (exons B0, B1, B2, C0, and C1). Exons A1 and A2 are used in a mutually exclusive manner and are 67% identical. The other exons can extend the RCK1-RCK2 linker by up to 41 residues. Electrophysiological recordings of all isoforms show that the A1 and A2 exons regulate activation kinetics and Ca(2+) sensitivity, but only if alternate exons are inserted at site B or C. Thus, RCK1 interacts with the RCK1-RCK2 linker, and the effect of exon variation on gating depends on the combination of alternate exons present in each isoform.

Targeting BK (big potassium) channels in epilepsy

INTRODUCTION

Epilepsies are disorders of neuronal excitability characterized by spontaneous and recurrent seizures. Ion channels are critical for regulating neuronal excitability and, therefore, can contribute significantly to epilepsy pathophysiology. In particular, large conductance, Ca2+-activated K+ (BKCa) channels play an important role in seizure etiology. These channels are activated by both membrane depolarization and increased intracellular Ca2+. This unique coupling of Ca2+ signaling to membrane depolarization is important in controlling neuronal hyperexcitability, as outward K+ current through BKCa channels hyperpolarizes neurons.

AREAS COVERED

BKCa channel structure-function and the role of these channels in epilepsy pathophysiology.

EXPERT OPINION

Loss-of-function BKCa channel mutations contribute to neuronal hyperexcitability that can lead to temporal lobe epilepsy, tonic-clonic seizures and alcohol withdrawal seizures. Similarly, BKCa channel blockade can trigger seizures and status epilepticus. Paradoxically, some mutations in BKCa channel subunit can give rise to channel gain-of-function that leads to development of idiopathic epilepsy (primarily absence epilepsy). Seizures themselves also enhance BKCa channel currents associated with neuronal hyperexcitability, and blocking BKCa channels suppresses generalized tonic-clonic seizures. Thus, both loss-of-function and gain-of-function BKCa channels might serve as molecular targets for drugs to suppress certain seizure phenotypes including temporal lobe seizures and absence seizures, respectively.

Structural correlates of selectivity and inactivation in potassium channels

Potassium channels are involved in a tremendously diverse range of physiological applications requiring distinctly different functional properties. Not surprisingly, the amino acid sequences for these proteins are diverse as well, except for the region that has been ordained the "selectivity filter". The goal of this review is to examine our current understanding of the role of the selectivity filter and regions adjacent to it in specifying selectivity as well as its role in gating/inactivation and possible mechanisms by which these processes are coupled. Our working hypothesis is that an amino acid network behind the filter modulates selectivity in channels with the same signature sequence while at the same time affecting channel inactivation properties. This article is part of a Special Issue entitled: Membrane protein structure and function.

Charge substitution for a deep-pore residue reveals structural dynamics during BK channel gating

The pore-lining amino acids of ion channel proteins reside on the interface between a polar (the pore) and a nonpolar environment (the rest of the protein). The structural dynamics of this region, which physically controls ionic flow, are essential components of channel gating. Using large-conductance, Ca²⁺-dependent K⁺ (BK) channels, we devised a systematic charge–substitution method to probe conformational changes in the pore region during channel gating. We identified a deep-pore residue (314 in hSlo1) as a marker of structural dynamics. We manipulated the charge states of this residue by substituting amino acids with different valence and pKa, and by adjusting intracellular pH. We found that the charged states of the 314 residues stabilized an open state of the BK channel. With models based on known structures of related channels, we postulate a dynamic rearrangement of the deep-pore region during BK channel opening/closing, which involves a change of the degree of pore exposure for 314.

Cysteine scanning and modification reveal major differences between BK channels and Kv channels in the inner pore region

BK channels are regulated by two distinct physiological signals, transmembrane potential and intracellular Ca(2+), each acting through independent modular sensor domains. However, despite a presumably central role in the coupling of sensor activation to channel gating, the pore-lining S6 transmembrane segment has not been systematically studied. Here, cysteine substitution and modification studies of the BK S6 point to substantial differences between BK and Kv channels in the structure and function of the S6-lined inner pore. Gating shifts caused by introduction of cysteines define a pattern and direction of free energy changes in BK S6 distinct from Shaker. Modification of BK S6 residues identifies pore-facing residues that occur at different linear positions along aligned BK and Kv S6 segments. Periodicity analysis suggests that one factor contributing to these differences may be a disruption of the BK S6 α-helix from the unique diglycine motif at the position of the Kv hinge glycine. State-dependent MTS accessibility reveals that, even in closed states, modification can occur. Furthermore, the inner pore of BK channels is much larger than that of K(+) channels with solved crystal structures. The results suggest caution in the use of Kv channel structures as templates for BK homology models, at least in the pore-gate domain.

Metal-driven operation of the human large-conductance voltage- and Ca2+-dependent potassium channel (BK) gating ring apparatus

Large-conductance voltage- and Ca(2+)-dependent K(+) (BK, also known as MaxiK) channels are homo-tetrameric proteins with a broad expression pattern that potently regulate cellular excitability and Ca(2+) homeostasis. Their activation results from the complex synergy between the transmembrane voltage sensors and a large (>300 kDa) C-terminal, cytoplasmic complex (the "gating ring"), which confers sensitivity to intracellular Ca(2+) and other ligands. However, the molecular and biophysical operation of the gating ring remains unclear. We have used spectroscopic and particle-scale optical approaches to probe the metal-sensing properties of the human BK gating ring under physiologically relevant conditions. This functional molecular sensor undergoes Ca(2+)- and Mg(2+)-dependent conformational changes at physiologically relevant concentrations, detected by time-resolved and steady-state fluorescence spectroscopy. The lack of detectable Ba(2+)-evoked structural changes defined the metal selectivity of the gating ring. Neutralization of a high-affinity Ca(2+)-binding site (the "calcium bowl") reduced the Ca(2+) and abolished the Mg(2+) dependence of structural rearrangements. In congruence with electrophysiological investigations, these findings provide biochemical evidence that the gating ring possesses an additional high-affinity Ca(2+)-binding site and that Mg(2+) can bind to the calcium bowl with less affinity than Ca(2+). Dynamic light scattering analysis revealed a reversible Ca(2+)-dependent decrease of the hydrodynamic radius of the gating ring, consistent with a more compact overall shape. These structural changes, resolved under physiologically relevant conditions, likely represent the molecular transitions that initiate the ligand-induced activation of the human BK channel.

The relationship between duration of initial alcohol exposure and persistence of molecular tolerance is markedly nonlinear

The neuronal calcium- and voltage-activated BK potassium channel is modulated by ethanol, and plays a role in behavioral tolerance in vertebrates and invertebrates. We examine the influence of temporal parameters of alcohol exposure on the characteristics of BK molecular tolerance in the ventral striatum, an important component of brain reward circuitry. BK channels in striatal neurons of C57BL/6J mice exhibited molecular tolerance whose duration was a function of exposure time. After 6 h exposure to 20 mm (0.09 mg%) ethanol, alcohol sensitivity was suppressed beyond 24 h after withdrawal, while after a 1 or 3 h exposure, sensitivity had significantly recovered after 4 h. This temporally controlled transition to persistent molecular tolerance parallels changes in BK channel isoform profile. After withdrawal from 6 h, but not 3 h alcohol exposure, mRNA levels of the alcohol-insensitive STREX (stress axis-regulated exon) splice variant were increased. Moreover, the biophysical properties of BK channels during withdrawal from 6 h exposure were altered, and match the properties of STREX channels exogenously expressed in HEK 293 cells. Our results suggest a temporally triggered shift in BK isoform identity. Once activated, the transition does not require the continued presence of alcohol. We next determined whether the results obtained using cultured striatal neurons could be observed in acutely dissociated striatal neurons, after alcohol administration in the living mouse. The results were in remarkable agreement with the striatal culture data, showing persistent molecular tolerance after injections producing 6 h of intoxication, but not after injections producing only 3 h of intoxication.

Thiol/Disulfide redox switches in the regulation of heme binding to proteins

This review focuses on thiol/disulfide redox switches that regulate heme binding to proteins and modulate their activities. The importance of redox switches in metabolic regulation and the general mechanism by which redox switches modulate activity are discussed. Methods are described to characterize heme-binding sites and to assess their physiological relevance. For thiol/disulfide interconversion to regulate activity of a system, the redox process must be reversible at the ambient redox potentials found within the cell; thus, methods (and their limitations) are discussed that can address the physiological relevance of a redox switch. We review recent results that define a mechanism for how thiol/disulfide redox switches that control heme binding can regulate the activities of an enzyme, heme oxygenase-2, and an ion channel, the BK potassium channel. The redox switches on these proteins are composed of different types of Cys-containing motifs that have opposite effects on heme affinity, yet have complementary effects on hypoxia sensing. Finally, a model is proposed to describe how the redox switches on heme oxygenase-2 and the BK channel form an interconnected system that is poised to sense oxygen levels in the bloodstream and to elicit the hypoxic response when oxygen levels drop below a threshold value.

Mechanism of action of lolitrem B, a fungal endophyte derived toxin that inhibits BK large conductance Ca²+-activated K+ channels

The aim of this study was to compare the mode of action of the commonly used BK inhibitor paxilline with that of the more recently discovered lolitrem B. Similarities and differences in characteristics of inhibition between the two compounds were investigated. We have previously shown that lolitrem B does not affect the BK channel G-V, in contrast to the rightward shift produced by paxilline. These different effects on the voltage-dependence of activation suggest different modes of action for these two compounds. In this study we show that inhibition by both paxilline and lolitrem B is characterized by an open state preference for BK (hSlo) channels. Both compounds had a 3-fold higher apparent affinity under conditions likely to favour the open state, suggesting they have a similar BK conformational preference for binding. Furthermore, both compounds had a calcium concentration-dependence to their inhibitory effects. The G-V shift induced by paxilline was calcium concentration-dependent.

Modulation of BK channel gating by the ß2 subunit involves both membrane-spanning and cytoplasmic domains of Slo1

Large-conductance, Ca(2+)- and voltage-sensitive K(+) (BK) channels regulate neuronal functions such as spike frequency adaptation and transmitter release. BK channels are composed of four Slo1 subunits, which contain the voltage-sensing and pore-gate domains in the membrane and Ca(2+) binding sites in the cytoplasmic domain, and accessory β subunits. Four types of BK channel β subunits (β1-β4) show differential tissue distribution and unique functional modulation, resulting in diverse phenotypes of BK channels. Previous studies show that both the β1 and β2 subunits increase Ca(2+) sensitivity, but different mechanisms may underline these modulations. However, the structural domains in Slo1 that are critical for Ca(2+)-dependent activation and targeted by these β subunits are not known. Here, we report that the N termini of both the transmembrane (including S0) and cytoplasmic domains of Slo1 are critical for β2 modulation based on the study of differential effects of the β2 subunit on two orthologs, mouse Slo1 and Drosophila Slo1. The N terminus of the cytoplasmic domain of Slo1, including the AC region (βA-αC) of the RCK1 (regulator of K(+) conductance) domain and the peptide linking it to S6, both of which have been shown previously to mediate the coupling between Ca(2+) binding and channel opening, is specifically required for the β2 but not for the β1 modulation. These results suggest that the β2 subunit modulates the coupling between Ca(2+) binding and channel opening, and, although sharing structural homology, the BK channel β subunits interact with structural domains in the Slo1 subunit differently to enhance channel activity.

Relative motion of transmembrane segments S0 and S4 during voltage sensor activation in the human BK(Ca) channel

Large-conductance voltage- and Ca(2+)-activated K(+) (BK(Ca)) channel α subunits possess a unique transmembrane helix referred to as S0 at their N terminus, which is absent in other members of the voltage-gated channel superfamily. Recently, S0 was found to pack close to transmembrane segments S3 and S4, which are important components of the BK(Ca) voltage-sensing apparatus. To assess the role of S0 in voltage sensitivity, we optically tracked protein conformational rearrangements from its extracellular flank by site-specific labeling with an environment-sensitive fluorophore, tetramethylrhodamine maleimide (TMRM). The structural transitions resolved from the S0 region exhibited voltage dependence similar to that of charge-bearing transmembrane domains S2 and S4. The molecular determinant of the fluorescence changes was identified in W203 at the extracellular tip of S4: at hyperpolarized potential, W203 quenches the fluorescence of TMRM labeling positions at the N-terminal flank of S0. We provide evidence that upon depolarization, W203 (in S4) moves away from the extracellular region of S0, lifting its quenching effect on TMRM fluorescence. We suggest that S0 acts as a pivot component against which the voltage-sensitive S4 moves upon depolarization to facilitate channel activation.

BK channel activation: structural and functional insights

The voltage- and Ca(2+)-activated K(+) (BK) channels are involved in the regulation of neurotransmitter release and neuronal excitability. Structurally, BK channels are homologous to voltage- and ligand-gated K(+) channels, having a voltage sensor and pore as the membrane-spanning domain and a cytosolic domain containing metal binding sites. Recently published electron cryomicroscopy (cryo-EM) and X-ray crystallographic structures of the BK channel provided the first glimpse into the assembly of these domains, corroborating the close interactions among these domains during channel gating that have been suggested by functional studies. This review discusses these latest findings and an emerging new understanding about BK channel gating and implications for diseases such as epilepsy, in which mutations in BK channel genes have been associated.

Ca2+-activated K+ channels: from protein complexes to function

Molecular research on ion channels has demonstrated that many of these integral membrane proteins associate with partner proteins, often versatile in their function, or even assemble into stable macromolecular complexes that ensure specificity and proper rate of the channel-mediated signal transduction. Calcium-activated potassium (K(Ca)) channels that link excitability and intracellular calcium concentration are responsible for a wide variety of cellular processes ranging from regulation of smooth muscle tone to modulation of neurotransmission and control of neuronal firing pattern. Most of these functions are brought about by interaction of the channels' pore-forming subunits with distinct partner proteins. In this review we summarize recent insights into protein complexes associated with K(Ca) channels as revealed by proteomic research and discuss the results available on structure and function of these complexes and on the underlying protein-protein interactions. Finally, the results are related to their significance for the function of K(Ca) channels under cellular conditions.

Ion sensing in the RCK1 domain of BK channels

BK-type K(+) channels are activated by voltage and intracellular Ca(2+), which is important in modulating muscle contraction, neural transmission, and circadian pacemaker output. Previous studies suggest that the cytosolic domain of BK channels contains two different Ca(2+) binding sites, but the molecular composition of one of the sites is not completely known. Here we report, by systematic mutagenesis studies, the identification of E535 as part of this Ca(2+) binding site. This site is specific for binding to Ca(2+) but not Cd(2+). Experimental results and molecular modeling based on the X-ray crystallographic structures of the BK channel cytosolic domain suggest that the binding of Ca(2+) by the side chains of E535 and the previously identified D367 changes the conformation around the binding site and turns the side chain of M513 into a hydrophobic core, providing a basis to understand how Ca(2+) binding at this site opens the activation gate of the channel that is remotely located in the membrane.

Inhibition of large-conductance Ca2+-activated K+ channels by nanomolar concentrations of Ag+

Silver has been widely used in various medical products because of its antibacterial properties. However, there is only limited information concerning silver-related cytotoxicity. In this study we show that Ag(+) at low nanomolar concentrations (<10 nM) strongly inhibits the activity of large-conductance Ca(2+)-activated K(+) channels (BK) (Slo1), a widely expressed and physiologically important potassium channel. The Ag(+) inhibition is caused by irreversible modification on cytosolically accessible parts of the BK channel. At least four intracellular cysteines are involved in this process. In addition, at least one of these key cysteines is not accessible to the bulkier thiolate-active reagent [2-(trimethylammonium)ethyl] methanethiosulfonate bromide. One of the cysteine-less constructs generated in this study shows gating properties similar to wild-type BK channel but with much lower Ag(+) sensitivity, in which the Ag(+) modification rate was decreased by approximately 20-fold. The results from the present study suggest a possible contribution of BK channel inhibition to the cytotoxicity of Ag(+) in humans and other species.

An epilepsy/dyskinesia-associated mutation enhances BK channel activation by potentiating Ca2+ sensing

Ca(2+)-activated BK channels modulate neuronal activities, including spike frequency adaptation and synaptic transmission. Previous studies found that Ca(2+)-binding sites and the activation gate are spatially separated in the channel protein, but the mechanism by which Ca(2+) binding opens the gate over this distance remains unknown. By studying an Asp-to-Gly mutation (D434G) associated with human syndrome of generalized epilepsy and paroxysmal dyskinesia (GEPD), we show that a cytosolic motif immediately following the activation gate S6 helix, known as the AC region, mediates the allosteric coupling between Ca(2+) binding and channel opening. The GEPD mutation inside the AC region increases BK channel activity by enhancing this allosteric coupling. We found that Ca(2+) sensitivity is enhanced by increases in solution viscosity that reduce protein dynamics. The GEPD mutation alters such a response, suggesting that a less flexible AC region may be more effective in coupling Ca(2+) binding to channel opening.

TRPC6 channels and their binding partners in podocytes: role in glomerular filtration and pathophysiology

Loss or dysfunction of podocytes is a major cause of glomerular kidney disease. Several genetic forms of glomerular disease are caused by mutations in genes that encode structural elements of the slit diaphragm or the underlying cytoskeleton of podocyte foot processes. The recent discovery that gain-of-function mutations in Ca(2+)-permeable canonical transient receptor potential-6 channels (TRPC6) underlie a subset of familial forms of focal segmental glomerulosclerosis (FSGS) has focused attention on the basic cellular physiology of podocytes. Several recent studies have examined the role of Ca(2+) dynamics in normal podocyte function and their possible contributions to glomerular disease. This review summarizes the properties of TRPC6 and related channels, focusing on their permeation and gating properties, the nature of mutations associated with familial FSGS, and the role of TRPC channels in podocyte cell biology as well as in glomerular pathophysiology. TRPC6 interacts with several proteins in podocytes, including essential slit diaphragm proteins and mechanosensitive large-conductance Ca(2+)-activated K(+) channels. The signaling dynamics controlling ion channel function and localization in podocytes appear to be quite complex.

BK-type calcium-activated potassium channels: coupling of metal ions and voltage sensing

Ion channels and lipid phosphatases adopt a transmembrane voltage sensor domain (VSD) that moves in response to physiological variations of the membrane potential to control their activities. However, the VSD movements and coupling to the channel or phosphatase activities may differ depending on various interactions between the VSD and its host molecules. BK-type voltage, Ca²(+) and Mg²(+) activated K(+) channels contain the VSD and a large cytosolic domain (CTD) that binds Ca²(+)and Mg²(+). VSD movements are coupled to BK channel opening with a unique allosteric mechanism and are modulated by Ca²(+) and Mg²(+) binding via the interactions among the channel pore, VSD and CTD. These properties are energetically advantageous for the pore to be controlled by multiple stimuli, revealing the adaptability of the VSD to its host molecules and showing the potential for intracellular signals to affect the VSD in order to modulate the function of its host molecules.

Coupling and cooperativity in voltage activation of a limited-state BK channel gating in saturating Ca2+

Voltage-dependent gating mechanisms of large conductance Ca²⁺ and voltage-activated (BK) channels were investigated using two-dimensional maximum likelihood analysis of single-channel open and closed intervals. To obtain sufficient data at negative as well as positive voltages, single-channel currents were recorded at saturating Ca²⁺ from BK channels mutated to remove the RCK1 Ca²⁺ and Mg²⁺ sensors. The saturating Ca²⁺ acting on the Ca²⁺ bowl sensors of the resulting BK_B channels increased channel activity while driving the gating into a reduced number of states, simplifying the model. Five highly constrained idealized gating mechanisms based on extensions of the Monod-Wyman-Changeux model for allosteric proteins were examined. A 10-state model without coupling between the voltage sensors and the opening/closing transitions partially described the voltage dependence of Po but not the single-channel kinetics. With allowed coupling, the model gave improved descriptions of Po and approximated the single-channel kinetics; each activated voltage sensor increased the opening rate approximately an additional 23-fold while having little effect on the closing rate. Allowing cooperativity among voltage sensors further improved the description of the data: each activated voltage sensor increased the activation rate of the remaining voltage sensors approximately fourfold, with little effect on the deactivation rate. The coupling factor was decreased in models with cooperativity from ∼23 to ∼18. Whether the apparent cooperativity among voltage sensors arises from imposing highly idealized models or from actual cooperativity will require additional studies to resolve. For both cooperative and noncooperative models, allowing transitions to five additional brief (flicker) closed states further improved the description of the data. These observations show that the voltage-dependent single-channel kinetics of BK_B channels can be approximated by highly idealized allosteric models in which voltage sensor movement increases Po mainly through an increase in channel opening rates, with limited effects on closing rates.

Structure of the gating ring from the human large-conductance Ca(2+)-gated K(+) channel

Large-conductance Ca(2+)-gated K(+) (BK) channels are essential for many biological processes such as smooth muscle contraction and neurotransmitter release. This group of channels can be activated synergistically by both voltage and intracellular Ca(2+), with the large carboxy-terminal intracellular portion being responsible for Ca(2+) sensing. Here we present the crystal structure of the entire cytoplasmic region of the human BK channel in a Ca(2+)-free state. The structure reveals four intracellular subunits, each comprising two tandem RCK domains, assembled into a gating ring similar to that seen in the MthK channel and probably representing its physiological assembly. Three Ca(2+) binding sites including the Ca(2+) bowl are mapped onto the structure based on mutagenesis data. The Ca(2+) bowl, located within the second RCK domain, forms an EF-hand-like motif and is strategically positioned close to the assembly interface between two subunits. The other two Ca(2+) (or Mg(2+)) binding sites, Asp 367 and Glu 374/Glu 399, are located on the first RCK domain. The Asp 367 site has high Ca(2+) sensitivity and is positioned in the groove between the amino- and carboxy-terminal subdomains of RCK1, whereas the low-affinity Mg(2+)-binding Glu 374/Glu 399 site is positioned on the upper plateau of the gating ring and close to the membrane. Our structure also contains the linker connecting the transmembrane and intracellular domains, allowing us to dock a voltage-gated K(+) channel pore of known structure onto the gating ring with reasonable accuracy and generate a structural model for the full BK channel.

The RCK1 domain of the human BKCa channel transduces Ca2+ binding into structural rearrangements

Large-conductance voltage- and Ca²⁺-activated K⁺ (BK_Ca) channels play a fundamental role in cellular function by integrating information from their voltage and Ca²⁺ sensors to control membrane potential and Ca²⁺ homeostasis. The molecular mechanism of Ca²⁺-dependent regulation of BK_Ca channels is unknown, but likely relies on the operation of two cytosolic domains, regulator of K⁺ conductance (RCK)1 and RCK2. Using solution-based investigations, we demonstrate that the purified BK_Ca RCK1 domain adopts an α/β fold, binds Ca²⁺, and assembles into an octameric superstructure similar to prokaryotic RCK domains. Results from steady-state and time-resolved spectroscopy reveal Ca²⁺-induced conformational changes in physiologically relevant [Ca²⁺]. The neutralization of residues known to be involved in high-affinity Ca²⁺ sensing (D362 and D367) prevented Ca²⁺-induced structural transitions in RCK1 but did not abolish Ca²⁺ binding. We provide evidence that the RCK1 domain is a high-affinity Ca²⁺ sensor that transduces Ca²⁺ binding into structural rearrangements, likely representing elementary steps in the Ca²⁺-dependent activation of human BK_Ca channels.

LRRC26 auxiliary protein allows BK channel activation at resting voltage without calcium

Large-conductance, voltage- and calcium-activated potassium (BK, or K(Ca)1.1) channels are ubiquitously expressed in electrically excitable and non-excitable cells, either as alpha-subunit (BKalpha) tetramers or together with tissue specific auxiliary beta-subunits (beta1-beta4). Activation of BK channels typically requires coincident membrane depolarization and elevation in free cytosolic Ca(2+) concentration ([Ca(2+)](i)), which are not physiological conditions for most non-excitable cells. Here we present evidence that in non-excitable LNCaP prostate cancer cells, BK channels can be activated at negative voltages without rises in [Ca(2+)](i) through their complex with an auxiliary protein, leucine-rich repeat (LRR)-containing protein 26 (LRRC26). LRRC26 modulates the gating of a BK channel by enhancing the allosteric coupling between voltage-sensor activation and the channel's closed-open transition. This finding reveals a novel auxiliary protein of a voltage-gated ion channel that gives an unprecedentedly large negative shift ( approximately -140 mV) in voltage dependence and provides a molecular basis for activation of BK channels at physiological voltages and calcium levels in non-excitable cells.

Allosteric interactions and the modular nature of the voltage- and Ca2+-activated (BK) channel

The high conductance voltage- and Ca(2+)-activated K(+) channel is one of the most broadly expressed channels in mammals. This channel is named BK for 'big K' because of its single-channel conductance that can be as large as 250 pS in 100 mm symmetrical K(+). BK channels increase their activity by membrane depolarization or an increase in cytosolic Ca(2+). One of the key features that defines the behaviour of BK channels is that neither Ca(2+) nor voltage is strictly necessary for channel activation. This and several other observations led to the idea that both Ca(2+) and voltage increase the open probability by an allosteric mechanism. In this type of mechanism, the processes of voltage sensor displacement, Ca(2+) binding and pore opening are independent equilibria that interact allosterically with each other. These allosteric interactions in BK channels reside in the structural characteristics of the BK channel in the sense that voltage and Ca(2+) sensors and the pore need to be contained in different structures or 'modules'. Through electrophysiological, mutagenesis, biochemical and fluorescence studies these modules have been identified and, more important, some of the interactions between them have been unveiled. In this review, we have covered the main advances achieved during the last few years in the elucidation of the structure of the BK channel and how this is related with its function as an allosteric protein.

Expression of BK-type calcium-activated potassium channel splice variants during chick cochlear development

The appearance of large-conductance, calcium-activated potassium (BK) current is a hallmark of functional maturation in auditory hair cells. Acquisition of this fast-activating current enables high-frequency, graded receptor potentials in all vertebrates and an electrical tuning mechanism in nonmammals. The gene encoding BK alpha subunits is highly alternatively spliced, and the resulting variations in channel isoforms may contribute to functional diversity at the onset of hearing. We examined the tissue specificity of nine BK alpha alternative exons and investigated changes in expression during chick cochlear development using quantitative polymerase chain reaction (qPCR). Each alternative was widely expressed in several tissues except for an insert near the C-terminus Ca(2+) sensing domain, which appeared brain-specific. The only alternative form in the membrane-bound core of the channel was expressed in brain and muscle but was undetected in cochlea. Of the remaining variants, three increased in expression prior to the onset of hearing and acquisition of BK currents. These three variants cause decreased Ca(2+) sensitivity or increased intracellular retention, traits that would not easily explain the advent of calcium-sensitive currents at embryonic day (E)18-19. Expression levels of other variants were mature and stable by E15, days before currents were acquired. Surface expression of C-terminal isoforms was examined using patch-clamp electrophysiology and immunocytochemistry. C-terminal variants that exhibit robust surface expression appeared in the membrane at E18, even though transcripts were unchanged during development starting from E12. These results indicate that delays in protein synthesis and trafficking/scaffolding of channel subunits underlie the late acquisition of BK currents in cochlear hair cells.

The RCK2 domain uses a coordination site present in Kir channels to confer sodium sensitivity to Slo2.2 channels

Slo2 Na(+)-activated potassium channels are widely expressed in neurons and other cells, such as kidney, heart, and skeletal muscle. Although their important physiological roles continue to be appreciated, molecular determinants responsible for sensing intracellular Na(+) remain unknown. Here we report identification of an Na(+) regulatory site, similar to an Na(+) coordination motif described in Kir channels, localized in the RCK2 domain of Slo2.2 channels. Molecular simulations of the homology-modeled Slo2.2 RCK2 domain provided structural insights into the organization of this Na(+) coordination site. Furthermore, free energy calculations reproduced the experimentally derived monovalent cation selectivity. Our results suggest that Slo2.2 and Kir channels share a similar mechanism to coordinate Na(+). The localization of an Na(+) sensor within the RCK2 domain of Slo2.2 further supports the role of RCK (regulators of conductance of K(+)) domains of Slo channels in coupling ion sensing to channel gating.

Structure of the human BK channel Ca2+-activation apparatus at 3.0 A resolution

High-conductance voltage- and Ca2+-activated K+ (BK) channels encode negative feedback regulation of membrane voltage and Ca2+ signaling, playing a central role in numerous physiological processes. We determined the x-ray structure of the human BK Ca2+ gating apparatus at a resolution of 3.0 angstroms and deduced its tetrameric assembly by solving a 6 angstrom resolution structure of a Na+-activated homolog. Two tandem C-terminal regulator of K+ conductance (RCK) domains from each of four channel subunits form a 350-kilodalton gating ring at the intracellular membrane surface. A sequence of aspartic amino acids that is known as the Ca2+ bowl, and is located within the second of the tandem RCK domains, creates four Ca2+ binding sites on the outer perimeter of the gating ring at the "assembly interface" between RCK domains. Functionally important mutations cluster near the Ca2+ bowl, near the "flexible interface" between RCK domains, and on the surface of the gating ring that faces the voltage sensors. The structure suggests that the Ca2+ gating ring, in addition to regulating the pore directly, may also modulate the voltage sensor.

Identification of a thiol/disulfide redox switch in the human BK channel that controls its affinity for heme and CO

Heme is a required prosthetic group in many electron transfer proteins and redox enzymes. The human BK channel, which is a large-conductance Ca(2+) and voltage-activated K(+) channel, is involved in the hypoxic response in the carotid body. The BK channel has been shown to bind and undergo inhibition by heme and activation by CO. Furthermore, evidence suggests that human heme oxygenase-2 (HO2) acts as an oxygen sensor and CO donor that can form a protein complex with the BK channel. Here we describe a thiol/disulfide redox switch in the human BK channel and biochemical experiments of heme, CO, and HO2 binding to a 134-residue region within the cytoplasmic domain of the channel. This region, called the heme binding domain (HBD) forms a linker segment between two Ca(2+)-sensing domains (called RCK1 and RCK2) of the BK channel. The HBD includes a CXXCH motif in which histidine serves as the axial heme ligand and the two cysteine residues can form a reversible thiol/disulfide redox switch that regulates affinity of the HBD for heme. The reduced dithiol state binds heme (K(d) = 210 nm) 14-fold more tightly than the oxidized disulfide state. Furthermore, the HBD is shown to tightly bind CO (K(d) = 50 nm) with the Cys residues in the CXXCH motif regulating affinity of the HBD for CO. This HBD is also shown to interact with heme oxygenase-2. We propose that the thiol/disulfide switch in the HBD is a mechanism by which activity of the BK channel can respond quickly and reversibly to changes in the redox state of the cell, especially as it switches between hypoxic and normoxic conditions.

Dual role of protein kinase C on BK channel regulation

Large conductance voltage- and Ca(2+)-activated potassium channels (BK channels) are important feedback regulators in excitable cells and are potently regulated by protein kinases. The present study reveals a dual role of protein kinase C (PKC) on BK channel regulation. Phosphorylation of S(695) by PKC, located between the two regulators of K(+) conductance (RCK1/2) domains, inhibits BK channel open-state probability. This PKC-dependent inhibition depends on a preceding phosphorylation of S(1151) in the C terminus of the channel alpha-subunit. Phosphorylation of only one alpha-subunit at S(1151) and S(695) within the tetrameric pore is sufficient to inhibit BK channel activity. We further detected that protein phosphatase 1 is associated with the channel, constantly counteracting phosphorylation of S(695). PKC phosphorylation at S(1151) also influences stimulation of BK channel activity by protein kinase G (PKG) and protein kinase A (PKA). Though the S(1151)A mutant channel is activated by PKA only, the phosphorylation of S(1151) by PKC renders the channel responsive to activation by PKG but prevents activation by PKA. Phosphorylation of S(695) by PKC or introducing a phosphomimetic aspartate at this position (S(695)D) renders BK channels insensitive to the stimulatory effect of PKG or PKA. Therefore, our findings suggest a very dynamic regulation of the channel by the local PKC activity. It is shown that this complex regulation is not only effective in recombinant channels but also in native BK channels from tracheal smooth muscle.

Closed-channel block of BK potassium channels by bbTBA requires partial activation

Blockade of large-conductance Ca²⁺-activated K⁺ (BK) channels by the bulky quaternary ammonium compound, N-(4-[benzoyl]benzyl)-N,N,N-tributylammonium (bbTBA), exhibits features consistent with blockade of both closed and open states. Here, we examine block of closed BK channels by bbTBA and how it may differ from block of open channels. Although our observations generally confirm earlier results, we describe three observations that are inconsistent with a model in which closed and open channels are equally accessible to blockade by bbTBA. First, block by bbTBA exhibits Ca²⁺-dependent features that are inconsistent with strictly state-independent block. Second, the steady-state voltage dependence of bbTBA block at negative potentials shows that any block of completely closed states either does not occur or is completely voltage independent. Third, determination of the fractional unblock by bbTBA at either low or high Ca²⁺ reveals deviations from a model in which open- and closed-state block is identical. The results support the view that bbTBA blockade of fully closed channels does not occur. We imagine two general types of explanation. First, a stronger voltage dependence of closed-channel block may minimize the contribution of closed-channel block at negative potentials. Second, voltage-dependent conformational changes among closed-channel states may permit block by bbTBA. The analysis supports the latter view, suggesting that bbTBA blockade of fully closed channels does not occur, but the ability of bbTBA to block a closed channel requires movement of one or more voltage sensors. Models in which block is coupled to voltage sensor movement can qualitatively account for (1) the ability of open-channel block to better fit block of conductance–voltage curves at high Ca²⁺; (2) the voltage dependence of fractional availability; and (3) the fractional unblock at different open probabilities. BK channels appear to undergo voltage-dependent conformational changes among closed states that are permissive for bbTBA block.

Block of mouse Slo1 and Slo3 K+ channels by CTX, IbTX, TEA, 4-AP and quinidine

pH-regulated Slo3 channels, perhaps exclusively expressed in mammalian sperm, may play a role in alkalization-mediated K⁺ fluxes associated with sperm capacitation. The Slo3 channel shares extensive homology with Ca²⁺- and voltage-regulated BK-type Slo1 K⁺ channels. Here, using heterologous expression in oocytes, we define distinctive differences in pharmacological properties of Slo3 and Slo1 currents, examine blockade in terms of distinct blocking models, and, for some blockers, use mutated constructs to evaluate determinants of block. Slo3 is resistant to block by the standard Slo1 blockers, iberiotoxin, charybdotoxin and extracellular TEA. Slo3 is relatively insensitive to extracellular 4-AP up to 100 mM, while Slo1 is blocked in a voltage-dependent fashion consistent with block on the extracellular side of the channel. Block of both Slo1 and Slo3 by cytosolic 4-AP can be described by open channel block, with Slo3 being ~10–15-fold more sensitive, but exhibiting weaker voltage-dependence of block. The cytosolic concentrations of 4-AP required to block Slo3 make it unlikely that the effects of 4-AP on volume regulation in mammalian sperm is mediated by Slo3. Quinidine was more effective in blocking Slo3 than Slo1. For Slo1, quinidine block was favored by depolarization, irrespective of the side of application. For Slo3, quinidine block was relieved by depolarization, irrespective of the side of application, with strong block by less than 10 μM quinidine at potentials near 0 mV. The unusual voltage-dependence of block of Slo3 by quinidine may result from preferential binding of quinidine to closed Slo3 channels. The quinidine concentrations effective in blocking Slo3 suggest, that in experiments that have examined quinidine effects on sperm, any Slo3 currents would be almost completely inhibited.

Single-nucleotide polymorphisms in vascular Ca2+-activated K+-channel genes and cardiovascular disease

In the cardiovascular system, Ca2+-activated K+-channels (KCa) are considered crucial mediators in the control of vascular tone and blood pressure by modulating the membrane potential and shaping Ca2+-dependent contraction. Vascular smooth muscle cells express the BKCa channel which fine-tunes contractility by providing a negative feedback on Ca2+-elevations. BKCa channel's ion-conducting alpha-subunit is encoded by the KCa1.1 gene, and the accessory and Ca2+-sensitivity modulating beta1-subunit is encoded by the KCNMB1 gene. Vascular endothelial cells express the calmodulin-gated KCa channels IKCa (encoded by the KCa3.1 gene) and SKCa (encoded by the KCa2.3 gene). These two channels mediate endothelial hyperpolarization and initiate the endothelium-derived hyperpolarizing factor-dilator response. Considering these essential roles of KCa in arterial function, mutations in KCa genes have been suspected to contribute to cardiovascular disease in humans. So far, DNA sequence analysis in the population and patient cohorts has identified single-nucleotide polymorphisms (SNPs) in the BKCa beta1-subunit gene as well as in the alpha-subunit gene (KCa1.1). Some of these SNPs produce amino acid exchanges and evoke alterations of channel functions ("gain-of-function" as well as "loss-of-function"). Moreover, the epidemiological studies showed that the presence of the E65K polymorphism in, e.g., BKCa beta1-subunit gene (producing a "gain-of-function") lowers the prevalence for severe hypertension and myocardial infarction. Other SNPs in the BKCa alpha-subunit gene and also in the KCa3.1 gene expressed in the endothelium have been suggested to increase the risk of cardiovascular disease. These findings from sequence analysis of human KCa genes, and epidemiological studies thus provide evidence that genetic variations and mutations in KCa channel genes contribute to human cardiovascular disease.

Zn2+ activates large conductance Ca2+-activated K+ channel via an intracellular domain

Zinc is an essential trace element and plays crucial roles in normal development, often as an integral structural component of transcription factors and enzymes. Recent evidence suggests that intracellular Zn(2+) functions as a signaling molecule, mediating a variety of important physiological phenomena. However, the immediate effectors of intracellular Zn(2+) signaling are not well known. We show here that intracellular Zn(2+) potently and reversibly activates large-conductance voltage- and Ca(2+)-activated Slo1 K(+) (BK) channels. The full effect of Zn(2+) requires His(365) in the RCK1 (regulator of conductance for K(+)) domain of the channel. Furthermore, mutation of two nearby acidic residues, Asp(367) and Glu(399), also reduced activation of the channel by Zn(2+), suggesting a possible structural arrangement for Zn(2+) binding by the aforementioned residues. Extracellular Zn(2+) activated Slo1 BK channels when coexpressed with Zn(2+)-permeable TRPM7 (transient receptor potential melastatin 7) channels. The results thus demonstrate that Slo1 BK channels represent a positive and direct effector of Zn(2+) signaling and may participate in sculpting cellular response to an increase in intracellular Zn(2+) concentration.

Modulation of the conductance-voltage relationship of the BK(Ca) channel by shortening the cytosolic loop connecting two RCK domains

Calcium-dependent gating of large-conductance calcium-activated potassium (BK(Ca)) channels is mediated by the intracellular carboxyl terminus, which contains two domains of regulator of K(+) conductance (RCK). In mammalian BK(Ca) channels, the two RCK domains are separated by a protein segment of 101 residues that is poorly conserved in evolution and predicted to have no regular secondary structures. We investigated the functional importance of this loop using a series of deletion mutations. We found that the length, rather than the specific sequence at the central region of the segment, is critical for the functionality of the channel. As the length of the loop is progressively shorted, the conductance-voltage relationship gradually shifts toward more positive voltages with a minimum length of 70 amino acids, in an apparent response to increased tension within the loop. Thus, the functional activity of the BK(Ca) channel can be modulated by altering the tension of this loop region.

The beta1 subunit of Na+/K+-ATPase interacts with BKCa channels and affects their steady-state expression on the cell surface

Large conductance Ca2+-activated K+ channels (BKCa) encoded by the Slo1 gene play a role in the physiological regulation of many cell types. Here, we show that the beta1 subunit of Na+/K+-ATPase (NKbeta1) interacts with the cytoplasmic COOH-terminal region of Slo1 proteins. Reduced expression of endogenous NKbeta1 markedly inhibits evoked BKCa currents with no apparent effect on their gating. In addition, NKbeta1 down-regulated cells show decreased density of Slo1 subunits on the cell surface.

Allosteric control of gating mechanisms revisited: the large conductance Ca2+-activated K+ channel

Large-conductance Ca(2+)-activated K(+) channels (BK) play a fundamental role in modulating membrane potential in many cell types. The gating of BK channels and its modulation by Ca(2+) and voltage has been the subject of intensive research over almost three decades, yielding several of the most complicated kinetic mechanisms ever proposed. A large number of open and closed states disposed, respectively, in two planes, named tiers, characterize these mechanisms. Transitions between states in the same plane are cooperative and modulated by Ca(2+). Transitions across planes are highly concerted and voltage-dependent. Here we reexamine the validity of the two-tiered hypothesis by restricting attention to the modulation by Ca(2+). Large single channel data sets at five Ca(2+) concentrations were simultaneously analyzed from a Bayesian perspective by using hidden Markov models and Markov-chain Monte Carlo stochastic integration techniques. Our results support a dramatic reduction in model complexity, favoring a simple mechanism derived from the Monod-Wyman-Changeux allosteric model for homotetramers, able to explain the Ca(2+) modulation of the gating process. This model differs from the standard Monod-Wyman-Changeux scheme in that one distinguishes when two Ca(2+) ions are bound to adjacent or diagonal subunits of the tetramer.

Comparative effects of H+ and Ca2+ on large-conductance Ca2+- and voltage-gated Slo1 K+ channels

Large-conductance Ca(2+)- and voltage-gated Slo1 BK channels are allosterically activated by depolarization and intracellular ligands such as Ca(2+). Of the two high-affinity Ca(2+) sensors present in the channel, the RCK1 sensor also mediates H(+)-dependent activation of the channel. In this study, we examined the comparative mechanisms of the channel activation by Ca(2+) and H(+). Steady-state macroscopic conductance-voltage measurements as well as single-channel openings at negative voltages where voltage-sensor activation is negligible showed that at respective saturating concentrations Ca(2+) is more effective in relative stabilization of the open conformation than H(+). Calculations using the Debye-Hückel formalism suggest that small structural changes in the RCK1 sensor, on the order of few angstroms, may accompany the H(+)-mediated opening of the channel. While the efficacy of H(+) in activation of the channel is less than that of Ca(2+), H(+) more effectively accelerates the activation kinetics when examined at the concentrations equipotent on macroscopic voltage-dependent activation. The RCK1 sensor therefore is capable of transducing the nature of the bound ligand and transmits qualitatively different information to the channel's permeation gate.

N-terminal inactivation domains of beta subunits are protected from trypsin digestion by binding within the antechamber of BK channels

N termini of auxiliary β subunits that produce inactivation of large-conductance Ca²⁺-activated K⁺ (BK) channels reach their pore-blocking position by first passing through side portals into an antechamber separating the BK pore module and the large C-terminal cytosolic domain. Previous work indicated that the β2 subunit inactivation domain is protected from digestion by trypsin when bound in the inactivated conformation. Other results suggest that, even when channels are closed, an inactivation domain can also be protected from digestion by trypsin when bound within the antechamber. Here, we provide additional tests of this model and examine its applicability to other β subunit N termini. First, we show that specific mutations in the β2 inactivation segment can speed up digestion by trypsin under closed-channel conditions, supporting the idea that the β2 N terminus is protected by binding within the antechamber. Second, we show that cytosolic channel blockers distinguish between protection mediated by inactivation and protection under closed-channel conditions, implicating two distinct sites of protection. Together, these results confirm the idea that β2 N termini can occupy the BK channel antechamber by interaction at some site distinct from the BK central cavity. In contrast, the β3a N terminus is digested over 10-fold more quickly than the β2 N terminus. Analysis of factors that contribute to differences in digestion rates suggests that binding of an N terminus within the antechamber constrains the trypsin accessibility of digestible basic residues, even when such residues are positioned outside the antechamber. Our analysis indicates that up to two N termini may simultaneously be protected from digestion. These results indicate that inactivation domains have sites of binding in addition to those directly involved in inactivation.

Plasticity and emerging role of BKCa channels in nociceptive control in neuropathic pain

Large-conductance Ca(2+)-activated K(+) (BK(Ca), MaxiK) channels are important for the regulation of neuronal excitability. Peripheral nerve injury causes plasticity of primary afferent neurons and spinal dorsal horn neurons, leading to central sensitization and neuropathic pain. However, little is known about changes in the BK(Ca) channels in the dorsal root ganglion (DRG) and spinal dorsal horn and their role in the control of nociception in neuropathic pain. Here we show that L5 and L6 spinal nerve ligation in rats resulted in a substantial reduction in both the mRNA and protein levels of BK(Ca) channels in the DRG but not in the spinal cord. Nerve injury primarily reduced the BK(Ca) channel immunoreactivity in small- and medium-sized DRG neurons. Furthermore, although the BK(Ca) channel immunoreactivity was decreased in the lateral dorsal horn, there was an increase in the BK(Ca) channel immunoreactivity present on dorsal horn neurons near the dorsal root entry zone. Blocking the BK(Ca) channel with iberiotoxin at the spinal level significantly reduced the mechanical nociceptive withdrawal threshold in control and nerve-injured rats. Intrathecal injection of the BK(Ca) channel opener [1,3-dihydro-1-[2-hydroxy-5-(trifluoromethyl)phenyl]-5-(trifluoromethyl)-2H-benzimidazol-2-one] dose dependently reversed allodynia and hyperalgesia in nerve-ligated rats but it had no significant effect on nociception in control rats. Our study provides novel information that nerve injury suppresses BK(Ca) channel expression in the DRG and induces a redistribution of BK(Ca) channels in the spinal dorsal horn. BK(Ca) channels are increasingly involved in the control of sensory input in neuropathic pain and may represent a new target for neuropathic pain treatment.