Molecular identification of Ca2+-activated K+ channels in parotid acinar cells

We used molecular biological and patch-clamp techniques to identify the Ca(2+)-activated K(+) channel genes in mouse parotid acinar cells. Two types of K(+) channels were activated by intracellular Ca(2+) with single-channel conductance values of 22 and 140 pS (in 135 mM external K(+)), consistent with the intermediate and maxi-K classes of Ca(2+)-activated K(+) channels, typified by the mIK1 (Kcnn4) and mSlo (Kcnma1) genes, respectively. The presence of mIK1 mRNA was established in acinar cells by in situ hybridization. The electrophysiological and pharmacological properties of heterologously expressed mIK1 channels matched those of the native current; thus the native, smaller conductance channel is likely derived from the mIK1 gene. We found that parotid acinar cells express a single, uncommon splice variant of the mSlo gene and that heterologously expressed channels of this Slo variant had a single-channel conductance indistinguishable from that of the native, large-conductance channel. However, the sensitivity of this expressed Slo variant to the scorpion toxin iberiotoxin was considerably different from that of the native current. RT-PCR analysis revealed the presence of two mSlo beta-subunits (Kcnmb1 and Kcnmb4) in parotid tissue. Comparison of the iberiotoxin sensitivity of the native current with that of parotid mSlo expressed with each beta-subunit in isolation and measurements of the iberiotoxin sensitivity of currents in cells from beta(1) knockout mice suggest that parotid acinar cells contain approximately equal numbers of homotetrameric channel proteins from the parotid variant of the Slo gene and heteromeric proteins composed of the parotid Slo variant in combination with the beta(4)-subunit.

The cytoplasmic tail of large conductance, voltage- and Ca2+-activated K+ (MaxiK) channel is necessary for its cell surface expression

The large conductance, voltage- and Ca(2+)-activated K(+) channel (MaxiK) is expressed in several renal segments and functions in cell volume regulation and flow-mediated K(+) secretion. Previously, we cloned two MaxiK channel isoforms, named rbslo1 and rbslo2, from rabbit renal cells. rbslo1 has a 58-amino acid insertion after the S8 hydrophobic domain, whereas rbslo2 is truncated and cannot be activated. Here we use the sequence differences between the two variants to examine their plasma membrane processing. Plasma membrane localization of rbslo1 and 2 expressed in HEK293 cells was assayed by electrophysiology, immunocytochemistry, and biochemistry studies. Consistent with its functional silence, rbslo2 localized primarily within the cytoplasm, presumably in the endoplasmic reticulum and Golgi region. Coexpression with MaxiK beta subunits did not alter the cellular localization of either rbslo1 or rbslo2. When rbslo1 and 2 are cotransfected in non-polarized cells, they colocalized primarily within the cell with only rbslo1 detected at the plasma membrane. When transfected into polarized, medullary-thick ascending limb (mTAL) cells, rbslo1 is expressed at the apical membrane whereas the majority of rbslo2 localized throughout the cytoplasm. Given the high degree of similarity between the two isoforms, we conclude that the cytoplasmic tail of rbslo1 is important for the cell surface expression of MaxiK channels.

Expression of Ca2+-activated BK channel mRNA and its splice variants in the rat cochlea

Voltage-activated K(+) channels are important for shaping the receptor potentials of cochlear hair cells. In particular, the functional maturation of inner hair cells in mice around the onset of hearing coincides with the expression of a large, fast K(+) conductance, probably mediated by Ca(2+)-activated K(+) (BK) channels. In hearing organs of lower vertebrates, frequency tuning depends on BK-type K(+) channels with different kinetics. Kinetics are varied by alternative splicing of the channels' alpha subunits and combination with modulating beta subunits. It is unclear whether similar mechanisms "fine tune" mammalian hair cells. We used various polymerase chain reaction (PCR) approaches to screen rat cochleae for splice variants of BK-type alpha subunits. We isolated mainly minimal variants and only occasionally splice variants with additional inserts. We conclude that alpha subunits with different kinetics are not substantially used in the rat cochlea. However, we isolated six variants differing in their extreme C-terminal sequences, which may be involved in the targeting of the channel protein. By using reverse transcriptase-PCR, we demonstrated also the expression of transcripts for several beta subunits. In situ hybridization experiments revealed strict coexpression of alpha with beta1 transcripts. In inner hair cells, strong labeling emerged shortly before the onset of hearing. Labeling of outer hair cells appeared later and generally weaker. Thus, our molecular data confirm electrophysiological results that suggested that BK channels underlie the large K(+) conductance in inner hair cells of mammals. Extensive splicing of BK channel transcripts, however, does not seem to be used in mammalian hair cells as is done in lower vertebrates.

Freshly isolated bovine coronary endothelial cells do not express the BK Ca channel gene

Recent reports have suggested that different types of Ca(2+)-activated K(+) channels may be selectively expressed either in the vascular endothelial cells (ECs) or smooth muscle cells (SMCs) of a single artery. In this study, we directly compared mRNA, protein and functional expression of the high-conductance Ca(2+)-activated K(+) (BK(Ca)) channel between freshly isolated ECs and SMCs from bovine coronary arteries. Fresh ECs and SMCs were enzymatically isolated, and their separation verified by immunofluorescent detection of alpha-actin and platelet/endothelium cell adhesion molecule (PECAM) proteins, respectively. Subsequently, studies using a sequence-specific antibody directed against the pore-forming alpha-subunit of the BK(Ca) channel only detected its expression in the SMCs, whereas PECAM-positive ECs were devoid of the alpha-subunit protein. Additionally, multicell RT-PCR performed using cDNA derived from either SMCs or ECs only detected mRNA encoding the BK(Ca) alpha-subunit in the SMCs. Finally, whole-cell recordings of outward K(+) current detected a prominent iberiotoxin-sensitive BK(Ca) current in SMCs that was absent in ECs, and the BK(Ca) channel opener NS 1619 only enhanced K(+) current in the SMCs. Thus, bovine coronary SMCs densely express BK(Ca) channels whereas adjacent ECs in the same artery appear to lack the expression of the BK(Ca) channel gene. These findings indicate a cell-specific distribution of Ca(2+)-activated K(+) channels in SMCs and ECs from a single arterial site.

Slo1 tail domains, but not the Ca2+ bowl, are required for the beta 1 subunit to increase the apparent Ca2+ sensitivity of BK channels

Functional large-conductance Ca(2+)- and voltage-activated K(+) (BK) channels can be assembled from four alpha subunits (Slo1) alone, or together with four auxiliary beta1 subunits to greatly increase the apparent Ca(2+) sensitivity of the channel. We examined the structural features involved in this modulation with two types of experiments. In the first, the tail domain of the alpha subunit, which includes the RCK2 (regulator of K(+) conductance) domain and Ca(2+) bowl, was replaced with the tail domain of Slo3, a BK-related channel that lacks both a Ca(2+) bowl and high affinity Ca(2+) sensitivity. In the second, the Ca(2+) bowl was disrupted by mutations that greatly reduce the apparent Ca(2+) sensitivity. We found that the beta1 subunit increased the apparent Ca(2+) sensitivity of Slo1 channels, independently of whether the alpha subunits were expressed as separate cores (S0-S8) and tails (S9-S10) or full length, and this increase was still observed after the Ca(2+) bowl was mutated. In contrast, beta1 subunits no longer increased Ca(2+) sensitivity when Slo1 tails were replaced by Slo3 tails. The beta1 subunits were still functionally coupled to channels with Slo3 tails, as DHS-I and 17 beta-estradiol activated these channels in the presence of beta1 subunits, but not in their absence. These findings indicate that the increase in apparent Ca(2+) sensitivity induced by the beta1 subunit does not require either the Ca(2+) bowl or the linker between the RCK1 and RCK2 domains, and that Slo3 tails cannot substitute for Slo1 tails. The beta1 subunit also induced a decrease in voltage sensitivity that occurred with either Slo1 or Slo3 tails. In contrast, the beta1 subunit-induced increase in apparent Ca(2+) sensitivity required Slo1 tails. This suggests that the allosteric activation pathways for these two types of actions of the beta1 subunit may be different.

Reciprocal modulation between the alpha and beta 4 subunits of hSlo calcium-dependent potassium channels

Large conductance Ca(2+)-dependent potassium (K(Ca) or maxi K) channels are composed of a pore-forming alpha subunit and an auxiliary beta subunit. We have shown that the brain-specific beta4 subunit modulates the voltage dependence, activation kinetics, and toxin sensitivity of the hSlo channel (Weiger, T. M., Holmqvist, M. H., Levitan, I. B., Clark, F. T., Sprague, S., Huang, W. J., Ge, P., Wang, C., Lawson, D., Jurman, M. E., Glucksmann, M. A., Silos-Santiago, I., DiStefano, P. S., and Curtis, R. (2000) J. Neurosci. 20, 3563-3570). We investigated here the N-linked glycosylation of the beta4 subunit and its effect on the modulation of the hSlo alpha subunit. When expressed alone in HEK293 cells, the beta4 subunit runs as a single molecular weight band on an SDS gel. However, when coexpressed with the hSlo alpha subunit, the beta4 subunit appears as two different molecular weight bands. Enzymatic deglycosylation or mutation of the N-linked glycosylation residues in beta4 converts it to a single lower molecular weight band, even in the presence of the hSlo alpha subunit, suggesting that the beta4 subunit can be present as an immature, core glycosylated form and a mature, highly glycosylated form. Blockage of protein transport from the endoplasmic reticulum to the Golgi compartment with brefeldin A abolishes the mature, highly glycosylated beta4 band. Glycosylation of the beta4 subunit is not required for its binding to the hSlo channel alpha subunit. It also is not necessary for cell membrane targeting of the beta4 subunit, as demonstrated by surface biotinylation experiments. However, the double glycosylation site mutant beta4 (beta4 N53A/N90A) protects the channel less against toxin blockade, as compared with the hSlo channel coexpressed with wild type beta4 subunit. Taken together, these data show that the pore-forming alpha subunit of the hSlo channel promotes N-linked glycosylation of its auxiliary beta4 subunit, and this in turn influences the modulation of the channel by the beta4 subunit.

Coupling of c-Src to large conductance voltage- and Ca2+-activated K+ channels as a new mechanism of agonist-induced vasoconstriction

The voltage-dependent and Ca(2+)-activated K(+) channel (MaxiK, BK) and the cellular proto-oncogene pp60(c-Src) (c-Src) are abundant proteins in vascular smooth muscle. The role of MaxiK channels as a vasorelaxing force is well established, but their role in vasoconstriction is unclear. Because Src participates in regulating vasoconstriction, we investigated whether c-Src inhibits MaxiK as a mechanism for agonist-induced vasoconstriction. Functional experiments in human and rat show that inhibitors of Src (Lavendustin A, PP2) but not inactive compounds (Lavendustin B, PP3) induce a pronounced relaxation of coronary or aortic smooth muscle precontracted with 5-hydroxytriptamine, phenylephrine, or Angiotensin II. Iberiotoxin, a MaxiK blocker, antagonizes the relaxation induced by Lavendustin A or PP2, indicating that c-Src inhibits the Iberiotoxin-sensitive component, likely MaxiK channels. In agreement, coronary muscle MaxiK currents were enhanced by Lavendustin A. To investigate the molecular mechanism of c-Src action on MaxiK channels, we transiently expressed its alpha subunit, hSlo, with or without c-Src in HEK293T cells. The voltage sensitivity of hSlo was right-shifted by approximately 16 mV. hSlo inhibition by c-Src is due to channel direct phosphorylation because: (i) excised patches exposed to protein tyrosine phosphatase (CD45) resulted in a partial reversal of the inhibitory effect by approximately 10 mV, and (ii) immunoprecipitated hSlo channels were recognized by an anti-phosphotyrosine Ab. Furthermore, coexpression of hSlo and c-Src demonstrate a striking colocalization in HEK293T cells. We propose that MaxiK channels via direct c-Src-dependent phosphorylation play a significant role supporting vasoconstriction after activation of G protein-coupled receptors by vasoactive substances and neurotransmitters.

Molecular basis of pimarane compounds as novel activators of large-conductance Ca(2+)-activated K(+) channel alpha-subunit

Effects of pimaric acid (PiMA) and eight closely related compounds on large-conductance K(+) (BK) channels were examined using human embryonic kidney (HEK) 293 cells, in which either the alpha subunit of BK channel (HEKBKalpha) or both alpha and beta1 (HEKBKalphabeta1) subunits were heterologously expressed. Effects of these compounds (10 microM) on the membrane potential of HEKBKalphabeta1 were monitored by use of DiBAC(4)(3), a voltage-sensitive dye. PiMA, isopimaric acid, sandaracoisopimaric acid, dihydropimaric acid, dihydroisopimaric acid, and dihydroisopimarinol induced substantial membrane hyperpolarization. The direct measurement of BKalphabeta1 opening under whole-cell voltage clamp showed that these six compounds activated BKalphabeta1 in a very similar concentration range (1-10 microM); in contrast, abietic acid, sclareol, and methyl pimarate had no effect. PiMA did not affect the charybdotoxin-induced block of macroscopic BKalphabeta1 current. Single channel recordings of BKalphabeta1 in inside-out patches showed that 10 microM PiMA did not change channel conductance but significantly increased its open probability as a result of increase in sensitivity to Ca(2+) and voltage. Because coexpression of the beta1 subunit did not affect PiMA-induced potentiation, the site of action for PiMA is suggested to be BKalpha subunit. PiMA was selective to BK over cloned small and intermediate Ca(2+) activated K(+) channels. In conclusion, PiMA (>1 microM) increases Ca(2+) and voltage-sensitivity of BKalpha when applied from either side of the cell membrane. The marked difference in potency as BK channel openers between PiMA and abietic acid, despite only very small differences in their chemical structures, may provide insight into the fundamental structure-activity relationship governing BKalpha activation.

Coupling between voltage sensor activation, Ca2+ binding and channel opening in large conductance (BK) potassium channels

To determine how intracellular Ca(2+) and membrane voltage regulate the gating of large conductance Ca(2+)-activated K(+) (BK) channels, we examined the steady-state and kinetic properties of mSlo1 ionic and gating currents in the presence and absence of Ca(2+) over a wide range of voltage. The activation of unliganded mSlo1 channels can be accounted for by allosteric coupling between voltage sensor activation and the closed (C) to open (O) conformational change (Horrigan, F.T., and R.W. Aldrich. 1999. J. Gen. Physiol. 114:305-336; Horrigan, F.T., J. Cui, and R.W. Aldrich. 1999. J. Gen. Physiol. 114:277-304). In 0 Ca(2+), the steady-state gating charge-voltage (Q(SS)-V) relationship is shallower and shifted to more negative voltages than the conductance-voltage (G(K)-V) relationship. Calcium alters the relationship between Q-V and G-V, shifting both to more negative voltages such that they almost superimpose in 70 microM Ca(2+). This change reflects a differential effect of Ca(2+) on voltage sensor activation and channel opening. Ca(2+) has only a small effect on the fast component of ON gating current, indicating that Ca(2+) binding has little effect on voltage sensor activation when channels are closed. In contrast, open probability measured at very negative voltages (less than -80 mV) increases more than 1,000-fold in 70 microM Ca(2+), demonstrating that Ca(2+) increases the C-O equilibrium constant under conditions where voltage sensors are not activated. Thus, Ca(2+) binding and voltage sensor activation act almost independently, to enhance channel opening. This dual-allosteric mechanism can reproduce the steady-state behavior of mSlo1 over a wide range of conditions, with the assumption that activation of individual Ca(2+) sensors or voltage sensors additively affect the energy of the C-O transition and that a weak interaction between Ca(2+) sensors and voltage sensors occurs independent of channel opening. By contrast, macroscopic I(K) kinetics indicate that Ca(2+) and voltage dependencies of C-O transition rates are complex, leading us to propose that the C-O conformational change may be described by a complex energy landscape.

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

Large conductance, Ca(2+)- and voltage-activated K(+) channels (BK) respond to two distinct physiological signals -- membrane voltage and cytosolic Ca(2+) (refs 1, 2). Channel opening is regulated by changes in Ca(2+) concentration spanning 0.5 micro M to 50 mM (refs 2-5), a range of Ca(2+) sensitivity unusual among Ca(2+)-regulated proteins. Although voltage regulation arises from mechanisms shared with other voltage-gated channels, the mechanisms of Ca(2+) regulation remain largely unknown. One potential Ca(2+)-regulatory site, termed the 'Ca(2+) bowl', has been located to the large cytosolic carboxy terminus. Here we show that a second region of the C terminus, the RCK domain (regulator of conductance for K(+) (ref. 12)), contains residues that define two additional regulatory effects of divalent cations. One site, together with the Ca(2+) bowl, accounts for all physiological regulation of BK channels by Ca(2+); the other site contributes to effects of millimolar divalent cations that may mediate physiological regulation by cytosolic Mg(2+) (refs 5, 13). Independent regulation by multiple sites explains the large concentration range over which BK channels are regulated by Ca(2+). This allows BK channels to serve a variety of physiological roles contingent on the Ca(2+) concentration to which the channels are exposed.

Mechanism of magnesium activation of calcium-activated potassium channels

Large-conductance (BK type) Ca(2+)-dependent K(+) channels are essential for modulating muscle contraction and neuronal activities such as synaptic transmission and hearing. BK channels are activated by membrane depolarization and intracellular Ca(2+) and Mg(2+) (refs 6-10). The energy provided by voltage, Ca(2+) and Mg(2+) binding are additive in activating the channel, suggesting that these signals open the activation gate through independent pathways. Here we report a molecular investigation of a Mg(2+)-dependent activation mechanism. Using a combined site-directed mutagenesis and structural analysis, we demonstrate that a structurally new Mg(2+)-binding site in the RCK/Rossman fold domain -- an intracellular structural motif that immediately follows the activation gate S6 helix -- is responsible for Mg(2+)-dependent activation. Mutations that impair or abolish Mg(2+) sensitivity do not affect Ca(2+) sensitivity, and vice versa. These results indicate distinct structural pathways for Mg(2+)- and Ca(2+)-dependent activation and suggest a possible mechanism for the coupling between Mg(2+) binding and channel opening.

Stepwise contribution of each subunit to the cooperative activation of BK channels by Ca2+

BK channels (Slo1) are widely distributed K+ channels that control Ca2+-dependent processes and cellular excitability. Their activation by intracellular Ca2+ (Ca(i)2+) is highly cooperative, with Hill coefficients of typically 2-5. To investigate the cooperativity contributed by each of the four alpha subunits that form the BK channel, we studied single channels comprised of mixtures of functional subunits and subunits with a mutation to disrupt a key site (Ca-bowl) required for activation by low concentrations of Ca(i)2+. As the number of functional subunits increased, we found a stepwise increase in the Hill coefficient of 0.3-0.8 per functional subunit and a stepwise decrease in the Ca(i)2+ required for half activation (K(d)). These results show directly that BK channels can open with 0, 1, 2, 3, or 4 functional Ca-bowls, and that each subunit with a functional Ca-bowl contributes a stepwise increase to both the cooperativity of activation and the apparent Ca2+ affinity. A model with 0-4 high-affinity allosteric activators and four low-affinity allosteric activators was examined. In this model, Ca2+ bindings were independent of one another and the cooperativity arose from the joint action of the allosteric activators on the open-closed equilibrium. Although this model described well the major features of the experimental data, some differences between the observed and predicted results indicated that additional factors not included in the model also contribute to the cooperativity.

Interacting effects of N-terminal variation and strex exon splicing on slo potassium channel regulation by calcium, phosphorylation, and oxidation

We have investigated the structural basis for the phenotype of a native rat Slo (rSlo) potassium channel (BK(Ca); KCNMA1) in a rat pituitary cell line, GH(4)C(1). Opposing regulation of these calcium- and voltage-activated potassium channels by cAMP- and cGMP-dependent protein kinases requires an alternatively spliced exon (strex) of 59 amino acids in the cytoplasmic C terminus of the pore-forming alpha subunit encoded by rslo. However, inclusion of this cysteine-rich exon produces a 10-fold increase in the sensitivity of the channels to inhibition by oxidation. Inclusion of the strex exon also increases channel sensitivity to stimulation by calcium, but responses in the physiological ranges of calcium and voltage require coassembly with beta(1) subunits. With strex present, however, beta(1) subunits only stimulated channels assembled from rSlo alpha subunits with a truncated N terminus beginning MDALI-. Thus N-terminal variation and strex exon splicing in rSlo interact to produce BK(Ca) channels with a physiologically relevant phenotype.

17beta-Estradiol upregulates distinct maxi-K channel transcripts in mouse uterus

The mouse maxi-K channel transcript undergoes alternative splicing to produce isoforms differing in sensitivity to intracellular regulators. We hypothesized that 17beta-estradiol could induce myometrial maxi-K channel transcripts to differentially splice. Polymerase chain reaction demonstrated two products at site D in mice injected with either 8.5 microg of 17beta-estradiol for 4 days or a vehicle control. Splicing of site D is known to modulate the sensitivity of the maxi-K channel to calcium and voltage. RNase protection analyses revealed that the alpha subunit transcript, and an exon encoding 59 amino acids at site D that enhances Ca(2+)- and voltage-sensitivity, are upregulated approximately 1.4-fold after 17beta-estradiol stimulation however, the insertless isoform of this transcript is enhanced approximately 5-fold. Immunoblotting demonstrates that the total maxi-K channel alpha subunit expression mimics transcript regulation. These findings verify that maxi-K channel transcripts are differentially spliced by 17beta-estradiol, which may contribute to stoichiometric changes in isoform expression during pregnancy.

Modulation of Drosophila slowpoke calcium-dependent potassium channel activity by bound protein kinase a catalytic subunit

Drosophila Slowpoke (dSlo) calcium-dependent potassium channels bind directly to the catalytic subunit of cAMP-dependent protein kinase (PKAc). We demonstrate here that coexpression of PKAc with dSlo in mammalian cells results in a dramatic decrease of dSlo channel activity. This modulation requires catalytically active PKAc but is not mediated by phosphorylation of S942, the only PKA consensus site in the dSlo C-terminal domain. dSlo binds to free PKAc but not to the PKA holoenzyme that includes regulatory subunits and is inactive. Activators of endogenous PKA that stimulate dSlo phosphorylation, but do not produce detectable PKAc binding to dSlo, do not modulate channel function. Furthermore, the catalytically inactive PKAc mutant does bind to dSlo but does not modulate channel activity. These results are consistent with the hypothesis that both binding of active PKAc to dSlo and phosphorylation of dSlo or some other protein are necessary for channel modulation.

Hypoxia inhibits human recombinant large conductance, Ca(2+)-activated K(+) (maxi-K) channels by a mechanism which is membrane delimited and Ca(2+) sensitive

Large conductance, Ca(2+)-activated K(+) (maxi-K ) channel activity was recorded in excised, inside-out patches from HEK 293 cells stably co-expressing the alpha- and beta-subunits of human brain maxi-K channels. At +50 mV, and in the presence of 300 nM Ca2+i, single channel activity was acutely and reversibly suppressed upon reducing P(O(2)) from 150 to > 40 mmHg by over 30 %. The hypoxia-evoked reduction in current was due predominantly to suppression in NP(o), although a minor component was attributable to reduced unitary conductance of 8-12 %. Hypoxia caused an approximate doubling of the time constant for activation but was without effect on deactivation. At lower levels of Ca2+i(30 and 100 nM), hypoxic inhibition did not reach significance. In contrast, 300 nM and 1 microM Ca2+i both sustained significant hypoxic suppression of activity over the entire activating voltage range. At these two Ca2+i levels, hypoxia evoked a positive shift in the activating voltage (by approximately 10 mV at 300 nM and approximately 25 mV at 1 microM). At saturating [Ca(2+)](i) (100 microM), hypoxic inhibition was absent. Distinguishing between hypoxia-evoked changes in voltage- and/or Ca2+i-sensitivity was achieved by evoking maximal channel activity using high depolarising potentials (up to +200 mV) in the presence of 300 nM or 100 microM Ca2+i or in its virtual absence (> 1 nM). Under these experimental conditions, hypoxia caused significant channel inhibition only in the presence of 300 nM Ca2+i. Thus, since regulation was observed in excised patches, maxi-K channel inhibition by hypoxia does not require soluble intracellular components and, mechanistically, is voltage independent and Ca2+i sensitive.

Steady-state and closed-state inactivation properties of inactivating BK channels

Calcium-dependent potassium (BK-type) Ca2+ and voltage-dependent K+ channels in chromaffin cells exhibit an inactivation that probably arises from coassembly of Slo1 alpha subunits with auxiliary beta subunits. One goal of this work was to determine whether the Ca2+ dependence of inactivation arises from any mechanism other than coupling of inactivation to the Ca2+ dependence of activation. Steady-state inactivation and the onset of inactivation were studied in inside-out patches and whole-cell recordings from rat adrenal chromaffin cells with parallel experiments on inactivating BK channels resulting from cloned alpha + beta2 subunits. In both cases, steady-state inactivation was shifted to more negative potentials by increases in submembrane [Ca2+] from 1 to 60 microM. At 10 and 60 microM Ca2+, the maximal channel availability at negative potentials was similar despite a shift in the voltage of half availability, suggesting there is no strictly Ca2+-dependent inactivation. In contrast, in the absence of Ca2+, depolarization to potentials positive to +20 mV induces channel inactivation. Thus, voltage-dependent, but not solely Ca2+-dependent, kinetic steps are required for inactivation to occur. Finally, under some conditions, BK channels are shown to inactivate as readily from closed states as from open states, indicative that a key conformational change required for inactivation precedes channel opening.

Ethylbromide tamoxifen, a membrane-impermeant antiestrogen, activates smooth muscle calcium-activated large-conductance potassium channels from the extracellular side

Smooth-muscle calcium-activated large-conductance potassium channels (BK channels) are activated by tamoxifen and 17-beta-estradiol. This increase in NP(o), the number of channels, N, multiplied by open probability, depends on the presence of the regulatory beta1-subunit. Furthermore, a previous study indicated that 17-beta-estradiol might bind an extracellular site on the beta1-subunit. Because tamoxifen and 17-beta-estradiol may share a common binding site, we hypothesized that tamoxifen activates BK channels through a site on the extracellular surface of the membrane. A membrane-impermeant analog of tamoxifen, ethylbromide tamoxifen, was synthesized and used to test this hypothesis in whole-cell, outside-out, cell-attached, and inside-out patches from canine colonic smooth muscle cells. Ethylbromide tamoxifen is positively charged and is therefore membrane-impermeant. In whole-cell experiments, ethylbromide tamoxifen increased K(+) current at potentials positive to +40 mV, which has previously been attributed to BK channels. Unlike tamoxifen, ethylbromide tamoxifen did not inhibit delayed rectifier current. In outside-out patches, ethylbromide tamoxifen increased BK channel NP(o) with an EC(50) value of 1 microM. Ethylbromide tamoxifen did not increase BK channel NP(o) in cell-attached or inside-out patches; however, subsequent addition of equimolar tamoxifen did. Both drugs diminished BK channel unitary conductance to a degree that paralleled the effect on NP(o), suggesting an additional interaction with the pore-forming alpha-subunit. An interaction of tamoxifen with the pore was supported by a right shift in the concentration-response curve for tetraethylammonium; similar results were evident with iberiotoxin and charybdotoxin block. Our data suggest that ethylbromide tamoxifen does not easily traverse the plasma membrane and that tamoxifen binding responsible for activation of BK channels is at an extracellular site. The tamoxifen binding site may be within the extracellular loop of the BK channel beta1-subunit or, alternatively, on an as-yet-unidentified mediator that has an extracellular binding site.

Prostaglandin E2 induces interaction between hSlo potassium channel and Syk tyrosine kinase in osteosarcoma cells

Prostaglandins (PGs) are important mediators of bone response to growth factors, hormones, inflammation, or mechanical strains. In this study, we show that in MG63 osteosarcoma cells, prostaglandin E2 (PGE2) produces the opening of a large conductance Ca2+-dependent K+ channel (BK). This PGE2-mediated channel opening induces the recruitment of various tyrosine-phosphorylated proteins on the hSlo alpha-subunit of BK. Because the C-terminal domain of hSlo encompasses an immunoreceptor tyrosine-based activation motif (ITAM), we show that the Syk nonreceptor tyrosine kinase, reported yet to be expressed mainly in hematopoietic cells, is expressed also in osteoblastic cells, and recruited on this ITAM after a PGE2-induced docking/activation process. We show that Syk/hSlo association is dependent of an upstream Src-related tyrosine kinase activity, in accord with the classical two-step model described for immune receptors. Finally, we provide evidence that this Syk/hSlo interaction does not affect the electrical features of BK channels in osteosarcoma cells. With these data, we would like to suggest the new notion that besides its conductance function, hSlo channel can behave in bone cells, as a true transduction protein intervening in the bone remodeling induced by PGE2.

Accurate quantitative RT-PCR for relative expression of Slo splice variants

Much interest has been shown in the use of multi-template reverse transcription-polymerase chain reaction (RT-PCR) as a quantitative instrument for low-abundance mRNAs. A desire to achieve finely-graded quantification of the stress- and hormone-related regulation of one splicing decision in an ion channel gene motivated us to test the reliability of simultaneous amplification of two splice variants with one pair of flanking constitutive primers. Unexpectedly indiscriminate heteroduplexing between the two amplification products, despite a large length difference, and their tight comigration with one homoduplex, mandated a rigorously-denaturing electrophoresis protocol. Conveniently, a new fluorescent dye with high affinity for single-stranded DNA has become available. Though the dye has a good dynamic range, we found that dye and gel saturation compounded by the length difference between products introduced an asymmetrical error into the calculation of relative abundance. Avoiding several pitfalls, dye calibration could be used to correct the error. We also found that differences in the amplification efficiency of the two templates were not constant, but dependent on the initial template ratio, requiring a non-linear correction. Together these improvements gave us very consistent quantitative results, and thus advance our analysis of hormonal mechanisms underlying the regulation of alternative splicing of an ion channel critically involved in stress responses.

Phosphorylation-dependent functional coupling of hSlo calcium-dependent potassium channel and its hbeta 4 subunit

The auxiliary beta4 subunit of the human slowpoke calciumdependent potassium (slo) channel is expressed predominantly in the brain. Co-expression of beta4 subunit with the slo channel alpha subunit in HEK293 and Chinese hamster ovary cells slows channel activation and deactivation and also shifts the voltage dependence of the channel to more depolarized potentials. We show here that the functional interaction between the hbeta4 subunit and the slo channel is influenced by the phosphorylation state of hbeta4. Treatment of cells with okadaic acid (OA) reduces the effect of hbeta4 on slo channel activation kinetics and voltage dependence but not on slo channel deactivation kinetics. The effect of OA can be blocked by mutating three putative serine/threonine phosphorylation sites in hbeta4 (Thr-11/Ser-17/Ser-210) to alanines, suggesting that OA potentiates phosphorylation of hbeta4 and thereby suppresses its functional coupling to the slo channel. Mutation of Ser-17 alone to a negatively charged residue (S17E) can mimic the effect of OA. Mutating all three phosphorylation sites in hbeta4 to negatively charged residues (T11D/S17E/S210E) not only suppresses the effect of hbeta4 on slo channel activation kinetics and voltage dependence, it also suppresses its effect on slo channel deactivation kinetics. Co-immunoprecipitation/Western blot experiments indicate that all of these hbeta4 mutants, as well as the wild-type hbeta4, bind to the slo channel. Taken together, these data suggest that phosphorylation of the beta4 subunit dynamically regulates the functional coupling between the beta4 subunit and the pore-forming alpha subunit of the slo channel. In addition, phosphorylation of different residues in hbeta4 differentially influences its effects on slo channel activation kinetics, deactivation kinetics, and voltage dependence.

Molecular BKology: the study of splicing and dicing

The activity of ion channels is regulated by several different mechanisms. Studies have been undertaken to determine how the large-conductance Ca(2+)-sensitive K(+) channel (the BK channel) is regulated. Fury et al. discuss how the presence or absence of alternatively spliced regions in the BK channel alpha subunit can act as a molecular switch by which different kinases activate the BK channel.

Nitric oxide enhances Ca(2+)-dependent K(+) channel activity in rat carotid body cells

The nitric oxide (NO) donor S-nitroso-acetylpenicillamine (SNAP) enhanced Ca(2+)-dependent K(+) channel activity in rat carotid body chemoreceptor cells. Ca(2+)-dependent K(+) channel activity was enhanced by SNAP in 38% (whole-cell configuration) and 67% (cell-attached mode) of the cells tested and was not affected by intracellular Ca(2+) chelation with BAPTA-AM. Enhancement of Ca(2+)-dependent K(+) channel activity by SNAP was blocked by the cGMP-dependent protein kinase G inhibitor 8-[(4-chlorophenyl)thio]-guanosine 3',5'-cyclic monophosphothioate Rp diastereomer (Rp-8-pCPT-cGMPS). NO thus enhances Ca(2+)-dependent K(+) channel activity through cGMP-dependent protein kinase G. The NO-mediated increase in Ca(2+)-dependent K(+) channel activity is likely to alter the function of carotid body chemoreceptor cells and could explain the decreased chemosensitivity of the carotid body in response to NO released from efferent nerves or vascular endothelial cells.

Ca(2+)-activated K+ channel inhibition by reactive oxygen species

We studied the effect of H(2)O(2) on the gating behavior of large-conductance Ca(2+)-sensitive voltage-dependent K(+) (K(V,Ca)) channels. We recorded potassium currents from single skeletal muscle channels incorporated into bilayers or using macropatches of Xenopus laevis oocytes membranes expressing the human Slowpoke (hSlo) alpha-subunit. Exposure of the intracellular side of K(V,Ca) channels to H(2)O(2) (4-23 mM) leads to a time-dependent decrease of the open probability (P(o)) without affecting the unitary conductance. H(2)O(2) did not affect channel activity when added to the extracellular side. These results provide evidence for an intracellular site(s) of H(2)O(2) action. Desferrioxamine (60 microM) and cysteine (1 mM) completely inhibited the effect of H(2)O(2), indicating that the decrease in P(o) was mediated by hydroxyl radicals. The reducing agent dithiothreitol (DTT) could not fully reverse the effect of H(2)O(2). However, DTT did completely reverse the decrease in P(o) induced by the oxidizing agent 5,5'-dithio-bis-(2-nitrobenzoic acid). The incomplete recovery of K(V,Ca) channel activity promoted by DTT suggests that H(2)O(2) treatment must be modifying other amino acid residues, e.g., as methionine or tryptophan, besides cysteine. Noise analysis of macroscopic currents in Xenopus oocytes expressing hSlo channels showed that H(2)O(2) induced a decrease in current mediated by a decrease both in the number of active channels and P(o).