N-terminal isoforms of the large-conductance Ca²⁺-activated K⁺ channel are differentially modulated by the auxiliary β1-subunit

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

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

Ionic Channels as Potential Therapeutic Targets for Erectile Dysfunction: A Review

Erectile dysfunction (ED) is a prevalent condition, especially in men over 40 years old, characterized by the inability to obtain and/or maintain penile erection sufficient for satisfactory sexual intercourse. Several psychological and/or organic factors are involved in the etiopathogenesis of ED. In this context, we gathered evidence of the involvement of Large-conductance, Ca2+-activated K+ channels (BKCa), Small-conductance, Ca2+-activated K+ channels (SKCa), KCNQ-encoded voltage-dependent K+ channels (KV7), Transient Receptor Potential channels (TRP), and Calcium-activated Chloride channels (CaCC) dysfunctions on ED. In addition, the use of modulating agents of these channels are involved in relaxation of the cavernous smooth muscle cell and, consequent penile erection, suggesting that these channels are promising therapeutic targets for the treatment of erectile dysfunction.

The dominant models of KCNJ11 E23K and KCNMB1 E65K are associated with essential hypertension (EH) in Asian: Evidence from a meta-analysis

BACKGROUND

The K channel, subfamily J, member-11 (KCNJ11) E23K and β1 subunit of large-conductance Ca-activated K channel (KCNMB1) E65K polymorphisms were shown to be associated with the risk of essential hypertension (EH). However, the results were inconclusive with relatively small sample size. Thus, we carried out a meta-analysis to investigate the genetic association between KCNJ11 E23K and KCNMB1 E65K polymorphisms and essential hypertension risk.

METHODS

Relative studies were collected using PubMed, Web of Science, the Cochrane Library databases, Chinese National Knowledge Infrastructure and Embase databases. Pooled odds ratios with 95% confidence intervals were used to assess the strength of associations.

RESULTS

The dominant models of KCNJ11 E23K (P = .006, OR [95%CI] = 0.45 [0.25, 0.79]) and KCNMB1 E65K (P = .04, OR [95%CI] = 0.91 [0.83, 1.00]) were significantly associated with essential hypertension risk. No significant association was detected between the allelic and recessive models of KCNJ11 E23K and KCNMB1 E65K and the susceptibility of EH. Subgroup analysis stratified by ethnicity showed that the dominant model of KCNMB1 E65K was associated with EH risk in Asian population (P = .003, OR [95%CI] = 0.83 [0.74, 0.94]), but not in Caucasian (P = .74, OR [95%CI] = 1.02 [0.89, 1.18]).

CONCLUSIONS

The dominant model of KCNJ11 E23K and KCNMB1 E65K might be susceptible factors for essential hypertension. To confirm this result, large-scale case-control studies with more subjects are necessary.

Oxytocin can regulate myometrial smooth muscle excitability by inhibiting the Na+ -activated K+ channel, Slo2.1

KEY POINTS

At the end of pregnancy, the uterus transitions from a quiescent state to a highly contractile state. This transition requires that the uterine (myometrial) smooth muscle cells increase their excitability, although how this occurs is not fully understood. We identified SLO2.1, a potassium channel previously unknown in uterine smooth muscle, as a potential significant contributor to the electrical excitability of myometrial smooth muscle cells. We found that activity of the SLO2.1 channel is negatively regulated by oxytocin via Gαq-protein-coupled receptor activation of protein kinase C. This results in depolarization of the uterine smooth muscle cells and calcium entry, which may contribute to uterine contraction. These findings provide novel insights into a previously unknown mechanism by which oxytocin may act to modulate myometrial smooth muscle cell excitability. Our findings also reveal a new potential pharmacological target for modulating uterine excitability.

ABSTRACT

During pregnancy, the uterus transitions from a quiescent state to a more excitable contractile state. This is considered to be at least partly a result of changes in the myometrial smooth muscle cell (MSMC) resting membrane potential. However, the ion channels controlling the myometrial resting membrane potential and the mechanism of transition to a more excitable state have not been fully clarified. In the present study, we show that the sodium-activated, high-conductance, potassium leak channel, SLO2.1, is expressed and active at the resting membrane potential in MSMCs. Additionally, we report that SLO2.1 is inhibited by oxytocin binding to the oxytocin receptor. Inhibition of SLO2.1 leads to membrane depolarization and activation of voltage-dependent calcium channels, resulting in calcium influx. The results of the present study reveal that oxytocin may modulate MSMC electrical activity by inhibiting SLO2.1 potassium channels.

The large-conductance voltage- and Ca2+ -activated K+ channel and its γ1-subunit modulate mouse uterine artery function during pregnancy

KEY POINTS

The uterine artery (UA) markedly vasodilates during pregnancy to direct blood flow to the developing fetus. Inadequate UA vasodilatation leads to intrauterine growth restriction and fetal death. The large-conductance voltage- and Ca2+ -activated K+ (BKCa ) channel promotes UA vasodilatation during pregnancy. We report that BKCa channel activation increases the UA diameter at late pregnancy stages in mice. Additionally, a BKCa channel auxiliary subunit, γ1, participates in this process by increasing channel activation and inducing UA vasodilatation at late pregnancy stages. Our results highlight the importance of the BKCa channel and its γ1-subunit for UA functional changes during pregnancy.

ABSTRACT

Insufficient vasodilatation of the uterine artery (UA) during pregnancy leads to poor utero-placental perfusion, contributing to intrauterine growth restriction and fetal loss. Activity of the large-conductance Ca2+ -activated K+ (BKCa ) channel increases in the UA during pregnancy, and its inhibition reduces uterine blood flow, highlighting a role of this channel in UA adaptation to pregnancy. The auxiliary γ1-subunit increases BKCa activation in vascular smooth muscle, but its role in pregnancy-associated UA remodelling is unknown. We explored whether the BKCa and its γ1-subunit contribute to UA remodelling during pregnancy. Doppler imaging revealed that, compared to UAs from wild-type (WT) mice, UAs from BKCa knockout (BKCa-/- ) mice had lower resistance at pregnancy day 14 (P14) but not at P18. Lumen diameters were twofold larger in pressurized UAs from P18 WT mice than in those from non-pregnant mice, but this difference was not seen in UAs from BKCa-/- mice. UAs from pregnant WT mice constricted 20-50% in response to the BKCa blocker iberiotoxin (IbTX), whereas UAs from non-pregnant WT mice only constricted 15%. Patch-clamp analysis of WT UA smooth muscle cells confirmed that BKCa activity increased over pregnancy, showing three distinct voltage sensitivities. The γ1-subunit transcript increased 7- to 10-fold during pregnancy. Furthermore, γ1-subunit knockdown reduced IbTX sensitivity in UAs from pregnant mice, whereas γ1-subunit overexpression increased IbTX sensitivity in UAs from non-pregnant mice. Finally, at P18, γ1-knockout (γ1-/- ) mice had smaller UA diameters than WT mice, and IbTX-mediated vasoconstriction was prevented in UAs from γ1-/- mice. Our results suggest that the γ1-subunit increases BKCa activation in UAs during pregnancy.

The unique N-terminal sequence of the BKCa channel α-subunit determines its modulation by β-subunits

Large conductance voltage- and Ca²⁺-activated K⁺ (BK_Ca) channels are essential regulators of membrane excitability in a wide variety of cells and tissues. An important mechanism of modulation of BK_Ca channel activity is its association with auxiliary subunits. In smooth muscle cells, the most predominant regulatory subunit of BK_Ca channels is the β1-subunit. We have previously described that BK_Ca channels with distinctive N-terminal ends (starting with the amino acid sequence MDAL, MSSN or MANG) are differentially modulated by the β1-subunit, but not by the β2. Here we extended our studies to understand how the distinct N-terminal regions differentially modulate channel activity by β-subunits. We recorded inside-out single-channel currents from HEK293T cells co-expressing the BK_Ca containing three N-terminal sequences with two β1-β2 chimeric constructs containing the extracellular loop of β1 or β2, and the transmembrane and cytoplasmic domains of β2 or β1, respectively. Both β chimeric constructs induced leftward shifts of voltage-activation curves of channels starting with MANG and MDAL, in the presence of 10 or 100 μM intracellular Ca²⁺. However, MSSN showed no shift of the voltage-activation, at the same Ca²⁺ concentrations. The presence of the extracellular loop of β1 in the chimera resembled results seen with the full β1 subunit, suggesting that the extracellular region of β1 might be responsible for the lack of modulation observed in MSSN. We further studied a poly-serine stretch present in the N-terminal region of MSSN and observed that the voltage-activation curves of BK_Ca channels either containing or lacking this poly-serine stretch were leftward shifted by β1-subunit in a similar way. Overall, our results provide further insights into the mechanism of modulation of the different N-terminal regions of the BK_Ca channel by β-subunits and highlight the extension of this region of the channel as a form of modulation of channel activity.

Posttranscriptional and Posttranslational Regulation of BK Channels

Large conductance calcium- and voltage-activated potassium (BK) channels are ubiquitously expressed and play an important role in the regulation of an eclectic array of physiological processes. Their diverse functional role requires channels with a wide variety of properties even though the pore-forming α-subunit is encoded by a single gene, KCNMA1. To achieve this, BK channels exploit some of the most fundamental posttranscriptional and posttranslational mechanisms that allow proteomic diversity to be generated from a single gene. These include mechanisms that diversify mRNA variants and abundance such as alternative pre-mRNA splicing, editing, and control by miRNA. The BK channel is also subject to a diverse array of posttranslational modifications including protein phosphorylation, lipidation, glycosylation, and ubiquitination to control the number, properties, and regulation of BK channels in specific cell types. Importantly, "cross talk" between these posttranscriptional and posttranslational modifications typically converge on disordered domains of the BK channel α-subunit. This allows both wide physiological diversity to be generated and a diversity of mechanisms to allow conditional regulation of BK channels and is emerging as an important determinant of BK channel function in health and disease.

Functional insights into modulation of BKCa channel activity to alter myometrial contractility

The large-conductance voltage- and Ca(2+)-activated K(+) channel (BKCa) is an important regulator of membrane excitability in a wide variety of cells and tissues. In myometrial smooth muscle, activation of BKCa plays essential roles in buffering contractility to maintain uterine quiescence during pregnancy and in the transition to a more contractile state at the onset of labor. Multiple mechanisms of modulation have been described to alter BKCa channel activity, expression, and cellular localization. In the myometrium, BKCa is regulated by alternative splicing, protein targeting to the plasma membrane, compartmentation in membrane microdomains, and posttranslational modifications. In addition, interaction with auxiliary proteins (i.e., β1- and β2-subunits), association with G-protein coupled receptor signaling pathways, such as those activated by adrenergic and oxytocin receptors, and hormonal regulation provide further mechanisms of variable modulation of BKCa channel function in myometrial smooth muscle. Here, we provide an overview of these mechanisms of BKCa channel modulation and provide a context for them in relation to myometrial function.