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

Hypoxic Regulation of the Large-Conductance, Calcium and Voltage-Activated Potassium Channel, BK

Hypoxia is a condition characterized by a reduction of cellular oxygen levels derived from alterations in oxygen balance. Hypoxic events trigger changes in cell-signaling cascades, oxidative stress, activation of pro-inflammatory molecules, and growth factors, influencing the activity of various ion channel families and leading to diverse cardiovascular diseases such as myocardial infarction, ischemic stroke, and hypertension. The large-conductance, calcium and voltage-activated potassium channel (BK) has a central role in the mechanism of oxygen (O₂) sensing and its activity has been related to the hypoxic response. BK channels are ubiquitously expressed, and they are composed by the pore-forming α subunit and the regulatory subunits β (β1–β4), γ (γ1–γ4), and LINGO1. The modification of biophysical properties of BK channels by β subunits underly a myriad of physiological function of these proteins. Hypoxia induces tissue-specific modifications of BK channel α and β subunits expression. Moreover, hypoxia modifies channel activation kinetics and voltage and/or calcium dependence. The reported effects on the BK channel properties are associated with events such as the increase of reactive oxygen species (ROS) production, increases of intracellular Calcium ([Ca²⁺]_i), the regulation by Hypoxia-inducible factor 1α (HIF-1α), and the interaction with hemeproteins. Bronchial asthma, chronic obstructive pulmonary diseases (COPD), and obstructive sleep apnea (OSA), among others, can provoke hypoxia. Untreated OSA patients showed a decrease in BK-β1 subunit mRNA levels and high arterial tension. Treatment with continuous positive airway pressure (CPAP) upregulated β1 subunit mRNA level, decreased arterial pressures, and improved endothelial function coupled with a reduction in morbidity and mortality associated with OSA. These reports suggest that the BK channel has a role in the response involved in hypoxia-associated hypertension derived from OSA. Thus, this review aims to describe the mechanisms involved in the BK channel activation after a hypoxic stimulus and their relationship with disorders like OSA. A deep understanding of the molecular mechanism involved in hypoxic response may help in the therapeutic approaches to treat the pathological processes associated with diseases involving cellular hypoxia.

Oligodendrocytes, BK channels and remyelination

Oligodendrocytes wrap multiple lamellae of their membrane, myelin, around axons of the central nervous system (CNS), to improve impulse conduction. Myelin synthesis is specialised and dynamic, responsive to local neuronal excitation. Subtle pathological insults are sufficient to cause significant neuronal metabolic impairment, so myelin preservation is necessary to safeguard neural networks. Multiple sclerosis (MS) is the most prevalent demyelinating disease of the CNS. In MS, inflammatory attacks against myelin, proposed to be autoimmune, cause myelin decay and oligodendrocyte loss, leaving neurons vulnerable. Current therapies target the prominent neuroinflammation but are mostly ineffective in protecting from neurodegeneration and the progressive neurological disability. People with MS have substantially higher levels of extracellular glutamate, the main excitatory neurotransmitter. This impairs cellular homeostasis to cause excitotoxic stress. Large conductance Ca2 +-activated K + channels (BK channels) could preserve myelin or allow its recovery by protecting cells from the resulting excessive excitability. This review evaluates the role of excitotoxic stress, myelination and BK channels in MS pathology, and explores the hypothesis that BK channel activation could be a therapeutic strategy to protect oligodendrocytes from excitotoxic stress in MS. This could reduce progression of neurological disability if used in parallel to immunomodulatory therapies.

Oligodendrocytes, BK channels and the preservation of myelin

Oligodendrocytes wrap multiple lamellae of their membrane, myelin, around axons of the central nervous system (CNS), to improve impulse conduction. Myelin synthesis is specialised and dynamic, responsive to local neuronal excitation. Subtle pathological insults are sufficient to cause significant neuronal metabolic impairment, so myelin preservation is necessary to safeguard neural networks. Multiple sclerosis (MS) is the most prevalent demyelinating disease of the CNS. In MS, inflammatory attacks against myelin, proposed to be autoimmune, cause myelin decay and oligodendrocyte loss, leaving neurons vulnerable. Current therapies target the prominent neuroinflammation but are mostly ineffective in protecting from neurodegeneration and the progressive neurological disability. People with MS have substantially higher levels of extracellular glutamate, the main excitatory neurotransmitter. This impairs cellular homeostasis to cause excitotoxic stress. Large conductance Ca2 +-activated K + channels (BK channels) could preserve myelin or allow its recovery by protecting cells from the resulting excessive excitability. This review evaluates the role of excitotoxic stress, myelination and BK channels in MS pathology, and explores the hypothesis that BK channel activation could be a therapeutic strategy to protect oligodendrocytes from excitotoxic stress in MS. This could reduce progression of neurological disability if used in parallel to immunomodulatory therapies.

Hydrogen Sulfide Relaxes Human Uterine Artery via Activating Smooth Muscle BKCa Channels

Opening of large conductance calcium-activated and voltage-dependent potassium (BK_Ca) channels hyperpolarizes plasma membranes of smooth muscle (SM) to cause vasodilation, underling a key mechanism for mediating uterine artery (UA) dilation in pregnancy. Hydrogen sulfide (H₂S) has been recently identified as a new UA vasodilator, yet the mechanism underlying H₂S-induced UA dilation is unknown. Here, we tested whether H₂S activated BK_Ca channels in human UA smooth muscle cells (hUASMC) to mediate UA relaxation. Multiple BK_Ca subunits were found in human UA in vitro and hUASMC in vitro, and high β1 and γ1 proteins were localized in SM cells in human UA. Baseline outward currents, recorded by whole-cell and single-channel patch clamps, were significantly inhibited by specific BK_Ca blockers iberiotoxin (IBTX) or tetraethylammonium, showing specific BK_Ca activity in hUASMC. H₂S dose (NaHS, 1–1000 µM)-dependently potentiated BK_Ca currents and open probability. Co-incubation with a Ca²⁺ blocker nifedipine (5 µM) or a chelator (ethylene glycol-bis (β-aminoethyl ether)-N,N,N′,N′-tetraacetic acid (EGTA), 5 mM) did not alter H₂S-potentiated BK_Ca currents and open probability. NaHS also dose-dependently relaxed phenylephrine pre-constricted freshly prepared human UA rings, which was inhibited by IBTX. Thus, H₂S stimulated human UA relaxation at least partially via activating SM BK_Ca channels independent of extracellular Ca²⁺.

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

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

Control of anterior pituitary cell excitability by calcium-activated potassium channels

In anterior pituitary endocrine cells, large (BK), small (SK) and intermediate (IK) conductance calcium activated potassium channels are key determinants in shaping cellular excitability in a cell type- and context-specific manner. Indeed, these channels are targeted by multiple signaling pathways that stimulate or inhibit cellular excitability. BK channels can, paradoxically, both promote electrical bursting as well as terminate bursting and spiking dependent upon intrinsic BK channel properties and proximity to voltage gated calcium channels in somatotrophs, lactotrophs and corticotrophs. In contrast, SK channels are predominantly activated by calcium released from intracellular IP3-sensitive calcium stores and mediate membrane hyperpolarization in cells including gonadotrophs and corticotrophs. IK channels are predominantly expressed in corticotrophs where they limit membrane excitability. A major challenge for the future is to determine the cell-type specific molecular composition of calcium-activated potassium channels and how they control anterior pituitary hormone secretion as well as other calcium-dependent processes.

Tagging of Endogenous BK Channels with a Fluorogen-Activating Peptide Reveals β4-Mediated Control of Channel Clustering in Cerebellum

BK channels are critical regulators of neuronal activity, controlling firing, neurotransmitter release, cerebellar function, and BK channel mutations have been linked to seizure disorders. Modulation of BK channel gating is well characterized, regulated by accessory subunit interactions, intracellular signaling pathways, and membrane potential. In contrast, the role of intracellular trafficking mechanisms in controlling BK channel function, especially in live cells, has been less studied. Fluorogen-activating peptides (FAPs) are well-suited for trafficking and physiological studies due to the binding of malachite green (MG)-based dyes with sub-nanomolar affinity to the FAP, resulting in bright, photostable, far-red fluorescence. Cell-excluded MG dyes enable the selective tagging of surface protein and tracking through endocytic pathways. We used CRISPR to insert the FAP at the extracellular N-terminus of BKα in the first exon of its native locus, enabling regulation by the native promoter elements and tag incorporation into multiple splice isoforms. Motor coordination was found to be normal; however, BK channel expression seems to be reduced in some locations. Alternate start site selection or post-translational proteolytic processing resulted in incomplete FAP tagging of the BKα proteins in brain tissues. In Purkinje cell somata, FAP revealed BK channel clustering previously only observed by electron microscopy. Measurement of these clusters in β4^+/- and β4^-/- mice showed that puncta number and cluster fluorescence intensity on the soma are reduced in β4^-/- knockout animals. This novel mouse line provides a versatile fluorescent platform for studying endogenous BK channels in living and fixed tissues. Future studies could apply this line to ex vivo neuronal cultures to study live-cell channel trafficking.

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.

Large-conductance Ca2+ -activated K+ channel activation by apical P2Y receptor agonists requires hydrocortisone in differentiated airway epithelium

KEY POINTS

Hydrocortisone (HC) is required for activation of large-conductance Ca²⁺ -activated K⁺ current (BK) by purinergic receptor agonists. HC reduces insertion of the stress-regulated exon (STREX) in the KCNMA1 gene, permitting protein kinase C (PKC)-dependent channel activation. Overlapping and unique purinergic signalling regions exist at the apical border of differentiated surface cells. BK channels localize in the cilia of surface cells.

ABSTRACT

In the present study we investigated the role of hydrocortisone (HC) on uridine-5'-triphosphate (UTP)-stimulated ion transport in differentiated, pseudostratified epithelia derived from normal human bronchial basal cells. The presence of a UTP-stimulated, paxilline-sensitive large-conductance Ca²⁺ -activated K⁺ (BK) current was demonstrated in control epithelia but was not stimulated in epithelia differentiated in the absence of HC (HC0). Addition of the BK channel opener NS11021 directly activated channels in control epithelia; however, under HC0 conditions, activation only occurred when UTP was added after NS11021. The PKC inhibitors GF109203x and Gö6983 blocked BK activation by UTP in control epithelia, suggesting that PKC-mediated phosphorylation plays a permissive role in purinoceptor-stimulated BK activation. Moreover, HC0 epithelia expressed significantly more KCNMA1 containing the stress-regulated exon (STREX), a splice-variant of the α-subunit that displays altered channel regulation by phosphorylation, compared to control epithelia. Furthermore, BK channels as well as purinergic receptors were shown to localize in unique and overlapping domains at the apical membrane of ciliated surface cells. These results establish a previously unrecognized role for glucocorticoids in regulation of BK channels in airway epithelial cells.

Molecular Determinants of BK Channel Functional Diversity and Functioning

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

Down-Regulation of Ca2+-Activated K⁺ Channel KCa1.1 in Human Breast Cancer MDA-MB-453 Cells Treated with Vitamin D Receptor Agonists

Vitamin D (VD) reduces the risk of breast cancer and improves disease prognoses. Potential VD analogs are being developed as therapeutic agents for breast cancer treatments. The large-conductance Ca²⁺-activated K⁺ channel K_Ca1.1 regulates intracellular Ca²⁺ signaling pathways and is associated with high grade tumors and poor prognoses. In the present study, we examined the effects of treatments with VD receptor (VDR) agonists on the expression and activity of K_Ca1.1 in human breast cancer MDA-MB-453 cells using real-time PCR, Western blotting, flow cytometry, and voltage-sensitive dye imaging. Treatments with VDR agonists for 72 h markedly decreased the expression levels of K_Ca1.1 transcripts and proteins in MDA-MB-453 cells, resulting in the significant inhibition of depolarization responses induced by paxilline, a specific K_Ca1.1 blocker. The specific proteasome inhibitor MG132 suppressed VDR agonist-induced decreases in K_Ca1.1 protein expression. These results suggest that K_Ca1.1 is a new downstream target of VDR signaling and the down-regulation of K_Ca1.1 through the transcriptional repression of K_Ca1.1 and enhancement of K_Ca1.1 protein degradation contribute, at least partly, to the antiproliferative effects of VDR agonists in breast cancer cells.

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.

Alternatively Spliced Isoforms of KV10.1 Potassium Channels Modulate Channel Properties and Can Activate Cyclin-dependent Kinase in Xenopus Oocytes

K_V10.1 is a voltage-gated potassium channel expressed selectively in the mammalian brain but also aberrantly in cancer cells. In this study we identified short splice variants of K_V10.1 resulting from exon-skipping events (E65 and E70) in human brain and cancer cell lines. The presence of the variants was confirmed by Northern blot and RNase protection assays. Both variants completely lacked the transmembrane domains of the channel and produced cytoplasmic proteins without channel function. In a reconstituted system, both variants co-precipitated with the full-length channel and induced a robust down-regulation of K_V10.1 current when co-expressed with the full-length form, but their effect was mechanistically different. E65 required a tetramerization domain and induced a reduction in the overall expression of full-length K_V10.1, whereas E70 mainly affected its glycosylation pattern. E65 triggered the activation of cyclin-dependent kinases in Xenopus laevis oocytes, suggesting a role in cell cycle control. Our observations highlight the relevance of noncanonical functions for the oncogenicity of K_V10.1, which need to be considered when ion channels are targeted for cancer therapy.

Oxidative Stress and Maxi Calcium-Activated Potassium (BK) Channels

All cells contain ion channels in their outer (plasma) and inner (organelle) membranes. Ion channels, similar to other proteins, are targets of oxidative impact, which modulates ion fluxes across membranes. Subsequently, these ion currents affect electrical excitability, such as action potential discharge (in neurons, muscle, and receptor cells), alteration of the membrane resting potential, synaptic transmission, hormone secretion, muscle contraction or coordination of the cell cycle. In this chapter we summarize effects of oxidative stress and redox mechanisms on some ion channels, in particular on maxi calcium-activated potassium (BK) channels which play an outstanding role in a plethora of physiological and pathophysiological functions in almost all cells and tissues. We first elaborate on some general features of ion channel structure and function and then summarize effects of oxidative alterations of ion channels and their functional consequences.

Phosphorylation of BK channels modulates the sensitivity to hydrogen sulfide (H2S)

Introduction: Gases, such as nitric oxide (NO), carbon monoxide (CO), or hydrogen sulfide (H₂S), termed gasotransmitters, play an increasingly important role in understanding of how electrical signaling of cells is modulated. H₂S is well-known to act on various ion channels and receptors. In a previous study we reported that H₂S increased calcium-activated potassium (BK) channel activity.

Aims: The goal of the present study is to investigate the modulatory effect of BK channel phosphorylation on the action of H₂S on the channel as well as to recalculate and determine the H₂S concentrations in aqueous sodium hydrogen sulfide (NaHS) solutions.

Methods: Single channel recordings of GH3, GH4, and GH4 STREX cells were used to analyze channel open probability, amplitude, and open dwell times. H₂S was measured with an anion selective electrode.

Results: The concentration of H₂S produced from NaHS was recalculated taking pH, temperature salinity of the perfusate, and evaporation of H₂S into account. The results indicate that from a concentration of 300 μM NaHS, only 11–13%, i.e., 34–41 μM is effective as H₂S in solution. GH3, GH4, and GH4 STREX cells respond differently to phosphorylation. BK channel open probability (Po) of all cells lines used was increased by H₂S in ATP-containing solutions. PKA prevented the action of H₂S on channel Po in GH4 and GH4 STREX, but not in GH3 cells. H₂S, high significantly increased Po of all PKG pretreated cells. In the presence of PKC, which lowers channel activity, H₂S increased channel Po of GH4 and GH4 STREX, but not those of GH3 cells. H₂S increased open dwell times of GH3 cells in the absence of ATP significantly. A significant increase of dwell times with H₂S was also observed in the presence of okadaic acid.

Conclusions: Our results suggest that phosphorylation by PKG primes the channels for H₂S activation and indicate that channel phosphorylation plays an important role in the response to H₂S.

Large conductance voltage- and Ca2+-gated potassium (BK) channel β4 subunit influences sensitivity and tolerance to alcohol by altering its response to kinases

Tolerance is a well described component of alcohol abuse and addiction. The large conductance voltage- and Ca(2+)-gated potassium channel (BK) has been very useful for studying molecular tolerance. The influence of association with the β4 subunit can be observed at the level of individual channels, action potentials in brain slices, and finally, drinking behavior in the mouse. Previously, we showed that 50 mm alcohol increases both α and αβ4 BK channel open probability, but only α BK develops acute tolerance to this effect. Currently, we explore the possibility that the influence of the β4 subunit on tolerance may result from a striking effect of β4 on kinase modulation of the BK channel. We examine the influence of the β4 subunit on PKA, CaMKII, and phosphatase modulation of channel activity, and on molecular tolerance to alcohol. We record from human BK channels heterologously expressed in HEK 293 cells composed of its core subunit, α alone (Insertless), or co-expressed with the β4 BK auxiliary subunit, as well as, acutely dissociated nucleus accumbens neurons using the cell-attached patch clamp configuration. Our results indicate that BK channels are strongly modulated by activation of specific kinases (PKA and CaMKII) and phosphatases. The presence of the β4 subunit greatly influences this modulation, allowing a variety of outcomes for BK channel activity in response to acute alcohol.

Oxidative modulation of voltage-gated potassium channels

SIGNIFICANCE

Voltage-gated K+ channels are a large family of K+-selective ion channel protein complexes that open on membrane depolarization. These K+ channels are expressed in diverse tissues and their function is vital for numerous physiological processes, in particular of neurons and muscle cells. Potentially reversible oxidative regulation of voltage-gated K+ channels by reactive species such as reactive oxygen species (ROS) represents a contributing mechanism of normal cellular plasticity and may play important roles in diverse pathologies including neurodegenerative diseases.

RECENT ADVANCES

Studies using various protocols of oxidative modification, site-directed mutagenesis, and structural and kinetic modeling provide a broader phenomenology and emerging mechanistic insights.

CRITICAL ISSUES

Physicochemical mechanisms of the functional consequences of oxidative modifications of voltage-gated K+ channels are only beginning to be revealed. In vivo documentation of oxidative modifications of specific amino-acid residues of various voltage-gated K+ channel proteins, including the target specificity issue, is largely absent.

FUTURE DIRECTIONS

High-resolution chemical and proteomic analysis of ion channel proteins with respect to oxidative modification combined with ongoing studies on channel structure and function will provide a better understanding of how the function of voltage-gated K+ channels is tuned by ROS and the corresponding reducing enzymes to meet cellular needs.

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

The large-conductance Ca(2+)-activated K(+) (BK(Ca)) channel is essential for maintaining the membrane in a hyperpolarized state, thereby regulating neuronal excitability, smooth muscle contraction, and secretion. The BK(Ca) α-subunit has three predicted initiation codons that generate proteins with N-terminal ends starting with the amino acid sequences MANG, MSSN, or MDAL. Because the N-terminal region and first transmembrane domain of the α-subunit are required for modulation by auxiliary β1-subunits, we examined whether β1 differentially modulates the N-terminal BK(Ca) α-subunit isoforms. In the absence of β1, all isoforms had similar single-channel conductances and voltage-dependent activation. However, whereas β1 did not modulate the voltage-activation curve of MSSN, β1 induced a significant leftward shift of the voltage activation curves of both the MDAL and MANG isoforms. These shifts, of which the MDAL was larger, occurred at both 10 μM and 100 μM Ca(2+). The β1-subunit increased the open dwell times of all three isoforms and decreased the closed dwell times of MANG and MDAL but increased the closed dwell times of MSSN. The distinct modulation of voltage activation by the β1-subunit may be due to the differential effect of β1 on burst duration and interburst intervals observed among these isoforms. Additionally, we observed that the related β2-subunit induced comparable leftward shifts in the voltage-activation curves of all three isoforms, indicating that the differential modulation of these isoforms was specific to β1. These findings suggest that the relative expression of the N-terminal isoforms can fine-tune BK(Ca) channel activity in cells, highlighting a novel mechanism of BK(Ca) channel regulation.

A BK (Slo1) channel journey from molecule to physiology

Calcium and voltage-activated potassium (BK) channels are key actors in cell physiology, both in neuronal and non-neuronal cells and tissues. Through negative feedback between intracellular Ca (2+) and membrane voltage, BK channels provide a damping mechanism for excitatory signals. Molecular modulation of these channels by alternative splicing, auxiliary subunits and post-translational modifications showed that these channels are subjected to many mechanisms that add diversity to the BK channel α subunit gene. This complexity of interactions modulates BK channel gating, modifying the energetic barrier of voltage sensor domain activation and channel opening. Regions for voltage as well as Ca (2+) sensitivity have been identified, and the crystal structure generated by the 2 RCK domains contained in the C-terminal of the channel has been described. The linkage of these channels to many intracellular metabolites and pathways, as well as their modulation by extracellular natural agents, has been found to be relevant in many physiological processes. This review includes the hallmarks of BK channel biophysics and its physiological impact on specific cells and tissues, highlighting its relationship with auxiliary subunit expression.

Molecular heterogeneity of large-conductance calcium-activated potassium channels in canine intracardiac ganglia

Large conductance calcium-activated potassium (BK) channels are widely expressed in the nervous system. We have recently shown that principal neurons from canine intracardiac ganglia (ICG) express a paxilline- and TEA-sensitive BK current, which increases neuronal excitability. In the present work, we further explore the molecular constituents of the BK current in canine ICG. We found that the β1 and β4 regulatory subunits are expressed in ICG. Single channel voltage-dependence at different calcium concentrations suggested that association of the BKα with a particular β subunit was not enough to explain the channel activity in this tissue. Indeed, we detected the presence of several splice variants of the BKα subunit. In conclusion, BK channels in canine ICG may result from the arrangement of different BKα splice variants, plus accessory β subunits. The particular combinations expressed in canine IC neurons likely rule the excitatory role of BK current in this tissue.

Splicing of the rSlo gene affects the molecular composition and drug response of Ca2+-activated K+ channels in skeletal muscle

The molecular composition and drug responses of calcium-activated K⁺ (BK) channels of skeletal muscle are unknown. Patch-clamp experiments combined with transcript scanning of the Kcnma1 gene encoding the alpha subunit of the BK channel were performed in rat slow-twitch soleus (Sol) and fast-twitch flexor digitorum brevis (FDB) skeletal muscles. Five splicing products of the Kcnma1 gene were isolated from Sol and FDB: the e17, e22, +29 aa, Slo27 and Slo0 variants. RT-PCR analysis demonstrated that the expression of e22 and Slo0 were 80–90% higher in FDB than Sol, whereas the expression of Slo27 was 60% higher in Sol than FDB, and the +29 aa variant was equally expressed in both muscle types. No beta 1-4 subunits were detected. In Sol, a large BK current with low Ca²⁺ sensitivity was recorded. The BK channel of Sol also showed a reduced response to BK channel openers, such as NS1619, acetazolamide and related drugs. In FDB, a reduced BK current with high Ca²⁺ sensitivity and an enhanced drug response was recorded. The total BK RNA content, which was 200% higher in Sol than in FDB, correlated with the BK currents in both muscles. Drug responses primarily correlated with e22 and Slo0 expression levels in FDB and to Slo27 expression in Sol muscle. In conclusion, phenotype-dependent BK channel biophysical and pharmacological properties correlated with the expression levels of the variants in muscles. These findings may be relevant to conditions affecting postural muscles, such as prolonged bed-rest, and to diseases affecting fast-twitch muscles, such as periodic paralysis. Down-regulation or up-regulation of the variants associated with pathological conditions may affect channel composition and drug responses.

Cerebroside-A provides potent neuroprotection after cerebral ischaemia through reducing glutamate release and Ca²⁺ influx of NMDA receptors

Excessive presynaptic glutamate release after cerebral ischaemia leads to neuronal death mainly through excessive calcium entry of N-methyl-D-aspartate receptors (NMDARs). Our recent study reported that cerebroside can open large-conductance Ca²⁺-activated K⁺ (BKCa) channels. The present study evaluated the effects of cerebroside-A (CS-A), a single molecule isolated from an edible mushroom, on brain injury after focal or global ischaemia in adult male mice and rats. We herein report that treatment with CS-A after 60-min middle cerebral artery occlusion dose-dependently reduced the cerebral infarction with at least a 6-h efficacious time-window, which was partially blocked by the BKCa channel blocker charybdotoxin (CTX). Treatment with CS-A after 20 min global cerebral ischaemia (four-vessel occlusion) significantly attenuated the death of pyramidal cells in hippocampal CA1 area, which was also sensitive to CTX. CS-A, by opening the BKCa channel, could prevent excessive glutamate release after oxygen-glucose deprivation (OGD). In addition, CS-A could inhibit NMDAR Ca²⁺ influx, which did not require the activation of the BKCa channel. Furthermore, CS-A blocked the OGD-induced NMDAR-dependent long-term potentiation in hippocampal CA1 region. These findings indicate that treatment with CS-A after stroke exerts potent neuroprotection through prevention of excessive glutamate release and reduction of Ca²⁺ influx through NMDARs.

Interactions between intracellular free Ca2+ and cyclic AMP in neuroendocrine cells

Calcium ions and cyclic adenosine monophosphate (cAMP) are virtually ubiquitous intracellular signaling molecules in mammalian cells. This paper will focus on the cross-talk between Ca(2+) and cAMP mobilizing signaling pathways and summarize the underlying molecular mechanisms. Subsequently, workings of adenohypophyseal corticotrope cells will be reviewed to highlight the physiological relevance of a Ca(2+) cAMP interactions in neuroendocrinology.

Identification and quantification of full-length BK channel variants in the developing mouse cochlea

Maxi-K(+) (BK) channel diversity is attributed to alternative splicing in the kcnma1 gene. The resultant variants manifest themselves in different cell types, tissues, and functions, such as excitation, metabolism, and signaling. Immunoelectron microscopy revealed immunogold particle labeling of BK in apical and basal regions of inner and outer hair cells, respectively. Additional labeling occurs in Deiters' cells and the inner mitochondrial membrane. Identification of full-length sequences reveals 27 BK variants from embryonic and postnatal mouse inner ear, per classification by tail motif, VYR, DEC, and ERL, and by exon usage. Three predicted start codons are found encoding MAN, MSS, and MDA, of which MDA shows the greatest expression through all stages in development, whereas MAN is undetectable. Complex splice sites occur between exons 9 and 10 and between 21 and 23. Spliced-in/out exons between 8 and 10 reveal a short fragment composed of exons 8 + 10, detectable on postnatal day (PD) 14 and PD30, and a longer fragment composed of exons 8 + 9 + 10 that is upregulated on embryonic day (ED) 14. Spliced-in exons 22 or 23 are expressed on ED14 but decrease over time; however, exon 22 increases again on PD34. Using tail-specific primers, qRT-PCR from ED14, PD4, -14, and -30 shows that BK-VYR and -ERL dominate expression on ED14, whereas DEC dominates after birth in all cochlear regions. The localization of BK and the changes in expression of its exons and tail types, by alternative splicing during development, may contribute to cochlear organization, acquisition of hearing, and intracellular signaling.

Baifuzi reduces transient ischemic brain damage through an interaction with the STREX domain of BKCa channels

Stroke is a long-term disability and one of the leading causes of death. However, no successful therapeutic intervention is available for the majority of stroke patients. In this study, we explored a traditional Chinese medicine Baifuzi (Typhonium giganteum Engl.). We show, at first, that the ethanol extract of Baifuzi exerts neuroprotective effects against brain damage induced by transient global or focal cerebral ischemia in rats and mice. Second, the extract activated large-conductance Ca(2+)-activated K(+) channel (BK(Ca)) channels, and BK(Ca) channel blockade suppressed the neuroprotection of the extract, suggesting that the BK(Ca) is the molecular target of Baifuzi. Third, Baifuzi cerebroside (Baifuzi-CB), purified from its ethanol extract, activated BK(Ca) channels in a manner similar to that of the extract. Fourth, the stress axis hormone-regulated exon (STREX) domain of the BK(Ca) channel directly interacted with Baifuzi-CB, and its deletion suppressed channel activation by Baifuzi-CB. These results indicate that Baifuzi-CB activated the BK(Ca) channel through its direct interaction with the STREX domain of the channel and suggests that Baifuzi-CB merits exploration as a potential therapeutic agent for treating brain ischemia.

Upregulation of STREX splice variant of the large conductance Ca2+-activated potassium (BK) channel in a rat model of mesial temporal lobe epilepsy

Functional properties of large conductance Ca(2+) activated potassium (BK) channels are determined by complex alternative splicing of the Kcnma1 gene encoding the alpha pore-forming subunit. Inclusion of the STREX exon in a C-terminal splice site is dynamically regulated and confers enhanced Ca(2+) sensitivity and channel inhibition via cAMP-dependent phosphorylation. Here, we describe a real time quantitative PCR (qPCR) approach to investigate relative changes in the expression of STREX and ZERO splice variants using a newly designed set of probes and primers for TaqMan-based qPCR analysis of cDNA from the rat dentate gyrus at different time points following pilocarpine-induced status epilepticus. Reduction in Kcnma1 gene expression is associated with a relative increase of STREX splice variant. Relative expression of STREX variant mRNA was increased at 10 days and at more than 1 month following status epilepticus. The biological consequences of seizure-related changes in alternative splicing of Kcnma1 deserve additional investigation.

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.

Temporal and spatial mouse brain expression of cereblon, an ionic channel regulator involved in human intelligence

A mild form of autosomal recessive, nonsyndromal intellectual disability (ARNSID) in humans is caused by a homozygous nonsense mutation in the cereblon gene (mutCRBN). Rodent crbn protein binds to the intracellular C-terminus of the large conductance Ca(2+)-activated K(+)channel (BK(Ca)). An mRNA variant (human SITE 2 INSERT or mouse strex) of the BK(Ca) gene (KCNMA1) that is normally expressed during embryonic development is aberrantly expressed in mutCRBN human lymphoblastoid cell lines (LCLs) as compared to wild-type (wt) LCLs. The present study analyzes the temporal and spatial distribution of crbn and kcnma1 mRNAs in the mouse brain by the quantitative real-time reverse transcriptase-polymerase chain reaction (qPCR). The spatial expression pattern of endogenous and exogenous crbn proteins is characterized by immunostaining. The results show that neocortical (CTX) crbn and kcnma1 mRNA expression increases from embryonic stages to adulthood. The strex mRNA variant is >3.5-fold higher in embryos and decreases rapidly postnatally. Mouse crbn mRNA is abundant in the cerebellum (CRBM), with less expression in the CTX, hippocampus (HC), and striatum (Str) in adult mice. The intracytoplasmic distribution of endogenous crbn protein in the mouse CRBM, CTX, HC, and Str is similar to the immunostaining pattern described previously for the BK(Ca) channel. Exogenous hemagglutinin (HA) epitope-tagged human wt- and mutCRBN proteins using cDNA transfection in HEK293T cell lines showed the same intracellular expression distribution as endogenous mouse crbn protein. The results suggest that mutCRBN may cause ARNSID by disrupting the developmental regulation of BK(Ca) in brain regions that are critical for memory and learning.

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.

BK channel and alcohol, a complicated affair

Alcohol is a fast acting molecule that alters behavior within a few minutes of absorption. Its rapid behavioral impact suggests early action on ion channels. Of all voltage-gated potassium ion channels, BK channels, a subcategory of potassium channels characterized by their large unitary conductance, and by their capacity of being activated synergistically by membrane potential and intracellular free calcium, are unique due to their high sensitivity to alcohol. In this review, we discuss BK channels structure and function, and how they help us understand the various ways BK channel mediates alcohol's effects on neuronal function and on behavior in the striatum.

The role of microRNAs in drug addiction: a big lesson from tiny molecules

Alcoholism is a multifactorial disease of unclear molecular underpinnings. Currently, we are witnessing a major shift in our understanding of the functional elements of the genome, which could help us to discover novel insights into the nature of alcoholism. In humans, the vast majority of the genome encodes non-protein-coding DNA with unclear function. Recent research has started to unveil this mystery by describing the functional relevance of microRNAs, and examining which genes are regulated by non-protein-coding DNA. Here, I describe alcohol regulation of microRNAs and provide examples of microRNAs that control the expression of alcohol-relevant genes. Emphasis is put on the potential of microRNAs in explaining the polygenic nature of alcoholism and prospects of microRNA research and future directions of this burgeoning field.

Convergent evolution of alternative splices at domain boundaries of the BK channel

Alternative splicing is a widespread mechanism for generating transcript diversity in higher eukaryotic genomes. The alternative splices of the large-conductance calcium-activated potassium (BK) channel have been the subject of a good deal of experimental functional characterization in the Arthropoda, Chordata, and Nematoda phyla. In this review, we examine a list of splices of the BK channel by manual curation of Unigene clusters mapped to mouse, human, chicken, Drosophila, and Caenorhabditis elegans genomes. We find that BK alternative splices do not appear to be conserved across phyla. Despite this lack of conservation, splices occur in both vertebrates and invertebrates at identical regions of the channel at experimentally established domain boundaries. The fact that, across phyla, unique splices occur at experimentally established domain boundaries suggests a prominent role for the convergent evolution of alternative splices that produce functional changes via changes in interdomain communication.

Modulation of BKCa channel gating by endogenous signaling molecules

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

Cloning of potassium channel splice variants from tissues and cells

Potassium channels display considerable functional diversity. Alternative pre-mRNA splicing represents one of the most powerful post-transcriptional mechanisms to create physiological diversity by generating multiple protein products from a single gene. Due to the modular nature of proteins, alternative splicing can profoundly modify potassium channel structure, function and regulation. Alternative pre-mRNA splicing is exploited by most genes but is particularly prevalent in single gene families as exemplified by the gene (KCNMA1), which encodes large conductance calcium- and voltage-gated potassium (BK) channel alpha-subunits. Importantly, alternative pre-mRNA splicing is kept under spatiotemporal control by circulating hormones and cellular activity, as well as being differentially modified during development and in different tissues. While the sequencing of numerous genomes has further demonstrated the importance of splicing in generating diversity from a limited genome size, a major challenge is to define splice variants that are expressed in tissues and their functional role. Here we describe strategies and protocols to experimentally define and isolate splice variant mRNA transcripts in multiple tissues and provide a platform to characterise the effect of splice variants on channel function and physiology.

Characterization of BK channel splice variants using membrane potential dyes

BACKGROUND AND PURPOSE

Large conductance calcium- and voltage-activated potassium (BK) channels are encoded by a single gene that displays extensive pre-mRNA splicing. Here we exploited a membrane potential assay to investigate the sensitivity of different BK splice variants to elevations in intracellular free calcium and their inhibition by the BK channel blocker paxilline.

EXPERIMENTAL APPROACH

Murine BK channel splice variants were expressed in human embryonic kidney 293 cells and their properties analysed in response to ionomycin-induced calcium influx in both fluorescent membrane potential (fluorescent-imaging plate reader) and patch clamp electrophysiological assays. The dose-dependent inhibition of distinct splice variants by the BK channel-specific blocker paxilline was also investigated.

KEY RESULTS

Ionomycin-induced calcium influx induced a robust hyperpolarization of human embryonic kidney 293 cells expressing distinct BK channel splice variants: stress regulated exon (STREX), e22 and ZERO. Splice variant expression resulted in membrane hyperpolarization that displayed a rank order of potency in response to calcium influx of STREX > e22 > ZERO. The BK channel inhibitor paxilline exhibited very similar potency on all three splice variants with IC(50)s in membrane potential assays of 0.35 +/- 0.04, 0.37 +/- 0.03 and 0.70 +/- 0.02 micromol x L(-1) for STREX, ZERO and e22 respectively.

CONCLUSIONS AND IMPLICATIONS

BK channel splice variants can be rapidly discriminated using membrane potential based assays, based on their sensitivity to calcium. BK channel splice variants are inhibited by the specific blocker paxilline with similar IC(50)s. Thus, paxilline may be used in functional assays to inhibit BK channel function, irrespective of the variant expressed.

Control of alternative pre-mRNA splicing by Ca(++) signals

Alternative pre-mRNA splicing is a common way of gene expression regulation in metazoans. The selective use of specific exons can be modulated in response to various manipulations that alter Ca(++) signals, particularly in neurons. A number of splicing factors have also been found to be controlled by Ca(++) signals. Moreover, pre-mRNA elements have been identified that are essential and sufficient to mediate Ca(++)-regulated splicing, providing model systems for dissecting the involved molecular components. In neurons, this regulation likely contributes to the fine-tuning of neuronal properties.

Dysregulation of large-conductance Ca2+-activated K+ channel expression in nonsyndromal mental retardation due to a cereblon p.R419X mutation

A nonsense mutation (R419X) in the human cereblon gene [mutation (mut) CRBN] causes a mild type of autosomal recessive nonsyndromal mental retardation (ARNSMR). CRBN, a cytosolic protein, regulates the assembly and neuronal surface expression of large-conductance Ca(2+)-activated K(+) channels (BK(Ca)) in brain regions involved in memory and learning. Using the real-time quantitative polymerase chain reaction, we show that mut CRBN disturbs the development of adult brain BK(Ca) isoforms. These changes are predicted to result in BK(Ca) channels with a higher intracellular Ca(2+) sensitivity, faster activation, and slower deactivation kinetics. Such alterations may contribute to cognitive impairments in patients with mild ARNSMR.

Regulation of STREX exon large conductance, calcium-activated potassium channels by the beta4 accessory subunit

Large conductance (BK-type) calcium-activated potassium channels utilize alternative splicing and association with accessory beta subunits to tailor BK channel properties to diverse cell types. Two important modulators of BK channel gating are the neuronal-specific beta4 accessory subunit (beta4) and alternative splicing at the stress axis hormone-regulated exon (STREX). Individually, these modulators affect the gating properties of the BK channel as well as its response to phosphorylation. In this study, the combined functional consequences of STREX and the mouse beta4 subunit on mouse BK channel biophysical properties were investigated in transfected HEK 293 cells. Surprisingly, we found that the combined effects of STREX and beta4 are non-additive and even opposite for some properties. At high calcium, beta4 and the STREX individually share properties that promote BK channel opening via slowing of deactivation. However, the combined effects are a speeding of deactivation and a decreased open probability. beta4 also inhibits BK channel opening by a slowing of activation. This effect occurs across calcium concentrations in the absence of STREX, but predominates only at low calcium for STREX containing channels. BK channel responses to phosphorylation status are also altered by the combination of the beta4 subunit and STREX. beta4/STREX channels show a slowing of activation kinetics following dephosphorylation whereas beta4 channels lacking STREX do not. In contrast, beta4 confers a speeding of activation in response to cyclic AMP-dependent phosphorylation in channels lacking STREX, but not in channels containing STREX. These results indicate that the combination of the beta4 subunit and STREX confers non-additive and unique properties to BK channels. Analysis of expression in brain slices suggests that STREX and beta4 mRNA overlap expression in the dentate gyrus of the hippocampus and the cerebellar Purkinje cells, suggesting that these unique properties of BK channels may underlie BK channel gating in these cells.

Alternatively spliced C-terminal domains regulate the surface expression of large conductance calcium-activated potassium channels

The Slo1 gene, also known as KCNMA1, encodes the pore-forming subunits of large-conductance Ca2+-activated K+ (BK(Ca)) channels. Products of this gene are widely expressed in vertebrate tissues, and occur in a large number (>or=20) of alternatively spliced variants that vary in their gating properties, susceptibility to modulation, and trafficking to the plasma membrane. Motifs in the large cytoplasmic C-terminal are especially important in determining the functional properties of BK(Ca) channels. Here we report that chick ciliary ganglion neurons express transcripts and proteins of two Slo1 splice variants that differ at the extreme C-terminal. We refer to these variants as VEDEC and QEDRL (or QEERL for the orthologous mammalian versions), after the five terminal amino acid residues in each isoform. Individual ciliary ganglion neurons preferentially express these variants in different subcellular compartments. Moreover, QEERL channels show markedly higher levels of constitutive expression on the plasma membrane than VEDEC channels in HEK293T and NG108-15 cells. However, growth factor treatment can stimulate surface expression of VEDEC channels to levels comparable to those seen with QEERL. In addition, we show that co-expression of a soluble protein composed of VEDEC C-terminal tail residues markedly increases cell surface expression of full-length VEDEC channels, suggesting that this region binds to proteins that cause retention of the these channels in intracellular stores.

An S6 mutation in BK channels reveals beta1 subunit effects on intrinsic and voltage-dependent gating

Large conductance, Ca(2+)- and voltage-activated K(+) (BK) channels are exquisitely regulated to suit their diverse roles in a large variety of physiological processes. BK channels are composed of pore-forming alpha subunits and a family of tissue-specific accessory beta subunits. The smooth muscle-specific beta1 subunit has an essential role in regulating smooth muscle contraction and modulates BK channel steady-state open probability and gating kinetics. Effects of beta1 on channel's gating energetics are not completely understood. One of the difficulties is that it has not yet been possible to measure the effects of beta1 on channel's intrinsic closed-to-open transition (in the absence of voltage sensor activation and Ca(2+) binding) due to the very low open probability in the presence of beta1. In this study, we used a mutation of the alpha subunit (F315Y) that increases channel openings by greater than four orders of magnitude to directly compare channels' intrinsic open probabilities in the presence and absence of the beta1 subunit. Effects of beta1 on steady-state open probabilities of both wild-type alpha and the F315Y mutation were analyzed using the dual allosteric HA model. We found that mouse beta1 has two major effects on channel's gating energetics. beta1 reduces the intrinsic closed-to-open equilibrium that underlies the inhibition of BK channel opening seen in submicromolar Ca(2+). Further, P(O) measurements at limiting slope allow us to infer that beta1 shifts open channel voltage sensor activation to negative membrane potentials, which contributes to enhanced channel opening seen at micromolar Ca(2+) concentrations. Using the F315Y alpha subunit with deletion mutants of beta1, we also demonstrate that the small N- and C-terminal intracellular domains of beta1 play important roles in altering channel's intrinsic opening and voltage sensor activation. In summary, these results demonstrate that beta1 has distinct effects on BK channel intrinsic gating and voltage sensor activation that can be functionally uncoupled by mutations in the intracellular domains.

Tissue-specific alternative splicing of BK channel transcripts in Drosophila

BK-type calcium-activated potassium channels are large conductance channels that respond to changes in intracellular calcium and membrane potential. These channels are used in a wide variety of cell types and have recently been linked to drug sensitivity and tolerance. In both Drosophila and mammals, BK channels are encoded by the slowpoke gene. The Drosophila slowpoke gene includes 14 alternative exons distributed among five sites of alternative splicing. Presumably, the purpose of alternative processing is to provide transcripts tailored to the needs of the cell. The slowpoke gene is expressed in nervous, muscle and epithelial tissues. To determine whether splicing is controlled in a tissue- and/or developmental-specific manner, we built tissue- and developmental-specific cDNA libraries that preserved the relative frequency of various slowpoke splice variants. These libraries were screened by colony hybridization using alternative exon-specific DNA probes to document the frequency of individual alternative exons in different developmental stages and distinct tissue types. We demonstrate that slowpoke transcripts undergo tissue- and developmental-specific splicing in Drosophila and some exons are diagnostic for specific tissues.

Increased large conductance calcium-activated potassium (BK) channel expression accompanied by STREX variant downregulation in the developing mouse CNS

Background

Large conductance calcium- and voltage activated potassium (BK) channels are important determinants of neuronal excitability through effects on action potential duration, frequency and synaptic efficacy. The pore- forming subunits are encoded by a single gene, KCNMA1, which undergoes extensive alternative pre mRNA splicing. Different splice variants can confer distinct properties on BK channels. For example, insertion of the 58 amino acid stress-regulated exon (STREX) insert, that is conserved throughout vertebrate evolution, encodes channels with distinct calcium sensitivity and regulation by diverse signalling pathways compared to the insertless (ZERO) variant. Thus, expression of distinct splice variants may allow cells to differentially shape their electrical properties during development. However, whether differential splicing of BK channel variants occurs during development of the mammalian CNS has not been examined.

Results

Using quantitative real-time polymerase chain reaction (RT-PCR) Taqman™ assays, we demonstrate that total BK channel transcripts are up regulated throughout the murine CNS during embryonic and postnatal development with regional variation in transcript levels. This upregulation is associated with a decrease in STREX variant mRNA expression and an upregulation in ZERO variant expression.

Conclusion

As BK channel splice variants encode channels with distinct functional properties the switch in splicing from the STREX phenotype to ZERO phenotype during embryonic and postnatal CNS development may provide a mechanism to allow BK channels to control distinct functions at different times of mammalian brain development.

BK channel beta1-subunit regulation of calcium handling and constriction in tracheal smooth muscle

The large-conductance, Ca2+-activated K+ (BK) channels are regulators of voltage-dependent Ca2+ entry in many cell types. The BK channel accessory beta1-subunit promotes channel activation in smooth muscle and is required for proper tone in the vasculature and bladder. However, although BK channels have also been implicated in airway smooth muscle function, their regulation by the beta1-subunit has not been investigated. Utilizing the gene-targeted mice for the beta1-subunit gene, we have investigated the role of the beta1-subunit in tracheal smooth muscle. In mice with the beta1-subunit-knockout allele, BK channel activity was significantly reduced in excised tracheal smooth muscle patches and spontaneous BK currents were reduced in whole tracheal smooth muscle cells. Knockout of the beta1-subunit resulted in an increase in resting Ca2+ levels and an increase in the sustained component of Ca2+ influx after cholinergic signaling. Tracheal constriction studies demonstrate that the level of constriction is the same with knockout of the beta1-subunit and BK channel block with paxillin, indicating that BK channels contribute little to airway relaxation in the absence of the beta1-subunit. Utilizing nifedipine, we found that the increased constriction caused by knockout of the beta1-subunit could be accounted for by an increased recruitment of L-type voltage-dependent Ca2+ channels. These results indicate that the beta1-subunit is required in airway smooth muscle for control of voltage-dependent Ca2+ influx during rest and after cholinergic signaling in BK channels.

BKCa Channels Activating at Resting Potential without Calcium in LNCaP Prostate Cancer Cells

Large-conductance Ca2+-dependent K+ (BK(Ca)) channels are activated by intracellular Ca2+ and membrane depolarization in an allosteric manner. We investigated the pharmacological and biophysical characteristics of a BK(Ca)-type K+ channel in androgen-dependent LNCaP (lymph node carcinoma of the prostate) cells with novel functional properties, here termed BK(L). K+ selectivity, high conductance, activation by Mg2+ or NS1619, and inhibition by paxilline and penitrem A largely resembled the properties of recombinant BK(Ca) channels. However, unlike conventional BK(Ca) channels, BK(L) channels activated in the absence of free cytosolic Ca2+ at physiological membrane potentials; the half-maximal activation voltage was shifted by about -100 mV compared with BK(Ca) channels. Half-maximal Ca2+-dependent activation was observed at 0.4 microM: for BK(L) (at -20 mV) and at 4.1 microM: for BK(Ca) channels (at +50 mV). Heterologous expression of hSlo1 in LNCaP cells increased the BK(L) conductance. Expression of hSlo-beta1 in LNCaP cells shifted voltage-dependent activation to values between that of BK(L) and BK(Ca) channels and reduced the slope of the P (open) (open probability)-voltage curve. We propose that LNCaP cells harbor a so far unknown type of BK(Ca) subunit, which is responsible for the BK(L) phenotype in a dominant manner. BK(L)-like channels are also expressed in the human breast cancer cell line T47D. In addition, functional expression of BK(L) in LNCaP cells is regulated by serum-derived factors, however not by androgens.

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

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

A cysteine-rich motif confers hypoxia sensitivity to mammalian large conductance voltage- and Ca-activated K (BK) channel alpha-subunits

Cellular responses to hypoxia are tissue-specific and dynamic. However, the mechanisms that underlie this differential sensitivity to hypoxia are unknown. Large conductance voltage- and Ca-activated K (BK) channels are important mediators of hypoxia responses in many systems. Although BK channels are ubiquitously expressed, alternative pre-mRNA splicing of the single gene encoding their pore-forming alpha-subunits provides a powerful mechanism for generating functional diversity. Here, we demonstrate that the hypoxia sensitivity of BK channel alpha-subunits is splice-variant-specific. Sensitivity to hypoxia is conferred by a highly conserved motif within an alternatively spliced cysteine-rich insert, the stress-regulated exon (STREX), within the intracellular C terminus of the channel. Hypoxic inhibition of the STREX variant is Ca-sensitive and reversible, and it rapidly follows the change in oxygen tension by means of a mechanism that is independent of redox or CO regulation. Hypoxia sensitivity was abolished by mutation of the serine (S24) residue within the STREX insert. Because STREX splice-variant expression is tissue-specific and dynamically controlled, alternative splicing of BK channels provides a mechanism to control the plasticity of cellular responses to hypoxia.

Functionally Diverse Complement of Large Conductance Calcium- and Voltage-activated Potassium Channel (BK) α-Subunits Generated from a Single Site of Splicing

The pore-forming alpha-subunits of large conductance calcium- and voltage-activated potassium (BK) channels are encoded by a single gene that undergoes extensive alternative pre-mRNA splicing. However, the extent to which differential exon usage at a single site of splicing may confer functionally distinct properties on BK channels is largely unknown. Here we demonstrated that alternative splicing at site of splicing C2 in the mouse BK channel C terminus generates five distinct splice variants: ZERO, e20, e21(STREX), e22, and a novel variant deltae23. Splice variants display distinct patterns of tissue distribution with e21(STREX) expressed at the highest levels in adult endocrine tissues and e22 at embryonic stages of mouse development. deltae23 is not functionally expressed at the cell surface and acts as a dominant negative of cell surface expression by trapping other BK channel splice variant alpha-subunits in the endoplasmic reticulum and perinuclear compartments. Splice variants display a range of biophysical properties. e21(STREX) and e22 variants display a significant left shift (>20 mV at 1 microM [Ca2+]i) in half-maximal voltage of activation compared with ZERO and e20 as well as considerably slower rates of deactivation. Splice variants are differentially sensitive to phosphorylation by endogenous cAMP-dependent protein kinase; ZERO, e20, and e22 variants are all activated, whereas e21 (STREX) is the only variant that is inhibited. Thus alternative pre-mRNA splicing from a single site of splicing provides a mechanism to generate a physiologically diverse complement of BK channel alpha-subunits that differ dramatically in their tissue distribution, trafficking, and regulation.

Cysteine modification alters voltage- and Ca(2+)-dependent gating of large conductance (BK) potassium channels

The Ca²⁺-activated K⁺ (BK) channel α-subunit contains many cysteine residues within its large COOH-terminal tail domain. To probe the function of this domain, we examined effects of cysteine-modifying reagents on channel gating. Application of MTSET, MTSES, or NEM to mSlo1 or hSlo1 channels changed the voltage and Ca²⁺ dependence of steady-state activation. These reagents appear to modify the same cysteines but have different effects on function. MTSET increases I_K and shifts the G_K–V relation to more negative voltages, whereas MTSES and NEM shift the G_K–V in the opposite direction. Steady-state activation was altered in the presence or absence of Ca²⁺ and at negative potentials where voltage sensors are not activated. Combinations of [Ca²⁺] and voltage were also identified where P_o is not changed by cysteine modification. Interpretation of our results in terms of an allosteric model indicate that cysteine modification alters Ca²⁺ binding and the relative stability of closed and open conformations as well as the coupling of voltage sensor activation and Ca²⁺ binding and to channel opening. To identify modification-sensitive residues, we examined effects of MTS reagents on mutant channels lacking one or more cysteines. Surprisingly, the effects of MTSES on both voltage- and Ca²⁺-dependent gating were abolished by replacing a single cysteine (C430) with alanine. C430 lies in the RCK1 (regulator of K⁺ conductance) domain within a series of eight residues that is unique to BK channels. Deletion of these residues shifted the G_K–V relation by >−80 mV. Thus we have identified a region that appears to strongly influence RCK domain function, but is absent from RCK domains of known structure. C430A did not eliminate effects of MTSET on apparent Ca²⁺ affinity. However an additional mutation, C615S, in the Haem binding site reduced the effects of MTSET, consistent with a role for this region in Ca²⁺ binding.