Ca2+-dependent inactivation of large conductance Ca2+-activated K+ (BK) channels in rat hippocampal neurones produced by pore block from an associated particle

Ca2+- and Voltage-Activated K+ (BK) Channels in the Nervous System: One Gene, a Myriad of Physiological Functions

BK channels are large conductance potassium channels characterized by four pore-forming α subunits, often co-assembled with auxiliary β and γ subunits to regulate Ca2+ sensitivity, voltage dependence and gating properties. BK channels are abundantly expressed throughout the brain and in different compartments within a single neuron, including axons, synaptic terminals, dendritic arbors, and spines. Their activation produces a massive efflux of K+ ions that hyperpolarizes the cellular membrane. Together with their ability to detect changes in intracellular Ca2+ concentration, BK channels control neuronal excitability and synaptic communication through diverse mechanisms. Moreover, increasing evidence indicates that dysfunction of BK channel-mediated effects on neuronal excitability and synaptic function has been implicated in several neurological disorders, including epilepsy, fragile X syndrome, mental retardation, and autism, as well as in motor and cognitive behavior. Here, we discuss current evidence highlighting the physiological importance of this ubiquitous channel in regulating brain function and its role in the pathophysiology of different neurological disorders.

Calcium-Activated K+ Channels (KCa) and Therapeutic Implications

Potassium channels are the most diverse and ubiquitous family of ion channels found in cells. The Ca²⁺ and voltage gated members form a subfamily that play a variety of roles in both excitable and non-excitable cells and are further classified on the basis of their single channel conductance to form the small conductance (SK), intermediate conductance (IK) and big conductance (BK) K⁺ channels.In this chapter, we will focus on the mechanisms underlying the gating of BK channels, whose function is modified in different tissues by different splice variants as well as the expanding array of regulatory accessory subunits including β, γ and LINGO subunits. We will examine how BK channels are modified by these regulatory subunits and describe how the channel gating is altered by voltage and Ca²⁺ whilst setting this in context with the recently published structures of the BK channel. Finally, we will discuss how BK and other calcium-activated channels are modulated by novel ion channel modulators and describe some of the challenges associated with trying to develop compounds with sufficient efficacy, potency and selectivity to be of therapeutic benefit.

Regulation of BK Channels by Beta and Gamma Subunits

Ca²⁺- and voltage-gated K⁺ channels of large conductance (BK channels) are expressed in a diverse variety of both excitable and inexcitable cells, with functional properties presumably uniquely calibrated for the cells in which they are found. Although some diversity in BK channel function, localization, and regulation apparently arises from cell-specific alternative splice variants of the single pore-forming α subunit ( KCa1.1, Kcnma1, Slo1) gene, two families of regulatory subunits, β and γ, define BK channels that span a diverse range of functional properties. We are just beginning to unravel the cell-specific, physiological roles served by BK channels of different subunit composition.

Voltage- and calcium-gated ion channels of neurons in the vertebrate retina

In this review, we summarize studies investigating the types and distribution of voltage- and calcium-gated ion channels in the different classes of retinal neurons: rods, cones, horizontal cells, bipolar cells, amacrine cells, interplexiform cells, and ganglion cells. We discuss differences among cell subtypes within these major cell classes, as well as differences among species, and consider how different ion channels shape the responses of different neurons. For example, even though second-order bipolar and horizontal cells do not typically generate fast sodium-dependent action potentials, many of these cells nevertheless possess fast sodium currents that can enhance their kinetic response capabilities. Ca2+ channel activity can also shape response kinetics as well as regulating synaptic release. The L-type Ca2+ channel subtype, CaV1.4, expressed in photoreceptor cells exhibits specific properties matching the particular needs of these cells such as limited inactivation which allows sustained channel activity and maintained synaptic release in darkness. The particular properties of K+ and Cl- channels in different retinal neurons shape resting membrane potentials, response kinetics and spiking behavior. A remaining challenge is to characterize the specific distributions of ion channels in the more than 100 individual cell types that have been identified in the retina and to describe how these particular ion channels sculpt neuronal responses to assist in the processing of visual information by the retina.

Regulation of BK Channels by Beta and Gamma Subunits

Ca²⁺- and voltage-gated K⁺ channels of large conductance (BK channels) are expressed in a diverse variety of both excitable and inexcitable cells, with functional properties presumably uniquely calibrated for the cells in which they are found. Although some diversity in BK channel function, localization, and regulation apparently arises from cell-specific alternative splice variants of the single pore-forming α subunit ( KCa1.1, Kcnma1, Slo1) gene, two families of regulatory subunits, β and γ, define BK channels that span a diverse range of functional properties. We are just beginning to unravel the cell-specific, physiological roles served by BK channels of different subunit composition.

Regulation mechanisms and implications of sperm membrane hyperpolarization

Mammalian sperm are unable to fertilize the egg immediately after ejaculation. In order to gain fertilization competence, they need to undergo a series of biochemical and physiological modifications inside the female reproductive tract, known as capacitation. Capacitation correlates with two essential events for fertilization: hyperactivation, an asymmetric and vigorous flagellar motility, and the ability to undergo the acrosome reaction. At a molecular level, capacitation is associated to: phosphorylation cascades, modification of membrane lipids, alkalinization of the intracellular pH, increase in the intracellular Ca²⁺ concentration and hyperpolarization of the sperm plasma membrane potential. Hyperpolarization is a crucial event in capacitation since it primes the sperm to undergo the exocytosis of the acrosome content, essential to achieve fertilization of the oocyte.

Calcium-activated BKCa channels govern dynamic membrane depolarizations of horizontal cells in rodent retina

KEY POINTS

Large conductance, Ca²⁺ -activated K⁺ (BK_Ca ) channels play important roles in mammalian retinal neurons, including photoreceptors, bipolar cells, amacrine cells and ganglion cells, but they have not been identified in horizontal cells. BK_Ca channel blockers paxilline and iberiotoxin, as well as Ca²⁺ free solutions and divalent cation Ca_v channel blockers, eliminate the outwardly rectifying current, while NS1619 enhances it. In symmetrical 150 mm K⁺ , single channels had a conductance close to 250 pS, within the range of all known BK_Ca channels. In current clamped horizontal cells, BK_Ca channels subdue depolarizing membrane potential excursions, reduce the average resting potential and decrease oscillations. The results show that BK_Ca channel activation puts a ceiling on horizontal cell depolarization and regulates the temporal responsivity of the cells.

ABSTRACT

Large conductance, calcium-activated potassium (BK_Ca ) channels have numerous roles in neurons including the regulation of membrane excitability, intracellular [Ca²⁺ ] regulation, and neurotransmitter release. In the retina, they have been identified in photoreceptors, bipolar cells, amacrine cells and ganglion cells, but have not been conclusively identified in mammalian horizontal cells. We found that outward current recorded between -30 and +60 mV is carried primarily in BK_Ca channels in isolated horizontal cells of rats and mice. Whole-cell outward currents were maximal at +50 mV and declined at membrane potentials positive to this value. This current was eliminated by the selective BK_Ca channel blocker paxilline (100 nm), iberiotoxin (10 μm), Ca²⁺ free solutions and divalent cation Ca_v channel blockers. It was activated by the BK_Ca channel activator NS1619 (30 μm). Single channel recordings revealed the conductance of the channels to be 244 ± 11 pS (n = 17; symmetrical 150 mm K⁺ ) with open probability being both voltage- and Ca²⁺ -dependent. The channels showed fast activation kinetics in response to Ca²⁺ influx and inactivation gating that could be modified by intracellular protease treatment, which suggests β subunit involvement. Under current clamp, block of BK_Ca current increased depolarizing membrane potential excursions, raising the average resting potential and producing oscillations. BK_Ca current activation with NS1619 inhibited oscillations and hyperpolarized the resting potential. These effects underscore the functional role of BK_Ca current in limiting depolarization of the horizontal cell membrane potential and suggest actions of these channels in regulating the temporal responsivity of the cells.

Gene-based association study of genes linked to hippocampal sclerosis of aging neuropathology: GRN, TMEM106B, ABCC9, and KCNMB2

Hippocampal sclerosis of aging (HS-Aging) is a common neurodegenerative condition associated with dementia. To learn more about genetic risk of HS-Aging pathology, we tested gene-based associations of the GRN, TMEM106B, ABCC9, and KCNMB2 genes, which were reported to be associated with HS-Aging pathology in previous studies. Genetic data were obtained from the Alzheimer's Disease Genetics Consortium, linked to autopsy-derived neuropathological outcomes from the National Alzheimer's Coordinating Center. Of the 3251 subjects included in the study, 271 (8.3%) were identified as an HS-Aging case. The significant gene-based association between the ABCC9 gene and HS-Aging appeared to be driven by a region in which a significant haplotype-based association was found. We tested this haplotype as an expression quantitative trait locus using 2 different public-access brain gene expression databases. The HS-Aging pathology protective ABCC9 haplotype was associated with decreased ABCC9 expression, indicating a possible toxic gain of function.

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.

BK Channels in the Central Nervous System

Large conductance Ca(2+)- and voltage-activated K(+) (BK) channels are widely distributed in the postnatal central nervous system (CNS). BK channels play a pleiotropic role in regulating the activity of brain and spinal cord neural circuits by providing a negative feedback mechanism for local increases in intracellular Ca(2+) concentrations. In neurons, they regulate the timing and duration of K(+) influx such that they can either increase or decrease firing depending on the cellular context, and they can suppress neurotransmitter release from presynaptic terminals. In addition, BK channels located in astrocytes and arterial myocytes modulate cerebral blood flow. Not surprisingly, both loss and gain of BK channel function have been associated with CNS disorders such as epilepsy, ataxia, mental retardation, and chronic pain. On the other hand, the neuroprotective role played by BK channels in a number of pathological situations could potentially be leveraged to correct neurological dysfunction.

BK channel inactivation gates daytime excitability in the circadian clock

Inactivation is an intrinsic property of several voltage-dependent ion channels, closing the conduction pathway during membrane depolarization and dynamically regulating neuronal activity. BK K⁺ channels undergo N-type inactivation via their β2 subunit, but the physiological significance is not clear. Here, we report that inactivating BK currents predominate during the day in the suprachiasmatic nucleus, the brain's intrinsic clock circuit, reducing steady-state current levels. At night inactivation is diminished, resulting in larger BK currents. Loss of β2 eliminates inactivation, abolishing the diurnal variation in both BK current magnitude and SCN firing, and disrupting behavioural rhythmicity. Selective restoration of inactivation via the β2 N-terminal ‘ball-and-chain' domain rescues BK current levels and firing rate, unexpectedly contributing to the subthreshold membrane properties that shift SCN neurons into the daytime ‘upstate'. Our study reveals the clock employs inactivation gating as a biophysical switch to set the diurnal variation in suprachiasmatic nucleus excitability that underlies circadian rhythm.

BK potassium channels have been previously shown to mediate SCN circadian firing, although the precise mechanisms are unclear. Here, using knockout and rescue approaches, the authors find that the ß2 ‘ball-and-chain' confers BK channel inactivation during the day, promoting SCN electrical upstate.

Transient BK outward current enhances motoneurone firing rates during Drosophila larval locomotion

KEY POINTS

We combine in situ electrophysiology with genetic manipulation in Drosophila larvae aiming to investigate the role of fast calcium-activated potassium currents for motoneurone firing patterns during locomotion. We first demonstrate that slowpoke channels underlie fast calcium-activated potassium currents in these motoneurones. By conducting recordings in semi-intact animals that produce crawling-like movements, we show that slowpoke channels are required specifically in motoneurones for maximum firing rates during locomotion. Such enhancement of maximum firing rates occurs because slowpoke channels prevent depolarization block by limiting the amplitude of motoneurone depolarization in response to synaptic drive. In addition, slowpoke channels mediate a fast afterhyperpolarization that ensures the efficient recovery of sodium channels from inactivation during high frequency firing. The results of the present study provide new insights into the mechanisms by which outward conductances facilitate neuronal excitability and also provide direct confirmation of the functional relevance of precisely regulated slowpoke channel properties in motor control.

ABSTRACT

A large number of voltage-gated ion channels, their interactions with accessory subunits, and their post-transcriptional modifications generate an immense functional diversity of neurones. Therefore, a key challenge is to understand the genetic basis and precise function of specific ionic conductances for neuronal firing properties in the context of behaviour. The present study identifies slowpoke (slo) as exclusively mediating fast activating, fast inactivating BK current (ICF ) in larval Drosophila crawling motoneurones. Combining in vivo patch clamp recordings during larval crawling with pharmacology and targeted genetic manipulations reveals that ICF acts specifically in motoneurones to sculpt their firing patterns in response to a given input from the central pattern generating (CPG) networks. First, ICF curtails motoneurone postsynaptic depolarizations during rhythmical CPG drive. Second, ICF is activated during the rising phase of the action potential and mediates a fast afterhyperpolarization. Consequently, ICF is required for maximal intraburst firing rates during locomotion, probably by allowing recovery from inactivation of fast sodium channels and decreased potassium channel activation. This contrasts the common view that outward conductances oppose excitability but is in accordance with reports on transient BK and Kv3 channel function in multiple types of vertebrate neurones. Therefore, our finding that ICF enhances firing rates specifically during bursting patterns relevant to behaviour is probably of relevance to all brains.

Age-related changes of inactivating BK channels in rat dorsal root ganglion neurons

The large-conductance, voltage- and Ca(2+)-activated K(+) channels (termed BK) are associated with age-related dysfunctions or diseases. Previously, with our colleagues, we reported that the rβ2-associated inactivating BK (BKi) channels play an essential role in rat dorsal root ganglion (DRG) neurons. However, the age-dependent changes in BKi channels are still elusive. Here, we identify three types of BK channels in small DRG neurons, the single exponential BKi, the double exponential BKi and the non-inactivating BK. Interestingly, compared to the increased occurrence of the non-inactivating BK, the presence of BKi channels declined with age. Furthermore, the peak amplitude of the single exponential BKi current increased from infancy to youth, but decreased from youth to old age. The inactivation time constant, however, did not change with age. The double exponential BKi also displayed age-related change in current amplitude with an intricate kinetics. Physiologically, the decay speed of the action potential was significantly increased in Youth, which correlated with the change of current amplitude of BKi channels. Collectively, these results reveal an age-related change pattern of BKi channels in small DRG neurons, providing potential mechanistic clues for different susceptibility to sensation in different ages.

Knockout of the BK β2 subunit abolishes inactivation of BK currents in mouse adrenal chromaffin cells and results in slow-wave burst activity

Chromaffin cells from mice lacking the BK β2 subunit show decreased action potential firing during current injection but an increase in spontaneous burst firing.

Rat and mouse adrenal medullary chromaffin cells (CCs) express an inactivating BK current. This inactivation is thought to arise from the assembly of up to four β2 auxiliary subunits (encoded by the kcnmb2 gene) with a tetramer of pore-forming Slo1 α subunits. Although the physiological consequences of inactivation remain unclear, differences in depolarization-evoked firing among CCs have been proposed to arise from the ability of β2 subunits to shift the range of BK channel activation. To investigate the role of BK channels containing β2 subunits, we generated mice in which the gene encoding β2 was deleted (β2 knockout [KO]). Comparison of proteins from wild-type (WT) and β2 KO mice allowed unambiguous demonstration of the presence of β2 subunit in various tissues and its coassembly with the Slo1 α subunit. We compared current properties and cell firing properties of WT and β2 KO CCs in slices and found that β2 KO abolished inactivation, slowed action potential (AP) repolarization, and, during constant current injection, decreased AP firing. These results support the idea that the β2-mediated shift of the BK channel activation range affects repetitive firing and AP properties. Unexpectedly, CCs from β2 KO mice show an increased tendency toward spontaneous burst firing, suggesting that the particular properties of BK channels in the absence of β2 subunits may predispose to burst firing.

Effects of amyloid β-peptide fragment 31-35 on the BK channel-mediated K⁺ current and intracellular free Ca²⁺ concentration of hippocampal CA1 neurons

The present study characterizes the effects of Aβ31-35, a short active fragment of amyloid β-peptide (Aβ), upon the BK channel-mediated K⁺ current and intracellular free Ca²⁺ concentration ([Ca²⁺]i) of freshly dissociated pyramidal cells from rat CA1 hippocampus by using whole-cell patch-clamp recording and single cell Ca²⁺ imaging techniques. The results show that: (1) in the presence of voltage- and ATP-gated K⁺ channel blockers application of 5.0 μM Aβ31-35 significantly diminished transient outward K⁺ current amplitudes at clamped voltages between 0 and 45mV; (2) under the same conditions [Ca²⁺]i was minimally affected by 5.0 μM but significantly increased by 12.5 μM and 25 μM Aβ31-35; and (3) when 25 μM of a larger fragment of the amyloid β-peptide, Aβ25-35, was applied, the results were similar to those obtained with the same concentration of Aβ31-35. These results indicate that Aβ31-35 is likely to be the shortest active fragment of the full Aβ sequence, and can be as effectively as the full-length Aβ peptide in suppressing BK-channel mediated K⁺ currents and significantly elevating [Ca²⁺]i in hippocampal CA1 neurons.

K(+) channels: function-structural overview

Potassium channels are particularly important in determining the shape and duration of the action potential, controlling the membrane potential, modulating hormone secretion, epithelial function and, in the case of those K(+) channels activated by Ca(2+), damping excitatory signals. The multiplicity of roles played by K(+) channels is only possible to their mammoth diversity that includes at present 70 K(+) channels encoding genes in mammals. Today, thanks to the use of cloning, mutagenesis, and the more recent structural studies using x-ray crystallography, we are in a unique position to understand the origins of the enormous diversity of this superfamily of ion channels, the roles they play in different cell types, and the relations that exist between structure and function. With the exception of two-pore K(+) channels that are dimers, voltage-dependent K(+) channels are tetrameric assemblies and share an extremely well conserved pore region, in which the ion-selectivity filter resides. In the present overview, we discuss in the function, localization, and the relations between function and structure of the five different subfamilies of K(+) channels: (a) inward rectifiers, Kir; (b) four transmembrane segments-2 pores, K2P; (c) voltage-gated, Kv; (d) the Slo family; and (e) Ca(2+)-activated SK family, SKCa.

Comprehensive RNA-Seq expression analysis of sensory ganglia with a focus on ion channels and GPCRs in Trigeminal ganglia

The specific functions of sensory systems depend on the tissue-specific expression of genes that code for molecular sensor proteins that are necessary for stimulus detection and membrane signaling. Using the Next Generation Sequencing technique (RNA-Seq), we analyzed the complete transcriptome of the trigeminal ganglia (TG) and dorsal root ganglia (DRG) of adult mice. Focusing on genes with an expression level higher than 1 FPKM (fragments per kilobase of transcript per million mapped reads), we detected the expression of 12984 genes in the TG and 13195 in the DRG. To analyze the specific gene expression patterns of the peripheral neuronal tissues, we compared their gene expression profiles with that of the liver, brain, olfactory epithelium, and skeletal muscle. The transcriptome data of the TG and DRG were scanned for virtually all known G-protein-coupled receptors (GPCRs) as well as for ion channels. The expression profile was ranked with regard to the level and specificity for the TG. In total, we detected 106 non-olfactory GPCRs and 33 ion channels that had not been previously described as expressed in the TG. To validate the RNA-Seq data, in situ hybridization experiments were performed for several of the newly detected transcripts. To identify differences in expression profiles between the sensory ganglia, the RNA-Seq data of the TG and DRG were compared. Among the differentially expressed genes (> 1 FPKM), 65 and 117 were expressed at least 10-fold higher in the TG and DRG, respectively. Our transcriptome analysis allows a comprehensive overview of all ion channels and G protein-coupled receptors that are expressed in trigeminal ganglia and provides additional approaches for the investigation of trigeminal sensing as well as for the physiological and pathophysiological mechanisms of pain.

Slo1 is the principal potassium channel of human spermatozoa

Mammalian spermatozoa gain competence to fertilize an oocyte as they travel through the female reproductive tract. This process is accompanied by an elevation of sperm intracellular calcium and a membrane hyperpolarization. The latter is evoked by K(+) efflux; however, the molecular identity of the potassium channel of human spermatozoa (hKSper) is unknown. Here, we characterize hKSper, reporting that it is regulated by intracellular calcium but is insensitive to intracellular alkalinization. We also show that human KSper is inhibited by charybdotoxin, iberiotoxin, and paxilline, while mouse KSper is insensitive to these compounds. Such unique properties suggest that the Slo1 ion channel is the molecular determinant for hKSper. We show that Slo1 is localized to the sperm flagellum and is inhibited by progesterone. Inhibition of hKSper by progesterone may depolarize the spermatozoon to open the calcium channel CatSper, thus raising [Ca(2+)] to produce hyperactivation and allowing sperm to fertilize an oocyte. DOI:http://dx.doi.org/10.7554/eLife.01009.001.

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.

Iberiotoxin-sensitive and -insensitive BK currents in Purkinje neuron somata

Purkinje cells have specialized intrinsic ionic conductances that generate high-frequency action potentials. Disruptions of their Ca or Ca-activated K (KCa) currents correlate with altered firing patterns in vitro and impaired motor behavior in vivo. To examine the properties of somatic KCa currents, we recorded voltage-clamped KCa currents in Purkinje cell bodies isolated from postnatal day 17-21 mouse cerebellum. Currents were evoked by endogenous Ca influx with approximately physiological Ca buffering. Purkinje somata expressed voltage-activated, Cd-sensitive KCa currents with iberiotoxin (IBTX)-sensitive (>100 nS) and IBTX-insensitive (>75 nS) components. IBTX-sensitive currents activated and partially inactivated within milliseconds. Rapid, incomplete macroscopic inactivation was also evident during 50- or 100-Hz trains of 1-ms depolarizations. In contrast, IBTX-insensitive currents activated more slowly and did not inactivate. These currents were insensitive to the small- and intermediate-conductance KCa channel blockers apamin, scyllatoxin, UCL1684, bicuculline methiodide, and TRAM-34, but were largely blocked by 1 mM tetraethylammonium. The underlying channels had single-channel conductances of ∼150 pS, suggesting that the currents are carried by IBTX-resistant (β4-containing) large-conductance KCa (BK) channels. IBTX-insensitive currents were nevertheless increased by small-conductance KCa channel agonists EBIO, chlorzoxazone, and CyPPA. During trains of brief depolarizations, IBTX-insensitive currents flowed during interstep intervals, and the accumulation of interstep outward current was enhanced by EBIO. In current clamp, EBIO slowed spiking, especially during depolarizing current injections. The two components of BK current in Purkinje somata likely contribute differently to spike repolarization and firing rate. Moreover, augmentation of BK current may partially underlie the action of EBIO and chlorzoxazone to alleviate disrupted Purkinje cell firing associated with genetic ataxias.

Functional upregulation of Ca(2+)-activated K(+) channels in the development of substantia nigra dopamine neurons

Many connections in the basal ganglia are made around birth when animals are exposed to a host of new affective, cognitive, and sensori-motor stimuli. It is thought that dopamine modulates cortico-striatal synapses that result in the strengthening of those connections that lead to desired outcomes. We propose that there must be a time before which stimuli cannot be processed into functional connections, otherwise it would imply an effective link between stimulus, response, and reward in uterus. Consistent with these ideas, we present evidence that early in development dopamine neurons are electrically immature and do not produce high-frequency firing in response to salient stimuli. We ask first, what makes dopamine neurons immature? and second, what are the implications of this immaturity for the basal ganglia? As an answer to the first question, we find that at birth the outward current is small (3nS-V), insensitive to Ca(2+), TEA, BK, and SK blockers. Rapidly after birth, the outward current increases to 15nS-V and becomes sensitive to Ca(2+), TEA, BK, and SK blockers. We make a detailed analysis of the kinetics of the components of the outward currents and produce a model for BK and SK channels that we use to reproduce the outward current, and to infer the geometrical arrangement of BK and Ca(2+) channels in clusters. In the first cluster, T-type Ca(2+) and BK channels are coupled within distances of ~20 nm (200 Å). The second cluster consists of L-type Ca(2+) and BK channels that are spread over distances of at least 60 nm. As for the second question, we propose that early in development, the mechanism of action selection is in a "locked-in" state that would prevent dopamine neurons from reinforcing cortico-striatal synapses that do not have a functional experiential-based value.

Persistence and storage of activity patterns in spiking recurrent cortical networks: modulation of sigmoid signals by after-hyperpolarization currents and acetylcholine

Many cortical networks contain recurrent architectures that transform input patterns before storing them in short-term memory (STM). Theorems in the 1970's showed how feedback signal functions in rate-based recurrent on-center off-surround networks control this process. A sigmoid signal function induces a quenching threshold below which inputs are suppressed as noise and above which they are contrast-enhanced before pattern storage. This article describes how changes in feedback signaling, neuromodulation, and recurrent connectivity may alter pattern processing in recurrent on-center off-surround networks of spiking neurons. In spiking neurons, fast, medium, and slow after-hyperpolarization (AHP) currents control sigmoid signal threshold and slope. Modulation of AHP currents by acetylcholine (ACh) can change sigmoid shape and, with it, network dynamics. For example, decreasing signal function threshold and increasing slope can lengthen the persistence of a partially contrast-enhanced pattern, increase the number of active cells stored in STM, or, if connectivity is distance-dependent, cause cell activities to cluster. These results clarify how cholinergic modulation by the basal forebrain may alter the vigilance of category learning circuits, and thus their sensitivity to predictive mismatches, thereby controlling whether learned categories code concrete or abstract features, as predicted by Adaptive Resonance Theory. The analysis includes global, distance-dependent, and interneuron-mediated circuits. With an appropriate degree of recurrent excitation and inhibition, spiking networks maintain a partially contrast-enhanced pattern for 800 ms or longer after stimuli offset, then resolve to no stored pattern, or to winner-take-all (WTA) stored patterns with one or multiple winners. Strengthening inhibition prolongs a partially contrast-enhanced pattern by slowing the transition to stability, while strengthening excitation causes more winners when the network stabilizes.

After-hyperpolarization currents and acetylcholine control sigmoid transfer functions in a spiking cortical model

Recurrent networks are ubiquitous in the brain, where they enable a diverse set of transformations during perception, cognition, emotion, and action. It has been known since the 1970's how, in rate-based recurrent on-center off-surround networks, the choice of feedback signal function can control the transformation of input patterns into activity patterns that are stored in short term memory. A sigmoid signal function may, in particular, control a quenching threshold below which inputs are suppressed as noise and above which they may be contrast enhanced before the resulting activity pattern is stored. The threshold and slope of the sigmoid signal function determine the degree of noise suppression and of contrast enhancement. This article analyses how sigmoid signal functions and their shape may be determined in biophysically realistic spiking neurons. Combinations of fast, medium, and slow after-hyperpolarization (AHP) currents, and their modulation by acetylcholine (ACh), can control sigmoid signal threshold and slope. Instead of a simple gain in excitability that was previously attributed to ACh, cholinergic modulation may cause translation of the sigmoid threshold. This property clarifies how activation of ACh by basal forebrain circuits, notably the nucleus basalis of Meynert, may alter the vigilance of category learning circuits, and thus their sensitivity to predictive mismatches, thereby controlling whether learned categories code concrete or abstract information, as predicted by Adaptive Resonance Theory.

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

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

Novel spliced variants of large-conductance Ca(2+)-activated K(+)-channel β2-subunit in human and rodent pancreas

Large-conductance Ca(2+)-activated K(+ )(BK) channel regulates action potential firing in pancreatic β-cells. We cloned novel spliced variants of the BK-channel β(2)-subunit (BKβ2b), which consisted of 36 amino acids including the N-terminal in the original human BKβ2 (BKβ2a), from human and rodent pancreas. Real-time PCR analysis showed the abundant expression of BKβ2b transcripts in human and rodent pancreas and also in the RINm5f insulinoma cell line. In addition, up-regulation of both BK-channel α-subunit (BKα) and BKβ2b transcripts was observed in pancreas tissues from diabetes mellitus patients. In HEK293 cells co-expressing BKα and BKβ2b, the inactivation of BK-channel currents, which is typical for BKα + BKβ2a, was not observed, and electrophysiological and pharmacological properties of BKα + BKβ2b were almost identical to those of BKα alone. In HEK293 cells stably expressing BKα, the transient co-expression of yellow fluorescence protein (YFP)-tagged BKβ2a proteins resulted in their distribution along the cell membrane. In contrast, the co-expression of YFP-tagged BKβ2b with BKα showed diffusely distributed fluorescence signals throughout the cell body. Taken together, the predominant splicing of BKβ2b versus that of BKβ2a presumably enhances the contribution of BK channels to membrane potential and may possibly be a factor modulating insulin secretion in a suppressive manner in pancreatic β-cells.

Ancillary subunits associated with voltage-dependent K+ channels

Since the first discovery of Kvbeta-subunits more than 15 years ago, many more ancillary Kv channel subunits were characterized, for example, KChIPs, KCNEs, and BKbeta-subunits. The ancillary subunits are often integral parts of native Kv channels, which, therefore, are mostly multiprotein complexes composed of voltage-sensing and pore-forming Kvalpha-subunits and of ancillary or beta-subunits. Apparently, Kv channels need the ancillary subunits to fulfill their many different cell physiological roles. This is reflected by the large structural diversity observed with ancillary subunit structures. They range from proteins with transmembrane segments and extracellular domains to purely cytoplasmic proteins. Ancillary subunits modulate Kv channel gating but can also have a great impact on channel assembly, on channel trafficking to and from the cellular surface, and on targeting Kv channels to different cellular compartments. The importance of the role of accessory subunits is further emphasized by the number of mutations that are associated in both humans and animals with diseases like hypertension, epilepsy, arrhythmogenesis, periodic paralysis, and hypothyroidism. Interestingly, several ancillary subunits have in vitro enzymatic activity; for example, Kvbeta-subunits are oxidoreductases, or modulate enzymatic activity, i.e., KChIP3 modulates presenilin activity. Thus different modes of beta-subunit association and of functional impact on Kv channels can be delineated, making it difficult to extract common principles underlying Kvalpha- and beta-subunit interactions. We critically review present knowledge on the physiological role of ancillary Kv channel subunits and their effects on Kv channel properties.

BKCa currents are enriched in a subpopulation of adult rat cutaneous nociceptive dorsal root ganglion neurons

The biophysical properties and distribution of voltage-dependent, Ca(2+) -modulated K(+) (BK(Ca)) currents among subpopulations of acutely dissociated DiI-labeled cutaneous sensory neurons from the adult rat were characterized with whole-cell patch-clamp techniques. BK(Ca) currents were isolated from total K(+) current with iberiotoxin, charybdotoxin or paxilline. There was considerable variability in biophysical properties of BK(Ca) currents. There was also variability in the distribution of BK(Ca) current among subpopulations of cutaneous dorsal root ganglia (DRG) neurons. While present in each of the subpopulations defined by cell body size, IB4 binding or capsaicin sensitivity, BK(Ca) current was present in the vast majority (> 90%) of small-diameter IB4+ neurons, but was present in only a minority of neurons in subpopulations defined by other criteria (i.e. small-diameter IB4-). Current-clamp analysis indicated that in IB4+ neurons, BK(Ca) currents contribute to the repolarization of the action potential and adaptation in response to sustained membrane depolarization, while playing little role in the determination of action potential threshold. Reverse transcriptase-polymerase chain reaction analysis of mRNA collected from whole DRG revealed the presence of multiple splice variants of the BK(Ca) channel alpha-subunit, rslo and all four of the accessory beta-subunits, suggesting that heterogeneity in the biophysical and pharmacological properties of BK(Ca) current in cutaneous neurons reflects, at least in part, the differential distribution of splice variants and/or beta-subunits. Because even a small decrease in BK(Ca) current appears to have a dramatic influence on excitability, modulation of this current may contribute to sensitization of nociceptive afferents observed following tissue injury.

BK channels modulate pre- and postsynaptic signaling at reciprocal synapses in retina

In the mammalian retina, A17 amacrine cells provide reciprocal inhibitory feedback to rod bipolar cells, thereby shaping the time course of visual signaling in vivo. Previous results indicate that A17 feedback can be triggered by Ca²⁺ influx through Ca²⁺ permeable AMPARs and can occur independently of voltage-gated Ca²⁺ (Ca_v) channels, whose presence and functional role in A17 dendrites have not been explored. Here, we combine electrophysiology, calcium imaging and immunohistochemistry to show that L-type Ca_v channels in rat A17 amacrine cells are located at the sites of reciprocal synaptic feedback, but their contribution to GABA release is diminished by large-conductance Ca²⁺-activated potassium (BK) channels, which suppress postsynaptic depolarization in A17s and limit Ca_v channel activation. We also show that BK channels, by limiting GABA release from A17s, regulate the flow of excitatory synaptic transmission through the rod pathway.

Identification and characterization of Ca2+-activated K+ channels in granulosa cells of the human ovary

BACKGROUND

Granulosa cells (GCs) represent a major endocrine compartment of the ovary producing sex steroid hormones. Recently, we identified in human GCs a Ca2+-activated K+ channel (K(Ca)) of big conductance (BK(Ca)), which is involved in steroidogenesis. This channel is activated by intraovarian signalling molecules (e.g. acetylcholine) via raised intracellular Ca2+ levels. In this study, we aimed at characterizing 1. expression and functions of K(Ca) channels (including BK(Ca) beta-subunits), and 2. biophysical properties of BK(Ca) channels.

METHODS

GCs were obtained from in vitro-fertilization patients and cultured. Expression of mRNA was determined by standard RT-PCR and protein expression in human ovarian slices was detected by immunohistochemistry. Progesterone production was measured in cell culture supernatants using ELISAs. Single channels were recorded in the inside-out configuration of the patch-clamp technique.

RESULTS

We identified two K(Ca) types in human GCs, the intermediate- (IK) and the small-conductance K(Ca) (SK). Their functionality was concluded from attenuation of human chorionic gonadotropin-stimulated progesterone production by K(Ca) blockers (TRAM-34, apamin). Functional IK channels were also demonstrated by electrophysiological recording of single K(Ca) channels with distinctive features. Both, IK and BK(Ca) channels were found to be simultaneously active in individual GCs. In agreement with functional data, we identified mRNAs encoding IK, SK1, SK2 and SK3 in human GCs and proteins of IK and SK2 in corresponding human ovarian cells. Molecular characterization of the BK(Ca) channel revealed the presence of mRNAs encoding several BK(Ca) beta-subunits (beta2, beta3, beta4) in human GCs. The multitude of beta-subunits detected might contribute to variations in Ca2+ dependence of individual BK(Ca) channels which we observed in electrophysiological recordings.

CONCLUSION

Functional and molecular studies indicate the presence of active IK and SK channels in human GCs. Considering the already described BK(Ca), they express all three K(Ca) types known. We suggest that the plurality and co-expression of different K(Ca) channels and BK(Ca) beta-subunits might allow differentiated responses to Ca2+ signals over a wide range caused by various intraovarian signalling molecules (e.g. acetylcholine, ATP, dopamine). The knowledge of ovarian K(Ca) channel properties and functions should help to understand the link between endocrine and paracrine/autocrine control in the human ovary.

The BK-mediated fAHP is modulated by learning a hippocampus-dependent task

Intrinsic excitability is a plastic property of cells that, along with synaptic changes, can be modulated by learning. Action potential (AP) height, width, and frequency are intrinsically controlled properties which rely on the activation of Na(+), Ca(2+), and K(+) channels in the dendritic, somatic, and axonal membranes. The fast after-hyperpolarization (fAHP) after an AP is partially responsible for determining the half-width and duration of the AP and thus Ca(2+) influx during the depolarization. In CA1 hippocampal pyramidal cells, the fAHP is carried by the voltage- and Ca(2+)-dependent BK channel. In addition to modulating the duration of the AP, the BK-mediated potassium current exerts control over the frequency of AP generation in response to a depolarizing input. These facts position BK-mediated effects to not only modulate immediate intraneuronal communication, but also to control longer-term Ca(2+)-dependent changes in the neuron, such as kinase activation, gene transcription, and synaptic plasticity. We examined how the BK-mediated fAHP was altered in hippocampal neurons after learning trace eyeblink conditioning. By using current clamp methods, it was found that the fAHP is reduced and the AP duration is increased in cells from conditioned animals. Additionally, in vitro and in vivo measures of firing frequency show that BK-channel blockade increases both evoked (in vitro) and spontaneous (in vivo) firing frequency of CA1 neurons, implicating the BK channel in the control of intrinsic excitability. These data indicate that the reduction of the BK-mediated fAHP is an essential part of the total increase of neuronal excitability known to accompany hippocampus-dependent learning.

Subunit-specific effect of the voltage sensor domain on Ca2+ sensitivity of BK channels

Large conductance Ca(2+)- and voltage-activated K(+) (BK) channels, composed of pore-forming alpha-subunits and auxiliary beta-subunits, play important roles in diverse physiological processes. The differences in BK channel phenotypes are primarily due to the tissue-specific expression of beta-subunits (beta1-beta4) that modulate channel function differently. Yet, the molecular basis of the subunit-specific regulation is not clear. In our study, we demonstrate that perturbation of the voltage sensor in BK channels by mutations selectively disrupts the ability of the beta1-subunit--but not that of the beta2-subunit--to enhance apparent Ca(2+) sensitivity. These mutations change the number of equivalent gating charges, the voltage dependence of voltage sensor movements, the open-close equilibrium of the channel, and the allosteric coupling between voltage sensor movements and channel opening to various degrees, indicating that they alter the conformation and movements of the voltage sensor and the activation gate. Similarly, the ability of the beta1-subunit to enhance apparent Ca(2+) sensitivity is diminished to various degrees, correlating quantitatively with the shift of voltage dependence of voltage sensor movements. In contrast, none of these mutations significantly reduces the ability of the beta2-subunit to enhance Ca(2+) sensitivity. These results suggest that the beta1-subunit enhances Ca(2+) sensitivity by altering the conformation and movements of the voltage sensor, whereas the similar function of the beta2-subunit is governed by a distinct mechanism.

Characterization of voltage-and Ca2+-activated K+ channels in rat dorsal root ganglion neurons

Auxiliary beta-subunits associated with pore-forming Slo1 alpha-subunits play an essential role in regulating functional properties of large-conductance, voltage- and Ca(2+)-activated K(+) channels commonly termed BK channels. Even though both noninactivating and inactivating BK channels are thought to be regulated by beta-subunits (beta1, beta2, beta3, or beta4), the molecular determinants underlying inactivating BK channels in native cells have not been extensively demonstrated. In this study, rbeta2 (but not rbeta3-subunit) was identified as a molecular component in rat lumbar L4-6 dorsal root ganglia (DRG) by RT-PCR responsible for inactivating large-conductance Ca(2+)-dependent K(+) currents (BK(i) currents) in small sensory neurons. The properties of native BK(i) currents obtained from both whole-cell and inside-out patches are very similar to inactivating BK channels produced by co-expressing mSlo1 alpha- and hbeta2-subunits in Xenopus oocytes. Intracellular application of 0.5 mg/ml trypsin removes inactivation of BK(i) channels, and the specific blockers of BK channels, charybdotoxin (ChTX) and iberiotoxin (IbTX), inhibit these BK(i) currents. Single BK(i) channel currents derived from inside-out patches revealed that one BK(i) channel contained three rbeta2-subunits (on average), with a single-channel conductance about 217 pS under 160 K(+) symmetrical recording conditions. Blockade of BK(i) channels by 100 nM IbTX augmented firing frequency, broadened action potential waveform and reduced after-hyperpolarization. We propose that the BK(i) channels in small diameter DRG sensory neurons might play an important role in regulating nociceptive input to the central nervous system (CNS).

Endocytic trafficking signals in KCNMB2 regulate surface expression of a large conductance voltage and Ca2+-activated K+ channel

Large conductance voltage and calcium-activated K(+) channels play critical roles in neuronal excitability and vascular tone. Previously, we showed that coexpression of the transmembrane beta2 subunit, KCNMB2, with the human pore-forming alpha subunit of the large conductance voltage and Ca(2+)-activated K(+) channel (hSlo) yields inactivating currents similar to those observed in hippocampal neurons [Hicks GA, Marrion NV (1998) Ca(2+)-dependent inactivation of large conductance Ca(2+)-activated K(+) (BK) channels in rat hippocampal neurones produced by pore block from an associated particle. J Physiol (Lond) 508 (Pt 3):721-734; Wallner M, Meera P, Toro L (1999b) Molecular basis of fast inactivation in voltage and Ca(2+)-activated K(+) channels: A transmembrane beta-subunit homolog. Proc Natl Acad Sci U S A 96:4137-4142]. Herein, we report that coexpression of beta2 subunit with hSlo can also modulate hSlo surface expression levels in HEK293T cells. We found that, when expressed alone, beta2 subunit appears to reach the plasma membrane but also displays a distinct intracellular punctuated pattern that resembles endosomal compartments. beta2 Subunit coexpression with hSlo causes two biological effects: i) a shift of hSlo's intracellular expression pattern from a relatively diffuse to a distinct punctated cytoplasmic distribution overlapping beta2 expression; and ii) a decrease of hSlo surface expression that surpassed an observed small decrease in total hSlo expression levels. beta2 Site-directed mutagenesis studies revealed two putative endocytic signals at the C-terminus of beta2 that can control expression levels of hSlo. In contrast, a beta2 N-terminal consensus endocytic signal had no effect on hSlo expression levels. Thus, beta2 subunit not only can influence hSlo currents but also has the ability to limit hSlo surface expression levels via an endocytic mechanism. This new mode of beta2 modulation of hSlo may depend on particular coregulatory mechanisms in different cell types.

Co-assembly of N-type Ca2+ and BK channels underlies functional coupling in rat brain

Activation of large conductance Ca(2+)-activated potassium (BK) channels hastens action potential repolarisation and generates the fast afterhyperpolarisation in hippocampal pyramidal neurons. A rapid coupling of Ca(2+) entry with BK channel activation is necessary for this to occur, which might result from an identified coupling of Ca(2+) entry through N-type Ca(2+) channels to BK channel activation. This selective coupling was extremely rapid and resistant to intracellular BAPTA, suggesting that the two channel types are close. Using reciprocal co-immunoprecipitation, we found that N-type channels were more abundantly associated with BK channels than L-type channels (Ca(V)1.2) in rat brain. Expression of only the pore-forming alpha-subunits of the N-type (Ca(V)2.2) and BK (Slo(27)) channels in a non-neuronal cell-line gave robust macroscopic currents and reproduced the interaction. Co-expression of Ca(V)2.2/Ca(V)beta(3) subunits with Slo(27) channels revealed rapid functional coupling. By contrast, extremely rare examples of rapid functional coupling were observed with co-expression of Ca(V)1.2/Ca(V)beta(3) and Slo(27) channels. Action potential repolarisation in hippocampal pyramidal neurons was slowed by the N-type channel blocker omega-conotoxin GVIA, but not by the L-type channel blocker isradipine. These data showed that selective functional coupling between N-type Ca(2+) and BK channels provided rapid activation of BK channels in central neurons.

Mechanisms of two modulatory actions of the channel-binding protein Slob on the Drosophila Slowpoke calcium-dependent potassium channel

Slob57 is an ion channel auxiliary protein that binds to and modulates the Drosophila Slowpoke calcium-dependent potassium channel (dSlo). We reported recently that residues 1–39 of Slob57 comprise the key domain that both causes dSlo inactivation and shifts its voltage dependence of activation to more depolarized voltages. In the present study we show that removal of residues 2–6 from Slob57 abolishes the inactivation, but the ability of Slob57 to rightward shift the voltage dependence of activation of dSlo remains. A synthetic peptide corresponding in sequence to residues 1–6 of Slob57 blocks dSlo in a voltage- and dose-dependent manner. Two Phe residues and at least one Lys residue in this peptide are required for the blocking action. These data indicate that the amino terminus of Slob57 directly blocks dSlo, thereby leading to channel inactivation. Further truncation to residue Arg¹⁶ eliminates the modulation of voltage dependence of activation. Thus these two modulatory actions of Slob57 are independent. Mutation within the calcium bowl of dSlo greatly reduces its calcium sensitivity (Bian, S., I. Favre, and E. Moczydlowski. 2001. Proc. Natl. Acad. Sci. USA. 98:4776–4781). We found that Slob57 still causes inactivation of this mutant channel, but does not shift its voltage dependence of activation. This result confirms further the independence of the inactivation and the voltage shift produced by Slob57. It also suggests that the voltage shift requires high affinity Ca²⁺ binding to an intact calcium bowl. Furthermore, Slob57 inhibits the shift in the voltage dependence of activation of dSlo evoked by Ca²⁺, and this inhibition by Slob57 is greater at higher free Ca²⁺ concentrations. These results implicate distinct calcium-dependent and -independent mechanisms in the modulation of dSlo by Slob.

Inhibition of BKCachannel activity by association with calcineurin in rat brain

Large conductance calcium-activated potassium (BK(Ca)) channels are regulated by a number of different protein kinases and phosphatases. The close association of enzymes and channel have been shown to underlie many examples of modulation. However, only the association of protein kinase A with the BK(Ca) channel has been detailed [Tian et al. (2003)J. Biol. Chem., 278, 8669-8677]. We have found using reciprocal immunoprecipitations that the BK(Ca) channel associates with the calcium/calmodulin-dependent phosphatase calcineurin, in Wistar rat brain. A HA-tagged construct of the carboxyl terminus of rSlo(27), a variant of the BK(Ca) channel that is abundant in the hippocampus [Ha et al. (2000)Eur. J. Biochem., 267, 910-9218], was found to associate only with the B subunit of calcineurin. This data suggests that the majority of the interaction of the BK(Ca) channel with calcineurin is mediated by the B subunit of the phosphatase. This was confirmed by using glutathione-S-transferase (GST) fusion proteins of the linker regions between the S7-S10 hydrophobic domains in the carboxyl terminus of rSlo(27), where only the B subunit of calcineurin interacted with regions between S7 and S9 of the channel. Addition of a constitutively active calcineurin (CaN(420)) to inside-out membrane patches excised from cultured hippocampal neurons resulted in a dramatic reduction in BK(Ca) channel open probability, with only very short-duration events being apparent. These data suggest that BK(Ca) channel activity is inhibited by calcineurin, an effect mediated by the association of the calcineurin B subunit with the carboxyl terminus of the channel.

The amino terminus of Slob, Slowpoke channel binding protein, critically influences its modulation of the channel

The Drosophila Slowpoke calcium-dependent potassium channel (dSlo) binding protein Slob was discovered by a yeast two-hybrid screen using the carboxy-terminal tail region of dSlo as bait. Slob binds to and modulates the dSlo channel. We have found that there are several Slob proteins, resulting from multiple translational start sites and alternative splicing, and have named them based on their molecular weights (in kD). The larger variants, which are initiated at the first translational start site and are called Slob71 and Slob65, shift the voltage dependence of dSlo activation, measured by the whole cell conductance-voltage relationship, to the left (less depolarized voltages). Slob53 and Slob47, initiated at the third translational start site, also shift the dSlo voltage dependence to the left. In contrast, Slob57 and Slob51, initiated at the second translational start site, shift the conductance-voltage relationship of dSlo substantially to more depolarized voltages, cause an apparent dSlo channel inactivation, and increase the deactivation rate of the channel. These results indicate that the amino-terminal region of Slob plays a critical role in its modulation of dSlo.

Dissociation enzyme effects on the potassium currents of inner hair cells isolated from guinea-pig cochlea

Tetraethylammonium (TEA)-sensitive potassium currents in the cochlear inner hair cells (IHCs) possess the kinetics of fast inactivation. Some enzymes using for IHCs dissociation affect these inactivation kinetics. IHCs were dissociated from guinea-pig cochlea by 1 mg/ml trypsin or 0.25 mg/ml protease VIII, and the properties of the K+ currents were compared using conventional whole-cell voltage-clamp recordings. TEA-sensitive potassium currents showed fast inactivation kinetics in both trypsin-dissociated cells and protease VIII-dissociated cells. The time constant of the inactivation phase in trypsin-treated cells was similar to that in protease VIII-treated cells. However, the rate of inactivation (compared by the ratio between the steady-state current and initial peak current) in protease VIII-treated cells was larger than that in trypsin-treated cells. In protease VIII-dissociated cells, the time constant of recovery from inactivation elucidated by paired-pulse protocol was 3.5 ms. Papain is another enzyme that is sometimes used for dissociating IHCs, so effects of papain were observed. Extracellular papain application (8 unit/ml) demonstrated a slight increase of the outward potassium currents.

Somatic localization of a specific large-conductance calcium-activated potassium channel subtype controls compartmentalized ethanol sensitivity in the nucleus accumbens

Alcohol is an addictive drug that targets a variety of ion channels and receptors. To address whether the effects of alcohol are compartment specific (soma vs dendrite), we examined the effects of ethanol (EtOH) on large-conductance calcium-activated potassium channels (BK) in cell bodies and dendrites of freshly isolated neurons from the rat nucleus accumbens (NAcc), a region known to be critical for the development of addiction. Compartment-specific drug action was indeed observed. Clinically relevant concentrations of EtOH increased somatic but not dendritic BK channel open probability. Electrophysiological single-channel recordings and pharmacological analysis of the BK channel in excised patches from each region indicated a number of differences, suggestive of a compartment-specific expression of the beta4 subunit of the BK channel, that might explain the differential alcohol sensitivity. These parameters included activation kinetics, calcium dependency, and toxin blockade. Reverse transcription-PCR showed that both BK channel beta1 and beta4 subunit mRNAs are found in the NAcc, although the signal for beta1 is significantly weaker. Immunohistochemistry revealed that beta1 subunits were found in both soma and dendrites, whereas beta4 appeared restricted to the soma. These findings suggest that the beta4 subunit may confer EtOH sensitivity to somatic BK channels, whereas the absence of beta4 in the dendrite results in insensitivity to the drug. Consistent with this idea, acute EtOH potentiated alphabeta4 BK currents in transfected human embryonic kidney cells, whereas it failed to alter alphabeta1 BK channel-mediated currents. Finally, an EtOH concentration (50 mm) that increased BK channel open probability strongly decreased the duration of somatic-generated action potential in NAcc neurons.

Ca2+-activated K+ (BK) channel inactivation contributes to spike broadening during repetitive firing in the rat lateral amygdala

In many neurons, trains of action potentials show frequency-dependent broadening. This broadening results from the voltage-dependent inactivation of K+ currents that contribute to action potential repolarisation. In different neuronal cell types these K+ currents have been shown to be either slowly inactivating delayed rectifier type currents or rapidly inactivating A-type voltage-gated K+ currents. Recent findings show that inactivation of a Ca2+-dependent K+ current, mediated by large conductance BK-type channels, also contributes to spike broadening. Here, using whole-cell recordings in acute slices, we examine spike broadening in lateral amygdala projection neurons. Spike broadening is frequency dependent and is reversed by brief hyperpolarisations. This broadening is reduced by blockade of voltage-gated Ca2+ channels and BK channels. In contrast, broadening is not blocked by high concentrations of 4-aminopyridine (4-AP) or alpha-dendrotoxin. We conclude that while inactivation of BK-type Ca2+-activated K+ channels contributes to spike broadening in lateral amygdala neurons, inactivation of another as yet unidentified outward current also plays a role.

Conditional protein phosphorylation regulates BK channel activity in rat cerebellar Purkinje neurons

Large conductance calcium- and voltage-activated potassium (BK) channels are widely expressed in the mammalian central nervous system. Although the activity of BK channels in endocrine and vascular cells is regulated by protein kinases and phosphatases associated with the channel complex, direct evidence for such modulation in neurons is largely lacking. Single-channel analysis from inside-out patches isolated from the soma of dissociated rat cerebellar Purkinje neurons demonstrated that the activity of BK channels is regulated by multiple endogenous protein kinases and protein phosphatases in the membrane patch. The majority of BK channels were non-inactivating and displayed a 'low' activity phenotype determined at +40 mV and 1 muM intracellular free calcium. These channels were activated by cAMP-dependent protein kinase (PKA) associated with the patch and the extent of PKA activation was limited by an opposing endogenous type 2A-like protein phosphatase (PP2A). Importantly, PKA activation was dependent upon the prior phosphorylation status of the BK channel complex dynamically controlled by protein kinase C (PKC) and protein phosphatase 1 (PP1). In contrast, Purkinje cells also displayed a low proportion of non-inactivating BK channels with a 'high' activity under the same recording conditions and these channels were inhibited by endogenous PKA. Our data suggest that: (1) multiple endogenous protein kinases and phosphatases functionally couple to the BK channel complex to allow conditional modulation of BK channel activity in neurons, and (2) native, phenotypically distinct, neuronal BK channels are differentially sensitive to PKA-dependent phosphorylation.

Effects of trypsin on large-conductance Ca2+-activated K+ channels of guinea-pig outer hair cells

High-conductance Ca(2+)-activated K(+) (BK(Ca)) channels from isolated adult guinea-pig outer hair cells were studied in inside-out membrane patches. They had a 300 pS unitary conductance and were inhibited by tetraethyl ammonium (1 mM), iberiotoxin (33 nM) and charybdotoxin (50 nM). In symmetrical 144 mM KCl their K(+) permeability (P(K)) was 5.4 x 10(-13) cm(3)/s; this was reduced to around 4.5 x 10(-13) cm(3)/s with 160 mM Na(+) in place of K(+) on either internal or external membrane surface. BK(Ca) channels from trypsin-isolated hair cells had a high open probability, that depended on both membrane voltage (16 mV/e-fold change) and the concentration of calcium ions at their intracellular surface ([Ca(2+)](i)). The Hill coefficient was 3-4. About 50% of BK(Ca) channels from mechanically isolated outer hair cells had similar characteristics; the remainder had the same high conductance but a low open probability. Trypsin (<0.5 mg/ml) applied to the intracellular face of these 'inactive' channels markedly increased their open probability. It is possible that exposure to trypsin during cell isolation removes an inactivating beta subunit. This would account for the absence of 'inactive' BK(Ca) channels in trypsin-isolated cells.

Variants of the KCNMB3 regulatory subunit of maxi BK channels affect channel inactivation

The steady-state and kinetic properties of the KCNMB3 regulatory subunits associated with calcium-activated potassium channels (BK channels) are presented. BK channels containing four sequence variants (V1-V4) in the four different isoforms of the beta-subunit (beta3a, beta3b, beta3c, and beta3d) were expressed in Xenopus oocytes. Reconstituted BK channel inactivation ranged from none to around 90% inactivation. In particular, channels expressing the beta3b-V4 variant displayed a right shift in the potassium current voltage-dependence of activation and inactivated to about 30% of the maximum conductance, compared with wild-type beta3b channels that showed no inactivation. When the membrane potential was depolarized, BK channels inactivated with a very rapid time course (approximately 2-6 ms). This same variant was previously demonstrated to show subtly higher incidence in patients with idiopathic epilepsy (IE) compared with controls, especially when combined with variant V2 (combined heterozygotes). Furthermore, the gene maps to a region containing a susceptibility factor for this disorder. Taken together, these data suggest that neurons expressing BK channels composed of the beta3b-V4 variant subunit may experience reduced levels of inhibition and may therefore play permissive roles in high levels of neuronal activity that is characteristic of epilepsy.

Calcium influx via L- and N-type calcium channels activates a transient large-conductance Ca2+-activated K+ current in mouse neocortical pyramidal neurons

Ca2+-activated K+ currents and their Ca2+ sources through high-threshold voltage-activated Ca2+ channels were studied using whole-cell patch-clamp recordings from freshly dissociated mouse neocortical pyramidal neurons. In the presence of 4-aminopyridine, depolarizing pulses evoked transient outward currents and several components of sustained currents in a subgroup of cells. The fast transient current and a component of the sustained currents were Ca2+ dependent and sensitive to charybdotoxin and iberiotoxin but not to apamin, suggesting that they were mediated by large-conductance Ca2+-activated K+ (BK) channels. Thus, mouse neocortical neurons contain both inactivating and noninactivating populations of BK channels. Blockade of either L-type Ca2+ channels by nifedipine or N-type Ca2+ channels by omega-conotoxin GVIA reduced the fast transient BK current. These data suggest that the transient BK current is activated by Ca2+ entry through both N- and L-type Ca2+ channels. The physiological role of the fast transient BK current was also examined using current-clamp techniques. Iberiotoxin broadened action potentials (APs), indicating a role of BK current in AP repolarization. Similarly, both the extracellular Ca2+ channel blocker Cd2+ and the intracellular Ca2+ chelator BAPTA blocked the transient component of the outward current and broadened APs in a subgroup of cells. Our results indicate that the outward current in pyramidal mouse neurons is composed of multiple components. A fast transient BK current is activated by Ca2+ entry through high-threshold voltage-activated Ca2+ channels (L- and N-type), and together with other voltage-gated K+ currents, this transient BK current plays a role in AP repolarization.

Calcium influx via L- and N-type calcium channels activates a transient large-conductance Ca2+-activated K+ current in mouse neocortical pyramidal neurons

Ca2+-activated K+ currents and their Ca2+ sources through high-threshold voltage-activated Ca2+ channels were studied using whole-cell patch-clamp recordings from freshly dissociated mouse neocortical pyramidal neurons. In the presence of 4-aminopyridine, depolarizing pulses evoked transient outward currents and several components of sustained currents in a subgroup of cells. The fast transient current and a component of the sustained currents were Ca2+ dependent and sensitive to charybdotoxin and iberiotoxin but not to apamin, suggesting that they were mediated by large-conductance Ca2+-activated K+ (BK) channels. Thus, mouse neocortical neurons contain both inactivating and noninactivating populations of BK channels. Blockade of either L-type Ca2+ channels by nifedipine or N-type Ca2+ channels by omega-conotoxin GVIA reduced the fast transient BK current. These data suggest that the transient BK current is activated by Ca2+ entry through both N- and L-type Ca2+ channels. The physiological role of the fast transient BK current was also examined using current-clamp techniques. Iberiotoxin broadened action potentials (APs), indicating a role of BK current in AP repolarization. Similarly, both the extracellular Ca2+ channel blocker Cd2+ and the intracellular Ca2+ chelator BAPTA blocked the transient component of the outward current and broadened APs in a subgroup of cells. Our results indicate that the outward current in pyramidal mouse neurons is composed of multiple components. A fast transient BK current is activated by Ca2+ entry through high-threshold voltage-activated Ca2+ channels (L- and N-type), and together with other voltage-gated K+ currents, this transient BK current plays a role in AP repolarization.

The large conductance Ca2+-activated potassium channel (pSlo) of the cockroach Periplaneta americana: structure, localization in neurons and electrophysiology

Voltage-activated, Ca2+-sensitive K+ channels (BK or maxi K,Ca channels) play a major role in the control of neuronal excitability. We have cloned pSlo, the BK channel alpha subunit of the cockroach Periplaneta americana. The amino acid sequence of pSlo shows 88% identity to dSlo from Drosophila. There are five alternatively spliced positions in pSlo showing differential expression in various tissues. A pSlo-specific antibody prominently stained the octopaminergic dorsal unpaired median (DUM) neurons and peptidergic midline neurons in Periplaneta abdominal ganglia. HEK293 cells expressing pSlo exhibit K+ channels of 170 pS conductance. They have a tendency for brief closures, exhibit subconductance states and show slight inward rectification. Activation kinetics and voltage dependence are controlled by cytoplasmic [Ca2+]. In contrast to dSlo, pSlo channels are sensitive to charybdotoxin and iberiotoxin. Mutagenesis at two positions (E254 and Q285) changed blocking efficacy of charybdotoxin. In contrast to pSlo expressed in HEK293 cells, native IbTx-sensitive K,Ca currents in DUM and in peptidergic neurons, exhibited rapid, partial inactivation. The fast component of the K,Ca current partly accounts for the repolarization and the early after-hyperpolarization of the action potential. By means of Ca2+-induced repolarization, BK channels may reduce the risk of Ca2+ overload in cockroach neurons. Interestingly, the neurons expressing pSlo were also found to express taurine, a messenger that is likely to limit overexcitation by an autocrine mechanism in mammalian central neurons.

Beta-agkistrodotoxin inhibits large-conductance calcium-activated potassium channels in rat hippocampal CA1 pyramidal neurons

Effect of beta-agkistrodotoxin (beta-AgTx), a presynaptic neurotoxin purified from snake venom, on large-conductance calcium-activated potassium channels (BK(Ca)) was studied in rat hippocampal CA1 pyramidal neurons using inside-out configuration of patch-clamp technique. The results showed that in equimolar K+ (150 mM) and 1 microM intracellular Ca2+ conditions, internal application of beta-AgTx inhibited the activity of BK(Ca) by reducing open probability (P(o)) of the channels in a concentration-dependent manner. High concentration (74 nM) of beta-AgTx completely eliminated opening of the channels. However, 37 nM beta-AgTx (at -40 mV) decreased P(o) from 0.49+/-0.07 to 0.03+/-0.03, switched two open time constants (0.51+/-0.32 and 8.77+/-1.63 ms) to be a single time constant of 0.46+/-0.40 ms. The results indicate that inhibition of BK(Ca) by beta-AgTx may account for the facilitatory phase of the toxin on acetylcholine release from nerve terminals.

Calcium-induced transitions between the spontaneous miniature outward and the transient outward currents in retinal amacrine cells

Spontaneous miniature outward currents (SMOCs) occur in a subset of retinal amacrine cells at membrane potentials between -60 and -40 mV. At more depolarized potentials, a transient outward current (I(to)) appears and SMOCs disappear. Both SMOCs and the I(to) are K(+) currents carried by BK channels. They both arise from Ca(2+) influx through high voltage-activated (HVA) Ca(2+) channels, which stimulates release of internal Ca(2+) from caffeine- and ryanodine-sensitive stores. An increase in Ca(2+) influx resulted in an increase in SMOC frequency, but also led to a decline in SMOC mean amplitude. This reduction showed a temporal dependence: the effect being greater in the latter part of a voltage step. Thus, Ca(2+) influx, although required to generate SMOCs, also produced a negative modulation of their amplitudes. Increasing Ca(2+) influx also led to a decline in the first latency to SMOC occurrence. A combination of these effects resulted in the disappearance of SMOCs, along with the concomitant appearance of the I(to) at high levels of Ca(2+) influx. Therefore, low levels of Ca(2+) influx, arising from low levels of activation of the HVA Ca(2+) channels, produce randomly occurring SMOCs within the range of -60 to -40 mV. Further depolarization leads to greater activation of the HVA Ca(2+) channels, larger Ca(2+) influx, and the disappearance of discontinuous SMOCs, along with the appearance of the I(to). Based on their characteristics, SMOCs in retinal neurons may function as synaptic noise suppressors at quiescent glutamatergic synapses.