scholarly journals Mechanism of Increased BK Channel Activation from a Channel Mutation that Causes Epilepsy

2009 ◽  
Vol 133 (3) ◽  
pp. 283-294 ◽  
Author(s):  
Bin Wang ◽  
Brad S. Rothberg ◽  
Robert Brenner
Keyword(s):  
Action Potential ◽  
Action Potentials ◽  
Dissociation Constants ◽  
Bk Channels ◽  
Bk Channel ◽  
Gain Of Function ◽  
Gating Model ◽  
Voltage Dependent ◽  
Human Mutation ◽  
Opposing Effects

Concerted depolarization and Ca2+ rise during neuronal action potentials activate large-conductance Ca2+- and voltage-dependent K+ (BK) channels, whose robust K+ currents increase the rate of action potential repolarization. Gain-of-function BK channels in mouse knockout of the inhibitory β4 subunit and in a human mutation (αD434G) have been linked to epilepsy. Here, we investigate mechanisms underlying the gain-of-function effects of the equivalent mouse mutation (αD369G), its modulation by the β4 subunit, and potential consequences of the mutation on BK currents during action potentials. Kinetic analysis in the context of the Horrigan-Aldrich allosteric gating model revealed that changes in intrinsic and Ca2+-dependent gating largely account for the gain-of-function effects. D369G causes a greater than twofold increase in the closed-to-open equilibrium constant (6.6e−7→1.65e−6) and an approximate twofold decrease in Ca2+-dissociation constants (closed channel: 11.3→5.2 µM; open channel: 0.92→0.54 µM). The β4 subunit inhibits mutant channels through a slowing of activation kinetics. In physiological recording solutions, we established the Ca2+ dependence of current recruitment during action potential–shaped stimuli. D369G and β4 have opposing effects on BK current recruitment, where D369G reduces and β4 increases K1/2 (K1/2 μM: αWT 13.7, αD369G 6.3, αWT/β4 24.8, and αD369G/β4 15.0). Collectively, our results suggest that the D369G enhancement of intrinsic gating and Ca2+ binding underlies greater contributions of BK current in the sharpening of action potentials for both α and α/β4 channels.

scholarly journals BK current contributions to action potentials across the first postnatal week reflect age dependent changes in BK current kinetics in rat hippocampal neurons

2019 ◽  
Author(s):  
Michael Hunsberger ◽  
Michelle Mynlieff
Keyword(s):  
Action Potential ◽  
Action Potentials ◽  
Hippocampal Neurons ◽  
Expression Patterns ◽  
Bk Channels ◽  
Bk Channel ◽  
Action Potential Firing ◽  
Developmental Trends ◽  
Channel Expression ◽  
Old Rats

AbstractThe large conductance calcium-activated potassium (BK) channel is a critical regulator of neuronal action potential firing and follows two distinct trends in early postnatal development: an increase in total expression and a shift from the faster activating STREX isoform to the slower ZERO isoform. We analyzed the functional consequences of developmental trends in BK channel expression in hippocampal neurons isolated from neonatal rats aged one to seven days. Following overnight cultures, action potentials were recorded using whole-cell patch clamp electrophysiology. This population of neurons undergoes a steady increase in excitability during this time and the effect of blockade of BK channel activity with 100 nM iberiotoxin, changes as the neurons mature. BK currents contribute significantly more to single action potentials in neurons of one-day old rats (with BK blockade extending action potential duration by 0.46±0.12 ms) than in those of seven-day old rats (with BK blockade extending action potential duration by 0.17±0.05 ms). BK currents also contribute consistently to maintain firing rates in neurons of one-day old rats throughout extended action potential firing; BK blockade evenly depresses action potentials frequency across action potential trains. In neurons from seven-day old rats, BK blockade initially increases firing frequency and then progressively decreases frequency as firing continues, ultimately depressing neuronal firing rates to a greater extent than in the neurons from one day old animals. These results are consistent with a transition from low expression of a fast activating BK isoform (STREX) to high expression of a slower activating isoform (ZERO).New and NoteworthyThis work describes the early developmental trends of BK channel activity. Early developmental trends in expression of BK channels, both total expression and relative isoform expression, have been previously reported, but little work describes the effect of these changes in expression patterns on excitability. Here, we show that early changes in BK channel expression patterns lead to changes in the role of BK channels in determining the action potential waveform and neuronal excitability.

scholarly journals Inhibition of Ca2+-activated large-conductance K+ channel activity alters synaptic AMPA receptor phenotype in mouse cerebellar stellate cells

2011 ◽  
Vol 106 (1) ◽  
pp. 144-152 ◽  
Author(s):  
Yu Liu ◽  
Iaroslav Savtchouk ◽  
Shoana Acharjee ◽  
Siqiong June Liu
Keyword(s):  
Potassium Channels ◽  
Action Potential ◽  
Action Potential Duration ◽  
Action Potentials ◽  
Channel Blocker ◽  
Stellate Cells ◽  
Bk Channels ◽  
Bk Channel ◽  
K Channel ◽  
Duration Of Action

Many fast-spiking inhibitory interneurons, including cerebellar stellate cells, fire brief action potentials and express α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)-type glutamate receptors (AMPAR) that are permeable to Ca2+ and do not contain the GluR2 subunit. In a recent study, we found that increasing action potential duration promotes GluR2 gene transcription in stellate cells. We have now tested the prediction that activation of potassium channels that control the duration of action potentials can suppress the expression of GluR2-containing AMPARs at stellate cell synapses. We find that large-conductance Ca2+-activated potassium (BK) channels mediate a large proportion of the depolarization-evoked noninactivating potassium current in stellate cells. Pharmacological blockade of BK channels prolonged the action potential duration in postsynaptic stellate cells and altered synaptic AMPAR subtype from GluR2-lacking to GluR2-containing Ca2+-impermeable AMPARs. An L-type channel blocker abolished an increase in Ca2+ entry that was associated with spike broadening and also prevented the BK channel blocker-induced switch in AMPAR phenotype. Thus blocking BK potassium channels prolongs the action potential duration and increases the expression of GluR2-containing receptors at the synapse by enhancing Ca2+ entry in cerebellar stellate cells.

scholarly journals BK channel inhibition by strong extracellular acidification

eLife ◽  
2018 ◽  
Vol 7 ◽  
Author(s):  
Yu Zhou ◽  
Xiao-Ming Xia ◽  
Christopher J Lingle
Keyword(s):  
Bk Channels ◽  
Bk Channel ◽  
Extracellular Acidification ◽  
Channel Inhibition ◽  
Wide Range ◽  
The Family ◽  
Voltage Dependent ◽  
Intracellular Organelles ◽  
Extracellular Side ◽  
Key Residues

Mammalian BK-type voltage- and Ca2+-dependent K+ channels are found in a wide range of cells and intracellular organelles. Among different loci, the composition of the extracellular microenvironment, including pH, may differ substantially. For example, it has been reported that BK channels are expressed in lysosomes with their extracellular side facing the strongly acidified lysosomal lumen (pH ~4.5). Here we show that BK activation is strongly and reversibly inhibited by extracellular H+, with its conductance-voltage relationship shifted by more than +100 mV at pHO 4. Our results reveal that this inhibition is mainly caused by H+ inhibition of BK voltage-sensor (VSD) activation through three acidic residues on the extracellular side of BK VSD. Given that these key residues (D133, D147, D153) are highly conserved among members in the voltage-dependent cation channel superfamily, the mechanism underlying BK inhibition by extracellular acidification might also be applicable to other members in the family.

scholarly journals LRRC52 regulates BK channel function and localization in mouse cochlear inner hair cells

2019 ◽  
Vol 116 (37) ◽  
pp. 18397-18403 ◽  
Author(s):  
Christopher J. Lingle ◽  
Pedro L. Martinez-Espinosa ◽  
Aizhen Yang-Hood ◽  
Luis E. Boero ◽  
Shelby Payne ◽  
...  
Keyword(s):  
Hair Cells ◽  
Glutamate Release ◽  
Nerve Fibers ◽  
Bk Channels ◽  
Bk Channel ◽  
Membrane Potentials ◽  
Macromolecular Complex ◽  
Inner Hair Cells ◽  
Channel Function ◽  
Voltage Dependent

The perception of sound relies on sensory hair cells in the cochlea that convert the mechanical energy of sound into release of glutamate onto postsynaptic auditory nerve fibers. The hair cell receptor potential regulates the strength of synaptic transmission and is shaped by a variety of voltage-dependent conductances. Among these conductances, the Ca2+- and voltage-activated large conductance Ca2+-activated K+channel (BK) current is prominent, and in mammalian inner hair cells (IHCs) displays unusual properties. First, BK currents activate at unprecedentedly negative membrane potentials (−60 mV) even in the absence of intracellular Ca2+elevations. Second, BK channels are positioned in clusters away from the voltage-dependent Ca2+channels that mediate glutamate release from IHCs. Here, we test the contributions of two recently identified leucine-rich-repeat–containing (LRRC) regulatory γ subunits, LRRC26 and LRRC52, to BK channel function and localization in mouse IHCs. Whereas BK currents and channel localization were unaltered in IHCs fromLrrc26knockout (KO) mice, BK current activation was shifted more than +200 mV in IHCs fromLrrc52KO mice. Furthermore, the absence of LRRC52 disrupted BK channel localization in the IHCs. Given that heterologous coexpression of LRRC52 with BK α subunits shifts BK current gating about −90 mV, to account for the profound change in BK activation range caused by removal of LRRC52, we suggest that additional factors may help define the IHC BK gating range. LRRC52, through stabilization of a macromolecular complex, may help retain some other components essential both for activation of BK currents at negative membrane potentials and for appropriate BK channel positioning.

scholarly journals Comparative gain-of-function effects of the KCNMA1-N999S mutation on human BK channel properties

2020 ◽  
Vol 123 (2) ◽  
pp. 560-570 ◽  
Author(s):  
Hans J. Moldenhauer ◽  
Katia K. Matychak ◽  
Andrea L. Meredith
Keyword(s):  
Action Potential ◽  
Potassium Channel ◽  
Antiepileptic Drug ◽  
Bk Channel ◽  
Gain Of Function ◽  
Double Mutation ◽  
Brain Physiology ◽  
Number Of Patients ◽  
Calcium Activated Potassium Channel ◽  
Channel Properties

KCNMA1, encoding the voltage- and calcium-activated potassium channel, has a pivotal role in brain physiology. Mutations in KCNMA1 are associated with epilepsy and/or dyskinesia (PNKD3). Two KCNMA1 mutations correlated with these phenotypes, D434G and N999S, were previously identified as producing gain-of-function (GOF) effects on BK channel activity. Three new patients have been reported harboring N999S, one carrying a second mutation, R1128W, but the effects of these mutations have not yet been reported under physiological K+ conditions or compared to D434G. In this study, we characterize N999S, the novel N999S/R1128W double mutation, and D434G in a brain BK channel splice variant, comparing the effects on BK current properties under a physiological K+ gradient with action potential voltage commands. N999S, N999S/R1128W, and D434G cDNAs were expressed in HEK293T cells and characterized by patch-clamp electrophysiology. N999S BK currents were shifted to negative potentials, with faster activation and slower deactivation compared with wild type (WT) and D434G. The double mutation N999S/R1128W did not show any additional changes in current properties compared with N999S alone. The antiepileptic drug acetazolamide was assessed for its ability to directly modulate WT and N999S channels. Neither the WT nor N999S channels were sensitive to the antiepileptic drug acetazolamide, but both were sensitive to the inhibitor paxilline. We conclude that N999S is a strong GOF mutation that surpasses the D434G phenotype, without mitigation by R1128W. Acetazolamide has no direct modulatory action on either WT or N999S channels, indicating that its use may not be contraindicated in patients harboring GOF KCNMA1 mutations. NEW & NOTEWORTHY KCNMA1-linked channelopathy is a new neurological disorder characterized by mutations in the BK voltage- and calcium-activated potassium channel. The epilepsy- and dyskinesia-associated gain-of-function mutations N999S and D434G comprise the largest number of patients in the cohort. This study provides the first direct comparison between D434G and N999S BK channel properties as well as a novel double mutation, N999S/R1128W, from another patient, defining the functional effects during an action potential stimulus.

BK channel β1-subunit regulation of calcium handling and constriction in tracheal smooth muscle

2006 ◽  
Vol 291 (4) ◽  
pp. L802-L810 ◽  
Author(s):  
Iurii Semenov ◽  
Bin Wang ◽  
Jeremiah T. Herlihy ◽  
Robert Brenner
Keyword(s):  
Smooth Muscle ◽  
Airway Smooth Muscle ◽  
Cell Types ◽  
Bk Channels ◽  
Bk Channel ◽  
Tracheal Smooth Muscle ◽  
Calcium Handling ◽  
Cholinergic Signaling ◽  
Smooth Muscle Function ◽  
Voltage Dependent

The large-conductance, Ca2+-activated K+ (BK) channels are regulators of voltage-dependent Ca2+ entry in many cell types. The BK channel accessory β1-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 β1-subunit has not been investigated. Utilizing the gene-targeted mice for the β1-subunit gene, we have investigated the role of the β1-subunit in tracheal smooth muscle. In mice with the β1-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 β1-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 β1-subunit and BK channel block with paxillin, indicating that BK channels contribute little to airway relaxation in the absence of the β1-subunit. Utilizing nifedipine, we found that the increased constriction caused by knockout of the β1-subunit could be accounted for by an increased recruitment of L-type voltage-dependent Ca2+ channels. These results indicate that the β1-subunit is required in airway smooth muscle for control of voltage-dependent Ca2+ influx during rest and after cholinergic signaling in BK channels.

scholarly journals Heme Regulates Allosteric Activation of the Slo1 BK Channel

2005 ◽  
Vol 126 (1) ◽  
pp. 7-21 ◽  
Author(s):  
Frank T. Horrigan ◽  
Stefan H. Heinemann ◽  
Toshinori Hoshi
Keyword(s):  
Divalent Cations ◽  
Ionic Currents ◽  
Bk Channels ◽  
Bk Channel ◽  
Voltage Sensor ◽  
Gating Currents ◽  
Calcium Dependent ◽  
Channel Opening ◽  
Activation Gate ◽  
Voltage Dependent

Large conductance calcium-dependent (Slo1 BK) channels are allosterically activated by membrane depolarization and divalent cations, and possess a rich modulatory repertoire. Recently, intracellular heme has been identified as a potent regulator of Slo1 BK channels (Tang, X.D., R. Xu, M.F. Reynolds, M.L. Garcia, S.H. Heinemann, and T. Hoshi. 2003. Nature. 425:531–535). Here we investigated the mechanism of the regulatory action of heme on heterologously expressed Slo1 BK channels by separating the influences of voltage and divalent cations. In the absence of divalent cations, heme generally decreased ionic currents by shifting the channel's G–V curve toward more depolarized voltages and by rendering the curve less steep. In contrast, gating currents remained largely unaffected by heme. Simulations suggest that a decrease in the strength of allosteric coupling between the voltage sensor and the activation gate and a concomitant stabilization of the open state account for the essential features of the heme action in the absence of divalent ions. At saturating levels of divalent cations, heme remained similarly effective with its influence on the G–V simulated by weakening the coupling of both Ca2+ binding and voltage sensor activation to channel opening. The results thus show that heme dampens the influence of allosteric activators on the activation gate of the Slo1 BK channel. To account for these effects, we consider the possibility that heme binding alters the structure of the RCK gating ring and thereby disrupts both Ca2+- and voltage-dependent gating as well as intrinsic stability of the open state.

scholarly journals The BK channel gating ring is strongly coupled to the voltage sensor

2018 ◽  
Author(s):  
Pablo Miranda ◽  
Miguel Holmgren ◽  
Teresa Giraldez
Keyword(s):  
Conformational Changes ◽  
Bk Channels ◽  
Bk Channel ◽  
Voltage Sensor ◽  
Voltage Sensitivity ◽  
Divalent Ions ◽  
Gating Currents ◽  
Open Probability ◽  
Voltage Dependent ◽  
Tightly Coupled

ABSTRACTThe open probability of large conductance voltage- and calcium-dependent potassium (BK) channels is regulated allosterically by changes in the transmembrane voltage and intracellular concentration of divalent ions (Ca2+ and Mg2+). The divalent cation sensors reside within the gating ring formed by eight Regulator of Conductance of Potassium (RCK) domains, two from each of the four channel subunits. Overall, the gating ring contains 12 sites that can bind Ca2+ with different affinities. Using patch-clamp fluorometry, we have shown robust changes in FRET signals within the gating ring in response to divalent ions and voltage, which do not directly track open probability. Only the conformational changes triggered through the RCK1 binding site are voltage-dependent in presence of Ca2+. Because the gating ring is outside the electric field, it must gain voltage sensitivity from coupling to the voltage-dependent channel opening, the voltage sensor or both. Here we demonstrate that alterations of voltage sensor dynamics known to shift gating currents produce a cognate shift in the gating ring voltage dependence, whereas changing BK channels’ relative probability of opening had little effect. These results strongly suggest that the conformational changes of the RCK1 domain of the gating ring are tightly coupled to the voltage sensor function, and this interaction is central to the allosteric modulation of BK channels.

Stimulation pulse characteristics and electrode configuration determine site of excitation in isolated mammalian skeletal muscle: implications for fatigue

2007 ◽  
Vol 103 (1) ◽  
pp. 359-368 ◽  
Author(s):  
Simeon P. Cairns ◽  
Eva R. Chin ◽  
Jean-Marc Renaud
Keyword(s):  
Skeletal Muscle ◽  
Action Potential ◽  
Soleus Muscle ◽  
Action Potentials ◽  
Surface Membrane ◽  
Electrical Field Stimulation ◽  
Mammalian Skeletal Muscle ◽  
Voltage Dependent ◽  
Flexor Digitorum Brevis ◽  
Edl Muscle

We examined whether electrical field stimulation with varying characteristics could excite isolated mammalian skeletal muscle through different sites. Supramaximal (20-V, 0.1-ms) pulse stimulation with transverse wire or parallel plate electrodes evoked similar forces in nonfatigued slow-twitch soleus and fast-twitch extensor digitorum longus (EDL) muscles from mice. d-tubocurarine shifted the twitch force-stimulation strength relationship toward higher pulse strengths with both electrode configurations in soleus muscle, suggesting that weaker pulses excite muscle via neuromuscular transmission. With wire stimulation, movement of the recording electrode along the muscle caused a delay between the stimulus artifact and the peak of the action potential, consistent with action potential propagation along the sarcolemma. TTX abolished all contractions evoked with 20-V, 0.1-ms pulses, suggesting that excitation occurred via voltage-dependent Na+ channels and, hence, muscle action potentials. TTX did not prevent force development with ≥0.4-ms pulses in soleus or 1-ms pulses in EDL muscle. Furthermore, myoplasmic Ca2+ (i.e., the fura 2 ratio) and sarcomere shortening were greater during tetanic stimulation with 2.0-ms than with 0.5-ms pulses in flexor digitorum brevis fibers from rats. TTX prevented all shortening and Ca2+ release with 0.5-ms, but not 2.0-ms, pulses, indicating that longer pulses can directly trigger Ca2+ release. Hence, proper interpretation of mechanistic studies requires precise understanding of how muscles are excited; otherwise, incorrect conclusions can be made. Using this new understanding, we showed that disrupted propagation of action potentials along the surface membrane is a major cause of fatigue in soleus muscle that is focally and continuously stimulated at 125 Hz.

Mechanisms underlying the modulation of arrhythmogenic events by components of ischemia in guinea pig cardiac myocytes

1994 ◽  
Vol 72 (4) ◽  
pp. 382-393 ◽  
Author(s):  
Qi-Ying Liu ◽  
Mario Vassalle
Keyword(s):  
Guinea Pig ◽  
Action Potential ◽  
Voltage Clamp ◽  
Action Potentials ◽  
Ventricular Myocytes ◽  
Resting Potential ◽  
Spontaneous Discharge ◽  
High K ◽  
Voltage Dependent ◽  
Dependent Mechanism

The effects of some components of ischemia on the oscillatory (Vos) and nonoscillatory (Vex) potentials and respective currents (Ios and Iex), as well as their mechanisms, were studied in guinea pig isolated ventricular myocytes by means of a single-microelectrode, discontinuous voltage clamp method. Repetitive activations induced not only Vos and Ios, but also Vex and Iex. A small decrease in resting potential caused an immediate increase in Vos followed by a gradual increase due to the longer action potential. Immediate and gradual increases in Ios also occurred during voltage clamp steps. A small depolarization increased Vos and Vex, and facilitated the induction of spontaneous discharge by fast drive. At Vh where INa is inactivated, depolarizing steps induced larger Ios and Iex, indicating the importance of the Na-independent Ca loading. High [K]odecreased the resting potential, but also Vos, Vex, Ios, Iex, and ICa. In high [K]o, depolarization still increased Vos and Vex. Norepinephrine (NE) enhanced Vos and Vex, and also Ios and Iex, during voltage clamp steps. High [K]o antagonized NE effects, and NE those of high [K]o. In conclusion, on depolarization, Vos and Ios immediately increase through a voltage-dependent mechanism; and then Vos and Ios gradually increase, apparently through an increased Ca load related to the longer action potentials and the Na–Ca exchange. The depolarization induced by Vex may contribute to increase Vos size. Vos and Vex are similarly influenced by different procedures that modify Ca load. The arrhythmogenic events are enhanced by the simultaneous presence of depolarization, faster rate, or NE. Instead, high [K]o decreases Vos and Vex by decreasing ICa and opposes the effects of NE. The voltage clamp results show that potentiation and antagonism between different components of ischemia are due primarily to changes in Ca loading and not to changes in action potential configuration.Key words: ischemia, arrhythmias, oscillatory and nonoscillatory potentials and currents, norepinephrine, potassium.

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