Phosphorylation of a constitutive serine inhibits BK channel variants containing the alternate exon "SRKR"

Paraoxonase 3 regulates the pore-forming α subunit of the large-conductance K+ channel

Paraoxonase 3 (PON3) is expressed in the aldosterone-sensitive distal nephron (ASDN) where the fine tuning of Na+ and K+ homeostasis in the kidney occurs. Flow-induced K+ secretion within intercalated cells (ICs) of the ASDN is mediated by the large-conductance K+ (BK) channels. We have previously shown that renal PON3 expression was altered by dietary K+ intake and that Pon3 knockout (KO) mice had lower plasma [K+]. These findings led us to hypothesize that PON3 may have a role in regulating renal K+ secretion by altering BK channel functional expression. The present study shows that both PON3 and the pore-forming α subunit of the BK channel (αBK) are expressed in ICs of mouse kidney and that the two proteins co-localize to the same cellular compartments when expressed in HEK293 cells. Using a biochemical approach, we show that PON3 interacts with αBK endogenously in the mouse kidney and when both proteins were co-expressed in HEK293 cells. We also examined the effects of PON3 on αBK expression and channel activity in HEK293 cells. We found that paxilline-sensitive BK currents were significantly reduced by PON3 expression, likely a consequence of lower surface abundance of αBK. Consistent with this finding, we observed a stronger αBK staining signal in ICs of Pon3 KO kidneys. Together, our data suggest that PON3 negatively regulates the functional expression of BK channels.

Structural mapping of patient-associated KCNMA1 gene variants

KCNMA1-linked channelopathy is a neurological disorder characterized by seizures, motor abnormalities, and neurodevelopmental disabilities. The disease mechanisms are predicted to result from alterations in KCNMA1-encoded BK K+ channel activity; however, only a subset of the patient-associated variants have been functionally studied. The localization of these variants within the tertiary structure or evaluation by pathogenicity algorithms has not been systematically assessed. In this study, 82 nonsynonymous patient-associated KCNMA1 variants were mapped within the BK channel protein. Fifty-three variants localized within cryoelectron microscopy-resolved structures, including 21 classified as either gain of function (GOF) or loss of function (LOF) in BK channel activity. Clusters of LOF variants were identified in the pore, the AC region (RCK1), and near the Ca2+ bowl (RCK2), overlapping with sites of pharmacological or endogenous modulation. However, no clustering was found for GOF variants. To further understand variants of uncertain significance (VUSs), assessments by multiple standard pathogenicity algorithms were compared, and new thresholds for sensitivity and specificity were established from confirmed GOF and LOF variants. An ensemble algorithm was constructed (KCNMA1 meta score (KMS)), consisting of a weighted summation of this trained dataset combined with a structural component derived from the Ca2+-bound and unbound BK channels. KMS assessment differed from the highest-performing individual algorithm (REVEL) at 10 VUS residues, and a subset were studied further by electrophysiology in HEK293 cells. M578T, E656A, and D965V (KMS+;REVEL-) were confirmed to alter BK channel properties in voltage-clamp recordings, and D800Y (KMS-;REVEL+) was assessed as benign under the test conditions. However, KMS failed to accurately assess K457E. These combined results reveal the distribution of potentially disease-causing KCNMA1 variants within BK channel functional domains and pathogenicity evaluation for VUSs, suggesting strategies for improving channel-level predictions in future studies by building on ensemble algorithms such as KMS.

Kcnma1 alternative splicing in mouse kidney: regulation during development and by dietary K+ intake

The pore-forming α-subunit of the large-conductance K+ (BK) channel is encoded by a single gene, KCNMA1. BK channel-mediated K+ secretion in the kidney is crucial for overall renal K+ homeostasis in both physiological and pathological conditions. BK channels achieve phenotypic diversity by various mechanisms, including substantial exon rearrangements at seven major alternative splicing sites. However, KCNMA1 alternative splicing in the kidney has not been characterized. The present study aims to identify the major splice variants of mouse Kcnma1 in whole kidney and distal nephron segments. We designed primers that specifically cross exons within each alternative splice site of mouse Kcnma1 and performed real-time quantitative RT-PCR (RT-qPCR) to quantify relative abundance of each splice variant. Our data suggest that Kcnma1 splice variants within mouse kidney are less diverse than in the brain. During postnatal kidney development, most Kcnma1 splice variants at site 5 and the COOH terminus increase in abundance over time. Within the kidney, the regulation of Kcnma1 alternative exon splicing within these two sites by dietary K+ loading is both site and sex specific. In microdissected distal tubules, the Kcnma1 alternative splicing profile, as well as its regulation by dietary K+, are distinctly different than in the whole kidney, suggesting segment and/or cell type specificity in Kcnma1 splicing events. Overall, our data provide evidence that Kcnma1 alternative splicing is regulated during postnatal development and may serve as an important adaptive mechanism to dietary K+ loading in mouse kidney.NEW & NOTEWORTHY We identified the major Kcnma1 splice variants that are specifically expressed in the whole mouse kidney or aldosterone-sensitive distal nephron segments. Our data suggest that Kcnma1 alternative splicing is developmentally regulated and subject to changes in dietary K+.

BK Channelopathies and KCNMA1-Linked Disease Models

Novel KCNMA1 variants, encoding the BK K+ channel, are associated with a debilitating dyskinesia and epilepsy syndrome. Neurodevelopmental delay, cognitive disability, and brain and structural malformations are also diagnosed at lower incidence. More than half of affected individuals present with a rare negative episodic motor disorder, paroxysmal nonkinesigenic dyskinesia (PNKD3). The mechanistic relationship of PNKD3 to epilepsy and the broader spectrum of KCNMA1-associated symptomology is unknown. This review summarizes patient-associated KCNMA1 variants within the BK channel structure, functional classifications, genotype-phenotype associations, disease models, and treatment. Patient and transgenic animal data suggest delineation of gain-of-function (GOF) and loss-of-function KCNMA1 neurogenetic disease, validating two heterozygous alleles encoding GOF BK channels (D434G and N999S) as causing seizure and PNKD3. This discovery led to a variant-defined therapeutic approach for PNKD3, providing initial insight into the neurological basis. A comprehensive clinical definition of monogenic KCNMA1-linked disease and the neuronal mechanisms currently remain priorities for continued investigation.

Calcium-Activated Big-Conductance (BK) Potassium Channels Traffic through Nuclear Envelopes into Kinocilia in Ray Electrosensory Cells

Electroreception through ampullae of Lorenzini in the little skate, Leucoraja erinacea, involves functional coupling between voltage-activated calcium channels (CaV1.3, cacna1d) and calcium-activated big-conductance potassium (BK) channels (BK, kcnma1). Whole-mount confocal microscopy was used to characterize the pleiotropic expression of BK and CaV1.3 in intact ampullae. BK and CaV1.3 are co-expressed in electrosensory cell plasma membranes, nuclear envelopes and kinocilia. Nuclear localization sequences (NLS) were predicted in BK and CaV1.3 by bioinformatic sequence analyses. The BK NLS is bipartite, occurs at an alternative splice site for the mammalian STREX exon and contains sequence targets for post-translational phosphorylation. Nuclear localization of skate BK channels was characterized in heterologously transfected HEK293 cells. Double-point mutations in the bipartite NLS (KR to AA or SVLS to AVLA) independently attenuated BK channel nuclear localization. These findings support the concept that BK partitioning between the electrosensory cell plasma membrane, nucleus and kinocilium may be regulated through a newly identified bipartite NLS.

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.

CACNA1C-Related Channelopathies

The CACNA1C gene encodes the pore-forming subunit of the CaV1.2 L-type Ca2+ channel, a critical component of membrane physiology in multiple tissues, including the heart, brain, and immune system. As such, mutations altering the function of these channels have the potential to impact a wide array of cellular functions. The first mutations identified within CACNA1C were shown to cause a severe, multisystem disorder known as Timothy syndrome (TS), which is characterized by neurodevelopmental deficits, long-QT syndrome, life-threatening cardiac arrhythmias, craniofacial abnormalities, and immune deficits. Since this initial description, the number and variety of disease-associated mutations identified in CACNA1C have grown tremendously, expanding the range of phenotypes observed in affected patients. CACNA1C channelopathies are now known to encompass multisystem phenotypes as described in TS, as well as more selective phenotypes where patients may exhibit predominantly cardiac or neurological symptoms. Here, we review the impact of genetic mutations on CaV1.2 function and the resultant physiological consequences.

CaV1.2 channelopathic mutations evoke diverse pathophysiological mechanisms

The first pathogenic mutation in CaV1.2 was identified in 2004 and was shown to cause a severe multisystem disorder known as Timothy syndrome (TS). The mutation was localized to the distal S6 region of the channel, a region known to play a major role in channel activation. TS patients suffer from life-threatening cardiac symptoms as well as significant neurodevelopmental deficits, including autism spectrum disorder (ASD). Since this discovery, the number and variety of mutations identified in CaV1.2 have grown tremendously, and the distal S6 regions remain a frequent locus for many of these mutations. While the majority of patients harboring these mutations exhibit cardiac symptoms that can be well explained by known pathogenic mechanisms, the same cannot be said for the ASD or neurodevelopmental phenotypes seen in some patients, indicating a gap in our understanding of the pathogenesis of CaV1.2 channelopathies. Here, we use whole-cell patch clamp, quantitative Ca2+ imaging, and single channel recordings to expand the known mechanisms underlying the pathogenesis of CaV1.2 channelopathies. Specifically, we find that mutations within the S6 region can exert independent and separable effects on activation, voltage-dependent inactivation (VDI), and Ca2+-dependent inactivation (CDI). Moreover, the mechanisms underlying the CDI effects of these mutations are varied and include altered channel opening and possible disruption of CDI transduction. Overall, these results provide a structure-function framework to conceptualize the role of S6 mutations in pathophysiology and offer insight into the biophysical defects associated with distinct clinical manifestations.

Effect of an autism-associated KCNMB2 variant, G124R, on BK channel properties

BK K⁺ channels are critical regulators of neuron and muscle excitability, comprised of a tetramer of pore-forming αsubunits from the KCNMA1 gene and cell- and tissue-selective β subunits (KCNMB1-4). Mutations in KCNMA1 are associated with neurological disorders, including autism. However, little is known about the role of neuronal BK channel β subunits in human neuropathology. The β2 subunit is expressed in central neurons and imparts inactivation to BK channels, as well as altering activation and deactivation gating. In this study, we report the functional effect of G124R, a novel KCNMB2 mutation obtained from whole-exome sequencing of a patient diagnosed with autism spectrum disorder. Residue G124, located in the extracellular loop between TM1 and TM2, is conserved across species, and the G124R missense mutation is predicted deleterious with computational tools. To investigate the pathogenicity potential, BK channels were co-expressed with β2^WT and β2^G124R subunits in HEK293T cells. BK/β2 currents were assessed from inside-out patches under physiological K⁺ conditions (140/6 mM K⁺ and 10 μM Ca²⁺) during activation and inactivation (voltage-dependence and kinetics). Using β2 subunits lacking inactivation (β2IR) revealed that currents from BK/β2IR^G124R channels activated 2-fold faster and deactivated 2-fold slower compared with currents from BK/β2IR^WT channels, with no change in the voltage-dependence of activation (V_1/2). Despite the changes in the BK channel opening and closing, BK/β2^G124R inactivation rates (τ_inact and τ_recovery), and the V_1/2 of inactivation, were unaltered compared with BK/β2^WT channels under standard steady-state voltage protocols. Action potential-evoked current was also unchanged. Thus, the mutant phenotype suggests the β2^G124R TM1-TM2 extracellular loop could regulate BK channel activation and deactivation kinetics. However, additional evidence is needed to validate pathogenicity for this patient-associated variant in KCNMB2.

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KCNMA1 channelopathy is a neurobehavioral disorder associated with seizures, dyskinesia, and intellectual disability.

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KCNMB2 encodes an accessory β subunit that confers inactivation to the KCNMA1 pore-forming α subunit BK channel.

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The KCNMB2-G124R variant, identified in an autistic individual, affects BK/β2 channel activation but not inactivation.

BK channel activation by L-type Ca2+ channels CaV1.2 and CaV1.3 during the subthreshold phase of an action potential

Mammalian circadian (24 h) rhythms are timed by the pattern of spontaneous action potential firing in the suprachiasmatic nucleus (SCN). This oscillation in firing is produced through circadian regulation of several membrane currents, including large-conductance Ca²⁺- and voltage-activated K⁺ (BK) and L-type Ca²⁺ channel (LTCC) currents. During the day steady-state BK currents depend mostly on LTCCs for activation, whereas at night they depend predominantly on ryanodine receptors (RyRs). However, the contribution of these Ca²⁺ channels to BK channel activation during action potential firing has not been thoroughly investigated. In this study, we used a pharmacological approach to determine that both LTCCs and RyRs contribute to the baseline membrane potential of SCN action potential waveforms, as well as action potential-evoked BK current, during the day and night, respectively. Since the baseline membrane potential is a major determinant of circadian firing rate, we focused on the LTCCs contributing to low voltage activation of BK channels during the subthreshold phase. For these experiments, two LTCC subtypes found in SCN (Ca_V1.2 and Ca_V1.3) were coexpressed with BK channels in heterologous cells, where their differential contributions could be separately measured. Ca_V1.3 channels produced currents that were shifted to more hyperpolarized potentials compared with Ca_V1.2, resulting in increased subthreshold Ca²⁺ and BK currents during an action potential command. These results show that although multiple Ca²⁺ sources in SCN can contribute to the activation of BK current during an action potential, specific BK-Ca_V1.3 partnerships may optimize the subthreshold BK current activation that is critical for firing rate regulation.NEW & NOTEWORTHY BK K⁺ channels are important regulators of firing. Although Ca²⁺ channels are required for their activation in excitable cells, it is not well understood how BK channels activate using these Ca²⁺ sources during an action potential. This study demonstrates the differences in BK current activated by Ca_V1.2 and Ca_V1.3 channels in clock neurons and heterologous cells. The results define how specific ion channel partnerships can be engaged during distinct phases of the action potential.

Neuronal Inactivity Co-opts LTP Machinery to Drive Potassium Channel Splicing and Homeostatic Spike Widening

Homeostasis of neural firing properties is important in stabilizing neuronal circuitry, but how such plasticity might depend on alternative splicing is not known. Here we report that chronic inactivity homeostatically increases action potential duration by changing alternative splicing of BK channels; this requires nuclear export of the splicing factor Nova-2. Inactivity and Nova-2 relocation were connected by a novel synapto-nuclear signaling pathway that surprisingly invoked mechanisms akin to Hebbian plasticity: Ca²⁺-permeable AMPA receptor upregulation, L-type Ca²⁺ channel activation, enhanced spine Ca²⁺ transients, nuclear translocation of a CaM shuttle, and nuclear CaMKIV activation. These findings not only uncover commonalities between homeostatic and Hebbian plasticity but also connect homeostatic regulation of synaptic transmission and neuronal excitability. The signaling cascade provides a full-loop mechanism for a classic autoregulatory feedback loop proposed ∼25 years ago. Each element of the loop has been implicated previously in neuropsychiatric disease.

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.

Ion Channels Controlling Circadian Rhythms in Suprachiasmatic Nucleus Excitability

Animals synchronize to the environmental day-night cycle by means of an internal circadian clock in the brain. In mammals, this timekeeping mechanism is housed in the suprachiasmatic nucleus (SCN) of the hypothalamus and is entrained by light input from the retina. One output of the SCN is a neural code for circadian time, which arises from the collective activity of neurons within the SCN circuit and comprises two fundamental components: 1) periodic alterations in the spontaneous excitability of individual neurons that result in higher firing rates during the day and lower firing rates at night, and 2) synchronization of these cellular oscillations throughout the SCN. In this review, we summarize current evidence for the identity of ion channels in SCN neurons and the mechanisms by which they set the rhythmic parameters of the time code. During the day, voltage-dependent and independent Na⁺ and Ca²⁺ currents, as well as several K⁺ currents, contribute to increased membrane excitability and therefore higher firing frequency. At night, an increase in different K⁺ currents, including Ca²⁺-activated BK currents, contribute to membrane hyperpolarization and decreased firing. Layered on top of these intrinsically regulated changes in membrane excitability, more than a dozen neuromodulators influence action potential activity and rhythmicity in SCN neurons, facilitating both synchronization and plasticity of the neural code.

Comparative gain-of-function effects of the KCNMA1-N999S mutation on human BK 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.

Effects of Single Nucleotide Polymorphisms in Human KCNMA1 on BK Current Properties

BK Ca2+-activated K+ channels are important regulators of membrane excitability. Multiple regulatory mechanisms tailor BK current properties across tissues, such as alternative splicing, posttranslational modifications, and auxiliary subunits. Another potential mechanism for modulating BK channel activity is genetic variation due to single nucleotide polymorphisms (SNPs). The gene encoding the human BK α subunit, KCNMA1, contains hundreds of SNPs. However, the variation in BK channel activity due to SNPs is not well studied. Here, we screened the effects of four SNPs (A138V, C495G, N599D, and R800W) on BK currents in HEK293T cells, selected based on predicted protein pathogenicity or disease linkage. We found that the SNPs C495G and R800W had the largest effects on BK currents, affecting the conductance-voltage relationship across multiple Ca2+ conditions in the context of two BK channel splice variants. In symmetrical K+, C495G shifted the V1/2 to more hyperpolarized potentials (by -15 to -20 mV) and accelerated activation, indicating C495G confers some gain-of-function properties. R800W shifted the V1/2 to more depolarized potentials (+15 to +35 mV) and slowed activation, conferring loss-of-function properties. Moreover, the C495G and R800W effects on current properties were found to persist with posttranslational modifications. In contrast, A138V and N599D had smaller and more variable effects on current properties. Neither application of alkaline phosphatase to patches, which results in increased BK channel activity attributed to channel dephosphorylation, nor bidirectional redox modulations completely abrogated SNP effects on BK currents. Lastly, in physiological K+, C495G increased the amplitude of action potential (AP)-evoked BK currents, while R800W had a more limited effect. However, the introduction of R800W in parallel with the epilepsy-linked mutation D434G (D434G/R800W) decreased the amplitude of AP-evoked BK currents compared with D434G alone. These results suggest that in a physiological context, C495G could increase BK activation, while the effects of the loss-of-function SNP R800W could oppose the gain-of-function effects of an epilepsy-linked mutation. Together, these results implicate naturally occurring human genetic variation as a potential modifier of BK channel activity across a variety of conditions.

Shaw and Shal voltage-gated potassium channels mediate circadian changes in Drosophila clock neuron excitability

As in mammals, Drosophila circadian clock neurons display rhythms of activity with higher action potential firing rates and more positive resting membrane potentials during the day. This rhythmic excitability has been widely observed but, critically, its regulation remains unresolved. We have characterized and modelled the changes underlying these electrical activity rhythms in the lateral ventral clock neurons (LNvs). We show that currents mediated by the voltage-gated potassium channels Shaw (Kv3) and Shal (Kv4) oscillate in a circadian manner. Disruption of these channels, by expression of dominant negative (DN) subunits, leads to changes in circadian locomotor activity and shortens lifespan. LNv whole-cell recordings then show that changes in Shaw and Shal currents drive changes in action potential firing rate and that these rhythms are abolished when the circadian molecular clock is stopped. A whole-cell biophysical model using Hodgkin-Huxley equations can recapitulate these changes in electrical activity. Based on this model and by using dynamic clamp to manipulate clock neurons directly, we can rescue the pharmacological block of Shaw and Shal, restore the firing rhythm, and thus demonstrate the critical importance of Shaw and Shal. Together, these findings point to a key role for Shaw and Shal in controlling circadian firing of clock neurons and show that changes in clock neuron currents can account for this. Moreover, with dynamic clamp we can switch the LNvs between morning-like and evening-like states of electrical activity. We conclude that changes in Shaw and Shal underlie the daily oscillation in LNv firing rate.

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.

Spliceosome factors target timeless (tim) mRNA to control clock protein accumulation and circadian behavior in Drosophila

Transcription-translation feedback loops that comprise eukaryotic circadian clocks rely upon temporal delays that separate the phase of active transcription of clock genes, such as Drosophila period (per) and timeless (tim), from negative feedback by the two proteins. However, our understanding of the mechanisms involved is incomplete. Through an RNA interference screen, we found that pre-mRNA processing 4 (PRP4) kinase, a component of the U4/U5.U6 triple small nuclear ribonucleoprotein (tri-snRNP) spliceosome, and other tri-snRNP components regulate cycling of the molecular clock as well as rest:activity rhythms. Unbiased RNA-Sequencing uncovered an alternatively spliced intron in tim whose increased retention upon prp4 downregulation leads to decreased TIM levels. We demonstrate that the splicing of tim is rhythmic with a phase that parallels delayed accumulation of the protein in a 24 hr cycle. We propose that alternative splicing constitutes an important clock mechanism for delaying the daily accumulation of clock proteins, and thereby negative feedback by them.

Editorial note: This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter).

Differential contribution of Ca2+ sources to day and night BK current activation in the circadian clock

Large conductance K⁺ (BK) channels are expressed widely in neurons, where their activation is regulated by membrane depolarization and intracellular Ca²⁺ (Ca²⁺_i). To enable this regulation, BK channels functionally couple to both voltage-gated Ca²⁺ channels (VGCCs) and channels mediating Ca²⁺ release from intracellular stores. However, the relationship between BK channels and their specific Ca²⁺ source for particular patterns of excitability is not well understood. In neurons within the suprachiasmatic nucleus (SCN)-the brain's circadian clock-BK current, VGCC current, and Ca²⁺_i are diurnally regulated, but paradoxically, BK current is greatest at night when VGCC current and Ca²⁺_i are reduced. Here, to determine whether diurnal regulation of Ca²⁺ is relevant for BK channel activation, we combine pharmacology with day and night patch-clamp recordings in acute slices of SCN. We find that activation of BK current depends primarily on three types of channels but that the relative contribution changes between day and night. BK current can be abrogated with nimodipine during the day but not at night, establishing that L-type Ca²⁺ channels (LTCCs) are the primary daytime Ca²⁺ source for BK activation. In contrast, dantrolene causes a significant decrease in BK current at night, suggesting that nighttime BK activation is driven by ryanodine receptor (RyR)-mediated Ca²⁺_i release. The N- and P/Q-type Ca²⁺ channel blocker ω-conotoxin MVIIC causes a smaller reduction of BK current that does not differ between day and night. Finally, inhibition of LTCCs, but not RyRs, eliminates BK inactivation, but the BK β2 subunit was not required for activation of BK current by LTCCs. These data reveal a dynamic coupling strategy between BK channels and their Ca²⁺ sources in the SCN, contributing to diurnal regulation of SCN excitability.

Novel description of the large conductance Ca2+-modulated K+ channel current, BK, during an action potential from suprachiasmatic nucleus neurons

The contribution of the large conductance, Ca²⁺‐modulated, voltage‐gated K⁺ channel current, I_BK, to the total current during an action potential (AP) from suprachiasmatic nucleus (SCN) neurons is described using a novel computational approach. An experimental recording of an SCN AP and the corresponding AP‐clamp recording of I_BK from the literature were both digitized. The AP data set was applied computationally to a kinetic model of I_BK that was based on results from a clone of the BK channel α subunit heterologolously expressed in Xenopus oocytes. The I_BK model result during an AP was compared with the AP‐clamp recording of I_BK. The comparison suggests that a change in the intracellular Ca²⁺ concentration does not have an immediate effect on BK channel kinetics. Rather, a delay of a few milliseconds may occur prior to the full effect of a change in Ca_i²⁺. As shown elsewhere, the β2 subunit of the BK channel in the SCN, which is present in the daytime along with the α subunit, shifts the BK channel activation curve leftward on the voltage axis relative to the activation curve of BK channels comprised of the α subunit alone. That shift may underlie the diurnal changes in electrical activity that occur in the SCN and it may also enhance the delay in the effect of a change in Ca_i²⁺ on BK kinetics reported here. The implication of these results for models of the AP for neurons in which BK channels are present is that an additional time dependent process may be required in the models, a process that describes the time dependence of the development of a change in the intracellular Ca²⁺ concentration on BK channel gating.

Novel Rbfox2 isoforms associated with alternative exon usage in rat cortex and suprachiasmatic nucleus

Transcriptome diversity in adult neurons is partly mediated by RNA binding proteins (RBPs), including the RBFOX factors. RBFOX3/NeuN, a neuronal maturity marker, is strangely depleted in suprachiasmatic nucleus (SCN) neurons, and may be compensated by a change in Rbfox2 expression. In this study, we found no superficial changes in Rbfox2 expression in the SCN, but mRNA population analysis revealed a distinct SCN transcript profile that includes multiple novel Rbfox2 isoforms. Of eleven isoforms in SCN and cerebral cortex that exhibit exon variation across two protein domains, we found a 3-fold higher abundance of a novel (‘−12–40’) C-terminal domain (CTD)-variant in the SCN. This isoform embraces an alternative reading frame that imparts a 50% change in CTD protein sequence, and functional impairment of exon 7 exclusion activity in a RBFOX2-target, the L-type calcium channel gene, Cacna1c. We have also demonstrated functional correlates in SCN gene transcripts; inclusion of Cacna1c exon 7, and also exclusion of both NMDA receptor gene Grin1 exon 4, and Enah exon 12, all consistent with a change in SCN RBFOX activity. The demonstrated regional diversity of Rbfox2 in adult brain highlights the functional adaptability of this RBP, enabling neuronal specialization, and potentially responding to disease-related neuronal dysfunction.

Molecular structure and function of big calcium-activated potassium channels in skeletal muscle: pharmacological perspectives

The large-conductance Ca2+-activated K+ (BK) channel is broadly expressed in various mammalian cells and tissues such as neurons, skeletal muscles (sarco-BK), and smooth muscles. These channels are activated by changes in membrane electrical potential and by increases in the concentration of intracellular calcium ion (Ca2+). The BK channel is subjected to many mechanisms that add diversity to the BK channel α-subunit gene. These channels are indeed subject to alternative splicing, auxiliary subunits modulation, posttranslational modifications, and protein-protein interactions. BK channels can be modulated by diverse molecules that may induce either an increase or decrease in channel activity. The linkage of these channels to many intracellular metabolites and pathways, as well as their modulation by extracellular natural agents, have been found to be relevant in many physiological processes. BK channel diversity is obtained by means of alternative splicing and modulatory β- and γ-subunits. The association of the α-subunit with β- or with γ-subunits can change the BK channel phenotype, functional diversity, and pharmacological properties in different tissues. In the case of the skeletal muscle BK channel (sarco-BK channel), we established that the main mechanism regulating BK channel diversity is the alternative splicing of the KCNMA1/slo1 gene encoding for the α-subunit generating different splicing isoform in the muscle phenotypes. This finding helps to design molecules selectively targeting the skeletal muscle subtypes. The use of drugs selectively targeting the skeletal muscle BK channels is a promising strategy in the treatment of familial disorders affecting muscular skeletal apparatus including hyperkalemia and hypokalemia periodic paralysis.

Understanding the conformational motions of RCK gating rings

A timely review of the structural basis of Ca²⁺-activated K⁺ channel modulation by regulator of conduction of K⁺ (RCK) domains

Regulator of conduction of K⁺ (RCK) domains are ubiquitous regulators of channel and transporter activity in prokaryotes and eukaryotes. In humans, RCK domains form an integral component of large-conductance calcium-activated K channels (BK channels), key modulators of nerve, muscle, and endocrine cell function. In this review, we explore how the study of RCK domains in bacterial and human channels has contributed to our understanding of the structural basis of channel function. This knowledge will be critical in identifying mechanisms that underlie BK channelopathies that lead to epilepsy and other diseases, as well as regions of the channel that might be successfully targeted to treat such diseases.

Functional Role of Mitochondrial and Nuclear BK Channels

BK channels are important for the regulation of many cell functions. The significance of plasma membrane BK channels in the control of action potentials, resting membrane potential, and neurotransmitter release is well established; however, the composition and functions of mitochondrial and nuclear BK (nBK) channels are largely unknown. In this chapter, we summarize the recent findings on the subcellular localization, biophysical, and pharmacological properties of mitochondrial and nBK channels and discuss their molecular identity and physiological functions.

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.

Posttranscriptional and Posttranslational Regulation of BK Channels

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

Recent advances in therapeutic strategies that focus on the regulation of ion channel expression

A number of different ion channel types are involved in cell signaling networks, and homeostatic regulatory mechanisms contribute to the control of ion channel expression. Profiling of global gene expression using microarray technology has recently provided novel insights into the molecular mechanisms underlying the homeostatic and pathological control of ion channel expression. It has demonstrated that the dysregulation of ion channel expression is associated with the pathogenesis of neural, cardiovascular, and immune diseases as well as cancers. In addition to the transcriptional, translational, and post-translational regulation of ion channels, potentially important evidence on the mechanisms controlling ion channel expression has recently been accumulated. The regulation of alternative pre-mRNA splicing is therefore a novel therapeutic strategy for the treatment of dominant-negative splicing disorders. Epigenetic modification plays a key role in various pathological conditions through the regulation of pluripotency genes. Inhibitors of pre-mRNA splicing and histone deacetyalase/methyltransferase have potential as potent therapeutic drugs for cancers and autoimmune and inflammatory diseases. Moreover, membrane-anchoring proteins, lysosomal and proteasomal degradation-related molecules, auxiliary subunits, and pharmacological agents alter the protein folding, membrane trafficking, and post-translational modifications of ion channels, and are linked to expression-defect channelopathies. In this review, we focused on recent insights into the transcriptional, spliceosomal, epigenetic, and proteasomal regulation of ion channel expression: Ca(2+) channels (TRPC/TRPV/TRPM/TRPA/Orai), K(+) channels (voltage-gated, KV/Ca(2+)-activated, KCa/two-pore domain, K2P/inward-rectifier, Kir), and Ca(2+)-activated Cl(-) channels (TMEM16A/TMEM16B). Furthermore, this review highlights expression of these ion channels in expression-defect channelopathies.

Calcium activated K⁺ channels in the electroreceptor of the skate confirmed by cloning. Details of subunits and splicing

Elasmobranchs detect small potentials using excitable cells of the ampulla of Lorenzini which have calcium-activated K(+) channels, first described in 1974. A distinctive feature of the outward current in voltage clamped ampullae is its apparent insensitivity to voltage. The sequence of a BK channel α isoform expressed in the ampulla of the skate was characterized. A signal peptide is present at the beginning of the gene. When compared to human isoform 1 (the canonical sequence), the largest difference was absence of a 59 amino acid region from the S8-S9 intra-cellular linker that contains the strex regulatory domain. The ampulla isoform was also compared with the isoform predicted in late skate embryos where strex was also absent. The BK voltage sensors were conserved in both skate isoforms. Differences between the skate and human BK channel included alternative splicing. Alternative splicing occurs at seven previously defined sites that are characteristic for BK channels in general and hair cells in particular. Skate BK sequences were highly similar to the Australian ghost shark and several other vertebrate species. Based on alignment of known BK sequences with the skate genome and transcriptome, there are at least two isoforms of Kcnma1α expressed in the skate. One of the β subunits (β4), which is known to decrease voltage sensitivity, was also identified in the skate genome and transcriptome and in the ampulla. These studies advance our knowledge of BK channels and suggest further studies in the ampulla and other excitable tissues.

Novel description of ionic currents recorded with the action potential clamp technique: application to excitatory currents in suprachiasmatic nucleus neurons

The traditional method of recording ionic currents in neurons has been with voltage-clamp steps. Other waveforms such as action potentials (APs) can be used. The AP clamp method reveals contributions of ionic currents that underlie excitability during an AP (Bean BP. Nat Rev Neurosci 8: 451-465, 2007). A novel usage of the method is described in this report. An experimental recording of an AP from the literature is digitized and applied computationally to models of ionic currents. These results are compared with experimental AP-clamp recordings for model verification or, if need be, alterations to the model. The method is applied to the tetrodotoxin-sensitive sodium ion current, INa, and the calcium ion current, ICa, from suprachiasmatic nucleus (SCN) neurons (Jackson AC, Yao GL, Bean BP. J Neurosci 24: 7985-7998, 2004). The latter group reported voltage-step and AP-clamp results for both components. A model of INa is constructed from their voltage-step results. The AP clamp computational methodology applied to that model compares favorably with experiment, other than a modest discrepancy close to the peak of the AP that has not yet been resolved. A model of ICa was constructed from both voltage-step and AP-clamp results of this component. The model employs the Goldman-Hodgkin-Katz equation for the current-voltage relation rather than the traditional linear dependence of this aspect of the model on the Ca(2+) driving force. The long-term goal of this work is a mathematical model of the SCN AP. The method is general. It can be applied to any excitable cell.

Current understanding of iberiotoxin-resistant BK channels in the nervous system

While most large-conductance, calcium-, and voltage-activated potassium channels (BK or Maxi-K type) are blocked by the scorpion venom iberiotoxin, the so-called “type II” subtype has the property of toxin resistance. This property is uniquely mediated by channel assembly with one member of the BK accessory β subunit family, the neuron-enriched β4 subunit. This review will focus on current understanding of iberiotoxin-resistant, β4-containing BK channel properties and their function in the CNS. Studies have shown that β4 dramatically promotes BK channel opening by shifting voltage sensor activation to more negative voltage ranges, but also slows activation to timescales that theoretically preclude BK ability to shape action potentials (APs). In addition, β4 membrane trafficking is regulated through an endoplasmic retention signal and palmitoylation. More recently, the challenge has been to understand the functional role of the iberiotoxin-resistant BK subtype utilizing computational modeling of neurons and neurophysiological approaches. Utilizing iberiotoxin-resistance as a footprint for these channels, they have been identified in dentate gyrus granule neurons and in purkinje neurons of the cerebellum. In these neurons, the role of these channels is largely consistent with slow-gated channels that reduce excitability either through an interspike conductance, such as in purkinje neurons, or by replacing fast-gating BK channels that otherwise facilitate high frequency AP firing, such as in dentate gyrus neurons. They are also observed in presynaptic mossy fiber terminals of the dentate gyrus and posterior pituitary terminals. More recent studies suggest that β4 subunits may also be expressed in some neurons lacking iberiotoxin-resistant BK channels, such as in CA3 hippocampus neurons. Ongoing research using novel, specific blockers and agonists of BK/β4, and β4 knockout mice, will continue to move the field forward in understanding the function of these channels.

BK channels regulate sinoatrial node firing rate and cardiac pacing in vivo

Large-conductance Ca(2+)- and voltage-activated K(+) (BK) channels play prominent roles in shaping muscle and neuronal excitability. In the cardiovascular system, BK channels promote vascular relaxation and protect against ischemic injury. Recently, inhibition of BK channels has been shown to lower heart rate in intact rodents and isolated hearts, suggesting a novel role in heart function. However, the underlying mechanism is unclear. In the present study, we recorded ECGs from mice injected with paxilline (PAX), a membrane-permeable BK channel antagonist, and examined changes in cardiac conduction. ECGs revealed a 19 ± 4% PAX-induced reduction in heart rate in wild-type but not BK channel knockout (Kcnma1(-/-)) mice. The heart rate decrease was associated with slowed cardiac pacing due to elongation of the sinus interval. Action potential firing recorded from isolated sinoatrial node cells (SANCs) was reduced by 55 ± 15% and 28 ± 9% by application of PAX (3 μM) and iberiotoxin (230 nM), respectively. Furthermore, baseline firing rates from Kcnma1(-/-) SANCs were 33% lower than wild-type SANCs. The slowed firing upon BK current inhibition or genetic deletion was due to lengthening of the diastolic depolarization phase of the SANC action potential. Finally, BK channel immunoreactivity and PAX-sensitive currents were identified in SANCs with HCN4 expression and pacemaker current, respectively, and BK channels cloned from SANCs recapitulated similar activation as the PAX-sensitive current. Together, these data localize BK channels to SANCs and demonstrate that loss of BK current decreases SANC automaticity, consistent with slowed sinus pacing after PAX injection in vivo. Furthermore, these findings suggest BK channels are potential therapeutic targets for disorders of heart rate.