Cloning of human pancreatic islet large conductance Ca(2+)-activated K+ channel (hSlo) cDNAs: evidence for high levels of expression in pancreatic islets and identification of a flanking genetic marker

Featuring how calcium channels and calmodulin affect glioblastoma behavior. A review article

Glioblastoma (GBM) is considered to be the most aggressive primary brain tumor with an extremely bad prognosis. Recurrence after treatment is a major problem with a survival rate for one year ranging about 39.7%. Ideal outcomes are still difficult to be achieved despite the recent treatment combinations. The ultimate capacity to regrow after resection is considered to be related to the availability of self-regenerating populations of stem cells. We made a literature review interpreting how calcium channels and calcium-regulated proteins mechanistically elaborate glioblastoma virulence in different ways. Calcium channels, and calcium-regulated proteins have shown diverse interconnected roles in shaping different aspects of GBM biology as indicated in some experimental studies. The beneficial prospective of those roles granting GBM different aggressive potentials pose variable applications in targeted therapy whether it is experimental or clinical trials.

Involvement of the γ1 subunit of the large-conductance Ca2+-activated K+ channel in the proliferation of human somatostatinoma cells

Pancreatic neuroendocrine tumors (pNETs) occur due to the abnormal growth of pancreatic islet cells and predominantly develop in the duodenal-pancreatic region. Somatostatinoma is one of the pNETs associated with tumors of pancreatic δ cells, which produce and secrete somatostatin. Limited information is currently available on the pathogenic mechanisms of somatostatinoma. The large-conductance Ca²⁺-activated K⁺ (BK_Ca) channel is expressed in several types of cancer cells and regulates cell proliferation, migration, invasion, and metastasis. In the present study, the functional expression of the BK_Ca channel was examined in a human somatostatinoma QGP-1 cell line. In QGP-1 cells, outward currents were elicited by membrane depolarization at pCa 6.5 (300 nM) in the pipette solution and inhibited by the specific BK_Ca channel blocker, paxilline. Paxilline-sensitive currents were detected, even at pCa 8.0 (10 nM) in the pipette solution, in QGP-1 cells. In addition to the α and β2-4 subunits of the BK_Ca channel, the novel regulatory γ1 subunit (BK_Caγ1) was co-localized with the α subunit in QGP-1 cells. Paxilline-sensitive currents at pCa 8.0 in the pipette solution were reduced by the siRNA knockdown of BK_Caγ1. Store-operated Ca²⁺ entry was smaller in BK_Caγ1 siRNA-treated QGP-1 cells. The proliferation of QGP-1 cells was attenuated by paxilline or the siRNA knockdown of BK_Caγ1. These results strongly suggest that BK_Caγ1 facilitates the proliferation of human somatostatinoma cells. Therefore, BK_Caγ1 may be a novel therapeutic target for somatostatinoma.

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.

GPR55-dependent stimulation of insulin secretion from isolated mouse and human islets of Langerhans

AIMS

The novel cannabinoid receptor GPR55 is expressed by rodent islets and it has been implicated in β-cell function in response to a range of ligands. This study evaluated the effects of GPR55 ligands on intracellular calcium ([Ca²⁺ ]_i ) levels and insulin secretion from islets isolated from GPR55 knockout (GPR55 ^-/- ) mice, age-matched wildtype (WT) mice and human pancreas.

MATERIALS AND METHODS

GPR55 expression was determined by Western blotting and fluorescent immunohistochemistry. Changes in [Ca²⁺ ]_i were measured by Fura-2 microfluorimetry. Dynamic insulin secretion was quantified by radioimmunoassay following perifusion of isolated islets. RhoA activity was monitored using a Rho binding domain pull down assay.

RESULTS

Western blotting indicated that MIN6 β-cells, mouse and human islets express GPR55 and its localization on human β-cells was demonstrated by fluorescent immunohistochemistry. The pharmacological GPR55 agonist O-1602 (10 μM) significantly stimulated [Ca²⁺ ]_i and insulin secretion from WT mouse islets and these stimulatory effects were abolished in islets isolated from GPR55 ^-/- mice. In contrast, while the putative endogenous GPR55 agonist lysophosphatidylinositol (LPI, 5 µM) and the GPR55 antagonist cannabidiol (CBD, 1 µM) also elevated [Ca²⁺ ]_i and insulin secretion, these effects were sustained in islets from GPR55 ^-/- mice. Stimulatory effects of O-1602 on [Ca²⁺ ]_i and insulin secretion were also observed in experiments using human islets, but O-1602 did not activate RhoA in MIN6 β-cells.

CONCLUSIONS

Our results therefore suggest that GPR55 plays an important role in the regulation of mouse and human islet physiology, but LPI and CBD exert stimulatory effects on islet function by a GPR55-independent pathway(s).

Ion channels and regulation of insulin secretion in human β-cells: a computational systems analysis

In mammals an increase in glucose leads to block of ATP dependent potassium channels in pancreatic β cells leading to membrane depolarization. This leads to the repetitive firing of action potentials that increases calcium influx and triggers insulin granule exocytosis. Several important differences between species in this process suggest that a dedicated human-oriented approach is advantageous as extrapolating from rodent data may be misleading in several respects. We examined depolarization-induced spike activity in pancreatic human islet-attached β-cells employing whole-cell patch-clamp methods. We also reviewed the literature concerning regulation of insulin secretion by channel activity and constructed a data-based computer model of human β cell function. The model couples the Hodgkin-Huxley-type ionic equations to the equations describing intracellular Ca²⁺ homeostasis and insulin release. On the basis of this model we employed computational simulations to better understand the behavior of action potentials, calcium handling and insulin secretion in human β cells under a wide range of experimental conditions. This computational system approach provides a framework to analyze the mechanisms of human β cell insulin secretion.

The BK channel: a vital link between cellular calcium and electrical signaling

Large-conductance Ca²⁺-activated K⁺ channels (BK channels) constitute an key physiological link between cellular Ca²⁺ signaling and electrical signaling at the plasma membrane. Thus these channels are critical to the control of action potential firing and neurotransmitter release in several types of neurons, as well as the dynamic control of smooth muscle tone in resistance arteries, airway, and bladder. Recent advances in our understanding of K⁺ channel structure and function have led to new insight toward the molecular mechanisms of opening and closing (gating) of these channels. Here we will focus on mechanisms of BK channel gating by Ca²⁺, transmembrane voltage, and auxiliary subunit proteins.

Palmitoylation and membrane association of the stress axis regulated insert (STREX) controls BK channel regulation by protein kinase C

Large-conductance, calcium- and voltage-gated potassium (BK) channels play an important role in cellular excitability by controlling membrane potential and calcium influx. The stress axis regulated exon (STREX) at splice site 2 inverts BK channel regulation by protein kinase A (PKA) from stimulatory to inhibitory. Here we show that palmitoylation of STREX controls BK channel regulation also by protein kinase C (PKC). In contrast to the 50% decrease of maximal channel activity by PKC in the insertless (ZERO) splice variant, STREX channels were completely resistant to PKC. STREX channel mutants in which Ser(700), located between the two regulatory domains of K(+) conductance (RCK) immediately downstream of the STREX insert, was replaced by the phosphomimetic amino acid glutamate (S700E) showed a ∼50% decrease in maximal channel activity, whereas the S700A mutant retained its normal activity. BK channel inhibition by PKC, however, was effectively established when the palmitoylation-mediated membrane-anchor of the STREX insert was removed by either pharmacological inhibition of palmitoyl transferases or site-directed mutagenesis. These findings suggest that STREX confers a conformation on BK channels where PKC fails to phosphorylate and to inhibit channel activity. Importantly, PKA which inhibits channel activity by disassembling the STREX insert from the plasma membrane, allows PKC to further suppress the channel gating independent from voltage and calcium. Our results present an important example for the cross-talk between ion channel palmitoylation and phosphorylation in regulation of cellular excitability.

BK channels affect glucose homeostasis and cell viability of murine pancreatic beta cells

AIMS/HYPOTHESIS

Evidence is accumulating that Ca(2+)-regulated K(+) (K(Ca)) channels are important for beta cell function. We used BK channel knockout (BK-KO) mice to examine the role of these K(Ca) channels for glucose homeostasis, beta cell function and viability.

METHODS

Glucose and insulin tolerance were tested with male wild-type and BK-KO mice. BK channels were detected by single-cell RT-PCR, cytosolic Ca(2+) concentration ([Ca(2+)](c)) by fura-2 fluorescence, and insulin secretion by radioimmunoassay. Electrophysiology was performed with the patch-clamp technique. Apoptosis was detected via caspase 3 or TUNEL assay.

RESULTS

BK channels were expressed in murine pancreatic beta cells. BK-KO mice were normoglycaemic but displayed markedly impaired glucose tolerance. Genetic or pharmacological deletion of the BK channel reduced glucose-induced insulin secretion from isolated islets. BK-KO and BK channel inhibition (with iberiotoxin, 100 nmol/l) broadened action potentials and abolished the after-hyperpolarisation in glucose-stimulated beta cells. However, BK-KO did not affect action potential frequency, the plateau potential at which action potentials start or glucose-induced elevation of [Ca(2+)](c). BK-KO had no direct influence on exocytosis. Importantly, in BK-KO islet cells the fraction of apoptotic cells and the rate of cell death induced by oxidative stress (H(2)O(2), 10-100 μmol/l) were significantly increased compared with wild-type controls. Similar effects were obtained with iberiotoxin. Determination of H(2)O(2)-induced K(+) currents revealed that BK channels contribute to the hyperpolarising K(+) current activated under conditions of oxidative stress.

CONCLUSIONS/INTERPRETATION

Ablation or inhibition of BK channels impairs glucose homeostasis and insulin secretion by interfering with beta cell stimulus-secretion coupling. In addition, BK channels are part of a defence mechanism against apoptosis and oxidative stress.

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.

Enhanced glucose tolerance by SK4 channel inhibition in pancreatic beta-cells

OBJECTIVE

Ca(2+)-regulated K(+) channels are involved in numerous Ca(2+)-dependent signaling pathways. In this study, we investigated whether the Ca(2+)-activated K(+) channel of intermediate conductance SK4 (KCa3.1, IK1) plays a physiological role in pancreatic beta-cell function.

RESEARCH DESIGN AND METHODS

Glucose tolerance and insulin sensitivity were determined in wild-type (WT) or SK4 knockout (SK4-KO) mice. Electrophysiological experiments were performed with the patch-clamp technique. The cytosolic Ca(2+) concentration ([Ca(2+)](c)) was determined by fura-2 fluorescence. Insulin release was assessed by radioimmunoassay, and SK4 protein was detected by Western blot analysis.

RESULTS

SK4-KO mice showed improved glucose tolerance, whereas insulin sensitivity was not altered. The animals were not hypoglycemic. Isolated SK4-KO beta-cells stimulated with 15 mmol/l glucose had an increased Ca(2+) action potential frequency, and single-action potentials were broadened. These alterations were coupled to increased [Ca(2+)](c). In addition, glucose responsiveness of membrane potential, [Ca(2+)](c), and insulin secretion were shifted to lower glucose concentrations. SK4 protein was expressed in WT islets. An increase in K(+) currents and concomitant membrane hyperpolarization could be evoked in WT beta-cells by the SK4 channel opener DCEBIO (100 micromol/l). Accordingly, the SK4 channel blocker TRAM-34 (1 micromol/l) partly inhibited K(Ca) currents and induced electrical activity at a threshold glucose concentration. In stimulated WT beta-cells, TRAM-34 further increased [Ca(2+)](c) and broadened action potentials similar to those seen in SK4-KO beta-cells. SK4 channels were found to substantially contribute to K(slow) (slowly activating K(+) current).

CONCLUSIONS

SK4 channels are involved in beta-cell stimulus-secretion coupling. Deficiency of SK4 current induces elevated beta-cell responsiveness and coincides with improved glucose tolerance in vivo. Therefore, pharmacologic modulation of these channels might provide an interesting approach for the development of novel insulinotropic drugs.

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

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

Action potentials and insulin secretion: new insights into the role of Kv channels

Coordinated electrical activity allows pancreatic beta-cells to respond to secretagogues with calcium entry followed by insulin secretion. Metabolism of glucose affects multiple membrane proteins including ion channels, transporters and pumps that collaborate in a cascade of electrical activity resulting in insulin release. Glucose induces beta-cell depolarization resulting in the firing of action potentials (APs), which are the primary electrical signal of the beta-cell. They are shaped by orchestrated activation of ion channels. Here we give an overview of the voltage-gated potassium (Kv) channels of the beta-cell, which are responsible in part for the falling phase of the AP, and how their regulation affects insulin secretion. beta cells contain several Kv channels allowing dynamic integration of multiple signals on repolarization of glucose-stimulated APs. Recent studies on Kv channel regulation by cAMP and arachidonic acid and on the Kv2.1 null mouse have greatly increased our understanding of beta-cell excitation-secretion coupling.

Expression and function of calcium-activated potassium channels in human glioma cells

Ca(2+)-activated K(+) (K(Ca)) channels are a unique family of ion channels because they are capable of directly communicating calcium signals to changes in cell membrane potential required for cellular processes including but not limited to cellular proliferation and migration. It is now possible to distinguish three families of K(Ca) channels based on differences in their biophysical and pharmacological properties as well as genomic sequence. Using a combination of biochemical, molecular, and biophysical approaches, we show that human tumor cells of astrocytic origin, i.e. glioma cells, express transcripts for all three family members of K(Ca) channels including BK, IK, and all three SK channel types (SK1, SK2, and SK3). The use of selective pharmacological inhibitors shows prominent expression of currents that are inhibited by the BK channel specific inhibitors iberiotoxin and paxilline. However, despite the presence of transcripts for IK and SK, neither clotrimazole, an inhibitor of IK channels, nor apamin, known to block most SK channels inhibited any current. The exclusive expression of functional BK channels was further substantiated by shRNA knockdown experiments, which selectively reduced iberiotoxin sensitive currents. Western blotting of patient biopsies with antibodies specific for all three KCa channel types further substantiated the exclusive expression of BK type KCa channels in vivo. This finding is in sharp contrast to other cancers that express primarily IK channels.

Relative contribution of chloride channels and transporters to regulatory volume decrease in human glioma cells

Primary brain tumors (gliomas) often present with peritumoral edema. Their ability to thrive in this osmotically altered environment prompted us to examine volume regulation in human glioma cells, specifically the relative contribution of Cl(-) channels and transporters to this process. After a hyposmotic challenge, cultured astrocytes, D54-MG glioma cells, and glioma cells from human patient biopsies exhibited a regulatory volume decrease (RVD). Although astrocytes were not able to completely reestablish their original prechallenge volumes, glioma cells exhibited complete volume recovery, sometimes recovering to a volume smaller than their original volumes (V(Post-RVD) < V(baseline)). In glioma cells, RVD was largely inhibited by treatment with a combination of Cl(-) channel inhibitors, 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) and Cd(2+) (V(Post-RVD) > 1.4*V(baseline)). Volume regulation was also attenuated to a lesser degree by the addition of R-(+)-[(2-n-butyl-6,7-dichloro-2-cyclopentyl-2,3-dihydro-1-oxo-1H-inden-5-yl)oxy]acetic acid (DIOA), a known K(+)-Cl(-) cotransporter (KCC) inhibitor. To dissect the relative contribution of channels vs. transporters in RVD, we took advantage of the comparatively high temperature dependence of transport processes vs. channel-mediated diffusion. Cooling D54-MG glioma cells to 15 degrees C resulted in a loss of DIOA-sensitive volume regulation. Moreover, at 15 degrees C, the channel blockers NPPB + Cd(2+) completely inhibited RVD and cells behaved like perfect osmometers. The calculated osmolyte flux during RVD under these experimental conditions suggests that the relative contribution of Cl(-) channels vs. transporters to this process is approximately 60-70% and approximately 30-40%, respectively. Finally, we identified several candidate proteins that may be involved in RVD, including the Cl(-) channels ClC-2, ClC-3, ClC-5, ClC-6, and ClC-7 and the transporters KCC1 and KCC3a.

Stimulatory action of internal protons on Slo1 BK channels

We investigated the internal pH-sensitivity of heterologously expressed hSlo1 BK channels. In the virtual absence of Ca(2+) and Mg(2+) to isolate the voltage-dependent gating transitions, low internal pH enhanced macroscopic hSlo1 currents by shifting the voltage-dependence of activation to more negative voltages. The activation time course was faster and the deactivation time course was slower with low pH. The estimated K(d) value of the stimulatory effect was approximately pH = 6.5 or 0.35 micro M. The stimulatory effect was maintained when the auxiliary subunit mouse beta1 was coexpressed. Treatment of the hSlo1 channel with the histidine modifying agent diethyl pyrocarbonate also enhanced the hSlo1 currents and greatly diminished the internal pH sensitivity, suggesting that diethyl pyrocarbonate and low pH may work on the same effector mechanism. High concentrations of Ca(2+) or Mg(2+) also masked the stimulatory effect of low internal pH. These results indicate that the acid-sensitivity of the Slo BK channel may involve the channel domain implicated in the divalent-dependent activation.

Cloning and characterization of glioma BK, a novel BK channel isoform highly expressed in human glioma cells

Voltage-dependent large-conductance Ca2+-activated K+ channels (BK channels) are widely expressed in excitable and nonexcitable cells. BK channels exhibit diverse electrophysiological properties, which are attributable in part to alternative splicing of their alpha-subunits. BK currents have been implicated in the growth control of glial cells, and BK channels with novel biophysical properties have recently been characterized in human glioma cells. Here we report the isolation, cloning, and functional characterization of glioma BK (gBK), a novel splice isoform of hSlo, the gene that encodes the alpha-subunits of human BK channels. The primary sequence of gBK is 97% identical to its closest homolog hbr5, but it contains an additional 34-amino-acid exon at splice site 2 in the C-terminal tail of BK channels. hSlo transcripts containing this novel exon are expressed ubiquitously in various normal tissues as well as in neoplasmic samples, suggesting that the novel exon may modulate important physiological functions of BK channels. Expression of gBK in Xenopus oocytes gives rise to iberiotoxin-sensitive (IbTX) currents, with an IC(50) for IbTX of 5.7 nm and a Hill coefficient of 0.76. Single gBK channels have a unitary conductance of similar250 pS, and the currents show significantly slower activation and higher Ca2+ sensitivity than hbr5. Ca2+ sensitivity was enhanced specifically at physiologically relevant [Ca2+]i (100-500 nm). Examination of biopsies from patients with malignant gliomas has revealed specific overexpression of BK channels in gliomas compared with nonmalignant human cortical tissues. Importantly, tumor malignancy grades have correlated positively with BK channel expression, suggesting an important role for the gBK channel in glioma biology.

Alternative splicing determines sensitivity of murine calcium-activated potassium channels to glucocorticoids

1. Large-conductance Ca(2+)- and voltage-activated potassium (BK) channels are important regulators of cellular excitability. Here, we present a patch-clamp electrophysiological analysis of splice-variant-specific regulation by the synthetic glucocorticoid dexamethasone (DEX) of BK channels consisting of cloned STREX or ZERO alpha-subunit variants expressed in human embryonic kidney (HEK 293) cells. 2. STREX channels in isolated membrane patches were inhibited by protein kinase A (PKA) and this was blocked on pre-treatment of intact cells with DEX (100 nM) for 2 h. 3. The effect of DEX required the synthesis of new mRNA and protein. Furthermore, it required protein phosphatase 2A (PP2A)-like activity intimately associated with the channels, as it was blocked by 10 nM okadaic acid but not by the specific protein phosphatase-1 inhibitor peptide PPI-2. 4. ZERO variant channels that lack the STREX insert were activated by PKA but were not influenced by DEX. ZERO channels containing a mutant STREX domain (S4(STREX)A) were also activated by PKA. Importantly, DEX blocked PKA activation of S4(STREX)A channels in a PP2A-dependent manner. 5. Taken together, the STREX domain is crucial for glucocorticoid regulation of BK channels through a PP2A-type enzyme. Moreover, glucocorticoids appear to induce a generic set of proteins in different types of cells, the actions of which depend on the expression of cell-specific targets.

Characterization of a novel 132-bp exon of the human maxi-K channel

The large-conductance Ca2+-activated voltage-dependent K+ channel (maxi-K channel) induces a significant repolarizing current that buffers cell excitability. This channel can derive its diversity by alternative splicing of its transcript-producing isoforms that differ in their sensitivity to voltage and intracellular Ca2+. We have identified a novel 132-bp exon of the maxi-K channel from human myometrial cells that encodes 44 amino acids within the first intracellular loop of the channel protein. Distribution analysis reveals that this exon is expressed predominantly in human smooth muscle tissues with the highest abundance in the uterus and aorta and resembles the previously reported distribution of the total maxi-K channel transcript. Single-channel K+ current measurements in fibroblasts transfected with the maxi-K channel containing this novel 132-bp exon demonstrate that the presence of this insert attenuates the sensitivity to voltage and intracellular Ca2+. Alternative splicing to introduce this 132-bp exon into the maxi-K channel may elicit another mode to modulate cell excitability.

A CaMK IV responsive RNA element mediates depolarization-induced alternative splicing of ion channels

Calcium regulation of gene expression is critical for the long-lasting activity-dependent changes in cellular electrical properties that underlie important physiological functions such as learning and memory. Cellular electrical properties are diversified through the extensive alternative splicing of ion channel pre-messenger RNAs; however, the regulation of splicing by cell signalling pathways has not been well explored. Here we show that depolarization of GH3 pituitary cells represses splicing of the STREX exon in BK potassium channel transcripts through the action of Ca2+/calmodulin-dependent protein kinases (CaMKs). Overexpressing constitutively active CaMK IV, but not CaMK I or II, specifically decreases STREX inclusion in the mRNA. This decrease is prevented by mutations in particular RNA repressor sequences. Transferring 54 nucleotides from the 3' splice site upstream of STREX to a heterologous gene is sufficient to confer CaMK IV repression on an otherwise constitutive exon. These experiments define a CaMK IV-responsive RNA element (CaRRE), which mediates the alternative splicing of ion channel pre-mRNAs. The CaRRE presents a unique molecular target for inducing long-term adaptive changes in cellular electrical properties. It also provides a model system for dissecting the effect of signal transduction pathways on alternative splicing.

Cloning and functional expression of two families of beta-subunits of the large conductance calcium-activated K+ channel

We report here a characterization of two families of calcium-activated K(+) channel beta-subunits, beta2 and beta3, which are encoded by distinct genes that map to 3q26.2-27. A single beta2 family member and four alternatively spliced variants of beta3 were investigated. These subunits have predicted molecular masses of 27. 1-31.6 kDa, share approximately 30-44% amino acid identity with beta1, and exhibit distinct but overlapping expression patterns. Coexpression of the beta2 or beta3a-c subunits with a BK alpha-subunit altered the functional properties of the current expressed by the alpha-subunit alone. The beta2 subunit rapidly and completely inactivated the current and shifted the voltage dependence for activation to more polarized membrane potentials. In contrast, coexpression of the beta3a-c subunits resulted in only partial inactivation of the current, and the beta3b subunit conferred an apparent inward rectification. Furthermore, unlike the beta1 and beta2 subunits, none of the beta3 subunits increased channel sensitivity to calcium or voltage. The tissue-specific expression of these beta-subunits may allow for the assembly of a large number of distinct BK channels in vivo, contributing to the functional diversity of native BK currents.

A point mutation in the maxi-K clone dSlo forms a high affinity site for charybdotoxin

This work investigated the interaction of CTX with two cloned analogues of the maxi-K channel, dSlo and hSlo. dSlo has been reported to be CTX insensitive. Single channel analysis revealed that dSlo was weakly blocked by the toxin, with a very high K(D) of 5.8 microM. The hSlo channels bound wild-type, recombinant CTX with high affinity and in a bimolecular fashion, and displayed a half-blocking concentration (K(D)) of 36 nM. A glutamate residue was substituted for the wild-type threonine at position 290 in dSlo. The mutant channel was expressed in COS-7 cells and reconstituted into lipid bilayers for single channel analysis. The mutant channel bound wild-type, recombinant CTX with high affinity and in a bimolecular fashion, and displayed a half-blocking concentration (K(D)) of 23 nM. Changing just one residue from threonine to glutamate at position 290 in dSlo changed the affinity of the channel's CTX-receptor over 100-fold. Kinetic analysis revealed that this large increase in affinity was due to decreasing the dissociation rate of the toxin. These results suggest that a CTX receptor does exist in the dSlo channel mouth and that the threonine at position 290 destabilizes the toxin on the binding site.

Molecular components of large conductance calcium-activated potassium (BK) channels in mouse pituitary corticotropes

Large-conductance calcium- and voltage- activated potassium (BK) channels play a fundamental role in the signaling pathways regulating mouse anterior pituitary corticotrope function. Here we describe the cloning and functional characterization of the components of mouse corticotrope BK channels. RT-PCR cloning and splice variant analysis of mouse AtT20 D16:16 corticotropes revealed robust expression of mslo transcripts encoding pore-forming alpha-subunits containing the mouse homolog of the 59-amino acid STREX-1 exon at splice site 2. RT-PCR and functional analysis, using the triterpenoid glycoside, DHS-1, revealed that native corticotrope BK channels are not functionally coupled to beta-subunits in vivo. Functional expression of the STREX-1 containing alpha-subunit in HEK 293 cells resulted in BK channels with calcium sensitivity, single-channel conductance, and inhibition by protein kinase A identical to that of native mouse corticotrope BK channels. This report represents the first corticotrope ion channel to be characterized at the molecular level and demonstrates that mouse corticotrope BK channels are composed of alpha-subunits expressing the mouse STREX-1 exon.

Human pancreatic islets express mRNA species encoding two distinct catalytically active isoforms of group VI phospholipase A2 (iPLA2) that arise from an exon-skipping mechanism of alternative splicing of the transcript from the iPLA2 gene on chromosome 22q13.1

An 85-kDa Group VI phospholipase A2 enzyme (iPLA2) that does not require Ca2+ for catalysis has recently been cloned from three rodent species. A homologous 88-kDa enzyme has been cloned from human B-lymphocyte lines that contains a 54-amino acid insert not present in the rodent enzymes, but human cells have not previously been observed to express catalytically active iPLA2 isoforms other than the 88-kDa protein. We have cloned cDNA species that encode two distinct iPLA2 isoforms from human pancreatic islet RNA and a human insulinoma cDNA library. One isoform is an 85-kDa protein (short isoform of human iPLA2 (SH-iPLA2)) and the other an 88-kDa protein (long isoform of human iPLA2 (LH-iPLA2)). Transcripts encoding both isoforms are also observed in human promonocytic U937 cells. Recombinant SH-iPLA2 and LH-iPLA2 are both catalytically active in the absence of Ca2+ and inhibited by a bromoenol lactone suicide substrate, but LH-iPLA2 is activated by ATP, whereas SH-iPLA2 is not. The human iPLA2 gene has been found to reside on chromosome 22 in region q13.1 and to contain 16 exons represented in the LH-iPLA2 transcript. Exon 8 is not represented in the SH-iPLA2 transcript, indicating that it arises by an exon-skipping mechanism of alternative splicing. The amino acid sequence encoded by exon 8 of the human iPLA2 gene is proline-rich and shares a consensus motif of PX5PX8HHPX12NX4Q with the proline-rich middle linker domains of the Smad proteins DAF-3 and Smad4. Expression of mRNA species encoding two active iPLA2 isoforms with distinguishable catalytic properties in two different types of human cells demonstrated here may have regulatory or functional implications about the roles of products of the iPLA2 gene in cell biologic processes.

Identification and localization of Ca(2+)-activated K+ channels in rat sciatic nerve

To understand the physiology of Schwann cells and myelinated nerve, we have been engaged in identifying K+ channels in sciatic nerve and determining their subcellular localization. In the present study, we examined the slo family of Ca(2+)-activated K+ channels, a class of channel that had not previously been identified in myelinated nerve. We have determined that these channels are indeed expressed in peripheral nerve, and have cloned rat homologues of slo that are more than 95% identical to the murine slo. We found that sciatic nerve RNA contained numerous alternatively spliced variants of the slo homologue, as has been seen in other tissues. We raised a polyclonal antibody against a peptide from the carboxyl terminal of the channels. Immunocytochemistry revealed that the channel proteins are in Schwann cells and are associated with canaliculi that run along the outer surface of the cells. They are also relatively concentrated near the node of Ranvier in the Schwann cell outer membrane. This staining pattern is quite similar to what we previously reported for the voltage-dependent K+ channel Kv 1.5. We did not observe staining of axons or connective tissue in the nerve and so it seems likely that most or all of the splicing variants are located in the Schwann cells. The localization of these channels also suggests that they may participate in maintaining the resting potential of the Schwann cells during K+ buffering.

RINm5f cells express inactivating BK channels whereas HIT cells express noninactivating BK channels

Large-conductance Ca2+- and voltage-activated BK-type K+ channels are expressed abundantly in normal rat pancreatic islet cells and in the clonal rat insulinoma tumor (RINm5f) and hamster insulinoma tumor (HIT) beta cell lines. Previous work has suggested that the Ca2+ sensitivity of BK channels in RIN cells is substantially less than that in HIT cells, perhaps contributing to differences between the cell lines in responsiveness to glucose in mediating insulin secretion. In both RIN cells and normal pancreatic beta cells, BK channels are thought to play a limited role in responses of beta cells to secretagogues and in the electrical activity of beta cells. Here we examine in detail the properties of BK channels in RIN and HIT cells using inside-out patches and whole cell recordings. BK channels in RIN cells exhibit rapid inactivation that results in an anomalous steady-state Ca2+ dependence of activation. In contrast, BK channels in HIT cells exhibit the more usual noninactivating behavior. When BK inactivation is taken into account, the Ca2+ and voltage dependence of activation of BK channels in RIN and HIT cells is essentially indistinguishable. The properties of BK channel inactivation in RIN cells are similar to those of inactivating BK channels (termed BKi channels) previously identified in rat chromaffin cells. Inactivation involves multiple, trypsin-sensitive cytosolic domains and exhibits a dependence on Ca2+ and voltage that appears to arise from coupling to channel activation. In addition, the rates of inactivation onset and recovery are similar to that of BKi channels in chromaffin cells. The charybdotoxin (CTX) sensitivity of BKi currents is somewhat less than that of the noninactivating BK variant. Action potential voltage-clamp waveforms indicate that BK current is activated only weakly by Ca2+ influx in RIN cells but more strongly activated in HIT cells even when Ca2+ current magnitude is comparable. Concentrations of CTX sufficient to block BKi current in RIN cells have no effect on action potential activity initiated by glucose or DC injection. Despite its abundant expression in RIN cells, BKi current appears to play little role in action potential activity initiated by glucose or DC injection in RIN cells, but BK current may play an important role in action potential repolarization in HIT cells.

Control of alternative splicing of potassium channels by stress hormones

Many molecular mechanisms for neural adaptation to stress remain unknown. Expression of alternative splice variants of Slo, a gene encoding calcium- and voltage-activated potassium channels, was measured in rat adrenal chromaffin tissue from normal and hypophysectomized animals. Hypophysectomy triggered an abrupt decrease in the proportion of Slo transcripts containing a "STREX" exon. The decrease was prevented by adrenocorticotropic hormone injections. In Xenopus oocytes, STREX variants produced channels with functional properties associated with enhanced repetitive firing. Thus, the hormonal stress axis is likely to control the excitable properties of epinephrine-secreting cells by regulating alternative splicing of Slo messenger RNA.

Inactivating BK channels in rat chromaffin cells may arise from heteromultimeric assembly of distinct inactivation-competent and noninactivating subunits

Inactivating and noninactivating variants of large-conductance, Ca2+-dependent, voltage-dependent BK-type channels are found in rat chromaffin cells and are largely segregated into different cells. Here we test the hypothesis that, within the population of cells that express inactivating BK current (BKi current), the BKi channels are largely heteromultimers composed of inactivation-competent subunits (bk(i)) and noninactivating subunits (bk(s)). Several independent types of evidence support this view. The gradual removal of inactivation by trypsin is consistent with the idea that in most cells and patches there are, on average, about two to three inactivation domains per channel. In addition, several aspects of blockade of BKi current by charybdotoxin (CTX) are consistent with the idea that BKi channels contain differing numbers (one to four) of relatively CTX-resistant bk(i) subunits. Finally, the frequency of occurrence of noninactivating BKs channels in patches with predominantly inactivating BKi channels is consistent with the binomial expectations of random, independent assembly of two distinct subunits, if most cells have, on average, about two to three bk(i) subunits per channel. These results suggest that the phenotypic properties of BKi currents and the resulting cellular electrical excitability may exhibit a continuum of behavior that arises simply from the differential expression of two distinct subunits.

A cysteine-rich domain defined by a novel exon in a slo variant in rat adrenal chromaffin cells and PC12 cells

cDNA libraries from rat chromaffin cells and PC12 cells were screened for homologs to the mouse mSlo gene that encodes a large conductance, calcium (Ca2+)- and voltage-activated potassium channel (BK channel). One Slo variant contained sequence encoding a cysteine-rich, 59-amino acid insert for a previously described site of alternative splicing. This insert is reminiscent of zinc-finger domains. The exon was found in RNA from pancreas, anterior pituitary, cerebellum, and hippocampus. Expression in Xenopus oocytes of a Slo construct containing this exon conferred a -30 to -20 mV shift of the conductance-voltage curve. A previously uncharacterized alternative splice junction near the C-terminal end of Slo was also identified. In contrast to BK channels in rat chromaffin cells, none of the Slo variants exhibited inactivation when expressed in Xenopus oocytes. PCR screening of chromaffin cell RNA failed to reveal a homolog of an accessory beta subunit known to influence Slo channel function. Furthermore, a beta-subunit-dependent Slo channel activator, dehydrosoyasaponin I, was without effect on chromaffin cell BK current. The results argue that an accessory subunit may not be a required component of the native chromaffin cell BK channel.