Increasing small conductance Ca2+-activated potassium channel activity reverses ischemia-induced impairment of long-term potentiation

Global cerebral ischemia following cardiac arrest and cardiopulmonary resuscitation (CA/CPR) causes injury to hippocampal CA1 pyramidal neurons and impairs cognition. Small conductance Ca(2+)-activated potassium channels type 2 (SK2), expressed in CA1 pyramidal neurons, have been implicated as potential protective targets. Here we showed that, in mice, hippocampal long-term potentiation (LTP) was impaired as early as 3 h after recovery from CA/CPR and LTP remained impaired for at least 30 days. Treatment with the SK2 channel agonist 1-Ethyl-2-benzimidazolinone (1-EBIO) at 30 min after CA provided sustained protection from plasticity deficits, with LTP being maintained at control levels at 30 days after recovery from CA/CPR. Minimal changes in glutamate release probability were observed at delayed times after CA/CPR, implicating post-synaptic mechanisms. Real-time quantitative reverse transcriptase-polymerase chain reaction indicated that CA/CPR did not cause a loss of N-methyl-D-aspartate (NMDA) receptor mRNA at 7 or 30 days after CA/CPR. Similarly, no change in synaptic NMDA receptor protein levels was observed at 7 or 30 days after CA/CPR. Further, patch-clamp experiments demonstrated no change in functional synaptic NMDA receptors at 7 or 30 days after CA/CPR. Electrophysiology recordings showed that synaptic SK channel activity was reduced for the duration of experiments performed (up to 30 days) and that, surprisingly, treatment with 1-EBIO did not prevent the CA/CPR-induced loss of synaptic SK channel function. We concluded that CA/CPR caused alterations in post-synaptic signaling that were prevented by treatment with the SK2 agonist 1-EBIO, indicating that activators of SK2 channels may be useful therapeutic agents to prevent ischemic injury and cognitive impairments.

SK2 and SK3 expression differentially affect firing frequency and precision in dopamine neurons

The firing properties of dopamine (DA) neurons in the substantia nigra (SN) pars compacta are strongly influenced by the activity of apamin-sensitive small conductance Ca(2+)-activated K(+) (SK) channels. Of the three SK channel genes expressed in central neurons, only SK3 expression has been identified in DA neurons. The present findings show that SK2 was also expressed in DA neurons. Immuno-electron microscopy (iEM) showed that SK2 was primarily expressed in the distal dendrites, while SK3 was heavily expressed in the soma and, to a lesser extent, throughout the dendritic arbor. Electrophysiological recordings of the effects of the SK channel blocker apamin on DA neurons from wild type and SK(-/-) mice show that SK2-containing channels contributed to the precision of action potential (AP) timing, while SK3-containing channels influenced AP frequency. The expression of SK2 in DA neurons may endow distinct signaling and subcellular localization to SK2-containing channels.

Selective cognitive deficits and reduced hippocampal brain-derived neurotrophic factor mRNA expression in small-conductance calcium-activated K+ channel deficient mice

Small-conductance calcium-activated K(+) channels 1-3 (SK1-3) are important for neuronal firing regulation and are considered putative CNS drug targets. For instance non-selective SK blockers improve performance in animal models of cognition. The SK subtype(s) involved herein awaits identification and the question is difficult to address pharmacologically due to the lack of subtype-selective SK-channel modulators. In this study, we used doxycycline-induced conditional SK3-deficient (T/T) mice to address the cognitive consequences of selective SK3 deficiency. In T/T mice SK3 protein is near-eliminated from the brain following doxycycline treatment. We tested T/T and wild type (WT) littermate mice in five distinct learning and memory paradigms. In Y-maze spontaneous alternations and five-trial inhibitory avoidance the performance of T/T mice was markedly inferior to WT mice. In contrast, T/T and WT mice performed equally well in passive avoidance, object recognition and the Morris water maze. Thus, some aspects of working/short-term memory are disrupted in T/T mice. Using in situ hybridization, we further found the cognitive deficits in T/T mice to be paralleled by reduced brain-derived neurotrophic factor (BDNF) mRNA expression in the dentate gyrus and CA3 of the hippocampus. BDNF mRNA levels in the frontal cortex were not affected. BDNF has been crucially implicated in many cognitive processes. Hence, the biological substrate for the cognitive impairments in T/T mice could conceivably entail reduced trophic support of the hippocampus.

SK3 K+ channel-deficient mice have enhanced dopamine and serotonin release and altered emotional behaviors

SK3 K(+) channels influence neuronal excitability and are present in 5-hydroxytryptamine (5-HT) and dopamine (DA) nuclei in the brain stem. We therefore hypothesized that SK3 channels affect 5-HT and DA neurotransmission and associated behaviors. To explore this, we used doxycycline-induced conditional SK3-deficient (T/T) mice. In microdialysis, T/T mice had elevated baseline levels of striatal extracellular DA and the metabolites dihydroxyphenylacetic acid and homovanillic acid. While baseline hippocampal extracellular 5-HT was unchanged in T/T mice, the 5-HT response to the 5-HT transporter inhibitor citalopram was enhanced. Furthermore, baseline levels of the 5-HT metabolite 5-hydroxyindoleacetic acid were elevated in T/T mice. T/T mice performed equally to wild type (WT) in most sensory and motor tests, indicating that SK3 deficiency does not lead to gross impairments. In the forced swim and tail suspension tests, the T/T mice displayed reduced immobility compared with WT, indicative of an antidepressant-like phenotype. Female T/T mice were more anxious in the zero maze. In contrast, anxiety-like behaviors in the open-field and four-plate tests were unchanged in T/T mice of both sexes. Home cage diurnal activity was also unchanged in T/T mice. However, SK3 deficiency had a complex effect on activity responses to novelty: T/T mice showed decreased, increased or unchanged activity responses to novelty, depending on sex and context. In summary, we report that SK3 deficiency leads to enhanced DA and 5-HT neurotransmission accompanied by distinct alterations in emotional behaviors.

Small-conductance, Ca(2+) -activated K+ channel 2 is the key functional component of SK channels in mouse urinary bladder

Small-conductance Ca(2+)-activated K(+) (SK) channels play an important role in regulating the frequency and in shaping urinary bladder smooth muscle (UBSM) action potentials, thereby modulating contractility. Here we investigated a role for the SK2 member of the SK family (SK1-3) utilizing: 1) mice expressing beta-galactosidase (beta-gal) under the direction of the SK2 promoter (SK2 beta-gal mice) to localize SK2 expression and 2) mice lacking SK2 gene expression (SK2(-/-) mice) to assess SK2 function. In SK2 beta-gal mice, UBSM staining was observed, but staining was undetected in the urothelium. Consistent with this, urothelial SK2 mRNA was determined to be 4% of that in UBSM. Spontaneous phasic contractions in wild-type (SK2(+/+)) UBSM strips were potentiated (259% of control) by the selective SK channel blocker apamin (EC(50) = 0.16 nM), whereas phasic contractions of SK2(-/-) strips were unaffected. Nerve-mediated contractions of SK2(+/+) UBSM strips were also increased by apamin, an effect absent in SK2(-/-) strips. Apamin increased the sensitivity of SK2(+/+) UBSM strips to electrical field stimulation, since pretreatment with apamin decreased the frequency required to reach a 50% maximal contraction (vehicle, 21 +/- 4 Hz, n = 6; apamin, 12 +/- 2 Hz, n = 7; P < 0.05). In contrast, the sensitivity of SK2(-/-) UBSM strips was unaffected by apamin. Here we provide novel insight into the molecular basis of SK channels in the urinary bladder, demonstrating that the SK2 gene is expressed in the bladder and that it is essential for the ability of SK channels to regulate UBSM contractility.

Small-conductance calcium-activated potassium currents in mouse hyperexcitable denervated skeletal muscle

1. Hyperexcitability in denervated skeletal muscle is associated with the expression of SK3, a small-conductance Ca2+-activated K+ channel (SK channel). SK currents were examined in dissociated fibres from flexor digitorum brevis (FDB) muscle using the whole-cell patch clamp configuration. 2. Depolarization activated a K+-selective, apamin-sensitive and iberiotoxin-insensitive current, detected as a tail current upon repolarization, in fibres from denervated but not innervated muscle. Dialysis of the fibres with 20 mM EGTA in the patch pipette solution eliminated the tail current, consistent with this current reflecting Ca2+-activated SK channels expressed only in denervated muscle. 3. Activation of SK tail currents depended on the duration of the depolarizing pulse, consistent with a rise in intracellular Ca2+ due to release from the sarcoplasmic reticulum (SR) and influx through voltage-gated Ca2+ channels. 4. The envelope of SK tail currents was diminished by 10 microM ryanodine for all pulse durations, whereas 2 mM cobalt reduced the SK tail current for pulses greater than 80 ms, demonstrating that Ca2+ release from the SR during short pulses primarily activated SK channels. 5. In current clamp mode with the resting membrane potential set at -70 mV, denervation decreased the action potential threshold by approximately 8 mV. Application of apamin increased the action potential threshold in denervated fibres to that measured in innervated fibres, suggesting that SK channel activity modulates the apparent action potential threshold. 6. These results are consistent with a model in which SK channel activity in the T-tubules of denervated skeletal muscle causes a local increase in K+ concentration that results in hyperexcitability.

ATP-dependent activation of the intermediate conductance, Ca2+-activated K+ channel, hIK1, is conferred by a C-terminal domain

We previously demonstrated that hIK1 is activated directly by ATP in excised, inside-out patches in a protein kinase A inhibitor 5-24 dependent manner, suggesting a role for phosphorylation in the regulation of this Ca(2+)-dependent channel. However, mutation of the single consensus cAMP-dependent protein kinase phosphorylation site (S334A) failed to modify the response of hIK1 to ATP (Gerlach, A. C., Gangopadhyay, N. N., and Devor, D. C. (2000) J. Biol. Chem. 275, 585-598). Here we demonstrate that ATP does not similarly activate the highly homologous Ca(2+)-dependent K(+) channels, hSK1, rSK2, and rSK3. To define the region of hIK1 responsible for the ATP-dependent regulation, we generated a series of hIK1 truncations and hIK1/rSK2 chimeras. ATP did not activate a chimera containing the N terminus plus S1-S4 from hIK1. In contrast, ATP activated a chimera containing the hIK1 C-terminal amino acids His(299)-Lys(427). Furthermore, truncation of hIK1 at Leu(414) resulted in an ATP-dependent channel, whereas larger truncations of hIK1 failed to express. Additional hIK1/rSK2 chimeras defined the minimal region of hIK1 required to confer complete ATP sensitivity as including amino acids Arg(355)-Ala(413). An alanine scan of all non-conserved serines and threonines within this region failed to alter the response of hIK1 to ATP, suggesting that hIK1 itself is not directly phosphorylated. Additionally, substitution of amino acids Arg(355)-Met(368) of hIK1 into the corresponding region of rSK2 resulted in an ATP-dependent activation, which was approximately 50% of that of hIK1. These results demonstrate that amino acids Arg(355)-Ala(413) within the C terminus of hIK1 confer sensitivity to ATP. Finally, we demonstrate that the ATP-dependent phosphorylation of hIK1 or an associated protein is independent of Ca(2+).

Structure of the gating domain of a Ca2+-activated K+ channel complexed with Ca2+/calmodulin

Small-conductance Ca2+-activated K+ channels (SK channels) are independent of voltage and gated solely by intracellular Ca2+. These membrane channels are heteromeric complexes that comprise pore-forming alpha-subunits and the Ca2+-binding protein calmodulin (CaM). CaM binds to the SK channel through the CaM-binding domain (CaMBD), which is located in an intracellular region of the alpha-subunit immediately carboxy-terminal to the pore. Channel opening is triggered when Ca2+ binds the EF hands in the N-lobe of CaM. Here we report the 1.60 A crystal structure of the SK channel CaMBD/Ca2+/CaM complex. The CaMBD forms an elongated dimer with a CaM molecule bound at each end; each CaM wraps around three alpha-helices, two from one CaMBD subunit and one from the other. As only the CaM N-lobe has bound Ca2+, the structure provides a view of both calcium-dependent and -independent CaM/protein interactions. Together with biochemical data, the structure suggests a possible gating mechanism for the SK channel.

ATP-dependent activation of the intermediate conductance, Ca2+-activated K+ channel, hIK1, is conferred by a C-terminal domain

We previously demonstrated that hIK1 is activated directly by ATP in excised, inside-out patches in a protein kinase A inhibitor 5-24 dependent manner, suggesting a role for phosphorylation in the regulation of this Ca(2+)-dependent channel. However, mutation of the single consensus cAMP-dependent protein kinase phosphorylation site (S334A) failed to modify the response of hIK1 to ATP (Gerlach, A. C., Gangopadhyay, N. N., and Devor, D. C. (2000) J. Biol. Chem. 275, 585-598). Here we demonstrate that ATP does not similarly activate the highly homologous Ca(2+)-dependent K(+) channels, hSK1, rSK2, and rSK3. To define the region of hIK1 responsible for the ATP-dependent regulation, we generated a series of hIK1 truncations and hIK1/rSK2 chimeras. ATP did not activate a chimera containing the N terminus plus S1-S4 from hIK1. In contrast, ATP activated a chimera containing the hIK1 C-terminal amino acids His(299)-Lys(427). Furthermore, truncation of hIK1 at Leu(414) resulted in an ATP-dependent channel, whereas larger truncations of hIK1 failed to express. Additional hIK1/rSK2 chimeras defined the minimal region of hIK1 required to confer complete ATP sensitivity as including amino acids Arg(355)-Ala(413). An alanine scan of all non-conserved serines and threonines within this region failed to alter the response of hIK1 to ATP, suggesting that hIK1 itself is not directly phosphorylated. Additionally, substitution of amino acids Arg(355)-Met(368) of hIK1 into the corresponding region of rSK2 resulted in an ATP-dependent activation, which was approximately 50% of that of hIK1. These results demonstrate that amino acids Arg(355)-Ala(413) within the C terminus of hIK1 confer sensitivity to ATP. Finally, we demonstrate that the ATP-dependent phosphorylation of hIK1 or an associated protein is independent of Ca(2+).

Control of electrical activity in central neurons by modulating the gating of small conductance Ca2+-activated K+ channels

In most central neurons, action potentials are followed by an afterhyperpolarization (AHP) that controls firing pattern and excitability. The medium and slow components of the AHP have been ascribed to the activation of small conductance Ca(2+)-activated potassium (SK) channels. Cloned SK channels are heteromeric complexes of SK alpha-subunits and calmodulin. The channels are activated by Ca(2+) binding to calmodulin that induces conformational changes resulting in channel opening, and channel deactivation is the reverse process brought about by dissociation of Ca(2+) from calmodulin. Here we show that SK channel gating is effectively modulated by 1-ethyl-2-benzimidazolinone (EBIO). Application of EBIO to cloned SK channels shifts the Ca(2+) concentration-response relation into the lower nanomolar range and slows channel deactivation by almost 10-fold. In hippocampal CA1 neurons, EBIO increased both the medium and slow AHP, strongly reducing electrical activity. Moreover, EBIO suppressed the hyperexcitability induced by low Mg(2+) in cultured cortical neurons. These results underscore the importance of SK channels for shaping the electrical response patterns of central neurons and suggest that modulating SK channel gating is a potent mechanism for controlling excitability in the central nervous system.

Structure and complex transcription pattern of the mouse SK1 K(Ca) channel gene, KCNN1

Small conductance calcium-gated K(+) channels (SK channels) are encoded by the three SK genes, SK1, SK2, and SK3. These channels likely contribute to slow synaptic afterhyperpolarizations of apamin-sensitive and apamin-insensitive types. SK channels are also widely expressed outside the nervous system. The mouse SK1 gene comprises at least 12 exons extending across 19.8 kb of genomic DNA. This gene encodes a complex pattern of alternatively spliced SK1 transcripts widely expressed among mouse tissues. These transcripts exhibit at least four distinct 5'-nucleotide sequence variants encoding at least two N-terminal amino acid sequences. Optional inclusion of exons 7 and 9, together with two alternate splice donor sites in exon 8, yields transcripts encoding eight variant C-terminal amino acid sequences for SK1. These include an altered putative S6 transmembrane span, modification of the C-terminal cytoplasmic domain binding site for calmodulin, and generation of two alternate predicted binding sites for PDZ domain-containing proteins. 20 of the 32 predicted mouse SK1 transcripts are expressed in brain at levels sufficient to allow consistent detection, and encode 16 SK1 polypeptide variants. Only four of these 16 polypeptides preserve the ability to bind calmodulin in a Ca(2+)-independent manner. Mouse SK1 also exhibits novel, strain-specific, length polymorphism of a polyglutamate repeat in the N-terminal cytoplasmic domain. The evolutionary conservation of this complex transcription pattern suggests a possible role in the tuning of SK1 channel function.

Acid-sensing ion channel 3 matches the acid-gated current in cardiac ischemia-sensing neurons

Cardiac afferents are sensory neurons that mediate angina, pain that occurs when the heart receives insufficient blood supply for its metabolic demand (ischemia). These neurons display enormous acid-evoked depolarizing currents, and they fire action potentials in response to extracellular acidification that accompanies myocardial ischemia. Here we show that acid-sensing ion channel 3 (ASIC3), but no other known acid-sensing ion channel, reproduces the functional features of the channel that underlies the large acid-evoked current in cardiac afferents. ASIC3 and the native channel are both especially sensitive to pH, interact similarly with Ca(2+), and gate rapidly between closed, open, and desensitized states. Particularly important is the ability of ASIC3 and the native channel to open at pH 7, a value reached in the first few minutes of a heart attack. The steep activation curve suggests that the channel opens when four protons bind. We propose that ASIC3, a member of the degenerin channel (of Caenorhabditis elegans)/epithelial sodium channel family of ion channels, is the sensor of myocardial acidity that triggers cardiac pain, and that it might be a useful pharmaceutical target for treating angina.

Respiration and parturition affected by conditional overexpression of the Ca2+-activated K+ channel subunit, SK3

In excitable cells, small-conductance Ca2+-activated potassium channels (SK channels) are responsible for the slow after-hyperpolarization that often follows an action potential. Three SK channel subunits have been molecularly characterized. The SK3 gene was targeted by homologous recombination for the insertion of a gene switch that permitted experimental regulation of SK3 expression while retaining normal SK3 promoter function. An absence of SK3 did not present overt phenotypic consequences. However, SK3 overexpression induced abnormal respiratory responses to hypoxia and compromised parturition. Both conditions were corrected by silencing the gene. The results implicate SK3 channels as potential therapeutic targets for disorders such as sleep apnea or sudden infant death syndrome and for regulating uterine contractions during labor.

Inhibition of the human intermediate conductance Ca(2+)-activated K(+) channel, hIK1, by volatile anesthetics

Ca(2+)-activated K(+) channels (K(Ca)) regulate a wide variety of cellular functions by coupling intracellular Ca(2+) concentration to membrane potential. There are three major groups of K(Ca) classified by their unit conductances: large (BK), intermediate (IK), and small (SK) conductance of channels. BK channel is gated by combined influences of Ca(2+) and voltage, while IK and SK channels are gated solely by Ca(2+). Volatile anesthetics inhibit BK channel activity by interfering with the Ca(2+) gating mechanism. However, the effects of anesthetics on IK and SK channels are unknown. Using cloned IK and SK channels, hIK1 and hSK1-3, respectively, we found that the currents of hIK1 were inhibited rapidly and reversibly by volatile anesthetics, whereas those of SK channels were not affected. The IC(50) values of the volatile anesthetics, halothane, sevoflurane, enflurane, and isoflurane for hIK1 inhibition were 0.69, 0.42, 1.01 and 1.03 mM, respectively, and were in the clinically used concentration range. In contrast to BK channel, halothane inhibition of hIK1 currents was independent of Ca(2+) concentration, suggesting that Ca(2+) gating mechanism is not involved. These results demonstrate that volatile anesthetics, such as halothane, enflurane, isoflurane, and sevoflurane, affect BK, IK, and SK channels in distinct ways.

SK2 encodes the apamin-sensitive Ca(2+)-activated K(+) channels in the human leukemic T cell line, Jurkat

T cells express two different types of voltage-independent Ca(2+)-activated K(+) channels with small (SK) and intermediate (IK) conductance that serve important roles in the activation of T lymphocytes. In contrast to the IK channels from T lymphocytes which are upregulated upon mitogen stimulation, SK channels of Jurkat T cells, a human leukemic T cell line, are constitutively expressed even in the absence of mitogenic stimulation. We have used patch-clamp recordings from transfected or injected mammalian cells to show that the cloned SK2 channel demonstrates the biophysical and pharmacological properties of the majority of K(Ca) channels in Jurkat T cells. The cloned and native channels are voltage-independent, Ca(2+)-activated, apamin-sensitive, show an equivalent voltage-dependent Ba(2+) block and possess a similar ion selectivity. In addition, we used the polymerase chain reaction to demonstrate the presence of SK2 mRNA in Jurkat T cells, whereas SK3 transcripts encoding the other cloned apamin-sensitive SK channel were not detected. These data suggest that the voltage-independent apamin-sensitive K(Ca) channel in Jurkat T cells represents the recently cloned SK2 channel.

The murine Bis1 seizure gene and the Kcnab2 gene encoding the beta2-subunit of the K+ channel are different

We tested the hypothesis that Bis1, a gene involved in seizure regulation in mice which has been localized to the distal part of chromosome 4, was the same as the gene Kcnab2, encoding the beta2-subunit for voltage-dependent K+ channels. Two facts suggested this hypothesis: Kcnab2 is located in the 3.1-cM confidence interval containing Bis1 and many studies have shown an involvement of K+ channels in the genesis of seizures. DNA sequence analysis of the coding sequence for Kcnab2 from JE/Le mice revealed no structural alterations which might affect Kcnab2 function. However, several nucleotide changes were observed.

Domains responsible for constitutive and Ca(2+)-dependent interactions between calmodulin and small conductance Ca(2+)-activated potassium channels

Small conductance Ca(2+)-activated potassium channels (SK channels) are coassembled complexes of pore-forming SK alpha subunits and calmodulin. We proposed a model for channel activation in which Ca2+ binding to calmodulin induces conformational rearrangements in calmodulin and the alpha subunits that result in channel gating. We now report fluorescence measurements that indicate conformational changes in the alpha subunit after calmodulin binding and Ca2+ binding to the alpha subunit-calmodulin complex. Two-hybrid experiments showed that the Ca(2+)-independent interaction of calmodulin with the alpha subunits requires only the C-terminal domain of calmodulin and is mediated by two noncontiguous subregions; the ability of the E-F hands to bind Ca2+ is not required. Although SK alpha subunits lack a consensus calmodulin-binding motif, mutagenesis experiments identified two positively charged residues required for Ca(2+)-independent interactions with calmodulin. Electrophysiological recordings of SK2 channels in membrane patches from oocytes coexpressing mutant calmodulins revealed that channel gating is mediated by Ca2+ binding to the first and second E-F hand motifs in the N-terminal domain of calmodulin. Taken together, the results support a calmodulin- and Ca(2+)-calmodulin-dependent conformational change in the channel alpha subunits, in which different domains of calmodulin are responsible for Ca(2+)-dependent and Ca(2+)-independent interactions. In addition, calmodulin is associated with each alpha subunit and must bind at least one Ca2+ ion for channel gating. Based on these results, a state model for Ca2+ gating was developed that simulates alterations in SK channel Ca2+ sensitivity and cooperativity associated with mutations in CaM.

Gene structure and chromosome mapping of the human small-conductance calcium-activated potassium channel SK1 gene (KCNN1)

Small-conductance, calcium-activated potassium channels contribute to the afterhyperpolarization in central neurons and other cell types. Because these channels regulate neuronal excitability, defects in their genes could cause excitability disorders. The human cDNA encoding one such channel, SK1 (KCNN1), was recently cloned. Here we describe the gene structure of KCNN1 and its localization by radiation hybrid mapping to chromosome 19p13.1.

Gating properties of single SK channels in hippocampal CA1 pyramidal neurons

The activation of small-conductance calcium-activated potassium channels (SK) has a profound effect on membrane excitability. In hippocampal pyramidal neurons, SK channel activation by Ca2+ entry from a preceding burst of action potentials generates the slow afterhyperpolarization (AHP). Stimulation of a number of receptor types suppresses the slow AHP, inhibiting spike frequency adaptation and causing these neurons to fire tonically. Little is known of the gating properties of native SK channels in CNS neurons. By using excised inside-out patches, a small-amplitude channel has been resolved that was half-activated by approximately 0.6 microM Ca2+ in a voltage-independent manner. The channel possessed a slope conductance of 10 pS and exhibited nonstationary gating. These properties are in accord with those of cloned SK channels. The measured Ca2+ sensitivity of hippocampal SK channels suggests that the slow AHP is generated by activation of SK channels from a local rise of intracellular Ca2+.

The HER-2/neu receptor tyrosine kinase gene encodes a secreted autoinhibitor

HER-2/neu (erbB-2) encodes an 185-kDa orphan receptor tyrosine kinase that is constitutively active as a dimer and displays potent oncogenic activity when overexpressed. Here we describe a secreted protein of approximately 68 kDa, designated herstatin, as the product of an alternative HER-2 transcript that retains intron 8. This alternative transcript specifies 340 residues identical to subdomains I and II from the extracellular domain of p185HER-2 followed by a unique C-terminal sequence of 79 aa encoded by intron 8. The recombinant product of the alternative transcript specifically binds to HER-2-transfected cells with a K(D) of approximately 14 nM and was chemically crosslinked to p185HER-2, whereas the intron encoded sequence alone also binds with high affinity to transfected cells and associates with p185 solubilized from cell extracts. The herstatin mRNA is expressed in normal human fetal kidney and liver, but is at reduced levels relative to p185HER-2 mRNA in carcinoma cells that contain an amplified HER-2 gene. Herstatin appears to be an inhibitor of p185HER-2, because it disrupts dimers, reduces tyrosine phosphorylation of p185, and inhibits the anchorage-independent growth of transformed cells that overexpress HER-2.

An alternative HER-2/neu transcript of 8 kb has an extended 3'UTR and displays increased stability in SKOV-3 ovarian carcinoma cells

HER-2/neu is a potent oncogene that predicts poor outcome when overexpressed in ovarian cancer. The SKOV-3 ovarian carcinoma cell line, one of the only models for HER2-driven ovarian cancer, expresses a major uncharacterized 8-kb alternative HER-2 transcript.

OBJECTIVES

The aim of this study was to characterize the structure and determine the origin of the alternative sequence and examine the possible role of the 8-kb alternative transcript in overexpression of the HER-2 gene.

METHODS

The structure of the 8-kb transcript was investigated using polymerase chain reaction (PCR) and nucleotide sequencing of cDNA clones. PCR analysis of genomic DNA was used to assess the origin of the 8-kb transcript. The stability of the 8-kb mRNA was assessed by Northern blot analysis of RNA extracted from cells treated with transcriptional inhibitors.

RESULTS

Similar 5'UTR and coding sequence but an extended 3'UTR were contained in the 8-kb compared to the well-characterized 4.5-kb HER-2 transcript. Genomic DNA had continuity between the novel 3'UTR sequence from the 8-kb transcript and adjacent HER-2 terminal exon sequence. The 8-kb transcript had a half-life of 13 h compared to 5.5 h for the 4.5-kb transcript (P<0.01).

CONCLUSIONS

The 8-kb transcript is generated from alternative polyadenylation site usage rather than gene rearrangement. Since the 8-kb transcript contains alternative sequence found at the 3' end of the normal HER-2 gene, it could be expressed in other cells. Increased stability of the 8-kb transcript may confer a selective advantage for SKOV-3 cells by providing enhanced HER-2 expression.

Bicuculline block of small-conductance calcium-activated potassium channels

Small-conductance calcium-activated potassium channels (SK channels) are gated solely by intracellular calcium ions and their activity is responsible for the slow afterhyperpolarization (AHP) that follows an action potential in many excitable cells. Brain slice studies commonly employ a methyl derivative of bicuculline (bicuculline-m), a GABAA (gamma-aminobutyric acid) receptor antagonist, to diminish the tonic inhibitory influences of GABAergic synapses, or to investigate the role of these synapses in specialized neural networks. However, recent evidence suggests that bicuculline-m may not be specific for GABAA receptors and may also block the slow AHP. Therefore, the effects of bicuculline-m on cloned apamin-sensitive SK2 and apamin-insensitive SK1 channels were examined following expression in Xenopus oocytes. The results show that at concentrations employed for slice recordings, bicuculline-m potently blocks both apamin-sensitive SK2 currents and apamin-insensitive SK1 currents when applied to outside-out patches. Apamin-insensitive SK1 currents run down in excised patches. The potency of bicuculline-m block also decreases with time after patch excision. Site-directed mutagenesis that changes two residues in the outer vestibule of the SK1 pore that confers apamin sensitivity also reduces run down of the current in patches, and endows stable sensitivity to bicuculline-m indistinguishable from SK2. Therefore, the use of bicuculline-m in slice recordings may mask apamin-sensitive slow AHPs that are important determinants of neuronal excitability. In addition, bicuculline-m-insensitive slow AHPs may indicate that the underlying channels have run down.

Skeletal muscle and small-conductance calcium-activated potassium channels

Skeletal muscle becomes hyperexcitable following denervation and when cultured in the absence of nerve cells. In these circumstances, the bee venom peptide toxin apamin, a blocker of small-conductance calcium-activated potassium (SK) channels, dramatically reduces the hyperexcitability. In this report, we show that SK3 channels are expressed in denervated skeletal muscle and in L6 cells. Action potentials evoked from normal innervated rat skeletal muscle did not exhibit an afterhyperpolarization, indicating a lack of SK channel activity; very low levels of apamin binding sites, SK3 protein, or SK3 mRNA were present. However, denervation resulted in apamin-sensitive afterhyperpolarizations and increased apamin binding sites, SK3 protein, and SK3 mRNA. Cultured rat L6 myoblasts and differentiated L6 myotubes contained similar levels of SK3 mRNA, although apamin-sensitive SK currents and apamin binding sites were detected only following myotube differentiation. Therefore, different molecular mechanisms govern SK3 expression levels in denervated muscle compared with muscle cells differentiated in culture.

Small-conductance calcium-activated potassium channels

SK channels play a fundamental role in all excitable cells. SK channels are potassium selective and are activated by an increase in the level of intracellular calcium, such as occurs during an action potential. Their activation causes membrane hyperpolarization, which inhibits cell firing and limits the firing frequency of repetitive action potentials. The intracellular calcium increase evoked by action potential firing decays slowly, allowing SK channel activation to generate a long-lasting hyperpolarization termed the slow afterhyperpolarization (sAHP). This spike-frequency adaptation protects the cell from the deleterious effects of continuous tetanic activity and is essential for normal neurotransmission. Slow AHPs can be classified into two groups, based on sensitivity to the bee venom toxin apamin. In general, apamin-sensitive sAHPs activate rapidly following a single action potential and decay with a time constant of approximately 150 ms. In contrast, apamin-insensitive sAHPs rise slowly and decay with a time constant of approximately 1.5 s. The basis for this kinetic difference is not yet understood. Apamin-sensitive and apamin-insensitive SK channels have recently been cloned. This chapter will compare with different classes of sAHPs, discuss the cloned SK channels and how they are gated by calcium ions, describe the molecular basis for their different pharmacologies, and review the possible role of SK channels in several pathological conditions.

Calmodulin is the Ca2+ sensor for Ca2+ -dependent inactivation of L-type calcium channels

Elevated intracellular Ca2+ triggers inactivation of L-type calcium channels, providing negative Ca2+ feedback in many cells. Ca2+ binding to the main alpha1c channel subunit has been widely proposed to initiate such Ca2+ -dependent inactivation. Here, we find that overexpression of mutant, Ca2+ -insensitive calmodulin (CaM) ablates Ca2+ -dependent inactivation in a "dominant-negative" manner. This result demonstrates that CaM is the actual Ca2+ sensor for inactivation and suggests that CaM is constitutively tethered to the channel complex. Inactivation is likely to occur via Ca2+ -dependent interaction of tethered CaM with an IQ-like motif on the carboxyl tail of alpha1c. CaM also binds to analogous IQ regions of N-, P/Q-, and R-type calcium channels, suggesting that CaM-mediated effects may be widespread in the calcium channel family.

Mechanism of calcium gating in small-conductance calcium-activated potassium channels

The slow afterhyperpolarization that follows an action potential is generated by the activation of small-conductance calcium-activated potassium channels (SK channels). The slow afterhyperpolarization limits the firing frequency of repetitive action potentials (spike-frequency adaptation) and is essential for normal neurotransmission. SK channels are voltage-independent and activated by submicromolar concentrations of intracellular calcium. They are high-affinity calcium sensors that transduce fluctuations in intracellular calcium concentrations into changes in membrane potential. Here we study the mechanism of calcium gating and find that SK channels are not gated by calcium binding directly to the channel alpha-subunits. Instead, the functional SK channels are heteromeric complexes with calmodulin, which is constitutively associated with the alpha-subunits in a calcium-independent manner. Our data support a model in which calcium gating of SK channels is mediated by binding of calcium to calmodulin and subsequent conformational alterations in the channel protein.

cDNA cloning and functional characterization of the mouse Ca2+-gated K+ channel, mIK1. Roles in regulatory volume decrease and erythroid differentiation

We have cloned from murine erythroleukemia (MEL) cells, thymus, and stomach the cDNA encoding the Ca2+-gated K+ (KCa) channel, mIK1, the mouse homolog of hIK1 (Ishii, T. M., Silvia, C., Hirschberg, B., Bond, C. T., Adelman, J. P., and Maylie, J. (1997) Proc. Natl. Acad. Sci.(U. S. A. 94, 11651-11656). mIK1 mRNA was detected at varied levels in many tissue types. mIK1 KCa channel activity expressed in Xenopus oocytes closely resembled the Kca of red cells (Gardos channel) and MEL cells in its single channel conductance, lack of voltage-sensitivity of activation, inward rectification, and Ca2+ concentration dependence. mIK1 also resembled the erythroid channel in its pharmacological properties, mediating whole cell and unitary currents sensitive to low nM concentrations of both clotrimazole (CLT) and its des-imidazolyl metabolite, 2-chlorophenyl-bisphenyl-methanol, and to low nM concentrations of iodocharybdotoxin. Whereas control oocytes subjected to hypotonic swelling remained swollen, mIK1 expression conferred on oocytes a novel, Ca2+-dependent, CLT-sensitive regulatory volume decrease response. Hypotonic swelling of voltage-clamped mIK1-expressing oocytes increased outward currents that were Ca2+-dependent, CLT-sensitive, and reversed near the K+ equilibrium potential. mIK1 mRNA levels in ES cells increased steadily during erythroid differentiation in culture, in contrast to other KCa mRNAs examined. Low nanomolar concentrations of CLT inhibited proliferation and erythroid differentiation of peripheral blood stem cells in liquid culture.

Characterization of three episodic ataxia mutations in the human Kv1.1 potassium channel

Episodic ataxia (EA) is a rare inherited neurological disorder due to mutation in the voltage-dependent Kv1.1 potassium channel. In nine unrelated families, a different missense point mutation at highly conserved positions has been reported. We have previously characterized six of the EA mutants. In this study, three recently identified mutations were introduced into the human Kv1.1 cDNA and expressed in Xenopus oocytes. Compared to wild type, T226A and T226M reduced the current amplitude by > 95%, shifted the voltage dependence by 15 mV, and slowed activation and deactivation kinetics. Currents from G311S were approximately 25% of wild type, less steeply voltage-dependent and had more pronounced C-type inactivation. These altered gating properties will reduce the delayed-rectifier potassium current which may underlie the symptoms of EA.

Calcium-activated potassium channels

Calcium-activated potassium channels are fundamental regulators of neuronal excitability, participating in interspike interval and spike-frequency adaptation. For large-conductance calcium-activated potassium (BK) channels, recent experiments have illuminated the fundamental biophysical mechanisms of gating, demonstrating that BK channels are voltage gated and calcium modulated. Structurally, BK channels have been shown to possess an extracellular amino-terminal domain, different from other potassium channels. Domains and residues involved in calcium-gating, and perhaps calcium binding itself, have been identified. For small- and intermediate-conductance calcium-activated potassium channels, SK and IK channels, clones have only recently become available, and they show that SK channels are a distinct subfamily of potassium channels. The biophysical properties of SK channels demonstrate that kinetic differences between apamin-sensitive and apamin-insensitive slow afterhyperpolarizations are not attributable to intrinsic gating differences between the two subtypes. Interestingly, SK and IK channels may prove effective drug targets for diseases such as myotonic muscular dystrophy and sickle cell anemia.

Gating of I(sK) channels expressed in Xenopus oocytes

The channel underlying the slow component of the voltage-dependent delayed outward rectifier K+ current, I(Ks), in heart is composed of the minK and KvLQT1 proteins. Expression of the minK protein in Xenopus oocytes results in I(Ks)-like currents, I(sK), due to coassembly with the endogenous XKvLQT1. The kinetics and voltage-dependent characteristics of I(sK) suggest a distinct mechanism for voltage-dependent gating. Currents recorded at 40 mV from holding potentials between -60 and -120 mV showed an unusual "cross-over," with the currents obtained from more depolarized holding potentials activating more slowly and deviating from the Cole-Moore prediction. Analysis of the current traces revealed two components with fast and slow kinetics that were not affected by the holding potential. Rather, the relative contribution of the fast component decreased with depolarized holding potentials. Deactivation and reactivation, after a short period of repolarization (100 ms), was markedly faster than the fast component of activation. These gating properties suggest a physiological mechanism by which cardiac I(Ks) may suppress premature action potentials.

Episodic ataxia mutations in Kv1.1 alter potassium channel function by dominant negative effects or haploinsufficiency

Subunits of the voltage-gated potassium channel Kv1.1 containing mutations responsible for episodic ataxia (EA), a human inherited neurological disease, were expressed in Xenopus oocytes. Five EA subunits formed functional homomeric channels with lower current amplitudes and altered gating properties compared with wild type. Two EA mutations located in the first cytoplasmic loop, R239S and F249I, yielded minimal or no detectable current, and Western blot analysis showed reduced protein levels. Coinjection of equal amounts of EA and wild-type mRNAs, mimicking the heterozygous condition, resulted in current amplitudes and gating properties that were intermediate between wild-type and EA homomeric channels, suggesting that heteromeric channels are formed with a mixed stoichiometry of EA and wild-type subunits. To examine the relative contribution of EA subunits in forming heteromeric EA and wild-type channels, each EA subunit was made insensitive to TEA, TEA-tagged, and coexpressed with wild-type subunits. TEA-tagged R239S and F249I induced the smallest shift in TEA sensitivity compared with homomeric wild-type channels, whereas the other TEA-tagged EA subunits yielded TEA sensitivities similar to coexpression of wild-type and TEA-tagged wild-type subunits. Taken together, these results show that the different mutations in Kv1.1 affect channel function and indicate that both dominant negative effects and haplotype insufficiency may result in the symptoms of EA.

Aromatic residues affecting permeation and gating in dSlo BK channels

Structural determinants of permeation in large unit conductance calcium-activated potassium channels (BK channels) were investigated. Y293 and F294 in the P-region of dSlo were substituted by tryptophans. Compared to wild-type channels, Y293W channels displayed reduced inward unitary currents while F294W channels exhibited normal inward current amplitudes but flickery kinetics. Both mutations produced changes in current/voltage relations under bi-ionic conditions. Sensitivity to block by external tetraethylammonium (TEA) was affected in both channels, and the voltage dependence of TEA block was increased in F294W channels. Both mutations also affected gating by shifting the half-maximal activation voltage of macroscopic conductance/voltage relations to more positive potentials, and eliminating a slow component of deactivation. The double mutant did not produce ionic currents. These data are consistent with a model in which Y293 contributes to a potassium-binding site close to the outer mouth of the dSlo pore, while F294 contributes to an energy barrier near this site.

dSLo interacting protein 1, a novel protein that interacts with large-conductance calcium-activated potassium channels

Large-conductance calcium-activated potassium channels (BK channels) are activated by depolarized membrane potential and elevated levels of intracellular calcium. BK channel activity underlies the fast afterhyperpolarization that follows an action potential and attenuates neurotransmitter and hormone secretion. Using a modified two-hybrid approach, the interaction trap, we have identified a novel protein from Drosophila, dSLIP1 (dSLo interacting protein), which specifically interacts with Drosophila and human BK channels and has partial homology to the PDZ domain of alpha1 syntrophin. The dSLIP1 and dSlo mRNAs are expressed coincidently throughout the Drosophila nervous system, the two proteins interact in vitro, and they may be coimmunoprecipitated from transfected cells. Coexpression of dSLIP1 with dSlo or hSlo BK channels in Xenopus oocytes results in reduced currents as compared with expression of BK channels alone; current amplitudes may be rescued by coexpression with the channel domain that interacts with dSLIP1. Single-channel recordings and immunostaining of transfected tissue culture cells suggest that dSLIP1 selectively reduces Slo BK currents by reducing the number of BK channels in the plasma membrane.

Gating of recombinant small-conductance Ca-activated K+ channels by calcium

Small-conductance Ca-activated K+ channels play an important role in modulating excitability in many cell types. These channels are activated by submicromolar concentrations of intracellular Ca2+, but little is known about the gating kinetics upon activation by Ca2+. In this study, single channel currents were recorded from Xenopus oocytes expressing the apamin-sensitive clone rSK2. Channel activity was detectable in 0.2 micro M Ca2+ and was maximal above 2 micro M Ca2+. Analysis of stationary currents revealed two open times and three closed times, with only the longest closed time being Ca dependent, decreasing with increasing Ca2+ concentrations. In addition, elevated Ca2+ concentrations resulted in a larger percentage of long openings and short closures. Membrane voltage did not have significant effects on either open or closed times. The open probability was approximately 0.6 in 1 micro M free Ca2+. A lower open probability of approximately 0.05 in 1 micro M Ca2+ was also observed, and channels switched spontaneously between behaviors. The occurrence of these switches and the amount of time channels spent displaying high open probability behavior was Ca2+ dependent. The two behaviors shared many features including the open times and the short and intermediate closed times, but the low open probability behavior was characterized by a different, long Ca2+-dependent closed time in the range of hundreds of milliseconds to seconds. Small-conductance Ca- activated K+ channel gating was modeled by a gating scheme consisting of four closed and two open states. This model yielded a close representation of the single channel data and predicted a macroscopic activation time course similar to that observed upon fast application of Ca2+ to excised inside-out patches.

Episodic ataxia type-1 mutations in the hKv1.1 cytoplasmic pore region alter the gating properties of the channel

Episodic ataxia type-1 is a rare human neurological syndrome which occurs during childhood and persists through the whole life of affected patients. Several heterozygous point mutations have been found in the coding sequence of the voltage-gated potassium channel gene hKv1.1 of different affected families. V408A and E325D mutations are located in the cytoplasmic putative pore region of hKv1.1 channels and profoundly alter their gating properties. V408A channels showed increased kinetic rates of activation, deactivation and C-type inactivation. Expression of E325D channels in Xenopus oocytes led to an approximately 13-fold current amplitude reduction and to a 52.4 mV positive shift in the voltage dependence of activation. Moreover, the E325D mutation altered the kinetics of activation, deactivation, C-type inactivation and channel open probability. Heteromeric channels composed of two wild-type and two mutated subunits, linked as dimers, showed gating properties intermediate between channels formed from four normal or four mutated subunits. The results demonstrate that the highly conserved residues Val408 and Glu325 play a pivotal role in several gating processes of a human potassium channel, and suggest a pathogenetic mechanism by which the impairment of the delayed-rectifier function of affected neurons is related to the type and number of mutated subunits which make up the hKv1.1 channels.

A human intermediate conductance calcium-activated potassium channel

An intermediate conductance calcium-activated potassium channel, hIK1, was cloned from human pancreas. The predicted amino acid sequence is related to, but distinct from, the small conductance calcium-activated potassium channel subfamily, which is approximately 50% conserved. hIK1 mRNA was detected in peripheral tissues but not in brain. Expression of hIK1 in Xenopus oocytes gave rise to inwardly rectifying potassium currents, which were activated by submicromolar concentrations of intracellular calcium (K0.5 = 0.3 microM). Although the K0.5 for calcium was similar to that of small conductance calcium-activated potassium channels, the slope factor derived from the Hill equation was significantly reduced (1.7 vs. 3. 5). Single-channel current amplitudes reflected the macroscopic inward rectification and revealed a conductance level of 39 pS in the inward direction. hIK1 currents were reversibly blocked by charybdotoxin (Ki = 2.5 nM) and clotrimazole (Ki = 24.8 nM) but were minimally affected by apamin (100 nM), iberiotoxin (50 nM), or ketoconazole (10 microM). These biophysical and pharmacological properties are consistent with native intermediate conductance calcium-activated potassium channels, including the erythrocyte Gardos channel.

Determinants of apamin and d-tubocurarine block in SK potassium channels

Small conductance calcium-activated potassium channels show a distinct pharmacology. Some, but not all, are blocked by the peptide toxin apamin, and apamin-sensitive channels are also blocked by d-tubocurarine. Cloned SK channels (small conductance calcium-activated potassium channel) recapitulate these properties. We have investigated the structural basis for these differences and found that two amino acid residues on either side of the deep pore are the primary determinants of sensitivity to apamin and differential block by d-tubocurarine. Therefore, the pharmacology of SK channels compared with other potassium channels correlates with structural differences in the outer pore region. However, introduction of a tyrosine residue in the position analogous to that which determines sensitivity to external tetraethylammonium for voltage-gated potassium channels endows SK channels with an equivalent tetraethylammonium sensitivity, indicating that the outer vestibules of the pores are similar. The pharmacology of channels formed in oocytes coinjected with SK1 and SK2 mRNAs, or with SK1-SK2 dimer mRNA, show that SK subunits may form heteromeric channels.

Heterooligomeric assembly of inward-rectifier K+ channels from subunits of different subfamilies: Kir2.1 (IRK1) and Kir4.1 (BIR10)

Activities of strong inward-rectifier K+ channels composed of Kir2. 1(84 M), Kir2.1(84T) and Kir4.1 subunits and weak inward-rectifier K+ channels composed of Kir4.1(E158N) subunits were measured from giant inside-out patches of Xenopus laevis oocytes. The conductance/voltage (g/V) relationship for block by intracellular spermine (SPM) was biphasic for both Kir2.1 channel types while it was monophasic for both Kir4.1 channel types. The release of blocking Mg2+ ions was slow for Kir2.1(84T) but virtually instantaneous for Kir2.1(84M) and both Kir4.1 channel types. Coexpression of Kir2.1(84T) and Kir4.1(E158N) resulted in heterooligomeric channels which were strongly rectifying, with a g/V relationship for SPM-evoked block that was significantly different from that of either parental homooligomeric channel type. Block by intracellular Mg2+ was markedly stronger than that for Kir4.1(E158N) channels, while release of the block was almost instantaneous, similar to that for Kir4.1(E158N) channels. This suggests preferential formation of a particular heterooligomer such as was recently proposed for subunits within the Kir3.0 family.

Characterization of two new preproGnRH mRNAs in the tree shrew: first direct evidence for mesencephalic GnRH gene expression in a placental mammal

Reproductive maturation and regulation is centrally orchestrated by gonadotropin-releasing hormone (GnRH). GnRH produced in the vertebrate hypothalamus acts on the pituitary to regulate gonadotropins. In nonplacental mammalian species, it has recently been shown that a second GnRH gene is expressed in mesencephalic cells. Here, we report the cDNA sequences and expression patterns for two distinct genes encoding the hypothalamic and mesencephalic GnRH forms in the brain of a placental mammal, the tree shrew (Tupaia glis belangeri). The novel mammalian GnRH form, designated here as [His5Trp7Tyr8]GnRH (often called chicken GnRH II), is expressed in neurons of the mesencephalon and is the first nonhypothalamic form to be isolated from a mammal. Its peptide sequence is identical to the form previously reported in fish, amphibians, reptiles, and birds, revealing that it has remained unchanged for 500 million years. In contrast, the sequences of the hypothalamic GnRH decapeptides vary by as much as 50% across vertebrate species. The remarkable sequence conservation of mesencephalic GnRH suggests that it has been highly constrained throughout evolution, perhaps indicating an important, conserved nongonadotropic role. The discovery and localization of two mRNAs encoding distinct GnRH forms in an advanced mammal suggest that other mammals, including primates, may also have a second GnRH gene with expression localized in the midbrain.

Small-conductance, calcium-activated potassium channels from mammalian brain

Members of a previously unidentified family of potassium channel subunits were cloned from rat and human brain. The messenger RNAs encoding these subunits were widely expressed in brain with distinct yet overlapping patterns, as well as in several peripheral tissues. Expression of the messenger RNAs in Xenopus oocytes resulted in calcium-activated, voltage-independent potassium channels. The channels that formed from the various subunits displayed differential sensitivity to apamin and tubocurare. The distribution, function, and pharmacology of these channels are consistent with the SK class of small-conductance, calcium-activated potassium channels, which contribute to the afterhyperpolarization in central neurons and other cell types.

Inward rectifier potassium channels. Cloning, expression and structure-function studies

A PCR-based cloning strategy was used to identify novel subunits of the two-transmembrane domain inward rectifier potassium channel family from rat brain, heart, and skeletal muscle. When expressed in Xenopus oocytes, two of these clones (Kir4.1 and Kir2.3) gave rise to inwardly rectifying potassium currents. Two-electrode voltage clamp commands to potentials negative to EK evoked inward potassium-selective currents which rapidly reached a peak amplitude and then relaxed to a steady-state level. Differences in the extent of current relaxation, the degree of rectification, and the voltage-dependent block by external cesium were detected. Two other members of this family (Kir5.1 and Kir3.4) did not produce macroscopic currents, when expressed by themselves, yet both subunits modified the currents when coexpressed with other specific members of the Kir family. Expression of chimeric subunits between Kir4.1 and either Kir5.1 or Kir3.4 suggested that the transmembrane domains determine the specificity of subunit heteropolymerization, while the C-terminal domains contribute to alterations in activation kinetics and rectification. Expression of covalently linked subunits demonstrated that the relative subunit positions, as well as stoichiometry, affect heteromeric channel activity.

Heteromeric channel formation and Ca(2+)-free media reduce the toxic effect of the weaver Kir 3.2 allele

Weaver mice have a severe hypoplasia of the cerebellum with an almost complete loss of the midline granule cells. Recent genetic studies of weaver mice have identified a mutation resulting in an amino acid substitution (G156S) in the pore of the inwardly rectifying potassium channel subunit Kir 3.2. When expressed in Xenopus oocytes the weaver mutation alters channel selectivity from a potassium-selective to a nonspecific cation-selective pore. In this study we confirm by cell-attached patch-clamp recording that the mutation produces a non-selective cation channel. We also demonstrate that the cell death induced by weaver expression may be prevented by elimination of calcium from the extracellular solution as well as by coexpression with the wild-type Kir 3.2 allele, or other members of the Kir 3.0 subfamily. These results suggest that the weaver defect in Kir 3.2 may cause cerebellar cell death by cell swelling and calcium overload. Cells which express the weaver subunit, but which normally survive, may do so because of heteromeric subunit assembly with wild-type subunits of the Kir 3.0 subfamily.

Muscarine-gated K+ channel: subunit stoichiometry and structural domains essential for G protein stimulation

Coexpression in Xenopus oocytes of the cloned cardiac inward rectifier subunits Kir 3.1 and Kir 3.4 results in G protein-stimulated channel activity closely resembling the muscarinic channel underlying the inwardly rectifying K+ current in atrial myocytes. To determine the stoichiometry and relative subunit positions within the channel, Kir 3.1 and Kir 3.4 were coexpressed in varying ratios with cloned G beta 1 gamma 2 subunits and also as tandemly linked tetramers with different relative subunit positions. The results reveal that the most efficient channel comprises two subunits of each type in an alternating array within the tetramer. To localize regions important for subunit coassembly and G protein sensitivity, chimeric subunits containing domains from either Kir 3.1, Kir 3.4, or the G protein-insensitive subunit Kir 4.1 were expressed. The results demonstrate that the transmembrane domains dictate the potentiation of the coassembled channels and that, although the NH4- or COOH-termini of both subunits alone can confer G protein sensitivity, both termini are required for maximal stimulation by G beta 1 gamma 2.

Wanderlust kinetics and variable Ca(2+)-sensitivity of dSlo [correction of Drosophila], a large conductance CA(2+)-activated K+ channel, expressed in oocytes

Cloned large conductance Ca2+-activated K+ channels (BK or maxi-K+ channels) from Drosophila (dSlo) were expressed in Xenopus oocytes and studied in excised membrane patches with the patch-clamp technique. Both a natural variant and a mutant that eliminated a putative cyclic AMP-dependent protein kinase phosphorylation site exhibited large, slow fluctuations in open probability with time. These fluctuations, termed "wanderlust kinetics," occurred with a time course of tens of seconds to minutes and had kinetic properties inconsistent with simple gating models. Wanderlust kinetics was still observed in the presence of 5mM caffeine or 50 nM thapsigargin, or when the Ca2+ buffering capacity of the solution was increased by the addition of 5 mM HEDTA, suggesting that the wanderlust kinetics did not arise from Ca2+ release from caffeine and thapsigargin sensitive internal stores in the excised patch. The slow changes in kinetics associated with wanderlust kinetics could be generated with a discrete-state Markov model with transitions among three or more kinetic modes with different levels of open probability. To average out the wanderlust kinetics, large amounts of data were analyzed and demonstrated up to a threefold difference in the [Ca2+]i required for an open probability of 0.5 among channels expressed from the same injected mRNA. These findings indicate that cloned dSlo channels in excised patches from Xenopus oocytes can exhibit large variability in gating properties, both within a single channel and among channels.

Subunit positional effects revealed by novel heteromeric inwardly rectifying K+ channels

Kir 4.1 is an inward rectifier potassium channel subunit isolated from rat brain which forms homomeric channels when expressed in Xenopus oocytes; Kir 5.1 is a structurally related subunit which does not. Co-injection of mRNAs encoding Kir 4.1 and Kir 5.1 resulted in potassium currents that (i) were much larger than those seen from expression of Kir 4.1 alone, (ii) increased rather than decreased during several seconds at strongly negative potentials and (iii) had an underlying unitary conductance of 43 pS rather than the 12 pS seen with Kir 4.1 alone. In contrast, the properties of Kir 1.1, 2.1, 2.3, 3.1, 3.2 or 3.4 were not altered by coexpression with Kir 5.1. Expression of a concatenated cDNA encoding two or four linked subunits produced currents with the properties of co-expressed Kir 4.1 and Kir 5.1 when the subunits were connected 4-5 or 4-5-4-5, but not when they were connected 4-4-5-5. The results indicate that Kir 5.1 associates specifically with Kir 4.1 to form heteromeric channels, and suggest that they do so normally in the subunit order 4-5-4-5. Further, the relative order of subunits within the channel contributes to their functional properties.