miR-448 regulates potassium voltage-gated channel subfamily A member 4 (KCNA4) in ischemia and heart failure

BACKGROUND

MicroRNA miR-448 mediates some of the effects of ischemia on arrhythmic risk. Potassium voltage-gated channel subfamily A member 4 (KCNA4) encodes a Kv1.4 current that opens in response to membrane depolarization and is essential for regulating the action potential duration in heart. KCNA4 has a miR-448 binding site.

OBJECTIVE

We investigated whether miR-448 was involved in the regulation of KCNA4 messenger RNA expression in ischemia.

METHODS

Quantitative real-time reverse-transcriptase polymerase chain reaction was used to investigate the expression of KCNA4 and miR-448. Pull-down assays were used to examine the interaction between miR-448 and KCNA4. miR-448 decoy and binding site mutation were used to examine the specificity of the effect for KCNA4.

RESULTS

The expression of KCNA4 is diminished in ischemia and human heart failure tissues with ventricular tachycardia. Previously, we have shown that miR-448 is upregulated in ischemia and inhibition can prevent arrhythmic risk after myocardial infarction. The 3'-untranslated region of KCNA4 has a conserved miR-448 binding site. miR-448 bound to this site directly and reduced KCNA4 expression and the transient outward potassium current. Inhibition of miR-448 restored KCNA4.

CONCLUSION

These findings showed a link between Kv1.4 downregulation and miR-448-mediated upregulation in ischemia, suggesting a new mechanism for the antiarrhythmic effect of miR-448 inhibition.

Amyloid-β Protein Precursor Regulates Electrophysiological Properties in the Hippocampus via Altered Kv1.4 Expression and Function in Mice

BACKGROUND

Amyloid-β protein precursor (AβPP) is enriched in neurons. However, the mechanism underlying AβPP regulation of neuronal activity is poorly understood. Potassium channels are critically involved in neuronal excitability. In hippocampus, A-type potassium channels are highly expressed and involved in determining neuronal spiking.

OBJECTIVE

We explored hippocampal local field potential (LFP) and spiking in the presence and absence of AβPP, and the potential involvement of an A-type potassium channel.

METHODS

We used in vivo extracellular recording and whole-cell patch-clamp recording to determine neuronal activity, current density of A-type potassium currents, and western blot to detect changes in related protein levels.

RESULTS

Abnormal LFP was observed in AβPP-/- mice, including reduced beta and gamma power, and increased epsilon and ripple power. The firing rate of glutamatergic neurons reduced significantly, in line with an increased action potential rheobase. Given that A-type potassium channels regulate neuronal firing, we measured the protein levels and function of two major A-type potassium channels and found that the post-transcriptional level of Kv1.4, but not Kv4.2, was significantly increased in the AβPP-/- mice. This resulted in a marked increase in the peak time of A-type transient outward potassium currents in both glutamatergic and gamma-aminobutyric acid-ergic (GABAergic) neurons. Furthermore, a mechanistic experiment using human embryonic kidney 293 (HEK293) cells revealed that the AβPP deficiency-induced increase in Kv1.4 may not involve protein-protein interaction between AβPP and Kv1.4.

CONCLUSION

This study suggests that AβPP modulates neuronal firing and oscillatory activity in the hippocampus, and Kv1.4 may be involved in mediating the modulation.

Altered gating of Kv1.4 in the nucleus accumbens suppresses motivation for reward

Deficient motivation contributes to numerous psychiatric disorders, including withdrawal from drug use, depression, schizophrenia, and others. Nucleus accumbens (NAc) has been implicated in motivated behavior, but it remains unclear whether motivational drive is linked to discrete neurobiological mechanisms within the NAc. To examine this, we profiled cohorts of Sprague-Dawley rats in a test of motivation to consume sucrose. We found that substantial variability in willingness to exert effort for reward was not associated with operant responding under low-effort conditions or stress levels. Instead, effort-based motivation was mirrored by a divergent NAc shell transcriptome with differential regulation at potassium and dopamine signaling genes. Functionally, motivation was inversely related to excitability of NAc principal neurons. Furthermore, neuronal and behavioral outputs associated with low motivation were linked to faster inactivation of a voltage-gated potassium channel, Kv1.4. These results raise the prospect of targeting Kv1.4 gating in psychiatric conditions associated with motivational dysfunction.

Interleukin 6 decreases nociceptor expression of the potassium channel KV1.4 in a rat model of hand-arm vibration syndrome

Chronic muscle pain is a prominent symptom of the hand-arm vibration syndrome (HAVS), an occupational disease induced by exposure to vibrating power tools, but the underlying mechanism remains unknown. We evaluated the hypothesis that vibration induces an interleukin 6 (IL-6)-mediated downregulation of the potassium voltage-gated channel subfamily A member 4 (KV1.4) in nociceptors leading to muscle pain. Adult male rats were submitted to a protocol of mechanical vibration of the right hind limb. Twenty-four hours after vibration, muscle hyperalgesia was observed, concomitant to increased levels of IL-6 in the gastrocnemius muscle and decreased expression of KV1.4 in the dorsal root ganglia. Local injection of neutralizing antibodies against IL-6 attenuated the muscle hyperalgesia induced by vibration, whereas antisense knockdown of this channel in the dorsal root ganglia mimicked the muscle hyperalgesia observed in the model of HAVS. Finally, knockdown of the IL-6 receptor signaling subunit glycoprotein 130 (gp130) attenuated both vibration-induced muscle hyperalgesia and downregulation of KV1.4. These results support the hypothesis that IL-6 plays a central role in the induction of muscle pain in HAVS. This likely occurs through intracellular signaling downstream to the IL-6 receptor subunit gp130, which decreases the expression of KV1.4 in nociceptors.

Antidepressive and anxiolytic effects of ostruthin, a TREK-1 channel activator

We screened a library of botanical compounds purified from plants of Vietnam for modulators of the activity of a two-pore domain K⁺ channel, TREK-1, and we identified a hydroxycoumarin-related compound, ostruthin, as an activator of this channel. Ostruthin increased whole-cell TREK-1 channel currents in 293T cells at a low concentration (EC₅₀ = 5.3 μM), and also activity of the TREK-2 channel (EC₅₀ = 3.7 mM). In contrast, ostruthin inhibited other K⁺ channels, e.g. human ether-à-go-go-related gene (HERG1), inward-rectifier (Kir2.1), voltage-gated (Kv1.4), and two-pore domain (TASK-1) at higher concentrations, without affecting voltage-gated potassium channel (KCNQ1 and 3). We tested the effect of this compound on mouse anxiety- and depression-like behaviors and found anxiolytic activity in the open-field, elevated plus maze, and light/dark box tests. Of note, ostruthin also showed antidepressive effects in the forced swim and tail suspension tests, although previous studies reported that inhibition of TREK-1 channels resulted in an antidepressive effect. The anxiolytic and antidepressive effect was diminished by co-administration of a TREK-1 blocker, amlodipine, indicating the involvement of TREK-1 channels. Administration of ostruthin suppressed the stress-induced increase in anti-c-Fos immunoreactivity in the lateral septum, without affecting immunoreactivity in other mood disorder-related nuclei, e.g. the amygdala, paraventricular nuclei, and dorsal raphe nucleus. Ostruthin may exert its anxiolytic and antidepressive effects through a different mechanism from current drugs.

Effects of ginseng-spikenard heart-nourishing capsule on inactivation of C-type Kv1.4 potassium channel

This research is to explore the effects of traditional Chinese medicine Ginseng-spikenard heart-nourishing capsule on the inactivation of c-type Kv1.4 channels (Kv1.4∆N) in Xenopus laevis oocytes with two-electrode voltageclamp technique. Defolliculated oocytes (stage V-VI) were injected with transcribed cRNAs of ferret Kv1.4δN channels. During recording, oocytes were continuously perfused with ND96 solution (control group) and solution prepared from Ginseng-spikenard heart-nourishing capsule (experimental group). Results found that, at the command potential of +50 mV, the current of experimental group was reduced to 48.33±4.0% of that in control group. The inactivation time constants in control and experimental groups were 2962.56±175.35 ms and 304.13±36.22ms, respectively (P<0.05, n=7). The recovery time of fKv1.4∆N channel after inactivation in control group and experimental groups was 987±68.39 ms and 1734.15±98.45 ms, respectively (P<0.05, n=5). Ginseng-spikenard heart-nourishing capsule can inhibit the Kv1.4δN channel, which may be one of the mechanisms of underlying antiarrhythmia.

Systemic administration of substance P recovers beta amyloid-induced cognitive deficits in rat: involvement of Kv potassium channels

Reduced levels of Substance P (SP), an endogenous neuropeptide endowed with neuroprotective and anti-apoptotic properties, have been found in brain and spinal fluid of Alzheimer's disease (AD) patients. Potassium (K(+)) channel dysfunction is implicated in AD development and the amyloid-β (Aβ)-induced up-regulation of voltage-gated potassium channel subunits could be considered a significant step in Aβ brain toxicity. The aim of this study was to evaluate whether SP could reduce, in vivo, Aβ-induced overexpression of Kv subunits. Rats were intracerebroventricularly infused with amyloid-β 25-35 (Aβ25-35, 20 µg) peptide. SP (50 µg/Kg, i.p.) was daily administered, for 7 days starting from the day of the surgery. Here we demonstrate that the Aβ infused rats showed impairment in cognitive performances in the Morris water maze task 4 weeks after Aβ25-35 infusion and that this impairing effect was prevented by SP administration. Kv1.4, Kv2.1 and Kv4.2 subunit levels were quantified in hippocampus and in cerebral cortex by Western blot analysis and immunofluorescence. Interestingly, SP reduced Kv1.4 levels overexpressed by Aβ, both in hippocampus and cerebral cortex. Our findings provide in vivo evidence for a neuroprotective activity of systemic administration of SP in a rat model of AD and suggest a possible mechanism underlying this effect.

Ablation of the androgen receptor gene modulates atrial electrophysiology and arrhythmogenesis with calcium protein dysregulation

Androgen deficiency is important in the pathophysiology of atrial fibrillation. Androgen regulates cardiac electrophysiology and calcium (Ca(2+)) homeostasis. The purpose of this study is to evaluate whether androgen receptor knockout (ARKO) can modulate atrial electrophysiology and arrhythmogenesis with modulation of Ca(2+) homeostasis proteins. We used conventional microelectrodes to study the action potential (AP) in left atrium (LA) tissues prepared from wild-type (WT) and ARKO mice (aged 6-10 months) before and after the administration of isoproterenol, hypocalcemic/hypercalcemic solutions, and ouabain. Echocardiography and Western blots were used to evaluate the cardiac function and expression levels of ionic channel proteins in WT and ARKO LAs. ARKO LAs had larger LA diameter with decreased LA fractional shortening than did WT LAs. In the current study, we found that ARKO LAs had a lower negative resting membrane potential and a greater 90% AP duration (APD) than did WT LAs. Isoproterenol increased the incidence and amplitude of delayed afterdepolarizations (DADs) in ARKO LAs but not in WT LAs. Hypocalcemic solutions prolonged APD in WT and ARKO LAs but increased DAD amplitude only in ARKO LAs. Hypercalcemic solutions shortened APD in ARKO LAs but not in WT LAs. Ouabain increased DAD amplitude in ARKO LAs but not in WT LAs. ARKO LAs expressed higher amounts of Ca(2+)/calmodulin-dependent protein kinase II, Na(+)/Ca(2+) exchanger, and phosphorylated phospholamban (Ser-16/Thr-17 site) and less Cav1.2, Kir2.1, Kir3.1, and Kv7.1 than WT LAs. These observations indicate that ARKO alters atrial electrophysiology with increased atrial arrhythmogenesis.

Acute inactivation of PSD-95 destabilizes AMPA receptors at hippocampal synapses

Postsynatptic density protein (PSD-95) is a 95 kDa scaffolding protein that assembles signaling complexes at synapses. Over-expression of PSD-95 in primary hippocampal neurons selectively increases synaptic localization of AMPA receptors; however, mice lacking PSD-95 display grossly normal glutamatergic transmission in hippocampus. To further study the scaffolding role of PSD-95 at excitatory synapses, we generated a recombinant PSD-95-4c containing a tetracysteine motif, which specifically binds a fluorescein derivative and allows for acute and permanent inactivation of PSD-95. Interestingly, acute inactivation of PSD-95 in rat hippocampal cultures rapidly reduced surface AMPA receptor immunostaining, but did not affected NMDA or transferrin receptor localization. Acute photoinactivation of PSD-95 in dissociated neurons causes ∼80% decrease in GluR2 surface staining observed by live-cell microscopy within 15 minutes of PSD-95-4c ablation. These results confirm that PSD-95 stabilizes AMPA receptors at postsynaptic sites and provides insight into the dynamic interplay between PSD-95 and AMPA receptors in live neurons.

Sigma-1 receptor alters the kinetics of Kv1.3 voltage gated potassium channels but not the sensitivity to receptor ligands

Sigma1 receptors (Sigma1R) are intracellular chaperone proteins that bind psychotropic drugs and also clinically used drugs such as ketamine and haloperidol. Co-expression of the Sigma1R has been reported to enhance the sensitivity of several voltage-gated ion channels to Sigma1R ligands. Kv1.3 is the predominant voltage-gated potassium channel expressed in T lymphocytes with a documented role in immune activation. To gain a better understanding of Sigma1R modulation of Kv ion channels, we investigated the effects of Sigma1R co-expression on Kv1.3 physiology and pharmacology in ion channels expressed in Xenopus oocytes. We also explored the protein domains of Kv1.3 necessary for protein:protein interaction between Kv1.3 and Sigma1R through co-immunoprecipitation studies. Slowly inactivating outward-going currents consistent with Kv1.3 expression were elicited on step depolarizations. The current characterized by E(rev), V(1/2), and slope factor remained unchanged when co-expressed with Sigma1R. Analysis of inactivation time constant revealed a faster Kv1.3 current decay when co-expressed with Sigma1R. However the sensitivity to Sigma1R ligands remained unaltered when co-expressed with the Sigma1R in contrast to the previously reported modulation of ligand sensitivity in closely related Kv1.4 and Kv1.5 voltage gated potassium channels. Co-immunoprecipitation assays of various Kv1.3 truncation constructs indicated that the transmembrane domain of the Kv1.3 protein was responsible for the protein:protein interaction with the Sigma1R. Sigma1R likely interacts with different domains of Kv ion channel family proteins resulting in distinct modulation of different channels.

Effect of the I(to) activator NS5806 on cloned K(V)4 channels depends on the accessory protein KChIP2

BACKGROUND AND PURPOSE

The compound NS5806 increases the transient outward current (I(to)) in canine ventricular cardiomyocytes and slows current decay. In human and canine ventricle, I(to) is thought to be mediated by K(V)4.3 and various ancillary proteins, yet, the exact subunit composition of I(to) channels is still debated. Here we characterize the effect of NS5806 on heterologously expressed putative I(to) channel subunits and other potassium channels.

EXPERIMENTAL APPROACH

Cloned K(V)4 channels were co-expressed with KChIP2, DPP6, DPP10, KCNE2, KCNE3 and K(V)1.4 in Xenopus laevis oocytes or CHO-K1 cells.

KEY RESULTS

NS5806 increased K(V)4.3/KChIP2 peak current amplitudes with an EC(50) of 5.3 +/- 1.5microM and significantly slowed current decay. KCNE2, KCNE3, DPP6 and DPP10 modulated K(V)4.3 currents and the response to NS5806, but current decay was slowed only in complexes containing KChIP2. The effect of NS5806 on K(V)4.2 was similar to that on K(V)4.3, and current decay was only slowed in presence of KChIP2. However, for K(V)4.1, the slowing of current decay by NS5806 was independent of KChIP2. K(V)1.4 was strongly inhibited by 10 microM NS5806 and K(V)1.5 was inhibited to a smaller extent. Effects of NS5806 on kinetics of currents generated by K(V)4.3/KChIP2/DPP6 with K(V)1.4 in oocytes could reproduce those on cardiac I(to) in canine ventricular myocytes. K(V)7.1, K(V)11.1 and K(ir)2 currents were unaffected by NS5806.

CONCLUSION AND IMPLICATIONS

NS5806 modulated K(V)4 channel gating depending on the presence of KChIP2, suggesting that NS5806 can potentially be used to address the molecular composition as well as the physiological role of cardiac I(to).

NMR assignments of a 48 kDa tetramer of the T1 domain of the mammalian voltage gated potassium channel Kv1.4

The N-terminal cytosolic T1 domain of the mammalian voltage gated potassium channel Kv1.4 is strongly involved in the tetramerization of the Kv1.4 subunit that is required for forming a functional ion channel. The T1 domain forms a stable tetramer of 48 kDa in solution that cannot be dissociated into monomers. In spite of the high molecular mass it was possible to completely assign the backbone and part of the side chain resonances by multidimensional NMR spectroscopy on uniformly (2)H, (13)C, (15)N enriched protein. The secondary structure analysis derived from the chemical shifts is in line with the expectations from X-ray structures of related proteins.

N type rapid inactivation in human Kv1.4 channels: functional role of a putative C-terminal helix

Voltage gated potassium channels are tetrameric membrane proteins, which have a central role in cellular excitability. Human Kv1.4 channels open on membrane depolarization and inactivate rapidly by a 'ball and chain' mechanism whose molecular determinants have been mapped to the cytoplasmic N terminus of the channel. Here we show that the other terminal end of the channel also plays a role in channel inactivation. Swapping the C-terminal residues of hKv1.4 with those from two non-inactivating channels (hKv1.1 and hKv1.2) affects the rates of inactivation, as well as the recovery of the channel from the inactivated state. Secondary structure predictions of the hKv1.4 sequence reveal a helical structure at its distal C-terminal. Complete removal or partial disruption of this helical region results in channels with remarkably slowed inactivation kinetics. The ionic selectivity and voltage-dependence of channel opening were similar to hKv1.4, indicative of an unperturbed channel pore. These results demonstrate that fast inactivation is modulated by structural elements in the C-terminus, suggesting that the process involves the concerted action of the N- and C-termini.

Regulation of antiarrhythmic drug propafenone effects on the c-type Kv1.4 potassium channel by PHo and K+

The effects of the antiarrhythmic drug propafenone at c-type kv1.4 channels in Xenopus laevis oocytes were studied with the two-electrode voltage-clamp technique. Defolliculated oocytes (stage V-VI) were injected with transcribed cRNAs of ferret Kv1.4 Delta N channels. During recording, oocytes were continuously perfused with control solution or propafenone. Propafenone decreased the currents during voltage steps. The block was voltage-, use-, and concentration- dependent manners. The block was increased with positive going potentials. The voltage dependence of block could be fitted with the sum of monoexponential and a linear function. Propafenone accelerated the inactivate of current during the voltage step. The concentration of half-maximal block (IC(50)) was 121 microM/L. With high, normal, and low extracellular potassium concentrations, the changes of IC(50) value had no significant statistical differences. The block of propafenone was PH- dependent in high-, normal- and low- extracellular potassium concentrations. Acidification of the extracellular solution to PH 6.0 increased the IC(50) values to 463 microM/L, alkalization to PH 8.0 reduced it to 58 microM/L. The results suggest that propafenone blocks the Kv1.4 Delta N channel in the open state and give some hints for an intracellular site of action.

Kv1 potassium channel C-terminus constant HRETE region: arginine substitution affects surface protein level and conductance level of subfamily members differentially

We have shown previously that truncating all of the variable cytoplasmic C-terminus of Kv1.1 potassium channels to G421stop had only a small inhibitory effect on their cell surface conductance density levels and cell surface protein levels. Here we investigated the role of a highly conserved cytoplasmic C-terminal charged region of five amino acids (HRETE) of the S6 transmembrane domain in the protein and conductance expression of Kv1.1, Kv1.2, and Kv1.4 channels. For Kv1.1 we found that E420stop, T419stop, and E418stop showed cell surface conductance densities and cell surface protein levels similar to full length control, whereas R417stop and H416stop exhibited essentially no conductance but their surface protein levels were similar to full length control. A bulky non-negatively charged hydrophilic amino acid at position 417 appeared to be critical for wild type gating of Kv1.1 because R417K and R417Q rescued conductance levels whereas R417A or R417E did not. The R417A mutation in the full length Kv1.1 also exhibited surface protein levels similar to control but it did not exhibit significant conductance. In contrast, mutation of the equivalent arginine to alanine in full length Kv1.2 and Kv1.4 appeared to have little or no effect on channel conductance but rather decreased cell surface protein levels by inducing partial high ER retention. These findings are consistent with the notion that the arginine amino acid in the HRETE region plays a different role in affecting conductance levels or cell surface protein levels of very closely related Kv1 potassium channels.

Re-expression of a developmentally restricted potassium channel in autoimmune demyelination: Kv1.4 is implicated in oligodendroglial proliferation

Mechanisms of lesion repair in multiple sclerosis are incompletely understood. To some degree, remyelination can occur, associated with an increase of proliferating oligodendroglial cells. Recently, the expression of potassium channels has been implicated in the control of oligodendrocyte precursor cell proliferation in vitro. We investigated the expression of Kv1.4 potassium channels in myelin oligodendrocyte glycoprotein-induced experimental autoimmune encephalomyelitis, a model of multiple sclerosis. Confocal microscopy revealed expression of Kv1.4 in AN2-positive oligodendrocyte precursor cells and premyelinating oligodendrocytes in vitro but neither in mature oligodendrocytes nor in the spinal cords of healthy adult mice. After induction of myelin oligodendrocyte glycoprotein-induced experimental autoimmune encephalomyelitis, Kv1.4 immunoreactivity was detected in or around lesions already during disease onset with a peak early and a subsequent decrease in the late phase of the disease. Kv1.4 expression was confined to 2',3'-cyclic nucleotide 3'-phosphodiesterase-positive oligodendroglial cells, which were actively proliferating and ensheathed naked axons. After a demyelinating episode, the number of Kv1.4 and 2',3'-cyclic nucleotide 3'-phosphodiesterase double-positive cells was greatly reduced in ciliary neurotrophic factor knockout mice, a model with impaired lesion repair. In summary, the re-expression of an oligodendroglial potassium channel may have a functional implication on oligodendroglial cell cycle progression, thus influencing tissue repair in experimental autoimmune encephalomyelitis and multiple sclerosis.

Temporomandibular joint inflammation decreases the voltage-gated K+ channel subtype 1.4-immunoreactivity of trigeminal ganglion neurons in rats

Voltage-gated K+ (Kv) channels are one of the important physiological regulators of the membrane potentials in excitable cells, including sensory ganglion neurons. The aim of the present study was to investigate whether temporomandibular joint (TMJ) inflammation alters expression of Kv channel subtype 1.4 (Kv1.4) of trigeminal ganglion (TRG) neurons innervating TMJ relating allodynia (pain caused by normally innoxious stimulation), by using both behavioral and immunohistochemical techniques. TMJ inflammation was induced by injection of Complete Freund's Adjuvant (CFA) into the rat TMJ. The threshold for escape from mechanical stimulation applied to the orofacial area in TMJ inflamed rats was significantly lower than that in naïve rats. TMJ afferents were identified by fluorogold (FG) labeling. The mean numbers of Kv1.4-/neurofilament (NF) 200(myelinated fiber marker) positive- and negative-immunoreactivities FG-labeled small-/medium-diameter TRG neurons in inflamed rats were significantly decreased when compared with those in the naïve rats. These findings suggest that TMJ inflammation reduces the expression of Kv1.4 subunits in the small-/medium sized (Adelta-/C-) TRG neurons and this may contribute to trigeminal inflammatory allodynia in TMJ disorder. These results lead us to suggest that Kv channel openers may be a potential therapeutic agents for prevention of mechanical allodynia.

Kv1.4 subunit expression is decreased in neurons of painful human pulp

Kv1.4, a subunit of voltage-gated K(+) channels, plays a large role in regulating neuronal excitability. The level of Kv1.4 expression is unknown in human sensory neurons innervating healthy or painful tissue. Therefore, we examined Kv1.4 immunoreactivity in axons innervating both clinically diagnosed asymptomatic and painful symptomatic human tooth pulp. Antibodies directed against Kv1.4 and PGP9.5, a protein marker for axons, was used to determine the proportion of PGP9.5 immunopositive tissue that was also immunopositive for Kv1.4. We report that on pulpal axons innervating symptomatic teeth Kv1.4 immunoreactivity, a correlate of decreased Kv1.4 expression, is significantly decreased (p < 0.0001), suggestive of a factor responsible for facilitating chronic dental pain and decreases in currents produced, such as I(A), in neurons innervating painful pulp.

Diabetes mellitus attenuates the repolarization reserve in mammalian heart

OBJECTIVE

In diabetes mellitus several cardiac electrophysiological parameters are known to be affected. In rodent experimental diabetes models changes in these parameters were reported, but no such data are available in other mammalian species including the dog. The present study was designed to analyse the effects of experimental type 1 diabetes on ventricular repolarization and its underlying transmembrane ionic currents and channel proteins in canine hearts.

METHODS AND RESULTS

Diabetes was induced by a single injection of alloxan, a subgroup of dogs received insulin substitution. After the development of diabetes (8 weeks) electrophysiological studies were performed using conventional microelectrodes, whole cell voltage clamp, and ECG. Expression of ion channel proteins was evaluated by Western blotting. The QTc interval and the ventricular action potential duration in diabetic dogs were moderately prolonged. This was accompanied by significant reduction in the density of the transient outward K+ current (I(to)) and the slow delayed rectifier K+ current (I(Ks)), to 54.6% and 69.3% of control, respectively. No differences were observed in the density of the inward rectifier K+ current (I(K1)), rapid delayed rectifier K+ current (I(Kr)), and L-type Ca2+ current (I(Ca)). Western blot analysis revealed a reduced expression of Kv4.3 and MinK (to 25+/-21% and 48+/-15% of control, respectively) in diabetic dogs, while other channel proteins were unchanged (HERG, MiRP1, alpha(1c)) or increased (Kv1.4, KChIP2, KvLQT1). Insulin substitution fully prevented the diabetes-induced changes in I(Ks), KvLQT1 and MinK, however, the changes in I(to), Kv4.3, and Kv1.4 were only partially diminished by insulin.

CONCLUSION

It is concluded that type 1 diabetes mellitus, although only moderately, lengthens ventricular repolarization, attenuates the repolarization reserve by decreasing I(to) and I(Ks) currents, and thereby may markedly enhance the risk of sudden cardiac death.

DPP10 is an inactivation modulatory protein of Kv4.3 and Kv1.4

Voltage-gated K+ channels exist in vivo as multiprotein complexes made up of pore-forming and ancillary subunits. To further our understanding of the role of a dipeptidyl peptidase-related ancillary subunit, DPP10, we expressed it with Kv4.3 and Kv1.4, two channels responsible for fast-inactivating K+ currents. Previously, DPP10 has been shown to effect Kv4 channels. However, Kv1.4, when expressed with DPP10, showed many of the same effects as Kv4.3, such as faster time to peak current and negative shifts in the half-inactivation potential of steady-state activation and inactivation. The exception was recovery from inactivation, which is slowed by DPP10. DPP10 expressed with Kv4.3 caused negative shifts in both steady-state activation and inactivation of Kv4.3, but no significant shifts were detected when DPP10 was expressed with Kv4.3 + KChIP2b (Kv channel interacting protein). DPP10 and KChIP2b had different effects on closed-state inactivation. At −60 mV, KChIP2b nearly abolishes closed-state inactivation in Kv4.3, whereas it developed to a much greater extent in the presence of DPP10. Finally, expression of a DPP10 mutant consisting of its transmembrane and cytoplasmic 58 amino acids resulted in effects on Kv4.3 gating that were nearly identical to those of wild-type DPP10. These data show that DPP10 and KChIP2b both modulate Kv4.3 inactivation but that their primary effects are on different inactivation states. Thus DPP10 may be a general modulator of voltage-gated K+ channel inactivation; understanding its mechanism of action may lead to deeper understanding of the inactivation of a broad range of K+ channels.

Clustering assay for studying the interaction of membrane proteins with PDZ domain proteins

Some membrane proteins must be clustered at target sites to efficiently perform their functions. PDZ domain-containing scaffold proteins bind to the tails of target membrane proteins and promote their localization and clustering on the cell surface. This chapter describes the experimental details of the clustering assay, using the interaction between potassium channels and PSD-95, an abundant PDZ domain protein in neuronal synapses, as a model.

Immunolocalization of the Ca2+-activated K+ channel Slo1 in axons and nerve terminals of mammalian brain and cultured neurons

Ca(2+)-activated voltage-dependent K(+) channels (Slo1, KCa1.1, Maxi-K, or BK channel) play a crucial role in controlling neuronal signaling by coupling channel activity to both membrane depolarization and intracellular Ca(2+) signaling. In mammalian brain, immunolabeling experiments have shown staining for Slo1 channels predominantly localized to axons and presynaptic terminals of neurons. We have developed anti-Slo1 mouse monoclonal antibodies that have been extensively characterized for specificity of staining against recombinant Slo1 in heterologous cells, and native Slo1 in mammalian brain, and definitively by the lack of detectable immunoreactivity against brain samples from Slo1 knockout mice. Here we provide precise immunolocalization of Slo1 in rat brain with one of these monoclonal antibodies and show that Slo1 is accumulated in axons and synaptic terminal zones associated with glutamatergic synapses in hippocampus and GABAergic synapses in cerebellum. By using cultured hippocampal pyramidal neurons as a model system, we show that heterologously expressed Slo1 is initially targeted to the axonal surface membrane, and with further development in culture, become localized in presynaptic terminals. These studies provide new insights into the polarized localization of Slo1 channels in mammalian central neurons and provide further evidence for a key role in regulating neurotransmitter release in glutamatergic and GABAergic terminals.

Pituitary adenylate cyclase activating polypeptide reduces expression of Kv1.4 and Kv4.2 subunits underlying A-type K(+) current in adult mouse olfactory neuroepithelia

A-type K(+) currents (I(A)) in olfactory receptor neurons have been characterized electrophysiologically but the molecular identities of the underlying channel subunits have not been determined. Using RT-PCR, immunoblot and immunohistochemistry, we found that the two candidate channel families underlying I(A), shaker and shal, are expressed in olfactory epithelia of Swiss Webster mice. Specifically, Kv1.4, the only I(A) candidate from the shaker family, and Kv4.2 and Kv4.3 from the shal family were expressed, but Kv4.1 mRNA was not amplified from the olfactory epithelia. Immunoblot and immunohistochemical studies confirmed the existence of Kv1.4 and Kv4.2/3 subunits. Furthermore, quantitative RT-PCR showed that pituitary adenylate cyclase activating polypeptide (PACAP) reduced the expression of Kv1.4 and Kv4.2 but did not reduce the already low expression of Kv4.3. The PACAP-induced reduction of Kv4.1 and Kv4.2 expression was completely blocked by inhibiting the phospholipase C (PLC) pathway but was still significantly downregulated by PACAP when the cyclic AMP pathway was inhibited. In addition, downstream of the PLC pathway, calcium mediated the reduction of both Kv1.4 and Kv4.2 expression and I(A) current density. Phosphokinase C (PKC) activation did not affect Kv1.4 and Kv4.2 mRNA expression, even though PKC reduced I(A) current density. Together with our previous studies, our data suggest that A-type K(+) currents in olfactory receptor neurons are composed of multiple K(+) channel subunits, among which Kv1.4 and Kv4.2 are subject to transcriptional modulation by PACAP. We also found that PACAP predominately uses a PLC-calcium pathway to modulate Kv4.1 and Kv4.2 expression. Modulation of A-type K(+) current expression may contribute to the previously observed neuroprotective effects of PACAP on olfactory receptor neurons.

Expression and biophysical properties of Kv1 channels in supragranular neocortical pyramidal neurones

Potassium channels are extremely diverse regulators of neuronal excitability. As part of an investigation into how this molecular diversity is utilized by neurones, we examined the expression and biophysical properties of native Kv1 channels in layer II/III pyramidal neurones from somatosensory and motor cortex. Single-cell RT-PCR, immunocytochemistry, and whole cell recordings with specific peptide toxins revealed that individual pyramidal cells express multiple Kv1 alpha-subunits. The most abundant subunit mRNAs were Kv1.1 > 1.2 > 1.4 > 1.3. All of these subunits were localized to somatodendritic as well as axonal cell compartments. These data suggest variability in the subunit complexion of Kv1 channels in these cells. The alpha-dendrotoxin (alpha-DTX)-sensitive current activated more rapidly and at more negative potentials than the alpha-DTX-insensitive current, was first observed at voltages near action potential threshold, and was relatively insensitive to holding potential. The alpha-DTX-sensitive current comprised about 10% of outward current at steady-state, in response to steps from -70 mV. From -50 mV, this percentage increased to approximately 20%. All cells expressed an alpha-DTX-sensitive current with slow inactivation kinetics. In some cells a transient component was also present. Deactivation kinetics were voltage dependent, such that deactivation was slow at potentials traversed by interspike intervals during repetitive firing. Because of its kinetics and voltage dependence, the alpha-DTX-sensitive current should be most important at physiological resting potentials and in response to brief stimuli. Kv1 channels should also be important at voltages near threshold and corresponding to interspike intervals.

Targeting of voltage-gated potassium channel isoforms to distinct cell surface microdomains

Voltage-gated potassium (Kv) channels regulate action potential duration in nerve and muscle; therefore changes in the number and location of surface channels can profoundly influence electrical excitability. To investigate trafficking of Kv2.1, 1.4 and 1.3 within the plasma membrane, we combined the expression of fluorescent protein-tagged Kv channels with live cell confocal imaging. Kv2.1 exhibited a clustered distribution in HEK cells similar to that seen in hippocampal neurons, whereas Kv1.4 and Kv1.3 were evenly distributed over the plasma membrane. Using FRAP, surface Kv2.1 displayed limited mobility; approximately 40% of the fluorescence recovered within 20 minutes of photobleach (M(f)=0.41+/-0.04). Recovery occurred not by diffusion from adjacent membrane but probably by transport of nascent channel from within the cell. By contrast, the Kv1 family members Kv1.4 and Kv1.3 were highly mobile, both showing approximately 80% recovery (Kv 1.4 M(f)=0.78+/-0.07; Kv1.3 M(f)=0.78+/-0.04; without correction for photobleach); unlike Kv2.1, recovery was consistent with diffusion of channel from membrane adjacent to the bleach region. Studies using PA-GFP-tagged channels were consistent with the FRAP results. Following photoactivation of a small region of plasma membrane PA-GFP-Kv2.1 remained restricted to the photoactivation ROI, while PA-GFP-Kv1.4 rapidly diffused throughout the cell surface. Additionally, PA-GFP-Kv2.1 moved into regions of the cell membrane not adjacent to the original photoactivation ROI. Sucrose density gradient analysis indicated that half of Kv2.1 is part of a large, macromolecular complex while Kv1.4 sediments as predicted for the tetrameric channel complex. Disruption of membrane cholesterol by cyclodextrin minimally altered Kv2.1 mobility (M(f)=0.32+/-0.03), but significantly increased surface cluster size by at least fourfold. By comparison, the mobility of Kv1.4 decreased following cholesterol depletion with no change in surface distribution. The mobility of Kv1.3 was slightly increased following cyclodextrin treatment. These results indicate that (1) Kv2.1, Kv1.4 and Kv1.3 exist in distinct compartments that exhibit different trafficking properties, (2) membrane cholesterol levels differentially modulate the trafficking and localization of Kv channels and (3) Kv2.1 expressed in HEK cells exhibits a surface distribution similar to that seen in native cells.

Influence of outer pore residue K533 on the inhibition of Kv1.4 potassium channels by n-alkyl sulphate anions

We have previously shown that although n-octyl sulphate (OS-) and n-dodecyl sulphate (DDS-) anions had similar effects on the kinetics and activation voltage dependence of RCK1 (Kv1.1), RCK4 (Kv1.4) and Shaker B channels expressed in Xenopus oocytes, both compounds produced a large decrease in the maximum conductance of RCK4 channels while significantly increasing the conductance of RCK1 and Shaker B. We suggested that this channel-specific inhibition might depend on the nature of the amino-acid residue corresponding to position 533 in RCK4. We now present data on the effects of n-alkyl sulphates on an RCK4 mutant in which the wild-type lysine at position 533 was changed to the corresponding tyrosine residue in RCK1. At a concentration of 15 microM, DDS- caused a 48% reduction in the wild-type current at 50 mV but a 32% increase in the mutant current. n-Hexyl sulphate and OS- had similar differential effects. The activation and inactivation kinetics of the mutant current were still accelerated by n-alkyl sulphates and 15 microM DDS- moved the conductance/voltage curves of both wild-type and mutant channels some 24 mV in the hyperpolarizing direction. The K533Y mutation thus had a selective effect on current inhibition by n-alkyl sulphates.