A590T mutation in KCNQ1 C-terminal helix D decreases IKs channel trafficking and function but not Yotiao interaction

KCNQ1 encodes the α subunit of the voltage-gated channel that mediates the cardiac slow delayed rectifier K(+) current (IKs). Here, we report a KCNQ1 allele encoding an A590T mutation [KCNQ1(A590T)] found in a 39-year-old female with a mild QT prolongation. A590 is located in the C-terminal α helical region of KCNQ1 that mediates subunit tetramerization, membrane trafficking, and interaction with Yotiao. This interaction is known to be required for the proper modulation of IKs by cAMP. Since previous studies reported that mutations in the vicinity of A590 impair IKs channel surface expression and function, we examined whether and how the A590T mutation affects the IKs channel. Electrophysiological measurements in HEK-293T cells showed that the A590T mutation caused a reduction in IKs density and a right-shift of the current-voltage relation of channel activation. Immunocytochemical and immunoblot analyses showed the reduced cell surface expression of KCNQ1(A590T) subunit and its rescue by coexpression of the wild-type KCNQ1 [KCNQ1(WT)] subunit. Moreover, KCNQ1(A590T) subunit interacted with Yotiao and had a cAMP-responsiveness comparable to that of KCNQ1(WT) subunit. These findings indicate that the A590 of KCNQ1 subunit plays important roles in the maintenance of channel surface expression and function via a novel mechanism independent of interaction with Yotiao.

Glycine/Serine polymorphism at position 38 influences KCNE1 subunit's modulatory actions on rapid and slow delayed rectifier K+ currents

BACKGROUND

 KCNE1 encodes a modulator of KCNH2 and KCNQ1 delayed rectifier K(+) current channels. KCNE1 mutations might cause long QT syndrome (LQTS) by impairing KCNE1 subunit's modulatory actions on these channels. There are major and minor polymorphismic KCNE1 variants whose 38(th) amino acids are glycine and serine [KCNE1(38G) and KCNE1(38S) subunits], respectively. Despite its frequent occurrence, the influence of this polymorphism on the K(+) channels' function is unclear.

METHODS AND RESULTS

 Patch-clamp recordings were obtained from human embryonic kidney -293T cells. KCNH2 channel current density in KCNE1(38S)-transfected cells was smaller than that in KCNE1(38G)-transfected cells by 34%. The voltage-sensitivity of the KCNQ1 channel current in KCNE1(38S)-transfected cells was lowered compared to that in KCNE1(38G)-transfected cells, with a +13mV shift in the half-maximal activation voltage. KCNH2 channel current density or KCNQ1 channel voltage-sensitivity was not different between KCNE1(38G)-transfected cells and cells transfected with both KCNE1(38G) and KCNE1(38S). Moreover, the KCNH2 channel current in KCNE1(38S)-transfected cells was more susceptible to E4031, a QT prolonging drug and a condition with hypokalemia, than that in KCNE1(38G)-transfected cells.

CONCLUSIONS

 Homozygous inheritance of KCNE1(38S) might cause a mild reduction of the delayed rectifier K(+) currents and might thereby increase an arrhythmogenic potential particularly in the presence of QT prolonging factors. By contrast, heterozygous inheritance of KCNE1(38G) and KCNE1(38S) might not affect the K(+) currents significantly.  (Circ J 2014; 78: 610-618).

[Activation of cerebellar B-type γ-aminobutyric acid receptor modulates optokinetic reflex adaptation]

The cerebellar cortex, the brain region responsible for motor coordination and learning expresses a high density of B-type γ-aminobutyric acid receptor (GABAbR). Previous in vitro and in situ studies indicated that cerebellar GABAbR may mediate multiple forms of inhibitory and excitatory modulation of cerebellar circuits. Nevertheless, the in vivo influence of cerebellar GABAbR activation is unclear. As the first step in addressing this issue, we examined how pharmacological activation of cerebellar GABAbR modulates optokinetic reflex (OKR), an involuntary cerebellum-dependent eye movement for stabilizing the retinal image against the drift of the visual scene. We injected baclofen, a GABAbR-selective agonist, or control saline into the cerebellar flocculi of adult mice and then performed 1-h OKR measurement sessions on two consecutive days. In the day 1 session, the baclofen (5 nM)-injected mice and control mice showed similar initial OKR gains and similar training-induced increases in the OKR gain (OKR adaptation). This result suggests that GABAbR activation does not affect cerebellar computation for executing OKR and formation of short-term memory for OKR adaptation. At the beginning of the day 2 session, the baclofen (5 nM or 50 μM)-injected mice showed an OKR gain higher than that achieved in the day 1 session while the control mice did not. This result suggests that GABAbR activation may facilitate the formation of OKR adaptation-related long-term memory. These findings provide a new insight into the functional architecture of the cerebellar circuits and indicate GABAbR to be a new target of pharmacological therapy against diseases with cerebellar dysfunction.

Characterization of a novel mutant KCNQ1 channel subunit lacking a large part of the C-terminal domain

A mutation of KCNQ1 gene encoding the alpha subunit of the channel mediating the slow delayed rectifier K(+) current in cardiomyocytes may cause severe arrhythmic disorders. We identified KCNQ1(Y461X), a novel mutant gene encoding KCNQ1 subunit whose C-terminal domain is truncated at tyrosine 461 from a man with a mild QT interval prolongation. We made whole-cell voltage-clamp recordings from HEK-293T cells transfected with either of wild-type KCNQ1 [KCNQ1(WT)], KCNQ1(Y461X), or their mixture plus KCNE1 auxiliary subunit gene. The KCNQ1(Y461X)-transfected cells showed no delayed rectifying current. The cells transfected with both KCNQ1(WT) and KCNQ1(Y461X) showed the delayed rectifying current that is thought to be mediated largely by homomeric channel consisting of KCNQ1(WT) subunit because its voltage-dependence of activation, activation rate, and deactivation rate were similar to the current in the KCNQ1(WT)-transfected cells. The immunoblots of HEK-293T cell-derived lysates showed that KCNQ1(Y461X) subunit cannot form channel tetramers by itself or with KCNQ1(WT) subunit. Moreover, immunocytochemical analysis in HEK-293T cells showed that the surface expression level of KCNQ1(Y461X) subunit was very low with or without KCNQ1(WT) subunit. These findings suggest that the massive loss of the C-terminal domain of KCNQ1 subunit impairs the assembly, trafficking, and function of the mutant subunit-containing channels, whereas the mutant subunit does not interfere with the functional expression of the homomeric wild-type channel. Therefore, the homozygous but not heterozygous inheritance of KCNQ1(Y461X) might cause major arrhythmic disorders. This study provides a new insight into the structure-function relation of KCNQ1 channel and treatments of cardiac channelopathies.

A simple machine vision-driven system for measuring optokinetic reflex in small animals

The optokinetic reflex (OKR) is useful to monitor the function of the visual and motor nervous systems. However, OKR measurement is not open to all because dedicated commercial equipment or detailed instructions for building in-house equipment is rarely offered. Here we describe the design of an easy-to-install/use yet reliable OKR measuring system including a computer program to visually locate the pupil and a mathematical procedure to estimate the pupil azimuth from the location data. The pupil locating program was created on a low-cost machine vision development platform, whose graphical user interface allows one to compose and operate the program without programming expertise. Our system located mouse pupils at a high success rate (~90 %), estimated their azimuth precisely (~94 %), and detected changes in OKR gain due to the pharmacological modulation of the cerebellar flocculi. The system would promote behavioral assessment in physiology, pharmacology, and genetics.

Abstract 049: Functional Characterization of KCNQ1 Channel Subunit With an A590T Mutation

Background: KCNQ1 encodes the alpha subunit of the voltage-gated K + channel that mediates the cardiac slow delayed rectifier K + current (I Ks ). A mutation, A590T, in KCNQ1 was incidentally identified in a 40 years old female. She had a mild QTc prolongation in electrocardiogram but has never experienced any cardiac events. A590 is located in the C-terminal domain forming a coiled-coil structure, which has been suggested as a pivotal component for subunit tetramerization and channel trafficking to the cell surface. The previously reported mutations around A590 result in markedly reduced cell surface expression and loss of functional channel. We, for the first time, examined whether and how the A590T mutation affects the I Ks channel function.

Methods: To assess the trafficking and channel function of KCNQ1(A590T) mutant subunit, we performed immunostaining, immunoblotting, and voltage-clamp measurements in HEK-293T cells transfected with wild-type or the A590T mutant KCNQ1 or their mixture (WT, A590T, and A590T/WT cells, respectively).

Results: The density of a depolarization-activated current in the A590T cells was smaller than that in the WT cells. The threshold, half-maximal activation, and saturating voltages of the depolarization-activated current in the A590T cells were more positive than those in the WT cells. The immunoreactivity against KCNQ1 subunit on the cell surface in the A590T cells is lower than in WT cells. The A590T/WT cells had a similar density of the depolarization-activated current and a similar level of immunoreactivity against the channel subunit to the WT cells. Furthermore, the immunoblotting detected subunit oligomers in the membrane fraction of the A590T cells while their densities were lower than those of the WT cells.

Conclusion: Although the A590T mutant subunit can form oligomers for itself, this subunit is not efficiently trafficked to the cell surface without the aid of the WT subunit. Thus, homozygous inheritance of the mutant KCNQ1 might be pathogenic. By contrast, the cells expressing both the mutant and wild-type KCNQ1 subunit had normal I Ks and cell surface expression, indicating that the heterozygous inheritance of the mutant KCNQ1 might not cause severe cardiac diseases.

A novel missense mutation causing a G487R substitution in the S2-S3 loop of human ether-à-go-go-related gene channel

INTRODUCTION

Mutations of human ether-à-go-go-related gene (hERG), which encodes a cardiac K(+) channel responsible for the acceleration of the repolarizing phase of an action potential and the prevention of premature action potential regeneration, often cause severe arrhythmic disorders. We found a novel missense mutation of hERG that results in a G487R substitution in the S2-S3 loop of the channel subunit [hERG(G487R)] from a family and determined whether this mutant gene could induce an abnormality in channel function.

METHODS AND RESULTS

We made whole-cell voltage-clamp recordings from HEK-293T cells transfected with wild-type hERG [hERG(WT)], hERG(G487R), or both. We measured hERG channel-mediated current as the "tail" of a depolarization-elicited current. The current density of the tail current and its voltage- and time-dependences were not different among all the cell groups. The time-courses of deactivation, inactivation, and recovery from inactivation and their voltage-dependences were not different among all the cell groups. Furthermore, we performed immunocytochemical analysis using an anti-hERG subunit antibody. The ratio of the immunoreactivity of the plasma membrane to that of the cytoplasm was not different between cells transfected with hERG(WT), hERG(G487R), or both.

CONCLUSION

 hERG(G487R) can produce functional channels with normal gating kinetics and cell-surface expression efficiency with or without the aid of hERG(WT). Therefore, neither the heterozygous nor homozygous inheritance of hERG(G487R) is thought to cause severe cardiac disorders. hERG(G487R) would be a candidate for a rare variant or polymorphism of hERG with an amino acid substitution in the unusual region of the channel subunit.

Use of co-expression of HGF and c-Met to predict peritoneal dissemination established by autocrine HGF/c-Met signaling in gastric cancer

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Background: Epithelial mesencymal transition (EMT) promotes facilitates migration and invasion of epithelial tumour cells. EMT is induced by growth factors implicated in theses process such as hepatocyte growth factor (HGF). Our aim of this study is whether HGF/c-Met pathway is associated with metastasis of gastric cancer (GC), especially in peritoneal dissemination (PD).

Methods: HGF and c-Met expression and EMT related molecules were evaluated using real-time PCR and immunohistochemistry in GC tissues. The role of HGF/c-Met pathway for EMT and anoikis was determined and c-Met TKI (SU11274) was tested for their ability to block HGF-induced biological effects in vitro and vivo.

Results: In HGF(-)c-Met(+) GC cells,recombinant HGF promoted EMT phenotype characterized by morphology, impaired E-cadherin and induction of Vimentin. HGF promoted cell growth, invasiveness, migration ability and inhibition of anoikis. SU11274 blocked HGF-induced EMT and the biological effects in vitro. In contrast of HGF(+)c-Met(+) GC cells, HGF exposure was not affected biological outcome of EMT and anoikis but SU11274 blocked biological effect as same as in HGF(-)c-Met(+) GC cells. In vivo, HGF(+)c-Met(+) GC cell line only established PD and SU11274 intraperitoneally caused an inhibition of PD growth. Clinically, HGF expression was significantly positive correlated with c-Met expression in GC specimens. Increased HGF and c-Met demonstrated a significantly associated with poor prognosis and can predict PD, respectively. Furthermore, HGF was one of the independent factors for predicting PD. Immunohistochemical analysis showed HGF and c-Met were predominantly co-expressed in cancer cell of both primary GC and PD.

Conclusions: We have demonstrated that HGF/c-Met pathway as an inducer of EMT and anoikis inhibition in GC cell. Co-expression of HGF and c-Met implicates its potential to promote PD in GC. Blocking the autocrine HGF/c-Met pathway may be clinically useful for the treatment of PD in GC.

No significant financial relationships to disclose.

A new multiple-drug applicator with minimal drug cross-talk, leakage, and consumption

The relative effects of multiple drugs give an important clue to dissect a neuronal mechanism and to seek for a candidate neurotherapeutical agent. Here we have devised a "flute" applicator which can deliver several drugs to a neural cell preparation. The applicator stands by, cleaning itself with bath perfusate and delivers drugs only during test applications. This minimizes drug cross-talk in and leakage from the applicator and drug consumption. Using the applicator, we successfully compared the relative effects of widely different doses of an agonist in single neurons. The flute applicator would be a useful tool for pharmacological analyses.