Functional significance of K+ channel β-subunit KCNE3 in auditory neurons

The potassium channels: Neurobiology and pharmacology of tinnitus

Tinnitus is a widespread public health issue that imposes a significant social burden. The occurrence and maintenance of tinnitus have been shown to be associated with abnormal neuronal activity in the auditory pathway. Based on this view, neurobiological and pharmacological developments in tinnitus focus on ion channels and synaptic neurotransmitter receptors in neurons in the auditory pathway. With major breakthroughs in the pathophysiology and research methodology of tinnitus in recent years, the role of the largest family of ion channels, potassium ion channels, in modulating the excitability of neurons involved in tinnitus has been increasingly demonstrated. More and more potassium channels involved in the neural mechanism of tinnitus have been discovered, and corresponding drugs have been developed. In this article, we review animal (mouse, rat, hamster, and guinea-pig), human, and genetic studies on the different potassium channels involved in tinnitus, analyze the limitations of current clinical research on potassium channels, and propose future prospects. The aim of this review is to promote the understanding of the role of potassium ion channels in tinnitus and to advance the development of drugs targeting potassium ion channels for tinnitus.

Kv7 channel inhibition increases hypoxic pulmonary vasoconstriction in endotoxemic mouse lungs

PURPOSE

Hypoxic pulmonary vasoconstriction (HPV) regulates regional pulmonary blood flow in order to match regional ventilation to preserve arterial oxygenation. HPV is impaired in patients with sepsis or acute respiratory distress syndrome (ARDS). Endotoxemic mice show reduced HPV and recent evidence suggests a central role of voltage gated potassium channel 7 (Kv7) in regulating HPV. Therefore, we tested the hypothesis if Kv7 is induced and inhibition of Kv7 increases HPV in endotoxemia.

MATERIALS AND METHODS

Isolated lungs of LPS-pretreated and untreated animals were perfused with and without specific inhibitors of Kv7 (linopirdine (LI) 0, 0.1, 1 and 10 µM) or Kv7.1 (HMR1556 100 nM). Pulmonary artery pressure (PAP) during normoxic (FiO₂ 0.21) as well as hypoxic (FiO₂ 0.01) ventilation were obtained. Expressions of Kv7 composing (KCNQ1-5) as well as auxiliary subunits (KCNE1-5) were measured in mouse lungs with and without endotoxemia.

RESULTS

HPV was impaired in lungs from LPS mice (16 ± 7% vs 105 ± 13% control, p < 0.05). Perfusion of control lungs with 10 µM LI or 100 nM HMR1556 did not affect HPV (LI 105 ± 12% vs 105 ± 13% vehicle, HMR1556 100 ± 6% vs 98 ± 26%, P = NS). In LPS mice perfusion with 10 µM LI (74.2 ± 7% vs. 16 ± 7% vehicle, P < 0.05) or HMR1556 100 nM augmented HPV (74 ± 28% vs. 15 ± 17% vehicle, P < 0.05). KCNQ1, 4 and 5 gene- and protein expressions as well as KCNE1, 2 and 4 gene expressions were unaltered in endotoxemic lungs. KCNE3 gene and protein expressions were increased in lungs of LPS treated mice (3.1 ± 1.3-fold and 1.8 ± 0.3-fold, respectively, P < 0.05 for both).

CONCLUSIONS

Endotoxemia does not alter KCNQ1, 4 and 5 gene and protein expressions but increases pulmonary KCNE3 gene and protein expression. In isolated perfused endotoxemic mouse lungs, perfusion with 10 µM LI or 100 nM HMR1556 augments HPV.

The local translation of KNa in dendritic projections of auditory neurons and the roles of KNa in the transition from hidden to overt hearing loss

Local and privileged expression of dendritic proteins allows segregation of distinct functions in a single neuron but may represent one of the underlying mechanisms for early and insidious presentation of sensory neuropathy. Tangible characteristics of early hearing loss (HL) are defined in correlation with nascent hidden hearing loss (HHL) in humans and animal models. Despite the plethora of causes of HL, only two prevailing mechanisms for HHL have been identified, and in both cases, common structural deficits are implicated in inner hair cell synapses, and demyelination of the auditory nerve (AN). We uncovered that Na⁺-activated K⁺ (K_Na) mRNA and channel proteins are distinctly and locally expressed in dendritic projections of primary ANs and genetic deletion of K_Na channels (Kcnt1 and Kcnt2) results in the loss of proper AN synaptic function, characterized as HHL, without structural synaptic alterations. We further demonstrate that the local functional synaptic alterations transition from HHL to increased hearing-threshold, which entails changes in global Ca²⁺ homeostasis, activation of caspases 3/9, impaired regulation of inositol triphosphate receptor 1 (IP₃R1), and apoptosis-mediated neurodegeneration. Thus, the present study demonstrates how local synaptic dysfunction results in an apparent latent pathological phenotype (HHL) and, if undetected, can lead to overt HL. It also highlights, for the first time, that HHL can precede structural synaptic dysfunction and AN demyelination. The stepwise cellular mechanisms from HHL to canonical HL are revealed, providing a platform for intervention to prevent lasting and irreversible age-related hearing loss (ARHL).

Early functional alterations in membrane properties and neuronal degeneration are hallmarks of progressive hearing loss in NOD mice

Presbycusis or age-related hearing loss (ARHL) is the most common sensory deficit in the human population. A substantial component of the etiology stems from pathological changes in sensory and non-sensory cells in the cochlea. Using a non-obese diabetic (NOD) mouse model, we have characterized changes in both hair cells and spiral ganglion neurons that may be relevant for early signs of age-related hearing loss (ARHL). We demonstrate that hair cell loss is preceded by, or in parallel with altered primary auditory neuron functions, and latent neurite retraction at the hair cell-auditory neuron synapse. The results were observed first in afferent inner hair cell synapse of type I neurites, followed by type II neuronal cell-body degeneration. Reduced membrane excitability and loss of postsynaptic densities were some of the inaugural events before any outward manifestation of hair bundle disarray and hair cell loss. We have identified profound alterations in type I neuronal membrane properties, including a reduction in membrane input resistance, prolonged action potential latency, and a decrease in membrane excitability. The resting membrane potential of aging type I neurons in the NOD, ARHL model, was significantly hyperpolarized, and analyses of the underlying membrane conductance showed a significant increase in K⁺ currents. We propose that attempts to alleviate some forms of ARHL should include early targeted primary latent neural degeneration for effective positive outcomes.

Sodium-activated potassium channels shape peripheral auditory function and activity of the primary auditory neurons in mice

Potassium (K⁺) channels shape the response properties of neurons. Although enormous progress has been made to characterize K⁺ channels in the primary auditory neurons, the molecular identities of many of these channels and their contributions to hearing in vivo remain unknown. Using a combination of RNA sequencing and single molecule fluorescent in situ hybridization, we localized expression of transcripts encoding the sodium-activated potassium channels K_Na1.1 (SLO2.2/Slack) and K_Na1.2 (SLO2.1/Slick) to the primary auditory neurons (spiral ganglion neurons, SGNs). To examine the contribution of these channels to function of the SGNs in vivo, we measured auditory brainstem responses in K_Na1.1/1.2 double knockout (DKO) mice. Although auditory brainstem response (wave I) thresholds were not altered, the amplitudes of suprathreshold responses were reduced in DKO mice. This reduction in amplitude occurred despite normal numbers and molecular architecture of the SGNs and their synapses with the inner hair cells. Patch clamp electrophysiology of SGNs isolated from DKO mice displayed altered membrane properties, including reduced action potential thresholds and amplitudes. These findings show that K_Na1 channel activity is essential for normal cochlear function and suggest that early forms of hearing loss may result from physiological changes in the activity of the primary auditory neurons.

Semaphorin 3A induces acute changes in membrane excitability in spiral ganglion neurons in vitro

The development and survival of spiral ganglion neurons (SGNs) are dependent on multiple trophic factors as well as membrane electrical activity. Semaphorins (Sema) constitute a family of membrane-associated and secreted proteins that have garnered significant attention as a potential SGN "navigator" during cochlea development. Previous studies using mutant mice demonstrated that Sema3A plays a role in the SGN pathfinding. The mechanisms, however, by which Sema3A shapes SGNs firing behavior are not known. In these studies, we found that Sema3A plays a novel role in regulating SGN resting membrane potential and excitability. Using dissociated SGN from pre-hearing (P3-P5) and post-hearing mice (P12-P15), we recorded membrane potentials using whole-cell patch clamp recording techniques in apical and basal SGN populations. Recombinant Sema3A was applied to examine the effects on intrinsic membrane properties and action potentials evoked by current injections. Apical and basal SGNs from newborn mice treated with recombinant Sema3A (100 ng/ml) displayed a higher resting membrane potential, higher threshold, decreased amplitude, and prolonged latency and duration of spikes. Although a similar phenomenon was observed in SGNs from post-hearing mice, the resting membrane potential was essentially indistinguishable before and after Sema3A exposure. Sema3A-mediated changes in membrane excitability were associated with a significant decrease in K⁺ and Ca²⁺ currents. Sema3A acts through linopirdine-sensitive K⁺ channels in apical, but not in the basal SGNs. Therefore, Sema3A induces differential effects in SGN membrane excitability that are dependent on age and location, and constitutes an additional early and novel effect of Sema3A SGNs in vitro.

Modulation of thalamocortical oscillations by TRIP8b, an auxiliary subunit for HCN channels

Hyperpolarization-activated cyclic nucleotide-gated cation (HCN) channels have important functions in controlling neuronal excitability and generating rhythmic oscillatory activity. The role of tetratricopeptide repeat-containing Rab8b-interacting protein (TRIP8b) in regulation of hyperpolarization-activated inward current, I h, in the thalamocortical system and its functional relevance for the physiological thalamocortical oscillations were investigated. A significant decrease in I h current density, in both thalamocortical relay (TC) and cortical pyramidal neurons was found in TRIP8b-deficient mice (TRIP8b-/-). In addition basal cAMP levels in the brain were found to be decreased while the availability of the fast transient A-type K+ current, I A, in TC neurons was increased. These changes were associated with alterations in intrinsic properties and firing patterns of TC neurons, as well as intrathalamic and thalamocortical network oscillations, revealing a significant increase in slow oscillations in the delta frequency range (0.5-4 Hz) during episodes of active-wakefulness. In addition, absence of TRIP8b suppresses the normal desynchronization response of the EEG during the switch from slow-wave sleep to wakefulness. It is concluded that TRIP8b is necessary for the modulation of physiological thalamocortical oscillations due to its direct effect on HCN channel expression in thalamus and cortex and that mechanisms related to reduced cAMP signaling may contribute to the present findings.

Regulation of human cardiac potassium channels by full-length KCNE3 and KCNE4

Voltage-gated potassium (Kv) channels comprise pore-forming α subunits and a multiplicity of regulatory proteins, including the cardiac-expressed and cardiac arrhythmia-linked transmembrane KCNE subunits. After recently uncovering novel, N-terminally extended (L) KCNE3 and KCNE4 isoforms and detecting their transcripts in human atrium, reported here are their functional effects on human cardiac Kv channel α subunits expressed in Xenopus laevis oocytes. As previously reported for short isoforms KCNE3S and KCNE4S, KCNE3L inhibited hERG; KCNE4L inhibited Kv1.1; neither form regulated the HCN1 pacemaker channel. Unlike KCNE4S, KCNE4L was a potent inhibitor of Kv4.2 and Kv4.3; co-expression of cytosolic β subunit KChIP2, which regulates Kv4 channels in cardiac myocytes, partially relieved Kv4.3 but not Kv4.2 inhibition. Inhibition of Kv4.2 and Kv4.3 by KCNE3L was weaker, and its inhibition of Kv4.2 abolished by KChIP2. KCNE3L and KCNE4L also exhibited subunit-specific effects on Kv4 channel complex inactivation kinetics, voltage dependence and recovery. Further supporting the potential physiological significance of the robust functional effects of KCNE4L on Kv4 channels, KCNE4L protein was detected in human atrium, where it co-localized with Kv4.3. The findings establish functional effects of novel human cardiac-expressed KCNE isoforms and further contribute to our understanding of the potential mechanisms influencing cardiomyocyte repolarization.

BmP02 Atypically Delays Kv4.2 Inactivation: Implication for a Unique Interaction between Scorpion Toxin and Potassium Channel

BmP02, a short-chain peptide with 28 residues from the venom of Chinese scorpion Buthus martensi Karsch, has been reported to inhibit the transient outward potassium currents (I_to) in rat ventricular muscle cells. However, it remains unclear whether BmP02 modulates the Kv4.2 channel, one of the main contributors to I_to. The present study investigated the effects of BmP02 on Kv4.2 kinetics and its underlying molecular mechanism. The electrophysiological recordings showed that the inactivation of Kv4.2 expressed in HEK293T cells was significantly delayed by BmP02 in a dose-response manner with EC₅₀ of ~850 nM while the peak current, activation and voltage-dependent inactivation of Kv4.2 were not affected. Meanwhile, the recovery from inactivation of Kv4.2 was accelerated and the deactivation was slowed after the application of BmP02. The site-directed mutagenesis combined with computational modelling identified that K347 and K353, located in the turret motif of the Kv4.2, and E4/E5, D20/D21 in BmP02 are key residues to interact with BmP02 through electrostatic force. These findings not only reveal a novel interaction between Kv4.2 channel and its peptidyl modulator, but also provide valuable information for design of highly-selective Kv4.2 modulators.

The Cardiac TBX5 Interactome Reveals a Chromatin Remodeling Network Essential for Cardiac Septation

Human mutations in the cardiac transcription factor gene TBX5 cause congenital heart disease (CHD), although the underlying mechanism is unknown. We report characterization of the endogenous TBX5 cardiac interactome and demonstrate that TBX5, long considered a transcriptional activator, interacts biochemically and genetically with the nucleosome remodeling and deacetylase (NuRD) repressor complex. Incompatible gene programs are repressed by TBX5 in the developing heart. CHD mis-sense mutations that disrupt the TBX5-NuRD interaction cause depression of a subset of repressed genes. Furthermore, the TBX5-NuRD interaction is required for heart development. Phylogenetic analysis showed that the TBX5-NuRD interaction domain evolved during early diversification of vertebrates, simultaneous with the evolution of cardiac septation. Collectively, this work defines a TBX5-NuRD interaction essential to cardiac development and the evolution of the mammalian heart, and when altered may contribute to human CHD.

Novel exon 1 protein-coding regions N-terminally extend human KCNE3 and KCNE4

The 5 human (h)KCNE β subunits each regulate various cation channels and are linked to inherited cardiac arrhythmias. Reported here are previously undiscovered protein-coding regions in exon 1 of hKCNE3 and hKCNE4 that extend their encoded extracellular domains by 44 and 51 residues, which yields full-length proteins of 147 and 221 residues, respectively. Full-length hKCNE3 and hKCNE4 transcript and protein are expressed in multiple human tissues; for hKCNE4, only the longer protein isoform is detectable. Two-electrode voltage-clamp electrophysiology revealed that, when coexpressed in Xenopus laevis oocytes with various potassium channels, the newly discovered segment preserved conversion of KCNQ1 by hKCNE3 to a constitutively open channel, but prevented its inhibition of Kv4.2 and KCNQ4. hKCNE4 slowing of Kv4.2 inactivation and positive-shifted steady-state inactivation were also preserved in the longer form. In contrast, full-length hKCNE4 inhibition of KCNQ1 was limited to 40% at +40 mV vs. 80% inhibition by the shorter form, and augmentation of KCNQ4 activity by hKCNE4 was entirely abolished by the additional segment. Among the genome databases analyzed, the longer KCNE3 is confined to primates; full-length KCNE4 is widespread in vertebrates but is notably absent from Mus musculus Findings highlight unexpected KCNE gene diversity, raise the possibility of dynamic regulation of KCNE partner modulation via splice variation, and suggest that the longer hKCNE3 and hKCNE4 proteins should be adopted in future mechanistic and genetic screening studies.-Abbott, G. W. Novel exon 1 protein-coding regions N-terminally extend human KCNE3 and KCNE4.

KCNE3 acts by promoting voltage sensor activation in KCNQ1

KCNE β-subunits assemble with and modulate the properties of voltage-gated K(+) channels. In the colon, stomach, and kidney, KCNE3 coassembles with the α-subunit KCNQ1 to form K(+) channels important for K(+) and Cl(-) secretion that appear to be voltage-independent. How KCNE3 subunits turn voltage-gated KCNQ1 channels into apparent voltage-independent KCNQ1/KCNE3 channels is not completely understood. Different mechanisms have been proposed to explain the effect of KCNE3 on KCNQ1 channels. Here, we use voltage clamp fluorometry to determine how KCNE3 affects the voltage sensor S4 and the gate of KCNQ1. We find that S4 moves in KCNQ1/KCNE3 channels, and that inward S4 movement closes the channel gate. However, KCNE3 shifts the voltage dependence of S4 movement to extreme hyperpolarized potentials, such that in the physiological voltage range, the channel is constitutively conducting. By separating S4 movement and gate opening, either by a mutation or PIP2 depletion, we show that KCNE3 directly affects the S4 movement in KCNQ1. Two negatively charged residues of KCNE3 (D54 and D55) are found essential for the effect of KCNE3 on KCNQ1 channels, mainly exerting their effects by an electrostatic interaction with R228 in S4. Our results suggest that KCNE3 primarily affects the voltage-sensing domain and only indirectly affects the gate.

KCNE1 and KCNE3: The yin and yang of voltage-gated K(+) channel regulation

The human KCNE gene family comprises five genes encoding single transmembrane-spanning ion channel regulatory subunits. The primary function of KCNE subunits appears to be regulation of voltage-gated potassium (Kv) channels, and the best-understood KCNE complexes are with the KCNQ1 Kv α subunit. Here, we review the often opposite effects of KCNE1 and KCNE3 on Kv channel biology, with an emphasis on regulation of KCNQ1. Slow-activating IKs channel complexes formed by KCNQ1 and KCNE1 are essential for human ventricular myocyte repolarization, while constitutively active KCNQ1-KCNE3 channels are important in the intestine. Inherited sequence variants in human KCNE1 and KCNE3 cause cardiac arrhythmias but by different mechanisms, and each is important for hearing in unique ways. Because of their contrasting effects on KCNQ1 function, KCNE1 and KCNE3 have proved invaluable tools in the mechanistic understanding of how channel gating can be manipulated, and each may also provide a window into novel insights and new therapeutic opportunities in K(+) channel pharmacology. Finally, findings from studies of Kcne1(-/-) and Kcne3(-/-) mouse lines serve to illustrate the complexity of KCNE biology and KCNE-linked disease states.

The genetics of Ménière's disease

Our understanding of the genetic basis of Ménière’s disease (MD) is still limited. Although the familial clustering and the geographical and racial differences in incidence strongly suggest a certain role for genetic factors in the development of MD, no convincing evidence for an association with any gene exists, at present. In this review, starting from rational bases for a genetic approach to MD, we explored the numerous reports published in literature and summarize the recent advances in understanding of the genetic fundaments of the disease.