Erg K+ channels modulate contractile activity in the bovine epididymal duct

ERG K+ channels mediate a major component of action potential repolarization in lymphatic muscle

Smooth muscle cells in the walls of collecting lymphatic vessels fire spontaneous action potentials (APs), which conduct rapidly over the muscle layer to initiate contractions that propel lymph. Several ion channels have been implicated in the currents underlying the AP spike and the preceding diastolic depolarization, but the molecular identities of K+ channels involved in AP repolarization are unknown. Based on previous studies of other rhythmically active smooth muscles, we hypothesized that ether-a-go-go related gene (ERG) K+ channels (Kv11) play an important role in repolarization of the AP in lymphatic muscle. Message for one or more ERG channel isoforms was detected by RT-PCR analysis of lymphatic vessels from mice, rats and humans. Membrane potential recordings in smooth muscle cells of rat and human lymphatics revealed that nanomolar concentrations of ERG-1 inhibitors (E-4031 and BeKm-1) prolonged the duration of the AP plateau (normally ~ 1 s in duration) and induced multiple spikes, whereas ERG-1 activators (ICA-105574 and RPR-260243) shortened the plateau and could completely inhibit spontaneous APs. At relatively high inhibitor concentrations, the AP plateau duration lasted as long as 24 s. ERG activators reversed the effects of ERG inhibitors and vice-versa. In pressure myograph studies, ERG channel inhibition prolonged the diastolic repolarization phase of the contraction cycle and reduced the frequency of spontaneous contractions. This is the first evidence for a specific K+ channel contributing to the AP in lymphatic muscle. Our results imply that lymphatic contractile dysfunction may occur in long QT type II patients with mutations that result in ERG channel loss-of-function or impaired trafficking of the channel to the cell membrane.

Key role for Kv11.1 (ether-a-go-go related gene) channels in rat bladder contractility

In addition, to their established role in cardiac myocytes and neurons, ion channels encoded by ether-a-go-go-related genes (ERG1-3 or kcnh2,3 and 6) (kcnh2) are functionally relevant in phasic smooth muscle. The aim of the study was to determine the expression and functional impact of ERG expression products in rat urinary bladder smooth muscle using quantitative polymerase chain reaction, immunocytochemistry, whole-cell patch-clamp and isometric tension recording. kcnh2 was expressed in rat bladder, whereas kcnh6 and kcnh3 expression were negligible. Immunofluorescence for the kcnh2 expression product Kv11.1 was detected in the membrane of isolated smooth muscle cells. Potassium currents with voltage-dependent characteristics consistent with Kv11.1 channels and sensitive to the specific blocker E4031 (1 μM) were recorded from isolated detrusor smooth muscles. Disabling Kv11.1 activity with specific blockers (E4031 and dofetilide, 0.2-20 μM) augmented spontaneous contractions to a greater extent than BKCa channel blockers, enhanced carbachol-driven activity, increased nerve stimulation-mediated contractions, and impaired β-adrenoceptor-mediated inhibitory responses. These data establish for the first time that Kv11.1 channels are key determinants of contractility in rat detrusor smooth muscle.

Prognostic role of hERG1 Potassium Channels in Neuroendocrine Tumours of the Ileum and Pancreas

hERG1 potassium channels are widely expressed in human cancers of different origins, where they affect several key aspects of cellular behaviour. The present study was designed to evaluate the expression and clinical relevance of hERG1 protein in cancer tissues from patients suffering from neuroendocrine tumours (NETs) of ileal (iNETs) and pancreatic (pNETs) origin, with available clinicopathological history and follow-up. The study was carried out by immunohistochemistry with an anti-hERG1 monoclonal antibody. In a subset of samples, a different antibody directed against the hERG1/β1 integrin complex was also used. The analysis showed for the first time that hERG1 is expressed in human NETs originating from either the ileum or the pancreas. hERG1 turned out to have a prognostic value in NETs, showing (i) a statistically significant positive impact on OS of patients affected by ileal NETs, regardless the TNM stage; (ii) a statistically significant positive impact on OS of patients affected by aggressive (TNM stage IV) disease, either ileal or pancreatic; (iii) a trend to a negative impact on OS of patients affected by less aggressive (TNM stage I-III) disease, either ileal or pancreatic. Moreover, in order to evaluate whether ERG1 was functionally expressed in a cellular model of pNET, the INS1E rat insulinoma cell line was used, and it emerged that blocking ERG1 with a specific inhibitor of the channel (E4031) turned out in a significant reduction in cell proliferation.

Dynamics and physiological meaning of complexes between ion channels and integrin receptors: the case of Kv11.1

The cellular functions are regulated by a complex interplay of diffuse and local signals. Experimental work in cell physiology has led to recognize that understanding a cell's dynamics requires a deep comprehension of local fluctuations of cytosolic regulators. Macromolecular complexes are major determinants of local signaling. Multi-enzyme assemblies limit the diffusion restriction to reaction kinetics by direct exchange of metabolites. Likewise, close coupling of ion channels and transporters modulate the ion concentration around a channel mouth or transporter binding site. Extreme signal locality is brought about by conformational coupling between membrane proteins, as is typical of mechanotransduction. A paradigmatic case is integrin-mediated cell adhesion. Sensing the extracellular microenvironment and providing an appropriate response is essential in growth and development and has innumerable pathological implications. The process involves bidirectional signal transduction by complex supra-molecular structures that link integrin receptors to ion channels and transporters, growth factor receptors, cytoskeletal elements and other regulatory elements. The dynamics of such complexes is only beginning to be understood. A thoroughly studied example is the association between integrin receptors and the voltage-gated K+ channels Kv11.1. These channels are widely expressed in early embryos, where their physiological roles are poorly understood and apparently different from the shaping of action potential firing in the adult. Hints about these roles come from studies in cancer cells, where Kv11.1 is often overexpressed and appears to re-assume functions, such as controlling cell proliferation/differentiation, apoptosis and migration. Kv11.1 is implicated in these processes through its linking to integrin subunits.

Increased Smooth Muscle Kv11.1 Channel Expression in Pulmonary Hypertension and Protective Role of Kv11.1 Channel Blocker Dofetilide

Kv11.1 potassium channels are essential for heart repolarization. Prescription medication that blocks Kv11.1 channels lengthens the ventricular action potential and causes cardiac arrhythmias. Surprisingly little is known about the Kv11.1 channel expression and function in the lung tissue. Here we report that Kv11.1 channels were abundantly expressed in the large pulmonary arteries (PAs) of healthy lung tissues from humans and rats. Kv11.1 channel expression was increased in the lungs of humans affected by chronic obstructive pulmonary disease-associated pulmonary hypertension and in the lungs of rats with pulmonary arterial hypertension (PAH). In healthy lung tissues from humans and rats, Kv11.1 channels were confined to the large PAs. In humans with chronic obstructive pulmonary disease-associated pulmonary hypertension and in rats with PAH, Kv11.1 channels were expressed in both the large and small PAs. The increase in Kv11.1 channel expression closely followed the time-course of the development of pulmonary vascular remodeling in PAH rats. Treatment of PAH rats with dofetilide, an Kv11.1 channel blocker approved by the US Food and Drug Administration for use in the treatment of arrythmia, inhibited PAH-associated pulmonary vascular remodeling. Taken together, the findings from this study uncovered a novel role of Kv11.1 channels in lung function and their potential as new drug targets in the treatment of pulmonary hypertension. The protective effect of dofetilide raises the possibility of repurposing this antiarrhythmic drug for the treatment of patients with pulmonary hypertension.

Proliferative Role of Kv11 Channels in Murine Arteries

K⁺ channels encoded by the ether-a-go-go related gene (ERG1 or KCNH2) are important determinants of the cardiac action potential. Expression of both cardiac isoforms (ERG1a and ERG1b) were identified in murine portal vein and distinctive voltage-gated K⁺ currents were recorded from single myocytes. The aim of the present study was to ascertain the expression and functional impact of ERG channels in murine arteries.

Methods: Quantitative RT-PCR was undertaken on RNA extracted from a number of murine arteries. Immunofluorescence was performed on single vascular smooth muscle cells using antibodies against the ERG1 expression product (Kv11.1). Single cell electrophysiology was performed on myocytes from portal vein and several different arteries, complimented by isometric tension recordings. Proliferation assays were undertaken on smooth muscle cells isolated from femoral arteries.

Results: ERG1 transcripts were detected in all murine blood vessels, and Kv11.1 immunofluorescence was observed in all smooth muscle cells. However, K⁺ currents with properties consistent with ERG channels were only recorded in portal vein myocytes. Moreover, ERG channel blockers (E4031 or dofetilide, 1 μM) failed to depolarize carotid arteries or produce contraction. Proliferation of arterial smooth muscle cells was associated with a marked increase in ERG1 expression and ERG blockers suppressed proliferation significantly.

Conclusions: These data reveal that arterial blood vessels express ERG channels that appear to be functional silent in contractile smooth muscle but contribute to proliferative response.

Time-lapse imaging as a tool to investigate contractility of the epididymal duct--effects of cGMP signaling

The well orchestrated function of epididymal smooth muscle cells ensures transit of spermatozoa through the epididymal duct during which spermatozoa acquire motility and fertilizing capacity. Relaxation of smooth muscle cells is mediated by cGMP signaling and components of this pathway are found within the male reproductive tract. Whereas contractile function of caudal parts of the rat epididymal duct can be examined in organ bath studies, caput and corpus regions are fragile and make it difficult to mount them in an organ bath. We developed an ex vivo time-lapse imaging-based approach to investigate the contractile pattern in these parts of the epididymal duct. Collagen-embedding allowed immobilization without impeding contractility or diffusion of drugs towards the duct and therefore facilitated subsequent movie analyses. The contractile pattern was made visible by placing virtual sections through the acquired image stack to track wall movements over time. By this, simultaneous evaluation of contractile activity at different positions of the observed duct segment was possible. With each contraction translating into a spike, drug-induced alterations in contraction frequency could be assessed easily. Peristaltic contractions were also detectable and throughout all regions in the proximal epididymis we found regular spontaneous contractile activity that elicited movement of intraluminal contents. Stimulating cGMP production by natriuretic peptide ANP or inhibiting degradation of cGMP by the phosphodiesterase 5 inhibitor sildenafil significantly reduced contractile frequency in isolated duct segments from caput and corpus. RT-PCR analysis after laser-capture microdissection localized the corresponding molecules to the smooth muscle layer of the duct. Our time-lapse imaging approach proved to be feasible to assess contractile function in all regions of the epididymal duct under near physiological conditions and provides a tool to evaluate acute (side) effects of drugs and to investigate various signaling pathways.

Kv7 and Kv11 channels in myometrial regulation

Ion channels play a key role in defining myometrial contractility. Modulation of ion channel populations is proposed to underpin gestational changes in uterine contractility associated with the transition from uterine quiescence to active labour. Of the myriad ion channels present in the uterus, this article will focus upon potassium channels encoded by the KCNQ genes and ether-à-go-go-related (ERG) genes. Voltage-gated potassium channels encoded by KCNQ and ERG (termed Kv7 and Kv11, respectively) are accepted as major determinants of neuronal excitability and the duration of the cardiac action potential. However, there is now growing appreciation that these ion channels have a major functional impact in vascular and non-vascular smooth muscle. Moreover, Kv7 channels may be potential therapeutic targets for the treatment of preterm labour.

hERG channel function: beyond long QT

To date, research on the human ether-a-go-go related gene (hERG) has focused on this potassium channel's role in cardiac repolarization and Long QT Syndrome (LQTS). However, growing evidence implicates hERG in a diversity of physiologic and pathological processes. Here we discuss these other functions of hERG, particularly their impact on diseases beyond cardiac arrhythmia.

K+ and Cl- channels and transporters in sperm function

To succeed in fertilization, spermatozoa must decode environmental cues which require a set of ion channels. Recent findings have revealed that K(+) and Cl(-) channels participate in some of the main sperm functions. This work reviews the evidence indicating the involvement of K(+) and Cl(-) channels in motility, maturation, and the acrosome reaction, and the advancement in identifying their molecular identity and modes of regulation. Improving our insight on how these channels operate will strengthen our ability to surmount some infertility problems, improve animal breeding, preserve biodiversity, and develop selective and secure male contraceptives.

Effects of the small molecule HERG activator NS1643 on Kv11.3 channels

NS1643 is one of the small molecule HERG (Kv11.1) channel activators and has also been found to increase erg2 (Kv11.2) currents. We now investigated whether NS1643 is also able to act as an activator of Kv11.3 (erg3) channels expressed in CHO cells. Activation of rat Kv11.3 current occurred in a dose-dependent manner and maximal current increasing effects were obtained with 10 µM NS1643. At this concentration, steady-state outward current increased by about 80% and the current increase was associated with a significant shift in the voltage dependence of activation to more negative potentials by about 15 mV. In addition, activation kinetics were accelerated, whereas deactivation was slowed. There was no significant effect on the kinetics of inactivation and recovery from inactivation. The strong current-activating agonistic effect of NS1643 did not result from a shift in the voltage dependence of Kv11.3 channel inactivation and was independent from external Na⁺ or Ca²⁺. At the higher concentration of 20 µM, NS1643 induced clearly less current increase. The left shift in the voltage dependence of activation reversed and the voltage sensitivity of activation dramatically decreased along with a slowing of Kv11.3 channel activation. These data show that, in comparison to other Kv11 family members, NS1643 exerts distinct effects on Kv11.3 channels with especially pronounced partial antagonistic effects at higher concentration.

hERG K+ Channels: Structure, Function, and Clinical Significance

The human ether-a-go-go related gene (hERG) encodes the pore-forming subunit of the rapid component of the delayed rectifier K+ channel, Kv11.1, which are expressed in the heart, various brain regions, smooth muscle cells, endocrine cells, and a wide range of tumor cell lines. However, it is the role that Kv11.1 channels play in the heart that has been best characterized, for two main reasons. First, it is the gene product involved in chromosome 7-associated long QT syndrome (LQTS), an inherited disorder associated with a markedly increased risk of ventricular arrhythmias and sudden cardiac death. Second, blockade of Kv11.1, by a wide range of prescription medications, causes drug-induced QT prolongation with an increase in risk of sudden cardiac arrest. In the first part of this review, the properties of Kv11.1 channels, including biogenesis, trafficking, gating, and pharmacology are discussed, while the second part focuses on the pathophysiology of Kv11.1 channels.

Histochemistry and cell biology: the annual review 2010

This review summarizes recent advances in histochemistry and cell biology which complement and extend our knowledge regarding various aspects of protein functions, cell and tissue biology, employing appropriate in vivo model systems in conjunction with established and novel approaches. In this context several non-expected results and discoveries were obtained which paved the way of research into new directions. Once the reader embarks on reading this review, it quickly becomes quite obvious that the studies contribute not only to a better understanding of fundamental biological processes but also provide use-oriented aspects that can be derived therefrom.

Gonadotropin-releasing hormone inhibits ether-à-go-go-related gene K+ currents in mouse gonadotropes

Secretion of LH from gonadotropes is initiated by a GnRH-induced increase in intracellular Ca(2+) concentration ([Ca(2+)](i)). This increase in [Ca(2+)](i) is the result of Ca(2+) release from intracellular stores and Ca(2+) influx through voltage-dependent Ca(2+) channels. Here we describe an ether-à-go-go-related gene (erg) K(+) current in primary mouse gonadotropes and its possible function in the control of Ca(2+) influx. To detect gonadotropes, we used a knock-in mouse strain, in which GnRH receptor-expressing cells are fluorescently labeled. Erg K(+) currents were recorded in 80-90% of gonadotropes. Blockage of erg currents by E-4031 depolarized the resting potential by 5-8 mV and led to an increase in [Ca(2+)](i), which was abolished by nifedipine. GnRH inhibited erg currents by a reduction of the maximal erg current and in some cells additionally by a shift of the activation curve to more positive potentials. In conclusion, the erg current contributes to the maintenance of the resting potential in gonadotropes, thereby securing a low [Ca(2+)](i) by restricting Ca(2+) influx. In addition, the erg channels are modulated by GnRH by an as-yet unknown signal cascade.

Loss of functional K+ channels encoded by ether-à-go-go-related genes in mouse myometrium prior to labour onset

There is a growing appreciation that ion channels encoded by the ether-à-go-go-related gene family have a functional impact in smooth muscle in addition to their accepted role in cardiac myocytes and neurones. This study aimed to assess the expression of ERG1-3 (KCNH1-3) genes in the murine myometrium (smooth muscle layer of the uterus) and determine the functional impact of the ion channels encoded by these genes in pregnant and non-pregnant animals. Quantitative RT-PCR did not detect message for ERG2 and 3 in whole myometrial tissue extracts. In contrast, message for two isoforms of mERG1 were readily detected with mERG1a more abundant than mERG1b. In isometric tension studies of non-pregnant myometrium, the ERG channel blockers dofetilide (1 microM), E4031 (1 microM) and Be-KM1 (100 nM) increased spontaneous contractility and ERG activators (PD118057 and NS1643) inhibited spontaneous contractility. In contrast, neither ERG blockade nor activation had any effect on the inherent contractility in myometrium from late pregnant (19 days gestation) animals. Moreover, dofetilide-sensitive K(+) currents with distinctive 'hooked' kinetics were considerably smaller in uterine myocytes from late pregnant compared to non-pregnant animals. Expression of mERG1 isoforms did not alter throughout gestation or upon delivery, but the expression of genes encoding auxillary subunits (KCNE) were up-regulated considerably. This study provides the first evidence for a regulation of ERG-encoded K(+) channels as a precursor to late pregnancy physiological activity.

Potent activation of large-conductance Ca2+-activated K+ channels by the diphenylurea 1,3-bis-[2-hydroxy-5-(trifluoromethyl)phenyl]urea (NS1643) in pituitary tumor (GH3) cells

1,3-Bis-[2-hydroxy-5-(trifluoromethyl)phenyl]urea (NS1643) is reported to be an activator of human ether-à-go-go-related gene current. However, it remains unknown whether it has any effects on other types of ion channels. The effects of NS1643 on ion currents and membrane potential were investigated in this study. NS1643 stimulated Ca(2+)-activated K(+) current [I(K(Ca))] in a concentration-dependent manner with an EC(50) value of 1.8 microM in pituitary tumor (GH(3)) cells. In inside-out recordings, this compound applied to the intracellular side of the detached channels stimulated large-conductance Ca(2+)-activated K(+) (BK(Ca)) channels with no change in single-channel conductance. It shifted the activation curve of BK(Ca) channels to less depolarized voltages without altering the gating charge of the channels. NS1643-stimulated channel activity depended on intracellular Ca(2+), and mean closed time during exposure to NS1643 was reduced. NS1643 (3 microM) had little or no effect on peak amplitude of ether-à-go-go-related gene-mediated K(+) current evoked by membrane hyperpolarization, although it increased the amplitude of late-sustained components of K(+) inward current, which was suppressed by paxilline but not by azimilide. NS1643 (3 microM) had no effect on L-type Ca(2+) current. This compound reduced repetitive firing of action potentials, and further application of paxilline attenuated its decrease in firing rate. In addition, NS1643 enhanced BK(Ca)-channel activity in human embryonic kidney 293T cells expressing alpha-hSlo. In summary, we clearly show that NS1643 interacts directly with the BK(Ca) channel to increase the amplitude of I(K(Ca)) in pituitary tumor (GH(3)) cells. The alpha-subunit of the channel may be a target for the action of this small compound.