Vasopressin stimulates action potential firing by protein kinase C-dependent inhibition of KCNQ5 in A7r5 rat aortic smooth muscle cells

Retigabine and gabapentin restore channel function and neuronal firing in a cellular model of an epilepsy-associated dominant-negative KCNQ5 variant

KCNQ5 encodes the voltage-gated potassium channel KV7.5, a member of the KV7 channel family, which conducts the M-current. This current is a potent regulator of neuronal excitability by regulating membrane potential in the subthreshold range of action potentials and mediating the medium and slow afterhyperpolarization. Recently, we have identified five loss-of-function variants in KCNQ5 in patients with genetic generalized epilepsy. Using the most severe dominant-negative variant (R359C), we set out to investigate pharmacological therapeutic intervention by KV7 channel openers on channel function and neuronal firing. Retigabine and gabapentin increased R359C-derived M-current amplitudes in HEK cells expressing homomeric or heteromeric mutant KV7.5 channels. Retigabine was most effective in restoring K+ currents. Ten μM retigabine was sufficient to reach the level of WT currents without retigabine, whereas 100 μM of gabapentin showed less than half of this effect and application of 50 μM ZnCl2 only significantly increased M-current amplitude in heteromeric channels. Overexpression of KV7.5-WT potently inhibited neuronal firing by increasing the M-current, whereas R359C overexpression had the opposite effect and additionally decreased the medium afterhyperpolarization current. Both aforementioned drugs and Zn2+ reversed the effect of R359C expression by reducing firing to nearly normal levels at high current injections. Our study shows that a dominant-negative variant with a complete loss-of-function in KV7.5 leads to largely increased neuronal firing which may explain a neuronal hyperexcitability in patients. KV7 channel openers, such as retigabine or gabapentin, could be treatment options for patients currently displaying pharmacoresistant epilepsy and carrying loss-of-function variants in KCNQ5.

Potential Therapeutic Targets for Hypotension in Duchenne Muscular Dystrophy

Duchenne Muscular Dystrophy (DMD) is marked by genetic mutations occurring in the DMD gene, which is widely expressed in the cardiovascular system. In addition to developing cardiomyopathy, patients with DMD have been reported to be susceptible to the development of symptomatic hypotension, although the mechanisms are unclear. Analysis of single-cell RNA sequencing data has identified potassium voltage-gated channel subfamily Q member 5 (KCNQ5) and possibly ryanodine receptor 2 (RyR2) as potential candidate hypotension genes whose expression is significantly upregulated in the vascular smooth muscle cells of DMD mutant mice. We hypothesize that heightened KCNQ5 and RyR2 expression contributes to decreased arterial blood pressure in patients with DMD. Exploring pharmacological approaches to inhibit the KCNQ5 and RyR2 channels holds promise in managing the systemic hypotension observed in individuals with DMD. This avenue of investigation presents new prospects for improving clinical outcomes for these patients.

Marked oestrous cycle-dependent regulation of rat arterial KV 7.4 channels driven by GPER1

BACKGROUND AND PURPOSE

Kcnq-encoded KV 7 channels (termed KV 7.1-5) regulate vascular smooth muscle cell (VSMC) contractility at rest and as targets of receptor-mediated responses. However, the current data are mostly derived from males. Considering the known effects of sex, the oestrous cycle and sex hormones on vascular reactivity, here we have characterised the molecular and functional properties of KV 7 channels from renal and mesenteric arteries from female Wistar rats separated into di-oestrus and met-oestrus (F-D/M) and pro-oestrus and oestrus (F-P/E).

EXPERIMENTAL APPROACH

RT-qPCR, immunocytochemistry, proximity ligation assay and wire myography were performed in renal and mesenteric arteries. Circulating sex hormone concentrations were determined by liquid chromatography-tandem mass spectrometry. Whole-cell electrophysiology was undertaken on cells expressing KV 7.4 channels in association with G-protein-coupled oestrogen receptor 1 (GPER1).

KEY RESULTS

The KV 7.2-5 activators S-1 and ML213 and the pan-KV 7 inhibitor linopirdine were more effective in arteries from F-D/M compared with F-P/E animals. In VSMCs isolated from F-P/E rats, exploratory evidence indicates reduced membrane abundance of KV 7.4 but not KV 7.1, KV 7.5 and Kcne4 when compared with cells from F-D/M. Plasma oestradiol was higher in F-P/E compared with F-D/M, and progesterone showed the converse pattern. Oestradiol/GPER1 agonist G-1 diminished KV 7.4 encoded currents and ML213 relaxations and reduced the membrane abundance of KV 7.4 and interaction between KV 7.4 and heat shock protein 90 (HSP90), in arteries from F-D/M but not F-P/E.

CONCLUSIONS AND IMPLICATIONS

GPER1 signalling decreased KV 7.4 membrane abundance in conjunction with diminished interaction with HSP90, giving rise to a 'pro-contractile state'.

Loss-of-function variants in the KCNQ5 gene are implicated in genetic generalized epilepsies

Background

De novo missense variants in KCNQ5, encoding the voltage-gated K⁺ channel K_V7.5, have been described to cause developmental and epileptic encephalopathy (DEE) or intellectual disability (ID). We set out to identify disease-related KCNQ5 variants in genetic generalized epilepsy (GGE) and their underlying mechanisms.

Methods

1292 families with GGE were studied by next-generation sequencing. Whole-cell patch-clamp recordings, biotinylation and phospholipid overlay assays were performed in mammalian cells combined with homology modelling.

Findings

We identified three deleterious heterozygous missense variants, one truncation and one splice site alteration in five independent families with GGE with predominant absence seizures; two variants were also associated with mild to moderate ID. All missense variants displayed a strongly decreased current density indicating a loss-of-function (LOF). When mutant channels were co-expressed with wild-type (WT) K_V7.5 or K_V7.5 and K_V7.3 channels, three variants also revealed a significant dominant-negative effect on WT channels. Other gating parameters were unchanged. Biotinylation assays indicated a normal surface expression of the variants. The R359C variant altered PI(4,5)P₂-interaction.

Interpretation

Our study identified deleterious KCNQ5 variants in GGE, partially combined with mild to moderate ID. The disease mechanism is a LOF partially with dominant-negative effects through functional deficits. LOF of K_V7.5 channels will reduce the M-current, likely resulting in increased excitability of K_V7.5-expressing neurons. Further studies on network level are necessary to understand which circuits are affected and how this induces generalized seizures.

Funding

DFG/FNR Research Unit FOR-2715 (Germany/Luxemburg), BMBF rare disease network Treat-ION (Germany), foundation ‘no epilep’ (Germany).

KCNQ5 activation by tannins mediates vasorelaxant effects of barks used in Native American botanical medicine

Tree and shrub barks have been used as folk medicine by numerous cultures across the globe for millennia, for a variety of indications, including as vasorelaxants and antispasmodics. Here, using electrophysiology and myography, we discovered that the KCNQ5 voltage-gated potassium channel mediates vascular smooth muscle relaxant effects of barks used in Native American folk medicine. Bark extracts (1%) from Birch, Cramp Bark, Slippery Elm, White Oak, Red Willow, White Willow, and Wild Cherry each strongly activated KCNQ5 expressed in Xenopus oocytes. Testing of a subset including both the most and the least efficacious extracts revealed that Red Willow, White Willow, and White Oak KCNQ-dependently relaxed rat mesenteric arteries; in contrast, Black Haw bark neither activated KCNQ5 nor induced vasorelaxation. Two compounds common to the active barks (gallic acid and tannic acid) had similarly potent and efficacious effects on both KCNQ5 activation and vascular relaxation, and this together with KCNQ5 modulation by other tannins provides a molecular basis for smooth muscle relaxation effects of Native American folk medicine bark extracts.

KCNQ Potassium Channels as Targets of Botanical Folk Medicines

Since prehistory, human species have depended on plants for both food and medicine. Even in countries with ready access to modern medicines, alternative treatments are still highly regarded and commonly used. Unlike modern pharmaceuticals, many botanical medicines are in widespread use despite a lack of safety and efficacy data derived from controlled clinical trials and often unclear mechanisms of action. Contributing to this are the complex and undefined composition and likely multifactorial mechanisms of action and multiple targets of many botanical medicines. Here, we review the newfound importance of the ubiquitous KCNQ subfamily of voltage-gated potassium channels as targets for botanical medicines, including basil, capers, cilantro, lavender, fennel, chamomile, ginger, and Camellia, Sophora, and Mallotus species. We discuss the implications for the traditional use of these plants for disorders such as seizures, hypertension, and diabetes and the molecular mechanisms of plant secondary metabolite effects on KCNQ channels.

Kv7 Channels and Excitability Disorders

Kv7.1-Kv7.5 (KCNQ1-5) K⁺ channels are voltage-gated K⁺ channels with major roles in neurons, muscle cells and epithelia where they underlie physiologically important K⁺ currents, such as neuronal M current and cardiac I_Ks. Specific biophysical properties of Kv7 channels make them particularly well placed to control the activity of excitable cells. Indeed, these channels often work as 'excitability breaks' and are targeted by various hormones and modulators to regulate cellular activity outputs. Genetic deficiencies in all five KCNQ genes result in human excitability disorders, including epilepsy, arrhythmias, deafness and some others. Not surprisingly, this channel family attracts considerable attention as potential drug targets. Here we will review biophysical properties and tissue expression profile of Kv7 channels, discuss recent advances in the understanding of their structure as well as their role in various neurological, cardiovascular and other diseases and pathologies. We will also consider a scope for therapeutic targeting of Kv7 channels for treatment of the above health conditions.

KV7 Channel Expression and Function Within Rat Mesenteric Endothelial Cells

Background and Purpose: Arterial diameter is dictated by the contractile state of the vascular smooth muscle cells (VSMCs), which is modulated by direct and indirect inputs from endothelial cells (ECs). Modulators of KCNQ-encoded k_V7 channels have considerable impact on arterial diameter and these channels are known to be expressed in VSMCs but not yet defined in ECs. However, expression of k_V7 channels in ECs would add an extra level of vascular control. This study aims to characterize the expression and function of K_V7 channels within rat mesenteric artery ECs. Experimental Approach: In rat mesenteric artery, KCNQ transcript and K_V7 channel protein expression were determined via RT-qPCR, immunocytochemistry, immunohistochemistry and immunoelectron microscopy. Wire myography was used to determine vascular reactivity. Key Results: KCNQ transcript was identified in isolated ECs and VSMCs. K_V7.1, K_V7.4 and K_V7.5 protein expression was determined in both isolated EC and VSMC and in whole vessels. Removal of ECs attenuated vasorelaxation to two structurally different K_V7.2-5 activators S-1 and ML213. K_IR2 blockers ML133, and BaCl₂ also attenuated S-1 or ML213-mediated vasorelaxation in an endothelium-dependent process. K_V7 inhibition attenuated receptor-dependent nitric oxide (NO)-mediated vasorelaxation to carbachol, but had no impact on relaxation to the NO donor, SNP. Conclusion and Implications: In rat mesenteric artery ECs, K_V7.4 and K_V7.5 channels are expressed, functionally interact with endothelial K_IR2.x channels and contribute to endogenous eNOS-mediated relaxation. This study identifies K_V7 channels as novel functional channels within rat mesenteric ECs and suggests that these channels are involved in NO release from the endothelium of these vessels.

Vascular Kv7 channels control intracellular Ca2+ dynamics in smooth muscle

Voltage-gated Kv7 (or KCNQ) channels control activity of excitable cells, including vascular smooth muscle cells (VSMCs), by setting their resting membrane potential and controlling other excitability parameters. Excitation-contraction coupling in muscle cells is mediated by Ca2+ but until now, the exact role of Kv7 channels in cytosolic Ca2+ dynamics in VSMCs has not been fully elucidated. We utilised microfluorimetry to investigate the impact of Kv7 channel activity on intracellular Ca2+ levels and electrical activity of rat A7r5 VSMCs and primary human internal mammary artery (IMA) SMCs. Both, direct (XE991) and G protein coupled receptor mediated (vasopressin, AVP) Kv7 channel inhibition induced robust Ca2+ oscillations, which were significantly reduced in the presence of Kv7 channel activator, retigabine, L-type Ca2+ channel inhibitor, nifedipine, or T-type Ca2+ channel inhibitor, NNC 55-0396, in A7r5 cells. Membrane potential measured using FluoVolt exhibited a slow depolarisation followed by a burst of sharp spikes in response to XE991; spikes were temporally correlated with Ca2+ oscillations. Phospholipase C inhibitor (edelfosine) reduced AVP-induced, but not XE991-induced Ca2+ oscillations. AVP and XE991 induced a large increase of [Ca2+]i in human IMA, which was also attenuated with retigabine, nifedipine and NNC 55-0396. RT-PCR, immunohistochemistry and electrophysiology suggested that Kv7.5 was the predominant Kv7 subunit in both rat and human arterial SMCs; CACNA1C (Cav1.2; L-type) and CACNA1 G (Cav3.1; T-type) were the most abundant voltage-gated Ca2+ channel gene transcripts in both types of VSMCs. This study establishes Kv7 channels as key regulators of Ca2+ signalling in VSMCs with Kv7.5 playing a dominant role.

Heteromeric Channels Formed From Alternating Kv7.4 and Kv7.5 α-Subunits Display Biophysical, Regulatory, and Pharmacological Characteristics of Smooth Muscle M-Currents

Smooth muscle cells of the vasculature, viscera, and lungs generally express multiple α-subunits of the Kv7 voltage-gated potassium channel family, with increasing evidence that both Kv7.4 and Kv7.5 can conduct “M-currents” that are functionally important for the regulation of smooth muscle contractility. Although expression systems demonstrate that functional channels can form as homomeric tetramers of either Kv7.4 or Kv7.5 α-subunits, there is evidence that heteromeric channel complexes, containing some combination of Kv7.4 and Kv7.5 α-subunits, may represent the predominant configuration natively expressed in some arterial myocytes, such as rat mesenteric artery smooth muscle cells (MASMCs). Our previous work has suggested that Kv7.4/Kv7.5 heteromers can be distinguished from Kv7.4 or Kv7.5 homomers based on their biophysical, regulatory, and pharmacological characteristics, but it remains to be determined how Kv7.4 and Kv7.5 α-subunits combine to produce these distinct characteristics. In the present study, we constructed concatenated dimers or tetramers of Kv7.4 and Kv7.5 α-subunits and expressed them in a smooth muscle cell line to determine if a particular α-subunit configuration can exhibit the features previously reported for natively expressed Kv7 currents in MASMCs. Several unique characteristics of native smooth muscle M-currents were reproduced under conditions that constrain channel formation to a Kv7.4:Kv7.5 stoichiometry of 2:2, with alternating Kv7.4 and Kv7.5 α-subunits within a tetrameric structure. Although other subunit arrangements/combinations are not ruled out, the findings provide new insights into the oligomerization of α-subunits and the ways in which Kv7.4/Kv7.5 subunit assembly can affect smooth muscle signal transduction and pharmacological responses to Kv7 channel modulating drugs.

Remodeling of Kv7.1 and Kv7.5 Expression in Vascular Tumors

Voltage-dependent potassium (Kv) channels contribute to the excitability of nerves and muscles. In addition, Kv participates in several cell functions, including cell cycle progression and proliferation. Kv channel remodeling has been associated with neoplastic cell growth and cancer. Kv7 channels are expressed in blood vessels, and they participate in the maintenance of vascular tone and are implicated in myocyte proliferation. Although evidence links Kv7 remodeling to different types of cancer, its expression in vascular tumors has never been studied. Endothelium-derived vascular neoplasms range from indolent lesions to highly aggressive and metastasizing cancers. Here, we show that Kv7.1 and Kv7.5 are evenly distributed in tunicas as well as the endothelium of healthy veins and arteries. The layered structure of vessels is lost in vascular tumors. By studying eight vascular tumors with different origins and characteristics, we found that Kv7.1 and Kv7.5 expression was changed in vascular cancers. While both channels were generally downregulated, Kv7.5 expression was clearly correlated with neoplastic malignancy. The vascular tumors did not contract; therefore, the role of Kv7 channels is probably related to proliferation rather than controlling vascular tone. Our results identify vascular Kv7 channels as targets for cancer detection and anticancer therapies.

Structural Determinants of Kv7.5 Potassium Channels That Confer Changes in Phosphatidylinositol 4,5-Bisphosphate (PIP2) Affinity and Signaling Sensitivities in Smooth Muscle Cells

Smooth muscle cells express Kv7.4 and Kv7.5 voltage-dependent potassium channels, which have each been implicated as regulators of smooth muscle contractility, though they display different sensitivities to signaling via cAMP/protein kinase A (PKA) and protein kinase C (PKC). We expressed chimeric channels composed of different components of the Kv7.4 and Kv7.5 α-subunits in vascular smooth muscle cells to determine which components are essential for enhancement or inhibition of channel activity. Forskolin, an activator of the cAMP/PKA pathway, increased wild-type Kv7.5 but not wild-type Kv7.4 current amplitude. Replacing the amino terminus of Kv7.4 with the amino terminus of Kv7.5 conferred partial responsiveness to forskolin. In contrast, swapping carboxy-terminal phosphatidylinositol 4,5-bisphosphate (PIP₂) binding domains, or the entire C terminus, was without effect on the forskolin response, but the latter conferred responsiveness to arginine-vasopressin (an inhibitory PKC-dependent response). Serine-to-alanine mutation at position 53 of the Kv7.5 amino terminus abrogated its ability to confer forskolin sensitivity to Kv7.4. Forskolin treatment reduced the sensitivity of Kv7.5 channels to Ciona intestinalis voltage-sensing phosphatase (Ci-VSP)-induced PIP₂ depletion, whereas activation of PKC with phorbol-12-myristate-13-acetate potentiated the Ci-VSP-induced decline in Kv7.5 current amplitude. Our findings suggest that PKA-dependent phosphorylation of serine 53 on the amino terminus of Kv7.5 increases its affinity for PIP₂, whereas PKC-dependent phosphorylation of the Kv7.5 carboxy terminus is associated with a reduction in PIP₂ affinity; these changes in PIP₂ affinity have corresponding effects on channel activity. Resting affinities for PIP₂ differ for Kv7.4 and Kv7.5 based on differential responsiveness to Ci-VSP activation and different rates of current rundown in ruptured patch recordings. SIGNIFICANCE STATEMENT: Kv7.4 and Kv7.5 channels are known signal transduction intermediates and drug targets for regulation of smooth muscle tone. The present studies identify distinct functional domains that confer differential sensitivities of Kv7.4 and Kv7.5 to stimulatory and inhibitory signaling and reveal structural features of the channel subunits that determine their biophysical properties. These findings may improve our understanding of the roles of these channels in smooth muscle physiology and disease, particularly in conditions where Kv7.4 and Kv7.5 are differentially expressed.

KCNQ5 activation is a unifying molecular mechanism shared by genetically and culturally diverse botanical hypotensive folk medicines

Botanical folk medicines have been used throughout human history to treat common disorders such as hypertension, often with unknown underlying mechanisms. Here, we discovered that hypotensive folk medicines from a genetically diverse range of plant species each selectively activated the vascular-expressed KCNQ5 potassium channel, a feature lacking in the modern synthetic pharmacopeia, whereas nonhypotensive plant extracts did not. Analyzing constituents of the hypotensive Sophora flavescens root, we found that the quinolizidine alkaloid aloperine is a KCNQ-dependent vasorelaxant that potently and isoform-selectively activates KCNQ5 by binding near the foot of the channel voltage sensor. Our findings reveal that KCNQ5-selective activation is a defining molecular mechanistic signature of genetically diverse traditional botanical hypotensives, transcending plant genus and human cultural boundaries. Discovery of botanical KCNQ5-selective potassium channel openers may enable future targeted therapies for diseases including hypertension and KCNQ5 loss-of-function encephalopathy.

Perivascular adipose tissue and the dynamic regulation of Kv 7 and Kir channels: Implications for resistant hypertension

Resistant hypertension is defined as high blood pressure that remains uncontrolled despite treatment with at least three antihypertensive drugs at adequate doses. Resistant hypertension is an increasingly common clinical problem in older age, obesity, diabetes, sleep apnea, and chronic kidney disease. Although the direct vasodilator minoxidil was introduced in the early 1970s, only recently has this drug been shown to be particularly effective in a subgroup of patients with treatment-resistant or uncontrolled hypertension. This pharmacological approach is interesting from a mechanistic perspective as minoxidil is the only clinically used K⁺ channel opener today, which targets a subclass of K⁺ channels, namely K_ATP channels in VSMCs. Beside K_ATP channels, two other classes of VSMC K⁺ channels could represent novel effective targets for treatment of resistant hypertension, namely K_v 7 (KCNQ) and inward rectifier potassium (K_ir 2.1) channels. Interestingly, these channels are unique among VSMC potassium channels. First, both have been implicated in the control of microvascular tone by perivascular adipose tissue. Second, they exhibit biophysical properties strongly controlled and regulated by membrane voltage, but not intracellular calcium. This review focuses on K_v 7 (K_v 7.1-5) and K_ir (K_ir 2.1) channels in VSMCs as potential novel drug targets for treatment of resistant hypertension, particularly in comorbid conditions such as obesity and metabolic syndrome.

Kv7 potassium channels as signal transduction intermediates in the control of microvascular tone

Potassium channels are recognized as important regulators of cellular functions in most, if not all cell types. These cellular proteins assemble to form gated pores in the plasma membrane, which serve to regulate the flow of potassium ions (K⁺ ) from the cytosol to the extracellular space. In VSMCs, the open state of potassium channels enables the efflux of K⁺ and thereby establishes a negative resting voltage across the plasma membrane that inhibits the opening of VSCCs. Under these conditions, cytosolic Ca²⁺ concentrations are relatively low and Ca²⁺ -dependent contraction is inhibited. Recent research has identified Kv7 family potassium channels as important contributors to resting membrane voltage in VSMCs, with much of the research focusing on the effects of drugs that specifically activate or block these channels to produce corresponding effects on VSMC contraction and vascular tone. Increasingly, evidence is emerging that these channels are not just good drug targets-they are also essential intermediates in vascular signal transduction, mediating vasoconstrictor or vasodilator responses to a variety of physiological stimuli. This review will summarize recent research findings that support a crucial function of Kv7 channels in both positive (vasoconstrictive) and negative (vasorelaxant) regulation of microvascular tone.

Mechanisms of PKA-Dependent Potentiation of Kv7.5 Channel Activity in Human Airway Smooth Muscle Cells

β-adrenergic receptor (βAR) activation promotes relaxation of both vascular and airway smooth muscle cells (VSMCs and ASMCs, respectively), though the signaling mechanisms have not been fully elucidated. We previously found that the activity of Kv7.5 voltage-activated potassium channels in VSMCs is robustly enhanced by activation of βARs via a mechanism involving protein kinase A (PKA)-dependent phosphorylation. We also found that enhancement of Kv7 channel activity in ASMCs promotes airway relaxation. Here we provide evidence that Kv7.5 channels are natively expressed in primary cultures of human ASMCs and that they conduct currents which are robustly enhanced in response to activation of the βAR/cyclic adenosine monophosphate (cAMP)/PKA pathway. MIT Scansite software analysis of putative PKA phosphorylation sites on Kv7.5 identified 8 candidate serine or threonine residues. Each residue was individually mutated to an alanine to prevent its phosphorylation and then tested for responses to βAR activation or to stimuli that elevate cAMP levels. Only the mutation of serine 53 (S53A), located on the amino terminus of Kv7.5, significantly reduced the increase in Kv7.5 current in response to these stimuli. A phospho-mimic mutation (S53D) exhibited characteristics of βAR-activated Kv7.5. Serine-to-alanine mutations of 6 putative PKA phosphorylation sites on the Kv7.5 C-terminus, individually or in combination, did not significantly reduce the enhancement of the currents in response to forskolin treatment (to elevate cAMP levels). We conclude that phosphorylation of S53 on the amino terminus of Kv7.5 is essential for PKA-dependent enhancement of channel activity in response to βAR activation in vascular and airway smooth muscle cells.

Dysregulation of the Renin-Angiotensin System and the Vasopressinergic System Interactions in Cardiovascular Disorders

Purpose of Review

In many instances, the renin-angiotensin system (RAS) and the vasopressinergic system (VPS) are jointly activated by the same stimuli and engaged in the regulation of the same processes.

Recent Findings

Angiotensin II (Ang II) and arginine vasopressin (AVP), which are the main active compounds of the RAS and the VPS, interact at several levels. Firstly, Ang II, acting on AT1 receptors (AT1R), plays a significant role in the release of AVP from vasopressinergic neurons and AVP, stimulating V1a receptors (V1aR), regulates the release of renin in the kidney. Secondly, Ang II and AVP, acting on AT1R and V1aR, respectively, exert vasoconstriction, increase cardiac contractility, stimulate the sympathoadrenal system, and elevate blood pressure. At the same time, they act antagonistically in the regulation of blood pressure by baroreflex. Thirdly, the cooperative action of Ang II acting on AT1R and AVP stimulating both V1aR and V2 receptors in the kidney is necessary for the appropriate regulation of renal blood flow and the efficient resorption of sodium and water. Furthermore, both peptides enhance the release of aldosterone and potentiate its action in the renal tubules.

Summary

In this review, we (1) point attention to the role of the cooperative action of Ang II and AVP for the regulation of blood pressure and the water-electrolyte balance under physiological conditions, (2) present the subcellular mechanisms underlying interactions of these two peptides, and (3) provide evidence that dysregulation of the cooperative action of Ang II and AVP significantly contributes to the development of disturbances in the regulation of blood pressure and the water-electrolyte balance in cardiovascular diseases.

Downregulation of KCNQ5 expression in the rat pulmonary vasculature of nitrofen-induced congenital diaphragmatic hernia

PURPOSE

Pulmonary hypertension (PH) is a common complication of congenital diaphragmatic hernia (CDH). Voltage-gated potassium channels KCNQ1, KCNQ4, and KCNQ5 are expressed by rodent pulmonary artery smooth muscle cells, contributing to their vascular tone. We hypothesized that KCNQ1, KCNQ4, and KCNQ5 expression is altered in the pulmonary vasculature of nitrofen-induced CDH rats.

METHODS

After ethical approval (REC913b), time-pregnant rats received nitrofen or vehicle on gestational day (D)9. D21 fetuses were divided into CDH and control group (n=22). QRT-PCR and western blotting were performed to determine gene and protein expression of KCNQ1, KCNQ4, and KCNQ5. Confocal microscopy was used to detect these proteins in the pulmonary vasculature.

RESULTS

Relative mRNA level of KCNQ5 (p=0.025) was significantly downregulated in CDH lungs compared to controls. KCNQ1 (p=0.052) and KCNQ4 (p=0.574) expression was not altered. Western blotting confirmed the decreased pulmonary KCNQ5 protein expression in CDH lungs. Confocal-microscopy detected a markedly diminished KCNQ5 expression in pulmonary vasculature of CDH fetuses.

CONCLUSIONS

Downregulated pulmonary expression of KCNQ5 in CDH lungs suggests that this potassium channel may play an important role in the development of PH in this model. KCNQ5 channel activator drugs may be a potential therapeutic target for the treatment of PH in CDH.

LEVEL OF EVIDENCE

2b (Centre for Evidence-Based Medicine, Oxford).

Novel treatment strategies for smooth muscle disorders: Targeting Kv7 potassium channels

Smooth muscle cells provide crucial contractile functions in visceral, vascular, and lung tissues. The contractile state of smooth muscle is largely determined by their electrical excitability, which is in turn influenced by the activity of potassium channels. The activity of potassium channels sustains smooth muscle cell membrane hyperpolarization, reducing cellular excitability and thereby promoting smooth muscle relaxation. Research over the past decade has indicated an important role for Kv7 (KCNQ) voltage-gated potassium channels in the regulation of the excitability of smooth muscle cells. Expression of multiple Kv7 channel subtypes has been demonstrated in smooth muscle cells from viscera (gastrointestinal, bladder, myometrial), from the systemic and pulmonary vasculature, and from the airways of the lung, from multiple species, including humans. A number of clinically used drugs, some of which were developed to target Kv7 channels in other tissues, have been found to exert robust effects on smooth muscle Kv7 channels. Functional studies have indicated that Kv7 channel activators and inhibitors have the ability to relax and contact smooth muscle preparations, respectively, suggesting a wide range of novel applications for the pharmacological tool set. This review summarizes recent findings regarding the physiological functions of Kv7 channels in smooth muscle, and highlights potential therapeutic applications based on pharmacological targeting of smooth muscle Kv7 channels throughout the body.

Nano-Bioelectronics

Nano-bioelectronics represents a rapidly expanding interdisciplinary field that combines nanomaterials with biology and electronics and, in so doing, offers the potential to overcome existing challenges in bioelectronics. In particular, shrinking electronic transducer dimensions to the nanoscale and making their properties appear more biological can yield significant improvements in the sensitivity and biocompatibility and thereby open up opportunities in fundamental biology and healthcare. This review emphasizes recent advances in nano-bioelectronics enabled with semiconductor nanostructures, including silicon nanowires, carbon nanotubes, and graphene. First, the synthesis and electrical properties of these nanomaterials are discussed in the context of bioelectronics. Second, affinity-based nano-bioelectronic sensors for highly sensitive analysis of biomolecules are reviewed. In these studies, semiconductor nanostructures as transistor-based biosensors are discussed from fundamental device behavior through sensing applications and future challenges. Third, the complex interface between nanoelectronics and living biological systems, from single cells to live animals, is reviewed. This discussion focuses on representative advances in electrophysiology enabled using semiconductor nanostructures and their nanoelectronic devices for cellular measurements through emerging work where arrays of nanoelectronic devices are incorporated within three-dimensional cell networks that define synthetic and natural tissues. Last, some challenges and exciting future opportunities are discussed.

Kv7.5 Potassium Channel Subunits Are the Primary Targets for PKA-Dependent Enhancement of Vascular Smooth Muscle Kv7 Currents

Kv7 (KCNQ) channels, formed as homo- or heterotetramers of Kv7.4 and Kv7.5 α-subunits, are important regulators of vascular smooth muscle cell (VSMC) membrane voltage. Recent studies demonstrate that direct pharmacological modulation of VSMC Kv7 channel activity can influence blood vessel contractility and diameter. However, the physiologic regulation of Kv7 channel activity is still poorly understood. Here, we study the effect of cAMP/protein kinase A (PKA) activation on whole cell K(+) currents through endogenous Kv7.5 channels in A7r5 rat aortic smooth muscle cells or through Kv7.4/Kv7.5 heteromeric channels natively expressed in rat mesenteric artery smooth muscle cells. The contributions of specific α-subunits are further dissected using exogenously expressed human Kv7.4 and Kv7.5 homo- or heterotetrameric channels in A7r5 cells. Stimulation of Gαs-coupled β-adrenergic receptors with isoproterenol induced PKA-dependent activation of endogenous Kv7.5 currents in A7r5 cells. The receptor-mediated enhancement of Kv7.5 currents was mimicked by pharmacological agents that increase [cAMP] (forskolin, rolipram, 3-isobutyl-1-methylxanthine, and papaverine) or mimic cAMP (8-bromo-cAMP); the 2- to 4-fold PKA-dependent enhancement of currents was also observed with exogenously expressed Kv7.5 channels. In contrast, exogenously-expressed heterotetrameric Kv7.4/7.5 channels in A7r5 cells or native mesenteric artery smooth muscle Kv7.4/7.5 channels were only modestly enhanced, and homo-tetrameric Kv7.4 channels were insensitive to this regulatory pathway. Correspondingly, proximity ligation assays indicated that isoproterenol induced PKA-dependent phosphorylation of exogenously expressed Kv7.5 channel subunits, but not of Kv7.4 subunits. These results suggest that signal transduction-mediated responsiveness of vascular smooth muscle Kv7 channel subunits to cAMP/PKA activation follows the order of Kv7.5 >> Kv7.4/Kv7.5 > Kv7.4.

K(V)7.4 channels participate in the control of rodent renal vascular resting tone

AIM

We tested the hypothesis that K(V)7 channels contribute to basal renal vascular tone and that they participate in agonist-induced renal vasoconstriction or vasodilation.

METHODS

KV 7 channel subtypes in renal arterioles were characterized by immunofluorescence. Renal blood flow (RBF) was measured using an ultrasonic flow probe. The isometric tension of rat interlobar arteries was examined in a wire myograph. Mice afferent arteriolar diameter was assessed utilizing the perfused juxtamedullary nephron technique.

RESULTS

Immunofluorescence revealed that K(V)7.4 channels were expressed in rat afferent arterioles. The K(V)7 blocker XE991 dose-dependently increased the isometric tension of rat interlobar arteries and caused a small (approx. 4.5%) RBF reduction in vivo. Nifedipine abolished these effects. Likewise, XE991 reduced mouse afferent arteriolar diameter by approx. 5%. The K(V)7.2-5 stimulator flupirtine dose-dependently relaxed isolated rat interlobar arteries and increased (approx. 5%) RBF in vivo. The RBF responses to NE or Ang II administration were not affected by pre-treatment with XE991 or flupirtine. XE991 pre-treatment caused a minor augmentation of the acetylcholine-induced increase in RBF, while flupirtine pre-treatment did not affect this response.

CONCLUSION

It is concluded that K(V)7 channels, via nifedipine sensitive channels, have a role in the regulation of basal renal vascular tone. There is no indication that K(V)7 channels have an effect on agonist-induced renal vasoconstriction while there is a small effect on acetylcholine-induced vasodilation.

Activation of KV7 channels stimulates vasodilatation of human placental chorionic plate arteries

INTRODUCTION

Potassium (K(+)) channels are key regulators of vascular smooth muscle cell (VSMC) excitability. In systemic small arteries, Kv7 channel expression/activity has been noted and a role in vascular tone regulation demonstrated. We aimed to demonstrate functional Kv7 channels in human fetoplacental small arteries.

METHODS

Human placental chorionic plate arteries (CPAs) were obtained at term. CPA responses to Kv7 channel modulators was determined by wire myography. Presence of Kv7 channel mRNA (encoded by KCNQ1-5) and protein expression were assessed by RT-PCR and immunohistochemistry/immunofluorescence, respectively.

RESULTS

Kv7 channel blockade with linopirdine increased CPA basal tone and AVP-induced contraction. Pre-contracted CPAs (AVP; 80 mM K(+) depolarization solution) exhibited significant relaxation to flupirtine, retigabine, the acrylamide (S)-1, and (S) BMS-204352, differential activators of Kv7.1 - Kv7.5 channels. All CPAs assessed expressed KCNQ1 and KCNQ3-5 mRNA; KCNQ2 was expressed only in a subset of CPAs. Kv7 protein expression was confirmed in intact CPAs and isolated VSMCs.

DISCUSSION

Kv7 channels are present and active in fetoplacental vessels, contributing to vascular tone regulation in normal pregnancy. Targeting these channels may represent a therapeutic intervention in pregnancies complicated by increased vascular resistance.

Cardiovascular responses to retigabine in conscious rats--under normotensive and hypertensive conditions

BACKGROUND AND PURPOSE

Retigabine is a recently approved antiepileptic agent which activates Kv7.2-7.5 potassium channels. It is emerging that these channels have an important role in vascular regulation, but the vascular effects of retigabine in the conscious state are unknown. Hence, in the present study we assessed the regional haemodynamic responses to retigabine in conscious rats.

EXPERIMENTAL APPROACH

Male Sprague Dawley rats were chronically instrumented with pulsed Doppler flow probes to measure regional haemodynamic responses to retigabine under control conditions and during acute hypertension induced by infusion of angiotensin II and arginine vasopressin. Further experiments were performed, using the β-adrenoceptor antagonists CGP 20712A, ICI 118551 and propranolol, to elucidate the roles of β-adrenoceptors in the responses to retigabine in vivo and in vitro.

KEY RESULTS

Under normotensive conditions, retigabine induced dose-dependent hypotension and hindquarters vasodilatation, with small, transient renal and mesenteric vasodilatations. In the acutely hypertensive state, the renal and mesenteric, but not hindquarters, vasodilatations were enhanced. The response of the hindquarters vascular bed to retigabine was mediated, in part, by β₂-adrenoceptors. However, in vitro experiments confirmed that retigabine did not act as a β-adrenoceptor agonist.

CONCLUSIONS AND IMPLICATIONS

We demonstrated that retigabine causes regionally specific vasodilatations, which are different under normotensive and hypertensive conditions, and are, in part, mediated by β₂-adrenoceptors in some vascular beds but not in others. These results broadly support previous findings and further indicate that Kv7 channels are a potential therapeutic target for the treatment of vascular diseases associated with inappropriate vasoconstriction.

KV 7 channels are involved in hypoxia-induced vasodilatation of porcine coronary arteries

BACKGROUND AND PURPOSE

Hypoxia causes vasodilatation of coronary arteries, but the underlying mechanisms are poorly understood. We hypothesized that hypoxia reduces intracellular Ca(2+) concentration ([Ca(2+)](i)) by opening of K channels and release of H₂S.

EXPERIMENTAL APPROACH

Porcine coronary arteries without endothelium were mounted for measurement of isometric tension and [Ca(2+)](i), and the expression of voltage-gated K channels K(V)7 channels (encoded by KCNQ genes) and large-conductance calcium-activated K channels (K(Ca)1.1) was examined. Voltage clamp assessed the role of K(V)7 channels in hypoxia.

KEY RESULTS

Gradual reduction of oxygen concentration from 95 to 1% dilated the precontracted coronary arteries and this was associated with reduced [Ca(2+)](i) in PGF(2α) (10 μM)-contracted arteries whereas no fall in [Ca(2+)](i) was observed in 30 mM K-contracted arteries. Blockers of ATP-sensitive voltage-gated potassium channels and K(Ca)1.1 inhibited hypoxia-induced dilatation in PGF2α -contracted arteries; this inhibition was more marked in the presence of the K(v)7 channel blockers, XE991 and linopirdine, while a K(V)7.1 blocker, failed to change hypoxic vasodilatation. XE991 also inhibited H₂S- and adenosine-induced vasodilatation. PCR revealed the expression of K(V)7.1, K(V)7.4, K(V)7.5 and K(Ca)1.1 channels, and K(Ca)1.1, K(V)7.4 and K(V)7.5 were also identified by immunoblotting. Voltage clamp studies showed the XE991-sensitive current was more marked in hypoxic conditions.

CONCLUSION

The K(V)7.4 and K(V)7.5 channels, which we identified in the coronary arteries, appear to have a major role in hypoxia-induced vasodilatation. The voltage clamp results further support the involvement of K(V)7 channels in this vasodilatation. Activation of these K(V)7 channels may be induced by H₂S and adenosine.

Differential activation of vascular smooth muscle Kv7.4, Kv7.5, and Kv7.4/7.5 channels by ML213 and ICA-069673

Recent research suggests that smooth muscle cells express Kv7.4 and Kv7.5 voltage-activated potassium channels, which contribute to maintenance of their resting membrane voltage. New pharmacologic activators of Kv7 channels, ML213 (N-mesitybicyclo[2.2.1]heptane-2-carboxamide) and ICA-069673 N-(6-chloropyridin-3-yl)-3,4-difluorobenzamide), have been reported to discriminate among channels formed from different Kv7 subtypes. We compared the effects of ML213 and ICA-069673 on homomeric human Kv7.4, Kv7.5, and heteromeric Kv7.4/7.5 channels exogenously expressed in A7r5 vascular smooth muscle cells. We found that, despite its previous description as a selective activator of Kv7.2 and Kv7.4, ML213 significantly increased the maximum conductance of homomeric Kv7.4 and Kv7.5, as well as heteromeric Kv7.4/7.5 channels, and induced a negative shift of their activation curves. Current deactivation rates decreased in the presence of the ML213 (10 μM) for all three channel combinations. Mutants of Kv7.4 (W242L) and Kv7.5 (W235L), previously found to be insensitive to another Kv7 channel activator, retigabine, were also insensitive to ML213 (10 μM). In contrast to ML213, ICA-069673 robustly activated Kv7.4 channels but was significantly less effective on homomeric Kv7.5 channels. Heteromeric Kv7.4/7.5 channels displayed intermediate responses to ICA-069673. In each case, ICA-069673 induced a negative shift of the activation curves without significantly increasing maximal conductance. Current deactivation rates decreased in the presence of ICA-069673 in a subunit-specific manner. Kv7.4 W242L responded to ICA-069673-like wild-type Kv7.4, but a Kv7.4 F143A mutant was much less sensitive to ICA-069673. Based on these results, ML213 and ICA-069673 likely bind to different sites and are differentially selective among Kv7.4, Kv7.5, and Kv7.4/7.5 channel subtypes.

Functional assembly of Kv7.1/Kv7.5 channels with emerging properties on vascular muscle physiology

OBJECTIVE

Voltage-dependent K(+) (Kv) channels from the Kv7 family are expressed in blood vessels and contribute to cardiovascular physiology. Although Kv7 channel blockers trigger muscle contractions, Kv7 activators act as vasorelaxants. Kv7.1 and Kv7.5 are expressed in many vessels. Kv7.1 is under intense investigation because Kv7.1 blockers fail to modulate smooth muscle reactivity. In this study, we analyzed whether Kv7.1 and Kv7.5 may form functional heterotetrameric channels increasing the channel diversity in vascular smooth muscles.

APPROACH AND RESULTS

Kv7.1 and Kv7.5 currents elicited in arterial myocytes, oocyte, and mammalian expression systems suggest the formation of heterotetrameric complexes. Kv7.1/Kv7.5 heteromers, exhibiting different pharmacological characteristics, participate in the arterial tone. Kv7.1/Kv7.5 associations were confirmed by coimmunoprecipitation, fluorescence resonance energy transfer, and fluorescence recovery after photobleaching experiments. Kv7.1/Kv7.5 heterotetramers were highly retained at the endoplasmic reticulum. Studies in HEK-293 cells, heart, brain, and smooth and skeletal muscles demonstrated that the predominant presence of Kv7.5 stimulates release of Kv7.1/Kv7.5 oligomers out of lipid raft microdomains. Electrophysiological studies supported that KCNE1 and KCNE3 regulatory subunits further increased the channel diversity. Finally, the analysis of rat isolated myocytes and human blood vessels demonstrated that Kv7.1 and Kv7.5 exhibited a differential expression, which may lead to channel diversity.

CONCLUSIONS

Kv7.1 and Kv7.5 form heterotetrameric channels increasing the diversity of structures which fine-tune blood vessel reactivity. Because the lipid raft localization of ion channels is crucial for cardiovascular physiology, Kv7.1/Kv7.5 heteromers provide efficient spatial and temporal regulation of smooth muscle function. Our results shed light on the debate about the contribution of Kv7 channels to vasoconstriction and hypertension.

Kv7.2 regulates the function of peripheral sensory neurons

The Kv7 (KCNQ) family of voltage-gated K(+) channels regulates cellular excitability. The functional role of Kv7.2 has been hampered by the lack of a viable Kcnq2-null animal model. In this study, we generated homozygous Kcnq2-null sensory neurons using the Cre-Lox system; in these mice, Kv7.2 expression is absent in the peripheral sensory neurons, whereas the expression of other molecular components of nodes (including Kv7.3), paranodes, and juxtaparanodes is not altered. The conditional Kcnq2-null animals exhibit normal motor performance but have increased thermal hyperalgesia and mechanical allodynia. Whole-cell patch recording technique demonstrates that Kcnq2-null sensory neurons have increased excitability and reduced spike frequency adaptation. Taken together, our results suggest that the loss of Kv7.2 activity increases the excitability of primary sensory neurons.

One man's side effect is another man's therapeutic opportunity: targeting Kv7 channels in smooth muscle disorders

Retigabine is a first in class anticonvulsant that has recently undergone clinical trials to test its efficacy in epileptic patients. Retigabine's novel mechanism of action - activating Kv7 channels - suppresses neuronal activity to prevent seizure generation by hyperpolarizing the membrane potential and suppressing depolarizing surges. However, Kv7 channels are not expressed exclusively in neurones and data generated over the last decade have shown that Kv7 channels play a key role in various smooth muscle systems of the body. This review discusses the potential of targeting Kv7 channels in the smooth muscle to treat diseases such as hypertension, bladder instability, constipation and preterm labour.

Differential protein kinase C-dependent modulation of Kv7.4 and Kv7.5 subunits of vascular Kv7 channels

The Kv7 family (Kv7.1-7.5) of voltage-activated potassium channels contributes to the maintenance of resting membrane potential in excitable cells. Previously, we provided pharmacological and electrophysiological evidence that Kv7.4 and Kv7.5 form predominantly heteromeric channels and that Kv7 activity is regulated by protein kinase C (PKC) in response to vasoconstrictors in vascular smooth muscle cells. Direct evidence for Kv7.4/7.5 heteromer formation, however, is lacking. Furthermore, it remains to be determined whether both subunits are regulated by PKC. Utilizing proximity ligation assays to visualize single molecule interactions, we now show that Kv7.4/Kv.7.5 heteromers are endogenously expressed in vascular smooth muscle cells. Introduction of dominant-negative Kv7.4 and Kv7.5 subunits in mesenteric artery myocytes reduced endogenous Kv7 currents by 84 and 76%, respectively. Expression of an inducible protein kinase Cα (PKCα) translocation system revealed that PKCα activation is sufficient to suppress endogenous Kv7 currents in A7r5 rat aortic and mesenteric artery smooth muscle cells. Arginine vasopressin (100 and 500 pm) and the PKC activator phorbol 12-myristate 13-acetate (1 nm) each inhibited human (h) Kv7.5 and hKv7.4/7.5, but not hKv7.4 channels expressed in A7r5 cells. A decrease in hKv7.5 and hKv7.4/7.5 current densities was associated with an increase in PKC-dependent phosphorylation of the channel proteins. These findings provide further evidence for a differential regulation of Kv7.4 and Kv7.5 channel subunits by PKC-dependent phosphorylation and new mechanistic insights into the role of heteromeric subunit assembly for regulation of vascular Kv7 channels.

Role of KCNQ channels in skeletal muscle arteries and periadventitial vascular dysfunction

KCNQ channels have been identified in arterial smooth muscle. However, their role in vasoregulation and chronic vascular diseases remains elusive. We tested the hypothesis that KCNQ channels contribute to periadventitial vasoregulation in peripheral skeletal muscle arteries by perivascular adipose tissue and that they represent novel targets to rescue periadventitial vascular dysfunction. Two models, spontaneously hypertensive rats and New Zealand obese mice, were studied using quantitative polymerase chain reaction, the patch-clamp technique, membrane potential measurements, myography of isolated vessels, and blood pressure telemetry. In rat Gracilis muscle arteries, anticontractile effects of perivascular fat were inhibited by the KCNQ channel blockers XE991 and linopirdine but not by other selective K(+) channel inhibitors. Accordingly, XE991 and linopirdine blocked noninactivating K(+) currents in freshly isolated Gracilis artery smooth muscle cells. mRNAs of several KCNQ channel subtypes were detected in those arteries, with KCNQ4 channels being dominant. In spontaneously hypertensive rats, the anticontractile effect of perivascular fat in Gracilis muscle arteries was largely reduced compared with Wistar rats. However, the vasodilator effects of KCNQ channel openers and mRNA expression of KCNQ channels were normal. Furthermore, KCNQ channel openers restored the diminished anticontractile effects of perivascular fat in spontaneously hypertensive rats. Moreover, KCNQ channel openers reduced arterial blood pressure in both models of hypertension independent of ganglionic blockade. Thus, our data suggest that KCNQ channels play a pivotal role in periadventitial vasoregulation of peripheral skeletal muscle arteries, and KCNQ channel opening may be an effective mechanism to improve impaired periadventitial vasoregulation and associated hypertension.

Oroxylin a, but not vasopressin, ameliorates cardiac dysfunction of endotoxemic rats

The mortality in septic patients with myocardial dysfunction is higher than those without it. Beneficial effects of flavonoid oroxylin A (Oro-A) on endotoxemic hearts were evaluated and compared with that of arginine vasopressin (AVP) which is used to reverse hypotension in septic patients. Endotoxemia in rats was induced by one-injection of lipopolysaccharides (LPS, 10 mg/kg, i.p.), and hearts were isolated 5-hrs or 16-hrs later. Isolated hearts with constant-pressure or constant-flow mode were examined by Langendorff technique. Rate and force of contractions of isolated atrial and ventricular strips were examined by tissue myography. Isolated endotoxemic hearts were characterized by decreased or increased coronary flow (CF) in LPS-treated-for-5hr and LPS-treated-for-16-hr groups, respectively, with decreased inotropy in both groups. Oro-A-perfusion ameliorated while AVP-perfusion worsened the decreased CF and inotropy in both preparations. Oro-A and AVP, however, did not affect diminished force or rate of contraction of atrial and ventricular strips of endotoxemic hearts. Oro-A-induced CF increase was not affected following coronary endothelium-denudation with saponin. These results suggest that Oro-A ameliorates LPS-depressed cardiac functions by increasing CF, leading to positive inotropy. In contrast, AVP aggravates cardiac dysfunction by decreasing CF. Oro-A is a potentially useful candidate for treating endotoxemia complicated with myocardial dysfunction.

Kv7.5 is the primary Kv7 subunit expressed in C-fibers

Kv7 (KCNQ) potassium channel openers (enhancers) decrease neuropathic pain in experimental models. Here we show that C-fibers, and their associated small-diameter neurons in the dorsal root ganglia (both IB4- and TrkA-positive), expressed Kv7.5. In contrast, C-fibers did not express detectable levels of Kv7.2 or Kv7.3, which are instead localized to nodes of Ranvier and the cell bodies of large sensory neurons. These data suggest that Kv7.5 provides the primary M current in nociceptive neurons.

The KCNE Tango - How KCNE1 Interacts with Kv7.1

The classical tango is a dance characterized by a 2/4 or 4/4 rhythm in which the partners dance in a coordinated way, allowing dynamic contact. There is a surprising similarity between the tango and how KCNE β-subunits "dance" to the fast rhythm of the cell with their partners from the Kv channel family. The five KCNE β-subunits interact with several members of the Kv channels, thereby modifying channel gating via the interaction of their single transmembrane-spanning segment, the extracellular amino terminus, and/or the intracellular carboxy terminus with the Kv α-subunit. Best studied is the molecular basis of interactions between KCNE1 and Kv7.1, which, together, supposedly form the native cardiac I(Ks) channel. Here we review the current knowledge about functional and molecular interactions of KCNE1 with Kv7.1 and try to summarize and interpret the tango of the KCNEs.

Vasodilator signals from perivascular adipose tissue

Visceral fat has been linked to metabolic disturbances and increased risk for cardiovascular disease and type 2 diabetes. Recent studies propose a paracrine role for periadventitial adipose tissue in the control of arterial vascular tone. This regulation depends on the anatomical integrity of the vessels and involves a transferable mediator(s) (adipokine) released from either periadventitial adipocytes or perivascular adipose tissue. Although a number of adipokines with vasoactive properties have been identified, a still unidentified adipocyte-derived relaxing factor (ADRF) plays a major role in the periadventitial vasoregulation of visceral arteries, such as the aorta and mesenteric arteries. ADRF is released by visceral periadventitial adipocytes and primarily produces endothelium-independent vasorelaxation by opening voltage-dependent (K(v) ) K(+) channels in the plasma membrane of smooth muscle cells. At least in part, KCNQ (K(v) 7) channels could represent the subtype of K(v) channels involved. Glibenclamide-sensitive K(ATP) channels are not involved or play a minor role. The 'third gas', namely H(2) S, could represent ADRF. Alterations in the paracrine control of arterial tone by visceral periadventitial adipose tissue have been found in animal models of hypertension and metabolic disease. ADRF, or perhaps its putative targets, might represent exciting new targets for the development of drugs for treatment of cardiovascular and metabolic disorders.

LINKED ARTICLES

This article is part of a themed section on Fat and Vascular Responsiveness. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.165.issue-3.

Driving With No Brakes: Molecular Pathophysiology of Kv7 Potassium Channels

Kv7 potassium channels regulate excitability in neuronal, sensory, and muscular cells. Here, we describe their molecular architecture, physiological roles, and involvement in genetically determined channelopathies highlighting their relevance as targets for pharmacological treatment of several human disorders.

Kv7 potassium channels in airway smooth muscle cells: signal transduction intermediates and pharmacological targets for bronchodilator therapy

Expression and function of Kv7 (KCNQ) voltage-activated potassium channels in guinea pig and human airway smooth muscle cells (ASMCs) were investigated by quantitative reverse transcriptase polymerase chain reaction (qRT-PCR), patch-clamp electrophysiology, and precision-cut lung slices. qRT-PCR revealed expression of multiple KCNQ genes in both guinea pig and human ASMCs. Currents with electrophysiological and pharmacological characteristics of Kv7 currents were measured in freshly isolated guinea pig and human ASMCs. In guinea pig ASMCs, Kv7 currents were significantly suppressed by application of the bronchoconstrictor agonists methacholine (100 nM) or histamine (30 μM), but current amplitudes were restored by addition of a Kv7 channel activator, flupirtine (10 μM). Kv7 currents in guinea pig ASMCs were also significantly enhanced by another Kv7.2-7.5 channel activator, retigabine, and by celecoxib and 2,5-dimethyl celecoxib. In precision-cut human lung slices, constriction of airways by histamine was significantly reduced in the presence of flupirtine. Kv7 currents in both guinea pig and human ASMCs were inhibited by the Kv7 channel blocker XE991. In human lung slices, XE991 induced robust airway constriction, which was completely reversed by addition of the calcium channel blocker verapamil. These findings suggest that Kv7 channels in ASMCs play an essential role in the regulation of airway diameter and may be targeted pharmacologically to relieve airway hyperconstriction induced by elevated concentrations of bronchoconstrictor agonists.

Activation of vascular KCNQ (Kv7) potassium channels reverses spasmogen-induced constrictor responses in rat basilar artery

BACKGROUND AND PURPOSE

Cerebral vasospasm is the persistent constriction of large conduit arteries in the base of the brain. This pathologically sustained contraction of the arterial myocytes has been attributed to locally elevated concentrations of vasoconstrictor agonists (spasmogens). We assessed the presence and function of KCNQ (K(v) 7) potassium channels in rat basilar artery myocytes, and determined the efficacy of K(v) 7 channel activators in relieving spasmogen-induced basilar artery constriction.

EXPERIMENTAL APPROACH

Expression and function of K(v) 7 channels in freshly isolated basilar artery myocytes were evaluated by reverse transcriptase polymerase chain reaction and whole-cell electrophysiological techniques. Functional responses to K(v) 7 channel modulators were studied in intact artery segments using pressure myography.

KEY RESULTS

All five mammalian KCNQ subtypes (KCNQ1-5) were detected in the myocytes. K(v) currents were attributed to K(v) 7 channel activity based on their voltage dependence of activation (V(0.5) ∼-34 mV), lack of inactivation, enhancement by flupirtine (a selective K(v) 7 channel activator) and inhibition by 10,10-bis(pyridin-4-ylmethyl)anthracen-9-one (XE991; a selective K(v) 7 channel blocker). XE991 depolarized the myocytes and constricted intact basilar arteries. Celecoxib, a clinically used anti-inflammatory drug, not only enhanced K(v) 7 currents but also inhibited voltage-sensitive Ca(2+) currents. In arteries pre-constricted with spasmogens, both celecoxib and flupirtine were more effective in dilating artery segments than was nimodipine, a selective L-type Ca(2+) channel blocker.

CONCLUSIONS AND IMPLICATIONS

K(v) 7 channels are important determinants of basilar artery contractile status. Targeting the K(v) 7 channels using flupirtine or celecoxib could provide a novel strategy to relieve basilar artery constriction in patients with cerebral vasospasm.

LINKED ARTICLES

To view two letters to the Editor regarding this article visit http://dx.doi.org/10.1111/j.1476-5381.2011.01454.x and http://dx.doi.org/10.1111/j.1476-5381.2011.01457.x.

Systemic peripheral artery relaxation by KCNQ channel openers and hydrogen sulfide

BACKGROUND

Perivascular adipose tissue secretes an adipocyte-derived relaxing factor (ADRF) that opens voltage-dependent K (Kv) channels in peripheral arteries. We studied the role of KCNQ-type Kv channels and tested the hypothesis that hydrogen sulfide (H2S) could be an ADRF.

METHODS

We performed isometric contraction studies on systemic arteries of rats and mice.

RESULTS

In mesenteric arteries and aortas without perivascular adipose tissue, the KCNQ channel openers retigabine, VRX0530727, VRX0621238, and VRX0621688 produced concentration-dependent vasorelaxation; VRX0621688 was the most potent vasodilator. The KCNQ inhibitor XE991 (30 micromol/l) blocked the effects of both the drugs and ADRF. Inhibitors of cystathionine gamma lyase (CSE) beta-cyano-L-alanine (BCA, 5 mmol/l) and 4-propargyl glycine (PPG, 10 mmol/l) also blocked the relaxations. CSE is expressed in perivascular adipose tissue and endogenously generates H2S. The H2S donor NaHS produced concentration-dependent vasorelaxation, which was also blocked by XE991. The vasodilatory capacities of retigabine, VRX0530727, VRX0621238, and VRX0621688 were preserved following inhibition of H2S generation in perivascular fat.

CONCLUSION

We suggest that KCNQ channel opening is a powerful mechanism to produce vasorelaxation of systemic arteries in rats and mice. Furthermore, KCNQ channels play a major role in the paracrine control of vascular tone by perivascular adipose tissue, which is at least in part mediated or modulated by H2S. In conditions of reduced H2S release from perivascular adipose tissue, these paracrine effects can be mimicked by synthetic KCNQ channel openers.

Diclofenac distinguishes among homomeric and heteromeric potassium channels composed of KCNQ4 and KCNQ5 subunits

KCNQ4 and KCNQ5 potassium channel subunits are expressed in vascular smooth muscle cells, although it remains uncertain how these subunits assemble to form functional channels. Using patch-clamp techniques, we compared the electrophysiological characteristics and effects of diclofenac, a known KCNQ channel activator, on human KCNQ4 and KCNQ5 channels expressed individually or together in A7r5 rat aortic smooth muscle cells. The conductance curves of the overexpressed channels were fitted by a single Boltzmann function in each case (V(0.5) values: -31, -44, and -38 mV for KCNQ4, KCNQ5, and KCNQ4/5, respectively). Diclofenac (100 μM) inhibited KCNQ5 channels, reducing maximum conductance by 53%, but increased maximum conductance of KCNQ4 channels by 38%. The opposite effects of diclofenac on KCNQ4 and KCNQ5 could not be attributed to the presence of a basic residue (lysine) in the voltage-sensing domain of KCNQ5, because mutation of this residue to neutral glycine (the residue present in KCNQ4) resulted in a more effective block of the channel. Differences in deactivation rates and distinct voltage-dependent effects of diclofenac on channel activation and deactivation observed with each of the subunit combinations (KCNQ4, KCNQ5, and KCNQ4/5) were used as diagnostic tools to evaluate native KCNQ currents in vascular smooth muscle cells. A7r5 cells express only KCNQ5 channels endogenously, and their responses to diclofenac closely resembled those of the overexpressed KCNQ5 currents. In contrast, mesenteric artery myocytes, which express both KCNQ4 and KCNQ5 channels, displayed whole-cell KCNQ currents with properties and diclofenac responses characteristic of overexpressed heteromeric KCNQ4/5 channels.

Participation of KCNQ (Kv7) potassium channels in myogenic control of cerebral arterial diameter

KCNQ gene expression was previously shown in various rodent blood vessels, where the products of KCNQ4 and KCNQ5, Kv7.4 and Kv7.5 potassium channel subunits, respectively, have an influence on vascular reactivity. The aim of this study was to determine if small cerebral resistance arteries of the rat express KCNQ genes and whether Kv7 channels participate in the regulation of myogenic control of diameter. Quantitative reverse transcription polymerase chain reaction (QPCR) was undertaken using RNA isolated from rat middle cerebral arteries (RMCAs) and immunocytochemistry was performed using Kv7 subunit-specific antibodies and freshly isolated RMCA myocytes. KCNQ4 message was more abundant than KCNQ5 = KCNQ1, but KCNQ2 and KCNQ3 message levels were negligible. Kv7.1, Kv7.4 and Kv7.5 immunoreactivity was present at the sarcolemma of freshly isolated RMCA myocytes. Linopirdine (1 microm) partially depressed, whereas the Kv7 activator S-1 (3 and/or 20 microm) enhanced whole-cell Kv7.4 (in HEK 293 cells), as well as native RMCA myocyte Kv current amplitude. The effects of S-1 were voltage-dependent, with progressive loss of stimulation at potentials of >15 mV. At the concentrations employed linopirdine and S-1 did not alter currents due to recombinant Kv1.2/Kv1.5 or Kv2.1/Kv9.3 channels (in HEK 293 cells) that are also expressed by RMCA myocytes. In contrast, another widely used Kv7 blocker, XE991 (10 microm), significantly attenuated native Kv current and also reduced Kv1.2/Kv1.5 and Kv2.1/Kv9.3 currents. Pressurized arterial myography was performed using RMCAs exposed to intravascular pressures of 10-100 mmHg. Linopirdine (1 microm) enhanced the myogenic response at 20 mmHg, whereas the activation of Kv7 channels with S-1 (20 microm) inhibited myogenic constriction at >20 mmHg and reversed the increased myogenic response produced by suppression of Kv2-containing channels with 30 nm stromatoxin (ScTx1). These data reveal a novel contribution of KCNQ gene products to the regulation of myogenic control of cerebral arterial diameter and suggest that Kv7 channel activating drugs may be appropriate candidates for the development of an effective therapy to ameliorate cerebral vasospasm.

New tricks for old dogs: KCNQ expression and role in smooth muscle

Ion channels encoded by the KCNQ gene family (K(v)7.1-7.5) are major determinants of neuronal membrane potential and the cardiac action potential. This key physiological role is highlighted by the existence of a number of hereditary disorders caused by mutations to KCNQ genes. Recently, KCNQ gene expression has been identified in vascular and non-vascular smooth muscles. In addition, experiments with an array of pharmacological modulators of KCNQ channels have supported a crucial role for these channels in regulating smooth muscle contractility. This article will provide an overview of present understanding in this nascent area of KCNQ research and will offer guidance as to future directions.

Novel Actions of Nonsteroidal Anti-Inflammatory Drugs on Vascular Ion Channels: Accounting for Cardiovascular Side Effects and Identifying New Therapeutic Applications

Non-steroidal anti-inflammatory drugs (NSAIDs) are commonly used medications for the treatment of both acute and chronic pain. Selective cyclooxygenase-2 (COX-2) inhibitors, such as celecoxib (Celebrex(®)), rofecoxib (Vioxx(®)), and diclofenac, have been among the most widely prescribed NSAIDs because they prevent the generation of prostaglandins involved in inflammation and pain, but avoid some of the gastrointestinal complications associated with less selective COX-1/COX-2 inhibitors. In 2004, rofecoxib (Vioxx(®)) was voluntarily withdrawn from the market because of adverse cardiovascular side effects. This led to an explosion of research into the cardiovascular effects of the 'coxibs', which revealed differential cardiovascular risk profiles among the members of this drug class. The differential risk profiles may relate to the tendency of some of the drugs to elevate blood pressure (BP). An important component of BP regulation is dependent on the contractile state of vascular smooth muscle cells (VSMCs), which is controlled to a large extent by the activities of KCNQ (Kv7 family) potassium channels and L-type calcium channels. Our recently published data indicate that celecoxib, but not rofecoxib or diclofenac, at therapeutically relevant concentrations, acts as a Kv7 potassium channel activator and a calcium channel blocker, causing relaxation of VSMCs and decreasing vascular tone. These vasorelaxant ion channel effects may account for the differential cardiovascular risk profiles among the different COX-2 inhibitors. We further speculate that these properties may be exploited for therapeutic benefit in the treatment of cardiovascular diseases or other medical conditions.

Differential effects of selective cyclooxygenase-2 inhibitors on vascular smooth muscle ion channels may account for differences in cardiovascular risk profiles

Celecoxib, rofecoxib, and diclofenac are clinically used cyclooxygenase-2 (COX-2) inhibitors, which have been under intense scrutiny because long-term rofecoxib (Vioxx; Merck, Whitehouse Station, NJ) treatment was found to increase the risk of adverse cardiovascular events. A differential risk profile for these drugs has emerged, but the underlying mechanisms have not been fully elucidated. We investigated the effects of celecoxib, rofecoxib, and diclofenac on ionic currents and calcium signaling in vascular smooth muscle cells (VSMCs) using patch-clamp techniques and fura-2 fluorescence and on arterial constriction using pressure myography. Celecoxib, but not rofecoxib or diclofenac, dramatically enhanced KCNQ (K(v)7) potassium currents and suppressed L-type voltage-sensitive calcium currents in A7r5 rat aortic smooth muscle cells (native KCNQ currents or overexpressed human KCNQ5 currents) and freshly isolated rat mesenteric artery myocytes. The effects of celecoxib were concentration-dependent within the therapeutic concentration range, and were reversed on washout. Celecoxib, but not rofecoxib, also inhibited calcium responses to vasopressin in A7r5 cells and dilated intact or endothelium-denuded rat mesenteric arteries. A celecoxib analog, 2,5-dimethyl-celecoxib, which does not inhibit COX-2, mimicked celecoxib in its enhancement of vascular KCNQ5 currents, suppression of L-type calcium currents, and vasodilation. We conclude that celecoxib inhibits calcium responses in VSMCs by enhancing KCNQ5 currents and suppressing L-type calcium currents, which ultimately reduces vascular tone. These effects are independent of its COX-2 inhibitory actions and may explain the differential risk of cardiovascular events in patients taking different drugs of this class.

Molecular and functional characterization of Kv7 K+ channel in murine gastrointestinal smooth muscles

Members of the K(v)7 voltage-gated K(+) channel family are important determinants of cardiac and neuronal membrane excitability. Recently, we and others have shown that K(v)7 channels are also crucial regulators of smooth muscle activity. The aim of the present study was to assess the K(v)7 expression in different parts of the murine gastrointestinal (GI) tract and to assess their functional roles by use of pharmacological agents. Of KCNQ/K(v)7 members, both KCNQ4/K(v)7.4 and KCNQ5/K(v)7.5 genes and proteins were the most abundantly expressed K(v)7 channels in smooth muscles throughout the GI tract. Immunohistochemical staining also revealed that K(v)7.4 and K(v)7.5 but not K(v)7.1 were expressed in the circular muscle layer of the colon. In segments of distal colon circular muscle exhibiting spontaneous phasic contractions, the nonselective K(v)7 blockers XE991 and linopirdine increased the integral of tension. Increases in the integral of tension were also observed under conditions of neuronal blockade. Similar effects, although less marked, were observed in the proximal colon. As expected, the K(v)7.1-selective blocker chromanol 293B had no effect in either type of segment. These data show that K(v)7.x especially K(v)7.4 and K(v)7.5 are expressed in different regions of the murine gastrointestinal tract and blockers of K(v)7 channels augment inherent contractile activity. Drugs that selectively block K(v)7.4/7.5 might be promising therapeutics for the treatment of motility disorders such as constipation associated with irritable bowel syndrome.