Computational and functional analyses of a small-molecule binding site in ROMK

Interaction of ROMK2 channel with lipid kinases DGKE and AGK: Potential channel activation by localized anionic lipid synthesis

In this study, we utilized enzyme-catalyzed proximity labeling with the engineered promiscuous biotin ligase Turbo-ID to identify the proxisome of the ROMK2 channel. This channel resides in various cellular membrane compartments of the cell including the plasma membrane, endoplasmic reticulum and mitochondria. Within mitochondria, ROMK2 has been suggested as a pore-forming subunit of mitochondrial ATP-regulated potassium channel (mitoKATP). We found that ROMK2 proxisome in addition to previously known protein partners included two lipid kinases: acylglycerol kinase (AGK) and diacylglycerol kinase ε (DGKE), which are localized in mitochondria and the endoplasmic reticulum, respectively. Through co-immunoprecipitation, we confirmed that these two kinases are present in complexes with ROMK2 channels. Additionally, we found that the products of AGK and DGKE, lysophosphatidic acid (LPA) and phosphatidic acid (PA), stimulated the activity of ROMK2 channels in artificial lipid bilayers. Our molecular docking studies revealed the presence of acidic lipid binding sites in the ROMK2 channel, similar to those previously identified in Kir2 channels. Based on these findings, we propose a model wherein localized lipid synthesis, mediated by channel-bound lipid kinases, contributes to the regulation of ROMK2 activity within distinct intracellular compartments, such as mitochondria and the endoplasmic reticulum.

Pharmacological Characterization of a Recombinant Mitochondrial ROMK2 Potassium Channel Expressed in Bacteria and Reconstituted in Planar Lipid Bilayers

In the inner mitochondrial membrane, several potassium channels that play a role in cell life and death have been identified. One of these channels is the ATP-regulated potassium channel (mitoK_ATP). The ROMK2 potassium channel is a potential molecular component of the mitoK_ATP channel. The current study aimed to investigate the pharmacological modulation of the activity of the ROMK2 potassium channel expressed in Escherichia coli bacteria. ROMK2 was solubilized in polymer nanodiscs and incorporated in planar lipid bilayers. The impact of known mitoK_ATP channel modulators on the activity of the ROMK2 was characterized. We found that the ROMK2 channel was activated by the mitoK_ATP channel opener diazoxide and blocked by mitoK_ATP inhibitors such as ATP/Mg²⁺, 5-hydroxydecanoic acid, and antidiabetic sulfonylurea glibenclamide. These results indicate that the ROMK2 potassium protein may be a pore-forming subunit of mitoK_ATP and that the impact of channel modulators is not related to the presence of accessory proteins.

Alternative Targets for Modulators of Mitochondrial Potassium Channels

Mitochondrial potassium channels control potassium influx into the mitochondrial matrix and thus regulate mitochondrial membrane potential, volume, respiration, and synthesis of reactive oxygen species (ROS). It has been found that pharmacological activation of mitochondrial potassium channels during ischemia/reperfusion (I/R) injury activates cytoprotective mechanisms resulting in increased cell survival. In cancer cells, the inhibition of these channels leads to increased cell death. Therefore, mitochondrial potassium channels are intriguing targets for the development of new pharmacological strategies. In most cases, however, the substances that modulate the mitochondrial potassium channels have a few alternative targets in the cell. This may result in unexpected or unwanted effects induced by these compounds. In our review, we briefly present the various classes of mitochondrial potassium (mitoK) channels and describe the chemical compounds that modulate their activity. We also describe examples of the multidirectional activity of the activators and inhibitors of mitochondrial potassium channels.

Kir Channel Molecular Physiology, Pharmacology, and Therapeutic Implications

For the past two decades several scholarly reviews have appeared on the inwardly rectifying potassium (Kir) channels. We would like to highlight two efforts in particular, which have provided comprehensive reviews of the literature up to 2010 (Hibino et al., Physiol Rev 90(1):291-366, 2010; Stanfield et al., Rev Physiol Biochem Pharmacol 145:47-179, 2002). In the past decade, great insights into the 3-D atomic resolution structures of Kir channels have begun to provide the molecular basis for their functional properties. More recently, computational studies are beginning to close the time domain gap between in silico dynamic and patch-clamp functional studies. The pharmacology of these channels has also been expanding and the dynamic structural studies provide hope that we are heading toward successful structure-based drug design for this family of K⁺ channels. In the present review we focus on placing the physiology and pharmacology of this K⁺ channel family in the context of atomic resolution structures and in providing a glimpse of the promising future of therapeutic opportunities.

Next-generation inward rectifier potassium channel modulators: discovery and molecular pharmacology

Inward rectifying potassium (Kir) channels play important roles in both excitable and nonexcitable cells of various organ systems and could represent valuable new drug targets for cardiovascular, metabolic, immune, and neurological diseases. In nonexcitable epithelial cells of the kidney tubule, for example, Kir1.1 (KCNJ1) and Kir4.1 (KCNJ10) are linked to sodium reabsorption in the thick ascending limb of Henle's loop and distal convoluted tubule, respectively, and have been explored as novel-mechanism diuretic targets for managing hypertension and edema. G protein-coupled Kir channels (Kir3) channels expressed in the central nervous system are critical effectors of numerous signal transduction pathways underlying analgesia, addiction, and respiratory-depressive effects of opioids. The historical dearth of pharmacological tool compounds for exploring the therapeutic potential of Kir channels has led to a molecular target-based approach using high-throughput screen (HTS) of small-molecule libraries and medicinal chemistry to develop "next-generation" Kir channel modulators that are both potent and specific for their targets. In this article, we review recent efforts focused specifically on discovery and improvement of target-selective molecular probes. The reader is introduced to fluorescence-based thallium flux assays that have enabled much of this work and then provided with an overview of progress made toward developing modulators of Kir1.1 (VU590, VU591), Kir2.x (ML133), Kir3.X (ML297, GAT1508, GiGA1, VU059331), Kir4.1 (VU0134992), and Kir7.1 (ML418). We discuss what is known about the small molecules' molecular mechanisms of action, in vitro and in vivo pharmacology, and then close with our view of what critical work remains to be done.

Solubilization, purification, and functional reconstitution of human ROMK potassium channel in copolymer styrene-maleic acid (SMA) nanodiscs

Expression, purification, and functional reconstitution of mammalian ion channels are often challenging. Heterologous expression of mammalian channels in bacteria can be advantageous due to unrelated protein environment and the lack of risk of copurification of endogenous proteins, e.g., accessory channel subunits that can influence the channel activity. Also, direct recording of channel activity could be challenging due to their intracellular localization like in the case of mitochondrial channels. The activity of purified channels can be characterized at the single-molecule level by electrophysiological techniques, such as planar lipid bilayers (PLB). In this work, we describe a simple approach to accomplish PLB recording of the activity of single renal outer medullary potassium channels ROMK expressed in E. coli. We focused on the ROMK2 isoform that is present at low levels in the mitochondria and can be responsible for mitoK_ATP activity. We screened for the best construct to express the codon-optimized ROMK proteins with a 6xHis tag for protein purification. The strategy involved the use of optimal styrene-maleic acid (SMA) copolymer, which forms so-called polymer nanodiscs, to solubilize and purify ROMK-containing SMA lipid particles (SMALPs), which were amenable for fusion with PLB. Reconstituted ROMK channels exhibited ion selectivity, rectification, and pharmacological properties, which are in agreement with previous work on ROMK channels.

Screening Technologies for Inward Rectifier Potassium Channels: Discovery of New Blockers and Activators

K⁺ channels play a critical role in maintaining the normal electrical activity of excitable cells by setting the cell resting membrane potential and by determining the shape and duration of the action potential. In nonexcitable cells, K⁺ channels establish electrochemical gradients necessary for maintaining salt and volume homeostasis of body fluids. Inward rectifier K⁺ (Kir) channels typically conduct larger inward currents than outward currents, resulting in an inwardly rectifying current versus voltage relationship. This property of inward rectification results from the voltage-dependent block of the channels by intracellular polyvalent cations and makes these channels uniquely designed for maintaining the resting potential near the K⁺ equilibrium potential (E_K). The Kir family of channels consist of seven subfamilies of channels (Kir1.x through Kir7.x) that include the classic inward rectifier (Kir2.x) channel, the G-protein-gated inward rectifier K⁺ (GIRK) (Kir3.x), and the adenosine triphosphate (ATP)-sensitive (K_ATP) (Kir 6.x) channels as well as the renal Kir1.1 (ROMK), Kir4.1, and Kir7.1 channels. These channels not only function to regulate electrical/electrolyte transport activity, but also serve as effector molecules for G-protein-coupled receptors (GPCRs) and as molecular sensors for cell metabolism. Of significance, Kir channels represent promising pharmacological targets for treating a number of clinical conditions, including cardiac arrhythmias, anxiety, chronic pain, and hypertension. This review provides a brief background on the structure, function, and pharmacology of Kir channels and then focuses on describing and evaluating current high-throughput screening (HTS) technologies, such as membrane potential-sensitive fluorescent dye assays, ion flux measurements, and automated patch clamp systems used for Kir channel drug discovery.

Global knockout of ROMK potassium channel worsens cardiac ischemia-reperfusion injury but cardiomyocyte-specific knockout does not: Implications for the identity of mitoKATP

The renal-outer-medullary‑potassium (ROMK) channel, mutated in Bartter's syndrome, regulates ion exchange in kidney, but its extra-renal functions remain unknown. Additionally, ROMK was postulated to be the pore-forming subunit of the mitochondrial ATP-sensitive K+ channel (mitoKATP), a mediator of cardioprotection. Using global and cardiomyocyte-specific knockout mice (ROMK-GKO and ROMK-CKO respectively), we characterize the effects of ROMK knockout on mitochondrial ion handling, the response to pharmacological KATP channel modulators, and ischemia/reperfusion (I/R) injury. Mitochondria from ROMK-GKO hearts exhibited a lower threshold for Ca2+-triggered permeability transition pore (mPTP) opening but normal matrix volume changes during oxidative phosphorylation. Isolated perfused ROMK-GKO hearts exhibited impaired functional recovery and increased infarct size when I/R was preceded by an ischemic preconditioning (IPC) protocol. Because ROMK-GKO mice exhibited severe renal defects and cardiac remodeling, we further characterized ROMK-CKO hearts to avoid confounding systemic effects. Mitochondria from ROMK-CKO hearts had unchanged matrix volume responses during oxidative phosphorylation and still swelled upon addition of a mitoKATP opener, but exhibited a lower threshold for mPTP opening, similar to GKO mitochondria. Nevertheless, I/R induced damage was not exacerbated in ROMK-CKO hearts, either ex vivo or in vivo. Lastly, we examined the response of ROMK-CKO hearts to ex vivo I/R injury with or without IPC and found that IPC still protected these hearts, suggesting that cardiomyocyte ROMK does not participate significantly in the cardioprotective pathway elicited by IPC. Collectively, our findings from these novel strains of mice suggest that cardiomyocyte ROMK is not a central mediator of mitoKATP function, although it can affect mPTP activation threshold.

Discovery of Small Molecule Renal Outer Medullary Potassium (ROMK) Channel Inhibitors: A Brief History of Medicinal Chemistry Approaches To Develop Novel Diuretic Therapeutics

The renal outer medullary potassium (ROMK) channel is a member of the inwardly rectifying family of potassium (Kir, Kir1.1) channels. It is primarily expressed in two regions of the kidney, the cortical collecting duct (CCD) and the thick ascending loop of Henle (TALH). At the CCD it tightly regulates potassium secretion while controlling potassium recycling in TALH. As loss-of-function mutations lead to salt wasting and low blood pressure, it has been surmised that inhibitors of ROMK would represent a target for new and improved diuretics for the treatment of hypertension and heart failure. In this review, we discuss and provide an overview of the medicinal chemistry approaches toward the development of small molecule ROMK inhibitors over the past decade.

Discovery and in Vitro Optimization of 3-Sulfamoylbenzamides as ROMK Inhibitors

Inhibitors of the renal outer medullary potassium channel (ROMK) show promise as novel mechanism diuretics, with potentially lower risk of diuretic-induced hypokalemia relative to current thiazide and loop diuretics. Here, we report the identification of a novel series of 3-sulfamoylbenzamide ROMK inhibitors. Starting from HTS hit 4, this series was optimized to provide ROMK inhibitors with good in vitro potencies and well-balanced ADME profiles. In contrast to previously reported small-molecule ROMK inhibitors, members of this series were demonstrated to be highly selective for inhibition of human over rat ROMK and to be insensitive to the N171D pore mutation that abolishes inhibitory activity of previously reported ROMK inhibitors.

The therapeutic potential of targeting the Kir1.1 (renal outer medullary K+) channel

K_ir1.1 (renal outer medullary K⁺) channels are potassium channels expressed almost exclusively in the kidney and play a role in the body's electrolyte and water balance. Potassium efflux through K_ir1.1 compliments the role of transporters and sodium channels that are the targets of known diuretics. Consequently, loss-of-function mutations in men and rodents are associated with salt wasting and low blood pressure. On this basis, K_ir1.1 inhibitors may have value in the treatment of hypertension and heart failure. Efforts to develop small molecule K_ir1.1 inhibitors produced MK-7145, which entered into clinical trials. The present manuscript describes the structure-activity relationships associated with this scaffold alongside other preclinical K_ir1.1 blockers.

Role of inward rectifier potassium channels in salivary gland function and sugar feeding of the fruit fly, Drosophila melanogaster

The arthropod salivary gland is of critical importance for horizontal transmission of pathogens, yet a detailed understanding of the ion conductance pathways responsible for saliva production and excretion is lacking. A superfamily of potassium ion channels, known as inward rectifying potassium (Kir) channels, is overexpressed in the Drosophila salivary gland by 32-fold when compared to the whole body mRNA transcripts. Therefore, we aimed to test the hypothesis that pharmacological and genetic depletion of salivary gland specific Kir channels alters the efficiency of the gland and reduced feeding capabilities using the fruit fly Drosophila melanogaster as a model organism that could predict similar effects in arthropod disease vectors. Exposure to VU041, a selective Kir channel blocker, reduced the volume of sucrose consumption by up to 3.2-fold and was found to be concentration-dependent with an EC₅₀ of 68μM. Importantly, the inactive analog, VU937, was shown to not influence feeding, suggesting the reduction in feeding observed with VU041 is due to Kir channel inhibition. Next, we performed a salivary gland specific knockdown of Kir1 to assess the role of these channels specifically in the salivary gland. The genetically depleted fruit flies had a reduction in total volume ingested and an increase in the time spent feeding, both suggestive of a reduction in salivary gland function. Furthermore, a compensatory mechanism appears to be present at day 1 of RNAi-treated fruit flies, and is likely to be the Na⁺-K⁺-2Cl^- cotransporter and/or Na⁺-K⁺-ATPase pumps that serve to supplement the inward flow of K⁺ ions, which highlights the functional redundancy in control of ion flux in the salivary glands. These findings suggest that Kir channels likely provide, at least in part, a principal potassium conductance pathway in the Drosophila salivary gland that is required for sucrose feeding.

Pore Polarity and Charge Determine Differential Block of Kir1.1 and Kir7.1 Potassium Channels by Small-Molecule Inhibitor VU590

VU590 was the first publicly disclosed, submicromolar-affinity (IC₅₀ = 0.2 μM), small-molecule inhibitor of the inward rectifier potassium (Kir) channel and diuretic target, Kir1.1. VU590 also inhibits Kir7.1 (IC₅₀ ∼ 8 μM), and has been used to reveal new roles for Kir7.1 in regulation of myometrial contractility and melanocortin signaling. Here, we employed molecular modeling, mutagenesis, and patch clamp electrophysiology to elucidate the molecular mechanisms underlying VU590 inhibition of Kir1.1 and Kir7.1. Block of both channels is voltage- and K⁺-dependent, suggesting the VU590 binding site is located within the pore. Mutagenesis analysis in Kir1.1 revealed that asparagine 171 (N171) is the only pore-lining residue required for high-affinity block, and that substituting negatively charged residues (N171D, N171E) at this position dramatically weakens block. In contrast, substituting a negatively charged residue at the equivalent position in Kir7.1 enhances block by VU590, suggesting the VU590 binding mode is different. Interestingly, mutations of threonine 153 (T153) in Kir7.1 that reduce constrained polarity at this site (T153C, T153V, T153S) make wild-type and binding-site mutants (E149Q, A150S) more sensitive to block by VU590. The Kir7.1-T153C mutation enhances block by the structurally unrelated inhibitor VU714 but not by a higher-affinity analog ML418, suggesting that the polar side chain of T153 creates a barrier to low-affinity ligands that interact with E149 and A150. Reverse mutations in Kir1.1 suggest that this mechanism is conserved in other Kir channels. This study reveals a previously unappreciated role of membrane pore polarity in determination of Kir channel inhibitor pharmacology.

Selective Small Molecule Activators of TREK-2 Channels Stimulate Dorsal Root Ganglion c-Fiber Nociceptor Two-Pore-Domain Potassium Channel Currents and Limit Calcium Influx

The two-pore-domain potassium (K2P) channel TREK-2 serves to modulate plasma membrane potential in dorsal root ganglia c-fiber nociceptors, which tunes electrical excitability and nociception. Thus, TREK-2 channels are considered a potential therapeutic target for treating pain; however, there are currently no selective pharmacological tools for TREK-2 channels. Here we report the identification of the first TREK-2 selective activators using a high-throughput fluorescence-based thallium (Tl+) flux screen (HTS). An initial pilot screen with a bioactive lipid library identified 11-deoxy prostaglandin F2α as a potent activator of TREK-2 channels (EC50 ≈ 0.294 μM), which was utilized to optimize the TREK-2 Tl+ flux assay (Z' = 0.752). A HTS was then performed with 76 575 structurally diverse small molecules. Many small molecules that selectively activate TREK-2 were discovered. As these molecules were able to activate single TREK-2 channels in excised membrane patches, they are likely direct TREK-2 activators. Furthermore, TREK-2 activators reduced primary dorsal root ganglion (DRG) c-fiber Ca2+ influx. Interestingly, some of the selective TREK-2 activators such as 11-deoxy prostaglandin F2α were found to inhibit the K2P channel TREK-1. Utilizing chimeric channels containing portions of TREK-1 and TREK-2, the region of the TREK channels that allows for either small molecule activation or inhibition was identified. This region lies within the second pore domain containing extracellular loop and is predicted to play an important role in modulating TREK channel activity. Moreover, the selective TREK-2 activators identified in this HTS provide important tools for assessing human TREK-2 channel function and investigating their therapeutic potential for treating chronic pain.

ML418: The First Selective, Sub-Micromolar Pore Blocker of Kir7.1 Potassium Channels

The inward rectifier potassium (Kir) channel Kir7.1 (KCNJ13) has recently emerged as a key regulator of melanocortin signaling in the brain, electrolyte homeostasis in the eye, and uterine muscle contractility during pregnancy. The pharmacological tools available for exploring the physiology and therapeutic potential of Kir7.1 have been limited to relatively weak and nonselective small-molecule inhibitors. Here, we report the discovery in a fluorescence-based high-throughput screen of a novel Kir7.1 channel inhibitor, VU714. Site-directed mutagenesis of pore-lining amino acid residues identified glutamate 149 and alanine 150 as essential determinants of VU714 activity. Lead optimization with medicinal chemistry generated ML418, which exhibits sub-micromolar activity (IC50 = 310 nM) and superior selectivity over other Kir channels (at least 17-fold selective over Kir1.1, Kir2.1, Kir2.2, Kir2.3, Kir3.1/3.2, and Kir4.1) except for Kir6.2/SUR1 (equally potent). Evaluation in the EuroFins Lead Profiling panel of 64 GPCRs, ion-channels, and transporters for off-target activity of ML418 revealed a relatively clean ancillary pharmacology. While ML418 exhibited low CLHEP in human microsomes which could be modulated with lipophilicity adjustments, it showed high CLHEP in rat microsomes regardless of lipophilicity. A subsequent in vivo PK study of ML418 by intraperitoneal (IP) administration (30 mg/kg dosage) revealed a suitable PK profile (Cmax = 0.20 μM and Tmax = 3 h) and favorable CNS distribution (mouse brain/plasma Kp of 10.9 to support in vivo studies. ML418, which represents the current state-of-the-art in Kir7.1 inhibitors, should be useful for exploring the physiology of Kir7.1 in vitro and in vivo.

Targeting renal epithelial channels for the control of insect vectors

Three small molecules were identified in high throughput screens that 1) block renal inward rectifier potassium (Kir) channels of Aedes aegypti expressed in HEK cells and Xenopus oocytes, 2) inhibit the secretion of KCl but not NaCl in isolated Malpighian tubules, and after injection into the hemolymph, 3) inhibit KCl excretion in vivo, and 4) render mosquitoes flightless or dead within 24h. Some mosquitoes had swollen abdomens at death consistent with renal failure. VU625, the most potent and promising small molecule for development as mosquitocide, inhibits AeKir1-mediated currents with an IC50 less than 100 nM. It is highly selective for AeKir1 over mammalian Kir channels, and it affects only 3 of 68 mammalian membrane proteins. These results document 1) renal failure as a new mode-of-action for mosquitocide development, 2) renal Kir channels as molecular target for inducing renal failure, and 3) the promise of the discovery and development of new species-specific insecticides.

Inhibitors of the renal outer medullary potassium channel: a patent review

INTRODUCTION

Hypertension represents a substantial cardiovascular risk factor. Among anti-hypertensive drugs, diuretics play an important role. Nevertheless, they present adverse effects such as hypokalemia or hyperkalemia. In this panorama, inhibitors of the renal outer medullary potassium (ROMK) channels are emerging because they are predicted to give a diuretic/natriuretic activity higher than that provided by loop diuretics, without hypokaliemic and hyperkaliemic side effects.

AREAS COVERED

This article reviews the current literature, including all the patents published in the field of inhibitors of the ROMK channels for the treatment of hypertension, heart failure and correlated diseases. The patent examination has been carried out using electronic databases Espacenet.

EXPERT OPINION

Although anti-hypertensive drugs armamentarium enumerates a plethora of therapeutic classes, including diuretics, the novel class of ROMK inhibitors may find a place in this crowded market, because of the diuretic/natriuretic effects, devoid of worrying influence on potassium balance. The patent examination highlights, as a strength, the individuation of a successful template: almost all the compounds show noteworthy potency. However, only few selected compounds underwent an in vivo investigation of diuretic and anti-hypertensive activities, and no data on the hERG channel are given in these patents.