Inhibiting the clathrin-mediated endocytosis pathway rescues KIR2.1 downregulation by pentamidine

The network of cardiac KIR2.1: its function, cellular regulation, electrical signaling, diseases and new drug avenues

The functioning of the human heart relies on complex electrical and communication systems that coordinate cardiac contractions and sustain rhythmicity. One of the key players contributing to this intricate system is the KIR2.1 potassium ion channel, which is encoded by the KCNJ2 gene. KIR2.1 channels exhibit abundant expression in both ventricular myocytes and Purkinje fibers, exerting an important role in maintaining the balance of intracellular potassium ion levels within the heart. And by stabilizing the resting membrane potential and contributing to action potential repolarization, these channels have an important role in cardiac excitability also. Either gain- or loss-of-function mutations, but also acquired impairments of their function, are implicated in the pathogenesis of diverse types of cardiac arrhythmias. In this review, we aim to elucidate the system functions of KIR2.1 channels related to cellular electrical signaling, communication, and their contributions to cardiovascular disease. Based on this knowledge, we will discuss existing and new pharmacological avenues to modulate their function.

Fungal antitumor protein D1 is internalized via endocytosis and inhibits non-small cell lung cancer proliferation through MAPK signaling pathway

Lung cancer has the highest mortality among cancer-related deaths worldwide. Among lung cancers, non-small cell lung cancer (NSCLC) is the most common histological type. In the previous research, we isolated a protein (D1) from Boletus bicolor that inhibits the proliferation of NSCLC cell lines. In this study, we elucidated the internalization mechanism and antitumor mechanism of protein D1 in A549 cells. Protein D1 has a strong inhibitory effect on A549 cells. It binds to secretory carrier membrane protein 3 on the A549 cell membrane and enters A549 cells by clathrin-mediated endocytosis. In vitro, protein D1 activates mitogen-activated protein kinase (MAPK) signaling pathway. JNK and p38MAPK are the biological targets for protein D1. In vivo, protein D1 inhibits the tumor growth of NSCLC xenografts by inducing apoptosis and inhibiting cell proliferation. Protein D1 alters the expression of genes related to apoptosis, cell cycle, and MAPK signaling pathway in tumor cells.

N-(3-Oxododecanoyl) Homoserine Lactone Is a Generalizable Plasma Membrane Lipid-Ordered Domain Modifier

A mammalian plasma membrane is a structure on which several layers of complexity are built. The first order of complexity comes from the heterogeneity of lipid-ordered domains. Gangliosides in concert with cholesterol are preferentially packed on the outer leaflet and form lipid-ordered domains, commonly known as lipid rafts. The formation and dynamics of these domains impact nearly all membrane protein functions and are an intensely studied topic. However, tools suited for lipid domain alteration are extremely limited. Currently, methyl-β-cyclodextrin (MβCD) appears to be the most common way to disrupt lipid domains, which is believed to operate via cholesterol extraction. This significantly limits our ability in membrane biophysics research. Previously, we found that N-(3-oxo-dodecanoyl) homoserine lactone (3oc), a small signaling chemical produced by Pseudomonas aeruginosa, is highly efficient in altering lipid-ordered domains. In this study, 3oc was compared with MβCD in a series of biochemical, biophysical, and cell biological analyses. Per molarity, 3oc is more efficient than MβCD in domain alteration and appears to better retain membrane lipids after treatment. This finding will provide an essential reagent in membrane biophysics research.

Quantitative Analysis of the Cytoskeleton's Role in Inward Rectifier K IR 2.1 Forward and Backward Trafficking

Alteration of the inward rectifier current I_K1, carried by K_IR2.1 channels, affects action potential duration, impacts resting membrane stability and associates with cardiac arrhythmias. Congenital and acquired K_IR2.1 malfunction frequently associates with aberrant ion channel trafficking. Cellular processes underlying trafficking are intertwined with cytoskeletal function. The extent to which the cytoskeleton is involved in K_IR2.1 trafficking processes is unknown. We aimed to quantify the dependence of K_IR2.1 trafficking on cytoskeleton function. GFP or photoconvertible Dendra2 tagged K_IR2.1 constructs were transfected in HEK293 or HeLa cells. Photoconversion of the Dendra2 probe at the plasma membrane and subsequent live imaging of trafficking processes was performed by confocal laser-scanning microscopy. Time constant of green fluorescent recovery (τg,s) represented recruitment of new K_IR2.1 at the plasma membrane. Red fluorescent decay (τr,s) represented internalization of photoconverted K_IR2.1. Patch clamp electrophysiology was used to quantify I_KIR2.₁. Biochemical methods were used for cytoskeleton isolation and detection of K_IR2.1-cytoskeleton interactions. Cytochalasin B (20 μM), Nocodazole (30 μM) and Dyngo-4a (10 nM) were used to modify the cytoskeleton. Chloroquine (10 μM, 24 h) was used to impair K_IR2.1 breakdown. Cytochalasin B and Nocodazole, inhibitors of actin and tubulin filament formation respectively, strongly inhibited the recovery of green fluorescence at the plasma membrane suggestive for inhibition of K_IR2.1 forward trafficking [τg,s 13 ± 2 vs. 131 ± 31* and 160 ± 40* min, for control, Cytochalasin B and Nocodazole, respectively (*p < 0.05 vs. control)]. Dyngo-4a, an inhibitor of dynamin motor proteins, strongly slowed the rate of photoconverted channel internalization, whereas Nocodazole and Cytochalasin B had less effect [τr,s 20 ± 2 vs. 87 ± 14*, 60 ± 16 and 64 ± 20 min (*p < 0.05 vs. control)]. Cytochalasin B treatment (20 μM, 24 h) inhibited I_KIR2.₁. Chloroquine treatment (10 μM, 24 h) induced intracellular aggregation of K_IR2.1 channels and enhanced interaction with the actin/intermediate filament system (103 ± 90 fold; p < 0.05 vs. control). Functional actin and tubulin cytoskeleton systems are essential for forward trafficking of K_IR2.1 channels, whereas initial backward trafficking relies on a functional dynamin system. Chronic disturbance of the actin system inhibits K_IR2.1 currents. Internalized K_IR2.1 channels become recruited to the cytoskeleton, presumably in lysosomes.

Syncytium cell growth increases Kir2.1 contribution in human iPSC-cardiomyocytes

Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) enable cardiotoxicity testing and personalized medicine. However, their maturity is of concern, including relatively depolarized resting membrane potential and more spontaneous activity compared with adult cardiomyocytes, implicating low or lacking inward rectifier potassium current (I_k1). Here, protein quantification confirms Kir2.1 expression in hiPSC-CM syncytia, albeit several times lower than in adult heart tissue. We find that hiPSC-CM culture density influences Kir2.1 expression at the mRNA level (potassium inwardly rectifying channel subfamily J member 2) and at the protein level and its associated electrophysiology phenotype. Namely, all-optical cardiac electrophysiology and pharmacological treatments reveal reduction of spontaneous and irregular activity and increase in action potential upstroke in denser cultures. Blocking I_k1-like currents with BaCl₂ increased spontaneous frequency and blunted action potential upstrokes during pacing in a dose-dependent manner only in the highest-density cultures, in line with I_k1’s role in regulating the resting membrane potential. Our results emphasize the importance of syncytial growth of hiPSC-CMs for more physiologically relevant phenotype and the power of all-optical electrophysiology to study cardiomyocytes in their multicellular setting.

NEW & NOTEWORTHY We identify cell culture density and cell-cell contact as an important factor in determining the expression of a key ion channel at the transcriptional and the protein levels, KCNJ2/Kir2.1, and its contribution to the electrophysiology of human induced pluripotent stem cell-derived cardiomyocytes. Our results indicate that studies on isolated cells, out of tissue context, may underestimate the cellular ion channel properties being characterized.

Cell-type-specific regulation of neuronal intrinsic excitability by macroautophagy

The basal ganglia are a group of subcortical nuclei that contribute to action selection and reinforcement learning. The principal neurons of the striatum, spiny projection neurons of the direct (dSPN) and indirect (iSPN) pathways, maintain low intrinsic excitability, requiring convergent excitatory inputs to fire. Here, we examined the role of autophagy in mouse SPN physiology and animal behavior by generating conditional knockouts of Atg7 in either dSPNs or iSPNs. Loss of autophagy in either SPN population led to changes in motor learning but distinct effects on cellular physiology. dSPNs, but not iSPNs, required autophagy for normal dendritic structure and synaptic input. In contrast, iSPNs, but not dSPNs, were intrinsically hyperexcitable due to reduced function of the inwardly rectifying potassium channel, Kir2. These findings define a novel mechanism by which autophagy regulates neuronal activity: control of intrinsic excitability via the regulation of potassium channel function.

Disease Associated Mutations in KIR Proteins Linked to Aberrant Inward Rectifier Channel Trafficking

The ubiquitously expressed family of inward rectifier potassium (K_IR) channels, encoded by KCNJ genes, is primarily involved in cell excitability and potassium homeostasis. Channel mutations associate with a variety of severe human diseases and syndromes, affecting many organ systems including the central and peripheral neural system, heart, kidney, pancreas, and skeletal muscle. A number of mutations associate with altered ion channel expression at the plasma membrane, which might result from defective channel trafficking. Trafficking involves cellular processes that transport ion channels to and from their place of function. By alignment of all K_IR channels, and depicting the trafficking associated mutations, three mutational hotspots were identified. One localized in the transmembrane-domain 1 and immediately adjacent sequences, one was found in the G-loop and Golgi-export domain, and the third one was detected at the immunoglobulin-like domain. Surprisingly, only few mutations were observed in experimentally determined Endoplasmic Reticulum (ER)exit-, export-, or ER-retention motifs. Structural mapping of the trafficking defect causing mutations provided a 3D framework, which indicates that trafficking deficient mutations form clusters. These “mutation clusters” affect trafficking by different mechanisms, including protein stability.

Altering integrin engagement regulates membrane localization of Kir2.1 channels

How ion channels localize and distribute on the cell membrane remains incompletely understood. We show that interventions that vary cell adhesion proteins and cell size also affect the membrane current density of inward-rectifier K⁺ channels (K_ir2.1; encoded by KCNJ2) and profoundly alter the action potential shape of excitable cells. By using micropatterning to manipulate the localization and size of focal adhesions (FAs) in single HEK293 cells engineered to stably express K_ir2.1 channels or in neonatal rat cardiomyocytes, we establish a robust linear correlation between FA coverage and the amplitude of K_ir2.1 current at both the local and whole-cell levels. Confocal microscopy showed that K_ir2.1 channels accumulate in membrane proximal to FAs. Selective pharmacological inhibition of key mediators of protein trafficking and the spatially dependent alterations in the dynamics of K_ir2.1 fluorescent recovery after photobleaching revealed that the K_ir2.1 channels are transported to the cell membrane uniformly, but are preferentially internalized by endocytosis at sites that are distal from FAs. Based on these results, we propose adhesion-regulated membrane localization of ion channels as a fundamental mechanism of controlling cellular electrophysiology via mechanochemical signals, independent of the direct ion channel mechanogating.

Identification of a PEST Sequence in Vertebrate KIR2.1 That Modifies Rectification

K_IR2.1 potassium channels, producing inward rectifier potassium current (I_K1), are important for final action potential repolarization and a stable resting membrane potential in excitable cells like cardiomyocytes. Abnormal K_IR2.1 function, either decreased or increased, associates with diseases such as Andersen-Tawil syndrome, long and short QT syndromes. K_IR2.1 ion channel protein trafficking and subcellular anchoring depends on intrinsic specific short amino acid sequences. We hypothesized that combining an evolutionary based sequence comparison and bioinformatics will identify new functional domains within the C-terminus of the K_IR2.1 protein, which function could be determined by mutation analysis. We determined PEST domain signatures, rich in proline (P), glutamic acid (E), serine (S), and threonine (T), within K_IR2.1 sequences using the “epestfind” webtool. WT and ΔPEST K_IR2.1 channels were expressed in HEK293T and COS-7 cells. Patch-clamp electrophysiology measurements were performed in the inside-out mode on excised membrane patches and the whole cell mode using AxonPatch 200B amplifiers. K_IR2.1 protein expression levels were determined by western blot analysis. Immunofluorescence microscopy was used to determine K_IR2.1 subcellular localization. An evolutionary conserved PEST domain was identified in the C-terminus of the K_IR2.1 channel protein displaying positive PEST scores in vertebrates ranging from fish to human. No similar PEST domain was detected in K_IR2.2, K_IR2.3, and K_IR2.6 proteins. Deletion of the PEST domain in California kingsnake and human K_IR2.1 proteins (ΔPEST), did not affect plasma membrane localization. Co-expression of WT and ΔPEST K_IR2.1 proteins resulted in heterotetrameric channel formation. Deletion of the PEST domain did not increase protein stability in cycloheximide assays [T½ from 2.64 h (WT) to 1.67 h (ΔPEST), n.s.]. WT and ΔPEST channels, either from human or snake, produced typical I_K1, however, human ΔPEST channels displayed stronger intrinsic rectification. The current observations suggest that the PEST sequence of K_IR2.1 is not associated with rapid protein degradation, and has a role in the rectification behavior of I_K1 channels.

Hypoxic stress upregulates Kir2.1 expression by a pathway including hypoxic-inducible factor-1α and dynamin2 in brain capillary endothelial cells

Brain capillary endothelial cells (BCECs) play a central role in maintenance of blood-brain barrier (BBB) function and, therefore, are essential for central nervous system homeostasis and integrity. Although brain ischemia damages BCECs and causes disruption of BBB, the related influence of hypoxia on BCECs is not well understood. Hypoxic stress can upregulate functional expression of specific K+ currents in endothelial cells, e.g., Kir2.1 channels without any alterations in the mRNA level, in t-BBEC117, a cell line derived from bovine BCECs. The hyperpolarization of membrane potential due to Kir2.1 channel upregulation significantly facilitates cell proliferation. In the present study, the mechanisms underlying the hypoxia-induced Kir2.1 upregulation was examined. We emphasize the involvement of dynamin2, a protein known to be involved in a number of surface expression pathways. Hypoxic culture upregulated dynamin2 expression in t-BBEC117 cells. The inhibition of dynamin2 by Dynasore canceled hypoxia-induced upregulation of Kir2.1 currents by reducing surface expression. On the contrary, Kir2.1 currents and proteins in t-BBEC117 cultured under normoxia were increased by overexpression of dynamin2, but not by dominant-negative dynamin2. Molecular imaging based on bimolecular fluorescence complementation, double-immunostaining, and coimmunoprecipitation assays revealed that dynamin2 can directly bind to the Kir2.1 channel. Moreover, hypoxic culture downregulated hypoxic-inducible factor-1α (HIF-1α) expression. Knockdown of HIF-1α increased dynamin2 expression in t-BBEC117 cells, in both normoxic and hypoxic culture conditions. In summary, our results demonstrated that hypoxia downregulates HIF-1α, increases dynamin2 expression, and facilitates Kir2.1 surface expression, resulting in hyperpolarization of membrane potential and subsequent increase in Ca2+ influx in BCECs.

The immature electrophysiological phenotype of iPSC-CMs still hampers in vitro drug screening: Special focus on IK1

Preclinical drug screens are not based on human physiology, possibly complicating predictions on cardiotoxicity. Drug screening can be humanised with in vitro assays using human induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs). However, in contrast to adult ventricular cardiomyocytes, iPSC-CMs beat spontaneously due to presence of the pacemaking current I_f and reduced densities of the hyperpolarising current I_K1. In adult cardiomyocytes, I_K1 finalises repolarisation by stabilising the resting membrane potential while also maintaining excitability. The reduced I_K1 density contributes to proarrhythmic traits in iPSC-CMs, which leads to an electrophysiological phenotype that might bias drug responses. The proarrhythmic traits can be suppressed by increasing I_K1 in a balanced manner. We systematically evaluated all studies that report strategies to mature iPSC-CMs and found that only few studies report I_K1 current densities. Furthermore, these studies did not succeed in establishing sufficient I_K1 levels as they either added too little or too much I_K1. We conclude that reduced densities of I_K1 remain a major flaw in iPSC-CMs, which hampers their use for in vitro drug screening.

PA-6 inhibits inward rectifier currents carried by V93I and D172N gain-of-function KIR2.1 channels, but increases channel protein expression

BACKGROUND

The inward rectifier potassium current IK1 contributes to a stable resting membrane potential and phase 3 repolarization of the cardiac action potential. KCNJ2 gain-of-function mutations V93I and D172N associate with increased IK1, short QT syndrome type 3 and congenital atrial fibrillation. Pentamidine-Analogue 6 (PA-6) is an efficient (IC50 = 14 nM with inside-out patch clamp methodology) and specific IK1 inhibitor that interacts with the cytoplasmic pore region of the KIR2.1 ion channel, encoded by KCNJ2. At 10 μM, PA-6 increases wild-type (WT) KIR2.1 expression in HEK293T cells upon chronic treatment. We hypothesized that PA-6 will interact with and inhibit V93I and D172N KIR2.1 channels, whereas impact on channel expression at the plasma membrane requires higher concentrations.

METHODS

Molecular modelling was performed with the human KIR2.1 closed state homology model using FlexX. WT and mutant KIR2.1 channels were expressed in HEK293 cells. Patch-clamp single cell electrophysiology measurements were performed in the whole cell and inside-out mode of the patch clamp method. KIR2.1 expression level and localization were determined by western blot analysis and immunofluorescence microscopy, respectively.

RESULTS

PA-6 docking in the V93I/D172N double mutant homology model of KIR2.1 demonstrated that mutations and drug-binding site are >30 Å apart. PA-6 inhibited WT and V93I outward currents with similar potency (IC50 = 35.5 and 43.6 nM at +50 mV for WT and V93I), whereas D172N currents were less sensitive (IC50 = 128.9 nM at +50 mV) using inside-out patch-clamp electrophysiology. In whole cell mode, 1 μM of PA-6 inhibited outward IK1 at -50 mV by 28 ± 36%, 18 ± 20% and 10 ± 6%, for WT, V93I and D172N channels respectively. Western blot analysis demonstrated that PA-6 (5 μM, 24 h) increased KIR2.1 expression levels of WT (6.3 ± 1.5 fold), and V93I (3.9 ± 0.9) and D172N (4.8 ± 2.0) mutants. Immunofluorescent microscopy demonstrated dose-dependent intracellular KIR2.1 accumulation following chronic PA-6 application (24 h, 1 and 5 μM).

CONCLUSIONS

1) KCNJ2 gain-of-function mutations V93I and D172N in the KIR2.1 ion channel do not impair PA-6 mediated inhibition of IK1, 2) PA-6 elevates KIR2.1 protein expression and induces intracellular KIR2.1 accumulation, 3) PA-6 is a strong candidate for further preclinical evaluation in treatment of congenital SQT3 and AF.

Class III antiarrhythmic drugs amiodarone and dronedarone impair KIR 2.1 backward trafficking

Drug‐induced ion channel trafficking disturbance can cause cardiac arrhythmias. The subcellular level at which drugs interfere in trafficking pathways is largely unknown. K_IR2.1 inward rectifier channels, largely responsible for the cardiac inward rectifier current (I_K1), are degraded in lysosomes. Amiodarone and dronedarone are class III antiarrhythmics. Chronic use of amiodarone, and to a lesser extent dronedarone, causes serious adverse effects to several organs and tissue types, including the heart. Both drugs have been described to interfere in the late‐endosome/lysosome system. Here we defined the potential interference in K_IR2.1 backward trafficking by amiodarone and dronedarone. Both drugs inhibited I_K1 in isolated rabbit ventricular cardiomyocytes at supraclinical doses only. In HK‐KWGF cells, both drugs dose‐ and time‐dependently increased K_IR2.1 expression (2.0 ± 0.2‐fold with amiodarone: 10 μM, 24 hrs; 2.3 ± 0.3‐fold with dronedarone: 5 μM, 24 hrs) and late‐endosomal/lysosomal K_IR2.1 accumulation. Increased K_IR2.1 expression level was also observed in the presence of Na_v1.5 co‐expression. Augmented K_IR2.1 protein levels and intracellular accumulation were also observed in COS‐7, END‐2, MES‐1 and EPI‐7 cells. Both drugs had no effect on K_v11.1 ion channel protein expression levels. Finally, amiodarone (73.3 ± 10.3% P < 0.05 at −120 mV, 5 μM) enhanced I_KIR2.1 upon 24‐hrs treatment, whereas dronedarone tended to increase I_KIR2.1 and it did not reach significance (43.8 ± 5.5%, P = 0.26 at −120 mV; 2 μM). We conclude that chronic amiodarone, and potentially also dronedarone, treatment can result in enhanced I_K1 by inhibiting K_IR2.1 degradation.

Structure-activity relationships of pentamidine-affected ion channel trafficking and dofetilide mediated rescue

BACKGROUND AND PURPOSE

Drug interference with normal hERG protein trafficking substantially reduces the channel density in the plasma membrane and thereby poses an arrhythmic threat. The chemical substructures important for hERG trafficking inhibition were investigated using pentamidine as a model drug. Furthermore, the relationship between acute ion channel block and correction of trafficking by dofetilide was studied.

EXPERIMENTAL APPROACH

hERG and K(IR)2.1 trafficking in HEK293 cells was evaluated by Western blot and immunofluorescence microscopy after treatment with pentamidine and six pentamidine analogues, and correction with dofetilide and four dofetilide analogues that displayed different abilities to inhibit IKr . Molecular dynamics simulations were used to address mode, number and type of interactions between hERG and dofetilide analogues.

KEY RESULTS

Structural modifications of pentamidine differentially affected plasma membrane levels of hERG and K(IR)2.1. Modification of the phenyl ring or substituents directly attached to it had the largest effect, affirming the importance of these chemical residues in ion channel binding. PA-4 had the mildest effects on both ion channels. Dofetilide corrected pentamidine-induced hERG, but not K(IR)2.1 trafficking defects. Dofetilide analogues that displayed high channel affinity, mediated by pi-pi stacks and hydrophobic interactions, also restored hERG protein levels, whereas analogues with low affinity were ineffective.

CONCLUSIONS AND IMPLICATIONS

Drug-induced trafficking defects can be minimized if certain chemical features are avoided or 'synthesized out'; this could influence the design and development of future drugs. Further analysis of such features in hERG trafficking correctors may facilitate the design of a non-blocking corrector for trafficking defective hERG proteins in both congenital and acquired LQTS.

Altered cortical GABAA receptor composition, physiology, and endocytosis in a mouse model of a human genetic absence epilepsy syndrome

Patients with generalized epilepsy exhibit cerebral cortical disinhibition. Likewise, mutations in the inhibitory ligand-gated ion channels, GABAA receptors (GABAARs), cause generalized epilepsy syndromes in humans. Recently, we demonstrated that heterozygous knock-out (Hetα1KO) of the human epilepsy gene, the GABAAR α1 subunit, produced absence epilepsy in mice. Here, we determined the effects of Hetα1KO on the expression and physiology of GABAARs in the mouse cortex. We found that Hetα1KO caused modest reductions in the total and surface expression of the β2 subunit but did not alter β1 or β3 subunit expression, results consistent with a small reduction of GABAARs. Cortices partially compensated for Hetα1KO by increasing the fraction of residual α1 subunit on the cell surface and by increasing total and surface expression of α3, but not α2, subunits. Co-immunoprecipitation experiments revealed that Hetα1KO increased the fraction of α1 subunits, and decreased the fraction of α3 subunits, that associated in hybrid α1α3βγ receptors. Patch clamp electrophysiology studies showed that Hetα1KO layer VI cortical neurons exhibited reduced inhibitory postsynaptic current peak amplitudes, prolonged current rise and decay times, and altered responses to benzodiazepine agonists. Finally, application of inhibitors of dynamin-mediated endocytosis revealed that Hetα1KO reduced base-line GABAAR endocytosis, an effect that probably contributes to the observed changes in GABAAR expression. These findings demonstrate that Hetα1KO exerts two principle disinhibitory effects on cortical GABAAR-mediated inhibitory neurotransmission: 1) a modest reduction of GABAAR number and 2) a partial compensation with GABAAR isoforms that possess physiological properties different from those of the otherwise predominant α1βγ GABAARs.