A novel small molecule selective activator of homomeric GIRK4 channels

Structural determinants of the direct inhibition of GIRK channels by Sigma-1 receptor antagonist

G-protein-gated inward rectifier K+ (GIRK) channels play a critical role in the regulation of the excitability of cardiomyocytes and neurons and include GIRK1, GIRK2, GIRK3 and GIRK4 subfamily members. BD1047 dihydrobromide (BD1047) is one of the representative antagonists of the multifunctional Sigma-1 receptor (S1R). In the analysis of the effect of BD1047 on the regulation of Gi-coupled receptors by S1R using GIRK channel as an effector, we observed that BD1047, as well as BD1063, directly inhibited GIRK currents even in the absence of S1R and in a voltage-independent manner. Thus, we aimed to clarify the effect of BD1047 on GIRK channels and identify the structural determinants. By electrophysiological recordings in Xenopus oocytes, we observed that BD1047 directly inhibited GIRK channel currents, producing a much stronger inhibition of GIRK4 compared to GIRK2. It also inhibited ACh-induced native GIRK current in isolated rat atrial myocytes. Chimeric and mutagenesis studies of GIRK2 and GIRK4 combined with molecular docking analysis demonstrated the importance of Leu77 and Leu84 within the cytoplasmic, proximal N-terminal region and Glu147 within the pore-forming region of GIRK4 for inhibition by BD1047. The activator of GIRK channels, ivermectin, competed with BD1047 at Leu77 on GIRK4. This study provides us with a novel inhibitor of GIRK channels and information for developing pharmacological treatments for GIRK4-associated diseases.

Ivermectin Exerts Anticonvulsant Effects Against Status Epilepticus Induced by Lithium-Pilocarpine in Rats via GABAA Receptor and Neuroinflammation Modulation: Possible Interaction of Opioidergic Pathways and KATP Channel with Nitrergic System

Status epilepticus (SE) is a critical medical emergency marked by persistent or rapidly repeating seizures, posing a threat to life. Using the lithium-pilocarpine-induced SE model, we decide to evaluate the anti-seizure effects of ivermectin as a positive allosteric modulator of GABAA receptor and the underlying mechanisms involved. Lithium chloride was injected intraperitoneally at a dose of 127 mg/kg, followed by the administration of pilocarpine at a dose of 60 mg/kg after a 20-h interval in order to induce SE. Subsequently, the rats received varying amounts of ivermectin (0.3, 1, 3, 5, and 10 mg/kg, i.p.) 30 min before the onset of SE. To study the underlying molecular mechanisms, we had pharmacological interventions of diazepam (1 mg/kg), glibenclamide and nicorandil as ATP-sensitive potassium channel blocker and opener (both 1 mg/kg, i.p.), naltrexone and morphine, as opioid receptor antagonist and agonist (1 mg/kg and 0.5 mg/kg, i.p., respectively). In addition, three nitric oxide inhibitors, namely, L-NAME (10 mg/kg, i.p.), 7-NI (30 mg/kg, i.p.), and aminoguanidine (100 mg/kg, i.p.), were administered to the rats in the experiment. Finally, we use ELISA and western blotting, respectively, to examine the amounts of pro-inflammatory cytokines (TNF-α and IL-1β), nitrite, and GABAA receptors in the rat hippocampal tissue. The study found that ivermectin, at doses of 3, 5, and 10 mg/kg, exerts anti-seizure effects and decrease Racine's scale SE score. Interestingly glibenclamide and naltrexone reduced the anti-seizure effects of ivermectin, and from other hand diazepam, nicorandil, morphine, L-NAME, 7-NI, and aminoguanidine, enhance the effects when co-administrated with subeffective dose of ivermectin. Additionally, the study found that ivermectin decreased the elevated levels of TNF-α and IL-1β following SE, while increased the reduced expression of GABAA receptors. Overall, these findings suggest that ivermectin has anti-seizure effects in a SE seizure which may be mediated by the modulation of GABAergic, opioidergic, and nitrergic pathways and KATP channels.

Identification of Potential Modulators of a Pathogenic G Protein-Gated Inwardly Rectifying K+ Channel 4 Mutant: In Silico Investigation in the Context of Drug Discovery for Hypertension

Genetic abnormalities have been associated with primary aldosteronism, a major cause of secondary hypertension. This includes mutations in the KCNJ5 gene, which encodes G protein-gated inwardly rectifying K+ channel 4 (GIRK4). For example, the substitution of glycine with glutamic acid gives rise to the pathogenic GIRK4G151E mutation, which alters channel selectivity, making it more permeable to Na+ and Ca2+. While tertiapin and tertiapin-Q are well-known peptide inhibitors of the GIRK4WT channel, clinically, there is a need for the development of selective modulators of mutated channels, including GIRK4G151E. Using in silico methods, including homology modeling, protein-peptide docking, ligand-binding site prediction, and molecular docking, we aimed to explore potential modulators of GIRK4WT and GIRK4G151E. Firstly, protein-peptide docking was performed to characterize the binding site of tertiapin and its derivative to the GIRK4 channels. In accordance with previous studies, the peptide inhibitors preferentially bind to the GIRK4WT channel selectivity filter compared to GIRK4G151E. A ligand-binding site analysis was subsequently performed, resulting in the identification of two potential regions of interest: the central cavity and G-loop gate. Utilizing curated chemical libraries, we screened over 700 small molecules against the central cavity of the GIRK4 channels. Flavonoids, including luteolin-7-O-rutinoside and rutin, and the macrolides rapamycin and troleandomycin bound strongly to the GIRK4 channels. Similarly, xanthophylls, particularly luteoxanthin, bound to the central cavity with a strong preference towards the mutated GIRK4G151E channel compared to GIRK4WT. Overall, our findings suggest potential lead compounds for further investigation, particularly luteoxanthin, that may selectively modulate GIRK4 channels.

Ion channels regulate energy homeostasis and the progression of metabolic disorders: Novel mechanisms and pharmacology of their modulators

The progression of metabolic diseases, featured by dysregulated metabolic signaling pathways, is orchestrated by numerous signaling networks. Among the regulators, ion channels transport ions across the membranes and trigger downstream signaling transduction. They critically regulate energy homeostasis and pathogenesis of metabolic diseases and are potential therapeutic targets for treating metabolic disorders. Ion channel blockers have been used to treat diabetes for decades by stimulating insulin secretion, yet with hypoglycemia and other adverse effects. It calls for deeper understanding of the largely elusive regulatory mechanisms, which facilitates the identification of new therapeutic targets and safe drugs against ion channels. In the article, we critically assess the two principal regulatory mechanisms, protein-channel interaction and post-translational modification on the activities of ion channels to modulate energy homeostasis and metabolic disorders through multiple novel mechanisms. Moreover, we discuss the multidisciplinary methods that provide the tools for elucidation of the regulatory mechanisms mediating metabolic disorders by ion channels. In terms of translational perspective, the mechanistic analysis of recently validated ion channels that regulate insulin resistance, body weight control, and adverse effects of current ion channel antagonists are discussed in details. Their small molecule modulators serve as promising new drug candidates to combat metabolic disorders.

Lipopolysaccharide-induced sepsis impairs M2R-GIRK signaling in the mouse sinoatrial node

Sepsis has emerged as a global health burden associated with multiple organ dysfunction and 20% mortality rate in patients. Numerous clinical studies over the past two decades have correlated the disease severity and mortality in septic patients with impaired heart rate variability (HRV), as a consequence of impaired chronotropic response of sinoatrial node (SAN) pacemaker activity to vagal/parasympathetic stimulation. However, the molecular mechanism(s) downstream to parasympathetic inputs have not been investigated yet in sepsis, particularly in the SAN. Based on electrocardiography, fluorescence Ca2+ imaging, electrophysiology, and protein assays from organ to subcellular level, we report that impaired muscarinic receptor subtype 2-G protein-activated inwardly-rectifying potassium channel (M2R-GIRK) signaling in a lipopolysaccharide-induced proxy septic mouse model plays a critical role in SAN pacemaking and HRV. The parasympathetic responses to a muscarinic agonist, namely IKACh activation in SAN cells, reduction in Ca2+ mobilization of SAN tissues, lowering of heart rate and increase in HRV, were profoundly attenuated upon lipopolysaccharide-induced sepsis. These functional alterations manifested as a direct consequence of reduced expression of key ion-channel components (GIRK1, GIRK4, and M2R) in the mouse SAN tissues and cells, which was further evident in the human right atrial appendages of septic patients and likely not mediated by the common proinflammatory cytokines elevated in sepsis.

PI(4,5)P2 regulates the gating of NaV1.4 channels

Voltage-gated sodium (NaV) channels are densely expressed in most excitable cells and activate in response to depolarization, causing a rapid influx of Na+ ions that initiates the action potential. The voltage-dependent activation of NaV channels is followed almost instantaneously by fast inactivation, setting the refractory period of excitable tissues. The gating cycle of NaV channels is subject to tight regulation, with perturbations leading to a range of pathophysiological states. The gating properties of most ion channels are regulated by the membrane phospholipid, phosphatidylinositol (4,5) bisphosphate (PI(4,5)P2). However, it is not known whether PI(4,5)P2 modulates the activity of NaV channels. Here, we utilize optogenetics to activate specific, membrane-associated phosphoinositide (PI)-phosphatases that dephosphorylate PI(4,5)P2 while simultaneously recording NaV1.4 channel currents. We show that dephosphorylating PI(4,5)P2 left-shifts the voltage-dependent gating of NaV1.4 to more hyperpolarized membrane potentials, augments the late current that persists after fast inactivation, and speeds the rate at which channels recover from fast inactivation. These effects are opposed by exogenous diC8PI(4,5)P2. We provide evidence that PI(4,5)P2 is a negative regulator that tunes the gating behavior of NaV1.4 channels.

PI(4,5)P2 and Cholesterol: Synthesis, Regulation, and Functions

Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) is the most abundant membrane phosphoinositide and cholesterol is an essential component of the plasma membrane (PM). Both lipids play key roles in a variety of cellular functions including as signaling molecules and major regulators of protein function. This chapter provides an overview of these two important lipids. Starting from a brief description of their structure, synthesis, and regulation, the chapter continues to describe the primary functions and signaling processes in which PI(4,5)P₂ and cholesterol are involved. While PI(4,5)P₂ and cholesterol can act independently, they often act in concert or affect each other's impact. The chapters in this volume on "Cholesterol and PI(4,5)P₂ in Vital Biological Functions: From Coexistence to Crosstalk" focus on the emerging relationship between cholesterol and PI(4,5)P₂ in a variety of biological systems and processes. In this chapter, the next section provides examples from the ion channel field demonstrating that PI(4,5)P₂ and cholesterol can act via common mechanisms. The chapter ends with a discussion of future directions.

Anticonvulsant effects of ivermectin on pentylenetetrazole- and maximal electroshock-induced seizures in mice: the role of GABAergic system and KATP channels

Introduction

Ivermectin (IVM) is an antiparasitic medicine that exerts its function through glutamate-gated chloride channels and GABA_A receptors predominantly. There is paucity of information on anti-seizure activity of IVM. Moreover, the probable pharmacological mechanisms underlying this phenomenon have not been identified.

Materials and methods

In this study, pentylenetetrazole (PTZ)-induced clonic seizures and maximal electroshock (MES)-induced tonic-clonic seizure models, respectively in mice was utilized to inquire whether IVM could alter clonic seizure threshold (CST) and seizure susceptibility. To assess the underlying mechanism behind the anti-seizure activity of IVM, we used positive and negative allosteric modulators of GABA_A (diazepam and flumazenil, respectively) as well as K_ATP channel opener and closer (cromakalim and glibenclamide, respectively). Data are provided as mean ± S.E.M. After the performance of the variance homogeneity test, a one-way and two-way analysis of variance was used. Fisher's exact test was performed in case of MES. P-value less than 0.05 considered statistically significant.

Results

and Discussion: Our data showed that IVM (0.5, 1, 5, and 10 mg/kg, i.p.) increased CST. Furthermore, flumazenil 0.25 mg/kg, i.p. and glibenclamide 1 mg/kg, i.p., could inhibit the anticonvulsant effects of IVM. Supplementary, an ineffective dose of diazepam 0.02 mg/kg, i.p. or cromakalim 10 μg/kg, i.p. were able to enhance the anticonvulsant effects of IVM. Besides, we figure out that the IVM (1 and 5 mg/kg, i.p.) could delay the onset of first clonic seizure and also might decrease the frequency of clonic seizures induced by PTZ (85 mg/kg, i.p.). Finally, IVM could prevent the incidence and death in MES-induced tonic-clonic seizures.

Conclusion

Based on the obtained results, it can be concluded that IVM may exert anticonvulsant effects against PTZ- and MES-induced seizures in mice that might be mediated by GABA_A receptors and K_ATP channels.

•

Ivermectin exerts anticonvulsant effects on PTZ-induced clonic seizures.

•

Ivermectin prevents MES-induced tonic-clonic seizures in mice.

•

Ivermectin has the most anticonvulsant effects in doses of 1 and 5 mg/kg in mice.

•

These anticonvulsant effects may be mediated through the GABAergic system.

•

ATP-sensitive potassium channels could play a role in these anti-seizure effects.

Neuronal G protein-gated K+ channels

G protein-gated inwardly rectifying K⁺ (GIRK/Kir3) channels exert a critical inhibitory influence on neurons. Neuronal GIRK channels mediate the G protein-dependent, direct/postsynaptic inhibitory effect of many neurotransmitters including γ-aminobutyric acid (GABA), serotonin, dopamine, adenosine, somatostatin, and enkephalin. In addition to their complex regulation by G proteins, neuronal GIRK channel activity is sensitive to PIP₂, phosphorylation, regulator of G protein signaling (RGS) proteins, intracellular Na⁺ and Ca²⁺, and cholesterol. The application of genetic and viral manipulations in rodent models, together with recent progress in the development of GIRK channel modulators, has increased our understanding of the physiological and behavioral impact of neuronal GIRK channels. Work in rodent models has also revealed that neuronal GIRK channel activity is modified, transiently or persistently, by various stimuli including exposure drugs of abuse, changes in neuronal activity patterns, and aversive experience. A growing body of preclinical and clinical evidence suggests that dysregulation of GIRK channel activity contributes to neurological diseases and disorders. The primary goals of this review are to highlight fundamental principles of neuronal GIRK channel biology, mechanisms of GIRK channel regulation and plasticity, the nascent landscape of GIRK channel pharmacology, and the potential relevance of GIRK channels to the pathophysiology and treatment of neurological diseases and disorders.