Identification of a G-Protein-Independent Activator of GIRK Channels

Hypnotic treatment improves sleep architecture and EEG disruptions and rescues memory deficits in a mouse model of fragile X syndrome

Fragile X syndrome (FXS) is associated with disrupted cognition and sleep abnormalities. Sleep loss negatively impacts cognitive function, and one untested possibility is that disrupted cognition in FXS is exacerbated by abnormal sleep. We tested whether ML297, a hypnotic acting on G-protein-activated inward-rectifying potassium (GIRK) channels, could reverse sleep phenotypes and disrupted memory in Fmr1-/y mice. Fmr1-/y mice exhibit reduced non-rapid eye movement (NREM) sleep and fragmented NREM architecture, altered sleep electroencephalogram (EEG) oscillations, and reduced EEG coherence between cortical areas; these are partially reversed following ML297 administration. Treatment following contextual fear or spatial learning restores disrupted memory consolidation in Fmr1-/y mice. During memory recall, Fmr1-/y mice show an altered balance of activity among hippocampal principal neurons vs. parvalbumin-expressing interneurons; this is partially reversed by ML297. Because sleep disruption could impact neurophysiological phenotypes in FXS, augmenting sleep may improve disrupted cognition in this disorder.

Direct modulation of G protein-gated inwardly rectifying potassium (GIRK) channels

Ion channels play a pivotal role in regulating cellular excitability and signal transduction processes. Among the various ion channels, G-protein-coupled inwardly rectifying potassium (GIRK) channels serve as key mediators of neurotransmission and cellular responses to extracellular signals. GIRK channels are members of the larger family of inwardly-rectifying potassium (Kir) channels. Typically, GIRK channels are activated via the direct binding of G-protein βγ subunits upon the activation of G-protein-coupled receptors (GPCRs). GIRK channel activation requires the presence of the lipid signaling molecule, phosphatidylinositol 4,5-bisphosphate (PIP2). GIRK channels are also modulated by endogenous proteins and other molecules, including RGS proteins, cholesterol, and SNX27 as well as exogenous compounds, such as alcohol. In the last decade or so, several groups have developed novel drugs and small molecules, such as ML297, GAT1508 and GiGA1, that activate GIRK channels in a G-protein independent manner. Here, we aim to provide a comprehensive overview focusing on the direct modulation of GIRK channels by G-proteins, PIP2, cholesterol, and novel modulatory compounds. These studies offer valuable insights into the underlying molecular mechanisms of channel function, and have potential implications for both basic research and therapeutic development.

Research hotspots and frontiers of alcohol and epilepsy: A bibliometric analysis

PURPOSE

Alcohol is implicated in epileptogenesis and seizures attack. An increasing number of studies about alcohol and epilepsy have been published. We aimed to assess research trends and hot spots in the field of alcohol and epilepsy.

PATIENTS AND METHODS

Literature concerning alcohol and epilepsy was systemically searched through the Web of Science database. Collaborative maps were quantitatively analyzed by using the VOSviewer and CiteSpace tools.

RESULTS

A total of 1578 papers about the field of alcohol and epilepsy were taken into analysis, which was written by 6840 authors from 2153 institutions in 85 countries, published in 676 journals, and cited 79 667 references from 10 750 journals. The United States was the leading country and had close ties with others. The University of Toronto was the most productive institution. Alcoholism-clinical and experimental research was the fastest-growing journal. Richard J. Bodnar was the author contributing the most literature. Analysis of keywords showed epilepsy, alcohol, seizures, alcohol withdrawal, and management were common themes.

CONCLUSION

The presented study conducted the first bibliometric analysis of the field of alcohol and epilepsy, which will provide insights into the latest progress, evolution paths, frontier research hot spots, and future research trends in the field.

Upregulated GIRK2 Counteracts Ethanol-Induced Changes in Excitability and Respiration in Human Neurons

Genome-wide association studies (GWAS) of electroencephalographic endophenotypes for alcohol use disorder (AUD) has identified noncoding polymorphisms within the KCNJ6 gene. KCNJ6 encodes GIRK2, a subunit of a G-protein-coupled inwardly rectifying potassium channel that regulates neuronal excitability. We studied the effect of upregulating KCNJ6 using an isogenic approach with human glutamatergic neurons derived from induced pluripotent stem cells (male and female donors). Using multielectrode arrays, population calcium imaging, single-cell patch-clamp electrophysiology, and mitochondrial stress tests, we find that elevated GIRK2 acts in concert with 7-21 d of ethanol exposure to inhibit neuronal activity, to counteract ethanol-induced increases in glutamate response, and to promote an increase intrinsic excitability. Furthermore, elevated GIRK2 prevented ethanol-induced changes in basal and activity-dependent mitochondrial respiration. These data support a role for GIRK2 in mitigating the effects of ethanol and a previously unknown connection to mitochondrial function in human glutamatergic neurons.

KCNJ3 is a novel candidate gene for autosomal dominant pure hereditary spastic paraplegia identified using whole genome sequencing

Hereditary spastic paraplegia (HSP) is a group of familial diseases characterized by progressive corticospinal tract degeneration. Clinically, patients present with lower-limb spasticity and weakness. To date, more than 80 genetic HSP types have been identified. Despite advances in molecular genetics, novel HSP gene discoveries are ongoing, with a low genetic diagnostic yield. In this study, we aimed to determine pathogenic variants in a family with HSP, which was not diagnosed through conventional genetic testing. We clinically characterized a large family and conducted whole genome sequencing (WGS) analysis of four affected and three unaffected individuals in the family to identify the genetic cause of HSP. This family had autosomal dominant pure (uncomplicated) late childhood-onset HSP. The patients' symptoms accelerated between the ages of 20 and 30. Brain magnetic resonance images typically showed white matter changes, a thin corpus callosum, and cerebellar atrophy. We identified a heterozygous missense variant, KCNJ3 c.1297T>G (p.Leu433Val), through WGS and family genetic analysis, confirmed by Sanger sequencing. We suggest that the identification of KCNJ3 c.1297T>G (p.Leu433Val) constitutes the discovery of a potential novel gene responsible for HSP in this family. This is the first study to report the possible role of a KCNJ3 variant in HSP pathogenesis. Our findings further expand the phenotypic and genotypic spectrum of HSP.

De novo variants in KCNJ3 are associated with early-onset epilepsy

BACKGROUND

KCNJ3 encodes a subunit of G-protein-coupled inwardly rectifying potassium channels, which are important for cellular excitability and inhibitory neurotransmission. However, the genetic basis of KCNJ3 in epilepsy has not been determined. This study aimed to identify the pathogenic KCNJ3 variants in patients with epilepsy.

METHODS

Trio exome sequencing was performed to determine potential variants of epilepsy. Individuals with KCNJ3 variants were recruited for this study. Detailed clinical information and genetic data were obtained and systematically reviewed. Whole-cell patch-clamp recordings were performed to evaluate the functional consequences of the identified variants.

RESULTS

Two de novo missense variants (c.998T>C (p.Leu333Ser) and c.938G>A (p. Arg313Gln)) in KCNJ3 were identified in two unrelated families with epilepsy. The variants were absent from the gnomAD database and were assumed to be damaging or probably damaging using multiple bioinformatics tools. They were both located in the C-terminal domain. The amino acid residues were highly conserved among various species. Clinically, the seizures occurred at a young age and were under control after combined treatment. Electrophysiological analysis revealed that the KCNJ3 Leu333Ser and Arg313Gln variants significantly compromised the current activities and exhibited loss-of-function (LOF) effects.

CONCLUSION

Our findings suggest that de novo LOF variants in KCNJ3 are associated with early-onset epilepsy. Genetic testing of KCNJ3 in patients with epilepsy may serve as a strategy for precision medicine.

Upregulated GIRK2 counteracts ethanol-induced changes in excitability & respiration in human neurons

Genome-wide association analysis (GWAS) of electroencephalographic endophenotypes for alcohol use disorder (AUD) has identified non-coding polymorphisms within the KCNJ6 gene. KCNJ6 encodes GIRK2, a subunit of a G protein-coupled inwardly-rectifying potassium channel that regulates neuronal excitability. How changes in GIRK2 affect human neuronal excitability and the response to repeated ethanol exposure is poorly understood. Here, we studied the effect of upregulating KCNJ6 using an isogenic approach with human glutamatergic neurons derived from induced pluripotent stem cells (male and female donors). Using multi-electrode-arrays, population calcium imaging, single-cell patch-clamp electrophysiology, and mitochondrial stress tests, we find that elevated GIRK2 acts in concert with 7-21 days of ethanol exposure to inhibit neuronal activity, to counteract ethanol-induced increases in glutamate response, and to promote an increase intrinsic excitability. Furthermore, elevated GIRK2 prevented ethanol-dependent changes in basal and activity-dependent mitochondrial respiration. These data support a role for GIRK2 in mitigating the effects of ethanol and a previously unknown connection to mitochondrial function in human glutamatergic neurons.

SIGNIFICANCE STATEMENT

Alcohol use disorder (AUD) is a major health problem that has worsened since COVID, affecting over 100 million people worldwide. While it is known that heritability contributes to AUD, specific genes and their role in neuronal function remain poorly understood, especially in humans. In the current manuscript, we focused on the inwardly-rectifying potassium channel GIRK2, which has been identified in an AUD-endophenotype genome-wide association study. We used human excitatory neurons derived from healthy donors to study the impact of GIRK2 expression. Our results reveal that elevated GIRK2 counteracts ethanol-induced increases in glutamate response and intracellular calcium, as well as deficits in activity-dependent mitochondrial respiration. The role of GIRK2 in mitigating ethanol-induced hyper-glutamatergic and mitochondrial offers therapeutic promise for treating AUD.

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.

The Relevance of GIRK Channels in Heart Function

Among the large number of potassium-channel families implicated in the control of neuronal excitability, G-protein-gated inwardly rectifying potassium channels (GIRK/Kir3) have been found to be a main factor in heart control. These channels are activated following the modulation of G-protein-coupled receptors and, although they have been implicated in different neurological diseases in both human and animal studies of the central nervous system, the therapeutic potential of different subtypes of these channel families in cardiac conditions has remained untapped. As they have emerged as a promising potential tool to treat a variety of conditions that disrupt neuronal homeostasis, many studies have started to focus on these channels as mediators of cardiac dynamics, thus leading to research into their implication in cardiovascular conditions. Our aim is to review the latest advances in GIRK modulation in the heart and their role in the cardiovascular system.

Experimental and Clinical Biomarkers for Progressive Evaluation of Neuropathology and Therapeutic Interventions for Acute and Chronic Neurological Disorders

This article describes commonly used experimental and clinical biomarkers of neuronal injury and neurodegeneration for the evaluation of neuropathology and monitoring of therapeutic interventions. Biomarkers are vital for diagnostics of brain disease and therapeutic monitoring. A biomarker can be objectively measured and evaluated as a proxy indicator for the pathophysiological process or response to therapeutic interventions. There are complex hurdles in understanding the molecular pathophysiology of neurological disorders and the ability to diagnose them at initial stages. Novel biomarkers for neurological diseases may surpass these issues, especially for early identification of disease risk. Validated biomarkers can measure the severity and progression of both acute neuronal injury and chronic neurological diseases such as epilepsy, migraine, Alzheimer's disease, Parkinson's disease, Huntington's disease, traumatic brain injury, amyotrophic lateral sclerosis, multiple sclerosis, and other brain diseases. Biomarkers are deployed to study progression and response to treatment, including noninvasive imaging tools for both acute and chronic brain conditions. Neuronal biomarkers are classified into four core subtypes: blood-based, immunohistochemical-based, neuroimaging-based, and electrophysiological biomarkers. Neuronal conditions have progressive stages, such as acute injury, inflammation, neurodegeneration, and neurogenesis, which can serve as indices of pathological status. Biomarkers are critical for the targeted identification of specific molecules, cells, tissues, or proteins that dramatically alter throughout the progression of brain conditions. There has been tremendous progress with biomarkers in acute conditions and chronic diseases affecting the central nervous system.

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.

A novel small molecule selective activator of homomeric GIRK4 channels

G protein–sensitive inwardly rectifying potassium (GIRK) channels are important pharmaceutical targets for neuronal, cardiac, and endocrine diseases. Although a number of GIRK channel modulators have been discovered in recent years, most lack selectivity. GIRK channels function as either homomeric (i.e., GIRK2 and GIRK4) or heteromeric (e.g., GIRK1/2, GIRK1/4, and GIRK2/3) tetramers. Activators, such as ML297, ivermectin, and GAT1508, have been shown to activate heteromeric GIRK1/2 channels better than GIRK1/4 channels with varying degrees of selectivity but not homomeric GIRK2 and GIRK4 channels. In addition, VU0529331 was discovered as the first homomeric GIRK channel activator, but it shows weak selectivity for GIRK2 over GIRK4 (or G4) homomeric channels. Here, we report the first highly selective small-molecule activator targeting GIRK4 homomeric channels, 3hi2one-G4 (3-[2-(3,4-dimethoxyphenyl)-2-oxoethyl]-3-hydroxy-1-(1-naphthylmethyl)-1,3-dihydro-2H-indol-2-one). We show that 3hi2one-G4 does not activate GIRK2, GIRK1/2, or GIRK1/4 channels. Using molecular modeling, mutagenesis, and electrophysiology, we analyzed the binding site of 3hi2one-G4 formed by the transmembrane 1, transmembrane 2, and slide helix regions of the GIRK4 channel, near the phosphatidylinositol-4,5-bisphosphate binding site, and show that it causes channel activation by strengthening channel–phosphatidylinositol-4,5-bisphosphate interactions. We also identify slide helix residue L77 in GIRK4, corresponding to residue I82 in GIRK2, as a major determinant of isoform-specific selectivity. We propose that 3hi2one-G4 could serve as a useful pharmaceutical probe in studying GIRK4 channel function and may also be pursued in drug optimization studies to tackle GIRK4-related diseases such as primary aldosteronism and late-onset obesity.

AsKC11, a Kunitz Peptide from Anemonia sulcata, Is a Novel Activator of G Protein-Coupled Inward-Rectifier Potassium Channels

(1) Background: G protein-coupled inward-rectifier potassium (GIRK) channels, especially neuronal GIRK1/2 channels, have been the focus of intense research interest for developing drugs against brain diseases. In this context, venom peptides that selectively activate GIRK channels can be seen as a new source for drug development. Here, we report on the identification and electrophysiological characterization of a novel activator of GIRK1/2 channels, AsKC11, found in the venom of the sea anemone Anemonia sulcata. (2) Methods: AsKC11 was purified from the sea anemone venom by reverse-phase chromatography and the sequence was identified by mass spectrometry. Using the two-electrode voltage-clamp technique, the activity of AsKC11 on GIRK1/2 channels was studied and its selectivity for other potassium channels was investigated. (3) Results: AsKC11, a Kunitz peptide found in the venom of A. sulcata, is the first peptide shown to directly activate neuronal GIRK1/2 channels independent from Gi/o protein activity, without affecting the inward-rectifier potassium channel (IRK1) and with only a minor effect on K_V1.6 channels. Thus, AsKC11 is a novel activator of GIRK channels resulting in larger K⁺ currents because of an increased chord conductance. (4) Conclusions: These discoveries provide new insights into a novel class of GIRK activators.

Envisioning the role of inwardly rectifying potassium (Kir) channel in epilepsy

Epilepsy is a devastating neurological disorder characterized by recurrent seizures attributed to the disruption of the dynamic excitatory and inhibitory balance in the brain. Epilepsy has emerged as a global health concern affecting about 70 million people worldwide. Despite recent advances in pre-clinical and clinical research, its etiopathogenesis remains obscure, and there are still no treatment strategies modifying disease progression. Although the precise molecular mechanisms underlying epileptogenesis have not been clarified yet, the role of ion channels as regulators of cellular excitability has increasingly gained attention. In this regard, emerging evidence highlights the potential implication of inwardly rectifying potassium (Kir) channels in epileptogenesis. Kir channels consist of seven different subfamilies (Kir1-Kir7), and they are highly expressed in both neuronal and glial cells in the central nervous system. These channels control the cell volume and excitability. In this review, we discuss preclinical and clinical evidence on the role of the several subfamilies of Kir channels in epileptogenesis, aiming to shed more light on the pathogenesis of this disorder and pave the way for future novel therapeutic approaches.

Encephalopathy-causing mutations in Gβ1 (GNB1) alter regulation of neuronal GIRK channels

Mutations in the GNB1 gene, encoding the Gβ₁ subunit of heterotrimeric G proteins, cause GNB1 Encephalopathy. Patients experience seizures, pointing to abnormal activity of ion channels or neurotransmitter receptors. We studied three Gβ₁ mutations (K78R, I80N and I80T) using computational and functional approaches. In heterologous expression models, these mutations did not alter the coupling between G protein-coupled receptors to G_i/o, or the Gβγ regulation of the neuronal voltage-gated Ca²⁺ channel Ca_V2.2. However, the mutations profoundly affected the Gβγ regulation of the G protein-gated inwardly rectifying potassium channels (GIRK, or Kir3). Changes were observed in Gβ₁ protein expression levels, Gβγ binding to cytosolic segments of GIRK subunits, and in Gβγ function, and included gain-of-function for K78R or loss-of-function for I80T/N, which were GIRK subunit-specific. Our findings offer new insights into subunit-dependent gating of GIRKs by Gβγ, and indicate diverse etiology of GNB1 Encephalopathy cases, bearing a potential for personalized treatment.

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GIRK channels are key players affected by GNB1 mutations under study (K78R and I80N/T)

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Effects of mutations (LoF or GoF) are channel subunit composition-specific

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The findings help to understand the GNB1 encephalopathy and to devise treatments

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The results yield new insights into mechanisms of Gβγ regulation of GIRKs

Characterization of VU0468554, a New Selective Inhibitor of Cardiac G Protein-Gated Inwardly Rectifying K+ Channels

G protein-gated inwardly rectifying K⁺ (GIRK) channels are critical mediators of excitability in the heart and brain. Enhanced GIRK-channel activity has been implicated in the pathogenesis of supraventricular arrhythmias, including atrial fibrillation. The lack of selective pharmacological tools has impeded efforts to investigate the therapeutic potential of cardiac GIRK-channel interventions in arrhythmias. Here, we characterize a recently identified GIRK-channel inhibitor, VU0468554. Using whole-cell electrophysiological approaches and primary cultures of sinoatrial nodal cells and hippocampal neurons, we show that VU0468554 more effectively inhibits the cardiac GIRK channel than the neuronal GIRK channel. Concentration-response experiments suggest that VU0468554 inhibits Gβγ-activated GIRK channels in noncompetitive and potentially uncompetitive fashion. In contrast, VU0468554 competitively inhibits GIRK-channel activation by ML297, a GIRK-channel activator containing the same chemical scaffold as VU0468554. In the isolated heart model, VU0468554 partially reversed carbachol-induced bradycardia in hearts from wild-type mice but not Girk4^-/- mice. Collectively, these data suggest that VU0468554 represents a promising new pharmacological tool for targeting cardiac GIRK channels with therapeutic implications for relevant cardiac arrhythmias. SIGNIFICANCE STATEMENT: Although cardiac GIRK-channel inhibition shows promise for the treatment of supraventricular arrhythmias, the absence of subtype-selective channel inhibitors has hindered exploration into this therapeutic strategy. This study utilizes whole-cell patch-clamp electrophysiology to characterize the new GIRK-channel inhibitor VU0468554 in human embryonic kidney 293T cells and primary cultures. We report that VU0468554 exhibits a favorable pharmacodynamic profile for cardiac over neuronal GIRK channels and partially reverses GIRK-mediated bradycardia in the isolated mouse heart model.

Gaussian accelerated molecular dynamics (GaMD): principles and applications

Gaussian accelerated molecular dynamics (GaMD) is a robust computational method for simultaneous unconstrained enhanced sampling and free energy calculations of biomolecules. It works by adding a harmonic boost potential to smooth biomolecular potential energy surface and reduce energy barriers. GaMD greatly accelerates biomolecular simulations by orders of magnitude. Without the need to set predefined reaction coordinates or collective variables, GaMD provides unconstrained enhanced sampling and is advantageous for simulating complex biological processes. The GaMD boost potential exhibits a Gaussian distribution, thereby allowing for energetic reweighting via cumulant expansion to the second order (i.e., "Gaussian approximation"). This leads to accurate reconstruction of free energy landscapes of biomolecules. Hybrid schemes with other enhanced sampling methods, such as the replica exchange GaMD (rex-GaMD) and replica exchange umbrella sampling GaMD (GaREUS), have also been introduced, further improving sampling and free energy calculations. Recently, new "selective GaMD" algorithms including the ligand GaMD (LiGaMD) and peptide GaMD (Pep-GaMD) enabled microsecond simulations to capture repetitive dissociation and binding of small-molecule ligands and highly flexible peptides. The simulations then allowed highly efficient quantitative characterization of the ligand/peptide binding thermodynamics and kinetics. Taken together, GaMD and its innovative variants are applicable to simulate a wide variety of biomolecular dynamics, including protein folding, conformational changes and allostery, ligand binding, peptide binding, protein-protein/nucleic acid/carbohydrate interactions, and carbohydrate/nucleic acid interactions. In this review, we present principles of the GaMD algorithms and recent applications in biomolecular simulations and drug design.

Structural insights into GIRK2 channel modulation by cholesterol and PIP2

G-protein-gated inwardly rectifying potassium (GIRK) channels are important for determining neuronal excitability. In addition to G proteins, GIRK channels are potentiated by membrane cholesterol, which is elevated in the brains of people with neurodegenerative diseases such as Alzheimer’s dementia and Parkinson’s disease. The structural mechanism of cholesterol modulation of GIRK channels is not well understood. In this study, we present cryo-electron microscopy (cryoEM) structures of GIRK2 in the presence and absence of the cholesterol analog cholesteryl hemisuccinate (CHS) and phosphatidylinositol 4,5-bisphosphate (PIP₂). The structures reveal that CHS binds near PIP₂ in lipid-facing hydrophobic pockets of the transmembrane domain. Our structural analysis suggests that CHS stabilizes PIP₂ interaction with the channel and promotes engagement of the cytoplasmic domain onto the transmembrane region. Mutagenesis of one of the CHS binding pockets eliminates cholesterol-dependent potentiation of GIRK2. Elucidating the structural mechanisms underlying cholesterol modulation of GIRK2 channels could facilitate the development of therapeutics for treating neurological diseases.

Ion channels are important in determining neuronal excitability. Elevated cholesterol levels found in some neurodegenerative diseases can affect the function of ion channels. Mathiharan et al. take a structural and functional approach to identifying physical sites for cholesterol, and they provide details on how cholesterol potentiates ion channel activity.

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.

Comparison of K+ Channel Families

K⁺ channels enable potassium to flow across the membrane with great selectivity. There are four K⁺ channel families: voltage-gated K (K_v), calcium-activated (K_Ca), inwardly rectifying K (K_ir), and two-pore domain potassium (K_2P) channels. All four K⁺ channels are formed by subunits assembling into a classic tetrameric (4x1P = 4P for the K_v, K_Ca, and K_ir channels) or tetramer-like (2x2P = 4P for the K_2P channels) architecture. These subunits can either be the same (homomers) or different (heteromers), conferring great diversity to these channels. They share a highly conserved selectivity filter within the pore but show different gating mechanisms adapted for their function. K⁺ channels play essential roles in controlling neuronal excitability by shaping action potentials, influencing the resting membrane potential, and responding to diverse physicochemical stimuli, such as a voltage change (K_v), intracellular calcium oscillations (K_Ca), cellular mediators (K_ir), or temperature (K_2P).

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.

Therapeutic potential of targeting G protein-gated inwardly rectifying potassium (GIRK) channels in the central nervous system

G protein-gated inwardly rectifying potassium channels (Kir3/GirK) are important for maintaining resting membrane potential, cell excitability and inhibitory neurotransmission. Coupled to numerous G protein-coupled receptors (GPCRs), they mediate the effects of many neurotransmitters, neuromodulators and hormones contributing to the general homeostasis and particular synaptic plasticity processes, learning, memory and pain signaling. A growing number of behavioral and genetic studies suggest a critical role for the appropriate functioning of the central nervous system, as well as their involvement in many neurologic and psychiatric conditions, such as neurodegenerative diseases, mood disorders, attention deficit hyperactivity disorder, schizophrenia, epilepsy, alcoholism and drug addiction. Hence, GirK channels emerge as a very promising tool to be targeted in the current scenario where these conditions already are or will become a global public health problem. This review examines recent findings on the physiology, function, dysfunction, and pharmacology of GirK channels in the central nervous system and highlights the relevance of GirK channels as a worthful potential target to improve therapies for related diseases.

Advances in Targeting GIRK Channels in Disease

G protein-gated inwardly rectifying potassium (GIRK) channels are essential regulators of cell excitability in the brain. While they are implicated in a variety of neurological diseases in both human and animal model studies, their therapeutic potential has been largely untapped. Here, we review recent advances in the development of small molecule compounds that specifically modulate GIRK channels and compare them with first-generation compounds that exhibit off-target activity. We describe the method of discovery of these small molecule modulators, their chemical features, and their effects in vivo. These studies provide a promising outlook on the future development of subunit-specific GIRK modulators to regulate neuronal excitability in a brain region-specific manner.

VU0606170, a Selective Slack Channels Inhibitor, Decreases Calcium Oscillations in Cultured Cortical Neurons

Malignant migrating partial seizures of infancy is a rare, devastating form of epilepsy most commonly associated with gain-of-function mutations in the potassium channel, Slack. Not only is this condition almost completely pharmacoresistant, there are not even selective drug-like tools available to evaluate whether inhibition of these overactivated, mutant Slack channels may represent a viable path forward toward new antiepileptic therapies. Therefore, we used a high-throughput thallium flux assay to screen a drug-like, 100 000-compound library in search of inhibitors of both wild-type and a disease-associated mutant Slack channel. Using this approach, we discovered VU0606170, a selective Slack channel inhibitor with low micromolar potency. Critically, VU0606170 also proved effective at significantly decreasing the firing rate in overexcited, spontaneously firing cortical neuron cultures. Taken together, our data provide compelling evidence that selective inhibition of Slack channel activity can be achieved with small molecules and that inhibition of Slack channel activity in neurons produces efficacy consistent with an antiepileptic effect. Thus, the identification of VU0606170 provides a much-needed tool for advancing our understanding of the role of the Slack channel in normal physiology and disease as well as its potential as a target for therapeutic intervention.