Identification of native atrial G-protein-regulated inwardly rectifying K+ (GIRK4) channel homomultimers

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.

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.

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.

Genetic Complexity of Sinoatrial Node Dysfunction

The pacemaker cells of the cardiac sinoatrial node (SAN) are essential for normal cardiac automaticity. Dysfunction in cardiac pacemaking results in human sinoatrial node dysfunction (SND). SND more generally occurs in the elderly population and is associated with impaired pacemaker function causing abnormal heart rhythm. Individuals with SND have a variety of symptoms including sinus bradycardia, sinus arrest, SAN block, bradycardia/tachycardia syndrome, and syncope. Importantly, individuals with SND report chronotropic incompetence in response to stress and/or exercise. SND may be genetic or secondary to systemic or cardiovascular conditions. Current management of patients with SND is limited to the relief of arrhythmia symptoms and pacemaker implantation if indicated. Lack of effective therapeutic measures that target the underlying causes of SND renders management of these patients challenging due to its progressive nature and has highlighted a critical need to improve our understanding of its underlying mechanistic basis of SND. This review focuses on current information on the genetics underlying SND, followed by future implications of this knowledge in the management of individuals with SND.

Muscarinic agonists inhibit the ATP-dependent potassium current and suppress the ventricle-Purkinje action potential dispersion

Activation of the parasympathetic nervous system has been reported to have an antiarrhythmic role during ischemia-reperfusion injury by decreasing the arrhythmia triggers. Furthermore, it was reported that the parasympathetic neurotransmitter acetylcholine is able to modulate the ATP-dependent potassium current (I K-ATP), a crucial current activated during hypoxia. However, the possible significance of this current modulation in the antiarrhythmic mechanism is not fully clarified. Action potentials were measured using the conventional microelectrode technique from canine left ventricular papillary muscle and free-running Purkinje fibers, under normal and hypoxic conditions. Ionic currents were measured using the whole-cell configuration of the patch-clamp method. Acetylcholine at 5 μmol/L did not influence the action potential duration (APD) either in Purkinje fibers or in papillary muscle preparations. In contrast, it significantly lengthened the APD and suppressed the Purkinje-ventricle APD dispersion when it was administered after 5 μmol/L pinacidil application. Carbachol at 3 μmol/L reduced the pinacidil-activated I K-ATP under voltage-clamp conditions. Acetylcholine lengthened the ventricular action potential under simulated ischemia condition. In this study, we found that acetylcholine inhibits the I K-ATP and thus suppresses the ventricle-Purkinje APD dispersion. We conclude that parasympathetic tone may reduce the arrhythmogenic substrate exerting a complex antiarrhythmic mechanism during hypoxic conditions.

Cardiac transmembrane ion channels and action potentials: cellular physiology and arrhythmogenic behavior

Cardiac arrhythmias are among the leading causes of mortality. They often arise from alterations in the electrophysiological properties of cardiac cells and their underlying ionic mechanisms. It is therefore critical to further unravel the pathophysiology of the ionic basis of human cardiac electrophysiology in health and disease. In the first part of this review, current knowledge on the differences in ion channel expression and properties of the ionic processes that determine the morphology and properties of cardiac action potentials and calcium dynamics from cardiomyocytes in different regions of the heart are described. Then the cellular mechanisms promoting arrhythmias in congenital or acquired conditions of ion channel function (electrical remodeling) are discussed. The focus is on human-relevant findings obtained with clinical, experimental, and computational studies, given that interspecies differences make the extrapolation from animal experiments to human clinical settings difficult. Deepening the understanding of the diverse pathophysiology of human cellular electrophysiology will help in developing novel and effective antiarrhythmic strategies for specific subpopulations and disease conditions.

Cryo-EM analysis of PIP2 regulation in mammalian GIRK channels

G-protein-gated inward rectifier potassium (GIRK) channels are regulated by G proteins and PIP2. Here, using cryo-EM single particle analysis we describe the equilibrium ensemble of structures of neuronal GIRK2 as a function of the C8-PIP2 concentration. We find that PIP2 shifts the equilibrium between two distinguishable structures of neuronal GIRK (GIRK2), extended and docked, towards the docked form. In the docked form the cytoplasmic domain, to which Gβγ binds, becomes accessible to the cytoplasmic membrane surface where Gβγ resides. Furthermore, PIP2 binding reshapes the Gβγ binding surface on the cytoplasmic domain, preparing it to receive Gβγ. We find that cardiac GIRK (GIRK1/4) can also exist in both extended and docked conformations. These findings lead us to conclude that PIP2 influences GIRK channels in a structurally similar manner to Kir2.2 channels. In Kir2.2 channels, the PIP2-induced conformational changes open the pore. In GIRK channels, they prepare the channel for activation by Gβγ.

GPCR-dependent biasing of GIRK channel signaling dynamics by RGS6 in mouse sinoatrial nodal cells

How G protein-coupled receptors (GPCRs) evoke specific biological outcomes while utilizing a limited array of G proteins and effectors is poorly understood, particularly in native cell systems. Here, we examined signaling evoked by muscarinic (M2R) and adenosine (A1R) receptor activation in the mouse sinoatrial node (SAN), the cardiac pacemaker. M2R and A1R activate a shared pool of cardiac G protein-gated inwardly rectifying K+ (GIRK) channels in SAN cells from adult mice, but A1R-GIRK responses are smaller and slower than M2R-GIRK responses. Recordings from mice lacking Regulator of G protein Signaling 6 (RGS6) revealed that RGS6 exerts a GPCR-dependent influence on GIRK-dependent signaling in SAN cells, suppressing M2R-GIRK coupling efficiency and kinetics and A1R-GIRK signaling amplitude. Fast kinetic bioluminescence resonance energy transfer assays in transfected HEK cells showed that RGS6 prefers Gαo over Gαi as a substrate for its catalytic activity and that M2R signals preferentially via Gαo, while A1R does not discriminate between inhibitory G protein isoforms. The impact of atrial/SAN-selective ablation of Gαo or Gαi2 was consistent with these findings. Gαi2 ablation had minimal impact on M2R-GIRK and A1R-GIRK signaling in SAN cells. In contrast, Gαo ablation decreased the amplitude and slowed the kinetics of M2R-GIRK responses, while enhancing the sensitivity and prolonging the deactivation rate of A1R-GIRK signaling. Collectively, our data show that differences in GPCR-G protein coupling preferences, and the Gαo substrate preference of RGS6, shape A1R- and M2R-GIRK signaling dynamics in mouse SAN cells.

miR-221 and -222 target CACNA1C and KCNJ5 leading to altered cardiac ion channel expression and current density

MicroRNAs (miRs) contribute to different aspects of cardiovascular pathology, among others cardiac hypertrophy and atrial fibrillation. The aim of our study was to evaluate the impact of miR-221/222 on cardiac electrical remodeling. Cardiac miR expression was analyzed in a mouse model with altered electrocardiography parameters and severe heart hypertrophy. Next generation sequencing revealed 14 differentially expressed miRs in hypertrophic hearts, with miR-221 and -222 being the strongest regulated miR-cluster. This increase was restricted to cardiomyocytes and not observed in cardiac fibroblasts. Additionally, we evaluated the change of miR-221/222 in vivo in two models of pharmacologically induced heart hypertrophy (angiotensin II, isoprenaline), thereby demonstrating a stimulus-induced increase in miR-221/222 in vivo by angiotensin II but not by isoprenaline. Whole transcriptome analysis by RNA-seq and qRT-PCR validation revealed an enriched number of downregulated mRNAs coding for proteins located in the T-tubule, which are also predicted targets for miR-221/222. Among those, mRNAs were the L-type Ca2+ channel subunits as well as potassium channel subunits. We confirmed that both miRs target the 3'-untranslated regions of Cacna1c and Kcnj5. Furthermore, enhanced expression of these miRs reduced L-type Ca2+ channel and Kcnj5 channel abundance and function, which was analyzed by whole-cell patch clamp recordings or Western blot and flux measurements, respectively. miR-221 and -222 contribute to the regulation of L-type Ca2+ channels as well as Kcnj5 channels and, therefore, potentially contribute to disturbed cardiac excitation generation and propagation. Future studies will have to evaluate the pathophysiological and clinical relevance of aberrant miR-221/222 expression for electrical remodeling.

The intriguing effect of ethanol and nicotine on acetylcholine-sensitive potassium current IKAch: Insight from a quantitative model

Recent experimental work has revealed unusual features of the effect of certain drugs on cardiac inwardly rectifying potassium currents, including the constitutively active and acetylcholine-induced components of acetylcholine-sensitive current (I_KAch). These unusual features have included alternating susceptibility of the current components to activation and inhibition induced by ethanol or nicotine applied at various concentrations, and significant correlation between the drug effect and the current magnitude measured under drug-free conditions. To explain these complex drug effects, we have developed a new type of quantitative model to offer a possible interpretation of the effect of ethanol and nicotine on the I_KAch channels. The model is based on a description of I_KAch as a sum of particular currents related to the populations of channels formed by identical assemblies of different α-subunits. Assuming two different channel populations in agreement with the two reported functional I_KAch-channels (GIRK1/4 and GIRK4), the model was able to simulate all the above-mentioned characteristic features of drug-channel interactions and also the dispersion of the current measured in different cells. The formulation of our model equations allows the model to be incorporated easily into the existing integrative models of electrical activity of cardiac cells involving quantitative description of I_KAch. We suppose that the model could also help make sense of certain observations related to the channels that do not show inward rectification. This new ionic channel model, based on a concept we call population type, may allow for the interpretation of complex interactions of drugs with ionic channels of various types, which cannot be done using the ionic channel models available so far.

Systematic Analysis of Transcriptomic Profile of the Effects of Low Dose Atropine Treatment on Scleral Fibroblasts using Next-Generation Sequencing and Bioinformatics

This study has two novel findings: it is not only the first to deduct potential genes involved in scleral growth repression upon atropine instillation from a prevention point of view, but also the first to demonstrate that only slight changes in scleral gene expression were found after atropine treatment as side effects and safety reasons of the eye drops are of concern. The sclera determines the final ocular shape and size, constituting of scleral fibroblasts as the principal cell type and the major regulator of extracellular matrix. The aim of our study was to identify differentially expressed genes and microRNA regulations in atropine-treated scleral fibroblasts that are potentially involved in preventing the onset of excessive ocular growth using next-generation sequencing and bioinformatics approaches. Differentially expressed genes were functionally enriched in anti-remodeling effects, comprising of structural changes of extracellular matrix and metabolic pathways involving cell differentiation. Significant canonical pathways were correlated to inhibition of melatonin degradation, which was compatible with our clinical practice as atropine eye drops are instilled at night. Validation of the dysregulated genes with previous eye growth-related arrays and through microRNA-mRNA interaction predictions revealed the association of hsa-miR-2682-5p-KCNJ5 and hsa-miR-2682-5p-PRLR with scleral anti-remodeling and circadian rhythmicity. Our findings present new insights into understanding the anti-myopic effects of atropine, which may assist in prevention of myopia development.

Atrial GIRK Channels Mediate the Effects of Vagus Nerve Stimulation on Heart Rate Dynamics and Arrhythmogenesis

Diminished parasympathetic influence is central to the pathogenesis of cardiovascular diseases, including heart failure and hypertension. Stimulation of the vagus nerve has shown promise in treating cardiovascular disease, prompting renewed interest in understanding the signaling pathway(s) that mediate the vagal influence on cardiac physiology. Here, we evaluated the contribution of G protein-gated inwardly rectifying K+ (GIRK/Kir3) channels to the effect of vagus nerve stimulation (VNS) on heart rate (HR), HR variability (HRV), and arrhythmogenesis in anesthetized mice. As parasympathetic fibers innervate both atria and ventricle, and GIRK channels contribute to the cholinergic impact on atrial and ventricular myocytes, we collected in vivo electrocardiogram recordings from mice lacking either atrial or ventricular GIRK channels, during VNS. VNS decreased HR and increased HRV in control mice, in a muscarinic receptor-dependent manner. This effect was preserved in mice lacking ventricular GIRK channels, but was nearly completely absent in mice lacking GIRK channels in the atria. In addition, atrial-specific ablation of GIRK channels conferred resistance to arrhythmic episodes induced by VNS. These data indicate that atrial GIRK channels are the primary mediators of the impact of VNS on HR, HRV, and arrhythmogenesis in the anesthetized mouse.

Expression and relevance of the G protein-gated K+ channel in the mouse ventricle

The atrial G protein-gated inwardly rectifying K⁺ (GIRK) channel is a critical mediator of parasympathetic influence on cardiac physiology. Here, we probed the details and relevance of the GIRK channel in mouse ventricle. mRNAs for the atrial GIRK channel subunits (GIRK1, GIRK4), M2 muscarinic receptor (M₂R), and RGS6, a negative regulator of atrial GIRK-dependent signaling, were detected in mouse ventricle at relatively low levels. The cholinergic agonist carbachol (CCh) activated small GIRK currents in adult wild-type ventricular myocytes that exhibited relatively slow kinetics and low CCh sensitivity; these currents were absent in ventricular myocytes from Girk1^-/- or Girk4^-/- mice. While loss of GIRK channels attenuated the CCh-induced shortening of action potential duration and suppression of ventricular myocyte excitability, selective ablation of GIRK channels in ventricle had no effect on heart rate, heart rate variability, or electrocardiogram parameters at baseline or after CCh injection. Additionally, loss of ventricular GIRK channels did not impact susceptibility to ventricular arrhythmias. These data suggest that the mouse ventricular GIRK channel is a GIRK1/GIRK4 heteromer, and show that while it contributes to the cholinergic suppression of ventricular myocyte excitability, this influence does not substantially impact cardiac physiology or ventricular arrhythmogenesis in the mouse.

Potassium channels in the sinoatrial node and their role in heart rate control

Potassium currents determine the resting membrane potential and govern repolarisation in cardiac myocytes. Here, we review the various currents in the sinoatrial node focussing on their molecular and cellular properties and their role in pacemaking and heart rate control. We also describe how our recent finding of a novel ATP-sensitive potassium channel population in these cells fits into this picture.

Macrolides selectively inhibit mutant KCNJ5 potassium channels that cause aldosterone-producing adenoma

Aldosterone-producing adenomas (APAs) are benign tumors of the adrenal gland that constitutively produce the salt-retaining steroid hormone aldosterone and cause millions of cases of severe hypertension worldwide. Either of 2 somatic mutations in the potassium channel KCNJ5 (G151R and L168R, hereafter referred to as KCNJ5MUT) in adrenocortical cells account for half of APAs worldwide. These mutations alter channel selectivity to allow abnormal Na+ conductance, resulting in membrane depolarization, calcium influx, aldosterone production, and cell proliferation. Because APA diagnosis requires a difficult invasive procedure, patients often remain undiagnosed and inadequately treated. Inhibitors of KCNJ5MUT could allow noninvasive diagnosis and therapy of APAs carrying KCNJ5 mutations. Here, we developed a high-throughput screen for rescue of KCNJ5MUT-induced lethality and identified a series of macrolide antibiotics, including roxithromycin, that potently inhibit KCNJ5MUT, but not KCNJ5WT. Electrophysiology demonstrated direct KCNJ5MUT inhibition. In human aldosterone-producing adrenocortical cancer cell lines, roxithromycin inhibited KCNJ5MUT-induced induction of CYP11B2 (encoding aldosterone synthase) expression and aldosterone production. Further exploration of macrolides showed that KCNJ5MUT was similarly selectively inhibited by idremcinal, a macrolide motilin receptor agonist, and by synthesized macrolide derivatives lacking antibiotic or motilide activity. Macrolide-derived selective KCNJ5MUT inhibitors thus have the potential to advance the diagnosis and treatment of APAs harboring KCNJ5MUT.

Familial hyperaldosteronism type III

Primary aldosteronism is the most common form of endocrine hypertension. This disorder comprises both sporadic and familial forms. Four familial forms of primary aldosteronism (FH-I to FH-IV) have been described. FH-III is caused by germline mutations in KCNJ5, encoding the potassium channel Kir3.4 (also called GIRK4). These mutations alter the selectivity filter of the channel and lead to abnormal ion currents with loss of potassium selectivity, sodium influx and consequent increased intracellular calcium that causes excessive aldosterone biosynthesis. To date, eleven families have been reported, carrying six different mutations. Although the clinical features are variable, FH-III patients often display severe hyperaldosteronism with an early onset, associated with hypokalemia and diabetes insipidus-like symptoms. In most cases FH-III patients are resistant to pharmacological therapy and require bilateral adrenalectomy to control symptoms. In the present manuscript, we review the genetics and pathological basis of FH-III, the diagnostic work-up, clinical features and therapeutic management. Finally, we will describe a new case of FH-III of an Italian patient carrying a Gly151Arg mutation.

Atrial fibrillation: Therapeutic potential of atrial K+ channel blockers

Despite the epidemiological scale of atrial fibrillation, current treatment strategies are of limited efficacy and safety. Ideally, novel drugs should specifically correct the pathophysiological mechanisms responsible for atrial fibrillation with no other cardiac or extracardiac actions. Atrial-selective drugs are directed toward cellular targets with sufficiently different characteristics in atria and ventricles to modify only atrial function. Several potassium (K⁺) channels with either predominant expression in atria or distinct electrophysiological properties in atria and ventricles can serve as atrial-selective drug targets. These channels include the ultra-rapidly activating, delayed outward-rectifying Kv1.5 channel conducting I_Kur, the acetylcholine-activated inward-rectifying Kir3.1/Kir3.4 channel conducting I_K,ACh, the Ca²⁺-activated K⁺ channels of small conductance (SK) conducting I_SK, and the two pore domain K⁺ (K2P) channels TWIK-1, TASK-1 and TASK-3 that are responsible for voltage-independent background currents I_TWIK-1, I_TASK-1, and I_TASK-3. Here, we briefly review the characteristics of these K⁺ channels and their roles in atrial fibrillation. The antiarrhythmic potential of drugs targeting the described channels is discussed as well as their putative value in treatment of atrial fibrillation.

Genetics of primary hyperaldosteronism

Hypertension is a common medical condition and affects approximately 20% of the population in developed countries. Primary aldosteronism is the most common form of secondary hypertension and affects 8-13% of patients with hypertension. The two most common causes of primary aldosteronism are aldosterone-producing adenoma and bilateral adrenal hyperplasia. Familial hyperaldosteronism types I, II and III are the known genetic syndromes, in which both adrenal glands produce excessive amounts of aldosterone. However, only a minority of patients with primary aldosteronism have one of these syndromes. Several novel susceptibility genes have been found to be mutated in aldosterone-producing adenomas: KCNJ5, ATP1A1, ATP2B3, CTNNB1, CACNA1D, CACNA1H and ARMC5 This review describes the genes currently known to be responsible for primary aldosteronism, discusses the origin of aldosterone-producing adenomas and considers the future clinical implications based on these novel insights.

The GIRK1 subunit potentiates G protein activation of cardiac GIRK1/4 hetero-tetramers

G protein gated inward rectifier potassium (GIRK) channels are gated by direct binding of G protein beta-gamma subunits (Gβγ), signaling lipids, and intracellular Na(+). In cardiac pacemaker cells, hetero-tetramer GIRK1/4 channels and homo-tetramer GIRK4 channels play a central role in parasympathetic slowing of heart rate. It is known that the Na(+) binding site of the GIRK1 subunit is defective, but the functional difference between GIRK1/4 hetero-tetramers and GIRK4 homo-tetramers remains unclear. Here, using purified proteins and the lipid bilayer system, we characterize Gβγ and Na(+) regulation of GIRK1/4 hetero-tetramers and GIRK4 homo-tetramers. We find in GIRK4 homo-tetramers that Na(+) binding increases Gβγ affinity and thereby increases the GIRK4 responsiveness to G protein stimulation. GIRK1/4 hetero-tetramers are not activated by Na(+), but rather are in a permanent state of high responsiveness to Gβγ, suggesting that the GIRK1 subunit functions like a GIRK4 subunit with Na(+) permanently bound.

NCI-H295R cell line as in vitro model of hyperaldosteronism lacks functional KCNJ5 (GIRK4; Kir3.4) channels

As a major cause of aldosterone producing adenomas, numerous gain-of-function mutations in the KCNJ5 gene (encoding the K(+) channel subunit GIRK4) have been identified. The human adrenocortical carcinoma cell line NCI-H295R is the most frequently used cellular model for in vitro studies related to regulation of aldosterone-synthesis. Because of the undefined role of KCNJ5 (GIRK4) in regulating synthesis of aldosterone, we aimed at identifying basal and G protein-activated GIRK4 currents in this paradigmatic cell line. The GIRK-specific blocker Tertiapin-Q did not affect basal current. Neither loading of the cells with GTP-γ-S via the patch-clamp pipette nor agonist stimulation of an infected A1-adenosine receptor resulted in activation of GIRK current. In cells co-infected with KCNJ5, robust activation of basal and adenosine-activated inward-rectifying current was observed. Although GIRK4 protein can be detected in Western blots of H295R homogenates, we suggest that GIRK4 in aldosterone-producing cells does not form functional G(βγ)-activated channels.

Unifying Mechanism of Controlling Kir3 Channel Activity by G Proteins and Phosphoinositides

The question that started with the pioneering work of Otto Loewi in the 1920s, to identify how stimulation of the vagus nerve decreased heart rate, is approaching its 100th year anniversary. In the meantime, we have learned that the neurotransmitter acetylcholine acting through muscarinic M2 receptors activates cardiac potassium (Kir3) channels via the βγ subunits of G proteins, an important effect that contributes to slowing atrial pacemaker activity. Concurrent stimulation of M1 or M3 receptors hydrolyzes PIP2, a signaling phospholipid essential to maintaining Kir3 channel activity, thus causing desensitization of channel activity and protecting the heart from overinhibition of pacemaker activity. Four mammalian members of the Kir3 subfamily, expressed in heart, brain, endocrine organs, etc., are modulated by a plethora of stimuli to regulate cellular excitability. With the recent great advances in ion channel structural biology, three-dimensional structures of Kir3 channels with PIP2 and the Gβγ subunits are now available. Mechanistic insights have emerged that explain how modulatory control of activity feeds into a core mechanism of channel-PIP2 interactions to regulate the conformation of channel gates. This complex but beautiful system continues to surprise us for almost 100 years with an apparent wisdom in its intricate design.

Mechanisms underlying the beneficial effects of Kaiyu Granule for depression

The proprietary Chinese medicine preparation Kaiyu Granule is made of bupleurum, nutgrass galingale rhizome, szechwan lovage rhizome, turmeric root tuber, white peony alba, cape jasmine fruit, fried semen ziziphi jujubae, and prepared liquorice root. It is a common recipe for the clinical treatment of depression in China. In this study, after 21 days of unpredictable stress exposure, Wistar rats exhibited similar behavioral changes to patients with depression. Moreover, G-protein-coupled inwardly rectifying K⁺ channel 1 mRNA and protein expression were significantly reduced in rat hippocampal CA1 and CA3 regions. However, G-protein-coupled inwardly rectifying K⁺ channel 1 mRNA, protein expression, and rat behavior were clearly better after administration of 12, 8, or 4 g/kg of Kaiyu Granule when depression model rats underwent stress. 12 g/kg of Kaiyu Granule had the most obvious effects on the increased expression of G-protein-coupled inwardly rectifying K⁺ channel 1 mRNA and protein in rat hippocampal CA1 and CA3 regions. These results suggested that Kaiyu Granule improved depression by affecting G-protein-coupled inwardly rectifying K⁺ channel 1 expression in the rat hippocampus.

Role of KCNJ5 in familial and sporadic primary aldosteronism

Primary aldosteronism is characterised by the dysregulation of aldosterone production and comprises both sporadic forms, caused by an aldosterone-producing adenoma or bilateral adrenal hyperplasia, and familial forms (familial hyperaldosteronism types I, II and III). The two principal physiological regulators of aldosterone synthesis are angiotensin II and serum K(+), which reverse the high resting K(+) conductance and hyperpolarized membrane potential of adrenal glomerulosa cells. The resulting membrane depolarization causes the opening of voltage-gated Ca(2+) channels and an increase in intracellular Ca(2+) that stimulates aldosterone biosynthesis. Point mutations in the KCNJ5 gene, which encodes the G-protein-activated inward rectifier K(+) channel 4 (GIRK4), have been implicated in the pathogenesis of both sporadic and familial forms of primary aldosteronism. These mutations interfere with the selectivity filter of GIRK4 causing Na(+) entry, cell depolarization and Ca(2+) channel opening, resulting in constitutive aldosterone production. Seven families with familial hyperaldosteronism caused by KCNJ5 germline mutations have so far been described, and multicentre studies have reported KCNJ5 mutations in approximately 40% of sporadic aldosterone-producing adenomas. Herein, we review the role of GIRK4 in adrenal pathophysiology and provide an overview of the clinical and biochemical phenotypes resulting from KCNJ5 mutations in patients with sporadic and familial primary aldosteronism.

Genetic variations in the KCNJ5 gene in primary aldosteronism patients from Xinjiang, China

BACKGROUND

Primary aldosteronism (PA) is the most common endocrine form of secondary hypertension, and one of the most common subtypes of sporadic PA is aldosterone-producing adenoma (APA). Recently, two somatic mutations of the KCNJ5 gene were implicated in APA, and two germline mutations were associated with familial hyperaldosteronism III.

OBJECTIVES

This case-control study was designed to investigate the relationship between genetic variations in the KCNJ5 gene and sporadic PA patients in Xinjiang, China.

METHODS

Five common single nucleotide polymorphisms (SNPs) of the KCNJ5 gene (rs6590357, rs4937391, rs3740835, rs2604204, and rs11221497) were detected in patients with sporadic PA (n = 235) and essential hypertension (EH; n=913) by the TaqMan polymerase chain reaction method.

RESULTS

The EH group and the PA group showed significant differences in the distributions of genotypes and alleles of rs4937391 and rs2604204 in total and male subjects (P<0.05), as well as rs3740835 in male subjects (P<0.05). However, only the association between the rs2604204 genotype and male sporadic PA remained significant after Bonferroni's correction (P<0.01). Furthermore, logistic regression analysis demonstrated that the CC genotype of rs2604204 was a risk factor for male patients with sporadic PA, after adjusting for age and body mass index (odds ratio=2.228, 95% CI: 1.300-3.819, P=0.004).

CONCLUSION

The genetic variant rs2604204 of KCNJ5 is associated with sporadic PA in Chinese males, suggesting that KCNJ5 may be involved in the pathogenesis of sporadic PA in these particular patients.

Molecular basis of the facilitation of the heterooligomeric GIRK1/GIRK4 complex by cAMP dependent protein kinase

G-protein activated inwardly rectifying K⁺ channels (GIRKs) of the heterotetrameric GIRK1/GIRK4 composition mediate I_K + ACh in atrium and are regulated by cAMP dependent protein kinase (PKA). Phosphorylation of GIRK1/GIRK4 complexes promotes the activation of the channel by the G-protein Gβγ-dimer (“heterologous facilitation”). Previously we reported that 3 serines/threonines (S/Ts) within the GIRK1 subunit are phosphorylated by the catalytic subunit of PKA (PKA-cs) in-vitro and are responsible for the acute functional effects exerted by PKA on the homooligomeric GIRK1^F137S (GIRK1^⁎) channel. Here we report that homooligomeric GIRK4^WT and GIRK4^S143T (GIRK4^⁎) channels are clearly regulated by PKA phosphorylation. Heterooligomeric channels of the GIRK1^{S385CS401CT407C}/GIRK4^WT composition, where the GIRK1 subunit is devoid of PKA mediated phosphorylation, exhibited reduced but still significant acute effects (reduction during agonist application was ≈ 49% compared to GIRK1^WT/GIRK4^WT). Site directed mutagenesis of truncated cytosolic regions of GIRK4 revealed four serines/threonines (S/Ts) that were heavily phosphorylated by PKA-cs in vitro. Two of them were found to be responsible for the acute effects exerted by PKA in vivo, since the effect of cAMP injection was reduced by ≈ 99% in homooligomeric GIRK4^{⁎T199CS412C} channels. Coexpression of GIRK1^WT/GIRK4^T199CS412C reduced the acute effect by ≈ 65%. Only channels of the GIRK1^{S385CS401CT407C}/GIRK4^T199CS412C composition were practically devoid of PKA mediated effects (reduction by ≈ 97%), indicating that both subunits contribute to the heterologous facilitation of I_K + ACh.

Inhibition of glucose-stimulated insulin secretion by KCNJ15, a newly identified susceptibility gene for type 2 diabetes

Potassium inwardly rectifying channel, subfamily J, member 15 (KCNJ15) is a type 2 diabetes-associated risk gene, and Kcnj15 overexpression suppresses insulin secretion in rat insulinoma (INS1) cells. The aim of the current study was to characterize the role of Kcnj15 by knockdown of this gene in vitro and in vivo. Human islet cells were used to determine the expression of KCNJ15. Expression of KCNJ15 mRNA in islets was higher in subjects with type 2 diabetes. In INS1 cells, Kcnj15 expression was induced by high glucose-containing medium. Regulation of Kcnj15 by glucose and its effect on insulin secretion were analyzed in INS1 cells and in normal mice and diabetic mice by the inactivation of Kcnj15 using small interfering RNA. Knockdown of Kcnj15 increased the insulin secretion in vitro and in vivo. KCNJ15 and Ca(2+)-sensing receptor (CsR) interact in the kidney. Binding of Kcnj15 with CsR was also detected in INS1 cells. In conclusion, downregulation of Kcnj15 leads to increased insulin secretion in vitro and in vivo. The mechanism to regulate insulin secretion involves KCNJ15 and CsR.

Identification of selective agonists and antagonists to g protein-activated inwardly rectifying potassium channels: candidate medicines for drug dependence and pain

G protein-activated inwardly rectifying K⁺ (GIRK) channels have been known to play a key role in the rewarding and analgesic effects of opioids. To identify potent agonists and antagonists to GIRK channels, we examined various compounds for their ability to activate or inhibit GIRK channels. A total of 503 possible compounds with low molecular weight were selected from a list of fluoxetine derivatives at Pfizer Japan Inc. We screened these compounds by a Xenopus oocyte expression system. GIRK1/2 and GIRK1/4 heteromeric channels were expressed on Xenopus laevis oocytes at Stage V or VI. A mouse IRK2 channel, which is another member of inwardly rectifying potassium channels with similarity to GIRK channels, was expressed on the oocytes to examine the selectivity of the identified compounds to GIRK channels. For electrophysiological analyses, a two-electrode voltage clamp method was used. Among the 503 compounds tested, one compound and three compounds were identified as the most effective agonist and antagonists, respectively. All of these compounds induced only negligible current responses in the oocytes expressing the IRK2 channel, suggesting that these compounds were selective to GIRK channels. These effective and GIRK-selective compounds may be useful possible therapeutics for drug dependence and pain.

Naringin directly activates inwardly rectifying potassium channels at an overlapping binding site to tertiapin-Q

BACKGROUND

G protein-coupled inwardly rectifying potassium (K(IR) 3) channels are important proteins that regulate numerous physiological processes including excitatory responses in the CNS and the control of heart rate. Flavonoids have been shown to have significant health benefits and are a diverse source of compounds for identifying agents with novel mechanisms of action.

EXPERIMENTAL APPROACH

The flavonoid glycoside, naringin, was evaluated on recombinant human K(IR) 3.1-3.4 and K(IR) 3.1-3.2 expressed in Xenopus oocytes using two-electrode voltage clamp methods. In addition, we evaluated the activity of naringin alone and in the presence of the K(IR) 3 channel blocker tertiapin-Q (0.5 nM, 1 nM and 3 nM) at recombinant K(IR) 3.1-3.4 channels. Site-directed mutagenesis was used to identify amino acids within the M1-M2 loop of the K(IR) 3.1(F137S) mutant channel important for naringin's activity.

KEY RESULTS

Naringin (100 µM) had minimal effect on uninjected oocytes but activated K(IR) 3.1-3.4 and K(IR) 3.1-3.2 channels. The activation by naringin of K(IR) 3.1-3.4 channels was inhibited by tertiapin-Q in a competitive manner. An alanine-scan performed on the K(IR) 3.1(F137S) mutant channel, replacing one by one aromatic amino acids within the M1-M2 loop, identified tyrosines 148 and 150 to be significantly contributing to the affinity of naringin as these mutations reduced the activity of naringin by 20- and 40-fold respectively.

CONCLUSIONS AND IMPLICATIONS

These results show that naringin is a direct activator of K(IR) 3 channels and that tertiapin-Q shares an overlapping binding site on the K(IR) 3.1-3.4. This is the first example of a ligand that activates K(IR) 3 channels by binding to the extracellular M1-M2 linker of the channel.

Common polymorphisms in KCNJ5 [corrected] are associated with early-onset lone atrial fibrillation in Caucasians

OBJECTIVES

The aim of this study was to screen lone atrial fibrillation (AF) patients for mutations in the genes KCNJ2, KCNJ3 and KCNJ5, all encoding potassium channels. Furthermore, we wanted to replicate the prior association of two single-nucleotide polymorphisms (SNPs) in KCNJ5, C171T (rs6590357) and G810T (rs7118824), with lone AF in Han Chinese.

METHODS

We sequenced the coding region and splice site of KCNJ2, KCNJ3 and KCNJ5 in 187 early-onset lone-AF patients screening for mutations and counting SNP frequencies for the two noted SNPs in KCNJ5.

RESULTS

No mutations were found in KCNJ2, KCNJ3 or KCNJ5. Both genotype distribution and allele frequencies of the SNPs rs6590357 and rs7118824 significantly differed between the AF and control group (p(genotype) = 0.0067, p(allele) = 0.0021 and p(genotype) = 0.014, p(allele) = 0.0101, respectively). On allele level, the OR for lone AF for rs6590357 was 1.77 (95% CI 1.16-2.73, p = 0.009) and for rs7118824 it was 1.71 (95% CI 1.13-2.57, p = 0.01) in a model adjusted for age and gender.

CONCLUSIONS

Our findings indicate that rs6590357 and rs7118824 in KCNJ5 are associated with early-onset lone AF in Caucasians. No mutations were found in the exon or splice site of KCNJ2, KCNJ3 or KCNJ5.

K+ channel mutations in adrenal aldosterone-producing adenomas and hereditary hypertension

Endocrine tumors such as aldosterone-producing adrenal adenomas (APAs), a cause of severe hypertension, feature constitutive hormone production and unrestrained cell proliferation; the mechanisms linking these events are unknown. We identify two recurrent somatic mutations in and near the selectivity filter of the potassium (K(+)) channel KCNJ5 that are present in 8 of 22 human APAs studied. Both produce increased sodium (Na(+)) conductance and cell depolarization, which in adrenal glomerulosa cells produces calcium (Ca(2+)) entry, the signal for aldosterone production and cell proliferation. Similarly, we identify an inherited KCNJ5 mutation that produces increased Na(+) conductance in a Mendelian form of severe aldosteronism and massive bilateral adrenal hyperplasia. These findings explain pathogenesis in a subset of patients with severe hypertension and implicate loss of K(+) channel selectivity in constitutive cell proliferation and hormone production.

Evidence for oligomerization between GABAB receptors and GIRK channels containing the GIRK1 and GIRK3 subunits

The stimulation of inhibitory neurotransmitter receptors, such as γ-aminobutyric acid type B (GABA(B) ) receptors, activates G protein-gated inwardly-rectifying K(+) (GIRK) channels, which influence membrane excitability. There is now evidence suggesting that G protein-coupled receptors and G protein-gated inwardly-rectifying K(+) [GIRK/family 3 of inwardly-rectifying K(+) (Kir3)] channels do not diffuse freely within the plasma membrane, but instead there are direct protein-protein interactions between them. Here, we used bioluminescence resonance energy transfer, co-immunoprecipitation, confocal and electron microscopy techniques to investigate the oligomerization of GABA(B) receptors with GIRK channels containing the GIRK3 subunit, whose contribution to functional channels is still unresolved. Co-expression of GABA(B) receptors and GIRK channels in human embryonic kidney-293 cells in combination with co-immunoprecipitation experiments established that the metabotropic receptor forms stable complexes with GIRK channels. Using bioluminescence resonance energy transfer, we have shown that, in living cells under physiological conditions, GABA(B) receptors interact directly with GIRK1/GIRK3 heterotetramers. In addition, we have provided evidence that the receptor-effector complexes are also found in vivo and identified that the cerebellar granule cells are one neuron population where the interaction probably takes place. Altogether, our data show that signalling complexes containing GABA(B) receptors and GIRK channels are formed shortly after biosynthesis, probably in the endoplasmic reticulum and/or endoplasmic reticulum/Golgi apparatus complex, suggesting that this might be a general feature of receptor-effector ion channel signal transduction and supporting a channel-forming role for the GIRK3 subunit.

Identification of a Kir3.4 mutation in congenital long QT syndrome

Congenital long QT syndrome (LQTS) is a hereditary disorder that leads to sudden cardiac death secondary to fatal cardiac arrhythmias. Although many genes for LQTS have been described, the etiology remains unknown in 30%-40% of cases. In the present study, a large Chinese family (four generations, 49 individuals) with autosomal-dominant LQTS was clinically evaluated. Genome-wide linkage analysis was performed by using polymorphic microsatellite markers to map the genetic locus, and positional candidate genes were screened by sequencing for mutations. The expression pattern and functional characteristics of the mutated protein were investigated by western blotting and patch-clamp electrophysiology. The genetic locus of the LQTS-associated gene was mapped to chromosome 11q23.3-24.3. A heterozygous mutation (Kir3.4-Gly387Arg) was identified in the G protein-coupled, inwardly rectifying potassium channel subunit Kir3.4, encoded by the KCNJ5 gene. The Kir3.4-Gly387Arg mutation was present in all nine affected family members and absent in 528 ethnically matched controls. Western blotting of human cardiac tissue demonstrated significant Kir3.4 expression levels in the cardiac ventricles. Heterologous expression studies with Kir3.4-Gly387Arg revealed a loss-of-function electrophysiological phenotype resulting from reduced plasma membrane expression. Our findings suggest a role for Kir3.4 in the etiology of LQTS.

Inwardly rectifying potassium channels: their structure, function, and physiological roles

Inwardly rectifying K(+) (Kir) channels allow K(+) to move more easily into rather than out of the cell. They have diverse physiological functions depending on their type and their location. There are seven Kir channel subfamilies that can be classified into four functional groups: classical Kir channels (Kir2.x) are constitutively active, G protein-gated Kir channels (Kir3.x) are regulated by G protein-coupled receptors, ATP-sensitive K(+) channels (Kir6.x) are tightly linked to cellular metabolism, and K(+) transport channels (Kir1.x, Kir4.x, Kir5.x, and Kir7.x). Inward rectification results from pore block by intracellular substances such as Mg(2+) and polyamines. Kir channel activity can be modulated by ions, phospholipids, and binding proteins. The basic building block of a Kir channel is made up of two transmembrane helices with cytoplasmic NH(2) and COOH termini and an extracellular loop which folds back to form the pore-lining ion selectivity filter. In vivo, functional Kir channels are composed of four such subunits which are either homo- or heterotetramers. Gene targeting and genetic analysis have linked Kir channel dysfunction to diverse pathologies. The crystal structure of different Kir channels is opening the way to understanding the structure-function relationships of this simple but diverse ion channel family.

Identification of KCNJ15 as a susceptibility gene in Asian patients with type 2 diabetes mellitus

Recent advances in genome research have enabled the identification of new genomic variations that are associated with type 2 diabetes mellitus (T2DM). Via fine mapping of SNPs in a candidate region of chromosome 21q, the current study identifies potassium inwardly-rectifying channel, subfamily J, member 15 (KCNJ15) as a new T2DM susceptibility gene. KCNJ15 is expressed in the beta cell of the pancreas, and a synonymous SNP, rs3746876, in exon 4 (C566T) of this gene, with T allele frequency among control subjects of 3.1%, showed a significant association with T2DM affecting lean individuals in three independent Japanese sample sets (p = 2.5 x 10(-7), odds ratio [OR] = 2.54, 95% confidence interval [CI] = 1.76-3.67) and with unstratified T2DM (p = 6.7 x 10(-6), OR = 1.76, 95% CI = 1.37-2.25). The diabetes risk allele frequency was, however, very low among Europeans in whom no association between this variant and T2DM could be shown. Functional analysis in human embryonic kidney 293 cells demonstrated that the risk allele of the synonymous SNP in exon 4 increased KCNJ15 expression via increased mRNA stability, which resulted in the higher expression of protein as compared to that of the nonrisk allele. We also showed that KCNJ15 is expressed in human pancreatic beta cells. In conclusion, we demonstrated a significant association between a synonymous variant in KCNJ15 and T2DM in lean Japanese patients with T2DM, suggesting that KCNJ15 is a previously unreported susceptibility gene for T2DM among Asians.

Elucidation of the gating of the GIRK channel using a spectroscopic approach

The traditional view of G protein-coupled receptor (GPCR)-mediated signalling puts the players in this signalling cascade, namely the GPCR, the G protein and its effector, as individual components in space, where the signalling specificity is obtained mainly by the interaction of the GPCR and the Galpha subunits of the G protein. A question is then raised as to how fidelity in receptor signalling is achieved, given that many systems use the same components of the G protein signalling machinery. One possible mechanism for obtaining the specific flow of the downstream signals, from the activated G protein to its specific effector target, in a timely manner, is compartmentalization, a spatial arrangement of the complex in a rather restricted space. Here we review our recent findings related to these issues, using the G protein-coupled potassium channel (GIRK) as a model effector and fluorescence-based approaches to reveal how the signalling complex is arranged and how the G protein exerts its action to activate the GIRK channel in intact cells.

Heteromeric assembly of inward rectifier channel subunit Kir2.1 with Kir3.1 and with Kir3.4

Heteromultimerization of different pore-forming subunits is known to contribute to the diversity of inward rectifier K(+) channels. We examined if the subunits belonging to different subfamilies Kir2 and Kir3 can co-assemble to form heteromultimers in heterologous expression systems. We observed co-immunoprecipitation of Kir2.1 and Kir3.1 as well as Kir2.1 and Kir3.4 in HEK293T cells. Furthermore, analyses of subcellular localization using confocal microscopy revealed that co-expression of Kir2.1 promoted the cell surface localization of Kir3.1 and Kir3.4 in HEK293T cells. In electrophysiological experiments, co-expression of Kir2.1 with Kir3.1 and/or Kir3.4 in Xenopus oocytes and HEK293T cells did not yield currents with distinguishable features. However, co-expression of a dominant-negative Kir2.1 with the wild-type Kir3.1/3.4 decreased the Kir3.1/3.4 current amplitude in Xenopus oocytes. The results indicate that Kir2.1 is capable of forming heteromultimeric channels with Kir3.1 and with Kir3.4.

Generation of a constitutive Na+-dependent inward-rectifier current in rat adult atrial myocytes by overexpression of Kir3.4

Apart from gating by interaction with betagamma subunits from heterotrimeric G proteins upon stimulation of appropriate receptors, Kir.3 channels have been shown to be gated by intracellular Na+. However, no information is available on how Na+-dependent gating affects endogenous Kir3.1/Kir3.4 channels in mammalian atrial myocytes. We therefore studied how loading of adult atrial myocytes from rat hearts via the patch pipette filling solution with different concentrations of Na+ ([Na+]pip) affects Kir3 current. Surprisingly, in a range between 0 and 60 mm, Na+ neither had an effect on basal inward-rectifier current nor on the current activated by acetylcholine. Overexpression of Kir3.4 in adult atrial myocytes forced by adenoviral gene transfer results in formation of functional homomeric channels that interact with betagamma subunits upon activation of endogenous muscarinic receptors. These channels are activated at [Na+]pip >or= 15 mm, resulting in a receptor-independent basal inward rectifier current (I bir). I bir was neither affected by pertussis toxin nor by GDP-beta-S, suggesting G-protein-independent activation. PIP(2) depletion via endogenous PLC-coupled alpha1 adrenergic receptors causes inhibition of endogenous Kir3.1/3.4 channel currents by about 75%. In contrast, inhibition of Na+-activated I bir amounts to < 20%. The effect of the Kir3 channel blocker tertiapin-Q can be described using an IC50 of 12 nm (endogenous I K(ACh)) and 0.61 nm (I bir). These data clearly identify I bir as a homotetrameric Kir3.4 channel current with novel properties of regulation and pharmacology. Ibir shares some properties with a basal current recently described in atrial myocytes from an animal model of atrial fibrillation (AF) and AF patients.

Dynamic Integration of α-Adrenergic and Cholinergic Signals in the Atria

Numerous heptahelical receptors use activation of heterotrimeric G proteins to convey a multitude of extracellular signals to appropriate effector molecules in the cell. Both high specificity and correct integration of these signals are required for reliable cell function. Yet the molecular machineries that allow each cell to merge information flowing across different receptors are not well understood. Here we demonstrate that G protein-regulated inwardly rectifying K(+) (GIRK) channels can operate as dynamic integrators of alpha-adrenergic and cholinergic signals in atrial myocytes. Acting at the last step of the cholinergic signaling cascade, these channels are activated by direct interactions with betagamma subunits of the inhibitory G proteins (G betagamma), and efficiently translate M(2) muscarinic acetylcholine receptor (M2R) activation into membrane hyperpolarization. The parallel activation of alpha-adrenergic receptors imposed a distinctive "signature" on the function of M2R-activated GIRK1/4 channels, affecting both the probability of G betagamma binding to the channel and its desensitization. This modulation of channel function was correlated with a parallel depletion of G beta and protein phosphatase 1 from the oligomeric GIRK1 complexes. Such plasticity of the immediate GIRK signaling environment suggests that multireceptor integration involves large protein networks undergoing dynamic changes upon receptor activation.

Gene expression profiling of human cardiac potassium and sodium channels

BACKGROUND

The native cardiac ion currents and the action potential itself are the results of the concerted action of several different ion channels. The electrophysiological properties of cardiac cells are determined by the composition of ion channels and by their absolute abundance and proportional ratio.

METHODS

Our aim in this study was to compare the gene expression level of a representative panel of cardiac ion channels with each other and to compare the same channels in the atrium and ventricle of the human heart using quantitative real-time PCR analysis.

RESULTS

We obtained a significant difference in the gene expression levels in 21 of 35 channels between atrium and ventricle of healthy human hearts. Further, we found that the expression levels of Kv1.5 and Kv2.1 transcripts in the ventricle were very high, and that mRNAs for Kv1.7 and Kv3.4 are highly abundant in both the atrium and ventricle, which might indicate a functional role of these ion channel subunits in the formation of action potential in the human ventricle and both in the atrium and ventricle, respectively.

CONCLUSIONS

This is the first report on the expression of several ion channel subunits, such as Kv1.7, Kv3.3 or Kv3.4 in human cardiomyocytes. The expression levels of these genes are comparable with that of well known ion channel subunits. Therefore, it is reasonable to assume, that these ion channel subunits may contribute to native currents in the human myocardium.

Abnormal expression of the G-protein-activated inwardly rectifying potassium channel 2 (GIRK2) in hippocampus, frontal cortex, and substantia nigra of Ts65Dn mouse: a model of Down syndrome

Ts65Dn, a mouse model of Down syndrome (DS), demonstrates abnormal hippocampal synaptic plasticity and behavioral abnormalities related to spatial learning and memory. The molecular mechanisms leading to these impairments have not been identified. In this study, we focused on the G-protein-activated inwardly rectifying potassium channel 2 (GIRK2) gene that is highly expressed in the hippocampus region. We studied the expression pattern of GIRK subunits in Ts65Dn and found that GIRK2 was overexpressed in all analyzed Ts65Dn brain regions. Interestingly, elevated levels of GIRK2 protein in the Ts65Dn hippocampus and frontal cortex correlated with elevated levels of GIRK1 protein. This suggests that heteromeric GIRK1-GIRK2 channels are overexpressed in Ts65Dn hippocampus and frontal cortex, which could impair excitatory input and modulate spike frequency and synaptic kinetics in the affected regions. All GIRK2 splicing isoforms examined were expressed at higher levels in the Ts65Dn in comparison to the diploid hippocampus. The pattern of GIRK2 expression in the Ts65Dn mouse brain revealed by in situ hybridization and immunohistochemistry was similar to that previously reported in the rodent brain. However, in the Ts65Dn mouse a strong immunofluorescent staining of GIRK2 was detected in the lacunosum molecular layer of the CA3 area of the hippocampus. In addition, tyrosine hydroxylase containing dopaminergic neurons that coexpress GIRK2 were more numerous in the substantia nigra compacta and ventral tegmental area in the Ts65Dn compared to diploid controls. In summary, the regional localization and the increased brain levels coupled with known function of the GIRK channel may suggest an important contribution of GIRK2 containing channels to Ts65Dn and thus to DS neurophysiological phenotypes.

Mutation of critical GIRK subunit residues disrupts N- and C-termini association and channel function

The subfamily of G-protein-linked inwardly rectifying potassium channels (GIRKs) is coupled to G-protein receptors throughout the CNS and in the heart. We used mutational analysis to address the role of a specific hydrophobic region of the GIRK1 subunit. Deletion of the GIRK1 C-terminal residues 330-384, as well as the point mutation I331R, resulted in a decrease in channel function when coexpressed with GIRK4 in oocytes and in COS-7 cells. Surface protein expression of GIRK1 I331R coexpressed with GIRK4 was comparable with wild type, indicating that subunits assemble and are correctly localized to the membrane. Subsequent mutation of homologous residues in both the GIRK4 subunit and Kir2.1 (Gbetagamma-independent inward rectifier) also resulted in a decrease in channel function. Intracellular domain associations resulted in the coimmunoprecipitation of the GIRK1 N and C termini and GIRK4 N and C termini. The point mutation I331R in the GIRK1 C terminus or L337R in the GIRK4 C terminus decreased the association between the N and C termini. Mutation of a GIRK1 N-terminal hydrophobic residue, predicted structurally to interact with the C-terminal domain, also resulted in a decrease in channel function and termini association. We hypothesize that the hydrophobic nature of this GIRK1 subunit region is critical for interaction between adjacent termini and is permissive for channel gating. In addition, the homologous mutation in cytoplasmic domains of Kir2.1 (L330R) did not disrupt association, suggesting that the overall structural integrity of this region is critical for inward rectifier function.

The distribution of small and intermediate conductance calcium-activated potassium channels in the rat sensory nervous system

Small (SK) and intermediate (IK) conductance calcium-activated potassium channels are candidate ion channels for the regulation of excitability in nociceptive neurones. We have used unique peptide-directed antisera to describe the immunocytochemical distribution of the known isoforms of these ion channels in dorsal root ganglia (DRG) and spinal cord of the rat. These investigations sought to characterize further the phenotype and hence possible functions of nociceptive neurone subpopulations in the rat. In addition, using Western blotting, we sought to determine the level of protein expression of SK and IK channels in sensory nervous tissues following induction of inflammation (Freund's Complete Adjuvant (FCA) arthritis model) or nerve injury (chronic constriction injury model). We show that SK1, SK2, SK3 and IK1 are all expressed in DRG and spinal cord. Morphometric analysis revealed that SK1, SK2 and IK1 were preferentially localized to neurones having cell bodies <1000 microm2 (putative nociceptors) in DRG. Dual labeling immunocytochemistry showed that these ion channels co-localize with both CGRP and IB4, known markers of nociceptor sub-populations. SK2 was localized almost exclusively in the superficial laminae of the spinal cord dorsal horn, the region in which many sensory afferents terminate; the distribution of SK1 and IK1 was more widespread in spinal cord, although some preferential labeling within the dorsal horn was observed in the case of IK1. Here we show evidence for a distinctive pattern of expression for certain members of the calcium-activated potassium channel family in the rat DRG.