Genomic structure and expression of human KCNJ9 (Kir3.3/GIRK3)

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.

KIR2DL2, KIR2DL5A and KIR2DL5B Genes Induce Susceptibility to Dengue Virus Infection, while KIR3DL3 and KIR2DS5 Confer Protection

BACKGROUND AND OBJECTIVES

Dengue fever (DF), an emerging and re-emerging viral disease, is a major public health problem. The aim of this study was to investigate the influence of KIRs genes polymorphism and KIRs genotypes in susceptibility to dengue virus infection and disease severity in a population from Burkina Faso through a case-control study.

METHODS

KIRs genes determination was performed using PCR-SSP in 50 patients infected by dengue virus (DENV) and 54 Healthy controls (HC) subjects who had never been infected.

RESULTS

Data analysis showed significant association between frequencies of three KIR genes and dengue virus infection (DF): KIR2DL2 (OR: 7.32; IC: 2.87-18.65; P < 0.001); KIR2DL5A (OR: 15.00, IC: 5.68-39.59; P < 0.001) and KIR2DL5B (OR: 11.43; IC: 4.42-29; P < 0.001). While, KIR3DL3 (OR: 0.13, IC: 0.052-0.32; P < 0.001) and KIR2DS5 (OR: 0.12; IC: 0.04-0.30; P < 0.001) were associated with protection against DF. KIR2DL4 (OR: 9.75; IC95%: 1.33-70.97; p: 0.03) and KIRD3DL1 (OR: 12.00; IC95%: 1.60-90.13; p: 0.02) were associated with an increased risk in the development of secondary dengue infection (SDI).

CONCLUSION

The results suggest a contribution of KIR2DL2, KIR2DL5A, and KIR2DL5B genes in the susceptibility of DF development. In contrast, KIR3DL3 and KIR2DS5 were associated with protection against DF development by enhancing both innate and acquired immune responses.

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.

KIR channels in the microvasculature: Regulatory properties and the lipid-hemodynamic environment

Basal tone and perfusion control is set in cerebral arteries by the sensing of pressure and flow, key hemodynamic stimuli. These forces establish a contractile foundation within arterial networks upon which local neurovascular stimuli operate. This fundamental process is intimately tied to arterial V_M and the rise in cytosolic [Ca²⁺] by the graded opening of voltage-operated Ca²⁺ channels. Arterial V_M is in turn controlled by a dynamic interaction among several resident ion channels, K_IR being one of particular significance. As the name suggests, K_IR displays strong inward rectification, retains a small outward component, potentiated by extracellular K⁺ and blocked by micromolar Ba²⁺. Cerebrovascular K_IR is unique from other K⁺ currents as it is present in both smooth muscle and endothelium yet lacking in classical regulatory modulation. Such observations have fostered the view that K_IR is nothing more than a background conductance, activated by extracellular K⁺ and which passively facilitates dilation. Recent work in cell model systems has; however, identified two membrane lipids, phosphatidylinositol 4,5-bisphosphate (PIP₂) and cholesterol, that interact with K_IR2.x, to stabilize the channel in the preferred open or silent state, respectively. Translating this unique form of regulation, recent studies have demonstrated that specific lipid-protein interactions enable unique K_IR populations to sense distinct hemodynamic stimuli and set basal tone. This review summarizes the current knowledge of vascular K_IR channels and how the lipid and hemodynamic impact their activity.

Potassium channels in pancreatic duct epithelial cells: their role, function and pathophysiological relevance

Pancreatic ductal epithelial cells play a fundamental role in HCO3 (-) secretion, a process which is essential for maintaining the integrity of the pancreas. Although several studies have implicated impaired HCO3 (-) and fluid secretion as a triggering factor in the development of pancreatitis, the mechanism and regulation of HCO3 (-) secretion is still not completely understood. To date, most studies on the ion transporters that orchestrate ductal HCO3 (-) secretion have focussed on the role of Cl(-)/HCO3 (-) exchangers and Cl(-) channels, whereas much less is known about the role of K(+) channels. However, there is growing evidence that many types of K(+) channels are present in ductal cells where they have an essential role in establishing and maintaining the electrochemical driving force for anion secretion. For this reason, strategies that increase K(+) channel function may help to restore impaired HCO3 (-) and fluid secretion, such as in pancreatitis, and therefore provide novel directions for future pancreatic therapy. In this review, our aims are to summarize the types of K(+) channels found in pancreatic ductal cells and to discuss their individual roles in ductal HCO3 (-) secretion. We will also describe how K(+) channels are involved in pathophysiological conditions and discuss how they could act as new molecular targets for the development of therapeutic approaches to treat pancreatic diseases.

Physiology and pathophysiology of potassium channels in gastrointestinal epithelia

Epithelial cells of the gastrointestinal tract are an important barrier between the "milieu interne" and the luminal content of the gut. They perform transport of nutrients, salts, and water, which is essential for the maintenance of body homeostasis. In these epithelia, a variety of K(+) channels are expressed, allowing adaptation to different needs. This review provides an overview of the current literature that has led to a better understanding of the multifaceted function of gastrointestinal K(+) channels, thereby shedding light on pathophysiological implications of impaired channel function. For instance, in gastric mucosa, K(+) channel function is a prerequisite for acid secretion of parietal cells. In epithelial cells of small intestine, K(+) channels provide the driving force for electrogenic transport processes across the plasma membrane, and they are involved in cell volume regulation. Fine tuning of salt and water transport and of K(+) homeostasis occurs in colonic epithelia cells, where K(+) channels are involved in secretory and reabsorptive processes. Furthermore, there is growing evidence for changes in epithelial K(+) channel expression during cell proliferation, differentiation, apoptosis, and, under pathological conditions, carcinogenesis. In the future, integrative approaches using functional and postgenomic/proteomic techniques will help us to gain comprehensive insights into the role of K(+) channels of the gastrointestinal tract.

Adrenaline-induced hyperpolarization of mouse pancreatic islet cells is mediated by G protein-gated inwardly rectifying potassium (GIRK) channels

Insulin secretion inhibitors (ISI) such as adrenaline and somatostatin act on the pancreatic beta-cell by a number of mechanisms, one of which is plasma membrane hyperpolarization. Despite the ample evidence for this effect, the principal underlying channels have not been identified thus far. The G protein-gated inwardly rectifying potassium (Kir3.x/GIRK) channels, which are responsible for hyperpolarization in other excitable tissues, are likely candidates. In this paper, we show that GIRK channels are expressed and functional in mouse pancreatic islet cells. Reverse transcription polymerase chain reaction analysis revealed all four GIRK gene products in islet tissue. Immunofluorescent labeling of pancreatic sections demonstrated exclusive islet localization of all GIRK subunits, in part within insulin-expressing cells. Using the whole-cell configuration of the patch clamp technique, we found that the application of tertiapin-Q, a selective inhibitor of the GIRK channels, abolishes adrenaline-mediated inward currents and strongly attenuates adrenaline-induced hyperpolarization in a reversible manner. These results imply that GIRK channels are responsible for a major part of the electrical response to adrenaline in islet cells and suggest a role for these channels in pancreatic physiology.

Adrenaline-induced hyperpolarization of mouse pancreatic islet cells is mediated by G protein-gated inwardly rectifying potassium (GIRK) channels

Insulin secretion inhibitors (ISI) such as adrenaline and somatostatin act on the pancreatic beta-cell by a number of mechanisms, one of which is plasma membrane hyperpolarization. Despite the ample evidence for this effect, the principal underlying channels have not been identified thus far. The G protein-gated inwardly rectifying potassium (Kir3.x/GIRK) channels, which are responsible for hyperpolarization in other excitable tissues, are likely candidates. In this paper, we show that GIRK channels are expressed and functional in mouse pancreatic islet cells. Reverse transcription polymerase chain reaction analysis revealed all four GIRK gene products in islet tissue. Immunofluorescent labeling of pancreatic sections demonstrated exclusive islet localization of all GIRK subunits, in part within insulin-expressing cells. Using the whole-cell configuration of the patch clamp technique, we found that the application of tertiapin-Q, a selective inhibitor of the GIRK channels, abolishes adrenaline-mediated inward currents and strongly attenuates adrenaline-induced hyperpolarization in a reversible manner. These results imply that GIRK channels are responsible for a major part of the electrical response to adrenaline in islet cells and suggest a role for these channels in pancreatic physiology.

A simple nucleic acid hybridization/latex agglutination assay for the rapid detection of polymerase chain reaction amplicons

We have developed a new method for the detection of nucleic acid hybridization, based on a simple latex agglutination test that can be evaluated by the unaided eye. Nucleic acid, e.g., a polymerase chain reaction (PCR) product, is denatured and incubated with polystyrene beads carrying covalently bound complementary oligonucleotide sequences. Hybridization of the nucleic acids leads to aggregation of the latex particles, thereby verifying the presence of target sequence. The test is performed at room temperature, and results are available within 10 min. As a proof of principle, the hybridization/latex agglutination assay was applied to the detection of purified PCR fragments either specific for Salmonella spp. or a synthetic sequence, and to the detection of Salmonella enterica in artificially contaminated chicken samples. A few nanograms of purified PCR fragments were detectable. In artificially contaminated chicken samples, 3 colony-forming units (cfu)/25 g were detected in one of three replicates, and 30 cfu/25 g were detected in both of two replicates when samples for PCR were taken directly from primary enrichment, demonstrating the practical applicability of this test system. Even multiplex detection might be achievable. This novel kind of assay could be useful for a range of applications where hybridization of nucleic acids, e.g., PCR fragments, is to be detected.

Free fatty acid receptor 1 (FFA(1)R/GPR40) and its involvement in fatty-acid-stimulated insulin secretion

Free fatty acids (FFA) have generally been proposed to regulate pancreatic insulin release by an intracellular mechanism involving inhibition of CPT-1. The recently de-orphanized G-protein coupled receptor, FFA(1)R/GPR40, has been shown to be essential for fatty-acid-stimulated insulin release in MIN6 mouse insulinoma cells. The CPT-1 inhibitor, 2-bromo palmitate (2BrP), was investigated for its ability to interact with mouse FFA(1)R/GPR40. It was found to inhibit phosphatidyl inositol hydrolysis induced by linoleic acid (LA) (100 muM in all experiments) in HEK293 cells transfected with FFA(1)R/GPR40 and in the MIN6 subclone, MIN6c4. 2BrP also inhibited LA-stimulated insulin release from mouse pancreatic islets. Mouse islets were subjected to antisense intervention by treatment with a FFA(1)R/GPR40-specific morpholino oligonucleotide for 48 h. Antisense treatment of islets suppressed LA-stimulated insulin release by 50% and by almost 100% when islets were pretreated with LA for 30 min before applying the antisense. Antisense treatment had no effect on tolbutamide-stimulated insulin release. Confocal microscopy using an FFA(1)R/GPR40-specific antibody revealed receptor expression largely localized to the plasma membrane of insulin-producing cells. Pretreating the islets with LA for 30 min followed by antisense oligonucleotide treatment for 48 h reduced the FFA(1)R/GPR40 immunoreactivity to background levels. The results demonstrate that FFA(1)R/GPR40 is inhibited by the CPT-1 inhibitor, 2BrP, and confirm that FFA(1)R/GPR40 is indeed necessary, at least in part, for fatty-acid-stimulated insulin release.

Dopamine D2-like receptors are expressed in pancreatic beta cells and mediate inhibition of insulin secretion

Dopamine signaling is mediated by five cloned receptors, grouped into D1-like (D1 and D5) and D2-like (D2, D3 and D4) families. We identified by reverse transcription-PCR the presence of dopamine receptors from both families in INS-1E insulin-secreting cells as well as in rodent and human isolated islets. D2 receptor expression was confirmed by immunodetection revealing localization on insulin secretory granules of INS-1E and primary rodent and human beta cells. We then tested potential effects mediated by the identified receptors on beta cell function. Dopamine (10 microM) and the D2-like receptor agonist quinpirole (5 microM) inhibited glucose-stimulated insulin secretion tested in several models, i.e. INS-1E beta cells, fluorescence-activated cell-sorted primary rat beta cells, and pancreatic islets of rat, mouse, and human origin. Insulin exocytosis is controlled by metabolism coupled to cytosolic calcium changes. Measurements of glucose-induced mitochondrial hyperpolarization and ATP generation showed that dopamine and D2-like agonists did not inhibit glucose metabolism. On the other hand, dopamine decreased cell membrane depolarization as well as cytosolic calcium increases evoked by glucose stimulation in INS-1E beta cells. These results show for the first time that dopamine receptors are expressed in pancreatic beta cells. Dopamine inhibited glucose-stimulated insulin secretion, an effect that could be ascribed to D2-like receptors. Regarding the molecular mechanisms implicated in dopamine-mediated inhibition of insulin release, our results point to distal steps in metabolism-secretion coupling. Thus, the role played by dopamine in glucose homeostasis might involve dopamine receptors, expressed in pancreatic beta cells, modulating insulin release.

Human adipose cells have voltage-dependent potassium currents

The whole-cell patch-clamp method was used to study the membrane electrical properties of human adipocyte cells obtained by differentiating from precursors of human abdominal and mammary tissues. All differentiated cells exhibited outward currents with sigmoidal activation kinetics. The outward currents showed activation thresholds between -20 to -30 mV and slow inactivation. The ionic channels underlying the macroscopic current were highly selective for K(+). Their selectivity was for typical K(+) channels with relative permeabilities of K(+)>NH4+>Cs(+)>Na(+). No evidence of any other type of voltage-gated channel was found. The potassium currents ( I(KV)) were blocked reversibly by tetraethylammonium and barium. The IC(50) value and Hill coefficient of tetraethylammonium inhibition of I(KV) were 0.56 m M and 1.17 respectively. These results demonstrate that human adipose cells have voltage-dependent potassium currents.

Molecular screening and association studies of retinoid-related orphan receptor gamma (RORC): a positional and functional candidate for type 2 diabetes

The retinoid-related orphan receptor gamma (RORC) is a member of the nuclear hormone superfamily which maps to the 1q21-q23 region. Linkage of type 2 diabetes (T2DM) to this region is well replicated. Several factors argue that RORC is a strong candidate for T2DM susceptibility within this region. RORC may form heterodimers with peroxisome-proliferator activated receptor gamma, it is expressed at high levels in skeletal muscle, and expression is induced in adipocytes during differentiation. To test the hypothesis that sequence variation in RORC is a risk factor for T2DM, we screened approximately 21kb of DNA for sequence variation, including 11 exons of the RORC gene, a region 1-kb upstream (5' flanking region), intronic regions flanking the exons, and the entire 3' untranslated region (UTR). Screening was performed using single strand conformation polymorphism (SSCP) analysis in Caucasian individuals of northern European ancestry and in African American individuals. We detected 11 single nucleotide polymorphisms (SNPs), ranging from the promoter region to intron 10. We also confirmed 2 SNPs from public databases that were in regions not included in our screening. Only 1 SNP was nonsynonymous, resulting in Ala to Gly at residue 464 (exon 10). All other SNPs were noncoding. One SNP (intron 3) was unique to Caucasians, and three SNPs (Ala464Gly, intron 2, intron 6) were specific to African American subjects. We typed 7 SNPs spanning the gene from the promoter to 3' UTR in unrelated cases with T2DM and controls of Northern European ancestry. We also tested linkage of a microsatellite within the RORC gene. Modest evidence for linkage (LOD=1.47) was seen on two-point analysis, but no linkage to the RORC region was found on multipoint analysis. However, transmission of the microsatellite alleles from parents to affected offspring showed a trend to deviate from the expected 50% (p=0.078). No association of any other SNP with T2DM was found, but the Ala454Gly variant was 3-fold more common among African American patients with diabetes than in controls. SNPs 1, 2 and 4 were in strong linkage disequilibrium (D>0.85) and may constitute a haplotype block. Our data suggest that RORC cannot explain the linkage of T2DM in this region. The role of the unusual Ala454Gly variant will require a much larger study size to evaluate.

Identification of a single QTL, Mptp1, for susceptibility to MPTP-induced substantia nigra pars compacta neuron loss in mice

The loss of substantia nigra pars compacta (SNpc) neurons seen in idiopathic Parkinson's disease is hypothesized to result from a genetic susceptibility to an unknown environmental toxin. MPTP has been used as a prototypical toxin, since exposure to this drug results in variable SNpc cell death in several vertebrate species, including man and mouse. Previously, we have shown that C57BL/6J mice are sensitive to this compound, while Swiss-Webster mice are resistant. In this study, we intercrossed these mouse strains to map quantitative trait loci (QTL) for MPTP sensitivity. Using genome wide PCR analysis, we found that a single major QTLs, Mptp1, located near the distal end of chromosome 1 between D1Mit113 and D1Mit293, accounts for the majority of the strain sensitivity to MPTP.

Mutation screening and association of human retinoid X receptor gamma variation with lipid levels in familial type 2 diabetes

Both type 2 diabetes (T2DM) and familial combined hyperlipidemia have been mapped to human chromosome 1q21-q24. This region includes the retinoid X receptor gamma (RXRgamma), which is a strong candidate for both glucose and lipid metabolism. Retinoid X receptors form heterodimers with a variety of nuclear receptors, including peroxisome-proliferator-activated receptors alpha and gamma (PPARalpha and PPARgamma), and are synergistic targets for drugs that alter glucose and lipid metabolism. We hypothesized that RXRgamma variation could explain the linkage of diabetes and lipid disorders to this region. We screened each of the 10 exons, the flanking intronic sequences, the 3' untranslated region, and the 5' flanking region. We identified 14 variants, none of which altered the coding sequence. Of the 10 variants examined in a diabetes case-control study, three showed nominal (p < 0.05) associations with T2DM. We subsequently typed four variants in all members of the 63 multiplex families used in our previous linkage analysis. No individual variant showed excess transmission to offspring with T2DM using a transmission disequilibrium test and only a single rare haplotype showed evidence of an association with T2DM. Likewise, neither individual variants nor haplotypes were associated with either fasting or post-challenge glucose in non-diabetic subjects. In contrast, three of the four variants were associated with fasting free fatty acid (FFA) levels (p = 0.024-0.00044) and two variants were associated with triglyceride levels (p < 0.05). These findings were supported by the association of several haplotypes with FFA and triglyceride levels. RXRgamma haplotypes were also associated with several measures of pancreatic beta-cell function, consistent with the proposed role of lipid metabolism in insulin secretion. These data suggest that RXRgamma may contribute to disordered lipid metabolism in members of familial T2DM kindreds, but this gene is unlikely to explain the linkage of T2DM with this region.

Analysis of linkage disequilibrium between polymorphisms in the KCNJ9 gene with type 2 diabetes mellitus in Pima Indians

The KCNJ9 gene encodes a G-protein-coupled inwardly rectifying potassium channel and is located within a region on human chromosome 1 that has been linked with type 2 diabetes mellitus in Pima Indians and Caucasians. To assess the potential contribution of genetic alterations within KCNJ9 to diabetes susceptibility in the Pimas, we have genotyped 11 single nucleotide polymorphisms (SNPs) in 50 Pimas with diabetes and 50 Pimas over the age of 45 without diabetes and in 51 sib pairs, discordant for the disease, who were characterized by decreased allele sharing at the chromosomal location of the maximum LOD score. We detected three SNP clusters exhibiting distinct linkage disequilibria. Polymorphisms in intron 2, exon 3, and the 3'-UTR were in statistically significant linkage disequilibrium with diabetes in the case-control group (P = 0.006), but not the sibling pairs (P = 0.097). A weak association with diabetes was also found in the original linkage set comprising 1150 Pimas (odds ratio = 0.64/P = 0.079 for a dominant model and OR = 0.67/P = 0.005 for a recessive model). However, no effect on linkage was detected following adjustment for one of the most strongly associated SNPs in the entire original linkage set. Our results indicate that variants in KCNJ9 are associated with diabetes in Pimas but do not account for the linkage of 1q with diabetes in this population.