Subunit interactions in the assembly of neuronal Kir3.0 inwardly rectifying K+ channels

Preassembly of specific Gβγ subunits at GABAB receptors through auxiliary KCTD proteins accelerates channel gating

GABAB receptors (GBRs) are G protein-coupled receptors for GABA, the main inhibitory neurotransmitter in the brain. GBRs regulate fast synaptic transmission by gating Ca2+ and K+ channels via the Gβγ subunits of the activated G protein. It has been demonstrated that auxiliary GBR subunits, the KCTD proteins, shorten onset and rise time and increase desensitization of receptor-induced K+ currents. KCTD proteins increase desensitization of K+ currents by scavenging Gβγ from the channel, yet the mechanism responsible for the rapid activation of K+ currents has remained elusive. In this study, we demonstrate that KCTD proteins preassemble Gβγ at GBRs. The preassembly obviates the need for diffusion-limited G protein recruitment to the receptor, thereby accelerating G protein activation and, as a result, K+ channel activation. Preassembly of Gβγ at the receptor relies on the interaction of KCTD proteins with a loop protruding from the seven-bladed propeller of Gβ subunits. The binding site is shared between Gβ1 and Gβ2, limiting the interaction of KCTD proteins to these particular Gβ isoforms. Substituting residues in the KCTD binding site of Gβ1 with those from Gβ3 hinders the preassembly of Gβγ with GBRs, delays onset and prolongs rise time of receptor-activated K+ currents. The KCTD-Gβ interface, therefore, represents a target for pharmacological modulation of channel gating by GBRs.

Soluble amyloid-β precursor peptide does not regulate GABAB receptor activity

Amyloid-β precursor protein (APP) regulates neuronal activity through the release of secreted APP (sAPP) acting at cell surface receptors. APP and sAPP were reported to bind to the extracellular sushi domain 1 (SD1) of GABA_B receptors (GBRs). A 17 amino acid peptide (APP17) derived from APP was sufficient for SD1 binding and shown to mimic the inhibitory effect of sAPP on neurotransmitter release and neuronal activity. The functional effects of APP17 and sAPP were similar to those of the GBR agonist baclofen and blocked by a GBR antagonist. These experiments led to the proposal that sAPP activates GBRs to exert its neuronal effects. However, whether APP17 and sAPP influence classical GBR signaling pathways in heterologous cells was not analyzed. Here, we confirm that APP17 binds to GBRs with nanomolar affinity. However, biochemical and electrophysiological experiments indicate that APP17 does not influence GBR activity in heterologous cells. Moreover, APP17 did not regulate synaptic GBR localization, GBR-activated K⁺ currents, neurotransmitter release, or neuronal activity in vitro or in vivo. Our results show that APP17 is not a functional GBR ligand and indicate that sAPP exerts its neuronal effects through receptors other than GBRs.

Kir3 channel blockade in the cerebellar cortex suppresses performance of classically conditioned Purkinje cell responses

In the eyeblink conditioning paradigm, cerebellar Purkinje cells learn to respond to the conditional stimulus with an adaptively timed pause in its spontaneous firing. Evidence suggests that the pause is elicited by glutamate released from parallel fibers and acting on metabotropic receptors (mGluR7) which initiates a delayed-onset suppression of firing. We suggested that G protein activation of hyperpolarizing K_ir3 channels (or ‘GIRK’, G protein-coupled inwardly-rectifying K⁺ channels) could be part of such a mechanism. Application of the K_ir3 antagonist Tertiapin-LQ locally in the superficial layers of the cerebellar cortex in decerebrate ferrets suppressed normal performance of Purkinje cell pause responses to the conditional stimulus. Importantly, there was no detectable effect on spontaneous firing. These findings suggest that intact functioning of K_ir3 channels in the cerebellar cortex is required for normal conditioned Purkinje cell responses.

Screening Technologies for Inward Rectifier Potassium Channels: Discovery of New Blockers and Activators

K⁺ channels play a critical role in maintaining the normal electrical activity of excitable cells by setting the cell resting membrane potential and by determining the shape and duration of the action potential. In nonexcitable cells, K⁺ channels establish electrochemical gradients necessary for maintaining salt and volume homeostasis of body fluids. Inward rectifier K⁺ (Kir) channels typically conduct larger inward currents than outward currents, resulting in an inwardly rectifying current versus voltage relationship. This property of inward rectification results from the voltage-dependent block of the channels by intracellular polyvalent cations and makes these channels uniquely designed for maintaining the resting potential near the K⁺ equilibrium potential (E_K). The Kir family of channels consist of seven subfamilies of channels (Kir1.x through Kir7.x) that include the classic inward rectifier (Kir2.x) channel, the G-protein-gated inward rectifier K⁺ (GIRK) (Kir3.x), and the adenosine triphosphate (ATP)-sensitive (K_ATP) (Kir 6.x) channels as well as the renal Kir1.1 (ROMK), Kir4.1, and Kir7.1 channels. These channels not only function to regulate electrical/electrolyte transport activity, but also serve as effector molecules for G-protein-coupled receptors (GPCRs) and as molecular sensors for cell metabolism. Of significance, Kir channels represent promising pharmacological targets for treating a number of clinical conditions, including cardiac arrhythmias, anxiety, chronic pain, and hypertension. This review provides a brief background on the structure, function, and pharmacology of Kir channels and then focuses on describing and evaluating current high-throughput screening (HTS) technologies, such as membrane potential-sensitive fluorescent dye assays, ion flux measurements, and automated patch clamp systems used for Kir channel drug discovery.

Chloroform is a potent activator of cardiac and neuronal Kir3 channels

Chloroform has been used over decades in anesthesia before it was replaced by other volatile anesthetics like halothane or sevoflurane. Some of the reasons were inadmissible side effects of chloroform like bradycardia or neural illness. In the present study, we identified members of the G protein-activated inwardly rectifying potassium channel family (Kir3) expressed in Xenopus oocytes as potential common molecular targets for both the neural and cardiac effects of chloroform. Millimolar concentration currents representing a 1:10000 dilution of commercially available chloroform were used in laboratories that augment neuronal Kir3.1/3.2 currents as well as cardiac Kir3.1/3.4. This effect was selective and only observed in currents from Kir3 subunits but not in currents from Kir2 subunits. Augmentation of atrial Kir3.1/3.4 currents leads to an effective drop of the heart rate and a reduction in contraction force in isolated mouse atria.

Down syndrome

Down syndrome (DS; Trisomy 21) is the most common chromosomal disorder in humans. It has numerous associated neurologic phenotypes including intellectual disability, sleep apnea, seizures, behavioral problems, and dementia. With improved access to medical care, people with DS are living longer than ever before. As more individuals with DS reach old age, the necessity for further life span research is essential and cannot be overstated. There is currently a scarcity of information on common medical conditions encountered as individuals with DS progress into adulthood and old age. Conflicting information and uncertainty about the relative risk of dementia for adults with DS is a source of distress for the DS community that creates a major obstacle to proper evaluation and treatment. In this chapter, we discuss the salient neurologic phenotypes of DS, including Alzheimer's disease (AD), and current understanding of their biologic bases and management.

Evidence that increased Kcnj6 gene dose is necessary for deficits in behavior and dentate gyrus synaptic plasticity in the Ts65Dn mouse model of Down syndrome

Down syndrome (DS), trisomy 21, is caused by increased dose of genes present on human chromosome 21 (HSA21). The gene-dose hypothesis argues that a change in the dose of individual genes or regulatory sequences on HSA21 is necessary for creating DS-related phenotypes, including cognitive impairment. We focused on a possible role for Kcnj6, the gene encoding Kir3.2 (Girk2) subunits of a G-protein-coupled inwardly-rectifying potassium channel. This gene resides on a segment of mouse Chromosome 16 that is present in one extra copy in the genome of the Ts65Dn mouse, a well-studied genetic model of DS. Kir3.2 subunit-containing potassium channels serve as effectors for a number of postsynaptic metabotropic receptors including GABAB receptors. Several studies raise the possibility that increased Kcnj6 dose contributes to synaptic and cognitive abnormalities in DS. To assess directly a role for Kcnj6 gene dose in cognitive deficits in DS, we produced Ts65Dn mice that harbor only 2 copies of Kcnj6 (Ts65Dn:Kcnj6++- mice). The reduction in Kcnj6 gene dose restored to normal the hippocampal level of Kir3.2. Long-term memory, examined in the novel object recognition test with the retention period of 24h, was improved to the level observed in the normosomic littermate control mice (2N:Kcnj6++). Significantly, both short-term and long-term potentiation (STP and LTP) was improved to control levels in the dentate gyrus (DG) of the Ts65Dn:Kcnj6++- mouse. In view of the ability of fluoxetine to suppress Kir3.2 channels, we asked if fluoxetine-treated DG slices of Ts65Dn:Kcnj6+++ mice would rescue synaptic plasticity. Fluoxetine increased STP and LTP to control levels. These results are evidence that increased Kcnj6 gene dose is necessary for synaptic and cognitive dysfunction in the Ts65Dn mouse model of DS. Strategies aimed at pharmacologically reducing channel function should be explored for enhancing cognition in DS.

Mitragynine and its potential blocking effects on specific cardiac potassium channels

Mitragyna speciosa Korth is known for its euphoric properties and is frequently used for recreational purposes. Several poisoning and fatal cases involving mitragynine have been reported but the underlying causes remain unclear. Human ether-a-go-go-related gene (hERG) encodes the cardiac IKr current which is a determinant of the duration of ventricular action potentials and QT interval. On the other hand, IK1, a Kir current mediated by Kir2.1 channel and IKACh, a receptor-activated Kir current mediated by GIRK channel are also known to be important in maintaining the cardiac function. This study investigated the effects of mitragynine on the current, mRNA and protein expression of hERG channel in hERG-transfected HEK293 cells and Xenopus oocytes. The effects on Kir2.1 and GIRK channels currents were also determined in the oocytes. The hERG tail currents following depolarization pulses were inhibited by mitragynine with an IC50 value of 1.62μM and 1.15μM in the transfected cell line and Xenopus oocytes, respectively. The S6 point mutations of Y652A and F656A attenuated the inhibitor effects of mitragynine, indicating that mitragynine interacts with these high affinity drug-binding sites in the hERG channel pore cavity which was consistent with the molecular docking simulation. Interestingly, mitragynine does not affect the hERG expression at the transcriptional level but inhibits the protein expression. Mitragynine is also found to inhibit IKACh current with an IC50 value of 3.32μM but has no significant effects on IK1. Blocking of both hERG and GIRK channels may cause additive cardiotoxicity risks.

Tandem pore domain halothane-inhibited K+ channel subunits THIK1 and THIK2 assemble and form active channels

Despite a high level of sequence homology, tandem pore domain halothane-inhibited K(+) channel 1 (THIK1) produces background K(+) currents, whereas THIK2 is silent. This lack of activity is due to a unique combination of intracellular retention and weak basal activity in the plasma membrane. Here, we designed THIK subunits containing dominant negative mutations (THIK1(DN) and THIK2(DN)). THIK2(DN) mutant inhibits THIK1 currents, whereas THIK1(DN) inhibits an activated form of THIK2 (THIK2-A155P-I158D). In situ proximity ligation assays and Förster/fluorescence resonance energy transfer (FRET) experiments support a physical association between THIK1 and THIK2. Next, we expressed covalent tandems of THIK proteins to obtain expression of pure heterodimers. Td-THIK1-THIK2 (where Td indicates tandem) produces K(+) currents of amplitude similar to Td-THIK1-THIK1 but with a noticeable difference in the current kinetics. Unlike Td-THIK2-THIK2 that is mainly detected in the endoplasmic reticulum, Td-THIK1-THIK2 distributes at the plasma membrane, indicating that THIK1 can mask the endoplasmic reticulum retention/retrieval motif of THIK2. Kinetics and unitary conductance of Td-THIK1-THIK2 are intermediate between THIK1 and THIK2. Altogether, these results show that THIK1 and THIK2 form active heteromeric channels, further expanding the known repertoire of K(+) channels.

Up-regulation of GABA(B) receptor signaling by constitutive assembly with the K+ channel tetramerization domain-containing protein 12 (KCTD12)

GABA_B receptors are the G-protein coupled receptors (GPCRs) for GABA, the main inhibitory neurotransmitter in the central nervous system. Native GABA_B receptors comprise principle and auxiliary subunits that regulate receptor properties in distinct ways. The principle subunits GABA_B1a, GABA_B1b, and GABA_B2 form fully functional heteromeric GABA_B(1a,2) and GABA_B(1b,2) receptors. Principal subunits regulate forward trafficking of the receptors from the endoplasmic reticulum to the plasma membrane and control receptor distribution to axons and dendrites. The auxiliary subunits KCTD8, -12, -12b, and -16 are cytosolic proteins that influence agonist potency and G-protein signaling of GABA_B(1a,2) and GABA_B(1b,2) receptors. Here, we used transfected cells to study assembly, surface trafficking, and internalization of GABA_B receptors in the presence of the KCTD12 subunit. Using bimolecular fluorescence complementation and metabolic labeling, we show that GABA_B receptors associate with KCTD12 while they reside in the endoplasmic reticulum. Glycosylation experiments support that association with KCTD12 does not influence maturation of the receptor complex. Immunoprecipitation and bioluminescence resonance energy transfer experiments demonstrate that KCTD12 remains associated with the receptor during receptor activity and receptor internalization from the cell surface. We further show that KCTD12 reduces constitutive receptor internalization and thereby increases the magnitude of receptor signaling at the cell surface. Accordingly, knock-out or knockdown of KCTD12 in cultured hippocampal neurons reduces the magnitude of the GABA_B receptor-mediated K⁺ current response. In summary, our experiments support that the up-regulation of functional GABA_B receptors at the neuronal plasma membrane is an additional physiological role of the auxiliary subunit KCTD12.

Opposite effects of KCTD subunit domains on GABA(B) receptor-mediated desensitization

GABA(B) receptors assemble from principle and auxiliary subunits. The principle subunits GABA(B1) and GABA(B2) form functional heteromeric GABA(B(1,2)) receptors that associate with homotetramers of auxiliary KCTD8, -12, -12b, or -16 (named after their K(+) channel tetramerization domain) subunits. These auxiliary subunits constitute receptor subtypes with distinct functional properties. KCTD12 and -12b generate desensitizing receptor responses while KCTD8 and -16 generate largely non-desensitizing receptor responses. The structural elements of the KCTDs underlying these differences in desensitization are unknown. KCTDs are modular proteins comprising a T1 tetramerization domain, which binds to GABA(B2), and a H1 homology domain. KCTD8 and -16 contain an additional C-terminal H2 homology domain that is not sequence-related to the H1 domains. No functions are known for the H1 and H2 domains. Here we addressed which domains and sequence motifs in KCTD proteins regulate desensitization of the receptor response. We found that the H1 domains in KCTD12 and -12b mediate desensitization through a particular sequence motif, T/NFLEQ, which is not present in the H1 domains of KCTD8 and -16. In addition, the H2 domains in KCTD8 and -16 inhibit desensitization when expressed C-terminal to the H1 domains but not when expressed as a separate protein in trans. Intriguingly, the inhibitory effect of the H2 domain is sequence-independent, suggesting that the H2 domain sterically hinders desensitization by the H1 domain. Evolutionary analysis supports that KCTD12 and -12b evolved desensitizing properties by liberating their H1 domains from antagonistic H2 domains and acquisition of the T/NFLEQ motif.

Functional consequences of the interactions among the neural cell adhesion molecule NCAM, the receptor tyrosine kinase TrkB, and the inwardly rectifying K+ channel KIR3.3

Cell adhesion molecules and neurotrophin receptors are crucial for the development and the function of the nervous system. Among downstream effectors of neurotrophin receptors and recognition molecules are ion channels. Here, we provide evidence that G protein-coupled inwardly rectifying K(+) channel Kir3.3 directly binds to the neural cell adhesion molecule (NCAM) and neurotrophin receptor TrkB. We identified the binding sites for NCAM and TrkB at the C-terminal intracellular domain of Kir3.3. The interaction between NCAM, TrkB, and Kir3.3 was supported by immunocytochemical co-localization of Kir3.3, NCAM, and/or TrkB at the surface of hippocampal neurons. Co-expression of TrkB and Kir3.1/3.3 in Xenopus oocytes increased the K(+) currents evoked by Kir3.1/3.3 channels. This current enhancement was reduced by the concomitant co-expression with NCAM. Both surface fluorescence measurements of microinjected oocytes and cell surface biotinylation of transfected CHO cells indicated that the cell membrane localization of Kir3.3 is regulated by TrkB and NCAM. Furthermore, the level of Kir3.3, but not of Kir3.2, at the plasma membranes was reduced in TrkB-deficient mice, supporting the notion that TrkB regulates the cell surface expression of Kir3.3. The premature expression of developmentally late appearing Kir3.1/3.3 in hippocampal neurons led to a reduction of NCAM-induced neurite outgrowth. Our observations indicate a decisive role for the neuronal K(+) channel in regulating NCAM-dependent neurite outgrowth and attribute a physiologically meaningful role to the functional interplay of Kir3.3, NCAM, and TrkB in ontogeny.

Native GABA(B) receptors are heteromultimers with a family of auxiliary subunits

GABA(B) receptors are the G-protein-coupled receptors for gamma-aminobutyric acid (GABA), the main inhibitory neurotransmitter in the brain. They are expressed in almost all neurons of the brain, where they regulate synaptic transmission and signal propagation by controlling the activity of voltage-gated calcium (Ca(v)) and inward-rectifier potassium (K(ir)) channels. Molecular cloning revealed that functional GABA(B) receptors are formed by the heteromeric assembly of GABA(B1) with GABA(B2) subunits. However, cloned GABA(B(1,2)) receptors failed to reproduce the functional diversity observed with native GABA(B) receptors. Here we show by functional proteomics that GABA(B) receptors in the brain are high-molecular-mass complexes of GABA(B1), GABA(B2) and members of a subfamily of the KCTD (potassium channel tetramerization domain-containing) proteins. KCTD proteins 8, 12, 12b and 16 show distinct expression profiles in the brain and associate tightly with the carboxy terminus of GABA(B2) as tetramers. This co-assembly changes the properties of the GABA(B(1,2)) core receptor: the KCTD proteins increase agonist potency and markedly alter the G-protein signalling of the receptors by accelerating onset and promoting desensitization in a KCTD-subtype-specific manner. Taken together, our results establish the KCTD proteins as auxiliary subunits of GABA(B) receptors that determine the pharmacology and kinetics of the receptor response.

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.

Pre-synaptic GABA receptors inhibit glutamate release through GIRK channels in rat cerebral cortex

Neuronal G protein-gated inwardly rectifying potassium (GIRK) channels mediate the slow inhibitory effects of many neurotransmitters post-synaptically. However, no evidence exists that supports that GIRK channels play any role in the inhibition of glutamate release by GABA(B) receptors. In this study, we show for the first time that GABA(B) receptors operate through two mechanisms in nerve terminals from the cerebral cortex. As shown previously, GABA(B) receptors reduces glutamate release and the Ca(2+) influx mediated by N-type Ca(2+) channels in a mode insensitive to the GIRK channel blocker tertiapin-Q and consistent with direct inhibition of this voltage-gated Ca(2+) channel. However, by means of weak stimulation protocols, we reveal that GABA(B) receptors also reduce glutamate release mediated by P/Q-type Ca(2+) channels, and that these responses are reversed by the GIRK channel blocker tertiapin-Q. Consistent with the functional interaction between GABA(B) receptors and GIRK channels at nerve terminals we demonstrate by immunogold electron immunohistochemistry that pre-synaptic boutons of asymmetric synapses co-express GABA(B) receptors and GIRK channels, thus suggesting that the functional interaction of these two proteins, found at the post-synaptic level, also occurs at glutamatergic nerve terminals.

Knock-in mice lacking the PDZ-ligand motif of mGluR7a show impaired PKC-dependent autoinhibition of glutamate release, spatial working memory deficits, and increased susceptibility to pentylenetetrazol

The metabotropic glutamate receptor 7 (mGluR7) is widely expressed throughout the brain and primarily localized at presynaptic active zones, where it is thought to regulate neurotransmitter release. Protein interacting with C kinase 1 (PICK1), a postsynaptic density protein-95/disc-large tumor suppressor protein/zonula occludens-1 (PDZ)-domain protein, binds to the three C-terminal amino acids (-LVI) of the predominant mGluR7 splice variant, mGluR7a, and has been implicated in the synaptic clustering of this receptor. Here, we generated knock-in mice in which the C-terminal LVI coding sequence of exon 10 of the mGluR7 gene was replaced by three alanine codons (-AAA). Immunoprecipitation showed that the PICK1-mGluR7a interaction is disrupted in mGluR7a(AAA/AAA) mice. However, the synaptic localization of mGluR7a was not altered in cultured hippocampal neurons and brain sections prepared from the knock-in animals. In cerebellar granule cell cultures, the group III mGluR agonist l-AP-4 decreased the frequency of spontaneous excitatory currents in neurons derived from wild-type but not mGluR7a(AAA/AAA) mice, consistent with the interaction between mGluR7a and PICK1 being required for protein kinase C-mediated inhibition of glutamate release. At the behavioral level, the mGluR7a(AAA/AAA) mice showed no deficits in motor coordination, pain sensitivity, and anxiety but exhibited significant defects in hippocampus-dependent spatial working memory. In addition, they displayed a high susceptibility to the convulsant drug pentylenetetrazole. Together, these results indicate that PICK1 binding to the C-terminal region of mGluR7a is crucial for agonist-triggered presynaptic signaling in vivo.

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.

Expression of inwardly rectifying potassium channel subunits in native human retinal pigment epithelium

Previously, we demonstrated that the inwardly rectifying K(+) (Kir) channel subunit Kir7.1 is highly expressed in bovine and human retinal pigment epithelium (RPE). The purpose of this study was to determine whether any of the 14 other members of the Kir gene family are expressed in native human RPE. Conventional reverse transcription-polymerase chain reaction (RT-PCR) analysis indicated that in addition to Kir7.1, seven other Kir channel subunits (Kir1.1, Kir2.1, Kir2.2, Kir3.1, Kir3.4, Kir4.2 and Kir6.1) are expressed in the RPE, whereas in neural retina, all 14 of the Kir channel subunits examined are expressed. The identities of RT-PCR products in the RPE were confirmed by DNA sequencing. Real-time RT-PCR analysis showed, however, that transcripts of these channels are significantly less abundant than Kir7.1 in the RPE. Western blot analysis of the Kir channel subunits detected in the RPE by RT-PCR revealed the expression of Kir2.1, Kir3.1, Kir3.4, Kir4.2, Kir6.1, and possibly Kir2.2, but not Kir1.1, in both human RPE and neural retina. Our results indicate that human RPE expresses at least five other Kir channel subtypes in addition to Kir7.1, suggesting that multiple members of the Kir channel family may function in this epithelium.

Cell type-specific subunit composition of G protein-gated potassium channels in the cerebellum

G protein-gated inwardly rectifying potassium (GIRK/Kir3) channels regulate cellular excitability and neurotransmission. In this study, we used biochemical and morphological techniques to analyze the cellular and subcellular distributions of GIRK channel subunits, as well as their interactions, in the mouse cerebellum. We found that GIRK1, GIRK2, and GIRK3 subunits co-precipitated with one another in the cerebellum and that GIRK subunit ablation was correlated with reduced expression levels of residual subunits. Using quantitative RT-PCR and immunohistochemical approaches, we found that GIRK subunits exhibit overlapping but distinct expression patterns in various cerebellar neuron subtypes. GIRK1 and GIRK2 exhibited the most widespread and robust labeling in the cerebellum, with labeling particularly prominent in granule cells. A high degree of molecular diversity in the cerebellar GIRK channel repertoire is suggested by labeling seen in less abundant neuron populations, including Purkinje neurons (GIRK1/GIRK2/GIRK3), basket cells (GIRK1/GIRK3), Golgi cells (GIRK2/GIRK4), stellate cells (GIRK3), and unipolar brush cells (GIRK2/GIRK3). Double-labeling immunofluorescence and electron microscopies showed that GIRK subunits were mainly found at post-synaptic sites. Altogether, our data support the existence of rich GIRK molecular and cellular diversity, and provide a necessary framework for functional studies aimed at delineating the contribution of GIRK channels to synaptic inhibition in the cerebellum.

Heterogeneous Distribution of Kir3 Potassium Channel Proteins Within Dopaminergic Neurons in the Mesencephalon of the Rat Brain

1. Dopaminergic neurons in the substantia nigra pars compacta (SNc) and the ventral tegmental area (VTA) of the ventral mesencephalon play an important role in the regulation of the parallel basal ganglia loops.2. We have raised affinity-purified polyclonal rabbit antibodies specific for all four members of the Kir3 family of inwardly rectifying potassium channels (Kir3.1-Kir3.4) to investigate the distribution of the channel proteins in the dopaminergic neurons of the rat mesencephalon at light and electron microscopic level. In addition, immunocytochemical double labeling with tyrosine hydroxylase (TH), a marker of dopaminergic neurons, were performed.3. All Kir3 channels were present in this region. However, the individual proteins showed differential cellular and subcellular distributions.4. Kir3.1 immunoreactivity was found in SNc fibers and some neurons of the substantia nigra pars reticulata (SNr). Few Kir3.3-positive neurons were found in the SNc. However, a strong Kir3.3 signal was identified in the SNr neuropil. Weak Kir3.4 staining was detected in neuronal somata as well as in dendritic fibers of both parts of the SN.5. In the VTA, Kir3.1, Kir3.3, and Kir3.4 showed only weak staining of neuropil structures. The distribution of the Kir3.2 channel protein was especially striking with strong labeling in the SNc and in the lateral but not central VTA.6. Our results suggest that the heterogeneously distributed Kir3.2 channel proteins could help to discriminate the dopaminergic neurons of VTA and SNc.

Gender-dependent regulation of G-protein-gated inwardly rectifying potassium current in dorsal raphe neurons in knock-out mice devoid of the 5-hydroxytryptamine transporter

Agonists at G-protein-coupled receptors in neurons of the dorsal raphe nucleus (DRN) of knock-out mice devoid of the serotonin transporter (5-HTT(-/-)) exhibit lower efficacy to inhibit cellular discharge than in wild-type counterparts. Using patch-clamp whole-cell recordings, we found that a G-protein-gated inwardly rectifying potassium (GIRK) current is involved in the inhibition of spike discharge induced by 5-HT1A agonists (5-carboxamidotryptamine (5-CT) and (+/-)-2-dipropylamino-8-hydroxy-1,2,3,4-tetrahydronaphthalene hydrobromide (8-OH-DPAT); 50 nM-30 microM) in both wild-type and 5-HTT(-/-) female and male mice. These effects were mimicked by 5'-guanylyl-imido-diphosphate (Gpp(NH)p; 400 microM) dialysis into cells with differences between genders. The 5-HTT(-/-) knock-out mutation reduced the current density induced by Gpp(NH)p in females but not in males. These data suggest that the decreased response of 5-HT1A receptors to agonists in 5-HTT(-/-) mutants reflects notably alteration in the coupling between G-proteins and GIRK channels in females but not in males. Accordingly, gender differences in central 5-HT neurotransmission appear to depend-at least in part-on sex-related variations in corresponding receptor-G protein signaling mechanisms.

NovelDrosophilatwo-pore domain K+channels: rescue of channel function by heteromeric assembly

Ten genes with essential structural features of two-pore domain potassium channels were identified in the genome of Drosophila melanogaster. Two Drosophila two-pore domain potassium subunits displayed substantial amino acid similarity to human TWIK-related acid-sensitive K(+) (TASK) channels (38-43%), whereas all others were less than 26% similar to any human homolog. The cDNAs of Drosophila TASK (dTASK)-6 and dTASK-7 channels were isolated from adult fruit flies. In Northern blots dTASK transcripts were found predominantly in the head fraction of adult flies and whole-mount brain in situ hybridizations showed strongly overlapping expression patterns of both dTASK isoforms in the antennal lobes. When heterologously expressed in Drosophila Schneider 2 cells, dTASK-6 gave rise to rapidly activating K(+)-selective currents that steeply depended on external pH. Structural elements in the extracellular M1-P1 loop of dTASK-6 were found to be involved in proton sensation. In contrast to mammalian TASK channels, the pH sensitivity was independent of extracellular histidines adjacent to the GYG selectivity filter (His98). As revealed by mutational analysis, functional expression of dTASK-7 was prevented by two nonconserved amino acids (Ala92-Met93) in the pore domain. When these two residues were replaced by conserved Thr92-Thr93, typical K(+)-selective leak currents were generated that were insensitive to changes in external pH. Nonfunctional wildtype dTASK-7 channels appeared to form heteromeric assemblies with dTASK-6. Following cotransfection of dTASK-6 and wildtype dTASK-7 (or when engineered as concatemers), K(+) currents were observed that were smaller in amplitude, harbored slower activation kinetics and were considerably less inhibited by local anesthetics as compared with dTASK-6. Thus, pore-loop residues in dTASK-7 changed functional and pharmacological properties in heteromeric dTASK channels.

GABA(B) receptor-mediated modulation of glutamate signaling in cerebellar Purkinje cells

Since Purkinje cells are the sole output neurons of the cerebellar cortex, the postsynaptic integration of excitatory and inhibitory synaptic inputs in this cell type is a pivotal step for cerebellar motor information processing. In Purkinje cells, Gi/o protein-coupled B-type gamma-aminobutyric acid receptor (GABABR) is expressed at the annuli of the dendritic spines that are innervated by the glutamatergic terminals of parallel fibers. The subcellular localization of GABABR suggests the possibility of postsynaptic interplay between GABABR and glutamate signaling. It has recently been demonstrated that GABABR indeed modulates alpha amino-3-hydroxy-5-methyl-4-isoxalone propionate-type ionotropic glutamate receptor (AMPAR)-mediated and type-1 metabotropic glutamate receptor (mGluR1)-mediated signaling. Interestingly, GABABR exerts modulatory actions not only via the classical Gi/o protein-dependent signaling cascade but also via a Gi/o protein-independent interaction between GABABR and mGluR1. In this review, we compare the physiological nature, underlying mechanisms, and possible functional significance of these modulatory actions of GABABR.

Compartment-dependent colocalization of Kir3.2-containing K+ channels and GABAB receptors in hippocampal pyramidal cells

G-protein-coupled inwardly rectifying K+ channels (Kir3 channels) coupled to metabotropic GABAB receptors are essential for the control of neuronal excitation. To determine the distribution of Kir3 channels and their spatial relationship to GABAB receptors on hippocampal pyramidal cells, we used a high-resolution immunocytochemical approach. Immunoreactivity for the Kir3.2 subunit was most abundant postsynaptically and localized to the extrasynaptic plasma membrane of dendritic shafts and spines of principal cells. Quantitative analysis of immunogold particles for Kir3.2 revealed an enrichment of the protein around putative glutamatergic synapses on dendritic spines, similar to that of GABA(B1). Consistent with this observation, a high degree of coclustering of Kir3.2 and GABA(B1) was revealed around excitatory synapses by the highly sensitive SDS-digested freeze-fracture replica immunolabeling. In contrast, in dendritic shafts receptors and channels were found to be mainly segregated. These results suggest that Kir3.2-containing K+ channels on dendritic spines preferentially mediate the effect of GABA, whereas channels on dendritic shafts are likely to be activated by other neurotransmitters as well. Thus, Kir3 channels, localized to different subcellular compartments of hippocampal principal cells, appear to be differentially involved in synaptic integration in pyramidal cell dendrites.

Molecular and cellular diversity of neuronal G-protein-gated potassium channels

Neuronal G-protein-gated potassium (GIRK) channels mediate the inhibitory effects of many neurotransmitters. Although the overlapping distribution of GIRK subunits suggests that channel composition varies in the CNS, little direct evidence supports the existence of structural or functional diversity in the neuronal GIRK channel repertoire. Here we show that the GIRK channels linked to GABAB receptors differed in two neuron populations. In the substantia nigra, GIRK2 was the principal subunit, and it was found primarily in dendrites of neurons in the substantia nigra pars compacta (SNc). Baclofen evoked prominent barium-sensitive outward current in dopamine neurons of the SNc from wild-type mice, but this current was completely absent in neurons from GIRK2 knock-out mice. In the hippocampus, all three neuronal GIRK subunits were detected. The loss of GIRK1 or GIRK2 was correlated with equivalent, dramatic reductions in baclofen-evoked current in CA1 neurons. Virtually all of the barium-sensitive component of the baclofen-evoked current was eliminated with the ablation of both GIRK2 and GIRK3, indicating that channels containing GIRK3 contribute to the postsynaptic inhibitory effect of GABAB receptor activation. The impact of GIRK subunit ablation on baclofen-evoked current was consistent with observations that GIRK1, GIRK2, and GABAB receptors were enriched in lipid rafts isolated from mouse brain, whereas GIRK3 was found primarily in higher-density membrane fractions. Altogether, our data show that different GIRK channel subtypes can couple to GABAB receptors in vivo. Furthermore, subunit composition appears to specify interactions between GIRK channels and organizational elements involved in channel distribution and efficient receptor coupling.

The neuropeptide head activator is a high-affinity ligand for the orphan G-protein-coupled receptor GPR37

The neuropeptide head activator (HA) is a mitogen for mammalian cell lines of neuronal or neuroendocrine origin. HA signalling is mediated by a G-protein-coupled receptor (GPCR). Orphan GPCRs with homology to peptide receptors were screened for HA interaction. Electrophysiological recordings in frog oocytes and in mammalian cell lines as well as Ca2+ mobilisation assays revealed nanomolar affinities of HA to GPR37. HA signal transduction through GPR37 was mediated by an inhibitory G protein and required Ca2+ influx through a channel of the transient receptor potential (TRP) family. It also required activation of Ca2+-dependent calmodulin kinase and phosphoinositide 3-kinase. Respective inhibitors blocked HA signalling and HA-induced mitosis in GPR37-expressing cells. HA treatment resulted in internalisation of GPR37. Overexpression of GPR37 led to aggregate formation, retention of the receptor in the cytoplasm and low survival rates of transfected cells, confirming the notion that misfolded GPR37 contributes to cell death, as observed in Parkinson's disease.

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.

Atria selective prolongation by NIP-142, an antiarrhythmic agent, of refractory period and action potential duration in guinea pig myocardium

NIP-142 is a novel benzopyran compound that was shown to prolong the atrial effective refractory period and terminate experimental atrial fibrillation in the dog. In the present study, we examined the effects of NIP-142 on isolated guinea pig myocardium and on the G-protein-coupled inwardly rectifying potassium channel current (acetylcholine-activated potassium current; I(KACh)) expressed in Xenopus oocytes. NIP-142 (10 and 100 microM) concentration-dependently prolonged the refractory period and action potential duration in the atrium but not in the ventricle. E-4031 and 4-aminopyridine prolonged action potential duration in both left atrium and right ventricle. Prolongation by NIP-142 of the atrial action potential duration was observed at stimulation frequencies between 0.5 and 5 Hz. In contrast, the prolongation by E-4031 was not observed at higher frequencies. Tertiapin, a blocker of I(KACh), prolonged action potential duration in the atrium but not in the ventricle. NIP-142 completely reversed the carbachol-induced shortening of atrial action potential duration. NIP-142 (1 to 100 microM), as well as tertiapin (0.1 to 100 nM), concentration-dependently blocked I(KACh) expressed in Xenopus oocytes; the blockade by NIP-142 was not affected by membrane voltage. In conclusion, NIP-142 was shown to prolong atrial refractory period and action potential duration through blockade of I(KACh) which may possibly explain its previously described antiarrhythmic activity. NIP-142 has pharmacological properties that are different from classical class III antiarrhythmic agents such as atria specificity and lack of reverse frequency dependence, and thus appears promising for the treatment of supraventricular arrhythmia.

GABAergic activation of an inwardly rectifying K+ current in mouse cerebellar Purkinje cells

Cerebellar Purkinje cells integrate motor information conveyed by excitatory synaptic inputs from parallel and climbing fibres. Purkinje cells abundantly express B-type G-protein-coupled gamma-aminobutyric acid receptors (GABABR) that are assumed to mediate major responses, including postsynaptic modulation of the synaptic inputs. However, the identity and function of effectors operated by GABABR are not fully elucidated. Here we characterized an inwardly rectifying current activated by baclofen (Ibacl), a GABABR agonist, in cultured mouse Purkinje cells using a ruptured-patch whole-cell technique. Ibacl is operated by GABABR via Gi/o-proteins, as it is not inducible in pertussis-toxin-pretreated cells. Ibacl is carried by K+ because its reversal potential shifts with the equilibrium potential of K+. Ibacl is blocked by 10(-3) M Ba2+ or Cs+, and 10(-8) M tertiapin-Q. Upon the onset and offset of a hyperpolarizing step, Ibacl is activated and deactivated, respectively, with double-exponential time courses (time constants, <1 ms and 30-80 ms). Based on similarities in the above properties, G-protein-coupled inwardly rectifying K+ (GIRK) channels are thought to be responsible for Ibacl. Perforated-patch recordings from cultured Purkinje cells demonstrate that Ibacl hyperpolarizes the resting potential and the peak level achieved by glutamate-evoked potentials initiated in the dendrites. Moreover, cell-attached recordings from Purkinje cells in cerebellar slices demonstrate that Ibacl impedes spontaneous firing. Therefore, Ibacl may reduce the postsynaptic and intrinsic excitability of Purkinje cells under physiological conditions. These findings give a new insight into the role of GABABR signalling in cerebellar information processing.

Genetic alterations of the murine serotonergic gene pathway: the neurodevelopmental basis of anxiety

The relative contribution of genetic and environmental factors in the configuration of behavioral differences is among the most prolonged and contentious controversies in intellectual history. Although current views emphasize the joint influence of genes and environmental sources during early brain development, the physiological complexities of multiple gene-gene and gene-environment interactions in the developmental neurobiology of fear and anxiety remain elusive. Variation in genes coding for proteins that control serotonin (5-hydroxytryptamine, 5-HT) system development and plasticity, establish 5-HT neuron identity, and modulate 5-HT receptor-mediated signal transduction as well as cellular pathways have been implicated in the genetics of anxiety and related disorders. This review selects anxiety and avoidance as paradigmatic traits and behaviors, and it focuses on mouse models that have been modified by deletion of genes coding for key players of serotonergic neurotransmission. In particular, pertinent approaches regarding phenotypic changes in mice bearing inactivation mutations of 5-HT receptors, 5-HT transporter, and monoamine oxidase A and other genes related to 5-HT signaling will be discussed and major findings highlighted.

Axonal sorting of Kir3.3 defines a GABA-containing neuron in the CA3 region of rodent hippocampus

Hippocampal interneurons comprise a heterogeneous group of locally acting GABAergic neurons. In addition to their variability in cotransmitter content and receptor profile, they express a variety of potassium channels that specify their individual properties. Here we describe a new type of large GABA-containing neuron in rodent hippocampus that is characterized by an axonal sorting of the potassium channel Kir3.3. The parent cell bodies of the Kir3.3-positive axons are located in CA3, as assessed by primary cultures derived from hippocampal subareas. At postnatal day 14 these neurons appear at the border between stratum oriens and stratum pyramidale of CA3, from where their axons pass through stratum pyramidale to join the mossy fiber tract. In adult hippocampus, high levels of Kir3.3 channel protein exist in axons that run with the mossy fiber tract. Kir3.3 and the vesicular GABA transporter could be identified in subpopulations of large synaptic terminals that probably derive from Kir3.3 neurons. Axonal sorting of Kir3.3 appears to be typical of a group of large inhibitory neurons, including Purkinje cells and a novel type of interneuron in CA3. Kir3.3 neurons might modulate the activity of CA3 circuitries and consequently memory processing in the hippocampus.

Molecular basis for the inhibition of G protein-coupled inward rectifier K(+) channels by protein kinase C

G protein-coupled inward rectifier K(+) (GIRK) channels regulate cellular excitability and neurotransmission. The GIRK channels are activated by a number of inhibitory neurotransmitters through the G protein betagamma subunit (G(betagamma)) after activation of G protein-coupled receptors and inhibited by several excitatory neurotransmitters through activation of phospholipase C. If the inhibition is produced by PKC, there should be PKC phosphorylation sites in GIRK channel proteins. To identify the PKC phosphorylation sites, we performed systematic mutagenesis analysis on GIRK4 and GIRK1 subunits expressed in Xenopus oocytes. Our data showed that the heteromeric GIRK1/GIRK4 channels were inhibited by a PKC activator phorbol 12-myristate 13-acetate (PMA) through reduction of single channel open-state probability. Direct application of the catalytic subunit of PKC to excised patches had a similar inhibitory effect. This inhibition was greatly eliminated by mutation of Ser-185 in GIRK1 and Ser-191 in GIRK4 that remained G protein sensitive. The PKC-dependent phosphorylation seems to mediate the channel inhibition by the excitatory neurotransmitter substance P (SP) as specific PKC inhibitors and mutation of these PKC phosphorylation sites abolished the SP-induced inhibition of GIRK1/GIRK4 channels. Thus, these results indicate that the PKC-dependent phosphorylation underscores the inhibition of GIRK channels by SP, and Ser-185 in GIRK1 and Ser-191 in GIRK4 are the PKC phosphorylation sites.

Contribution of Kir3.1, Kir3.2A and Kir3.2C subunits to native G protein-gated inwardly rectifying potassium currents in cultured hippocampal neurons

G protein-gated inwardly rectifying potassium (GIRK) channels are found in neurons, atrial myocytes and neuroendocrine cells. A characteristic feature is their activation by stimulation of Gi/o-coupled receptors. In central neurons, for example, they are activated by adenosine and GABA and, as such, they play an important role in neurotransmitter-mediated regulation of membrane excitability. The channels are tetrameric assemblies of Kir3.x subunits (Kir3.1-3.4 plus splice variants). In this study I have attempted to identify the channel subunits which contribute to the native GIRK current recorded from primary cultured rat hippocampal pyramidal neurons. Reverse transcriptase-polymerase chain reaction revealed the expression of mRNA for Kir3.1, 3.2A, 3.2C and 3.3 subunits and confocal immunofluorescence microscopy was used to investigate their expression patterns. Diffuse staining was observed on both cell somata and dendrites for Kir3.1 and Kir3.2A yet that for Kir3.2C was weaker and punctate. Whole-cell patch clamp recordings were used to record GIRK currents from hippocampal pyramidal neurons which were identified on the basis of inward rectification, dependence of reversal potential on external potassium concentration and sensitivity to tertiapin. The GIRK currents were enhanced by the stimulation of a number of Gi/o-coupled receptors and were inhibited by pertussis toxin. In order to ascertain which Kir3.x subunits were responsible for the native GIRK current I compared the properties with those of the cloned Kir3.1 + 3.2A and Kir3.1 + 3.2C channels heterologously expressed in HEK293 cells.

Anxiety-related traits in mice with modified genes of the serotonergic pathway

The neurobiology of anxiety is complex, reflecting the cumulative physiological effects of multiple genes. These genes are interactive with each other and with the environment in which they are expressed. Variation in genes coding for proteins that control serotonin (5-HT) system development and plasticity, establish 5-HT neuron identity, and modulate 5-HT receptor-mediated signal transduction and cellular pathways have been implicated in the genetics of anxiety and related disorders. Here, we selected anxiety and avoidance as paradigmatic traits and behavior and cover both traditional studies with inbred murine strains and selected lines which have been modified by gene knockout technologies. The design of a mouse model partially or completely lacking a gene of interest during all stages of development (constitutive knockout) or in a spatio-temporal context (conditional knockout) is among the prime strategies directed at elucidating the role of genetic factors in fear and anxiety. In many cases, knockout mice have been able to confirm what has already been anticipated based on pharmacological studies. In other instances, knockout studies have changed views of the relevance of 5-HT homeostasis in brain development and plasticity as well as processes underlying emotional behavior. In this review, we discuss the pertinent literature regarding phenotypic changes in mice bearing inactivation mutations of 5-HT receptors, 5-HT transporter, monoamine oxidase A and other components of the serotonergic pathway. Finally, we attempt to identify future directions of genetic manipulation in animal models to advance our understanding of brain dysregulation characteristic of anxiety disorders.

Inwardly rectifying Kir3.1 subunit knockdown impairs learning and memory in an olfactory associative task in rat

Inward-rectifier potassium channels gated by the direct action of G proteins are activated or inhibited by numerous neurotransmitters and they modulate neuronal excitability. Using an olfactory associative task, the effect of Kir3.1 subunit knockdown was tested on learning and memory. Repeated intracerebroventricular injections of antisense oligodeoxyribonucleotide to the Kir3.1 subunit significantly reduced hippocampal expression of its mRNA target determined by Western blotting. The antisense knockdown had no effect on locomotor and drinking activity or on attention processes. The reduction in Kir3.1 subunit impaired the learning of the odor associations and the procedural side of the task. This reduction correlated with the performance impairment. The results suggest that Kir3.1 channel activity is implicated in the memory processes.

Inhibition of G-protein-coupled inward rectifying K+ channels by intracellular acidosis

G-protein-coupled inward rectification K(+) (GIRK) channels play an important role in modulation of synaptic transmission and cellular excitability. The GIRK channels are regulated by diverse intra- and extracellular signaling molecules. Previously, we have shown that GIRK1/GIRK4 channels are activated by extracellular protons. The channel activation depends on a histidine residue in the M1-H5 linker and may play a role in neurotransmission. Here, we show evidence that the heteromeric GIRK1/GIRK4 channels are inhibited by intracellular acidification. This inhibition was produced by selective decrease in the channel open probability with a modest drop in the single-channel conductance. The inhibition does not seem to require G-proteins as it was seen in two G-protein coupling-defective GIRK mutants and in excised patches in the absence of exogenous G-proteins. Three histidine residues in intracellular domains were critical for the inhibition. Individual mutation of His-64, His-228, or His-352 in GIRK4 abolished or greatly diminished the inhibition in homomeric GIRK4. Mutations of any of these histidine residues in GIRK4 or their counterparts in GIRK1 were sufficient to eliminate the pH(i) sensitivity of the heteromeric GIRK1/GIRK4 channels. Thus, the molecular and biophysical bases for the inhibition of GIRK channels by intracellular protons are illustrated. Because of the inequality of the pH(i) and pH(o) in most cells and their relatively independent controls by cellular versus systemic mechanisms, such pH(i) sensitivity may allow these channels to regulate cellular excitability in certain physiological and pathophysiological conditions when intracellular acidosis occurs.

The neural cell adhesion molecule regulates cell-surface delivery of G-protein-activated inwardly rectifying potassium channels via lipid rafts

Mice deficient in the neural cell adhesion molecule (NCAM) exhibit increased anxiety and anxiolytic sensitivity to serotonin 5-HT1A receptor agonists. Here, we investigate the relationship between NCAM and 5-HT1A receptor signaling pathways modulating G-protein-activated inwardly rectifying K+ (Kir3) channels. When studying this relationship in cultured hippocampal neurons, we observed that in cells from NCAM-deficient mice, inwardly rectifying K+ (Kir3) currents were increased compared with wild-type controls. Analysis of this modulatory mechanism in Xenopus oocytes and Chinese hamster ovary (CHO) cells revealed that the recombinantly expressed major transmembrane isoforms NCAM140 and NCAM180 specifically reduced inward currents generated by neuronal Kir3.1/3.2 and Kir3.1/3.3 but not by cardiac Kir3.1/3.4 channels. Using fluorescence measurements and surface biotinylation assays, we show that this effect was caused by a reduced surface localization of Kir3 channels. Furthermore, expression of flag-tagged Kir3 channels in cultured neurons of NCAM-deficient mice resulted in a higher transport of these channels into neurites and a higher cell-surface localization compared with wild-type neurons. Neuronal Kir3 channels and NCAM isoforms are associated with cholesterol-rich microdomains (lipid rafts) in CHO cells and in isolated brain membranes. Mutational and pharmacological disruption of the lipid raft association of NCAM140 normalizes surface delivery of channels. We conclude that the transmembrane isoforms of NCAM reduce the transport of Kir3 channels to the cell surface via lipid rafts. Thus, regulation of Kir3 channel activity by NCAM may represent a novel mechanism controlling long-term excitability of neurons.

Diverse trafficking patterns due to multiple traffic motifs in G protein-activated inwardly rectifying potassium channels from brain and heart

G protein-activated inwardly rectifying potassium channels (Kir3, GIRK) provide an important mechanism for neurotransmitter regulation of membrane excitability. GIRK channels are tetramers containing various combinations of Kir3 subunits (Kir3.1--Kir3.4). We find that different combinations of Kir3 subunits exhibit a surprisingly complex spectrum of trafficking phenotypes. Kir3.2 and Kir3.4, but not Kir3.1, contain ER export signals that are important for plasma membrane expression of Kir3.1/Kir3.2 and Kir3.1/Kir3.4 heterotetramers, the GIRK channels found in the brain and the heart, respectively. Additional motifs in Kir3.2 and Kir3.4 control the trafficking between endosome and plasma membrane. In contrast, the Kir3.3 subunit potently inhibits plasma membrane expression by diverting the heterotetrameric channels to lysosomes. Such rich trafficking behaviors provide a mechanism for dynamic regulation of GIRK channel density in the plasma membrane.

Structural characterization of the mouse Girk genes

Cardiac and neuronal G protein-gated potassium (K(G)) channels are activated by neurotransmitters such as acetylcholine, opioids, and dopamine. K(G) channel activation leads to an inhibition of synaptic transmission. K(G) channels are tetrameric complexes formed by assembly of G protein-gated, inwardly-rectifying potassium (K(+)) channel (GIRK) subunits. Four mammalian GIRK subunits (GIRK1-4) have been identified. In this study, we identify key features of the four mouse Girk genes including sequence, intron/exon structures, alternative splicing events, and candidate transcriptional start points. The mouse Girk genes are organized similarly, each containing four to seven exons. While the mouse Girk1 and Girk2 genes are relatively large (>100 kb), mouse Girk3 and Girk4 genes are compact (<20 kb). Multiple mRNA variants of Girk1, Girk3, and Girk4 were identified, existing by virtue of alternative splicing and/or usage of distinct transcription initiation sites. These findings should facilitate future studies aimed at understanding the transcriptional regulation of K(G) channels and their potential involvement in disease.

Interaction of the C-terminal tail region of the metabotropic glutamate receptor 7 with the protein kinase C substrate PICK1

Group III metabotropic glutamate receptors (mGluRs) are highly enriched in the presynaptic terminals of glutamatergic synapses where they mediate feedback inhibition of neurotransmitter release. Here, we used the yeast two-hybrid system to identify a direct interaction of the C-terminal tail region of mGluR7 with the rat homologue of the protein kinase C substrate PICK1. This interaction is specifically mediated by the very C-terminal amino acids of the receptor and can be reconstituted in human embryonic kidney 293 cells by transfection of full-length mGluR7 and PICK1 cDNAs. Quantitative beta-galactosidase assays revealed that among the different group III mGluRs, mGluR7 is the major PICK1 binding partner although other subfamily members can also interact with PICK1. These data indicate that PDZ domain-containing proteins might contribute to the presynaptic localization of group III mGluRs.

Residues and mechanisms for slow activation and Ba2+ block of the cardiac muscarinic K+ channel, Kir3.1/Kir3.4

Mechanisms and residues responsible for slow activation and Ba(2+) block of the cardiac muscarinic K(+) channel, Kir3.1/Kir3.4, were investigated using site-directed mutagenesis. Mutagenesis of negatively charged residues located throughout the pore of the channel (in H5, M2, and proximal C terminus) reduced or abolished slow activation. The strongest effects resulted from mutagenesis of residues in H5 close to the selectivity filter; mutagenesis of residues in M2 and proximal C terminus equivalent to those identified as important determinants of the activation kinetics of Kir2.1 was less effective. In giant patches, slow activation was present in cell-attached patches, lost on excision of the patch, and restored on perfusion with polyamine. Mutagenesis of residues in H5 and M2 close to the selectivity filter also decreased Ba(2+) block of the channel. A critical residue for Ba(2+) block was identified in Kir3.4. Mutagenesis of the equivalent residue in Kir3.1 failed to have as pronounced an effect on Ba(2+) block, suggesting an asymmetry of the channel pore. It is concluded that slow activation is principally the result of unbinding of polyamines from negatively charged residues close to the selectivity filter of the channel and not an intrinsic gating mechanism. Ba(2+) block involves an interaction with the same residues.

Functional and biochemical evidence for G-protein-gated inwardly rectifying K+ (GIRK) channels composed of GIRK2 and GIRK3

G-protein-gated inwardly rectifying K(+) (GIRK) channels are widely expressed in the brain and are activated by at least eight different neurotransmitters. As K(+) channels, they drive the transmembrane potential toward E(K) when open and thus dampen neuronal excitability. There are four mammalian GIRK subunits (GIRK1-4 or Kir 3.1-4), with GIRK1 being the most unique of the four by possessing a long carboxyl-terminal tail. Early studies suggested that GIRK1 was an integral component of native GIRK channels. However, more recent data indicate that native channels can be either homo- or heterotetrameric complexes composed of several GIRK subunit combinations. The functional implications of subunit composition are poorly understood at present. The purpose of this study was to examine the functional and biochemical properties of GIRK channels formed by the co-assembly of GIRK2 and GIRK3, the most abundant GIRK subunits found in the mammalian brain. To examine the properties of a channel composed of these two subunits, we co-transfected GIRK2 and GIRK3 in CHO-K1 cells and assayed the cells for channel activity by patch clamp. The most significant difference between the putative GIRK2/GIRK3 heteromultimeric channel and GIRK1/GIRKx channels at the single channel level was an approximately 5-fold lower sensitivity to activation by Gbetagamma. Complexes containing only GIRK2 and GIRK3 could be immunoprecipitated from transfected cells and could be purified from native brain tissue. These data indicate that functional GIRK channels composed of GIRK2 and GIRK3 subunits exist in brain.

The metabotropic GABAB receptor directly interacts with the activating transcription factor 4

G protein-coupled receptors regulate gene expression by cellular signaling cascades that target transcription factors and their recognition by specific DNA sequences. In the central nervous system, heteromeric metabotropic gamma-aminobutyric acid type B (GABA(B)) receptors through adenylyl cyclase regulate cAMP levels, which may control transcription factor binding to the cAMP response element. Using yeast-two hybrid screens of rat brain libraries, we now demonstrate that GABA(B) receptors are engaged in a direct and specific interaction with the activating transcription factor 4 (ATF-4), a member of the cAMP response element-binding protein /ATF family. As confirmed by pull-down assays, ATF-4 associates via its conserved basic leucine zipper domain with the C termini of both GABA(B) receptor (GABA(B)R) 1 and GABA(B)R2 at a site which serves to assemble these receptor subunits in heterodimeric complexes. Confocal fluorescence microscopy shows that GABA(B)R and ATF-4 are strongly coclustered in the soma and at the dendritic membrane surface of both cultured hippocampal neurons as well as retinal amacrine cells in vivo. In oocyte coexpression assays short term signaling of GABA(B)Rs via G proteins was only marginally affected by the presence of the transcription factor, but ATF-4 was moderately stimulated in response to receptor activation in in vivo reporter assays. Thus, inhibitory metabotropic GABA(B)Rs may regulate activity-dependent gene expression via a direct interaction with ATF-4.

Brain localization and behavioral impact of the G-protein-gated K+ channel subunit GIRK4

Neuronal G-protein-gated potassium (K(G)) channels are activated by several neurotransmitters and constitute an important mode of synaptic inhibition in the mammalian nervous system. K(G) channels are composed of combinations of four subunits termed G protein-gated inwardly rectifying K(+) channels (GIRK). All four GIRK subunits are expressed in the brain, and there is a general consensus concerning the expression patterns of GIRK1, GIRK2, and GIRK3. The localization pattern of GIRK4, however, remains controversial. In this study, we exploit the negative background of mice lacking a functional GIRK4 gene to identify neuronal populations that contain GIRK4 mRNA. GIRK4 mRNA was detected in only a few regions of the mouse brain, including the deep cortical pyramidal neurons, the endopiriform nucleus and claustrum of the insular cortex, the globus pallidus, the ventromedial hypothalamic nucleus, parafascicular and paraventricular thalamic nuclei, and a few brainstem nuclei (e.g., the inferior olive and vestibular nuclei). Mice lacking GIRK4 were viable and appeared normal and did not display gross deficiencies in locomotor activity, visual tasks, and pain perception. Furthermore, GIRK4-deficient mice performed similarly to wild-type controls in the passive avoidance paradigm, a test of aversive learning. GIRK4 knock-out mice did, however, exhibit impaired performance in the Morris water maze, a test of spatial learning and memory.

Neuronal inwardly rectifying K(+) channels differentially couple to PDZ proteins of the PSD-95/SAP90 family

Several signaling proteins clustered at the postsynaptic density specialization in neurons harbor a conserved C-terminal PDZ domain recognition sequence (X-S/T-X-V/I) that mediates binding to members of the PSD-95/SAP90 protein family. This motif is also present in the C termini of some inwardly rectifying K(+) (Kir) channels. Constitutively active Kir2 channels as well as G protein-gated Kir3 channels, which are fundamental for neuronal excitability, were analyzed as candidates for binding to PSD-95/SAP90 family members. Therefore C termini of Kir2.1(+), Kir2.3(+), Kir2.4(-), Kir3.1(-), Kir3.2(+), Kir3.3(+) and Kir3.4(-) subunits (+, motif present; -, motif absent) were used as baits in the yeast two-hybrid assay to screen for in vivo interaction with PDZ domains 1-3 of PSD-95/SAP90. In contrast to Kir2.1 and Kir2.3, all Kir3 fragments failed to bind PSD-95 in this assay, which was supported by the lack of coimmunoprecipitation and colocalization of the entire proteins in mammalian cells. A detailed analysis of interaction domains demonstrated that the C-terminal motif in Kir3 channels is insufficient for binding PDZ domains. Kir2.1 and Kir2.3 subunits on the other hand coprecipitate with PSD-95. When coexpressed in a bicistronic internal ribosome entry site expression vector in HEK-293 cells macroscopic and elementary current analysis revealed that PSD-95 suppressed the activity of Kir2.3 channels by >50%. This inhibitory action of PSD-95, which predominantly affects the single-channel conductance, is likely attributable to a molecular association with additional internal interaction sites in the Kir2.3 protein.

Co-expression of human Kir3 subunits can yield channels with different functional properties

To date, no comprehensive study has been done on all combinations of the human homologues of the Kir3.0 channel family, and the human homologue of Kir3.3 has not yet been identified. To obtain support for the contention that most of the functional data on non-human Kir3.0 channels can be extrapolated to human channels, we have cloned the human homologues of the Kir3.0 family, including the yet unidentified human Kir3.3, and the human Kir4.1. The expression pattern of these channels in various human brain areas and peripheral tissues, analysed by Northern blot analysis, allows for the existence of various homomeric and heteromeric forms of human Kir3.0 channels. Expression studies of all possible combinations in Xenopus oocytes indicated that in homomeric Kir3.2c and heteromeric Kir3.1/3.2c channels mediate, in our studies, inward currents with largest amplitude of any other Kir3.0 channel combinations, followed by heteromeric Kir3.1/3.4 and homomeric Kir4.1 channels. Channel combinations which include Kir3.3 are detrimental to the formation of functional channels. The co-expression experiments with different Kir channel subunits indicate the selective formation of certain channel combinations, suggesting that channel specificity is not solely dependent on spatial and temporal regulation of Kir subunit expression.

Calmodulin dependence of presynaptic metabotropic glutamate receptor signaling

Glutamatergic neurotransmission is controlled by presynaptic metabotropic glutamate receptors (mGluRs). A subdomain in the intracellular carboxyl-terminal tail of group III mGluRs binds calmodulin and heterotrimeric guanosine triphosphate-binding protein (G protein) betagamma subunits in a mutually exclusive manner. Mutations interfering with calmodulin binding and calmodulin antagonists inhibit G protein-mediated modulation of ionic currents by mGluR 7. Calmodulin antagonists also prevent inhibition of excitatory neurotransmission via presynaptic mGluRs. These results reveal a novel mechanism of presynaptic modulation in which Ca(2+)-calmodulin is required to release G protein betagamma subunits from the C-tail of group III mGluRs in order to mediate glutamatergic autoinhibition.