Effector contributions to G beta gamma-mediated signaling as revealed by muscarinic potassium channel gating

α7 nicotinic acetylcholine receptors induce long-term synaptic enhancement in the dorsal but not ventral hippocampus

Agents that positively modulate the activity of α7nAChRs are used as cognitive enhancers and for the treatment of hippocampus-dependent functional decline. However, it is not known whether the expression and the effects of α7nAChRs apply to the entire longitudinal axis of the hippocampus equally. Given that cholinergic system-involving hippocampal functions are not equally distributed along the hippocampus, we comparatively examined the expression and the effects of α7nAChRs on excitatory synaptic transmission between the dorsal and the ventral hippocampal slices from adult rats. We found that α7nAChRs are equally expressed in the CA1 field of the two segments of the hippocampus. However, activation of α7nAChRs by their highly selective agonist PNU 282987 induced a gradually developing increase in field excitatory postsynaptic potential only in the dorsal hippocampus. This long-term potentiation was not reversed upon application of nonselective nicotinic receptor antagonist mecamylamine, but the induction of potentiation was prevented by prior blockade of α7nAChRs by their antagonist MG 624. In contrast to the long-term synaptic plasticity, we found that α7nAChRs did not modulate short-term synaptic plasticity in either the dorsal or the ventral hippocampus. These results may have implications for the role that α7nAChRs play in specifically modulating functions that depend on the normal function of the dorsal hippocampus. We propose that hippocampal functions that rely on a direct α7 nAChR-mediated persistent enhancement of glutamatergic synaptic transmission are preferably supported by dorsal but not ventral hippocampal synapses.

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

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

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

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

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

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

A Collision Coupling Model Governs the Activation of Neuronal GIRK1/2 Channels by Muscarinic-2 Receptors

The G protein-activated Inwardly Rectifying K⁺-channel (GIRK) modulates heart rate and neuronal excitability. Following G-Protein Coupled Receptor (GPCR)-mediated activation of heterotrimeric G proteins (Gαβγ), opening of the channel is obtained by direct binding of Gβγ subunits. Interestingly, GIRKs are solely activated by Gβγ subunits released from Gα_i/o-coupled GPCRs, despite the fact that all receptor types, for instance Gα_q-coupled, are also able to provide Gβγ subunits. It is proposed that this specificity and fast kinetics of activation stem from pre-coupling (or pre-assembly) of proteins within this signaling cascade. However, many studies, including our own, point towards a diffusion-limited mechanism, namely collision coupling. Here, we set out to address this long-standing question by combining electrophysiology, imaging, and mathematical modeling. Muscarinic-2 receptors (M2R) and neuronal GIRK1/2 channels were coexpressed in Xenopus laevis oocytes, where we monitored protein surface expression, current amplitude, and activation kinetics. Densities of expressed M2R were assessed using a fluorescently labeled GIRK channel as a molecular ruler. We then incorporated our results, along with available kinetic data reported for the G-protein cycle and for GIRK1/2 activation, to generate a comprehensive mathematical model for the M2R-G-protein-GIRK1/2 signaling cascade. We find that, without assuming any irreversible interactions, our collision coupling kinetic model faithfully reproduces the rate of channel activation, the changes in agonist-evoked currents and the acceleration of channel activation by increased receptor densities.

Mutual action by Gγ and Gβ for optimal activation of GIRK channels in a channel subunit-specific manner

The tetrameric G protein-gated K⁺ channels (GIRKs) mediate inhibitory effects of neurotransmitters that activate G_i/o-coupled receptors. GIRKs are activated by binding of the Gβγ dimer, via contacts with Gβ. Gγ underlies membrane targeting of Gβγ, but has not been implicated in channel gating. We observed that, in Xenopus oocytes, expression of Gγ alone activated homotetrameric GIRK1* and heterotetrameric GIRK1/3 channels, without affecting the surface expression of GIRK or Gβ. Gγ and Gβ acted interdependently: the effect of Gγ required the presence of ambient Gβ and was enhanced by low doses of coexpressed Gβ, whereas excess of either Gβ or Gγ imparted suboptimal activation, possibly by sequestering the other subunit “away” from the channel. The unique distal C-terminus of GIRK1, G1-dCT, was important but insufficient for Gγ action. Notably, GIRK2 and GIRK1/2 were not activated by Gγ. Our results suggest that Gγ regulates GIRK1* and GIRK1/3 channel’s gating, aiding Gβ to trigger the channel’s opening. We hypothesize that Gγ helps to relax the inhibitory effect of a gating element (“lock”) encompassed, in part, by the G1-dCT; GIRK2 acts to occlude the effect of Gγ, either by setting in motion the same mechanism as Gγ, or by triggering an opposing gating effect.

Emergence of ion channel modal gating from independent subunit kinetics

Many ion channels exhibit a slow stochastic switching between distinct modes of gating activity. This feature of channel behavior has pronounced implications for the dynamics of ionic currents and the signaling pathways that they regulate. A canonical example is the inositol 1,4,5-trisphosphate receptor (IP3R) channel, whose regulation of intracellular Ca(2+) concentration is essential for numerous cellular processes. However, the underlying biophysical mechanisms that give rise to modal gating in this and most other channels remain unknown. Although ion channels are composed of protein subunits, previous mathematical models of modal gating are coarse grained at the level of whole-channel states, limiting further dialogue between theory and experiment. Here we propose an origin for modal gating, by modeling the kinetics of ligand binding and conformational change in the IP3R at the subunit level. We find good agreement with experimental data over a wide range of ligand concentrations, accounting for equilibrium channel properties, transient responses to changing ligand conditions, and modal gating statistics. We show how this can be understood within a simple analytical framework and confirm our results with stochastic simulations. The model assumes that channel subunits are independent, demonstrating that cooperative binding or concerted conformational changes are not required for modal gating. Moreover, the model embodies a generally applicable principle: If a timescale separation exists in the kinetics of individual subunits, then modal gating can arise as an emergent property of channel behavior.

A Quantitative Model of the GIRK1/2 Channel Reveals That Its Basal and Evoked Activities Are Controlled by Unequal Stoichiometry of Gα and Gβγ

G protein-gated K⁺ channels (GIRK; Kir3), activated by Gβγ subunits derived from G_i/o proteins, regulate heartbeat and neuronal excitability and plasticity. Both neurotransmitter-evoked (I_evoked) and neurotransmitter-independent basal (I_basal) GIRK activities are physiologically important, but mechanisms of I_basal and its relation to I_evoked are unclear. We have previously shown for heterologously expressed neuronal GIRK1/2, and now show for native GIRK in hippocampal neurons, that I_basal and I_evoked are interrelated: the extent of activation by neurotransmitter (activation index, R_a) is inversely related to I_basal. To unveil the underlying mechanisms, we have developed a quantitative model of GIRK1/2 function. We characterized single-channel and macroscopic GIRK1/2 currents, and surface densities of GIRK1/2 and Gβγ expressed in Xenopus oocytes. Based on experimental results, we constructed a mathematical model of GIRK1/2 activity under steady-state conditions before and after activation by neurotransmitter. Our model accurately recapitulates I_basal and I_evoked in Xenopus oocytes, HEK293 cells and hippocampal neurons; correctly predicts the dose-dependent activation of GIRK1/2 by coexpressed Gβγ and fully accounts for the inverse I_basal-R_a correlation. Modeling indicates that, under all conditions and at different channel expression levels, between 3 and 4 Gβγ dimers are available for each GIRK1/2 channel. In contrast, available Gα_i/o decreases from ~2 to less than one Gα per channel as GIRK1/2's density increases. The persistent Gβγ/channel (but not Gα/channel) ratio support a strong association of GIRK1/2 with Gβγ, consistent with recruitment to the cell surface of Gβγ, but not Gα, by GIRK1/2. Our analysis suggests a maximal stoichiometry of 4 Gβγ but only 2 Gα_i/o per one GIRK1/2 channel. The unique, unequal association of GIRK1/2 with G protein subunits, and the cooperative nature of GIRK gating by Gβγ, underlie the complex pattern of basal and agonist-evoked activities and allow GIRK1/2 to act as a sensitive bidirectional detector of both Gβγ and Gα.

Many neurotransmitters and hormones inhibit the electric activity of excitable cells (such as cardiac cells and neurons) by activating a K⁺ channel, GIRK (G protein-gated Inwardly Rectifying K⁺ channel). GIRK channels also possess constitutive “basal” activity which contributes to regulation of neuronal and cardiac excitability and certain disorders, but the mechanism of this activity and its interrelation with the neurotransmitter-evoked activity are poorly understood. In this work we show that key features of basal and neurotransmitter-evoked activities are similar in cultured hippocampal neurons and in two model systems (mammalian HEK293 cells and Xenopus oocytes). Using experimental data of the neuronal GIRK1/2 channel function upon changes in GIRK and G protein concentrations, we constructed a mathematical model that quantitatively accounts for basal and evoked activity, and for the inverse correlation between the two. Our analysis suggests a novel and unexpected mechanism of interaction of GIRK1/2 with the G protein subunits, where the tetrameric GIRK channel can assemble with 4 molecules of the Gβγ subunits but only 2 molecules of Gα. GIRK is a prototypical effector of Gβγ, and the unequal stoichiometry of interaction with G protein subunits may have general implications for G protein signaling.

Membrane channels as integrators of G-protein-mediated signaling

A variety of extracellular stimuli regulate cellular responses via membrane receptors. A well-known group of seven-transmembrane domain-containing proteins referred to as G protein-coupled receptors, directly couple with the intracellular GTP-binding proteins (G proteins) across cell membranes and trigger various cellular responses by regulating the activity of several enzymes as well as ion channels. Many specific populations of ion channels are directly controlled by G proteins; however, indirect modulation of some channels by G protein-dependent phosphorylation events and lipid metabolism is also observed. G protein-mediated diverse modifications affect the ion channel activities and spatio-temporally regulate membrane potentials as well as of intracellular Ca(2+) concentrations in both excitatory and non-excitatory cells. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé.

Two distinct aspects of coupling between Gα(i) protein and G protein-activated K+ channel (GIRK) revealed by fluorescently labeled Gα(i3) protein subunits

G protein-activated K(+) channels (Kir3 or GIRK) are activated by direct interaction with Gβγ. Gα is essential for specific signaling and regulates basal activity of GIRK (I(basal)) and kinetics of the response elicited by activation by G protein-coupled receptors (I(evoked)). These regulations are believed to occur within a GIRK-Gα-Gβγ signaling complex. Fluorescent energy resonance transfer (FRET) studies showed strong GIRK-Gβγ interactions but yielded controversial results regarding the GIRK-Gα(i/o) interaction. We investigated the mechanisms of regulation of GIRK by Gα(i/o) using wild-type Gα(i3) (Gα(i3)WT) and Gα(i3) labeled at three different positions with fluorescent proteins, CFP or YFP (xFP). Gα(i3)xFP proteins bound the cytosolic domain of GIRK1 and interacted with Gβγ in a guanine nucleotide-dependent manner. However, only an N-terminally labeled, myristoylated Gα(i3)xFP (Gα(i3)NT) closely mimicked all aspects of Gα(i3)WT regulation except for a weaker regulation of I(basal). Gα(i3) labeled with YFP within the Gα helical domain preserved regulation of I(basal) but failed to restore fast I(evoked). Titrated expression of Gα(i3)NT and Gα(i3)WT confirmed that regulation of I(basal) and of the kinetics of I(evoked) of GIRK1/2 are independent functions of Gα(i). FRET and direct biochemical measurements indicated much stronger interaction between GIRK1 and Gβγ than between GIRK1 and Gα(i3). Thus, Gα(i/o)βγ heterotrimer may be attached to GIRK primarily via Gβγ within the signaling complex. Our findings support the notion that Gα(i/o) actively regulates GIRK. Although regulation of I(basal) is a function of Gα(i)(GDP), our new findings indicate that regulation of kinetics of I(evoked) is mediated by Gα(i)(GTP).

Relaxation gating of the acetylcholine-activated inward rectifier K+ current is mediated by intrinsic voltage sensitivity of the muscarinic receptor

Normal heart rate variability is critically dependent upon the G-protein-coupled, acetylcholine (ACh)-activated inward rectifier K+ current, I(KACh). A unique feature of I(KACh) is the so-called ‘relaxation' gating property that contributes to increased current at hyperpolarized membrane potentials. I(KACh) relaxation refers to a slow decrease or increase in current magnitude with depolarization or hyperpolarization, respectively. The molecular mechanism underlying this perplexing gating behaviour remains unclear. Here, we consider a novel explanation for I(KACh) relaxation based upon the recent finding that G-protein-coupled receptors (GPCRs) are intrinsically voltage sensitive and that the muscarinic agonists acetylcholine (ACh) and pilocarpine (Pilo) manifest opposite voltage-dependent I(KACh) modulation. We show that Pilo activation of I(KACh) displays relaxation characteristics opposite to that of ACh. We explain the opposite effects of ACh and Pilo using Markov models of I(KACh) that incorporate ligand-specific, voltage-dependent parameters. Based on experimental and computational findings, we propose a novel molecular mechanism to describe the enigmatic relaxation gating process: I(KACh) relaxation represents a voltage-dependent change in agonist affinity as a consequence of a voltage-dependent conformational change in the muscarinic receptor.

Emerging roles for G protein-gated inwardly rectifying potassium (GIRK) channels in health and disease

G protein-gated inwardly rectifying potassium (GIRK) channels hyperpolarize neurons in response to activation of many different G protein-coupled receptors and thus control the excitability of neurons through GIRK-mediated self-inhibition, slow synaptic potentials and volume transmission. GIRK channel function and trafficking are highly dependent on the channel subunit composition. Pharmacological investigations of GIRK channels and studies in animal models suggest that GIRK activity has an important role in physiological responses, including pain perception and memory modulation. Moreover, abnormal GIRK function has been implicated in altering neuronal excitability and cell death, which may be important in the pathophysiology of diseases such as epilepsy, Down's syndrome, Parkinson's disease and drug addiction. GIRK channels may therefore prove to be a valuable new therapeutic target.

Base of pore loop is important for rectification, activation, permeation, and block of Kir3.1/Kir3.4

The Kir3.1/Kir3.4 channel is an inward rectifier, agonist-activated K(+) channel. The location of the binding site within the channel pore that coordinates polyamines (and is thus responsible for inward rectification) and the location of the gate that opens the channel in response to agonist activation is unclear. In this study, we show, not surprisingly, that mutation of residues at the base of the selectivity filter in the pore loop and second transmembrane domain weakens Cs(+) block and decreases selectivity (as measured by Rb(+) and spermine permeation). However, unexpectedly, the mutations also weaken inward rectification and abolish agonist activation of the channel. In the wild-type channel and 34 mutant channels, there are significant (p < 0.05) correlations among the K(D) for Cs(+) block, Rb(+) and spermine permeation, inward rectification, and agonist activation. The significance of these findings is discussed. One possible conclusion is that the selectivity filter is responsible for inward rectification and agonist activation as well as permeation and block.

Kinetic modeling of Na(+)-induced, Gbetagamma-dependent activation of G protein-gated K(+) channels

G protein-activated K(+)(GIRK) channels are activated by numerous neurotransmitters that act on Gi/o proteins, via a direct interaction with the Gbetagamma subunit of G proteins. In addition, GIRK channels are positively regulated by intracellular Na(+) via a direct interaction (fast pathway) and via a GGbetagamma-dependent mechanism (slow pathway). The slow modulation has been proposed to arise from the recently described phenomenon of Na(+)-induced reduction of affinity of interaction between GalphaGDP and Gbetagamma subunits of G proteins. In this scenario, elevated Na(+) enhances basal dissociation of G protein heterotrimers, elevating free cellular Gbetagamma and activating GIRK. However, it is not clear whether this hypothesis can account for the quantitative and kinetic aspects of the observed regulation. Here, we report the development of a quantitative model of slow, Na(+)-dependent, G protein-mediated activation of GIRK. Activity of GIRK1F137S channels, which are devoid of direct interaction with Na(+), was measured in excised membrane patches and used as an indicator of free GGbetagamma levels. The change in channel activity was used to calculate the Na(+)-dependent change in the affinity of G protein subunit interaction. Under a wide range of initial conditions, the model predicted that a relatively small decrease in the affinity of interaction of GalphaGDP and GGbetagamma (about twofold under most conditions) accounts for the twofold activation of GIRK induced by Na(+), in agreement with biochemical data published previously. The model also correctly described the slow time course of Na(+) effect and explained the previously observed enhancement of Na(+)-induced activation of GIRK by coexpressed Galphai3. This is the first quantitative model that describes the basal equilibrium between free and bound G protein subunits and its consequences on regulation of a GGbetagamma effector.

G-protein-gated TRP-like cationic channel activated by muscarinic receptors: effect of potential on single-channel gating

There is little information about the mechanisms by which G-protein–coupled receptors gate ion channels although many ionotropic receptors are well studied. We have investigated gating of the muscarinic cationic channel, which mediates the excitatory effect of acetylcholine in smooth muscles, and proposed a scheme consisting of four pairs of closed and open states. Channel kinetics appeared to be the same in cell-attached or outside-out patches whether the channel was activated by carbachol application or by intracellular dialysis with GTPγS. Since in the latter case G-proteins are permanently active, it is concluded that the cationic channel is the major determinant of its own gating, similarly to the K_ACh channel (Ivanova-Nikolova, T.T., and G.E. Breitwieser. 1997. J. Gen. Physiol. 109:245–253). Analysis of adjacent-state dwell times revealed connections between the states that showed features conserved among many other ligand-gated ion channels (e.g., nAChR, BK_Ca channel). Open probability (P_O) of the cationic channel was increased by membrane depolarization consistent with the prominent U-shaped I-V relationship of the muscarinic whole-cell current at negative potentials. Membrane potential affected transitions within each closed-open state pair but had little effect on transitions between pairs; thus, the latter are likely to be caused by interactions of the channel with its ligands, e.g., Ca²⁺ and Gαo-GTP. Channel activity was highly heterogeneous, as was evident from the prominent cycling behavior when P_O was measured over 5-s intervals. This was related to the variable frequency of openings (as in the K_ACh channel) and, especially, to the number of long openings between consecutive long shuttings. Analysis of the underlying Markov chain in terms of probabilities allowed us to evaluate the contribution of each open state to the integral current (from shortest to longest open state: 0.1, 3, 24, and 73%) as P_O increased 525-fold in three stages.

The pore helix is involved in stabilizing the open state of inwardly rectifying K+ channels

Ion channels can be gated by various extrinsic cues, such as voltage, pH, and second messengers. However, most ion channels display extrinsic cue-independent transitions as well. These events represent spontaneous conformational changes of the channel protein. The molecular basis for spontaneous gating and its relation to the mechanism by which channels undergo activation gating by extrinsic cue stimulation is not well understood. Here we show that the proximal pore helix of inwardly rectifying (Kir) channels is partially responsible for determining spontaneous gating characteristics, affecting the open state of the channel by stabilizing intraburst openings as well as the bursting state itself without affecting K(+) ion-channel interactions. The effect of the pore helix on the open state of the channel is qualitatively similar to that of two well-characterized mutations at the second transmembrane domain (TM2), which stabilize the channel in its activated state. However, the effects of the pore helix and the TM2 mutations on gating were additive and independent of each other. Moreover, in sharp contrast to the two TM2 mutations, the pore helix mutation did not affect the functionality of the agonist-responsive gate. Our results suggest that in Kir channels, the bottom of the pore helix and agonist-induced conformational transitions at the TM2 ultimately stabilize via different pathways the open conformation of the same gate.

Single channel analysis of the regulation of GIRK1/GIRK4 channels by protein phosphorylation

G-Protein activated, inwardly rectifying potassium channels (GIRKs) are important effectors of G-protein beta/gamma-subunits, playing essential roles in the humoral regulation of cardiac activity and also in higher brain functions. G-protein activation of channels of the GIRK1/GIRK4 heterooligomeric composition is controlled via phosphorylation by cyclic AMP dependent protein kinase (PKA) and dephosphorylation by protein phosphatase 2A (PP(2)A). To study the molecular mechanism of this unprecedented example of G-protein effector regulation, single channel recordings were performed on isolated patches of plasma membranes of Xenopus laevis oocytes. Our study shows that: (i) The open probability (P(o)) of GIRK1/GIRK4 channels, stimulated by coexpressed m(2)-receptors, was significantly increased upon addition of the catalytic subunit of PKA to the cytosolic face of an isolated membrane patch. (ii) At moderate concentrations of recombinant G(beta1/gamma2), used to activate the channel, P(o) was significantly reduced in patches treated with PP(2)A, when compared to patches with PKA-cs. (iii) Several single channel gating parameters, including modal gating behavior, were significantly different between phosphorylated and dephosphorylated channels, indicating different gating behavior between the two forms of the protein. Most of these changes were, however, not responsible for the marked difference in P(o) at moderate G-protein concentrations. (iv) An increase of the frequency of openings (f(o)) and a reduction of dwell time duration of the channel in the long-lasting C(5) state was responsible for facilitation of GIRK1/GIRK4 channels by protein phosphorylation. Dephosphorylation by PP(2)A led to an increase of G(beta1/gamma2) concentration required for full activation of the channel and hence to a reduction of the apparent affinity of GIRK1/GIRK4 for G(beta1/gamma2). (v) Although possibly not directly the target of protein phosphorylation/dephosphorylation, the last 20 C-terminal amino acids of the GIRK1 subunit are required for the reduction of apparent affinity for the G-protein by PP(2)A, indicating that they constitute an essential part of the off-switch.

Regulation of the inward rectifying properties of G-protein-activated inwardly rectifying K+ (GIRK) channels by Gbeta gamma subunits

Gbetagamma subunits are known to bind to and activate G-protein-activated inwardly rectifying K(+) channels (GIRK) by regulating their open probability and bursting behavior. Studying G-protein regulation of either native GIRK (I(KACh)) channels in feline atrial myocytes or heterologously expressed GIRK1/4 channels in Chinese hamster ovary cells and HEK 293 cells uncovered a novel Gbetagamma subunit mediated regulation of the inwardly rectifying properties of these channels. I(KACh) activated by submaximal concentrations of acetylcholine exhibited a approximately 2.5-fold stronger inward rectification than I(KACh) activated by saturating concentrations of acetylcholine. Similarly, the inward rectification of currents through GIRK1/4 channels expressed in HEK cells was substantially weakened upon maximal stimulation with co-expressed Gbetagamma subunits. Analysis of the outward current block underlying inward rectification demonstrated that the fraction of instantaneously blocked channels was reduced when Gbetagamma was over-expressed. The Gbetagamma induced weakening of inward rectification was associated with reduced potencies for Ba(2+) and Cs(+) to block channels from the extracellular side. Based on these results we propose that saturation of the channel with Gbetagamma leads to a conformational change within the pore of the channel that reduced the potency of extracellular cations to block the pore and increased the fraction of channels inert to a pore block in outward direction.

Phasic and tonic attenuation of EPSPs by inward rectifier K+ channels in rat hippocampal pyramidal cells

We made whole-cell recordings from CA1 pyramidal cells of hippocampal slices in combination with brief dendritic glutamate pulses to study the role of constitutive inwardly rectifying K+ channels (IRK, Kir2.0) and G-protein-activated inwardly rectifying K+ channels (GIRK, Kir3.0) in the processing of excitatory inputs. Phasic activation of GIRK channels by baclofen (20 microM) produced a reversible reduction of glutamate-evoked postsynaptic potentials (GPSPs), our equivalent of EPSPs, by about one-third. Conversely, tertiapin (30 nM), a selective inhibitor of GIRK channels, and Ba2+ (200 microM), a non-selective blocker of inwardly rectifying K+ channels, enhanced GPSPs and, in voltage-clamp experiments, reduced the underlying K+ conductances, indicating a functionally significant background GIRK conductance, in addition to constitutive IRK channel activity. When examined after suppression of endogenous adenosinergic inhibition, using either adenosine deaminase or the selective A1 receptor antagonist, 1,3-dipropyl-8-cyclopentylxanthine, tertiapin failed to influence either the GPSPs or the inwardly rectifying K+ conductance. Voltage-clamp recordings from acutely isolated CA1 pyramidal cells not exposed to ambient adenosine exhibited no response to tertiapin, whereas Ba2+ was still capable of reducing hyperpolarizing inward rectification. Our data indicate that in hippocampal pyramidal cells, two components of the inwardly rectifying K+ conductance can be identified, which together exert a tonic modulation of excitatory synaptic input: one arises from constitutive putative IRK channels, the other is mediated by the background activity of GIRK channels that results from the tonic activation of A1 receptors by ambient adenosine.

G(alpha)(i) controls the gating of the G protein-activated K(+) channel, GIRK

GIRK (Kir3) channels are activated by neurotransmitters coupled to G proteins, via a direct binding of G(beta)(gamma). The role of G(alpha) subunits in GIRK gating is elusive. Here we demonstrate that G(alpha)(i) is not only a donor of G(beta)(gamma) but also regulates GIRK gating. When overexpressed in Xenopus oocytes, GIRK channels show excessive basal activity and poor activation by agonist or G(beta)(gamma). Coexpression of G(alpha)(i3) or G(alpha)(i1) restores the correct gating parameters. G(alpha)(i) acts neither as a pure G(beta)(gamma) scavenger nor as an allosteric cofactor for G(beta)(gamma). It inhibits only the basal activity without interfering with G(beta)(gamma)-induced response. Thus, GIRK is regulated, in distinct ways, by both arms of the G protein. G(alpha)(i) probably acts in its GDP bound form, alone or as a part of G(alpha)(beta)(gamma) heterotrimer.

Multiple pertussis toxin-sensitive G-proteins can couple receptors to GIRK channels in rat sympathetic neurons when expressed heterologously, but only native G(i)-proteins do so in situ

Although many G-protein-coupled neurotransmitter receptors are potentially capable of modulating both voltage-dependent Ca(2+) channels (I(Ca)) and G-protein-gated K(+) channels (I(GIRK)), there is a substantial degree of selectivity in the coupling to one or other of these channels in neurons. Thus, in rat superior cervical ganglion (SCG) neurons, M(2) muscarinic acetylcholine receptors (mAChRs) selectively activate I(GIRK) whereas M(4) mAChRs selectively inhibit I(Ca). One source of selectivity might be that the two receptors couple preferentially to different G-proteins. Using antisense depletion methods, we found that M(2) mAChR-induced activation of I(GIRK) is mediated by G(i) whereas M(4) mAChR-induced inhibition of I(Ca) is mediated by G(oA). Experiments with the beta gamma-sequestering peptides alpha-transducin and beta ARK1(C-ter) indicate that, although both effects are mediated by G-protein beta gamma subunits, the endogenous subunits involved in I(GIRK) inhibition differ from those involved in I(Ca) inhibition. However, this pathway divergence does not result from any fundamental selectivity in receptor-G-protein-channel coupling because both I(GIRK) and I(Ca) modulation can be rescued by heterologously expressed G(i) or G(o) proteins after the endogenously coupled alpha-subunits have been inactivated with Pertussis toxin (PTX). We suggest instead that the divergence in the pathways activated by the endogenous mAChRs results from a differential topographical arrangement of receptor, G-protein and ion channel.

The Stoichiometry of Gbeta gamma binding to G-protein-regulated inwardly rectifying K+ channels (GIRKs)

G-protein-coupled inwardly rectifying K(+) (GIRK; Kir3.x) channels are the primary effectors of numerous G-protein-coupled receptors. GIRK channels decrease cellular excitability by hyperpolarizing the membrane potential in cardiac cells, neurons, and secretory cells. Although direct regulation of GIRKs by the heterotrimeric G-protein subunit Gbetagamma has been extensively studied, little is known about the number of Gbetagamma binding sites per channel. Here we demonstrate that purified GIRK (Kir 3.x) tetramers can be chemically cross-linked to exogenously purified Gbetagamma subunits. The observed laddering pattern of Gbetagamma attachment to GIRK4 homotetramers was consistent with the binding of one, two, three, or four Gbetagamma molecules per channel tetramer. The fraction of channels chemically cross-linked to four Gbetagamma molecules increased with increasing Gbetagamma concentrations and approached saturation. These results suggest that GIRK tetrameric channels have four Gbetagamma binding sites. Thus, GIRK (Kir 3.x) channels, like the distantly related cyclic nucleotide-gated channels, are tetramers and exhibit a 1:1 subunit/ligand binding stoichiometry.

Yeast screen for constitutively active mutant G protein-activated potassium channels

GIRK2 is a major contributor to G protein-activated inward rectifier potassium channels in the mammalian brain. How GIRK channels open upon contact with Gbetagamma remains unknown. Using a yeast genetic screen to select constitutively active mutants from a randomly mutagenized GIRK2 library, we identified five gating mutations at four residues in the transmembrane domain. Further mutagenesis indicates that GIRK channel opening involves a rotation of the transmembrane segments, bringing one of these residues (V188) to a pore-lining position in the open conformation. Combined with double-mutant studies, these findings suggest that GIRK channels gate by moving from the open conformation inferred from our yeast study of Kir2.1 to a closed conformation perhaps resembling the known KcsA structure.

Single channel studies of inward rectifier potassium channel regulation by muscarinic acetylcholine receptors

Negative regulation of the heartbeat rate involves the activation of an inwardly rectifying potassium current (I(KACh)) by G protein-coupled receptors such as the m2 muscarinic acetylcholine receptor. Recent studies have shown that this process involves the direct binding of G(betagamma) subunits to the NH(2)- and COOH-terminal cytoplasmic domains of the proteins termed GIRK1 and GIRK4 (Kir3.1 and Kir3.4/CIR), which mediate I(KACh). Because of the very low basal activity of native I(KACh), it has been difficult to determine the single channel effect of G(betagamma) subunit binding on I(KACh) activity. Through analysis of a novel G protein-activated chimeric inward rectifier channel that displays increased basal activity relative to I(KACh), we find that single channel activation can be explained by a G protein-dependent shift in the equilibrium of open channel transitions in favor of a bursting state of channel activity over a long-lived closed state.

Slow modal gating of single G protein-activated K+ channels expressed in Xenopus oocytes

The slow kinetics of G protein-activated K+ (GIRK) channels expressed in Xenopus oocytes were studied in single-channel, inside-out membrane patches. Channels formed by GIRK1 plus GIRK4 subunits, which are known to form the cardiac acetylcholine (ACh)-activated GIRK channel (KACh), were activated by a near-saturating dose of G protein betagamma subunits (Gbetagamma; 20 nM). The kinetic parameters of the expressed GIRK1/4 channels were similar to those of cardiac KACh. GIRK1/4 channels differed significantly from channels formed by GIRK1 with the endogenous oocyte subunit GIRK5 (GIRK1/5) in some of their kinetic parameters and in a 3-fold lower open probability, Po. The unexpectedly low Po (0.025) of GIRK1/4 was due to the presence of closures of hundreds of milliseconds; the channel spent approximately 90 % of the time in the long closed states. GIRK1/4 channels displayed a clear modal behaviour: on a time scale of tens of seconds, the Gbetagamma-activated channels cycled between a low-Po mode (Po of about 0.0034) and a bursting mode characterized by an approximately 30-fold higher Po and a different set of kinetic constants (and, therefore, a different set of channel conformations). The available evidence indicates that the slow modal transitions are not driven by binding and unbinding of Gbetagamma. The GTPgammaS-activated Galphai1 subunit, previously shown to inhibit GIRK channels, substantially increased the time spent in closed states and apparently shifted the channel to a mode similar, but not identical, to the low-Po mode. This is the first demonstration of slow modal transitions in GIRK channels. The detailed description of the slow gating kinetics of GIRK1/4 may help in future analysis of mechanisms of GIRK gating.

Cytoplasmic unsaturated free fatty acids inhibit ATP-dependent gating of the G protein-gated K(+) channel

This study reports the identification of an endogenous inhibitor of the G protein-gated (K(ACh)) channel and its effect on the K(ACh) channel kinetics. In the presence of acetylcholine in the pipette, K(ACh) channels in inside-out atrial patches were activated by applying GTP to the cytoplasmic side of the membrane. In these patches, addition of physiological concentration of intracellular ATP (4 mM) upregulated K(ACh) channel activity approximately fivefold and induced long-lived openings. However, such ATP-dependent gating is normally not observed in cell-attached patches, indicating that an endogenous substance that inhibits the ATP effect is present in the cell. We searched for such an inhibitor in the cell. ATP-dependent gating of the K(ACh) channel was inhibited by the addition of the cytosolic fraction of rat atrial or brain tissues. The lipid component of the cytosolic fraction was found to contain the inhibitory activity. To identify the lipid inhibitor, we tested the effect of approximately 40 different lipid molecules. Among the lipids tested, only unsaturated free fatty acids such as oleic, linoleic, and arachidonic acids (0.2-2 microM) reversibly inhibited the ATP-dependent gating of native K(ACh) channels in atrial cells and hippocampal neurons, and of recombinant K(ACh) channels (GIRK1/4 and GIRK1/2) expressed in oocytes. Unsaturated free fatty acids also inhibited phosphatidylinositol-4, 5-bisphosphate (PIP(2))-induced changes in K(ACh) channel kinetics but were ineffective against ATP-activated background K(1) channels and PIP(2)-activated K(ATP) channels. These results show that during agonist-induced activation, unsaturated free fatty acids in the cytoplasm help to keep the cardiac and neuronal K(ACh) channels downregulated by antagonizing their ATP-dependent gating. The opposing effects of ATP and free fatty acids represent a novel regulatory mechanism for the G protein-gated K(+) channel.

Voltage-dependent inhibition of the muscarinic cationic current in guinea-pig ileal cells by SK&F 96365

The effects of SK&F 96365 on cationic current evoked either by activating muscarinic receptors with carbachol or by intracellularly applied GTPgammaS (in the absence of carbachol) were studied using patch-clamp recording techniques in single guinea-pig ileal smooth muscle cells. SK&F 96365 reversibly inhibited the muscarinic receptor cationic current in a concentration-, time- and voltage-dependent manner producing concomitant alteration of the steady-state I-V relationship shape which could be explained by assuming that increasing membrane positivity increased the affinity of the blocker. The inhibition was similar for both carbachol- and GTPgammaS-evoked currents suggesting that the cationic channel rather than the muscarinic receptor was the primary site of the SK&F 96365 action. Increased membrane positivity induced additional rapid inhibition of the cationic current by SK&F 96365 which was more slowly relieved during membrane repolarization. Both the inhibition and disinhibition time course could be well fitted by a single exponential function with the time constants decreasing with increasing positivity for the inhibition (e-fold per about 12 mV) and approximately linearly decreasing with increasing negativity for the disinhibition. At a constant SK&F 96365 concentration, the degree of cationic current inhibition was a sigmoidal function of the membrane potential with a potential of half-maximal increase positive to about +30 mV and a slope factor of about -13 mV. Increasing the duration of voltage steps at -80 or at 80 mV, increased the percentage inhibition; the degree of inhibition was almost identical at both potentials providing evidence that the same cationic channel was responsible for the cationic current both at negative and at positive potentials. It is concluded that the distinctive and unique mode of SK&F 96365 action on the muscarinic receptor cationic channel is a valuable tool in future molecular biology studies of this channel.

Gbetagamma binding increases the open time of IKACh: kinetic evidence for multiple Gbetagamma binding sites

IKACh is an inwardly rectifying potassium channel that plays an important role in the regulation of mammalian heart rate. IKACh is activated by direct interaction with Gbetagamma subunits of pertussis toxin-sensitive heterotrimeric G-proteins. The stoichiometry of the Gbetagamma/channel complex is currently unknown, and kinetic analysis of the channel behavior has led to conflicting conclusions. Here, we analyze the kinetics of the native IKACh channel in inside-out cardiomyocyte patches activated directly by Gbetagamma. We conclude that the channel has at least two open states and that binding of Gbetagamma prolongs its mean open time duration. These findings imply the existence of at least two binding sites on the channel complex for Gbetagamma. We also show that the duration of the channel opening is negatively correlated with the duration of subsequent channel closing, which further constrains the possible kinetic models. A simple qualitative model describing the kinetic behavior of IKACh is presented.

Muscarinic K+ channel in the heart. Modal regulation by G protein beta gamma subunits

The membrane-delimited activation of muscarinic K+ channels by G protein beta gamma subunits plays a prominent role in the inhibitory synaptic transmission in the heart. These channels are thought to be heterotetramers comprised of two homologous subunits, GIRK1 and CIR, both members of the family of inwardly rectifying K+ channels. Here, we demonstrate that muscarinic K+ channels in neonatal rat atrial myocytes exhibit four distinct gating modes. In intact myocytes, after muscarinic receptor activation, the different gating modes were distinguished by differences in both the frequency of channel opening and the mean open time of the channel, which accounted for a 76-fold increase in channel open probability from mode 1 to mode 4. Because of the tetrameric architecture of the channel, the hypothesis that each of the four gating modes reflects binding of a different number of Gbeta gamma subunits to the channel was tested, using recombinant Gbeta1 gamma5. Gbeta1 gamma5 was able to control the equilibrium between the four gating modes of the channel in a manner consistent with binding of Gbeta gamma to four equivalent and independent sites in the protein complex. Surprisingly, however, Gbeta1 gamma5 lacked the ability to stabilize the long open state of the channel that is responsible for the augmentation of the mean open time in modes 3 and 4 after muscarinic receptor stimulation. The modal regulation of muscarinic K+ channel gating by Gbeta gamma provides the atrial cells with at least two major advantages: the ability to filter out small inputs from multiple membrane receptors and yet the ability to create the gradients of information necessary to control the heart rate with great precision.

Signalling via the G protein-activated K+ channels

The inwardly rectifying K+ channels of the GIRK (Kir3) family, members of the superfamily of inwardly rectifying K+ channels (Kir), are important physiological tools to regulate excitability in heart and brain by neurotransmitters, and the only ion channels conclusively shown to be activated by a direct interaction with heterotrimeric G protein subunits. During the last decade, especially since their cloning in 1993, remarkable progress has been made in understanding the structure, mechanisms of gating, activation by G proteins, and modulation of these channels. However, much of the molecular details of structure and of gating by G protein subunits and other factors, mechanisms of modulation and desensitization, and determinants of specificity of coupling to G proteins, remain unknown. This review summarizes both the recent advances and the unresolved questions now on the agenda in GIRK studies.

A C-terminal peptide of the GIRK1 subunit directly blocks the G protein-activated K+ channel (GIRK) expressed in Xenopus oocytes

1. In order to find out the functional roles of cytosolic regions of a G protein-activated, inwardly rectifying potassium channel subunit we studied block of GIRK channels, expressed in Xenopus laevis oocytes, by synthetic peptides in isolated inside-out membrane patches. 2. A peptide (DS6) derived from the very end of the C-terminus of GIRK1 reversibly blocked GIRK activity with IC50 values of 7.9 +/- 2.0 or 3.5 +/- 0.5 micrograms ml-1 (corresponding to 3.7 +/- 0.9 or 1.7 +/- 0.2 mumol l-1) for GIRK1/GIRK5 or GIRK1/GIRK4 channels, respectively. 3. Dose dependency studies of GIRK activation by purified beta gamma subunits of the G protein (G beta gamma) showed that DS6 block of GIRK channels is not the result of competition of the peptide with functional GIRK channels for the available G beta gamma. 4. Burst duration of GIRK channels was reduced, whereas long closed times between bursts were markedly increased, accounting for the channel block observed. 5. Block by the DS6 peptide was slightly voltage dependent, being stronger at more negative potentials. 6. These data support the hypothesis that the distal part of the carboxy-terminus of GIRK1 is a part of the intrinsic gate that keeps GIRK channels closed in the absence of G beta gamma.

RGS proteins reconstitute the rapid gating kinetics of gbetagamma-activated inwardly rectifying K+ channels

G protein-gated inward rectifier K+ (GIRK) channels mediate hyperpolarizing postsynaptic potentials in the nervous system and in the heart during activation of Galpha(i/o)-coupled receptors. In neurons and cardiac atrial cells the time course for receptor-mediated GIRK current deactivation is 20-40 times faster than that observed in heterologous systems expressing cloned receptors and GIRK channels, suggesting that an additional component(s) is required to confer the rapid kinetic properties of the native transduction pathway. We report here that heterologous expression of "regulators of G protein signaling" (RGS proteins), along with cloned G protein-coupled receptors and GIRK channels, reconstitutes the temporal properties of the native receptor --> GIRK signal transduction pathway. GIRK current waveforms evoked by agonist activation of muscarinic m2 receptors or serotonin 1A receptors were dramatically accelerated by coexpression of either RGS1, RGS3, or RGS4, but not RGS2. For the brain-expressed RGS4 isoform, neither the current amplitude nor the steady-state agonist dose-response relationship was significantly affected by RGS expression, although the agonist-independent "basal" GIRK current was suppressed by approximately 40%. Because GIRK activation and deactivation kinetics are the limiting rates for the onset and termination of "slow" postsynaptic inhibitory currents in neurons and atrial cells, RGS proteins may play crucial roles in the timing of information transfer within the brain and to peripheral tissues.