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

Angiotensin receptors and α1B-adrenergic receptors regulate native IK(ACh) and phosphorylation-deficient GIRK4 (S418A) channels through different PKC isoforms

Signaling of G protein-activated inwardly rectifying K+ (GIRK) channels is an important mechanism of the parasympathetic regulation of the heart rate and cardiac excitability. GIRK channels are inhibited during stimulation of Gq-coupled receptors (GqPCRs) by depletion of phosphatidyl-4,5-bisphosphate (PIP2) and/or channel phosphorylation by protein kinase C (PKC). The GqPCR-dependent modulation of GIRK currents in terms of specific PKC isoform activation was analyzed in voltage-clamp experiments in rat atrial myocytes and in CHO or HEK 293 cells. By using specific PKC inhibitors, we identified the receptor-activated PKC isoforms that contribute to phenylephrine- and angiotensin-induced GIRK channel inhibition. We demonstrate that the cPKC isoform PKCα significantly contributes to GIRK inhibition during stimulation of wildtype α1B-adrenergic receptors (α1B-ARs). Deletion of the α1B-AR serine residues S396 and S400 results in a preferential regulation of GIRK activity by PKCβ. As a novel finding, we report that the AT1-receptor-induced GIRK inhibition depends on the activation of the nPKC isoform PKCε whereas PKCα and PKCβ do not mainly participate in the angiotensin-mediated GIRK reduction. Expression of the dominant negative (DN) PKCε prolonged the onset of GIRK inhibition and significantly reduced AT1-R desensitization, indicating that PKCε regulates both GIRK channel activity and the strength of the receptor signal via a negative feedback mechanism. The serine residue S418 represents an important phosphorylation site for PKCε in the GIRK4 subunit. To analyze the functional impact of this PKC phosphorylation site for receptor-specific GIRK channel modulation, we monitored the activity of a phosphorylation-deficient (GIRK4 (S418A)) GIRK4 channel mutant during stimulation of α1B-ARs or AT1-receptors. Mutation of S418 did not impede α1B-AR-mediated GIRK inhibition, suggesting that S418 within the GIRK4 subunit is not subject to PKCα-induced phosphorylation. Furthermore, activation of angiotensin receptors induced pronounced GIRK4 (S418A) channel inhibition, excluding that this phosphorylation site contributes to the AT1-R-induced GIRK reduction. Instead, phosphorylation of S418 has a facilitative effect on GIRK activity that was abolished in the GIRK4 (S418A) mutant. To summarize, the present study shows that the receptor-dependent regulation of atrial GIRK channels is attributed to the GqPCR-specific activation of different PKC isoforms. Receptor-specific activated PKC isoforms target distinct phosphorylation sites within the GIRK4 subunit, resulting in differential regulation of GIRK channel activity with either facilitative or inhibitory effects on GIRK currents.

Neuromedin B excites central lateral amygdala neurons and reduces cardiovascular output and fear-potentiated startle

Neuromedin B (NMB) and gastrin-releasing peptide (GRP) are the two mammalian analogs in the bombesin peptide family that exert a variety of actions including emotional processing, appetitive behaviors, cognition, and tumor growth. The bombesin-like peptides interact with three receptors: the NMB-preferring bombesin 1 (BB1) receptors, the GRP-preferring bombesin 2 (BB2) receptors and the orphan bombesin 3 (BB3) receptors. Whereas, injection of bombesin into the central amygdala reduces satiety and modulates blood pressure, the underlying cellular and molecular mechanisms have not been determined. As administration of bombesin induces the expression of Fos in the lateral nucleus of the central amygdala (CeL) which expresses BB1 receptors, we probed the effects of NMB on CeL neurons using in vitro and in vivo approaches. We showed that activation of the BB1 receptors increased action potential firing frequency recorded from CeL neurons via inhibition of the inwardly rectifying K+ (Kir) channels. Activities of phospholipase Cβ and protein kinase C were required, whereas intracellular Ca2+ release was unnecessary for BB1 receptor-elicited potentiation of neuronal excitability. Application of NMB directly into the CeA reduced blood pressure and heart rate and significantly reduced fear-potentiated startle. We may provide a cellular and molecular mechanism whereby bombesin-like peptides modulate anxiety and fear responses in the amygdala.

The effects of serotonergic psychedelics in synaptic and intrinsic properties of neurons in layer II/III of the orbitofrontal cortex

RATIONALE

Serotonergic psychedelics show promise in the treatment of psychiatric disorders, including obsessive-compulsive disorder. Dysfunction of the orbitofrontal cortex (OFc) has been implicated in the pathophysiology of compulsive behavior, which might be a key region for the efficacy of psychedelics. However, the effects of psychedelics on the neural activities and local excitation/inhibition (E/I) balance in the OFc are unclear.

OBJECTIVES

This study aimed to investigate how 25C-NBOMe, a substituted phenethylamine psychedelic, regulated the synaptic and intrinsic properties of neurons in layer II/III of the OFc.

METHODS

Acute brain slices containing the OFc of adult male Sprague Dawley rats were used for ex vivo whole-cell recording. The synaptic and intrinsic properties of neurons were monitored using voltage and current clamps, respectively. Electrically evoked action potential (eAP) was used to measure synaptic-driven pyramidal activity.

RESULTS

25C-NBOMe enhanced spontaneous neurotransmission at glutamatergic synapses but diminished that in GABAergic synapses through the 5-HT_2A receptor. 25C-NBOMe also increased both evoked excitatory currents and evoked action potentials. Moreover, 25C-NBOMe promoted the excitability of pyramidal neurons but not fast-spiking neurons. Either inhibiting G protein-gated inwardly rectifying potassium channels or activating protein kinase C significantly obstructed the facilitative effect of 25C-NBOMe on the intrinsic excitability of pyramidal neurons.

CONCLUSIONS

This work reveals the multiple roles of 25C-NBOMe in modulating synaptic and neuronal function in the OFc, which collectively promotes local E/I ratios.

Consequences of somatic mutations of GIRK1 detected in primary malign tumors on expression and function of G-protein activated, inwardly rectifying, K+ channels

A search in the GDC Data Portal revealed 304 documented somatic mutations of the KCNJ3 gene in primary tumors (out of 10.202 cases). Most affected tumor types were carcinomas from uterus, skin and lung, while breast cancer exerted the lowest number of somatic mutations. We focused our research on 15 missense mutations within the region between TM1 and TM2, comprising the pore helix and ion selectivity signature. Expression was measured by confocal laser scan microscopy of eGFP tagged GIRK1 subunits, expressed with and without GIRK4 in oocytes of Xenopus laevis. GIRK ion currents were activated via coexpressed m2Rs and measured by the Two Electrode Voltage Clamp technique. Magnitude of the total GIRK current, as well as the fraction of current inducible by the agonist, were measured. Ion selectivity was gauged by assessment of the PNa+/PK+ ratio, calculated by the GIRK current reversal potential in extracellular media at different Na+ and K+ concentrations. None of the tested mutations was able to form functional GIRK1 homooligomeric ion channels. One of the mutations, G145A, which locates directly to the ion selectivity signature, exerted an increased PNa+/PK+ ratio. Generally, the missense mutations studied can be categorized into three groups: (i) normal/reduced expression accompanied by reduced/absent function (S132Y, F136L, E139K, G145A, R149Q, R149P, G178D, S185Y, Q186R), (ii) normal/increased expression as well as increased function (E140M, A142T, M184I) and (iii) miniscule expression but increased function relative to expression levels (I151N, G158S). We conclude, that gain of function mutations, identical or similar to categories (ii) and (iii), may potentially be involved in genesis and progression of malignancies in tissues that exert a high rate of occurrence of somatic mutations of KCNJ3.

Ionic signalling mechanisms involved in neurokinin-3 receptor-mediated augmentation of fear-potentiated startle response in the basolateral amygdala

The tachykinin peptides include substance P (SP), neurokinin A and neurokinin B, which interact with three G-protein-coupled neurokinin receptors, NK1Rs, NK2Rs and NK3Rs, respectively. Whereas high densities of NK3Rs have been detected in the basolateral amygdala (BLA), the functions of NK3Rs in this brain region have not been determined. We found that activation of NK3Rs by application of the selective agonist, senktide, persistently excited BLA principal neurons. NK3R-elicited excitation of BLA neurons was mediated by activation of a non-selective cation channel and depression of the inwardly rectifying K+ (Kir) channels. With selective channel blockers and knockout mice, we further showed that NK3R activation excited BLA neurons by depressing the G protein-activated inwardly rectifying K+ (GIRK) channels and activating TRPC4 and TRPC5 channels. The effects of NK3Rs required the functions of phospholipase Cβ (PLCβ), but were independent of intracellular Ca2+ release and protein kinase C. PLCβ-mediated depletion of phosphatidylinositol 4,5-bisphosphate was involved in NK3R-induced excitation of BLA neurons. Microinjection of senktide into the BLA of rats augmented fear-potentiated startle (FPS) and this effect was blocked by prior injection of the selective NK3R antagonist SB 218795, suggesting that activation of NK3Rs in the BLA increased FPS. We further showed that TRPC4/5 and GIRK channels were involved in NK3R-elicited facilitation of FPS. Our results provide a cellular and molecular mechanism whereby NK3R activation excites BLA neurons and enhances FPS. KEY POINTS: Activation of NK3 receptors (NK3Rs) facilitates the excitability of principal neurons in rat basolateral amygdala (BLA). NK3R-induced excitation is mediated by inhibition of GIRK channels and activation of TRPC4/5 channels. Phospholipase Cβ and depletion of phosphatidylinositol 4,5-bisphosphate are necessary for NK3R-mediated excitation of BLA principal neurons. Activation of NK3Rs in the BLA facilitates fear-potentiated startle response. GIRK channels and TRPC4/5 channels are involved in NK3R-mediated augmentation of fear-potentiated startle.

Neuronal G protein-gated K+ channels

G protein-gated inwardly rectifying K⁺ (GIRK/Kir3) channels exert a critical inhibitory influence on neurons. Neuronal GIRK channels mediate the G protein-dependent, direct/postsynaptic inhibitory effect of many neurotransmitters including γ-aminobutyric acid (GABA), serotonin, dopamine, adenosine, somatostatin, and enkephalin. In addition to their complex regulation by G proteins, neuronal GIRK channel activity is sensitive to PIP₂, phosphorylation, regulator of G protein signaling (RGS) proteins, intracellular Na⁺ and Ca²⁺, and cholesterol. The application of genetic and viral manipulations in rodent models, together with recent progress in the development of GIRK channel modulators, has increased our understanding of the physiological and behavioral impact of neuronal GIRK channels. Work in rodent models has also revealed that neuronal GIRK channel activity is modified, transiently or persistently, by various stimuli including exposure drugs of abuse, changes in neuronal activity patterns, and aversive experience. A growing body of preclinical and clinical evidence suggests that dysregulation of GIRK channel activity contributes to neurological diseases and disorders. The primary goals of this review are to highlight fundamental principles of neuronal GIRK channel biology, mechanisms of GIRK channel regulation and plasticity, the nascent landscape of GIRK channel pharmacology, and the potential relevance of GIRK channels to the pathophysiology and treatment of neurological diseases and disorders.

PKC regulation of ion channels: The involvement of PIP2

Ion channels are integral membrane proteins whose gating has been increasingly shown to depend on the presence of the low-abundance membrane phospholipid, phosphatidylinositol (4,5) bisphosphate. The expression and function of ion channels is tightly regulated via protein phosphorylation by specific kinases, including various PKC isoforms. Several channels have further been shown to be regulated by PKC through altered surface expression, probability of channel opening, shifts in voltage dependence of their activation, or changes in inactivation or desensitization. In this review, we survey the impact of phosphorylation of various ion channels by PKC isoforms and examine the dependence of phosphorylated ion channels on phosphatidylinositol (4,5) bisphosphate as a mechanistic endpoint to control channel gating.

Kir Channel Molecular Physiology, Pharmacology, and Therapeutic Implications

For the past two decades several scholarly reviews have appeared on the inwardly rectifying potassium (Kir) channels. We would like to highlight two efforts in particular, which have provided comprehensive reviews of the literature up to 2010 (Hibino et al., Physiol Rev 90(1):291-366, 2010; Stanfield et al., Rev Physiol Biochem Pharmacol 145:47-179, 2002). In the past decade, great insights into the 3-D atomic resolution structures of Kir channels have begun to provide the molecular basis for their functional properties. More recently, computational studies are beginning to close the time domain gap between in silico dynamic and patch-clamp functional studies. The pharmacology of these channels has also been expanding and the dynamic structural studies provide hope that we are heading toward successful structure-based drug design for this family of K⁺ channels. In the present review we focus on placing the physiology and pharmacology of this K⁺ channel family in the context of atomic resolution structures and in providing a glimpse of the promising future of therapeutic opportunities.

Roles of PLCβ, PIP2 , and GIRK channels in arginine vasopressin-elicited excitation of CA1 pyramidal neurons

Arginine vasopressin (AVP) is a hormone exerting vasoconstrictive and antidiuretic action in the periphery and serves as a neuromodulator in the brain. Although the hippocampus receives vasopressinergic innervation and AVP has been shown to facilitate the excitability of CA1 pyramidal neurons, the involved ionic and signaling mechanisms have not been determined. Here we found that AVP excited CA1 pyramidal neurons by activation of V_1a receptors. Functions of G proteins and phospholipase Cβ (PLCβ) were required for AVP-elicited excitation of CA1 pyramidal neurons, whereas intracellular Ca²⁺ release and protein kinase C were unnecessary. PLCβ-mediated depletion of phosphatidylinositol 4,5-bisphosphate (PIP₂ ) was required for AVP-elicited excitation of CA1 pyramidal neurons. AVP augmented the input resistance and increased the time constants of CA1 pyramidal neurons. AVP induced an inward current in K⁺ -containing intracellular solution, whereas no inward currents were observed with Cs⁺ -containing intracellular solution. AVP-sensitive currents showed inward rectification with a reversal potential close to the K⁺ reversal potential, suggesting the involvement of inwardly rectifying K⁺ channels. AVP-induced currents were sensitive to the micromolar concentration of Ba²⁺ and tertiapin-Q, whereas application of ML 133, a selective Kir2 channel blocker had no effects, suggesting that AVP excited CA1 pyramidal neurons by depressing G protein-gated inwardly rectifying K⁺ channels. Activation of V_1a receptors in the CA1 region facilitated glutamatergic transmission onto subicular pyramidal neurons, suggesting that AVP modulates network activity in the brain. Our results may provide one of the cellular and molecular mechanisms to explain the in vivo physiological functions of AVP.

Ionic and signaling mechanisms involved in neurotensin-mediated excitation of central amygdala neurons

Neurotensin (NT) serves as a neuromodulator in the brain where it regulates a variety of physiological functions. Whereas the central amygdala (CeA) expresses NT peptide and NTS1 receptors and application of NT has been shown to excite CeA neurons, the underlying cellular and molecular mechanisms have not been determined. We found that activation of NTS1 receptors increased the neuronal excitability of the lateral nucleus (CeL) of CeA. Both phospholipase Cβ (PLCβ) and phosphatidylinositol 4,5-bisphosphate (PIP₂) depletion were required, whereas intracellular Ca²⁺ release and PKC were unnecessary for NT-elicited excitation of CeL neurons. NT increased the input resistance and time constants of CeL neurons, suggesting that NT excites CeL neurons by decreasing a membrane conductance. Depressions of the inwardly rectifying K⁺ (Kir) channels including both the Kir2 subfamily and the GIRK channels were required for NT-elicited excitation of CeL neurons. Activation of NTS1 receptors in the CeL led to GABAergic inhibition of medial nucleus of CeA neurons, suggesting that NT modulates the network activity in the amygdala. Our results may provide a cellular and molecular mechanism to explain the physiological functions of NT in vivo.

Human adrenal glomerulosa cells express K2P and GIRK potassium channels that are inhibited by ANG II and ACTH

In whole cell patch clamp recordings, it was discovered that normal human adrenal zona glomerulosa (AZG) cells express members of the three major families of K⁺ channels. Among these are a two-pore (K2P) leak-type and a G protein-coupled, inwardly rectifying (GIRK) channel, both inhibited by peptide hormones that stimulate aldosterone secretion. The K2P current displayed properties identifying it as TREK-1 (KCNK2). This outwardly rectifying current was activated by arachidonic acid and inhibited by angiotensin II (ANG II), adrenocorticotrophic hormone (ACTH), and forskolin. The activation and inhibition of TREK-1 was coupled to AZG cell hyperpolarization and depolarization, respectively. A second K2P channel, TASK-1 (KCNK3), was expressed at a lower density in AZG cells. Human AZG cells also express inwardly rectifying K⁺ current(s) (K_IR) that include quasi-instantaneous and time-dependent components. This is the first report demonstrating the presence of K_IR in whole cell recordings from AZG cells of any species. The time-dependent current was selectively inhibited by ANG II, and ACTH, identifying it as a G protein-coupled (GIRK) channel, most likely K_IR3.4 (KCNJ5). The quasi-instantaneous K_IR current was not inhibited by ANG II or ACTH and may be a separate non-GIRK current. Finally, AZG cells express a voltage-gated, rapidly inactivating K⁺ current whose properties identified as K_V1.4 (KCNA4), a conclusion confirmed by Northern blot. These findings demonstrate that human AZG cells express K2P and GIRK channels whose inhibition by ANG II and ACTH is likely coupled to depolarization-dependent secretion. They further demonstrate that human AZG K⁺ channels differ fundamentally from the widely adopted rodent models for human aldosterone secretion.

Ethanol inhibition of lateral orbitofrontal cortex neuron excitability is mediated via dopamine D1/D5 receptor-induced release of astrocytic glycine

Recent findings from this laboratory demonstrate that ethanol reduces the intrinsic excitability of orbitofrontal cortex (OFC) neurons via activation of strychnine-sensitive glycine receptors. Although the mechanism linking ethanol to the release of glycine is currently unknown, astrocytes are a source of neurotransmitters including glycine and activation of dopamine D1-like receptors has been reported to enhance extracellular levels of glycine via a functional reversal of the astrocytic glycine transporter GlyT1. We recently reported that like ethanol, dopamine or a D1/D5 receptor agonist increases a tonic current in lateral OFC (lOFC) neurons. Therefore, in this study, we used whole-cell patch-clamp electrophysiology to examine whether ethanol inhibition of OFC spiking involves the release of glycine from astrocytes and whether this release is dopamine receptor dependent. Ethanol, applied acutely, decreased spiking of lOFC neurons and this effect was blocked by antagonists of GlyT1, the norepinephrine transporter or D1-like but not D2-like receptors. Ethanol enhanced the tonic current of OFC neurons and occluded the effect of dopamine suggesting that ethanol and dopamine may share a common pathway. Altering astrocyte function by suppressing intracellular astrocytic calcium signaling or blocking the astrocyte-specific Kir4.1 potassium channels reduced but did not completely abolish ethanol inhibition of OFC neuron firing. However, when both astrocytic calcium signaling and Kir4.1 channels were inhibited, ethanol had no effect on firing. Ethanol inhibition was also prevented by inhibitors of phospholipase C and conventional isoforms of protein kinase C (cPKC) previously shown to block D1R-induced GlyT1 reversal and PKC inhibition of Kir4.1 channels. Finally, the membrane potential of OFC astrocytes was depolarized by bath application of a Kir4.1 blocker, a D1 agonist or ethanol and ethanol effect was blocked by a D1 antagonist. Together, these findings suggest that acute ethanol inhibits OFC neuron excitability via a D1 receptor-mediated dysregulation of astrocytic glycine transport.

Activation of V1a vasopressin receptors excite subicular pyramidal neurons by activating TRPV1 and depressing GIRK channels

Arginine vasopressin (AVP) is a nonapeptide that serves as a neuromodulator in the brain and a hormone in the periphery that regulates water homeostasis and vasoconstriction. The subiculum is the major output region of the hippocampus and an integral component in the networks that processes sensory and motor cues to form a cognitive map encoding spatial, contextual, and emotional information. Whereas the subiculum expresses high densities of AVP-binding sites and AVP has been shown to increase the synaptic excitability of subicular pyramidal neurons, the underlying cellular and molecular mechanisms have not been determined. We found that activation of V_1a receptors increased the excitability of subicular pyramidal neurons via activation of TRPV1 channels and depression of the GIRK channels. V_1a receptor-induced excitation of subicular pyramidal neurons required the function of phospholipase Cβ, but was independent of intracellular Ca²⁺ release. Protein kinase C was responsible for AVP-mediated depression of GIRK channels, whereas degradation of phosphatidylinositol 4,5-bisphosphate was involved in V_1a receptor-elicited activation of TRPV1 channels. Our results may provide one of the cellular and molecular mechanisms to explain the physiological functions of AVP in the brain.

The Signaling Pathways Involved in the Anticonvulsive Effects of the Adenosine A1 Receptor

Adenosine acts as an endogenous anticonvulsant and seizure terminator in the brain. Many of its anticonvulsive effects are mediated through the activation of the adenosine A₁ receptor, a G protein-coupled receptor with a wide array of targets. Activating A₁ receptors is an effective approach to suppress seizures. This review gives an overview of the neuronal targets of the adenosine A₁ receptor focusing in particular on signaling pathways resulting in neuronal inhibition. These include direct interactions of G protein subunits, the adenyl cyclase pathway and the phospholipase C pathway, which all mediate neuronal hyperpolarization and suppression of synaptic transmission. Additionally, the contribution of the guanyl cyclase and mitogen-activated protein kinase cascades to the seizure-suppressing effects of A₁ receptor activation are discussed. This review ends with the cautionary note that chronic activation of the A₁ receptor might have detrimental effects, which will need to be avoided when pursuing A₁ receptor-based epilepsy therapies.

The antitussive cloperastine improves breathing abnormalities in a Rett Syndrome mouse model by blocking presynaptic GIRK channels and enhancing GABA release

Rett Syndrome (RTT) is an X-linked neurodevelopmental disorder caused mainly by mutations in the MECP2 gene. One of the major RTT features is breathing dysfunction characterized by periodic hypo- and hyperventilation. The breathing disorders are associated with increased brainstem neuronal excitability, which can be alleviated with GABA agonists. Since neuronal hypoexcitability occurs in the forebrain of RTT models, it is necessary to find pharmacological agents with a relative preference to brainstem neurons. Here we show evidence for the improvement of breathing disorders of Mecp2-disrupted mice with the brainstem-acting drug cloperastine (CPS) and its likely neuronal targets. CPS is an over-the-counter cough medicine that has an inhibitory effect on brainstem neuronal networks. In Mecp2-disrupted mice, CPS (30 mg/kg, i.p.) decreased the occurrence of apneas/h and breath frequency variation. GIRK currents expressed in HEK cells were inhibited by CPS with IC₅₀ 1 μM. Whole-cell patch clamp recordings in locus coeruleus (LC) and dorsal tegmental nucleus (DTN) neurons revealed an overall inhibitory effect of CPS (10 μM) on neuronal firing activity. Such an effect was reversed by the GABA_A receptor antagonist bicuculline (20 μM). Voltage clamp studies showed that CPS increased GABAergic sIPSCs in LC cells, which was blocked by the GABA_B receptor antagonist phaclofen. Functional GABAergic connections of DTN neurons with LC cells were shown. These results suggest that CPS improves breathing dysfunction in Mecp2-null mice by blocking GIRK channels in synaptic terminals and enhancing GABA release.

Oxytocin receptors excite lateral nucleus of central amygdala by phospholipase Cβ- and protein kinase C-dependent depression of inwardly rectifying K+ channels

KEY POINTS

Activation of oxytocin receptors (OXTRs) facilitates neuronal excitability in rat lateral nucleus of central amygdala (CeL). OXTR-induced excitation is mediated by inhibition of inwardly rectifying K⁺ (Kir) channels. Phospholipase Cβ is necessary for OXTR-mediated excitation of CeL neurons and depression of Kir channels. OXTR-elicited depression of Kir channels and excitation of CeL neurons require the function of Ca²⁺ -dependent protein kinase C.

ABSTRACT

Oxytocin (OXT) is a nonapeptide that exerts anxiolytic effects in the brain. The amygdala is an important structure involved in the modulation of fear and anxiety. A high density of OXT receptors (OXTRs) has been detected in the capsular (CeC) and lateral (CeL) nucleus of the central amygdala (CeA). Previous studies have demonstrated that activation of OXTRs induces remarkable increases in neuronal excitability in the CeL/C. However, the signalling and ionic mechanisms underlying OXTR-induced facilitation of neuronal excitability have not been determined. We found that activation of OXTRs in the CeL increased action potential firing frequency recorded from neurons in this region via inhibition of the inwardly rectifying K⁺ channels. The functions of phospholipase Cβ and protein kinase C were required for OXTR-induced augmentation of neuronal excitability. Our results provide a cellular and molecular mechanism whereby activation of OXTRs exerts anxiolytic effects.

Jingshu Keli and its Components Notoginsenoside R1 and Ginsenoside Rb1 Alleviate the Symptoms of Cervical Myelopathy through Kir3.1 Mediated Mechanisms

Background & Objective::

Cervical Spondylotic Myelopathy (CSM) is one of the most serious spinal cord disorders in adults. Pharmacological modulation of ion channels is a common strategy to interfere with CSM and prevent neuronal damage.

Methods:

Here, we investigated the effects of Jingshu Keli (JSKL), a traditional Chinese herbal formula, on CSM-related gait abnormality, mechanical allodynia and thermal hyperalgesia, and assessed the neuronal mechanisms of JSKL on cultured brainstem cells. Behavioral tests and patch clamp recordings were performed to make this assessment.

Results:

In our study, we found that JSKL significantly recovered the gait performance (P<0.001) and decreased the levels of mechanical pain in 18.9% (P<0.01) and thermal pain in 18.1% (P<0.05). Further investigation suggested that JSKL and its containing ginsenoside Rb1 (GRb1), notoginsenoside R1 (NGR1) reduced the action potential frequency in 38.5%, 27.2%, 25.9%, and hyperpolarized resting membrane potential in 15.0%, 13.8%, 12.1%, respectively. Kir channels, not KV channels and KCa channels, were the major intermediate factors achieving treatment effects. Finally, immunostaining results showed that the phosphorylation of Kir3.1 was promoted, whereas the total expression level did not change.

Conclusion:

Our study reveals a novel strategy of treating CSM by using Traditional Chinese Medicines (TCMs) containing active components.

Biased signaling of Ca2+-sensing receptors in cardiac myocytes regulates GIRK channel activity

Ca²⁺-sensing receptors (CaSRs) belong to the class C of G protein-coupled receptors and are activated by extracellular Ca²⁺. CaSRs display biased G protein signaling by coupling to different classes of heterotrimeric G proteins depending on agonist and cell type. In this study we used fluorescent biosensors to directly analyze G protein coupling to CaSRs and downstream signaling in living cells. In HEK 293 cells, CaSRs displayed biased signaling: elevation of extracellular Ca²⁺ or application of the alternative agonist spermine caused activation of G_i- and G_q-proteins. Adult cardiac myocytes express endogenous CaSRs, which have been implicated in regulating Ca²⁺ signaling and contractility. Biased signaling of CaSRs has not been investigated in these cells. To evaluate efficiencies of G_i- and G_q-signaling via CaSRs in rat atrial myocytes, we measured G protein-activated K⁺ (GIRK) channels. Activation of GIRK requires binding of Gβγ subunits released from G_i proteins, whereas G_q-signaling results in inhibition of GIRK channel activity. Stimulation of CaSRs by Ca²⁺ or spermine failed to directly activate G_i and GIRK channels. When GIRK channels were pre-activated via endogenous M₂ receptors, stimulation of CaSRs caused pronounced inhibition of GIRK currents. This effect was specific to CaSR activation: GIRK current inhibition was sensitive to NPS-2143, a negative allosteric modulator of CaSRs, and abrogated by FR900359, a direct inhibitor of G_q. GIRK current inhibition was also sensitive to the PKC inhibitor chelerythrine, suggesting that following activation of CaSR and G_q, GIRK currents are modulated by PKC phosphorylation. We conclude from this data that cardiac CaSRs do not activate G_i and affect GIRK currents preferentially via the G_q/PKC pathway.

Selective binding of a toxin and phosphatidylinositides to a mammalian potassium channel

G-protein-gated inward rectifying potassium channels (GIRKs) require Gβγ subunits and phosphorylated phosphatidylinositides (PIPs) for gating. Although studies have provided insight into these interactions, the mechanism of how these events are modulated by Gβγ and the binding affinity between PIPs and GIRKs remains poorly understood. Here, native ion mobility mass spectrometry is employed to directly monitor small molecule binding events to mouse GIRK2. GIRK2 binds the toxin tertiapin Q and PIPs selectively and with significantly higher affinity than other phospholipids. A mutation in GIRK2 that causes a rotation in the cytoplasmic domain, similarly to Gβγ-binding to the wild-type channel, revealed differences in the selectivity towards PIPs. More specifically, PIP isoforms known to weakly activate GIRKs have decreased binding affinity. Taken together, our results reveal selective small molecule binding and uncover a mechanism by which rotation of the cytoplasmic domain can modulate GIRK•PIP interactions.

Protein kinase C activity is a protective modifier of Purkinje neuron degeneration in cerebellar ataxia

Among the many types of neurons expressing protein kinase C (PKC) enzymes, cerebellar Purkinje neurons are particularly reliant on appropriate PKC activity for maintaining homeostasis. The importance of PKC enzymes in Purkinje neuron health is apparent as mutations in PRKCG (encoding PKCγ) cause cerebellar ataxia. PRKCG has also been identified as an important node in ataxia gene networks more broadly, but the functional role of PKC in other forms of ataxia remains unexplored, and the mechanisms by which PKC isozymes regulate Purkinje neuron health are not well understood. Here, we investigated how PKC activity influences neurodegeneration in inherited ataxia. Using mouse models of spinocerebellar ataxia type 1 (SCA1) and 2 (SCA2) we identify an increase in PKC-mediated substrate phosphorylation in two different forms of inherited cerebellar ataxia. Normalizing PKC substrate phosphorylation in SCA1 and SCA2 mice accelerates degeneration, suggesting that the increased activity observed in these models is neuroprotective. We also find that increased phosphorylation of PKC targets limits Purkinje neuron membrane excitability, suggesting that PKC activity may support Purkinje neuron health by moderating excitability. These data suggest a functional role for PKC enzymes in ataxia gene networks, and demonstrate that increased PKC activity is a protective modifier of degeneration in inherited cerebellar ataxia.

GIRK1-mediated inwardly rectifying potassium current suppresses the epileptiform burst activities and the potential antiepileptic effect of ML297

G protein-gated inwardly rectifying potassium (GIRK) channels are important inhibitory regulators of neuronal excitability in central nervous system, and the impairment of GIRK channel function has been reported to be associated with the susceptibility of epilepsy. However, the dynamics of GIRK channels in the pathogenesis of epilepsy are still unclear. In this study, our results showed that cyclothiazide, a potent convulsant, dose dependently increased the epileptiform bursting activities and suppressed the baclofen induced GIRK currents. In addition, TPQ, a selective GIRK antagonist, significantly decreased the total inwardly rectifying potassium (Kir) current, and increased the neuronal epileptiform activities. In contrast, ML297, a potent and selective GIRK channel agonist, reversed the cyclothiazide induced decrease of GIRK currents and the increase of neuronal excitability in cultured hippocampal neurons. Further investigation revealed that GIRK1, but not GIRK2, played a key role in suppressing epileptic activities. Finally, in pilocarpine mice seizure model, we demonstrated that ML297 significantly suppressed the seizure behavior. In summary, our current results indicate that GIRK channels, especially GIRK1-containing channels, are involved in epileptic activities and ML297 has a potential antiepileptic effect.

Hemispheric differences in the number of parvalbumin-positive neurons in subdivisions of the rat basolateral amygdala complex

The amygdala is a bilateral temporal lobe brain region which plays an important role in emotional processing. Past studies on the amygdala have shown hemispheric differences in amygdalar processes and responses associated with specific pain and fear behaviors. Despite the functional differences in the amygdala, few studies have been performed to characterize whether anatomical differences exist between the left and right amygdala. Parvalbumin (PV) is a phenotypic marker for an inhibitory interneuronal population in cortical brain structures such as the basolateral amygdala complex (BLC). This study examined the number of PV-positive neurons in the left and right BLC of adult, male Long-Evans rats using unbiased stereology. Coronal sections through the rostral-caudal extent of the BLC were immunohistochemically-stained for PV and the optical fractionator method was used to obtain an unbiased estimate of the number of PV-positive neurons in subdivisions through the BLC. The lateral and basolateral amygdala divisions of the BLC were analyzed, were subdivided into the dorsolateral, ventrolateral and ventromedial and the posterior, anterior and ventral subdivisions, respectively. The results indicate that there are significantly more PV-positive neurons in the left basolateral amygdala compared to the right, with a significant difference specifically in the posterior subdivision. This difference in PV neuronal number could help explain the distinct hemispheric roles of the BLC in the behavioral processing following exposure to painful and fearful stimuli.

Calcium Release from Stores Inhibits GIRK

Synaptic transmission is mediated by ionotropic and metabotropic receptors that together regulate the rate and pattern of action potential firing. Metabotropic receptors can activate ion channels and modulate other receptors and channels. The present paper examines the interaction between group 1 mGluR-mediated calcium release from stores and GABA_B/D2-mediated GIRK currents in rat dopamine neurons of the Substantia Nigra. Transient activation of mGluRs decreased the GIRK current evoked by GABA_B and D2 receptors, although less efficaciously for D2. The mGluR-induced inhibition of GIRK current peaked in 1 s and recovered to baseline after 5 s. The inhibition was dependent on release of calcium from stores, was larger for transient than for tonic currents, and was unaffected by inhibitors of PLC, PKC, PLA₂, or calmodulin. This inhibition of GABA_B IPSCs through release of calcium from stores is a postsynaptic mechanism that may broadly reduce GIRK-dependent inhibition of many central neurons.

Desensitization and Tolerance of Mu Opioid Receptors on Pontine Kölliker-Fuse Neurons

Acute desensitization of mu opioid receptors is thought to be an initial step in the development of tolerance to opioids. Given the resistance of the respiratory system to develop tolerance, desensitization of neurons in the Kölliker-Fuse (KF), a key area in the respiratory circuit, was examined. The activation of G protein-coupled inwardly rectifying potassium current was measured using whole-cell voltage-clamp recordings from KF and locus coeruleus (LC) neurons contained in acute rat brain slices. A saturating concentration of the opioid agonist [Met⁵]-enkephalin (ME) caused significantly less desensitization in KF neurons compared with LC neurons. In contrast to LC, desensitization in KF neurons was not enhanced by activation of protein kinase C or in slices from morphine-treated rats. Cellular tolerance to ME and morphine was also lacking in KF neurons from morphine-treated rats. The lack of cellular tolerance in KF neurons correlates with the relative lack of tolerance to the respiratory depressant effect of opioids.

The ICl,swell inhibitor DCPIB blocks Kir channels that possess weak affinity for PIP2

Inwardly rectifying K(+) (Kir) channels are important contributors to the resting membrane potential and regulate cellular excitability. The activity of Kir channels depends critically on the phospholipid PIP2. Several modulators of the activity of Kir channels alter the apparent affinity of the channel to PIP2. Channels with high apparent affinity to PIP2 may not respond to a given modulator, but mutations that decrease such affinity can render the channel susceptible to modulation. Here, we identify a known inhibitor of the swelling-activated Cl(-) current, DCPIB, as an effective inhibitor of a number of Kir channels both in native cardiac cells and in heterologous expression systems. We show that the apparent affinity to PIP2 determines whether DCPIB will serve as an efficient blocker of Kir channels. These effects are consistent with a model in which DCPIB competes with PIP2 for a common binding site.

A Critical Gating Switch at a Modulatory Site in Neuronal Kir3 Channels

Inwardly rectifying potassium channels enforce tight control of resting membrane potential in excitable cells. The Kir3.2 channel, a member of the Kir3 subfamily of G-protein-activated potassium channels (GIRKs), plays several roles in the nervous system, including key responsibility in the GABAB pathway of inhibition, in pain perception pathways via opioid receptors, and is also involved in alcoholism. PKC phosphorylation acts on the channel to reduce activity, yet the mechanism is incompletely understood. Using the heterologous Xenopus oocyte system combined with molecular dynamics simulations, we show that PKC modulation of channel activity is dependent on Ser-196 in Kir3.2 such that, when this site is phosphorylated, the channel is less sensitive to PKC inhibition. This reduced inhibition is dependent on an interaction between phospho-Ser (SEP)-196 and Arg-201, reducing Arg-201 interaction with the sodium-binding site Asp-228. Neutralization of either SEP-196 or Arg-201 leads to a channel with reduced activity and increased sensitivity to PKC inhibition. This study clarifies the role of Ser-196 as an allosteric modulator of PKC inhibition and suggests that the SEP-196/Arg-201 interaction is critical for maintaining maximal channel activity.

SIGNIFICANCE STATEMENT

The inwardly rectifying potassium 3.2 (Kir3.2) channel is found principally in neurons that regulate diverse brain functions, including pain perception, alcoholism, and substance addiction. Activation or inhibition of this channel leads to changes in neuronal firing and chemical message transmission. The Kir3.2 channel is subject to regulation by intracellular signals including sodium, G-proteins, ethanol, the phospholipid phosphatidylinositol bis-phosphate, and phosphorylation by protein kinases. Here, we take advantage of the recently published structure of Kir3.2 to provide an in-depth molecular view of how phosphorylation of a specific residue previously thought to be the target of PKC promotes channel gating and acts as an allosteric modulator of PKC-mediated inhibition.

GIRK Channels: A Potential Link Between Learning and Addiction

The ability of drug-associated cues to reinitiate drug craving and seeking, even after long periods of abstinence, has led to the hypothesis that addiction represents a form of pathological learning, in which drugs of abuse hijack normal learning and memory processes to support long-term addictive behaviors. In this chapter, we review evidence suggesting that G protein-gated inwardly rectifying potassium (GIRK/Kir3) channels are one mechanism through which numerous drugs of abuse can modulate learning and memory processes. We will examine the role of GIRK channels in two forms of experience-dependent long-term changes in neuronal function: homeostatic plasticity and synaptic plasticity. We will also discuss how drug-induced changes in GIRK-mediated signaling can lead to changes that support the development and maintenance of addiction.

Firing modes of dopamine neurons drive bidirectional GIRK channel plasticity

G-protein-coupled inwardly rectifying potassium (GIRK) channels contribute to the resting membrane potential of many neurons, including dopamine (DA) neurons in the ventral tegmental area (VTA). VTA DA neurons are bistable, firing in two modes: one characterized by bursts of action potentials, the other by tonic firing at a lower frequency. Here we provide evidence that these firing modes drive bidirectional plasticity of GIRK channel-mediated currents. In acute midbrain slices of mice, we observed that in vitro burst activation of VTA DA neurons potentiated GIRK currents whereas tonic firing depressed these currents. This plasticity was not specific to the metabotropic receptor activating the GIRK channels, as direct activation of GIRK channels by nonhydrolyzable GTP also potentiated the currents. The plasticity of GIRK currents required NMDA receptor and CaMKII activation, and involved protein trafficking through specific PDZ domains of GIRK2c and GIRK3 subunit isoforms. Prolonged tonic firing may thus enhance the probability to switch into burst-firing mode, which then potentiates GIRK currents and favors the return to baseline. In conclusion, activity-dependent GIRK channel plasticity may represent a slow destabilization process favoring the switch between the two firing modes of VTA DA neurons.

Cardiac potassium channel subtypes: new roles in repolarization and arrhythmia

About 10 distinct potassium channels in the heart are involved in shaping the action potential. Some of the K+ channels are primarily responsible for early repolarization, whereas others drive late repolarization and still others are open throughout the cardiac cycle. Three main K+ channels drive the late repolarization of the ventricle with some redundancy, and in atria this repolarization reserve is supplemented by the fairly atrial-specific KV1.5, Kir3, KCa, and K2P channels. The role of the latter two subtypes in atria is currently being clarified, and several findings indicate that they could constitute targets for new pharmacological treatment of atrial fibrillation. The interplay between the different K+ channel subtypes in both atria and ventricle is dynamic, and a significant up- and downregulation occurs in disease states such as atrial fibrillation or heart failure. The underlying posttranscriptional and posttranslational remodeling of the individual K+ channels changes their activity and significance relative to each other, and they must be viewed together to understand their role in keeping a stable heart rhythm, also under menacing conditions like attacks of reentry arrhythmia.

K(+) channels: function-structural overview

Potassium channels are particularly important in determining the shape and duration of the action potential, controlling the membrane potential, modulating hormone secretion, epithelial function and, in the case of those K(+) channels activated by Ca(2+), damping excitatory signals. The multiplicity of roles played by K(+) channels is only possible to their mammoth diversity that includes at present 70 K(+) channels encoding genes in mammals. Today, thanks to the use of cloning, mutagenesis, and the more recent structural studies using x-ray crystallography, we are in a unique position to understand the origins of the enormous diversity of this superfamily of ion channels, the roles they play in different cell types, and the relations that exist between structure and function. With the exception of two-pore K(+) channels that are dimers, voltage-dependent K(+) channels are tetrameric assemblies and share an extremely well conserved pore region, in which the ion-selectivity filter resides. In the present overview, we discuss in the function, localization, and the relations between function and structure of the five different subfamilies of K(+) channels: (a) inward rectifiers, Kir; (b) four transmembrane segments-2 pores, K2P; (c) voltage-gated, Kv; (d) the Slo family; and (e) Ca(2+)-activated SK family, SKCa.

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

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

Regulatory mechanisms underlying the modulation of GIRK1/GIRK4 heteromeric channels by P2Y receptors

The muscarinic K(+) channel (I (K,ACh)) is a heterotetramer composed of GIRK1 (Kir3.1) and GIRK4 (Kir3.4) subunits of a G protein-coupled inwardly rectifying channel, and plays an important role in mediating electrical responses to the vagal stimulation in the heart. I (K,ACh) displays biphasic changes (activation followed by inhibition) through the stimulation of the purinergic P2Y receptors, but the regulatory mechanism involved in these modulation of I (K,ACh) by P2Y receptors remains to be fully elucidated. Various P2Y receptor subtypes and GIRK1/GIRK4 (I (GIRK)) were co-expressed in Chinese hamster ovary cells, and the effect of stimulation of P2Y receptor subtypes on I (GIRK) were examined using the whole-cell patch-clamp method. Extracellular application of 10 μM ATP induced a transient activation of I (GIRK) through the P2Y(1) receptor, which was completely abolished by pretreatment with pertussis toxin. ATP initially caused an additive transient increase in ACh-activated I (GIRK) (via M(2) receptor), which was followed by subsequent inhibition. This inhibition of I (GIRK) by ATP was attenuated by co-expression of regulator of G-protein signaling 2, or phosphatidylinositol-4-phosphate-5-kinase, or intracellular phosphatidylinositol 4,5-bisphosphate loading, but not by the exposure to protein kinase C inhibitors. P2Y(4) stimulation also persistently suppressed the ACh-activated I (GIRK). In addition, I (GIRK) evoked by the stimulation of the P2Y(4) receptor exhibited a transient activation, but that evoked by the stimulation of P2Y(2) or P2Y(12) receptor showed a rather persistent activation. These results reveal (1) that P2Y(1) and P2Y(4) are primarily coupled to the G(q)-phospholipase C-pathway, while being weakly linked to G(i/o), and (2) that P2Y(2) and P2Y(12) involve G(i/o) activation.

Differential Protein Kinase C Isoform Regulation and Increased Constitutive Activity of Acetylcholine-Regulated Potassium Channels in Atrial Remodeling

Rationale:

Atrial fibrillation (AF) causes atrial-tachycardia remodeling (ATR), with enhanced constitutive acetylcholine-regulated K + current (I KAChC ) contributing to action potential duration shortening and AF promotion. The underlying mechanisms are unknown.

Objective:

To evaluate the role of protein-kinase C (PKC) isoforms in ATR-induced I KAChC activation.

Methods and Results:

Cells from ATR-dogs (400-bpm atrial pacing for 1 week) were compared to control dog cells. In vitro tachypaced (TP; 3 Hz) canine atrial cardiomyocytes were compared to parallel 1-Hz paced cells. I KAChC single-channel activity was assessed in cell-attached and cell-free (inside-out) patches. Protein expression was assessed by immunoblot. In vitro TP activated I KAChC , mimicking effects of in vivo ATR. Discrepant effects of PKC activation and inhibition between control and ATR cells suggested isoform-selective effects and altered PKC isoform distribution. Conventional PKC isoforms (cPKC; including PKCα) inhibited, whereas novel isoforms (including PKCε) enhanced, acetylcholine-regulated K + current (I KACh ) in inside-out patches. TP and ATR downregulated PKCα (by 33% and 37%, respectively) and caused membrane translocation of PKCε, switching PKC predominance to the stimulatory novel isoform. TP increased [Ca 2+ ] i at 2 hours by 30%, with return to baseline at 24 hours. Buffering [Ca 2+ ] i during TP with the cell-permeable Ca 2+ chelator BAPTA-AM (1 μmol/L) or inhibiting the Ca 2+ -dependent protease calpain with PD150606 (20 μmol/L) prevented PKCα downregulation and TP enhancement of I KAChC . PKCε inhibition with a cell-permeable peptide inhibitor suppressed TP/ATR-induced I KAChC activation, whereas cPKC inhibition enhanced I KAChC activity in 1-Hz cells.

Conclusions:

PKC isoforms differentially modulate I KACh , with conventional Ca 2+ -dependent isoforms inhibiting and novel isoforms enhancing activity. ATR causes a rate-dependent PKC isoform switch, with Ca 2+ /calpain-dependent downregulation of inhibitory PKCα and membrane translocation of stimulatory PKCε, enhancing I KAChC . These findings provide novel insights into mechanisms underlying I KAChC dysregulation in AF.

The intra-molecular activation mechanisms of the dimeric metabotropic glutamate receptor 1 differ depending on the type of G proteins

Metabotropic glutamate receptor 1 (mGlu1) functions as a homodimer and activates not only the Gq but also the Gi/o and Gs pathways. Because of the dimeric configuration, different pathways could be activated either through the glutamate-bound subunit (cis-activation) and/or the other one (trans-activation). We here examined whether the intra-molecular activation mechanisms in the mGlu1 differ depending on the type of coupling G proteins, using various combinations of mGlu1 constructs that lack glutamate binding and/or G-protein coupling. The cis- or trans-activation alone was confirmed to trigger the Gq-coupled intracellular Ca(2+) transient. In contrast, the Gi/o-coupled G protein-dependent inward rectifying potassium (GIRK) channels were not activated either through the cis- or trans-activation alone. When one subunit of dimeric mGlu1 lacked the G-protein coupling, a significant decrease in the glutamate-induced GIRK current density was also observed. As the G protein-coupling-deficient subunit did not decrease the cell surface expression of mGlu1 and the Gq-coupled Ca(2+) transient, it was suggested that the coupling deficiency in one subunit of mGlu1 attenuates the Gi/o but not Gq coupling. In conclusion, multiple G-protein signaling was differentially activated by different intra-molecular activation mechanisms of the dimeric mGlu1.

A genetically encoded tool kit for manipulating and monitoring membrane phosphatidylinositol 4,5-bisphosphate in intact cells

Background

Most ion channels are regulated by phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P₂) in the cell membrane by diverse mechanisms. Important molecular tools to study ion channel regulation by PtdIns(4,5)P₂ in living cells have been developed in the past. These include fluorescent PH-domains as sensors for Förster resonance energy transfer (FRET), to monitor changes in plasma membrane_. For controlled and reversible depletion of PtdIns(4,5)P₂, voltage-sensing phosphoinositide phosphatases (VSD) have been demonstrated as a superior tool, since they are independent of cellular signaling pathways. Combining these methods in intact cells requires multiple transfections. We used self-cleaving viral 2A-peptide sequences for adenovirus driven expression of the PH-domain of phospholipase-Cδ1 (PLCδ1) fused to ECFP and EYFP respectively and Ciona intestinalis VSP (Ci-VSP), from a single open reading frame (ORF) in adult rat cardiac myocytes.

Methods and Results

Expression and correct targeting of ECFP-PH-PLCδ1_, EYFP-PH-PLCδ1, and Ci-VSP from a single tricistronic vector containing 2A-peptide sequences first was demonstrated in HEK293 cells by voltage-controlled FRET measurements and Western blotting. Adult rat cardiac myocytes expressed Ci-VSP and the two fluorescent PH-domains within 4 days after gene transfer using the vector integrated into an adenoviral construct. Activation of Ci-VSP by depolarization resulted in rapid changes in FRET ratio indicating depletion of PtdIns(4,5)P₂ in the plasma membrane. This was paralleled by inhibition of endogenous G protein activated K⁺ (GIRK) current. By comparing changes in FRET and current, a component of GIRK inhibition by adrenergic receptors unrelated to depletion of PtdIns(4,5)P₂ was identified.

Conclusions

Expression of a FRET sensor pair and Ci-VSP from a single ORF provides a useful approach to study regulation of ion channels by phosphoinositides in cell lines and transfection-resistant postmitotic cells. Generally, adenoviral constructs containing self-cleaving 2A-peptide sequences are highly suited for simultaneous transfer of multiple genes in adult cardiac myocytes.

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.

Substance P inhibits GABAB receptor signalling in the ventral tegmental area

Substance P (SP) and its receptors are involved in anxiety-related behaviours and regulate the intake of drugs of abuse and alcohol. Within the midbrain ventral tegmental area (VTA), a region that is clearly involved in the control of these behaviours, SP is released by stress and has been shown to trigger relapse. SP activates neurokinin (NK) receptors, which excites midbrain dopamine (DA) neurons and leads to increased DA in target regions. In this study, we have investigated the mechanisms underlying SP actions in the VTA, specifically investigating interactions between SP and GABA(B) receptors. We show that in VTA neurons, NK receptor activation closes an inwardly rectifying potassium channel, and moreover inhibits GABA(B) receptor-mediated transmission through an interaction that depends upon phospholipase C (PLC), intracellular calcium and protein kinase C (PKC).

Regulation of HERG (KCNH2) potassium channel surface expression by diacylglycerol

The HERG (KCNH2) channel is a voltage-sensitive potassium channel mainly expressed in cardiac tissue, but has also been identified in other tissues like neuronal and smooth muscle tissue, and in various tumours and tumour cell lines. The function of HERG has been extensively studied, but it is still not clear what mechanisms regulate the surface expression of the channel. In the present report, using human embryonic kidney cells stably expressing HERG, we show that diacylglycerol potently inhibits the HERG current. This is mediated by a protein kinase C-evoked endocytosis of the channel protein, and is dependent on the dynein-dynamin complex. The HERG protein was found to be located only in early endosomes and not lysosomes. Thus, diacylglycerol is an important lipid participating in the regulation of HERG surface expression and function.

Tyrosine phosphorylation of Kir3 following kappa-opioid receptor activation of p38 MAPK causes heterologous desensitization

Prior studies showed that tyrosine 12 phosphorylation in the N-terminal, cytoplasmic domain of the G-protein-gated inwardly rectifying potassium channel, K(ir)3.1 facilitates channel deactivation by increasing intrinsic GTPase activity of the channel. Using a phosphoselective antibody directed against this residue (pY12), we now report that partial sciatic nerve ligation increased pY12-K(ir)3.1-immunoreactivity (ir) in the ipsilateral dorsal horn of wild-type mice, but not in mice lacking the kappa-opioid receptor (KOR) or lacking the G-protein receptor kinase 3 (GRK3) genes. Treatment of AtT-20 cells stably expressing KOR-GFP with the selective KOR agonist U50,488 increased both phospho-p38-ir and pY12-K(ir)3.1-ir. The U50,488-induced increase in pY12-K(ir)3.1-ir was blocked by the p38 inhibitor SB203580. Cells expressing KOR(S369A)-GFP did not increase either phospho-p38-ir or pY12-K(ir)3.1-ir following U50,488 treatment. Whole cell voltage clamp of AtT-20 cells expressing KOR-GFP demonstrated that p38 activation by U50,488 reduced somatostatin-evoked K(ir)3 currents. This heterologous desensitization was blocked by SB203580 and was not evident in cells expressing KOR(S369A)-GFP. Tyrosine phosphorylation of K(ir)3.1 was likely mediated by p38 MAPK activation of Src kinase. U50,488 also increased (pY418)Src-ir; this increase was blocked by SB203580 and not evident in KOR(S369A)-GFP expressing AtT20 cells; the Src inhibitor PP2 blocked the U50,488-induced increase in pY12-K(ir)3.1-ir; and the heterologous desensitization of K(ir)3 currents was blocked by PP2. These results suggest that KOR causes phosphorylation of Y12-K(ir)3.1 and channel inhibition through a GRK3-, p38 MAPK- and Src-dependent mechanism. Reduced inward potassium current following nerve ligation would increase dorsal horn neuronal excitability and may contribute to the neuropathic pain response.

The N-terminal domain of Slack determines the formation and trafficking of Slick/Slack heteromeric sodium-activated potassium channels

Potassium channels activated by intracellular Na(+) ions (K(Na)) play several distinct roles in regulating the firing patterns of neurons, and, at the single channel level, their properties are quite diverse. Two known genes, Slick and Slack, encode K(Na) channels. We have now found that Slick and Slack subunits coassemble to form heteromeric channels that differ from the homomers in their unitary conductance, kinetic behavior, subcellular localization, and response to activation of protein kinase C. Heteromer formation requires the N-terminal domain of Slack-B, one of the alternative splice variants of the Slack channel. This cytoplasmic N-terminal domain of Slack-B also facilitates the localization of heteromeric K(Na) channels to the plasma membrane. Immunocytochemical studies indicate that Slick and Slack-B subunits are coexpressed in many central neurons. Our findings provide a molecular explanation for some of the diversity in reported properties of neuronal K(Na) channels.

Mass spectrometric analysis reveals a functionally important PKA phosphorylation site in a Kir3 channel subunit

Phosphorylation of the Kir3 channel by cAMP-dependent protein kinase (PKA) potentiates activity and strengthens channel-PIP(2) interactions, whereas phosphorylation by protein kinase C (PKC) exerts the opposite effects (Keselman et al., Channels 1:113-123, 2007; Lopes et al., Channels 1:124-134, 2007). Unequivocal identification of phosphorylated residues in ion channel proteins has been difficult, but recent advances in mass spectrometry techniques have allowed precise identification of phosphorylation sites (Park et al., Science 313:976-979, 2006). In this study, we utilized mass spectrometry to identify phosphorylation sites within the Kir3.1 channel subunit. We focused on the Kir3.1 C-terminal cytosolic domain that has been reported to be regulated by several modulators. In vitro phosphorylation by PKA exhibited a convincing signal upon treatment with a phosphoprotein stain. The phosphorylated C terminus was subjected to mass spectrometric analysis using matrix-assisted lased desorption/ionization-time of flight mass spectroscopy (MS). Peptides whose mass underwent a shift corresponding to addition of a phosphate group were then subjected to tandem MS (MS/MS) in order to confirm the modification and determine its precise location. Using this approach, we identified S385 as an in vitro phosphorylation site. Mutation of this residue to alanine resulted in a reduced sensitivity of Kir3.1* currents to H89 and Forskolin, confirming an in vivo role for this novel site of the Kir3.1 channel subunit in its regulation by PKA.

Modulation of the heteromeric Kir4.1-Kir5.1 channel by multiple neurotransmitters via Galphaq-coupled receptors

The heteromeric Kir4.1-Kir5.1 channel is a candidate sensing molecule for central CO(2) chemoreception. Since central CO(2) chemoreception is subject to neural modulations, we performed studies to test the hypothesis that the Kir4.1-Kir5.1 channel is modulated by the neurotransmitters critical for respiratory control, including serotonin (5-HT), substance-P (SP), and thyrotropin releasing hormone (TRH). The heteromeric Kir4.1-Kir5.1 channel was strongly inhibited by SP, TRH, and 5-HT when expressed in Xenopus oocytes, whereas these neurotransmitters had no effect on the homomeric Kir4.1 channel. Such an inhibition was dose-dependent and relied on specific G(alphaq)-protein-coupled receptors and protein kinase C (PKC). No direct interaction of the channel with G-proteins was found. Channel sensitivity to CO(2)/pH was not compromised with the inhibition by these neurotransmitters, as the channel remained to be inhibited by acidic pH following an exposure to the neurotransmitters. The firing rate of CO(2)-sensitive brainstem neurons cultured in microelectrode arrays was augmented by SP or a 5-HT2A receptor agonist, which was blocked by PKC inhibitors suggesting that PKC underscores the inhibitory effect of SP and 5-HT in cultured brainstem neurons as well. Immunostaining showed that both Kir4.1 and Kir5.1 proteins were co-localized in the cultured brainstem neurons. These results therefore indicate that the heteromeric Kir4.1-Kir5.1 channel is modulated by the neurotransmitters critical for respiratory control, suggesting a novel neuromodulatory mechanism for the chemosensitivity of brainstem neurons to elevated PCO(2) and acidic pH.