Heartfelt crosstalk: desensitization of the GIRK current

Immunoreactivity of Kir3.1, muscarinic receptors 2 and 3 on the brainstem, vagus nerve and heart tissue under experimental demyelination

AIMS

Demyelination affects the propogation of neuronal action potential by slowing down the progression. This process results in a neuro-impairment like Multiple Sclerosis (MS). Evidence show that MS also contributes to involvement of the autonomic system. In the molecular approach to this involvement, we aimed to observe muscarinic ACh receptor 2-3 (mAChR2-3), and inwardly rectifying potassium channel 3.1 (Kir3.1) immunoreactivities on the brainstem, vagus nerve, and heart under cuprizone model.

MAIN METHODS

Wistar albino rats were randomly divided into 8 groups; duplicating 4 groups as male and female: control groups (n = 3 +3), Cuprizone groups (n = 12 +12), sham groups (n = 4 +4), and carboxy-methyl-cellulose groups (n = 3 +3). Cuprizone-fed rats underwent demyelination via Luxol fast blue (LFB) staining of the hippocampus (Gyrus dentatus and Cornu Ammonis) and cortex. Immunohistochemistry analysis followed to the pathologic measurement of the brainstem, vagus nerve, and heart for mAChR2, mAChR3 and Kir3.1 proteins KEY FINDINGS: A significant demyelination was observed in the hippocampus and cortex tissues of rats in the female and male cuprizone groups. Myelin basic protein immunoreactivity demonstrated that cuprizone groups, in both males and females, had down-regulation in the hippocampus and cortex areas. The weights of the cuprizone-fed rats significantly decreased over six weeks. Dilated blood vessels and neuronal degeneration were severe in the hippocampus and cortex of the cuprizone groups. In the female cuprizone group, expression of mAChR2 and mAChR2 was significantly increased in the brainstem, atrium/ventricle of heart, and left/right sections of vagus nerve. Kir3.1 channels were also up-regulated in the left vagus nerve and heart sections of the female cuprizone group SIGNIFICANCE: Especially in our data where female-based significant results were obtained reveal that demyelination may lead to significant mAChR2, mAChR3 and Kir3.1 changes in brainstem, vagus nerve, and heart. A high immunoreactive response to demyelination at cholinergic centers may be a new target.

Neural Subtype-dependent Cholinergic Modulation of Neural Activities by Activation of Muscarinic 2 Receptors and G Protein-activated Inwardly Rectifying Potassium Channel in Rat Periaqueductal Gray Neurons

Acetylcholine plays a pivotal role in the regulation of functions such as pain and the sleep and wake cycle by modulating neural activities of the ventrolateral periaqueductal gray (vlPAG). Electrophysiological studies have shown that cholinergic effects are inconsistent among recorded neurons, particularly in the depolarization and hyperpolarization of the resting membrane potential (RMP). This discrepancy may be due to the neural subtype-dependent cholinergic modulation of the RMP. To examine this possibility, we performed whole-cell patch-clamp recordings from subtype-identified neurons using vesicular GABA transporter (VGAT)-Venus × ChAT-TdTomato rats and elucidated cellular mechanisms of cholinergic effects on the RMP. The application of carbachol hyperpolarized the RMP of cholinergic neurons in a dose-dependent manner but had much less of an effect on other neural subtypes, including GABAergic/glycinergic and glutamatergic neurons. Cholinergic hyperpolarization was accompanied by a decrease in input resistance. These cholinergic effects were blocked by AF-DX384 or gallamine and were mimicked by arecaidine but-2-ynyl ester tosylate, suggesting that the carbachol-induced hyperpolarization of the RMP in cholinergic neurons is mediated via M₂ receptors. Tertiapin suppressed the carbachol-induced G protein-activated inwardly rectifying potassium channel (GIRK) currents and hyperpolarization of the RMP in cholinergic neurons. Intracellular application of GDP-β-S blocked the carbachol-induced hyperpolarization of the RMP. Neostigmine slowly hyperpolarized the RMP in cholinergic neurons. These results suggest that neural firing of vlPAG cholinergic neurons is suppressed by GIRK currents induced via M₂ receptor activation, and this negative feedback regulation of cholinergic neuronal activities can be induced by acetylcholine, which is intrinsically released in the vlPAG.

On the mechanism of GIRK2 channel gating by phosphatidylinositol bisphosphate, sodium, and the Gβγ dimer

G protein-gated inwardly rectifying K⁺ (GIRK) channels belong to the inward-rectifier K⁺ (Kir) family, are abundantly expressed in the heart and the brain, and require that phosphatidylinositol bisphosphate is present so that intracellular channel-gating regulators such as Gβγ and Na⁺ ions can maintain the channel-open state. However, despite high-resolution structures (GIRK2) and a large number of functional studies, we do not have a coherent picture of how Gβγ and Na⁺ ions control gating of GIRK2 channels. Here, we utilized computational modeling and all-atom microsecond-scale molecular dynamics simulations to determine which gates are controlled by Na⁺ and Gβγ and how each regulator uses the channel domain movements to control gate transitions. We found that Na⁺ ions control the cytosolic gate of the channel through an anti-clockwise rotation, whereas Gβγ stabilizes the transmembrane gate in the open state through a rocking movement of the cytosolic domain. Both effects alter the way in which the channel interacts with phosphatidylinositol bisphosphate and thereby stabilizes the open states of the respective gates. These studies of GIRK channel dynamics present for the first time a comprehensive structural model that is consistent with the great body of literature on GIRK channel function.

Lipid signaling to membrane proteins: From second messengers to membrane domains and adapter-free endocytosis

Hilgemann et al. explain how lipid signaling to membrane proteins involves a hierarchy of mechanisms from lipid binding to membrane domain coalescence.

Lipids influence powerfully the function of ion channels and transporters in two well-documented ways. A few lipids act as bona fide second messengers by binding to specific sites that control channel and transporter gating. Other lipids act nonspecifically by modifying the physical environment of channels and transporters, in particular the protein–membrane interface. In this short review, we first consider lipid signaling from this traditional viewpoint, highlighting innumerable Journal of General Physiology publications that have contributed to our present understanding. We then switch to our own emerging view that much important lipid signaling occurs via the formation of membrane domains that influence the function of channels and transporters within them, promote selected protein–protein interactions, and control the turnover of surface membrane.

Second generation S1P pathway modulators: research strategies and clinical developments

Multiple Sclerosis (MS) is a chronic autoimmune disorder affecting the central nervous system (CNS) through demyelination and neurodegeneration. Until recently, major therapeutic treatments have relied on agents requiring injection delivery. In September 2010, fingolimod/FTY720 (Gilenya, Novartis) was approved as the first oral treatment for relapsing forms of MS. Fingolimod causes down-modulation of S1P1 receptors on lymphocytes which prevents the invasion of autoaggressive T cells into the CNS. In astrocytes, down-modulation of S1P1 by the drug reduces astrogliosis, a hallmark of MS, thereby allowing restoration of productive astrocyte communication with other neural cells and the blood brain barrier. Animal data further suggest that the drug directly supports the recovery of nerve conduction and remyelination. In human MS, such mechanisms may explain the significant decrease in the number of inflammatory markers on brain magnetic resonance imaging in recent clinical trials, and the reduction of brain atrophy by the drug. Fingolimod binds to 4 of the 5 known S1P receptor subtypes, and significant efforts were made over the past 5 years to develop next generation S1P receptor modulators and determine the minimal receptor selectivity needed for maximal therapeutic efficacy in MS patients. Other approaches considered were competitive antagonists of the S1P1 receptor, inhibitors of the S1P lyase to prevent S1P degradation, and anti-S1P antibodies. Below we discuss the current status of the field, and the functional properties of the most advanced compounds. This article is part of a Special Issue entitled New Frontiers in Sphingolipid Biology.

The role of G proteins in assembly and function of Kir3 inwardly rectifying potassium channels

Kir3 channels (also known as GIRK channels) are important regulators of electrical excitability in both cardiomyocytes and neurons. Much is known regarding the assembly and function of these channels and the roles that their interacting proteins play in controlling these events. Further, they are one of the best studied effectors of heterotrimeric G proteins in general and Gβγ subunits in particular. However, our understanding of the roles of multiple Gβγ binding sites on Kir3 channels is still rudimentary. We discuss potential roles for Gβγ in channel assembly and trafficking in addition to their known role in cellular signaling.

The potent inhibitory effect of tipepidine on marble-burying behavior in mice

Our previous study revealed that centrally acting non-narcotic antitussives inhibited G-protein-coupled inwardly rectifying K(+) (GIRK) channel currents in brain neurons, and that the tipepidine antitussives had a novel antidepressive-like effect on rats. Furthermore, the antitussives revealed multiplexed ameliorating actions on intractable brain disease models. This study evaluated the therapeutic potential of tipepidine in obsessive-compulsive disorder (OCD) subjects using marble-burying behavior (MBB) tests in mice. In fact, OCD is classified as an anxiety disorder characterized by obsession or compulsion. Although selective 5-HT reuptake inhibitors (SSRIs) are considered first choice agents for the pharmacological treatment of OCD, 50% of patients with OCD failed to respond to SSRIs. The burying of harmless objects such as marbles by mice might reflect the formation of compulsive behavior. The results show that tipepidine reduced MBB in a dose-dependent manner. The effect of tipepidine was significant even at a dosage as small as 5 mg/kg. The tipepidine at 10 mg/kg s.c. nearly abolished MBB without reducing the locomotor activity in mice. It is particularly interesting that the dopamine D₂ antagonist or 5-HT(1A) antagonist partly inhibited the effect of tipepidine on MBB. The results suggest that tipepidine has more of a potent inhibitory effect on MBB, compared with known drugs used for the treatment of OCD, and that the tipepidine action mechanism might differ from that of known drugs.

Acute alcohol action and desensitization of ligand-gated ion channels

Ethanol exerts its biological actions through multiple receptors, including ion channels. Ion channels that are sensitive to pharmacologically relevant ethanol concentrations constitute a heterogeneous set, including structurally unrelated proteins solely sharing the property that their gating is regulated by a ligand(s). Receptor desensitization is almost universal among these channels, and its modulation by ethanol may be a crucial aspect of alcohol pharmacology and effects in the body. We review the evidence documenting interactions between ethanol and ionotropic receptor desensitization, and the contribution of this interaction to overall ethanol action on channel function. In some cases, such as type 3 serotonin, nicotinic acetylcholine, GABA-A, and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors, ethanol actions on apparent desensitization play a significant role in acute drug action on receptor function. In a few cases, mutagenesis helped to identify different areas within a receptor protein that differentially sense n-alcohols, resulting in differential modulation of receptor desensitization. However, desensitization of a receptor is linked to a variety of biochemical processes that may alter protein conformation, such as the lipid microenvironment, post-translational channel modification, and channel subunit composition, the relative contribution of these processes to ethanol interactions with channel desensitization remains unclear. Understanding interactions between ethanol and ionotropic receptor desensitization may help to explain different ethanol actions 1) when ethanol is evaluated in vitro on cloned channel proteins, 2) under physiological or pathological conditions or in distinct cell domains with modified ligand concentration and/or receptor conformation. Finally, receptor desensitization is likely to participate in molecular and, possibly, behavioral tolerance to ethanol, which is thought to contribute to the risk of alcoholism.

Sphingosine 1-phosphate receptors in health and disease: mechanistic insights from gene deletion studies and reverse pharmacology

Sphingosine 1-phosphate (S1P) is a bioactive sphingolipid that is critically involved in the embryonic development of the cardiovascular and central nervous systems. In the adult, S1P can produce cytoskeletal re-arrangements in many cell types to regulate immune cell trafficking, vascular homeostasis and cell communication in the central nervous system. S1P is contained in body fluids and tissues at different concentrations, and excessive production of the pleiotropic mediator at inflammatory sites may participate in various pathological conditions. Gene deletion studies and reverse pharmacology (techniques aiming to identify both ligands and function of receptors) provided evidence that many effects of S1P are mediated via five G-protein-coupled S1P receptor subtypes, and novel therapeutic strategies based on interaction with these receptors are being initiated. The prototype S1P receptor modulator, FTY720 (fingolimod), targets four of the five S1P receptor subtypes and may act at several levels to modulate lymphocyte trafficking via lymphocytic and endothelial S1P1 and, perhaps, other inflammatory processes through additional S1P receptor subtypes. A recently completed Phase II clinical trial suggested that the drug may provide an effective treatment of relapsing-remitting multiple sclerosis. FTY720 is currently being evaluated in larger-scale, longer-term, Phase III studies. This review provides an overview on S1P activities and S1P receptor function in health and disease, and summarizes the clinical experience with FTY720 in transplantation and multiple sclerosis.

FTY720, an immunomodulatory sphingolipid mimetic: translation of a novel mechanism into clinical benefit in multiple sclerosis

FTY720 (fingolimod; 2-amino-2[2-(4-octylphenyl)ethyl]-1,3-propanediol, Novartis) is the prototype of a new generation of immunomodulators. The drug is the result of extensive chemical derivatisation based on the natural product myriocin, isolated from the ascomycete Isaria sinclairii. FTY720 bears structural similarity to sphingosine, a naturally occurring sphingolipid. As with sphingosine, FTY720 is effectively phosphorylated by sphingosine kinases in vivo and the phosphorylated drug targets G-protein-coupled receptors for sphingosine-1-phosphate (S1P). Gene deletion and reverse pharmacology studies have shown that FTY720 acts at S1P1 receptors on lymphocytes and the endothelium, thereby inhibiting the egress of T- and B cells from secondary lymphoid organs into the blood and their recirculation to inflamed tissues. Animal studies suggest that this novel mechanism translates into effective treatments for several autoimmune diseases and a recently completed Phase II clinical trial highlighted FTY720 as a potential therapy for relapsing-remitting multiple sclerosis.

FTY720: sphingosine 1-phosphate receptor-1 in the control of lymphocyte egress and endothelial barrier function

The novel immunomodulator FTY720 is effective in experimental models of transplantation and autoimmunity, and is currently undergoing Phase III clinical trials for prevention of kidney graft rejection. In contrast to conventional immunosuppressants, FTY720 does not impair T- and B-cell activation, proliferation and effector function, but interferes with cell traffic between lymphoid organs and blood. The molecular basis for the mode of action of the drug has only recently been established. FTY720, after phosphorylation, acts as a high-affinity agonist at the G protein-coupled sphingosine 1-phosphate receptor-1 (S1P(1)) on thymocytes and lymphocytes, thereby inducing aberrant internalization of the receptor. This renders the cells unresponsive to the serum lipid sphingosine 1-phosphate (S1P), depriving them from an obligatory signal to egress from lymphoid organs. As a consequence, lymphocytes are unable to recirculate to peripheral inflammatory tissues and graft sites but remain functional in the lymphoid compartment. In addition to the effects on lymphocyte recirculation, the drug acts on endothelial cells and preserves vascular integrity by enhancing adherens junction assembly and endothelial barrier function. The available data establish S1P(1) as a key target for FTY720, and further point to therapeutically relevant effects of the drug on lymphocytes and vascular endothelium.

Acetylcholine-induced phosphatidylinositol 4,5-bisphosphate depletion does not cause short-term desensitization of G protein-gated inwardly rectifying K+ current in mouse atrial myocytes

Depletion of phosphatidylinositol 4,5-bisphosphate (PIP(2)) induced by phenylephrine or endothelin causes the inhibition of acetylcholine-activated K(+) current (I(KACh)) in atrial myocytes. In the present study, we have investigated the hypothesis that muscarinic receptor induced PIP(2) depletion also causes inhibition of I(KACh), resulting in desensitization. We confirmed the expression of G(q)-coupled muscarinic receptors in mouse atrial myocytes using reverse transcriptase-polymerase chain reaction. The involvement of M(1) and M(3) receptors in desensitization is examined using specific antagonists, 4-DAMP and pirenzepine, but they significantly reduced peak I(KACh), implying nonspecific M(2) blockade. When ACh-induced phosphoinositide depletion was specifically inhibited using PLCbeta1 knock-out mice, the extent of desensitization during 4 min was 47.5 +/- 3.2%, which was not different from that in wild type (46.8 +/- 2.1%). Phenylephrine-induced phosphoinositide hydrolysis and phenylephrine-induced inhibition of I(KACh) were not affected by PLCbeta1 knock-out. To facilitate PIP(2) depletion, replenishment of PIP(2) was blocked by wortmannin. Wortmannin did not affect the desensitization and the recovery from desensitization. These results suggest that PIP(2) depletion by acetylcholine does not contribute to short-term desensitization of I(KACh). The differential regulation of I(KACh) by different phospholipase C-linked receptors may imply that receptor co-localization is required for PIP(2) to act as a signaling molecule.

Cell death in weaver mouse cerebellum

Mice with the weaver mutation exhibit an uneven weave to their gait, ataxia, mild locomotor hyperactivity and, occasionally, tonic-clonic seizures. A single amino acid mutation in a G-protein coupled, inwardly rectifying K+ channel, GIRK2, gives rise to the symptoms seen in the weaver mice. Two areas of the brain are primarily affected. Cerebellar granule cell neurons die soon after birth and dopaminergic neurons are severely depleted in the substantia nigra. In this article we review recent studies of wild-type and mutant GIRK channels found in native cells or introduced into expression systems. We also review two models that explain some of the details leading to the neuronal cell death observed in weaver mice.

Phosphoinositides as key regulators of synaptic function

Phosphoinositides have recently emerged as key regulators of a variety of synaptic processes, including neurosecretory vesicle targeting, exo-endocytosis, and ion channel modulation. These pleiotropic activities derive from their ability to serve either as membrane targeting sites for cytosolic factors, as allosteric ligands, or as nucleation points for coat proteins and cytoskeletal elements. This versatility depends upon the existence of highly diversified enzymatic machinery for their synthesis and degradation, which governs, both temporally and spatially, their appearance in the microenvironment of the synapse.