Gaba-b receptor coupling to girk channels can be modulated by rgs protein

GABA-B receptor coupling to GIRK channels can be modulated by RGS protein Drug dependence continues to be a major public health problem worldwide. Death rate due to abused drug and alcohol in United State alone is about 100,000 persons per year (1). Recently, it is reported that GABA-B receptor agonist such as Baclofen can reduce desire for addictive drugs (2-3). GABA is a major inhibitory neurotransmitter present ubiquitously in brain that acts through the ionotropic GABAA receptor and the G protein coupled GABA-B receptor. The GABA-B receptor is coupled to Gi/Go G-proteins which modulate the action of inward-rectifier potassium (Kir3/GIRK) and voltage gated calcium (Cav) channels (4-5). The functional form of the GABA-B receptor is a heterodimer composed of the GABA-B1subunit which binds ligand, and the GABA-B2 subunit which activates Gi/Go (6). Activation of the GABA-B receptor mediates opening of GIRK channels which generate a slow inhibitory postsynaptic potential thereby decreasing the neuronal excitability (5). In the conventional G protein signaling paradigm, activation of GPCR by ligand leads to the liberation of Gβγ
subunits of the trimeric G protein, by promoting the exchange of GDP/GTP associated with the Gα subunit. Binding
of Gβγ to GIRK leads to activation/opening of the channel (7). However, recent studies have suggested that GPCRs
and their cognate G proteins as well as their immediate downstream effectors, including ion channels and adenylyl
cyclase, are organized as macromolecular complexes (8). Therefore there is a direct protein–protein interaction
between GPCRs, G proteins and ion channels in a signaling complex (9, 11). Recently, Regulator of G protein
Signaling (RGS) proteins were also identified as members of the signaling complex surrounding GPCRs (10). These
observations lead to the hypothesis that heterodimeric GABA B receptor modulation of GIRK channels may
require oligomerization with its effectors and RGS proteins.
Direct interaction of GABA-B1 with GIRK1, GIRK2 channels and Gβγ subunits was initially detected by Martin David et al. who suggested that the interaction between GABA-B1 and GIRK channel does not require ligand activation of receptor rather such interaction is constitutive (11). To further investigate direct interaction between GABA-B receptor and effector proteins within its signaling pathway, Fowler et al used fluorescence resonance energy transfer (FRET) combined with total internal reflection fluorescence (TIRF) microscopy (12). They demonstrated direct interactions between RGS4 and GABAB1 and GABAB2 subunits. They were also able to measure significant FRET between GABA-B receptor and different GIRK subunits (12). Ciruela et al validated these findings and extended the model of direct interaction between GABA-B receptors and their functionally coupled GIRK channels (13). To investigate the functional significance of these signaling complexes in neurons relevant to the physiology of addiction, Labouebe et al. focused on RGS protein modulation of GABA-B receptor/GIRK coupling in the ventral tegmental area (VTA) (14-15). By treating dopaminergic (DA) neurons with phosphatidylinositol (3,4,5)-trisphosphate, a non-specific inhibitor of RGS proteins, they observed a decrease in the EC50 of baclofen activation of GIRK channel activity. They also found that RGS2 is selectively expressed in DA neurons which indicated that RGS2 may be involved in modulating the GABAB-GIRK coupling efficiency in the DA neurons of VTA (15). This was confirmed with experiments with RGS2–/– mice, which also show a higher GABAB-GIRK coupling efficiency in DA neurons of the VTA but only when GIRK3 is also co-expressed. These data suggest that RGS2 may specifically mediate GABA-B receptor modulation of GIRK3 channel activity. To investigate the modulation of RGS expression with respect to drug addiction, Labouebe et al. treated normal mice chronically with GHB and morphine and observed a decrease in EC50 value for activation of Baclofen induced current in DA neurons caused by reduction of RGS2 mRNA level (15). In conclusion, these data suggest that the formation of higher order heterocomplex may be a central feature of GABAB receptor signaling, and that RGS2 may modulate GABAB signaling in DA neurons of the VTA. 1. Richard Saitz (2007) Treatment of Alcohol and Other Drug Dependence. Liver transplantation13, S59-S64 2. Joyce Besheer, Veronique Lepoutre, Clyde W. Hodge. (2004) GABA (B) receptor agonists reduce operant ethanol self-administration and enhance ethanol sedation in C57BL/6J mice Psychopharmacology 174(3), 358-66. 3. Colombo G, Addolorato G, Agabio R, Carai MA, Pibiri F, Serra S, Vacca G, Gessa GL. (2004) Role of GABA (B) receptor in alcohol dependence: reducing effect of baclofen on alcohol intake and alcohol motivational properties in rats and amelioration of alcohol withdrawal syndrome and alcohol craving in human alcoholics Neurotox Res. 6(5), 403-14. 4. Bernhard Bettler, Klemens Kaupmann, Johannes Mosbacher and Martin Gassmann. (2004) Molecular Structure and Physiological Functions of GABAB Receptors Physiol Rev 84, 835–867 5. Claire L. Padgetta, Paul A. Slesinger (2010) GABAB Receptor Coupling to G-proteins and Ion Channels 6. Jones, K. A. et al. (1998) GABAB receptors function as a heteromeric assembly of the subunits GABABR1 7. Reuveny E, Slesinger PA, Inglese J, Morales JM, Iñiguez-Lluhi JA, Lefkowitz RJ, Bourne HR, Jan YN, Jan LY (1994) Activation of the cloned muscarinic potassium channel by G protein beta gamma subunits. Nature, 370, 143–146 8. Hewavitharana T, Wedegaertner PB. (2011) Non-canonical signaling and localizations of heterotrimeric G 9. Sinead M. Clancya, Catherine E. Fowlera, Melissa Finleya, Ka Fai Suena, Christine Arrabita, Frédérique Bertona, Tohru Kosazab, Patrick J. Caseyc, Paul A. Slesinger. (2005) Pertussis-toxin-sensitive G alpha subunits selectively bind to C-terminal domain of neuronal GIRK channels: evidence for a heterotrimeric G-protein-channel complex. Mol.Cell. Neurosci., 28, 375–389. 10. John R. Hepler (2003) RGS Protein and G Protein Interactions: A Little Help from Their Friends. Mol 11. Martin David, Maxime Richer, Aida M. Mamarbachi, Louis R. Villeneuve, Denis J. Dupré, Terence E. Hebert (2006) Interactions between GABA-B1 receptors and kir 3 inwardly rectifying potassium channels. Cell. Signal. 18, 2172–2181. 12. Fowler, C.E., Aryal, P., Suen, K.F. & Slesinger, P.A. (2007) Evidence for association of GABA (B) receptors with Kir3 channels and regulators of G protein signalling (RGS4) proteins. J. Physiol., 580, 51–65 13. Francisco Ciruela, Victor Fernandez-Duenas, Kristoffer Sahlholm, Laura Fernandez-Alacid, Joel C. Nicolau, Masahiko Watanabe and Rafael Lujan. (2010) Evidence for oligomerization between GABAB receptors and GIRK channels containing the GIRK1 and GIRK3 subunits. European J. of Neuroscience, 32, 1265-1277 14. Marta Lomazzi, Paul A. Slesinger, Christian Lüscher (2008) Addictive drugs modulate GIRK-channel signaling by regulating RGS proteins. Trends Pharmacol Sci 29(11), 544-9. 15. Gwenae¨l Laboue`, Marta Lomazzi, Hans G Cruz, Cyril Creton, Rafael Luja´n, Meng Li, Yuchio Yanagawa, Kunihiko Obata, Masahiko Watanabe, Kevin Wickman, Stephanie B Boyer, Paul A Slesinger & Christian Lu¨scher (2007) RGS2 modulates coupling between GABAB receptors and GIRK channels in dopamine neurons of the ventral tegmental area. Nat. Neurosci. 10, 1559–1568.

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