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Cyclic AMP–dependent protein kinase phosphorylation facilitates GABAB receptor–effector coupling

Abstract

GABA (γ-aminobutyric acid)B receptors are heterodimeric G protein–coupled receptors that mediate slow synaptic inhibition in the central nervous system. Here we show that the functional coupling of GABABR1/GABABR2 receptors to inwardly rectifying K+ channels rapidly desensitizes. This effect is alleviated after direct phosphorylation of a single serine residue (Ser892) in the cytoplasmic tail of GABABR2 by cyclic AMP (cAMP)–dependent protein kinase (PKA). Basal phosphorylation of this residue is evident in rat brain membranes and in cultured neurons. Phosphorylation of Ser892 is modulated positively by pathways that elevate cAMP concentration, such as those involving forskolin and β-adrenergic receptors. GABAB receptor agonists reduce receptor phosphorylation, which is consistent with PKA functioning in the control of GABAB-activated currents. Mechanistically, phosphorylation of Ser892 specifically enhances the membrane stability of GABAB receptors. We conclude that signaling pathways that activate PKA may have profound effects on GABAB receptor–mediated synaptic inhibition. These results also challenge the accepted view that phosphorylation is a universal negative modulator of G protein–coupled receptors.

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Figure 1: Cyclic AMP reduces attenuation of K+ channel activation after repetitive exposure to baclofen in cultured hippocampal neurons.
Figure 2: A GST-GABABR2 fusion protein is a substrate for PKA.
Figure 3: Recombinant GABABR2 is phosphorylated on Ser892 in transiently transfected COS cells.
Figure 4: cAMP reduces attenuation of GABA-activated K+ channel currents in HEK-293 cells requires phosphorylation of Ser892.
Figure 5: GABAB receptor activation, Kir3 channel activation and G protein coupling are unaffected by the phosphorylation of Ser892.
Figure 6: The cell-surface stability of GABAB receptor heterodimers is affected by Ser892.
Figure 7: Native GABABR2 subunits are phosphorylated in the rat CNS.
Figure 8: Stimulation of cAMP-dependent signaling pathways enhances GABABR2 phosphorylation.

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Acknowledgements

We thank C. Sinjin, F. Bedford, J. Kittler and V. Tretter for supplying cultures of cortical and hippocampal neurons, and J. Pitcher for critically reading the manuscript. A.C. was supported by The Wellcome Trust.

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Correspondence to Stephen J. Moss.

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Couve, A., Thomas, P., Calver, A. et al. Cyclic AMP–dependent protein kinase phosphorylation facilitates GABAB receptor–effector coupling. Nat Neurosci 5, 415–424 (2002). https://doi.org/10.1038/nn833

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