Fairfax Benjamin P, Pitcher Julie A, Scott Mark G H, Calver Andrew R, Pangalos Menelas N, Moss Stephen J, Couve Andrés
Medical Research Council Laboratory of Molecular Cell Biology and Department of Pharmacology, University College London, London WC1E 6BT, United Kingdom.
J Biol Chem. 2004 Mar 26;279(13):12565-73. doi: 10.1074/jbc.M311389200. Epub 2004 Jan 5.
GABA(B) receptors are heterodimeric G protein-coupled receptors that mediate slow synaptic inhibition in the central nervous system. The dynamic control of the cell surface stability of GABA(B) receptors is likely to be of fundamental importance in the modulation of receptor signaling. Presently, however, this process is poorly understood. Here we demonstrate that GABA(B) receptors are remarkably stable at the plasma membrane showing little basal endocytosis in cultured cortical and hippocampal neurons. In addition, we show that exposure to baclofen, a well characterized GABA(B) receptor agonist, fails to enhance GABA(B) receptor endocytosis. Lack of receptor internalization in neurons correlates with an absence of agonist-induced phosphorylation and lack of arrestin recruitment in heterologous systems. We also demonstrate that chronic exposure to baclofen selectively promotes endocytosis-independent GABA(B) receptor degradation. The effect of baclofen can be attenuated by activation of cAMP-dependent protein kinase or co-stimulation of beta-adrenergic receptors. Furthermore, we show that increased degradation rates are correlated with reduced receptor phosphorylation at serine 892 in GABA(B)R2. Our results support a model in which GABA(B)R2 phosphorylation specifically stabilizes surface GABA(B) receptors in neurons. We propose that signaling pathways that regulate cAMP levels in neurons may have profound effects on the tonic synaptic inhibition by modulating the availability of GABA(B) receptors.
GABA(B)受体是异二聚体G蛋白偶联受体，介导中枢神经系统中的缓慢突触抑制。GABA(B)受体细胞表面稳定性的动态控制在受体信号调节中可能至关重要。然而，目前对这一过程了解甚少。在这里，我们证明GABA(B)受体在质膜上非常稳定，在培养的皮层和海马神经元中几乎没有基础内吞作用。此外，我们表明，暴露于巴氯芬（一种特征明确的GABA(B)受体激动剂）不能增强GABA(B)受体内吞作用。神经元中受体内化的缺乏与激动剂诱导的磷酸化缺失以及异源系统中抑制蛋白募集的缺乏相关。我们还证明，长期暴露于巴氯芬选择性地促进不依赖内吞作用的GABA(B)受体降解。巴氯芬的作用可通过激活cAMP依赖性蛋白激酶或共同刺激β-肾上腺素能受体而减弱。此外，我们表明降解率的增加与GABA(B)R2中丝氨酸892处受体磷酸化的减少相关。我们的结果支持一个模型，其中GABA(B)R2磷酸化特异性地稳定神经元表面的GABA(B)受体。我们提出，调节神经元中cAMP水平的信号通路可能通过调节GABA(B)受体的可用性对强直性突触抑制产生深远影响。
