GABA _B receptors do not internalize after baclofen treatment, possibly due to a lack of β-arrestin association: Study with a real-time visualizing assay

摘要

The mechanism of agonist-induced GABA _B receptor (GABA _BR) internalization is not well understood. To investigate this process, we focused on the interaction of GABA _BR with β-arrestins, which are key proteins in the internalization of most of the G protein-coupled receptors, and the agonist-induced GABA BR internalization and the interaction of GABA _BR with β-arrestin1 and β-arrestin2 were investigated in real time using GABA _BR and β-arrestins both of which were fluorescent protein-tagged. We then compared these profiles with those of μ-opioid receptors (μOR), well-studied receptors that associate and cointernalize with β-arrestins. When stimulated by the specific GABA _BR agonist baclofen, GABA _BR composed of GABA _(B1a)R (GB _(1a)R) and fluorescent protein-tagged GABA _(B2)R-Venus (GB _2R-V) formed functional GABA _BR; they elicited G protein-activated inwardly rectifying potassium channels as well as nontagged GABA _BR. In cells coexpressing GB _(1a)R, GB _2R-V, and β-arrestin1-Cerulean (βarr1-C) or β-arrestin2-Cerulean (βarr2-C), real-time imaging studies showed that baclofen treatment neither internalized GB 2R-V nor mobilized βarr1-C or βarr2-C to the cell surface. This happened regardless of the presence of G protein-coupled receptor kinase 4 (GRK4), which forms a complex with GABA _BR and causes GABA _BR desensitization. On the other hand, in cells coexpressing μOR-Venus, GRK2, and βarr1-C or βarr2-C, the μOR molecule formed μOR/βarr1 or μOR/βarr2 complexes on the cell surface, which were then internalized into the cytoplasm in a time-dependent manner. Fluorescence resonance energy transfer assay also indicated scarce association of GB _2R-V and β-arrestins-C with or without the stimulation of baclofen, while robust association of μOR-V with β-arrestins-C was detected after μOR activation. These findings suggest that GABA _BRs failure to undergo agonist-induced internalization results in part from its failure to interact with β-arrestins.