Blackwell Publishing Journal Backfiles 1879-2005
In the present study, we investigate the role of specific cytoplasmic tail (CT) regions of the D1A receptor in mediating dopamine (DA)-induced phosphorylation, desensitization and endocytosis. Results obtained in human embryonic kidney (HEK) cells expressing the wild-type (WT) or truncation forms (Δ425, Δ379 and Δ351) of the D1A receptor show that sequences located downstream of Gly379 regulate DA-mediated phosphorylation-dependent desensitization of D1A receptors. However, the longer truncation mutant Δ351 failed to undergo detectable DA-induced phosphorylation while exhibiting DA-induced desensitization features similar to the shorter truncation mutant Δ379. These data potentially suggest a novel role for a receptor phosphorylation-independent process in the DA-promoted D1A subtype desensitization. Our immunofluorescence data also suggest that sequences located between Cys351 and Gly379 play an important role in DA-mediated receptor endocytosis. Additionally, time-course studies were done in intact cells expressing WT or truncation receptors to measure the observed rate constant for adenylyl cyclase (AC) activation or kobs, a parameter linked to the receptor-G protein coupling status. In agreement with the desensitization data, Δ425- and Δ379-expressing cells exhibit an increase of kobs in comparison with WT-expressing cells. Nevertheless, Δ351-expressing cells, which harbor similar desensitization features of Δ379-expressing cells, display no change in kobs when compared with WT-expressing cells. Our results suggest that a defective DA-induced endocytosis may hamper Δ351 resensitization and concomitant increase in kobs. Thus, our study showing that specific D1A receptor CT sequences regulate DA-induced phosphorylation, desensitization, and endocytosis highlights the underlying molecular complexity of signaling at dopaminergic synapses.
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