Homologous regulation of the heptahelical D1A receptor responsiveness: specific cytoplasmic tail regions mediate dopamine‐induced phosphorylation, desensitization and endocytosis

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Abstract
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 orkobs, a parameter linked to the receptor‐G protein coupling status. In agreement with the desensitization data, Δ425‐ and Δ379‐expressing cells exhibit an increase ofkobsin comparison with WT‐expressing cells. Nevertheless, Δ351‐expressing cells, which harbor similar desensitization features of Δ379‐expressing cells, display no change inkobswhen compared with WT‐expressing cells. Our results suggest that a defective DA‐induced endocytosis may hamper Δ351 resensitization and concomitant increase inkobs. 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.