Intracellular sorting studies of wild‐type and truncated forms of the D1 dopaminergic receptor unravel a chaperone‐like effect of antipsychotic drugs

The D1 dopaminergic receptor (D1R) is a seven‐transmembrane protein linked to stimulatory heterotrimeric GTP‐binding proteins. Here, we investigate the role of the cytoplasmic tail (CT) of D1R in regulating intracellular sorting properties of the receptor. We show that wild‐type (WT) D1R and its truncated and phosphorylation‐deficient form (Δ351) were sorted at the plasma membrane of transfected embryonic kidney 293 (HEK293) cells. Antibody feeding and co‐localization studies indicate that similar to WT, Δ351 internalizes through a clathrin‐, CT‐ and receptor phosphorylation‐independent pathway following dopamine (DA) exposure. However, in the absence of DA, Δ351 displays a strong intracellular staining unlike WT as assessed using immunofluorescence studies. Our confocal microscopy analysis shows that Δ351 co‐localizes with the endoplasmic reticulum (ER) markers, calnexin and calreticulin. Intriguingly, we observe an increase in the mature form of Δ351 and a complete rescue of ER‐retained Δ351 following an overnight treatment of transfected HEK293 cells with the inverse agonist flupentixol (Flu), an antipsychotic drug. These data strongly suggest that Flu displays a chaperone‐like effect on ER‐retained Δ351. Overall, our studies underpin a pivotal role of the CT of D1R in regulating the ontogeny and sorting to cell surface of this dopaminergic receptor subtype. Supported by CIHR (MOP‐81341; MT).