Differential G protein activation by the long and short isoforms of the dopamine D₂ receptor

Background and Purpose: The dopamine D₂ receptor is expressed as a short (D2S) and a long (D2L) isoform with 29 additional amino acids in the third intracellular loop. The D2S isoform shows higher presynaptic expression than the D2L isoform, and decreased D2S expression has recently been linked to an increased risk for schizophrenia. Here, we present the first investigation, at receptor isoform level, of kinetic differences in the G protein activation profiles of the D2S, compared with the D2L isoform. Experimental Approach: We employed a NanoBRET-based approach to G protein dissociation to interrogate the time-resolved coupling profile of 3×HA-tagged D2L and D2S to Gα_i/o/z proteins in vitro. Key Results: Using dopamine as a D₂ receptor agonist, we observed a more pronounced activation of Gα_o and Gα_z than Gα_i proteins by D2L compared with D2S. This differentiation was not observed for D2S, which activated Gα_o and Gα_z with lower efficacy than D2L. These signalling differences were preserved on second messenger level and were not due to differences in receptor expression. Expanding to a set of seven full and partial D₂ receptor agonists showed these effects were not restricted to dopamine but rather a mutual, receptor-associated property. Contrasting this trend, we found that D2S activated G proteins faster than D2L upon full receptor activation. Conclusion and Implications: The findings highlight that both D2L and D2S are mechanistically able to activate all non-visual Gα_i/o proteins. Thereby, they add to previous reports about isoform-specificity to certain Gα_i/o proteins observed in specific cell types.