GPR37 surface expression enhancement via N-terminal truncation or protein-protein interactions.

GPR37, also known as the parkin-associated endothelin-like receptor (Pael-R), is an orphan G-protein-coupled receptor (GPCR) that exhibits poor plasma membrane expression when expressed in most cell types. We sought to find ways to enhance GPR37 trafficking to the cell surface to facilitate studies of GPR37 functional activity in heterologous cells. In truncation studies, we found that removing the GPR37 N-terminus (NT) dramatically enhanced the receptor's plasma membrane insertion. Further studies on sequential NT truncations revealed that removal of the first 210 amino acids increased the level of surface expression nearly as much as removal of the entire NT. In studies examining the effects of coexpression of GPR37 with a variety of other GPCRs, we observed significant increases in the level of GPR37 surface expression when the receptor was coexpressed with adenosine receptor A(2A)R or dopamine receptor D(2)R. Co-immunoprecipitation experiments revealed that full-length GPR37 and, to a greater extent, the truncated GPR37 were capable of robustly associating with D(2)R, resulting in modestly altered D(2)R affinity for both agonists and antagonists. In studies examining potential interactions of GPR37 with PDZ scaffolds, we observed a specific interaction between GPR37 and syntenin-1, which resulted in a dramatic increase in the level of GPR37 surface expression in HEK-293 cells. These findings reveal three independent approaches (N-terminal truncation, coexpression with other receptors, and coexpression with syntenin-1) by which GPR37 surface trafficking in heterologous cells can be greatly enhanced to facilitate functional studies with this orphan receptor.