Homo- and hetero-oligomerization of thyrotropin-releasing hormone (TRH) receptor subtypes -: Differential regulation of β-arrestins 1 and 2

G-protein-coupled receptors (GPCRs) are regulated by a complex network of mechanisms such as oligomerization and internalization. Using the GPCR subtypes for thyrotropin-releasing hormone (TRHR1 and TRHR2), the aim of this study was to determine if subtype-specific differences exist in the trafficking process. If so, we wished to determine the impact of homo- and hetero-oligomerization on TRHR subtype trafficking as a potential mechanism for the differential cellular responses induced by TRH. Expression of either beta-arrestin 1 or 2 promoted TRHR1 internalization. In contrast, only beta-arrestin 2 could enhance TRHR2 internalization. The preference for beta-arrestin 2 by TRHR2 was supported by the impairment of TRHR2 trafficking in mouse embryonic fibroblasts (MEFs) from either a beta-arrestin 2 knockout or a beta-arrestin 1/2 knockout, while TRHR1 trafficking was only abolished in MEFs lacking both beta-arrestins. The differential beta-arrestin-dependence of TRHR2 was directly measured in live cells using bioluminescence resonance energy transfer (BRET). Both BRET and confocal microscopy were also used to demonstrate that TRHR subtypes form hetero-oligomers. In addition, these hetero-oligomers have altered internalization kinetics compared with the homo-oligomer. The formation of TRHR1/2 heteromeric complexes increased the interaction between TRHR2 and beta-arrestin 1. This may be due to conformational differences between TRHR1/2 hetero-oligomers versus TRHR2 homo-oligomers as a mutant TRHR1 (TRHR1 C335Stop) that does not interact with beta-arrestins, could also enhance TRHR2H/beta-arrestin 1 interaction. This study demonstrates that TRHR subtypes are differentially regulated by the beta-arrestins and also provides the first evidence that the interactions of TRHRs with beta-arrestin may be altered by hetero-oligomer formation.