Structural determinants critical for localization and signaling within the seventh transmembrane domain of the type 1 corticotropin releasing hormone receptor: lessons from the receptor variant R1d.

The type 1 CRH receptor (CRH-R1) plays a fundamental role in homeostatic adaptation to stressful stimuli. CRH-R1 gene activity is regulated through alternative splicing and generation of various CRH-R1 mRNA variants. One such variant is the CRH-R1d, which has 14 amino acids missing from the putative seventh transmembrane domain due to exon 13 deletion, a splicing event common to other members of the B1 family of G protein-coupled receptors. In this study, using overexpression of recombinant receptors in human embryonic kidney 293 and myometrial cells, we showed by confocal microscopy that in contrast to CRH-R1alpha, the R1d variant is primarily retained in the cytoplasm, although some cell membrane expression is also evident. Use of antibodies against the CRH-R1 C terminus in nonpermeabilized cells showed that membrane-expressed CRH-R1d contains an extracellular C terminus. Interestingly, treatment of CRH-R1d-expressing cells with CRH (100 nM) for 45-60 min elicited functional responses associated with a significant reduction of plasma membrane receptor expression, redistribution of intracellular receptors, and increased receptor degradation. Site-directed mutagenesis studies identified the cassette G356-F358 within transmembrane domain 7 as crucial for CRH-R1alpha stability to the plasma membrane because deletion of this cassette caused substantial intracellular localization of CRH-R1 alpha. Most importantly, coexpression studies between CRH-R1d and CRH-R2beta demonstrated that the CRH-R2beta could partially rescue CRH-R1d membrane expression, and this was associated with a significant attenuation of urocotrin II-induced cAMP production and ERK1/2 and p38MAPK activation, suggesting that CRH-R1d might specifically induce heterologous impairment of CRH-R2 signaling responses. This mechanism appears to involve accelerated CRH-R2beta endocytosis.