A novel spliced variant of the type 1 corticotropin-releasing hormone receptor with a deletion in the seventh transmembrane domain present in the human pregnant term myometrium and fetal membranes

CRH exerts its actions via activation of specific G protein-coupled recepto rs, which exist in two types, CRH-R1 and CRH-RP, and arise from different g enes with multiple spliced variants. RT-PCR amplification of CRH receptor s equences from human myometrium and fetal membranes yielded cDNAs that encod e a novel CRH-R type 1 spliced variant. This variant (CRH-R1d) is present i n the human pregnant myometrium at term only, which suggests a physiologica lly important role at the end of human pregnancy and labor. The amino acid sequence of CRH-R1d is identical to the CRH-R1 alpha receptor except that i t contains an exon deletion resulting in the absence of 14 amino acids in t he predicted seventh transmembrane domain. Binding studies in HEK-293 cells stably expressing the CRH-R1d or CRH-R1 alpha receptors revealed that the deletion does not change the binding characteristics of the variant recepto r. In contrast, studies on the G protein activation demonstrated that CRH-R 1d is not well coupled to the four subtypes of G proteins (G(s), G(i), G(o) , G(q)) that CRH-R1 alpha can activate. These data suggest that although th e deleted segment is not important for CRH binding, it plays a crucial role in CRH receptor signal transduction. Second messenger studies of the varia nt receptor showed that CRH and CRH-like peptides can stimulate the adenyla te cyclase system, with reduced sensitivity and potency by 10-fold compared with the CRH-R1 alpha. Furthermore, CRH failed to stimulate inositol trisp hosphate production. Coexpression studies between the CRH-R1d or CRH-R1 alp ha showed that this receptor does not play a role as a dominant negative re ceptor for CRH.