The carboxyterminal peptide of chorionic gonadotropin facilitates activation of the marmoset LH receptor

Luteinizing hormone (LH) and chorionic gonadotropin (CG) are heterodimeric glycoprotein hormones acting on the luteinizing hormone receptor (LHR). In the LHR, which is genomically encoded by eleven exons, exon 10 encodes for the hinge region and its elimination impairs LH action, while CG maintains normal activity. The two gonadotropins differ in the carboxyterminal peptide (CTP) present in CG but absent in LH. Since the marmoset monkey (Callithrix jacchus) LHR naturally lacks exon 10 (LHR type II), we generated two recombinant marmoset gonadotropin preparations, one consisting of the wild type CG and one of truncated CG lacking the CTP (CG (-CTP)). After calibration in a mouse Leydig cell bioassay against the WHO LH80/522 standard, the ED (50) of the CG preparation on a COS7 cell line permanently expressing the marmoset LHR was 4.25 +/- 0.21 IU/L (n = 3). Stimulation of the COS7 cell line with equipotent concentrations of CG and CG (-CTP) resulted in significantly different formation of cAMP (two-way ANOVA, p < 0.001). In particular, cAMP production stimulated by CG (-CTP) was 3 - 4 times lower compared to CG at the saturating CG concentration (8 IU/L). We conclude, supplementing one current model of LHR activation, that exon 10 might play a permissive role in releasing the constraint of the receptor upon hormone binding, resulting in receptor activation. We speculate that, when exon 10 is lacking, the CTP can overcome its absence and facilitates the opening of the receptor, resulting in normal activation.