Involvement of the ser-glu-pro motif in ligand species-dependent desensitisation of the rat gonadotrophin-releasing hormone receptor.

There are two forms of gonadotrophin-releasing hormone (GnRH), GnRH-I and GnRH-II, in the vertebrate brain. Both GnRH-I and GnRH-II are thought to interact with the type-I GnRH receptor (GnRHR). The present study attempted to demonstrate whether GnRH-I and GnRH-II induce differential desensitisation of GnRHR and to identify the motif involved. Time course inositol phosphate (IP) accumulation assay reveals that, in cells expressing the wild-type rat GnRHR, GnRH-I induced continuous increase in IP production, whereas GnRH-II-induced IP production rate at later time points (30-120 min after ligand treatment) became attenuated. However, in cells expressing the mutant receptor in which the Ser-Glu-Pro (SEP) motif in extracellular loop 3 was replaced by Pro-Glu-Val (PEV), IP accumulation rates at later time points were more decreased by GnRH-I than GnRH-II. Ca2+ responses to repetitive GnRH applications reveal that GnRH-II desensitised the wild-type receptor faster than GnRH-I, whereas the opposite situation was observed in the PEV mutant. In addition, cell surface loss of GFP-tagged wild-type receptor was more facilitated by GnRH-II than GnRH-I, whereas that of the GFP-tagged PEV mutant receptor was more enhanced by GnRH-I than GnRH-II. The present study indicates that the SEP motif is potentially responsible for ligand species-dependent receptor desensitisation. Together, these results suggest that GnRH-I and GnRH-II may have different effects on mammalian type-I GnRHR via modulation of desensitisation rates.