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a Department of Physiology, University of Turku, 20520 Turku, Finland
The gonadotropin receptors, i.e., those of LH and FSH (FSHR), are pivotal elements in the regulation of gonadal function. Recently, extensive efforts have been made to elucidate the structure-function relationship of these receptors as well as the modulatory mechanism(s) of their function. In the present study, we report 1) characterization of the mouse (m) FSHR cDNA coding sequence and 2) the functional consequences of coexpression of several splice variants of the mFSHR. In addition, we evaluate 3) the impact on mFSHR function of a C566T transition in exon 7 of the coding sequence, a substitution analogous to the inactivating mutation in the human FSHR gene responsible for a hereditary form of hypergonadotropic ovarian failure. Molecular cloning of the mFSHR cDNA was carried out by reverse transcription-polymerase chain reaction (RT-PCR) using 129/Sv mouse testicular RNA and primers complementary to the rat or the partially characterized mouse FSHR sequence. Overlapping partial fragments of receptor cDNA were amplified, sequenced, and engineered to produce the entire cDNA coding sequence, subcloned into the pSG5 expression vector. Using a similar approach, 4 different receptor splice variants, selectively lacking exons 2, 2 and 5, 5 and 6, and 2, 5, and 6 of the coding region, were cloned. Finally, PCR-based site-directed mutagenesis was used to generate the C566T mutant of mFSHR. Sequence analysis showed an open reading frame of 2076 base pairs for the mFSHR cDNA, predicting a putative 17-amino acid signal peptide and a 675-amino acid mature receptor protein, and overall sequence homology of 94% with rat, 87% with human, and 85–84% with bovine, and ovine FSHRs. Functional expression in human embryonic kidney (HEK 293) and mouse granulosa (KK-1) cells demonstrated for the cloned receptor high-affinity binding to recombinant human (rh) FSH and ability to elicit cAMP, inositol trisphosphate (IP3), and progesterone responses. In contrast, transient transfection studies showed that despite successful transcription, the exon-lacking FSHR variants were unable to bind rhFSH either in intact or in solubilized HEK 293 cells, or to elicit cAMP or progesterone responses in KK-1 cells. Furthermore, cotransfections of the splice variants in the context of an ovarian cell line stably expressing the full-length mFSHR failed to demonstrate modulatory effects on the holoreceptor function. Finally, transient expression of the C566T mFSHR mutant in HEK 293 cells revealed that, in accordance with observations on human FSHR, this substitution profoundly impaired the ligand binding and cAMP and IP3 responses to rhFSH stimulation. In conclusion, the present data indicate that, despite extensive splicing of the mFSHR message, a potential role of the exon-lacking receptor transcripts in modulating FSH actions is unlikely. In addition, we provide evidence for mFSHR inactivation by a C566T transition in exon 7 of the coding sequence, thus paving the way for further development of animal models of hypergonadotropic ovarian failure.
2 Correspondence: Ilpo Huhtaniemi, Department of Physiology, University of Turku, Kiinamyllynkaty 10, 20520 Turku, Finland. FAX: 358 2 2502610; ilpo.huhtaniemi{at}utu.fi
3 Current address: Department of Physiology, University of Córdoba, Avda Menéndez Pidal s/n, 14004 Córdoba, Spain.
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