Differential expression and function of the human chorionic gonadotropin beta subunits

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Human chorionic gonadotropin is known as a central pregnancy-supporting hormone playing an essential role in early human pregnancy. Besides its classical endocrine function in supporting corpus luteum survival to maintain progesterone production several new paracrine roles promoting embryo implantation and placentation have been described lately. hCG is active as a highly glycosylated heterodimer. There are six genes coding for the hCG beta-subunit (CGB) with CGB3, 5, 7 and 8 being most relevant in pregnancy. CGB3, 5 and 8 encode identical proteins secreted by the placenta and some tumour types. CGB7 differs in only three amino acids from CGB3/5/8 but shows a substantially different expression pattern. In a recent report I demonstrated that the transcription factor p53 selectively induces CGB7 expression by binding directly to a consensus site in the CGB7 promoter region. Additionally, recent findings from our group provide evidence that CGB7 is produced by the secretory endometrium during the implantation window. This observation emphasizes the importance of the maternally-produced CGB for the first time. The different expression patterns of the CGB variants raise the question of functional differences which has not yet been addressed. In the proposed project I wish to investigate whether CGB7 and CGB3/5/8 actually exert different functions. hCG is known to modulate the maternal immune response. Hence, the influence of the CGB isoforms on peripheral blood mononuclear cells will be studied. Additionally, CGB7 and CGB3/5/8 will be compared with regard to their impact on the cell cycle, apoptosis and invasion as well as their signal transduction properties. Considering that glycosyl groups constitute about 30 % of the hCG molecular weight and that glycosylation is essential for hCG function, one focus of the proposed work will be on identifying the glycosylation patterns of the different CGB isoforms. According to the hypothesis we developed from the latest results, CGB7 synthesised by maternal tissue supports implantation and thus can serve as an implantation marker. To determine the clinical significance of hCG in more detail, the association of CGB with successful embryo implantation will be analysed. For this purpose, endometrial paraffin sections of patients from the Centre for Reproductive Medicine at the University of Leipzig will be employed. For immunohistochemical expression analysis of the CGB variants, monoclonal antibodies specific to the individual CGB variants will be developed. The proposed experiments aim to address the question what functional differences result from the differences in amino acid sequence and glycosylation patterns of CGB7 versus CGB3/5/8. The results are fundamental for understanding the events during early human pregnancy and have the potential to improve diagnosis and possibly even therapy in reproductive medicine.