Study On The Function Of Lgr4 In The Female Reproductive System

The main functions of ovary are reproduction and endocrine. Ovary is responsible for many physiological activities, such as the maturing of germ cell, the process of estrous cycle, the fluctation of mood, the maintenance of bone system et al. At the beginning of the puberty, the primordial follicle is activated in response of many factors. The follicles begin to grow. And the granulosa cells start to proliferate from single layer to multi-layer. When the follicle reaches the stage of pre-antral follicle, the development begins to be gonadotropin dependant. Under the stimulating of gonadotropins, the follicle developed quickly to the stage of pre-ovulatory follicle. After the surge of LH, ovulation takes place. The cumulus-oocyte complex is released from ovary to oviduct.2-4 hours after ovulation, the granulosa cells exit from cell cycle to differentiate to luteal cells after the surge of LH. Then the corpus luteum forms. The main function of the corpus luteum is to secrete progesterone which plays critical role in the growth of the gland in uterus endometrium, congestion uterus, endometrial hyperplasia and implantation of embryos. Progesterone can cooperate with estradiol to regulate the development of breads. So the hypotrophy of the corpus luteum leads to infertility or miscarriage.During the development of follicle, the G protein coupled receptors, especially the leucine rich G protein receptors play important roles. According to the conservation, the LGRs can be divided into 3 sub-groups:FSHR, LHR, TSHR; LGR4, LGR5, LGR6; LGR7, LGR8. According to previous reports, the first and the third sub-groups lead essential roles in reproduction system. Based on the homology, it is meaningful to study the function and mechanism of the orphan receptor Lgr4 in female reproduction system.Since first cloned in 1998, a lot of biological functions of Lgr4 are reported in recent years. Lgr4 knockout mouse models suggest the gene of Lgr4 refers to the development of embryo, eye, kidney, bones, hair follicle and gall bladder. A recent research showed Lgr4 could regulate the differentiation of Panth cell. And Lgr4 also refers to the maintenance of the stem cell in intestine. However, few report of Lgr4 in female reproductive system, especially in ovary, is found.In this paper, we use the Lgr4 gene trap mouse model to study the function and mechanism of Lgr4 in female reproductive system, including ovary and oviduct.1 The expression of Lgr4 in female reproductive systemThe Lgr4 knockout mouse was produced by the method of gene trap. A beta-gal reporter gene was inserted into the region between the first and the second exon of Lgr4. As this reporter gene was regulated by Lgr4 promoter and enhancers, the expression of the beta-gal gene reflexes the expression of Lgr4 in vivo. Through lacZ staining, we found Lgr4 expressed in the luteal cell. The expression of Lgr4 increased as the development of corpus luteum 4-6 hours after the surge of LH, and decreased as the regression of corpus leum. We also detected the expression of Lgr4 in the epithelial cell in reproductive tract, but minor in the muscle layer in uterus.2 The function of Lgr4 in ovaryBased on the previous data that Lgr4 female was infertile, we studied the function of Lgr4 in ovary. Our research showed the Lgr4 knockout mouse’s progesterone level was lower than the WT. And the defect of the synthesis of progesterone affected the estrous cycle. In the productive tract, the mutant mouse’s ciliated and secretory cells were less than the wt. Transmission electron microscope assay showed the defected morphology of secretory cell, which suggested the deficiency of the development of secretory cell in the oviduct tract.3 The mechanism of Lgr4 in ovaryCompared with wt, lgr4 mutant mouse’s steroid synthesis related and luteinization genes were down regulated in vivo. We tested the Egfr/Erk pathway, and found this signal pathway was specifically blocked in luteal cell in vivo. We also constructed Erk1-/-;Erk2f/f;Pr-Cre mouse and found block of Erk pathway specifically in luteal cells affected the formation of corpus luteum. Interestingly, the defect of Lgr4 mutant luteal cell can be rescued by HB-EGF. Under the stimulation of HB-EGF, not only the steroid genes but also other luteinization genes restored.We firstly found Lgr4 could regulate the synthesis of progesterone via Egfr/Erk signal pathway. And we also showed the significance of Egfr/Erk signal pathway in the function and maintenance of corpus luteum. Together, our research provieded theoretical basis for the treatment of infertility or miscarriage that were caused by the hypotrophy of corpus luteum.