Studies of Lin28a in regulation of ovarian development and function in mice

Abstract

Infertility has a prevalence of 15% in couples at childbearing age around the world. Impaired ovarian function causes infertility in women, but its etiology is not fully clear. Recent studies suggest that Lin28a, an RNA binding protein, may play a role in ovarian follicle development. Let-7 is a microRNA that regulates Lin28a expression, and the Lin28a/Let-7 axis is involved in multiple disease regulation. Moreover, high progesterone has been reported to retard follicle development. The hypothesis of this study is that overexpression of Lin28a causes premature primordial follicle activation and increased progesterone production, leading to impaired fertility. This study aimed to investigate the function of Lin28a in regulating ovarian function and development using a transgenic mouse overexpressing Lin28a. In the first part of the study, we confirmed a higher Lin28a expression in Lin28a transgenic (Tg) mice than twild type mice on postnatal day (PD)7, PD14, PD21, 2-month, 3-month, 4-month and 5-month old. The litter size and oocyte number were significantly decreased in Lin28a Tg mice from 4-month old. In line with this, the number of primordial follicles was significantly decreased, while that of the early secondary follicles was markedly increased in the 3-month and 4-month old Lin28a Tg mice. At 5-month old, the number of primordial and early secondary follicles were both decreased in Lin28a Tg mice. This was accompanied by increase in apoptosis in the ovary. Akt/mTOR and Wnt signaling pathway are known to be crucial in follicle activation. At molecular level, the Akt/mTOR pathway was activated and the expression of active β-catenin was significantly enhanced in the ovary of Lin28a Tg mice. Interestingly, we found that Let-7 negatively regulated Wnt activation; Wnt signaling positively regulated Akt/mTOR activity leading to primordial follicle activation in the ovary. In the second part of the study, we found PMSG and hCG stimulation induced Lin28a expression and elevated serum progesterone level in female wild type mice on PD23. The expression of Lin28a in the ovary was also elevated on day 2 and day 7 of pregnancy in 3-month old wild type pregnant females, suggesting that Lin28a played a role in the function of corpus luteum. The expression of molecules involved in progesterone synthesis including p27/p21, cyp11a1 and StAR and the serum progesterone level were significantly increased in Lin28a Tg mice. Overexpression of Lin28a enhanced the expression of p27/p21, cyp11a1 and StAR; while overexpression of Let-7 had the opposite effect. hCG activated the PI3K/Akt/mTOR and the MAPK pathway and enhanced the expression of Lin28a and p27/p21; while blocking of these pathways with inhibitors suppressed the expression of Lin28a and p27/p21 in treated human granulosa cell line. High serum progesterone level suppressed follicle development in vitro. In summary, premature follicle activation of Lin28a Tg mice is mediated by Let-7 through Akt/mTOR and Wnt-signaling pathway which in turn induced p27/p21, cyp11a1 and StAR expression, increased progesterone synthesis resulting in reduction of fertility in the female mice. This study provides new insights for a role of Lin28a/let-7 axis in the etiology of ovarian dysfunction in infertility.