Mechanistic target of rapamycin (MTOR) signaling during ovulation in mice

Abstract

A complex network of endocrine/paracrine signals regulates granulosa-cell function in ovarian follicles. Mechanistic target of rapamycin (MTOR) has recently emerged as a master intracellular integrator of extracellular signals and nutrient availability. The objectives of the present study were to characterize the expression pattern and kinase activity of MTOR during follicular and corpus luteum development, and to examine how inhibition of MTOR kinase activity affects preovulatory maturation of ovarian follicles. MTOR expression was constitutive throughout follicular and corpus luteum development. Gonadotropins induced MTOR kinase activity in the ovary, which was inhibited by rapamycin treatment (10 µg/g body weight, intraperitoneal injection). Inhibition of human chorionic gonadotropin (hCG)-induced MTOR activity during preovulatory follicle maturation did not change key events of ovulation. Granulosa cells of rapamycin-treated mice showed reduced MTOR kinase activity at 1 and 4 hr post-hCG and overexpression of hCG-induced ovulation genes at 4 hr post-hCG. Overexpression of these ovulatory genes was associated with hyper-activation of extracellular signal-regulated kinase 1/2 (ERK1/2), which occurred in response to inhibition of MTOR with rapamycin and suggested that MTOR may function as a negative regulator of the mitogen-activated protein kinase (MAPK) pathway. Indeed, simultaneous inhibition of MTOR and ERK1/2 activities during preovulatory follicle maturation caused anovulation. Inhibition of hCG-induced ERK1/2 activity alone suppressed MTOR kinase activity, indicating that MAPK pathway is upstream of MTOR. Thus, normal ovulation appears to be a result of complex interactions between MTOR and MAPK signaling pathways in granulosa cells of ovulating follicles in mice.

Ovarian proteomic study reveals the possible molecular mechanism for hyperprolificacy of Small Tail Han sheep

“…Interestingly, though the expressions of eIF4B and S6 were higher in Dorset sheep than in Han sheep, the protein content of mitogen-activated protein kinase 1 (MAPK1), the upper stream regulator of mTOR pathway - also known as extracellular signal-regulated kinase 2 (ERK2), in Dorset group was significantly lower than in both Han groups. ERK1/2 plays an important role in animal ovulation, and the higher expression level of ERK1/2 in Han sheep may contribute to its higher ovulation rate compared with Dorset sheep32. Our proteomic results show that the roles of ribosome and mTOR pathway in the differences of sheep fecundity between Dorset and Han sheep need further study.…”

Metabolic control of oocyte development: linking maternal nutrition and reproductive outcomes

“…In addition, accumulating evidence suggests that the metabolic mTOR/PTEN/PI3K signaling pathways play crucial roles during follicular growth and oocyte development in mice [230, 231]. Genetic data have shown that all primordial follicles become depleted in early adulthood of mice lacking PTEN in oocytes, causing premature ovarian failure [232].…”

Mechanistic target of rapamycin is activated in bovine granulosa cells after LH surge but is not essential for ovulation

“…The mTOR pathway has been implicated in many cellular processes including protein translation and signalling from hormones, nutrients and growth factors (Huang & Manning, ; Kim, Goraksha‐Hicks, Li, Neufeld, & Guan, ). In this study, we observed that GnRH treatment increases mTOR activity in granulosa cells as revealed by the increase in p‐S6K (Siddappa et al., ) and RHEB (Guertin & Sabatini, ; Inoki, Li, Xu, & Guan, ) mRNA levels. This demonstrates that mTOR activity is inducted by gonadotropins in granulosa cells from preovulatory follicles of monovulatory species as previously observed in polyovulatory species (Siddappa et al., ; Yu et al., ).…”

“…This suggests the mTOR may participate in the regulation of EREG expression after LH surge. However, a previous study in mice observed that rapamycin injection could not prevent the upregulation of EREG mRNA induced by hCG treatment (Siddappa et al., ). This suggests that mTOR effect on EREG regulation may differ in monovulatory and polyovulatory species.…”

“…Inhibition of mTOR activity with rapamycin reduced granulosa cell proliferation at G1 phase and decreased ovulation rates in mice (Yu, Yaba, Kasiman, Thomson, & Johnson, ). In another study, rapamycin treatment did not affect ovulation in hCG‐stimulated mice albeit downregulation of S6K (Siddappa et al., ). In bovine oocyte culture, mTOR inhibition prevents meiosis progression (Mayer, Wrenzycki, & Tomek, ), but its effect on the bovine ovulatory process has not been investigated.…”