Rapamycin rescues the poor developmental capacity of aged porcine oocytes. Lee SE 2014 et al.
Unfertilized oocytes age inevitably after ovulation, which limits their fertilizable life span and embryonic development. Rapamycin affects mammalian target of rapamycin (mTOR) expression and cytoskeleton reorganization during oocyte meiotic maturation. The goal of this study was to examine the effects of rapamycin treatment on aged porcine oocytes and their in vitro development. Rapamycin treatment of aged oocytes for 24 h (68 h in vitro maturation ; 44 h+10 ?M rapamycin/24 h, 47.52?5.68) or control oocytes (44 h IVM; 42.14?4.40) significantly increased the development rate and total cell number compared with untreated aged oocytes (68 h IVM, 22.04?5.68) (p<0.05). Rapamycin treatment of aged IVM oocytes for 24 h also rescued aberrant spindle organization and chromosomal misalignment, blocked the decrease in the level of phosphorylated-p44/42 mitogen-activated protein kinase (MAPK), and increased the mRNA expression of cytoplasmic maturation factor genes (MOS, BMP15, GDF9, and CCNB1) compared with untreated, 24 h-aged IVM oocytes (p<0.05). Furthermore, rapamycin treatment of aged oocytes decreased reactive oxygen species (ROS) activity and DNA fragmentation (p<0.05), and downregulated the mRNA expression of mTOR compared with control or untreated aged oocytes. By contrast, rapamycin treatment of aged oocytes increased mitochondrial localization (p<0.05) and upregulated the mRNA expression of autophagy (BECN1, ATG7, MAP1LC3B, ATG12, GABARAP, and GABARAPL1), anti-apoptosis (BCL2L1 and BIRC5; p<0.05), and development (NANOG and SOX2; p<0.05) genes, but it did not affect the mRNA expression of pro-apoptosis genes (FAS and CASP3) compared with the control. This study demonstrates that rapamycin treatment can rescue the poor developmental capacity of aged porcine oocytes.
Caloric Restriction Promotes the Reserve of Follicle Pool in Adult Female Rats by Inhibiting the Activation of Mammalian Target of Rapamycin Signaling. Li L 2014 et al.
Caloric restriction (CR) is known to increase the number of primordial follicles and prolong the reproductive life span. However, how CR modulates follicular development is not well understood. In the present study, we examined the effects of CR on follicular development in rats and investigated the underlying mechanism. After 10 weeks of CR or high-fat diet, ovarian follicles at different developmental stages were examined by histological analysis. Plasma levels of luteinizing hormone (LH), follicle-stimulating hormone (FSH), and estrogen (ESG) were measured, and the levels of mammalian target of rapamycin (mTOR), p70S6 kinase (p70S6K), and phosphorylated p70S6K in the ovary were detected by Western blot. The results showed that the reserve of follicle pool in CR rats was increased, accompanied by decreased level of phosphorylated p70S6K in the ovary, and decreased serum LH, FSH, and ESG levels. Taken together, these results suggest that CR may suppress ovarian follicular development and enhance the follicle pool reserve by inhibiting mTOR signaling.
Rapamycin-sensitive mTORC1 signaling is involved in physiological primordial follicle activation in mouse ovary. Tong Y 2013 et al.
In mammals, resting female oocytes reside in primordial ovarian follicles. An individual primordial follicle may stay quiescent for a protracted period of time before initiating follicular growth, also termed 'activation'. Female reproductive capacity is sustained by the gradual, streamlined activation of the entire population of primordial follicles, but this process also results in reproductive senescence in older animals. Based on the recent findings that genetically triggered, excessive mTORC1 (mammalian target of rapamycin complex 1) activation in mouse oocytes leads to accelerated primordial follicle activation, we examined the necessity of mTORC1 signaling in physiological primordial follicle activation. We found that induction of oocyte mTORC1 activity is associated with early follicular growth in neonatal mouse ovaries. Pharmacological inhibition of mTORC1 activity in vivo by rapamycin treatment leads to a marked, but partial, suppression of primordial follicle activation. The suppressive effect of rapamycin on primordial follicle activation was reproduced in cultured ovaries. While rapamycin did not apparently affect several plausible cellular targets in neonatal mouse ovaries, such as mTORC2, AKT, or cyclin-dependent kinase (CDK) inhibitor p27-KIP1, its inhibitory effect on Cyclin A2 gene expression implies that mTORC1 signaling in oocytes may engage a Cyclin A/CDK regulatory network that promotes primordial follicle activation. The current work strengthens the concept that mTORC1-dependent events in the oocytes of primordial follicles may represent potential targets for intervention in humans to slow the depletion of the ovarian reserve. Mol. Reprod. Dev. ? 2013 Wiley Periodicals, Inc.
/////////////////////////Inactivation of mTor arrests bovine oocytes in M I stage, despite reversible inhibition of 4E-BP1 phosphorylation. Mayer S 2014 et al.
The mammalian target of rapamycin (mTor), a Ser/Thr protein kinase, is implicated in the phosphorylation-triggered inactivation of translation repressors, the so called eukaryotic initiation factor 4E (eIF4E)-binding proteins (4E-BPs). Previous observations in porcine and bovine oocytes revealed an increasing phosphorylation of 4E-BP1 during meiotic maturation. The factor is hypophosphorylated in the germinal vesicle (GV) stage and highest phosphorylated in the metaphase II (M II). In the present approach we intended to block 4E-BP1 phosphorylation specifically to impair initiation of translation and elucidate effects on resumption of meiosis. Torin2, which acts as an active-site mTor inhibitor, reduces 4E-BP1 phosphorylation without any effect on eIF4E and arrests up to 60% of the oocytes in the M I stage. Effects of Torin2 treatment, analyzed by site-specific substrate phosphorylation, were also observed at protein kinase B (Akt, PKB) and cyclin dependent kinases (CDKs). However, if at all, only minor side effects were found at protein kinase A, C (PKA, PKC), ATM/ATR (Ataxia telangiectasia mutated/AT and Rad3-related protein) and the mitogen activated protein kinases (MAPK) ERK1, 2. The inhibition of 4E-BP1 phosphorylation by Torin2 is reversible when cultivating oocytes for additional 24?h in Torin2-free medium. However, even so, oocytes persist in the M I stage. This may indicate the necessity of spatiotemporally regulated translation during meiosis, which cannot be restored later. In conclusion, Torin2 enables an effective and specific inhibition of 4E-BP1 phosphorylation and such an approach may be valuable to investigate maturation specific protein synthesis in more detail. Mol. Reprod. Dev. ? 2014 Wiley Periodicals, Inc.
Luteinizing Hormone/Human Chorionic Gonadotropin-Mediated Activation of mTORC1 Signaling Is Required for Androgen Synthesis by Theca-Interstitial Cells. Palaniappan M et al. LH triggers the biosynthesis of androgens in the theca-interstitial (T-I) cells of ovary through the activation of a cAMP-dependent pathway. We have previously shown that LH/human chorionic gonadotropin (hCG) activates mammalian target of rapamycin complex 1 (mTORC1) signaling network, leading to cell proliferation. In the present study, we provide evidence that the LH/hCG-mediated activation of the mTORC1 signaling cascade is involved in the regulation of steroidogenic enzymes in androgen biosynthesis. Treatment with LH/hCG increased the expression of downstream targets of mTORC1, ribosomal protein S6 kinase 1, and eukaryotic initiation factor 4E as well as steroidogenic enzymes. LH/hCG-mediated stimulation of the steroidogenic enzyme mRNA was blocked by the mTORC1 inhibitor, rapamycin. This inhibitory effect was selective because rapamycin failed to block hCG-mediated increase in the expression of Star mRNA levels. Furthermore, pharmacological targeting of mTORC1 with rapamycin also blocked LH/hCG- or forskolin-induced expression of cAMP response element-binding protein (CREB) and steroidogenic enzymes (P450 side-chain cleavage enzyme, 3?hydroxysteroid dehydrogenase type 1, and 17a-hydroxylase/17,20 lyase) but produced no effect on steroidogenic acute regulatory protein levels. These results were further confirmed by demonstrating that the knockdown of mTOR using small interfering RNA selectively abrogated the LH/hCG-induced increase in steroidogenic enzyme expression, without affecting steroidogenic acute regulatory protein expression. LH/hCG-stimulated androgen production was also blocked by rapamycin. Furthermore, the pharmacological inhibition of mTORC1 or ribosomal protein S6 kinase 1 signaling prevented the LH/hCG-induced phosphorylation of CREB. Chromatin immunoprecipitation assays revealed the association of CREB with the proximal promoter of the Cyp17a1 gene in response to hCG, and this association was reduced by rapamycin treatment. Taken together, our findings show for the first time that LH/hCG-mediated activation of androgen biosynthesis is regulated by the mTORC1 signaling pathway in T-I cells.
mTOR Controls Ovarian Follicle Growth by Regulating Granulosa Cell Proliferation. Yu J et al. We have shown that inhibition of mTOR in granulosa cells and ovarian follicles results in compromised granulosa proliferation and reduced follicle growth. Further analysis here using spontaneously immortalized rat granulosa cells has revealed that mTOR pathway activity is enhanced during M-phase of the cell cycle. mTOR specific phosphorylation of p70S6 kinase and 4E-BP, and expression of Raptor are all enhanced during M-phase. The predominant effect of mTOR inhibition by the specific inhibitor Rapamycin (RAP) was a dose-responsive arrest in the G1 cell cycle stage. The fraction of granulosa cells that continued to divide in the presence of RAP exhibited a dose-dependent increase in aberrant mitotic figures known as anaphase bridges. Strikingly, estradiol consistently decreased the incidence of aberrant mitotic figures. In mice treated with RAP, the mitotic index was reduced compared to controls, and a similar increase in aberrant mitotic events was noted. RAP injected during a superovulation regime resulted in a dose-dependent reduction in the numbers of eggs ovulated. Implications for the real-time regulation of follicle growth and dominance, including the consequences of increased numbers of aneuploid granulosa cells, are discussed.
Cross Talk Between Estradiol and mTOR Kinase in the Regulation of Ovarian Granulosa Proliferation. Yu J et al. Treatment of ovarian granulosa cells and follicles with the mammalian target of rapamycin (mTOR) kinase inhibitor results in biphasic effects where nanomolar rapamycin (RAP) results in reduced proliferation, mitotic anomalies, and attenuated follicle growth, while the picomolar RAP results in accelerated follicle growth. Here, we tested whether such effects are specific to RAP or could be mimicked by 2 alternative mTOR inhibitors, everolimus (EV) and temsirolimus (TEM), and whether these effects were dependent on the presence of estradiol (E2). Spontaneously immortalized rat granulosa cells (SIGCs) were cultured in dose curves of RAP, EV, TEM, or vehicle with or without E2. Proliferation and phosphorylation of mTOR targets p70S6 kinase and 4E-binding protein (BP) were determined. Cell cycle gene array analysis and confirmatory quantitative reverse transcriptase polymerase chain reaction were performed upon cells treated with picomolar RAP versus controls. Nanomolar RAP, EV, and TEM reduced SIGC proliferation and decreased phospho-p70 and 4E-BP. Picomolar concentrations accelerated proliferation without affecting mTOR substrate phosphorylation. Acceleration of growth by picomolar inhibitor required E2. Picomolar drug treatment altered the transcription of cell cycle regulators, increasing Integrin beta 1 and calcineurin expression, and decreasing inhibin alpha, Chek1, p16ARF, p27/Kip1, and Sestrin2 expression. At nanomolar concentrations, mTOR inhibitors attenuated granulosa proliferation. Accelerated growth and alterations in cell cycle gene transcription found with picomolar concentrations required E2 within the intrafollicular concentration range. The low concentrations of inhibitors required to increase granulosa proliferation suggest a novel use to support the growth of ovarian follicles.
A putative mitotic checkpoint dependent on mTOR function controls cell proliferation and survival in ovarian granulosa cells. Yaba A et al. The conserved target of rapamycin (TOR) proteins are involved in sensing nutrient levels and/or stress and the resultant control of cell growth, size, and survival. The authors assess mammalian TOR (mTOR) kinase expression in the mouse ovary and also the expression of its cofactors, Raptor, Rictor, and LST8. In granulosa cells, mTOR demonstrates high cytoplasmic/perinuclear expression. The kinase-active serine 2448-phosphorylated form of mTOR (P-mTOR) is present at very high levels during the M-phase. P-mTOR was enriched on or near the mitotic spindle and also near the contractile ring during cytokinesis. Rapamycin inhibition of mTOR resulted in both reduced granulosa cell proliferation and reduced follicle growth in vitro, each in a dose-dependent fashion. Follicles cultured in rapamycin did not undergo atresia. mTOR inhibition results in a reduction in granulosa cell proliferation, supporting a model in which stress and nutritional cues may directly influence ovarian follicle growth.
/////////////////Fertilization-Induced Autophagy in Mouse Embryos Is Independent of mTORC1. Yamamoto A 2014 et al.
Autophagy is a dynamically regulated intracellular degradation system that is important for cellular processes such as amino acid production during starvation and intracellular quality control. Previously, we reported that autophagy is suppressed in oocytes, but is rapidly up-regulated after fertilization. During this period, autophagy is thought to be important for the generation of amino acids from the bulk degradation of maternal proteins that have accumulated during oogenesis. However, the mechanism of autophagy induction after fertilization is presently unknown. In most cell types, autophagy is negatively controlled by mammalian target of rapamycin complex 1 (mTORC1), which is typically regulated by amino acids and insulin or related growth factors. In this study, we determined the role of mTORC1 in fertilization-induced autophagy. On the basis of the phosphorylation status of mTORC1 substrates, we found that mTORC1 activity was relatively high in metaphase II (MII) oocytes, but was rapidly decreased within 3 h of fertilization. However, chemical inhibition of mTORC1 by Torin1 or PP242 in MII oocytes or fertilized embryos did not induce autophagy. In addition, activation of mTORC1 by cycloheximide did not inhibit fertilization-induced autophagy in fertilized embryos. By contrast, the phosphatidylinositol 3-kinase inhibitors wortmannin and LY294002 effectively suppressed autophagy in these embryos. These data suggest that, even though autophagy induction and post-fertilization mTORC1 activity are inversely correlated with each other, as observed in other cell types, mTORC1 suppression is neither essential nor sufficient for fertilization-induced autophagy, highlighting a unique feature of the regulation mechanism of autophagy-mediated intracellular turnover in early embryos.
Mechanistic target of rapamycin (MTOR) signaling during ovulation in mice. Siddappa D 2014 et al.
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??g/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 1h and 4h post-hCG and overexpression of hCG-induced ovulation genes at 4h 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 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. Mol. Reprod. Dev. ? 2014 Wiley Periodicals, Inc.
Distribution and association of mTOR with its cofactors, raptor and rictor, in cumulus cells and oocytes during meiotic maturation in mice. Kogasaka Y et al. Mammalian target of rapamycin (mTOR), a Ser/Thr protein kinase, is the catalytic component of 2 distinct signaling complexes, mTOR-raptor complex (mTORC1) and mTOR-rictor complex (mTORC2). Recently, studies have demonstrated mitosis-specific roles for mTORC1, but the functions and expression dynamics of mTOR complexes during meiotic maturation remain unclear. In the present study, to evaluate the roles of respective mTOR complexes in maternal meiosis and compare them with those in mitosis, we sought to elucidate the spatiotemporal immunolocalization of mTOR, the kinase-active Ser2448- and Ser2481-phosphorylated mTOR, and raptor and rictor during cumulus-cell mitosis and oocyte meiotic maturation in mice. mTOR principally accumulated around the chromosomes and on the spindle. Phosphorylated mTOR (Ser2448 and Ser2481) exhibited elevated fluorescence intensities in the cytoplasm and punctate localization adjacent to the chromosomes, on the spindle poles, and on the midbody during mitotic and meiotic maturation, suggesting functional homology of mTOR between the 2 cell-division systems, despite their mechanistically distinctive spindles. Raptor colocalized with mTOR during both types of cell division, indicating that mTORC1 is predominantly associated with these events. Mitotic rictor uniformly distributed through the cytoplasm, and meiotic rictor localized around the spindle poles of metaphase-I oocytes, suggesting functional divergence of mTORC2 between mitosis and female meiosis. Based on the general function of mTORC2 in the organization of the actin cytoskeleton, we propose that mTORC1 controls spindle function during mitosis and meiosis, while mTORC2 contributes to actin-dependent asymmetric division during meiotic maturation in mice. Mol. Reprod. Dev. ? 2013 Wiley Periodicals, Inc.
FSH activation of HIF-1 by the PI3-kinase/AKT/Rheb/mTOR pathway is necessary for induction of select protein markers of follicular differentiation.
Alam H, et al sought to elucidate the role of AKT in follicle-stimulating hormone (FSH)-mediated granulosa cell (GC) differentiation. Their results define a signaling pathway in GCs whereby the inactivating phosphorylation of tuberin downstream of phosphatidylinositol-3 (PI3)-kinase/AKT activity leads to ras homolog enriched in brain (Rheb) and subsequent mammalian target of rapamycin (mTOR) activation. mTOR then stimulates translation by phosphorylating p70 S6 kinase and, consequently, the 40S ribosomal protein S6. Activation of this pathway is required for FSH-mediated induction of several follicular differentiation markers including luteinizing-hormone receptor (LHR), inhibin-alpha, microtubule-associated protein 2D, and the PKA type IIbeta regulatory subunit. FSH also promotes activation of the transcription factor hypoxia-inducible factor-1 (HIF-1). FSH-stimulated HIF-1 activity is inhibited by the PI3-kinase inhibitor LY294002, the Rheb inhibitor FTI-277, and the mTOR inhibitor rapamycin. Finally, the authors find that the FSH-mediated up-regulation of reporter activities for LHR, inhibin-alpha, and vascular endothelial growth factor is dependent upon HIF-1 activity as a dominant negative form of HIF-1alphainterferes with the up-regulation of these genes. These results show that FSH enhances HIF-1 activity downstream of the PI3-kinase/AKT/Rheb/mTOR pathway in GCs and that HIF-1 activity is necessary for FSH to induce multiple follicular differentiation markers.
Role of the PI3-Kinase and ERK Pathways in the Induction of HIF-1 Activity and the HIF-1 Target VEGF in Ovarian Granulosa Cells in response to Follicle Stimulating Hormone. Alam H et al. FSH stimulation of granulosa cells (GCs) results in increased hypoxia-inducible factor (HIF)-1alpha protein levels and HIF-1 activity that is necessary for up-regulation of certain FSH target genes including vascular endothelial growth factor (VEGF). We report that the role of the phosphatidylinositol (PI)-3-kinase/AKT pathway in increasing HIF-1alpha protein in FSH-stimulated GCs extends beyond an increase in mammalian target of rapamycin (mTOR)-stimulated translation. FSH increases phosphorylation of the AKT target mouse double minute 2 (MDM2); a phosphomimetic mutation of MDM2 is sufficient to induce HIF-1 activity. The PI3-kinase/AKT target forkhead box-containing protein O subfamily 1 (FOXO1) also effects the accumulation of HIF-1alpha as evidenced by the ability of a constitutively active FOXO1 mutant to inhibit the induction by FSH of HIF-1alpha protein and HIF-1 activity. Activation of the PI3-kinase/AKT pathway in GCs by IGF-1 is sufficient to induce HIF-1alpha protein but surprisingly not HIF-1 activity. HIF-1 activity also appears to require a PD98059-sensitive protein (kinase) activity stimulated by FSH that is both distinct from mitogen-activated extracellular signal-regulated kinase (ERK) kinase1/2 or 5 and independent of the PI3-kinase/AKT pathway. These results indicate that FSH-stimulated HIF-1 activation leading to up-regulation of targets such as VEGF requires not only PI3-kinase/AKT-mediated activation of mTOR as well as phosphorylation of FOXO1 and possibly MDM2 but also a protein (kinase) activity that is inhibited by the classic ERK kinase inhibitor PD98059 but is not ERK1/2 or 5. Thus, regulation of HIF-1 activity in GCs by FSH under normoxic conditions is complex and requires input from multiple signaling pathways.