AMPK regulation of mouse oocyte meiotic resumption in vitro

Ser149 is another potential PKA phosphorylation target of Cdc25B in G2/M transition of fertilized mouse eggs

It is well documented that protein kinase A (PKA) acts as a negative regulator of M phase promoting factor (MPF) by phosphorylating cell division cycle 25 homolog B (Cdc25B) in mammals. However, the molecular mechanism remains unclear. In this study, we identified PKA phosphorylation sites in vitro by LC-MS/MS analysis, including Ser(149), Ser(229), and Ser(321) of Cdc25B, and explored the role of Ser(149) in G(2)/M transition of fertilized mouse eggs. The results showed that the overexpressed Cdc25B-S149A mutant initiated efficient MPF activation by direct dephosphorylation of Cdc2-Tyr(15), resulting in triggering mitosis prior to Cdc25B-WT. Conversely, overexpression of the phosphomimic Cdc25B-S149D mutant showed no significant difference in comparison with the control groups. Furthermore, we found that Cdc25B-Ser(149) was phosphorylated at G(1) and S phases, whereas dephosphorylated at G(2) and M phases, and the phosphorylation of Cdc25B-Ser(149) was modulated by PKA in vivo. In addition, we examined endogenous and exogenous Cdc25B, which were expressed mostly in the cytoplasm at the G(1) and S phases and translocated to the nucleus at the G(2) phase. Collectively, our findings provide evidence that Ser(149) may be another potential PKA phosphorylation target of Cdc25B in G(2)/M transition of fertilized mouse eggs and Cdc25B as a direct downstream substrate of PKA in mammals, which plays important roles in the regulation of early development of mouse embryos.

Regulation of the G2/M transition in rodent oocytes

Regulation of maturation in meiotically competent mammalian oocytes is a complex process involving the carefully coordinated exchange of signals between the somatic and germ cell compartments of the ovarian follicle via paracrine and cell-cell coupling pathways. This review highlights recent advances in our understanding of how such signaling controls both meiotic arrest and gonadotropin-triggered meiotic resumption in competent oocytes and relates them to the historical context. Emphasis will be on rodent systems, where many of these new findings have taken place. A regulatory scheme is then proposed that integrates this information into an overall framework for meiotic regulation that demonstrates the complex interplay between different follicular compartments.

Role of AMPK throughout meiotic maturation in the mouse oocyte: evidence for promotion of polar body formation and suppression of premature activation

This study was conducted to assess the role of AMPK in regulating meiosis in mouse oocytes from the germinal vesicle stage to metaphase II. Exposure of mouse cumulus cell-enclosed oocytes (CEO) and denuded oocytes (DO) during spontaneous maturation in vitro to AMPK-activating agents resulted in augmentation of the rate and frequency of polar body formation. Inhibitors of AMPK had an opposite, inhibitory effect. In addition, the AMPK inhibitor, compound C (Cmpd C) increased the frequency of oocyte activation. The stimulatory action of the AMPK-activating agent, AICAR, and the inhibitory action of Cmpd C were diminished if exposure was delayed, indicating an early action of AMPK on polar body formation. The frequency of spontaneous and Cmpd C-induced activation in CEO was reduced as the period of hormonal priming was increased, and AMPK stimulation eliminated the activation response. Immunostaining of oocytes with antibody to active AMPK revealed an association of active kinase with chromatin, spindle poles, and midbody during maturation. Immunolocalization of the α1 catalytic subunit of AMPK showed an association with condensed chromatin and the meiotic spindle but not in the spindle poles or midbody; α2 stained only diffusely throughout the oocyte. These data suggest that AMPK is involved in a regulatory capacity throughout maturation and helps promote the completion of meiosis while suppressing premature activation.

Pharmacological analyses of protein kinases regulating egg maturation in marine nemertean worms: a review and comparison with Mammalian eggs

For development to proceed normally, animal eggs must undergo a maturation process that ultimately depends on phosphorylations of key regulatory proteins. To analyze the kinases that mediate these phosphorylations, eggs of marine nemertean worms have been treated with pharmacological modulators of intracellular signaling pathways and subsequently probed with immunoblots employing phospho-specific antibodies. This article both reviews such analyses and compares them with those conducted on mammals, while focusing on how egg maturation in nemerteans is affected by signaling pathways involving cAMP, mitogen-activated protein kinases, Src-family kinases, protein kinase C isotypes, AMP-activated kinase, and the Cdc2 kinase of maturation-promoting factor.

Diet-induced obesity model: abnormal oocytes and persistent growth abnormalities in the offspring

Associations between maternal obesity and adverse fetal outcomes are well documented, but the mechanisms involved are largely unknown. Most previous work has focused on postconceptional events, however, our laboratory has shown pre- and periconceptional aberrations in maternal glucose metabolism have adverse effects on oocytes and embryos that carry on to the fetus. To demonstrate effects of maternal obesity in the pre- and periconceptional periods, we compared reproductive tissues from diet-induced obese female mice to those of control mice. Ovaries were either stained for follicular apoptosis or dissected and evaluated for oocyte size and meiotic maturation. Mice were also mated and followed for reproductive outcomes including preimplantation embryonic IGF-I receptor (IGF-IR) immunostaining, midgestation fetal growth, and midgestational placental IGF receptor 2 (Igf2r) mRNA. Delivered pups were followed for growth and development of markers of metabolic syndrome. Compared with controls, obese mice had significantly more apoptotic ovarian follicles, smaller and fewer mature oocytes, decreased embryonic IGF-IR staining, smaller fetuses, increased placental Igf2r mRNA, and smaller pups. All weaned pups were fed a regular diet. At 13 wk pups delivered from obese mice were significantly larger, and these pups demonstrated glucose intolerance and increased cholesterol and body fat suggesting early development of a metabolic-type syndrome. Together, our findings suggest maternal obesity has adverse effects as early as the oocyte and preimplantation embryo stage and that these effects may contribute to lasting morbidity in offspring, underscoring the importance of optimal maternal weight and nutrition before conception.

Stimulators of AMP-activated kinase (AMPK) inhibit seawater- but not cAMP-induced oocyte maturation in a marine worm: Implications for interactions between cAMP and AMPK signaling

Previous studies have shown that elevations in intraoocytic cAMP prevent mammalian oocytes from maturing, whereas cAMP degradation allows these oocytes to begin maturation, as evidenced by the onset of oocyte nuclear disassembly (="germinal vesicle breakdown", GVBD). Moreover, such cAMP degradation not only reduces cAMP levels but also generates AMP, which in turn can stimulate AMP-activated kinase (AMPK), a well-documented inducer of GVBD in mice. Alternatively, in some marine invertebrates, intraoocytic cAMP triggers, rather than blocks, GVBD, and whether AMPK up- or downregulates maturation in these species has not been tested. Thus, AMPK was monitored in the nemertean worm Cerebratulus during GVBD stimulated by seawater (SW) or cAMP elevators. In oocytes lacking surrounding follicle cells, AMPK activity was initially elevated in immature oocytes but subsequently reduced during SW- or cAMP-induced GVBD, given that the catalytic alpha-subunit of AMPK in maturing oocytes displayed a decreased stimulatory phosphorylation at T172 and an increased inhibitory phosphorylation at S485/491. Accordingly, AMPK-mediated phosphorylation of acetyl-CoA carboxylase, a known target of active AMPK, also declined during maturation. Moreover, treatments with either ice-cold calcium-free seawater (CaFSW) or AMPK agonists dissolved in SW maintained AMPK activity and inhibited GVBD. Conversely, adding cAMP elevators to CaFSW- or SW-solutions of AMPK activators restored GVBD while promoting S485/491 phosphorylation and AMPK deactivation. Collectively, such findings not only demonstrate for the first time that intraoocytic AMPK can block GVBD in the absence of surrounding follicle cells, but these results also provide evidence for a novel GVBD-regulating mechanism involving AMPK deactivation by cAMP-mediated S485/491 phosphorylation.

Activation of AMP-activated protein kinase may not be involved in AICAR- and metformin-mediated meiotic arrest in bovine denuded and cumulus-enclosed oocytes in vitro

The adenosine monophosphate-activated protein kinase (AMPK) activators, 5'-aminoimidazole-4-carboxamide 1-β-D-ribofuranoside (AICAR) and metformin (MET), inhibit resumption of meiosis in bovine cumulus-enclosed oocytes (CEO) and denuded oocytes (DO). The objectives of this study were to: (1) examine the effects of AMPK inhibitors on bovine oocyte meiosis in vitro; and (2) determine if AICAR or MET activates oocyte and/or cumulus cell AMPK. The AMPK inhibitor compound C (CC; 0.5, 1, 5, and 10 μM) did not reverse the inhibitory effects of AICAR (1 mM) and MET (2 mM) on bovine oocyte meiosis. Additionally, CC (5 and 10 μM) inhibited meiosis (p < 0.05) in CEO and DO cultured for 7 h. Okadaic acid (1 μM) reversed the inhibitory effect of MET (2 mM) and CC (5 μM; p < 0.05) but not of AICAR (1 mM). Phosphorylation of the alpha subunit of AMPK on Thr172 is required for activation. Based on western blot analysis, AICAR, MET and CC did not affect Thr172 phosphorylation levels in DO and oocytes from complexes (p > 0.05). In cumulus cells, Thr172 phosphorylation decreased after 3 h of culture (p < 0.05), regardless of the presence of AMPK modulators in the culture medium. Higher concentrations of AICAR (2 mM) and MET (10 mM) did not affect Thr172 phosphorylation, but phosphorylation on Ser79 of ACC, a substrate of AMPK, was increased in response to MET (p < 0.05). In conclusion, we inferred that the inhibitory effect of AICAR and MET on bovine oocyte meiosis was probably not mediated through activation of AMPK. Moreover, these compounds probably inhibited meiosis through different pathways.

Prophase I arrest and progression to metaphase I in mouse oocytes: comparison of resumption of meiosis and recovery from G2-arrest in somatic cells

Mammalian oocytes are arrested at prophase I until puberty when luteinizing hormone (LH) induces resumption of meiosis of follicle-enclosed oocytes. Resumption of meiosis is tightly coupled with regulating cyclin-dependent kinase 1 (CDK1) activity. Prophase I arrest depends on inhibitory phosphorylation of CDK1 and anaphase-promoting complex-(APC-CDH1)-mediated regulation of cyclin B levels. Prophase I arrest is maintained by endogenously produced cyclic adenosine monophosphate (cAMP), which activates protein kinase A (PKA) that in turn phosphorylates (and activates) the nuclear kinase WEE2. In addition, PKA-mediated phosphorylation of the phosphatase CDC25B results in its cytoplasmic retention. The combined effect maintains low levels of CDK1 activity that are not sufficient to initiate resumption of meiosis. LH triggers synthesis of epidermal growth factor-like factors in mural granulosa cells and leads to reduced cGMP transfer from cumulus cells to oocytes via gap junctions that couple the two cell types. cGMP inhibits oocyte phosphodiesterase 3A (PDE3A) and a decline in oocyte cGMP results in increased PDE3A activity. The ensuing decrease in oocyte cAMP triggers maturation by alleviating the aforementioned phosphorylations of WEE2 and CDC25B. As a direct consequence CDC25B translocates into the nucleus. The resulting activation of CDK1 also promotes extrusion of WEE2 from the nucleus thereby providing a positive amplification mechanism for CDK1 activation. Other kinases, e.g. protein kinase B, Aurora kinase A and polo-like kinase 1, also participate in resumption of meiosis. Mechanisms governing meiotic prophase I arrest and resumption of meiosis share common features with DNA damage-induced mitotic G2-checkpoint arrest and checkpoint recovery, respectively. These common features include CDC14B-dependent activation of APC-CDH1 in prophase I arrested oocytes or G2-arrested somatic cells, and CDC25B-dependent cell cycle resumption in both oocytes and somatic cells.

Maternal diabetes and oocyte quality

Maternal diabetes has been demonstrated to adversely affect preimplantation embryo development and pregnancy outcomes. Emerging evidence has implicated that these effects are associated with compromised oocyte competence. Several developmental defects during oocyte maturation in diabetic mice have been reported over past decades. Most recently, we further identified the structural, spatial and metabolic dysfunction of mitochondria in oocytes from diabetic mice, suggesting the impaired oocyte quality. These defects in the oocyte may be maternally transmitted to the embryo and then manifested later as developmental abnormalities in preimplantation embryo, congenital malformations, and even metabolic disease in the offspring. In this paper, we briefly review the effects of maternal diabetes on oocyte quality, with a particular emphasis on the mitochondrial dysfunction. The possible connection between dysfunctional oocyte mitochondria and reproductive failure of diabetic females, and the mechanism(s) by which maternal diabetes exerts its effects on the oocyte are also discussed.

cAMP pulsing of denuded mouse oocytes increases meiotic resumption via activation of AMP-activated protein kinase

cAMP plays a critical role in the control of oocyte maturation, as a high level of cAMP maintains oocyte arrest at the first meiotic prophase. Yet this study shows that pulsing meiotically arrested denuded oocytes (DO) with cAMP induces oocyte maturation through the activation of AMP-activated protein kinase (PRKA). Short-term (3 h) pulsing of meiotically arrested oocytes with forskolin, an adenyl cyclase (AC) activator, increased oocyte cAMP, led to elevated AMP, and induced oocyte meiotic resumption compared to oocytes continuously cultured in the control medium with or without forskolin. Western analysis showed that germinal vesicle (GV)-stage oocytes after forskolin pulsing contained increased levels of phospho-acetyl CoA carboxylase (pACACA), a primary substrate of PRKA. Pulsing oocytes with the phosphodiesterase (PDE)-sensitive cAMP analog, 8-bromo-cAMP (8-Br-cAMP), also increased pACACA and pPRKA levels in GV-stage oocytes and induced oocyte meiotic resumption. Moreover, the PRKA inhibitors, compound C and araA, prevented 8-Br-cAMP pulsing-induced maturation. The lack of effect on meiotic induction and PRKA activation when oocytes were pulsed with the PDE-resistant activators of cAMP-dependent protein kinase, Sp-cAMP-AM and Sp-5,6-DCI-cBIMPS, suggests that cAMP degradation is required for pulsing-induced maturation. Pulsing oocytes with the exchange protein directly activated by cAMP (Epac)-specific activator, 8-CPT-2'-O-Me-cAMP, had no stimulatory effect on oocyte maturation, suggesting Epac is not involved in the pulsing-induced maturation. Taken together, these data support the idea that a transient increase in oocyte cAMP can induce meiotic resumption via activation of PRKA.

Fatty acid oxidation and meiotic resumption in mouse oocytes

We have examined the potential role of fatty acid oxidation (FAO) in AMP-activated protein kinase (AMPK)-induced meiotic maturation. Etomoxir and malonyl CoA, two inhibitors of carnitine palmitoyl transferase-1 (CPT1), and thus FAO, blocked meiotic induction in dbcAMP-arrested cumulus cell-enclosed oocytes (CEO) and denuded oocytes (DO) by the AMPK activator, AICAR. C75, an activator of CPT1 and FAO, stimulated meiotic resumption in CEO and DO. This effect was insensitive to the AMPK inhibitor, compound C, indicating an action downstream of AMPK. Palmitic acid or carnitine also promoted meiotic resumption in DO in the presence of AICAR. Since C75 also suppresses the activity of fatty acid synthase (FAS), we tested another FAS inhibitor, cerulenin. Cerulenin stimulated maturation in arrested oocytes, but to a lesser extent, exhibited significantly slower kinetics and was effective in CEO but not DO. Moreover, etomoxir completely blocked C75-induced maturation but was ineffective in cerulenin-treated oocytes, suggesting that the meiosis-inducing action of C75 is through activation of FAO within the oocyte, while that of cerulenin is independent of FAO and acts within the cumulus cells. Finally, we determined that long chain, but not short chain, fatty acyl carnitine derivatives were stimulatory to oocyte maturation. Palmitoyl carnitine stimulated maturation in both CEO and DO, with rapid kinetics in DO; this effect was blocked by mercaptoacetate, a downstream inhibitor of FAO. These results indicate that activation of AMPK stimulates meiotic resumption in mouse oocytes by eliminating a block to FAO.

Anti-hypoxic effect of ginsenoside Rbl on neonatal rat cardiomyocytes is mediated through the specific activation of glucose transporter-4 ex vivo

AIM

The aim of this study was to investigate whether Gs-Rbl relieves the CoCl(2)-induced apoptosis of hypoxic neonatal rat cardiomyocytes and in which the role of glucose transporter-4 (GLUT-4).

METHODS

Gs-Rbl (0, 10, 50, 100, 200, 400, and 500 micromol/L), adenine 9-beta-D-arabinofuranoside (ara A, 500 micromol/L; AMPK inhibitor) and wortmannin (0.5 micromol/L; PI3K inhibitor) only in combination with 200 micromol/L Gs-Rbl were administered in hypoxic cardiomyocytes, which were induced by 500 micromol/L CoCl(2) for 12 h. Then, the apoptotic rate (AR), 2-[(3)H]-deoxy-D-glucose (2-[(3)H]-DG) uptake, and the expression of GLUT-4 (including in plasma membrane, PM), phospho-AMPKalpha (Thr172), AMPKalpha and Akt in cells were assayed.

RESULTS

Compared with simple hypoxia (0 micromol/L Gs-Rbl), Gs-Rb1 greater than 10 micromol/L significantly decreased the apoptotic rate (P<0.01) and significantly increased 2-[(3)H]-DG uptake (P<0.01), GLUT-4 content in cells and PM (P<0.01), AMPK activity (P<0.01) and Akt (P<0.01) levels in a dose-dependent manner. AMPK activity was completely suppressed by ara-A, just as Akt was suppressed by wortmannin. The AR, glucose uptake and GLUT-4 levels in cells and PM were partly down-regulated by ara-A or wortmannin.

CONCLUSION

Gs-Rb1 may protect neonatal rat cardiomyocytes from apoptosis induced by CoCl(2). The anti-apoptotic effect of Gs-Rb1 may occur by improving glucose uptake, in which GLUT-4 translocation and expression played a key role. Both the AMPK and the PI3K/Akt pathways may take part in the anti-hypoxic efficacy of Gs-Rb1.

The Potential Applications of Peroxisome Proliferator-Activated Receptor delta Ligands in Assisted Reproductive Technology

Peroxisome proliferator-activated receptor δ (PPARδ, also known as PPARβ) has ubiquitous distribution and extensive biological functions. The reproductive function of PPARδ was first revealed in the uterus at the implantation site. Since then, PPARδ and its ligand have been discovered in all reproductive tissues, including the gametes and the preimplantation embryos. PPARδ in preimplantation embryos is normally activated by oviduct-derived PPARδ ligand. PPARδ activation is associated with an increase in embryonic cell proliferation and a decrease in programmed cell death (apoptosis). On the other hand, the role of PPARδ and its ligand in gamete formation and function is less well understood. This review will summarize the reproductive functions of PPARδ and project its potential applications in assisted reproductive technology.

CDC25B acts as a potential target of PRKACA in fertilized mouse eggs

Protein kinase A (PRKACA) has been documented as a pivotal regulator in meiosis and mitosis arrest. Although our previous work has established that PRKACA regulates cell cycle progression of mouse fertilized eggs by inhibiting M-phase promoting factor (MPF), little is known about the intermediate factor between PRKACA and MPF in the mitotic cell cycle. In this study, we investigated the role of the PRKACA/CDC25B pathway on the early development of mouse fertilized eggs. Overexpression of unphosphorylatable CDC25B mutant (Cdc25b-S321A or Cdc25b-S229A/S321A) rapidly caused G2-phase eggs to enter mitosis. Microinjection of either Cdc25b-WT or Cdc25b-S229A mRNA also promoted G2/M transition, but much less efficiently than Cdc25b-S321A and Cdc25b-S229A/S321A. Moreover, mouse fertilized eggs overrode the G2 arrest by microinjection of either Cdc25b-S321A or Cdc25b-S229A/S321A mRNA, which efficiently resulted in MPF activation by directly dephosphorylating CDC2A-Tyr15, despite culture under conditions that maintained exogenous dibutyryl cAMP. Using a highly specific antibody against phospho-Ser321 of CDC25B in Western blotting, we showed that CDC25B-Ser321 was phosphorylated at the G1 and S phases, whereas Ser321 was dephosphorylated at the G2 and M phases in vivo. Our findings identify CDC25B as a potential target of PRKACA and show that PRKACA regulates G2/M transition by phosphorylating CDC25B-Ser321 but not CDC25B-Ser229 on the first mitotic division of mouse fertilized eggs.

Crosstalk between the AMP-activated kinase and insulin signaling pathways rescues murine blastocyst cells from insulin resistance

Maternal insulin resistance results in poor pregnancy outcomes. In vivo and in vitro exposure of the murine blastocyst to high insulin or IGF1 results in the down-regulation of the IGF1 receptor (IGF1R). This in turn leads to decreased glucose uptake, increased apoptosis, as well as pregnancy resorption and growth restriction. Recent studies have shown that blastocyst activation of AMP-activated protein kinase (AMPK) reverses these detrimental effects; however, the mechanism was not clear. The objective of this study was to determine how AMPK activation rescues the insulin-resistant blastocyst. Using trophoblast stem (TS) cells derived from the blastocyst, insulin resistance was recreated by transfecting with siRNA to Igf1r and down-regulating expression of the protein. These cells were then exposed to AMPK activators 5-aminoimidazole-4-carboxamide riboside and phenformin, and evaluated for apoptosis, insulin-stimulated 2-deoxyglucose uptake, PI3-kinase activity, and levels of phospho-AKT, phospho-mTor, and phospho-70S6K. Surprisingly, disrupted insulin signaling led to decreased AMPK activity in TS cells. Activators reversed these effects by increasing the AMP/ATP ratio. Moreover, this treatment increased insulin-stimulated 2-deoxyglucose transport and cell survival, and led to an increase in PI3-kinase activity, as well as increased P-mTOR and p70S6K levels. This study is the first to demonstrate significant crosstalk between the AMPK and insulin signaling pathways in embryonic cells, specifically the enhanced response of PI3K/AKT/mTOR to AMPK activation. Decreased insulin signaling also resulted in decreased AMPK activation. These findings provide mechanistic targets in the AMPK signaling pathway that may be essential for improved pregnancy success in insulin-resistant states.

alpha2 But not alpha1 AMP-activated protein kinase mediates oxidative stress-induced inhibition of retinal pigment epithelium cell phagocytosis of photoreceptor outer segments

Oxidative stress causes retinal pigment epithelium (RPE) cell dysfunction and is a major risk factor leading to the development of dry-type age-related macular degeneration. Taking pharmacological and genetic approaches, we address the mechanisms by which sublethal oxidative stress inhibits RPE cell phagocytosis. Sublethal oxidative stress dose-dependently inhibited RPE cell phagocytosis of photoreceptor outer segments (POS) and activated AMP-activated protein kinase (AMPK) as determined by increased Thr172 and Ser79 phosphorylation of AMPKalpha and its substrate acetyl-CoA carboxylase, respectively. Similar to oxidative stress, 5-aminoimidazole-4-carboxamide riboside (AICAR), a pharmacological activator of AMPK, inhibited RPE cell phagocytosis of POS in a dose-dependent manner. Inhibition of RPE cell phagocytosis by AICAR was fully reversed by blockade of AICAR translocation into cells by dipyridamole or inhibition of AICAR conversion to ZMP by adenosine kinase inhibitor 5-iodotubercidin. In agreement, AICAR-induced activation of AMPK was abolished by preincubation with dipyridamole or 5-iodotubercidin. Knock-out experiments further revealed that alpha2 but not alpha1 AMPK was involved in RPE cell phagocytosis and that activation of alpha2 AMPK contributed to the inhibition of RPE cell phagocytosis by oxidative stress. Inhibition of RPE cell phagocytosis by activation of alpha2 AMPK was associated with a dramatic increase in acetyl-CoA carboxylase phosphorylation. In comparison, AMPK had no role in oxidative stress-induced breakdown of RPE barrier function. Taken together, reduction in POS load under oxidative stress might direct RPE cells to a self-protected status. Thus, activating AMPK could have therapeutic potential in treating dry macular degeneration.

The role of peroxisome proliferator-activated receptors in the development and physiology of gametes and preimplantation embryos

In several species, a family of nuclear receptors, the peroxisome proliferator-activated receptors (PPARs) composed of three isotypes, is expressed in somatic cells and germ cells of the ovary as well as the testis. Invalidation of these receptors in mice or stimulation of these receptors in vivo or in vitro showed that each receptor has physiological roles in the gamete maturation or the embryo development. In addition, synthetic PPAR gamma ligands are recently used to induce ovulation in women with polycystic ovary disease. These results reveal the positive actions of PPAR in reproduction. On the other hand, xenobiotics molecules (in herbicides, plasticizers, or components of personal care products), capable of activating PPAR, may disrupt normal PPAR functions in the ovary or the testis and have consequences on the quality of the gametes and the embryos. Despite the recent data obtained on the biological actions of PPARs in reproduction, relatively little is known about PPARs in gametes and embryos. This review summarizes the current knowledge on the expression and the function of PPARs as well as their partners, retinoid X receptors (RXRs), in germ cells and preimplantation embryos. The effects of natural and synthetic PPAR ligands will also be discussed from the perspectives of reproductive toxicology and assisted reproductive technology.

Dorsomorphin inhibits BMP signals required for embryogenesis and iron metabolism

Bone morphogenetic protein (BMP) signals coordinate developmental patterning and have essential physiological roles in mature organisms. Here we describe the first known small-molecule inhibitor of BMP signaling-dorsomorphin, which we identified in a screen for compounds that perturb dorsoventral axis formation in zebrafish. We found that dorsomorphin selectively inhibits the BMP type I receptors ALK2, ALK3 and ALK6 and thus blocks BMP-mediated SMAD1/5/8 phosphorylation, target gene transcription and osteogenic differentiation. Using dorsomorphin, we examined the role of BMP signaling in iron homeostasis. In vitro, dorsomorphin inhibited BMP-, hemojuvelin- and interleukin 6-stimulated expression of the systemic iron regulator hepcidin, which suggests that BMP receptors regulate hepcidin induction by all of these stimuli. In vivo, systemic challenge with iron rapidly induced SMAD1/5/8 phosphorylation and hepcidin expression in the liver, whereas treatment with dorsomorphin blocked SMAD1/5/8 phosphorylation, normalized hepcidin expression and increased serum iron levels. These findings suggest an essential physiological role for hepatic BMP signaling in iron-hepcidin homeostasis.

AMP-activated protein kinase is involved in hormone-induced mouse oocyte meiotic maturation in vitro

We have previously shown that AMP-activated protein kinase (AMPK) can induce the resumption of meiosis in mouse oocytes maintained in meiotic arrest in vitro. The present study was carried out to determine whether AMPK activation is involved in hormone-induced maturation. Follicle-stimulating hormone (FSH) and the EGF-like peptide, amphiregulin (AR), are potent inducers of maturation in cumulus cell-enclosed oocytes (CEO). Within 3 h of FSH treatment, phospho-acetyl CoA carboxylase (ACC) levels were increased in germinal vesicle (GV)-stage oocytes when compared to non-stimulated controls and remained elevated throughout 9 h of culture, indicating AMPK activation. A similar response to AR was observed after 6 h of culture. Using anti-PT172 antibody (binds only to activated AMPK), Western analysis demonstrated active AMPK in both FSH- or AR-treated GV-stage oocytes within 6 h. The AMPK inhibitors, compound C and adenine 9-beta-d-arabinofuranoside (araA), blocked FSH- or AR-induced meiotic resumption and ACC phosphorylation, further supporting a causal role for AMPK in hormone-induced meiotic resumption. Immunocytochemistry using anti-PT172-AMPK antibody showed an increased diffuse cytoplasmic staining and more intense punctate staining in the germinal vesicles of oocytes following treatment with the AMPK activator 5-aminoimidazole-4-carboxamide-1-beta-d-ribofuranoside (AICAR) or with FSH or AR, and this staining was eliminated by compound C or a blocking peptide for the anti-PT172 antibody. Staining of oocytes from hCG-stimulated mice with the anti-PT172 antibody also showed pronounced label in the germinal vesicles within 1-2 h. Furthermore, in oocytes from all groups, active AMPK was always observed in association with the condensed chromosomes of maturing oocytes. Taken together, these results support a role for AMPK in FSH and AR-induced maturation in vitro and hCG-induced maturation in vivo.

Possible role of 5'AMP-activated protein kinase in the metformin-mediated arrest of bovine oocytes at the germinal vesicle stage during in vitro maturation

The 5'AMP-activated protein kinase (AMPK) activation is involved in the meiotic maturation of oocytes in the ovaries of mice and pigs. However, its effects on the oocyte appear to be species-specific. We investigated the patterns of AMPK and mitogen-activated protein kinases (MAPK3/1) phosphorylation during bovine in vitro maturation (IVM) and the effects of metformin, an AMPK activator, on oocyte maturation in cumulus-oocyte complexes (COCs) and denuded bovine oocytes (DOs). In bovine COCs, PRKAA Thr172 phosphorylation decreased, whereas MAPK3/1 phosphorylation increased in both oocytes and cumulus cells during IVM. Metformin (5 and 10 mM) arrested oocytes at the GV stage in COCs but not in DOs. In COCs, this arrest was associated with the inhibition of cumulus cell expansion, an increase in PRKAA Thr172 phosphorylation, and a decrease in MAPK3/1 phosphorylation in both oocytes and cumulus cells. However, the addition of compound C (10 muM), an inhibitor of AMPK, accelerated the initiation of the GV breakdown (GVBD) process without any alteration of MAPK3/1 phosphorylation in oocytes from bovine COCs. Metformin decreased AURKA and CCNB1 protein levels in oocytes. Moreover, after 1 h of IVM, metformin decreased RPS6 phosphorylation and increased EEF2 phosphorylation, suggesting that protein synthesis rates were lower in oocytes from metformin-treated COCs. Most oocytes were arrested after the GVBD stage following the treatment of COCs with the MEK inhibitor, U0126 (100 micromoles). Thus, in bovine COCs, metformin blocks meiotic progression at the GV stage, activates PRKAA, and inhibits MAPK3/1 phosphorylation in both the oocytes and cumulus cells during IVM. Moreover, cumulus cells were essential for the effects of metformin on bovine oocyte maturation, whereas MAPK3/1 phosphorylation was not.

Regulation of adiponectin and its receptors in rat ovary by human chorionic gonadotrophin treatment and potential involvement of adiponectin in granulosa cell steroidogenesis

In mammals, adiponectin and its receptors (AdipoR1 and AdipoR2) mRNAs are expressed in various tissues. However, the cellular expression and the role of adiponectin system have never been investigated in rat ovary. Here, we report the presence of adiponectin, AdipoR1 and AdipoR2 in rat ovaries, and we have investigated its role in granulosa cells. Using RT-PCR and western blot, we show that the mRNAs and proteins for adiponectin, AdipoR1 and AdipoR2 are found in the ovaries. Immunohistochemistry localized adiponectin, AdipoR1 and AdipoR2 in theca-interstitial T-I cells, corpus luteum, oocyte and less abundantly in granulosa cells. In the KGN human granulosa cell line, adiponectin mRNA and protein were undetectable; AdipoR2 was weakly expressed, whereas AdipoR1 was clearly present. Human chorionic gonadotrophin (hCG) injection (48 h) after pregnant mare serum gonadotrophin (PMSG) injection (24 h) in immature rats increased the level of adiponectin (protein) by about threefold (P< 0.05) and those of AdipoR1 by threefold (mRNA,P< 0.05) and 1.5-fold (protein,P< 0.05) in ovary, whereas the mRNA and protein levels of AdipoR2 were unchanged. Interestingly, hCG injection (48 h) after the PMSG treatment (24 h) decreased plasma adiponectin levels and increased insulin plasma levels.In vitroin primary rat granulosa cells, human adiponectin recombinant (5 μg/ml) in the presence or absence of follicle-stimulating hormone (10−8M, 48 h) had no effect on the steroidogenesis. However, it increased progesterone secretion (P< 0.05) by about twofold and oestradiol production (P< 0.05) by about 1.6-fold in response to insulin-like growth factor-I (IGF-I) (10−8M). Furthermore, it improved IGF-I-induced IGF-I receptor-β subunit tyrosine phosphorylation and ERK1/2 phosphorylation. In basal state, human adiponectin recombinant also increased rapidly but transiently the ERK1/2, p38 and Akt phosphorylations, whereas it increased more lately the adenosine 5′-monophosphate-activated protein kinase (AMPK) phosphorylation. Thus, AdipoR1 and AdipoR2 are regulated by hCG treatment in rat ovary and adiponectin enhances IGF-I-induced steroidogenesis in granulosa cells.

Induction of meiotic maturation in mouse oocytes by adenosine analogs

In this study we have examined the meiosis-inducing influence of adenosine analogs in mouse oocytes. When a varied group of nucleosides and nucleotides were tested on overnight cultures of hypoxanthine-arrested, cumulus cell-enclosed oocytes (CEO), halogenated adenosine nucleosides, but not native adenosine, exhibited a significant meiosis-inducing capability. When tested under a variety of conditions, meiotic induction by 8-bromo-adenosine (8-Br-Ado) and a second adenosine analog, methylmercaptopurine riboside (MMPR), was especially potent in denuded oocytes (DO) compared to CEO and was not dependent on the type of inhibitor chosen to maintain meiotic arrest. Germinal vesicle breakdown (GVB) was stimulated with rapid kinetics and was preceded by an increase in AMP-activated protein kinase (AMPK) activity. Moreover, compound C, an inhibitor of AMPK, blocked the meiosis-inducing activities of both adenosine analogs. When tested for an effect on meiotic progression to metaphase II (MII) in spontaneously maturing CEO, 8-Br-Ado and the AMPK activator, 5-aminoimidazole-4-carboxamide 1-beta-D-ribofuranoside (AICAR), increased the percentage of MII-stage oocytes, but MMPR decreased this number. Adenosine and inhibitors of de novo purine synthesis had no effect on the completion of maturation, while compound C suppressed this process. These results support the proposition that oocyte AMPK mediates the positive influence of AICAR and 8-Br-Ado on both the initiation and completion of meiotic maturation. The role of AMPK in MMPR action is less clear.

Adenosine 5'-monophosphate kinase-activated protein kinase (PRKA) activators delay meiotic resumption in porcine oocytes

Adenosine monophosphate-activated kinase (PRKA) is a serine/threonine kinase that functions as a metabolic switch in a number of physiological functions. The present study was undertaken to assess the role of this kinase in nuclear maturation of porcine oocytes. RT-PCR and immunoblotting revealed the expression of the PRKAA1 subunit in granulosa cells, cumulus-oocyte complexes (COC), and denuded oocytes (DO). Porcine COC and DO contained transcripts that corresponded to the expected sizes of the designed primers for PRKAB1 and PRKAG1. The PRKAA2 subunit was detected in granulosa cells and COC, whereas the PRKAG3 subunit was not detected in granulosa cells, COC or DO, whereas it was detected in the heart. The PRKAA1 protein was detected in granulosa cells, COC, DO, and zona pellucida (ZP). In the presence of the pharmacological activator of PRKA 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranosyl 5'-monophosphate (ZMP), COC were transiently maintained in meiotic arrest in a fully reversible manner. This inhibitory effect was not observed in DO. Other known PRKA activators, 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside (AICAR) and metformin, also blocked meiotic resumption in COC. In contrast to mouse oocytes, in which PRKA activators reverse the inhibitory effect of PDE3 inhibitors, this combination still blocked meiotic resumption in porcine COC. These results demonstrate that the meiotic resumption of porcine COC is transiently blocked by PRKA activators in a dose-dependent manner, and that this effect is dependent on PRKA activity in cumulus cells. The present study describes a new role for PRKA in regulating meiotic resumption in COC and strongly suggests that cumulus cells play an essential role in the control of porcine oocyte maturation through the PRKA metabolic switch.

Differing mechanisms of cAMP- versus seawater-induced oocyte maturation in marine nemertean worms I. The roles of serine/threonine kinases and phosphatases

Unlike in most animals, oocytes of marine nemertean worms initiate maturation (=germinal vesicle breakdown, GVBD) following an increase, rather than a decrease, in intraoocytic cAMP. To analyze how serine/threonine (Ser/Thr) kinase cascades involving mitogen-activated protein kinase (MAPK), maturation-promoting factor (MPF), cAMP-dependent protein kinase (PKA), and phosphatidylinositol 3-kinase (PI3K) regulate nemertean GVBD, oocytes of Cerebratulus sp. were treated with pharmacological modulators and stimulated with cAMP-elevating drugs or seawater (SW) alone. Both cAMP elevators and SW triggered GVBD while activating MAPK, its target p90Rsk, and MPF. Similarly, neither cAMP- nor SW-induced GVBD was affected by several Ser/Thr phosphatase inhibitors, and both stimuli apparently accelerated GVBD via a MAPK-independent, PI3K-dependent mechanism. However, inhibitors of Raf-1, a kinase that activates MAPK kinase, blocked GVBD and MAPK activation during SW-, but not cAMP-induced maturation. In addition, MPF blockers more effectively reduced GVBD and MAPK activity in SW versus in cAMP-elevating treatments. Moreover, the two maturation-inducing stimuli yielded disparate patterns of PKA-related MAPK activations and phosphorylations of putative PKA substrates. Collectively, such findings suggest that in maturing oocytes of Cerebratulus sp., Ser/Thr kinase cascades differ during cAMP- versus SW-induced GVBD in several ways, including MAPK activation modes, MPF-feedback loops, and PKA-related signaling pathways. Additional differences in cAMP- versus SW-induced oocyte maturation are also described in the accompanying study that deals with the roles of tyrosine kinase signaling during GVBD.

Effects of metformin on bovine granulosa cells steroidogenesis: possible involvement of adenosine 5' monophosphate-activated protein kinase (AMPK)

In mammals, IGFs are important for the proliferation and steroidogenesis of ovarian cells. Metformin is an insulin sensitizer molecule used for the treatment of the infertility of women with polycystic ovary syndrome. It is, however, unclear whether metformin acts on ovarian cells. Adenosine 5' monophosphate-activated protein kinase (AMPK) is involved in metformin action in various cell types. We investigated the effects of metformin on bovine granulosa cell steroidogenesis in response to IGF1 and FSH, and studied AMPK in bovine ovaries. In granulosa cells from small follicles, metformin (10 mM) reduced production of both progesterone and estradiol and decreased the abundance of HSD3B, CYP11A1, and STAR proteins in presence or absence of FSH (10(-8) M) and IGF1 (10(-8) M). In cows, the different subunits of AMPK are expressed in various ovarian cells including granulosa and theca cells, corpus luteum, and oocytes. In bovine granulosa cells from small follicles, metformin, like AICAR (1 mM) a pharmaceutical activator of AMPK, increased phosphorylation of both Thr172 of AMPK alpha and Ser 79 of ACACA (Acetyl-CoA Carboxylase). Both metformin and AICAR treatment reduced progesterone and estradiol secretion in presence or absence of FSH and IGF1. Metformin decreased phosphorylation levels of MAPK3/MAPK1 and MAPK14 in a dose- and time-dependent manner. The adenovirus-mediated production of dominant negative AMPK abolished the effects of metformin on secretion of progesterone and estradiol and on MAPK3/MAPK1 phosphorylation but not on MAPK14 phosphorylation. Thus, in bovine granulosa cells, metformin decreases steroidogenesis and MAPK3/MAPK1 phosphorylation through AMPK activation.

Meiotic induction by heat stress in mouse oocytes: involvement of AMP-activated protein kinase and MAPK family members

In this study, we examined the effect of heat pulsing on oocyte maturation and assessed the possible role of stress-activated enzymes during heat stress-induced meiotic maturation. Denuded oocytes from immature eCG-primed mice were pulsed for 30 min at increasing temperatures from 40 degrees C to 43 degrees C in dibutyryl cAMP-containing medium and were subsequently cultured at 37 degrees C for a total incubation time of 17-18 h. Oocytes exposed to 42 degrees C showed the greatest stimulation of maturation, with no effect at 43 degrees C. A heat pulse did not compromise progression to metaphase II as observed by polar body (PB) formation. The AMP-activated protein kinase (PRKA) inhibitors compound C and Ara-A each blocked the meiosis-stimulating effects of heat. Western blots showed that acetyl-CoA carboxylase, an important substrate of PRKA, was phosphorylated in heat-treated germinal vesicle-stage oocytes, indicating activation of PRKA before maturation. The mitogen-activated protein 2 kinase (MAP2K1) inhibitor PD98059 also prevented heat-induced maturation, but this effect was unrelated to MAPK1/3 activation, which was not observed until after germinal vesicle breakdown (GVB). Phosphorylated MAPK14 was not detected in the oocyte under any experimental condition, and only high concentrations of the MAPK14 inhibitor SB203580 blocked heat-stimulated maturation, suggesting that MAPK14 is not involved in meiotic induction. MAPK8/9 was activated by heat, and the MAPK8/9 inhibitor SP600125, but not JUN N-terminal kinase I, blocked heat-induced maturation. Heat treatment transiently suppressed GVB and PB formation in spontaneously maturing oocytes by a mechanism that is apparently different from its meiosis-inducing action. Collectively, these data show that an acute heat pulse stimulates GVB in meiotically arrested oocytes and suggest that this effect is mediated through the activation of PRKA.

Folliculin encoded by the BHD gene interacts with a binding protein, FNIP1, and AMPK, and is involved in AMPK and mTOR signaling

Birt-Hogg-Dubé syndrome, a hamartoma disorder characterized by benign tumors of the hair follicle, lung cysts, and renal neoplasia, is caused by germ-line mutations in the BHD(FLCN) gene, which encodes a tumor-suppressor protein, folliculin (FLCN), with unknown function. The tumor-suppressor proteins encoded by genes responsible for several other hamartoma syndromes, LKB1, TSC1/2, and PTEN, have been shown to be involved in the mammalian target of rapamycin (mTOR) signaling pathway. Here, we report the identification of the FLCN-interacting protein, FNIP1, and demonstrate its interaction with 5' AMP-activated protein kinase (AMPK), a key molecule for energy sensing that negatively regulates mTOR activity. FNIP1 was phosphorylated by AMPK, and its phosphorylation was reduced by AMPK inhibitors, which resulted in reduced FNIP1 expression. AMPK inhibitors also reduced FLCN phosphorylation. Moreover, FLCN phosphorylation was diminished by rapamycin and amino acid starvation and facilitated by FNIP1 overexpression, suggesting that FLCN may be regulated by mTOR and AMPK signaling. Our data suggest that FLCN, mutated in Birt-Hogg-Dubé syndrome, and its interacting partner FNIP1 may be involved in energy and/or nutrient sensing through the AMPK and mTOR signaling pathways.

Berberine-stimulated glucose uptake in L6 myotubes involves both AMPK and p38 MAPK

Berberine is a plant alkaloid used in traditional Chinese medicine and has been reported to have antihyperglycemic activity in NIDDM patients. However, the molecular basis for this action is yet to be elucidated. Here we investigate the effects and signaling pathways of berberine on L6 rat skeletal muscles. Our study demonstrates that berberine stimulates glucose uptake in a time- and dose-dependent manner. Intriguingly, berberine-stimulated glucose uptake does not vary as insulin concentration increases, and could not be blocked by the PI 3-kinase inhibitor wortmannin. Berberine only weakly stimulates the phosphorylation of Akt/PKB, a key molecule in the insulin signaling pathway, but strongly promotes the phosphorylation of AMPK and p38 MAPK. The effects of berberine are not a result of pro-oxidant action, but a consequence of an increased cellular AMP:ATP ratio. Moreover, berberine-stimulated glucose uptake is inhibited by the AMPK inhibitor Compound C and the p38 MAPK inhibitor SB202190. Inhibition of AMPK reduces p38 MAPK phosphorylation, suggesting that AMPK lies upstream of p38 MAPK. These results suggest that berberine circumvents insulin signaling pathways and stimulates glucose uptake through the AMP-AMPK-p38 MAPK pathway, which may account for the antihyperglycemic effects of this drug.