Progesterone induction of phospholipid methylation and arachidonic acid turnover during the first meiotic division in amphibian oocytes

Lipid Signaling During Gamete Maturation

Cell lipids are differentially distributed in distinct organelles and within the leaflets of the bilayer. They can further form laterally defined sub-domains within membranes with important signaling functions. This molecular and spatial complexity offers optimal platforms for signaling with the associated challenge of dissecting these pathways especially that lipid metabolism tends to be highly interconnected. Lipid signaling has historically been implicated in gamete function, however the detailed signaling pathways involved remain obscure. In this review we focus on oocyte and sperm maturation in an effort to consolidate current knowledge of the role of lipid signaling and set the stage for future directions.

Expression Profiles of the Progesterone Receptor, Cyclooxygenase-2, Growth Differentiation Factor 9, and Bone Morphogenetic Protein 15 Transcripts in the Canine Oviducts during the Oestrous Cycle

Simple Summary

The oestrous cycle in canines is specifically more extended than that in other mammals. This implies that the oocytes do not reach maturity within the ovarian follicle but undergo final maturation in the oviducts. Besides oocyte maturation, the oviduct provides the necessary milieu for fertilization and preimplantation embryonic development. Consequently, the oviductal environment presumably changes in the postovulatory period and throughout the entire reproductive cycle to provide a suitable condition for supporting different functions. In this study, we evaluated the gene expression of different genes associated with oocyte-embryo development, such as progesterone receptor, cyclooxygenase-2, growth differentiation factor 9, and bone morphogenetic protein 15 in the canine oviductal cells at different phases of the oestrous cycle. Using quantitative PCR (qPCR) analysis in bitch oviductal cells, this study revealed the ovarian cycle’s influence on the oviductal essential transcripts in the bitch. It also assessed the influence of the ovulated cumulus-oocytes complexes on the expression of GDF-9 and BMP-15 genes. Thus, the oestrous-cycle-dependent gene expression pattern of PR, COX-2, GDF-9, BMP-15 in the canine oviduct was found to execute the oviductal cell interactions necessary for the development and function of the canine reproductive tract.

Abstract

The gene expression in the canine oviduct, where oocyte maturation, fertilization, and early embryonic development occur, is still elusive. This study determined the oviductal expression of (PR), cyclooxygenase-2 (COX-2), growth differentiation factor 9 (GDF-9), and bone morphogenetic protein 15 (BMP-15) during the canine oestrous cycle. Samples were collected from bitches at anoestrus (9), proestrus (7), oestrus (8), and dioestrus (11), after routine ovariohysterectomy and the ovarian surface structures and plasma progesterone concentration evaluated the physiological status of each donor. The oviductal cells were isolated and pooled. Total RNA was isolated, and gene expression was assessed by qPCR followed by analysis using the t-test and ANOVA. The PR mRNA increased (P < 0.05) from the anoestrus to dioestrus with the plasma progesterone concentration (r = 0.8). COX-2 mRNA expression was low in the anoestrus and proestrus, and negligible in the oestrus, while it was around 10-fold higher (P < 0.05) in the dioestrus. The GDF-9 mRNA was expressed during all phases of the oestrous cycle and was most abundant (P < 0.05) during oestrus phase. The BMP-15 mRNA decreased (P < 0.05) in the anoestrus and proestrus phases. Thus, the transcripts were differentially expressed in a stage-dependent manner, suggesting the importance of oestrous cycle regulation for successful reproduction in dogs.

The distinct stage-specific effects of 2-(p-amylcinnamoyl)amino-4-chlorobenzoic acid on the activation of MAP kinase and Cdc2 kinase in Xenopus oocyte maturation

2-(p-amylcinnamoyl)amino-4-chlorobenzoic acid (PACA), pharmacological inhibitor of phospholipase A(2) (PLA(2)), inhibits epinephrine-stimulated thromboxane production in human platelets. In this study, we investigated the effect of PACA on meiotic maturation individually in stages V and VI oocytes. PACA prevented the maturation in stage V but merely delayed the process in stage VI oocytes. This was associated with the strong inhibition of Mos synthesis at both stages. Besides, PACA-induced inhibition of MAPK activation was evident in stage V but not in stage VI oocytes. PACA also inhibited the activation of Cdc2 kinase (Cdc2) in stage V but merely delayed the process in stage VI oocytes. Furthermore, 5 microM and higher concentrations of PACA completely inhibited the activation of MAPK and Cdc2 only in stage V, not in stage VI, oocytes. Moreover, we propose PACA as a new tool for the study of Xenopus oocyte maturation, which can also play a unique role for the studies of the stage-specific activation of MAPK and Cdc2.

Low-density caveolae-like membrane from Xenopus laevis oocytes is enriched in Ras

Detergent-free discontinuous sucrose density gradient centrifugation was used to resolve low- and high-density membrane fractions from Xenopus laevis oocytes. Compared to high-density membrane, low-density oocyte membrane is enriched two-fold in cholesterol and highly enriched in ganglioside GM1. Protein immunoblotting of membrane fractions from whole cells with polyclonal anti-human caveolin antibody detected multiple bands, including a distinctive triad with apparent molecular weights of 21, 33, and 48 kDa. To more clearly determine which of these caveolin-like protein(s) is associated with the oocyte plasma membrane, microdissection was used to separate external membrane (cortical preparations containing plasma membrane) from intracellular membrane. Cortical membrane preparations displayed a single 21-kDa caveolin-like protein in low-density membrane. Internal oocyte membrane displayed the higher molecular weight bands of 33 and 48 kDa and a lesser amount of the 21-kDa protein in low-density membrane fractions. Monoclonal anti-human Ras antibody detected a single 23-kDa immunoblot band that is enriched an average of eight-fold in low-density membrane fractions prepared from whole cells. This is the first report of caveolin-associated, low-density membrane in amphibian oocytes, and is consistent with a role for caveolin and caveolae-like microdomains in oocyte signal transduction.

Actions of progesterone on human sperm: a model of non-genomic effects of steroids

Non-genomic actions of steroids have been extensively studied in the last few years. Among these actions, the non-genomic effect of progesterone (P) on human spermatozoa appears to be very promising, in view of the dramatic effect of this steroid on intracellular calcium, activation of tyrosine kinase, and induction of acrosome reaction. We have shown that the ability of spermatozoa to respond to P increases during the process of capacitation and is not counteracted by the P-receptor antagonist RU486 nor by the GABAA antagonists bicuculline and picrotoxin. We have also shown that P increases tyrosine phosphorylation of a sperm protein of about 97 kDa, suggesting activation of tyrosine kinase(s). In addition, we found that P induces a perturbation of sperm membrane phospholipid metabolism resulting in an increase of synthesis of platelet-activating factor and liberation of arachidonic acid. Results of these biochemical studies indicate that P is able to stimulate several signal transduction pathways in human sperm. We have also investigated responsiveness to P in sperm of oligozoospermic subjects as well as of men undergoing an in vitro fertilization (IVF) program. Our results show that the percentage increases of intracellular calcium and acrosome reaction in response to P is significantly reduced in oligozoospermic men as well as in subjects with reduced fertilization rate. Moreover, in the latter subjects response to P is highly significant correlated to fertilization rate of oocytes. These studies indicate that a biochemical alteration of sperm in their capacity to respond to P might be responsible for reduced fertilizing ability.

Roles of second-messenger systems and neuronal activity in the regulation of lordosis by neurotransmitters, neuropeptides, and estrogen: a review

Many neurotransmitters and neuropeptides can affect the rodent feminine sexual behavior, lordosis, when administered in the ventromedial hypothalamus (VMH), midbrain central gray (MCG), or other brain regions. A survey of the electrophysiological and biochemical actions of these neural agents revealed that there is a very consistent association between lordosis facilitation with both the activation of the phosphoinositide (PI) pathway and the excitation of VMH and MCG neurons. In contrast, lordosis inhibition is associated, less consistently, with alterations of the adenylate cyclase (AC) system and the inhibition of neuronal activity. The findings that lordosis could be facilitated by going beyond membrane receptors and directly activating the PI pathway, suggest that this second-messenger pathway is a common mediator for the lordosis-facilitating agents. Furthermore, as in the case of stimulating membrane receptors, direct activation of this common mediator also requires estrogen priming for lordosis facilitation. Therefore, it is likely that the PI pathway is modulated by estrogen in the permissive action of estrogen priming. Indeed, a literature review shows that estrogen can affect selective isozymes of key enzyme families of the PI pathway at various levels. Such selective modulations, at several levels, could easily alter the course of a PI cascade; thence, the eventual functional outcome. These findings prompt us to propose that estrogen enables lordosis to be facilitated by a selective modulation of the PI pathway.

Stimulation of platelet-activating factor synthesis by progesterone and A23187 in human spermatozoa

The presence of platelet-activating factor (PAF) has been demonstrated recently in mammalian spermatozoa, together with evidence for a role of this phospholipid in enhancing sperm motility and fertilizing ability. To investigate whether PAF synthesis and release occurs in human spermatozoa following incubation with stimuli that induce acrosome reaction, spermatozoa were incubated with progesterone and A23187, two known inducers of the exocytotic event. PAF synthesis (remodelling pathway) was assessed by [3H]acetate incorporation into PAF. Treatment of spermatozoa with progesterone and A23187 resulted in an increase of [3H]acetate incorporation into PAF. Most of the newly synthesized [3H]PAF formed in response to acrosome reaction was found in the supernatant, suggesting a release of the phospholipid from spermatozoa. PAF-like material extracted from human spermatozoa was able to induce aggregation of rabbit platelets and showed identical retention time and the same ion m/e values as authentic PAF when analysed with g.c.-m.s. Lyso-PAF:acetyl-CoA acetyltransferase (EC 2.3.1.67) activity in human spermatozoa was also studied and showed similar kinetic parameters to those described for other cell systems. Stimulation of spermatozoa with progesterone and A23187 induced an increase of [3H]arachidonic acid release, suggesting an activation of phospholipase A. In conclusion, our results demonstrated increased production and release of PAF in human sperm following stimulation with progesterone and A23187 and suggest a role for this phospholipid in the activation of spermatozoa.

Steroid action at the plasma membrane: progesterone stimulation of phosphatidylcholine-specific phospholipase C following release of the prophase block in amphibian oocytes

Progesterone, acting at the amphibian oocyte plasma membrane, triggers the progression of the prophase oocyte nucleus through the first meiotic metaphase. We previously reported a transient increase in 1,2-diacylglycerol (1,2-DG) within the first 1-2 min after exposure of Rana pipiens oocytes to progesterone. We have now investigated the source of the 1,2-DG, using this highly synchronous oocyte population. Phospholipid pools of intact prophase-arrested oocytes were labeled with [3H]glycerol, [methyl-3H]choline chloride or 1-O-[3H]octadecyl-sn-glycero-3-phosphocholine (lyso platelet activating factor, lysoPAF). [3H]LysoPAF is selectively taken up into the plasma membrane of the intact oocyte and esterified to form the [3H]alkyl-analogue of phosphatidylcholine (PC). Intact oocytes and/or isolated plasma membranes were then stimulated with progesterone and the changes in [3H]DG, [methyl-3H]phosphocholine and [3H]phospholipids were monitored as a function of time. Progesterone induced a transient increase in [3H]glycerol-derived DG, [methyl-3H]phosphocholine and [3H]alkyl-2-acylglycerol from [3H]alkyl-PC within the first 2 min, indicating activation of a PC-specific phospholipase C. Different pulse-labeling conditions indicate a biphasic rise in [3H]DG from [3H]glycerol-labeled oocytes; the first rise (1-2 min) when phospholipid labeling in the plasma membrane is enriched followed by an approximately 3-fold larger rise at 5-15 min when phospholipids of intracellular membranes are preferentially labeled. An early transient increase in [3H]DG or [3H]alkyl-2-acylglycerol was also seen when progesterone and/or guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S) were added to isolated plasma-vitelline membranes prepared from oocytes prelabeled with either [3H]glycerol or [3H]lysoPAF. Progesterone thus appears to activate a G-protein-linked PC-specific phospholipase C in the oocyte plasma membrane which is followed by much larger DG release from intracellular membranes. The transient character of the hydrolysis suggests that this may represent a mechanism for transducing a membrane event into a meiotic signal.

Progesterone-induced second messengers at the onset of meiotic maturation in the amphibian oocyte: interrelationships between phospholipid N-methylation, calcium and diacylglycerol release, and inositol phospholipid turnover

The steady-state turnover in phospholipid N-methylation, 1,2-diacylglycerol and inositol phospholipids in prophase-arrested Rana pipiens oocytes was compared with changes occurring in these pathways immediately following progesterone induction of the first meiotic division. Oocytes were preincubated with [3H-methyl]methionine, [3H]glycerol, [3H]myo-inositol or [3H]arachidonic acid. Ca2+ efflux was measured in oocytes preloaded with 45Ca2+. Membrane phospholipids and cytosolic levels of radiolabeled 1,2-diacylglycerol (DAG), inositol bis- (InsP2), tris- (InsP3), and tetrakisphosphate (InsP4) were monitored immediately following induction with progesterone. A transient increase in both N-methylation of ethanolamine phospholipids and in [3H]DAG coincides with a release of 45Ca2+ from the oocyte surface during the first minute. At least 80% of the total phospholipid N-methylation is associated with the plasma membrane. 45Ca2+ and [3H]DAG release occur prior to a rise in intracellular InsP3, the latter beginning 2-3 min after exposure to the hormone and reaching a maximum by 15-30 min. Progesterone induces rapid and successive changes in ethanolamine, choline, and inositol-containing phospholipids, which represent three of the four major phospholipid classes found in membranes. The maintenance of higher levels of DAG and InsP3 during the first 90 min might be expected to sustain the previously observed increase in protein kinase C activity.

Progesterone induces transient changes in plasma membrane fluidity of amphibian oocytes during the first meiotic division

Progesterone acts at the surface of the amphibian oocyte to induce resumption of the meiotic divisions. Progesterone binding leads to a transient dose-dependent decrease in the fluidity (increase in order parameter) of the Rana oocyte plasma membrane, which was detected by electron spin resonance in isolated plasma membranes using either 5- or 16-DOXYL stearic acid probes. The 5-DOXYL probe, which inserts into the membrane with the spin label nearest the surface, showed an increase in the order parameter within minutes, a maximum change by 2 h, and a return to control levels by 6 h. The order parameter for the 16-DOXYL probe, which reflects the fluidity deeper within the plasma membrane, increased slowly and remained elevated during the first meiotic division. RU 38486, a synthetic steroid that blocks progesterone receptors, prevents progesterone-induced fluidity changes. These findings indicate that the binding of progesterone to its receptor changes the oocyte plasma membrane structure resulting in a differential decrease in mobility near the membrane surface compared to that deeper in the membrane.

Effect of defolliculation and 17 alpha-hydroxy, 20 beta-dihydroprogesterone on cyclic AMP level in full-grown oocytes of the rainbow trout, Salmo gairdneri

The changes in cAMP were followed in trout oocytes incubated in vitro after defolliculation performed by either enzymatic or manual dissection. Both defolliculation methods induced a highly significant rise in oocyte cAMP level (4.5 times the basal level of control [follicle-enclosed oocytes], after 6 h). Treatment of defolliculated oocytes with 17 alpha-hydroxy, 20 beta-dihydroprogesterone (17 alpha, 20 beta-OH-P) (10(-6) M), which induced oocyte maturation (germinal vesicle breakdown [GVBD]) was able, first, to interrupt the increase of oocyte cAMP level promoted by defolliculation and then to lower this level significantly down to values that still remained higher than folliculated controls. Very low concentrations of 17 alpha, 20 beta-OH-P (1.38-55.6 10(-9) M), or physiological doses of testosterone (0.35 10(-6) M, in the range found in vivo before ovulation) were able to induce a similar decrease of oocyte cAMP level without inducing GVBD. Under the same experimental conditions estradiol (0.35 10(-6) M) exhibited no action. These results suggest that some factor(s) originating in the follicle (FIF), inhibit the oocytes' tendency to accumulate cAMP before the final surge of 17 alpha, 20 beta-OH-P. This factor might be a follicular steroid such as testosterone or nonmaturing concentrations of 17 alpha, 20 beta-OH-P. Moreover our data favour the hypothesis that the final surge of 17 alpha, 20 beta-OH-P could induce distinct intraoocyte mechanisms: the first induces an irreversible blockage of cAMP level before the inhibitory action of the FIF is suppressed by ovulation, and the second mechanism leads to GVBD.

Calcium-dependent phosphorylation of the amphibian oocyte plasma membrane: an early event in initiating the meiotic divisions

Plasma membranes isolated from Rana oocytes showed a 7-10 fold increase in the Ca2+-dependent phosphorylation of endogenous protein following exposure to meiotic stimuli (progesterone, insulin) either in vivo or in-vitro. Exogenous phosphatidylmonomethylethanolamine (PME) was effective in stimulating Ca2+-dependent membrane phosphorylation and also induced meiosis. Induction of phosphorylation was blocked by the protease inhibitor leupeptin, as are all other responses to meiotic stimuli. Phosphatidylserine was inactive when added to intact oocytes, but stimulated membrane phosphorylation nearly 15-fold when added to isolated membranes. The results indicate a link between phospholipid methylation and protein kinase C activation.