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

The rapid activation of cPKCβII by progesterone results in the negative regulation of Ca 2+ influx in human resting T cells

BACKGROUND

Progesterone-stimulated rapid suppression of phytohemagglutinin (PHA)-activated sustained membrane Ca 2+ influx is revealed by Mn 2+ quenching fura-2 fluorescence. Ca 2+ influx suppression results in immunosuppression of T-cell proliferation. Downregulation of protein kinase C (PKC) activity by phorbol 12-myristate 13-acetate (PMA) enhances the PHA-activated increase in sustained intracellular Ca 2+ concentration ([Ca 2+ ] i ) via Ca 2+ influx in T cells. Conventional PKC (cPKC) inhibitors also enhance the [Ca 2+ ] i increase in resting T cells caused by progesterone. This study explores whether cPKC activation by progesterone results in suppression of Ca 2+ influx in resting T cells.

METHODS

Progesterone, its analogs (R5020/Org OD 02-0), and plasma membrane-impermeable progesterone-bovine serum albumin conjugate were used to stimulate human resting T cells. Inhibitors and PKC downregulation by PMA were used to investigate whether cPKC affects Ca 2+ influx.

RESULTS

Progesterone and analogs dose-dependently suppressed Ca 2+ influx in T cells. One cPKC inhibitor, Ro318220, attenuated Ca 2+ influx suppression, and enhanced the increase in [Ca 2+ ] i caused by progesterone and analogs. U73122 did not affect Ca 2+ influx suppression but did decrease the [Ca 2+ ] i increase. Ca 2+ influx suppression was not attenuated by the cPKCα/βI isoform-selective inhibitor, Go6976, nevertheless, a cPKCβI/βII isoform-selective inhibitor, LY333531 did. Ca 2+ influx suppression was attenuated by the cPKCβII-specific inhibitor CGP53353. After PKC downregulated by PMA, Ca 2+ influx suppression by progesterone and analogs was almost abolished in parallel with a massive reduction in cPKCβII expression. This suggests cPKCβII activation by progesterone and analogs mediate Ca 2+ influx suppression in resting T cells.

CONCLUSION

Nongenomic membrane activation of cPKCβII by progesterone causes immunosuppression via negative regulation of Ca 2+ influx into human resting T cells. This prevents resting T-cell activation and proliferation, which protects the fetus from maternal immune attack while decreasing maternal autoimmune disease flare-ups during pregnancy. Thus, cPKCβII modulators might provide a new therapeutic approach to balancing T-cell tolerance and immunity.

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.

Nongenomic steroid- and ceramide-induced maturation in amphibian oocytes involves functional caveolae-like microdomains associated with a cytoskeletal environment

Stimulation of full-grown amphibian oocytes with progesterone initiates a nontranscriptional signaling pathway that converges in the activation of Cdc2/cyclin B and reentry into meiosis. We observed that cholesterol depletion mediated by methyl-beta-cyclodextrin (MbetaCD) inhibited meiotic maturation, suggesting involvement of membrane rafts. In the present study, we further characterized caveolae-like membranes from Rhinella arenarum oocytes biochemically and functionally. The identification by mass spectrometry of a nonmuscle myosin heavy-chain associated with caveolar membranes showed evidence of direct involvement of the underlying cytoskeletal environment in the structure of oocyte rafts. Biophysical analysis using the fluorescent probe Laurdan revealed that MbetaCD-mediated cholesterol depletion affected membrane lipid order. In line with this finding, cholesterol removal also affected the localization of the raft marker lipid GM1. Results demonstrated that ceramide is an effective inducer of maturation that alters the distribution of the raft markers caveolin-1, SRC, and GM1, while progesterone seems not to affect membrane microdomain integrity. Cholesterol depletion had a greater effect on ceramide-induced maturation, thus suggesting that ceramide is an inducer more vulnerable to changes in the plasma membrane. MbetaCD treatment delayed tyrosine phosphorylation and MAPK activation in progesterone-induced maturation. Functional studies regarding tyrosine phosphorylation raise the possibility that the hormone receptor is located in the nonraft membrane in the absence of ligand and that it translocates to the caveola when it binds to progesterone. The presence of raft markers and the finding of signaling molecules from MAPK cascade functionally associated to oocyte light membranes suggest that this caveolae-rich fraction efficiently recreates, in part, maturation signaling.

Progesterone modulation of transmembrane helix-helix interactions between the alpha-subunit of Na/K-ATPase and phospholipid N-methyltransferase in the oocyte plasma membrane

BACKGROUND

Progesterone binding to the surface of the amphibian oocyte initiates the meiotic divisions. Our previous studies with Rana pipiens oocytes indicate that progesterone binds to a plasma membrane site within the external loop between the M1 and M2 helices of the alpha-subunit of Na/K-ATPase, triggering a cascade of lipid second messengers and the release of the block at meiotic prophase. We have characterized this site, using a low affinity ouabain binding isoform of the alpha1-subunit.

RESULTS

Preparations of isolated plasma membranes from Rana oocytes demonstrate that physiological levels of progesterone (or the non-metabolizable progestin R5020) successively activate phosphatidylethanolamine-N-methyltransferase (PE-NMT) and sphingomyelin synthase within seconds. Inhibition of PE-NMT blocks the progesterone induction of meiosis in intact oocytes, whereas its initial product, phosphatidylmonomethylethanolamine (PME), can itself initiate meiosis in the presence of the inhibitor. Published X-ray crystallographic data on Na/K-ATPase, computer-generated 3D projections, heptad repeat analysis and hydrophobic cluster analysis of the transmembrane helices predict that hydrophobic residues L, V, V, I, F and Y of helix M2 of the alpha1-subunit interact with F, L, G, L, L and F, respectively, of helix M3 of PE-NMT.

CONCLUSION

We propose that progesterone binding to the first external loop of the alpha1-subunit facilitates specific helix-helix interactions between integral membrane proteins to up-regulate PE-NMT, and, that successive interactions between two or more integral plasma membrane proteins induce the signaling cascades which result in completion of the meiotic divisions.

Progesterone binding to the alpha1-subunit of the Na/K-ATPase on the cell surface: insights from computational modeling

Progesterone triggers the resumption of meiosis in the amphibian oocyte through a signaling system at the plasma membrane. Analysis of [(3)H]ouabain and [(3)H]progesterone binding to the plasma membrane of the Rana pipiens oocyte indicates that progesterone competes with ouabain for a low affinity ouabain binding site on a 112kDa alpha1-subunit of the membrane Na/K-ATPase. Published amino acid sequences from both low and high affinity ouabain binding alpha1-subunits are compared, together with published site-directed mutagenesis studies of ouabain binding. We propose that the progesterone binding site is located in the external loop (23 amino acids) between the M1-M2 transmembrane helices. Analysis of loop topology and the countercurrent hydrophobicity/polarity gradients within the M1-M2 loop further suggest that the polar beta and hydrophobic alpha surfaces of the planar progesterone molecule interact with opposite sides of the amino acid loop. The 19-angular methyl group of progesterone is essential for activity; it could bind to the C-terminal region of the M1-M2 loop. Maximum biological activity requires formation of hydrogen-bond networks between the 3-keto group of progesterone and Arg(118), Asp(129) and possibly Glu(122-124) in the C-terminal region of the loop. The 20-keto group hydrogen may in turn hydrogen bond to Cys(111) near the M1 helix. Peptide flexibility undergoes a maximal transition near the midway point in the M1-M2 loop, suggesting that folding occurs within the loop, which further stabilizes progesterone binding.

The steroid-binding subunit of the Na/K-ATPase as a progesterone receptor on the amphibian oocyte plasma membrane

Progesterone acts at a plasma membrane receptor on the Rana oocyte to initiate meiosis. A cascade of lipid messengers occurs within seconds, followed by sequential changes in membrane phospholipid composition. We now show that progesterone binding to the plasma membrane increases continuously over the first 4 h. Subsequently, about 60% of the total plasma membrane and > 90% of membrane-bound progesterone, ouabain binding sites, and Na/K-ATPase activity are internalized. Until the completion of membrane internalization, oocytes must be continuously exposed to nanomolar concentrations of exogenous progesterone for meiosis to continue. The membrane-bound progesterone remains unchanged, whereas microinjected [(3)H]progesterone is rapidly metabolized. We find that progesterone and the plant steroid ouabain compete for one of two ouabain binding sites on the oocyte surface. Ouabain blocks progesterone action and inhibits subsequent meiosis if added at any time during the first 4-5 h. Western blots of SDS/PAGE extracts of isolated oocyte plasma membranes contain a -110 kDa band which binds an antibody to the steroid-binding c-terminal domain in rat and human PR. The number of binding sites and K(d) for progesterone binding to the plasma membrane is comparable to those for low-affinity ouabain binding to the alpha-subunit of the Na/K-ATPase (112 kDa). Our results suggest that progesterone binding to the ouabain binding site on the N-terminal region of the alpha-subunit of Na/K-ATPase may modulate early plasma membrane events over the first 4-6 h. Progesterone may thus act in part through the plasma membrane Na/K-ATPase signaling system.

Effect of oestrone on membrane fluidity of erythrocytes is mediated by a nitric oxide-dependent pathway: An electron paramagnetic resonance study

1. It has been recognized that hormone replacement therapy (HRT) may have a beneficial effect on protection against cardiovascular diseases. Oestrone is the major component of conjugated equiline oestrogens, which are commonly used in HRT. The present study was performed in order to investigate the effects of oestrone on the membrane fluidity of erythrocytes by means of an electron paramagnetic resonance (EPR) and spin-labelling method. 2. In an in vitro study, oestrone significantly decreased the order parameter (S) for 5-nitroxide stearate (5-NS) and the peak height ratio (ho/h-1) for 16-nitroxide stearate (16-NS) obtained from EPR spectra of erythrocyte membranes. This finding indicated that oestrone may increase the membrane fluidity and improve the membrane microviscosity of erythrocytes. 3. The effect of oestrone was significantly potentiated by the nitric oxide (NO) donor s-nitroso-N-acetylpenicillamine and the cGMP analogue 8-bromo-cGMP. 4. In contrast, the change in membrane fluidity induced by oestrone was antagonized by the NO synthase inhibitors NG-nitro-l-arginine methyl ester and asymmetric dimethyl-l-arginine. 5. The results of the present study show that oestrone significantly increases membrane fluidity and improves the rigidity of cell membranes, which is partially mediated by a NO- and cGMP-dependent pathway. Furthermore, the data may be consistent with the hypothesis that oestrone could have a beneficial effect on the rheological behaviour of erythrocytes and have a crucial role in the regulation of the microcirculation.

Interaction of albumin with the endothelial cell surface

Endothelial cells (EC) are covered with cell-borne proteoglycans and glycoproteins. Blood plasma proteins (e.g., albumin) adsorb to this glycocalyx forming a complex endothelial surface layer (ESL). We determined the molecular mobility of albumin by electron spin resonance (ESR) in the presence and absence of ECs to analyze interactions with the ESL. Albumin was spin labeled with 5- or 12-4,4-dimethyloxazolidine-N-oxyl (DOXYL)-stearic acid yielding information on the mobility of the molecular surface (5-DOXYL) or the entire protein (12-DOXYL). EC cultures grown on glass coverslips were immersed in labeled albumin and placed in the temperature-regulated cavity of an ESR spectrometer. Alternatively, ECs were labeled and then exposed to native albumin. At 37 degrees C, rotational correlation times determined by modified saturation transfer ESR (ST-ESR) were 26 and 48 ns for 5-DOXYL- and 12-DOXYL-labeled albumin, respectively. Presence of ECs increased rotational correlation time values for 5-DOXYL-stearic acid to 37 ns but not for 12-DOXYL-stearic acid. Albumin was able to completely take up the label from labeled EC within 2 min. The present study shows that modified ST-ESR can be used to determine the mobility of biological macromolecules interacting with cellular surfaces. Reduction in albumin surface mobility in the presence of EC at unchanged mobility of protein proper and fast removal of labeled fatty acids from EC membranes indicate rapid transient interactions between albumin surface and ESL but no rigid incorporation of albumin into a macromolecular network that would interfere with its transport function for poorly water-soluble substances.

The first product of phospholipid N-methylation, phosphatidylmonomethylethanolamine, is a lipid mediator for progesterone action at the amphibian oocyte plasma membrane

Progesterone has been shown to act at plasma membrane receptors on the amphibian oocyte to trigger a cascade of changes in membrane phospholipids and to initiate the G(2)/M transition of the first meiotic division. The earliest event (0-1 min) is the transient N-methylation of phosphatidylethanolamine (PE) to form phosphatidylmonomethylethanolamine (PME), demonstrated using [(3)H]glycerol to prelabel oocyte plasma membrane PE. [(3)H]Glycerol-labeled PME rises 10-fold within the 1-2 min after exposure to progesterone and accounts for conversion of about 50% of the [3H]Glycerol-labeled PE. [(3)H]PME levels slowly decline over the following 10-30 min. [(3)H] or [(14)C] labeled fatty acid experiments showed that newly formed PME is enriched in linoleic or palmitic, but not in arachidonic acid, indicating that specific PE pools undergo progesterone-induced N-methylation. Two plasma membrane changes: activation of serine protease, and Ca(2+) release from the oocyte surface coincide with PME formation; both are prevented by pretreatment of oocytes with the N-methylation inhibitor, 2-methylaminoethane. Media containing PME micelles release both protease and Ca(2+) from intact oocytes within the first 1-2 min. The immediate downstream metabolites of PME, PDE and PC, do not induce serine protease activity or Ca(2+) release. We conclude that progesterone initially activates N-methyltransferase in the oocyte plasma membrane, and that the first product, PME, is responsible for activation of serine protease in the plasma membrane and the release of Ca(2+) from the oocyte surface.

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.

Hormonal regulation of sodium/sulfate co-transport in renal epithelial cells

Serum sulfate concentrations are elevated in infants, young children, and pregnant women due, at least in part, to increased renal sulfate reabsorption. Little is known about the effects of hormones, particularly those involved in growth, development, and pregnancy, on renal sulfate reabsorption. The objective of this investigation was to examine the effects of growth hormone (GH), insulin-like growth factor 1 (IGF-1), progesterone (PG), and 17beta-estradiol (EST) on renal sodium/sulfate co-transport. 35S-sulfate uptake was determined in Madin-Darby canine kidney (MDCK)/NaSi-1 cells (MDCK cells that have been stably transfected with rat sodium/sulfate co-transporter (NaSi-1) cDNA) and in opossum kidney (OK) cells. NaSi-1 mRNA was determined by RT-PCR and protein levels by ELISA. GH (0.1 nM) significantly increased the sodium/sulfate co-transport in MDCK/NaSi-1 cells up to 35%. IGF-1 induced a concentration-related stimulation of the sodium/sulfate co-transport with a maximal response observed at 1000 nM (59% increase). Sodium-dependent sulfate uptake was significantly increased when cells were preincubated with 10 nM PG, 10 nM EST, or 10 nM PG/10 nM EST up to 41%, 46%, or 39%, respectively. OK cells exhibited endogenous sodium-dependent sulfate transport; significantly increased sodium/sulfate co-transport was also observed in OK cells that were preincubated with GH, IGF-1, and PG/EST, although not with EST alone. The NaSi-1 mRNA and NaSi-1 protein levels were significantly increased in MDCK/NaSi-1 cells treated with 0.1 nM GH, 100 nM IGF-1, 10 nM PG, and/or 10 nM EST compared with control. These results suggest that the increased renal sulfate reabsorption that occurs in neonates, young and pregnant humans, and animals could be mediated by the increased steady-state levels of NaSi-1 mRNA produced by the higher plasma concentrations of GH, IGF-1, or PG/EST.

Cyclic AMP binding to the amphibian oocyte plasma membrane: possible interrelationship between meiotic arrest and membrane fluidity

Cyclic AMP, which maintains the vertebrate oocyte in prophase arrest under physiological conditions, exhibits specific and saturable binding to the cytoplasmic face of the prophase-arrested Rana pipiens oocyte plasma membrane. Scatchard type analyses of [3H]cAMP binding to isolated plasma membranes indicate a single class of binding sites with a Kd = 19.3 +/- 7.0 nM at cAMP concentrations below 10(-6) M and additional low affinity site(s) and/or non-specific binding at concentrations above 10(-6) M. Photoaffinity labeling of prophase oocyte plasma membranes with [32P]-8-N3cAMP demonstrates cAMP/cGMP-displacable binding of 8-N3[32P]cAMP to a 100-110 kDa peptide doublet. Plasma membrane fluidity was monitored by electron spin resonance in isolated plasma-vitelline membranes using a 5-doxyl stearic acid probe. Exogenous dibutyryl cAMP (dbcAMP) produces an increase in membrane fluidity within minutes and blocks and/or reverses the progesterone-induced decrease in plasma membrane fluidity. The dbcAMP concentration that produced half-maximal fluidity increase (10 microM) corresponds to the half-maximal inhibiting dose of dbcAMP for progesterone induction of meiosis. Cholera toxin, which elevates intracellular cAMP and blocks meiosis, also increases membrane fluidity and inhibits progesterone-induced decrease in membrane fluidity. Elevated levels of intracellular cAMP thus appear to maintain meiotic arrest by binding to specific plasma membrane site(s) and maintaining the plasma membrane in a relatively fluid state. The progesterone-induced fall in intracellular cAMP first reported in Rana thus appears to be responsible for the progesterone-induced increase in membrane fluidity and further suggests that the change in membrane order is essential for the resumption of the meiotic divisions.

Steroid-induced perturbations of membranes and its relevance to sperm acrosome reaction

The interaction of progesterone, 17-alpha-hydroxyprogesterone, testosterone and estradiol with membrane vesicles prepared from phosphatidylserine (PS), from the total lipids of human and hamster spermatozoa, from the lipids of hamster spermatozoal plasma and acrosomal membrane and with the native membranes of hamster spermatozoa have been investigated by 90 degrees light scattering and fluorescence spectroscopy. The results indicate that progesterone decreases the fluidity of membranes, aggregates membrane vesicles, induces fusion of membrane vesicles and also renders them permeable to hydrophilic molecules like carboxyfluorescein. But, testosterone and estradiol at the same concentration had very little effect on membrane fluidity, membrane aggregation, fusion and leakage. The above membrane perturbing activities of the steroids is discussed in light of the recent findings that progesterone induces acrosome reaction in human and hamster spermatozoa [11,18].