Identification and characterization of functional nongenomic progesterone receptors on human sperm membrane

Vaginal progesterone for preventing preterm birth and adverse perinatal outcomes in singleton gestations with a short cervix: a meta-analysis of individual patient data

BACKGROUND

The efficacy of vaginal progesterone for preventing preterm birth and adverse perinatal outcomes in singleton gestations with a short cervix has been questioned after publication of the OPPTIMUM study.

OBJECTIVE

To determine whether vaginal progesterone prevents preterm birth and improves perinatal outcomes in asymptomatic women with a singleton gestation and a midtrimester sonographic short cervix.

STUDY DESIGN

We searched MEDLINE, EMBASE, LILACS, and CINAHL (from their inception to September 2017); Cochrane databases; bibliographies; and conference proceedings for randomized controlled trials comparing vaginal progesterone vs placebo/no treatment in women with a singleton gestation and a midtrimester sonographic cervical length ≤25 mm. This was a systematic review and meta-analysis of individual patient data. The primary outcome was preterm birth <33 weeks of gestation. Secondary outcomes included adverse perinatal outcomes and neurodevelopmental and health outcomes at 2 years of age. Individual patient data were analyzed using a 2-stage approach. Pooled relative risks with 95% confidence intervals were calculated. Quality of evidence was assessed using the GRADE methodology.

RESULTS

Data were available from 974 women (498 allocated to vaginal progesterone, 476 allocated to placebo) with a cervical length ≤25 mm participating in 5 high-quality trials. Vaginal progesterone was associated with a significant reduction in the risk of preterm birth <33 weeks of gestation (relative risk, 0.62; 95% confidence interval, 0.47-0.81; P = .0006; high-quality evidence). Moreover, vaginal progesterone significantly decreased the risk of preterm birth <36, <35, <34, <32, <30, and <28 weeks of gestation; spontaneous preterm birth <33 and <34 weeks of gestation; respiratory distress syndrome; composite neonatal morbidity and mortality; birthweight <1500 and <2500 g; and admission to the neonatal intensive care unit (relative risks from 0.47-0.82; high-quality evidence for all). There were 7 (1.4%) neonatal deaths in the vaginal progesterone group and 15 (3.2%) in the placebo group (relative risk, 0.44; 95% confidence interval, 0.18-1.07; P = .07; low-quality evidence). Maternal adverse events, congenital anomalies, and adverse neurodevelopmental and health outcomes at 2 years of age did not differ between groups.

CONCLUSION

Vaginal progesterone decreases the risk of preterm birth and improves perinatal outcomes in singleton gestations with a midtrimester sonographic short cervix, without any demonstrable deleterious effects on childhood neurodevelopment.

Influence of Dexamethasone on Some Reproductive Hormones and Uterine Progesterone Receptor Localization in Pregnant Yankasa Sheep in Semiarid Zones of Nigeria

Dexamethasone is widely used in both veterinary and human medical practices. However, it seems to cause some deleterious effects on pregnancy probably by causing changes in the reproductive hormone levels and their corresponding receptor concentrations. This study investigated the effects of dexamethasone on these parameters. Twenty healthy adult Yankasa sheep comprising 18 ewes and 2 rams were used for this study. Pregnancies were achieved by natural mating after estrus synchronization. Dexamethasone was administered at 0.25 mg/kg body weight on days 1, 3, and 5 during first trimester; days 51, 53, and 55 during second trimester; and days 101, 103, and 105 during the third trimester. Blood samples were collected biweekly for hormonal assay. Uterine biopsies were harvested through caesarean section for immunohistochemical analysis. Results showed that dexamethasone significantly (p < 0.05) decreased progesterone concentrations and caused abortion in Yankasa sheep but had no significant (p > 0.05) effect on estrogen, while progesterone receptors (PR) were upregulated. The abortion could probably be due to decreased progesterone concentrations as a consequence of the adverse effects on placenta. The PR upregulation may be a compensatory mechanism to increase progesterone sensitivity. It was concluded that dexamethasone should not be used in advanced pregnancy in Yankasa sheep.

Effects of dexamethasone on progesterone and estrogen profiles and uterine progesterone receptor localization during pregnancy in Sahel goat in Semi-Arid region

BACKGROUND

Despite the widespread use of dexamethasone in veterinary and human medicine, it is reported to cause some severe pregnancy related side effects like abortion in some animals. The mechanism of the response is not clear but seems to be related to interspecies and/or breed difference in response which may involve alterations in the concentrations of some reproductive hormones.

METHODS

Twenty Sahel goats comprising 18 does and 2 bucks were used for this study. Pregnancies were achieved by natural mating after synchronization. Repeated dexamethasone injections were given at 0.25 mg/kg body weight. Blood samples were collected biweekly for hormonal assay. Uterine biopsies were harvested at days 28 and day 78 of gestation through caesarean section for immunohistochemical analysis using 3 pregnant does randomly selected from each group at each instant. Data were expressed as Means ± Standard Deviations and analyzed using statistical soft ware package, GraphPad Instant, version 3.0 (2003) and progesterone receptor (PR) were scored semi-quantitatively.

RESULTS

Dexamethasone treatments had no significant (p > 0.05) effect on progesterone and estrogen concentrations in pregnant Sahel goats but up regulated PR from 2+ to 3+ in second trimester.

CONCLUSION

As dexamethasone adverse effect on placenta is an established fact, the lack of effect on progesterone level in this study may be due to the fact that unlike other species whose progesterone production during pregnancy is placenta - dependent, in goats is corpus luteum - dependent. Consequently dexamethasone adverse effect on placenta reported in literatures did not influence progesterone levels during pregnancy in Sahel goat. The up regulation of progesterone receptor (PR) in Sahel goat gravid uterus is a beneficial effects and that dexamethasone can safely be used in corpus luteum - dependent progesterone secreting pregnant animal species like Sahel goat and camel. Therefore source of progesterone secretions during pregnancy should be considered in clinical application of dexamethasone in pregnancy.

Expression of a mitochondrial progesterone receptor in human spermatozoa correlates with a progestin-dependent increase in mitochondrial membrane potential

The hyperactivation of human spermatozoa necessary for fertilization requires a substantial increase in cellular energy production. The factors responsible for increasing cellular energy remain poorly defined. This article proposes a role for a novel mitochondrial progesterone receptor (PR-M) in modulation of mitochondrial activity. Basic science studies demonstrate a 38 kDa protein with western blot analysis, consistent with PR-M; whereas imaging studies with confocal and immunoelectron microscopy demonstrate a PR on the mitochondria. Treatment with a PR-specific progestin shows increased mitochondrial membrane potential, not related to induction of an acrosome reaction. The increase in mitochondrial membrane potential was inhibited by a specific PR antagonist, but not affected by an inhibitor to the progesterone-dependent Catsper voltage-activated channel. In conclusion, these studies suggest expression of a novel mitochondrial PR in human spermatozoa with a progestin-dependent increase in mitochondrial activity. This mechanism may serve to enhance cellular energy production as the spermatozoa traverse the female genital tract being exposed to increasing concentrations of progesterone.

2-APB-potentiated channels amplify CatSper-induced Ca(2+) signals in human sperm

Ca²⁺_i signalling is pivotal to sperm function. Progesterone, the best-characterized agonist of human sperm Ca²⁺_i signalling, stimulates a biphasic [Ca²⁺]_i rise, comprising a transient and subsequent sustained phase. In accordance with recent reports that progesterone directly activates CatSper, the [Ca²⁺]_i transient was detectable in the anterior flagellum (where CatSper is expressed) 1–2 s before responses in the head and neck. Pre-treatment with 5 μM 2-APB (2-aminoethoxydiphenyl borate), which enhances activity of store-operated channel proteins (Orai) by facilitating interaction with their activator [STIM (stromal interaction molecule)] ‘amplified’ progesterone-induced [Ca²⁺]_i transients at the sperm neck/midpiece without modifying kinetics. The flagellar [Ca²⁺]_i response was unchanged. 2-APB (5 μM) also enhanced the sustained response in the midpiece, possibly reflecting mitochondrial Ca²⁺ accumulation downstream of the potentiated [Ca²⁺]_i transient. Pre-treatment with 50–100 μM 2-APB failed to potentiate the transient and suppressed sustained [Ca²⁺]_i elevation. When applied during the [Ca²⁺]_i plateau, 50–100 μM 2-APB caused a transient fall in [Ca²⁺]_i, which then recovered despite the continued presence of 2-APB. Loperamide (a chemically different store-operated channel agonist) enhanced the progesterone-induced [Ca²⁺]_i signal and potentiated progesterone-induced hyperactivated motility. Neither 2-APB nor loperamide raised pH_i (which would activate CatSper) and both compounds inhibited CatSper currents. STIM and Orai were detected and localized primarily to the neck/midpiece and acrosome where Ca²⁺ stores are present and the effects of 2-APB are focussed, but store-operated currents could not be detected in human sperm. We propose that 2-APB-sensitive channels amplify [Ca²⁺]_i elevation induced by progesterone (and other CatSper agonists), amplifying, propagating and providing spatio-temporal complexity in [Ca²⁺]_i signals of human sperm.

Rapid steroid hormone actions initiated at the cell surface and the receptors that mediate them with an emphasis on recent progress in fish models

In addition to the classic genomic mechanism of steroid action mediated by activation of intracellular nuclear receptors, there is now extensive evidence that steroids also activate receptors on the cell surface to initiate rapid intracellular signaling and biological responses that are often nongenomic. Recent progress in our understanding of rapid, cell surface-initiated actions of estrogens, progestins, androgens and corticosteroids and the identities of the membrane receptors that act as their intermediaries is briefly reviewed with a special emphasis on studies in teleost fish. Two recently discovered novel proteins with seven-transmembrane domains, G protein-coupled receptor 30 (GPR30), and membrane progestin receptors (mPRs) have the ligand binding and signaling characteristics of estrogen and progestin membrane receptors, respectively, but their functional significance is disputed by some researchers. GPR30 is expressed on the cell surface of fish oocytes and mediates estrogen inhibition of oocyte maturation. mPRα is also expressed on the oocyte cell surface and is the intermediary in progestin induction of oocyte maturation in fish. Recent results suggest there is cross-talk between these two hormonal pathways and that there is reciprocal down-regulation of GPR30 and mPRα expression by estrogens and progestins at different phases of oocyte development to regulate the onset of oocyte maturation. There is also evidence in fish that mPRs are involved in progestin induction of sperm hypermotility and anti-apoptotic actions in ovarian follicle cells. Nonclassical androgen and corticosteroid actions have also been described in fish models but the membrane receptors mediating these actions have not been identified.

Vaginal progesterone in women with an asymptomatic sonographic short cervix in the midtrimester decreases preterm delivery and neonatal morbidity: a systematic review and metaanalysis of individual patient data

OBJECTIVE

To determine whether the use of vaginal progesterone in asymptomatic women with a sonographic short cervix (≤ 25 mm) in the midtrimester reduces the risk of preterm birth and improves neonatal morbidity and mortality.

STUDY DESIGN

Individual patient data metaanalysis of randomized controlled trials.

RESULTS

Five trials of high quality were included with a total of 775 women and 827 infants. Treatment with vaginal progesterone was associated with a significant reduction in the rate of preterm birth <33 weeks (relative risk [RR], 0.58; 95% confidence interval [CI], 0.42-0.80), <35 weeks (RR, 0.69; 95% CI, 0.55-0.88), and <28 weeks (RR, 0.50; 95% CI, 0.30-0.81); respiratory distress syndrome (RR, 0.48; 95% CI, 0.30-0.76); composite neonatal morbidity and mortality (RR, 0.57; 95% CI, 0.40-0.81); birthweight <1500 g (RR, 0.55; 95% CI, 0.38-0.80); admission to neonatal intensive care unit (RR, 0.75; 95% CI, 0.59-0.94); and requirement for mechanical ventilation (RR, 0.66; 95% CI, 0.44-0.98). There were no significant differences between the vaginal progesterone and placebo groups in the rate of adverse maternal events or congenital anomalies.

CONCLUSION

Vaginal progesterone administration to asymptomatic women with a sonographic short cervix reduces the risk of preterm birth and neonatal morbidity and mortality.

In vitro assessment of some sperm function following exposure to levonorgestrel in human fallopian tubes

BACKGROUND

The mechanism of action of levonorgestrel (LNG) as emergency contraception (EC) remains a subject of debate and its effect on sperm function has been only partially explained. The aim of this study was to assess whether LNG at a similar dose to those found in serum following oral intake for EC could affect spermatozoa when exposed to human fallopian tubes in vitro.

METHODS

Fifteen mini-laparotomies were performed, the side on which ovulation occurred was recorded, and both tubes were removed and perfused with a suspension containing 1 × 10(6) motile spermatozoa, with or without LNG. Following 4-hour incubation, the tubes were sectioned to separate the isthmus and the ampulla. Each segment was flushed and the material was evaluated to quantify the number of motile sperm, the number of spermatozoa adhering to the oviductal epithelium and the acrosome reaction (AR) rate.

RESULTS

The addition of LNG did not significantly alter the number of recovered motile spermatozoa either at the isthmus or at the ampulla, nor did it have any effect on the number of recovered spermatozoa adhered to the human tubal epithelium. Furthermore, LNG did not affect the AR rate. No significant differences were found even when the side on which ovulation occurred was taken into account.

CONCLUSIONS

In a similar dose to that observed in serum following oral intake for EC, LNG had no effect on the number of motile spermatozoa recovered from the human fallopian tubes in vitro, on their adhesion to the tubal epithelium, distribution or AR rate. The possible effect of LNG as EC on sperm function remains poorly understood.

Depletion of calcium stores contributes to progesterone-induced attenuation of calcium signaling of G protein-coupled receptors

Progesterone non-genomically attenuates the calcium signaling of the human oxytocin receptor and several other Galpha(q) protein-coupled receptors. High progesterone concentrations are found in the endometrium during pregnancy opposing the responsiveness of the underlying myometrium to labor-inducing hormones. Here, we demonstrate that within minutes, progesterone inhibits oxytocin- and bradykinin-induced contractions of rat uteri, calcium responses induced by platelet-activating factor in the human endometrial cell line MFE-280, and oxytocin-induced calcium signals in PHM1-31 immortalized pregnant human myometrial cells. Using human embryonic kidney (HEK293) cells as model system, we analyzed the molecular mechanisms underlying these effects. Our data indicate that progesterone rapidly depletes intracellular calcium stores. The resulting desensitization of the cells might contribute to the quiescence of the uterus during pregnancy.

Conserved estrogen binding and signaling functions of the G protein-coupled estrogen receptor 1 (GPER) in mammals and fish

Recent studies by several research groups have shown that G protein estrogen receptor-1 (GPER) formerly known as GPR30, mediates 17beta-estradiol (E2) activation of signal transduction pathways in a variety of human cancer cells and displays E2 binding typical of a membrane estrogen receptor. However, the importance of GPER as an estrogen receptor has been questioned by Otto and co-workers. Some of the pitfalls in investigating the functions of recombinant steroid membrane receptors that may explain the negative results of these investigators are discussed. The characteristics of GPER have also been investigated in a teleost fish, Atlantic croaker, where it has been shown to mediate E2 inhibition of oocyte maturation. Investigations on newly discovered homologous proteins from distantly related vertebrate groups are valuable for determining their fundamental, evolutionarily conserved functions. Therefore, the functions of croaker and human GPERs were compared. The comparisons show that croaker and human GPER have very similar estrogen binding characteristics, typical of estrogen membrane receptors, and activate the same estrogen signaling pathways via stimulatory G proteins (Gs) resulting in increased cAMP production. These results suggest that the estrogen binding and estrogen signaling functions of GPER arose early in vertebrate evolution, prior to the divergence of the teleosts from the tetrapods, more than 200 million years ago. The finding that estrogen membrane signaling through GPER has been conserved for such a long period in two distantly related vertebrate groups, mammals and fish, suggests that this is a fundamental function of GPER in vertebrates, and likely its major physiological role.

Progesterone attenuates oestrogen neuroprotection via downregulation of oestrogen receptor expression in cultured neurones

Recent findings indicate that progesterone can attenuate the beneficial neural effects of oestrogen. In the present study, we investigated the hypothesis that progesterone can modulate oestrogen actions by regulating the expression and activity of oestrogen receptors, ERalpha and ERbeta. Our studies in cultured neurones demonstrate that progesterone decreases the expression of both ERalpha and ERbeta and, as a consequence, also reduces both ER-dependent transcriptional activity and neuroprotection. These results identify a potential mechanism by which progesterone antagonises neural oestrogen actions, a finding that may have important implications for hormone therapy in postmenopausal women.

Effects of progesterone administration on infarct volume and functional deficits following permanent focal cerebral ischemia in rats

Recent experimental evidence indicates that progesterone (PROG) protects against various models of brain injury, including ischemic stroke. Most human studies of pharmacologic treatments for acute cerebral stroke have failed despite initial success in animal models. To simulate better the typical human stroke without reperfusion, the present study was conducted to examine the efficacy of PROG on infarct volume and functional outcome in a permanent model of stroke, using direct cauterization of the middle cerebral artery (MCA). Twenty-four male adult Sprague-Dawley rats underwent pMCAO by electro-coagulation and sham operation. After induction of permanent MCA occlusion (pMCAO), the rats received an initial intraperitoneal injection of PROG (8 mg/kg) or vehicle at 1 h post-occlusion followed by subcutaneous injections at 6, 24 and 48 h. Functional deficits were tested on the rotarod and grip-strength meter at 24, 48 and 72 h after pMCAO. The rats were killed 72 h after surgery and isolated brain was sectioned into coronal slices and stained with 2, 3, 5-triphenyltetrazolium chloride (TTC). PROG-treated rats showed a substantial reduction (54.05%) in the volume of the infarct (% contralateral hemisphere) compared to vehicle controls. In addition there was a significant improvement in ability to remain on an accelerating rotarod and increased grip strength observed in the pMCAO rats treated with PROG compared to vehicle. Taken together, these data indicate that PROG is beneficial in one of the best-characterized models of stroke, and may warrant further testing in future clinical trials for human stroke.

Nonclassical mechanisms of progesterone action in the brain: II. Role of calmodulin-dependent protein kinase II in progesterone-mediated signaling in the hypothalamus of female rats

In addition to the activation of classical progestin receptor-dependent genomic pathway, progesterone (P) can activate nonclassical, membrane-initiated signaling pathways in the brain. We recently demonstrated rapid P activation of second-messenger kinases, protein kinase A, and protein kinase C in the ventromedial nucleus (VMN) and preoptic area (POA) of rat brain. To determine whether P can activate yet another Ca+2 dependent kinase, we examined the rapid P modulation of calcium and calmodulin-dependent protein kinase II (CaMKII) in the VMN and POA in female rats. A rapid P-initiated activation of CaMKII basal activity was observed in the VMN but not the POA at 30 min. Estradiol benzoate (EB) priming enhanced this CaMKII basal activity in both the VMN and POA. CaMKII protein levels and phosphorylation of Thr-286 moiety on CaMKII, however, remained unchanged with EB and/or P treatments, suggesting that the changes in the CaMKII kinase activity are due to rapid P modulation of the kinase activity and not its synthesis or autoactivation. Furthermore, intracerebroventricular (icv) administration of a CaMKII-specific inhibitor, KN-93, 30 min prior to the P infusion, in EB-primed, ovariectomized female rats inhibited CaMKII activation but not protein kinase A and protein kinase C activities. Interestingly, icv administration of KN-93 30 min prior to P infusion (icv) resulted in a reduction but not total inhibition of P-facilitated lordosis response in EB-primed female rats. These observations suggest a redundancy or, alternately, a hierarchy in the P-regulated activation of kinase signaling cascades in female reproductive behavior.

Progesterone from the cumulus cells is the sperm chemoattractant secreted by the rabbit oocyte cumulus complex

Sperm chemotaxis in mammals have been identified towards several female sources as follicular fluid (FF), oviduct fluid, and conditioned medium from the cumulus oophorus (CU) and the oocyte (O). Though several substances were confirmed as sperm chemoattractant, Progesterone (P) seems to be the best chemoattractant candidate, because: 1) spermatozoa express a cell surface P receptor, 2) capacitated spermatozoa are chemotactically attracted in vitro by gradients of low quantities of P; 3) the CU cells produce and secrete P after ovulation; 4) a gradient of P may be kept stable along the CU; and 5) the most probable site for sperm chemotaxis in vivo could be near and/or inside the CU. The aim of this study was to verify whether P is the sperm chemoattractant secreted by the rabbit oocyte-cumulus complex (OCC) in the rabbit, as a mammalian animal model. By means of videomicroscopy and computer image analysis we observed that only the CU are a stable source of sperm attractants. The CU produce and secrete P since the hormone was localized inside these cells by immunocytochemistry and in the conditioned medium by enzyme immunoassay. In addition, rabbit spermatozoa express a cell surface P receptor detected by western blot and localized over the acrosomal region by immunocytochemistry. To confirm that P is the sperm chemoattractant secreted by the CU, the sperm chemotactic response towards the OCC conditioned medium was inhibited by three different approaches: P from the OCC conditioned medium was removed with an anti-P antibody, the attractant gradient of the OCC conditioned medium was disrupted by a P counter gradient, and the sperm P receptor was blocked with a specific antibody. We concluded that only the CU but not the oocyte secretes P, and the latter chemoattract spermatozoa by means of a cell surface receptor. Our findings may be of interest in assisted reproduction procedures in humans, animals of economic importance and endangered species.

Characteristics of membrane progestin receptor alpha (mPRalpha) and progesterone membrane receptor component 1 (PGMRC1) and their roles in mediating rapid progestin actions

Rapid, progestin actions initiated at the cell surface that are often nongenomic have been described in a variety of reproductive tissues, but until recently the identities of the membrane receptors mediating these nonclassical progestins actions remained unclear. Evidence has been obtained in the last 4-5 years for the involvement of two types of novel membrane proteins unrelated to nuclear steroid receptors, progesterone membrane receptors (mPRs) and progesterone receptor membrane component 1 (PGMRC1), in progestin signaling in several vertebrate reproductive tissues and in the brain. The mPRs, (M(W) approximately 40 kDa) initially discovered in fish ovaries, comprise at least three subtypes, alpha, beta and gamma and belong to the seven-transmembrane progesterone adiponectin Q receptor (PAQR) family. Both recombinant and wildtype mPRs display high affinity (K(d) approximately 5 nM), limited capacity, displaceable and specific progesterone binding. The mPRs are directly coupled to G proteins and typically activate pertussis-sensitive inhibitory G proteins (G(i)), to down-regulate adenylyl cyclase activity. Recent studies suggest the alpha subtype (mPRalpha) has important physiological functions in variety of reproductive tissues. The mPRalpha is an intermediary in progestin induction of oocyte maturation and stimulation of sperm hypermotility in fish. In mammals, the mPRalphas have been implicated in progesterone regulation of uterine function in humans and GnRH secretion in rodents. The single-transmembrane protein PGMRC1 (M(W) 26-28 kDa) was first purified from porcine livers and its cDNA was subsequently cloned from porcine smooth muscle cells and a variety of other tissues by different investigators. PGMRC1 and the closely-related PGMRC2 belong to the membrane-associated progesterone receptor (MAPR) family. The PGMRC1 protein displays moderately high binding affinity for progesterone which is 2- to 10-fold greater than that for testosterone and glucocorticoids, and also can bind to other molecules such as heme, cholesterol metabolites and proteins. The signal transduction pathways induced by binding of progesterone to PGMRC1 have not been described to date, although motifs for tyrosine kinase, kinase binding, SH2 and SH3 have been predicted from the amino acid sequence. Evidence has been obtained that PGMRC1 mediates the antiapoptotic affects of progesterone in rat granulosa cells. The PGMRC1 protein may also be an intermediary in the progesterone induction of the acrosome reaction in mammalian sperm. Despite these recent advances, many aspects of progestin signaling through these two families of novel membrane proteins remain unresolved. Biochemical characterization of the receptors has been hampered by rapid degradation of the partially purified proteins. A major technical challenge has been to express sufficient amounts of the recombinant receptors on the plasma membranes in eukaryotic systems to permit investigations of their progestin binding and signal transduction characteristics. Additional basic information on the molecular and cellular mechanisms by which mPRs and PGMRC1 interact with progestins, signal transductions pathways and other proteins will be required to establish a comprehensive model of nontraditional progestin actions mediated through these novel proteins.

Nongenomic steroid action: controversies, questions, and answers

Steroids may exert their action in living cells by several ways: 1). the well-known genomic pathway, involving hormone binding to cytosolic (classic) receptors and subsequent modulation of gene expression followed by protein synthesis. 2). Alternatively, pathways are operating that do not act on the genome, therefore indicating nongenomic action. Although it is comparatively easy to confirm the nongenomic nature of a particular phenomenon observed, e.g., by using inhibitors of transcription or translation, considerable controversy exists about the identity of receptors that mediate these responses. Many different approaches have been employed to answer this question, including pharmacology, knock-out animals, and numerous biochemical studies. Evidence is presented for and against both the participation of classic receptors, or proteins closely related to them, as well as for the involvement of yet poorly understood, novel membrane steroid receptors. In addition, clinical implications for a wide array of nongenomic steroid actions are outlined.

Encoding of progesterone stimulus intensity by intracellular [Ca2+] ([Ca2+]i) in human spermatozoa

Progesterone induces a biphasic Ca(2+) influx and consequent acrosome reaction in human spermatozoa. We have used two procedures to vary the stimulus (dosage and prior receptor desensitization) to investigate the encoding of stimulus strength by intracellular [Ca(2+)] ([Ca(2+)](i)). Acrosome reaction and amplitude (but not kinetics) of the transient [Ca(2+)](i) response (population measurement) showed sigmoidal dose sensitivity over the range 0.3 nM-3 microM, saturating at approximately 300 nM (ED(50) approximately 30 nM). The amplitude of the sustained response saturated at 3 microM. Single-cell imaging showed that the amplitudes of both transient and sustained [Ca(2+)](i) responses were highly dose-dependent, but that their frequency of occurrence and kinetics were largely dose-independent. Fluorimetric measurements confirmed that progesterone-induced [Ca(2+)](i) influx was subject to desensitization, with second and subsequent applications of 3 microM progesterone being ineffective. However, sequential additions of 3 nM, 30 nM and 3 microM progesterone generated transient [Ca(2+)](i) responses at each concentration, the amplitude and duration of the response to 3 microM progesterone being reduced compared with non-pretreated cells. Single-cell imaging revealed that pretreatment had no effect on the proportion of responsive cells, but single-cell responses, similarly to population responses, were smaller and markedly reduced in duration, consistent with an effect of desensitization on a late component of the [Ca(2+)](i) transient. We conclude that the strength of the progesterone stimulus, when varied by dosage or by desensitization, is encoded by an analogue [Ca(2+)](i) signal. Dose dependency of the acrosome reaction is therefore determined not by the number of progesterone-responsive cells but by variation in the probability of exocytosis in a 'constant' responsive population.

Chemical modification and structural analysis of the progesterone membrane binding protein from porcine liver membranes

In addition to the classical genomic steroid actions on modulation of transcription and protein synthesis, rapid, nongenomic effects have been described for various steroids. These effects on cellular signaling and function are supposed to be transmitted by membrane binding sites unrelated to the classical intracellular receptors. Recently, a high affinity progesterone membrane binding protein (mPR) has been characterized in porcine liver membranes. In the present study, amino acid residues that are essential for progesterone binding to porcine liver microsomal mPR have been identified by the use of protein modifying reagents. Among all reagents tested, agents with specificity for carboxyl groups, methionine and tryptophan such as N,N'-dicyclohexylcarbodiimide, chloramine T and N-bromosuccinimide induced a reduction in [3H]progesterone binding. To evaluate the presence of essential disulfide bridges, porcine liver microsomes were incubated with the disulfide reducing agent dithiothreitol (DTT) and [3H]progesterone binding was measured. This treatment also resulted in a reduction of binding activity with an IC50 of 20 mM for DTT. Western-blotting analysis in the presence or absence of the reducing agent suggested that mPR--in its binding state--consists of at least two identical subunits with an apparent molecular mass of 28 kDa which are linked by a disulfide bridge. In conclusion, in the present study evidence for an involvement of carboxyl-, tryptophan- and methionine residues in [3H]progesterone binding to porcine liver microsomes is given. In addition, it is shown that mPR can form disulfide-linked homodimers.