Diradylglycerols stimulate phospholipase A2 and subsequent exocytosis in ram spermatozoa. Evidence that the effect is not mediated via protein kinase C

Involvement of Protein cAMP-dependent Kinase, Phospholipase A2 and Phospholipase C in Sperm Acrosome Reaction of Chinchilla lanigera

The mechanisms involved in fertilization are the centre of attention in order to determine the conditions required to reproduce in vitro the events that take place in vivo, with special interest in endangered species. Previous data from mouse sperm, where acrosome reaction (AR) occurs more often in the interstitium of the cumulus oophorus, contribute to strengthen the use of progesterone as a physiological inducer of this process. We studied the participation of protein kinase A (PKA), phospholipases A2 and C (PLA2 , PLC) in the AR induced by progesterone from Chinchilla epididymal spermatozoa. The addition of db-cAMP to the incubation medium caused an increase of 58% in the AR, while the use of H89 (30 μm), a PKA inhibitor, reflected a decrease of 40% in the percentage of reacted gametes. The assays conducted with arachidonic acid showed a maximum increase of 23% in the AR. When gametes were pre-incubated with PLA2 inhibitors, a dose-dependent inhibitory effect was observed. The addition of phorbol12-myristate13-acetate (10 μm) revealed higher percentages of AR induction (60%). When PLC was inhibited with neomycin and U73122, a dose-dependent decrease in AR percentages was observed. Combined inhibition of PKA, PLA2 and PLC, AR values similar to control were obtained. This work shows evidence, for the first time in Chinchilla, that progesterone activates the AC/cAMP/PKA system as well as sperm phospholipases and that these signalling pathways participate jointly and cooperatively in AR. These results contribute to the understanding of the complex regulation that is triggered in sperm after the effect of progesterone.

MARCKS protein is phosphorylated and regulates calcium mobilization during human acrosomal exocytosis

Acrosomal exocytosis is a calcium-regulated exocytosis that can be triggered by PKC activators. The involvement of PKC in acrosomal exocytosis has not been fully elucidated, and it is unknown if MARCKS, the major substrate for PKC, participates in this exocytosis. Here, we report that MARCKS is expressed in human spermatozoa and localizes to the sperm head and the tail. Calcium- and phorbol ester-triggered acrosomal exocytosis in permeabilized sperm was abrogated by different anti-MARCKS antibodies raised against two different domains, indicating that the protein participates in acrosomal exocytosis. Interestingly, an anti-phosphorylated MARCKS antibody was not able to inhibit secretion. Similar results were obtained using recombinant proteins and phospho-mutants of MARCKS effector domain (ED), indicating that phosphorylation regulates MARCKS function in acrosomal exocytosis. It is known that unphosphorylated MARCKS sequesters PIP₂. This phospholipid is the precursor for IP₃, which in turn triggers release of calcium from the acrosome during acrosomal exocytosis. We found that PIP₂ and adenophostin, a potent IP₃-receptor agonist, rescued MARCKS inhibition in permeabilized sperm, suggesting that MARCKS inhibits acrosomal exocytosis by sequestering PIP₂ and, indirectly, MARCKS regulates the intracellular calcium mobilization. In non-permeabilized sperm, a permeable peptide of MARCKS ED also inhibited acrosomal exocytosis when stimulated by a natural agonist such as progesterone, and pharmacological inducers such as calcium ionophore and phorbol ester. The preincubation of human sperm with the permeable MARCKS ED abolished the increase in calcium levels caused by progesterone, demonstrating that MARCKS regulates calcium mobilization. In addition, the phosphorylation of MARCKS increased during acrosomal exocytosis stimulated by the same activators. Altogether, these results show that MARCKS is a negative modulator of the acrosomal exocytosis, probably by sequestering PIP₂, and that it is phosphorylated during acrosomal exocytosis.

Diacylglycerol stimulates acrosomal exocytosis by feeding into a PKC- and PLD1-dependent positive loop that continuously supplies phosphatidylinositol 4,5-bisphosphate

Acrosomal exocytosis involves a massive fusion between the outer acrosomal and the plasma membranes of the spermatozoon triggered by stimuli that open calcium channels at the plasma membrane. Diacylglycerol has been implicated in the activation of these calcium channels. Here we report that this lipid promotes the efflux of intraacrosomal calcium and triggers exocytosis in permeabilized human sperm, implying that diacylglycerol activates events downstream of the opening of plasma membrane channels. Furthermore, we show that calcium and diacylglycerol converge in a signaling pathway leading to the production of phosphatidylinositol 4,5-bisphosphate (PIP(2)). Addition of diacylglycerol promotes the PKC-dependent activation of PLD1. Rescue experiments adding phosphatidic acid or PIP(2) and direct measurement of lipid production suggest that both PKC and PLD1 promote PIP(2) synthesis. Inhibition of different steps of the pathway was reverted by adenophostin, an agonist of IP(3)-sensitive calcium channels, indicating that PIP(2) is necessary to keep these channels opened. However, phosphatidic acid, PIP(2), or adenophostin could not trigger exocytosis by themselves, indicating that diacylglycerol must also activate another factor. We found that diacylglycerol and phorbol ester stimulate the accumulation of the GTP-bound form of Rab3A. Together our results indicate that diacylglycerol promotes acrosomal exocytosis by i) maintaining high levels of IP(3) - an effect that depends on a positive feedback loop leading to the production of PIP(2) - and ii) stimulating the activation of Rab3A, which in turn initiates a cascade of protein interactions leading to the assembly of SNARE complexes and membrane fusion.

Diacylglycerols, multivalent membrane modulators

Diacylglycerols are second messengers confined to biomembranes and, although relatively simple molecules from the structural point of view, they are able of triggering a surprisingly wide range of biological responses. Diacylglycerols are recognized by a well conserved protein motif, such as the C1 domain. This domain was observed for the first time in protein kinases C but is now known to be present in many other proteins. The effect of diacylglycerols is not limited to binding to C1 domains and they are able to alter the biophysical properties of biomembranes and hence modulate the activity of membrane associated proteins and also facilitate some processes like membrane fusion.

GABA, progesterone and zona pellucida activation of PLA2 and regulation by MEK-ERK1/2 during acrosomal exocytosis in guinea pig spermatozoa

We investigated whether GABA activates phospholipase A2 (PLA2) during acrosomal exocytosis, and if the MEK-ERK1/2 pathway modulates PLA2 activation initiated by GABA, progesterone or zona pellucida (ZP). In guinea pig spermatozoa prelabelled with [14C]arachidonic acid or [14C]choline chloride, GABA stimulated a decrease in phosphatidylcholine (PC), and release of arachidonic acid and lysoPC, during exocytosis. These lipid changes are indicative of PLA2 activation and appear essential for exocytosis since inclusion of aristolochic acid (a PLA2 inhibitor) abrogated them, along with exocytosis. GABA activation of PLA2 seems to be mediated, at least in part, by diacylglycerol (DAG) and protein kinase C since inclusion of the DAG kinase inhibitor R59022 enhanced PLA2 activity and exocytosis stimulated by GABA, whereas exposure to staurosporine decreased both. GABA-, progesterone- and ZP-induced release of arachidonic acid and exocytosis were prevented by U0126 and PD98059 (MEK inhibitors). Taken together, our results suggest that PLA2 plays a fundamental role in agonist-stimulated exocytosis and that MEK-ERK1/2 are involved in PLA2 regulation during this process.

Progesterone primes zona pellucida-induced activation of phospholipase A2 during acrosomal exocytosis in guinea pig spermatozoa

We investigated, using guinea-pig spermatozoa as a model, whether phospholipase A2 (PLA2) is involved in progesterone or zona pellucida (ZP)-stimulated acrosomal exocytosis, if progesterone enhances ZP-induced activation of PLA2, and mechanisms underlying PLA2 regulation. Spermatozoa were capacitated and labeled in low Ca2+ medium with [14C]choline chloride or [14C]arachidonic acid, washed, and then exposed to millimolar Ca2+ and progesterone and/or ZP. Each agonist stimulated decrease of phosphatidylcholine (PC) and release of arachidonic acid and lysoPC, indicative of PLA2 activation. Aristolochic acid (a PLA2 inhibitor) abrogated lipid changes and exocytosis, indicating that these lipid changes are essential for exocytosis. Exposure of spermatozoa to submaximal concentrations of both progesterone and ZP resulted in a synergistic increase of arachidonic acid and lysoPC releases, and exocytosis, suggesting that, under natural conditions, both agonists interact to bring about acrosomal exocytosis. Progesterone-induced PLA2 activation appears to be mediated by a GABA(A)-like receptor, because bicuculline (a GABA(A) receptor antagonist) blocked arachidonic acid release and exocytosis. In agreement with this, GABA mimicked progesterone actions. ZP-induced activation of PLA2 seemed to be transduced via G(i) proteins because pertussis toxin blocked arachidonic acid release and acrosomal exocytosis. PLA2 may be regulated by PKC because progesterone- or ZP-induced release of arachidonic acid was blocked by the PKC inhibitors staurosporine or chelerythrine chloride. PLA2 could also be regulated by the cAMP-PKA pathway; inclusion of the PKA inhibitor 14-22 amide or H-89 led to a reduction in arachidonic acid release or exocytosis after progesterone or ZP. Taken together, these results suggest that PLA2 plays an essential role in progesterone or ZP-stimulated exocytosis with progesterone priming ZP action.

Zona pellucida binding ability and responsiveness to ionophore challenge of cryopreserved dog spermatozoa after different periods of capacitation in vitro

The present study was aimed to evaluate the functional status of cryopreserved dog spermatozoa after different periods (2, 8 and 24 h) of capacitation in vitro. Sperm motility, viability and binding capacity to the zona pellucida of canine oocytes derived from frozen-thawed ovaries were evaluated at each time point. Sperm viability was assessed by flow cytometry using the Ca(2+)-sensitive indicator Fluo 3 AM and PI, to simultaneously detect the proportion of live spermatozoa and the existence of live sperm subpopulations with different intracellular Ca(2+) content. In addition, the acrosome reaction frequency in ionophore-treated aliquots of spermatozoa incubated in capacitating (CCM) versus non-capacitating (NCM) medium, were evaluated by using eosin-nigrosin staining at the same time intervals. The number of spermatozoa bound to the zona pellucida decreased in about 50% (from 18.61 +/- 14.40 to 7.7 +/- 6.97) when sperm incubation was prolonged from 2 to 8h, however, sperm motility, viability and the subpopulation of live spermatozoa with higher intracellular Ca(2+) concentration decreased in lower extent (10-15%). In CCM-incubated samples, the rate of acrosomal exocytosis in response to ionophore challenge was high (>80%), independently of the evaluation period. NCM-incubated sperm were not affected by ionophore treatment, however, their intracellular Ca(2+) concentration was no different than that observed in CCM-incubated spermatozoa. It was concluded that, after being capacitated, motile and viable spermatozoa seem to lose their ability to bind to the zona pellucida, but this loss is not accompanied by a reduced response to ionophore challenge and it may not be related with changes in the intracellular Ca(2+) concentration of spermatozoa.

Ceramide Enhances Acrosomal Exocytosis Triggered by Calcium and the Calcium Ionophore A23187 in Boar Spermatozoa

Mammalian spermatozoa must undergo acrosomal exocytosis prior to penetration of the oocyte at fertilization. The mechanisms underlying acrosomal exocytosis have not yet been fully elucidated. This study explored the possible involvement of ceramide in exocytosis of the boar sperm acrosome. Ejaculated boar spermatozoa, stored with the Beltsville TS extender at 17 degrees C for up to 3 days, were washed and preincubated for 10 min with C2-ceramide, an analogue of endogenous ceramide, C2-dihydroceramide (C2-DH-ceramide), a negative control to C2-ceramide, or with (1S,2R)-D-erythro-2-(N-myristoylamino)-1-phenyl-1-propanol (D-erythro-MAPP), an inhibitor of alkaline ceramidase, followed by incubation and stimulation with 3 mM Ca2+ and 0.3 microM A23187 (Ca2+/A23187) at 37 degrees C in air in a water bath. Spermatozoa fixed at specific intervals were examined, and the % of acrosomal exocytosis was monitored. Stimulation of spermatozoa with Ca2+/A23187 resulted in a time-dependent increase. There were no obvious changes at 5 min, but this was followed by a rapid increase at 10 min, reaching nearly a maximum level after 15 min or more of incubation. Preincubation with C2-ceramide or D-erythro-MAPP enhanced acrosomal exocytosis triggered by Ca2+/A23187 in a dose-dependent manner, whereas C2-DH-ceramide was without effect. These results suggest the possibility that ceramide may be involved in the mechanisms underlying acrosomal exocytosis.

Zona pellucida induces activation of phospholipase A2 during acrosomal exocytosis in guinea pig spermatozoa

Phospholipase A(2) (PLA(2)) is activated in spermatozoa in response to progesterone and Ca(2+) ionophores, but to our knowledge, no study has yet reported zona pellucida (ZP)-induced activation of PLA(2). We investigated whether PLA(2) is involved in ZP-stimulated acrosomal exocytosis, if Ca(2+) is required for activation of PLA(2), and signal transduction pathways modulating PLA(2) using guinea pig sperm as a model. Spermatozoa were capacitated and labeled in low-Ca(2+) medium with [(14)C]choline chloride or [(14)C]arachidonic acid and were then exposed to millimolar Ca(2+) and various reagents and stimulated with ZP. Precapacitated spermatozoa exposed to millimolar Ca(2+) and stimulated with ZP experienced increases in arachidonic acid (AA) and lysophosphatidylcholine (lysoPC) levels and a parallel decrease in phosphatidylcholine level; these changes are indicative of PLA(2) activation. Simulation with ZP also led to acrosomal exocytosis in a high proportion of spermatozoa. Lipid changes and exocytosis were prevented if spermatozoa were exposed to aristolochic acid, a PLA(2) inhibitor, before treatment with ZP. Stimulation with ZP in medium without added Ca(2+) or in medium with millimolar Ca(2+) and EGTA or La(3+) resulted in no lipid changes or exocytosis. Pretreatment with pertussis toxin, a G(i) protein inhibitor, before stimulation with ZP blocked the release of AA and lysoPC as well as acrosomal exocytosis. Exposure of spermatozoa to the diacylglycerol (DAG) kinase inhibitor R59022 before ZP stimulation led to a significant increase in generation of lysoPC and exocytosis. Taken together, these results indicate very strongly that PLA(2) plays an essential role in ZP-induced exocytosis in spermatozoa, that PLA(2) activation requires Ca(2+) internalization, and that PLA(2) activation is regulated by signal transduction pathways involving G proteins and DAG.

Seasonal variation of goat seminal plasma proteins

The present study describes the investigation of seasonal changes in seminal plasma proteins of Saanen goats under natural conditions in south Brazil. Proteins were isolated by liquid chromatography on heparin Sepharose CL-6B column and characterized by polyacrylamide gel electrophoresis (PAGE). Important differences were observed in the pattern of heparin-affinity proteins (HAPs), such as a band of 178 kDa unique to the breeding season; a decrease in 119 kDa proteins; and an increase in proteins ranging from 73 to 104 kDa. HAP caused deterioration of sperm motility and acrosome breakage in media containing and not containing skimmed milk; the effect was most remarkable with the proteins from the nonbreeding season. Furthermore, HAP presented phospholipase A2 (PLA2) activity, which was 4.4-fold higher in nonbreeding season than in breeding season. Binding sites for HAP were identified in the sperm surface, particularly at the middle piece of the spermatozoa. These results indicate that proteins from goat seminal plasma are under seasonal control and associated with sperm function during breeding and nonbreeding seasons.

Cryopreservation-induced acrosomal vesiculation in live spermatozoa from cynomolgus monkeys (Macaca fascicularis)

BACKGROUND

Cryopreserved spermatozoa are known to undergo accelerated capacitation and require a shorter incubation time for fertilization. However, details of their acrosomal membranes following cryopreservation remain unclear.

METHODS

Percoll density gradient centrifugation was used to remove dead spermatozoa; thus >90% live spermatozoa were recovered after cryopreservation, and acrosomal status was compared among non-incubated and incubated fresh and cryopreserved spermatozoa.

RESULTS

Transmission election microscopy (TEM) using microwave methods and fluorescein isothiocyanate-conjugated Pisum sativum agglutinin (FITC-PSA) staining revealed that 21.1 and 61.6% respectively of non-incubated, cryopreserved spermatozoa were intact, whereas 97.6% (TEM) or 91.9% (FITC-PSA) of non-incubated fresh spermatozoa were intact. TEM revealed that 28.8% of the cryopreserved spermatozoa were swollen, and probably included among those counted as intact by FITC-PSA staining. The non-incubated cryopreserved spermatozoa had fused plasma and outer acrosomal membranes, and 36.4% of them had vesiculation when observed by TEM. FITC-PSA staining indicated that 22% of the live spermatozoa were acrosome reacted.

CONCLUSIONS

Acceleration of the acrosome reaction was evident by both TEM and FITC-PSA. Incubation of cryopreserved spermatozoa for 2 h accelerated vesiculation to a state similar to that of fresh spermatozoa that had been incubated for 8 h. These results reveal that in cryopreserved spermatozoa, the process of acrosome reaction begins before incubation.

Dynamics of the mammalian sperm plasma membrane in the process of fertilization

Sexual reproduction requires the fusion of sperm cell and oocyte during fertilization to produce the diploid zygote. In mammals complex changes in the plasma membrane of the sperm cell are involved in this process. Sperm cells have unusual membranes compared to those of somatic cells. After leaving the testes, sperm cells cease plasma membrane lipid and protein synthesis, and vesicle mediated transport. Biophysical studies reveal that lipids and proteins are organized into lateral regions of the sperm head surface. A delicate reorientation and modification of plasma membrane molecules take place in the female tract when sperm cells are activated by so-called capacitation factors. These surface changes enable the sperm cell to bind to the extra cellular matrix of the egg (zona pellucida, ZP). The ZP primes the sperm cell to initiate the acrosome reaction, which is an exocytotic process that makes available the enzymatic machinery required for sperm penetration through the ZP. After complete penetration the sperm cell meets the plasma membrane of the egg cell (oolemma). A specific set of molecules is involved in a disintegrin-integrin type of anchoring of the two gametes which is completed by fusion of the two gamete plasma membranes. The fertilized egg is activated and zygote formation preludes the development of a new living organism. In this review we focus on the involvement of processes that occur at the sperm plasma membrane in the sequence of events that lead to successful fertilization. For this purpose, dynamics in adhesive and fusion properties, molecular composition and architecture of the sperm plasma membrane, as well as membrane derived signalling are reviewed.

Correlation between protein kinase C alpha activity and membrane phase behavior

Lipid activation of protein kinase C alpha (PKC alpha) was studied by using a model mixture containing 1, 2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), 1, 2-dimyristoyl-sn-glycero-3-phosphoserine (DMPS), and 1, 2-dimyristoyl-sn-glycerol (1,2-DMG). This lipid mixture was physically characterized by differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), and 31P-nuclear magnetic resonance (31P-NMR). Based on these techniques, a phase diagram was constructed by keeping a constant DMPC/DMPS molar ratio of 4:1 and changing the concentration of 1,2-DMG. This phase diagram displayed three regions and two compounds: compound 1 (C1), with 45 mol% 1,2-DMG, and compound 2 (C2), with 60 mol% 1,2-DMG. When the phase diagram was elaborated in the presence of Ca2+ and Mg2+, at concentrations similar to those used in the PKC alpha activity assay, the boundaries between the regions changed slightly and C1 had 35 mol% 1,2-DMG. The activity of PKC alpha was studied at several temperatures and at different concentrations of 1,2-DMG, with a maximum of activity reached at 30 mol% 1,2-DMG and lower values at higher concentrations. In the presence of Ca2+ and Mg2+, maximum PKC alpha activity occurred at concentrations of 1,2-DMG that were close to the boundary in the phase diagram between region 1, where compound C1 and the pure phospholipid coexisted in the gel phase, and region 2, where compounds C1 and C2 coexisted. These results suggest that the membrane structure corresponding to a mixture of 1,2-DMG/phospholipid complex and free phospholipid is better able to support the activity of PKC alpha than the 1,2-DMG/phospholipid complex alone.

Nongenomic effects of 1alpha,25-dihydroxyvitamin D(3)

The hormonally active form of vitamin D, 1alpha,25-dihydroxyvitamin D(3), is the key molecule of the vitamin D endocrine system, which produces biological effects in about 30 target cell systems. Growing experimental evidence supports the hypothesis that these biological effects can be generated both by a signal transduction mechanism involving a nuclear receptor (nVDR) that modulates gene transcription, and via a nongenomic receptor located in the plasma membrane (mVDR), which modulates a complex signaling system involving the rapid opening of Ca(2+) channels. Some data reviewed herein also indicate that crosstalk between genomic and nongenomic pathways operates in several cell types, and suggest that the physiological role of the rapid, nongenomic actions might involve the regulation of hormone-mediated gene activation.

The phase behavior of aqueous dispersions of unsaturated mixtures of diacylglycerols and phospholipids

The phase behavior of mixtures of 1-palmitoyl-2-oleoyl-sn-glycerol (1,2-POG) with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoserine (POPS) was studied by using DSC, small-angle X-ray diffraction and 31P-NMR. The results have been used to construct phase diagrams for both type of mixtures, in the 0-45 degreesC range. It is concluded that 1, 2-POG form complexes in the gel phases with both POPC and POPS. In the case of POPC, two complexes are postulated, the first one at a 1, 2-POG/POPC molar ratio of 40:60, and the second one at 70:30, defining three different regions in the phase diagram. Two eutectic points are proposed to occur: one at a very low 1,2-POG concentration and the other at a 1,2-POG concentration slightly lower than 70%. In the case of the 1,2-POG/POPS mixtures, the pattern was similar, but the first complex was seen to happen at a higher concentration, about 50 mol% of 1,2-POG, whereas the second was found at 80 mol% of 1,2-POG. This indicated a bigger presence of 1,2-POG in the complexes with POPS than with POPC. In the first region of the phase diagram, i.e. at concentrations of 1,2-POG lower than that required for the formation of the first complex, and at temperatures above the phase transition, lamellar phases were seen in all the cases. In region 2 of the phase diagram, i.e. at concentrations where the first and the second complexes coexist, a mixture of lamellar and non-lamellar phases was observed. Finally, at high concentrations of 1,2-POG, non-lamellar phases were detected as predominant, these phases being of an isotropic nature, according to 31P-NMR. An important conclusion of this study is that, using unsaturated lipids, similar to those found in biological membranes, it has been shown that diacylglycerols are found separated in domains, and that this process starts at very low concentrations of diacylglycerols. The formation of separated domains enriched in diacylglycerol is biologically relevant as it will allow them to have important effects on the membrane structure besides the fact that their concentration in the biomembrane is relatively low.

The cell biology of fertilization

Research into the cell biology of mammalian fertilization has been stimulated by the desire to provide a theoretical framework for the development of novel approaches to contraception and the need to understand the cellular basis of human infertility. The results of such studies have revealed a complex cascade of interactions initiated by the contact between capacitated spermatozoa on the oocyte-cumulus complex and culminating in sperm-oocyte fusion. In this review we shall examine our current understanding of the fertilization process, highlighting the strategic importance of recent findings and key areas where information is lacking.

Diacylglycerol species as messengers and substrates for phosphatidylcholine re-synthesis during Ca2+-dependent exocytosis in boar spermatozoa

We have investigated pathways of lipid metabolism in spermatozoa and generation of various metabolites with potential messenger functions during exocytosis stimulated with A23187/Ca2+. Stimulation of boar spermatozoa resulted in a considerable rapid increase in saturated/unsaturated 1,2-diacylglycerol (1,2-SU-DAG) and, concomitantly, a substantial reduction in disaturated 1,2-diacylglycerol (1,2-DS-DAG), and in phosphatidylcholine (PC). These changes preceded the onset of exocytosis. Phosphatidic acid was sometimes generated in parallel, but usually rose later, suggesting that 1,2-SU-DAG may be formed directly by phospholipase C action. Lipid changes observed in stimulated spermatozoa that have been prelabelled with several lipid precursors ([14C]palmitic acid, [14C]glycerol, [14C]choline, or [14C]arachidonic acid) suggested the existence of a unique process involving the utilization of the high basal levels of 1,2-DS-DAG to form 1,2-SU-DAG, with the latter being subsequently employed to replenish the PC pool. An ensuing generation of lysoPC and arachidonic acid, which paralleled the occurrence of exocytosis, revealed that the newly synthesized PC was hydrolyzed by phospholipase A2. The highest levels of 1,2-SU-DAG, minimum levels of 1,2-DS-DAG, and the regeneration of the PC pool were tightly coupled to the beginning of visible exocytosis. These results suggest that changes in these lipid metabolites may be fundamental processes during acrosomal exocytosis occurring in response to physiological agonists.

Inflammatory activation of arachidonic acid signaling in murine P388D1 macrophages via sphingomyelin synthesis

Ceramide has emerged as an important lipid messenger for many cellular processes triggered via surface receptors. In the present study, inflammatory activation of P388D1 macrophages with bacterial lipopolysaccharide (LPS) and platelet-activating factor (PAF) stimulated a transient accumulation of ceramide. Moreover, cell-permeable ceramide mimicked LPS/PAF in triggering arachidonate mobilization in these cells. LPS/PAF-induced ceramide synthesis did not result from sphingomyelinase activation but from increased de novo synthesis. Participation of this pathway in arachidonate signaling was detected since fumonisin B1, an inhibitor of de novo ceramide synthesis, was able to inhibit the LPS/PAF-induced response. These studies have uncovered a new role for sphingolipid metabolism in cellular signaling and constitute evidence that products of the sphingomyelin biosynthetic pathway may serve a specific role in signal transduction by influencing the activity of the novel Group V secretory phospholipase A2.

Progesterone and the zona pellucida activate different transducing pathways in the sequence of events leading to diacylglycerol generation during mouse sperm acrosomal exocytosis

We tested the involvement of protein tyrosine kinase and G-protein transducing pathways in the formation of diacylglycerol (DAG) during exocytosis in mouse spermatozoa. In capacitated spermatozoa, stimulation with solubilized zona pellucida (ZP) or progesterone led to the formation of DAG and to exocytosis of the acrosomal granule. Stimulation of DAG formation and exocytosis by ZP were inhibited in a concentration-dependent fashion by pre-exposure to tyrphostin A48, a protein tyrosine kinase inhibitor. These ZP-induced responses were also reduced in a concentration-dependent manner by prior incubation with pertussis toxin, a G-protein (Gi class) inhibitor. On the other hand, generation of DAG and exocytosis triggered by progesterone were inhibited if spermatozoa were preincubated with different concentrations of tyrphostin A48, but were not affected by pre-exposure to pertussis toxin. Progesterone acts on at least two novel surface receptors, one being a gamma-aminobutyric acid (GABA) type A (GABAA)-like receptor. Transducing mechanisms coupled to this receptor were tested directly by stimulating spermatozoa with GABA. Treatment of capacitated spermatozoa with GABA resulted in DAG formation and exocytosis. These responses were not seen when cells were preincubated with tyrphostin A48. Pertussis toxin, however, did not affect the generation of DAG and exocytosis triggered by GABA, in agreement with results obtained using progesterone. Taken together, these results indicate that DAG formation during acrosomal exocytosis is differentially regulated by transducing pathways activated by oocyte-associated agonists.

Bicarbonate/CO2 induces rapid activation of phospholipase A2 and renders boar spermatozoa capable of undergoing acrosomal exocytosis in response to progesterone

We tested whether capacitation, a phenomenon that renders spermatozoa capable of undergoing acrosomal exocytosis, may be completed rapidly after a short exposure to bicarbonate/CO2. We found that, in the presence of Ca2+, a 10-min exposure of boar spermatozoa to bicarbonate led to a partial activation of phospholipase A2, primed spermatozoa for a major subsequent activation of this enzyme upon stimulation with progesterone and furthermore rendered spermatozoa capable of undergoing exocytosis in response to this steroid. These results suggest that capacitation may be completed in a relatively short period of time and open up new possibilities for unravelling molecular mechanisms underlying this process.

rab 3-peptide stimulates exocytosis of the ram sperm acrosome via interaction with cyclic AMP and phospholipase A2 metabolites

Acrosomal exocytosis triggered with A23187/Ca2+ was enhanced by rab3AL, a synthetic peptide corresponding to the effector domain of the small GTP-binding protein rab3. Exocytosis was further enhanced when spermatozoa were also exposed to dibutyryl-cAMP, but was prevented when H-89, a protein kinase A (PKA) inhibitor, was included. The action of rab3AL was not on, or upstream of, phospholipase A2 (PLA2). Inhibition of exocytosis by the PLA2 inhibitor aristolochic acid was overcome by rab3AL when it was included together with lysophosphatidylcholine; this effect was prevented by H-89. These results suggest a functional coupling between rab3 protein, metabolites generated by PLA2, and cAMP-activated PKA, in the final steps leading to membrane fusion during acrosomal exocytosis.

Epidermal growth factor stimulates hydrolysis of phosphatidylinositol 4,5-bisphosphate, generation of diacylglycerol and exocytosis in mouse spermatozoa

Mouse spermatozoa stimulated with epidermal growth factor (EGF) or zona pellucida (ZP) experienced phosphatidylinositol 4,5-bisphosphate hydrolysis, diacylglycerol (DAG) generation and acrosomal exocytosis. The agonists showed additive effects but the action of EGF is likely to be mediated by a distinct receptor because maximal stimulation achieved with EGF was enhanced further by ZP. Generation of DAG and exocytosis stimulated by EGF were inhibited by tyrphostin A48, indicating that tyrosine kinase activity mediates EGF action. On the other hand, pertussis toxin did not affect the EGF-induced formation of DAG or exocytosis, ruling out the involvement of sperm Gi-like proteins. These results indicate that EGF could be an important co-factor in the initiation of exocytosis in spermatozoa.

Lipids trigger changes in the elasticity of the cytoskeleton in plant cells: a cell optical displacement assay for live cell measurements

An assay has been developed to quantitatively measure the tension and elasticity of the cytoskeleton in living plant cells. The cell optical displacement assay (CODA) uses a focused laser beam to optically trap and displace transvacuolar and cortical strands through a defined distance within the cell. Results from these experiments provide evidence for the classification of at least two rheologically distinct cytoskeletal assemblies, cortical and transvacuolar, that differ in their tension and response to both signaling molecules and reagents that perturb the cytoskeleton. It is further demonstrated that the tension of the transvacuolar strands can be significantly decreased by the addition of either linoleic acid, 1,2 dioctanoyl-sn-glycerol, or 1,3 dioctanoylglycerol. These decreases in tension could also be induced by lowering the cytoplasmic pH. In contrast, addition of Ca2+, Mg2+, or the ionophore A23187 to the cells caused a considerable increase in the tension of the transvacuolar strands. The data provides evidence that: (a) linoleic acid may be a signaling molecule in plant cells; (b) diacylglycerol functions as a signaling molecule through a protein kinase C-independent pathway mediated by PLA2; and (c) Ca2+ and pH have regulatory roles for controlling cytoskeleton tension and organization.