Diacylglycerol and phosphatidate production and the exocytosis of the sperm acrosome

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.

Phospholipid composition of human sperm and seminal plasma in relation to sperm fertility

The phospholipid and fatty acid composition of sperm was studied in 8 healthy and 16 infertile men. Infertile men randomly formed from the patients with normal semen parameters according to WHO criterion. Therefore, all semen parameters of infertile patients were similar to the same characteristics of the semen of healthy men, except the abnormal forms. The amount of abnormal forms in infertile men was significantly higher than in healthy men. Sperm from infertile men show a drastic loss of phosphatidyl ethanolamine. At the same time, the significant increase of phosphatidyl serine in the sperm and seminal plasma of sterile patients was found. Lysophosphatidyl serine in the sperm of the infertile men was detected. Fatty acid composition of the semen of infertile men was altered. The levels of stearic and n-3 polyunsaturated fatty acids (eicosopentaenoic and docosahexaenoic acids) was dramatically lowered, but the values of some n-6 polyunsaturated fatty acids (linolenic and docosatetraenoic) acids increased. There was significant positive correlation between docosahexaenoic acid and sperm motility (r = .82, p < .001) and negative correlation between linolenic acid and spermatozoa motility (r = -0.58. p < .05). Infertility of men with normal semen quality can originate from the disorder of sperm lipid metabolism. The drastic loss of phosphatidyl ethanolamine and n-3 polyunsaturated fatty acids with simultaneous enhancement of phosphatidyl serine and some n-6 polyunsaturated fatty acids in sperm could be an important cause of male infertility.

Incorporating lipids into boar sperm decreases chilling sensitivity but not capacitation potential

Fresh boar sperm were incubated with small unilamellar liposomes composed of either the total lipids extracted from head plasma membranes (HPM) of fresh boar sperm or selected lipids (SL) of five defined phospholipids with specific acyl chains. To optimize fusion, liposomes with 2 mol% octadecyl rhodamine fluorophore in Beltsville Thawing Solution +/- 1 mM CaCl(2) were incubated at 35 degrees C with 1;ts 10(7) or 10(8) spermatozoa/ml and monitored over 60 min, using flow cytometry and fluorescence microscopy. The HPM fused to both sperm concentrations faster than SL but was equivalent by 30 min (10(8) sperm/ml) or 60 min (10(7) sperm/ml; 57.5 +/- 3% and 67.1 +/- 8% sperm fused to HPM and SL, respectively) +/- Ca(2+). Neither HPM nor SL affected onset of capacitation or spontaneous or ionophore-induced acrosome reactions at 0 or 3 h (chlortetracycline and fluorescein isothiocyanate-Pisum sativum agglutinin; n = 3). During cooling and after cryopreservation (n = 4 ejaculates), SL but not HPM significantly improved sperm motility and viability (Sybr14/propidium iodide staining) +/- 20% egg yolk, but egg yolk alone was more effective than SL alone. Liposomes of complex composition can fuse to boar sperm without harming in vitro capacitation or acrosome reaction and reduce sperm chilling sensitivity.

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.

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.

Phospholipid metabolism in boar spermatozoa and role of diacylglycerol species in the de novo formation of phosphatidylcholine

We have investigated pathways of lipid metabolism in boar spermatozoa sperm cells incubated for up to 3 days with [14C]palmitic acid, [14C]glycerol, [14C]choline, or [14C]arachidonic acid or incorporated these precursors into diglycerides and/or phospholipids. When spermatozoa were incubated with [14C]palmitic acid or [14C]glycerol, there was first an incorporation into phosphatidic acid, followed by labelling of 1,2-diacylglycerol (DAG) and then phosphatidylcholine (PC). This indicates that the de novo pathway of phospholipid synthesis is active in these cells. However, not all DAG was converted to PC. A pool of di-saturated DAG, which represented a considerable proportion of the high basal levels of DAG, accumulated the majority of label. Another DAG pool, containing saturated fatty acids in position 1 and unsaturated fatty acids in position 2 and representing the remaining basal DAG, was in equilibrium with PC. When spermatozoa were incubated with [14C]arachidonic acid, there was a considerable incorporation of label into PC, which indicates the presence of an active deacylation/ reacylation cycle. The behaviour of certain lipid pools varied depending on the temperature at which spermatozoa were incubated. For example, in the presence of [14C]palmitic acid or [14C]arachidonic acid, there was more incorporation of label into PC when spermatozoa were incubated at 25 degrees C than when incubated at 17 degrees C. Taken together, these results indicate that spermatozoa have an active lipid synthetic capacity. It may therefore be possible to design methods to evaluate the metabolic activity of boar spermatozoa based on the incorporation of lipid precursors under standardized conditions.

Lipid mixing in dimyristoyl phosphatidylcholine-dimyristoyl glycerol dispersions: spin label ESR studies

The lipid transfer and mixing properties in hydrated dispersions of dimyristoyl phosphatidylcholine (DMPC)/dimyristoyl glycerol (DMG) binary mixtures have been investigated by using electron spin resonance spectroscopy of the spin-labelled lipid components. The assay system is based on the reduction in spectral broadening from spin-spin interactions that takes place by dilution of the spin-labelled lipid on transfer to a 9-fold excess of dispersions that contain no spin label. Lipid dispersions with DMPC:DMG compositions of 70:30, 40:60 and 20:80 mol/mol, for which the fluid phases have lamellar, inverted hexagonal and isotropic structures, respectively, have been studied. Essentially no transfer of spin-labelled lipid takes place for any of the lipid mixtures in the lamellar gel phase, or in dispersions of DMPC alone at all temperatures studied. The greatest degrees of transfer are found in the fluid phase of the DMPC/DMG mixtures. In general, the extent of lipid transfer is greater for the diacylglycerol component than for the phosphatidylcholine component. The extent of transfer of phosphatidylcholine is very low in the fluid lamellar phase of the 70:30 mol/mol DMPC/DMG, as compared with that of diacylglycerol. Only in the case of the 40:60 mol/mol DMPC/DMG mixture, in the inverted hexagonal phase, are the extents of transfer comparable for both phosphatidylcholine and diacylglycerol components, indicating a bulk transfer of lipid within the dispersions. The largest extent of transfer is found for diacylglycerol in the 20:80 mol/mol DMPC/DMG mixture in the isotropic phase.

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.

Role for Ca2+ channels in the signal transduction pathway leading to acrosomal exocytosis in human spermatozoa

Progesterone interaction with human spermatozoa promotes a rise in intracellular Ca2+ and can trigger acrosomal exocytosis in capacitated cells. We have used nifedipine, a 1,4-dihydropyridine Ca2+ channel antagonist, to investigate the possibility that Ca2+ channels play a role in the progesterone-stimulated exocytotic response. Cells were assessed biochemically for the generation of diacylglycerol (DAG) and microscopically for acrosome loss using chlortetracycline fluorescence. When motile cells were preincubated for 5 hr using culture conditions similar to those used for successful human in vitro fertilization, a short exposure to progesterone significantly stimulated DAG formation and acrosomal exocytosis. The addition of nifedipine (10 and 100 nM), either at time 0 or just prior to progesterone introduction, significantly inhibited both DAG formation and exocytosis, suggesting that Ca2+ channels are involved in the responses observed. Treatment of capacitated cells with a synthetic permeant DAG stimulated exocytosis irrespective of whether nifedipine was present, indicating that Ca2+ channels function prior to DAG generation. The possibility that an influx of Na+, as well as Ca2+, might be involved in the exocytotic pathway was investigated using the monovalent cation ionophores monensin and nigericin. Both significantly stimulated DAG generation and acrosome loss, but the prior inclusion of nifedipine significantly inhibited all responses. These results strongly suggest that the entry of Ca2+ through Ca2+ channels, with characteristics similar to those of L-type, voltage-sensitive Ca2+ channels found in cardiac and skeletal muscle, is a crucial step in the sequence of events leading to exocytosis in progesterone-stimulated human spermatozoa. An influx of Na+ also may play a role, but at a point prior to the opening of Ca2+ channels.

Diacylglycerol and alkylacylglycerol stimulate ram sperm phospholipase A2

1. We have investigated the susceptibility of ram sperm phospholipase A2 (PLA2) to stimulation by diacyl- and alkylacylglycerols and by monoacyl- and monoalkylglycerols. 2. PLA2 activity in sonicates from ram spermatozoa was enhanced when 1-stearoyl-2-arachidonoyl-sn-glycerol, the diacylglycerol usually generated by polyphosphoinositide breakdown, was added to a radioactive phosphatidylcholine substrate; the effect was time- and Ca(2+)-dependent. 3. Both diacyl- and alkylacylglycerol considerably enhanced PLA2 activity; 1-O-hexadecyl-2-O-methyl-rac-glycerol, however, only showed slight stimulatory ability. 4. The monoradylglycerols 1-monohexadecanoyl-rac-glycerol, 2-monohexadecanoylglycerol, and 1-O-hexadecyl-sn-glycerol had very little effect on the enzyme's activity. 5. Exposure of spermatozoa to 1-oleoyl-2-acetyl-sn-glycerol (OAG) or 1-O-hexadecyl-2-acetyl-rac-glycerol (1-O-C16/2-C2), when cells were stimulated with the ionophore A23187 and Ca2+, resulted in higher PLA2 activity in sperm sonicates. Furthermore, parallel experiments showed that exocytosis was enhanced if spermatozoa were treated with A23187/Ca2+ and either OAG or 1-O-C16/2-C2. Since both diacyl- and alkylacylglycerols increased PLA2 activity and exocytosis, stimulation of PLA2 activity by these diglycerides may take place independently from protein kinase C activation.

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

We tested the hypothesis that the role of diacylglycerol (DAG) in sperm acrosomal exocytosis is related to the activation of phospholipase A2, and that this effect is not mediated via protein kinase C. Treatment of [14C]arachidonic acid-labelled ram spermatozoa with Ca2+ and the ionophore A23187 stimulated both liberation of arachidonic acid and acrosomal exocytosis. No changes in [14C]DAG or [14C]monoacylglycerol were found after stimulation of spermatozoa, thus suggesting that arachidonic acid may be released exclusively via phospholipase A2. An increase in the endogenous levels of diradylglycerols (DRGs), resulting from exposure either to the DAG kinase inhibitor R 59022 or to exogenous 1-oleoyl-2-acetyl-sn-glycerol or 1,2-dioctanoyl-sn-glycerol, led to an increase in both phospholipase A2 activity and exocytosis when cells were stimulated with A23187 and Ca2+. Addition of DRGs that do not stimulate protein kinase C(1,3-dioctanoylglycerol, 1-O-hexadecyl-2-acetyl-rac-glycerol) also resulted in an increase in phospholipase A2 activity and exocytosis. On the other hand, phorbol esters (phorbol 12,13-dibutyrate; phorbol 12-myristate 13-acetate) did not enhance enzyme activity or exocytosis. Finally, exposure to 1-O-hexadecyl-2-O-methyl-rac-glycerol, a compound known to inhibit protein kinase C, did not affect phospholipase A2 activity or acrosomal exocytosis. We therefore conclude that in spermatozoa the messenger role of DAG is related to the activation of phospholipase A2, which in turn would generate an array of metabolites directly or indirectly involved in bringing about exocytosis of the acrosome.

Phospholipase D and exocytosis of the ram sperm acrosome

We have investigated whether phospholipase D (PLD) is involved in events leading to acrosomal exocytosis. Ram spermatozoa pre-labelled with [3H]alkyl-lysophosphatidylcholine and stimulated with the ionophore A23187 (1 microM) and Ca2+ (3 mM) in the presence of ethanol, showed a slow time-dependent increase in [3H]phosphatidic acid and [3H]phosphatidylethanol (PEt), the latter being clear evidence of PLD activity. Unlabelled cells similarly treated underwent acrosomal exocytosis. However, [3H]PEt formation was inhibited by high Ca2+ concentrations, although such conditions result in maximal acrosomal exocytosis. Treatment with A23187/Ca2+ led to a fast generation of [3H]alkyl-diglyceride and an increase in 1,2-diacylglycerol mass, which preceded [3H]PEt formation. The rises in [3H]alkyl-diglyceride and 1,2-diacylglycerol mass took place regardless of the presence or absence of ethanol. Inclusion of propranolol, a phosphatidic acid phosphohydrolase inhibitor, did not affect the early rise of labelled or unlabelled 1,2-diglycerides either. Stimulation of spermatozoa with A23187/Ca2+ in the presence of either ethanol or propranolol did not affect the occurrence of acrosomal exocytosis. Taken together, these results indicate that although Ca2+ entry triggers a late activation of PLD, this enzyme is not involved in the early generation of diglycerides. Moreover, they suggest that PLD does not make a substantial contribution in events leading to exocytosis of the sperm acrosome. Therefore, generation of diglycerides may take place primarily via phospholipase C.

Phospholipase A2 activity and exocytosis of the ram sperm acrosome: regulation by bivalent cations

Previous work has shown that the sequence leading to exocytosis of the sperm acrosome involves at least three Ca(2+)-requiring processes, the first one probably represented by breakdown of the polyphosphoinositides and the final one by membrane fusion. We have investigated whether phospholipase A2 (PLA2) represents the intermediate Ca(2+)-requiring event by stimulating ram spermatozoa with the ionophore A23187 and various bivalent cations. Spermatozoa prelabelled with [14C]arachidonic acid and treated with ionophore and millimolar Ca2+ showed a considerable release of arachidonic acid; parallel sperm samples similarly treated underwent acrosomal exocytosis. Mn2+ was capable of completely substituting for Ca2+, even if residual Ca2+ in the system was chelated with EGTA: both arachidonic acid release and acrosomal exocytosis took place after treatment with A23187, EGTA and Mn2+. Neither Mg2+ nor Ba2+ promoted arachidonic acid release or exocytosis. The effects of Sr2+ were more complex and allowed us to probe the sequence of events leading to membrane fusion. Both arachidonic acid release and exocytosis occurred after treatment with A23187 and Sr2+ but none of these responses were seen if EGTA was also included. These results suggest that residual micromolar Ca2+ is either needed for Sr2+ to fully promote PLA2 activity, or that micromolar Ca2+ is needed for one or more upstream events that may in turn serve to activate PLA2. Evidence for or against the first possibility was sought by examining PLA2 activity in sperm sonicates. Enzyme activity was maximal in the presence of any bivalent cation and it was not reduced (in the case of Sr2+) or only reduced slightly (Mg2+, Mn2+, Ba2+) if residual Ca2+ was chelated with EGTA; this indicates that Sr2+ can promote PLA2 activity in the total absence of Ca2+. The second possibility was explored by treating spermatozoa with A23187 for 5 min (to allow for complete phosphoinositide breakdown; Roldan and Harrison (1989) Biochem. J. 259, 397-406), and then adding EGTA and Sr2+. This resulted in neither arachidonic acid release nor exocytosis, thus indicating that another as yet unidentified Ca(2+)-dependent event may occur before PLA2 activation.

1,2-Dioleoylglycerol promotes calcium-induced fusion in phospholipid vesicles

The effect of 1,2-dioleoyglycerol (1,2-DOG) on the promotion of Ca(2+)-induced fusion of phosphatidylserine/phosphatidylcholine (PS/PC) vesicles was studied. 1,2-DOG is able to induce the mixing of membrane lipids at concentrations of 10 mol% without mixing of vesicular contents. At concentrations of 20 mol% or higher, 1,2-DOG promotes fusion, lipid and content mixing, of LUV composed of an equimolar mixture of PS and PC, which otherwise are unable to fuse in the presence of Ca2+. Fusion was demonstrated by fluorescence assays monitoring mixing of aqueous vesicular contents and mixing of membrane lipids. Studies by Fourier transform infrared spectroscopy provided evidence for a fusion mechanism different to that of Ca(2+)-induced fusion of pure PS vesicles. Final equilibrium structures were characterized by 31P-NMR and freeze-fracture electron microscopy. Ca(2+)-induced fusion of 1,2-DOG containing vesicles is accompanied by the formation of isotropic structures which are shown to correspond to structures with lipidic particle morphology. The possible fusion mechanisms and implications are discussed.

A transient rise in intracellular Ca2+ is a precursor reaction to the zona pellucida-induced acrosome reaction in mouse sperm and is blocked by the induced acrosome reaction inhibitor 3-quinuclidinyl benzilate

The acrosome reaction induced by the zona pellucida in mouse sperm has been shown to proceed in two stages experimentally distinguishable by the fluorescent probe chlortetracycline. Entry into the first stage of sperm bound to isolated, structurally intact zonae pellucidae is blocked by the compound 3-quinuclidinyl benzilate. In this study, we show, utilizing the fluorescent Ca2+ indicator fluo-3, that the first stage of the zona-induced acrosome reaction is characterized by an increase in intracellular Ca2+, followed by a decrease as the acrosome reaction proceeds. This calcium transient is completely suppressed by 3-quinuclidinyl benzilate. We conclude that the Ca2+ transient is induced by the zona pellucida and is required for the zona-induced acrosome reaction. Blockage of this sperm intracellular Ca2+ transient provides a mechanism for the inhibitory action of 3-quinuclidinyl benzilate on the zona-induced acrosome reaction in mouse sperm.

The role of diacylglycerol in the exocytosis of the sperm acrosome. Studies using diacylglycerol lipase and diacylglycerol kinase inhibitors and exogenous diacylglycerols

When ram spermatozoa were treated with Ca2+ and the ionophore A23187 to induce acrosomal exocytosis, a rise in diacylglycerol (DAG) mass was observed, concomitant with a rapid breakdown of [32P]P1-labelled phosphatidylinositol 4,5-bisphosphate and phosphatidylinositol 4-phosphate and a rise in [32P]Pi-labelled phosphatidate. Inclusion of the DAG lipase inhibitor RHC 80267 resulted in further but biphasic increases in DAG; there was an increasing accumulation of DAG with concentrations of RHC 80267 up to 10 microM, whereas higher concentrations produced lessening accumulation. Inclusion of RHC 80267 in the ionophore induction system also resulted in significant accelerations of the onset of exocytosis. In spermatozoa stimulated with Ca2+/A23187 and the DAG kinase inhibitor R59022, a similar increase in DAG levels together with stimulation of acrosomal exocytosis were observed. Preincubation of spermatozoa with sn-1-oleoyl-2-acetylglycerol, rac-1-oleoyl-2-acetylglycerol, sn-1,2-dioctanoylglycerol and sn-1,3-dioctanoylglycerol before treatment with Ca2+/A23187 resulted in a dose-dependent stimulation of exocytosis by all these isomers. Neomycin inhibited Ca2+/A23187-induced generation of DAG together with polyphosphoinositide breakdown, as well as acrosomal exocytosis. Inclusion of exogenous DAG, however, overcame the inhibitory effect of neomycin on exocytosis. Our results suggest that DAG has a key role in acrosomal exocytosis and that it acts as a messenger rather than as a substrate from which other active metabolites are generated. The lack of stereospecificity shown by the exogenous DAGs implies that DAG does not act by stimulating protein kinase C, but the metabolite's actual target in the sperm cell is as yet unclear.

Interactions of saturated diacylglycerols with phosphatidylcholine bilayers: A 2H NMR study

The interactions of a series of saturated diacylglycerols (DAGs) with fatty acid side chain lengths of 6-14 carbons with multilamellar phospholipid bilayers consisting either of dipalmitoylphosphatidylcholine (DPPC) or of a mixture of DPPC and bovine liver phosphatidylcholine (BL-PC) extracts were studied by 2H NMR spectrometry. We found that the perturbation induced by the DAGs into the bilayer structure strongly depends on the length of the DAG fatty acid side chain. Shorter chain 1,2-sn-dihexanoylglycerol and, to a larger degree, 1,2-sn-dioctanoylglycerol (diC8) induce transverse perturbation of the bilayer structure: the order parameters of the phospholipid side chains are increased by the intercalating DAG molecules in the region adjacent to the phospholipid headgroups and decreased toward the terminal methyls, corresponding to the bilayer interior. The longer chain DAGs (C greater than or equal to 12) studied in this and previous [De Boeck & Zidovetzki (1989) Biochemistry 28, 7439] work induce lateral phase separation of the lipids into DAG-enriched gellike domains and relatively DAG-free regions in the liquid-crystalline phase. Each of the DAGs studied induces a decrease in the area per phospholipid molecule, and a corresponding increase in the lateral surface pressure of the bilayers.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of protein kinase C and its neuronal substrates dephosphin, B-50, and MARCKS in neurotransmitter release

This article focuses on the role of protein phosphorylation, especially that mediated by protein kinase C (PKC), in neurotransmitter release. In the first part of the article, the evidence linking PKC activation to neurotransmitter release is evaluated. Neurotransmitter release can be elicited in at least two manners that may involve distinct mechanisms: Evoked release is stimulated by calcium influx following chemical or electrical depolarization, whereas enhanced release is stimulated by direct application of phorbol ester or fatty acid activators of PKC. A markedly distinct sensitivity of the two pathways to PKC inhibitors or to PKC downregulation suggests that only enhanced release is directly PKC-mediated. In the second part of the article, a framework is provided for understanding the complex and apparently contrasting effects of PKC inhibitors. A model is proposed whereby the site of interaction of a PKC inhibitor with the enzyme dictates the apparent potency of the inhibitor, since the multiple activators also interact with these distinct sites on the enzyme. Appropriate PKC inhibitors can now be selected on the basis of both the PKC activator used and the site of inhibitor interaction with PKC. In the third part of the article, the known nerve terminal substrates of PKC are examined. Only four have been identified, tyrosine hydroxylase, MARCKS, B-50, and dephosphin, and the latter two may be associated with neurotransmitter release. Phosphorylation of the first three of these proteins by PKC accompanies release. B-50 may be associated with evoked release since antibodies delivered into permeabilized synaptosomes block evoked, but not enhanced release. Dephosphin and its PKC phosphorylation may also be associated with evoked release, but in a unique manner. Dephosphin is a phosphoprotein concentrated in nerve terminals, which, upon stimulation of release, is rapidly dephosphorylated by a calcium-stimulated phosphatase (possibly calcineurin [CN]). Upon termination of the rise in intracellular calcium, dephosphin is phosphorylated by PKC. A priming model of neurotransmitter release is proposed where PKC-mediated phosphorylation of such a protein is an obligatory step that primes the release apparatus, in preparation for a calcium influx signal. Protein dephosphorylation may therefore be as important as protein phosphorylation in neurotransmitter release.

Diacylglycerol stimulates the Ca2(+)-dependent phospholipase A2 of ram spermatozoa

We have investigated phospholipase A2 (PLA2) activity in sonicates from ram spermatozoa and have analyzed the enzyme's susceptibility both to various inhibitors and to stimulation by diacylglycerols (DAGs). Ram sperm PLA2 activity was Ca2(+)-dependent and was inhibited by dexamethasone, chloracysine and compounds Ro 31-4493 and Ro 31-4639; mepacrine, however, did not inhibit PLA2 activity. Addition of three different 1,2-DAGs (dioctanoyl-sn-glycerol, oleoyl-acetyl-sn-glycerol and dioleoyl-sn-glycerol) markedly increased PLA2 activity; moreover, both 1,2- and 1,3-isomers enhanced enzyme activity. Since ram spermatozoa lack active protein kinase C, the target of DAG in most cells, our results suggest that stimulation of PLA2 activity by DAG may play an important role in intracellular signalling in the sperm cell.