Calcium-dependent polyphosphoinositide hydrolysis is associated with exocytosis in vitro

Critical role of cortical vesicles in dissecting regulated exocytosis: overview of insights into fundamental molecular mechanisms

Regulated exocytosis is one of the defining features of eukaryotic cells, underlying many conserved and essential functions. Definitively assigning specific roles to proteins and lipids in this fundamental mechanism is most effectively accomplished using a model system in which distinct stages of exocytosis can be effectively separated. Here we discuss the establishment of sea urchin cortical vesicle fusion as a model to study regulated exocytosis-a system in which the docked, release-ready, and late Ca(2+)-triggered steps of exocytosis are isolated and can be quantitatively assessed using the rigorous coupling of functional and molecular assays. We provide an overview of the insights this has provided into conserved molecular mechanisms and how these have led to and integrate with findings from other regulated exocytotic cells.

Phosphoinositides: tiny lipids with giant impact on cell regulation

Phosphoinositides (PIs) make up only a small fraction of cellular phospholipids, yet they control almost all aspects of a cell's life and death. These lipids gained tremendous research interest as plasma membrane signaling molecules when discovered in the 1970s and 1980s. Research in the last 15 years has added a wide range of biological processes regulated by PIs, turning these lipids into one of the most universal signaling entities in eukaryotic cells. PIs control organelle biology by regulating vesicular trafficking, but they also modulate lipid distribution and metabolism via their close relationship with lipid transfer proteins. PIs regulate ion channels, pumps, and transporters and control both endocytic and exocytic processes. The nuclear phosphoinositides have grown from being an epiphenomenon to a research area of its own. As expected from such pleiotropic regulators, derangements of phosphoinositide metabolism are responsible for a number of human diseases ranging from rare genetic disorders to the most common ones such as cancer, obesity, and diabetes. Moreover, it is increasingly evident that a number of infectious agents hijack the PI regulatory systems of host cells for their intracellular movements, replication, and assembly. As a result, PI converting enzymes began to be noticed by pharmaceutical companies as potential therapeutic targets. This review is an attempt to give an overview of this enormous research field focusing on major developments in diverse areas of basic science linked to cellular physiology and disease.

Calcium signalling in early embryos

The onset of development in most species studied is triggered by one of the largest and longest calcium transients known to us. It is the most studied and best understood aspect of the calcium signals that accompany and control development. Its properties and mechanisms demonstrate what embryos are capable of and thus how the less-understood calcium signals later in development may be generated. The downstream targets of the fertilization calcium signal have also been identified, providing some pointers to the probable targets of calcium signals further on in the process of development. In one species or another, the fertilization calcium signal involves all the known calcium-releasing second messengers and many of the known calcium-signalling mechanisms. These calcium signals also usually take the form of a propagating calcium wave or waves. Fertilization causes the cell cycle to resume, and therefore fertilization signals are cell-cycle signals. In some early embryonic cell cycles, calcium signals also control the progress through each cell cycle, controlling mitosis. Studies of these early embryonic calcium-signalling mechanisms provide a background to the calcium-signalling events discussed in the articles in this issue.

Calcium at fertilization and in early development

Fertilization calcium waves are introduced, and the evidence from which we can infer general mechanisms of these waves is presented. The two main classes of hypotheses put forward to explain the generation of the fertilization calcium wave are set out, and it is concluded that initiation of the fertilization calcium wave can be most generally explained in invertebrates by a mechanism in which an activating substance enters the egg from the sperm on sperm-egg fusion, activating the egg by stimulating phospholipase C activation through a src family kinase pathway and in mammals by the diffusion of a sperm-specific phospholipase C from sperm to egg on sperm-egg fusion. The fertilization calcium wave is then set into the context of cell cycle control, and the mechanism of repetitive calcium spiking in mammalian eggs is investigated. Evidence that calcium signals control cell division in early embryos is reviewed, and it is concluded that calcium signals are essential at all three stages of cell division in early embryos. Evidence that phosphoinositide signaling pathways control the resumption of meiosis during oocyte maturation is considered. It is concluded on balance that the evidence points to a need for phosphoinositide/calcium signaling during resumption of meiosis. Changes to the calcium signaling machinery occur during meiosis to enable the production of a calcium wave in the mature oocyte when it is fertilized; evidence that the shape and structure of the endoplasmic reticulum alters dynamically during maturation and after fertilization is reviewed, and the link between ER dynamics and the cytoskeleton is discussed. There is evidence that calcium signaling plays a key part in the development of patterning in early embryos. Morphogenesis in ascidian, frog, and zebrafish embryos is briefly described to provide the developmental context in which calcium signals act. Intracellular calcium waves that may play a role in axis formation in ascidian are discussed. Evidence that the Wingless/calcium signaling pathway is a strong ventralizing signal in Xenopus, mediated by phosphoinositide signaling, is adumbrated. The central role that calcium channels play in morphogenetic movements during gastrulation and in ectodermal and mesodermal gene expression during late gastrulation is demonstrated. Experiments in zebrafish provide a strong indication that calcium signals are essential for pattern formation and organogenesis.

A wave of IP3 production accompanies the fertilization Ca2+ wave in the egg of the frog, Xenopus laevis: theoretical and experimental support

The fertilization Ca2+ wave in Xenopus laevis is a single, large wave of elevated free Ca2+ that is initiated at the point of sperm-egg fusion and traverses the entire width of the egg. This Ca2+ wave involves an increase in inositol-1,4,5-trisphosphate (IP3) resulting from the interaction of the sperm and egg, which then results in the activation of the endoplasmic reticulum Ca2+ release machinery. The extraordinarily large size of this cell (1.2 mm diameter) together with the small surface region of sperm-receptor activation makes special demands on the IP3-dependent Ca2+ mobilizing machinery. We propose a detailed model of the fertilization Ca2+ wave in Xenopus eggs that requires an accompanying wave of IP3 production. While the Ca2+ wave is initiated by a localized increase of IP3 near the site of sperm-egg fusion, the Ca2+ wave propagates via IP3 production correlated with the Ca2+ wave-possibly via Ca(2+)-mediated PLC activation. Such a Ca(2+)-mediated IP(3) production wave has not been required previously to explain the fertilization Ca2+ wave in eggs; we argue this is necessary to explain the observed IP3 dynamics in Xenopus eggs. To test our hypothesis, we have measured the IP3 levels from 20 nl "sips" of the egg cortex during wave propagation. We were unable to detect the low IP3 levels in unfertilized eggs, but after fertilization, [IP3] ranged from 175 to 430 nM at the sperm entry point and from 120 to 700 nM 90 degrees away once the Ca2+ wave passed that region about 2 min after fertilization. Prior to the Ca2+ wave reaching that region the IP3 levels were undetectable. Since significant IP3 could not diffuse to this region from the sperm entry point within 2 min, this observation is consistent with a regenerative wave of IP3 production.

Decrease in acetylcholine-induced current by neomycin in PC12 cells

The effects of neomycin, one of the aminoglycoside antibiotics, on the acetylcholine (ACh)-induced current (I(ACh)) were studied in pheochromocytoma cells by using the whole-cell clamp technique. The I(ACh) proved to be generated through neuronal nicotinic receptor. ACh (30 microM) induced an inward current at a holding potential of -80 mV. When cells were treated with neomycin (0.01-1 mM) and ACh (30 microM) simultaneously, an inhibitory effect of neomycin on the peak of I(ACh) was found. This effect was fast, reversible, and concentration dependent. Pretreatment with neomycin for 3-8 min had no effect on the inhibition of I(ACh) induced by neomycin. External application of 0.1 mM neomycin neither shifted the dose-response curve of the peak I(ACh) to the right (dissociation constant (K(d)) = 16.5 microM) nor affected its coefficient (1.8) but inhibited the curve amplitudes by approximately 33%. Stimulated protein kinase C activation by using an exogenous activator produced inhibition of I(ACh), while using protein kinase C inhibitor (PKCI 19-31) had no effect on the inhibition of I(ACh) induced by neomycin. These results suggest that neomycin has an inhibitory effect on I(ACh) without the involvement of phospholipase C. It indicates that neomycin binds to a specific site on the cell membrane, probably on the neuronal nicotinic receptor-coupled channel, and inhibits the I(ACh) in a noncompetitive manner, thus controlling the immediate catecholamine release from the sympathetic cells.

Fluorescently labeled neomycin as a probe of phosphatidylinositol-4, 5-bisphosphate in membranes

Phosphatidylinositol-4,5-bisphosphate (PI(4,5)P(2)), a minor component of the plasma membrane, is important in signal transduction, exocytosis, and ion channel activation. Thus fluorescent probes suitable for monitoring the PI(4,5)P(2) distribution in living cells are valuable tools for cell biologists. We report here three experiments that show neomycin labeled with either fluorescein or coumarin can be used to detect PI(4,5)P(2) in model phospholipid membranes. First, addition of physiological concentrations of PI(4,5)P(2) (2%) to lipid vesicles formed from mixtures of phosphatidylcholine (PC) and phosphatidylserine (PS) enhances the binding of labeled neomycin significantly (40-fold for 5:1 PC/PS vesicles). Second, physiological concentrations of inositol-1,4,5-trisphosphate (10 microM I(1,4,5)P(3)) cause little translocation of neomycin from PC/PS/PI(4,5)P(2) membranes to the aqueous phase, whereas the same concentrations of I(1,4,5)P(3) cause significant translocation of the green fluorescent protein/phospholipase C-delta pleckstrin homology (GFP-PH) constructs from membranes (Hirose et al., Science, 284 (1999) 1527). Third, fluorescence microscopy observations confirm that one can distinguish between PC/PS vesicles containing either 0 or 2% PI(4, 5)P(2) by exposing a mixture of the vesicles to labeled neomycin. Thus fluorescently labeled neomycin could complement GFP-PH constructs to investigate the location of PI(4,5)P(2) in cell membranes.

Calcium release and subsequent development induced by modification of sulfhydryl groups in porcine oocytes

The mechanism of Ca2+ release induced by modification of sulfhydryl groups and the subsequent activation of porcine oocytes were investigated. Thimerosal, a sulfhydryl-oxidizing compound, induced Ca2+ oscillation in matured oocytes. In thimerosal-preincubated oocytes, the amount of Ca2+ released after microinjection of inositol 1,4,5-trisphosphate (InsP3) or ryanodine increased strikingly, indicating that thimerosal potentiated both InsP3- and ryanodine-sensitive Ca2+ release pathways. Thimerosal also enhanced the sensitivity of oocytes to microinjected Ca2+ so that in pretreated oocytes a Ca2+ injection triggered a larger transient. Heparin at concentrations that normally block the InsP3-induced Ca2+ release were without effect; higher doses significantly increased the time leading up to the first spike. The thimerosal-induced Ca2+ release could not be blocked by procaine, and it did not require the formation of InsP3 since preinjection with neomycin did not prevent the oscillation. Immunocytochemistry revealed that thimerosal treatment destroyed the meiotic spindle, preventing further development, an effect that could be reversed by dithiothreitol. The combined thimerosal/dithiothreitol treatment triggered second polar body extrusion in 50% of the oocytes, and as a result of this activation scheme approximately 15% of the in vitro- and approximately 60% of the in vivo-matured oocytes developed to blastocyst during a 7-day culture in vitro.

Structure and functional properties of diacylglycerols in membranes

1. 1,2-Diacyl-sn-glycerols (DAG) are minor components of cell membranes (about 1 mole% of the lipids) and yet they are potent regulators of both the physical properties of the lipid bilayer and the catalytic behaviour of several membrane-related enzymes. 2. In the pure state DAG's present a considerable polymorphism, with several crystalline phases in addition to the neat fluid phase. The most stable crystalline phase is the so-called beta' phase, a monoclinic crystalline form with orthorhombic perpendicular subcell chain packing, in which both acyl chains lie parallel to each other in a hairpinlike configuration about the sn-1 and sn-2 glycerol carbon atoms. The molecules are organized in a bilayer, with the glycerol backbone roughly parallel to the plane of the bilayer, and the acyl chains tilted at approximately 60 degrees with respect to that plane. Acyl chain unsaturation, and particularly a single cis unsaturation, impairs chain packing in mixed-chain DAG's, and this results in an increased number of metastable crystalline phases. 3. DAG's mix with phospholipids in fluid bilayers when their melting temperature is below or close enough to the melting temperature of the bilayer system. When incorporated in phospholipid bilayers, the conformation of DAG is such that the glycerol backbone is nearly perpendicular to the bilayer, with the sn-1 chain extending from the glycerol Cl carbon into the hydrophobic matrix of the bilayer and the sn-2 chain first extending parallel to the bilayer surface, then making a 90 degrees bend at the position of the sn-1 carbonyl to become parallel to the sn-1 chain. DAG's are located in phospholipid bilayers about two CH2 units deeper than the adjacent phospholipids. DAG's mix nonideally with phospholipids, giving rise to in-plane separations of DAG-rich and -poor domains, even in the fluid state. DAG molecules also increase the separation between phospholipid headgroups, and decrease the hydration of the bilayer surface. Also, because the transversal section of the DAG headgroup is small when compared to that of the acyl chains, DAG favours the (negative) curvature of the lipid monolayers, and DAG-phospholipid mixtures tend to convert into inverted nonlamellar hexagonal or cubic phases. 4. A number of membrane enzyme activities are modulated (activated) by DAG, most notably protein kinase C, phospholipases and other enzymes of lipid metabolism. Protein kinase C activation (and perhaps that of other enzymes as well) occurs as the combined result of a number of DAG-induced modifications of lipid bilayers that include: changes in lipid headgroup conformation, interspacing and hydration, changes in the bilayer propensity to form inverted nonlamellar phases, and lateral phase separations of DAG-rich and -poor domains. Among the DAG-activated enzymes, phospholipases C show the peculiarity of yielding the activator DAG as their reaction product, and this allows the self-induced transition from a low- to a high-activity status. 5. DAG's induce or enhance membrane fusion in a number of ways, mainly through partial dehydration of the bilayer surface, increase in lipid monolayer curvature and perhaps lateral phase separation. DAG-increased fusion rates have been demonstrated in several instances of cation-induced fusion of model membranes, as well as in Ca(2+)-induced fusion of chromaffin granules with plasma membrane vesicles. Also phospholipase C has been shown to induce vesicle aggregation and fusion through the catalytic generation of DAG in the bilayers. A rather general property of DAG is that it promotes vesicular or interparticle aggregation. 6. In the living cell, DAG is often generated through phospholipid degradation in response to an extracellular agonist binding a specific receptor in the cell surface. DAG is said to act as an intracellular second messenger. (ABSTRACT TRUNCATED)

Biochemical and functional studies of cortical vesicle fusion: the SNARE complex and Ca2+ sensitivity

Cortical vesicles (CV) possess components critical to the mechanism of exocytosis. The homotypic fusion of CV centrifuged or settled into contact has a sigmoidal Ca2+ activity curve comparable to exocytosis (CV-PM fusion). Here we show that Sr2+ and Ba2+ also trigger CV-CV fusion, and agents affecting different steps of exocytotic fusion block Ca2+, Sr2+, and Ba2+-triggered CV-CV fusion. The maximal number of active fusion complexes per vesicle, <n\>Max, was quantified by NEM inhibition of fusion, showing that CV-CV fusion satisfies many criteria of a mathematical analysis developed for exocytosis. Both <n\>Max and the Ca2+ sensitivity of fusion complex activation were comparable to that determined for CV-PM fusion. Using Ca2+-induced SNARE complex disruption, we have analyzed the relationship between membrane fusion (CV-CV and CV-PM) and the SNARE complex. Fusion and complex disruption have different sensitivities to Ca2+, Sr2+, and Ba2+, the complex remains Ca2+- sensitive on fusion-incompetent CV, and disruption does not correlate with the quantified activation of fusion complexes. Under conditions which disrupt the SNARE complex, CV on the PM remain docked and fusion competent, and isolated CV still dock and fuse, but with a markedly reduced Ca2+ sensitivity. Thus, in this system, neither the formation, presence, nor disruption of the SNARE complex is essential to the Ca2+-triggered fusion of exocytotic membranes. Therefore the SNARE complex alone cannot be the universal minimal fusion machine for intracellular fusion. We suggest that this complex modulates the Ca2+ sensitivity of fusion.

Detection of phospholipase Cgamma in sea urchin eggs

Phosphorylation on tyrosine and turnover of polyphosphoinositide metabolism are rapidly stimulated after fertilization. However, the interconnection between these pathways remains to be determined. In the present paper it is demonstrated that eggs of two different sea urchin species contain tyrosine phosphorylated proteins with calcium-sensitive phospholipase C activity. We have investigated whether phospholipase Cgamma (PLCgamma), characteristic of tyrosine kinase receptors, could be responsible for this activity. Western blot and immunocytochemistry performed with antibodies directed against PLCgamma revealed the presence of this protein in cortical regions. It was also observed that PLCgamma displayed calcium-sensitive activity. The present results suggest that PLCgamma may be part of the cascade of events leading to the calcium signal responsible for egg activation at fertilization.

A mammalian sperm cytosolic phospholipase C activity generates inositol trisphosphate and causes Ca2+ release in sea urchin egg homogenates

Injection of sperm extracts triggers Ca2+ oscillations in mammalian eggs similar to those seen at fertilisation. Here, we show that addition of sperm extracts to sea urchin egg homogenates causes Ca2+ release and inositol 1,4,5-trisphosphate (InsP3) production. Furthermore depleting homogenates of phosphatidylinositol lipids using a phosphatidylinositol-specific phospholipase C blocked the sperm extract from causing InsP3 production and a Ca2+ rise. A response could be recovered by the addition of phosphatidylinositol 4,5-bisphosphate to either sperm extracts or egg homogenates. These data indicate that sperm extracts contain an InsP3-generating phospholipase C which may play a role in Ca2+ release at fertilisation.

Measured effects of diacylglycerol on structural and elastic properties of phospholipid membranes

Diacylglycerol, a biological membrane second messenger, is a strong perturber of phospholipid planar bilayers. It converts multibilayers to the reverse hexagonal phase (HII), composed of highly curved monolayers. We have used x-ray diffraction and osmotic stress of the HII phase to measure structural dimensions, spontaneous curvature, and bending moduli of dioleoylphosphatidylethanolamine (DOPE) monolayers doped with increasing amounts of dioleoylglycerol (DOG). The diameter of the HII phase cylinders equilibrated in excess water decreases significantly with increasing DOG content. Remarkably, however, all structural dimensions at any specific water/lipid ratio that is less than full hydration are insensitive to DOG. By plotting structural parameters of the HII phase with changing water content in a newly defined coordinate system, we show that the elastic deformation of the lipid monolayers can be described as bending around a pivotal plane of constant area. This dividing surface includes 30% of the lipid volume independent of the DOG content (polar heads and a small fraction of hydrocarbon chains). As the mole fraction of DOG increases to 0.3, the radius of spontaneous curvature defined for the pivotal surface decreases from 29 A to 19 A, and the bending modulus increases from approximately 11 to 14 (+/-0.5) kT. We derive the conversion factors and estimate the spontaneous curvatures and bending moduli for the neutral surface which, unlike the pivotal plane parameters, are intrinsic properties that apply to other deformations and geometries. The spontaneous curvature of the neutral surface differs from that of the pivotal plane by less than 10%, but the difference in the bending moduli is up to 40%. Our estimate shows that the neutral surface bending modulus is approximately 9kT and practically does not depend on the DOG content.

Phospholipase C activates protein kinase C during induction of slow Na current in Xenopus oocytes

Protein phosphorylation by protein kinase C (PKC) has recently been shown to be a key event in the induction of the slow inward Na current observed during sustained depolarization of the Xenopus oocyte membrane. The present work investigates the possible pathways leading to PKC activation. PKC is activated by a series of phospholipid metabolites, such as diacylglycerol (DAG) and arachidonic acid produced by phospholipases C (PLC) and A2 (PLA2) respectively. To test whether PKC activation was dependent upon the phospholipid metabolites produced either by PLC or by PLA2, enzyme activity was reduced using selective inhibitors. Results indicated that inhibition of PLA2 activity and inhibition of the enzymes involved in the arachidonic acid cascade failed to affect Na current amplitude. On the other hand, PLC inhibition caused a marked decrease of Na current amplitude. In another series of experiments, Na current was fully restored, in spite of PLC inhibition, by directly enhancing PKC activity with a powerful activator phorbol 12-myristate 13-acetate. These data strongly suggest that PLC is involved in PKC activation during Na channel induction.

Phosphatidylinositol hydrolysis is involved in production of Ca(2+)-dependent currents, but not non-selective cation currents, by muscarine in chromaffin cells

Whether phosphatidylinositol hydrolysis and a subsequent Ca2+ mobilization are responsible for muscarine-induced transient outward currents (IO) and non-selective cation currents (INS) in the guinea-pig chromaffin cell was investigated using the perforated patch method. IO, but not INS, failed to be reproduced in Ca(2+)-free solution and was markedly reduced by prior exposure to caffeine under Ca(2+)-free conditions or by addition to normal solution of cyclopiazonic acid (CPA), a Ca2+ ATPase inhibitor. Application of CPA in Ca(2+)-free solution, however, suppressed INS by about 50% in 73% of the cells tested. Bath application of 1.5 mM neomycin, a phospholipase C inhibitor, induced the time-dependent decline of IO with near abolition at 20 min or less, whereas it produced a time-independent decrease of INS and an inwardly rectifying K+ current. INS in the presence or absence of neomycin was well fitted to rectangular hyperbolas with the same ED50 of 2.17 microM, but with a 33% smaller maximum amplitude in the former, indicating a non-competitive inhibition by neomycin. We conclude that, while phosphatidylinositol hydrolysis mediates the production of IO, it does not mediate that of INS by muscarine.

Inhibitors of phospholipid intracellular signaling as antiproliferative agents

The improved understanding of oncogenesis and the involvement of oncogenes and tumor suppressor genes, has led to a rational approach of specific target-directed anti-cancer drug development. Cancer genes have been found to be important not only in the control of cell proliferation but also in the mediation of processes such as drug resistance, metastasis, neo-vascularization (angiogenesis), and apoptosis. These are all important targets in their own right and the development of drugs against specific "upstream" targets in oncogenic or growth factor signal transduction cascades it may be possible to inhibit multiple "downstream" targets. Ultimately, to test the hypothesis that signaling pathways offer good targets for anticancer drug development will take several years of careful clinical study and we cannot say at this time whether the approach will work. There are a small number of compounds in the early stages of clinical development as anticancer agents that may act by inhibiting growth factor signaling pathways. In all cases the activity of the compounds on intracellular signaling pathways was discovered after their identification as antiproliferative agents. There are also compounds in preclinical development that have been specifically developed as inhibitors of growth factor signaling, although their selectivity for tumor cells compared to normal tissue remains to be investigated fully in appropriate animal tumor models. It is possible that a single antisignaling drug by itself may not have the power to completely inhibit tumor growth and a combination of drugs may be needed. It may also take a combination of drugs to prevent the emergence of resistance. Clearly there are several challenges to developing this new class of anticancer drugs, and there will undoubtedly be others that must be faced.

Signalling pathways as targets for anticancer drug development

Intracellular signalling pathways mediating the effects of oncogenes on cell growth and transformation offer novel targets for the development of anticancer drugs. With this approach, it may be sufficient to target a component of the signalling pathway activated by the oncogene rather than the oncogene product itself. In this review, the abilities of some antiproliferative drugs to inhibit signalling targets are considered. There are some anticancer drugs already in clinical trial that may act by inhibiting signalling targets, as well as drugs in preclinical development. Some problems that may be encountered in developing this new class of anticancer drugs are discussed.

Regulation of the fertilization current in ascidian oocytes by intracellular second messengers

Neomycin, injected into ascidian oocytes to a final concentration of 10-50 mM, inhibits both the fertilization current and the surface contraction, showing that phosphoinositide hydrolysis is required for these early activation events. Sperm-activated fertilization currents are not inhibited in the presence of 100 micrograms/ml intracellular heparin, suggesting that these currents are not directly gated by InsP3. The sulfhydryl reagent thimerosal at 100 microM, in contrast, significantly increases the fertilization current presumably by sensitizing the channel receptor. Since heparin inhibits the surface contraction, InsP3 receptors are shown to play a role in the propagation of the activation response in ascidian oocyte. Depleting intracellular calcium stores by microinjecting 50 mM EGTA into oocytes does not activate fertilization channels; however, subsequent fertilization of these EGTA loaded oocytes leads to a significantly larger and faster fertilization current. Thus in contrast to somatic cells studied to date, second messenger operated plasma membrane channels in ascidian oocytes are not gated by calcium released from intracellular stores.

Inhibitors of phosphatidylinositol signalling as antiproliferative agents

Intracellular signalling pathways mediating the effects of oncogenes on cell growth and transformation offer novel targets for the development of anticancer drugs. With this approach it may be sufficient to target a component of the signalling pathway activated by the oncogene rather than the oncogene product itself. Phosphatidylinositol (PtdIns) is a key component of two growth factor signalling pathways. It acts as a substrate for PtdIns specific phospholipase C (PtdInsPLC) and for PtdIns-3-kinase. In this review the antiproliferative properties of some inhibitors of PtdInsPLC and PtdIns-3-kinase are considered. There are some compounds already in clinical trial as anticancer drugs that may act by inhibiting PtdIns signalling, as well as several compounds in preclinical development. Some problems that may be encountered in developing this new class of anticancer drugs are discussed.

Inositol 1,4,5-trisphosphate mass changes from fertilization through first cleavage in Xenopus laevis

After fertilization in Xenopus laevis, inositol 1,4,5-trisphosphate (IP3) mass increased from 53 to 261 fmol/cell and returned to near basal by 10 min after insemination. IP3 was also elevated over control egg levels during first mitosis and first cleavage. Because IP3 levels and the fertilization calcium wave decline at about the same time and because calcium ionophore or pricking the egg increased IP3, the fertilization calcium wave may be due to calcium-induced IP3 production. In addition, the onset of sperm motility was associated with an increase, whereas the acrosomal reaction was accompanied by a decrease in IP3 mass. Combining our published data with this report, the first chronology of the levels of IP3 from the induction of meiosis (maturation) through fertilization and cleavage in one cellular system is summarized. These data suggest an in vivo dose response for IP3 and calcium release. A small (17 fmol/cell) IP3 change during the induction of meiosis may not be associated with a calcium change. Larger IP3 changes at cleavage (40 fmol/cell) and mitosis (125 fmol/cell) are associated with localized small calcium increases, whereas the largest IP3 change (208 fmol/cell) is associated with the large calcium increase at fertilization.

Characterization of phosphoinositide kinases in normal and sickle anaemia red cells

PtdIns and PtdInsP kinases from normal erythrocyte (AA) membranes and sickle cell anaemia erythrocyte (SS) membranes have been characterized. PtdIns kinase was studied in native membranes under conditions in which PtdInsP kinase and PtdInsP phosphatase do not express any activity. Kinetic analysis of the AA and SS PtdIns kinases indicate similar Km values for PtdIns and ATP but higher Vmax values for SS PtdIns kinase. PtdInsP kinase was partially purified from erythrocyte ghosts by NaCl extraction. The kinetic parameters of PtdInsP kinase determined under these conditions were similar in AA and SS NaCl extracts. These data suggest the presence of some effector of PtdIns kinase in SS cell membranes, resulting in a greater activity of the enzyme. This leads consequently, to increase the PtdIns4P pool and to activate PtdInsP kinase, in agreement with our previous observations of a greater [32P]Pi incorporation in both polyphosphoinositides in SS cells relatively to AA cells.

Neomycin blocks dihydropyridine-insensitive Ca2+ influx in bovine adrenal chromaffin cells

There is evidence that bovine adrenal chromaffin cells are provided with both dihydropyridine-sensitive and -resistant voltage-sensitive Ca2+ influx pathways. Although recent electrophysiological work indicates that the dihydropyridine-resistant pathway is partially mediated by omega-conotoxin-sensitive and -insensitive Ca2+ channels, the pharmacological sensitivity of the latter channels remains elusive. We have now found that combined incubations with nitrendipine (1 microM) and neomycin (0.5 mM) reduced high K+ (50 mM)-evoked intracellular Ca2+ concentration ([Ca2+]i) transients to a larger extent than each drug separately. [Ca2+]i was measured using the fluorescent intracellular Ca2+ indicator fura-2. Neomycin (0.05-2 mM) reduced high K(+)-evoked 45Ca2+ uptake in a dose-dependent manner (IC50 = 0.09 mM). In the presence of nitrendipine (1 microM), the minimal neomycin concentration necessary for total blockade of 45Ca2+ uptake was reduced to 0.3 mM. Moreover, in the absence of nitrendipine the 45Ca2+ uptake remaining in 0.3 mM neomycin (26% of maximum) was similar to the fractional inhibition by nitrendipine alone (29%). Neomycin (0.05-2 mM) inhibited the [Ca2+]i transient induced by the L-type Ca2+ channel agonist Bay K 8644 (1 microM) much more extensively at 2 mM than at 0.3 mM (percent inhibition = 59% and 15%, respectively). Neomycin (0.05-2 mM) blocked high K(+)-evoked noradrenaline and adrenaline release in a dose-dependent fashion (IC50 = 0.8-1.1 mM), the blockade efficiency being enhanced in the presence of 1 microM nitrendipine (IC50 = 0.17-0.19 mM). It is concluded that neomycin (< or = 0.3 mM) blocks preferentially the dihydropyridine-insensitive Ca2+ influx pathway of the chromaffin cell. Moreover, both the dihydropyridine-sensitive and the dihydropyridine-resistant, neomycin-sensitive Ca2+ influx pathways contribute strongly to depolarization-evoked catecholamine secretion.

Thimerosal reveals calcium-induced calcium release in unfertilised sea urchin eggs

The fertilisation calcium wave in sea urchin eggs triggers the onset of development. The wave is an explosive increase in intracellular free calcium concentration ([Ca2+i]) that begins at the point of sperm entry and crosses the egg in about 20 s. Thimerosal is a sulphydryl reagent that sensitises calcium release from intracellular stores in a variety of cell types. Treatment of unfertilised eggs with thimerosal causes a slow increase [Ca2+i] that results eventually in a large, spontaneous calcium transient and egg activation. At shorter times after thimerosal treatment, egg activation and the calcium transient can be triggered by calcium influx through voltage-gated calcium channels, a form of calcium-induced/calcium release (CICR). Thimerosal treatment also reduces the latency of the fertilisation calcium response and increases the velocity of the fertilisation wave. These results indicate that thimerosal can unmask CICR in sea urchin eggs and suggest that the ryanodine receptor channel based CICR may contribute to explosive calcium release during the fertilisation wave.

Inositol-lipid-specific phospholipase C isoenzymes and their differential regulation by receptors

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Sperm-induced currents at fertilization in sea urchin eggs injected with EGTA and neomycin

Membrane currents were measured in single voltage-clamped sea urchin eggs (Lytechinus pictus and Lytechinus variegatus) that were injected with either EGTA or neomycin and inseminated. Although egg activation and the fertilization calcium wave were prevented by injection of either of these compounds, sperm attached and still elicited inward currents. Sperm-induced currents in EGTA-injected eggs had an abrupt onset, quickly reached a maximum, and then slowly declined in amplitude. Sperm incorporation occurred readily in EGTA-injected eggs. Similar results were obtained with another calcium chelator, BAPTA. In neomycin-injected eggs, sperm-induced currents generally had an abrupt onset and, in contrast to EGTA-injected eggs, the currents usually cut off rapidly. Sperm failed to enter the neomycin-injected eggs and the duration of sperm-induced currents in neomycin-injected eggs was markedly dependent upon the voltage-clamp holding potential, with shorter duration currents occurring at -70 than at -20 mV. The lability of the initial interaction between sperm and egg at negative holding potentials may explain why activation often fails when the egg membrane is voltage clamped at these potentials (Lynn et al., Dev. Biol. 128, 305-323, 1988).

Internal calcium release and activation of sea urchin eggs by cGMP are independent of the phosphoinositide signaling pathway

We show that microinjecting cyclic GMP (cGMP) into unfertilized sea urchin eggs activates them by stimulating a rise in the intracellular free calcium ion concentration ([Ca2+]i). The increase in [Ca2+]i is similar in both magnitude and duration to the transient that activates the egg at fertilization. It is due to mobilization of calcium from intracellular stores but is not prevented by the inositol trisphosphate (InsP3) antagonist heparin. Furthermore, cGMP does not stimulate the eggs Na+/H+ antiport when the [Ca2+]i transient is blocked by the calcium chelator bis-(O-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA), suggesting that cGMP does not activate eggs by interacting with the their phosphoinositide signaling pathway. However, the [Ca2+]i increase and activation are prevented in eggs in which the InsP3-sensitive calcium stores have been emptied by the prior microinjection of the InsP3 analogue inositol 1,4,5-trisphosphorothioate. These data indicate that cGMP activates eggs by stimulating the release of calcium from an InsP3-sensitive calcium store via a novel, though unidentified, route independent of the InsP3 receptor.

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.

Phosphoprotein inhibition of calcium-stimulated exocytosis in sea urchin eggs

We have investigated the role of protein phosphorylation in the control of exocytosis in sea urchin eggs by treating eggs with a thio-analogue of ATP. ATP gamma S (adenosine 5'-O-3-thiotriphosphate) is a compound which can be used as a phosphoryl donor by protein kinases, leading to irreversible protein thiophosphorylation (Gratecos, D., and E.H. Fischer. 1974. Biochem. Biophys. Res. Commun. 58:960-967). Microinjection of ATP gamma S inhibits cortical granule exocytosis, but has no effect on the sperm-egg signal transduction mechanisms which normally cause exocytosis by generating an increase in [Ca2+]i. ATP gamma S requires cytosolic factors for its inhibition of cortical granule exocytosis: it does not affect exocytosis when applied directly to the isolated exocytotic apparatus. Our data suggest that ATP gamma S irreversibly inhibits exocytosis via thiophosphorylation of proteins associated with the egg cortex. We have identified two thiophosphorylated proteins (33 and 27 kD) that are associated with the isolated exocytotic apparatus. They may mediate the inhibition of exocytosis by ATP gamma S. In addition, we show that okadaic acid, an inhibitor of phosphoprotein phosphatases, prevents cortical granule exocytosis at fertilization without affecting calcium mobilization. Like ATP gamma S, okadaic acid has no effect on exocytosis in vitro. Our results suggest that an inhibitory phosphoprotein can obstruct calcium-stimulated exocytosis in sea urchin eggs; on the other hand, they do not readily support the idea that a protein phosphatase is an essential component of the mechanism controlling exocytosis.

Multiple intracellular signals coordinate structural dynamics in the sea urchin egg cortex at fertilization

Fertilization of the sea urchin egg is accompanied by a sequence of structural changes in the egg cortex that include exocytosis, endocytosis, and microvillar growth. This architectural reorganization is coordinated by two intracellular signals: a rapid, transient rise in cytosolic free calcium and a slower, longer lasting increase in cytoplasmic pH. In this report we provide ultrastructural views of these events in quick-frozen eggs and discuss their relationship to the calcium and pH signals.

Gamete interactions and the fate of sperm organelles in fertilized echinoderm eggs

Investigations of gamete fusion, sperm entry and the fate of the sperm nucleus, plasma membrane, mitochondrion, and axonemal complex in fertilized echinoderm eggs are reviewed. The timing of gamete fusion with respect to the onset of electrical activity characteristic of the activated egg and the affects of fixation conditions on the stability of fusing membranes are discussed. Observations from investigations using cationized ferritin labeled gametes and immunogold cytochemistry to demonstrate the mixing of sperm plasma membrane components within the egg plasma membrane, in particular along the surface of the fertilization cone, are compared with results from studies in somatic cells. Transformations of the sperm nucleus into a male pronucleus, consisting of sperm nuclear envelope breakdown, chromatin dispersion, and formation of a pronuclear envelope, are correlated with recent biochemical observation of similar processes in other cellular systems. Fates of the sperm mitochondrion and axonemal complex are examined.

Guanosine 5'-thiotriphosphate may stimulate phosphoinositide messenger production in sea urchin eggs by a different route than the fertilizing sperm

We show that microinjecting guanosine-5'-thiotriphosphate (GTP gamma S) into unfertilized sea urchin eggs generates an intracellular free calcium concentration [( Ca]i) transient apparently identical in magnitude and duration to the calcium transient that activates the egg at fertilization. The GTP gamma S-induced transient is blocked by prior microinjection of the inositol trisphosphate (InsP3) antagonist heparin. GTP gamma S injection also causes stimulation of the egg's Na+/H+ antiporter via protein kinase C, even in the absence of a [Ca]i increase. These data suggest that GTP gamma S acts by stimulating the calcium-independent production of the phosphoinositide messengers InsP3 and diacylglycerol (DAG). However, the fertilization [Ca]i transient is not affected by heparin, nor can the sperm cause calcium-independent stimulation of protein kinase C. It seems that the bulk of InsP3 and DAG production at fertilization is triggered by the [Ca]i transient, not by the sperm itself. GDP beta S, a G-protein antagonist, does not affect the fertilization [Ca]i transient. Our findings do not support the idea that signal transduction at fertilization operates via a G-protein linked directly to a plasma membrane sperm receptor.

Neomycin does not interfere with the inositol phospholipid metabolism, but blocks binding of alpha-thrombin to intact human platelets

Neomycin was demonstrated to inhibit the binding of thrombin to intact human platelets. The effects of neomycin on both thrombin binding and thrombin-induced changes in inositol phospholipid metabolism could be reproduced by the thrombin antagonist hirudin. We propose that neomycin inhibits thrombin-induced platelet activation by interference with the cellular receptor.

A synthetic peptide of the pseudosubstrate domain of protein kinase C blocks cytoplasmic alkalinization during activation of the sea urchin egg

Multiple second messenger pathways have been proposed for transduction of the sperm-egg fusion event during fertilization of sea urchin eggs. Cytoplasmic alkalinization due to increased Na(+)-H+ antiport has been causally linked to many of the metabolic events during fertilization. Two possible second messenger pathways coupling sperm-egg fusion and antiporter activity are activation of protein kinase C (PKC) and Ca2(+)-calmodulin kinase. A selective inhibitor of PKC is PKC(19-36), a synthetic peptide of the pseudosubstrate domain of the kinase. Injection of PKC(19-36) into unfertilized sea urchin eggs blocked cytoplasmic alkalinization during activation by phorbol 12-myristate 13-acetate, a PKC agonist. The rise in pH during fertilization was partially blocked by PKC(19-36), which suggested that multiple pathways regulate the antiporter during fertilization. The use of fluorescein chromophores to measure intracellular pH in sea urchin eggs is also discussed.

Micromolar free calcium exposes ouabain-binding sites in digitonin-permeabilized Xenopus laevis oocytes

As demonstrated previously, digitonin-permeabilized Xenopus oocytes have a large internal pool of sodium pumps which are inaccessible to cytosolic ouabain [Schmalzing, Kröner & Passow (1989) Biochem. J. 260, 395-399]. Access to internal ouabain-binding sites required permeabilization of inner membranes with SDS. In the present study, micromolar free Ca2+ was found to stimulate ouabain binding in the digitonin-permeabilized cells (K0.5 0.5 microM-Ca2+, h 1.9, average of seven experiments) without disrupting intracellular membranes. Sustained incubation at 9 microM-Ca2+ was as effective as SDS in inducing access to the ouabain-binding sites of the internal sodium pumps. Omission of either Mg2+ or ATP completely abolished the Ca2+ effect. Half-maximal stimulation by Ca2+ required approx. 0.4 mM-MgATP. Of a variety of nucleotides tested, none was as effective as ATP (rank order ATP greater than ADP greater than ATP[S] (adenosine 5'-[gamma-thio]triphosphate) greater than CTP greater than UTP greater than ITP = XTP greater than GTP). Pi, AMP, cyclic AMP, cyclic GMP, GTP[S] (guanosine 5'-[gamma-thio]triphosphate) and a stable ATP analogue p[NH]ppA (adenosine 5'-[beta gamma-imido]triphosphate), were ineffective. The metalloendoproteinase inhibitor carbobenzoxy-Gly-Phe-amide reduced the Ca2+ effect by some 50%. Inhibitors of chymotrypsin and the Ca2+ proteinase calpain had no effect. Ca2+ ionophores (A23187 and ionomycin) and the polycations neomycin and polymixin B blocked the Ca2+ response entirely. Neomycin also abolished a Ca2(+)-independent stimulation of ouabain binding by the wasp venom mastoparan. The requirements for increasing the accessibility of ouabain-binding sites are remarkably similar to those for exocytosis in secretory cells, suggesting that oocytes and eggs possess a Ca2(+)-regulated pathway for the plasma membrane insertion of sodium pumps.

The initiation of development at fertilization

As seen, important advances have now been made in understanding the beginning of development at fertilization. Free calcium and pHi level changes result from a sperm-mediated breakdown of PPI with production of IP3. The resultant calcium increase, either alone or in concert with diacylglycerol, activates the Na(+)-H+ exchange and a consequent cytosolic pHi level increase. The calcium increase is responsible for the NADP change (via NAD kinase) and possibly the change in G6PD. These two changes could be involved solely in producing NADPH for fertilization membrane hardening or these changes could also have some role in the later initiation of DNA synthesis. The finding that other enzymes assayed in permeabilized cells also evince large changes in activity suggests that a global change may be occurring with important portents for cell activity. The role of calcium in furthering subsequent synthetic events, however, is unclear since no calcium target has yet been described that is necessary for the subsequent specific synthesis of proteins, as cyclins, or for the initiation of DNA synthesis. The pHi level increase, in concert with increased calcium, might be sufficient to start off protein synthesis and subsequent cyclin accumulation. However, the pHi level increase, independently of protein synthesis, can initiate new DNA synthesis. These independent events converge in the putative activation of MPF by cyclin, which then starts off the first mitotic cycle. Other independent events are associated with the sperm entry, cortical modifications, fertilization membrane elevation and the numerous changes leading to the fusion of the sperm and egg nucleus in the egg center. Fertilization represents one of the best studied examples of how a covert developmental program is made overt by an external messenger. The challenges for the near future are to explain how sperm-egg contact leads to PPI hydrolysis and how pHi level changes (and Cai level changes?) lead to the initiation of the cell cycle. The challenge for the distant future is describing how this program is set up during oogenesis.

Functional heterogeneity of polyphosphoinositides in human erythrocytes

After labelling of erythrocytes with [32P]P1 for 23 h, the specific radioactivities of the phosphomonoester groups of PtdIns4P and of PtdIns(4,5)P2 approached equilibrium values which were close to that of the gamma-phosphate of ATP (78-85%), showing that almost all of these phosphate groups were metabolically active. Phosphoinositidase C (PIC) activation, using Ca2+ and the ionophore A23187, of 32P-prelabelled erythrocytes was used to investigate a possible functional heterogeneity of the phosphoinositides. Hydrolysis of PtdIns(4,5)P2, measured from its radioactivity, decreased as function of the time of prelabelling up to a constant value equal to that measured from its content. In contrast, hydrolysis of PtdIns4P, determined both from radioactivity and from content, was always the same. These data suggest that newly labelled molecules of PtdIns(4,5)P2, initially accessible to PIC, then moved towards a PIC-resistant pool. This was further confirmed by measuring the fraction of labelled PtdIns(4,5)P2 molecules accessible to PIC after a prelabelling period of 5 min and different times of reincubation. Hydrolysis by PIC was also measured in erythrocytes in which the phosphoinositide content had been modified by activation (Mg2+-enriched cells) or inhibition (ATP-depleted cells) of the phosphoinositide kinases. The sizes of the PIC-resistant pools of polyphosphoinositides were not affected by these treatments, indicating that the kinases (and the phosphatases) act on the PIC-sensitive pools. This was also shown by the decrease in the production of Ins(1,4,5)P3 upon PIC activation in ATP-depleted erythrocytes. A model is presented in which the PIC-sensitive pools of polyphosphoinositides are those which are accessible to the kinases and the phosphatases and are rapidly turned over.

Effects of protein kinase C stimulation and free Ca2+ rise in mammalian egg activation

Protein phosphorylation activity, chromosome segregation, and cortical granule exocytosis (CGE) have been studied in mouse eggs activated parthenogenetically by specific PKC stimulators such as 4 beta-phorbol 12-myristate 13-acetate (PMA) and 1-oleyl-2-acetylglycerol (OAG), or by agents inducing an immediate increase in cytosolic calcium concentration ([Ca2+]i) such as ethanol and Ca-ionophore A23187. When protein phosphorylation activity of mouse eggs was analyzed 10 min after different activation treatments, the phosphorylation of a 32 kDa polypeptide was a feature common to all different parthenogenetic agents used. The appearance of such labeling was independent of an increasing [Ca2+]i, as indicated by direct measurements of 1) cytosolic Ca2+ concentration with fura-2 and 2) exogenous Ca2+ entrance into activated eggs. Emission of the second polar body was blocked in PMA-elicited parthenogenones, whereas it was apparently normal in OAG-treated eggs, unless the eggs were continuously exposed to OAG. CGE was almost immediate in ethanol-activated eggs, but in PMA-treated cells, it occurred significantly later, with a timing corresponding to that found for the appearance of sustained Ca2+ oscillations in this system. Here, we propose that in mammalian eggs 1) PKC stimulation represents an early regulatory step in egg activation; 2) this kinase activity is turned off before the second meiotic cleavage; and 3) cytosolic free Ca2+ rise is essential for CGE occurrence.

Polyphosphoinositide breakdown and subsequent exocytosis in the Ca2+/ionophore-induced acrosome reaction of mammalian spermatozoa

An investigation was made of the modifications in phospholipids that occur during the exocytotic event known as the 'sperm acrosome reaction'. Phospholipids were prelabelled with 32P, and exocytosis was induced with Ca2+ and the ionophore A23187. When incubated with [32P]Pi in various media suitable for supporting sperm survival or fertilization in vitro, spermatozoa from all five species examined (ram, boar, guinea pig, mouse and human) incorporated 32P rapidly into the components of the phosphoinositide cycle. There were differences both between species and between media with respect to the actual rate of incorporation of label, and also between species with respect to other phospholipids labelled. Treatment of spermatozoa with Ca2+ and A23187 to induce the acrosome reaction resulted in a rapid breakdown of phosphatidylinositol 4, 5-bisphosphate and phosphatidylinositol 4-phosphate, which was complete within 3 min; there was also a great increase in labelling of phosphatidate. Occurrence of acrosome reactions in the sperm population was only observed after 5-10 min and reached a maximum response of greater than 90% after more than 30 min. The phosphoinositide breakdown was related to subsequent exocytosis: after EGTA/ionophore treatment, neither inositide breakdown nor exocytosis took place; however, later addition of Ca2+ resulted in immediate inositide breakdown, and exocytosis followed, with a delay relative to Ca2+ addition exactly similar to that following standard Ca2+/ionophore treatment. Neomycin inhibited both inositide breakdown and subsequent exocytosis provided it was added together with Ca2+ and ionophore; however, if the drug was added 3 min after Ca2+ and ionophore (by which time inositide breakdown was already complete), exocytosis was not inhibited. Ca2+ seemed to have several consecutive roles in the acrosome reaction. Low (micromolar) levels of free Ca2+ were needed both for phosphoinositide breakdown and for an event downstream of this breakdown; no other bivalent cation could substitute for Ca2+ in either event, and inositide breakdown was actually inhibited by Mg2+. In addition, millimolar levels of Ca2+ were needed for later stages of exocytosis, although this requirement could be satisfied by Sr2+. We conclude that breakdown of polyphosphoinositides is an essential early process after Ca2+ entry in the chain of events that lead to exocytosis in the mammalian sperm acrosome reaction.

Binding of neomycin to phosphatidylinositol 4,5-bisphosphate (PIP2)

Schacht (Schacht, J. (1976) J. Neurochem. 27, 1119-1124) demonstrated that neomycin, an aminoglycoside antibiotic, binds with high affinity to phosphatidylinositol 4,5-bisphosphate (PIP2). We investigated the binding of neomycin to PIP2 by making electrophoretic mobility measurements with multilamellar bilayer vesicles and surface potential measurements with monolayers. The bilayers and monolayers were formed from mixtures of PIP2 and egg phosphatidylcholine (PC) in 0.1 M KCl at pH 7. Neomycin does not bind to PC; 10(-3) M neomycin affects neither the zeta potential of PC vesicles nor the surface potential of PC monolayers. In contrast, 10(-6) M neomycin reduces the magnitude of the zeta potential of PC/PIP2 vesicles (5, 9, and 17 mol% PIP2) and the surface potential of monolayers (17 mol% PIP2) to less than 50% of their initial values. The electrophoretic mobility results indicate that neomycin forms an electroneutral complex with PIP2; high concentrations (greater than 10(-4) M) of neomycin reduce the zeta potential of the PC/PIP2 vesicles to zero. We could describe our data with the Gouy-Chapman-Stern theory assuming the intrinsic association constant of the 1:1 neomycin-PIP2 complex is 10(5) M-1. Neomycin is widely used in cell biology to interfere with the generation of second messengers; we discuss the relevance of our results to these studies. Specifically, 10(-6) M neomycin binds greater than 50% of the PIP2 in a bilayer or monolayer but 10(-5)-10(-3) M neomycin is required to affect the turnover of PIP2 in permeabilized platelets, mast cells, and sea urchin eggs. This result is consistent with a hypothesis that most of the PIP2 in the inner leaflet of these plasma membranes is not accessible to neomycin because it is associated with proteins.