Binding of phospholipase C delta 1 to phospholipid vesicles

Sphingolipids and glycerophospholipids - The "ying and yang" of lipotoxicity in metabolic diseases

Sphingolipids in general and ceramides in particular, contribute to pathophysiological mechanisms by modifying signalling and metabolic pathways. Here, we present the available evidence for a bidirectional homeostatic crosstalk between sphingolipids and glycerophospholipids, whose dysregulation contributes to lipotoxicity induced metabolic stress. The initial evidence for this crosstalk originates from simulated models designed to investigate the biophysical properties of sphingolipids in plasma membrane representations. In this review, we reinterpret some of the original findings and conceptualise them as a sort of "ying/yang" interaction model of opposed/complementary forces, which is consistent with the current knowledge of lipid homeostasis and pathophysiology. We also propose that the dysregulation of the balance between sphingolipids and glycerophospholipids results in a lipotoxic insult relevant in the pathophysiology of common metabolic diseases, typically characterised by their increased ceramide/sphingosine pools.

Functional analysis of duplicated genes and N-terminal splice variant of phospholipase C-δ1 in Paralichthys olivaceus

Phosphoinositide-specific phospholipase C δ (PLC δ) plays an important role in many cellular responses and is involved in the production of second messenger. Here, we describe the presence of novel N-terminal extended alternative splice form of PLC-δ1B in Paralichthys olivaceus, which differs from the reported mammalian PLC-δ1 isoform. The two variants PoPLC-δ1B-Lf and PoPLC-δ1B-Sf share exon 3 (including the PH domain) to exon 16, but differ at the exon 1 (Short form: Sf) and novel exon 2 (Long form: Lf) of the transcript. For the characterization of the novel duplicated gene variant of PLC-δ1B in P. olivaceus, tissue-specific expression with RT-PCR and real-time PCR, and purification and enzymatic characterization of native and recombinant proteins of all the three-types of PLC-δ1 isoforms (PoPLC-δ1A, PoPLC-δ1B-Lf and PoPLC-δ1B-Sf) of P. olivaceus were studied. The PoPLC-δ1A was ubiquitously distributed in gill, kidney and spleen. The PoPLC-δ1B-Lf gene was widely detected in various tissues, especially in the digestive system, while PoPLC-δ1B-Sf was highly expressed in the stomach. The recombinant PoPLC-δ1A, PoPLC-δ1B-Lf and PoPLC-δ1B-Sf proteins were expressed as a histidine-tagged fusion protein in Escherichia coli. The PLC activity of the PoPLC-δ1 isoform proteins showed a concentration-dependent activity to phosphatidylinositol (PI) and phosphatidylinositol 4,5-bisphosphate (PIP(2)). In addition, U73122, the PLC inhibitor, effectively inhibited PLC activities of PoPLC-δ1A, PoPLC-δ1B-Lf and PoPLC-δ1B-Sf proteins. However, PoPLC-δ1A and PoPLC-δ1B-Lf were sensitive at pH 7.5, while PoPLC-δ1B-Sf was relatively sensitive at pH 7. These results might be useful for the study of phospholipase C-mediated signal transduction in fish.

Phospholipase Cζ binding to PtdIns(4,5)P2 requires the XY-linker region

Phospholipase C-zeta (PLCζ) is a strong candidate for the mammalian sperm-derived factor that triggers the Ca²⁺ oscillations required for egg activation at fertilization. PLCζ lacks a PH domain, which targets PLCδ1 to the phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P₂) substrate in the plasma membrane. Previous studies failed to detect PLCζ in the plasma membrane, hence the means of PLCζ binding to PtdIns(4,5)P₂ is unclear. We find that the PLCζ XY linker, but not the C2 domain, exhibits robust binding to PtdIns(4,5)P₂ or to liposomes containing near-physiological levels of PtdIns(4,5)P₂. The role of positively charged residues within the XY linker was addressed by sequentially substituting alanines for three lysine residues, K374, K375 and K377. Microinjection of these mutants into mouse eggs enabled their Ca²⁺ oscillation-inducing activities to be compared with wild-type PLCζ. The XY-linker mutant proteins were purified and the in vitro PtdIns(4,5)P₂ hydrolysis and binding properties were monitored. Successive reduction of net positive charge within the PLCζ XY linker significantly affects both in vivo Ca²⁺-oscillation-inducing activity and in vitro PtdIns(4,5)P₂ interaction of mouse PLCζ. Our data suggest that positively charged residues within the XY linker play an important role in the PLCζ interaction with PtdIns(4,5)P₂, a crucial step in generating the Ca²⁺ activation signal that is essential for fertilization in mammals.

Molecular modeling of the membrane targeting of phospholipase C pleckstrin homology domains

Phospholipases C (PLCs) reversibly associate with membranes to hydrolyze phosphatidylinositol-4, 5-bisphosphate (PI[4,5]P(2)) and comprise four main classes: beta, gamma, delta, and epsilon. Most eukaryotic PLCs contain a single, N-terminal pleckstrin homology (PH) domain, which is thought to play an important role in membrane targeting. The structure of a single PLC PH domain, that from PLCdelta1, has been determined; this PH domain binds PI(4,5)P(2) with high affinity and stereospecificity and has served as a paradigm for PH domain functionality. However, experimental studies demonstrate that PH domains from different PLC classes exhibit diverse modes of membrane interaction, reflecting the dissimilarity in their amino acid sequences. To elucidate the structural basis for their differential membrane-binding specificities, we modeled the three-dimensional structures of all mammalian PLC PH domains by using bioinformatic tools and calculated their biophysical properties by using continuum electrostatic approaches. Our computational analysis accounts for a large body of experimental data, provides predictions for those PH domains with unknown functions, and indicates functional roles for regions other than the canonical lipid-binding site identified in the PLCdelta1-PH structure. In particular, our calculations predict that (1). members from each of the four PLC classes exhibit strikingly different electrostatic profiles than those ordinarily observed for PH domains in general, (2). nonspecific electrostatic interactions contribute to the membrane localization of PLCdelta-, PLCgamma-, and PLCbeta-PH domains, and (3). phosphorylation regulates the interaction of PLCbeta-PH with its effectors through electrostatic repulsion. Our molecular models for PH domains from all of the PLC classes clearly demonstrate how a common structural fold can serve as a scaffold for a wide range of surface features and biophysical properties that support distinctive functional roles.

Effects of hyperoxia and acrylonitrile on the phospholipase C isozyme protein levels in rat heart and brain

We previously showed that hyperoxia exerts oxidative stress on the rat cerebral cortex, and the protein levels of phospholipase C (PLC) -beta1 and -delta1, but not PLC-gamma1, were changed. Acrylonitrile (ACN) appears to induce astrocytomas through induction of oxidative stress on the rat brain selectively. This study compared hyperoxia or ACN treatments of rats with respect to lipid peroxidation and PLC levels in the heart and cerebral cortex. Treatment of rats with ACN promoted lipid peroxidation in the heart and cerebral cortex, the percent increase above control being greater in the cortex than heart. Hyperoxia did not cause significant increases in lipid peroxidation in the cerebral cortex or heart. In the ACN-treated cerebral cortex, significant increases in the PLC-beta1 and -delta1 in the cytosol, and PLC-gamma1 in the cytosolic and particulate fractions, and lysate were observed. In the rat heart, in which PLC-beta1 could not be detected, PLC-gamma1 and -delta1 were increased and decreased in the cytosolic and particulate fractions, respectively, by hyperoxia. In addition, the expression level of PLC-gamma1 was decreased in the lysate by the treatment. In the heart treated with ACN, there was no change in the level of PLC-gamma1, while PLC-delta1 was elevated in all fractions. These findings suggested that the expression levels of PLC isozymes are altered by hyperoxia and ACN, but there are apparent differences in these altered levels between the different levels of oxidative stress, and between the organs.

Structure, function, and control of phosphoinositide-specific phospholipase C

Phosphoinositide-specific phospholipase C (PLC) subtypes beta, gamma, and delta comprise a related group of multidomain phosphodiesterases that cleave the polar head groups from inositol lipids. Activated by all classes of cell surface receptor, these enzymes generate the ubiquitous second messengers inositol 1,4, 5-trisphosphate and diacylglycerol. The last 5 years have seen remarkable advances in our understanding of the molecular and biological facets of PLCs. New insights into their multidomain arrangement and catalytic mechanism have been gained from crystallographic studies of PLC-delta(1), while new modes of controlling PLC activity have been uncovered in cellular studies. Most notable is the realization that PLC-beta, -gamma, and -delta isoforms act in concert, each contributing to a specific aspect of the cellular response. Clues to their true biological roles were also obtained. Long assumed to function broadly in calcium-regulated processes, genetic studies in yeast, slime molds, plants, flies, and mammals point to specific and conditional roles for each PLC isoform in cell signaling and development. In this review we consider each subtype of PLC in organisms ranging from yeast to mammals and discuss their molecular regulation and biological function.

Protein kinase C-gamma phorbol-binding domain involved in protein-protein interaction

Protein kinase C-gamma (PKC-gamma) contains two cysteine-rich regions (Cys1, Cys2) responsible for interaction with phospholipids. However, previous experiments suggested that, only Cys1 represents the high affinity site involved in diacylglycerol-dependent activation of PKC-gamma. This raises the question whether Cys2 might participate in other functions of the PKC-gamma regulatory domain. The purpose of our studies was to examine the ability of Cys2 domain to bind cellular proteins. The Cys2 domain (residues 92-173) was expressed as a fusion protein with glutathione-S-transferase (GST) in Escherichia coli and purified. In order to investigate protein-protein interaction of Cys2 domain we used affinity column and an overlay assay. Our results demonstrate that the Cys2 domain of PKC-gamma binds several proteins from rat brain extracts. In the absence of phospholipids the Cys2 domain binds some proteins in the cytosolic fraction of rat brain, but no binding was detected with the proteins extracted from particulate fraction. Ca2+ at 1 microM concentration potentiated binding of cellular proteins to Cys2 domain. In the absence of Ca2+ the Cys2 domain binds proteins in the cytosolic fraction of rat brain in the presence of phosphatidylserine and to the lesser extend in the presence of phosphatidylinositol but neither phosphatidylcholine nor phosphatidylethanolamine. These results suggest that the Cys2 domain of PKC-gamma has the ability to interact with two classes of proteins. One class binds the Cys2 domain in the phosphatidylserine dependent fashion, and the other proteins bind Cys-2 domain in the Ca2+ dependent and phospholipid independent manner.

Binding of phosphatidylinositol-specific phospholipase C to phospholipid interfaces, determined by fluorescence resonance energy transfer

Dissociation constants for binding of phosphatidylinositol-specific phospholipase C from Bacillus cereus (bcPI-PLC) and the mammalian phosphatidylinositol-specific phospholipase C-delta(1) to lipid interfaces containing phosphoinositol, phosphocholine, and phosphomethanol head groups were determined by fluorescence resonance energy transfer. Dansyl-labeled lipid probes were used as acceptors, with intrinsic tryptophan of the enzyme as the donor. Titration of protein into lipid provided data from which K(d) and N, the limiting number of lipid molecules per protein bound, were calculated by non-linear regression analysis of exact binding equations. These results were compared with apparent K(m) values from kinetic data. K(d) values in the low microM range in terms of lipid monomers or low nM range in terms of binding sites were calculated with good fits of experimental data to theoretical binding curves. bcPI-PLC binds with high affinity to PI interfaces, slightly lower affinity to PC interfaces, and much lower affinity to PM interfaces. The mammalian enzyme also binds with high affinity to PI interfaces, but shows little or no binding with PC interfaces under similar concentration conditions. These K(d) values correlate reasonably with apparent K(m) values from kinetic experiments.

Phospholipase C-delta3 binds with high specificity to phosphatidylinositol 4,5-bisphosphate and phosphatidic acid in bilayer membranes

In order to acquire an understanding of phospholipase C-delta3 (PLC-delta3) action on substrate localized in lipid membrane we have studied the binding of human recombinant PLC-delta3 to large, unilamellar phospholipid vesicles (LUVs). PLC-delta3 bound weakly to vesicles composed of phosphatidylcholine (PtdCho) or PtdCho plus phosphatidylethanolamine (PtdEtn) or phosphatidylinositol (PtdIns). The enzyme bound strongly to LUVs composed of PtdEtn + PtdCho and phosphatidylinositol 4,5-bisphosphate (PtdInsP2). The binding affinity (molar partition coefficient) of PLC-delta3 to PtdEtn + PtdCho + PtdInsP2 vesicles was 7.7 x 105 m-1. High binding of PLC-delta3 was also observed for LUVs composed of phosphatidic acid (PA). Binding of PLC-delta3 to phosphatidylserine (PtdSer) vesicles was less efficient. Calculated molar partition coefficient for binding of PLC-delta3 to PA and PtdSer vesicles was 1.6 x 104 m-1 and 9.4 x 102 m-1, respectively. Presence of PA in the LUVs containing PtdInsP2 considerably enhanced the binding of PLC-delta3 to the phospholipid membrane. Binding of PLC-delta3 to phospholipid vesicles was not dependent on Ca2+ presence. In the liposome assay PA caused a concentration-dependent increase in activity of PLC-delta3. The stimulatory effect of PA on PLC-delta3 was calcium-dependent. At Ca2+ concentrations lower than 1 microm, no effect of PA on the activity of PLC-delta3 was observed. PA enhanced PLC-delta3 activity by increasing the Vmax and lowering Km for PtdInsP2. As the mol fraction of PA increased from 0-40 mol% the enzyme Vmax increased 2.3-fold and Km decreased threefold. Based on the results presented, we assume that PA supports binding of PLC-delta3 to lipid membranes by interaction with the PH domain of the enzyme. The stimulatory effect of PA depends on calcium-dependent interaction with the C2 domain of PLC-delta3. We propose that binding of PLC-delta3 to PA may serve as a mechanism for dynamic membrane association and modulation of PLC-delta3 activity.

Development of an assay for phospholipase C using column-reconstituted, extruded phospholipid vesicles

The reconstitution of heterotrimeric G proteins into phospholipid vesicles has been widely used for the measurement of PLC-beta activity in vitro. We have developed an improved and sensitive method for the assay of PLC-beta activity. This approach involves reconstitution of purified betagamma dimers into extruded phospholipid vesicles containing phosphatidylinositol 4, 5-bisphosphate and using a gel-filtration technique to separate the reconstituted vesicles from monodispersed betagamma dimers and the detergent used to solubilize G proteins. The method provides physical information about the partitioning of betagamma dimers into phospholipid vesicles and was used to examine the effect of different prenyl groups on the gamma subunits in the activation of PLC-beta. The beta1gamma1 dimer (containing the farnesyl group) and the beta1gamma2 dimer (containing the geranylgeranyl group) were purified from baculovirus-infected Sf9 insect cells and were found to partition equally into phospholipid vesicles. The beta1gamma2 dimer is more potent and effective in stimulating PLC-beta activity than the beta1gamma1 dimer. The EC50 values of betagamma dimers for the activation of PLC-beta determined with this method were lower than those determined by previous methodology, showing that betagamma subunits have a subnanomolar affinity for PLC-beta.

Pleckstrin homology domains: a common fold with diverse functions

Pleckstrin homology (PH) motifs are approximately 100 amino-acid residues long and have been identified in nearly 100 different eukaryotic proteins, many of which participate in cell signaling and cytoskeletal regulation. Despite minimal sequence homology, the three-dimensional structures are remarkably conserved. This review gives an overview of the PH domain architecture and examines the best-studied examples in an attempt to understand their function.

Regulation of phospholipase C-delta by GTP-binding proteins-rhoA as an inhibitory modulator

The regulation of Phospholipase C (PLC)delta activity remains obscure. These studies show that PLCdelta1 activity is significantly enhanced by both guanosine thiotriphosphate (GTPgammaS) and Clostridium botulinum exoenzyme C3 (C3) but not by aluminium fluoride. C3 ADP ribosylated a 21-kDa protein in the PLCdelta1 preparation and Western blotting identified rhoA in these samples. RhoA acts as an inhibitory modulator of PLCdelta activity.

The effect of different molecular species of sphingomyelin on phospholipase C delta 1 activity

Bovine brain sphingomyelin was separated into different molecular species using a reverse phase column. PLC delta 1 was inhibited by all molecular species of sphingomyelin. The extent of this inhibition was dependent on the hydrophobicity. Based on fatty acid analysis, we conclude that the inhibition of PLC delta 1 depends on the chain length and degree of unsaturation of the fatty acid moiety of SM. N-palmitoyl-D-sphingomyelin and N-stearoyl-D-sphingomyelin inhibited PLC delta 1 less then N-oleoyl-D-sphingomyelin. In the absence of Ca2+ (1 mM EGTA) all tested molecular species of SM inhibited weakly the enzyme. The sensitivity of PLC delta 1 to inhibition by SM increased with increasing Ca2+ concentration. The shape of calcium curve differed for molecular species with saturated and unsaturated fatty acids. Inhibition of PLC delta 1 by N-palmitoyl-D-sphingomyelin and N-stearoyl-D-sphingomyelin reached a maximum at 0.2 microM Ca2+, while inhibition by N-oleoyl-D-sphingomyelin reached maximum at 2 microM Ca2+. PLC delta 1 is more sensitive to inhibition by SM when it is maximally activated by spermine and calcium and the extent of this inhibition depends on the length and degree of fatty acid unsaturation of the molecular species.

Structural requirements of phospholipase C delta1 for regulation by spermine, sphingosine and sphingomyelin

We studied the relationship between sphingomyelin, calcium, spermine and sphingosine in regulation of phospholipase C (PLC) delta1 activity. Inhibition of PLC delta1 by sphingomyelin was promoted by spermine and Ca2+ and was partially abolished by sphingosine. The effect of sphingosine and spermine entirely depended on Ca2+. In the absence of Ca2+, no effect of these substances on PLC delta1 activity was observed. Using deletion mutants and active fragments of PLC delta1 generated by limited proteolysis, we have studied the structural requirements of the enzyme for regulation by these compounds. The deletion mutant of PLC delta1 lacking the first 58 amino acids and the mutant lacking the entire pleckstrin homology (PH) domain were fully active in the detergent assay, and their activities were affected by spermine, sphingosine, Ca2+ and sphingomyelin to the same extent as the native enzyme. The limited proteolysis of PLC delta1 generated two fragments of 40 kDa and 30 kDa, which formed a stable active complex. The relationship between Ca2+ concentration and enzymatic activity was almost identical for the native PLC delta1 and the proteolytic complex. The activity of the proteolytic complex formed by the 40 kDa and 30 kDa peptides was not affected by spermine and sphingosine. Sphingomyelin inhibited the complex slightly less than the native PLC delta1, and this inhibition was not promoted by spermine. These observations suggest that for activation of PLC delta1 by spermine and sphingosine, the region spanning domains of high conservation, named X and Y, must be intact. In contrast, the PH domain and the intact spanning region of the X and Y domains are not essential for inhibition of PLC delta1 by sphingomyelin.

Structural and mechanistic features of phospholipases C: effectors of inositol phospholipid-mediated signal transduction

The production of the intracellular second messengers inositol (1,4,5)-trisphosphate (InsP3) and sn 1,2-diacylglycerol (DG) in response to a wide variety of extracellular primary messengers is achieved by an extended family of inositol phospholipid phosphodiesterases termed phospholipases C (PLC, E.C. 3.1.4.11). This family has been the subject of extensive research and it is clear that the different isoenzymes exhibit some common characteristics (e.g., interactions with substrates) and other distinctive features (e.g., modes of regulation). The recent description of the X-ray crystal structure of a mammalian PLC has served to clarify much about the behaviour of the PLCs, emphasising the "modular" structure of these enzymes. The main focus of this review will concern the specific adaptations of PLC molecules which make them efficient lipid-metabolising enzymes. We also describe what is known about how these enzymes interact with their lipid substrates, which will serve as a basis for considering how PLCs may be activated.

Phosphoinositide binding specificity among phospholipase C isozymes as determined by photo-cross-linking to novel substrate and product analogs

We tested for the presence of high-affinity phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] and PI(3,4,5)P3 binding sites in four phospholipase C (PLC) isozymes (delta1, beta1, beta2, and beta3), by probing these proteins with analogs of inositol phosphates, D-Ins(1,4,5)P3, D-Ins(1,3,4,5)P4, and InsP6, and polyphosphoinositides PI(4,5)P2 and PI(3,4,5)P3, which contain a photoactivatable benzoyldihydrocinnamide moiety. Only PLC-delta1 was specifically radiolabeled. More than 90% of the label was found in tryptic and chymotryptic fragments which reacted with antisera against the pleckstrin homology (PH) domain, whereas less than 5% was recovered in fragments that encompassed the catalytic core. In separate experiments, the isolated delta1-PH domain was also specifically labeled. Equilibrium binding of D-Ins(1,4,5)P3 to PLC-delta1 indicated the presence of a single, high-affinity binding site; binding of D-Ins(1,4,5)P3 to PLC-beta1, -beta2, or -beta3 was not detected. The catalytic activity of PLC-delta1 was inhibited by the product D-Ins(1,4,5)P3, whereas no inhibition of PLC-beta1, -beta2, or -beta3 activity was observed. These results demonstrate that the PH domain is the sole high-affinity PI(4,5)P2 binding site of PLC-delta1 and that a similar site is not present in PLC-beta1, -beta2, or -beta3. The data are consistent with the idea that the PH domain of PLC-delta1, but not the beta isozymes, directs the catalytic core to membranes enriched in PI(4,5)P2 and is subject to product inhibition.

Effect of sphingomyelin and its metabolites on the activity of human recombinant PLC delta 1

In an attempt to obtain sufficient quantities of pure phospholipase C delta 1 (PLC delta 1) necessary for structural and kinetic studies, human fibroblast PLC delta 1 was cloned in the pPROEX-1 vector, expressed in E. coli cells as a (6xHis) fusion protein and purified to homogeneity. From 11 of E. coli culture 21 mg of pure PLC delta 1 was obtained by a two-step purification procedure, which includes Ni(2+)-NAT agarose and Mono S cation exchange chromatography. Catalytic properties of recombinant PLC delta 1 with respect to activation by spermine and calcium ions and inhibition by sphingomyelin were similar to or identical to PLC delta 1 purified from rat liver. Calcium activation of PLC delta 1 was dependent on the presence of spermine. Half-maximal activity was attained at 250 and 170 nM of free Ca2+ in the presence and absence of spermine, respectively. Sphingomyelin and lysosphingomyelin were mixed type inhibitors with respect to PIP2. Ceramide inhibits PLC delta 1 very weakly. GM1, which is a ceramide bound glucosidically to the oligosaccharide moiety, was a strong non-competitive inhibitor of PLC delta 1. In the absence of spermine, sphingosine and phytosphingosine weakly activated PLC delta 1. The results indicate that the effect of sphingomyelin and its metabolites on PLC delta 1 activity depends on the presence of spermine. It is postulated that, among other factors, in vivo, activity of PLC delta 1 may depend on the turnover of sphingomyelin.

Regulation of phospholipase C delta1 by sphingosine

Sphingosine, which is on the pathway of sphingomyelin degradation, activates phospholipase C (PLC) delta1 moderately. In the liposome assay effect of sphingosine on PLC delta1 activity depends on KCl concentration. Stimulation of PLC delta1 by sphingosine increased as the KCl concentration is increased from 0 to 100 mM, and then diminished with the increasing KCl. In the liposome assay sphingosine diminishes inhibition of PLC delta1 by sphingomyelin. To determine the domain of PLC delta1 which interacts with sphingosine active proteolytic fragments of PLC delta1 were generated by trypsin digestion of the native enzyme. Sphingosine affects the activity of PLC delta1 fragment which lacked the amino-terminal domain (first 60 amino acids) but not the active fragment that has cleaved the domain spanning the X and Y region of PLC delta1. These observations indicate that for interaction of sphingosine with PLC delta1 intact domain that span regions of conservation, designated as X and Y is necessary. When the activity of PLC delta1 was assayed with PIP2 in the erythrocyte membrane as substrate, sphingosine strongly inhibited PLC delta1. The other homolog of sphingosine 4-hydroxysphinganine (phytosphingosine) inhibited PLC delta1 to much lesser extent. The activity of PLC delta1 was inhibited by 68% and 22% in the presence of 20 microM sphingosine and phytosphingosine, respectively. This inhibition was completely abolished by deoxycholate at a concentration of 1.5 mM. These observations suggest that sphingosine may regulate activity of PLC delta1 in the cell.

Regulation of eukaryotic phosphatidylinositol-specific phospholipase C and phospholipase D

This review focuses on two phospholipase activities involved in eukaryotic signal transduction. The action of the phosphatidylinositol-specific phospholipase C enzymes produces two well-characterized second messengers, inositol 1,4,5-trisphosphate and diacylglycerol. This discussion emphasizes recent advances in elucidation of the mechanisms of regulation and catalysis of the various isoforms of these enzymes. These are especially related to structural information now available for a phospholipase C delta isozyme. Phospholipase D hydrolyzes phospholipids to produce phosphatidic acid and the respective head group. A perspective of selected past studies is related to emerging molecular characterization of purified and cloned phospholipases D. Evidence for various stimulatory agents (two small G protein families, protein kinase C, and phosphoinositides) suggests complex regulatory mechanisms, and some studies suggest a role for this enzyme activity in intracellular membrane traffic.

Structural views of phosphoinositide-specific phospholipase C: signalling the way ahead

Recent structural studies of mammalian phosphoinositide-specific phospholipase C (PI-PLC) have begun to shed light on the mechanism whereby this family of effector enzymes is able to hydrolyze phospholipid substrates to yield second messengers. PI-PLC isozymes employ a variety of modules (PH domain, EF-hand domain, SH2 domain, SH3 domain and C2 domain) that are common in proteins involved in signal transduction to reversibly interact with membranes and protein components of the signalling pathways.

Inhibition of phospholipase C-delta 1 catalytic activity by sphingomyelin

We measured the ability of sphingomyelin (SPM) to inhibit phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] hydrolysis catalyzed by human phospholipase C-delta 1 (PLC-delta 1) in model membranes and detergent phospholipid mixed micelles. SPM strongly inhibited PLC-delta 1 catalytic activity measured in large unilamellar vesicles (LUVs) composed of egg phosphatidylcholine (PC), PI(4,5)P2, and SPM from brain or egg. At 37 or 45 degrees C, the rate of PI(4,5)P2 hydrolysis in PC/SPM/PI(4,5)P2 vesicles (15:80:5 mol:mol) was less than 25% of that observed in PC/PI(4,5)P2 vesicles (95:5). By contrast, catalysis was only weakly inhibited by equivalent concentrations of the SPM analog, 3-deoxy-2-O-stearoyl-SPM, which lacks hydrogen bond-donating groups at the C-3 and C-2 positions of the sphingolipid backbone. Inhibition by SPM was not observed in detergent/phospholipid mixed micelles. The binding affinity of PLC-delta 1 for vesicles containing PC and PI(4,5)P2 was slightly diminished by inclusion of SPM in the lipid mixture, but not enough to account for the decreased rate of catalysis. We could find no evidence of specific binding of the enzyme to SPM, which argues against a simple negative allosteric mechanism. To understand the cause of inhibition, the effects of SPM and 3-deoxy-2-O-stearoyl-SPM on the bulk properties of the substrate bilayers were examined. Increasing the mole fraction of SPM altered the fluorescence emission spectra of two sets of head group probes, 6-lauronyl(N,N-dimethylamino)naphthalene and N-[5-(dimethylamino)naphthalene-1-sulfonyl]-1,2-dihexadecanoyl-sn- glycero-3-phosphoethanolamine, that are sensitive to water content at the membrane/solution interface. Results obtained with both probes suggested a reduction in hydration with increasing SPM content. Vesicles containing 3-deoxy-2-O-stearoyl-SPM produced intermediate changes. Our results are most consistent with a model in which SPM inhibits PLC by increasing interlipid hydrogen bonding and by decreasing membrane hydration; both factors raise the energy barrier for activation of PLC-delta 1 at the membrane/protein microinterface.

Phosphatidylinositol 4,5-bisphosphate binding to the pleckstrin homology domain of phospholipase C-delta1 enhances enzyme activity

The pleckstrin homology (PH) domain is a newly recognized protein module believed to play an important role in signal transduction. While the tertiary structures of several PH domains have been determined, some co-complexed with ligands, the function of this domain remains elusive. In this report, the PH domain located in the N terminus of human phospholipase C-delta1 (PLCdelta1) was found to regulate enzyme activity. The hydrolysis of phosphatidylinositol (PI) was stimulated by phosphatidylinositol 4,5-bisphosphate (PIP2) in a dose-dependent manner with an EC50 = 1 microM (0.3 mol%), up to 9-fold higher when 5 microM (1.5 mol%) of PIP2 was incorporated into the PI/phosphatidylserine (PS)/phosphatidylcholine (PC) vesicles (30 microM of PI with a molar ratio of PI:PS:PC = 1:5:5). Stimulation was specific for PIP2, since other anionic phospholipids including phosphatidylinositol 4-phosphate had no stimulatory effect. PIP2-mediated stimulation was, however, inhibited by inositol 1,4, 5-triphosphate (IP3) in a dose-dependent manner, suggesting a modulatory role for this inositol. When a nested set of PH domain deletions up to 70 amino acids from the N terminus of PLCdelta1 were constructed, the deletion mutant enzymes all catalyzed the hydrolysis of the micelle forms of PI and PIP2 with specific activities comparable with those of the wild type enzyme. However, the stimulatory effect of PIP2 was greatly diminished when more than 20 amino acid residues were deleted from the N terminus. To identify the specific residues involved in PIP2-mediated enzyme activation, amino acids with functional side chains between residues 20 and 40 were individually changed to glycine. While all these mutations had little effect on the ability of the enzyme to catalyze the hydrolysis of PI or PIP2 micelles, the catalytic activity of mutants K24G, K30G, K32G, R38G, or W36G was markedly unresponsive to PIP2. Analysis of PIP2-stimulated PI hydrolysis by a dual substrate binding model of catalysis revealed that the micellar dissociation constant (Ks) of PLCdelta1 for the PI/PS/PC vesicles was reduced from 558 microM to 53 microM, and the interfacial Michaelis constant (Km) was reduced from 0.21 to 0.06 by PIP2. The maximum rate of PI hydrolysis (Vmax) was not affected by PIP2. These results demonstrate that a major function of the PH domain of PLCdelta1 is to modulate enzyme activity. Further, our results identify PIP2 as a functional ligand for a PH domain and suggest a general mechanism for the regulation of other proteins by PIP2.

Changes in platelet phospholipase C protein level and activity in Alzheimer's disease

We have previously demonstrated that PLC-delta was abnormally accumulated in autopsied brains with Alzheimer's disease (AD). As nonneuronal tissue involvement in AD is also suggested and PLC activity is reduced in AD platelets, we examined the changes of the protein level of PLC-delta and its enzyme activity in platelets taken from patients with AD and age-matched controls. PLC-delta in human platelets was detected as a 72 kDa protein using a specific antibody against PLC-delta. Western blots revealed that the protein level of PLC-delta was significantly higher in the cytosolic fraction prepared from AD platelets compared to controls. We investigated the activity of PLC-delta which hydrolyzes phosphatidylinositol and found that the PLC-delta activity in the cytosolic fraction from AD platelets was significantly reduced compared to the control. This finding that the enzyme activity per PLC-delta molecule is reduced in AD platelets is consistent with the study using Alzheimer brains. These results suggest that aberrant phosphoinositide metabolism is present in nonneuronal tissues as well as the brains of patients with AD.

Stimulation and binding of myocardial phospholipase C by phosphatidic acid

Exposure of adult ventricular myocytes to exogenous natural phosphatidic acid results in the production of inositol phosphates by unknown mechanism(s). We characterized stimulation of myocytic phosphoinositide-specific phospholipase C (PLC) by synthetic dioleoyl phosphatidic acid (PA) as a potential mechanism for modulation of inositol phosphate production. Our data demonstrate that exogenous PA, at 10(-8)-10(-5) M, caused a concentration-dependent increase in inositol 1,4,5-trisphosphate in adult rabbit ventricular myocytes. PA also caused a concentration-dependent increase in in vitro activity of myocytic PLC in the presence or absence of ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA). PLC-delta 1, the predominant isozyme of PLC expressed in adult rabbit ventricular myocytes, bound to liposomes of PA with high affinity in the presence of EGTA. The phosphomonoester group of PA was critical to in vitro stimulation of myocytic PLC activity and high-affinity binding of PLC-delta 1. We propose that binding of PLC-delta 1 to phosphatidic acid may be a novel mechanism for dynamic membrane association and modulation of PLC in adult ventricular myocytes.

Ligation of the alpha 2-macroglobulin signalling receptor on macrophages induces protein phosphorylation and an increase in cytosolic pH

We have recently described an alpha 2-macroglobulin (alpha 2M) signalling receptor which is distinct from the low-density lipoprotein-related protein/alpha 2M receptor (LRP/alpha 2MR). Ligation of the macrophage signalling receptor by alpha 2M-methylamine stimulates production of several second messengers and involves a pertussis toxin-insensitive G-protein. We now report that binding of alpha 2M-methylamine, or the cloned M(r) = 20,000 receptor-binding fragment from rat alpha 1M, to macrophage alpha 2M signalling receptors induces protein phosphorylation. By use of a monoclonal antibody to phospholipase C gamma l (PLC gamma l) we were able to identify it as one target for protein phosphorylation. Phosphorylation was time and concentration dependent, being optimal at about 60 s of incubation and a 100-200 nM ligand concentration. By use of a second monoclonal antibody directed against phosphotyrosine, we were able to demonstrate that at least a portion of the label was incorporated into one or more tyrosine residues. PLC gamma l phosphorylation was then studied in membrane preparations at 4 degrees C in order to minimize serine or threonine modification. Preincubation of macrophage membranes with genistein, a tyrosine kinase inhibitor, drastically reduced phosphorylation of PLC gamma l. Receptor-associated protein, which blocks alpha 2M binding to LRP/alpha 2MR but not to the alpha 2M signalling receptor, had no effect on alpha 2M-methylamine-induced tyrosine phosphorylation of PLC gamma l. Binding of lactoferrin to LRP/alpha 2MR failed to induce phosphorylation of PLC gamma l, further supporting the hypothesis that the alpha 2M signalling receptor and LRP/alpha 2MR are distinct entities. Growth factors which induce tyrosine phosphorylation typically cause a rise in cytosolic pH. Binding of a2M-methylamine to macrophages also gradually increased the intracellular pH in a concentration-dependent manner, being optimal at a 200 nM ligand concentration. The increase in pH was amiloride sensitive. We propose that receptor-recognized forms of a2M may function like growth factors with regard to macrophage regulation.

Glutamate-induced antigenic changes of phospholipase C-delta in cultured cortical neurons

Phosphoinositide-specific phospholipase C (PLC) is a key enzyme in signal transduction. It was previously demonstrated that an antibody to an isozyme of PLC, PLC-delta, produces intense staining of neurofibrillary tangles (NFT), the neurites surrounding senile plaque (SP) cores and neuropil threads in the brains of patients with Alzheimer's disease (AD). Although the etiology of neuronal degeneration in AD is still to be defined, excitotoxic glutamate might be a candidate. In the present study, an anti-PLC-delta antibody was used to examine the influence of glutamate on PLC-delta immunoreactivity in cultured rat cortical neurons. Exposure to glutamate caused the death of cultured cortical neurons and exhibited increased immunostaining with the anti-PLC-delta antibody. Subtoxic doses of glutamate also increased PLC-delta immunoreactivity in a dose-dependent manner. Both glutamate-induced neuronal degeneration and the increases in PLC-delta immunoreactivity were prevented by removal of extracellular Ca2+ or the application of an N-methyl-D-aspartate (NMDA) receptor antagonist, MK-801. The glutamate-induced increase in PLC-delta immunoreactivity was also prevented by N omega-nitro-L-arginine, a nitric oxide (NO) synthase inhibitor. These results suggest that NO formation secondary to Ca2+ influx by NMDA receptor activation leads to similar modifications of PLC-delta to those seen in AD.

Cloning and identification of amino acid residues of human phospholipase C delta 1 essential for catalysis

In vitro single point mutagenesis, inositol phospholipid hydrolysis, and substrate protection experiments were used to identify catalytic residues of human phosphatidylinositide-specific phospholipase C delta 1 (PLC delta 1) isolated from a human aorta cDNA library. Invariant amino acid residues containing a functional side chain in the highly conserved X region were changed by in vitro mutagenesis. Most of the mutant enzymes were still able to hydrolyze inositol phospholipid with activity ranging from 10 to 100% of levels in the wild type enzyme. Exceptions were mutants with the conversion of Arg338 to Leu (R338L), Glu341 to Gly (E341G), or His356 to Leu (H356L), which made the enzyme severely defective in hydrolyzing inositol phospholipid. Phospholipid vesicle binding experiments showed that these three cleavage-defective mutant forms of PLC delta 1 could specifically bind to phosphatidylinositol 4,5-bisphosphate (PIP2) with an affinity similar to that of wild type enzyme. Western blotting analysis of trypsin-treated enzyme-PIP2 complexes revealed that a 67-kDa major protein fragment survived trypsin digestion if the wild type enzyme, E341G, or H356L mutant PLC delta 1 was preincubated with 7.5 microM PIP2, whereas if it was preincubated with 80 microM PIP2, the size of major protein surviving was comparable to that of intact enzyme. However, mutant enzyme R338L was not protected from trypsin degradation by PIP2 binding. These observations suggest that PLC delta 1 can recognize PIP2 through a high affinity and a low affinity binding site and that residues Glu341 and His356 are not involved in either high affinity or low affinity PIP2 binding but rather are essential for the Ca(2+)-dependent cleavage activity of PLC.