Phospholipase D and membrane traffic. Potential roles in regulated exocytosis, membrane delivery and vesicle budding

Localization and regulation of phospholipase D2 by ARF6

Phospholipase D (PLD) and ADP-ribosylation factor 6 (ARF6) have been implicated in vesicular trafficking and rearrangement of the actin cytoskeleton. We have explored the co-localization of rat PLD1b and rat PLD2 with wild type and mutant forms of ARF6 in HeLa cells and studied their activation by ARF6 and the role of the actin cytoskeleton. GFP-tagged PLD1 had a similar pattern to multivesicular and late endosomes and the trans-Golgi apparatus, but not to other organelles. When wild type or dominant negative ARF6 and PLD1 or PLD2 were co-expressed, they had a similar localization in cytosolic particles and at the cell periphery. In contrast, dominant active ARF6 caused cell shrinkage and had a similar localization with PLD1 and PLD2 in dense structures, containing the trans-Golgi apparatus and actin. Disruption of the actin cytoskeleton with cytochalasin D did not induce the formation of these structures. To determine, if ARF6 selectively activated PLD1 or PLD2, wild type and mutant forms of the ARF isoform were transfected together with PLD1 or PLD2. Wild type ARF6 did not affect either PLD isozyme, but dominant active ARF6 selectively activated PLD2 and dominant negative ARF6 selectively inhibited PLD2. In contrast, dominant active ARF1 or Rac1 stimulated both PLD isozymes but the ARF1 effect on PLD2 was very small. Cytochalasin D did not affect the activation of PLD by phorbol ester. The localizations of PLD and ARF6 were also analyzed by fractionation after methyl-beta-cyclodextrin extraction to deplete cholesterol. The results showed that all PLD isoforms and ARF6 mutants existed in the membrane fraction, but only wild type ARF6 was dependent on the presence of cholesterol. These experiments showed that wild type ARF6 had a similar location with PLD isoforms on cell staining, but it did not colocalize with PLD isoforms in fractionation experiments. It is proposed that activated ARF6 translocates to the cholesterol independent microdomain and then activates PLD2 there. It is further concluded that PLD2 is selectively activated by ARF6 in vivo and that disruption of the actin cytoskeleton does not affect this activation.

An essential role for phospholipase D in the activation of protein kinase C and degranulation in mast cells

Activation of phospholipase D (PLD) and protein kinase C (PKC) as well as calcium mobilization are essential signals for degranulation of mast cells. However, the exact role of PLD in degranulation remains undefined. In this study we have tested the hypothesis that the PLD product, phosphatidic acid, and diacylglycerides generated therefrom might promote activation of PKC. Studies were conducted in two rodent mast cell lines that were stimulated with Ag via FcepsilonRI and a pharmacologic agent, thapsigargin. Diversion of production of phosphatidic acid to phosphatidylbutanol (the transphosphatidylation reaction) by addition of l-butanol suppressed both the translocation of diacylglyceride-dependent isoforms of PKC to the membrane and degranulation. Tertiary-butanol, which is not a substrate for the transphosphatidylation, had a minimal effect on PKC translocation and degranulation, and 1-butanol itself had no effect on PKC translocation when PKC was stimulated directly with phorbol ester, 12-O-tetradecanoylphorbol-13-acetate. Also, in cells transfected with small inhibitory RNAs directed against PLD1 and PLD2, activation of PLD, generation of diacylglycerides, translocation of PKC, and degranulation were all suppressed. Phorbol ester, which did not stimulate degranulation by itself, restored degranulation when used in combination with thapsigargin whether PLD function was disrupted with 1-butanol or the small inhibitory RNAs. However, degranulation was not restored when cells were costimulated with Ag and phorbol ester. These results suggested that the production of phosphatidic acid by PLD facilitates activation of PKC and, in turn, degranulation, although additional PLD-dependent processes appear to be critical for Ag-mediated degranulation.

Ca(2+)-independent phospholipase A2 participates in the vesicular transport of milk proteins

Changes in the lipid composition of intracellular membranes are believed to take part in the molecular processes that sustain traffic between organelles of the endocytic and exocytic transport pathways. Here, we investigated the participation of the calcium-independent phospholipase A2 in the secretory pathway of mammary epithelial cells. Treatment with bromoenol lactone, a suicide substrate which interferes with the production of lysophospholipids by the calcium-independent phospholipase A2, resulted in the reduction of milk proteins secretion. The inhibitor slowed down transport of the caseins from the endoplasmic reticulum to the Golgi apparatus and affected the distribution of p58 and p23, indicating that the optimal process of transport of these proteins between the endoplasmic reticulum, the endoplasmic reticulum/Golgi intermediate compartment and/or the cis-side of the Golgi was dependent upon the production of lysolipids. Moreover, bromoenol lactone was found to delay the rate of protein transport from the trans-Golgi network to the plasma membrane. Concomitantly, membrane-bound structures containing casein accumulated in the juxtanuclear Golgi region. We concluded from these results that efficient formation of post-Golgi carriers also requires the phospholipase activity. These data further support the participation of calcium-independent phospholipase A2 in membrane trafficking and shed a new light on the tubulo/vesicular transport of milk protein through the secretory pathway.

Phospholipase d signaling and extracellular signal-regulated kinase-1 and -2 phosphorylation (activation) are required for maximal phorbol ester-induced transglutaminase activity, a marker of keratinocyte differentiation

Protein kinase C (PKC)-activating 12-O-tetradecanoylphorbol 13-acetate (TPA) stimulates phospholipase D (PLD) activity in primary mouse epidermal keratinocytes. PLD catalyzes the hydrolysis of phosphatidylcholine to yield phosphatidic acid (PA), which can be dephosphorylated to produce PKC-activating diacylglycerol. In the presence of small amounts of a primary alcohol, PLD can instead produce novel phosphatidylalcohols at the expense of PA and diacylglycerol. Here, we have demonstrated that inhibiting PLD signal generation with 1-butanol reduced TPA-stimulated transglutaminase activity, a marker of keratinocyte differentiation. On the other hand, the structurally related tertiary alcohol tert-butanol, which cannot be used by PLD, had no effect on TPA-induced transglutaminase activity. Since TPA activates all conventional and novel PKC isoforms directly, yet cannot overcome 1-butanol-mediated inhibition, this result suggests that PLD mediates its effects on transglutaminase activity (and keratinocyte differentiation) through an effector enzyme system distinct from the conventional or novel PKC isoenzymes. Data in the literature suggest that PA can recruit Raf-1 to the membrane, where it can be activated and initiate the mitogen-activated protein kinase cascade that culminates in activation of extracellular signal-regulated kinase (ERK)-1 and -2. Indeed, we found that inhibition of ERK-1/2 phosphorylation (activation) inhibited TPA-induced transglutaminase activity. However, inhibition of PLD-mediated signal generation had only a small effect on TPA-elicited ERK-1/2 phosphorylation (activation), whereas inhibition of ERK-1/2 did not affect PLD activation, suggesting that these two pathways likely operate largely in parallel. Thus, our results suggest the independent involvement of the PLD and ERK-1/2 pathways in mediating transglutaminase activity and keratinocyte differentiation.

Inhibition of Muscarinic Receptor-linked Phospholipase D Activation by Association with Tubulin

Mammalian phospholipase D (PLD) is considered a key enzyme in the transmission signals from various receptors including muscarinic receptors. PLD activation is a rapid and transient process, but a negative regulator has not been found that inhibits signal-dependent PLD activation. Here, for the first time, we report that tubulin binding to PLD2 is an inhibition mechanism for muscarinic receptor-linked PLD2 activation. Tubulin was identified in an immunoprecipitated PLD2 complex from COS-7 cells by peptide mass fingerprinting. The direct interaction between PLD2 and tubulin was found to be mediated by a specific region of PLD2 (amino acids 476-612). PLD2 was potently inhibited (IC50 <10 nM) by tubulin binding in vitro. In cells, the interaction between PLD2 and tubulin was increased by the microtubule disrupting agent nocodazole and reduced by the microtubule stabilizing agent Taxol. Moreover, PLD2 activity was found to be inversely correlated with the level of monomeric tubulin. In addition, we found that interaction with and the inhibition of PLD2 by monomeric tubulin is important for the muscarinic receptor-linked PLD signaling pathway. Interaction between PLD2 and tubulin was increased only after 1-2 min of carbachol stimulation when carbachol-stimulated PLD2 activity was decreased. The expression of the tubulin binding region of PLD2 blocked the later decrease in carbachol-induced PLD activity by masking tubulin binding. Taken together, these results indicate that an increase in local membrane monomeric tubulin concentration inhibits PLD2 activity, and provides a novel mechanism for the inhibition of muscarinic receptor-induced PLD2 activation by interaction with tubulin.

Phosphatidic acid and tumor necrosis factor-alpha induce the expression of CD83 through mitogen activated protein kinase pathway in a CD34+ hematopoietic progenitor cell line, KG1

To elucidate the signaling pathways involved in the expression of CD83, which is linked to the differentiation and maturation states of dendritic cells, we examined the effect of phosphatidic acid (PA) on the expression of CD83 in KG1, a CD34(+) hematopoietic progenitor cell. In the presence of tumor necrosis factor (TNF)-alpha, PA but not lyso-PA up-regulated CD83 on KG1 cells. Moreover, PA and TNF-alpha-induced expression of CD83 was slightly increased by propranolol, an inhibitor of PA phosphohydrolase but was unaffected by phospholipase A2 inhibitor. PA and TNF-alpha increased the phosphorylation of extracellular signal-regulated kinase (ERK)-1/2, p38-kinase, and c-Jun N-terminal kinase (JNK) by Western blotting. However, the up-regulation of CD83 by PA/TNF-alpha on KG1 was significantly abrogated by PD98059, a specific inhibitor of ERK kinase, but was enhanced by SP600125, a JNK inhibitor. Bis-indolylmaleimide, an inhibitor of protein kinase C, partially blocked the up-regulation of CD83 and ERK phosphorylation induced by PA and TNF-alpha. Moreover, the incubation of KG1 cells with phorbol ester and TNF-alpha for 5 days increased the protein level of phospholipase D. These results suggest that PA and TNF-alpha induce the up-regulation of CD83 and that their action is regulated by ERK and JNK.

Mechanism of Membrane Binding of the Phospholipase D1 PX Domain

Mammalian phospholipases D (PLD), which catalyze the hydrolysis of phosphatidylcholine to phosphatidic acid (PA), have been implicated in various cell signaling and vesicle trafficking processes. Mammalian PLD1 contains two different membrane-targeting domains, pleckstrin homology and Phox homology (PX) domains, but the precise roles of these domains in the membrane binding and activation of PLD1 are still unclear. To elucidate the role of the PX domain in PLD1 activation, we constructed a structural model of the PX domain by homology modeling and measured the membrane binding of this domain and selected mutants by surface plasmon resonance analysis. The PLD1 PX domain was found to have high phosphoinositide specificity, i.e. phosphatidylinositol 3,4,5-trisphosphate (PtdIns-(3,4,5)P(3)) >> phosphatidylinositol 3-phosphate > phosphatidylinositol 5-phosphate >> other phosphoinositides. The PtdIns(3,4,5)P(3) binding was facilitated by the cationic residues (Lys(119), Lys(121), and Arg(179)) in the putative binding pocket. Consistent with the model structure that suggests the presence of a second lipid-binding pocket, vesicle binding studies indicated that the PLD1 PX domain could also bind with moderate affinity to PA, phosphatidylserine, and other anionic lipids, which were mediated by a cluster of cationic residues in the secondary binding site. Simultaneous occupancy of both binding pockets synergistically increases membrane affinity of the PX domain. Electrostatic potential calculations suggest that a highly positive potential near the secondary binding site may facilitate the initial adsorption of the domain to the anionic membrane, which is followed by the binding of PtdIns(3,4,5)P(3) to its binding pocket. Collectively, our results suggest that the interaction of the PLD1 PX domain with PtdIns(3,4,5)P(3) and/or PA (or phosphatidylserine) may be an important factor in the spatiotemporal regulation and activation of PLD1.

Coupling actin and membrane dynamics during calcium-regulated exocytosis: a role for Rho and ARF GTPases

Release of neurotransmitters and hormones occurs by calcium-regulated exocytosis, a process that shares many similarities in neurons and neuroendocrine cells. Exocytosis is confined to specific regions in the plasma membrane, where actin remodelling, lipid modifications and protein-protein interactions take place to mediate vesicle/granule docking, priming and fusion. The spatial and temporal coordination of the various players to form a "fast and furious" machinery for secretion remain poorly understood. ARF and Rho GTPases play a central role in coupling actin dynamics to membrane trafficking events in eukaryotic cells. Here, we review the role of Rho and ARF GTPases in supplying actin and lipid structures required for synaptic vesicle and secretory granule exocytosis. Their possible functional interplay may provide the molecular cues for efficient and localized exocytotic fusion.

Expression and regulation of phospholipase D isoenzymes in human melanoma cells and primary melanocytes

Phospholipase D (PLD) is a highly regulated enzyme involved in lipid-mediated signal transduction processes affecting vesicular trafficking and cytoskeletal reorganization. It is regulated by protein kinase C, adenosine diphosphate (ADP)-ribosylation factors and Rho family proteins, and both protein kinase C and Rho family proteins have been implicated in the metastatic potential of melanoma. We analysed PLD in four human melanoma cell lines and in primary human melanocytes. Melanoma cell lines showed phosphatidylcholine-hydrolysing, phosphatidylinositol 4,5-bisphosphate-dependent PLD activity, which was activated by phorbol ester and a non-hydrolysable guanosine triphosphate (GTP) analogue in a dose-dependent and synergistic manner, whereas primary melanocytes exhibited only low PLD activity compared with the melanoma cell lines. As determined by reverse transcription polymerase chain reaction, both splicing variants of PLD1, PLD1a and PLD1b, and the isoenzyme PLD2, are expressed in melanoma cells and melanocytes. Western blot analysis showed that PLD1 expression was low in primary melanocytes in contrast to melanoma cells, which is in agreement with our finding of low activity. Interestingly, Rho protein mRNA was elevated in all melanoma cell lines. We conclude that in human melanoma cells, the PLD activity that is stimulated by phorbol ester requires ADP-ribosylation factor, protein kinase C and Rho proteins for full activity, and most probably represents the isoenzyme PLD1.

Activation of RBL-2H3 mast cells is dependent on tyrosine phosphorylation of phospholipase D2 by Fyn and Fgr

Both phospholipase D1 (PLD1) and PLD2 regulate degranulation when RBL-2H3 cells are stimulated via the immunoglobulin E receptor, Fc epsilon RI. However, the activation mechanism for PLD2 is unclear. As reported here, PLD2 but not PLD1 is phosphorylated through the Src kinases, Fyn and Fgr, and this phosphorylation appears to regulate PLD2 activation and degranulation. For example, only hemagglutinin-tagged PLD2 was tyrosine phosphorylated in antigen-stimulated cells that had been made to express HA-PLD1 and HA-PLD2. This phosphorylation was blocked by a Src kinase inhibitor or by small interfering RNAs directed against Fyn and Fgr and was enhanced by overexpression of Fyn and Fgr but not by other Src kinases. The phosphorylation and activity of PLD2 were further enhanced by the tyrosine phosphatase inhibitor, Na(3)VO(4). Mutation of PLD2 at tyrosines 11, 14, 165, or 470 partially impaired, and mutation of all tyrosines blocked, PLD2 phosphorylation and activation, although two of these mutations were detrimental to PLD2 function. PLD2 phosphorylation preceded degranulation, both events were equally sensitive to inhibition of Src kinase activity, and both were enhanced by coexpression of PLD2 and the Src kinases. The findings provide the first description of a mechanism for activation of PLD2 in a physiological setting and of a role for Fgr in Fc epsilon RI-mediated signaling.

Structural Determinants of PLD2 Inhibition by α-Synuclein

The presynaptic protein alpha-synuclein has been implicated in both neuronal plasticity and neurodegenerative disease, but its normal function remains unclear. We described the induction of an amphipathic alpha-helix at the N terminus (exons 2-4) of alpha-synuclein upon exposure to phospholipid vesicles, and hypothesized that lipid-binding might serve as a functional switch by stabilizing alpha-synuclein in an active (alpha-helical) conformation. Others have shown that alpha and beta-synucleins inhibit phospholipase D (PLD), an enzyme involved in lipid-mediated signaling cascades and vesicle trafficking. Here, we report that all three naturally occurring synuclein isoforms (alpha, beta, and gamma-synuclein) are similarly effective inhibitors of PLD2 in vitro, as is the Parkinson's disease-associated mutant A30P. The PD-associated mutant A53T, however, is a more potent inhibitor of PLD2 than is wild-type alpha-synuclein. We analyze mutations of the alpha-synuclein protein to identify critical determinants of human PLD2 inhibition in vitro. Deletion of residues 56-102 (exon 4) decreases PLD2 inhibition significantly; this activity of exon 4 may require adoption of an alpha-helical conformation, as mutations that disrupt alpha-helicity also abrogate inhibition. Deletion of C-terminal residues 130-140 (exon 6) completely abolishes inhibitory activity. In addition, PLD2 inhibition is blocked by phosphorylation at serine 129 or at tyrosine residues 125 and 136, or by mutations that mimic phosphorylation at these sites. We conclude that PLD2 inhibition by alpha-synuclein is mediated by a lipid-stabilized alpha-helical structure in exon 4 and also by residues within exon 6, and that this inhibition can be modulated by phosphorylation of specific residues in exons 5 and 6.

Munc-18-1 Inhibits Phospholipase D Activity by Direct Interaction in an Epidermal Growth Factor-reversible Manner

Mammalian phospholipase D (PLD) has been reported to be a key enzyme for epidermal growth factor (EGF)-induced cellular signaling, however, the regulatory mechanism of PLD is still unclear. In this report, we found that Munc-18-1 is a potent negative regulator of PLD in the basal state and that its inhibition is abolished by EGF stimulation. We investigated PLD-binding proteins obtained from rat brain extract, and identified a 67-kDa protein as Munc-18-1 by peptide-mass finger-printing. The direct association between PLD and Munc-18-1 was confirmed by in vitro binding analysis using the purified proteins, and their binding sites were identified as the phox homology domain of PLD and multiple sites of Munc-18-1. PLD activity was potently inhibited by Munc-18-1 in vitro (IC50 = 2-5 nm), and the cotransfection of COS-7 cells with Munc-18-1 and PLD inhibited basal PLD activity in vivo. In the basal state, Munc-18-1 coprecipitated with PLD and colocalized with PLD2 at the plasma membrane of COS-7 cells. EGF treatment triggered the dissociation of Munc-18-1 from PLD when PLD was activated by EGF. The dissociation of the endogenous interaction between Munc-18-1 and PLD, and the activation of PLD by EGF were also observed in primary cultured chromaffin cells. These results suggest that Munc-18-1 is a potent negative regulator of basal PLD activity and that EGF stimulation abolishes this interaction.

Phospholipase D2 localizes to the plasma membrane and regulates angiotensin II receptor endocytosis

Phospholipase D (PLD) is a key facilitator of multiple types of membrane vesicle trafficking events. Two PLD isoforms, PLD1 and PLD2, exist in mammals. Initial studies based on overexpression studies suggested that in resting cells, human PLD1 localized primarily to the Golgi and perinuclear vesicles in multiple cell types. In contrast, overexpressed mouse PLD2 was observed to localize primarily to the plasma membrane, although internalization on membrane vesicles was observed subsequent to serum stimulation. A recent report has suggested that the assignment of PLD2 to the plasma membrane is in error, because the endogenous isoform in rat secretory cells was imaged and found to be present primarily in the Golgi apparatus. We have reexamined this issue by using a monoclonal antibody specific for mouse PLD2, and find, as reported initially using overexpression studies, that endogenous mouse PLD2 is detected most readily at the plasma membrane in multiple cell types. In addition, we report that mouse, rat, and human PLD2 when overexpressed all similarly localize to the plasma membrane in cell lines from all three species. Finally, studies conducted using overexpression of wild-type active or dominant-negative isoforms of PLD2 and RNA interference-mediated targeting of PLD2 suggest that PLD2 functions at the plasma membrane to facilitate endocytosis of the angiotensin II type 1 receptor.

Gonococcal phospholipase d modulates the expression and function of complement receptor 3 in primary cervical epithelial cells

CR3-mediated endocytosis is a primary mechanism by which Neisseria gonorrhoeae elicits membrane ruffling and cellular invasion of the cervical epithelia. Our data indicate that, upon infection of cervical epithelia, N. gonorrhoeae specifically releases proteins, including a phospholipase D (PLD) homolog, which facilitate membrane ruffling. To elucidate the function of gonococcal PLD in infection of the cervical epithelia, we constructed an N. gonorrhoeae PLD mutant. By comparative association and/or invasion assays, we demonstrated that PLD mutant gonococci are impaired in their ability to adhere to and to invade primary cervical cells. This defect can be rescued by the addition of supernatants obtained from wild-type-infected cell monolayers but not by exogenously added Streptomyces PLD. The decreased level of total cell association (i.e., adherence and invasion) observed for mutant gonococci is, in part, attributed to the inability of these bacteria to recruit CR3 to the cervical cell surface with extended infection. Using electron microscopy, we demonstrate that gonococcal PLD may be necessary to potentiate membrane ruffling and clustering of gonococci on the cervical cell surface. These data may be indicative of the inability of PLD mutant gonococci to recruit CR3 to the cervical cell surface. Alternatively, in the absence of gonococcal PLD, signal transduction events required for CR3 clustering may not be activated. Collectively, our data indicate that PLD augments CR3-mediated gonococcus invasion of and survival within cervical epithelia.

Rice Phospholipase D Isoforms Show Differential Cellular Location and Gene Induction

Phospholipase D (PLD) has emerged as an important enzyme involved in signal transduction, stress responses, protein trafficking, and membrane metabolism. This report describes the cloning and characterization of three novel PLD genes from rice, designated RPLD3, RPLD4 and RPLD5. The rice PLDs, including the previously isolated RPLD1 and RPLD2, are similar to PLD subfamilies of Arabidopsis: Based on sequence homology and domain conservation, RPLD1 is most similar to the PLDalpha subfamily of PLDs while RPLD5 most closely resembles the PLDdelta type. RPLD2, 3 and 4 represent a unique subfamily, although they are most similar to PLDalpha. RPLD1 is located on chromosome 1, RPLD5 on chromosome 3, and RPLD2, RPLD3, and RPLD4 are tandemly arrayed on chromosome 5. Transcriptional analysis reveals that RPLD1, present in healthy rice vegetative tissues, is induced rapidly but transiently in wounded leaf tissues. RPLD2, also induced by wounding, is present at lower levels but for a more prolonged duration than RPLD1. Immunolocalization with peptide specific antibodies to each of the five PLDs was used to demonstrate that the isoforms have overlapping but distinct patterns of distribution in healthy rice cells. RPLD1 was detected in mesophyll cell wall, membranes, and chloroplasts, whereas RPLD3 and RPLD4 were located predominantly in the chloroplasts. Labeling of RPLD2 and RPLD5 was sparse, and was most concentrated in the secondary walls of xylem (RPLD2) and guard cells (RPLD2 and RPLD5). This combined information on structural features, expression profiles, and cellular localization will assist the basis for dissection of PLD isoform function in rice.

Regulation of phospholipase D by P2X7 receptors in submandibular ductal cells

ATP (1 mM) increased the phospholipase D (PLD) activity of rat submandibular gland (RSMG) ductal cells in a concentration-dependent and calcium-sensitive manner. The response to ATP was reproduced by benzoylbenzoyl-ATP (Bz-ATP, 100 microM) and also partly by adenosine 5'-(gamma-thio)triphosphate (ATPgammaS, 1 mM). A similar stimulation was observed in control mice (P2X7R+/+ mice) but not in mice lacking the P2X7 receptors (P2X7R-/- mice). Oxidized ATP and Coomassie blue or the addition of magnesium or nickel to the incubation medium inhibited the response to ATP. The stimulation of PLD by purinergic agonist was inhibited by about 50% by calphostin C and chelerythrine, two protein kinase C (PKC) inhibitors. The stimulation of PLD by Bz-ATP and by o-tetradecanoylphorbol 13-acetate (TPA), a phorbol ester which activates PKC, were not additive. From these results we can conclude that the activation of P2X7 receptors in RSMG ductal cells is coupled to the activation of a PLD. This activation is partly mediated by protein kinase C.

Regulation of exocytosis in adrenal chromaffin cells: focus on ARF and Rho GTPases

Neurons and neuroendocrine cells release transmitters and hormones by exocytosis, a highly regulated process in which secretory vesicles or granules fuse with the plasma membrane to release their contents in response to a calcium trigger. Several stages have been recognized in exocytosis. After recruitment and docking at the plasma membrane, vesicles/granules enter a priming step, which is then followed by the fusion process. Cortical actin remodelling accompanies the exocytotic reaction, but the links between actin dynamics and trafficking events remain poorly understood. Here, we review the action of Rho and ADP-ribosylation factor (ARF) GTPases within the exocytotic pathway in adrenal chromaffin cells. Rho proteins are well known for their pivotal role in regulating the actin cytoskeleton. ARFs were originally identified as regulators of vesicle transport within cells. The possible interplay between these two families of GTPases and their downstream effectors provides novel insights into the mechanisms that govern exocytosis.

ADP-ribosylation factor-dependent phospholipase D activation by the M3 muscarinic receptor

G protein-coupled receptors can potentially activate phospholipase D (PLD) by a number of routes. We show here that the native M3 muscarinic receptor in 1321N1 cells and an epitope-tagged M3 receptor expressed in COS7 cells substantially utilize an ADP-ribosylation factor (ARF)-dependent route of PLD activation. This pathway is activated at the plasma membrane but appears to be largely independent of G, phospholipase C, Ca2+ q/11, protein kinase C, tyrosine kinases, and phosphatidyl inositol 3-kinase. We report instead that it involves physical association of ARF with the M3 receptor as demonstrated by co-immunoprecipitation and by in vitro interaction with a glutathione S-transferase fusion protein of the receptor's third intracellular loop domain. Experiments with mutant constructs of ARF1/6 and PLD1/2 indicate that the M3 receptor displays a major ARF1-dependent route of PLD1 activation with an additional ARF6-dependent pathway to PLD1 or PLD2. Examples of other G protein-coupled receptors assessed in comparison display alternative pathways of protein kinase C- or ARF6-dependent activation of PLD2.

The GTP-binding protein RhoA mediates Na,K-ATPase exocytosis in alveolar epithelial cells

The purpose of this study was to define the role of the Rho family of small GTPases in the beta-adrenergic regulation of the Na,K-ATPase in alveolar epithelial cells (AEC). The beta-adrenergic receptor agonist isoproterenol (ISO) increased the Na,K-ATPase protein abundance at the plasma membrane and activated RhoA in a time-dependent manner. AEC pretreated with mevastatin, a specific inhibitor of prenylation, or transfected with the dominant negative RhoAN19, prevented ISO-mediated Na,K-ATPase exocytosis to the plasma membrane. The ISO-mediated activation of RhoA in AEC occurred via beta2-adrenergic receptors and involved Gs-PKA as demonstrated by incubation with the protein kinase A (PKA)-specific inhibitors H89 and PKI (peptide specific inhibitor), and Gi, as incubation with pertussis toxin or cells transfected with a minigene vector for Gi inhibited the ISO-mediated RhoA activation. However, cells transfected with minigene vectors for G12 and G13 did not prevent RhoA activation by ISO. Finally, the ISO-mediated Na,K-ATPase exocytosis was regulated by the Rho-associated kinase (ROCK), as preincubation with the specific inhibitor Y-27632 or transfection with dominant negative ROCK, prevented the increase in Na,K-ATPase at the plasma membrane. Accordingly, ISO regulates Na,K-ATPase exocytosis in AEC via the activation of beta2-adrenergic receptor, Gs, PKA, Gi, RhoA, and ROCK.

A phospholipase D-dependent process forms lipid droplets containing caveolin, adipocyte differentiation-related protein, and vimentin in a cell-free system

We developed a microsome-based, cell-free system that assembles newly formed triglyceride (TG) into spherical lipid droplets. These droplets were recovered in the d </= 1.055 g/ml fraction by gradient ultracentrifugation and were similar in size and appearance to those isolated from rat adipocytes and 3T3-L1 cells. Caveolin 1 and 2, vimentin, adipocyte differentiation-related protein, and the 78-kDa glucose regulatory protein were identified on the droplets from the cell-free system. The caveolin was soluble in 1% Triton X-100, as was the caveolin on lipid droplets from 3T3-L1 cells. The lipid droplets from the cell-free system, like those from 3T3-L1 cells, contained TG, diacylglycerol, phosphatidylcholine, phosphatidylethanolamine, and phosphatidylserine. The assembly of these TG-containing structures was dependent on the rate of TG biosynthesis and required an activator present in the 160,000 x g supernatant from homogenized rat adipocytes. The activator induced phospholipase D (PLD) activity, and its effect on the release of the TG-containing structures from the microsomes was inhibited by 1-butanol (but not 2-butanol) or 2,3-diphosphoglycerate. The activator could be replaced by a constitutively active PLD or phosphatidic acid. These results indicate that PLD and the formation of phosphatidic acid are important in the assembly of the TG-containing structures.

Molecular basis of platelet granule secretion

The energy-dependent release of granule contents from activated platelets is a well-established component of normal hemostasis and thrombosis. A role for membrane fusion in this process has been presumed for decades, but only recently have the mechanisms of platelet membrane fusion been investigated at the molecular level. Such studies have demonstrated that platelet membrane fusion is controlled by lipid components of the membrane bilayer, by transmembrane proteins termed SNARE proteins, and by chaperone proteins that interact with SNARE proteins. This core membrane fusion machinery is controlled by activation-dependent changes in cytoskeletal organization, intracellular calcium levels, kinase activity, and intracellular protease activity. Through these mechanisms, interactions of ligands with their cognate cell-surface receptors are transmitted to the membrane fusion machinery to facilitate membrane fusion and secretion of granule contents from platelets.

Cell signaling in yeast sporulation

Gametogenesis is essential for the propagation of all sexually reproducing organisms and consists of halving the chromosome number through meiosis, and the subsequent packaging of the haploid products into gametes. Meiosis and gamete formation must be tightly coupled to ensure the formation of viable progeny; perturbations result in infertility, inviability, and birth defects. In the yeast Saccharomyces cerevisiae, sexual reproduction occurs via sporulation and is similar in many respects to gametogenesis in mammals. An increasing number of conserved signaling molecules have been shown to be essential for yeast sporulation; recent studies reveal molecular insights into how these molecules regulate this intricate differentiation program.

The direct interaction of phospholipase C-gamma 1 with phospholipase D2 is important for epidermal growth factor signaling

The epidermal growth factor (EGF) receptor has an important role in cellular proliferation, and the enzymatic activity of phospholipase C (PLC)-gamma1 is regarded to be critical for EGF-induced mitogenesis. In this study, we report for the first time a phospholipase complex composed of PLC-gamma1 and phospholipase D2 (PLD2). PLC-gamma1 is co-immunoprecipitated with PLD2 in COS-7 cells. The results of in vitro binding analysis and co-immunoprecipitation analysis in COS-7 cells show that the Src homology (SH) 3 domain of PLC-gamma1 binds to the proline-rich motif within the Phox homology (PX) domain of PLD2. The interaction between PLC-gamma1 and PLD2 is EGF stimulation-dependent and potentiates EGF-induced inositol 1,4,5-trisphosphate (IP(3)) formation and Ca(2+) increase. Mutating Pro-145 and Pro-148 within the PX domain of PLD2 to leucines disrupts the interaction between PLC-gamma1 and PLD2 and fails to potentiate EGF-induced IP(3) formation and Ca(2+) increase. However, neither PLD2 wild type nor PLD2 mutant affects the EGF-induced tyrosine phosphorylation of PLC-gamma1. These findings suggest that, upon EGF stimulation, PLC-gamma1 directly interacts with PLD2 and this interaction is important for PLC-gamma1 activity.

Activation of phospholipase D is not mediated by direct phosphorylation on tyrosine residues

The activation of phospholipase D (PLD) in PC12/PC2 pheochromocytoma cells involves a tyrosine kinase. However, it is not clear whether this is due to direct phosphorylation of the enzyme or some other intermediary protein. In this manuscript, we examined this issue by two methods: (1) immunoprecipitation of phosphotyrosine containing proteins and assay of phospholipase D; (2) overexpression of HA-phospholipase D2 and susbsequent immunoprecipitation. The only agent that caused phosphorylation of phospholipase D on tyrosine residues was the phosphatase inhibitor, peroxyvanadate. Other agents that activate phospholipase D, including bradykinin, ionomycin, and phorbol dibutyrate did not cause phosphorylation of the enzyme. In addition, there was a lack of correlation between the peroxyvanadate-mediated phosphorylation and activation of phospholipase D, both in terms of time course and concentration dependence. These data demonstrate that phospholipase D is directly phosphorylated on tyrosine residues. However, phosphorylation of tyrosine residues does not correlate with activation of the enzyme.

Mastoparan selectively activates phospholipase D2 in cell membranes

Both known isoforms of phospholipase (PL) D, PLD1 and PLD2, require phosphatidylinositol 4,5-bisphosphate for activity. However, PLD2 is fully active in the presence of this phospholipid, whereas PLD1 activation is dependent on additional factors such as ADP-ribosylation factor-1 (ARF-1) and protein kinase Calpha. We find that mastoparan, an activator of G(i) and mast cells, stimulates an intrinsic PLD activity, most likely PLD2, in fractions enriched in plasma membranes from rat basophilic leukemia 2H3 mast cells. Overexpression of PLD2, but not of PLD1, results in a large increase in the mastoparan-inducible PLD activity in membrane fractions, particularly those enriched in plasma membranes. As in previous studies, expressed PLD2 is localized primarily in the plasma membrane and PLD1 in granule membranes. Studies with pertussis toxin and other agents indicate that mastoparan stimulates PLD2 independently of G(i), ARF-1, protein kinase C, and calcium. Kinetic studies indicate that mastoparan interacts synergistically with phosphatidylinositol 4,5-bisphosphate and that oleate, itself a weak stimulant of PLD2 at low concentrations, is a competitive inhibitor of mastoparan stimulation of PLD2. Therefore, mastoparan may be useful for investigating the regulation of PLD2, particularly in view of the well studied molecular interactions of mastoparan with certain other strategic signaling proteins.

Inhibition of formyl-methionyl-leucyl-phenylalanine-stimulated phospholipase D activation in rat neutrophils by the synthetic isoquinoline DMDI

The expression of phospholipase D (PLD) isoenzymes in neutrophils was investigated using reverse transcription-polymerase chain reaction analysis. Amplification products of predicted size were obtained from rat neutrophils with nucleotide sequences corresponding to PLD1a and PLD2. 1-(3',4'-Dimethoxybenzyl)-6,7-dichloroisoquinoline (DMDI) inhibited the formyl-methionyl-leucyl-phenylalanine (fMLP)-stimulated PLD activation in rat neutrophils. The underlying cellular signaling mechanism of DMDI inhibition was investigated. The fMLP-induced protein tyrosine phosphorylation and the membrane translocation of ADP-ribosylation factor (ARF) and Rho A in neutrophils was attenuated by DMDI in a concentration-dependent manner. However, neither the membrane association of protein kinase C-alpha and -beta isoenzymes in fMLP-stimulated cells nor the GTPgammaS- and phorbol 12-myristate 13-acetate-stimulated membrane translocation of ARF and Rho A in a cell-free system was affected significantly by DMDI. These results indicate that the expression of PLD1a and PLD2 mRNA in neutrophils. Attenuation of protein tyrosine phosphorylation and the membrane association of ARF and Rho A probably play a concerted role in the inhibition of PLD by DMDI in rat neutrophils in response to fMLP.

Pulmonary phosphatidic acid phosphatase and lipid phosphate phosphohydrolase

The lung contains two distinct forms of phosphatidic acid phosphatase (PAP). PAP1 is a cytosolic enzyme that is activated through fatty acid-induced translocation to the endoplasmic reticulum, where it converts phosphatidic acid (PA) to diacylglycerol (DAG) for the biosynthesis of phospholipids and neutral lipids. PAP1 is Mg(2+) dependent and sulfhydryl reagent sensitive. PAP2 is a six-transmembrane-domain integral protein localized to the plasma membrane. Because PAP2 degrades sphingosine-1-phosphate (S1P) and ceramide-1-phosphate in addition to PA and lyso-PA, it has been renamed lipid phosphate phosphohydrolase (LPP). LPP is Mg(2+) independent and sulfhydryl reagent insensitive. This review describes LPP isoforms found in the lung and their location in signaling platforms (rafts/caveolae). Pulmonary LPPs likely function in the phospholipase D pathway, thereby controlling surfactant secretion. Through lowering the levels of lyso-PA and S1P, which serve as agonists for endothelial differentiation gene receptors, LPPs regulate cell division, differentiation, apoptosis, and mobility. LPP activity could also influence transdifferentiation of alveolar type II to type I cells. It is considered likely that these lipid phosphohydrolases have critical roles in lung morphogenesis and in acute lung injury and repair.

Regulation of phospholipase D by muscarinic receptors in rat submandibular ductal cells

The muscarinic agonist carbachol stimulated phospholipase D (PLD) in rat submandibular gland (RSMG) ductal cells in a time and concentration-dependent manner. This effect was inhibited by chelation of extracellular calcium with ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA). PLD could also be activated by epinephrine and AlF(4)(-), two polyphosphoinositide-specific phospholipase C (PPI-PLC) activators, and by the phorbol ester o-tetradecanoylphorbol 13-acetate (TPA) which activates protein kinase C (PKC). Ionomycin and thapsigargin only slightly increased PLD activity. Ortho-vanadate, a tyrosine phosphatase inhibitor, also stimulated PLD activity. Both carbachol and o-vanadate increased the formation of inositol phosphates and the tyrosine phosphorylation of at least two proteins (55-60 and 120 kDa). Calphostin C (a PKC inhibitor), U73122 (a PPI-PLC inhibitor) and genistein (a tyrosine kinase inhibitor) blocked the activation of PLD, of PLC and the phosphorylation of tyrosyl residues in response to carbachol and vanadate. Taken together, these results suggest that rat submandibular gland ductal cells express a calcium-dependent PLD activity. This enzyme is regulated by carbachol via a PLC-PKC-tyrosine kinase pathway.

Roles of phospholipase D in apoptosis and pro-survival

Accumulating evidence has recognized phospholipase D (PLD) as an important element in signal transduction of cell responses, including proliferation and differentiation, However, its role in pro-apoptotic, anti-apoptotic or pro-survival signaling is not well-understood. Involvement of PLD in these signaling mechanisms is considered to differ depending on the cell type and the extracellular stimulus.

Localization of phospholipase D1 to caveolin-enriched membrane via palmitoylation: implications for epidermal growth factor signaling

Phospholipase D (PLD) has been suggested to mediate epidermal growth factor (EGF) signaling. However, the molecular mechanism of EGF-induced PLD activation has not yet been elucidated. We investigated the importance of the phosphorylation and compartmentalization of PLD1 in EGF signaling. EGF treatment of COS-7 cells transiently expressing PLD1 stimulated PLD1 activity and induced PLD1 phosphorylation. The EGF-induced phosphorylation of threonine147 was completely blocked and the activity of PLD1 attenuated by point mutations (S2A/T147A/S561A) of PLD1 phosphorylation sites. The expression of a dominant negative PKCalpha mutant by adenovirus-mediated gene transfer greatly inhibited the phosphorylation and activation of PLD1 induced by EGF in PLD1-transfected COS-7 cells. EGF-induced PLD1 phosphorylation occurred primarily in the caveolin-enriched membrane (CEM) fraction, and the kinetics of PLD1 phosphorylation in the CEM were strongly correlated with PLD1 phosphorylation in the total membrane. Interestingly, EGF-induced PLD1 phosphorylation and activation and the coimmunoprecipitation of PLD1 with caveolin-1 and the EGF receptor in the CEM were significantly attenuated in the palmitoylation-deficient C240S/C241S mutant, which did not localize to the CEM. Immunocytochemical analysis revealed that wild-type PLD1 colocalized with caveolin-1 and the EGF receptor and that phosphorylated PLD1 was localized exclusively in the plasma membrane, although some PLD1 was also detected in vesicular structures. Transfection of wild-type PLD1 but not of C240S/C241S mutant increased EGF-induced raf-1 translocation to the CEM and ERK phosphorylation. This study shows, for the first time, that EGF-induced PLD1 phosphorylation and activation occur in the CEM and that the correct localization of PLD1 to the CEM via palmitoylation is critical for EGF signaling.

Serine/threonine protein kinases synergistically regulate phospholipase D1 and 2 and secretion in RBL-2H3 mast cells

The role of phospholipase (PL) D in secretion was examined in RBL-2H3 mast cells which contain both PLD1 and 2. The effects of pharmacologic stimulants and inhibitors of Ca(2+)/calmodulin-dependent kinase II, protein kinase C, and protein kinase A suggested that all three kinases synergistically stimulate PLD and, when associated with a calcium signal, secretion as well to indicate a possible linkage between these two events. Overexpression of either PLD1 or 2 markedly enhanced the activation of PLD by pharmacologic stimulants as well as antigen and both isoforms thus appear co-ordinately regulated. As the expressed PLD1 was associated with secretory granules and PLD2 with the plasma membrane, the two isoforms may serve distinct but complementary functions in secretion.

Identification of phospholipase D from cabbage as N-terminally acetylated PLD2

Recently, the genes of two isoenzymes of phospholipase D from white cabbage (PLD1 and PLD2) with molecular masses of 91.7 and 91.9 kDa, respectively, have been sequenced and expressed in Escherichia coli [Schäffner, I., Rücknagel, K.-P., Mansfeld, J., and Ulbrich-Hofmann, R. (2002). Eur. J. Lipid Sci. Technol. 104: 79-87]. Both enzymes are highly homologous (91% identity) and behave very similarly. Phospholipase D purified from white cabbage leaves (PLDcab) is compared with the two recombinant enzymes in sodium dodecylsulfate and native polyacrylamide gel electrophoresis, isoelectric focusing, N-terminal sequencing, and mass spectrometry after tryptic digestion. As a result, PLDcab clearly can be assigned to PLD2. In contrast to recombinant PLD2, however, PLDcab is N-terminally acetylated.

Characterization and regulation of yeast Ca2+-dependent phosphatidylethanolamine-phospholipase D activity

An unconventional phospholipase D (PLD) activity was identified recently in Saccharomyces cerevisiae which is Ca2+-dependent, preferentially hydrolyses phosphatidylethanolamine (PtdEtn) and phosphatidylserine and does not catalyse a transphosphatidylation with primary short-chain alcohols. We have characterized the cytosolic and membrane-bound forms of the yeast PtdEtn-PLD and examined the regulation of its activity under certain growth, nutritional and stress conditions. Both forms of PtdEtn-PLD activity were similarly activated by Ca2+ ions in a biphasic manner. Likewise, other divalent cations affected both cytosolic and membrane-bound forms to the same extent. The yeast PtdEtn-PLD activity was found to interact with immobilized PtdEtn in a Ca2+-dependent manner. The partially purified cytosolic form and the salt-extracted membrane-bound form of yeast PtdEtn-PLD exhibited a similar elution pattern on size-exclusion chromatography, coeluting as low apparent molecular weight peaks. PtdEtn-PLD activity was stimulated, along with Spo14p/Pld1p activity, upon dilution of stationary phase cultures in glucose, acetate and galactose media, but PtdEtn-PLD activation was less pronounced. Interestingly, PtdEtn-PLD activity was found to be elevated by approximately 40% in sec14ts mutants at the restrictive temperature, whereas in other sec mutants it remained unaffected. The activity of PtdEtn-PLD was reduced by 30-40% upon addition to the medium of inositol (75 micro m) in either wild-type yeast or spo14Delta mutants and this effect was seen regardless of the presence of choline, suggesting that transcription of the PtdEtn-PLD gene is down-regulated by inositol. Finally, exposure of yeast cells to H2O2 resulted in a transient increase in PtdEtn-PLD activity followed by a profound, nearly 90% decrease in activity. In conclusion, our results indicate that yeast PtdEtn-PLD activity is highly regulated: the enzyme is acutely activated upon entry into the cell cycle and following inactivation of sec14ts, and is inhibited under oxidative stress conditions. The implications of these findings are discussed.

Alterations in metabolism and gene expression in brain regions during cuprizone-induced demyelination and remyelination

Exposure of mice to the copper chelator, cuprizone, results in CNS demyelination. There is remyelination after removal of the metabolic insult. We present brain regional studies identifying corpus callosum as particularly severely affected; 65% of cerebroside is lost after 6 weeks of exposure. We examined recovery of cerebroside and ability to synthesize cerebroside and cholesterol following removal of the toxicant. The temporal pattern for concentration of myelin basic protein resembled that of cerebroside. We applied Affymetrix GeneChip technology to corpus callosum to identify temporal changes in levels of mRNAs during demyelination and remyelination. Genes coding for myelin structural components were greatly down-regulated during demyelination and up-regulated during remyelination. Genes related to microglia/macrophages appeared in a time-course (peaking at 6 weeks) correlating with phagocytosis of myelin and repair of lesions. mRNAs coding for many cytokines had peak expression at 4 weeks, compatible with intercellular signaling roles. Of interest were other genes with temporal patterns correlating with one of the three above patterns, but of function not obviously related to demyelination/remyelination. The ability to correlate gene expression with known pathophysiological events should help in elucidating further function of such genes as related to demyelination/remyelination.

Neutral sphingomyelinase 1 deficiency in the mouse causes no lipid storage disease

Sphingomyelin is a major lipid in the bilayer of subcellular membranes of eukaryotic cells. Different sphingomyelinases catalyze the initial step in the catabolism of sphingomyelin, the hydrolysis to phosphocholine and ceramide. Sphingomyelinases have been postulated to generate ceramide as a lipophilic second messenger in intracellular signaling pathways involved in cell proliferation, differentiation, or apoptosis. To elucidate the function of the first cloned Mg(2+)-dependent, neutral sphingomyelinase (nSMase 1) in sphingomyelin catabolism and its potential role in signaling processes in a genetic and molecular approach, we have generated an nSMase 1-null mutant mouse line by gene targeting. The nSMase 1-deficient mice show an inconspicuous phenotype and no accumulation or changed metabolism of sphingomyelin or other lipids, despite grossly reduced nSMase activity in all organs except brain. We also addressed the recent proposal that nSMase 1 possesses lysophospholipase C activity. The unaltered metabolism of lysophosphatidylcholine or lyso-platelet-activating factor excludes the proposed role of nSMase 1 as a lysophospholipase C.

Novel mechanism for regulation of epidermal growth factor receptor endocytosis revealed by protein kinase A inhibition

Current models put forward that the epidermal growth factor receptor (EGFR) is efficiently internalized via clathrin-coated pits only in response to ligand-induced activation of its intrinsic tyrosine kinase and is subsequently directed into a lysosomal-proteasomal degradation pathway by mechanisms that include receptor tyrosine phosphorylation and ubiquitylation. Herein, we report a novel mechanism of EGFR internalization that does not require ligand binding, receptor kinase activity, or ubiquitylation and does not direct the receptor into a degradative pathway. Inhibition of basal protein kinase A (PKA) activity by H89 and the cell-permeable substrate peptide Myr-PKI induced internalization of 40-60% unoccupied, inactive EGFR, and its accumulation into early endosomes without affecting endocytosis of transferrin and mu-opioid receptors. This effect was abrogated by interfering with clathrin function. Thus, the predominant distribution of inactive EGFR at the plasma membrane is not simply by default but involves a PKA-dependent restrictive condition resulting in receptor avoidance of endocytosis until it is stimulated by ligand. Furthermore, PKA inhibition may contribute to ligand-induced EGFR endocytosis because epidermal growth factor inhibited 26% of PKA basal activity. On the other hand, H89 did not alter ligand-induced internalization of EGFR but doubled its half-time of down-regulation by retarding its segregation into degradative compartments, seemingly due to a delay in the receptor tyrosine phosphorylation and ubiquitylation. Our results reveal that PKA basal activity controls EGFR function at two levels: 1) residence time of inactive EGFR at the cell surface by a process of "endocytic evasion," modulating the accessibility of receptors to stimuli; and 2) sorting events leading to the down-regulation pathway of ligand-activated EGFR, determining the length of its intracellular signaling. They add a new dimension to the fine-tuning of EGFR function in response to cellular demands and cross talk with other signaling receptors.

Distinct signaling pathways mediate cardiomyocyte phospholipase D stimulation by endothelin-1 and thrombin

Several G protein-coupled receptors which stimulate phospholipase C (PLC) also activate phospholipase D (PLD) in cardiomyocytes. Here, we characterized PLD activation in neonatal rat cardiomyocytes by the PLC-stimulatory thrombin receptor PAR1, in comparison to the endothelin-1 receptor ET(A)R, which induces PLD stimulation by activation of protein kinase C (PKC) delta and epsilon. Similar to ET(A)R, activation of PAR1 induced PLD stimulation, which, however, was insensitive to PKC inhibition. Furthermore, in contrast to ET(A)R, PLD stimulation by PAR1 was suppressed by overexpression of regulators of G protein signaling specific for G(12)-type G proteins and treatment with brefeldin A, an inhibitor of guanine nucleotide exchange factors for ADP-ribosylation factor (ARF) GTPases. On the other hand, inactivation of Rho GTPases by Clostridium difficile toxin B and treatment with general tyrosine kinase inhibitors suppressed PAR1- and ET(A)R- as well as phorbol ester-induced PLD stimulation and was associated with a fall in the cellular level of phosphatidylinositol 4,5-bisphosphate (PIP(2)). We conclude that, in contrast to ET(A)R-PLD coupling, PAR1-induced cardiomyocyte PLD stimulation is PKC-independent and mediated by G(12)-type G proteins and ARF GTPases, while Rho and tyrosine kinases regulate PLD stimulation by either receptor, apparently by controlling the cellular level of PIP(2), a common regulator of PLD activity.

Collapsin response mediator protein-2 inhibits neuronal phospholipase D(2) activity by direct interaction

Although the functional significance of neuronal phospholipase D (PLD) is being recognized, little is known about its regulatory role in neuronal cells. To elucidate the regulatory mechanism of neuronal PLD, we investigated PLD(2)-binding neuronal protein from rat brain cytosol. During the fractionation of rat brain cytosol by four-column chromatography, a 62-kDa PLD(2)-interacting protein was detected by PLD(2) overlay assay and identified as collapsin response mediator protein-2 (CRMP-2), which controls neuronal axon guidance and outgrowth. Using bacterially expressed glutathione S-transferase fusion proteins, we found that two regions (amino acids 65-192 (the phagocytic oxidase domain) and 724-825) of PLD(2) and a single region (amino acids 243-300) of CRMP-2 are required for the direct binding of both proteins. A co-immunoprecipitation study in COS-7 cells also showed an in vivo interaction between CRMP-2 and PLD(2). Interestingly, CRMP-2 was found to potently inhibit PLD(2) activity in a concentration-dependent manner (IC(50) = 30 nm). Overexpression studies also showed that CRMP-2 is an in vivo inhibitor of PLD(2) in PC12 cells. Moreover, increasing the concentration of semaphorin 3A, one of the repulsive axon guidance cues, showed that PLD(2) activity can be inhibited in PC12 cells. Immunocytochemistry further revealed that PLD(2) is co-localized with CRMP-2 in the distal tips of neurites, its possible action site, in differentiated PC12 cells. Taken together, our results indicate that CRMP-2 may interact directly with and inhibit neuronal PLD(2), suggesting that this inhibitory mode of regulation may play a role in neuronal pathfinding during the developmental stage.

Cell-permeable ceramides preferentially inhibit coated vesicle formation and exocytosis in Chinese hamster ovary compared with Madin-Darby canine kidney cells by preventing the membrane association of ADP-ribosylation factor

Differential effects of acetyl(C2-) ceramide (N-acetylsphingosine) were studied on coated vesicle formation from Golgi-enriched membranes of Chinese hamster ovary (CHO) and Madin-Darby canine kidney (MDCK) cells. C2-ceramide blocked the translocation of ADP-ribosylation factor-1 (ARF-1) and protein kinase C-alpha (PKC-alpha) to the membranes from CHO cells, but not those of MDCK cells. Consequently, C2-ceramide blocked the stimulation of phospholipase D1 (PLD1) by the cytosol and guanosine 5'-[gamma-thio]triphosphate (GTP[S]) in membranes from CHO cells. Basal specific activity of PLD1 and the concentration of ARF-1 were 3-4 times higher in Golgi-enriched membranes from MDCK cells compared with CHO cells. Moreover, PLD1 activity in MDCK cells was stimulated less by cytosol and GTP[S]. PLD2 was not detectable in the Golgi-enriched membranes. Incubation of intact CHO cells or their Golgi-enriched membranes with C2-ceramide also inhibited COP1 vesicle formation by membranes from CHO, but not MDCK, cells. Specificity was demonstrated, since dihydro-C2-ceramide had no significant effect on ARF-1 translocation, PLD1 activation or vesicle formation in membranes from both cell types. C2-ceramide also decreased the secretion of virus-like particles to a greater extent in CHO compared with MDCK cells, whereas dihydro-C2-ceramide had no significant effect. The results demonstrate a biological effect of C2-ceramide in CHO cells by decreasing ARF-1 and PKC-alpha binding to Golgi-enriched membranes, thereby preventing COP1 vesicle formation.

A role for phospholipase D1 in neurotransmitter release

Phosphatidic acid produced by phospholipase D (PLD) as a result of signaling activity is thought to play a role in membrane vesicle trafficking, either as an intracellular messenger or as a cone-shaped lipid that promotes membrane fusion. We recently described that, in neuroendocrine cells, plasma membrane-associated PLD1 operates at a stage of Ca(2+)-dependent exocytosis subsequent to cytoskeletal-mediated recruitment of secretory granules to exocytotic sites. We show here that PLD1 also plays a crucial role in neurotransmitter release. Using purified rat brain synaptosomes subjected to hypotonic lysis and centrifugation, we found that PLD1 is associated with the particulate fraction containing the plasma membrane. Immunostaining of rat cerebellar granule cells confirmed localization of PLD1 at the neuronal plasma membrane in zones specialized for neurotransmitter release (axonal neurites, varicosities, and growth cone-like structures). To determine the potential involvement of PLD1 in neurotransmitter release, we microinjected catalytically inactive PLD1(K898R) into Aplysia neurons and analyzed its effects on evoked acetylcholine (ACh) release. PLD1(K898R) produced a fast and potent dose-dependent inhibition of ACh release. By analyzing paired-pulse facilitation and postsynaptic responses evoked by high-frequency stimulations, we found that the exocytotic inhibition caused by PLD1(K898R) was not the result of an alteration in stimulus-secretion coupling or in vesicular trafficking. Analysis of the fluctuations in amplitude of the postsynaptic responses revealed that the PLD1(K898R) blocked ACh release by reducing the number of active presynaptic-releasing sites. Our results provide evidence that PLD1 plays a major role in neurotransmission, most likely by controlling the fusogenic status of presynaptic release sites.

The exocytotic signaling pathway induced by nerve growth factor in the presence of lyso-phosphatidylserine in rat peritoneal mast cells involves a type D phospholipase

Nerve growth factor (NGF) has been previously shown to induce exocytosis in rat peritoneal mast cells (RPMCs) in the presence of lyso-phosphatidylserine (lysoPS) by interacting with high-affinity NGF receptors of the TrkA-type. In RPMCs, type D phosphatidylcholine-selective phospholipases (PLDs) have been postulated to be involved in some exocytotic signaling pathways induced by different agonists. The aim of the present study was to assess a putative functional role of PLD for NGF/lysoPS-induced exocytosis in RPMCs. In 1-[14C]palmitoyl-2-lyso-3-phosphatidylcholine-labelled RPMCs, NGF/lysoPS stimulated the formation of diacylglycerol (DAG) and, in the presence of ethanol (1% [v/v]), phosphatidylethanol (PEtOH). These data indicate PLD-activation by NGF/lysoPS in RPMCs. Preincubation of RPMCs for 2 min with ethanol, an inhibitor of PLD-derived DAG-formation, dose-dependently (IC(50): 0.6% [v/v]) and agonist-selectively inhibited the NGF/lysoPS induced release of [3H]serotonin ([3H]5-HT) in [3H]5-HT-loaded RPMCs, confirming the functional importance of PLD-action. Exocytosis and PEtOH-production was potently inhibited by the broad-spectrum serine/threonine kinase inhibitor staurosporine and activated by the protein kinase C(PKC)-activator PMA (phorbol-12-myristate-13-acetate) suggesting a role for PKC as mediator for NGF/lysoPS-induced activation of PLD.

Phospholipase D1b and D2a generate structurally identical phosphatidic acid species in mammalian cells

Mammalian cells contain different phospholipase D enzymes (PLDs) whose distinct physiological roles are poorly understood and whose products have not been characterized. The development of porcine aortic endothelial (PAE) cell lines able to overexpress PLD-1b or -2a under the control of an inducible promoter has enabled us to characterize both the substrate specificity and the phosphatidic acid (PtdOH) product of these enzymes under controlled conditions. Liquid chromatography-MS analysis showed that PLD1b- and PLD2a-transfected PAE cells, as well as COS7 and Rat1 cells, generate similar PtdOH and, in the presence of butan-1-ol, phosphatidylbutanol (PtdBut) profiles, enriched in mono- and di-unsaturated species, in particular 16:0/18:1. Although PtdBut mass increased, the species profile did not change in cells stimulated with ATP or PMA. Overexpression of PLD made little difference to basal or stimulated PtdBut formation, indicating that activity is tightly regulated in vivo and that factors other than just PLD protein levels limit hydrolytic function. In vitro assays using PLD-enriched lysates showed that the enzyme could utilize both phosphatidylcholine and, much less efficiently, phosphatidylethanolamine, with slight selectivity towards mono- and di-unsaturated species. Phosphatidylinositol was not a substrate. Thus PLD1b and PLD2a hydrolyse a structurally similar substrate pool to generate an identical PtdOH product enriched in mono- and di-unsaturated species that we propose to function as the intracellular messenger forms of this lipid.

Involvement of phospholipase D in sphingosine 1-phosphate-induced activation of phosphatidylinositol 3-kinase and Akt in Chinese hamster ovary cells overexpressing EDG3

Phospholipase D (PLD), phosphatidylinositol 3-kinase (PI3K), and Akt are known to be involved in cellular signaling related to proliferation and cell survival. In this report, we provide evidence that PLD links sphingosine 1-phosphate (S1P)-induced activation of the G protein-coupled EDG3 receptor to stimulation of PI3K and its downstream effector Akt in Chinese hamster ovary (CHO) cells. S1P stimulation of EDG3-overexpressing CHO cells but not vector-transfected cells induced activation of PLD, PI3K, and Akt in a time- and dose-dependent manner. Akt phosphorylation was prevented by the PI3K inhibitors wortmannin and LY294002 (2-(4-monrpholinyl)-8-phenyl-4H-1-benzopyran-4-one), indicating that Akt activation was dependent on PI3K. S1P-induced activation of PI3K and Akt was abrogated by 1-butanol, which inhibited S1P-induced accumulation of phosphatidic acid by serving as a phosphatidyl group acceptor in the transphosphatidylation reaction catalyzed by PLD, whereas both PI3K and Akt activation were not inhibited by 2-butanol without such reaction. Co-expression of wild-type PLD2 with myc-Akt resulted in increased Akt activation in response to S1P. In contrast, co-expression of a catalytically inactive mutant of PLD2 eliminated the S1P-induced Akt activation. The treatment of EDG3-expressing CHO cells with exogenous Streptomyces chromofuscus PLD, which caused an accumulation of phosphatidic acid, resulted in increases in PI3K activity and the phosphorylation of Akt, the latter of which was completely abolished by LY294002. Furthermore, S1P-induced membrane ruffling, which was dependent on PI3K and Rac, was inhibited by 1-butanol, but not by 2-butanol. These results demonstrate that PLD participates in the activation of PI3K and Akt stimulation of EDG3 receptor.

Expression and regulation of phospholipase D during neuronal differentiation of PC12 cells

To assess a possible role for phospholipase D (PLD) in PC12 cell signal transduction and differentiation, we have investigated the expression of PLD in PC12 cells and found that the differentiation factor, nerve growth factor (NGF) increased PLD1 protein expression and phorbol 12-myristate 13 acetate (PMA)-induced PLD activity. During neuronal differentiation, this effect showed correlation to the protein expression levels of classical protein kinase C (PKC) isozymes, PKC-alpha and -beta II, but there was no significant increase in the protein level of RhoA, another regulatory factor for PLD activation. Interestingly, PLD1 was associated with PKC-alpha or beta II, and its association gradually increased as NGF-induced neuronal differentiation progressed. PKC inhibitor, Ro-31-8220, caused a significant inhibition of neurite outgrowth and PLD activity. Furthermore, PLD1 was constitutively associated with the Shc adaptor molecule, the overexpression of which is known to induce PLD activity and to induce neurite outgrowth. Taken together, the data in this study suggests that PLD1 is closely implicated in neuronal differentiation of PC12 cells.

ADP ribosylation factor 6 binding to phosphatidylinositol 4,5-bisphosphate-containing vesicles creates defects in the bilayer structure: an electron spin resonance study

The effects of binding of myristoylated ADP ribosylation factor 6 (myr-ARF6), an activator of phospholipase D (PLD), to a model membrane were investigated using an electron spin resonance (ESR) labeling technique. Initial studies were conducted in vesicles composed of 1-palmitoyl-2-oleoyl phosphatidylethanolamine, dipalmitoylphosphatidylcholine, phosphatidylinositol 4,5-biphosphate (PIP(2)), and cholesterol. Recombinant ARF6 binding significantly enhances defects in both the headgroup and acyl-chain regions of the membrane, which are revealed by the emergence of sharp components in the spectra from a headgroup label, 1,2-dipalmitoylphosphatidyl-2,2,6,6-tetramethyl-1-piperidinyloxy-choline (DPPTC), and a chain label, 10PC, after myr-ARF6 binding. Binding of non-myristoylated ARF6 (non-ARF6) shows markedly reduced effects. Interestingly, no change in spectra from DPPTC was observed upon myr-ARF6 binding when PIP(2) in the vesicles was replaced by other negatively charged lipids, including phosphatidylinositol, phosphatidylserine, and phosphatidylglycerol, even when normalized for charge. The production of the sharp peak appears to be a specific event, because another GTP binding protein, CDC42, which binds PIP(2) and activates PLD, fails to induce changes in vesicle structure. These results suggest a previously unappreciated role for ARF in mediating a protein/lipid interaction that produces defects in lipid bilayers. This function may serve as an initial event in destabilizing membrane structure for subsequent membrane fusion or biogenesis of vesicles.

SPO14 separation-of-function mutations define unique roles for phospholipase D in secretion and cellular differentiation in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, phospholipase D (PLD), encoded by the SPO14 gene, catalyzes the hydrolysis of phosphatidylcholine, producing choline and phosphatidic acid. SPO14 is essential for cellular differentiation during meiosis and is required for Golgi function when the normal secretory apparatus is perturbed (Sec14-independent secretion). We isolated specific alleles of SPO14 that support Sec14-independent secretion but not sporulation. Identification of these separation-of-function alleles indicates that the role of PLD in these two physiological processes is distinct. Analyses of the mutants reveal that the corresponding proteins are stable, phosphorylated, catalytically active in vitro, and can localize properly within the cell during meiosis. Surprisingly, the separation-of-function mutations map to the conserved catalytic region of the PLD protein. Choline and phosphatidic acid molecular species profiles during Sec14-independent secretion and meiosis reveal that while strains harboring one of these alleles, spo14S-11, hydrolyze phosphatidylcholine in Sec14-independent secretion, they fail to do so during sporulation or normal vegetative growth. These results demonstrate that Spo14 PLD catalytic activity and cellular function can be differentially regulated at the level of phosphatidylcholine hydrolysis.

Phospholipase D1: a key factor for the exocytotic machinery in neuroendocrine cells

Phospholipase D (PLD) has been proposed to mediate cytoskeletal remodeling and vesicular trafficking along the secretory pathway. We recently described the activation of an ADP ribosylation factor-regulated PLD at the plasma membrane of chromaffin cells undergoing secretagogue-stimulated exocytosis. We show here that the isoform involved is PLD1b, and, using a real-time assay for individual cells, that PLD activation and exocytosis are closely correlated. Moreover, overexpressed PLD1, but not PLD2, increases stimulated exocytosis in a phosphatidylinositol 4,5-bisphosphate-dependent manner, whereas catalytically inactive PLD1 inhibits it. These results provide the first direct evidence that PLD1 is an important component of the exocytotic machinery in neuroendocrine cells.