Phospholipase D stimulates release of nascent secretory vesicles from the trans-Golgi network

Fragmentation of the Golgi apparatus. A role for beta III spectrin and synthesis of phosphatidylinositol 4,5-bisphosphate

Phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)) synthesis has been implicated in maintaining the function of the Golgi apparatus. Here we demonstrate that the inhibition of PtdIns(4,5)P(2) synthesis in vitro in response to primary alcohol treatment and the kinetics of Golgi fragmentation in vivo were very rapid and tightly coupled. Preloading Golgi membranes with short chain phosphatidic acid abrogated the alcohol-mediated inhibition of PtdIns(4,5)P(2) synthesis in vitro. We also show that fragmentation of the Golgi apparatus in response to diminished PtdIns(4,5)P(2) synthesis correlated with both the phosphorylation of a Golgi form of beta III spectrin, a PtdIns(4,5)P(2)-interacting protein, and changes in its intracellular redistribution. The data are consistent with a model suggesting that the decreased PtdIns(4,5)P(2) synthesis and the phosphorylation state of beta III spectrin modulate the structural integrity of the Golgi apparatus.

Phospholipase D2 is localized to the rims of the Golgi apparatus in mammalian cells

Phospholipase D (PLD) hydrolyzes phosphatidylcholine to generate phosphatidic acid, a molecule known to have multiple physiological roles, including release of nascent secretory vesicles from the trans-Golgi network. In mammalian cells two forms of the enzyme, PLD1 and PLD2, have been described. We recently demonstrated that PLD1 is localized to the Golgi apparatus, nuclei, and to a lesser extent, plasma membrane. Due to its low abundance, the intracellular localization of PLD2 has been characterized only indirectly through overexpression of chimeric proteins. Using antibodies specific to PLD2, together with immunofluorescence microscopy, herein we demonstrate that a significant fraction of endogenous PLD2 localized to the perinuclear Golgi region and was also distributed throughout cells in dense cytoplasmic puncta; a fraction of which colocalized with caveolin-1 and the plasma membrane. On treatment with brefeldin A, PLD2 translocated into the nucleus in a manner similar to PLD1, suggesting a potential role in nuclear signaling. Most significantly, cryoimmunogold electron microscopy demonstrated that in pituitary GH(3) cells >90% of PLD2 present in the Golgi apparatus was localized to cisternal rims and peri-Golgi vesicles exclusively. The data are consistent with a model whereby PLD2 plays a role in Golgi vesicular transport.

The role of phosphatidic acid in the regulation of the Ras/MEK/Erk signaling cascade

Phosphatidic acid (PA) is an important second messenger produced by the activation of numerous cell surface receptors. Recent data have suggested that PA regulates multiple cellular processes. This review addresses primarily the role of PA in the regulation of the Erk1/2 cascade pathway. A model for the regulation of Erk1/2 phosphorylation by cell surface receptors is presented. According to this model, agonists stimulate the binding of GTP to Ras and the activation of phospholipase D to generate phosphatidic acid. PA promotes the binding of cRaf-1 kinase to the membrane, where it interacts with Ras.GTP and other regulatory components of the pathway. Ras-Raf complexes remain bound to the surface of endosomes, where scaffolding complexes involving Ras, cRaf-1, MEK and Erk are formed. Complete activation and coupling of the cascade requires endocytosis, a process that is also modulated by PA.

Continual production of phosphatidic acid by phospholipase D is essential for antigen-stimulated membrane ruffling in cultured mast cells

Phospholipase Ds (PLDs) are regulated enzymes that generate phosphatidic acid (PA), a putative second messenger implicated in the regulation of vesicular trafficking and cytoskeletal reorganization. Mast cells, when stimulated with antigen, show a dramatic alteration in their cytoskeleton and also release their secretory granules by exocytosis. Butan-1-ol, which diverts the production of PA generated by PLD to the corresponding phosphatidylalcohol, was found to inhibit membrane ruffling when added together with antigen or when added after antigen. Inhibition by butan-1-ol was completely reversible because removal of butan-1-ol restored membrane ruffling. Measurements of PLD activation by antigen indicate a requirement for continual PA production during membrane ruffling, which was maintained for at least 30 min. PLD1 and PLD2 are both expressed in mast cells and green fluorescent protein-tagged proteins were used to identify PLD2 localizing to membrane ruffles of antigen-stimulated mast cells together with endogenous ADP ribosylation factor 6 (ARF6). In contrast, green fluorescent protein-PLD1 localized to intracellular vesicles and remained in this location after stimulation with antigen. Membrane ruffling was independent of exocytosis of secretory granules because phorbol 12-myristate 13-acetate increased membrane ruffling in the absence of exocytosis. Antigen or phorbol 12-myristate 13-acetate stimulation increased both PLD1 and PLD2 activity when expressed individually in RBL-2H3 cells. Although basal activity of PLD2-overexpressing cells is very high, membrane ruffling was still dependent on antigen stimulation. In permeabilized cells, antigen-stimulated phosphatidylinositol(4,5)bisphosphate synthesis was dependent on both ARF6 and PA generated from PLD. We conclude that both activation of ARF6 by antigen and a continual PLD2 activity are essential for local phosphatidylinositol(4,5)bisphosphate generation that regulates dynamic actin cytoskeletal rearrangements.

Regulation of ADL6 activity by its associated molecular network

Plant dynamin-like proteins consist of a group of high molecular weight GTPase with diverse structural arrangements and cellular localizations. In addition, unlike animal dynamins, there was no evidence for the involvement of any plant dynamin-like protein in clathrin-mediated vesicle trafficking. In this study we demonstrate that ADL6 (Arabidopsis dynamin-like protein 6), due to its domain arrangement, behaves similarly to the animal dynamins. The association of ADL6 with clathrin-coated vesicles was demonstrated by co-fractionation and immunocytochemical studies. ADL6 also interacted via its C-terminus with gamma-adaptin, an adaptor protein of clathrin-coated vesicles. Our results suggest that ADL6 participates in clathrin-mediated vesicle trafficking originating from the Golgi. In addition, our studies demonstrate that ADL6 intrinsic GTPase activity is regulated by its association with acidic phospholipids and an SH3 (Src homology 3)-containing protein.

Casein kinase I regulates membrane binding by ARF GAP1

ARF GAP1, a 415-amino acid GTPase activating protein (GAP) for ADP-ribosylation factor (ARF) contains an amino-terminal 115-amino acid catalytic domain and no other recognizable features. Amino acids 203-334 of ARF GAP1 were sufficient to target a GFP-fusion protein to Golgi membranes in vivo. When overexpressed in COS-1 cells, this protein domain inhibited protein transport between the ER and Golgi and, in vitro, competed with the full-length ARF GAP1 for binding to membranes. Membrane binding by ARF GAP1 in vitro was increased by a factor in cytosol and this increase was inhibited by IC261, an inhibitor selective for casein kinase Idelta (CKIdelta), or when cytosol was treated with antibody to CKIdelta. The noncatalytic domain of ARF GAP1 was phosphorylated both in vivo and in vitro by CKI. IC261 blocked membrane binding by ARF GAP1 in vivo and inhibited protein transport in the early secretory pathway. Overexpression of a catalytically inactive CKIdelta also inhibited the binding of ARF GAP1 to membranes and interfered with protein transport. Thus, a CKI isoform is required for protein traffic through the early secretory pathway and can modulate the amount of ARF GAP1 that can bind to membranes.

Similarities in the induction of post-Golgi vesicles by the vaccinia virus F13L protein and phospholipase D

Intracellular mature vaccinia virions are wrapped by cisternae, derived from virus-modified trans-Golgi or endosomal membranes, and then transported via microtubules to the cell periphery. Two viral proteins, encoded by the F13L and B5R open reading frames, are essential for the membrane-wrapping step. Previous transfection studies indicated that F13L induces the formation of post-Golgi vesicles that incorporate the B5R protein and that this activity depends on an intact F13L phospholipase motif. Here we show that the F13L protein has a general effect on the trafficking of integral membrane proteins from the Golgi apparatus, as both the vaccinia virus A36R protein and the vesicular stomatitis virus G protein also colocalized with the F13L protein in vesicles. In addition, increased expression of cellular phospholipase D, which has a similar phospholipase motif as, but little amino acid sequence identity with, F13L, induced post-Golgi vesicles that contained B5R and A36R proteins. Butanol-1, which prevents the formation of phosphatidic acid by phospholipase D and specifically inhibits phospholipase D-mediated vesicle formation, also inhibited F13L-induced vesicle formation, whereas secondary and tertiary alcohols had no effect. Moreover, inhibition of phospholipase activity by butanol-1 also reduced plaque size and decreased the formation of extracellular vaccinia virus without affecting the yield of intracellular mature virus. Phospholipase D, however, could not complement a vaccinia virus F13L deletion mutant, indicating that F13L has additional virus-specific properties. Taken together, these data support an important role for F13L in inducing the formation of vesicle precursors of the vaccinia virus membrane via phospholipase activity or activation.

Imaging secretory vesicles by fluorescent protein insertion in propeptide rather than mature secreted peptide

We combined confocal and live-cell imaging with a novel molecular strategy aimed at revealing mechanisms underlying glucose-regulated insulin vesicle secretion. The 'Ins-C-GFP' reporter monitors secretory peptide targeting, trafficking, and exocytosis without directly tagging the mature secreted peptide. We trapped a green fluorescent protein (GFP) reporter in equimolar quantity within the secretory vesicle by fusing it within the C peptide of proinsulin which only after nascent vesicle sealing and acidification is cleaved from the mature secreted A and B chains of insulin. Ins-C-GFP expression in mouse islets without fail exhibited punctate distribution of green fluorescence by confocal microscopy. Ins-C-GFP colocalized GFP with insulin at vesicle dense cores by immuno-electron microscopy. Glucose stimulation decreased vesicle fluorescence coordinately with enhanced secretion from islets of C-GFP detected by anti-GFP Western blots, and of insulin detected by anti-insulin radioimmunoassay. An insulin secretagogue with a red fluorescent label, glibenclamide BODIPY TR, was applied to islets expressing Ins-C-GFP. The stimulus response was imaged as a rise in red secretagogue leading to marked loss in green granules. Since neuropeptides as well as peptide hormones are processed from propeptides after sealing of secretory granules, vesicle trapping likely is widely applicable for studies on targeting, trafficking, and regulated release of secretory peptides.

Estrogen lowers Alzheimer beta-amyloid generation by stimulating trans-Golgi network vesicle biogenesis

Estrogen reduces the risk of Alzheimer's disease in post-menopausal women, beta-amyloid (Abeta) burden in animal models of Alzheimer's disease, and secretion of Abeta from neuronal cultures. The biological basis for these effects remains unknown. Here, utilizing cell-free systems derived from both neuroblastoma cells and primary neurons, we demonstrate that 17beta-estradiol (17beta-E2) stimulates formation of vesicles containing the beta-amyloid precursor protein (betaAPP) from the trans-Golgi network (TGN). Accelerated betaAPP trafficking precludes maximal Abeta generation within the TGN. 17beta-E2 appears to modulate TGN phospholipid levels, particularly those of phosphatidylinositol, and to recruit soluble trafficking factors, such as Rab11, to the TGN. Together, these results suggest that estrogen may exert its anti-Abeta effects by regulating betaAPP trafficking within the late secretory pathway. These results suggest a novel mechanism through which 17beta-E2 may act in estrogen-responsive tissues and illustrate how altering the kinetics of the transport of a protein can influence its metabolic fate.

Antigen-stimulated activation of phospholipase D1b by Rac1, ARF6, and PKCalpha in RBL-2H3 cells

Phospholipase D (PLD) activity can be detected in response to many agonists in most cell types; however, the pathway from receptor occupation to enzyme activation remains unclear. In vitro PLD1b activity is phosphatidylinositol 4,5-bisphosphate dependent via an N-terminal PH domain and is stimulated by Rho, ARF, and PKC family proteins, combinations of which cooperatively increase this activity. Here we provide the first evidence for the in vivo regulation of PLD1b at the molecular level. Antigen stimulation of RBL-2H3 cells induces the colocalization of PLD1b with Rac1, ARF6, and PKCalpha at the plasma membrane in actin-rich structures, simultaneously with cooperatively increasing PLD activity. Activation is both specific and direct because dominant negative mutants of Rac1 and ARF6 inhibit stimulated PLD activity, and surface plasmon resonance reveals that the regulatory proteins bind directly and independently to PLD1b. This also indicates that PLD1b can concurrently interact with a member from each regulator family. Our results show that in contrast to PLD1b's translocation to the plasma membrane, PLD activation is phosphatidylinositol 3-kinase dependent. Therefore, because inactive, dominant negative GTPases do not activate PLD1b, we propose that activation results from phosphatidylinositol 3-kinase-dependent stimulation of Rac1, ARF6, and PKCalpha.

Pharmacological importance of phospholipase D and phosphatidic acid in the regulation of the mitogen-activated protein kinase cascade

The stimulation of cells with many extracellular agonists leads to the activation of phospholipase (PL)D. PLD metabolizes phosphatidylcholine to generate phosphatidic acid (PA). Neither the mechanism through which cell surface receptors regulate PLD activation nor the functional consequences of PLD activity in mitogenic signaling are completely understood. PLD is activated by protein kinase C, phospholipids, and small GTPases of the ADP-ribosylation factor and Rho families, but the mechanisms linking cell surface receptors to the activation of PLD still require detailed analysis. Furthermore, the latest data on the functional consequences of the generation of cellular PA suggest an important role for this lipid in the regulation of membrane traffic and on the activation of the mitogen-activated protein kinase cascade. This review addresses these issues, examining some novel models for the physiological role of PLD and PA and discussing their potential usefulness as specific targets for the development of new therapies.

A novel phospholipase A1 with sequence homology to a mammalian Sec23p-interacting protein, p125

p125, a mammalian Sec23p-interacting protein, exhibits sequence homology with bovine testis phosphatidic acid-preferring phospholipase A(1). In this study, we identified and characterized a new homologue of p125, KIAA0725p. KIAA0725p exhibited remarkable sequence similarity with p125 throughout the entire sequence determined but lacked an N-terminal proline-rich, Sec23p-interacting region. In vitro binding analysis showed that KIAA0725p does not bind to Sec23p. KIAA0725p possessed phospholipase A(1) activity preferentially for phosphatidic acid. We examined the effects of overexpression of KIAA0725p on the morphology of organelles. Overexpression of KIAA0725p, like that of p125, caused dispersion of the endoplasmic reticulum-Golgi intermediate compartment and Golgi apparatus. Different from the case of p125, overexpression of KIAA0725p resulted in dispersion of tethering proteins located in the Golgi region and caused aggregation of the endoplasmic reticulum. Our results indicate that KIAA0725p is a new member of the phosphatidic acid-preferring phospholipase A(1) protein family and suggest that the cellular function of KIAA0725p is different from that of p125.

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.

Fragmentation and re-assembly of the Golgi apparatus in vitro. A requirement for phosphatidic acid and phosphatidylinositol 4,5-bisphosphate synthesis

Recent work from our laboratory demonstrated that phosphatidic acid (PA) and phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)), are required to maintain the structural integrity of the Golgi apparatus. To investigate the role of these lipids in regulating Golgi structure and function, we developed a novel assay to follow the release of post-Golgi vesicles. Isolated rat liver Golgi membranes were incubated with [(3)H]CMP sialic acid to radiolabel endogenous soluble and membrane glycoproteins present in the late Golgi and trans-Golgi network. The release of post-Golgi secretory vesicles was determined by measuring incorporation of (3)H-labeled proteins into a medium speed supernatant. Vesicle budding was dependent on temperature, cytosol, energy and time. Electron microscopy of Golgi fractions prior to and after incubation demonstrated that the stacked Golgi cisternae generated a heterogeneous population of vesicles (50- to 350-nm diameter). Inhibition of phospholipase D-mediated PA synthesis, by incubation with 1-butanol, resulted in the complete fragmentation of the Golgi membranes in vitro into 50- to 100-nm vesicles; this correlated with diminished PtdIns(4,5)P(2) synthesis. Following alcohol washout, PA synthesis resumed and in the presence of cytosol PtdIns(4,5)P(2) synthesis was restored. Most significantly, under these conditions the fragmented Golgi elements reformed into flattened cisternae and the re-assembled Golgi supported vesicle release. These data demonstrate that inositol phospholipid synthesis is essential for the structure and function of the Golgi apparatus.

Regulation of constitutive protein transit by phospholipase D in HT29-cl19A cells

Phospholipase D (PLD) plays a central role in the control of vesicle budding and protein transit. We previously showed that in resting epithelial HT29-cl19A cells, PLD is implicated in the control of constitutive protein transit, from the trans-Golgi network to the plasma membrane, and that phorbol ester stimulation of protein transit is correlated with PLD activation (Auger, R., Robin, P., Camier, B., Vial, G., Rossignol, B., Tenu, J.-P., and Raymond, M.-N. (1999) J. Biol. Chem. 274, 28652-28659). In this paper we demonstrate that: 1) PLD is not implicated in the earliest phases of protein transit; 2) PLD controls apical but not basolateral protein transit; 3) HT29-cl19A cells express PLD1b and PLD2a mRNAs and proteins; 4) the expression of a catalytically inactive mutant of PLD2 (mPLD2-K758R) significantly inhibited apical constitutive protein transit whereas expression of a catalytically inactive mutant of PLD1 (hPLD1b-K898R) prevented increases in the rate of apical transit as triggered by phorbol esters; 5) PLD2 appears to be located in a perinuclear region containing the Golgi whereas PLD1, which is scattered in the cytoplasm in resting cells, is translocated to the plasma membrane after phorbol ester stimulation. Taken together, these data lead to the conclusion that in HT29-cl19A cells, both PLDs regulate protein transit between the trans-Golgi network and the apical plasma membrane, but that they do so at different steps in the pathway.

Transformation of cells overexpressing a tyrosine kinase by phospholipase D1 and D2

Phospholipase D (PLD) activity is elevated in response to most mitogenic signals. Two mammalian PLD genes (PLD1 and PLD2) have been cloned and their gene products have been characterized. PLD1 is a downstream target of the Ras/RalA GTPase cascade implicated in mitogenic and oncogenic signaling. Consistent with a role in mitogenic signaling, elevated expression of PLD1 transforms cells overexpressing the epidermal growth factor (EGF) receptor (EGFR). However, PLD2 colocalizes with the EGFR in caveolin-enriched light membrane microdomains. We therefore investigated whether PLD2 could also contribute to the transformation of cells overexpressing a tyrosine kinase. We report here that elevated expression of PLD2 transforms rat fibroblasts overexpressing either the EGFR or c-Src. Since overexpression of a tyrosine kinase is a common genetic alteration in several human cancers, these data suggest that elevation of either PLD1 or PLD2 may contribute to the progression to a malignant phenotype in cells with elevated tyrosine kinase activity.

Phospholipase D1 is required for efficient mating projection formation in Saccharomyces cerevisiae

Phospholipase D1 (PLD1) is an important enzyme involved in lipid signal transduction in eukaryotes. A role for PLD1 in signaling in Saccharomyces cerevisiae was examined. Pheromone response in yeast is controlled by a well-characterized protein kinase cascade. Loss of PLD1 activity was found to impair pheromone-induced changes in cellular morphology that result in formation of mating projections. The rate at which projections appeared following pheromone treatment was delayed, suggesting that PLD1 facilitates the execution of a rate-limiting step in morphogenesis. Mutants were found to be less sensitive to pheromone, again arguing that PLD1 is acting at a rate-limiting step. The fact that morphogenesis is most dramatically affected indicates that PLD1 functions primarily in the morphogenic branch of the pheromone response pathway.

Alpha synuclein aggregation: is it the toxic gain of function responsible for neurodegeneration in Parkinson's disease?

Protein aggregation appears to be the common denominator in a series of distinct neurodegenerative diseases yet its role in the associated neuronal pathology in these various conditions remains elusive. In Parkinson's disease, localization of alpha synuclein aggregates within intracellular Lewy body occlusions represent a major hallmark of this disorder and suggest that such aggregation may play a causative role in the resulting dopaminergic cell loss. In this Viewpoint article, recent data is reviewed related to how alpha synuclein aggregation may occur, what cellular events might be responsible, and how this may interfere with normal cellular function(s). It appears likely that while aggregation of alpha synuclein may interfere with its normal function in the cell, this is not the primary cause of the related neurodegeneration.

GLUT-4 translocation in skeletal muscle studied with a cell-free assay: involvement of phospholipase D

GLUT-4-containing membranes immunoprecipitated from insulin-stimulated rat skeletal muscle produce the phospholipase D (PLD) product phosphatidic acid. In vitro stimulation of PLD in crude membrane with ammonium sulfate (5 mM) resulted in transfer of GLUT-4 (3.0-fold vs. control) as well as transferrin receptor proteins from large to small membrane structures. The in vitro GLUT-4 transfer could be blocked by neomycin (a PLD inhibitor), and neomycin also reduced insulin-stimulated glucose transport in intact incubated soleus muscles. Furthermore, protein kinase B(beta) (PKB(beta)) was found to associate with the GLUT-4 protein and was transferred to small vesicles in response to ammonium sulfate in vitro. Finally, addition of cytosolic proteins, prepared from basal skeletal muscle, and GTP nucleotides to an enriched GLUT-4 membrane fraction resulted in in vitro transfer of GLUT-4 to small membranes (6.8-fold vs. unstimulated control). The cytosol and nucleotide-induced GLUT-4 transfer could be blocked by neomycin and N-ethylmaleimide. In conclusion, we have developed a cell-free assay that demonstrates in vitro GLUT-4 transfer. This transfer may suggest release of GLUT-4-containing vesicles from donor GLUT-4 membranes involving PLD activity and binding of PKB(beta) to GLUT-4.

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.

Vaccinia virus F13L protein with a conserved phospholipase catalytic motif induces colocalization of the B5R envelope glycoprotein in post-Golgi vesicles

The wrapping of intracellular mature vaccinia virions by modified trans-Golgi or endosomal cisternae to form intracellular enveloped virions is dependent on at least two viral proteins encoded by the B5R and F13L open reading frames. B5R is a type I integral membrane glycoprotein, whereas F13L is an unglycosylated, palmitylated protein with a motif that is conserved in a superfamily of phospholipid-metabolizing enzymes. Microscopic visualization of the F13L protein was achieved by fusing it to the enhanced green fluorescent protein (GFP). F13L-GFP was functional when expressed by a recombinant vaccinia virus in which it replaced the wild-type F13L gene or by transfection of uninfected cells with a plasmid vector followed by infection with an F13L deletion mutant. In uninfected or infected cells, F13L-GFP was associated with Golgi cisternae and post-Golgi vesicles containing the LAMP 2 late endosomal-lysosomal marker. Association of F13L-GFP with vesicles was dependent on an intact phospholipase catalytic motif and sites of palmitylation. The B5R protein was also associated with LAMP2-containing vesicles when F13L-GFP was coexpressed, but was largely restricted to Golgi cisternae in the absence of F13L-GFP or when the F13L moiety was mutated. We suggest that the F13L protein, like its human phospholipase D homolog, regulates vesicle formation and that this process is involved in intracellular enveloped virion membrane formation.

alpha-Synuclein occurs in lipid-rich high molecular weight complexes, binds fatty acids, and shows homology to the fatty acid-binding proteins

alpha-Synuclein (alphaS) is a 140-residue neuronal protein that forms insoluble cytoplasmic aggregates in Parkinson's disease (PD) and several other neurodegenerative disorders. Two missense mutations (A53T and A30P) are linked to rare forms of familial PD. The normal function of alphaS is unknown, and cultured cell systems that model its modification from soluble monomers to aggregated forms have not been reported. Through a systematic centrifugal fractionation of mesencephalic neuronal cell lines and transgenic mouse brains expressing wild-type or A53T human alphaS, we observed unusual, previously unrecognized species of alphaS that migrate well above the 17-kDa monomeric form in denaturing gels. Incubation at 65 degrees C of high-speed cytosols from cells or brains revealed a modified alphaS species migrating at approximately 36 kDa and an extensive higher molecular mass alphaS-reactive smear. Extraction of the cytosols with chloroform/methanol or with a resin (Lipidex 1000) that binds fatty acids resulted in a similar pattern of higher molecular mass alphaS forms. On the basis of this effect of delipidation, we reexamined the primary structure of alphaS and detected a motif at the N and C termini that is homologous to a fatty acid-binding protein signature. In accord, we found that purified human alphaS binds oleic acid, with an apparent K(d) of 12.5 microM. We also observed an enhanced association of A53T alphaS with microsomal membranes in both mesencephalic cells and transgenic mouse brains. We conclude that alphaS has biochemical properties and a structural motif that suggest it is a novel member of the fatty acid-binding protein family and may thus transport fatty acids between the aqueous and membrane phospholipid compartments of the neuronal cytoplasm.

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.

Trans-Golgi network sorting

The trans-Golgi network (TGN) is a major secretory pathway sorting station that directs newly synthesized proteins to different subcellular destinations. The TGN also receives extracellular materials and recycled molecules from endocytic compartments. In this review, we summarize recent progress on understanding TGN structure and the dynamics of trafficking to and from this compartment. Protein sorting into different transport vesicles requires specific interactions between sorting motifs on the cargo molecules and vesicle coat components that recognize these motifs. Current understanding of the various targeting signals and vesicle coat components that are involved in TGN sorting are discussed, as well as the molecules that participate in retrieval to this compartment in both yeast and mammalian cells. Besides proteins, lipids and lipid-modifying enzymes also participate actively in the formation of secretory vesicles. The possible mechanisms of action of these lipid hydrolases and lipid kinases are discussed. Finally, we summarize the fundamentally different apical and basolateral cell surface delivery mechanisms and the current facts and hypotheses on protein sorting from the TGN into the regulated secretory pathway in neuroendocrine cells.

Phospholipase D activity is required for actin stress fiber formation in fibroblasts

Phospholipase D (PLD) is a ubiquitously expressed enzyme of ill-defined function. In order to explore its cellular actions, we inactivated the rat PLD1 (rPLD1) isozyme by tagging its C terminus with a V5 epitope (rPLD1-V5). This was stably expressed in Rat-2 fibroblasts to see if it acted as a dominant-negative mutant for PLD activity. Three clones that expressed rPLD1-V5 were selected (Rat2V16, Rat2V25, and Rat2V29). Another clone (Rat2V20) that lost expression of rPLD1-V5 was also obtained. In the three clones expressing rPLD1-V5, PLD activity stimulated by phorbol myristate acetate (PMA) or lysophosphatidic acid (LPA) was reduced by ~50%, while the PLD activity of Rat2V20 cells was normal. Changes in the actin cytoskeleton in response to LPA or PMA were examined in these clones. All three clones expressing rPLD1-V5 failed to form actin stress fibers after treatment with LPA. However, Rat2V20 cells formed stress fibers in response to LPA to the same extent as wild-type Rat-2 cells. In contrast, there was no significant change in membrane ruffling induced by PMA in the cells expressing rPLD1-V5. Since Rho is an activator both of rPLD1 and stress fiber formation, the activation of Rho was monitored in wild-type Rat-2 cells and Rat2V25 cells, but no significant difference was detected. The phosphorylation of vimentin mediated by Rho-kinase was also intact in Rat2V25 cells. Rat2V25 cells also showed normal vinculin-containing focal adhesions. However, the translocation of alpha-actinin to the cytoplasm and to the detergent-insoluble fraction in Rat2V25 cells was reduced. These results indicate that PLD activity is required for LPA-induced rearrangement of the actin cytoskeleton to form stress fibers and that PLD might be involved in the cross-linking of actin filaments mediated by alpha-actinin.

Phosphatidic acid activates a wound-activated MAPK in Glycine max

Many plant species demonstrate a systemic increase in phosphatidic acid (PA) levels after being wounded (Lee et al., 1997). To understand the role of PA in wound signal transduction, we investigated if PA can activate protein kinases in soybean (Glycine max L.). We found that a MAPK is activated in soybean seedlings in both wounded and neighboring unwounded leaves. The wound-activated soybean kinase is specifically recognized by an antibody against the alfalfa MAPK, SIMK. When PA production is inhibited with n-butanol, an inhibitor of phospholipase D, the wound-induced activation of the MAPK is suppressed, suggesting that an elevation in PA levels is essential for its activation. Supporting this is the observation that exogenous PA activates the MAPK in suspension-cultured soybean cells. Activation of the 49 kDa MAPK occurs almost exclusively by PA, as other lipids are unable to or can only weakly activate the kinase. PA-induced activation of the MAPK is not a direct effect on the kinase but is mediated by upstream kinases. Our results suggest that PA acts as a second messenger in wound-induced MAPK signaling in plants.

Toxoplasma gondii ADP-ribosylation factor 1 mediates enhanced release of constitutively secreted dense granule proteins

Toxoplasma gondii dense granules are morphologically similar to dense matrix granules in specialized secretory cells, yet are secreted in a constitutive, calcium-independent fashion. We previously demonstrated that secretion of dense granule proteins in permeabilized parasites was augmented by the non-hydrolyzable GTP analogue guanosine 5'-3-O-(thio)triphosphate (GTPgammaS) (Chaturvedi, S., Qi, H., Coleman, D. L., Hanson, P., Rodriguez, A., and Joiner, K. A. (1998) J. Biol. Chem. 274, 2424-2431). As now demonstrated by pharmacological and electron microscopic approaches, GTPgammaS enhanced release of dense granule proteins in the permeabilized cell system. To investigate the role of ADP-ribosylation factor 1 (ARF1) in this process, a cDNA encoding T. gondii ARF1 (TgARF1) was isolated. Endogenous and transgenic TgARF1 localized to the Golgi of T. gondii, but not to dense granules. An epitope-tagged mutant of TgARF1 predicted to be impaired in GTP hydrolysis (Q71L) partially dispersed the Golgi signal, with localization to scattered vesicles, whereas a mutant impaired in nucleotide binding (T31N) was cytosolic in location. Both mutants caused partial dispersion of a Golgi/trans-Golgi network marker. TgARF1 mutants inhibited delivery of the secretory reporter, Escherichia coli alkaline phosphatase, to dense granules, precluding an in vivo assessment of the role of TgARF1 in release of intact dense granules. To circumvent this limitation, recombinant TgARF1 was purified using two separate approaches, and used in the permeabilized cell assay. TgARF1 protein purified on a Cibacron G3 column and able to bind GTP stimulated dense granule secretion in the permeabilized cell secretion assay. These results are the first to show that ARF1 can augment release of constitutively secreted vesicles at the target membrane.

Intracellular localization of phospholipase D1 in mammalian cells

Phospholipase D (PLD) hydrolyzes phosphatidylcholine to generate phosphatidic acid. In mammalian cells this reaction has been implicated in the recruitment of coatomer to Golgi membranes and release of nascent secretory vesicles from the trans-Golgi network. These observations suggest that PLD is associated with the Golgi complex; however, to date, because of its low abundance, the intracellular localization of PLD has been characterized only indirectly through overexpression of chimeric proteins. We have used highly sensitive antibodies to PLD1 together with immunofluorescence and immunogold electron microscopy as well as cell fractionation to identify the intracellular localization of endogenous PLD1 in several cell types. Although PLD1 had a diffuse staining pattern, it was enriched significantly in the Golgi apparatus and was also present in cell nuclei. On fragmentation of the Golgi apparatus by treatment with nocodazole, PLD1 closely associated with membrane fragments, whereas after inhibition of PA synthesis, PLD1 dissociated from the membranes. Overexpression of an hemagglutinin-tagged form of PLD1 resulted in displacement of the endogenous enzyme from its perinuclear localization to large vesicular structures. Surprisingly, when the Golgi apparatus collapsed in response to brefeldin A, the nuclear localization of PLD1 was enhanced significantly. Our data show that the intracellular localization of PLD1 is consistent with a role in vesicle trafficking from the Golgi apparatus and suggest that it also functions in the cell nucleus.

Characterization of cytohesin-1 monoclonal antibodies: expression in neutrophils and during granulocytic maturation of HL-60 cells

ADP-ribosylation factors (Arf) are small GTP-binding proteins involved in vesicular transport and the activation of phospholipase D (PLD). The conversion of Arf-GDP to Arf-GTP is promoted in vivo by guanine nucleotide exchange factors such as ARNO or cytohesin-1. In order to examine the expression of ARNO and cytohesin-1 in human granulocytes, we generated specific polyclonal and monoclonal antibodies (mAbs). We also overexpressed GFP-ARNO and GFP-cytohesin-1 in RBL-2H3 cells to characterize the specificity and the ability of cytohesin-1 mAbs to immunoprecipitate cytohesin-1. Among the hybridomas secreting cytohesin-1 mAbs, only the clones 2E11, 1E4, 3C8, 6F5, 4C7, 7A3 and 8F7 were found to be specific for cytohesin-1. Furthermore, mAb 2E11 immunoprecipitated GFP-cytohesin-1 but not GFP-ARNO under native conditions. In contrast, mAbs 5D8, 4C3, 2G8, 6G11, 4C3, 6D4, 7B4 and 6F8 detected both cytohesin-1 and ARNO as monitored by immunoblotting. Although mAb 6G11 detected both proteins, this antibody immunoprecipitated GFP-ARNO but not GFP-cytohesin-1 under native conditions. Another antibody, mAb 10A12, also selectively immunoprecipitated GFP-ARNO under native conditions, but the epitope recognized by this mAb is unlikely to be linear as no signal was obtained by immunoblotting. Immunoprecipitation with a cytohesin-1 polyclonal antibody and blotting with cytohesin-1 specific mAbs revealed that cytohesin-1 is highly expressed in neutrophils. Cytohesin-1 can be detected in HL-60 cells but the endogenous protein levels were low in undifferentiated cells. Using the specific cytohesin-1 mAb 2E11 we observed a marked increase in levels of cytohesin-1 expression during dibutyryl-cyclic AMP-induced granulocytic differentiation of HL-60 cells. These data suggest that cytohesin-1, which may have important functions in neutrophil physiology, can be useful as a potential marker for granulocytic differentiation.

Enzymatic characterization of phospholipase D of protozoan Tetrahymena cells

Phospholipase D (PLD), which is present in plant, bacterial, and mammalian cells, has been proposed to be involved in a number of cellular processes including transmembrane signaling and membrane deterioration. We demonstrated the existence of evolutionally related PLD activity in the unicellular eukaryotic protozoan Tetrahymena. The partial characterization of this enzyme showed that PLD in Tetrahymena cells was a neutral phospholipase, which catalyzed both transphosphatidylation and hydrolysis reac tions. The activity was markedly stimulated by phosphatidylinositol 4, 5-bisphosphate (PIP2) but was insensitive to phorbol 12-myristate 13-acetate (PMA) and guanosine 5'-3-O-(thio)triphosphate (GTPgammaS), suggesting that it is a PIP2-dependent PLD and that protein kinase C (PKC) and GTP-binding proteins are not implicated in the regulation of this enzyme. For its maximal activity Ca2+ was not required. This enzyme was also capable of hydrolyzing phosphatidylcholine (PC) but not phosphatidylethanolamine (PE), implying that PC was a preferred substrate. Subcellular fractionation showed that PLD-like activity localized mainly to the membrane fraction, especially microsomes. As an initial step to explore the functions of PLD in Tetrahymena, the PLD-like activity was determined during the different culture phases, and it was found to be significantly and transiently elevated in the early logarithmic phase, indicating its possible role in the development of Tetrahymena.

Role for phospholipase D in receptor-mediated endocytosis

In response to epidermal growth factor (EGF), the EGF receptor is endocytosed and degraded. A substantial lag period exists between endocytosis and degradation, suggesting that endocytosis is more than a simple negative feedback. Phospholipase D (PLD), which has been implicated in vesicle formation in the Golgi, is activated in response to EGF and other growth factors. We report here that EGF receptor endocytosis is dependent upon PLD and the PLD1 regulators, protein kinase C alpha and RalA. EGF-induced receptor degradation is accelerated by overexpression of either wild-type PLD1 or PLD2 and retarded by overexpression of catalytically inactive mutants of either PLD1 or PLD2. EGF-induced activation of mitogen-activated protein kinase, which is dependent upon receptor endocytosis, is also dependent upon PLD. These data suggest a role for PLD in signaling that facilitates receptor endocytosis.

Regulation of expression of phospholipase D1 and D2 by PEA-15, a novel protein that interacts with them

Phospholipase D (PLD), a signal-transducing membrane-associated enzyme, is implicated in diverse processes including apoptosis, ERK activation, and glucose transport. Prior studies have identified specific PLD activators and repressors that directly regulate its enzymatic activity. Using two-hybrid screens, we have identified PEA-15 as a PLD interactor that unexpectedly functions to alter its level of expression. PEA-15 is a widely expressed death effector domain-containing phosphoprotein involved in signal transduction, apoptosis, ERK activation, and glucose transport. The PLD1-interacting site on PEA-15 consists of part of the death effector domain domain plus additional C-terminal flanking sequences, whereas the PEA-15-interacting site on PLD1 overlaps the previously identified RhoA-interacting site. PEA-15 did not affect basal or stimulated in vitro PLD1 enzymatic activation. However, co-expression of PEA-15 increased levels of PLD1 activity. This increased activation correlated with higher PLD1 protein expression levels, as marked by faster accumulation and longer persistence of PLD1 when PEA-15 was present. PEA-15 similarly increased protein expressions level of PLD2 and co-immunoprecipitated with it. These results suggest that PEA-15 may stabilize PLD or act as a PLD chaperone. The common involvement of PEA-15 and PLD in apoptosis, ERK activation, and glucose transport additionally suggests functional significance.

Actin assembly at membranes controlled by ARF6

The small GTPase, ADP-ribosylation factor-6 (ARF6), has been implicated in regulating membrane traffic and remodeling cortical F-actin. Using real-time video analysis of actin assembly in living cells, we investigated the function and mechanism of ARF6 in control of actin assembly. Expression of an activated form of ARF6 that mimicks the GTP-bound form of the GTPase induced actin assembly resulting in the movement of vesicle-like particles, some of which contain markers for pinosomes. Activated ARF6 also stimulated actin assembly at foci on the ventral surface of the cell and stimulated fluid phase pinocytosis. Particle motility induced by ARF6 involved Arp2/3 complex, tyrosine kinase activity, phospholipase D (PLD) and D3-phosphoinositides, but not phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2). We conclude that ARF6 regulates actin assembly for pinosome motility and at foci on the ventral cell surface.

Activation of phospholipase D by metabotropic glutamate receptor agonists in rat cerebrocortical synaptosomes

The pharmacological profile of metabotropic glutamate receptor (mGluR) activation of phospholipase D (PLD), and the associated signalling pathways, were examined in rat cerebrocortical synaptosomes. The assay was conducted using a transphosphatidylation reaction in synaptosomes which were pre-labelled with either [(3)H]-arachidonic acid or [(32)P]-orthophosphate. The mGluR agonists (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S, 3R-ACPD) and (RS)-3,5-dihydroxyphenylglycine (DHPG), both activated PLD, while phorbol 12,13-dibutyrate (PDBu) treatment caused receptor-independent activation of PLD and had an additive effect on 1S,3R-ACPD induced PLD activity. A protein kinase C (PKC) inhibitor, GF109203X, failed to antagonize mGluR receptor-coupled PLD activity. We could not detect any increase in the products of PI (phosphoinositide)-specific phospholipase C (PI-PLC), inositol(1,4, 5)trisphosphate or diacylglycerol, by 1S, 3R-ACPD at 15 s. However, diacylglycerol increased monophasically in response to mGluR agonists and remained elevated for at least 15 min. Phosphatidic acid phosphohydrolase (PAP) activity, which converts PA to DAG, was present in the synaptosomes. These data suggest that, in rat cerebrocortical synaptosomes, the 1S,3R-ACPD-sensitive mGluR is coupled to PLD through a mechanism that is independent of both PKC and PI-PLC.

Interaction of the type Ialpha PIPkinase with phospholipase D: a role for the local generation of phosphatidylinositol 4, 5-bisphosphate in the regulation of PLD2 activity

Phosphoinositides are localized in various intracellular compartments and can regulate a number of intracellular functions, such as cytoskeletal dynamics and membrane trafficking. Phospholipase Ds (PLDs) are regulated enzymes that hydrolyse phosphatidylcholine (PtdCho) to generate the putative second messenger phosphatidic acid (PtdOH). In vitro, PLDs have an absolute requirement for higher phosphorylated inositides, such as phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P(2)]. Whether this lipid is able to regulate the activity of PLD in vivo is contentious. To examine this hypothesis we studied the relationship between PLD and an enzyme critical for the intracellular synthesis of PtdIns(4,5)P(2): phosphatidylinositol 4-phosphate 5-kinase alpha (Type Ialpha PIPkinase). We find that both PLD1 and PLD2 interact with the Type Ialpha PIPkinase and that PLD2 activity in vivo can be regulated solely by the expression of this lipid kinase. Moreover, PLD2 is able to recruit the Type Ialpha PIPkinase to its intracellular location. We show that the physiological requirement of PLD enzymes for PtdIns(4,5)P(2) is critical and that PLD2 activity can be regulated solely by the levels of this key intracellular lipid.

Lipids, lipid modification and lipid-protein interaction in membrane budding and fission--insights from the roles of endophilin A1 and synaptophysin in synaptic vesicle endocytosis

Studies on the endocytosis of synaptic vesicles have provided two novel insights into the mechanism of vesicle formation from donor membranes, both of which concern lipids. One is the essential role of endophilin, a cytosolic protein converting lysophosphatidic acid by addition of the fatty acid arachidonate into phosphatidic acid. The other is the essential role of membrane cholesterol, which specifically interacts with synaptophysin, the major transmembrane protein of synaptic vesicles. These findings reveal novel modes of membrane lipid modification and lipid-protein interaction in vesicle biogenesis.

ADP-ribosylation factor 1 and its activation of phospholipase D are important for the assembly of very low density lipoproteins

The role of ADP-ribosylation factor 1 (ARF-1) in the assembly of very low density lipoproteins (VLDL) was investigated by expressing dominant-negative mutants in McA-RH7777 cells. Transient expression of ARF-1(T31N), a GDP-restrictive mutant, significantly inhibited apolipoprotein B-100 (apoB-100) VLDL production without influencing the biosynthesis of apoB-100 low density lipoproteins or total apoB production (indicating that it inhibited the second step of VLDL assembly) and without altering total protein production or biosynthesis of transferrin, phosphatidylcholine, or triglycerides. These effects were confirmed in stable inducible transfectants. In contrast, expression of an ARF-1 mutant lacking the N-terminal 17 amino acids, which has no myristoylation site and cannot interact with the microsomal membrane, did not affect VLDL assembly. Thus, active ARF-1 is needed for the second step of the process. To further explore these observations, we developed a cell-free system based on the postnuclear supernatant isolated from McA-RH7777 cells. In this system, 10-15% of the apoB-100 pool was converted to VLDL in a time- and temperature-dependent way. The assembly process was highly dependent on a heat-stable factor in the d > 1.21 g/ml infranatant of fetal calf serum; this factor was not present in low density lipoproteins or VLDL. Brefeldin A inhibited VLDL assembly in this system, as did a synthetic peptide (corresponding to N-terminal amino acids 2-17 of ARF-1) that displaces ARF-1 from the membrane. Thus, active ARF-1 is also needed for cell-free assembly of VLDL. Guanosine 5'-3-O-(thio)triphosphate also inhibited VLDL assembly in this system, indicating that the process requires ongoing hydrolysis of GTP. 1-Butanol, which inhibits the formation of phosphatidic acid (PA) and instead gives rise to phosphatidylbutanol, inhibited VLDL assembly, whereas 2-butanol, which does not inhibit PA formation, failed to do so. Thus, phospholipase D (PLD)-catalyzed formation of PA from phosphatidylcholine is essential for VLDL assembly. In support of this conclusion, exogenous PLD prevented brefeldin A from inhibiting the assembly process. Our results indicate that ARF-1 participates in the second step of VLDL assembly through a process that involves activation of PLD and production of PA.

Interleukin-1β Regulates Phospholipase D-1 Expression in Rat Pancreatic β-Cells

The cytokine interleukin (IL)-1beta induces a biphasic effect in rat pancreatic islets, with an early and transitory stimulation of insulin release followed by progressive functional suppression. To clarify the mechanisms involved in these effects, we have recently performed a differential display of messenger RNA (mRNA) by RT-PCR (DDRT-PCR) on rat beta-cells exposed for 6 or 24 h to IL-1beta. Among the different IL-1beta-induced genes, there was an early and transient increase in phospholipase D-1 (PLD1) expression. PLD1 can induce phosphatidic acid formation and subsequent activation of protein kinase C, a process which stimulates insulin release. In the present study, we characterized the regulation of PLD isoforms by IL-1beta in pancreatic beta-cells. By using different combinations of primers and RT-PCR, we observed that IL-1beta induces an early increase (2 and 6 h) in the expression of both alternatively spliced isoforms of PLD1 (PLD1alpha and 1b). Prolonged exposure to IL-1beta (12 and 24 h) caused a decrease of PLD1a mRNA expression compared with control beta-cells, and lead to a return of PLD1b mRNA to basal level. NG-methyl-L-arginine (LMA), a blocker of the inducible form of nitric oxide synthase (iNOS), prevented this late inhibitory effect of IL-1beta, suggesting that IL-1beta-induced decrease in PLD1a expression is NO-mediated. IL-1beta induced an early (2-6 h) and sustained (16-24 h) increase in PLD1a mRNA expression in insulin-producing RINm5F cells. This was paralleled by a cytokine-induced increase in PLD1 protein expression and enzyme activity. RINm5F cells, but not primary beta-cells, expressed PLD2, and the expression of this gene was not affected by IL-1beta. In conclusion, we have shown that the cytokine IL-1beta regulates PLD1 expression in primary and clonal beta-cells. The early induction of PLD1 probably contributes to the early stimulatory effects of IL-1beta on islet insulin release.

Inhibition of phospholipase D by amphiphysins

Two distinct proteins inhibiting phospholipase D (PLD) activity in rat brain cytosol were previously purified and identified as synaptojanin and AP180, which are specific to nerve terminals and associate with the clathrin coat. Two additional PLD-inhibitory proteins have now been purified and identified as the amphiphysins I and II, which forms a heterodimer that also associates with the clathrin coat. Bacterially expressed recombinant amphiphysins inhibited both PLD1 and PLD2 isozymes in vitro with a potency similar to that of brain amphiphysin (median inhibitory concentration of approximately 15 nm). Expressions of either amphiphysin in COS-7 cells reduced activity of endogenous PLD as well as exogenously expressed PLD1 and PLD2. Coprecipitation experiments suggested that the inhibitory effect of amphiphysins results from their direct interaction with PLDs. The NH(2) terminus of amphiphysin I was critical for both inhibition of and binding to PLD. Phosphatidic acid formed by signal-induced PLD is thought to be required for the assembly of clathrin-coated vesicles during endocytosis. Thus, the inhibition of PLD by amphiphysins, synaptojanin, and AP180 might play an important role in synaptic vesicle trafficking.

Phospholipid transfer proteins and physiological functions

Issues of how cells generate and maintain unique lipid compositions in distinct intracellular membrane systems remain the subject of much study. A ubiquitous class of soluble proteins capable of transporting phospholipid monomers from membrane to membrane across an aqueous milieu has been thought to define part of the mechanism by which lipids are sorted in cells. Progress in the study of these phospholipid transfer proteins (PLTPs) raises questions regarding their physiological functions in cells and the mechanisms by which these proteins execute them. It is now clear that across the eukaryotic kingdom, members of this protein family exert essential roles in the regulation of phospholipid metabolism and central aspects of phospholipid-mediated signaling. Indeed, it is now known that dysfunction of specific PLTPs defines the basis of inherited diseases in mammals, and this list is expected to grow. Phospholipid transfer proteins, their biochemical properties, and the emerging clues regarding their physiological functions are reviewed.

Activation of astroglial phospholipase D activity by phorbol ester involves ARF and Rho proteins

Primary cultures of rat cortical astrocytes express phospholipase D (PLD) isoforms 1 and 2 as determined by RT-PCR and Western blot. Basal PLD activity was strongly (10-fold) increased by 4beta-phorbol-12beta,13alpha-dibutyrate (PDB) (EC(50): 56 nM), an effect which was inhibited by Ro 31-8220 (0.1-1 microM), an inhibitor of protein kinase C (PKC), and by brefeldin A (10-100 microg/ml), an inhibitor of ADP-ribosylating factor (ARF) activation. Pretreatment of the cultures with Clostridium difficile toxin B-10463 (0.1-1 ng/ml), which inactivates small G proteins of the Rho family, led to a breakdown of the astroglial cytoskeleton; concomitantly, PLD activation by PDB was reduced by up to 50%. In contrast, inactivation of proteins of the Ras family by Clostridium sordellii lethal toxin 1522 did not affect PLD activation. In parallel experiments, serum-induced PLD activation was sensitive to brefeldin A, but not to Ro 31-8220 and not to clostridial toxins. We conclude that, in astrocytes, the PLD isoform which is activated by phorbol ester requires PKC, ARF and Rho proteins for full activity and probably represents PLD1.

A low molecular weight factor from dividing cells activates phospholipase D in caveolin-enriched membrane microdomains

Phospholipase D (PLD) activity is elevated in Ras-transformed NIH 3T3 cells. This difference in PLD activity between Ras-transformed and nontransformed parental cells disappeared in isolated membranes from these cells. In reconstitution experiments, heat-denatured cytosolic fractions from Ras-transformed, but not parental, NIH 3T3 cells elevated PLD activity in isolated membranes. This heat-resistant PLD-stimulating activity from the Ras-transformed cells was sensitive to proteases and passed through a 1-kDa MW cutoff membrane, suggesting that the factor is a peptide of less than 10 amino acids. The ability of this PLD-stimulating factor, designated PLD-SF, to elevate PLD activity in isolated membranes was restricted to the caveolin-enriched light membranes, where many signaling molecules are localized. PLD-SF was also elevated in v-Src- and v-Raf-transformed cells and in serum-stimulated NIH 3T3 cells. PLD-SF was detected in a variety of rat tissues but was highest in testes, where a large percentage of cells are dividing. A similar low molecular weight PLD-stimulating activity was found in actively dividing, but not stationary yeast, cells. The data here provide evidence for a highly conserved PLD-stimulating peptide that is elevated in response to mitogenic stimuli.

Inhibition of phosphatidic acid synthesis alters the structure of the Golgi apparatus and inhibits secretion in endocrine cells

In mammalian cells, activation of a Golgi-associated phospholipase D by ADP-ribosylation factor results in the hydrolysis of phosphatidylcholine to form phosphatidic acid (PA). This reaction stimulates the release of nascent secretory vesicles from the trans-Golgi network of endocrine cells. To understand the role of PA in mediating secretion, we have exploited the transphosphatidylation activity of phospholipase D. Rat anterior pituitary GH3 cells, which secrete growth hormone and prolactin, were treated with 1-butanol resulting in the synthesis of phosphatidylbutanol rather than PA. Under these conditions transport from the ER through the Golgi apparatus and secretion of polypeptide hormones were inhibited quantitatively. Furthermore, the in vitro synthesis of phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)) by Golgi membranes was inhibited quantitatively. Most significantly, in the presence of 1-butanol the architecture of the Golgi apparatus was disrupted, resulting in its disassembly and fragmentation. Removal of the alcohol resulted in the rapid restoration of Golgi structure and secretion of growth hormone and prolactin. Our results suggest that PA stimulation of PtdIns(4,5)P(2) synthesis is required for maintaining the structural integrity and function of the Golgi apparatus.

Phospholipase D

Phospholipase D is an ubiquitous enzyme that hydrolyzes phosphatidylcholine to phosphatidic acid and choline. Its cellular actions are related to the production of phosphatidic acid and include alterations to cell growth, shape, and secretion. There are two mammalian phospholipase D genes whose products (PLD1 and PLD2) are alternatively spliced. Both forms have two highly conserved HKD motifs that are essential for catalysis and dimerization. PLD1 is regulated in vitro and in vivo by protein kinase C and small GTPases of the Rho and ARF families, whereas PLD2 shows a higher basal activity with little or no response to these proteins. The cellular locations and specific functions of the two PLD isoforms remain to be established.

ARF1 regulates pH-dependent COP functions in the early endocytic pathway

Coat proteins of the COP family were recently shown by us and others to be involved in membrane transport in the endocytic pathway, in addition to their known functions in the biosynthetic pathway. We have also shown that membrane association of endosomal COPs depends on the acidic endosomal pH, in contrast to biosynthetic COPs. In this paper, we report that both membrane recruitment of endosomal COPs and in vitro biogenesis of transport intermediates destined for late endosomes, depend on a cytosolic factor, which we identified as the small GTP-binding protein ARF1. Our data indicate that ARF1 does not act via activation of an endosomal phospholipase D. We also find that ARF1 membrane association is regulated by the endosomal pH, and that this controls the pH-dependent association of endosomal COPs. These studies thus show that ARF1 regulates COP functions in the endocytic pathway, and indicate that ARF1 acts as the cytosolic component of a transmembrane pH-sensing mechanism.

A role for phospholipase D in GLUT4 glucose transporter translocation

Based on recent studies showing that phospholipase D (PLD)1 is associated with intracellular membranes and promotes membrane budding from the trans-Golgi, we tested its possible role in the membrane trafficking of GLUT4 glucose transporters. Using immunofluorescence confocal microscopy, expressed Myc epitope-tagged PLD1 was found to associate with intracellular vesicular structures by a mechanism that requires its N-terminal pleckstrin homology domain. Partial co-localization with expressed GLUT4 fused to green fluorescent protein in both 3T3-L1 adipocytes and Chinese hamster ovary cells was evident. Furthermore, microinjection of purified PLD into cultured adipocytes markedly potentiated the effect of a submaximal concentration of insulin to stimulate GLUT4 translocation to cell surface membranes. Insulin stimulated PLD activity in cells expressing high levels of insulin receptors but no such insulin effect was detected in 3T3-L1 adipocytes. Taken together, these results are consistent with the hypothesis that PLD1 associated with GLUT4-containing membranes acts in a constitutive manner to promote the mechanism of GLUT4 translocation by insulin.

Multiple forms of phospholipase D in plants: the gene family, catalytic and regulatory properties, and cellular functions

Multiple Phospholipase D (PLD) genes have been identified in plants and encode isoforms with distinct regulatory and catalytic properties. Elucidation of the genetic and biochemical heterogeneity has provided important clues as to the regulation and function of this family of enzymes. Polyphosphoinositides, Ca(2+), and G-proteins are possible cellular regulators for PLD activation. PLD-mediated hydrolysis of membrane lipids increases in response to various stresses. Recent studies suggest that PLD plays a role in the signaling and production of hormones involved in plant stress responses.

Phospholipase D-dependent and -independent mechanisms are involved in milk protein secretion in rabbit mammary epithelial cells

Phospholipase D has been implicated in membrane traffic in the secretory pathway of yeast and of some mammalian cell lines. Here we investigated the involvement of phospholipase D in protein transport at various steps of the secretory pathway of mammary epithelial cells. Treatment of rabbit mammary explants with butanol, which blocks the formation of phosphatidic acid, decreased the secretion of caseins and to a lesser extent that of whey acidic protein. Butanol interfered with both the endoplasmic reticulum to Golgi complex transport of the caseins and secretory vesicle formation from the trans-Golgi network. In contrast, the transport of whey acidic protein to the Golgi was less affected. Activation of protein kinase C enhanced the overall secretion of both markers and interestingly, this stimulation of secretion was maintained for whey acidic protein in the presence of butanol. Transphosphatidylation assays demonstrated the existence of a constitutive phospholipase D activity which was stimulated by the activation of protein kinase C. We conclude that phospholipase D plays a role in casein transport from the endoplasmic reticulum to the Golgi and in the secretory vesicle formation from the trans-Golgi network. Moreover, our results suggest a differential requirement for phospholipase D in the secretion of caseins and that of whey acidic protein.

Inhibition of phosphatidylinositol 4,5-bisphosphate-stimulated phospholipase D2 activity by Ser/Thr phosphorylation

Treatment of HeLa cells overexpressing PLD2 with the Ser/Thr-specific protein phosphatase inhibitor, okadaic acid, augmented spontaneous phosphorylation of PLD2 with concomitant inhibition of phosphatidylinositol 4,5-bisphosphate (PIP(2))-stimulated PLD2 activity. Dephosphorylation of the immunoprecipitated, spontaneously phosphorylated PLD2 in COS-7 cells by catalytic subunit of protein phosphatase 1gamma1 resulted in the stimulation of the PLD2 catalytic activity. These observations suggest that Ser/Thr phosphorylation regulates PLD2 activity.