Partial purification and characterization of Arf-sensitive phospholipase D from porcine brain

The wide world of non-mammalian phospholipase D enzymes

Phospholipase D (PLD) hydrolyses phosphatidylcholine (PtdCho) to produce free choline and the critically important lipid signaling molecule phosphatidic acid (PtdOH). Since the initial discovery of PLD activities in plants and bacteria, PLDs have been identified in a diverse range of organisms spanning the taxa. While widespread interest in these proteins grew following the discovery of mammalian isoforms, research into the PLDs of non-mammalian organisms has revealed a fascinating array of functions ranging from roles in microbial pathogenesis, to the stress responses of plants and the developmental patterning of flies. Furthermore, studies in non-mammalian model systems have aided our understanding of the entire PLD superfamily, with translational relevance to human biology and health. Increasingly, the promise for utilization of non-mammalian PLDs in biotechnology is also being recognized, with widespread potential applications ranging from roles in lipid synthesis, to their exploitation for agricultural and pharmaceutical applications.

Emerging Prospects for Combating Fungal Infections by Targeting Phosphatidylinositol Transfer Proteins

The emergence of fungal “superbugs” resistant to the limited cohort of anti-fungal agents available to clinicians is eroding our ability to effectively treat infections by these virulent pathogens. As the threat of fungal infection is escalating worldwide, this dwindling response capacity is fueling concerns of impending global health emergencies. These developments underscore the urgent need for new classes of anti-fungal drugs and, therefore, the identification of new targets. Phosphoinositide signaling does not immediately appear to offer attractive targets due to its evolutionary conservation across the Eukaryota. However, recent evidence argues otherwise. Herein, we discuss the evidence identifying Sec14-like phosphatidylinositol transfer proteins (PITPs) as unexplored portals through which phosphoinositide signaling in virulent fungi can be chemically disrupted with exquisite selectivity. Recent identification of lead compounds that target fungal Sec14 proteins, derived from several distinct chemical scaffolds, reveals exciting inroads into the rational design of next generation Sec14 inhibitors. Development of appropriately refined next generation Sec14-directed inhibitors promises to expand the chemical weaponry available for deployment in the shifting field of engagement between fungal pathogens and their human hosts.

Concerted regulation of actin polymerization during constitutive secretion by cortactin and PKD2

Constitutive secretion from the trans-Golgi-network (TGN) is facilitated by a concerted regulation of vesicle biogenesis and fission processes. The protein kinase D family (PKD) has been previously described to enhance vesicle fission by modifying the lipid environment. PKD also phosphorylates the actin regulatory protein cortactin at S298 to impair synergistic actin polymerization. We here report additional functions for PKD2 (also known as PRKD2) and cortactin in the regulation of actin polymerization during the fission of transport carriers from the TGN. Phosphorylation of cortactin at S298 impairs the interaction between WIP (also known as WIPF1) and cortactin. WIP stabilizes the autoinhibited conformation of N-WASP (also known as WASL). This leads to an inhibition of synergistic Arp2/3-complex-dependent actin polymerization at the TGN. PKD2 activity at the TGN is controlled by active CDC42-GTP which directly activates N-WASP, inhibits PKD2 and shifts the balance to non-S298-phosphorylated cortactin, which can in turn sequester WIP from N-WASP. Consequently, synergistic actin polymerization at the TGN and constitutive secretion are enhanced.

Mammalian phospholipase D: Function, and therapeutics

Despite being discovered over 60 years ago, the precise role of phospholipase D (PLD) is still being elucidated. PLD enzymes catalyze the hydrolysis of the phosphodiester bond of glycerophospholipids producing phosphatidic acid and the free headgroup. PLD family members are found in organisms ranging from viruses, and bacteria to plants, and mammals. They display a range of substrate specificities, are regulated by a diverse range of molecules, and have been implicated in a broad range of cellular processes including receptor signaling, cytoskeletal regulation and membrane trafficking. Recent technological advances including: the development of PLD knockout mice, isoform-specific antibodies, and specific inhibitors are finally permitting a thorough analysis of the in vivo role of mammalian PLDs. These studies are facilitating increased recognition of PLD's role in disease states including cancers and Alzheimer's disease, offering potential as a target for therapeutic intervention.

Phospholipase D and the Mitogen Phosphatidic Acid in Human Disease: Inhibitors of PLD at the Crossroads of Phospholipid Biology and Cancer

Lipids are key building blocks of biological membranes and are involved in complex signaling processes such as metabolism, proliferation, migration, and apoptosis. Extracellular signaling by growth factors, stress, and nutrients is transmitted through receptors that activate lipid-modifying enzymes such as the phospholipases, sphingosine kinase, or phosphoinositide 3-kinase, which then modify phospholipids, sphingolipids, and phosphoinositides. One such important enzyme is phospholipase D (PLD), which cleaves phosphatidylcholine to yield phosphatidic acid and choline. PLD isoforms have dual role in cells. The first involves maintaining cell membrane integrity and cell signaling, including cell proliferation, migration, cytoskeletal alterations, and invasion through the PLD product PA, and the second involves protein-protein interactions with a variety of binding partners. Increased evidence of elevated PLD expression and activity linked to many pathological conditions, including cancer, neurological and inflammatory diseases, and infection, has motivated the development of dual- and isoform-specific PLD inhibitors. Many of these inhibitors are reported to be efficacious and safe in cells and mouse disease models, suggesting the potential for PLD inhibitors as therapeutics for cancer and other diseases. Current knowledge and ongoing research of PLD signaling networks will help to evolve inhibitors with increased efficacy and safety for clinical studies.

Cofilin-mediated Neuronal Apoptosis via p53 Translocation and PLD1 Regulation

Amyloid β (Aβ) accumulation is an early event in the pathogenesis of Alzheimer’s disease (AD), leading to mitochondrial and synaptic dysfunction, tau accumulation, and eventual neuronal death. While the p53 apoptotic pathway has clearly been associated with Aβ deposits and neuronal apoptosis, the critical upstream factors contributing to p53 activation in AD are not well understood. We have previously shown that cofilin activation plays a pivotal role in Aβ-induced mitochondrial and synaptic dysfunction. In this study, we show that activated cofilin (S3A) preferentially forms a complex with p53 and promotes its mitochondrial and nuclear localization, resulting in transcription of p53-responsive genes and promotion of apoptosis. Conversely, reduction of endogenous cofilin by knockdown or genetic deficiency inhibits mitochondrial and nuclear translocation of p53 in cultured cells and in APP/PS1 mice. This cofilin-p53 pro-apoptotic pathway is subject to negative regulation by PLD1 thorough cofilin inactivation and inhibition of cofilin/p53 complex formation. Finally, activated cofilin is unable to induce apoptosis in cells genetically lacking p53. These findings taken together indicate that cofilin coopts and requires the nuclear and mitochondrial pro-apoptotic p53 program to induce and execute apoptosis, while PLD1 functions in a regulatory multi-brake capacity in this pathway.

Targeting phospholipase D in cancer, infection and neurodegenerative disorders

Phospholipase D (PLD) enzymes are one source of receptor-generated phosphatidic acid (PtdOH),which may subsequently be metabolized to diacylglycerol (DAG) and lysophosphatidic acid. There are other pathways that lead to PtdOH generation, but differences in pathways and in the acyl composition of the products seem to provide some specificity.

Both direct and indirect inhibitors of PLD activity have been identified despite a long-held suspicion that this pathway was undruggable. The identification of raloxifene and halopemide as direct inhibitors was followed by the systematic development of isoenzyme-preferring compounds that have been used to further differentiate the functions of PLD1 and PLD2.

PLD2 in host cells has been associated with viral entry processes and innate immune response pathways such that inhibition blocks efficient infection. This PLD2 pathway has been linked to autophagy via AKT kinases.

As a potential target in antiretroviral therapy, PLD1 works through the CAD enzyme (which contains carbamoyl aspartate synthase, aspartate transcarbamylase and dihydro-orotase domains) to modulate pyrimidine biosynthesis.

PLD activity and expression have been shown to be upregulated in several types of human cancers, in which PLD enzymes function downstream of a variety of known oncogenes. Inhibition of PtdOH production has a marked effect on tumorigenesis and malignant invasion.

PLD1, PLD2 and PLD3 have each been suggested to have a role in Alzheimer disease and other neurodegenerative conditions, but a mechanism has not yet emerged to explain the roles of these proteins in central nervous system pathophysiology.

Lipid second messengers such as phosphatidic acid (PtdOH) have a role in a wide range of pathological processes, and phospholipase D (PLD) enzymes are one of the major sources of signal-activated PtdOH generation. In this Review, Brown, Thomas and Lindsley discuss the development of PLD inhibitors, with a focus on isoform-specific inhibitors, and their potential applications in the treatment of cancer, neurodegeneration and infection.

Lipid second messengers have essential roles in cellular function and contribute to the molecular mechanisms that underlie inflammation, malignant transformation, invasiveness, neurodegenerative disorders, and infectious and other pathophysiological processes. The phospholipase D (PLD) isoenzymes PLD1 and PLD2 are one of the major sources of signal-activated phosphatidic acid (PtdOH) generation downstream of a variety of cell-surface receptors, including G protein-coupled receptors (GPCRs), receptor tyrosine kinases (RTKs) and integrins. Recent advances in the development of isoenzyme-selective PLD inhibitors and in molecular genetics have suggested that PLD isoenzymes in mammalian cells and pathogenic organisms may be valuable targets for the treatment of several human diseases. Isoenzyme-selective inhibitors have revealed complex inter-relationships between PtdOH biosynthetic pathways and the role of PtdOH in pathophysiology. PLD enzymes were once thought to be undruggable owing to the ubiquitous nature of PtdOH in cell signalling and concerns that inhibitors would be too toxic for use in humans. However, recent promising discoveries suggest that small-molecule isoenzyme-selective inhibitors may provide novel compounds for a unique approach to the treatment of cancers, neurodegenerative disorders and other afflictions of the central nervous system, and potentially serve as broad-spectrum antiviral and antimicrobial therapeutics.

Phospholipase D signaling pathways and phosphatidic acid as therapeutic targets in cancer

Phospholipase D is a ubiquitous class of enzymes that generates phosphatidic acid as an intracellular signaling species. The phospholipase D superfamily plays a central role in a variety of functions in prokaryotes, viruses, yeast, fungi, plants, and eukaryotic species. In mammalian cells, the pathways modulating catalytic activity involve a variety of cellular signaling components, including G protein-coupled receptors, receptor tyrosine kinases, polyphosphatidylinositol lipids, Ras/Rho/ADP-ribosylation factor GTPases, and conventional isoforms of protein kinase C, among others. Recent findings have shown that phosphatidic acid generated by phospholipase D plays roles in numerous essential cellular functions, such as vesicular trafficking, exocytosis, autophagy, regulation of cellular metabolism, and tumorigenesis. Many of these cellular events are modulated by the actions of phosphatidic acid, and identification of two targets (mammalian target of rapamycin and Akt kinase) has especially highlighted a role for phospholipase D in the regulation of cellular metabolism. Phospholipase D is a regulator of intercellular signaling and metabolic pathways, particularly in cells that are under stress conditions. This review provides a comprehensive overview of the regulation of phospholipase D activity and its modulation of cellular signaling pathways and functions.

Effect of mixed-phospholipid layer on phospholipase D reaction-induced vesicle rupture

Spherical phospholipid bilayers, or vesicles, were prepared layer by layer using a double-emulsion technique, which allows the outer layer of the vesicles to be formed with two phospholipids that have different head groups: phosphatidylcholine (PC) and phosphatidylethanolamine. At the outer layer of the vesicles, the phospholipase D (PLD) catalyzed for the conversion of PC to phosphatidic acid. The reaction caused by PLD induced the curvature change of the vesicles, which eventually led to the rupture of the vesicles. Before the investigation, the ratio of dioleoylphosphatidylethanolamine to oleoylhydroxyphosphatidylethanolamine was found as a condition such that the vesicles made with the mixed lipids were as stable as those made with pure dioleoylphosphatidylcholine. Response time from the PLD injection to vesicle rupture was monitored by the composition of the outer layer by the fluorescence intensity change of pH-sensitive dye encapsulated in the vesicles. The response time began to be slowed at approximately 30 % PC. The response times for the compositions were associated with the surface density of PC at the outer layer. These results also seem to be determined by the size of PLD, specifically the PLD active site.

Regulation of the Golgi complex by phospholipid remodeling enzymes

The mammalian Golgi complex is a highly dynamic organelle consisting of stacks of flattened cisternae with associated coated vesicles and membrane tubules that contribute to cargo import and export, intra-cisternal trafficking, and overall Golgi architecture. At the morphological level, all of these structures are continuously remodeled to carry out these trafficking functions. Recent advances have shown that continual phospholipid remodeling by phospholipase A (PLA) and lysophospholipid acyltransferase (LPAT) enzymes, which deacylate and reacylate Golgi phospholipids, respectively, contributes to this morphological remodeling. Here we review the identification and characterization of four cytoplasmic PLA enzymes and one integral membrane LPAT that participate in the dynamic functional organization of the Golgi complex, and how some of these enzymes are integrated to determine the relative abundance of COPI vesicle and membrane tubule formation. This article is part of a Special Issue entitled Lipids and Vesicular Transport.

Understanding of the roles of phospholipase D and phosphatidic acid through their binding partners

Phospholipase D (PLD) is a phosphatidyl choline (PC)-hydrolyzing enzyme that generates phosphatidic acid (PA), a lipid second messenger that modulates diverse intracellular signaling. Through interactions with signaling molecules, both PLD and PA can mediate a variety of cellular functions, such as, growth/proliferation, vesicle trafficking, cytoskeleton modulation, development, and morphogenesis. Therefore, systemic approaches for investigating PLD networks including interrelationship between PLD and PA and theirs binding partners, such as proteins and lipids, can enhance fundamental knowledge of roles of PLD and PA in diverse biological processes. In this review, we summarize previously reported protein-protein and protein-lipid interactions of PLD and PA and their binding partners. In addition, we describe the functional roles played by PLD and PA in these interactions, and provide PLD network that summarizes these interactions. The PLD network suggests that PLD and PA could act as a decision maker and/or as a coordinator of signal dynamics. This viewpoint provides a turning point for understanding the roles of PLD-PA as a dynamic signaling hub.

The exquisite regulation of PLD2 by a wealth of interacting proteins: S6K, Grb2, Sos, WASp and Rac2 (and a surprise discovery: PLD2 is a GEF)

Phospholipase D (PLD) catalyzes the conversion of the membrane phospholipid phosphatidylcholine to choline and phosphatidic acid (PA). PLD's mission in the cell is two-fold: phospholipid turnover with maintenance of the structural integrity of cellular/intracellular membranes and cell signaling through PA and its metabolites. Precisely, through its product of the reaction, PA, PLD has been implicated in a variety of physiological cellular functions, such as intracellular protein trafficking, cytoskeletal dynamics, chemotaxis of leukocytes and cell proliferation. The catalytic (HKD) and regulatory (PH and PX) domains were studied in detail in the PLD1 isoform, but PLD2 was traditionally studied in lesser detail and much less was known about its regulation. Our laboratory has been focusing on the study of PLD2 regulation in mammalian cells. Over the past few years, we have reported, in regards to the catalytic action of PLD, that PA is a chemoattractant agent that binds to and signals inside the cell through the ribosomal S6 kinases (S6K). Regarding the regulatory domains of PLD2, we have reported the discovery of the PLD2 interaction with Grb2 via Y169 in the PX domain, and further association to Sos, which results in an increase of de novo DNA synthesis and an interaction (also with Grb2) via the adjacent residue Y179, leading to the regulation of cell ruffling, chemotaxis and phagocytosis of leukocytes. We also present the complex regulation by tyrosine phosphorylation by epidermal growth factor receptor (EGF-R), Janus Kinase 3 (JAK3) and Src and the role of phosphatases. Recently, there is evidence supporting a new level of regulation of PLD2 at the PH domain, by the discovery of CRIB domains and a Rac2-PLD2 interaction that leads to a dual (positive and negative) effect on its enzymatic activity. Lastly, we review the surprising finding of PLD2 acting as a GEF. A phospholipase such as PLD that exists already in the cell membrane that acts directly on Rac allows a quick response of the cell without intermediary signaling molecules. This provides only the latest level of PLD2 regulation in a field that promises newer and exciting advances in the next few years.

Evidence for two CRIB domains in phospholipase D2 (PLD2) that the enzyme uses to specifically bind to the small GTPase Rac2

Phospholipase D (PLD) and small GTPases are vital to cell signaling. We report that the Rac2 and the PLD2 isoforms exist in the cell as a lipase-GTPase complex that enables the two proteins to elicit their respective functionalities. A strong association between the two molecules was demonstrated by co-immunoprecipitation and was confirmed in living cells by FRET with CFP-Rac2 and YFP-PLD2 fluorescent chimeras. We have identified the amino acids in PLD2 that define a specific binding site to Rac2. This site is composed of two CRIB (Cdc42-and Rac-interactive binding) motifs that we have named "CRIB-1" and "CRIB-2" in and around the PH domain in PLD2. Deletion mutants PLD2-ΔCRIB-1/2 negate co-immunoprecipitation with Rac2 and diminish the FRET signal in living cells. The PLD2-Rac2 association was further confirmed in vitro using affinity-purified recombinant proteins. Binding was saturable with an apparent K(d) of 3 nm and was diminished with PLD2-ΔCRIB mutants. Furthermore, PLD2 bound more efficiently to Rac2-GTP than to Rac2-GDP or to a GDP-constitutive Rac2-N17 mutant. Increasing concentrations of recombinant Rac2 in vitro and in vivo during cell adhesion inhibit PLD2. Conversely, Rac2 activity is increased in the presence of PLD2-WT but not in PLD2-ΔCRIB. We propose that in activated cells PLD2 affects Rac2 in an initial positive feedback, but as Rac2-GTP accumulates in the cell, this constitutes a "termination signal" leading to PLD2 inactivation.

The role of phospholipase D in osteoblast response to titanium surface microstructure

Biomaterial surface properties such as microtopography and energy can change cellular responses at the cell-implant interface. Phospholipase D (PLD) is required for the differentiation of osteoblast-like MG63 cells on machined and grit-blasted titanium surfaces. Here, we determined if PLD is also required on microstructured/high-energy substrates and the mechanism involved. shRNAs for human PLD1 and PLD2 were used to silence MG63 cells. Wild-type and PLD1 or PLD1/2 silenced cells were cultured on smooth-pretreatment surfaces (PT); grit-blasted, acid-etched surfaces (SLA); and SLA surfaces modified to have higher surface energy (modSLA). PLD was inhibited with ethanol or activated with 24,25-dihydroxyvitamin-D(3) [24R,25(OH)(2)D(3)]. As surface roughness/energy increased, PLD mRNA and activity increased, cell number decreased, osteocalcin and osteoprotegerin increased, and protein kinase C (PKC) and alkaline phosphatase specific activities increased. Ethanol inhibited PLD and reduced surface effects on these parameters. There was no effect on these parameters after knockdown of PLD1, but PLD1/2 double knockdown had effects comparableto PLD inhibition. 24R,25(OH)(2)D(3) increased PLD activity and the production of osteocalcin and osteoprotegerin, but decreased cell number on the rough/high-energy surfaces. These results confirm that surface roughness/energy-induced PLD activity is required for osteoblast differentiation and that PLD2 is the main isoform involved in this pathway. PLD is activated by 24R,25(OH)(2)D(3) in a surface-dependent manner and inhibition of PLD reduces the effects of surface microstructure/energy on PKC, suggesting that PLD mediates the stimulatory effect of microstructured/high-energy surfaces via PKC-dependent signaling.

Toward a model for Arf GTPases as regulators of traffic at the Golgi

In this review, I summarize the likely roles played by ADP-ribosylation factor (Arf) proteins in the regulation of membrane traffic at the Golgi, from the perspective of the GTPase. The most glaring limitations to the development of a coherent molecular model are highlighted; including incomplete information on the initiation of Arf activation, identification of the "accessory proteins" required for carrier maturation and scission, and those required for directed traffic and fusion at the destination membrane. Though incomplete, the molecular model of carrier biogenesis has developed rapidly in recent years and promises richness in understanding this essential process.

Early stages of Golgi vesicle and tubule formation require diacylglycerol

We have investigated the role for diacylglycerol (DAG) in membrane bud formation in the Golgi apparatus. Addition of propranolol to specifically inhibit phosphatidate phosphohydrolase (PAP), an enzyme responsible for converting phosphatidic acid into DAG, effectively prevents formation of membrane buds. The effect of PAP inhibition on Golgi membranes is rapid and occurs within 3 min. Removal of the PAP inhibitor then results in a rapid burst of buds, vesicles, and tubules that peaks within 2 min. The inability to form buds in the presence of propranolol does not appear to be correlated with a loss of ARFGAP1 from Golgi membranes, as knockdown of ARFGAP1 by RNA interference has little or no effect on actual bud formation. Rather, knockdown of ARFGAP1 results in an increase in membrane buds and a decrease of vesicles and tubules suggesting it functions in the late stages of scission. How DAG promotes bud formation is discussed.

Regulation of phosphatidylinositol 4-phosphate 5-kinase activity by partner proteins

The remarkably versatile phospholipid, phosphatidylinositol 4,5-bisphosphate [PI(4,5)P(2)], plays crucial roles in signal transduction, actin cytoskeleton reorganization, clathrin-dependent endocytosis, and regulation of membrane morphology. In mammalian cells, PI(4,5)P(2) is synthesized predominantly by phosphatidylinositol 4-phosphate [PI(4)P] 5-kinase (PIP5K) through phosphorylation of PI(4)P at the D-5 position of the inositol ring. PIP5K is composed of three isoforms, PIP5Kalpha, beta, and gamma, and three splicing variants of the gamma isozyme. Although the PIP5Kgamma splicing variant PIP5Kgamma661 appears to be very specifically activated by talin, which plays a crucial role in focal adhesion formation, and the adaptor complex AP-2, the regulation of activities of other PIP5K isozymes is not fully understood at present. To understand the activation mechanism and the physiological function specific to each PIP5K isozyme, it is required to identify a specific activator of each PIP5K isozyme. This chapter describes common assays used to measure interaction and activation of PIP5K isozymes by activators thus far identified. In addition, procedures for preparation of PIP5K isozymes and activators are described.

Understanding phospholipase D (PLD) using leukocytes: PLD involvement in cell adhesion and chemotaxis

Phospholipase D (PLD) is an enzyme that catalyzes the conversion of membrane phosphatidylcholine to choline and phosphatidic acid (PA; a second messenger). PLD is expressed in nearly all types of leukocytes and has been associated with phagocytosis, degranulation, microbial killing, and leukocyte maturation. With the application of recently developed molecular tools (i.e., expression vectors, silencing RNA, and specific antibodies), the demonstration of a key role for PLD in those and related cellular actions has contributed to a better awareness of its importance. A case in point is the recent findings that RNA interference-mediated depletion of PLD results in impaired leukocyte adhesion and chemotaxis toward a gradient of chemokines, implying that PLD is necessary for leukocyte movement. We forecast that based on results such as those, leukocytes may prove to be useful tools to unravel still-unresolved mechanistic issues in the complex biology of PLD. Three such issues are considered here: first, whether the cellular actions of PLD are mediated entirely by PA (the product of its enzymatic reaction) or whether PLD by itself interacts with other protein signaling molecules; second, the current difficulty of defining a "PA consensus site" in the various intracellular protein targets of PA; and third, the resolution of specific PLD location (upstream or downstream) in a particular effector signaling cascade. There are reasons to expect that leukocytes and their leukemic cell line counterparts will continue yielding invaluable information to cell biologists to resolve standing molecular and functional issues concerning PLD.

Effect of aroclor 1248 and two pure PCB congeners on phospholipase D activity in rat renal tubular cell cultures

This paper elucidates the effect of different polychlorinated biphenyls (PCBs) on the phospholipase D (PLD) activity in soluble and particulate fractions of rat renal proximal tubular culture cells. Treatment with Aroclor 1248 (a commercial PCB mixture) caused a marked increase in the activity of PLD in intact renal tubular cells. The PLD activity was increased by Aroclor 1248 in the particulate fraction while the enzyme activity was unaffected in the soluble fraction. This work also shows that PCB 153 (2,2',4,4',5,5'-hexachlorobiphenyl, a di-ortho-substituted nonplanar congener) can increase the activity of PLD only in the particulate fraction. The exposure of cell cultures to PCB 77 (3,3',4,4'-tetrachlorobiphenyl, a non-ortho-substituted planar congener) does not alter PLD activity. These results suggest that PCB effects are structure dependent. Therefore, in order to clarify the molecular mechanism of activation of PLD by PCBs, the contents of immunoreactive PLD were examined by immunoblot analysis. Renal tubular cells expressed a PLD protein of 120 kDa corresponding with the PLD1 mammalian isoform in both the particulate and the soluble fraction. Aroclor 1248, PCB 153, and PCB 77 do not induce changes in the levels of PLD protein. These data indicate that PCBs, particularly nonplanar congeners, increase PLD activity. Moreover, these changes could not be demonstrated in the enzyme content in rat renal tubular cell cultures.

The differential regulation of phosphatidylinositol 4-phosphate 5-kinases and phospholipase D1 by ADP-ribosylation factors 1 and 6

Phosphatidylinositol 4-phosphate 5-kinases [PtdIns4P5Ks] synthesise the majority of cellular phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P(2)] and phospholipase D1 (PLD1) synthesises large amounts of phosphatidic acid (PtdOH). The activities of PtdIns4P5Ks and PLDs are thought to be coupled during cell signalling in order to support large simultaneous increases in both PtdIns(4,5)P(2) and PtdOH, since PtdOH activates PtdIns4P5Ks and PLD1 requires PtdIns(4,5)P(2) as a cofactor. However, little is known about the control of such a system. Membrane recruitment of ADP-ribosylation factors (Arfs) activates both PtdIns4P5Ks and PLDs, but it is not known if each enzyme is controlled in series by different Arfs or in parallel by a single form. We show through pull-down and vesicle sedimentation interaction assays that PtdIns4P5K activation may be facilitated by Arf-enhanced membrane association. However PtdIns4P5Ks discriminate poorly between near homogeneously myristoylated Arf1 and Arf6 although examples of all three known active isoforms (mouse alpha>beta, gamma) respond to these G-proteins. Conversely PLD1 genuinely prefers Arf1 and so the two lipid metabolising enzymes are differentially controlled. We propose that isoform selective Arf/PLD interaction and not Arf/PtdIns4P5K will be the critical trigger in the formation of distinct, optimal triples of Arf/PLDs/PtdIns4P5Ks and be the principle regulator of any coupled increases in the signalling lipids PtdIns(4,5)P(2) and PtdOH.

Direct Modulation of Phospholipase D Activity by Gβγ

Phospholipase D-mediated hydrolysis of phosphatidylcholine is stimulated by protein kinase C and the monomeric G proteins Arf, RhoA, Cdc42, and Rac1, resulting in complex regulation of this enzyme. Using purified proteins, we have identified a novel inhibitor of phospholipase D activity, Gbetagamma subunits of heterotrimeric G proteins. G protein-coupled receptor activation alters affinity between Galpha and Gbetagamma subunits, allowing subsequent interaction with distinct effectors. Gbeta1gamma1 inhibited phospholipase D1 and phospholipase D2 activity, and both Gbeta1gamma1 and Gbeta1gamma2 inhibited stimulated phospholipase D1 activity in a dosedependent manner in reconstitution assays. Reconstitution assays suggest this interaction occurs through the amino terminus of phospholipase D, because Gbeta1gamma1 is unable to inhibit an amino-terminally truncated phospholipase D construct, PLD1.d311, which like full-length phospholipase D isoforms, requires phosphatidylinositol-4,5-bisphosphate for activity. Furthermore, a truncated protein consisting of the amino-terminal region of phospholipase D containing the phox/pleckstrin homology domains was found to interact with Gbeta1gamma1, unlike the PLD1.d311 recombinant protein, which lacks this domain. In vivo, expressed recombinant Gbeta1gamma2 was also found to inhibit phospholipase D activity under basal and stimulated conditions in MDA-MB-231 cells, which natively express both phospholipase D1 and phospholipase D2. These data demonstrate that Gbetagamma directly regulates phospholipase D activity in vitro and suggest a novel mechanism to negatively regulate phospholipase D signaling in vivo.

Differential Requirement for Phospholipase D/Spo14 and Its Novel Interactor Sma1 for Regulation of Exocytotic Vesicle Fusion in Yeast Meiosis

During sporulation and meiosis of budding yeast a developmental program determines the formation of the new plasma membranes of the spores. This process of prospore membrane (PSM) formation leads to the formation of meiotic daughter cells, the spores, within the lumen of the mother cell. It is initiated at the spindle pole bodies during meiosis II. Spore formation, but not meiotic cell cycle progression, requires the function of phospholipase D (PLD/Spo14). Here we show that PLD/Spo14 forms a complex with Sma1, a meiotically expressed protein essential for spore formation. Detailed analysis revealed that both proteins are required for early steps of prospore membrane assembly but with distinct defects in the respective mutants. In the Deltaspo14 mutant the initiation of PSM formation is blocked and aggregated vesicles of homogenous size are detected at the spindle pole bodies. In contrast, initiation of PSM formation does occur in the Deltasma1 mutant, but the enlargement of the membrane is impaired. During PSM growth both Spo14 and Sma1 localize to the membrane, and localization of Spo14 is independent of Sma1. Biochemical analysis revealed that Sma1 is not necessary for PLD activity per se and that PLD present in a complex with Sma1 is highly active. Together, our results suggest that yeast PLD is involved in two distinct but essential steps during the regulated vesicle fusion necessary for the assembly of the membranous encapsulations of the spores.

Arf1-dependent PLD1 is localized to oleic acid-induced lipid droplets in NIH3T3 cells

Phospholipase D (PLD) is known to play a role in vesicle transport through the hydrolysis of phosphatidylcholine (PC) to produce the bioactive lipid, phosphatidic acid. Lipid droplets (LDs) are surrounded by a monolayer of phospholipids, including PC and its lyso derivative, and exhibit a number of signaling proteins. Our recent report suggests that the association of adipose differentiation-related protein (ADRP) to LDs is regulated by an ADP-ribosylation factor 1 (Arf1)-dependent mechanism. In the present study, we found an increase in PLD activity accompanied with LD formation in oleic acid-treated NIH3T3 cells. Brefeldin A, an inhibitor of ARF-GEFs, suppressed both PLD activation and LD formation in oleic acid-treated cells. PLD1, but not PLD2, was found to exist in LDs by immunocytochemical analysis. Furthermore, co-existence of PLD1, Arf1, and ADRP was observed in the LD-enriched subcellular fractions obtained from oleic acid-treated NIH3T3 cells by Western blot analysis. PLD1 activity in the LD-enriched fractions was stimulated by exogenously added Arf1. Although LDs were induced in either PLD1- or PLD2-overexpressing CHO cells by oleic acid treatment, the stimulation of PLD activity was observed only in PLD1-CHO cells. Taken together, the data suggest that the activation of Arf1-dependent PLD1 occurs in LDs and may be involved in their physiological function.

Isoform-selective effects of the depletion of ADP-ribosylation factors 1-5 on membrane traffic

The ADP-ribosylation factors (Arfs) are six proteins within the larger Arf family and Ras superfamily that regulate membrane traffic. Arfs all share numerous biochemical activities and have very similar specific activities. The use of dominant mutants and brefeldin A has been important to the discovery of the cellular functions of Arfs but lack specificity between Arf isoforms. We developed small interference RNA constructs capable of specific depletion of each of the cytoplasmic human Arfs to examine the specificity of Arfs in live cells. No single Arf was required for any step of membrane traffic examined in HeLa cells. However, every combination of the double knockdowns of Arf1, Arf3, Arf4, and Arf5 yielded a distinct pattern of defects in secretory and endocytic traffic, demonstrating clear specificity for Arfs at multiple steps. These results suggest that the cooperation of two Arfs at the same site may be a general feature of Arf signaling and provide candidates at several cellular locations that when paired with data on the localization of the many different Arf guanine nucleotide exchange factors, Arf GTPase activating proteins, and effectors will aid in the description of the mechanisms of specificity in this highly conserved and primordial family of regulatory GTPases.

Lipid imaging by gold cluster time-of-flight secondary ion mass spectrometry: application to Duchenne muscular dystrophy

Imaging with time-of-flight secondary ion mass spectrometry (TOF-SIMS) has expanded very rapidly with the development of gold cluster ion sources (Au(3+)). It is now possible to acquire ion density maps (ion images) on a tissue section without any treatment and with a lateral resolution of few micrometers. In this article, we have taken advantage of this technique to study the degeneration/regeneration process in muscles of a Duchenne muscular dystrophy model mouse. Specific distribution of different lipid classes (fatty acids, triglycerides, phospholipids, tocopherol, coenzyme Q9, and cholesterol) allows us to distinguish three different regions on a mouse leg section: one is destroyed, another is degenerating (oxidative stress and deregulation of the phosphoinositol cycle), and the last one is stable. TOF-SIMS imaging shows the ability to localize directly on a tissue section a great number of lipid compounds that reflect the state of the cellular metabolism.

Novel functions of the phospholipase D2-Phox homology domain in protein kinase Czeta activation

It has been established that protein kinase Czeta (PKCzeta) participates in diverse signaling pathways and cellular functions in a wide variety of cells, exhibiting properties relevant to cellular survival and proliferation. Currently, however, the regulation mechanism of PKCzeta remains elusive. Here, for the first time, we determine that phospholipase D2 (PLD2) enhances PKCzeta activity through direct interaction in a lipase activity-independent manner. This interaction of the PLD2-Phox homology (PX) domain with the PKCzeta-kinase domain also induces the activation loop phosphorylation of PKCzeta and downstream signal stimulation, as measured by p70 S6 kinase phosphorylation. Furthermore, only the PLD2-PX domain directly stimulates PKCzeta activity in vitro, and it is necessary for the formation of the ternary complex with phosphoinositide-dependent kinase 1 and PKCzeta. The mutant that substitutes the triple lysine residues (Lys101, Lys102, and Lys103) within the PLD2-PX domain with alanine abolishes interaction with the PKCzeta-kinase domain and activation of PKCzeta. Moreover, breast cancer cell viability is significantly affected by PLD2 silencing. Taken together, these results suggest that the PLD2-mediated PKCzeta activation is induced by its PX domain performing both direct activation of PKCzeta and assistance of activation loop phosphorylation. Furthermore, we find it is an important factor in the survival of breast cancer cells.

Preparation of myristoylated Arf1 and Arf6

Arf proteins are members of the Arf family of small Ras-like GTP binding proteins. Six Arfs, grouped into three classes, have been identified in mammalian cells and three members have been identified in yeasts. Arf1 and Arf6, more extensively studied than other Arfs, have been found to affect membrane traffic and actin remodeling. A structural feature that distinguishes Arfs from other Ras superfamily members is an N-terminal alpha-helix, extending from the basic G-protein fold, which is cotranslationally myristoylated. Both the helix and the myristate affect biochemical properties of Arfs, including nucleotide exchange, membrane association, and interaction with some effector proteins. Preparation of myristoylated Arf for in vitro studies of Arf function requires consideration of both the reaction yielding myristoylated protein and the properties of the modified Arfs. Here, we describe methods that yield homogeneous preparations of myristoylated Arf1 and Arf6.

Activation of phospholipase D by osmotic cell swelling

In response to osmotic cell swelling, Intestine 407 cells react with a rapid and transient activation of phospholipase D (PLD). To investigate the role of PLD during the regulatory volume decrease, cells were treated with 1-butanol resulting in a depletion of PLD substrates. Activation of volume-regulated anion channels, but not the cell swelling-induced release of taurine, was largely inhibited in the presence of low concentrations of 1-butanol. In addition, hypotonicity-induced exocytosis, ATP release and subsequent endocytosis were found to be largely abrogated. The results support a model of cell volume regulation in which PLD plays an essential role in the cell swelling-induced vesicle cycling and in the activation of volume-sensitive anion channels.

Phosphoinositides in Constitutive Membrane Traffic

Proteins that make, consume, and bind to phosphoinositides are important for constitutive membrane traffic. Different phosphoinositides are concentrated in different parts of the central vacuolar pathway, with phosphatidylinositol 4-phosphate predominate on Golgi, phosphatidylinositol 4,5-bisphosphate predominate at the plasma membrane, phosphatidylinositol 3-phosphate the major phosphoinositide on early endosomes, and phosphatidylinositol 3,5-bisphosphate found on late endocytic organelles. This spatial segregation may be the mechanism by which the direction of membrane traffic is controlled. Phosphoinositides increase the affinity of membranes for peripheral membrane proteins that function for sorting protein cargo or for the docking and fusion of transport vesicles. This implies that constitutive membrane traffic may be regulated by the mechanisms that control the activity of the enzymes that produce and consume phosphoinositides. Although the lipid kinases and phosphatases that function in constitutive membrane traffic are beginning to be identified, their regulation is poorly understood.

A rac-like small G-protein from Brassica campestris activates a PKC-dependent phospholipase D

A cDNA clone encoding a rac-like small GTP binding protein was isolated from a cDNA library of Chinese cabbage (Brassica campestris L. ssp. pekinensis) flower buds and named Brac1. The Brac1 cDNA contains an open reading frame encoding 198 amino acid residues with an estimated molecular mass of 21,690 Da and this coding region has conserved residues and motifs unique to the Rho subfamily of proteins. The deduced amino acid sequence of the Brac1 protein is closely related to that of Arabidopsis thaliana Arac3 (91%), but it shares relatively little homology with other members of the Ras superfamily (about 30% identity). To further characterize Brac1, a pGBrac1 expression vector consisting of PCR-amplified Brac1 cDNA plus glutathione S-transferase (GST) and pBKS(+)II was used to purify the protein. Using a PEI-cellulose/TLC plate, GTPase activity of this protein was confirmed and competition binding studies, using the guanine nucleotides, ATP, UTP and CTP, revealed that the di- and triphosphate forms of guanine nucleotides strongly bind Brac1. Membrane-bound PLD activity was synergistically enhanced by Brac1 in the presence of protein kinase C, but not in the presence of ARF (ADP-ribosylation factor). Genomic analysis indicated that Brac1 belongs to a multigene family. Brac1 transcripts were expressed in all the organs of Brassica, but were especially prevalent in flower buds.

Roles of phosphoinositides and of Spo14p (phospholipase D)-generated phosphatidic acid during yeast sporulation

During yeast sporulation, internal membrane synthesis ensures that each haploid nucleus is packaged into a spore. Prospore membrane formation requires Spo14p, a phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2]-stimulated phospholipase D (PLD), which hydrolyzes phosphatidylcholine (PtdCho) to phosphatidic acid (PtdOH) and choline. We found that both meiosis and spore formation also require the phosphatidylinositol (PtdIns)/PtdCho transport protein Sec14p. Specific ablation of the PtdIns transport activity of Sec14p was sufficient to impair spore formation but not meiosis. Overexpression of Pik1p, a PtdIns 4-kinase, suppressed the sec14-1 meiosis and spore formation defects; conversely, pik1-ts diploids failed to undergo meiosis and spore formation. The PtdIns(4)P 5-kinase, Mss4p, also is essential for spore formation. Use of phosphoinositide-specific GFP-PH domain reporters confirmed that PtdIns(4,5)P2 is enriched in prospore membranes. sec14, pik1, and mss4 mutants displayed decreased Spo14p PLD activity, whereas absence of Spo14p did not affect phosphoinositide levels in vivo, suggesting that formation of PtdIns(4,5)P2 is important for Spo14p activity. Spo14p-generated PtdOH appears to have an essential role in sporulation, because treatment of cells with 1-butanol, which supports Spo14p-catalyzed PtdCho breakdown but leads to production of Cho and Ptd-butanol, blocks spore formation at concentrations where the inert isomer, 2-butanol, has little effect. Thus, rather than a role for PtdOH in stimulating PtdIns(4,5)P2 formation, our findings indicate that during sporulation, Spo14p-mediated PtdOH production functions downstream of Sec14p-, Pik1p-, and Mss4p-dependent PtdIns(4,5)P2 synthesis.

Disruption of lipid order by short-chain ceramides correlates with inhibition of phospholipase D and downstream signaling by FcepsilonRI

Specialized plasma membrane domains known as lipid rafts participate in signal transduction and other cellular processes, and their liquid-ordered properties appear to be important for their function. We investigated the possibility of using amphiphiles to disrupt lipid rafts and thereby inhibit IgE-FcepsilonRI signaling. We find that short-chain ceramides - C2-ceramide and C6-ceramide - decrease plasma membrane lipid order and reduce the extent of fluorescence resonance energy transfer between lipid-raft-associated molecules on intact cells; by contrast, biologically inactive C2-dihydroceramide does neither. Structural perturbations by these ceramides parallel their inhibitory effects on antigen-stimulated Ca2+ mobilization in RBL mast cells in the presence and absence of extracellular Ca2+. Similar inhibition of Ca2+ mobilization is caused by n-butanol, which prevents phosphatidic acid production by phospholipase D, but not by t-butanol, which does not prevent phosphatidic acid production. These results and previously reported effects of short-chain ceramides on phospholipase D activity prompted us to compare the effects of C2-ceramide, C2-dihydroceramide and C16-ceramide on phospholipase D1 and phospholipase D2 activities in vitro. We find that the effects of these ceramides on phospholipase D1 activity strongly correlate with their effects on antigen-stimulated Ca2+ mobilization and with their disruption of lipid order. Our results indicate that phospholipase D activity is upstream of antigen-stimulated Ca2+ mobilization in these cells, and they demonstrate that ceramides can serve as useful probes for investigating roles of plasma membrane structure and phospholipase D activity in cellular signaling.

Regulation of phospholipase D

Structural studies of plant and bacterial members of the phospholipase D (PLD) superfamily are providing information about the role of the conserved HKD domains in the structure of the catalytic center and the catalytic mechanism of mammalian PLD isozymes (PLD1 and PLD2). Mutagenesis and sequence comparison studies have also defined the presence of pleckstrin homology and phox homology domains in the N-terminus and have demonstrated that a conserved sequence at the C-terminus is required for catalysis. The N- and C-terminal regions of PLD1 also contain interaction sites for protein kinase C, which can directly activate the enzyme through a non-phosphorylating mechanism. Small G proteins of the Rho and ADP-ribosylation factor families also directly regulate the enzyme, with RhoA binding to a sequence in the C-terminus. Certain tyrosine kinases and members of the Ras subfamily of small G proteins can activate the enzyme, but the mechanisms appear to be indirect. The mechanisms by which agonists activate PLD in vivo probably involve multiple pathways.

Selective estrogen receptor (ER) modulators differentially regulate phospholipase D catalytic activity in ER-negative breast cancer cells

Recent successes in the pharmacotherapeutic treatment of breast cancer are associated with the use of selective estrogen receptor modulators. Two commonly prescribed pharmaceuticals in this class, tamoxifen and raloxifene, have been shown to have effects through estrogen receptor (ER)-independent mechanisms. Hyperactivation of phospholipase D (PLD) in certain tumor-derived cell lines have been reported, and recent findings suggest a role for PLD in transformation and metastasis. In the present study, we compare the effects of tamoxifen and raloxifene on PLD in the ER-positive mammary epithelial cell line MCF-12A, and the ER-negative, highly tumorigenic mammary carcinoma cell line MDA-MB-231. Our data demonstrate that tamoxifen and raloxifene have differential effects on PLD catalytic activity. Tamoxifen stimulates PLD in both ER-positive and -negative cells in vivo, whereas raloxifene inhibits PLD activity in these same cell types. In addition, we show that the active metabolite 4-OH-tamoxifen can be used to pharmacologically discriminate the two isoforms of PLD, through a stimulatory effect on PLD1 and an inhibitory effect on PLD2. Using recombinant PLD1, we show stimulation by tamoxifen requires a factor present in Sf21 insect cells that is not required for inhibition of PLD1 by raloxifene. Furthermore, tamoxifen stimulation and raloxifene inhibition of PLD activities are independent of the amino-terminal portion of PLD1 (amino acids 1-324). Knowledge of the mechanisms of action of these drugs on PLD may provide insights into the pharmacological action of these drugs and the role of PLD in some cancers.

Specificity of Rho insert-mediated activation of phospholipase D1

Members of the Rho subfamily of GTP-binding proteins regulate phospholipase D1 (PLD1) activity and signaling. In previous work, we demonstrated that binding of the Rho family member Cdc42 to PLD1 and the subsequent stimulation of its enzymatic activity are distinct events. Deletion of the insert helix from Cdc42 does not interfere with its switch I-mediated, GTP-dependent binding to PLD1 but inhibits Cdc42-stimulated PLD1 activity. To understand the mechanism of the insert-mediated activation of PLD1 by Cdc42 and to develop reagents to study Cdc42-activated PLD1 in cellular signaling events, we have undertaken a mutational analysis of the Rho insert region of Cdc42 and examined the specificity of the insert helix requirement in the other Rho family members, RhoA and Rac1. Here, we identify a critical residue, serine 124, in the Cdc42 insert helix central to its activation mechanism. Further, we examine this activation mechanism with respect to other members of the Rho family and demonstrate that each Rho protein activates PLD by distinct mechanisms, potentially allowing for unique signaling outcomes in the cell.

Mechanisms of regulation of phospholipase D1 and D2 by the heterotrimeric G proteins G13 and Gq

Our earlier studies of rat brain phospholipase D1 (rPLD1) showed that the enzyme could be activated in cells by alpha subunits of the heterotrimeric G proteins G(13) and G(q). Recently, we showed that rPLD1 is modified by Ser/Thr phosphorylation and palmitoylation. In this study, we first investigated the roles of these post-translational modifications on the activation of rPLD1 by constitutively active Galpha(13)Q226L and Galpha(q)Q209L. Mutations of Cys(240) and Cys(241) of rPLD1, which abolish both post-translational modifications, did not affect the ability of either Galpha(13)Q226L or Galpha(q)Q209L to activate rPLD1. However, the RhoA-insensitive mutants, rPLD1(K946A,K962A) and rPLD1(K962Q), were not activated by Galpha(13)Q226L, although these mutant enzymes responded to phorbol ester and Galpha(q)Q209L. On the contrary, the PKC-insensitive mutant rPLD1(DeltaN168), which lacks the first 168 amino acids of rPLD1, responded to Galpha(13)Q226L but not to Galpha(q)Q209L. In addition, we found that rPLD2 was strongly activated by Galpha(q)Q209L and phorbol ester. However, surprisingly, the enzymatic activity of rPLD2 was suppressed by Galpha(13)Q226L and constitutively active V14RhoA in COS-7 cells. Abolition of the post-translational modifications of rPLD2 did not alter the effects of Galpha(q)Q209L or Galpha(13)Q226L. The suppressive effect of Galpha(13)Q226L on rPLD2 was reversed by dominant negative N19RhoA and the C3 exoenzyme of Clostridium botulinum, further supporting a role for RhoA. In summary, Galpha(13) activation of rPLD1 in COS-7 cells is mediated by Rho, while Galpha(q) activation requires PKC. rPLD2 is activated by Galpha(q), but is inhibited by Galpha(13). Neither Ser/Thr phosphorylation nor palmitoylation is required for these effects.

Phosphorylation-dependent regulation of phospholipase D2 by protein kinase C delta in rat Pheochromocytoma PC12 cells

Many studies have shown that protein kinase C (PKC) is an important physiological regulator of phospholipase D (PLD). However, the role of PKC in agonist-induced PLD activation has been mainly investigated with a focus on the PLD1, which is one of the two PLD isoenzymes (PLD1 and PLD2) cloned to date. Since the expression of PLD2 significantly enhanced phorbol 12-myristate 13-acetate (PMA)- or bradykinin-induced PLD activity in rat pheochromocytoma PC12 cells, we investigated the regulatory mechanism of PLD2 in PC12 cells. Two different PKC inhibitors, GF109203X and Ro-31-8220, completely blocked PMA-induced PLD2 activation. In addition, specific inhibition of PKC delta by rottlerin prevented PLD2 activation in PMA-stimulated PC12 cells. Concomitant with PLD2 activation, PLD2 became phosphorylated upon PMA or bradykinin treatment of PC12 cells. Moreover, rottlerin blocked PMA- or bradykinin-induced PLD2 phosphorylation in PC12 cells. Expression of a kinase-deficient mutant of PKC delta using adenovirus-mediated gene transfer inhibited the phosphorylation and activation of PLD2 induced by PMA in PC12 cells, suggesting the phosphorylation-dependent regulation of PLD2 mediated by PKC delta kinase activity in PC12 cells. PKC delta co-immunoprecipitated with PLD2 from PC12 cell extracts, and associated with PLD2 in vitro in a PMA-dependent manner. Phospho-PLD2 immunoprecipitated from PMA-treated PC12 cells and PLD2 phosphorylated in vitro by PKC delta were resolved by two-dimensional phosphopeptide mapping and compared. At least seven phosphopeptides co-migrated, indicating the direct phosphorylation of PLD2 by PKC delta inside the cells. Immunocytochemical studies of PC12 cells revealed that after treatment with PMA, PKC delta was translocated from the cytosol to the plasma membrane where PLD2 is mainly localized. These results suggest that PKC delta-dependent direct phosphorylation plays an important role in the regulation of PLD2 activity in PC12 cells.

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.

Isolation and characterization of a 66-kDa protein from rat liver plasma membrane with RhoA-stimulated phospholipase D activity

A 66-kDa molecular weight protein with phospholipase D activity was solubilized and partially purified from rat liver plasma membrane. The activity and regulation of this phospholipase D have been characterized. Immunoblot analyses indicated that the enzyme was distinct from hPLD1 and PLD2, but was recognized by an antibody to the 12 terminal amino acids of PLD1. PLD activity was stimulated by 1-100 microM Ca(2+) and Mg(2+) and displayed a pH optimum of 7.5. Activity was inhibited by both saturated and unsaturated fatty acids. This PLD was activated in an ATP-independent manner by the PKC isozymes alpha and betaII but not activated by other PKC isozymes. It was also stimulated by the small G-proteins RhoA and ARF. RhoA stimulated the greatest activation, followed by ARF and PKC(alpha). This enzyme was further activated in a synergistic manner when combinations of PKC(alpha) and RhoA or ARF were used. This enzyme displayed a greater response activation by RhoA than to activation by ARF. While a potential breakdown product of PLD1, activation by RhoA indicates that the PLD characterized here is distinct from the other PLDs cloned or isolated to date.

Localization of mRNAs for subfamily of guanine nucleotide-exchange proteins (GEP) for ARFs (ADP-ribosylation factors) in the brain of developing and mature rats under normal and postaxotomy conditions

ADP-ribosylation factors (ARFs) play important roles in vesicular trafficking and cytoskeletal regulation and its activation depends on guanine nucleotide-exchange proteins (GEPs). By way of in situ hybridization histochemistry, the localization of mRNAs for subfamily members of low-molecular-weight ARF-GEPs in the rat brain was studied at embryonic and postnatal stages. In the embryonic brain, the gene expression for msec7-1 was distinct in the ventricular zone while that for msec7-1, -3 and EFA6 in the mantle zone. In early postnatal brain, the expression for msec7-1, -2, -3 and EFA6 was seen widely in various loci of the gray matter with different intensity, and the expression of msec7-1 and -2 mRNAs was evident in the cerebellar external granule cell layer. In the adult brain, the gene expression for the four ARF-GEPs decreased more or less in most gray matter and the distinct expression was maintained mainly in the hippocampal and dentate neuronal layers and cerebellar cortex. The expression of EFA6 mRNA was also evident in the molecular layer of the hippocampus and dentate gyrus. No obvious gene expression for cytohesin-4 and ARF-GEP100 was detected in the brain at any stages of development. The present findings suggest that ARF-GEPs are differentially involved in some processes essential to neuronal differentiation and maturation in association with ARFs.

Differential binding of traffic-related proteins to phosphatidic acid- or phosphatidylinositol (4,5)- bisphosphate-coupled affinity reagents

Phosphatidic acid (PA) is an important bioactive lipid, but its molecular targets remain unknown. To identify such targets, we have synthesized and coupled PA to an agarose-based matrix, Affi-Gel 10. Using this matrix as an affinity reagent, we have identified a substantial number of potential PA-binding proteins from brain cytosol. One class of such proteins is known to be involved in intracellular traffic and it included coatomer, ADP-ribosylation factor (Arf), N-ethylmaleimide-sensitive factor (NSF), and kinesin. Binding of these proteins to PA beads was suppressed by soluble PA, and it occurred preferentially over binding to beads coupled to phosphatidylinositol (4,5)-bisphosphate. For coatomer, Arf, and NSF, we verified direct binding to PA beads using purified proteins. For recombinant Arf1 and Arf6, binding to PA required myristoylation. In addition, for NSF and Arf6, an ATPase and a GTPase, respectively, binding to PA beads was extremely sensitive to the nucleotide state of the protein. Binding to PA may be a property linking together distinct participants in one complete round of membrane transport from a donor to an acceptor compartment.

Interaction of phospholipase D1 with a casein-kinase-2-like serine kinase

Phospholipase D (PLD)1 was phosphorylated in vivo and by an associated kinase in vitro following immunoprecipitation. Both phosphorylation events were greatly reduced in a catalytically inactive point mutant in which the serine residue at position 911 was converted into alanine (S911A). The kinase could be enriched from detergent-extracted brain membranes and bind and phosphorylate PLD1 that was immunoprecipitated from COS-7 cells. Using in-gel kinase assays we determined that the size of the kinase is approximately 40 kDa and that PLD1 is more effective than S911A in binding the kinase. Preliminary analysis of the phosphorylation sites on PLD1 suggested that the kinase belongs to the casein kinase 2 (CK2) family. Consistent with this, we found that the kinase could utilize GTP, and could be inhibited by heparin and 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB). Membrane fractions from Chinese hamster ovary (CHO) cell lines that inducibly express PLD1 contained an endogenous kinase activity that phosphorylated PLD1 using GTP and was inhibited by DRB. Direct evidence that the kinase is CK2 came from observations that immunoprecipitates using PLD1 antibodies contained immunoreactive CK2alpha, and immunoprecipitates using CK2alpha antibodies contained immunoreactive PLD1. Co-expression of PLD1 in COS-7 cells with the two recombinant CK2 subunits, alpha or beta, suggests that the association of PLD1 with the kinase is through the beta subunit. Supporting this, phosphorylation of PLD1 by purified recombinant CK2alpha was enhanced by purified recombinant CK2beta. Assays measuring PLD1 catalytic activity following phosphorylation by CK2 suggest that this phosphorylation event does not influence PLD1-mediated hydrolysis of phosphatidylcholine in vitro.

The C terminus of mammalian phospholipase D is required for catalytic activity

The activity of phospholipase D (PLD) is regulated by a variety of hormonal stimuli and provides a mechanistic pathway for response of cells to extracellular stimuli. The two identified mammalian PLD enzymes possess highly homologous C termini, which are required for catalytic activity. Mutational analysis of PLD1 and PLD2 reveals that modification of as little as the C-terminal threonine or the addition of a single alanine attenuates activity of the enzyme. Protein folding appears to be intact because mutant enzymes express to similar levels in Sf9 cells and addition of peptides representing the C-terminal amino acids, including the simple hexamer PMEVWT, restores partial activity to several of the mutants. Analysis of several mutants suggests a requirement for the hydrophobic reside at the -2-position but not an absolute requirement for the hydroxyl side chain of threonine at the C terminus. The inability of peptides amidated at their C termini to effect restoration of activity indicates the involvement of the C-terminal alpha carboxyl group in functional activity of these enzymes. The ability of peptides to restore activity to PLD enzymes mutated at the C terminus suggests a flexible interaction of this portion of the molecule with a catalytic core constructed on conserved HKD motifs. Participation of these C termini residues in either stabilization of the catalytic site or the enzymatic reaction itself remains to be determined. This requirement for the C terminus provides an excellent potential site for interaction with regulatory proteins that may either enhance or down-regulate the activity of these enzymes in vitro.

Presence of a phospholipase D (PLD) distinct from PLD1 or PLD2 in human neutrophils: immunobiochemical characterization and initial purification

Utilizing the transphosphatidylation reaction catalyzed by phospholipase D (PLD) in the presence of a primary alcohol and the short-chain phospholipid PC8, we have characterized the enzyme from human neutrophils. A pH optimum of 7.8-8.0 was determined. PIP(2), EDTA/EGTA, and ATP were found to enhance basal PLD activity in vitro. Inhibitory elements were: oleate, Triton X-100, n-octyl-beta-glucopyranoside, divalent cations, GTPgammaS and H(2)O(2). The apparent K(m) for the butanol substrate was 0.1 mM and the V(max) was 6.0 nmol mg(-1) h(-1). Immunochemical analysis by anti-pan PLD antibodies revealed a neutrophil PLD of approximately 90 kDa and other bands recognized minimally by anti-PLD1 or anti-PLD2 antibodies. The 90-kDa protein is tyrosine-phosphorylated upon cell stimulation with GM-CSF and formyl-Met-Leu-Phe. Protein partial purification using column liquid chromatography was performed after cell subfractionation. Based on the enzyme's regulatory and inhibitory factors, and its molecular weight, these data indicate an enzyme isoform that might be different from the mammalian PLD1/2 forms described earlier. The present results lay the foundation for further purification of this granulocyte PLD isoform.