Cloning and expression analysis of murine phospholipase D1

Phospholipase D1 Ablation Disrupts Mouse Longitudinal Hippocampal Axis Organization and Functioning

Phosphatidic acid (PA) is a signaling lipid involved in the modulation of synaptic structure and functioning. Based on previous work showing a decreasing PA gradient along the longitudinal axis of the rodent hippocampus, we asked whether the dorsal hippocampus (DH) and the ventral hippocampus (VH) are differentially affected by PA modulation. Here, we show that phospholipase D1 (PLD1) is a major hippocampal PA source, compared to PLD2, and that PLD1 ablation affects predominantly the lipidome of the DH. Moreover, Pld1 knockout (KO) mice show specific deficits in novel object recognition and social interaction and disruption in the DH-VH dendritic arborization differentiation in CA1/CA3 pyramidal neurons. Also, Pld1 KO animals present reduced long-term depression (LTD) induction and reduced GluN2A and SNAP-25 protein levels in the DH. Overall, we observe that PLD1-derived PA reduction leads to differential lipid signatures along the longitudinal hippocampal axis, predominantly affecting DH organization and functioning.

Human PLD structures enable drug design and characterization of isoenzyme selectivity

Phospholipase D enzymes (PLDs) are ubiquitous phosphodiesterases that produce phosphatidic acid (PA), a key second messenger and biosynthetic building block. Although an orthologous bacterial Streptomyces sp. strain PMF PLD structure was solved two decades ago, the molecular basis underlying the functions of the human PLD enzymes (hPLD) remained unclear based on this structure due to the low homology between these sequences. Here, we describe the first crystal structures of hPLD1 and hPLD2 catalytic domains and identify novel structural elements and functional differences between the prokaryotic and eukaryotic enzymes. Furthermore, structure-based mutation studies and structures of inhibitor-hPLD complexes allowed us to elucidate the binding modes of dual and isoform-selective inhibitors, highlight key determinants of isoenzyme selectivity and provide a basis for further structure-based drug discovery and functional characterization of this therapeutically important superfamily of enzymes.

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 D2 promotes degradation of hypoxia-inducible factor-1α independent of lipase activity

Hypoxia-inducible factor-1α (HIF-1α) is a key transcriptional mediator that coordinates the expression of various genes involved in tumorigenesis in response to changes in oxygen tension. The stability of HIF-1α protein is determined by oxygen-dependent prolyl hydroxylation, which is required for binding of the von Hippel-Lindau protein (VHL), the recognition component of an E3 ubiquitin ligase that targets HIF-1α for ubiquitination and degradation. Here, we demonstrate that PLD2 protein itself interacts with HIF-1α, prolyl hydroxylase (PHD) and VHL to promote degradation of HIF-1α via the proteasomal pathway independent of lipase activity. PLD2 increases PHD2-mediated hydroxylation of HIF-1α by increasing the interaction of HIF-1α with PHD2. Moreover, PLD2 promotes VHL-dependent HIF-1α degradation by accelerating the association between VHL and HIF-1α. The interaction of the pleckstrin homology domain of PLD2 with HIF-1α also promoted degradation of HIF-1α and decreased expression of its target genes. These results indicate that PLD2 negatively regulates the stability of HIF-1α through the dynamic assembly of HIF-1α, PHD2 and VHL.

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.

Mechanism of enzymatic reaction and protein-protein interactions of PLD from a 3D structural model

The phospholipase D (PLD) superfamily catalyzes the hydrolysis of cell membrane phospholipids generating the key intracellular lipid second messenger phosphatidic acid. However, there is not yet any resolved structure either from a crystallized protein or from NMR of any mammalian PLDs. We propose here a 3D model of the PLD2 by combining homology and ab initio 3 dimensional structural modeling methods, and docking conformation. This model is in agreement with the biochemical and physiological behavior of PLD in cells. For the lipase activity, the N- and C-terminal histidines of the HKD motifs (His 442/His 756) form a catalytic pocket, which accommodates phosphatidylcholine head group (but not phosphatidylethanolamine or phosphatidyl serine). The model explains the mechanism of the reaction catalysis, with nucleophilic attacks of His 442 and water, the latter aided by His 756. Further, the secondary structure regions superimposed with bacterial PLD crystal structure, which indicated an agreement with the model. It also explains protein-protein interactions, such as PLD2-Rac2 transmodulation (with a 1:2 stoichiometry) and PLD2 GEF activity both relevant for cell migration, as well as the existence of binding sites for phosphoinositides such as PIP2. These consist of R236/W238 and R557/W563 and a novel PIP2 binding site in the PH domain of PLD2, specifically R210/R212/W233. In each of these, the polar inositol ring is oriented towards the basic amino acid Arginine. Since tumor-aggravating properties have been found in mice overexpressing PLD2 enzyme, the 3D model of PLD2 will be also useful, to a large extent, in developing pharmaceuticals to modulate its in vivo activity.

Phospholipase D in cell signaling: from a myriad of cell functions to cancer growth and metastasis

Phospholipase D (PLD) enzymes play a double vital role in cells: they maintain the integrity of cellular membranes and they participate in cell signaling including intracellular protein trafficking, cytoskeletal dynamics, cell migration, and cell proliferation. The particular involvement of PLD in cell migration is accomplished: (a) through the actions of its enzymatic product of reaction, phosphatidic acid, and its unique shape-binding role on membrane geometry; (b) through a particular guanine nucleotide exchange factor (GEF) activity (the first of its class assigned to a phospholipase) in the case of the mammalian isoform PLD2; and (c) through protein-protein interactions with a wide network of molecules: Wiskott-Aldrich syndrome protein (WASp), Grb2, ribosomal S6 kinase (S6K), and Rac2. Further, PLD interacts with a variety of kinases (PKC, FES, EGF receptor (EGFR), and JAK3) that are activated by it, or PLD becomes the target substrate. Out of these myriads of functions, PLD is becoming recognized as a major player in cell migration, cell invasion, and cancer metastasis. This is the story of the evolution of PLD from being involved in a large number of seemingly unrelated cellular functions to its most recent role in cancer signaling, a subfield that is expected to grow exponentially.

Cellular and physiological roles for phospholipase D1 in cancer

Phospholipase D enzymes have long been proposed to play multiple cell biological roles in cancer. With the generation of phospholipase D1 (PLD1)-deficient mice and the development of small molecule PLD-specific inhibitors, in vivo roles for PLD1 in cancer are now being defined, both in the tumor cells and in the tumor environment. We review here tools now used to explore in vivo roles for PLD1 in cancer and summarize recent findings regarding functions in angiogenesis and metastasis.

The hydrophobic amino acids involved in the interdomain association of phospholipase D1 regulate the shuttling of phospholipase D1 from vesicular organelles into the nucleus

Phospholipase D (PLD) catalyzes the hydrolysis of phosphatidylcholine to generate the lipid second messenger, phosphatidic acid. PLD is localized in most cellular organelles, where it is likely to play different roles in signal transduction. PLD1 is primarily localized in vesicular structures such as endosomes, lysosomes and autophagosomes. However, the factors defining its localization are less clear. In this study, we found that four hydrophobic residues present in the N-terminal HKD catalytic motif of PLD1, which is involved in intramolecular association, are responsible for vesicular localization. Site-directed mutagenesis of the residues dramatically disrupted vesicular localization of PLD1. Interestingly, the hydrophobic residues of PLD1 are also involved in the interruption of its nuclear localization. Mutation of the residues increased the association of PLD1 with importin-β, which is known to mediate nuclear importation, and induced the localization of PLD1 from vesicles into the nucleus. Taken together, these data suggest that the hydrophobic amino acids involved in the interdomain association of PLD1 are required for vesicular localization and disturbance of its nuclear localization.

Phospholipase D2: a pivotal player modulating RBL-2H3 mast cell structure

The current study examined the role of PLD2 in the maintenance of mast cell structure. Phospholipase D (PLD) catalyzes hydrolysis of phosphatidylcholine to produce choline and phosphatidic acid (PA). PLD has two isoforms, PLD1 and PLD2, which vary in expression and localization depending on the cell type. The mast cell line RBL-2H3 was transfected to overexpress catalytically active (PLD2CA) and inactive (PLD2CI) forms of PLD2. The results of this study show that PLD2CI cells have a distinct star-shaped morphology, whereas PLD2CA and RBL-2H3 cells are spindle shaped. In PLD2CI cells, the Golgi complex was also disorganized with dilated cisternae, and more Golgi-associated vesicles were present as compared with the PLD2CA and RBL-2H3 cells. Treatment with exogenous PA led to the restoration of the wild-type Golgi complex phenotype in PLD2CI cells. Conversely, treatment of RBL-2H3 and PLD2CA cells with 1% 1-Butanol led to a disruption of the Golgi complex. The distribution of acidic compartments, including secretory granules and lysosomes, was also modified in PLD2CI cells, where they concentrated in the perinuclear region. These results suggest that the PA produced by PLD2 plays an important role in regulating cell morphology in mast cells.

Cloning of PLD2 from baculovirus for studies in inflammatory responses

The enzyme PLD hydrolyzes phosphodiester bonds of lipids in cell membranes. Phosphatidic acid, a chief product of PLD enzymatic activity, is a pleiotropic second messenger with key roles in membrane trafficking, cell invasion, cell growth, and anti-apoptosis. We describe in the present study molecular, cellular, and physiological methods to understand the mechanism of how the PLD2 isozyme regulates the process of inflammation. We describe here (1) a method that details phospholipase D2 (PLD2) cloning in the pBac expression vector, (2) the large-scale infection of Sf21 insect cells for protein production, (3) protein purification by TALON cobalt metal affinity matrix and subsequent assessment of PLD2 protein and lipase activity, (4) application of purified PLD2 protein for the study of Rac2 GTPase biology involving GTP binding by a pull-down assay and GTP/GDP exchange activity, (5) a method of PLD2 expression that involves mammalian cells, (6) a physiological application as relates to adhesion, chemotaxis, and phagocytosis, and (7) a model that integrates the results of a PLD-GTPase interaction from the molecular to the physiological contexts.

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.

Dopamine-induced plasticity, phospholipase D (PLD) activity and cocaine-cue behavior depend on PLD-linked metabotropic glutamate receptors in amygdala

Cocaine-cue associations induce synaptic plasticity with long lasting molecular and cellular changes in the amygdala, a site crucial for cue-associated memory mechanisms. The underlying neuroadaptations can include marked alterations in signaling via dopamine (DA) receptors (DRs) and metabotropic glutamate (Glu) receptors (mGluRs). Previously, we reported that DR antagonists blocked forms of synaptic plasticity in amygdala slices of Sprague-Dawley rats withdrawn from repeated cocaine administration. In the present study, we investigated synaptic plasticity induced by exogenous DA and its dependence on mGluR signaling and a potential role for phospholipase D (PLD) as a downstream element linked to mGluR and DR signaling. Utilizing a modified conditioned place preference (CPP) paradigm as a functional behavioral measure, we studied the neurophysiological effects after two-weeks to the last cocaine conditioning. We recorded, electrophysiologically, a DR-induced synaptic potentiation in the basolateral to lateral capsula central amygdala (BLA-lcCeA) synaptic pathway that was blocked by antagonists of group I mGluRs, particularly, the PLD-linked mGluR. In addition, we observed 2–2.5 fold increase in PLD expression and 3.7-fold increase in basal PLD enzyme activity. The enhanced PLD activity could be further stimulated (9.3 fold) by a DA D1-like (D1/5R) receptor agonist, and decreased to control levels by mGluR1 and PLD-linked mGluR antagonists. Diminished CPP was observed by infusion of a PLD-linked mGluR antagonist, PCCG-13, in the amygdala 15 minutes prior to testing, two weeks after the last cocaine injection. These results imply a functional interaction between D1/5Rs, group I mGluRs via PLD in the amygdala synaptic plasticity associated with cocaine-cues.

Phospholipase D family member 4, a transmembrane glycoprotein with no phospholipase D activity, expression in spleen and early postnatal microglia

BACKGROUND

Phospholipase D (PLD) catalyzes conversion of phosphatidylcholine into choline and phosphatidic acid, leading to a variety of intracellular signal transduction events. Two classical PLDs, PLD1 and PLD2, contain phosphatidylinositide-binding PX and PH domains and two conserved His-x-Lys-(x)(4)-Asp (HKD) motifs, which are critical for PLD activity. PLD4 officially belongs to the PLD family, because it possesses two HKD motifs. However, it lacks PX and PH domains and has a putative transmembrane domain instead. Nevertheless, little is known regarding expression, structure, and function of PLD4.

METHODOLOGY/PRINCIPAL FINDINGS

PLD4 was analyzed in terms of expression, structure, and function. Expression was analyzed in developing mouse brains and non-neuronal tissues using microarray, in situ hybridization, immunohistochemistry, and immunocytochemistry. Structure was evaluated using bioinformatics analysis of protein domains, biochemical analyses of transmembrane property, and enzymatic deglycosylation. PLD activity was examined by choline release and transphosphatidylation assays. Results demonstrated low to modest, but characteristic, PLD4 mRNA expression in a subset of cells preferentially localized around white matter regions, including the corpus callosum and cerebellar white matter, during the first postnatal week. These PLD4 mRNA-expressing cells were identified as Iba1-positive microglia. In non-neuronal tissues, PLD4 mRNA expression was widespread, but predominantly distributed in the spleen. Intense PLD4 expression was detected around the marginal zone of the splenic red pulp, and splenic PLD4 protein recovered from subcellular membrane fractions was highly N-glycosylated. PLD4 was heterologously expressed in cell lines and localized in the endoplasmic reticulum and Golgi apparatus. Moreover, heterologously expressed PLD4 proteins did not exhibit PLD enzymatic activity.

CONCLUSIONS/SIGNIFICANCE

Results showed that PLD4 is a non-PLD, HKD motif-carrying, transmembrane glycoprotein localized in the endoplasmic reticulum and Golgi apparatus. The spatiotemporally restricted expression patterns suggested that PLD4 might play a role in common function(s) among microglia during early postnatal brain development and splenic marginal zone cells.

Targeting phospholipase D with small-molecule inhibitors as a potential therapeutic approach for cancer metastasis

Phospholipase D (PLD)1 and PLD2, the classic mammalian members of the PLD superfamily, have been linked over the past three decades to immune cell function and to cell biological processes required by cancer cells for metastasis. However, owing to the lack of effective small-molecule inhibitors, it has not been possible to validate these roles for the PLDs and to explore the possible utility of acute and chronic PLD inhibition in vivo. The first such inhibitors have recently been described and demonstrated to block neutrophil chemotaxis and invasion by breast cancer cells in culture, increasing the prospects for a new class of therapeutics for autoimmune disorders and several types of metastatic cancer.

Ischemic preconditioning upregulates expression of phospholipase D2 in the rat hippocampus

To investigate the possible involvement of phospholipase D2 (PLD2) in the induction of ischemic tolerance, we analyzed the distribution and time course of PLD2 expression in the rat hippocampus after a sublethal period of ischemia. Forebrain ischemia was induced by four-vessel occlusion for 3 min. Increased PLD2 immunoreactivity after this sublethal ischemia was observed in CA1 pyramidal neurons of the rat hippocampus. In tolerance-acquired CA1 neurons, PLD2 immunoreactivity was upregulated as early as 12 h post-ischemia and was most prominent at 1-3 days, with expression sustained for at least 7 days, as shown by a time course of immunoblotting and measurement of the enzymatic activity of PLD. PLD2 expression was also increased in ischemia-resistant CA3 neurons and dentate granule cells, although weaker staining intensity was noted. Further, we showed that, in cultured SK-N-BE(2)C human neuroblastoma cells, overexpression of PLD2 inhibited cell death by chemical hypoxia induced with potassium cyanide and deoxyglucose. These data suggest that upregulation of PLD2 might be involved in the neuroprotective mechanism of ischemic tolerance in the rat hippocampus.

The basic secretagogue compound 48/80 activates G proteins indirectly via stimulation of phospholipase D-lysophosphatidic acid receptor axis and 5-HT1A receptors in rat brain sections

The basic secretagogues, such as compound 48/80 (c48/80) and mastoparans, are widely used histamine-releasing agents and their mechanism of action is commonly attributed to a direct, receptor-bypassing property to activate the G(i/o) class of G proteins. We tested here whether c48/80 could directly stimulate [(35)S]guanosine-5'-[gamma-thio]triphosphate ([(35)S]GTPgammaS) binding to rat brain sections in an attempt to visualize the entire signaling pool of G(i/o) in its native neuroanatomical context. Instead of direct G(i/o) activation, c48/80 (100 microg ml(-1)) from various suppliers stimulated brain phospholipase D (PLD) activity, leading to the generation of endogenous phospholipids capable of activating brain white matter-enriched, G(i/o)-coupled lysophosphatidic acid (LPA) receptors. This response was sensitive to 1-butanol and was potently reversed by the LPA(1)/LPA(3) receptor-selective antagonist Ki16425 (IC(50) 59+/-13 nM, mean+/-s.e.m.), and showed age-dependent decline, closely reflecting known developmental regulation of the PLD-LPA(1) receptor axis in the CNS. In addition, c48/80 was found to modestly activate hippocampal 5-HT(1A) receptors in a pH-dependent and antagonist-sensitive manner. Consistent with the lack of direct G(i/o)-activating properties in brain sections, c48/80 showed no activity in classical membrane [(35)S]GTPgammaS binding assays. Instead, c48/80 from one particular manufacturer elicited non-specific effect in these assays, therefore challenging the previous interpretations regarding the compound's ability to activate G proteins directly. We conclude that c48/80 is not a receptor-bypassing general G protein activator but rather activates PLD, leading to generation of endogenous LPA receptor-activating phospholipids. This property may also contribute to the compound's ability to release histamine from mast cells.

Synergism between phospholipase D2 and sorbitol accumulation in diabetic cataract formation through modulation of Na,K-ATPase activity and osmotic stress

Phospholipase D (PLD), a highly regulated enzyme that generates the second messenger phosphatidic acid, functions in signal transduction, membrane trafficking and cytoskeletal reorganization. PLD is thought to be involved in the pathogenesis of diabetic complications by activating PKC. Since PKC and PLD are present in the lens we sought to determine if PLD plays a role in diabetic cataract development. We developed transgenic mice that overexpress PLD2, one of the two mammalian isoforms of PLD. These mice developed congenital nuclear cataracts, but not diabetic cataracts. Histological analysis revealed vacuole formation in the fiber cells, mediated potentially by the substantially increased Na,K-ATPase activity. In the presence of the aldose reductase overexpressing transgene that increases lens osmotic pressure, these double transgenic mice developed more severe congenital cataract and became susceptible to develop diabetic cataract. Together, these data suggest that increased PLD2 activity in the lens under hyperglycemic condition might impair its osmoregulatory mechanism and reduce its ability to cope with the osmotic stress triggered by sorbitol accumulation.

Phospholipase D1-promoted release of tissue plasminogen activator facilitates neurite outgrowth

Temporal lobe epilepsy (TLE) is the most common form of epilepsy, affecting approximately 1-2% of the population. Seizure events resulting from TLE are characterized by aberrant hippocampal mossy fiber sprouting and plastic responses that affect brain function. Seizure susceptibility is modulated by the enzyme tissue plasminogen activator (tPA), the normal physiological role of which includes promotion of synaptic reorganization in the mossy fiber pathway by initiating a proteolytic cascade that cleaves extracellular matrix components and influences neurite extension. tPA is concentrated at and selectively secreted from growth cones during excitatory events. However, the mechanisms underlying tPA release during seizure-induced synaptogenesis are not well understood. We examine here potential roles for the signaling enzyme phospholipase D1 (PLD1), which promotes regulated exocytosis in non-CNS cell types, and which we previously demonstrated increases in expression in hippocampal neurons during seizure-induced mossy fiber sprouting. We now show that overexpression of wild-type PLD1 in cultured neurons promotes tPA release and tPA-dependent neurite extension, whereas overexpression of an inactive PLD1 allele or pharmacological inhibition of PLD1 inhibits tPA release. Similarly, viral delivery of wild-type PLD1 into the hippocampus facilitates tPA secretion and mossy fiber sprouting in a seizure-inducing model, whereas the inactive PLD1 allele inhibits tPA release and elicits blunted and abnormal mossy fiber extension similar to that observed for tPA-/- mice. Together, these findings secretion and thus mossy fiber extension in the setting of elevated suggest that PLD1 functions endogenously to regulate tPA-/- neuronal stimulation, such as that seen in TLE.

Cytokines affect PDX-1 expression, insulin and proinsulin secretion from iNOS deficient murine islets

In rodent islets, exposure to interleukin-1beta (IL-1beta) and interferon-gamma (IFN-gamma) induces expression of inducible nitric oxide synthase (iNOS) and subsequent nitric oxide (NO) formation, which may inhibit islet function. However, cytokines may also induce NO-independent islet suppression. The present aim was to investigate the effect of cytokine exposure to iNOS deficient (iNOS-/-) mouse islets on various islet functions. Islets from iNOS-/- and wt mice exposed to IL-1beta or (IL-1beta + IFN-gamma) for 2-20 h showed different kinetics of glucose-stimulated insulin secretion. In iNOS-/- islets, IL-1beta at high glucose induced a delayed and prolonged stimulation of insulin secretion, and this was followed by an increase in phospholipase D mRNA expression. After 6 and 24 h, proinsulin convertase 1 and 2 (PC1 and PC2) mRNA expression was suppressed and proinsulin secretion increased from wt islets. In iNOS-/- islets, PC1 expression was recovered after 24 h, and there was no difference in proinsulin secretion. PDX-1 mRNA expression was suppressed independent of NO-formation. We conclude that cytokines induce both NO-dependent and NO-independent functional inhibition of murine beta-cells.

Intracellular signaling pathways that regulate behavioral responses to ethanol

Recent evidence indicates that ethanol modulates the function of specific intracellular signaling cascades, including those that contain cyclic adenosine 3', 5'-monophosphate (cAMP)-dependent protein kinase A (PKA), protein kinase C (PKC), the tyrosine kinase Fyn, and phospholipase D (PLD). In some cases, the specific components of these cascades appear to mediate the effects of ethanol, whereas other components indirectly modify responses to ethanol. Studies utilizing selective inhibitors and genetically modified mice have identified specific isoforms of proteins involved in responses to ethanol. The effects of ethanol on neuronal signaling appear restricted to certain brain regions, partly due to the restricted distribution of these proteins. This likely contributes specificity to ethanol's actions on behavior. This review summarizes recent work on ethanol and intracellular signal transduction, emphasizing studies that have identified specific molecular events that underlie behavioral responses to ethanol.

Sites on phospholipase D2 phosphorylated by PKCα

The phosphorylation sites in phospholipase D2 (PLD2) induced by activation of protein kinase Calpha (PKCalpha) in COS 7 cells were analyzed by mass spectrometry. Ser134, 146, and 243, and Thr72, 99/100, and 252 were identified. These sites were mutated to Ala and the double mutation of Ser243 and Thr252 eliminated the phosphorylation. However, the PLD2 activity, and the binding between PKCalpha and PLD2 were unaffected by the mutations. We conclude that phosphorylation of these residues is not required for PLD2 activation by PKCalpha, and that protein-protein interaction between PLD2 and PKCalpha is sufficient to activate PLD2.

The uncovering of a novel regulatory mechanism for PLD2: formation of a ternary complex with protein tyrosine phosphatase PTP1B and growth factor receptor-bound protein GRB2

The regulation of PLD2 activation is poorly understood at present. Transient transfection of COS-7 with a mycPLD2 construct results in elevated levels of PLD2 enzymatic activity and tyrosyl phosphorylation. To investigate whether this phosphorylation affects PLD2 enzymatic activity, anti-myc immunoprecipitates were treated with recombinant protein tyrosine phosphatase PTP1B. Surprisingly, lipase activity and PY levels both increased over a range of PTP1B concentrations. These increases occurred in parallel to a measurable PTP1B-associated phosphatase activity. Inhibitor studies demonstrated that an EGF-receptor type kinase is involved in phosphorylation. In a COS-7 cell line created in the laboratory that stably expressed myc-PLD2, PTP1B induced a robust (>6-fold) augmentation of myc-PLD2 phosphotyrosine content. The addition of growth factor receptor-bound protein 2 (Grb2) to cell extracts also elevated PY levels of myc-PLD (>10-fold). Systematic co-immunoprecipitation-immunoblotting experiments pointed at a physical association between PLD2, Grb2, and PTP1B in both physiological conditions and in overexpressed cells. This is the first report of a demonstration of the mammalian isoform PLD2 existing in a ternary complex with a protein tyrosine phosphatase, PTP1b, and the docking protein Grb2 which greatly enhances tyrosyl phosphorylation of the lipase.

RhoA-mediated Phospholipase D1 signaling is not required for the formation of stress fibers and focal adhesions

The small GTPase RhoA regulates a wide spectrum of cellular functions including transformation and cytoskeletal reorganization. A large number of proteins have been identified as targets of RhoA, but their specific roles in these processes are not clear. Phospholipase D (PLD) was shown to be one such target several years ago; more recent work from our laboratory and others has demonstrated that of the two mammalian PLD isozymes, PLD1 but not PLD2 is activated by RhoA and this activation proceeds through direct binding both in vitro and in vivo. In this study, using a series of RhoA mutants, we have defined a PLD1-specific interacting site on RhoA composed of the residues Asn41, Trp58 and Asp76, using the yeast two-hybrid system, co-immunoprecipitation, and a PLD in vivo assay. The results further substantiate our previous finding that RhoA activates PLD1 through direct interaction. These mutants were then used to investigate the role of PLD1 in the cytoskeletal reorganization stimulated by RhoA signaling. Our results show that PLD1 is not required for the RhoA-mediated stress fiber and focal adhesion formation. The lack of importance of PLD1 signaling in RhoA-mediated cytoskeletal reorganization is further supported by the observation that PLD1 depletion using an shRNA approach and tetracycline-induced overexpression of the wild-type and the catalytically inactive mutant of PLD1 in stable cell lines do not alter stress fiber and focal adhesion formation.

Novel case of dup(3q) syndrome due to a de novo interstitial duplication 3q24-q26.31 with minimal overlap to the dup(3q) critical region

Dup(3q) syndrome is characterized by typical facial features, mental and growth retardation, often with congenital heart defects. The syndrome has attracted special attention because of the clinical overlap with Cornelia de Lange syndrome (CDLS). Patients with dup(3q) syndrome are trisomic for segments of the long arm of chromosome 3, most often within the region 3q21 to 3qter. Most cases have arisen as unbalanced translocations and do involve other chromosomes also. A dup(3q) minimal region has been defined at 3q26.3-q27. We report here a 15-month-old boy with a de novo interstitial inverted duplication of 3q24-q26.31. Clinical evaluation revealed mild but typical features of dup(3q) syndrome. The duplication was characterized by conventional and molecular cytogenetics. The results allow further narrowing of the dup(3q) critical region at its distal end and suggest the existence of one or several major genes responsible for the dup(3q) syndrome in the proximal half of 3q26.31. Moreover, the results of fluorescence in situ hybridization (FISH) analysis with BAC probes suggest a disruption of the NLGN1 gene at the distal end of the duplication in 3q26.31 in the patient. The breakpoint within NLGN1 is unique for this patient, and the contribution of NLGN1 disruption to the phenotype of this patient remains unclear. Yet since NLGN1 is involved in synaptogenesis in the central nervous system, altered gene dosage is a good candidate for mental retardation as a recurrent feature of dup(3q) syndrome.

Phospholipase D (PLD) gene expression in human neutrophils and HL-60 differentiation

Human neutrophils exhibit a regulated phospholipase D (PLD) activity that can be measured biochemically in vitro. However, the precise expression pattern of PLD isoforms and their specific biological role(s) are not well understood. Neutrophil mRNA is intrinsically difficult to isolate as a result of the extremely high content of lytic enzymes in the cell's lysosomal granules. Reverse transcription coupled to polymerase chain reaction indicated that pure populations of human neutrophils had the CD16b(+)/CD115(-)/CD20(-)/CD3zeta(-)/interleukin-5 receptor alpha(-) phenotype. These cells expressed the following splice variants of the PLD1 isoform: PLD1a, PLD1b, PLD1a2, and PLD1b2. As for the PLD2 isoform, neutrophils expressed the PLD2a but not the PLD2b mRNA variant. The relative amount of PLD1/PLD2 transcripts exists in an approximate 4:1 ratio. The expression of PLD isoforms varies during granulocytic differentiation, as demonstrated in the promyelocytic leukemia HL-60 cell line. Further, the pattern of mRNA expression is dependent on the differentiation-inducing agent, 1.25% dimethyl sulfoxide causes a dramatic increase in PLD2a and PLD1b transcripts, and 300 nM all-trans-retinoic acid induced PLD1a expression. These results demonstrate for the first time that human neutrophils express five PLD transcripts and that the PLD genes undergo qualitative changes in transcription regulation during granulocytic differentiation.

Phospholipase D

Phospholipase D catalyses the hydrolysis of the phosphodiester bond of glycerophospholipids to generate phosphatidic acid and a free headgroup. Phospholipase D activities have been detected in simple to complex organisms from viruses and bacteria to yeast, plants, and mammals. Although enzymes with broader selectivity are found in some of the lower organisms, the plant, yeast, and mammalian enzymes are selective for phosphatidylcholine. The two mammalian phospholipase D isoforms are regulated by protein kinases and GTP binding proteins of the ADP-ribosylation and Rho families. Mammalian and yeast phospholipases D are also potently stimulated by phosphatidylinositol 4,5-bisphosphate. This review discusses the identification, characterization, structure, and regulation of phospholipase D. Genetic and pharmacological approaches implicate phospholipase D in a diverse range of cellular processes that include receptor signaling, control of intracellular membrane transport, and reorganization of the actin cytoskeleton. Most ideas about phospholipase D function consider that the phosphatidic acid product is an intracellular lipid messenger. Candidate targets for phospholipase-D-generated phosphatidic acid include phosphatidylinositol 4-phosphate 5-kinases and the raf protein kinase. Phosphatidic acid can also be converted to two other lipid mediators, diacylglycerol and lyso phosphatidic acid. Coordinated activation of these phospholipase-D-dependent pathways likely accounts for the pleitropic roles for these enzymes in many aspects of cell regulation.

Essential role for phospholipase D2 activation downstream of ERK MAP kinase in nerve growth factor-stimulated neurite outgrowth from PC12 cells

The signaling pathway that triggers morphological differentiation of PC12 cells is mediated by extracellular signal-regulated kinase (ERK), the classic mitogen-activated protein (MAP) kinase. However, mediators of the pathway downstream of ERK have not been identified. We show here that phospholipase D2 (PLD2), which generates the pleiotropic signaling lipid phosphatidic acid (PA), links ERK activation to neurite outgrowth in nerve growth factor (NGF)-stimulated PC12 cells. Increased expression of wild type PLD2 (WT-PLD2) dramatically elongated neurites induced by NGF stimulation or transient expression of the active form of MAP kinase-ERK kinase (MEK-CA). The response was activity-dependent, because it was inhibited by pharmacological suppression of the PLD-mediated PA production and by expression of a lipase-deficient PLD2 mutant. Furthermore, PLD2 was activated by MEK-CA, whereas NGF-stimulated PLD2 activation and hypertrophic neurite extension were blocked by an MEK-specific inhibitor. Taken together, these results provide evidence that PLD2 functions as a downstream signaling effector of ERK in the NGF signaling pathway, which leads to neurite outgrowth by PC12 cells.

Protein kinase Calpha translocates to the perinuclear region to activate phospholipase D1

The inhibition of phorbol ester activation of phospholipase D1 (PLD1) by protein kinase C (PKC) inhibitors has been considered proof of phosphorylation-dependent activation of PLD1 by PKCalpha. We studied the effect of the PKC inhibitors Ro-31-8220 and bisindolylmaleimide I on PLD1 activation and found that they inhibited the activation by interfering with PKCalpha binding to PLD1. Further studies showed that only unphosphorylated PKCalpha could bind to and activate PLD1 and that both inhibitors induced phosphorylation of PKCalpha. The phosphorylation status of either PLD1 or PKCalpha per se did not affect PLD1 activation in vitro. Immunofluorescence studies showed that PLD1 remained in the perinuclear region after phorbol ester treatment, whereas PKCalpha translocated from cytosol to both plasma membrane and perinuclear regions. Both Ro-31-8220 and bisindolylmaleimide I blocked the translocation of PKCalpha to the perinuclear region but not to the plasma membrane. Studies with okadaic acid suggested that phosphorylation regulated the relocation of PKCalpha from the plasma membrane to the perinuclear region. It is proposed that localization and interaction of PKCalpha with PLD1 in the perinuclear region is required for PLD1 activation and that PKC inhibitors inhibit this through phosphorylation of PKCalpha, which blocks its translocation.

Expression and regulation of phospholipase D isoforms in sphingosine and phorbol ester-stimulated glioma C6 cells

Previously we have reported that in glioma C6 cells, sphingosine stimulatory effect on phospholipase D (PLD) activity is independent of protein kinase C [Cell. Signal. 12 (2000) 399]. In this paper we have shown that this effect was also GTPgammaS independent and was completely inhibited by the plasma membrane methyl-beta-cyclodextrin cholesterol depletion what destroys caveolae structure. On the contrary, phorbol ester (12-O-tetradecanoylphorbol-13-acetate, TPA)-mediated PLD activity was enhanced by GTPgammaS and was only partially decreased by methyl-beta-cyclodextrin. We have also shown that TPA significantly increased expression of PLD1a and PLD1b mRNAs and had lower effect on PLD2 mRNA. Sphingosine only slightly increased expression of PLD mRNA isoforms and did not cause synergistic effect when applied together with TPA. These results indicate that TPA, but not sphingosine, stimulates transcriptional activity of PLD isoforms. We also suggest that TPA stimulates primarily PLD1, while sphingosine affects PLD2 activity. This last process might occur at plasma membrane lipid microdomains.

Retinoic acid-induced differentiation into astrocytes and glutamatergic neurons is associated with expression of functional and activable phospholipase D

Phospholipase D (PLD) activity in mammalian cells has been associated with cell proliferation and differentiation. Here, we investigated the expression of PLD during differentiation of pluripotent embryonal carcinoma cells (P19) into astrocytes and neurons. Retinoic acid (RA)-induced differentiation increased PLD1 and PLD2 mRNA levels and PLD activity that was responsive to phorbol myristate acetate. Various agonists of membrane receptors activated PLD in RA-differentiated cells. Glutamate was a potent activator of PLD in neurons but not in astrocytes, whereas noradrenaline and carbachol increased PLD activity only in astrocytes. P19 neurons but not astrocytes released glutamate in response to a depolarizing stimulus, confirming the glutamatergic phenotype of these neurons. These results indicate upregulation of PLD gene expression associated with RA-induced neural differentiation.

Phospholipase D2 functions as a downstream signaling molecule of MAP kinase pathway in L1-stimulated neurite outgrowth of cerebellar granule neurons

Stimulation of the neuronal cell adhesion molecule L1 in cerebellar granule neurons (CGNs) enhances neurite outgrowth and this response is inhibited by the primary alcohol ethanol. Because primary alcohols suppress the formation of the signaling lipid phosphatidic acid (PA) by phospholipase D (PLD), this observation prompted us to investigate whether PLD plays a role in the L1-mediated neurite outgrowth in CGNs. In the cerebellum of postnatal day 8 mice, PLD2 protein was abundantly expressed, while PLD1 expression was not detected. The L1-stimulated neurite outgrowth was inhibited by primary alcohols and by overexpression of lipase-deficient PLD2. Increases in cellular PA levels by direct PA application or overexpression of wild-type PLD2 mimicked the L1-dependent stimulation of neurite outgrowth. Furthermore, it was found that L1 stimulation in CGNs increased PLD activity concomitantly with phosphorylation of extracellular signal-regulated kinase (ERK), both of which were inhibited by the MAP kinase-ERK kinase (MEK) inhibitor. These results provide evidence that PLD2 functions as a downstream signaling molecule of ERK to mediate the L1-dependent neurite outgrowth of CGNs, a mechanism that may be related to alcohol-related neurodevelopmental disorders.

Down-regulation of melanogenesis by phospholipase D2 through ubiquitin proteasome-mediated degradation of tyrosinase

The involvement of phospholipase D (PLD) in the regulation of melanogenesis was examined. Treatment of B16 mouse melanoma cells with 12-O-tetradecanoylphorbol-13-acetate (TPA) resulted in the activation of PLD and a decrease in melanin content. 1-Butanol, but not 2-butanol, completely blocked the TPA-induced inhibition of melanogenesis, suggesting the involvement of PLD in this event. Reverse transcription-PCR and immunoblot analyses revealed the existence of both PLD isozymes, PLD1 and PLD2, in B16 cells. When PLD1 or PLD2 was introduced into those cells by an adenoviral gene-transfer technique, both PLD1 and PLD2 were activated by TPA. When PLD1 and PLD2 were overexpressed, PLD2 potently caused a decrease in melanin content, whereas the effect of PLD1 expression on melanin content was minimal. Over-expression of PLD2 itself did not affect protein kinase C activity, as assessed by the intracellular distribution and levels of expression of each isoform expressed in B16 cells. The effects of TPA on the down-regulation of basal or alpha-melanocyte-stimulating hormone-enhanced melanogenesis were almost completely blocked by expressing a lipase activity-negative mutant, LN-PLD2, but not by LN-PLD1. Further, the PLD2-induced decrease in melanin content was accompanied by a decrease in the amount and activity of tyrosinase, a key enzyme in melanogenesis, whereas the mRNA level of tyrosinase was unchanged by the over-expression of PLD2. Moreover, treatment with proteasome inhibitors completely blocked the PLD2-induced down-regulation of melanogenesis. Taken together, the present results indicate that the TPA-induced down-regulation of melanogenesis is mediated by PLD2 but not by PLD1 through the ubiquitin proteasome-mediated degradation of tyrosinase. This suggests that PLD2 may play an important role in regulating pigmentation in vivo.

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

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

Regulation of phospholipase D2 activity by protein kinase C alpha

It has been well documented that protein kinase C (PKC) plays an important role in regulation of phospholipase D (PLD) activity. Although PKC regulation of PLD1 activity has been studied extensively, the role of PKC in PLD2 regulation remains to be established. In the present study it was demonstrated that phorbol 12-myristate 13-acetate (PMA) induced PLD2 activation in COS-7 cells. PLD2 was also phosphorylated on both serine and threonine residues after PMA treatment. PKC inhibitors Ro-31-8220 and bisindolylmaleimide I inhibited both PMA-induced PLD2 phosphorylation and activation. However, Gö 6976, a PKC inhibitor relatively specific for conventional PKC isoforms, almost completely abolished PLD2 phosphorylation by PMA but only slightly inhibited PLD2 activation. Furthermore, time course studies showed that phosphorylation of PLD2 lagged behind its activation by PMA. Concentration curves for PMA action on PLD2 phosphorylation and activation also showed that PLD2 was activated by PMA at concentrations at which PMA didn't induce phosphorylation. A kinase-deficient mutant of PKCalpha stimulated PLD2 activity to an even higher level than wild type PKCalpha. Co-expression of wild type PKCalpha, but not PKCdelta, greatly enhanced both basal and PMA-induced PLD2 phosphorylation. A PKCdelta-specific inhibitor, rottlerin, failed to inhibit PMA-induced PLD2 phosphorylation and activation. Co-immunoprecipitation studies indicated an association between PLD2 and PKCalpha under basal conditions that was further enhanced by PMA. Time course studies of the effects of PKCalpha on PLD2 showed that as the phosphorylation of PLD2 increased, its activity declined. In summary, the data demonstrated that PLD2 is activated and phosphorylated by PMA and PKCalpha in COS-7 cells. However, the phosphorylation is not required for PKCalpha to activate PLD2. It is suggested that interaction rather than phosphorylation underscores the activation of PLD2 by PKC in vivo and that phosphorylation may contribute to the inactivation of the enzyme.

Immunohistochemical localization of phospholipase D2 in embryonic rat brain

The present study has characterized the cellular and temporal localization of the phospholipase D2 (PLD2) protein in the embryonic rat brain, using immunohistochemistry. PLD2 immunoreactivity was first observed in the choroid plexus and in the most ventricular zone of the lateral and third ventricles at embryonic day 15 (E15), followed by gradual restriction to the limited zone of ventricles at E20. In addition, PLD2 expression was high in the developing cerebral cortex and hippocampus. In the cortex, PLD2 expression was observed in the marginal zone from the earliest stage (E15) and then declined and had completely disappeared by E20. Double-labelling studies demonstrated co-expression of the anti-class III beta-tubulin antibody in most of the PLD2 immunoreactive cells. Therefore, our findings suggest that PLD2 may be involved in early developmental processes of some neuronal progenitors.

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

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

A potentially critical role of phospholipases in central nervous system ischemic, traumatic, and neurodegenerative disorders

Phospholipases are a diverse group of enzymes whose activation may be responsible for the development of injury following insult to the brain. Amongst the numerous isoforms of phospholipase proteins expressed in mammals are 19 different phospholipase A2's (PLA2s), classified functionally as either secretory, calcium dependent, or calcium independent, 11 isozymes belonging to three structural groups of PLC, and 3 PLD gene products. Many of these phospholipases have been identified in selected brain regions. Under normal conditions, these enzymes regulate the turnover of free fatty acids (FFAs) in membrane phospholipids affecting membrane stability, fluidity, and transport processes. The measurement of free fatty acids thus provides a convenient method to follow phospholipase activity and their regulation. Phospholipase activity is also responsible for the generation of an extensive list of intracellular messengers including arachidonic acid metabolites. Phospholipases are regulated by many factors including selective phosphorylation, intracellular calcium and pH. However, under abnormal conditions, excessive phospholipase activation, along with a decreased ability to resynthesize membrane phospholipids, can lead to the generation of free radicals, excitotoxicity, mitochondrial dysfunction, and apoptosis/necrosis. This review evaluates the critical contribution of the various phospholipases to brain injury following ischemia and trauma and in neurodegenerative diseases.

Functional analysis of a phosphatidic acid binding domain in human Raf-1 kinase: mutations in the phosphatidate binding domain lead to tail and trunk abnormalities in developing zebrafish embryos

Previously, we and others identified a 35-amino acid segment within human Raf-1 kinase that preferentially binds phosphatidic acid. The presence of phosphatidic acid was found to be necessary for the translocation of Raf-1 to the plasma membrane. We have now employed a combination of alanine-scanning and deletion mutagenesis to identify the critical amino acid residues in Raf-1 necessary for interaction with phosphatidic acid. Progressive mutations within a tetrapeptide motif (residues 398-401 of human Raf-1) reduced and finally eliminated binding of Raf-1 to phosphatidic acid. We then injected zebrafish embryos with RNA encoding wild-type Raf-1 kinase or a mutant version with triple alanine mutations in the tetrapeptide motif and followed the morphological fate of embryonic development. Embryos with mutant but not wild-type Raf-1 exhibited defects in posterior axis formation exemplified by bent trunk and tail structures. Molecular evidence for lack of signaling through mutated Raf-1 was obtained by aberrant in situ hybridization of the ntl (no tail) gene, which functions downstream of Raf-1. Our results demonstrate that a functional phosphatidate binding site is necessary for Raf-1 function in embryonic development.

The role of phospholipase D in Glut-4 translocation

Insulin-stimulated Glut-4 translocation is regulated through a complex pathway. Increasing attention is being paid to the role undertaken in this process by Phospholipase D, a signal transduction-activated enzyme that generates the lipid second-messenger phosphatidic acid. Phospholipase D facilitates Glut-4 translocation at potentially multiple steps in its outward movement. Current investigation is centered on Phospholipase D promotion of Glut-4-containing membrane vesicle trafficking and vesicle fusion into the plasma membrane, in part through activation of atypical protein kinase C isoforms.

Mechanisms of regulation of phospholipase D1 by protein kinase Calpha

It has been suggested that protein-protein interaction is important for protein kinase C (PKC) alpha to activate phospholipase D1 (PLD1). To determine the one or more sites on PKCalpha that are involved in binding to PLD1, fragments containing the regulatory domain, catalytic domain, and C1-C3 domain of PKCalpha were constructed and shown to be functional, but they all failed to bind and activate PLD1 in vivo and in vitro. A C-terminal 23-amino acid (aa) deletion mutant of PKCalpha was also found to be inactive. To define the binding/activation site(s) in the C terminus of PKCalpha, 1- to 11-aa deletion mutants were made in this terminus. Deletion of up to 9 aa did not alter the ability of PKCalpha to bind and activate PLDl, whereas a 10-aa deletion was inactive. The residue at position 10 was Phe(663). Mutations of this residue (F663D and F663A) caused loss of binding, activation, and phosphorylation of PLD1, indicating that Phe(663) is essential for these activities. Time course experiments showed that the activation of PLD1 by PMA was much faster than its phosphorylation, and its activity decreased as phosphorylation increased with time. Staurosporine, a PKC inhibitor, completely inhibited PLD1 phosphorylation in response to 4beta-phorbol 12-myristate 13-acetate PMA and blocked the later decrease in PLD activity. The same results were found with the D481E mutant of PKCalpha, which is unable to phosphorylate PLD1. These results indicate that neither the regulatory nor catalytic domains of PKCalpha alone can bind to or activate PLD1 and that a residue in the C terminus of PKCalpha (Phe(663)) is required for these effects. The initial activation of PLD1 by PMA is highly correlated with the binding of PKCalpha. Although PKCalpha can phosphorylate PLD1, this is a relatively slow process and is associated with inactivation of the enzyme.

Regulatory pathways in lacrimal gland epithelium

Tears are a complex fluid that continuously cover the exposed surface of the eye, namely the cornea and conjunctiva. Tears are secreted in response to the multitude of environmental stresses that can harm the ocular surface such as cold, mechanical stimulation, physical injury, noxious chemicals, as well as infections from various organisms. Tears also provide nutrients and remove waste from cells of the ocular surface. Because of the varied function of tears, tears are complex and are secreted by several different tissues. Tear secretion is under tight neural control allowing tears to respond rapidly to changing environmental conditions. The lacrimal gland is the main contributor to the aqueous portion of the tear film and the regulation of secretion from this gland has been well studied. Despite multiple redundencies in pathways to stimulate secretion from the lacrimal gland, defects can occur resulting in dry eye syndromes. These diseases can have deleterious effects on vision. In this review, we summarize the latest information regarding the regulatory pathways, which control secretion from the lacrimal gland, and their roles in the pathogenesis of dry eye syndromes.

Gelsolin-induced epithelial cell invasion is dependent on Ras-Rac signaling

Gelsolin is a widely distributed actin binding protein involved in controlling cell morphology, motility, signaling and apoptosis. The role of gelsolin in tumor progression, however, remains poorly understood. Here we show that expression of green fluorescent protein (GFP)-tagged gelsolin in MDCK-AZ, MDCKtsSrc or HEK293T cells promotes invasion into collagen type I. In organ culture assays, MDCK cells expressing gelsolin-GFP invaded pre-cultured chick heart fragments. Gelsolin expression inhibited E-cadherin-mediated cell aggregation but did not disrupt the E-cadherin-catenin complex. Co-expression of dominant-negative Rac1N17, but not RhoAN19 or Cdc42N17, counteracted gelsolin-induced invasion, suggesting a requirement for Rac1 activity. Increased ARF6, PLD or PIP5K 1alpha activity canceled out gelsolin-induced invasion. Furthermore, we found that invasion induced by gelsolin is dependent on Ras activity, acting through the PI3K-Rac pathway via the Ras guanine nucleotide exchange factor Sos-1. These findings establish a connection between gelsolin and the Ras oncogenic signaling pathway.

Metabolism of sphingosine 1-phosphate and lysophosphatidic acid: a genome wide analysis of gene expression in Drosophila

Lipids, in addition to being structural components of cell membranes, can act as signaling molecules. Bioactive lipids, such as sphingosine 1-phosphate (S1P) and lysophosphatidic acid (LPA), may act intracellularly as second messengers or be secreted and act as intercellular signaling molecules. Such molecules can affect a variety of cellular processes including apoptosis, proliferation, differentiation and motility. To investigate possible sources of bioactive lipids during development we have searched the Drosophila genome for homologs of genes involved in mammalian S1P and LPA metabolism. Here we report the developmental expression of 31 such genes by in situ hybridization to Drosophila embryos. Most show expression in specific tissues, with expression in the gut and nervous system being recurring patterns.

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.

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.