Immunolocalisation of phospholipase D1 on tubular vesicular membranes of endocytic and secretory origin

Interactome analysis of human phospholipase D and phosphatidic acid-associated protein network

Mammalian phospholipase D (PLD) enzyme family consists of six members. Among them, PLD1/2/6 catalyzes phosphatidic acid (PA) production, while PLD3/4/5 has no catalytic activities. Deregulation of the PLD-PA lipid signaling has been associated with various human diseases including cancer. However, a comprehensive analysis of the regulators and effectors for this crucial lipid metabolic pathway has not been fully achieved. Using a proteomic approach, we defined the protein interaction network for the human PLD family of enzymes and PA and revealed diverse cellular signaling events involving them. Through it, we identified PJA2 as a novel E3 ubiquitin ligase for PLD1 involved in control of the PLD1-mediated mammalian target of rapamycin signaling. Additionally, we showed that PA interacted with and positively regulated sphingosine kinase 1. Taken together, our study not only generates a rich interactome resource for further characterizing the human PLD-PA lipid signaling but also connects this important metabolic pathway with numerous biological processes.

•

Defining the interactome of human phospholipase D enzymes and phosphatidic acid.

•

PJA2 functions as an E3 ubiquitin ligase of phospholipase D1.

•

Phosphatidic acid interacts with and positively regulates sphingosine kinase 1.

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.

Selective Autophagy of Mitochondria on a Ubiquitin-Endoplasmic-Reticulum Platform

The dynamics and coordination between autophagy machinery and selective receptors during mitophagy are unknown. Also unknown is whether mitophagy depends on pre-existing membranes or is triggered on the surface of damaged mitochondria. Using a ubiquitin-dependent mitophagy inducer, the lactone ivermectin, we have combined genetic and imaging experiments to address these questions. Ubiquitination of mitochondrial fragments is required the earliest, followed by auto-phosphorylation of TBK1. Next, early essential autophagy proteins FIP200 and ATG13 act at different steps, whereas ULK1 and ULK2 are dispensable. Receptors act temporally and mechanistically upstream of ATG13 but downstream of FIP200. The VPS34 complex functions at the omegasome step. ATG13 and optineurin target mitochondria in a discontinuous oscillatory way, suggesting multiple initiation events. Targeted ubiquitinated mitochondria are cradled by endoplasmic reticulum (ER) strands even without functional autophagy machinery and mitophagy adaptors. We propose that damaged mitochondria are ubiquitinated and dynamically encased in ER strands, providing platforms for formation of the mitophagosomes.

Dopey1-Mon2 complex binds to dual-lipids and recruits kinesin-1 for membrane trafficking

Proteins are transported among eukaryotic organelles along the cytoskeleton in membrane carriers. The mechanism regarding the motility of carriers and the positioning of organelles is a fundamental question in cell biology that remains incompletely understood. Here, we find that Dopey1 and Mon2 assemble into a complex and localize to the Golgi, endolysosome and endoplasmic reticulum exit site. The Golgi localization of Dopey1 and Mon2 requires their binding to phosphatidylinositol-4-phosphate and phosphatidic acid, respectively, two lipids known for the biogenesis of membrane carriers and the specification of organelle identities. The N-terminus of Dopey1 further interacts with kinesin-1, a plus-end or centrifugal-direction microtubule motor. Dopey1-Mon2 complex functions as a dual-lipid-regulated cargo-adaptor to recruit kinesin-1 to secretory and endocytic organelles or membrane carriers for centrifugally biased bidirectional transport. Dopey1-Mon2 complex therefore provides an important missing link to coordinate the budding of a membrane carrier and subsequent bidirectional transport along the microtubule.

Proteins are transported among eukaryotic organelles along the cytoskeleton in membrane carriers. Here authors find that the Dopey1-Mon2 complex functions as a dual-lipid-regulated cargo-adaptor to recruit kinesin-1 to secretory and endocytic organelles or membrane carriers.

Phospholipase D inhibitors reduce human prostate cancer cell proliferation and colony formation

Background:

Phospholipases D1 and D2 (PLD1/2) hydrolyse cell membrane glycerophospholipids to generate phosphatidic acid, a signalling lipid, which regulates cell growth and cancer progression through effects on mTOR and PKB/Akt. PLD expression and/or activity is raised in breast, colorectal, gastric, kidney and thyroid carcinomas but its role in prostate cancer (PCa), the major cancer of men in the western world, is unclear.

Methods:

PLD1 protein expression in cultured PNT2C2, PNT1A, P4E6, LNCaP, PC3, PC3M, VCaP, 22RV1 cell lines and patient-derived PCa cells was analysed by western blotting. PLD1 protein localisation in normal, benign prostatic hyperplasia (BPH), and castrate-resistant prostate cancer (CRPC) tissue sections and in a PCa tissue microarray (TMA) was examined by immunohistochemistry. PLD activity in PCa tissue was assayed using an Amplex Red method. The effect of PLD inhibitors on PCa cell viability was measured using MTS and colony forming assays.

Results:

PLD1 protein expression was low in the luminal prostate cell lines (LNCaP, VCaP, 22RV1) compared with basal lines (PC3 and PC3M). PLD1 protein expression was elevated in BPH biopsy tissue relative to normal and PCa samples. In normal and BPH tissue, PLD1 was predominantly detected in basal cells as well in some stromal cells, rather than in luminal cells. In PCa tissue, luminal cells expressed PLD1. In a PCa TMA, the mean peroxidase intensity per DAB-stained Gleason 6 and 7 tissue section was significantly higher than in sections graded Gleason 9. In CRPC tissue, PLD1 was expressed prominently in the stromal compartment, in luminal cells in occasional glands and in an expanding population of cells that co-expressed chromogranin A and neurone-specific enolase. Levels of PLD activity in normal and PCa tissue samples were similar. A specific PLD1 inhibitor markedly reduced the survival of both prostate cell lines and patient-derived PCa cells compared with two dual PLD1/PLD2 inhibitors. Short-term exposure of PCa cells to the same specific PLD1 inhibitor significantly reduced colony formation.

Conclusions:

A new specific inhibitor of PLD1, which is well tolerated in mice, reduces PCa cell survival and thus has potential as a novel therapeutic agent to reduce prostate cancer progression. Increased PLD1 expression may contribute to the hyperplasia characteristic of BPH and in the progression of castrate-resistant PCa, where an expanding population of neuroendocrine-like cells express PLD1.

Enhanced imaging of lipid rich nanoparticles embedded in methylcellulose films for transmission electron microscopy using mixtures of heavy metals

•

Uranyl acetate/tungsten double stains are proposed for imaging lipid rich nanoparticle in TEM.

•

Combined with methylcellulose embedment, the technique enhances membrane contrast.

•

The technique works for liposomes, nanodiscs and bicelles.

•

The double staining should improve quantification of lipid rich nanoparticles.

Synthetic and naturally occurring lipid-rich nanoparticles are of wide ranging importance in biomedicine. They include liposomes, bicelles, nanodiscs, exosomes and virus particles. The quantitative study of these particles requires methods for high-resolution visualization of the whole population. One powerful imaging method is cryo-EM of vitrified samples, but this is technically demanding, requires specialized equipment, provides low contrast and does not reveal all particles present in a population. Another approach is classical negative stain-EM, which is more accessible but is difficult to standardize for larger lipidic structures, which are prone to artifacts of structure collapse and contrast variability. A third method uses embedment in methylcellulose films containing uranyl acetate as a contrasting agent. Methylcellulose embedment has been widely used for contrasting and supporting cryosections but only sporadically for visualizing lipid rich vesicular structures such as endosomes and exosomes. Here we present a simple methylcellulose-based method for routine and comprehensive visualization of synthetic lipid rich nanoparticles preparations, such as liposomes, bicelles and nanodiscs. It combines a novel double-staining mixture of uranyl acetate (UA) and tungsten-based electron stains (namely phosphotungstic acid (PTA) or sodium silicotungstate (STA)) with methylcellulose embedment. While the methylcellulose supports the delicate lipid structures during drying, the addition of PTA or STA to UA provides significant enhancement in lipid structure display and contrast as compared to UA alone. This double staining method should aid routine structural evaluation and quantification of lipid rich nanoparticles structures.

Nanoparticle suspensions enclosed in methylcellulose: a new approach for quantifying nanoparticles in transmission electron microscopy

Nanoparticles are of increasing importance in biomedicine but quantification is problematic because current methods depend on indirect measurements at low resolution. Here we describe a new high-resolution method for measuring and quantifying nanoparticles in suspension. It involves premixing nanoparticles in a hydrophilic support medium (methylcellulose) before introducing heavy metal stains for visualization in small air-dried droplets by transmission electron microscopy (TEM). The use of methylcellulose avoids artifacts of conventional negative stain-TEM by (1) restricting interactions between the nanoparticles, (2) inhibiting binding to the specimen support films and (3) reducing compression after drying. Methylcellulose embedment provides effective electron imaging of liposomes, nanodiscs and viruses as well as comprehensive visualization of nanoparticle populations in droplets of known size. These qualities facilitate unbiased sampling, rapid size measurement and estimation of nanoparticle numbers by means of ratio counting using a colloidal gold calibrant. Specimen preparation and quantification take minutes and require a few microliters of sample using only basic laboratory equipment and a standard TEM.

Probing the structure of the mechanosensitive channel of small conductance in lipid bilayers with pulsed electron-electron double resonance

Mechanosensitive channel proteins are important safety valves against osmotic shock in bacteria, and are involved in sensing touch and sound waves in higher organisms. The mechanosensitive channel of small conductance (MscS) has been extensively studied. Pulsed electron-electron double resonance (PELDOR or DEER) of detergent-solubilized protein confirms that as seen in the crystal structure, the outer ring of transmembrane helices do not pack against the pore-forming helices, creating an apparent void. The relevance of this void to the functional form of MscS in the bilayer is the subject of debate. Here, we report PELDOR measurements of MscS reconstituted into two lipid bilayer systems: nanodiscs and bicelles. The distance measurements from multiple mutants derived from the PELDOR data are consistent with the detergent-solution arrangement of the protein. We conclude, therefore, that the relative positioning of the transmembrane helices is preserved in mimics of the cell bilayer, and that the apparent voids are not an artifact of detergent solution but a property of the protein that will have to be accounted for in any molecular mechanism of gating.

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.

Role of phospholipase D in parathyroid hormone type 1 receptor signaling and trafficking

The role of phospholipase D (PLD) in the regulation of the traffic of the PTH type 1 receptor (PTH1R) was studied in Chinese hamster ovary cells stably transfected with a human PTH1R (CHO-R3) and in rat osteosarcoma 17/2.8 (ROS) cells. PTH(1-34) increased total PLD activity by 3-fold in CHO-R3 cells and by 2-fold in ROS cells. Overexpression of wild-type (WT) PLD1 and WT-PLD2 increased basal PLD activity in CHO-R3 but not in ROS cells. Ligand-stimulated PLD activity greatly increased in CHO-R3 cells transfected with WT-PLD1 and WT-PLD2. However, only WT-PLD2 expression increased PTH-dependent PLD activity in ROS cells. Expression of the catalytically inactive mutants R898K-PLD1 (DN-PLD1) and R758K-PLD2 (DN-PLD2) inhibited ligand-dependent PLD activity in both cell lines. PTH(1-34) induced internalization of the PTH1R with a concomitant increase in the colocalization of the receptor with PLD1 in intracellular vesicles and in a perinuclear, ADP ribosylation factor-1-positive compartment. The distribution of PLD1 and PLD2 remained unaltered after PTH treatment. Expression of DN-PLD1 had a small effect on endocytosis of the PTH1R; however, DN-PLD1 prevented accumulation of the PTH1R in the perinuclear compartment. Expression of DN-PLD2 significantly retarded ligand-induced PTH1R internalization in both cell lines. The differential effects of PLD1 and PLD2 on receptor traffic were confirmed using isoform-specific short hairpin RNA constructs. We conclude that PLD1 and PLD2 play distinct roles in regulating PTH1R traffic; PLD2 primarily regulates endocytosis, whereas PLD1 regulates receptor internalization and intracellular receptor traffic.

Involvement of Phospholipase D 1 and 2 in the subcellular localization and activity of formyl-peptide-receptors in the human colonic cell line HT29

Epithelial cells of the alimentary tract play a central role in the mucosal host defence against pathogens and in the recognition of agonists that interact with mucosal surfaces. In particular, the formyl peptide receptor (FPR) family and their three human subtypes: FPR, formyl-peptide-receptor-like-1 (FPRL1) and FPRL2, are involved in the host defence against pathogens that mediate epithelial responses thus upregulating inflammation. To elucidate the mechanisms by which FPR function, we examined the influence of phospholipase D (PLD) 1 and 2 on the activity and signal transduction of human enterocytes cell line HT29. PLD is a key enzyme involved in secretion, endocytosis and receptor signalling. We inhibited PLD1 and 2 by small interference RNA (siRNA) and determined the activity of formyl peptide receptors using Western blotting and cAMP level measurements. We then analyzed the distribution of formyl peptide receptors FPR, FPRL1 and FPRL2 compared to a control. In this study, we demonstrated that the depletion of PLD1 and 2 resulted in a marked reduction of formyl peptide receptor activity due to inhibited extracellular-signal regulated kinases 1/2 (ERK1/2), phosphorylation and cAMP level reduction. In addition, we observed an intracellular accumulation of FPR, FPRL1 and FPRL2 as a result of receptor recycling inhibition using fluorescence microscopy. The constitutive internalization rate was unaffected. Our results support the importance of PLD1 and 2 in formyl peptide receptor function and the role of endocytosis, receptor recycling and reactivation for receptor activity.

The influence of linoleic and linolenic acid on the activity and intracellular localisation of phospholipase D in COS-1 cells

Phospholipase D (PLD) is a receptor-regulated signalling enzyme involved in biological functions, such as exocytosis, phagocytosis, actin dynamics, membrane trafficking, and is considered to be essential for stimulated degranulation of cells. The purpose of our investigation was to examine how the fatty acid pattern of cellular membranes influences the activities and cellular distribution of the PLD1 and PLD2 isoforms. Expression of GFP-tagged PLD1 and PLD2 in COS-1 cells that were stimulated with mastoparan after cultivation in 20 μmol linoleic (C18:2n6) or linolenic (C18:3n3) acid for 4 d demonstrated that PLD1 dramatically alters its cellular distribution and is redistributed from intracellular vesicles to the cell surface. PLD2, on the other hand, maintains its localisation at the plasma membrane. The activity of PLD, which corresponds to PLD1 and PLD2, significantly increased two- to three-fold in the presence of the fatty acids. We conclude that linoleic acid and linolenic acid supplementation affect the intracellular trafficking of the PLD1 isoform and the activity of PLD most likely due to alterations in the membrane lipid environment conferred by the fatty acids.

Molecular mechanisms of PLD function in membrane traffic

The two mammalian phosphatidylcholine (PC)-selective phospholipase D (PLD) enzymes remove the choline head group from PC to produce phosphatidic acid (PA). PA stimulates phosphatidylinositol(4)phosphate 5-kinases, can function as a binding site for membrane proteins, is required for certain membrane fusion or fission events and is an important precursor for the production of diacylglycerol (DAG). Both PA and DAG are lipids that favor negatively curved membranes rather than planar bilayers and can reduce the energetic barrier to membrane fission and fusion. Recent data provide a mechanistic explanation for the role PLDs play in some aspects of membrane traffic and provide an explanation for why some membrane fusion reactions require PA and some do not. PLDs also act as guanosine triphosphatase-activating proteins for dynamin and may participate with dynamin in the process of vesicle fission.

Quantitative immunoelectron microscopy: alternative ways of assessing subcellular patterns of gold labeling

Using antibodies conjugated with colloidal gold particles, immunoelectron microscopy permits the high-resolution detection, localization, and quantification of one or more defined antigens in cellular compartments. These benefits reflect the properties of gold particles (they are electron dense, punctate, and available in different sizes) and the ability of transmission electron microscopy to resolve both particles and compartments. By relating gold marker to cellular fine structure and by taking into account the study design, three pertinent questions can be addressed. When studying a particular group of cells, we might ask: "What is the spatial distribution of gold particles between compartments within a group of cells?" and/or "Is the spatial distribution of gold particles within a group of cells random or due to preferential labeling of compartments?" When comparing two or more groups, a relevant question is: "Are there shifts in compartment labeling distributions in different groups of cells?" Recently, new ways of testing these basic questions have been developed. The efficiency and validity of all these methods rely on sampling, stereological, and statistical tools. Key processes include random selection of items at each sampling stage (specimen blocks, microscopical fields, etc.), stereological morphometry and/or unbiased counting, and statistical evaluation of a suitable null hypothesis (no difference in labeling between compartments or groups). This chapter reviews these new methods and illustrates their application with a consistent dataset.

Role of phospholipase D2 in the agonist-induced and constitutive endocytosis of G-protein coupled receptors

We have recently shown that the mu-opioid receptor [MOR1, also termed mu-opioid peptide (MOP) receptor] is associated with the phospholipase D2 (PLD2), a phospholipid-specific phosphodiesterase located in the plasma membrane. We further demonstrated that, in human embryonic kidney (HEK) 293 cells co-expressing MOR1 and PLD2, treatment with (D-Ala2, Me Phe4, Glyol5)enkephalin (DAMGO) led to an increase in PLD2 activity and an induction of receptor endocytosis, whereas morphine, which does not induce opioid receptor endocytosis, failed to activate PLD2. In contrast, a C-terminal splice variant of the mu-opioid receptor (MOR1D, also termed MOP(1D)) exhibited robust endocytosis in response to both DAMGO and morphine treatment. We report here that MOR1D also mediates an agonist-independent (constitutive) PLD2-activation facilitating agonist-induced and constitutive receptor endocytosis. Inhibition of PLD2 activity by over-expression of a dominant negative PLD2 (nPLD2) blocked the constitutive PLD2 activation and impaired the endocytosis of MOR1D receptors. Moreover, we provide evidence that the endocytotic trafficking of the delta-opioid receptor [DOR, also termed delta-opioid peptide (DOP) receptor] and cannabinoid receptor isoform 1 (CB1) is also mediated by a PLD2-dependent pathway. These data indicate the generally important role for PLD2 in the regulation of agonist-dependent and agonist-independent G protein-coupled receptor (GPCR) endocytosis.

Phospholipase D2 is required for efficient endocytic recycling of transferrin receptors

RNA interference-mediated depletion of phospholipase D2 (PLD2), but not PLD1, inhibited recycling of transferrin receptors in HeLa cells, whereas the internalization rate was unaffected by depletion of either PLD. Although reduction of both PLD isoforms inhibits PLD activity stimulated by phorbol 12-myristic 13-acetate, only depletion of PLD2 decreased nonstimulated activity. Cells with reduced PLD2 accumulated a greater fraction of transferrin receptors in a perinuclear compartment that was positive for Rab11, a marker of recycling endosomes. EFA6, an exchange factor for Arf6, has been proposed to stimulate the recycling of transferrin receptors. Thus, one consequence of EFA6 overexpression would be a reduction of the internal pool of receptors. We confirmed this observation in control HeLa cells; however, overexpression of EFA6 failed to decrease the internal pool of transferrin receptors that accumulate in cells previously depleted of PLD2. These observations suggest that either PLD2 is required for a constitutive Arf6-mediated recycling pathway or in the absence of PLD2 transferrin receptors accumulate in recycling endosomes that are not responsive to overexpression of EFA6.

Mechanism of Membrane Binding of the Phospholipase D1 PX Domain

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

A rapid method for assessing the distribution of gold labeling on thin sections

Particulate gold labeling on ultrathin sections is in widespread use for antigen localization at the EM level. To extend the usefulness of gold labeling technology, we are evaluating different methods for sampling and estimating quantities of gold labeling. Here we present a simple, rapid, and unbiased method for assessing the relative pool sizes of immunogold labeling distributed over different cell compartments. The method uses a sampling approach developed for stereology in which a regular array of microscopic fields or linear scans is positioned randomly on labeled sections. From these readouts, gold particles are counted and assigned to identifiable cell structures to construct a gold labeling frequency distribution of those labeled compartments. Here we use ultrathin cryosections labeled for a range of different proteins and for a signaling lipid. We show by scanning labeled sections at the electron microscope that counting 100-200 particles on each of two grids is sufficient to obtain a reproducible and rapid assessment of the pattern of labeling proportions over 10-16 compartments. If more precise estimates of labeling proportions over individual compartments are required (e.g., to achieve coefficients of error of 10-20%), then 100-200 particles need to be counted over each compartment of interest.

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.

Regulation of phospholipase D1 subcellular cycling through coordination of multiple membrane association motifs

The signaling enzyme phospholipase D1 (PLD1) facilitates membrane vesicle trafficking. Here, we explore how PLD1 subcellular localization is regulated via Phox homology (PX) and pleckstrin homology (PH) domains and a PI4,5P2-binding site critical for its activation. PLD1 localized to perinuclear endosomes and Golgi in COS-7 cells, but on cellular stimulation, translocated to the plasma membrane in an activity-facilitated manner and then returned to the endosomes. The PI4,5P2-interacting site sufficed to mediate outward translocation and association with the plasma membrane. However, in the absence of PX and PH domains, PLD1 was unable to return efficiently to the endosomes. The PX and PH domains appear to facilitate internalization at different steps. The PH domain drives PLD1 entry into lipid rafts, which we show to be a step critical for internalization. In contrast, the PX domain appears to mediate binding to PI5P, a lipid newly recognized to accumulate in endocytosing vesicles. Finally, we show that the PH domain-dependent translocation step, but not the PX domain, is required for PLD1 to function in regulated exocytosis in PC12 cells. We propose that PLD1 localization and function involves regulated and continual cycling through a succession of subcellular sites, mediated by successive combinations of membrane association interactions.

Role of the connecting peptide in insulin biosynthesis

In single-chain insulins (SCIs), the C terminus of the insulin B-chain is contiguous with the N terminus of the A-chain, connected by a short bioengineered linker sequence. SCIs have been proposed to offer potential benefit for gene therapy of diabetes (Lee, H. C., Kim, S. J., Kim, K. S., Shin, H. C., and Yoon, J. W. (2000) Nature 408, 483-488) yet relatively little is known about their folding, intracellular transport, or secretion from mammalian cells. Because SCIs can be considered as mutant proinsulin (with selective shortening of the 35-amino acid connecting peptide that normally includes two sets of flanking dibasic residues), they offer insights into understanding the role of the connecting peptide in insulin biosynthesis. Herein we have explored the relationship of the linker sequence to SCI biosynthesis, folding, and intracellular transport in transiently transfected HEK293 or Chinese hamster ovary cells or in stably transfected AtT20 cells. Despite previous reports that direct linkage of B- and A-chains produces a structure isomorphous with authentic two-chain insulin, we find that constructs with short linkers tend to be synthesized at lower levels, with a significant fraction of molecules exhibiting improper disulfide bonding. Nevertheless, disulfide-mispaired isoforms from a number of different SCI constructs are secreted. While this suggests that a novel folded state goes unrecognized by secretory pathway quality control, we find that misfolded SCIs are detected at higher levels in Chinese hamster ovary cells with artificially activated unfolded protein response mediated by inducible overexpression of active ATF-6. Such a maneuver allows analysis of more seriously misfolded mutants with further foreshortening of the linker sequence or loss (by mutation) of the insulin interchain disulfide bonds.

Hierarchy of membrane-targeting signals of phospholipase D1 involving lipid modification of a pleckstrin homology domain

The amino terminus of phospholipase D1 (PLD1) contains three potential membrane-interacting determinants: a phox homology (PX) domain, a pleckstrin homology (PH) domain and two adjacent cysteines at positions 240 and 241 within the PH domain that are fatty acylated in vivo. To understand how these determinants contribute to membrane localization, we have mutagenized critical residues of the PLD1 PH domain in the wild type or palmitate-free background in the intact protein, in a fragment that deletes the first 210 amino acids including the PX domain, and in the isolated PH domain. Mutants were expressed in COS-7 cells and examined for membrane residence, intracellular localization, palmitoylation, and catalytic activity. Our results are as follows. 1) Mutagenesis of critical residues of the PH domain results in redistribution of PLD1 from membranes to cytosol, independently of fatty acylation sites. Importantly, PH domain mutants in the wild type background showed greatly reduced fatty acylation, despite the presence of all relevant cysteines. 2) The isolated PH domain did not co-localize with PLD1 and was not palmitoylated. 3) The PX deletion mutant showed similar distribution and palmitoylation to the intact protein. Interestingly, PH domain mutants in this background showed significant palmitoylation and incomplete cytosolic redistribution. 4) PH domain mutants in the wild type or palmitate-free background maintained catalytic activity. We propose that membrane targeting of PLD1 involves a hierarchy of signals with a functional PH domain allowing fatty acylation leading to strong membrane binding. The PX domain may modulate function of the PH domain.

Phospholipid signaling in plants: holding on to phospholipase D

Plant cells contain various phospholipase-based signaling pathways. In fact, their repertoire of phospholipase D (PLD) molecules far outnumbers those of mammalian and yeast cells. Munnik and Musgrave take a broad look at PLD function in animal, yeast, and plant cells, and suggest that a PLD-based connection between membranes and microtubules is a biological property worth considering across species.