Proteins involved in lipid translocation in eukaryotic cells

Molecular responses of agroinfiltrated Nicotiana benthamiana leaves expressing suppressor of silencing P19 and influenza virus-like particles

The production of influenza vaccines in plants is achieved through transient expression of viral hemagglutinins (HAs), a process mediated by the bacterial vector Agrobacterium tumefaciens. HA proteins are then produced and matured through the secretory pathway of plant cells, before being trafficked to the plasma membrane where they induce formation of virus-like particles (VLPs). Production of VLPs unavoidably impacts plant cells, as do viral suppressors of RNA silencing (VSRs) that are co-expressed to increase recombinant protein yields. However, little information is available on host molecular responses to foreign protein expression. This work provides a comprehensive overview of molecular changes occurring in Nicotiana benthamiana leaf cells transiently expressing the VSR P19, or co-expressing P19 and an influenza HA. Our data identifies general responses to Agrobacterium-mediated expression of foreign proteins, including shutdown of chloroplast gene expression, activation of oxidative stress responses and reinforcement of the plant cell wall through lignification. Our results also indicate that P19 expression promotes salicylic acid (SA) signalling, a process dampened by co-expression of the HA protein. While reducing P19 level, HA expression also induces specific signatures, with effects on lipid metabolism, lipid distribution within membranes and oxylipin-related signalling. When producing VLPs, dampening of P19 responses thus likely results from lower expression of the VSR, crosstalk between SA and oxylipin pathways, or a combination of both outcomes. Consistent with the upregulation of oxidative stress responses, we finally show that reduction of oxidative stress damage through exogenous application of ascorbic acid improves plant biomass quality during production of VLPs.

Protein Adsorption on Solid Supported Membranes: Monitoring the Transport Activity of P-Type ATPases

P-type ATPases are a large family of membrane transporters that are found in all forms of life. These enzymes couple ATP hydrolysis to the transport of various ions or phospholipids across cellular membranes, thereby generating and maintaining crucial electrochemical potential gradients. P-type ATPases have been studied by a variety of methods that have provided a wealth of information about the structure, function, and regulation of this class of enzymes. Among the many techniques used to investigate P-type ATPases, the electrical method based on solid supported membranes (SSM) was employed to investigate the transport mechanism of various ion pumps. In particular, the SSM method allows the direct measurement of charge movements generated by the ATPase following adsorption of the membrane-bound enzyme on the SSM surface and chemical activation by a substrate concentration jump. This kind of measurement was useful to identify electrogenic partial reactions and localize ion translocation in the reaction cycle of the membrane transporter. In the present review, we discuss how the SSM method has contributed to investigate some key features of the transport mechanism of P-type ATPases, with a special focus on sarcoplasmic reticulum Ca²⁺-ATPase, mammalian Cu⁺-ATPases (ATP7A and ATP7B), and phospholipid flippase ATP8A2.

Biological membranes

Biological membranes allow life as we know it to exist. They form cells and enable separation between the inside and outside of an organism, controlling by means of their selective permeability which substances enter and leave. By allowing gradients of ions to be created across them, membranes also enable living organisms to generate energy. In addition, they control the flow of messages between cells by sending, receiving and processing information in the form of chemical and electrical signals. This essay summarizes the structure and function of membranes and the proteins within them, and describes their role in trafficking and transport, and their involvement in health and disease. Techniques for studying membranes are also discussed.

P4-ATPases: lipid flippases in cell membranes

Cellular membranes, notably eukaryotic plasma membranes, are equipped with special proteins that actively translocate lipids from one leaflet to the other and thereby help generate membrane lipid asymmetry. Among these ATP-driven transporters, the P4 subfamily of P-type ATPases (P4-ATPases) comprises lipid flippases that catalyze the translocation of phospholipids from the exoplasmic to the cytosolic leaflet of cell membranes. While initially characterized as aminophospholipid translocases, recent studies of individual P4-ATPase family members from fungi, plants, and animals show that P4-ATPases differ in their substrate specificities and mediate transport of a broader range of lipid substrates, including lysophospholipids and synthetic alkylphospholipids. At the same time, the cellular processes known to be directly or indirectly affected by this class of transporters have expanded to include the regulation of membrane traffic, cytoskeletal dynamics, cell division, lipid metabolism, and lipid signaling. In this review, we will summarize the basic features of P4-ATPases and the physiological implications of their lipid transport activity in the cell.

Fusion-competent state induced by a C-terminal HIV-1 fusion peptide in cholesterol-rich membranes

The replicative cycle of the human immunodeficiency virus type-1 begins after fusion of the viral and target-cell membranes. The envelope glycoprotein gp41 transmembrane subunit contains conserved hydrophobic domains that engage and perturb the merging lipid bilayers. In this work, we have characterized the fusion-committed state generated in vesicles by CpreTM, a synthetic peptide derived from the sequence connecting the membrane-proximal external region (MPER) and the transmembrane domain (TMD) of gp41. Pre-loading cholesterol-rich vesicles with CpreTM rendered them competent for subsequent lipid-mixing with fluorescently-labeled target vesicles. Highlighting the physiological relevance of the lasting fusion-competent state, the broadly neutralizing antibody 4E10 bound to the CpreTM-primed vesicles and inhibited lipid-mixing. Heterotypic fusion assays disclosed dependence on the lipid composition of the vesicles that acted either as virus or cell membrane surrogates. Lipid-mixing exhibited above all a critical dependence on the cholesterol content in those experiments. We infer that the fusion-competent state described herein resembles bona-fide perturbations generated by the pre-hairpin MPER-TMD connection within the viral membrane.

Progressive familial intrahepatic cholestasis

Progressive familial intrahepatic cholestasis (PFIC) is a group of rare disorders which are caused by defect in bile secretion and present with intrahepatic cholestasis, usually in infancy and childhood. These are autosomal recessive in inheritance. The estimated incidence is about 1 per 50,000 to 1 per 100,000 births, although exact prevalence is not known. These diseases affect both the genders equally and have been reported from all geographical areas. Based on clinical presentation, laboratory findings, liver histology and genetic defect, these are broadly divided into three types-PFIC type 1, PFIC type 2 and PFIC type 3. The defect is in ATP8B1 gene encoding the FIC1 protein, ABCB 11 gene encoding BSEP protein and ABCB4 gene encoding MDR3 protein in PFIC1, 2 and 3 respectively. The basic defect is impaired bile salt secretion in PFIC1/2 whereas in PFIC3, it is reduced biliary phospholipid secretion. The main clinical presentation is in the form of cholestatic jaundice and pruritus. Serum gamma glutamyl transpeptidase (GGT) is normal in patients with PFIC1/2 while it is raised in patients with PFIC3. Treatment includes nutritional support (adequate calories, supplementation of fat soluble vitamins and medium chain triglycerides) and use of medications to relieve pruritus as initial therapy followed by biliary diversion procedures in selected patients. Ultimately liver transplantation is needed in most patients as they develop progressive liver fibrosis, cirrhosis and end stage liver disease. Due to the high risk of developing liver tumors in PFIC2 patients, monitoring is recommended from infancy. Mutation targeted pharmacotherapy, gene therapy and hepatocyte transplantation are being explored as future therapeutic options.

Phospholipid scramblase-1-induced lipid reorganization regulates compensatory endocytosis in neuroendocrine cells

Calcium-regulated exocytosis in neuroendocrine cells and neurons is accompanied by the redistribution of phosphatidylserine (PS) to the extracellular space, leading to a disruption of plasma membrane asymmetry. How and why outward translocation of PS occurs during secretion are currently unknown. Immunogold labeling on plasma membrane sheets coupled with hierarchical clustering analysis demonstrate that PS translocation occurs at the vicinity of the secretory granule fusion sites. We found that altering the function of the phospholipid scramblase-1 (PLSCR-1) by expressing a PLSCR-1 calcium-insensitive mutant or by using chromaffin cells from PLSCR-1⁻/⁻ mice prevents outward translocation of PS in cells stimulated for exocytosis. Remarkably, whereas transmitter release was not affected, secretory granule membrane recapture after exocytosis was impaired, indicating that PLSCR-1 is required for compensatory endocytosis but not for exocytosis. Our results provide the first evidence for a role of specific lipid reorganization and calcium-dependent PLSCR-1 activity in neuroendocrine compensatory endocytosis.

Lipid nanotechnology

Nanotechnology is a multidisciplinary field that covers a vast and diverse array of devices and machines derived from engineering, physics, materials science, chemistry and biology. These devices have found applications in biomedical sciences, such as targeted drug delivery, bio-imaging, sensing and diagnosis of pathologies at early stages. In these applications, nano-devices typically interface with the plasma membrane of cells. On the other hand, naturally occurring nanostructures in biology have been a source of inspiration for new nanotechnological designs and hybrid nanostructures made of biological and non-biological, organic and inorganic building blocks. Lipids, with their amphiphilicity, diversity of head and tail chemistry, and antifouling properties that block nonspecific binding to lipid-coated surfaces, provide a powerful toolbox for nanotechnology. This review discusses the progress in the emerging field of lipid nanotechnology.

Transbilayer dynamics of phospholipids in the plasma membrane of the Leishmania genus

Protozoans of the Leishmania genus are the etiological agents of a wide spectrum of diseases commonly known as leishmaniases. Lipid organization of the plasma membrane of the parasite may mimic the lipid organization of mammalian apoptotic cells and play a role in phagocytosis and parasite survival in the mammal host. Here, we analyzed the phospholipid dynamics in the plasma membrane of both the L. (Leishmania) and the L. (Viannia) subgenera. We found that the activity and substrate specificity of the inward translocation machinery varied between Leishmania species. The differences in activity of inward phospholipid transport correlated with the different sensitivities of the various species towards the alkyl-phospholipid analogue miltefosine. Furthermore, all species exhibited a phospholipid scramblase activity in their plasma membranes upon stimulation with calcium ionophores. However, binding of annexin V to the parasite surface was only detected for a subpopulation of parasites during the stationary growth phase and only marginally enhanced by scramblase activation.

Phospholipid flippases Lem3p-Dnf1p and Lem3p-Dnf2p are involved in the sorting of the tryptophan permease Tat2p in yeast

The type 4 P-type ATPases are flippases that generate phospholipid asymmetry in membranes. In budding yeast, heteromeric flippases, including Lem3p-Dnf1p and Lem3p-Dnf2p, translocate phospholipids to the cytoplasmic leaflet of membranes. Here, we report that Lem3p-Dnf1/2p are involved in transport of the tryptophan permease Tat2p to the plasma membrane. The lem3Δ mutant exhibited a tryptophan requirement due to the mislocalization of Tat2p to intracellular membranes. Tat2p was relocalized to the plasma membrane when trans-Golgi network (TGN)-to-endosome transport was inhibited. Inhibition of ubiquitination by mutations in ubiquitination machinery also rerouted Tat2p to the plasma membrane. Lem3p-Dnf1/2p are localized to endosomal/TGN membranes in addition to the plasma membrane. Endocytosis mutants, in which Lem3p-Dnf1/2p are sequestered to the plasma membrane, also exhibited the ubiquitination-dependent missorting of Tat2p. These results suggest that Tat2p is ubiquitinated at the TGN and missorted to the vacuolar pathway in the lem3Δ mutant. The NH(2)-terminal cytoplasmic region of Tat2p containing ubiquitination acceptor lysines interacted with liposomes containing acidic phospholipids, including phosphatidylserine. This interaction was abrogated by alanine substitution mutations in the basic amino acids downstream of the ubiquitination sites. Interestingly, a mutant Tat2p containing these substitutions was missorted in a ubiquitination-dependent manner. We propose the following model based on these results; Tat2p is not ubiquitinated when the NH(2)-terminal region is bound to membrane phospholipids, but if it dissociates from the membrane due to a low level of phosphatidylserine caused by perturbation of phospholipid asymmetry in the lem3Δ mutant, Tat2p is ubiquitinated and then transported from the TGN to the vacuole.

Lipids of plant membrane rafts

Lipids tend to organize in mono or bilayer phases in a hydrophilic environment. While they have long been thought to be incapable of coherent lateral segregation, it is now clear that spontaneous assembly of these compounds can confer microdomain organization beyond spontaneous fluidity. Membrane raft microdomains have the ability to influence spatiotemporal organization of protein complexes, thereby allowing regulation of cellular processes. In this review, we aim at summarizing briefly: (i) the history of raft discovery in animals and plants, (ii) the main findings about structural and signalling plant lipids involved in raft segregation, (iii) imaging of plant membrane domains, and their biochemical purification through detergent-insoluble membranes, as well as the existing debate on the topic. We also discuss the potential involvement of rafts in the regulation of plant physiological processes, and further discuss the prospects of future research into plant membrane rafts.

A putative plant aminophospholipid flippase, the Arabidopsis P4 ATPase ALA1, localizes to the plasma membrane following association with a β-subunit

Plasma membranes in eukaryotic cells display asymmetric lipid distributions with aminophospholipids concentrated in the inner leaflet and sphingolipids in the outer leaflet. This unequal distribution of lipids between leaflets is, amongst several proposed functions, hypothesized to be a prerequisite for endocytosis. P4 ATPases, belonging to the P-type ATPase superfamily of pumps, are involved in establishing lipid asymmetry across plasma membranes, but P4 ATPases have not been identified in plant plasma membranes. Here we report that the plant P4 ATPase ALA1, which previously has been connected with cold tolerance of Arabidopsis thaliana, is targeted to the plasma membrane and does so following association in the endoplasmic reticulum with an ALIS protein β-subunit.

Mathematical modeling and validation of the ergosterol pathway in Saccharomyces cerevisiae

The de novo biosynthetic machinery for both sphingolipid and ergosterol production in yeast is localized in the endoplasmic reticulum (ER) and Golgi. The interconnections between the two pathways are still poorly understood, but they may be connected in specialized membrane domains, and specific knockouts strongly suggest that both routes have different layers of mutual control and are co-affected by drugs. With the goal of shedding light on the functional integration of the yeast sphingolipid-ergosterol (SL-E) pathway, we constructed a dynamic model of the ergosterol pathway using the guidelines of Biochemical Systems Theory (BST) (Savageau., J. theor. Biol., 25, 365–9, 1969). The resulting model was merged with a previous mathematical model of sphingolipid metabolism in yeast (Alvarez-Vasquez et al., J. theor. Biol., 226, 265–91, 2004; Alvarez-Vasquez et al., Nature433, 425–30, 2005). The S-system format within BST was used for analyses of consistency, stability, and sensitivity of the SL-E model, while the GMA format was used for dynamic simulations and predictions. Model validation was accomplished by comparing predictions from the model with published results on sterol and sterol-ester dynamics in yeast. The validated model was used to predict the metabolomic dynamics of the SL-E pathway after drug treatment. Specifically, we simulated the action of drugs affecting sphingolipids in the endoplasmic reticulum and studied changes in ergosterol associated with microdomains of the plasma membrane (PM).

Involvement of Golgi-associated retrograde protein complex in the recycling of the putative Dnf aminophospholipid flippases in yeast

It is widely accepted that phosphatidylethanolamine (PE) is enriched in the cytosolic leaflet of the eukaryotic plasma membranes. To identify genes involved in the establishment and regulation of the asymmetric distribution of PE on the plasma membrane, we screened the deletion strain collection of the yeast Saccharomyces cerevisiae for hypersensitive mutants to the lantibiotic peptide Ro09-0198 (Ro) that specifically binds to PE on the cell surface and inhibits cellular growth. Deletion mutants of VPS51, VPS52, VPS53, and VPS54 encoding the components of Golgi-associated retrograde protein (GARP) complex, YPT6 encoding a Rab family small GTPase that functions with GARP complex, RIC1 and RGP1 encoding its guanine nucleotide exchange factor (GEF), and TLG2 encoding t-SNARE exhibited hypersensitivity to Ro. The mutants deleted for VPS51, VPS52, VPS53, and VPS54 were impaired in the uptake of fluorescently labeled PE. In addition, aberrant intracellular localization of the EGFP-tagged Dnf2p, the putative inward-directed phospholipid translocase (flippase) of the plasma membrane, was observed in the mutant defective in the GARP complex, Ypt6p, its GEF proteins, or Tlg2p. Our results suggest that the GARP complex is involved in the recycling of Dnf flippases.

Flipping and flopping--lipids on the move

The rapid movement of polar lipids from one membrane leaflet to the other is facilitated by lipid flippases or translocases. Although their activity was first observed over 30 years ago, the structures, physiological roles, and molecular mechanisms of this group of proteins remain enigmatic. Lipid flippases maintain membrane lipid asymmetry, and in eukaryotes they are also intimately involved in membrane budding and vesicle trafficking. The ATP-dependent flippases are members of well-characterized protein families, whose other members transport nonlipid substrates across cell membranes. The P(4)-type ATPases carry out the inward translocation of phospholipids, and various ABC transporters are involved in outward lipid movement. The ATP-independent flippases move lipid substrates in both directions between membrane leaflets. With only a few exceptions, the molecular identity of these proteins is still unknown, despite their involvement in key biosynthetic pathways in both bacteria and eukaryotes. This review provides an overview of the different classes of flippases, and summarizes recent progress in their identification and functional characterization. The possible mechanisms of action of lipid flippases are discussed, and future directions explored.

Oxalate impairs aminophospholipid translocase activity in renal epithelial cells via oxidative stress: implications for calcium oxalate urolithiasis

PURPOSE

We evaluated the possible involvement of phospholipid transporters and reactive oxygen species in the oxalate induced redistribution of renal epithelial cell phosphatidylserine.

MATERIALS AND METHODS

Madin-Darby canine kidney cells were labeled with the fluorescent phospholipid NBD-PS in the inner or outer leaflet of the plasma membrane and then exposed to oxalate in the presence or absence of antioxidant. This probe was tracked using a fluorescent quenching assay to assess the bidirectional transmembrane movement of phosphatidylserine. Surface expressed phosphatidylserine was detected by annexin V binding assay. The cell permeable fluorogenic probe DCFH-DA was used to measure the intracellular reactive oxygen species level.

RESULTS

Oxalate produced a time and concentration dependent increase in phosphatidylserine, which may have resulted from impaired aminophospholipid translocase mediated, inward directed phosphatidylserine transport and from enhanced phosphatidylserine outward transport. Adding the antioxidant N-acetyl-L-cysteine significantly attenuated phosphatidylserine externalization by effectively rescuing aminophospholipid translocase activity.

CONCLUSIONS

To our knowledge our findings are the first to show that oxalate induced increased reactive oxygen species generation impairs aminophospholipid translocase activity and decreased aminophospholipid translocase activity has a role in hyperoxaluria promoted calcium oxalate urolithiasis by facilitating phosphatidylserine redistribution in renal epithelial cells.

Ca2+-activated transbilayer movement of plasma membrane phospholipids in Leishmania donovani during ionomycin or thapsigargin stimulation

The protozoan parasite Leishmania causes serious infections in humans all over the world. After being inoculated into the skin through the bite of an infected sandfly, Leishmania promastigotes must gain entry into macrophages to initiate a successful infection. Specific, surface exposed phospholipids have been implicated in Leishmania-macrophage interaction but the mechanisms controlling and regulating the plasma membrane lipid distribution remains to be elucidated. Here, we provide evidence for Ca(2+)-induced phospholipid scrambling in the plasma membrane of Leishmania donovani. Stimulation of parasites with ionomycin increases intracellular Ca(2+) levels and triggers exposure of phosphatidylethanolamine at the cell surface. We found that increasing intracellular Ca(2+) levels with ionomycin or thapsigargin induces rapid transbilayer movement of NBD-labelled phospholipids in the parasite plasma membrane that is bidirectional, independent of cellular ATP and not specific to the polar lipid head group. The findings suggest the presence of a Ca(2+)-dependent lipid scramblase activity in Leishmania parasites. Our studies further show that lipid scrambling is not activated by rapid exposure of promastigotes to higher physiological temperature that increases intracellular Ca(2+) levels.

Human TMEM30a promotes uptake of antitumor and bioactive choline phospholipids into mammalian cells

Antitumor alkylphospholipids initiate apoptosis in transformed HL-60 and Jurkat cells while sparing their progenitors. 1-O-Alkyl-2-carboxymethyl-sn-glycero-3-phosphocholine (Edelfosine) like other short-chained phospholipids--inflammatory platelet-activating factor (PAF) and apoptotic oxidatively truncated phospholipids--are proposed to have intracellular sites of action, yet a conduit for these choline phospholipids into mammalian cells is undefined. Edelfosine is also accumulated by Saccharomyces cerevisiae in a process requiring the membrane protein Lem3p, and the human genome contains a Lem3p homolog TMEM30a. We show that import of choline phospholipids into S. cerevisiae ΔLem3 is partially reconstituted by human TMEM30a and by Lem3p-TMEM30a chimeras, showing the proteins are orthologous. TMEM30a-GFP chimeras expressed in mammalian cells localized in plasma membranes, as well as internal organelles, and ectopic TMEM30a expression promoted uptake of exogenous choline and ethanolamine phospholipids. Short hairpin RNA knockdown of TMEM30a reduced fluorescent choline phospholipid and [(3)H]PAF import. This knockdown also reduced mitochondrial depolarization from exogenous Edelfosine or the mitotoxic oxidatively truncated phospholipid azelaoyl phosphatidylcholine, and the knockdown reduced apoptosis in response to these two phospholipids. These results show that extracellular choline phospholipids with short sn-2 residues can have intracellular roles and sites of metabolism because they are transport substrates for a TMEM30a phospholipid import system. Variation in this mechanism could limit sensitivity to short chain choline phospholipids such as Edelfosine, PAF, and proapoptotic phospholipids.

Cellular mechanisms underlying the formation of circulating microparticles

Microparticles (MPs) derived from platelets, monocytes, endothelial cells, red blood cells, and granulocytes may be detected in low concentrations in normal plasma and at increased levels in atherothrombotic cardiovascular diseases. The elucidation of the cellular mechanisms underlying the generation of circulating MPs is crucial for improving our understanding of their pathophysiological role in health and disease. The flopping of phosphatidylserine (PS) to the outer leaflet of the plasma membrane is the key event that will ultimately lead to the shedding of procoagulant MPs from activated or apoptotic cells. Research over the last few years has revealed important roles for calcium-, mitochondrial-, and caspase-dependent mechanisms leading to PS exposure. The study of Scott cells has unraveled different molecular mechanisms that may contribute to fine-tuning of PS exposure and MP release in response to a variety of specific stimuli. The pharmacological modulation of MP release may have a substantial therapeutic impact in the management of atherothrombotic vascular disorders. Because PS exposure is a key feature in pathological processes different from hemostasis and thrombosis, the most important obstacle in the field of MP-modulating drugs seems to be carefully targeting MP release to relevant cell types at an optimal level, so as to achieve a beneficial action and limit possible adverse effects.

Cell cholesterol homeostasis: mediation by active cholesterol

Recent evidence suggests that the major pathways mediating cell cholesterol homeostasis respond to a common signal: active membrane cholesterol. Active cholesterol is the fraction that exceeds the complexing capacity of the polar bilayer lipids. Increments in plasma membrane cholesterol exceeding this threshold have an elevated chemical activity (escape tendency) and redistribute via diverse transport proteins to both circulating plasma lipoproteins and intracellular organelles. Active cholesterol thereby prompts several feedback responses. It is the substrate for its own esterification and for the synthesis of regulatory side-chain oxysterols. It also stimulates manifold pathways that down-regulate the biosynthesis, curtail the ingestion and increase the export of cholesterol. Thus, the abundance of cell cholesterol is tightly coupled to that of its polar lipid partners through active cholesterol.

Extracellular protein disulfide isomerase regulates coagulation on endothelial cells through modulation of phosphatidylserine exposure

Tissue factor (TF) is the cellular receptor for plasma protease factor VIIa (FVIIa), and the TF-FVIIa complex initiates coagulation in both hemostasis and thrombosis. Cell surface-exposed TF is mainly cryptic and requires activation to fully exhibit the procoagulant potential. Recently, the protein disulfide isomerase (PDI) has been hypothesized to regulate TF decryption through the redox switch of an exposed disulfide in TF extracellular domain. In this study, we analyzed PDI contribution to coagulation using an in vitro endothelial cell model. In this model, extracellular PDI is detected by imaging and flow cytometry. Inhibition of cell surface PDI induces a marked increase in TF procoagulant function, whereas exogenous addition of PDI inhibits TF decryption. The coagulant effects of PDI inhibition were sensitive to annexin V treatment, suggesting exposure of phosphatidylserine (PS), which was confirmed by prothrombinase assays and direct labeling. In contrast, exogenous PDI addition enhanced PS internalization. Analysis of fluorescent PS revealed that PDI affects both the apparent flippase and floppase activities on endothelial cells. In conclusion, we identified a new mechanism for PDI contribution to coagulation on endothelial cells, namely, the regulation of PS exposure, where PDI acts as a negative regulator of coagulation.

Influence of nonequilibrium lipid transport, membrane compartmentalization, and membrane proteins on the lateral organization of the plasma membrane

Compositional lipid domains (lipid rafts) in plasma membranes are believed to be important components of many cellular processes. The mechanisms by which cells regulate the sizes, lifetimes, and spatial localization of these domains are rather poorly understood at the moment. We propose a robust mechanism for the formation of finite-sized lipid raft domains in plasma membranes, the competition between phase separation in an immiscible lipid system and active cellular lipid transport processes naturally leads to the formation of such domains. Simulations of a continuum model reveal that the raft size distribution is broad and the average raft size is strongly dependent on the rates of cellular and interlayer lipid transport processes. We demonstrate that spatiotemporal variations in the recycling may enable the cell to localize larger raft aggregates at specific parts along the membrane. Moreover, we show that membrane compartmentalization may further facilitate spatial localization of the raft domains. Finally, we demonstrate that local interactions with immobile membrane proteins can spatially localize the rafts and lead to further clustering.

Lipid asymmetry in plant plasma membranes: phosphate deficiency-induced phospholipid replacement is restricted to the cytosolic leaflet

As in other eukaryotes, plant plasma membranes contain sphingolipids, phospholipids, and free sterols. In addition, plant plasma membranes also contain sterol derivatives and usually <5 mol% of a galactolipid, digalactosyldiacylglycerol (DGDG). We earlier reported that compared to fully fertilized oats (Avena sativa), oats cultivated without phosphate replaced up to 70 mol% of the root plasma membrane phospholipids with DGDG. Here, we investigated the implications of a high DGDG content on membrane properties. The phospholipid-to-DGDG replacement almost exclusively occurred in the cytosolic leaflet, where DGDG constituted up to one-third of the lipids. In the apoplastic (exoplasmic) leaflet, as well as in rafts, phospholipids were not replaced by DGDG, but by acylated sterol glycosides. Liposome studies revealed that the chain ordering in free sterol/phospholipid mixtures clearly decreased when >5 mol% DGDG was included. As both the apoplastic plasma membrane leaflet (probably the major water permeability barrier) and rafts both contain only trace amounts of DGDG, we conclude that this lipid class is not compatible with membrane functions requiring a high degree of lipid order. By not replacing phospholipids site specifically with DGDG, negative functional effects of this lipid in the plasma membrane are avoided.-Tjellström, H., Hellgren, L. I., Wieslander, A., Sandelius, A. S. Lipid asymmetry in plant plasma membranes: phosphate deficiency-induced phospholipid replacement is restricted to the cytosolic leaflet.

Asymmetrical stress generated by the erythrocyte lipid flippase triggers multiple bud formation on the surface of spherical giant liposomes

Proteo-giant liposomes were electroformed from a mixture of lecithin vesicles and inside-out vesicles from erythrocytes. After addition of Mg-ATP in the vicinity of the proteo-giant liposomes, small buds appeared on the liposome surfaces, which--via an increase in lipids in the outer monolayer--demonstrated the active transport of lipids from the inner to the outer monolayer, indicating flippase activity.

Bioinformatic characterization of p-type ATPases encoded within the fully sequenced genomes of 26 eukaryotes

P-type ATPases play essential roles in numerous processes, which in humans include nerve impulse propagation, relaxation of muscle fibers, secretion and absorption in the kidney, acidification of the stomach and nutrient absorption in the intestine. Published evidence suggests that uncharacterized families of P-type ATPases with novel specificities exist. In this study, the fully sequenced genomes of 26 eukaryotes, including animals, plants, fungi and unicellular eukaryotes, were analyzed for P-type ATPases. We report the organismal distributions, phylogenetic relationships, probable topologies and conserved motifs of nine functionally characterized families and 13 uncharacterized families of these enzyme transporters. We have classified these proteins according to the conventions of the functional and phylogenetic IUBMB-approved transporter classification system ( www.tcdb.org , Saier et al. in Nucleic Acids Res 34:181-186, 2006; Nucleic Acids Res 37:274-278, 2009).

Activity of the bile salt export pump (ABCB11) is critically dependent on canalicular membrane cholesterol content

Mutations in ATP8B1 cause severe inherited liver disease. The disease is characterized by impaired biliary bile salt excretion (cholestasis), but the mechanism whereby impaired ATP8B1 function results in cholestasis is poorly understood. ATP8B1 is a type 4 P-type ATPase and is a flippase for phosphatidylserine. Atp8b1-deficient mice display a dramatic increase in the biliary extraction of cholesterol from the canalicular (apical) membrane of the hepatocyte. Here we studied the hypothesis that disproportionate cholesterol extraction from the canalicular membrane impairs the activity of the bile salt transporter, ABCB11, and as a consequence causes cholestasis. Using single pass liver perfusions, we show that not only ABCB11-mediated transport but also Abcc2-mediated transport were reduced at least 4-fold in Atp8b1 deficiency. We show that canalicular membranes of cholestatic Atp8b1-deficient mice have a dramatically reduced cholesterol to phospholipid ratio, i.e. 0.75 +/- 0.24 versus 2.03 +/- 0.71 for wild type. In vitro depletion of cholesterol from mouse liver plasma membranes using methyl-beta-cyclodextrin demonstrated a near linear relation between cholesterol content of the membranes and ATP-dependent taurocholate transport. Abcc2-mediated transport activity was not affected up to 30% of membrane cholesterol depletion but declined to negligible levels at 70% of membrane cholesterol depletion. These effects were reversible as cholesterol repletion of the liver membranes completely restored Abcb11- and Abcc2-mediated transport. Our data demonstrate that membrane cholesterol content is a critical determinant of ABCB11/ABCC2 transport activity, provide an explanation for the etiology of ATP8B1 disease, and suggest a novel mechanism protecting the canalicular membrane against luminal bile salt overload.

Phosphatidylserine targets single-walled carbon nanotubes to professional phagocytes in vitro and in vivo

Broad applications of single-walled carbon nanotubes (SWCNT) dictate the necessity to better understand their health effects. Poor recognition of non-functionalized SWCNT by phagocytes is prohibitive towards controlling their biological action. We report that SWCNT coating with a phospholipid "eat-me" signal, phosphatidylserine (PS), makes them recognizable in vitro by different phagocytic cells - murine RAW264.7 macrophages, primary monocyte-derived human macrophages, dendritic cells, and rat brain microglia. Macrophage uptake of PS-coated nanotubes was suppressed by the PS-binding protein, Annexin V, and endocytosis inhibitors, and changed the pattern of pro- and anti-inflammatory cytokine secretion. Loading of PS-coated SWCNT with pro-apoptotic cargo (cytochrome c) allowed for the targeted killing of RAW264.7 macrophages. In vivo aspiration of PS-coated SWCNT stimulated their uptake by lung alveolar macrophages in mice. Thus, PS-coating can be utilized for targeted delivery of SWCNT with specified cargoes into professional phagocytes, hence for therapeutic regulation of specific populations of immune-competent cells.

Lipid flip-flop driven mechanical and morphological changes in model membranes

We study, using dissipative particle dynamics simulations, the effect of active lipid flip-flop on model fluid bilayer membranes. We consider both cases of symmetric as well as asymmetric flip-flops. Symmetric flip-flop leads to a steady state of the membrane with an effective temperature higher than that of the equilibrium membrane and an effective surface tension lower than that of the equilibrium membrane. Asymmetric flip-flop leads to transient conformational changes in the membrane in the form of bud or blister formation, depending on the flip rate.

The putative aminophospholipid translocases, DNF1 and DNF2, are not required for 7-nitrobenz-2-oxa-1,3-diazol-4-yl-phosphatidylserine flip across the plasma membrane of Saccharomyces cerevisiae

The regulation of phosphatidylserine (PS) distribution across the plasma membrane of eukaryotic cells has been implicated in numerous cell functions (e.g. apoptosis and coagulation). In a recent study, fluorescent phospholipids labeled in the acyl chain with 7-nitrobenz-2-oxa-1, 3-diazol-4-yl (NBD) were used to identify two members of the P4 subfamily of P-type ATPases, Dnf1p and Dnf2p, that are necessary for the inward-directed transport of phospholipids across the plasma membrane (flip) of yeast ( Pomorski, T., Lombardi, R., Riezman, H., Devaux, P. F., Van Meer, G., and Holthuis, J. C. (2003) Mol. Biol. Cell 14, 1240-1254 ). Herein, we present evidence that the flip of NBD-labeled PS (NBD-PS) across the plasma membrane does not require the expression of Dnf1p or Dnf2p. In strains in which DNF1 and DNF2 are both deleted, the flip of NBD-PS is increased approximately 2-fold over that of the isogenic parent strain, whereas the flip of NBD-labeled phosphatidylcholine and NBD-labeled phosphatidylethanolamine are reduced to approximately 20 and approximately 50%, respectively. The mechanism responsible for NBD-PS flip is similar to that for NBD-labeled phosphatidylcholine and NBD-labeled phosphatidylethanolamine in its dependence on cellular ATP and the plasma membrane proton electrochemical gradient, as well as its regulation by the transcription factors Pdr1p and Pdr3p. Based on the observation that deletion or inactivation of all four members of the DRS2/DNF essential subfamily of P-type ATPases does not affect NBD-PS flip, we conclude that the activity reflected by NBD-PS internalization is not the essential function of the DRS2/DNF subfamily of P-type ATPases.

Coincident exposure of phosphatidylethanolamine and anionic phospholipids on the surface of irradiated cells

The major anionic phospholipid, phosphatidylserine (PS), and the neutral phospholipid, phosphatidylethanolamine (PE), are largely confined to the inner leaflet of the plasma membrane bilayer in mammalian cells under normal conditions. This asymmetry is lost when cells undergo apoptosis, become activated, or are exposed to irradiation, reactive oxygen species or certain drugs. It is not known whether exposure of anionic phospholipids (APLs) and PE occurs simultaneously or in the same region of the plasma membrane. Here we examined the coincidence of exposure of APLs and PE on the surface of bovine aortic endothelial cells and NS0 myeloma cells after irradiation. The cells were irradiated (5 Gy) and stained for APLs and PE using liposomes coated with either an Fab' fragment of a PS-binding antibody (bavituximab) or a PE-binding peptide (duramycin). Using live cell imaging and flow cytometry, we showed that irradiation leads to synchronous externalization of APLs and PE. The time course of appearance of APLs and PE on the cell surface was the same and the two phospholipid types remained colocalized over time. Distinct patches double positive for APLs and PE were visible. Larger areas of APLs and PE appeared to have detached from the cytoskeleton to form membrane blebs which protruded and drifted on the cell surface. We conclude that APLs and PE coincidently appear on the external leaflet of the plasma membrane of cells after irradiation. Probably, this is because PE and the major APL, PS, share common regulatory mechanisms of translocation.

Structure of pulmonary surfactant membranes and films: the role of proteins and lipid-protein interactions

The pulmonary surfactant system constitutes an excellent example of how dynamic membrane polymorphism governs some biological functions through specific lipid-lipid, lipid-protein and protein-protein interactions assembled in highly differentiated cells. Lipid-protein surfactant complexes are assembled in alveolar pneumocytes in the form of tightly packed membranes, which are stored in specialized organelles called lamellar bodies (LB). Upon secretion of LBs, surfactant develops a membrane-based network that covers rapidly and efficiently the whole respiratory surface. This membrane-based surface layer is organized in a way that permits efficient gas exchange while optimizing the encounter of many different molecules and cells at the epithelial surface, in a cross-talk essential to keep the whole organism safe from potential pathogenic invaders. The present review summarizes what is known about the structure of the different forms of surfactant, with special emphasis on current models of the molecular organization of surfactant membrane components. The architecture and the behaviour shown by surfactant structures in vivo are interpreted, to some extent, from the interactions and the properties exhibited by different surfactant models as they have been studied in vitro, particularly addressing the possible role played by surfactant proteins. However, the limitations in structural complexity and biophysical performance of surfactant preparations reconstituted in vitro will be highlighted in particular, to allow for a proper evaluation of the significance of the experimental model systems used so far to study structure-function relationships in surfactant, and to define future challenges in the design and production of more efficient clinical surfactants.

Protein kinases Fpk1p and Fpk2p are novel regulators of phospholipid asymmetry

Type 4 P-type ATPases (flippases) are implicated in the generation of phospholipid asymmetry in membranes by the inward translocation of phospholipids. In budding yeast, the DRS2/DNF family members Lem3p-Dnf1p/Dnf2p and Cdc50p-Drs2p are putative flippases that are localized, respectively, to the plasma membrane and endosomal/trans-Golgi network (TGN) compartments. Herein, we identified a protein kinase gene, FPK1, as a mutation that exhibited synthetic lethality with the cdc50Delta mutation. The kinase domain of Fpk1p exhibits high homology to plant phototropins and the fungus Neurospora crassa NRC-2, both of which have membrane-associated functions. Simultaneous disruption of FPK1 and its homolog FPK2 phenocopied the lem3Delta/dnf1Delta dnf2Delta mutants, exhibiting the impaired NBD-labeled phospholipid uptake, defects in the early endosome-to-TGN pathway in the absence of CDC50, and hyperpolarized bud growth after exposure of phosphatidylethanolamine at the bud tip. The fpk1Delta fpk2Delta mutation did not affect the subcellular localization of Lem3p-Dnf1p or Lem3p-Dnf2p. Further, the purified glutathione S-transferase (GST)-fused kinase domain of Fpk1p phosphorylated immunoprecipitated Dnf1p and Dnf2p to a greater extent than Drs2p. We propose that Fpk1p/Fpk2p are upstream activating protein kinases for Lem3p-Dnf1p/Dnf2p.

The proton electrochemical gradient across the plasma membrane of yeast is necessary for phospholipid flip

Recently, two members of the P4 family of P-type ATPases, Dnf1p and Dnf2p, were shown to be necessary for the internalization (flip) of fluorescent, 7-nitrobenz-2-oxa-1,3-diazol-4-yl(NBD)-labeled phospholipids across the plasma membrane of Saccharomyces cerevisiae. In the current study, we have demonstrated that ATP hydrolysis is not sufficient for phospholipid flip in the absence of the proton electrochemical gradient across the plasma membrane. This requirement was demonstrated by two independent means. First, collapse of the plasma membrane proton electrochemical gradient by the protonophore, carbonyl cyanide m-chlorophenylhydrazone (CCCP) almost completely blocked NBD-phospholipid flip while only moderately reducing the cytosolic ATP concentration. Second, strains with point mutations in PMA1, which encodes the plasma membrane proton pump that generates the proton electrochemical gradient, are defective in NBD-PC flip, whereas their cytosolic ATP content is actually increased. These results establish that the proton electrochemical gradient is required for NBD-phospholipid flip across the plasma membrane of yeast and raise the question whether it contributes an additional required driving force or whether it functions as a regulatory signal.

Nitrosative stress inhibits the aminophospholipid translocase resulting in phosphatidylserine externalization and macrophage engulfment: implications for the resolution of inflammation

Macrophage recognition of apoptotic cells depends on externalization of phosphatidylserine (PS), which is normally maintained within the cytosolic leaflet of the plasma membrane by aminophospholipid translocase (APLT). APLT is sensitive to redox modifications of its -SH groups. Because activated macrophages produce reactive oxygen and nitrogen species, we hypothesized that macrophages can directly participate in apoptotic cell clearance by S-nitrosylation/oxidation and inhibition of APLT causing PS externalization. Here we report that exposure of target HL-60 cells to nitrosative stress inhibited APLT, induced PS externalization, and enhanced recognition and elimination of "nitrosatively" modified cells by RAW 264.7 macrophages. Using S-nitroso-L-cysteine-ethyl ester (SNCEE) and S-nitrosoglutathione (GSNO) that cause intracellular and extracellular trans-nitrosylation of proteins, respectively, we found that SNCEE (but not GSNO) caused significant S-nitrosylation/oxidation of thiols in HL-60 cells. SNCEE also strongly inhibited APLT, activated scramblase, and caused PS externalization. However, SNCEE did not induce caspase activation or nuclear condensation/fragmentation suggesting that PS externalization was dissociated from the common apoptotic pathway. Dithiothreitol reversed SNCEE-induced S-nitrosylation, APLT inhibition, and PS externalization. SNCEE but not GSNO stimulated phagocytosis of HL-60 cells. Moreover, phagocytosis of target cells by lipopolysaccharide-stimulated macrophages was significantly suppressed by an NO. scavenger, DAF-2. Thus, macrophage-induced nitrosylation/oxidation plays an important role in cell clearance, and hence in the resolution of inflammation.

Yeast P4-ATPases Drs2p and Dnf1p are essential cargos of the NPFXD/Sla1p endocytic pathway

Drs2p family P-type ATPases (P4-ATPases) are required in multiple vesicle-mediated protein transport steps and are proposed to be phospholipid translocases (flippases). The P4-ATPases Drs2p and Dnf1p cycle between the exocytic and endocytic pathways, and here we define endocytosis signals required by these proteins to maintain a steady-state localization to internal organelles. Internalization of Dnf1p from the plasma membrane uses an NPFXD endocytosis signal and its recognition by Sla1p, part of an endocytic coat/adaptor complex with clathrin, Pan1p, Sla2p/End4p, and End3p. Drs2p has multiple endocytosis signals, including two NPFXDs near the C terminus and PEST-like sequences near the N terminus that may mediate ubiquitin (Ub)-dependent endocytosis. Drs2p localizes to the trans-Golgi network in wild-type cells and accumulates on the plasma membrane when both the Ub- and NPFXD-dependent endocytic mechanisms are inactivated. Surprisingly, the pan1-20 temperature-sensitive mutant is constitutively defective for Ub-dependent endocytosis but is not defective for NPFXD-dependent endocytosis at the permissive growth temperature. To sustain viability of pan1-20, Drs2p must be endocytosed through the NPFXD/Sla1p pathway. Thus, Drs2p is an essential endocytic cargo in cells compromised for Ub-dependent endocytosis. These results demonstrate an essential role for endocytosis in retrieving proteins back to the Golgi, and they define critical cargos of the NPFXD/Sla1p system.

Biophysical properties of lipids and dynamic membranes

The lipid bilayer is a 3D assembly with a rich variety of physical features that modulate cell signaling and protein function. Lateral and transverse forces within the membrane are significant and change rapidly as the membrane is bent or stretched and as new constituents are added, removed or chemically modified. Recent studies have revealed how differences in structure between the two leaflets of the bilayer and between different areas of the bilayer can interact together with membrane deformation to alter the activities of transmembrane channels and peripheral membrane binding proteins. Here, we highlight some recent reports that the physical properties of the membrane can help control the function of transmembrane proteins and the motor-dependent elongation of internal organelles, such as the endoplasmic reticulum.

Diseases of intramembranous lipid transport

The maintenance of transbilayer distribution of phospholipids is crucial for proper cell function. Intramembrane transport of lipids is mediated by three activities termed floppases, flippases, and scramblases. Members of the ATP-binding cassette transporter family and P-type ATPase superfamily have been implicated in the translocation of lipids. The importance of these activities is exemplified by several severe human inherited disorders that are caused by defects in intramembranous transport of lipids. In order to elucidate the molecular mechanisms that underlie these disorders, the combination of in vivo, biochemical, and structural analyses on intramembrane transporters is crucial.