Aminophospholipid asymmetry: A matter of life and death

Measurement of the binding parameters of annexin derivative-erythrocyte membrane interactions

Erythrocyte ghosts prepared from fresh blood expressed phosphatidylserine (PS) on the membrane surfaces in a rather stable fashion. The binding of fluorescein-5-isothiocyanate (FITC)-labeled annexin V (ANV) derivatives to these membranes was studied by titration with proteins and with calcium. Whereas the preaddition of ethylenediaminetetraacetic acid (EDTA) to reaction mixtures totally prevented membrane binding, Ca(2+)-dependent binding was only partially reversed by EDTA treatment, consistent with an initial Ca(2+)-dependent binding that became partially Ca(2+) independent. Data derived from saturation titration with ANV derivatives poorly fit the simple protein-membrane equilibrium binding equation and showed negative cooperativity of binding with increasing membrane occupancy. In contrast, calcium titration at low binding site occupancy resulted in excellent fit into the protein-Ca(2+)-membrane equilibrium binding equation. Calcium titrations of FITC-labeled ANV and ANV-6L15 (a novel ANV-Kunitz protease inhibitor fusion protein) yielded a Hill coefficient of approximately 4 in both cases. The apparent dissociation constant for ANV-6L15 was approximately 4-fold lower than that of ANV at 1.2-2.5mM Ca(2+). We propose that ANV-6L15 may provide improved detection of PS exposed on the membrane surfaces of pathological cells in vitro and in vivo.

The Role of Lipids in Retrovirus Replication

Retroviruses undergo several critical steps to complete a replication cycle. These include the complex processes of virus entry, assembly, and budding that often take place at the plasma membrane of the host cell. Both virus entry and release involve membrane fusion/fission reactions between the viral envelopes and host cell membranes. Accumulating evidence indicates important roles for lipids and lipid microdomains in virus entry and egress. In this review, we outline the current understanding of the role of lipids and membrane microdomains in retroviral replication.

Death mechanisms in epithelial cells following rotavirus infection, exposure to inactivated rotavirus or genome transfection

Intestinal epithelial cell death following rotavirus infection is associated with villus atrophy and gastroenteritis. Roles for both apoptosis and necrosis in cytocidal activity within rotavirus-infected epithelial cells have been proposed. Additionally, inactivated rotavirus has been reported to induce diarrhoea in infant mice. We further examined the death mechanisms induced in epithelial cell lines following rotavirus infection or inactivated rotavirus exposure. Monolayer integrity changes in MA104, HT-29 and partially differentiated Caco-2 cells following inactivated rotavirus exposure or RRV or CRW-8 rotavirus infection paralleled cell metabolic activity and viability reductions. MA104 cell exposure to rotavirus dsRNA also altered monolayer integrity. Inactivated rotaviruses induced delayed cell function losses that were unrelated to apoptosis. Phosphatidylserine externalization, indicating early apoptosis, occurred in MA104 and HT-29 but not in partially differentiated Caco-2 cells by 11 h after infection. Rotavirus activation of phosphatidylinositol 3-kinase partially protected MA104 and HT-29 cells from early apoptosis. In contrast, activation of the stress-activated protein kinase JNK by rotavirus did not influence apoptosis induction in these cells. RRV infection produced DNA fragmentation, indicating late-stage apoptosis, in fully differentiated Caco-2 cells only. These studies show that the apoptosis initiation and cell death mechanism induced by rotavirus infection depend on cell type and degree of differentiation. Early stage apoptosis resulting from rotavirus infection is probably counter-balanced by virus-induced phosphatidylinositol 3-kinase activation. The ability of inactivated rotaviruses and rotavirus dsRNA to perturb monolayer integrity supports a potential role for these rotavirus components in disease pathogenesis.

Autoimmunity and the clearance of dead cells

To maintain organismal homeostasis, phagocytes engulf dead cells, which are recognized as dead by virtue of a characteristic "eat me" signal exposed on their surface. The dead cells are then transferred to lysosomes, where their cellular components are degraded for reuse. Inefficient engulfment of dead cells activates the immune system, causing disease such as systemic lupus erythematosus, and if the DNA of the dead cells is not properly degraded, the innate immune response becomes activated, leading to severe anemia and chronic arthritis. Here, we discuss how the endogenous components of dead cells activate the immune system through both extracellular and intracellular pathways.

Implications for lipids during replication of enveloped viruses

Enveloped viruses, which include many medically important viruses such as human immunodeficiency virus, influenza virus and hepatitis C virus, are intracellular parasites that acquire lipid envelopes from their host cells. Success of replication is intimately linked to their ability to hijack host cell mechanisms, particularly those related to membrane dynamics and lipid metabolism. Despite recent progress, our knowledge of lipid mediated virus-host interactions remains highly incomplete. In addition, diverse experimental systems are used to study different stages of virus replication thus complicating comparisons. This review aims to present a unifying view of the widely diverse strategies used by enveloped viruses at distinct stages of their replication cycles.

Involvements of the lipid peroxidation product, HNE, in the pathogenesis and progression of Alzheimer's disease

Alzheimer's disease (AD) is an age-related neurodegenerative disorder. A number of hypotheses have been proposed to explain AD pathogenesis. One such hypothesis proposed to explain AD pathogenesis is the oxidative stress hypothesis. Increased levels of oxidative stress markers including the markers of lipid peroxidation such as acrolein, 4-hydroxy-2-trans-nonenal (HNE), malondialdehyde, etc. are found in brains of AD subjects. In this review, we focus principally on research conducted in the area of HNE in the central nervous system (CNS) of AD and mild cognitive impairment (MCI), and further, we discuss likely consequences of lipid peroxidation with respect to AD pathogenesis and progression. Based on the research conducted so far in the area of lipid peroxidation, it is suggested that lipid accessible antioxidant molecules could be a promising therapeutic approach to treat or slow progression of MCI and AD.

Current applications of nanotechnology for magnetic resonance imaging of apoptosis

Apoptosis, or programmed cell death, is a morphologically and biochemically distinct form of cell death, which together with proliferation plays an important role in tissue development and homeostasis. Insufficient apoptosis is important in the pathology of various disorders such as cancer and autoimmune diseases, whereas a high apoptotic activity is associated with myocardial infarction, neurodegenerative diseases, and advanced atherosclerotic lesions. Consequently, apoptosis is recognized as an important therapeutic target, which should be either suppressed, e.g., during an ischemic cardiac infarction, or promoted, e.g., in the treatment of cancerous lesions. Imaging tools to address location, amount, and time course of apoptotic activity non-invasively in vivo are therefore of great clinical use in the evaluation of such therapies. This chapter reviews current literature and new developments in the application of nanoparticles for non-invasive apoptosis imaging. Focus is on functionalized nanoparticle contrast agents for MR imaging and bimodal nanoparticle agents that combine magnetic and fluorescent properties.

Antiphosphatidylserine antibody combined with irradiation damages tumor blood vessels and induces tumor immunity in a rat model of glioblastoma

PURPOSE

The vascular targeting antibody bavituximab is being combined with chemotherapy in clinical trials in cancer patients. Bavituximab targets the membrane phospholipid, phosphatidylserine, complexed with beta2-glycoprotein I. Phosphatidylserine is normally intracellular but becomes exposed on the luminal surface of vascular endothelium in tumors. Phosphatidylserine exposure on tumor vessels is increased by chemotherapy and irradiation. Here, we determined whether treatment with the murine equivalent of bavituximab, 2aG4, combined with irradiation can suppress tumor growth in a rat model of glioblastoma.

EXPERIMENTAL DESIGN

F98 glioma cells were injected into the brains of syngeneic rats where they grow initially as a solid tumor and then infiltrate throughout the brain. Rats with established tumors were treated with 10 Gy whole brain irradiation and 2aG4.

RESULTS

Combination treatment doubled the median survival time of the rats, and 13% of animals were rendered disease free. Neither treatment given individually was as effective. We identified two mechanisms. First, irradiation induced phosphatidylserine exposure on tumor blood vessels and enhanced antibody-mediated destruction of tumor vasculature by monocytes/macrophages. Second, the antibody treatment induced immunity to F98 tumor cells, which are normally weakly immunogenic. Surviving rats were immune to rechallenge with F98 tumor cells. In vitro, 2aG4 enhanced the ability of dendritic cells (DCs) to generate F98-specific cytotoxic T cells. Phosphatidylserine exposure, which is induced on tumor cells by irradiation, likely suppresses tumor antigen presentation, and 2aG4 blocks this tolerogenic effect.

CONCLUSION

Bavituximab combined with radiotherapy holds promise as a vascular targeting and immune enhancement strategy for the treatment of human glioblastoma.

Murine Lupus Autoantibodies Identify Distinct Subsets of Apoptotic Bodies

The specific modification of autoantigens and their redistribution into blebs at the surface of apoptotic cells contribute to the induction of autoimmune responses. Blebs containing fragments of the apoptotic nucleus separate from the remainder of the cell to form membrane-bound sub-cellular particles (SCPs), otherwise known as apoptotic bodies. To determine whether apoptotic bodies containing nuclear antigens represent a defined subset of SCPs, we examined the heterogeneity of particles generated by Jurkat cells following synchronization of the cell cycle by serum withdrawal and inhibition of topoisomerase I by camptothecin. Particles were purified by filtration, incubated in the presence of antinucleosome or anti-cardiolipin autoantibodies, annexin V, and Sytox Orange and analyzed by flow cytometry and confocal microscopy. We demonstrate that nuclear autoantigens are associated with one clearly defined subset of SCPs that can be distinguished from other products of late apoptosis. Our experiments represent an important step towards characterizing the heterogeneity of SCPs that are generated in late apoptosis and identifying their contributions to tolerance and autoimmunity.

Cell-derived microparticles: a new challenge in neuroscience

Microparticles (MPs) are membrane fragments shed by cells activated by a variety of stimuli including serine proteases, inflammatory cytokines, growth factors, and stress inducers. MPs originating from platelets, leukocytes, endothelial cells, and erythrocytes are found in circulating blood at relative concentrations determined by the pathophysiological context. The procoagulant activity of MPs is their most characterized property as a determinant of thrombosis in various vascular and systemic diseases including myocardial infarction and diabetes. An increase in circulating MPs has also been associated with ischemic cerebrovascular accidents, transient ischemic attacks, multiple sclerosis, and cerebral malaria. Recent data indicate that besides their procoagulant components and identity antigens, MPs bear a number of bioactive effectors that can be disseminated, exchanged, and transferred via MPs cell interactions. Furthermore, as activated parenchymal cells may also shed MPs carrying identity antigens and biomolecules, MPs are now emerging as new messengers/biomarkers from a specific tissue undergoing activation or damage. Thus, detection of MPs of neurovascular origin in biological fluids such as CSF or tears, and even in circulating blood in case of blood-brain barrier leakage, would not only improve our comprehension of neurovascular pathophysiology, but may also constitute a powerful tool as a biomarker in disease prediction, diagnosis, prognosis, and follow-up.

Control of protein and sterol trafficking by antagonistic activities of a type IV P-type ATPase and oxysterol binding protein homologue

The oxysterol binding protein homologue Kes1p has been implicated in nonvesicular sterol transport in Saccharomyces cerevisiae. Kes1p also represses formation of protein transport vesicles from the trans-Golgi network (TGN) through an unknown mechanism. Here, we show that potential phospholipid translocases in the Drs2/Dnf family (type IV P-type ATPases [P4-ATPases]) are downstream targets of Kes1p repression. Disruption of KES1 suppresses the cold-sensitive (cs) growth defect of drs2Delta, which correlates with an enhanced ability of Dnf P4-ATPases to functionally substitute for Drs2p. Loss of Kes1p also suppresses a drs2-ts allele in a strain deficient for Dnf P4-ATPases, suggesting that Kes1p antagonizes Drs2p activity in vivo. Indeed, Drs2-dependent phosphatidylserine translocase (flippase) activity is hyperactive in TGN membranes from kes1Delta cells and is potently attenuated by addition of recombinant Kes1p. Surprisingly, Drs2p also antagonizes Kes1p activity in vivo. Drs2p deficiency causes a markedly increased rate of cholesterol transport from the plasma membrane to the endoplasmic reticulum (ER) and redistribution of endogenous ergosterol to intracellular membranes, phenotypes that are Kes1p dependent. These data suggest a homeostatic feedback mechanism in which appropriately regulated flippase activity in the Golgi complex helps establish a plasma membrane phospholipid organization that resists sterol extraction by a sterol binding protein.

Lipids and membrane microdomains in HIV-1 replication

Several critical steps in the replication cycle of human immunodeficiency virus type 1 (HIV-1) - entry, assembly and budding - are complex processes that take place at the plasma membrane of the host cell. A growing body of data indicates that these early and late steps in HIV-1 replication take place in specialized plasma membrane microdomains, and that many of the viral and cellular components required for entry, assembly, and budding are concentrated in these microdomains. In particular, a number of studies have shown that cholesterol- and sphingolipid-enriched microdomains known as lipid rafts play important roles in multiple steps in the virus replication cycle. In this review, we provide an overview of what is currently known about the involvement of lipids and membrane microdomains in HIV-1 replication.

Exogenous phosphatidylethanolamine induces apoptosis of human hepatoma HepG2 cells via the bcl-2/bax pathway

AIM: To investigate the signaling pathways implicated in phosphatidylethanolamine (PE)-induced apoptosis of human hepatoma HepG2 cells.

METHODS: Inhibitory effects of PE on human hepatoma HepG2 cells were detected by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. Cell cycle, apoptosis and mitochondrial transmembrane potential (ΔΨm) were analyzed by flow cytometry. Immunocytochemical assay and Western blotting were used to examine Bcl-2, Bax and caspase-3 protein levels in HepG2 cells treated with PE.

RESULTS: PE inhibited the growth of HepG2 cells in a dose- and time- dependent manner. It did not affect the cell cycle, but induced apoptosis. PE significantly decreased δΨm at 0.25, 0.5 and 1 mmol/L, respectively, suggesting that PE induces cell apoptosis by decreasing the mitochondrial transmembrane potential. The Bcl-2 expression level induced by different concentrations of PE was lower than that in control groups. However, the Bax expression level induced by PE was higher than that in the control group. Meanwhile, PE increased the caspase-3 expression in a dose- and time-dependent manner.

CONCLUSION: Exogenous PE induces apoptosis of human hepatoma HepG2 cells via the bcl-2/bax pathway.

Linking phospholipid flippases to vesicle-mediated protein transport

Type IV P-type ATPases (P4-ATPases) are a large family of putative phospholipid translocases (flippases) implicated in the generation of phospholipid asymmetry in biological membranes. P4-ATPases are typically the largest P-type ATPase subgroup found in eukaryotic cells, with five members in Saccharomyces cerevisiae, six members in Caenorhabditis elegans, 12 members in Arabidopsis thaliana and 14 members in humans. In addition, many of the P4-ATPases require interaction with a noncatalytic subunit from the CDC50 gene family for their transport out of the endoplasmic reticulum (ER). Deficiency of a P4-ATPase (Atp8b1) causes liver disease in humans, and studies in a variety of model systems indicate that P4-ATPases play diverse and essential roles in membrane biogenesis. In addition to their proposed role in establishing and maintaining plasma membrane asymmetry, P4-ATPases are linked to vesicle-mediated protein transport in the exocytic and endocytic pathways. Recent studies have also suggested a role for P4-ATPases in the nonvesicular intracellular trafficking of sterols. Here, we discuss the physiological requirements for yeast P4-ATPases in phospholipid translocase activity, transport vesicle budding and ergosterol metabolism, with an emphasis on Drs2p and its noncatalytic subunit, Cdc50p.

P4 ATPases - lipid flippases and their role in disease

P4 ATPases (type 4 P-type ATPases) are multispan transmembrane proteins that have been implicated in phospholipid translocation from the exoplasmic to the cytoplasmic leaflet of biological membranes. Studies in Saccharomyces cerevisiae have indicated that P4 ATPases are important in vesicle biogenesis and are required for vesicular trafficking along several intracellular vesicular transport routes. Although little is known about mammalian P4 ATPases, some members of this subfamily appear to be associated with human disease or mouse pathophysiology. ATP8B1, a phosphatidylserine translocase, is the most extensively studied mammalian P4 ATPase. This protein is important for maintaining the detergent resistant properties of the apical membrane of the hepatocyte. Mutations in ATP8B1 give rise to severe liver disease. Furthermore, a role for Atp8b3 in mouse sperm cell capacitation has been suggested, whereas deficiency of Atp10a and Atp10d leads to insulin resistance and obesity in mice. Here we review the present status on the pathophysiological consequences of P4 ATPase deficiency.

Mechanism and significance of P4 ATPase-catalyzed lipid transport: lessons from a Na+/K+-pump

Members of the P(4) subfamily of P-type ATPases are believed to catalyze phospholipid transport across membrane bilayers, a process influencing a host of cellular functions. Atomic structures and functional analysis of P-type ATPases that pump small cations and metal ions revealed a transport mechanism that appears to be conserved throughout the family. A challenging problem is to understand how this mechanism is adapted in P(4) ATPases to flip phospholipids. P(4) ATPases form oligomeric complexes with members of the CDC50 protein family. While formation of these complexes is required for P(4) ATPase export from the endoplasmic reticulum, little is known about the functional role of the CDC50 subunits. The Na(+)/K(+)-ATPase and closely-related H(+)/K(+)-ATPase are the only other P-type pumps that are oligomeric, comprising mandatory beta-subunits that are strikingly reminiscent of CDC50 proteins. Besides serving a role in the functional maturation of the catalytic alpha-subunit, the beta-subunit also contributes specifically to intrinsic transport properties of the Na(+)/K(+) pump. As beta-subunits and CDC50 proteins likely adopted similar structures to accomplish analogous tasks, current knowledge of the Na(+)/K(+)-ATPase provides a useful guide for understanding the inner workings of the P(4) ATPase class of lipid pumps.

Mechanisms of selective antimicrobial activity of gaegurin 4

Gaegurin 4 (GGN4), an antimicrobial peptide isolated from a Korean frog, is five times more potent against Gram-positive than Gram-negative bacteria, but has little hemolytic activity. To understand the mechanism of such cell selectivity, we examined GGN4-induced K(+) efflux from target cells, and membrane conductances in planar lipid bilayers. The K(+) efflux from Gram-positive M. luteus (2.5 microg/ml) was faster and larger than that from Gram-negative E. coli (75 microg/ml), while that from RBC was negligible even at higher concentration (100 microg/ml). GGN4 induced larger conductances in the planar bilayers which were formed with lipids extracted from Gram-positive B. subtilis than in those from E. coli (p<0.01), however, the effects of GGN4 were not selective in the bilayers formed with lipids from E. coli and red blood cells. Addition of an acidic phospholipid, phosphatidylserine to planar bilayers increased the GGN4-induced membrane conductance (p<0.05), but addition of phosphatidylcholine or cholesterol reduced it (p<0.05). Transmission electron microscopy revealed that GGN4 induced pore-like damages in M. luteus and dis-layering damages on the outer wall of E. coli. Taken together, the present results indicate that the selectivity of GGN4 toward Gram-positive over Gram-negative bacteria is due to negative surface charges, and interaction of GGN4 with outer walls. The selectivity toward bacteria over RBC is due to the presence of phosphatidylcholine and cholesterol, and the trans-bilayer lipid asymmetry in RBC. The results suggest that design of selective antimicrobial peptides should be based on the composition and topology of membrane lipids in the target cells.

Transmembrane voltage regulates binding of annexin V and lactadherin to cells with exposed phosphatidylserine

BACKGROUND

Cells expose phosphatidylserine during apoptosis. The voltage across the plasma membrane also decreases or disappears during apoptosis, but the physiological significance of this is unknown.

RESULTS

Here we show that transmembrane potential regulates membrane binding of two unrelated proteins that recognize exposed phosphatidylserine on apoptotic cells. In Jurkat T leukemia cells and K562 promyelocytic leukemia cells undergoing apoptosis, extracellular binding of annexin V was increased by decreasing membrane potential in a dose-dependent manner. Studies with phospholipid vesicles showed that the effect was mediated via an increase in binding affinity. The effect was independent of the apoptotic stimulus. The same phenomenon occurred with lactadherin, a structurally unrelated protein that also binds to apoptotic cells via phosphatidylserine and is essential for in vivo clearance of dying cells.

CONCLUSION

Alterations in membrane potential regulate the binding of annexin V and lactadherin to cell membranes, and may also influence the membrane binding of other classes of phosphatidylserine-binding proteins.

P2X7 receptors regulate multiple types of membrane trafficking responses and non-classical secretion pathways

Activation of the P2X7 receptor (P2X7R) triggers a remarkably diverse array of membrane trafficking responses in leukocytes and epithelial cells. These responses result in altered profiles of cell surface lipid and protein composition that can modulate the direct interactions of P2X7R-expressing cells with other cell types in the circulation, in blood vessels, at epithelial barriers, or within sites of immune and inflammatory activation. Additionally, these responses can result in the release of bioactive proteins, lipids, and large membrane complexes into extracellular compartments for remote communication between P2X7R-expressing cells and other cells that amplify or modulate inflammation, immunity, and responses to tissue damages. This review will discuss P2X7R-mediated effects on membrane composition and trafficking in the plasma membrane (PM) and intracellular organelles, as well as actions of P2X7R in controlling various modes of non-classical secretion. It will review P2X7R regulation of: (1) phosphatidylserine distribution in the PM outer leaflet; (2) shedding of PM surface proteins; (3) release of PM-derived microvesicles or microparticles; (4) PM blebbing; (5) cell-cell fusion resulting in formation of multinucleate cells; (6) phagosome maturation and fusion with lysosomes; (7) permeability of endosomes with internalized pathogen-associated molecular patterns; (8) permeability/integrity of mitochondria; (9) exocytosis of secretory lysosomes; and (10) release of exosomes from multivesicular bodies.

Polyethylene glycols interact with membrane glycerophospholipids: is this part of their mechanism for hypothermic graft protection?

Polyethylene glycol (PEG), a high-molecular-weight colloid present in new organ preservation solutions, protects against cold ischemia injuries leading to better graft function of transplanted organs. This protective effect cannot be totally explained by immuno-camouflaging property or signaling-pathway modifications. Therefore, we sought for an alternative mechanism dependent on membrane fluidity. Using the Langmuir-Pockles technique, we show here that PEGs interacted with lipid monolayers of defined composition or constituted by a renal cell lipid extract. High-molecular-weight PEGs stabilized the lipid monolayer at low surface pressure. Paradoxically, at high surface pressure, PEGs destabilized the monolayers. Hypothermia reduced the destabilization of saturated monolayer whereas unsaturated monolayer remained unaffected. Modification of ionic strength and pH induced a stronger stabilizing effect of PEG 35,000 Da which could explain its reported higher effectiveness on cold-induced injuries during organ transplantation. This study sheds a new light on PEG protective effects during organ preservation different from all classical hypotheses.

Mobilization of lysosomal calcium regulates the externalization of phosphatidylserine during apoptosis

A hallmark of apoptotic cells is the Ca2+-dependent appearance of phosphatidylserine (PS) at the cell surface as a result of its redistribution from the inner-to-outer plasma membrane leaflet. Although endoplasmic reticulum and mitochondrial Ca2+ are known to participate in apoptosis, their role in PS externalization has not been established. In this study, several organelle-specific fluorescent markers and Ca2+-sensitive probes were used to identify the source of Ca2+ critical to PS externalization. By employing Rhod-2AM, fluorescein-labeled high molecular weight dextran, and Calcium Green 1, we provide evidence that lysosomes respond to apoptotic stimuli by releasing their luminal Ca2+ to the cytosol. Cells treated with the cytosolic phospholipase A2 inhibitor, cPLA2alpha, had no effect on caspase activation but exhibited a significant decrease in lysosomal Ca2+ release and externalization of PS in response to apoptotic stimuli. Similarly, cells depleted of lysosomal Ca2+ underwent programmed cell death yet failed to externalize PS. These data indicate that although Ca2+ release from other intracellular organelles to the cytosol is adequate for apoptosis, the release of Ca2+ from lysosomes is critical for PS externalization.

Targeting inside-out phosphatidylserine as a therapeutic strategy for viral diseases

Targeting exposed anionic phospholipids on a spectrum of virus-infected cells can protect against lethal virus infections in vivo.

Aminoglycoside-induced phosphatidylserine externalization in sensory hair cells is regionally restricted, rapid, and reversible

The aminophospholipid phosphatidylserine (PS) is normally restricted to the inner leaflet of the plasma membrane. During certain cellular processes, including apoptosis, PS translocates to the outer leaflet and can be labeled with externally applied annexin V, a calcium-dependent PS-binding protein. In mouse cochlear cultures, annexin V labeling reveals that the aminoglycoside antibiotic neomycin induces rapid PS externalization, specifically on the apical surface of hair cells. PS externalization is observed within approximately 75 s of neomycin perfusion, first on the hair bundle and then on membrane blebs forming around the apical surface. Whole-cell capacitance also increases significantly within minutes of neomycin application, indicating that blebbing is accompanied by membrane addition to the hair cell surface. PS externalization and membrane blebbing can, nonetheless, occur independently. Pretreating hair cells with calcium chelators, a procedure that blocks mechanotransduction, or overexpressing a phosphatidylinositol 4,5-biphosphate (PIP2)-binding pleckstrin homology domain, can reduce neomycin-induced PS externalization, suggesting that neomycin enters hair cells via transduction channels, clusters PIP2, and thereby activates lipid scrambling. The effects of short-term neomycin treatment are reversible. After neomycin washout, PS is no longer detected on the apical surface, apical membrane blebs disappear, and surface-bound annexin V is internalized, distributing throughout the supranuclear cytoplasm of the hair cell. Hair cells can therefore repair, and recover from, neomycin-induced surface damage. Hair cells lacking myosin VI, a minus-end directed actin-based motor implicated in endocytosis, can also recover from brief neomycin treatment. Internalized annexin V, however, remains below the apical surface, thereby pinpointing a critical role for myosin VI in the transport of endocytosed material away from the periphery of the hair cell.

Magainin 2 in action: distinct modes of membrane permeabilization in living bacterial and mammalian cells

Interactions of cationic antimicrobial peptides with living bacterial and mammalian cells are little understood, although model membranes have been used extensively to elucidate how peptides permeabilize membranes. In this study, the interaction of F5W-magainin 2 (GIGKWLHSAKKFGKAFVGEIMNS), an equipotent analogue of magainin 2 isolated from the African clawed frog Xenopus laevis, with unfixed Bacillus megaterium and Chinese hamster ovary (CHO)-K1 cells was investigated, using confocal laser scanning microscopy. A small amount of tetramethylrhodamine-labeled F5W-magainin 2 was incorporated into the unlabeled peptide for imaging. The influx of fluorescent markers of various sizes into the cytosol revealed that magainin 2 permeabilized bacterial and mammalian membranes in significantly different ways. The peptide formed pores with a diameter of approximately 2.8 nm (< 6.6 nm) in B. megaterium, and translocated into the cytosol. In contrast, the peptide significantly perturbed the membrane of CHO-K1 cells, permitting the entry of a large molecule (diameter, >23 nm) into the cytosol, accompanied by membrane budding and lipid flip-flop, mainly accumulating in mitochondria and nuclei. Adenosine triphosphate and negatively charged glycosaminoglycans were little involved in the magainin-induced permeabilization of membranes in CHO-K1 cells. Furthermore, the susceptibility of CHO-K1 cells to magainin was found to be similar to that of erythrocytes. Thus, the distinct membrane-permeabilizing processes of magainin 2 in bacterial and mammalian cells were, to the best of our knowledge, visualized and characterized in detail for the first time.

Imaging of apoptosis

Cells can die by several pathways, such as accidental death, apoptosis, autophagy, pyroptosis, and oncosis. These are important in normal physiology and many disease states, such as cancer and cardiovascular disease. Specific biochemical changes occur in cells undergoing apoptosis that provide potential targets for molecular imaging agents. Several of these molecular steps have been evaluated to date, including phosphatidylserine exposure at the extracellular face of the plasma membrane, detected by proteins such as annexin V; caspase activation in the intracellular compartment, detected by labeled enzyme substrates or inhibitors; and mitochondrial membrane potential collapse, detected by reduced levels of phosphonium cations that normally accumulate in healthy mitochondria. Phase I clinical trials have been performed with 1 of these agents, annexin V. Future work will likely include development of new agents that detect targets not exploited by current agents, translational research on the significance of imaging the different forms of cell death, and further improvements in the techniques for labeling existing agents to improve sensitivity and reduce nonspecific background.

Droplet interface bilayers

Droplet interface bilayers (DIBs) provide a superior platform for the biophysical analysis of membrane proteins. The versatile DIBs can also form networks, with features that include built-in batteries and sensors.

Chemically induced phospholipid translocation across biological membranes

Chemical means of manipulating the distribution of lipids across biological membranes is of considerable interest for many biomedical applications as a characteristic lipid distribution is vital for numerous cellular functions. Here we employ atomic-scale molecular simulations to shed light on the ability of certain amphiphilic compounds to promote lipid translocation (flip-flops) across membranes. We show that chemically induced lipid flip-flops are most likely pore-mediated: the actual flip-flop event is a very fast process (time scales of tens of nanoseconds) once a transient water defect has been induced by the amphiphilic chemical (dimethylsulfoxide in this instance). Our findings are consistent with available experimental observations and further emphasize the importance of transient membrane defects for chemical control of lipid distribution across cell membranes.

Kidney injury molecule-1 is a phosphatidylserine receptor that confers a phagocytic phenotype on epithelial cells

Following injury, the clearance of apoptotic and necrotic cells is necessary for mitigation and resolution of inflammation and tissue repair. In addition to macrophages, which are traditionally assigned to this task, neighboring epithelial cells in the affected tissue are postulated to contribute to this process. Kidney injury molecule-1 (KIM-1 or TIM-1) is an immunoglobulin superfamily cell-surface protein not expressed by cells of the myeloid lineage but highly upregulated on the surface of injured kidney epithelial cells. Here we demonstrate that injured kidney epithelial cells assumed attributes of endogenous phagocytes. Confocal images confirm internalization of apoptotic bodies within KIM-1-expressing epithelial cells after injury in rat kidney tubules in vivo. KIM-1 was directly responsible for phagocytosis in cultured primary rat tubule epithelial cells and also porcine and canine epithelial cell lines. KIM-1 was able to specifically recognize apoptotic cell surface-specific epitopes phosphatidylserine, and oxidized lipoproteins, expressed by apoptotic tubular epithelial cells. Thus, KIM-1 is the first nonmyeloid phosphatidylserine receptor identified to our knowledge that transforms epithelial cells into semiprofessional phagocytes.

Massive Ca-induced membrane fusion and phospholipid changes triggered by reverse Na/Ca exchange in BHK fibroblasts

Baby hamster kidney (BHK) fibroblasts increase their cell capacitance by 25-100% within 5 s upon activating maximal Ca influx via constitutively expressed cardiac Na/Ca exchangers (NCX1). Free Ca, measured with fluo-5N, transiently exceeds 0.2 mM with total Ca influx amounting to approximately 5 mmol/liter cell volume. Capacitance responses are half-maximal when NCX1 promotes a free cytoplasmic Ca of 0.12 mM (Hill coefficient approximately 2). Capacitance can return to baseline in 1-3 min, and responses can be repeated several times. The membrane tracer, FM 4-64, is taken up during recovery and can be released at a subsequent Ca influx episode. Given recent interest in signaling lipids in membrane fusion, we used green fluorescent protein (GFP) fusions with phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)) and diacylglycerol (DAG) binding domains to analyze phospholipid changes in relation to these responses. PI(4,5)P(2) is rapidly cleaved upon activating Ca influx and recovers within 2 min. However, PI(4,5)P(2) depletion by activation of overexpressed hM1 muscarinic receptors causes only little membrane fusion, and subsequent fusion in response to Ca influx remains massive. Two results suggest that DAG may be generated from sources other than PI(4,5)P in these protocols. First, acylglycerols are generated in response to elevated Ca, even when PI(4,5)P(2) is metabolically depleted. Second, DAG-binding C1A-GFP domains, which are brought to the cell surface by exogenous ligands, translocate rapidly back to the cytoplasm in response to Ca influx. Nevertheless, inhibitors of PLCs and cPLA2, PI(4,5)P(2)-binding peptides, and PLD modification by butanol do not block membrane fusion. The cationic agents, FM 4-64 and heptalysine, bind profusely to the extracellular cell surface during membrane fusion. While this binding might reflect phosphatidylserine (PS) "scrambling" between monolayers, it is unaffected by a PS-binding protein, lactadherin, and by polylysine from the cytoplasmic side. Furthermore, the PS indicator, annexin-V, binds only slowly after fusion. Therefore, we suggest that the luminal surfaces of membrane vesicles that fuse to the plasmalemma may be rather anionic. In summary, our results provide no support for any regulatory or modulatory role of phospholipids in Ca-induced membrane fusion in fibroblasts.

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.

Vascular imaging of solid tumors in rats with a radioactive arsenic-labeled antibody that binds exposed phosphatidylserine

PURPOSE

We recently reported that anionic phospholipids, principally phosphatidylserine, become exposed on the external surface of vascular endothelial cells in tumors, probably in response to oxidative stresses present in the tumor microenvironment. In the present study, we tested the hypothesis that a chimeric monoclonal antibody that binds phosphatidylserine could be labeled with radioactive arsenic isotopes and used for molecular imaging of solid tumors in rats.

EXPERIMENTAL DESIGN

Bavituximab was labeled with (74)As (beta(+), T(1/2) 17.8 days) or (77)As (beta(-), T(1/2) 1.6 days) using a novel procedure. The radionuclides of arsenic were selected because their long half-lives are consistent with the long biological half lives of antibodies in vivo and because their chemistry permits stable attachment to antibodies. The radiolabeled antibodies were tested for the ability to image subcutaneous Dunning prostate R3227-AT1 tumors in rats.

RESULTS

Clear images of the tumors were obtained using planar gamma-scintigraphy and positron emission tomography. Biodistribution studies confirmed the specific localization of bavituximab to the tumors. The tumor-to-liver ratio 72 h after injection was 22 for bavituximab compared with 1.5 for an isotype-matched control chimeric antibody of irrelevant specificity. Immunohistochemical studies showed that the bavituximab was labeling the tumor vascular endothelium.

CONCLUSIONS

These results show that radioarsenic-labeled bavituximab has potential as a new tool for imaging the vasculature of solid tumors.

Role of C. elegans TAT-1 protein in maintaining plasma membrane phosphatidylserine asymmetry

The asymmetrical distribution of phospholipids on the plasma membrane is critical for maintaining cell integrity and physiology and for regulating intracellular signaling and important cellular events such as clearance of apoptotic cells. How phospholipid asymmetry is established and maintained is not fully understood. We report that the Caenorhabditis elegans P-type adenosine triphosphatase homolog, TAT-1, is critical for maintaining cell surface asymmetry of phosphatidylserine (PS). In animals deficient in tat-1, PS is abnormally exposed on the cell surface, and normally living cells are randomly lost through a mechanism dependent on PSR-1, a PS-recognizing phagocyte receptor, and CED-1, which contributes to recognition and engulfment of apoptotic cells. Thus, tat-1 appears to function in preventing appearance of PS in the outer leaflet of plasma membrane, and ectopic exposure of PS on the cell surface may result in removal of living cells by neighboring phagocytes.

Extracellular annexin A5: functions of phosphatidylserine-binding and two-dimensional crystallization

In normal healthy cells phosphatidylserine is located in the inner leaflet of the plasma membrane. However, on activated platelets, dying cells and under specific circumstances also on various types of viable leukocytes phosphatidylserine is actively externalized to the outer leaflet of the plasma membrane. Annexin A5 has the ability to bind in a calcium-dependent manner to phosphatidylserine and to form a membrane-bound two-dimensional crystal lattice. Based on these abilities various functions for extracellular annexin A5 on the phosphatidylserine-expressing plasma membrane have been proposed. In this review we describe possible mechanisms for externalization of annexin A5 and various processes in which extracellular annexin A5 may play a role such as blood coagulation, apoptosis, phagocytosis and formation of plasma membrane-derived microparticles. We further highlight the recent discovery of internalization of extracellular annexin A5 by phosphatidylserine-expressing cells.

Autoimmune diseases caused by defects in clearing dead cells and nuclei expelled from erythroid precursors

Apoptotic cells are recognized and subsequently engulfed by macrophages and immature dendritic cells. The engulfed dead cells are transported to the lysosomes of macrophages, and their components are degraded into amino acids and nucleotides for reuse. In mammals, macrophages also engulf nuclei expelled from erythroid precursors in the final stage of definitive erythropoiesis. Failure to swiftly engulf dead cells at the germinal centers of lymphoid organs causes systemic lupus erythematosus-type autoimmune diseases. In contrast, failure to efficiently degrade the DNA of dead cells or erythroid cell nuclei activates innate immunity, causing lethal anemia in the fetus and chronic arthritis in adults.

Copper-induced peroxidation of phosphatidylserine-containing liposomes is inhibited by nanomolar concentrations of specific antioxidants

Copper-induced peroxidation of liposomal palmitoyllinoleoyl-phosphatidylcholine (PLPC) is inhibited by alpha-tocopherol at micromolar concentrations. In our previous study we found that when the liposomes contain phosphatidylserine (PS), nanomolar concentrations of Toc were sufficient to inhibit peroxidation. In an attempt to gain understanding of the origin of this extreme antioxidative potency, we tested the antioxidative potency of 36 additional antioxidants and the dependence of their potency on the presence of PS in the liposomes. The results of these studies reveal that only 11 of the tested antioxidants possess similar antioxidative potency to that of Toc. These include trolox, butylated hydroxytoluene (BHT), curcumin, nordihydroguaiaretic acid (NDGA), diethylstilbestrol (DES), 2 of the 13 tested flavonoids (luteolin and 7,3',4'-trihydroxyflavone; T-414), alpha-naphthol, 1,5-, 1,6- and 1,7-dihydroxynaphthalenes (DHNs). Propyl gallate (PG), methyl syringate, rosmarinic acid, resveratrol, other flavonoids, as well as beta-naphthol, 1,2-, 1,3-, 1,4-, 2,3-, 2,6-, and 2,7-DHNs were either moderately antioxidative or pro-oxidative. For liposomes made of PLPC (250 microM) and PS (25 microM) the "lag" preceding copper-induced peroxidation (5 microM copper) was doubled upon addition of 30-130nM of the "super-active" antioxidants. We propose that the mechanism responsible for the extreme antioxidative potency against copper-induced peroxidation in PS-containing liposomes involves replenishment of the antioxidant in a ternary PS-copper-antioxidant complex. Based on structure-activity relationship of the 37 tested antioxidants, the "super-antioxidative potency" is attributed to the recycling of relatively stable semiquinone or semiquinone-like radicals.

Loss of phospholipid asymmetry and elevated brain apoptotic protein levels in subjects with amnestic mild cognitive impairment and Alzheimer disease

Oxidative stress, a hallmark of Alzheimer disease (AD), has been shown to induce lipid peroxidation and apoptosis disrupting cellular homeostasis. Normally, the aminophospholipid phosphatidylserine (PtdSer) is asymmetrically distributed on the cytosolic leaflet of the lipid bilayer. Under oxidative stress conditions, asymmetry is altered, characterized by the appearance of PtdSer on the outer leaflet, to initiate the first stages of an apoptotic process. PtdSer asymmetry is actively maintained by the ATP-dependent translocase flippase, whose function is inhibited if covalently bound by lipid peroxidation products, 4-hydroxynonenal (HNE) and acrolein, within the membrane bilayer in which they are produced. Additionally, pro-apoptotic proteins Bax and caspase-3 have been implemented in the oxidative modification of PtdSer resulting in subsequent asymmetric collapse, while anti-apoptotic protein Bcl-2 has been found to prevent this process. The current investigation focused on detection of PtdSer on the outer leaflet of the bilayer in synaptosomes from brain of subjects with AD and amnestic mild cognitive impairment (MCI), as well as expression levels of apoptosis-related proteins Bcl-2, Bax, and caspase-3. Fluorescence and Western blot analysis suggest PtdSer exposure on the outer leaflet is significantly increased in brain from subjects with MCI and AD contributing to early apoptotic elevation of pro- and anti-apoptotic proteins and finally neuronal loss. MCI is considered a possible transition point between normal cognitive aging and probable AD. Brain from subjects with MCI is reported to have increased levels of tissue oxidation; therefore, the results of this study could mark the progression of patients with MCI into AD. This study contributes to a model of apoptosis-specific oxidation of phospholipids consistent with the notion that PtdSer exposure is required for apoptotic-cell death.

Apoptosis effector mechanisms: a requiem performed in different keys

Apoptosis is the regulated form of cell death utilized by metazoans to remove unneeded, damaged, or potentially deleterious cells. Certain manifestations of apoptosis may be associated with the proteolytic activity of caspases. These changes are often held as hallmarks of apoptosis in dying cells. Consequently, many regard caspases as the central effectors or executioners of apoptosis. However, this "caspase-centric" paradigm of apoptotic cell death does not appear to be as universal as once believed. In fact, during apoptosis the efficacy of caspases may be highly dependent on the cytotoxic stimulus as well as genetic and epigenetic factors. An ever-increasing number of studies strongly suggest that there are effectors in addition to caspases, which are important in generating apoptotic signatures in dying cells. These seemingly caspase-independent effectors may represent evolutionarily redundant or failsafe mechanisms for apoptotic cell elimination. In this review, we will discuss the molecular regulation of caspases and various caspase-independent effectors of apoptosis, describe the potential context and/or limitations of these mechanisms, and explore why the understanding of these processes may have relevance in cancer where treatment is believed to engage apoptosis to destroy tumor cells.

Differences in membrane acyl phospholipid composition between an endothermic mammal and an ectothermic reptile are not limited to any phospholipid class

This study examined questions concerning differences in the acyl composition of membrane phospholipids that have been linked to the faster rates of metabolic processes in endotherms versus ectotherms. In liver, kidney, heart and brain of the ectothermic reptile, Trachydosaurus rugosus, and the endothermic mammal, Rattus norvegicus, previous findings of fewer unsaturates but a greater unsaturation index (UI) in membranes of the mammal versus those of the reptile were confirmed. Moreover, the study showed that the distribution of phospholipid head-group classes was similar in the same tissues of the reptile and mammal and that the differences in acyl composition were present in all phospholipid classes analysed, suggesting a role for the physical over the chemical properties of membranes in determining the faster rates of metabolic processes in endotherms. The most common phosphatidylcholine (PC) molecules present in all tissues (except brain) of the reptile were 16:0/18:1, 16:0/18:2, 18:0/18:2,18:1/18:1 and 18:1/18:2, whereas arachidonic acid (20:4), containing PCs 16:0/20:4, 18:0/20:4, were the common molecules in the mammal. The most abundant phosphatidylethanolamines (PE) used in the tissue of the reptile were 18:0/18:2, 18:0/20:4, 18:1/18:1, 18:1/18:2 and 18:1/20:4, compared to 16:0/18:2, 16:0/20:4, 16:0/22:6, 18:0/20:4, 18:0/22:6 and 18:1/20:4 in the mammal. UI differences were primarily due to arachidonic acid found in both PC and PEs, whereas docosahexaenoic acid (22:6) was a lesser contributor mainly within PEs and essentially absent in the kidney. The phospholipid composition of brain was more similar in the reptile and mammal compared to those of other tissues.

The bacterial peptide pheromone plantaricin A permeabilizes cancerous, but not normal, rat pituitary cells and differentiates between the outer and inner membrane leaflet

Plantaricin A (PlnA) is a 26-mer peptide pheromone with membrane-permeabilizing, strain-specific antibacterial activity, produced by Lactobacillus plantarum C11. We investigated the membrane-permeabilizing effects of PlnA on cultured cancerous and normal rat anterior pituitary cells using patch-clamp techniques and microfluorometry (fura-2). Cancerous cells displayed massive permeabilization within 5 s after exposure to 10-100 microM PlnA. The membrane depolarized to nearly 0 mV, and the membrane resistance decreased to a mere fraction of the initial value after less than 1 min. In outside-out membrane patches, 10 microM PlnA induced membrane currents reversing at 0 mV, which is compatible with an unspecific conductance increase. The D and L forms of the peptide had similar potency, indicating a nonchiral mechanism for the membrane-permeabilizing effect. Surprisingly, inside-out patches were insensitive to 1 mM PlnA. Primary cultures of normal rat anterior pituitary cells were also insensitive to the peptide. Thus, PlnA differentiates between plasma membranes and membrane leaflets. Microfluorometric recordings of [Ca(2+)](i) and cytosolic concentration of fluorochrome verified the rapid permeabilizing effect of PlnA on cancerous cells and the insensitivity of normal pituitary cells.