Beyond apoptosis: the mechanism and function of phosphatidylserine asymmetry in the membrane of activating mast cells

Shrinking gate fluorescence correlation spectroscopy yields equilibrium constants and separates photophysics from structural dynamics

Fluorescence correlation spectroscopy is a versatile tool for studying fast conformational changes of biomolecules especially when combined with Förster resonance energy transfer (FRET). Despite the many methods available for identifying structural dynamics in FRET experiments, the determination of the forward and backward transition rate constants and thereby also the equilibrium constant is difficult when two intensity levels are involved. Here, we combine intensity correlation analysis with fluorescence lifetime information by including only a subset of photons in the autocorrelation analysis based on their arrival time with respect to the excitation pulse (microtime). By fitting the correlation amplitude as a function of microtime gate, the transition rate constants from two fluorescence-intensity level systems and the corresponding equilibrium constants are obtained. This shrinking-gate fluorescence correlation spectroscopy (sg-FCS) approach is demonstrated using simulations and with a DNA origami-based model system in experiments on immobilized and freely diffusing molecules. We further show that sg-FCS can distinguish photophysics from dynamic intensity changes even if a dark quencher, in this case graphene, is involved. Finally, we unravel the mechanism of a FRET-based membrane charge sensor indicating the broad potential of the method. With sg-FCS, we present an algorithm that does not require prior knowledge and is therefore easily implemented when an autocorrelation analysis is carried out on time-correlated single-photon data.

Pathobiological functions and clinical implications of annexin dysregulation in human cancers

Annexins are an extensive superfamily of structurally related calcium- and phospholipid-binding proteins, largely conserved and widely distributed among species. Twelve human annexins have been identified, referred to as Annexin A1-13 (A12 remains as of yet unassigned), whose genes are spread throughout the genome on eight different chromosomes. According to their distinct tissue distribution and subcellular localization, annexins have been functionally implicated in a variety of biological processes relevant to both physiological and pathological conditions. Dysregulation of annexin expression patterns and functions has been revealed as a common feature in multiple cancers, thereby emerging as potential biomarkers and molecular targets for clinical application. Nevertheless, translation of this knowledge to the clinic requires in-depth functional and mechanistic characterization of dysregulated annexins for each individual cancer type, since each protein exhibits varying expression levels and phenotypic specificity depending on the tumor types. This review specifically and thoroughly examines the current knowledge on annexin dysfunctions in carcinogenesis. Hence, available data on expression levels, mechanism of action and pathophysiological effects of Annexin A1-13 among different cancers will be dissected, also further discussing future perspectives for potential applications as biomarkers for early diagnosis, prognosis and molecular-targeted therapies. Special attention is devoted to head and neck cancers (HNC), a complex and heterogeneous group of aggressive malignancies, often lately diagnosed, with high mortality, and scarce therapeutic options.

Quantitative Single-Molecule Measurements of Membrane Charges with DNA Origami Sensors

Charges in lipid head groups generate electrical surface potentials at cell membranes, and changes in their composition are involved in various signaling pathways, such as T-cell activation or apoptosis. Here, we present a DNA origami-based sensor for membrane surface charges with a quantitative fluorescence read-out of single molecules. A DNA origami plate is equipped with modifications for specific membrane targeting, surface immobilization, and an anionic sensing unit consisting of single-stranded DNA and the dye ATTO542. This unit is anchored to a lipid membrane by the dye ATTO647N, and conformational changes of the sensing unit in response to surface charges are read out by fluorescence resonance energy transfer between the two dyes. We test the performance of our sensor with single-molecule fluorescence microscopy by exposing it to differently charged large unilamellar vesicles. We achieve a change in energy transfer of ∼10% points between uncharged and highly charged membranes and demonstrate a quantitative relation between the surface charge and the energy transfer. Further, with autocorrelation analyses of confocal data, we unravel the working principle of our sensor that is switching dynamically between a membrane-bound state and an unbound state on the timescale of 1-10 ms. Our study introduces a complementary sensing system for membrane surface charges to previously published genetically encoded sensors. Additionally, the single-molecule read-out enables investigations of lipid membranes on the nanoscale with a high spatial resolution circumventing ensemble averaging.

Physiological Roles of Apoptotic Cell Clearance: Beyond Immune Functions

The clearance of apoptotic cells is known to be a critical step in maintaining tissue and organism homeostasis. This process is rapidly/promptly mediated by recruited or resident phagocytes. Phagocytes that engulf apoptotic cells have been closely linked to the release of anti-inflammatory cytokines to eliminate inflammatory responses. Defective clearance of apoptotic cells can cause severe inflammation and autoimmune responses due to secondary necrosis of apoptotic cells. Recently accumulated evidence indicates that apoptotic cells and their clearance have important physiological roles in addition to immune-related functions. Herein, we review the current understanding of the mechanisms and fundamental roles of apoptotic cell clearance and the beneficial roles of apoptotic cells in physiological processes such as differentiation and development.

CD36 Ectodomain Detects Apoptosis in Mammalian Cells

The cells that undergo apoptosis show phosphatidylserine (PS) on the cell membrane. The fluorescently labeled hCD36_ecto is staining and detecting apoptotic cells in a flow-based assay with several advantages over Annexin V. The human CD36 ectodomain (hCD36_ecto) is stable for a range of temperatures and experimental conditions and doesn't require Ca²⁺ for detecting apoptosis and specific towards PS compared to other lipids. The blocking with unlabeled hCD36_ecto reduces the staining of Annexin V-FITC for apoptotic cells, whereas R63A does not affect the binding of Annexin V- FITC to apoptotic cells. It indicates the role of CD36-PS interaction in detecting apoptotic cells. Dual-staining with hCD36_ecto-FITC/PI is universally detecting apoptosis in different nucleated cells or eryptosis in non-nucleated RBCs. Hence, our study highlights the utility of CD36 as a probe to detect apoptosis in mammalian cells. It might be a robust, economical reagent for the scientific community to facilitate their research.

A novel apoptosis probe, cyclic ApoPep-1, for in vivo imaging with multimodal applications in chronic inflammatory arthritis

Apoptosis plays an essential role in the pathophysiologic processes of rheumatoid arthritis. A molecular probe that allows spatiotemporal observation of apoptosis in vitro, in vivo, and ex vivo concomitantly would be useful to monitoring or predicting pathophysiologic stages. In this study we investigated whether cyclic apoptosis-targeting peptide-1 (_CApoPep-1) can be used as an apoptosis imaging probe in inflammatory arthritis. We tested the utility of _CApoPep-1 for detecting apoptotic immune cells in vitro and ex vivo using flow cytometry and immunofluorescence. The feasibility of visualizing and quantifying apoptosis using this probe was evaluated in a murine collagen-induced arthritis (CIA) model, especially after treatment. _CApoPep-1 peptide may successfully replace Annexin V for in vitro and terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay for ex vivo in the measurement of apoptotic cells, thus function as a sensitive probe enough to be used clinically. In vivo imaging in CIA mice revealed that _CApoPep-1 had 42.9 times higher fluorescence intensity than Annexin V for apoptosis quantification. Furthermore, it may be used as an imaging probe for early detection of apoptotic response in situ after treatment. The _CApoPep-1 signal was mostly co-localized with the TUNEL signal (69.6% of TUNEL⁺ cells) in defined cell populations in joint tissues of CIA mice. These results demonstrate that _CApoPep-1 is sufficiently sensitive to be used as an apoptosis imaging probe for multipurpose applications which could detect the same target across in vitro, in vivo, to ex vivo in inflammatory arthritis.

Plasma Membrane Integrates Biophysical and Biochemical Regulation to Trigger Immune Receptor Functions

Plasma membrane provides a biophysical and biochemical platform for immune cells to trigger signaling cascades and immune responses against attacks from foreign pathogens or tumor cells. Mounting evidence suggests that the biophysical-chemical properties of this platform, including complex compositions of lipids and cholesterols, membrane tension, and electrical potential, could cooperatively regulate the immune receptor functions. However, the molecular mechanism is still unclear because of the tremendous compositional complexity and spatio-temporal dynamics of the plasma membrane. Here, we review the recent significant progress of dynamical regulation of plasma membrane on immune receptors, including T cell receptor, B cell receptor, Fc receptor, and other important immune receptors, to proceed mechano-chemical sensing and transmembrane signal transduction. We also discuss how biophysical-chemical cues couple together to dynamically tune the receptor’s structural conformation or orientation, distribution, and organization, thereby possibly impacting their in-situ ligand binding and related signal transduction. Moreover, we propose that electrical potential could potentially induce the biophysical-chemical coupling change, such as lipid distribution and membrane tension, to inevitably regulate immune receptor activation.

Structural and functional consequences of reversible lipid asymmetry in living membranes

Maintenance of lipid asymmetry across the two leaflets of the plasma membrane (PM) bilayer is a ubiquitous feature of eukaryotic cells. Loss of this asymmetry has been widely associated with cell death. However, increasing evidence points to the physiological importance of non-apoptotic, transient changes in PM asymmetry. Such transient scrambling events are associated with a range of biological functions, including intercellular communication and intracellular signaling. Thus, regulation of interleaflet lipid distribution in the PM is a broadly important but underappreciated cellular process with key physiological and structural consequences. Here, we compile the mounting evidence revealing multifaceted, functional roles of PM asymmetry and transient loss thereof. We discuss the consequences of reversible asymmetry on PM structure, biophysical properties and interleaflet coupling. We argue that despite widespread recognition of broad aspects of membrane asymmetry, its importance in cell biology demands more in-depth investigation of its features, regulation, and physiological and pathological implications.

In vitro stress response induced by sulfur mustard in lung fibroblasts NHLF and human pulmonary epithelial cells A-549

Sulfur mustard [bis(2-chloroethyl) sulfide; SM] is a highly poisonous chemical warfare agent. The mechanism of its cytotoxicity affects several pathways, which cause cell damage or death. The main organ affected in case of exposure to both aerosol and vapor is lungs. The present study focuses on time- and concentration-dependent changes in human lung fibroblasts NHLF and lung epithelial cell line A-549. The cells were treated with SM at the concentrations of 5, 10 and 100 µM and signs of stress response were evaluated during 1-72 h post-treatment. Parameters for testing included cell viability and morphology, loss of transmembrane mitochondrial potential, apoptosis, oxidative stress, changes in the cell cycle, and ATM kinase activation. The cytotoxic effect of SM resulted in a time-dependent decrease in viability of A-459 associated with apoptosis more markedly than in NHLF. We did not observe any generation of reactive oxygen species by SM. SM at concentrations of 5 and 10 µM induced the S-phase cell cycle arrest at both cell lines. On the other hand, 100 µM caused nonspecific cell cycle arrest. ATM kinase was activated transiently. The results indicate that NHLF cells are less prone to toxic damage by SM in case of cell viability, apoptosis and loss of transmembrane mitochondrial potential. The analysis provides a time-related cytotoxic profile of A-549 and NHLF cells for further investigation into the prevention of SM toxic effects and their potential treatment.

Induction of Ordered Lipid Raft Domain Formation by Loss of Lipid Asymmetry

How lipid asymmetry impacts ordered lipid domain (raft) formation may yield important clues to how ordered domain formation is regulated in vivo. Under some conditions, a sphingomyelin (SM) and cholesterol-rich ordered domain in one leaflet induces ordered domain formation in the corresponding region of an opposite leaflet composed of unsaturated phosphatidylcholine (PC) and cholesterol. In other conditions, the formation of ordered domains in a SM and cholesterol-rich leaflet can be suppressed by an opposite leaflet containing unsaturated PC and cholesterol. To explore how PC unsaturation influences the balance between these behaviors, domain formation was studied in asymmetric and symmetric lipid vesicles composed of egg SM, cholesterol, and either unsaturated dioleoyl PC (DOPC) or 1-palmitoyl 2-oleoyl PC (POPC). The temperature dependence of ordered domain formation was measured using Förster resonance energy transfer, which detects nanodomains as well as large domains. In cholesterol-containing asymmetric SM+PC outside/PC inside vesicles, the PC-containing inner leaflet tended to destabilize ordered domain formation in the SM+PC-containing outer leaflet relative to ordered domain stability in cholesterol-containing symmetric SM/PC vesicles. Residual ordered domain formation was detected in cholesterol-containing asymmetric SM+DOPC outside/DOPC inside vesicles, but ordered domain formation was completely or almost completely suppressed by asymmetry in cholesterol-containing SM+POPC outside/POPC inside vesicles over the entire temperature range measured. Suppression of ordered domain formation in the latter vesicles was confirmed by fluorescence anisotropy measurements. Because mixtures of SM, POPC, and cholesterol form domains in symmetric vesicles, and this lipid composition mimics plasma membranes to a significant degree, it is possible that under some conditions in vivo the loss of lipid asymmetry could trigger ordered domain formation.

Apoptotic Bodies: Particular Extracellular Vesicles Involved in Intercellular Communication

In the last decade, a new method of cell-cell communication mediated by membranous extracellular vesicles (EVs) has emerged. EVs, including exosomes, microvesicles, and apoptotic bodies (ApoBDs), represent a new and important topic, because they are a means of communication between cells and they can also be involved in removing cellular contents. EVs are characterized by differences in size, origin, and content and different types have different functions. They appear as membranous sacs released by a variety of cells, in different physiological and patho-physiological conditions. Intringuingly, exosomes and microvesicles are a potent source of genetic information carriers between different cell types both within a species and even across a species barrier. New, and therefore still relatively poorly known vesicles are apoptotic bodies, on which numerous in-depth studies are needed in order to understand their role and possible function. In this review we would like to analyze their morpho-functional characteristics.

Apoptotic mimicry as a strategy for the establishment of parasitic infections: parasite- and host-derived phosphatidylserine as key molecule

The establishment of parasitic infection is dependent on the development of efficient strategies to evade the host defense mechanisms. Phosphatidylserine (PS) molecules are pivotal for apoptotic cell recognition and clearance by professional phagocytes. Moreover, PS receptors are able to trigger anti-inflammatory and immunosuppressive responses by phagocytes, either by coupled enzymes or through the induction of regulatory cytokine secretion. These PS-dependent events are exploited by parasites in a mechanism called apoptotic mimicry. Generally, apoptotic mimicry refers to the effects of PS recognition for the initiation and maintenance of pathogenic infections. However, in this context, PS molecules can be recognized on the surface of the infectious agent or in the surface of apoptotic host debris, leading to the respective denomination of classical and non-classical apoptotic mimicry. In this review, we discuss the role of PS in the pathogenesis of several human infections caused by protozoan parasites.

Compensatory endocytosis occurs after cortical granule exocytosis in mouse eggs

Compensatory endocytosis (CE) is one of the primary mechanisms through which cells maintain their surface area after exocytosis. Considering that in eggs massive exocytosis of cortical granules (CG) takes place after fertilization, the aim of this study was to evaluate the occurrence of CE following cortical exocytosis in mouse eggs. For this purpose, we developed a pulse-chase assay to detect CG membrane internalization. Results showed internalized labeling in SrCl₂ -activated and fertilized eggs when chasing at 37°C, but not at a nonpermissive temperature (4°C). The use of kinase and calcineurin inhibitors led us to conclude that this internal labeling corresponded to CE. Further experiments showed that CE in mouse eggs is dependent on actin dynamics and dynamin activity, and could be associated with a transient exposure of phosphatidylserine. Finally, CE was impaired in A23187 ionophore-activated eggs, highlighting once again the mechanistic differences between the activation methods. Altogether, these results demonstrate for the first time that egg activation triggers CE in mouse eggs after exocytosis of CG, probably as a plasma membrane homeostasis mechanism.

A novel 1,4-naphthoquinone-derived compound induces apoptotic cell death in breast cancer cells

Breast cancer is the most-diagnosed cancer type among women. The triple-negative subtype is an especially aggressive type of breast cancer. Although chemotherapy is almost the only option for the treatment of triple-negative breast cancer (TNBC), currently used chemotherapeutics are not effective enough, considering the poor survival rate of patients. Therefore, novel compounds need to be developed to improve survival rates. It has been known that quinonic compounds, which are found in nature, have antibacterial, antifungal, and antitumorigenic properties. Naphthoquinones are members of the quinone family and are widely used in research due to their promising properties. In this study, we evaluated the cytotoxic activity of a novel naphthoquinone-derived compound (1,4-naphthoquinone (1,4-NQ)) against two different breast cancer cells: a hormone-responsive cell line (MCF-7) and a triple-negative cell line (MDA-MB-231). As a result, 1,4-NQ decreased cell viability in both tested cell lines in a dose-dependent manner. Increased apoptotic markers (presence of pyknotic nuclei, annexin-V positivity, caspase 3/7 activity, and decreased mitochondrial membrane potential) and DNA damage were especially observed in MDA-MB-231 cells after treatment with the compound. Considering the promising cytotoxic effect of the compound, 1,4-NQ needs further evaluation as a potential candidate for the treatment of TNBC.

Full-Length P2X7 Structures Reveal How Palmitoylation Prevents Channel Desensitization

P2X receptors are trimeric, non-selective cation channels activated by extracellular ATP. The P2X₇ receptor subtype is a pharmacological target because of involvement in apoptotic, inflammatory, and tumor progression pathways. It is the most structurally and functionally distinct P2X subtype, containing a unique cytoplasmic domain critical for the receptor to initiate apoptosis and not undergo desensitization. However, lack of structural information about the cytoplasmic domain has hindered understanding of the molecular mechanisms underlying these processes. We report cryoelectron microscopy structures of full-length rat P2X₇ receptor in apo and ATP-bound states. These structures reveal how one cytoplasmic element, the C-cys anchor, prevents desensitization by anchoring the pore-lining helix to the membrane with palmitoyl groups. They show a second cytoplasmic element with a unique fold, the cytoplasmic ballast, which unexpectedly contains a zinc ion complex and a guanosine nucleotide binding site. Our structures provide first insights into the architecture and function of a P2X receptor cytoplasmic domain.

Mutagenesis of the ADAM17-phosphatidylserine-binding motif leads to embryonic lethality in mice

ADAM17, prominent member of the "Disintegrin and Metalloproteinase" (ADAM) family, controls vital cellular functions through cleavage of transmembrane substrates. Several of these play central roles in oncogenesis and inflammation, yet despite its importance, the mechanism by which ADAM17 is activated is not fully understood. We recently presented evidence that surface exposure of phosphatidylserine (PS) is the penultimate event required for sheddase activation, which occurs upon binding of a membrane-proximal, cationic binding motif to the anionic phospholipid headgroup. Here, we show that mutagenesis of the 3 amino acids constituting the PS-binding motif leads to embryonic lethality in mice. Heterozygotes showed no abnormalities. Primary hepatocytes and fibroblasts were analysed and found to express the mutant protease on the cell surface. However, PMA-stimulated release of ADAM17 substrates was completely abolished. The results directly support the novel concept of transiently externalised PS as essential trigger of extracellular protease function in vivo.

Modulation of Mast Cell Reactivity by Lipids: The Neglected Side of Allergic Diseases

Mast cells (MCs) have long been mainly regarded as effector cells in IgE-associated allergic disorders with potential immunoregulatory roles. Located close to the allergen entry sites in the skin and mucosa, MCs can capture foreign substances such as allergens, toxins, or noxious substances and are exposed to the danger signals produced by epithelial cells. MC reactivity shaped by tissue-specific factors is crucial for allergic responses ranging from local skin reactions to anaphylactic shock. Development of Th2 response leading to allergen-specific IgE production is a prerequisite for MC sensitization and induction of FcεRI-mediated MC degranulation. Up to now, IgE production has been mainly associated with proteins, whereas lipids present in plant pollen grains, mite fecal particles, insect venoms, or food have been largely overlooked regarding their immunostimulatory and immunomodulatory properties. Recent studies, however, have now demonstrated that lipids affect the sensitization process by modulating innate immune responses of epithelial cells, dendritic cells, and NK-T cells and thus crucially contribute to the outcome of sensitization. Whether and how lipids affect also MC effector functions in allergic reactions has not yet been fully clarified. Here, we discuss how lipids can affect MC responses in the context of allergic inflammation. Direct effects of immunomodulatory lipids on MC degranulation, changes in local lipid composition induced by allergens themselves and changes in lipid transport affecting MC reactivity are possible mechanisms by which the function of MC might be modulated.

Multiscale Simulations of Biological Membranes: The Challenge To Understand Biological Phenomena in a Living Substance

Biological membranes are tricky to investigate. They are complex in terms of molecular composition and structure, functional over a wide range of time scales, and characterized by nonequilibrium conditions. Because of all of these features, simulations are a great technique to study biomembrane behavior. A significant part of the functional processes in biological membranes takes place at the molecular level; thus computer simulations are the method of choice to explore how their properties emerge from specific molecular features and how the interplay among the numerous molecules gives rise to function over spatial and time scales larger than the molecular ones. In this review, we focus on this broad theme. We discuss the current state-of-the-art of biomembrane simulations that, until now, have largely focused on a rather narrow picture of the complexity of the membranes. Given this, we also discuss the challenges that we should unravel in the foreseeable future. Numerous features such as the actin-cytoskeleton network, the glycocalyx network, and nonequilibrium transport under ATP-driven conditions have so far received very little attention; however, the potential of simulations to solve them would be exceptionally high. A major milestone for this research would be that one day we could say that computer simulations genuinely research biological membranes, not just lipid bilayers.

Subtypes of tumour cell-derived small extracellular vesicles having differently externalized phosphatidylserine

Phosphatidylserine (PS) has skewed distributions in the plasma membrane and is preferentially located in the inner leaflet of normal cells. Tumour cells, however, expose PS at the outer leaflet of cell surfaces, thereby potentially modulating the bio-signalling of cells. Interestingly, exosomes - or, more properly, small extracellular vesicles (sEVs) - which are secreted from tumour cells, are enriched with externalized PS, have been proposed as being involved in the progression of cancers, and could be used as a marker for tumour diagnostics. However, the sEV fractions prepared from various methods are composed of different subtypes of vesicles, and knowledge about the subtypes enriched with exposed PS is still limited. Here, we differentiated sEVs from cancer cell lines by density gradient centrifugation and characterized the separated fractions by using gold-labelling of PS in atomic force microscopy, thrombin generation assay, size and zeta potential measurements, and western blot analysis. These analyses revealed a previously unreported PS⁺-enriched sEV subtype, which is characterized by a lower density than that of canonical exosomes (1.06 g/ml vs. 1.08 g/ml), larger size (122 nm vs. 105 nm), more negative zeta potential (-28 mV vs. -21 mV), and lower abundance of canonical exosomal markers. The identification of the PS-exposed subtype of sEVs will provide deeper insight into the role of EVs in tumour biology and enhance the development of EV-based tumour diagnosis and therapy.

Calcium-dependent, non-apoptotic, large plasma membrane bleb formation in physiologically stimulated mast cells and basophils

Large membrane derangements in the form of non-detaching blebs or membrane protrusions occur in a variety of cell stress and physiological situations and do not always reflect apoptotic processes. They have been studied in model mast cells under conditions of cell stress, but their potential physiological relevance to mast cell function and formation in primary mast cells or basophils have not been addressed. In the current study, we examine the large, non-detaching, non-apoptotic, membrane structures that form in model and primary mast cells under conditions of stimulation that are relevant to allergy, atopy and Type IV delayed hypersensitivity reactions. We characterized the inflation kinetics, dependency of formation upon external free calcium and striking geometric consistency of formation for large plasma membrane blebs (LPMBs). We describe that immunologically stimulated LPMBs in mast cells are constrained to form in locations where dissociation of the membrane-associated cytoskeleton occurs. Mast cell LPMBs decorate with wheat germ agglutinin, suggesting that they contain plasma membrane (PM) lectins. Electrophysiological capacitance measurements support a model where LPMBs are not being formed from internal membranes newly fused into the PM, but rather arise from stretching of the existing membrane, or inflation and smoothing of a micro-ruffled PM. This study provides new insights into the physiological manifestations of LPMB in response to immunologically relevant stimuli and in the absence of cell stress, death or apoptotic pathways.

Sensitivity of peripheral membrane proteins to the membrane context: A case study of phosphatidylserine and the TIM proteins

There is a diverse class of peripheral membrane-binding proteins that specifically bind phosphatidylserine (PS), a lipid that signals apoptosis or cell fusion depending on the membrane context of its presentation. PS-receptors are specialized for particular PS-presenting pathways, indicating that they might be sensitive to the membrane context. In this review, we describe a combination of thermodynamic, structural, and computational techniques that can be used to investigate the mechanisms underlying this sensitivity. As an example, we focus on three PS-receptors of the T-cell Immunoglobulin and Mucin containing (TIM) protein family, which we have previously shown to differ in their sensitivity to PS surface density.

Introducing Membrane Charge and Membrane Potential to T Cell Signaling

While membrane models now include the heterogeneous distribution of lipids, the impact of membrane charges on regulating the association of proteins with the plasma membrane is often overlooked. Charged lipids are asymmetrically distributed between the two leaflets of the plasma membrane, resulting in the inner leaflet being negatively charged and a surface potential that attracts and binds positively charged ions, proteins, and peptide motifs. These interactions not only create a transmembrane potential but they can also facilitate the formation of charged membrane domains. Here, we reference fields outside of immunology in which consequences of membrane charge are better characterized to highlight important mechanisms. We then focus on T cell receptor (TCR) signaling, reviewing the evidence that membrane charges and membrane-associated calcium regulate phosphorylation of the TCR–CD3 complex and discuss how the immunological synapse exhibits distinct patterns of membrane charge distribution. We propose that charged lipids, ions in solution, and transient protein interactions form a dynamic equilibrium during T cell activation.

Prognostic implications and procoagulant activity of phosphatidylserine exposure of blood cells and microparticles in patients with atrial fibrillation treated with pulmonary vein isolation

The present study aimed to evaluate the procoagulant effects of phosphatidylserine (PS) exposure on blood cells and microparticles (MPs), and examine its role in predicting early recurrence atrial fibrillation (ERAF) in patients with atrial fibrillation (AF) treated with pulmonary vein isolation (PVI). Blood samples were obtained from 40 healthy controls and 56 patients with AF at baseline (prior to PVI), and 0, 1 h, 1 day, 3 days and 7 days following PVI. The exposure of PS (PS⁺) to blood cells (platelets, erythrocytes and leukocytes) and MPs was detected using flow cytometry. The procoagulant activity was evaluated by coagulation time, and the formation of factor Xa (FXa) and thrombin. In addition, independent factors associated with PS⁺ blood cells and MPs, and significant predictors of ERAF following PVI were investigated by statistical analyses. The numbers of PS⁺ blood cells and MPs were significantly increased by PVI (P<0.01). A significant decrease in coagulation time, and increases in FXa and thrombin were exhibited in the PS⁺ blood cells and MPs from patients with AF treated with PVI, whereas these alterations were inhibited by either lactadherin or anti-tissue factor (P<0.01). The maximum power of the PVI was significantly associated with platelet-derived MPs, and high-sensitivity C-reactive protein (hs-CRP) was closely associated with leukocyte-derived MPs and endothelial-derived MPs (EMPs) (P<0.01). In addition, hs-CRP and EMPs >355/µl were identified as independent predictors of ERAF (P<0.05). The increased numbers of PS⁺ platelets, erythrocytes, leukocytes and MPs contributed to the procoagulant state of AF, and hs-CRP and EMPs were able to predict ERAF following PVI.

Insights into the Cytoadherence Phenomenon of Plasmodium vivax: The Putative Role of Phosphatidylserine

Plasmodium vivax is the most geographically widespread and the dominant human malaria parasite in most countries outside of sub-Saharan Africa and, although it was classically recognized to cause benign infection, severe cases and deaths caused by P. vivax have remarkably been reported. In contrast to Plasmodium falciparum, which well-known ability to bind to endothelium and placental tissue and form rosettes is related to severity of the disease, it has been a dogma that P. vivax is unable to undergo cytoadherent phenomena. However, some studies have demonstrated that red blood cells (RBCs) infected by P. vivax can cytoadhere to host cells, while the molecules participating in this host–parasite interaction are still a matter of speculation. In the present overview, we address the evidences currently supporting the adhesive profile of P. vivax and, additionally, discuss the putative role of phosphatidylserine—a cell membrane phospholipid with cytoadhesive properties that has been detected on the surface of Plasmodium-parasitized RBCs.

Genetic determinants of susceptibility to silver nanoparticle-induced acute lung inflammation in mice

Silver nanoparticles (AgNPs) are employed in a variety of consumer products; however, in vivo rodent studies indicate that AgNPs can cause lung inflammation and toxicity in a strain- and particle type-dependent manner, but mechanisms of susceptibility remain unclear. The aim of this study was to assess the variation in AgNP-induced lung inflammation and toxicity across multiple inbred mouse strains and to use genome-wide association (GWA) mapping to identify potential candidate susceptibility genes. Mice received doses of 0.25 mg/kg of either 20-nm citrate-coated AgNPs or citrate buffer using oropharyngeal aspiration. Neutrophils in bronchoalveolar lavage fluid (BALF) served as markers of inflammation. We found significant strain- and treatment-dependent variation in neutrophils in BALF. GWA mapping identified 10 significant single-nucleotide polymorphisms (false discovery rate, 15%) in 4 quantitative trait loci on mouse chromosomes 1, 4, 15, and 18, and Nedd4l (neural precursor cell expressed developmentally downregulated gene 4-like; chromosome 18), Ano6 (anocatmin 6; chromosome 15), and Rnf220 (Ring finger protein 220; chromosome 4) were considered candidate genes. Quantitative RT-PCR revealed significant inverse associations between mRNA levels of these genes and neutrophil influx. Nedd4l, Ano6, and Rnf220 are candidate susceptibility genes for AgNP-induced lung inflammation that warrant additional exploration in future studies.-Scoville, D. K., Botta, D., Galdanes, K., Schmuck, S. C., White, C. C., Stapleton, P. L., Bammler, T. K., MacDonald, J. W., Altemeier, W. A., Hernandez, M., Kleeberger, S. R., Chen, L.-C., Gordon, T., Kavanagh, T. J. Genetic determinants of susceptibility to silver nanoparticle-induced acute lung inflammation in mice.

10-Gingerol as an inducer of apoptosis through HTR1A in cumulus cells: In-vitro and in-silico studies

Objectives

Cumulus cells play a crucial role as essential mediators in the maturation of ova. Ginger contains 10-gingerol, which induces apoptosis in colon cancer cells. Based on this hypothesis, this study aimed to determine whether 10-gingerol is able to induce apoptosis in normal cells, namely, cumulus cells.

Methods

This study used an in vitro analysis by culturing Cumulus cells in M199 containing 10-gingerol in various concentrations (12, 16, and 20 μM) and later detected early apoptotic activity using an Annexin V-FITC detection kit.

Result

The in vitro data revealed that the number of apoptosis cells increased along with the period of incubation as follows: 12 μM (63.71% ± 2.192%); 16 μM (74.51% ± 4.596%); and 20 μM (78.795% ± 1.435%). The substance 10-gingerol induces apoptosis in cumulus cells by inhibiting HTR1A functions and inactivating GSK3B and AKT-1.

Conclusions

These findings indicate that further examination is warranted for 10-gingerol as a contraception agent.

Plasma phospholipids and fatty acid composition differ between liver biopsy-proven nonalcoholic fatty liver disease and healthy subjects

Background:

There is growing evidence that nonalcoholic fatty liver disease (NAFLD) is associated with perturbations in liver lipid metabolism. Liver phospholipid and fatty acid composition have been shown to be altered in NAFLD. However, detailed profiles of circulating lipids in the pathogenesis of NAFLD are lacking.

Objective:

Therefore, the objective of the present study was to examine circulating lipids and potential mechanisms related to hepatic gene expression between liver biopsy-proven simple steatosis (SS), nonalcoholic steatohepatitis (NASH) and healthy subjects.

Subjects:

Plasma phospholipid and fatty acid composition were determined in 31 healthy living liver donors as healthy controls (HC), 26 patients with simple hepatic steatosis (SS) and 20 with progressive NASH. Hepatic gene expression was analyzed by Illumina microarray in a subset of 22 HC, 16 SS and 14 NASH.

Results:

Concentrations of phosphatidylethanolamine (PE) increased relative to disease progression, HC<SS<NASH (170<210<250 μg ml⁻¹), and was significantly different (P<0.05) between HC and NASH. Circulating phosphatidylserine (PS) and phosphatidylinositol were higher in SS and NASH compared with HC (P<0.05), but there was no difference between SS and NASH. Fatty acid composition of phospholipids was also remodeled. In particular, docosahexaenoic and arachidonic acid were higher (P<0.05) in SS and NASH relative to HC in PS. Differentially expressed hepatic genes included ETNK1 and PLSCR1 that are involved in PE synthesis and PS transport, respectively.

Conclusions:

The present study demonstrates that there is a disruption in phospholipid metabolism that is present in SS, but more pronounced in NASH. Intervention studies targeted at lipid metabolism could benefit SS and NASH.

Calcium Influx of Mast Cells Is Inhibited by Aptamers Targeting the First Extracellular Domain of Orai1

Using the systematic evolution of ligands by exponential enrichment (SELEX) method, we identified oligonucleotides that bind to the first extracellular domain of the Orai1 protein with high affinities and high specificities. These ligands were isolated from a random single-strand DNA (ssDNA) library with 40 randomized sequence positions, using synthesized peptides with amino acid sequences identical to the first extracellular domain of the Orai1 protein as the targets for SELEX selection. Seven aptamers were obtained after 12 rounds of SELEX. An enzyme-linked oligonucleotide assay (ELONA) was performed to determine the affinities of the aptamers. Aptamer Y1 had the highest affinity (Kd = 1.72×10⁻⁸ mol/L) and was selected for functional experiments in mast cells. Using LAD2 cells with the human high-affinity IgE receptor and Ca²⁺ release activation channel (CRAC), we demonstrated that Aptamer Y1 blocked IgE-mediated β-hexosaminidase release from cells triggered by biotin-IgE and streptavidin. A specific binding assay showed that Aptamer Y1 not only bound the Orai1 peptide specifically but also that the Orai1 peptide did not bind significantly to other random oligonucleotide molecules. Furthermore, Aptamer Y1 regulation of intracellular Ca²⁺ mobilization was investigated by probing intracellular Ca²⁺ with a Fluo-4-AM fluorescent probe. We found that Aptamer Y1 inhibits Ca²⁺ influx into antigen-activated mast cells. These results indicate that the target of Aptamer Y1 in the degranulation pathway is upstream of Ca²⁺ influx. Therefore, these oligonucleotide agents represent a novel class of CRAC inhibitors that may be useful in the fight against allergic diseases.

Phosphatidylserine exposure is required for ADAM17 sheddase function

ADAM17, a prominent member of the 'Disintegrin and Metalloproteinase' (ADAM) family, controls vital cellular functions through cleavage of transmembrane substrates. Here we present evidence that surface exposure of phosphatidylserine (PS) is pivotal for ADAM17 to exert sheddase activity. PS exposure is tightly coupled to substrate shedding provoked by diverse ADAM17 activators. PS dependency is demonstrated in the following: (a) in Raji cells undergoing apoptosis; (b) in mutant PSA-3 cells with manipulatable PS content; and (c) in Scott syndrome lymphocytes genetically defunct in their capacity to externalize PS in response to intracellular Ca(2+) elevation. Soluble phosphorylserine but not phosphorylcholine inhibits substrate cleavage. The isolated membrane proximal domain (MPD) of ADAM17 binds to PS but not to phosphatidylcholine liposomes. A cationic PS-binding motif is identified in this domain, replacement of which abrogates liposome-binding and renders the protease incapable of cleaving its substrates in cells. We speculate that surface-exposed PS directs the protease to its targets where it then executes its shedding function.

N-terminal amphipathic helix of Amphiphysin can change the spatial distribution of immunoglobulin E receptors (FcεRI) in the RBL-2H3 mast cell synapse

Biomembranes undergo extensive shape changes as they perform vital cellular functions or become diseased. To understand the mechanisms by which lipids and proteins control membrane curvature during various processes, researchers have identified and engineered many curvature-inducing and curvature-sensing proteins and peptides. In this paper, a simple experiment was performed to show qualitatively how membrane remodeling by N-terminal amphipathic helix of Amphiphysin affects the spatial distribution of the transmembrane Fc receptor protein (FcεRI) in mast cells. Results indicate that an elevated concentration of amphipathic helices can interfere with the formation of a typical mast cell synapse.

Pseudorabies Virus US3 Protein Kinase Protects Infected Cells from NK Cell-Mediated Lysis via Increased Binding of the Inhibitory NK Cell Receptor CD300a

Several reports have indicated that natural killer (NK) cells are of particular importance in the innate response against herpesvirus infections. As a consequence, herpesviruses have developed diverse mechanisms for evading NK cells, although few such mechanisms have been identified for the largest herpesvirus subfamily, the alphaherpesviruses. The antiviral activity of NK cells is regulated by a complex array of interactions between activating/inhibitory receptors on the NK cell surface and the corresponding ligands on the surfaces of virus-infected cells. Here we report that the US3 protein kinase of the alphaherpesvirus pseudorabies virus (PRV) displays previously uncharacterized immune evasion properties: it triggers the binding of the inhibitory NK cell receptor CD300a to the surface of the infected cell, thereby providing increased CD300a-mediated protection of infected cells against NK cell-mediated lysis. US3-mediated CD300a binding was found to depend on aminophospholipid ligands of CD300a and on group I p21-activated kinases. These data identify a novel alphaherpesvirus strategy for evading NK cells and demonstrate, for the first time, a role for CD300a in regulating NK cell activity upon contact with virus-infected target cells.

IMPORTANCE

Herpesviruses have developed fascinating mechanisms to evade elimination by key elements of the host immune system, contributing to their ability to cause lifelong infections with recurrent reactivation events. Natural killer (NK) cells are central in the innate antiviral response. Here we report that the US3 protein kinase of the alphaherpesvirus pseudorabies virus displays a previously uncharacterized capacity for evasion of NK cells. Expression of US3 protects infected cells from NK cell-mediated lysis via increased binding of the inhibitory NK cell receptor CD300a. We show that this US3-mediated increase in CD300a binding depends on aminophospholipids and on cellular p21-activated kinases (PAKs). The identification of this novel NK cell evasion strategy may contribute to the design of improved herpesvirus vaccines and may also have significance for other PAK- and CD300a-modulating viruses and cancer cells.