Getting to the Outer Leaflet: Physiology of Phosphatidylserine Exposure at the Plasma Membrane

Structural mapping of fluorescently-tagged, functional nhTMEM16 scramblase in a lipid bilayer

Most members of the TransMEMbrane protein 16 (TMEM16) family are Ca²⁺-regulated scramblases that facilitate the bidirectional movement of phospholipids across membranes necessary for diverse physiological processes. The nhTMEM16 scramblase (from the fungus Nectria hematococca) is a homodimer with a large cytoplasmic region and a hydrophilic, membrane-exposed groove in each monomer. The groove provides the transbilayer conduit for lipids, but the mechanism by which Ca²⁺ regulates it is not clear. Because fusion of large protein tags at either the N or C terminus abolishes nhTMEM16 activity, we hypothesized that its cytoplasmic portion containing both termini may regulate lipid translocation via a Ca²⁺-dependent conformational change. To test this hypothesis, here we used fluorescence methods to map key distances within the nhTMEM16 homodimer and between its termini and the membrane. To this end, we developed functional nhTMEM16 variants bearing an acyl carrier protein (ACP) tag at one or both of the termini. These constructs were fluorescently labeled by ACP synthase-mediated insertion of CoA-conjugated fluorophores and reconstituted into vesicles containing fluorescent lipids to obtain the distance of closest approach between the labeled tag and the membrane via FRET. Fluorescence lifetime measurements with phasor analysis were used to determine the distance between the N and C termini of partnering monomers in the nhTMEM16 homodimer. We now report that the measured distances do not vary significantly between Ca²⁺-replete and EGTA-treated samples, indicating that whereas the cytoplasmic portion of the protein is important for function, it does not appear to regulate scramblase activity via a detectable conformational change.

Novel assay demonstrates that coronary artery disease patients have heightened procoagulant platelet response

Essentials Procoagulant platelets can be detected using GSAO in human whole blood. Stable coronary artery disease is associated with a heightened procoagulant platelet response. Agonist-induced procoagulant platelet response is not inhibited by aspirin alone. Collagen plus thrombin induced procoagulant platelet response is partially resistant to clopidogrel.

SUMMARY

Background Procoagulant platelets are a subset of highly activated platelets with a critical role in thrombin generation. Evaluation of their clinical utility in thrombotic disorders, such as coronary artery disease (CAD), has been thwarted by the lack of a sensitive and specific whole blood assay. Objectives We developed a novel assay, utilizing the cell death marker, GSAO [(4-(N-(S-glutathionylacetyl)amino)phenylarsonous acid], and the platelet activation marker, P-selectin, to identify procoagulant platelets in whole blood by flow cytometry. Patients/Methods Using this assay, we characterized the procoagulant platelet population in healthy controls and a cohort of patients undergoing elective coronary angiography. Results In patients with CAD, compared with patients without CAD, there was a heightened procoagulant platelet response to thrombin (25.2% vs. 12.2%), adenosine diphosphate (ADP) (7.8% vs. 2.7%) and thrombin plus collagen (27.2% vs. 18.3%). The heightened procoagulant platelet potential in CAD patients was not associated with other markers of platelet function, including aggregation, dense granule release and activation of α2b β3 integrin. Although dual antiplatelet therapy (DAPT) was associated with partial suppression of procoagulant platelets, this inhibitory effect on a patient level could not be predicted by aggregation response to ADP and was not fully suppressed by clopidogrel. Conclusions We report for the first time that procoagulant platelets can be efficiently detected in a few microliters of whole blood using the cell death marker, GSAO, and the platelet activation marker, P-selectin. A heightened procoagulant platelet response may provide insight into the thrombotic risk of CAD and help identify a novel target for antiplatelet therapies in CAD.

Antimicrobial peptides with selective antitumor mechanisms: prospect for anticancer applications

In the last several decades, there have been significant advances in anticancer therapy. However, the development of resistance to cancer drugs and the lack of specificity related to actively dividing cells leading to toxic side effects have undermined these achievements. As a result, there is considerable interest in alternative drugs with novel antitumor mechanisms. In addition to the recent approach using immunotherapy, an effective but much cheaper therapeutic option of pharmaceutical drugs would still provide the best choice for cancer patients as the first line treatment. Ribosomally synthesized cationic antimicrobial peptides (AMPs) or host defense peptides (HDP) display broad-spectrum activity against bacteria based on electrostatic interactions with negatively charged lipids on the bacterial surface. Because of increased proportions of phosphatidylserine (negatively charged) on the surface of cancer cells compared to normal cells, cationic amphipathic peptides could be an effective source of anticancer agents that are both selective and refractory to current resistance mechanisms. We reviewed herein the prospect for AMP application to cancer treatment, with a focus on modes of action of cationic AMPs.

Bioinformatic characterization of the Anoctamin Superfamily of Ca2+-activated ion channels and lipid scramblases

Our laboratory has developed bioinformatic strategies for identifying distant phylogenetic relationships and characterizing families and superfamilies of transport proteins. Results using these tools suggest that the Anoctamin Superfamily of cation and anion channels, as well as lipid scramblases, includes three functionally characterized families: the Anoctamin (ANO), Transmembrane Channel (TMC) and Ca²⁺-permeable Stress-gated Cation Channel (CSC) families; as well as four families of functionally uncharacterized proteins, which we refer to as the Anoctamin-like (ANO-L), Transmembrane Channel-like (TMC-L), and CSC-like (CSC-L1 and CSC-L2) families. We have constructed protein clusters and trees showing the relative relationships among the seven families. Topological analyses suggest that the members of these families have essentially the same topologies. Comparative examination of these homologous families provides insight into possible mechanisms of action, indicates the currently recognized organismal distributions of these proteins, and suggests drug design potential for the disease-related channel proteins.

Avenues to molecular imaging of dying cells: Focus on cancer

Successful treatment of cancer patients requires balancing of the dose, timing, and type of therapeutic regimen. Detection of increased cell death may serve as a predictor of the eventual therapeutic success. Imaging of cell death may thus lead to early identification of treatment responders and nonresponders, and to “patient‐tailored therapy.” Cell death in organs and tissues of the human body can be visualized, using positron emission tomography or single‐photon emission computed tomography, although unsolved problems remain concerning target selection, tracer pharmacokinetics, target‐to‐nontarget ratio, and spatial and temporal resolution of the scans. Phosphatidylserine exposure by dying cells has been the most extensively studied imaging target. However, visualization of this process with radiolabeled Annexin A5 has not become routine in the clinical setting. Classification of death modes is no longer based only on cell morphology but also on biochemistry, and apoptosis is no longer found to be the preponderant mechanism of cell death after antitumor therapy, as was earlier believed. These conceptual changes have affected radiochemical efforts. Novel probes targeting changes in membrane permeability, cytoplasmic pH, mitochondrial membrane potential, or caspase activation have recently been explored. In this review, we discuss molecular changes in tumors which can be targeted to visualize cell death and we propose promising biomarkers for future exploration.

Single-molecule analysis of phospholipid scrambling by TMEM16F

Transmembrane protein 16F (TMEM16F) is a Ca²⁺-dependent phospholipid scramblase that translocates phospholipids bidirectionally between the leaflets of the plasma membrane. Phospholipid scrambling of TMEM16F causes exposure of phosphatidylserine in activated platelets to induce blood clotting and in differentiated osteoblasts to promote bone mineralization. Despite the importance of TMEM16F-mediated phospholipid scrambling in various biological reactions, the fundamental features of the scrambling reaction remain elusive due to technical difficulties in the preparation of a platform for assaying scramblase activity in vitro. Here, we established a method to express and purify mouse TMEM16F as a dimeric molecule by constructing a stable cell line and developed a microarray containing membrane bilayers with asymmetrically distributed phospholipids as a platform for single-molecule scramblase assays. The purified TMEM16F was integrated into the microarray, and monitoring of phospholipid translocation showed that a single TMEM16F molecule transported phospholipids nonspecifically between the membrane bilayers in a Ca²⁺-dependent manner. Thermodynamic analysis of the reaction indicated that TMEM16F transported 4.5 × 10⁴ lipids per second at 25 °C, with an activation free energy of 47 kJ/mol. These biophysical features were similar to those observed with channels, which transport substrates by facilitating diffusion, and supported the stepping-stone model for the TMEM16F phospholipid scramblase.

Lupus-like autoimmune disease caused by a lack of Xkr8, a caspase-dependent phospholipid scramblase

Apoptotic cells expose phosphatidylserine (PtdSer) on their cell surface and are recognized by macrophages for clearance. Xkr8 is a scramblase that exposes PtdSer in a caspase-dependent manner. Here, we found that among the three Xkr members with caspase-dependent scramblase activity, mouse hematopoietic cells express only Xkr8. The PtdSer exposure of apoptotic thymocytes, splenocytes, and neutrophils was strongly reduced when Xkr8 was absent. While wild-type apoptotic lymphocytes and neutrophils were efficiently engulfed in vitro by phagocytes expressing Tim4 and MerTK, Xkr8-deficient apoptotic cells were hardly engulfed by these phagocytes. Accordingly, the number of apoptotic thymocytes in the thymus and neutrophils in the peritoneal cavity of the zymosan-treated mice was significantly increased in Xkr8-deficient mice. The percentage of CD62L^lo senescent neutrophils was increased in the spleen of Xkr8-null mice, especially after the treatment with granulocyte colony-stimulating factor. Xkr8-null mice on an MRL background showed high levels of autoantibodies, splenomegaly with high levels of effector CD4 T cells, and glomerulonephritis development with immune-complex deposition at glomeruli. These results indicate that the Xkr8-mediated PtdSer exposure in apoptotic lymphocytes and aged neutrophils is essential for their clearance, and its defect activates the immune system, leading to lupus-like autoimmune disease.

Apoptosis and Clearance of Apoptotic Cells

The human body generates 10-100 billion cells every day, and the same number of cells die to maintain homeostasis in our body. Cells infected by bacteria or viruses also die. The cell death that occurs under physiological conditions mainly proceeds by apoptosis, which is a noninflammatory, or silent, process, while pathogen infection induces necroptosis or pyroptosis, which activates the immune system and causes inflammation. Dead cells generated by apoptosis are quickly engulfed by macrophages for degradation. Caspases are a large family of cysteine proteases that act in cascades. A cascade that leads to caspase 3 activation mediates apoptosis and is responsible for killing cells, recruiting macrophages, and presenting an "eat me" signal(s). When apoptotic cells are not efficiently engulfed by macrophages, they undergo secondary necrosis and release intracellular materials that represent a damage-associated molecular pattern, which may lead to a systemic lupus-like autoimmune disease.

Phosphatidylinositol-(4, 5)-bisphosphate regulates calcium gating of small-conductance cation channel TMEM16F

TMEM16F, which is activated by elevation of intracellular calcium to trigger phospholipid scrambling and the collapse of lipid bilayer asymmetry to mediate important cellular functions such as blood coagulation, also generates a small-conductance calcium-activated cation current. How TMEM16F activation may be regulated is an open question. By recording TMEM16F Ca²⁺-activated current, we found that the TMEM16F Ca²⁺-response is desensitized by a brief exposure to high intracellular Ca²⁺, which is associated with depletion of phosphatidylinositol-(4, 5)-bisphosphate (PIP₂) from the inner leaflet of the membrane. Application of artificial or natural PIP₂ restores TMEM16F channel activity. PIP₂ modulation of TMEM16F requires the presence of several positively charged amino acids in its cytoplasmic N-terminal domain. TMEM16F interaction with PIP₂ works synergistically with membrane depolarization to facilitate Ca²⁺-gating of TMEM16F. Our study reveals the dependence of TMEM16F activity on phosphoinositides and provides one mechanism for TMEM16F activation to be strictly regulated in the cell membrane.

Platelet subpopulations remain despite strong dual agonist stimulation and can be characterised using a novel six-colour flow cytometry protocol

It is recognised that platelets respond differently to activation, where a subpopulation of platelets adopt a procoagulant phenotype while others are aggregatory. However, it has not been thoroughly tested whether these subpopulations will remain in maximally activated samples, or if they are merely a result of different platelet sensitivities to agonist activation. Here platelets were activated with gradually increasing concentrations of thrombin and/or the GPVI agonist cross-linked collagen-related peptide (CRP-XL). Platelet activation was investigated using a novel six-colour flow cytometry protocol evaluating exposure of phosphatidylserine, active conformation of the fibrinogen receptor α_IIbβ₃, α-granule and lysosomal release (P-selectin and LAMP-1 exposure), mitochondrial membrane integrity and platelet fragmentation. Upon activation by CRP-XL or thrombin+CRP-XL, platelets formed three differently sized subpopulations. Normal-sized platelets showed high exposure of aggregatory active α_IIbβ₃ and intact mitochondria, while the smaller platelets and platelet fragments showed high exposure of procoagulant phosphatidylserine. The distribution of platelets between the differently sized subpopulations remained stable despite high agonist concentrations. All three were still present after 30 and 60 min of activation, showing that all platelets will not have the same characteristics even after maximal stimulation. This suggests that platelet subpopulations with distinct activation patterns exist within the total platelet population.

Prochemerin cleavage by factor XIa links coagulation and inflammation

Chemerin is a chemoattractant and adipokine that circulates in blood as inactive prochemerin (chem163S). Chem163S is activated by a series of C-terminal proteolytic cleavages resulting in diverse chemerin forms with different levels of activity. We screened a panel of proteases in the coagulation, fibrinolytic, and inflammatory cascades to identify those that process prochemerin in plasma. Factor XIa (FXIa) cleaved chem163S, generating a novel chemerin form, chem162R, as an intermediate product, and chem158K, as the final product. Processing at Arg162 was not required for cleavage at Lys158 or regulation of chemerin bioactivity. Contact phase activation of human platelet-poor plasma by kaolin led to cleavage of chem163S, which was undetectable in FXI-depleted plasma and markedly enhanced in platelet-rich plasma (PRP). Contact phase activation by polyphosphate in PRP resulted in 75% cleavage of chem163S. This cleavage was partially inhibited by hirudin, which blocks thrombin activation of FXI. After activation of plasma, levels of the most potent form of chemerin, chem157S, as well as inactive chem155A, increased. Plasma levels of chem163S in FXI-deficient patients were significantly higher compared with a matched control group (91 ± 10 ng/mL vs 58 ± 3 ng/mL, n = 8; P < .01) and inversely correlated with the plasma FXI levels. Thus FXIa, generated on contact phase activation, cleaves chem163S to generate chem158K, which can be further processed to the most active chemerin form, providing a molecular link between coagulation and inflammation.

Ca2+ and lipid signals hold hands at endoplasmic reticulum-plasma membrane contact sites

Discovery of the STIM1 and Orai proteins as the principal components of store-operated Ca²⁺ entry has drawn attention to contact sites between the endoplasmic reticulum (ER) and the plasma membrane (PM). Such contacts between adjacent membranes of different cellular organelles, primarily between the mitochondria and the ER, had already been known as the sites where Ca²⁺ released from the ER can be efficiently channelled to the mitochondria and also where phosphatidylserine synthesis and transfer takes place. Recent studies have identified contact sites between virtually every organelle and the ER and the functional importance of these small specialized membrane domains is increasingly recognized. Most recent developments have highlighted the role of phosphatidylinositol 4-phosphate gradients as critical determinants of the non-vesicular transport of various lipids from the ER to other organelles such as the Golgi or PM. As we learn more about membrane contact sites it becomes apparent that Ca²⁺ is not only transported at these sites but also controls both the dynamics and the lipid transfer efficiency of these processes. Conversely, lipids are critical for regulating the Ca²⁺ entry process. This review will summarize some of the most exciting recent developments in this rapidly expanding research field.

Herpes simplex viruses activate phospholipid scramblase to redistribute phosphatidylserines and Akt to the outer leaflet of the plasma membrane and promote viral entry

Herpes simplex virus (HSV) entry is associated with Akt translocation to the outer leaflet of the plasma membrane to promote a complex signaling cascade. We hypothesized that phospholipid scramblase-1 (PLSCR1), a calcium responsive enzyme that flips phosphatidylserines between membrane leaflets, might redistribute Akt to the outside during entry. Confocal imaging, biotinylation of membrane proteins and flow cytometric analysis demonstrated that HSV activates PLSCR1 and flips phosphatidylserines and Akt to the outside shortly following HSV-1 or HSV-2 exposure. Translocation was blocked by addition of a cell permeable calcium chelator, pharmacological scramblase antagonist, or transfection with small interfering RNA targeting PLSCR1. Co-immunoprecipitation and proximity ligation studies demonstrated that PLSCR1 associated with glycoprotein L at the outer leaflet and studies with gL deletion viruses indicate that this interaction facilitates subsequent restoration of the plasma membrane architecture. Ionomycin, a calcium ionophore, also induced PLSCR1 activation resulting in Akt externalization, suggesting a previously unrecognized biological phenomenon.

Defining the mechanisms by which herpes simplex viruses (HSV) enter cells will facilitate the development of new strategies to prevent or treat infections and provide insights into cell biology. We report the novel observation that HSV activates the enzyme, scramblase, which redistributes phosphatidylserines, the major component of the inner leaflet of the plasma membrane, and the associated protein, Akt, between the inner and outer leaflet of the plasma membrane, to promote viral entry.

Mechanisms of Lipid Scrambling by the G Protein-Coupled Receptor Opsin

Several class-A G protein-coupled receptor (GPCR) proteins act as constitutive phospholipid scramblases catalyzing the transbilayer translocation of >10,000 phospholipids per second when reconstituted into synthetic vesicles. To address the molecular mechanism by which these proteins facilitate rapid lipid scrambling, we carried out large-scale ensemble atomistic molecular dynamics simulations of the opsin GPCR. We report that, in the process of scrambling, lipid head groups traverse a dynamically revealed hydrophilic pathway in the region between transmembrane helices 6 and 7 of the protein while their hydrophobic tails remain in the bilayer environment. We present quantitative kinetic models of the translocation process based on Markov State Model analysis. As key residues on the lipid translocation pathway are conserved within the class-A GPCR family, our results illuminate unique aspects of GPCR structure and dynamics while providing a rigorous basis for the design of variants of these proteins with defined scramblase activity.

The CDC50A extracellular domain is required for forming a functional complex with and chaperoning phospholipid flippases to the plasma membrane

Flippases are enzymes that translocate phosphatidylserine (PtdSer) and phosphatidylethanolamine (PtdEtn) from the outer to the inner leaflet in the lipid bilayer of the plasma membrane, leading to the asymmetric distribution of aminophospholipids in the membrane. One mammalian phospholipid flippase at the plasma membrane is ATP11C, a type IV P-type ATPase (P4-ATPase) that forms a heterocomplex with the transmembrane protein CDC50A. However, the structural features in CDC50A that support the function of ATP11C and other P4-ATPases have not been characterized. Here, using error-prone PCR-mediated mutagenesis of human CDC50A cDNA followed by functional screening and deep sequencing, we identified 14 amino acid residues that affect ATP11C's flippase activity. These residues were all located in CDC50A's extracellular domain and were evolutionarily well-conserved. Most of the mutations decreased CDC50A's ability to chaperone ATP11C and other P4-ATPases to their destinations. The CDC50A mutants failed to form a stable complex with ATP11C and could not induce ATP11C's PtdSer-dependent ATPase activity. Notably, one mutant variant could form a stable complex with ATP11C and transfer ATP11C to the plasma membrane, yet the ATP11C complexed with this CDC50A variant had very weak or little PtdSer- or PtdEtn-dependent ATPase activity. These results indicated that the extracellular domain of CDC50A has important roles both in CDC50A's ability to chaperone ATP11C to the plasma membrane and in inducing ATP11C's ATP hydrolysis-coupled flippase activity.

Microvesicle Involvement in Shiga Toxin-Associated Infection

Shiga toxin is the main virulence factor of enterohemorrhagic Escherichia coli, a non-invasive pathogen that releases virulence factors in the intestine, causing hemorrhagic colitis and, in severe cases, hemolytic uremic syndrome (HUS). HUS manifests with acute renal failure, hemolytic anemia and thrombocytopenia. Shiga toxin induces endothelial cell damage leading to platelet deposition in thrombi within the microvasculature and the development of thrombotic microangiopathy, mostly affecting the kidney. Red blood cells are destroyed in the occlusive capillary lesions. This review focuses on the importance of microvesicles shed from blood cells and their participation in the prothrombotic lesion, in hemolysis and in the transfer of toxin from the circulation into the kidney. Shiga toxin binds to blood cells and may undergo endocytosis and be released within microvesicles. Microvesicles normally contribute to intracellular communication and remove unwanted components from cells. Many microvesicles are prothrombotic as they are tissue factor- and phosphatidylserine-positive. Shiga toxin induces complement-mediated hemolysis and the release of complement-coated red blood cell-derived microvesicles. Toxin was demonstrated within blood cell-derived microvesicles that transported it to renal cells, where microvesicles were taken up and released their contents. Microvesicles are thereby involved in all cardinal aspects of Shiga toxin-associated HUS, thrombosis, hemolysis and renal failure.

Phospholipid flippase ATP11C is endocytosed and downregulated following Ca2+-mediated protein kinase C activation

We and others showed that ATP11A and ATP11C, members of the P4-ATPase family, translocate phosphatidylserine (PS) and phosphatidylethanolamine from the exoplasmic to the cytoplasmic leaflets at the plasma membrane. PS exposure on the outer leaflet of the plasma membrane in activated platelets, erythrocytes, and apoptotic cells was proposed to require the inhibition of PS-flippases, as well as activation of scramblases. Although ATP11A and ATP11C are cleaved by caspases in apoptotic cells, it remains unclear how PS-flippase activity is regulated in non-apoptotic cells. Here we report that the PS-flippase ATP11C, but not ATP11A, is sequestered from the plasma membrane via clathrin-mediated endocytosis upon Ca²⁺-mediated PKC activation. Importantly, we show that a characteristic di-leucine motif (SVRPLL) in the C-terminal cytoplasmic region of ATP11C becomes functional upon PKC activation. Moreover endocytosis of ATP11C is induced by Ca²⁺-signaling via Gq-coupled receptors. Our data provide the first evidence for signal-dependent regulation of mammalian P4-ATPase.

ATP11C is a flippase that uses ATP hydrolysis to translocate phospholipids at the plasma membrane. Here, the authors show that the activation of Ca2+-dependent protein kinase C increases ATP11C endocytosis thus downregulating phospholipid translocation.

Eryptosis in autoimmune haemolytic anaemia

OBJECTIVE

Haemolysis and anaemia related to autoimmune haemolytic anaemia (AIHA) of warm type (wAIHA) and of cold type (cAIHA) are believed to be solely due to antibody and/or complement-mediated destruction and clearance of red blood cells (RBCs). There is evidence that RBCs of affected patients may also undergo eryptosis, the suicidal death of RBCs.

METHOD

RBCs from 24 patients with wAIHA, 7 patients with chronic cAIHA and one patient with AIHA of mixed type were analysed for exposed phosphatidylserine (PS) by treatment with phycoerythrin-labelled Annexin V, and cell-associated fluorescence was measured using a MACSQuant flow cytometer.

RESULTS

PS-exposing RBCs were detected in 7 of 13 patients with clinically significant wAIHA. Haemolysis was mostly related to IgM or IgA autoantibodies (aab) in those patients. In contrast, PS exposure in 11 patients with wAIHA in complete remission was comparable to that in healthy blood donors. All patients with chronic cAIHA and the patient with AIHA of mixed type showed haemolytic activity and high numbers of PS-exposing RBCs. Patients with decompensated AIHA appear to respond to treatment with erythropoietin, which is a known inhibitor of eryptosis.

CONCLUSION

Eryptosis may frequently occur in AIHA related to IgM or IgA aab. Inhibition of eryptosis with erythropoietin may represent a new therapeutic option in the treatment of AIHA.

Lipids and ions traverse the membrane by the same physical pathway in the nhTMEM16 scramblase

From bacteria to mammals, different phospholipid species are segregated between the inner and outer leaflets of the plasma membrane by ATP-dependent lipid transporters. Disruption of this asymmetry by ATP-independent phospholipid scrambling is important in cellular signaling, but its mechanism remains incompletely understood. Using MD simulations coupled with experimental assays, we show that the surface hydrophilic transmembrane cavity exposed to the lipid bilayer on the fungal scramblase nhTMEM16 serves as the pathway for both lipid translocation and ion conduction across the membrane. Ca²⁺ binding stimulates its open conformation by altering the structure of transmembrane helices that line the cavity. We have identified key amino acids necessary for phospholipid scrambling and validated the idea that ions permeate TMEM16 Cl^- channels via a structurally homologous pathway by showing that mutation of two residues in the pore region of the TMEM16A Ca²⁺-activated Cl^- channel convert it into a robust scramblase.

Disease mutations reveal residues critical to the interaction of P4-ATPases with lipid substrates

Phospholipid flippases (P₄-ATPases) translocate specific phospholipids from the exoplasmic to the cytoplasmic leaflet of membranes. While there is good evidence that the overall molecular structure of flippases is similar to that of P-type ATPase ion-pumps, the transport pathway for the “giant” lipid substrate has not been determined. ATP8A2 is a flippase with selectivity toward phosphatidylserine (PS), possessing a net negatively charged head group, whereas ATP8B1 exhibits selectivity toward the electrically neutral phosphatidylcholine (PC). Setting out to elucidate the functional consequences of flippase disease mutations, we have identified residues of ATP8A2 that are critical to the interaction with the lipid substrate during the translocation process. Among the residues pinpointed are I91 and L308, which are positioned near proposed translocation routes through the protein. In addition we pinpoint two juxtaposed oppositely charged residues, E897 and R898, in the exoplasmic loop between transmembrane helices 5 and 6. The glutamate is conserved between PS and PC flippases, whereas the arginine is replaced by a negatively charged aspartate in ATP8B1. Our mutational analysis suggests that the glutamate repels the PS head group, whereas the arginine minimizes this repulsion in ATP8A2, thereby contributing to control the entry of the phospholipid substrate into the translocation pathway.

Regulation of cytokinesis during corticogenesis: focus on the midbody

Development of the cerebral cortices depends on tight regulation of cell divisions. In this system, stem and progenitor cells undergo symmetric and asymmetric divisions to ultimately produce neurons that establish the layers of the cortex. Cell division culminates with the formation of the midbody, a transient organelle that establishes the site of abscission between nascent daughter cells. During cytokinetic abscission, the final stage of cell division, one daughter cell will inherit the midbody remnant, which can then maintain or expel the remnant, but mechanisms and circumstances influencing this decision are unclear. This review describes the midbody and its constituent proteins, as well as the known consequences of their manipulation during cortical development. The potential functional relevance of midbody mechanisms is discussed.

Extracellular vesicles in renal disease

Extracellular vesicles, such as exosomes and microvesicles, are host cell-derived packages of information that allow cell-cell communication and enable cells to rid themselves of unwanted substances. The release and uptake of extracellular vesicles has important physiological functions and may also contribute to the development and propagation of inflammatory, vascular, malignant, infectious and neurodegenerative diseases. This Review describes the different types of extracellular vesicles, how they are detected and the mechanisms by which they communicate with cells and transfer information. We also describe their physiological functions in cellular interactions, such as in thrombosis, immune modulation, cell proliferation, tissue regeneration and matrix modulation, with an emphasis on renal processes. We discuss how the detection of extracellular vesicles could be utilized as biomarkers of renal disease and how they might contribute to disease processes in the kidney, such as in acute kidney injury, chronic kidney disease, renal transplantation, thrombotic microangiopathies, vasculitides, IgA nephropathy, nephrotic syndrome, urinary tract infection, cystic kidney disease and tubulopathies. Finally, we consider how the release or uptake of extracellular vesicles can be blocked, as well as the associated benefits and risks, and how extracellular vesicles might be used to treat renal diseases by delivering therapeutics to specific cells.

Phosphatidylserine Is the Signal for TAM Receptors and Their Ligands

Nature repeatedly repurposes, in that molecules that serve as metabolites, energy depots, or polymer subunits are at the same time used to deliver signals within and between cells. The preeminent example of this repurposing is ATP, which functions as a building block for nucleic acids, an energy source for enzymatic reactions, a phosphate donor to regulate intracellular signaling, and a neurotransmitter to control the activity of neurons. A series of recent studies now consolidates the view that phosphatidylserine (PtdSer), a common phospholipid constituent of membrane bilayers, is similarly repurposed for use as a signal between cells and that the ligands and receptors of the Tyro3/Axl/Mer (TAM) family of receptor tyrosine kinases (RTKs) are prominent transducers of this signal.

Recombinant Phospholipase D from Loxosceles gaucho Binds to Platelets and Promotes Phosphatidylserine Exposure

Spider envenomation, from the genus Loxosceles, is frequently reported as a cause of necrotic lesions in humans around the world. Among the many components found in the venom of Loxosceles genus, phospholipases D (PLDs) are the most investigated, since they can cause a massive inflammatory response, dermonecrosis, hemolysis and platelet aggregation, among other effects. Even though the PLDs induce strong platelet aggregation, there are no studies showing how the PLDs interact with platelets to promote this effect. Since many agonists must interact with specific receptors on the platelet membrane to induce aggregation, it is reasonable to expect that the PLDs may, in some way, also interact with platelets, to induce this activity. Therefore, to address this possibility, in this work, a recombinant PLD, called LgRec1, from L. gaucho was fused to enhanced green fluorescent protein (EGFP) and used as a probe to detect the interaction of LgRec1 to platelets, by fluorescence-activated cell sorter (FACS) and confocal microscopy. The preservation of biological activities of this chimera toxin was also analyzed. As a first, the results show that LgRec1 does not require plasma components to bind to platelets, although these components are necessary to LgRec1 to induce platelet aggregation. Also, the attachment of LgRec1 to human platelets’ cell membranes suggests that the exposure of phosphatidylserine (PS) may act as a scaffold for coagulation factors. Therefore, the results add new information about the binding of Loxosceles PLDs to platelets, which may help unravel how these toxins promote platelet aggregation.

Microvesicles in vascular homeostasis and diseases. Position Paper of the European Society of Cardiology (ESC) Working Group on Atherosclerosis and Vascular Biology

Microvesicles are members of the family of extracellular vesicles shed from the plasma membrane of activated or apoptotic cells. Microvesicles were initially characterised by their pro-coagulant activity and described as "microparticles". There is mounting evidence revealing a role for microvesicles in intercellular communication, with particular relevance to hemostasis and vascular biology. Coupled with this, the potential of microvesicles as meaningful biomarkers is under intense investigation. This Position Paper will summarise the current knowledge on the mechanisms of formation and composition of microvesicles of endothelial, platelet, red blood cell and leukocyte origin. This paper will also review and discuss the different methods used for their analysis and quantification, will underline the potential biological roles of these vesicles with respect to vascular homeostasis and thrombosis and define important themes for future research.

Poring over furrows

Cryo-electron microscopy reveals the structure of a chloride channel that is closely related to a protein that transports lipids.

Characterization of the scrambling domain of the TMEM16 family

The TMEM16 protein family has 10 members, each of which carries 10 transmembrane segments. TMEM16A and 16B are Ca²⁺-activated Cl^- channels. Several other members, including TMEM16F, promote phospholipid scrambling between the inner and outer leaflets of a cell membrane in response to intracellular Ca²⁺ However, the mechanism by which TMEM16 proteins translocate phospholipids in plasma membranes remains elusive. Here we show that Ca²⁺-activated, TMEM16F-supported phospholipid scrambling proceeds at 4 °C. Similar to TMEM16F and 16E, seven TMEM16 family members were found to carry a domain (SCRD; scrambling domain) spanning the fourth and fifth transmembrane segments that conferred scrambling ability to TMEM16A. By introducing point mutations into TMEM16F, we found that a lysine in the fourth transmembrane segment of the SCRD as well as an arginine in the third and a glutamic acid in the sixth transmembrane segment were important for exposing phosphatidylserine from the inner to the outer leaflet. However, their role in internalizing phospholipids was limited. Our results suggest that TMEM16 provides a cleft containing hydrophilic "stepping stones" for the outward translocation of phospholipids.

Elimination of the unnecessary: Intra- and extracellular signaling by anionic phospholipids

High fidelity of biological systems is frequently achieved by duplication of the essential intracellular machineries or, removal of the entire cell, which becomes unnecessary or even harmful in altered physiological environments. Carefully controlled removal of these cells, without damaging normal cells, requires precise signaling, and is critical to maintaining homeostasis. This review describes how two anionic phospholipids - phosphatidylserine (PS) and cardiolipin (CL) - residing in distinct compartments of the cell, signal removal of "the unnecessary" using several uniform principles. One of these principles is realized by collapse of inherent transmembrane asymmetry and the externalization of the signal on the outer membrane surface - mitochondria for CL and the plasma membrane for PS - to trigger mitophagy and phagocytosis, respectively. Release from damaged cells of intracellular structures with externalized CL or externalized PS triggers their elimination by phagocytosis. Another of these principles is realized by oxidation of polyunsaturated species of CL and PS. Highly specific oxidation of CL by cytochrome c serves as a signal for mitochondria-dependent apoptosis, while oxidation of externalized PS improves its effectiveness to trigger phagocytosis of effete cells.

Anoctamins/TMEM16 Proteins: Chloride Channels Flirting with Lipids and Extracellular Vesicles

Anoctamin (ANO)/TMEM16 proteins exhibit diverse functions in cells throughout the body and are implicated in several human diseases. Although the founding members ANO1 (TMEM16A) and ANO2 (TMEM16B) are Ca²⁺-activated Cl^- channels, most ANO paralogs are Ca²⁺-dependent phospholipid scramblases that serve as channels facilitating the movement (scrambling) of phospholipids between leaflets of the membrane bilayer. Phospholipid scrambling significantly alters the physical properties of the membrane and its landscape and has vast downstream signaling consequences. In particular, phosphatidylserine exposed on the external leaflet of the plasma membrane functions as a ligand for receptors vital for cell-cell communication. A major consequence of Ca²⁺-dependent scrambling is the release of extracellular vesicles that function as intercellular messengers by delivering signaling proteins and noncoding RNAs to alter target cell function. We discuss the physiological implications of Ca²⁺-dependent phospholipid scrambling, the extracellular vesicles associated with this activity, and the roles of ANOs in these processes.

P2X7 receptor activation causes phosphatidylserine exposure in canine erythrocytes

AIM

To determine if activation of the ATP-gated P2X7 receptor channel induces phosphatidylserine (PS) exposure in erythrocytes from multiple dog breeds.

METHODS

Peripheral blood was collected from 25 dogs representing 13 pedigrees and seven crossbreeds. ATP-induced PS exposure on canine erythrocytes in vitro was assessed using a flow cytometric Annexin V binding assay.

RESULTS

ATP induced PS exposure in erythrocytes from all dogs studied. ATP caused PS exposure in a concentration-dependent manner with an EC₅₀ value of 395 μmol/L. The non-P2X7 agonists, ADP or AMP, did not cause PS exposure. The P2X7 antagonist, AZ10606120, but not the P2X1 antagonist, NF449, blocked ATP-induced PS exposure.

CONCLUSION

The results indicate that ATP induces PS exposure in erythrocytes from various dog breeds and that this process is mediated by P2X7 activation.

TMEM16F-Mediated Platelet Membrane Phospholipid Scrambling Is Critical for Hemostasis and Thrombosis but not Thromboinflammation in Mice—Brief Report

Objective—

It is known that both platelets and coagulation strongly influence infarct progression after ischemic stroke, but the mechanisms and their interplay are unknown. Our aim was to assess the contribution of the procoagulant platelet surface, and thus platelet-driven thrombin generation, to the progression of thromboinflammation in the ischemic brain.

Approach and Results—

We present the characterization of a novel platelet and megakaryocyte-specific TMEM16F (anoctamin 6) knockout mouse. Reflecting Scott syndrome, platelets from the knockout mouse had a significant reduction in procoagulant characteristics that altered thrombin and fibrin generation kinetics. In addition, knockout mice showed significant defects in hemostasis and arterial thrombus formation. However, infarct volumes in a model of ischemic stroke were comparable with wild-type mice.

Conclusions—

Platelet TMEM16F activity contributes significantly to hemostasis and thrombosis but not cerebral thromboinflammation. These results highlight another key difference between the roles of platelets and coagulation in these processes.

Independent activation of distinct pores in dimeric TMEM16A channels

The TMEM16 family encompasses Ca²⁺-activated Cl^- channels (CaCCs) and lipid scramblases. These proteins are formed by two identical subunits, as confirmed by the recently solved crystal structure of a TMEM16 lipid scramblase. However, the high-resolution structure did not provide definitive information regarding the pore architecture of the TMEM16 channels. In this study, we express TMEM16A channels constituting two covalently linked subunits with different Ca²⁺ affinities. The dose-response curve of the heterodimer appears to be a weighted sum of two dose-response curves-one corresponding to the high-affinity subunit and the other to the low-affinity subunit. However, fluorescence resonance energy transfer experiments suggest that the covalently linked heterodimeric proteins fold and assemble as one molecule. Together these results suggest that activation of the two TMEM16A subunits likely activate independently of each other. The Ca²⁺ activation curve for the heterodimer at a low Ca²⁺ concentration range ([Ca²⁺] < 5 µM) is similar to that of the wild-type channel-the Hill coefficients in both cases are significantly greater than one. This suggests that Ca²⁺ binding to one subunit of TMEM16A is sufficient to activate the channel and that each subunit contains more than one Ca²⁺-binding site. We also take advantage of the I-V curve rectification that results from mutation of a pore residue to address the pore architecture of the channel. By introducing the pore mutation and the mutation that alters Ca²⁺ affinity in the same or different subunits, we demonstrate that activation of different subunits appears to be associated with the opening of different pores. These results suggest that the TMEM16A CaCC may also adopt a "double-barrel" pore architecture, similar to that found in CLC channels and transporters.

Neo1 and phosphatidylethanolamine contribute to vacuole membrane fusion in Saccharomyces cerevisiae

NEO1 is an essential gene in budding yeast and belongs to a highly conserved subfamily of P-type ATPase genes that encode phospholipid flippases. Inactivation of temperature sensitive neo1^ts alleles produces pleiomorphic defects in the secretory and endocytic pathways, including fragmented vacuoles. A screen for multicopy suppressors of neo1-2^ts growth defects yielded YPT7, which encodes a Rab7 homolog involved in SNARE-dependent vacuolar fusion. YPT7 suppressed the vacuole fragmentation phenotype of neo1-2, but did not suppress Golgi-associated protein trafficking defects. Neo1 localizes to Golgi and endosomal membranes and was only observed in the vacuole membrane, where Ypt7 localizes, in retromer mutants or when highly overexpressed in wild-type cells. Phosphatidylethanolamine (PE) has been implicated in Ypt7-dependent vacuolar membrane fusion in vitro and is a potential transport substrate of Neo1. Strains deficient in PE synthesis (psd1Δ psd2Δ) displayed fragmented vacuoles and the neo1-2 fragmented vacuole phenotype was also suppressed by overexpression of PSD2, encoding a phosphatidylserine decarboxylase that produces PE at endosomes. In contrast, neo1-2 was not suppressed by overexpression of VPS39, an effector of Ypt7 that forms a membrane contact site potentially involved in PE transfer between vacuoles and mitochondria. These results support the crucial role of PE in vacuole membrane fusion and implicate Neo1 in concentrating PE in the cytosolic leaflet of Golgi and endosomes, and ultimately the vacuole membrane.

Xkr8 phospholipid scrambling complex in apoptotic phosphatidylserine exposure

Xk-related protein (Xkr) 8, a protein carrying 10 transmembrane regions, is essential for scrambling phospholipids during apoptosis. Here, we found Xkr8 as a complex with basigin (BSG) or neuroplastin (NPTN), type I membrane proteins in the Ig superfamily. In BSG(-/-)NPTN(-/-) cells, Xkr8 localized intracellularly, and the apoptosis stimuli failed to expose phosphatidylserine, indicating that BSG and NPTN chaperone Xkr8 to the plasma membrane to execute its scrambling activity. Mutational analyses of BSG showed that the atypical glutamic acid in the transmembrane region is required for BSG's association with Xkr8. In cells exposed to apoptotic signals, Xkr8 was cleaved at the C terminus and the Xkr8/BSG complex formed a higher-order complex, likely to be a heterotetramer consisting of two molecules of Xkr8 and two molecules of BSG or NPTN, suggesting that this cleavage causes the formation of a larger complex of Xkr8-BSG/NPTN for phospholipid scrambling.

Role of Ca(2+) in the Stability and Function of TMEM16F and 16K

There are 10 transmembrane protein (TMEM) 16-family proteins in humans and mice. Among them, TMEM16F acts as a Ca(2+)-dependent phospholipid scramblase at the plasma membrane. However, how Ca(2+) activates TMEM16F's phospholipid-scramblase activity has not been elucidated. Here we found that in the presence of Ca(2+), TMEM16K (whose function is unknown) directly binds Ca(2+) to form a stable complex that can be detected by blue-native polyacrylamide gel electrophoresis. In the absence of Ca(2+), TMEM16K and TMEM16F aggregated, suggesting that their structure is stabilized by Ca(2+). Comprehensive mutagenesis of acidic residues in TMEM16K's cytoplasmic and transmembrane regions identified five residues that are critical for binding Ca(2+). These residues were well conserved between TMEM16F and 16K, and point mutations of these residues in TMEM16F reduced its ability to support Ca(2+)-dependent phospholipid scrambling. Our results suggest that Ca(2+) binds TMEM16F directly and induces conformational changes that support its stability and function.

Topoisomerase IIα poisoning and DNA double-strand breaking by chiral ruthenium(ii) complexes containing 2-furanyl-imidazo[4,5-f][1,10]phenanthroline derivatives

Some chiral ruthenium(ii) complexes bearing furan ligands were developed to act as topoisomerase IIα poisons and caused DNA double-strand damage that could lead to apoptosis.