A novel phosphatidylserine-binding peptide motif defined by an anti-idiotypic monoclonal antibody. Localization of phosphatidylserine-specific binding sites on protein kinase C and phosphatidylserine decarboxylase

A Peptide-Based Fluorescent Sensor for Anionic Phospholipids

Anionic phospholipids are key cell signal mediators. The distribution of these lipids on the cell membrane and intracellular organelle membranes guides the recruitment of signaling proteins leading to the regulation of cellular processes. Hence, fluorescent sensors that can detect anionic phospholipids within living cells can provide a handle into revealing molecular mechanisms underlying lipid-mediated signal regulation. A major challenge in the detection of anionic phospholipids is related to the presence of these phospholipids mostly in the inner leaflet of the plasma membrane and in the membranes of intracellular organelles. Hence, cell-permeable sensors would provide an advantage by enabling the rapid detection and tracking of intracellular pools of anionic phospholipids. We have developed a peptide-based, cell-permeable, water-soluble, and ratiometric fluorescent sensor that entered cells within 15 min of incubation via the endosomal machinery and showed punctate labeling in the cytoplasm. The probe could also be introduced into living cells via lipofection, which allows bypassing of endosomal uptake, to image anionic phospholipids in the cell membrane. We validated the ability of the sensor toward detection of intracellular anionic phospholipids by colocalization studies with a fluorescently tagged lipid and a protein-based anionic phospholipid sensor. Further, the sensor could image the externalization of anionic phospholipids during programmed cell death, indicating the ability of the probe toward detection of both intra- and extracellular anionic phospholipids based on the biological context.

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.

Aberrant environment and PS-binding to calnuc C-terminal tail drives exosomal packaging and its metastatic ability

The characteristic features of cancer cells are aberrant (acidic) intracellular pH and elevated levels of phosphatidylserine. The primary focus of cancer research is concentrated on the discovery of biomarkers directed towards early diagnosis and therapy. It has been observed that azoxymethane-treated mice demonstrate an increased expression of calnuc (a multi-domain, Ca2+- and DNA-binding protein) in their colon, suggesting it to be a good biomarker of carcinogenesis. We show that culture supernatants from tumor cells have significantly higher amounts of secreted calnuc compared to non-tumor cells, selectively packaged into exosomes. Exosomal calnuc is causal for epithelial-mesenchymal transition and atypical migration in non-tumor cells, which are key events in tumorigenesis and metastasis. In vitro studies reveal a significant affinity for calnuc towards phosphatidylserine, specifically to its C-terminal region, leading to the formation of 'molten globule' conformation. Similar structural changes are observed at acidic pH (pH 4), which demonstrates the role of the acidic microenvironment in causing the molten globule conformation and membrane interaction. On a precise note, we propose that the molten globule structure of calnuc caused by aberrant conditions in cancer cells to be the causative mechanism underlying its exosome-mediated secretion, thereby driving metastasis.

Apoptosis Imaging in Oncology by Means of Positron Emission Tomography: A Review

To date, a wide variety of potential PET-apoptosis imaging radiopharmaceuticals targeting apoptosis-induced cell membrane asymmetry and acidification, as well as caspase 3 activation (substrates and inhibitors) have been developed with the purpose of rapidly assessing the response to treatment in cancer patients. Many of these probes were shown to specifically bind to their apoptotic target in vitro and their uptake to be enhanced in the in vivo-xenografted tumours in mice treated by means of chemotherapy, however, to a significantly variable degree. This may, in part, relate to the tumour model used given the fact that different tumour cell lines bear a different sensitivity to a similar chemotherapeutic agent, to differences in the chemotherapeutic concentration and exposure time, as well as to the different timing of imaging performed post-treatment. The best validated cell membrane acidification and caspase 3 targeting radioligands, respectively ¹⁸F-ML-10 from the Aposense family and the radiolabelled caspase 3 substrate ¹⁸F-CP18, have also been injected in healthy individuals and shown to bear favourable dosimetric and safety characteristics. However, in contrast to, for instance, the ^99mTc-HYNIC-Annexin V, neither of both tracers was taken up to a significant degree by the bone marrow in the healthy individuals under study. Removal of white and red blood cells from the bone marrow through apoptosis plays a major role in the maintenance of hematopoietic cell homeostasis. The major apoptotic population in normal bone marrow are immature erythroblasts. While an accurate estimate of the number of immature erythroblasts undergoing apoptosis is not feasible due to their unknown clearance rate, their number is likely substantial given the ineffective quote of the erythropoietic process described in healthy subjects. Thus, the clinical value of both ¹⁸F-ML-10 and ¹⁸F-CP18 for apoptosis imaging in cancer patients, as suggested by a small number of subsequent clinical phase I/II trials in patients suffering from primary or secondary brain malignancies using ¹⁸F-ML-10 and in an ongoing trial in patients suffering from cancer of the ovaries using ¹⁸F-CP18, remains to be proven and warrants further investigation.

Radiolabeled Peptides for Molecular Imaging of Apoptosis

Apoptosis is a regulated cell death induced by extrinsic and intrinsic stimulants. Tracking of apoptosis provides an opportunity for the assessment of cardiovascular and neurodegenerative diseases as well as monitoring of cancer therapy at early stages. There are some key mediators in apoptosis cascade, which could be considered as specific targets for delivering imaging or therapeutic agents. The targeted radioisotope-based imaging agents are able to sensitively detect the physiological signal pathways which make them suitable for apoptosis imaging at a single-cell level. Radiopeptides take advantage of both the high sensitivity of nuclear imaging modalities and favorable features of peptide scaffolds. The aim of this study is to review the characteristics of those radiopeptides targeting apoptosis with different mechanisms.

Targeting phosphatidylserine for radionuclide-based molecular imaging of apoptosis

One major characteristic of programmed cell death (apoptosis) results in the increased expression of phosphatidylserine (PS) on the outer membrane of dying cells. Consequently, PS represents an excellent target for non-invasive imaging of apoptosis by single-photon emission computed tomography (SPECT) and positron emission tomography (PET). Annexin V is a 36 kDa protein which binds with high affinity to PS in the presence of Ca²⁺ ions. This makes radiolabeled annexins valuable apoptosis imaging agents for clinical and biomedical research applications for monitoring apoptosis in vivo. However, the use of radiolabeled annexin V for in vivo imaging of cell death has been met with a variety of challenges which have prevented its translation into the clinic. These difficulties include: complicated and time-consuming radiolabeling procedures, sub-optimal biodistribution, inadequate pharmacokinetics leading to poor tumour-to-blood contrast ratios, reliance upon Ca²⁺ concentrations in vivo, low tumor tissue penetration, and an incomplete understanding of what constitutes the best imaging protocol following induction of apoptosis. Therefore, new concepts and improved strategies for the development of PS-binding radiotracers are needed. Radiolabeled PS-binding peptides and various Zn(II) complexes as phosphate chemosensors offer an innovative strategy for radionuclide-based molecular imaging of apoptosis with PET and SPECT. Radiolabeled peptides and Zn(II) complexes provide several advantages over annexin V including better pharmacokinetics due to their smaller size, better availability, simpler synthesis and radiolabeling strategies as well as facilitated tissue penetration due to their smaller size and faster blood clearance profile allowing for optimized image contrast. In addition, peptides can be structurally modified to improve metabolic stability along with other pharmacokinetic and pharmacodynamic properties. The present review will summarize the current status of radiolabeled annexins, peptides and Zn(II) complexes developed as radiotracers for imaging apoptosis through targeting PS utilizing PET and SPECT imaging.

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.

Involvement of a putative substrate binding site in the biogenesis and assembly of phosphatidylserine decarboxylase 1 from Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae, the mitochondrial phosphatidylserine decarboxylase 1 (Psd1p) produces the largest amount of cellular phosphatidylethanolamine (PE). Psd1p is synthesized as a larger precursor on cytosolic ribosomes and then imported into mitochondria in a three-step processing event leading to the formation of an α-subunit and a β-subunit. The α-subunit harbors a highly conserved motif, which was proposed to be involved in phosphatidylserine (PS) binding. Here, we present a molecular analysis of this consensus motif for the function of Psd1p by using Psd1p variants bearing either deletions or point mutations in this region. Our data show that mutations in this motif affect processing and stability of Psd1p, and consequently the enzyme's activity. Thus, we conclude that this consensus motif is essential for structural integrity and processing of Psd1p.

Synthesis, gallium labelling and characterization of P04087, a functionalized phosphatidylserine-binding peptide

Background

Radiolabeled phosphatidylserine (PS)-binding peptides represent an innovative strategy for molecular imaging of apoptosis and thrombus. The hexapeptide PGDLSR was described as a selective and high affinity ligand for PS. In this work, we synthesized and evaluated a gallium labelled-PGDLSR peptide as a potential and selective radiopharmaceutical for nuclear imaging. PGDLSR-β-alanine-NODAGA (P04087) was prepared using Fmoc-based synthesis and then chelated with cold gallium, ⁶⁸Ga and ⁶⁷Ga. The affinity of Ga-P04087 for PS was evaluated by a competitive binding assay using biotinylated AnnexinV. The in vitro stability of the radiotracer was checked at room temperature and after incubation in human serum at 37 °C with and without a metalloprotease inhibitor. The in vivo binding of ⁶⁷Ga-P04087 to phosphatidylserine was evaluated in a rat model of infective endocarditis.

Results

PGDLSR was successfully prepared with a yield of 31%. P04087 was obtained with a yield of 28% and in high chemical purity (>95%). The radiochemical purities of ⁶⁷Ga-P04087 and ⁶⁸Ga-P04087 exceeded 98% in all cases. IC50 of P04087 and Ga-P04087 were in the same order of magnitude (10⁻⁷M). The radiolabelled product was stable for 24 h at room temperature, but was very rapidly degraded in human serum in the absence of a protease inhibitor, which had a stabilizing effect. No focal uptake could be detected visually in the cardiac area on SPECT images. On autoradiography however, a focal uptake of ⁶⁷Ga-P04087 in the valve area was present and histological slices demonstrated localization of peptide binding at the peripheral layer of vegetations.

Conclusion

In spite of the preservation of the peptide affinity to the PS after its conjugation to the NODAGA chelator, and of the presence of ⁶⁷Ga-P04087 uptake on autoradiography, the absence of detectable foci in vivo in the valve area may be attributed to both the low intensity of the signal and the presence of background activity originating from blood pool and surrounding tissues in the living animals. Further modifications are necessary to design a radiolabeled peptide with higher binding potencies to PS while possessing enhanced metabolic stability in vivo.

Cell biology, physiology and enzymology of phosphatidylserine decarboxylase

Phosphatidylethanolamine is one of the most abundant phospholipids whose major amounts are formed by phosphatidylserine decarboxylases (PSD). Here we provide a comprehensive description of different types of PSDs in the different kingdoms of life. In eukaryotes, type I PSDs are mitochondrial enzymes, whereas other PSDs are localized to other cellular compartments. We describe the role of mitochondrial Psd1 proteins, their function, enzymology, biogenesis, assembly into mitochondria and their contribution to phospholipid homeostasis in much detail. We also discuss briefly the cellular physiology and the enzymology of Psd2. This article is part of a Special Issue entitled: Lipids of Mitochondria edited by Guenther Daum.

Identification of the minimum pharmacophore of lipid-phosphatidylserine (PS) binding peptide-peptoid hybrid PPS1D1

We previously reported a unique peptide-peptoid hybrid, PPS1 that specifically recognizes lipid-phosphatidylserine (PS) and a few other negatively charged phospholipids, but not neutral phospholipids, on the cell membrane. The dimeric version of PPS1, i.e., PPS1D1 triggers strong cancer cell cytotoxicity and has been validated in lung cancer models both in vitro and in vivo. Given that PS and other negatively charged phospholipids are abundant in almost all tumor microenvironments, PPS1D1 is an attractive drug lead that can be developed into a globally applicable anti-cancer agent. Therefore, it is extremely important to identify the minimum pharmacophore of PPS1D1. In this study, we have synthesized alanine/sarcosine derivatives as well as truncated derivatives of PPS1D1. We performed ELISA-like competitive binding assay to evaluate the PS-recognition potential and standard MTS cell viability assay on HCC4017 lung cancer cells to validate the cell cytotoxicity effects of these derivatives. Our studies indicate that positively charged residues at the second and third positions, as well as four hydrophobic residues at the fifth through eighth positions, are imperative for the binding and activity of PPS1D1. Methionine at the first position was not essential, whereas the positively charged Nlys at the fourth position was minimally needed, as two derivatives that were synthesized replacing this residue were almost as active as PPS1D1.

Radiolabeled apoptosis imaging agents for early detection of response to therapy

Since apoptosis plays an important role in maintaining homeostasis and is associated with responses to therapy, molecular imaging of apoptotic cells could be useful for early detection of therapeutic effects, particularly in oncology. Radiolabeled annexin V compounds are the hallmark in apoptosis imaging in vivo. These compounds are reviewed from the genesis of apoptosis (cell death) imaging agents up to recent years. They have some disadvantages, including slow clearance and immunogenicity, because they are protein-based imaging agents. For this reason, several studies have been conducted in recent years to develop low molecule apoptosis imaging agents. In this review, radiolabeled phosphatidylserine targeted peptides, radiolabeled bis(zinc(II)-dipicolylamine) complex, radiolabeled 5-fluoropentyl-2-methyl-malonic acid (ML-10), caspase-3 activity imaging agents, radiolabeled duramycin, and radiolabeled phosphonium cation are reviewed as promising low-molecular-weight apoptosis imaging agents.

Polymeric micelles for apoptosis-targeted optical imaging of cancer and intraoperative surgical guidance

In a two-step strategy, an intraperitoneal (IP) injection of poly(ethylene glycol)-block-poly(ε-caprolactone) (PEG-b-PCL) micelles containing paclitaxel (PTX), cyclopamine (CYP), and gossypol (GSP) at 30, 30, and 30 mg/kg, respectively, debulked tumor tissues by 1.3-fold, based on loss of bioluminescence with <10% body weight change, and induced apoptosis in peritoneal tumors when used as neoadjuvant chemotherapy (NACT) in an ES-2-luc-bearing xenograft model for ovarian cancer. In a second step, a single intravenous (IV) injection of apoptosis-targeting GFNFRLKAGAKIRFGS-PEG-b-PCL micelles containing a near-infrared (NIR) fluorescence probe, DiR (1,1′-dioctadecyltetramethyl indotricarbocyanine iodide), resulted in increased peritoneal DiR accumulation in apoptosis-induced ES-2-luc tumor tissues (ex vivo) by 1.5-fold compared with DiR molecules delivered by methoxy PEG-b-PCL micelles (non-targeted) at 48 h after IV injection in a second step. As a result, a tandem of PEG-b-PCL micelles enabled high-resolution detection of ca. 1 mm diameter tumors, resulting in resection of approximately 90% of tumors, and a low peritoneal cancer index (PCI) of ca. 7. Thus, a tandem of PEG-b-PCL micelles used for NCAT and NIR fluorescence imaging of therapy-induced apoptosis for intraoperative surgical guidance may be a promising treatment strategy for metastatic ovarian cancer.

Illuminating the lipidome to advance biomedical research: peptide-based probes of membrane lipids

Systematic investigation of the lipidome will reveal new opportunities for disease diagnosis and intervention. However, lipidomic research has been hampered by the lack of molecular tools to track specific lipids of interest. Accumulating reports indicate lipid recognition can be achieved with properly constructed short peptides in addition to large proteins. This review summarizes the key developments of this area within the past decade. Select lantibiotic peptides present the best examples of low-molecular-weight probes of membrane lipids, displaying selectivity comparable to lipid-binding proteins. Designed peptides, through biomimetic approaches and combinational screening, have begun to demonstrate their potential for lipid tracking in cultured cells and even in living organisms. Biophysical characterization of these lipid-targeting peptides has revealed certain features critical for selective membrane binding, including preorganized scaffolds and the balance of polar and nonpolar interactions. The knowledge summarized herein should facilitate the development of molecular tools to target a variety of membrane lipids.

Role of Phospholipids in Endocytosis, Phagocytosis, and Macropinocytosis

Endocytosis, phagocytosis, and macropinocytosis are fundamental processes that enable cells to sample their environment, eliminate pathogens and apoptotic bodies, and regulate the expression of surface components. While a great deal of effort has been devoted over many years to understanding the proteins involved in these processes, the important contribution of phospholipids has only recently been appreciated. This review is an attempt to collate and analyze the rapidly emerging evidence documenting the role of phospholipids in clathrin-mediated endocytosis, phagocytosis, and macropinocytosis. A primer on phospholipid biosynthesis, catabolism, subcellular distribution, and transport is presented initially, for reference, together with general considerations of the effects of phospholipids on membrane curvature and charge. This is followed by a detailed analysis of the critical functions of phospholipids in the internalization processes and in the maturation of the resulting vesicles and vacuoles as they progress along the endo-lysosomal pathway.

Accelerating membrane insertion of peripheral proteins with a novel membrane mimetic model

Characterizing atomic details of membrane binding of peripheral membrane proteins by molecular dynamics (MD) has been significantly hindered by the slow dynamics of membrane reorganization associated with the phenomena. To expedite lateral diffusion of lipid molecules without sacrificing the atomic details of such interactions, we have developed a novel membrane representation, to our knowledge, termed the highly mobile membrane-mimetic (HMMM) model to study binding and insertion of various molecular species into the membrane. The HMMM model takes advantage of an organic solvent layer to represent the hydrophobic core of the membrane and short-tailed phospholipids for the headgroup region. We demonstrate that using these components, bilayer structures are formed spontaneously and rapidly, regardless of the initial position and orientation of the lipids. In the HMMM membrane, lipid molecules exhibit one to two orders of magnitude enhancement in lateral diffusion. At the same time, the membrane atomic density profile of the headgroup region produced by the HMMM model is essentially identical to those obtained for full-membrane models, indicating the faithful representation of the membrane surface by the model. We demonstrate the efficiency of the model in capturing spontaneous binding and insertion of peripheral proteins by using the membrane anchor (γ-carboxyglutamic-acid-rich domain; GLA domain) of human coagulation factor VII as a test model. Achieving full insertion of the GLA domain consistently in 10 independent unbiased simulations within short simulation times clearly indicates the robustness of the HMMM model in capturing membrane association of peripheral proteins very efficiently and reproducibly. The HMMM model will provide significant improvements to the current all-atom models by accelerating lipid dynamics to examine protein-membrane interactions more efficiently.

Processing and topology of the yeast mitochondrial phosphatidylserine decarboxylase 1

The inner mitochondrial membrane plays a crucial role in cellular lipid homeostasis through biosynthesis of the non-bilayer-forming lipids phosphatidylethanolamine and cardiolipin. In the yeast Saccharomyces cerevisiae, the majority of cellular phosphatidylethanolamine is synthesized by the mitochondrial phosphatidylserine decarboxylase 1 (Psd1). The biogenesis of Psd1 involves several processing steps. It was speculated that the Psd1 precursor is sorted into the inner membrane and is subsequently released into the intermembrane space by proteolytic removal of a hydrophobic sorting signal. However, components involved in the maturation of the Psd1 precursor have not been identified. We show that processing of Psd1 involves the action of the mitochondrial processing peptidase and Oct1 and an autocatalytic cleavage at a highly conserved LGST motif yielding the α- and β-subunit of the enzyme. The Psd1 β-subunit (Psd1β) forms the membrane anchor, which binds the intermembrane space-localized α-subunit (Psd1α). Deletion of a transmembrane segment in the β-subunit results in mislocalization of Psd1 and reduced enzymatic activity. Surprisingly, autocatalytic cleavage does not depend on proper localization to the inner mitochondrial membrane. In summary, membrane integration of Psd1 is crucial for its functionality and for maintenance of mitochondrial lipid homeostasis.

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

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

Control of protein kinase C activity, phorbol ester-induced cytoskeletal remodeling, and cell survival signals by the scaffolding protein SSeCKS/GRAVIN/AKAP12

The product of the SSeCKS/GRAVIN/AKAP12 gene ("SSeCKS") is a major protein kinase (PK) C substrate that exhibits tumor- and metastasis-suppressing activity likely through its ability to scaffold multiple signaling mediators such as PKC, PKA, cyclins, calmodulin, and Src. Although SSeCKS and PKCα bind phosphatidylserine, we demonstrate that phosphatidylserine-independent binding of PKC by SSeCKS is facilitated by two homologous SSeCKS motifs, EG(I/V)(T/S)XWXSFK(K/R)(M/L)VTP(K/R)K(K/R)X(K/R)XXXEXXXE(E/D) (amino acids 592-620 and 741-769). SSeCKS binding to PKCα decreased kinase activity and was dependent on the two PKC-binding motifs. SSeCKS scaffolding of PKC was increased in confluent cell cultures, correlating with significantly increased SSeCKS protein levels and decreased PKCα activity, suggesting a role for SSeCKS in suppressing PKC activation during contact inhibition. SSeCKS-null mouse embryo fibroblasts displayed increased relative basal and phorbol ester (phorbol 12-myristate 13-acetate)-induced PKC activity but were defective in phorbol 12-myristate 13-acetate-induced actin cytoskeletal reorganization and cell shape change; these responses could be rescued by the forced expression of full-length SSeCKS but not by an SSeCKS variant deleted of its PKC-binding domains. Finally, the PKC binding sites in SSeCKS were required to restore cell rounding and/or decreased apoptosis in phorbol ester-treated LNCaP, LNCaP-C4-2, and MAT-LyLu prostate cancer cells. Thus, PKC-mediated remodeling of the actin cytoskeleton is likely regulated by the ability of SSeCKS to control PKC signaling and activity through a direct scaffolding function.

Identification of novel anionic phospholipid binding domains in neutral sphingomyelinase 2 with selective binding preference

Sphingolipids such as ceramide are recognized as vital regulators of many biological processes. Neutral sphingomyelinase 2 (nSMase2) is one of the key enzymes regulating ceramide production. It was previously shown that the enzymatic activity of nSMase2 was dependent on anionic phospholipids (APLs). In this study, the structural requirements for APL-selective binding of nSMase2 were determined and characterized. Using lipid-protein overlay assays, nSMase2 interacted specifically and directly with several APLs, including phosphatidylserine and phosphatidic acid. Lipid-protein binding studies of deletion mutants identified two discrete APL binding domains in the N terminus of nSMase2. Further, mutagenesis experiments pinpointed the core sequences and major cationic amino acids in the domains that are necessary for the cooperative activation of nSMase2 by APLs. The first domain included the first amino-terminal hydrophobic segment and Arg-33, which were essential for nSMase2 to interact with APLs. The second binding domain was comprised of the second hydrophobic segment and Arg-92 and Arg-93. Moreover, mutation of one or both domains decreased APL binding and APL-dependent catalytic activity of nSMase2. Further, mutation of both domains in nSMase2 reduced its plasma membrane localization. Finally, these binding domains are also important for the capability of nSMase2 to rescue the defects of yeast lacking the nSMase homologue, ISC1. In conclusion, these data have identified the APL binding domains of nSMase2 for the first time. The analysis of interactions between nSMase2 and APLs will contribute to our understanding of signaling pathways mediated by sphingolipid metabolites.

Peptide-based imaging agents targeting phosphatidylserine for the detection of apoptosis

A 14-residue phosphatidylserine (PS)-binding peptide FNFRLKAGQKIRFG (PSBP-0) was scanned with Ala. In addition, a radiometal chelator (SAAC) was introduced at selected sites of the lead peptides. Substitution of the Gln(6) residue in PSBP-0 with Ala resulted in a significant increase in binding affinity to PS as determined by surface plasmon resonance sensorgrams. The binding affinity of the resulting peptide FNFRLKAGAKIRFG (PSBP-6, molecular mass = 1623 Da) to PS (K(d) ∼ 100 nM) increased 10-fold as compared to PSBP-0 (K(d) ∼ 1.38 μM). Introduction of SAAC-Re to the N terminus of PSBP-6 further increased the binding affinity of the resulting peptide SAAC(Re)-PSBP-6 (K(d) ∼ 26 nM). SAAC(Re)-PSBP-6 shows specific binding to apoptotic cells in cell-based assays. Biodistribution studies showed significantly higher uptake of SAAC((99 m)Tc)-PSBP-6 in B16/F10 melanoma treated with poly(L-glutamic acid)-paclitaxel than untreated tumors (4.06 ± 0.55% ID/g vs 1.61 ± 0.33% ID/g, P = 0.00011). SAAC((99 m)Tc)-PSBP-6 is a promising probe for noninvasive imaging of apoptotic cells.

Phosphatidylserine targeting for diagnosis and treatment of human diseases

Cells are able to execute apoptosis by activating series of specific biochemical reactions. One of the most prominent characteristics of cell death is the externalization of phosphatidylserine (PS), which in healthy cells resides predominantly in the inner leaflet of the plasma membrane. These features have made PS-externalization a well-explored phenomenon to image cell death for diagnostic purposes. In addition, it was demonstrated that under certain conditions viable cells express PS at their surface such as endothelial cells of tumor blood vessels, stressed tumor cells and hypoxic cardiomyocytes. Hence, PS has become a potential target for therapeutic strategies aiming at Targeted Drug Delivery. In this review we highlight the biomarker PS and various PS-binding compounds that have been employed to target PS for diagnostic purposes. We emphasize the 35 kD human protein annexin A5, that has been developed as a Molecular Imaging agent to measure cell death in vitro, and non-invasively in vivo in animal models and in patients with cardiovascular diseases and cancer. Recently focus has shifted from diagnostic towards therapeutic applications employing annexin A5 in strategies to deliver drugs to cells that express PS at their surface.

Phosphatidylserine targeting for diagnosis and treatment of human diseases

Cells are able to execute apoptosis by activating series of specific biochemical reactions. One of the most prominent characteristics of cell death is the externalization of phosphatidylserine (PS), which in healthy cells resides predominantly in the inner leaflet of the plasma membrane. These features have made PS-externalization a well-explored phenomenon to image cell death for diagnostic purposes. In addition, it was demonstrated that under certain conditions viable cells express PS at their surface such as endothelial cells of tumor blood vessels, stressed tumor cells and hypoxic cardiomyocytes. Hence, PS has become a potential target for therapeutic strategies aiming at Targeted Drug Delivery. In this review we highlight the biomarker PS and various PS-binding compounds that have been employed to target PS for diagnostic purposes. We emphasize the 35 kD human protein annexin A5, that has been developed as a Molecular Imaging agent to measure cell death in vitro, and non-invasively in vivo in animal models and in patients with cardiovascular diseases and cancer. Recently focus has shifted from diagnostic towards therapeutic applications employing annexin A5 in strategies to deliver drugs to cells that express PS at their surface.

Phosphatidylserine decarboxylases, key enzymes of lipid metabolism

Phosphatidylserine decarboxylases (PSDs) (E.C. 4.1.1.65) are enzymes which catalyze the formation of phosphatidylethanolamine (PtdEtn) by decarboxylation of phosphatidylserine (PtdSer). This enzymatic activity has been identified in both prokaryotic and eukaryotic organisms. PSDs occur as two types of proteins depending on their localization and the sequence of a conserved motif. Type I PSDs include enzymes of eukaryotic mitochondria and bacterial origin which contain the amino acid sequence LGST as a characteristic motif. Type II PSDs are found in the endomembrane system of eukaryotes and contain a typical GGST motif. These characteristic motifs are considered as autocatalytic cleavage sites where proenzymes are split into alpha- and beta-subunits. The S-residue set free by this cleavage serves as an attachment site of a pyruvoyl group which is required for the activity of the enzymes. Moreover, PSDs harbor characteristic binding sites for the substrate PtdSer. Substrate supply to eukaryotic PSDs requires lipid transport because PtdSer synthesis and decarboxylation are spatially separated. Targeting of PSDs to their proper locations requires additional intramolecular domains. Mitochondrially localized type I PSDs are directed to the inner mitochondrial membrane by N-terminal targeting sequences. Type II PSDs also contain sequences in their N-terminal extensions which might be required for subcellular targeting. Lack of PSDs causes various defects in different cell types. The physiological relevance of these findings and the central role of PSDs in lipid metabolism will be discussed in this review.

Gas6-mediated signaling is dependent on the engagement of its gamma-carboxyglutamic acid domain with phosphatidylserine

Gas6 is a vitamin K-dependent protein containing gamma-carboxyglutamic acid (Gla) at its N-terminus and a receptor binding domain at its C-terminus. Gas6-Axl binding is necessary but not sufficient to support endothelial cell survival as decarboxylated gas6 inhibits the pro-survival function of gas6 by binding and inhibiting Axl, even though decarboxylated gas6 cannot support endothelial cell survival itself. It is hypothesized that interactions between the Gla domain of gas6 and phosphatidylserine (PS), though not required for gas6 binding to Axl, are necessary for gas6-Axl function. In support of this hypothesis are results showing that (1) two specific inhibitors of Gla-PS interactions, namely soluble PS and Annexin V, abrogate gas6-mediated endothelial cell survival and (2) Soluble PS inhibits Akt activation, a downstream intracellular event triggered by gas6-Axl binding. In conclusion, we propose a heretofore unknown function of Gla, where Gla-PS binding on the N-terminus of gas6 is necessary for a gas6 function mediated through its binding to Axl via its C-terminus.

A new N-terminal recognition domain in caveolin-1 interacts with sterol carrier protein-2 (SCP-2)

Although plasma membrane domains, such as caveolae, provide an organizing principle for signaling pathways and cholesterol homeostasis in the cell, relatively little is known regarding specific mechanisms, whereby intracellular lipid-binding proteins are targeted to caveolae. Therefore, the interaction between caveolin-1 and sterol carrier protein-2 (SCP-2), a protein that binds and transfers both cholesterol and signaling lipids (e.g., phosphatidylinositides and sphingolipids), was examined by yeast two-hybrid, in vitro binding and fluorescence resonance energy transfer (FRET) analyses. Results of the in vivo and in vitro assays identified for the first time the N-terminal amino acids (aa) 1-32 amphipathic alpha helix of SCP-2 functionally interacted with caveolin-1. This interaction was independent of the classic caveolin-1 scaffolding domain, in which many signaling proteins interact. Instead, SCP-2 bound caveolin-1 through a new domain identified in the N-terminal domain of caveolin-1 between aa 34-40. Modeling studies suggested that electrostatic interactions between the SCP-2 N-terminal aa 1-32 amphipathic alpha-helical domain (cationic, positively charged face) and the caveolin-1 N-terminal aa 33-59 alpha helix (anionic, negatively charged face) may significantly contribute to this interaction. These findings provide new insights on how SCP-2 enhances cholesterol retention within the cell as well as regulates the distribution of signaling lipids, such as phosphoinositides and sphingolipids, at plasma membrane caveolae.

Negatively charged phospholipids suppress IFN-gamma production in T cells

The effect of phospholipids on IFN-gamma production in mouse T cells was investigated. Phosphatidylserine (PS), which has a negatively charged head group, completely inhibited IFN-gamma production in splenic naïve T cells and antigen-dependent IFN-gamma production in Th1 clone 42-6A cells, whereas other phospholipids, which have neutrally charged head group, had no effect. The structural requirements for IFN-gamma inhibitory effects by PS were investigated, and dimyristoyl-PS (C14: 0) and dipalmitoyl-PS (C16: 0) had no effect on IFN-gamma production, and interestingly, distearoyl-PS (18: 0) increased IFN-gamma production. Dioleoyl-PS (C18: 1), dilinoleoyl-PS (C18: 2), and oleoyl-lyso-PS (C18: 1) completely inhibited IFN-gamma production. To clarify this mechanism, we focused on the stability of IFN-gamma mRNA, and the treatment of splenic naïve T cells with PS brought about 40% reductions in IFN-gamma mRNA expression in the presence of actinomycin D. Collectively, IFN-gamma inhibitory effects by PS are highly dependent on the molecular structure of PS and involve the decreasing of the stability of IFN-gamma mRNA.

Phosphatidylserine receptor is required for the engulfment of dead apoptotic cells and for normal embryonic development in zebrafish

During development, the role of the phosphatidylserine receptor (PSR) in the removal of apoptotic cells that have died is poorly understood. We have investigated this role of PSR in developing zebrafish. Programmed cell death began during the shield stage, with dead cells being engulfed by a neighboring cell that showed a normal-looking nucleus and the nuclear condensation multi-micronuclei of an apoptotic cell. The zebrafish PSR engulfing receptor was cloned (zfpsr), and its nucleotide sequence was compared with corresponding sequences in Drosophila melanogaster (76% identity), human (74%), mouse (72%) and Caenorhabditis elegans (60%). The PSR receptor contained a jmjC domain (residues 143-206) that is a member of the cupin metalloenzyme superfamily, but in this case serves an as yet unknown function(s). psr knockdown by a PSR morpholino oligonucleotide led to accumulation of a large number of dead apoptotic cells in whole early embryo. These cells interfered with embryonic cell migration. In addition, normal development of the somite, brain, heart and notochord was sequentially disrupted up to 24 hours post-fertilization. Development could be rescued in defective embryos by injecting psr mRNA. These results are consistent with a PSR-dependent system in zebrafish embryos that engulfs apoptotic cells mediated by PSR-phagocytes during development, with the system assuming an important role in the normal development of tissues such as the brain, heart, notochord and somite.

Lysenin, a unique sphingomyelin-binding protein

Sphingomyelin plays complex structural and signaling functions in the plasma membrane. Of special interest is that hydrolysis of sphingomyelin to ceramide can modulate dynamics of membrane rafts, which serve as signaling platforms for various receptors. This review is focused on a recently discovered sphingomyelin-binding protein, lysenin, which can be used as a unique probe to trace distribution and turnover of sphingomyelin in cellular membranes. We analyze the primary and secondary structures of lysenin with respect to its interaction with the plasma membrane. The specificity of lysenin binding to sphingomyelin, revealed by both biochemical and cytochemical approaches, is discussed.

Structural requirements for selective binding of ISC1 to anionic phospholipids

Yeast ISC1 (Yer019w) encodes inositolphosphosphingolipid-phospholipase C and is activated by phosphatidylserine (PS) and cardiolipin (CL) (Sawai, H., Okamoto, Y., Lubert, C., Mao, C., Bielawska, A., Domae, M., and Hannun, Y. A. (2000) J. Biol. Chem. 275, 39793-39798). In this study, the structural requirements for anionic phospholipid-selective binding of ISC1 were determined using site-directed and deletion mutants. FLAG-tagged Isc1p was activated by PS, CL, and phosphatidylglycerol (PG) in a dose-dependent manner. Using lipid-protein overlay assays, Isc1p interacted specifically and directly with PS/CL/PG. Lipid-protein binding studies of a series of deletion mutants demonstrated that the second transmembrane domain (TMII) and the C terminus were required for PS binding. Moreover, the TMII and the C terminus domain were sufficient to impart PS binding to a heterologous protein, green fluorescence protein. In addition, mutations of positively charged amino acid residues at the C terminus of ISC1 reduced the activating effects of PS, suggesting involvement of these amino acids in interaction with PS/CL/PG and in the activation of the enzyme. Finally, when separate fragments containing the N terminus-TMI and TMII-C terminus were expressed heterologously, enzyme activity was reconstituted, demonstrating that the interaction of the N terminus and the C terminus is required for activity of Isc1p. These results raise the hypothesis that in the presence of PS/CL/PG, the catalytic domain in the N terminus of Isc1p is "pulled" to the membrane to interact with substrate. These studies provide unique insights into the properties of ISC1 and define a novel mechanism for activation of enzymes by lipids cofactors.

Phosphatidylserine binding of class B scavenger receptor type I, a phagocytosis receptor of testicular sertoli cells

Testicular Sertoli cells phagocytose apoptotic spermatogenic cells in a manner depending on the membrane phospholipid phosphatidylserine (PS) expressed at the surface of the latter cell type. Our previous studies have indicated that class B scavenger receptor type I (SR-BI) is responsible for the PS-mediated phagocytosis by Sertoli cells. We examined here whether SR-BI binds directly to PS. A cell line acquired the ability to bind to PS-exposing apoptotic cells and to incorporate PS-containing liposomes when it was forced to express SR-BI. Furthermore, the extracellular domain of rat SR-BI fused with human Fc (SRBIecd-Fc) bound to PS with a dissociation equilibrium constant of 2.4 x 10(-7) m in a cell-free solid-phase assay, whereas other phospholipids including phosphatidylethanolamine, phosphatidylinositol, and phosphatidylcholine were poor binding targets. The binding activity was enhanced when CaCl(2) was included in the assay or when SRBIecd-Fc was pre-treated with N-glycanase. A portion of the extracellular domain spanning amino acid positions 33 and 191 (numbered with respect to the amino terminus) fused with Fc (SRBI33-191-Fc) showed activity and phospholipid specificity equivalent to those of SRBIecd-Fc. Finally, SRBI33-191-Fc bound to the surface of apoptotic cells with externalized PS, and the injection of SRBI33-191-Fc into the seminiferous tubules of live mice increased the number of apoptotic spermatogenic cells. These results allowed us to conclude that SR-BI is a phagocytosis-inducing PS receptor of Sertoli cells.

Additional binding sites for anionic phospholipids and calcium ions in the crystal structures of complexes of the C2 domain of protein kinase calpha

The C2 domain of protein kinase Calpha (PKCalpha) corresponds to the regulatory sequence motif, found in a large variety of membrane trafficking and signal transduction proteins, that mediates the recruitment of proteins by phospholipid membranes. In the PKCalpha isoenzyme, the Ca2+-dependent binding to membranes is highly specific to 1,2-sn-phosphatidyl-l-serine. Intrinsic Ca2+ binding tends to be of low affinity and non-cooperative, while phospholipid membranes enhance the overall affinity of Ca2+ and convert it into cooperative binding. The crystal structure of a ternary complex of the PKCalpha-C2 domain showed the binding of two calcium ions and of one 1,2-dicaproyl-sn-phosphatidyl-l-serine (DCPS) molecule that was coordinated directly to one of the calcium ions. The structures of the C2 domain of PKCalpha crystallised in the presence of Ca2+ with either 1,2-diacetyl-sn-phosphatidyl-l-serine (DAPS) or 1,2-dicaproyl-sn-phosphatidic acid (DCPA) have now been determined and refined at 1.9 A and at 2.0 A, respectively. DAPS, a phospholipid with short hydrocarbon chains, was expected to facilitate the accommodation of the phospholipid ligand inside the Ca2+-binding pocket. DCPA, with a phosphatidic acid (PA) head group, was used to investigate the preference for phospholipids with phosphatidyl-l-serine (PS) head groups. The two structures determined show the presence of an additional binding site for anionic phospholipids in the vicinity of the conserved lysine-rich cluster. Site-directed mutagenesis, on the lysine residues from this cluster that interact directly with the phospholipid, revealed a substantial decrease in C2 domain binding to vesicles when concentrations of either PS or PA were increased in the absence of Ca2+. In the complex of the C2 domain with DAPS a third Ca2+, which binds an extra phosphate group, was identified in the calcium-binding regions (CBRs). The interplay between calcium ions and phosphate groups or phospholipid molecules in the C2 domain of PKCalpha is supported by the specificity and spatial organisation of the binding sites in the domain and by the variable occupancies of ligands found in the different crystal structures. Implications for PKCalpha activity of these structural results, in particular at the level of the binding affinity of the C2 domain to membranes, are discussed.

Glyceraldehyde-3-phosphate dehydrogenase is phosphorylated by protein kinase Ciota /lambda and plays a role in microtubule dynamics in the early secretory pathway

The small GTPase Rab2 immunolocalizes to vesicular tubular clusters (VTCs) that function as transport complexes carrying cargo between the endoplasmic reticulum and the Golgi complex. Our previous studies showed that Rab2 promotes vesicle formation from VTCs and that the released vesicles are enriched in beta-coat protein, protein kinase C iota/lambda (PKCiota/lambda), glyceraldehyde-3-phosphate dehydrogenase (GAPDH), and the recycling protein p53/gp58. Because PKCiota/lambda kinase activity was necessary for vesicle formation, a search was initiated to identify the substrate(s) that potentiate Rab2 function within VTCs. In this study, we found that PKCiota/lambda phosphorylates GAPDH. Moreover, GAPDH interacts directly with the PKCiota/lambda regulatory domain. Based on numerous observations that show (beta-COP) GAPDH associates with cytoskeletal elements, we examined the role of phospho-GAPDH in promoting microtubule (MT) binding to membrane. Using a quantitative microsomal binding assay, we found that membrane association of beta-tubulin was dependent on phospho-GAPDH and was blocked by reagents that interfere with Rab2-dependent GAPDH membrane recruitment or with PKCiota/lambda kinase activity. Furthermore, normal rat kidney cells transfected with a constitutively activated form of Rab2 (Q65L) or with our anti-GAPDH polyclonal antibody displayed a dramatic change in MT organization. These combined results suggest that Rab2 stimulated PKCiota/lambda and GAPDH recruitment to VTCs, and the subsequent PKCiota/lambda phosphorylation of GAPDH ultimately influences MT dynamics in the early secretory pathway.

Mechanisms of transport across cell membranes of complexes contained in antitumour drugs

Various mechanism of antitumour drug transport across cell membranes has been described. Particular attention has been paid to a passive transport, active transport and multidrug resistance of complexes contained in antitumour drugs. A drug supply to the target site depends on the blood circulation within the tumour, on characteristic drug diffusion in the tissue, and also on binding protein. The physiologic transfer of hydrophilic compounds across the membrane is usually intermediated by means of a specific receptor or a carrier in that membrane, which facilitates the transport of compounds to and from the cell. Some drugs, e.g. doxorubicin and annamycin, can pass across the membrane by intermediacy of liposomes which exhibit a great activity in penetrating into tumour cells. The efficiency of antitumour drugs is limited by the appearance of resistance, i.e. by the lack of sensitivity of the cell to the administered drug. The presence in the membrane of specific proteins belonging to the ABC carriers group is postulated in a resistance theory; they would be responsible for 'pumping out' lipophilic drug molecules from the cell. Participation of high-energy ATP molecule is required by P-glycoprotein (Pgp) and by MRP protein described in this paper for their action. The mechanisms that are responsible for the cell resistance to drugs have been presented by analysing the resistance to antimetabolites, particularly to folate and fluoropyrimidine analogues, to alkylating agents, e.g. cisplatinum, and to heterocyclic compounds being responsible for so-called multidrug resistance.

Transmembrane topography of plasma membrane constituents in mung bean (Vigna radiata L.) hypocotyl cells. I. Transmembrane distribution of phospholipids

The transmembrane distribution of phospholipids (PLs) in the plasma membrane (PM) of mung bean (Vigna radiata L.) hypocotyl cells was investigated using annexin V-fluorescein isothiocyanate, porcine pancreas phospholipase A(2), and (31)P-nuclear magnetic resonance (NMR) spectroscopy. Phosphatidylserine was not located on the cell surface of mung bean protoplasts. However, phosphatidylcholine, phosphatidylethanolamine and phosphatidic acid were found to be almost symmetrically distributed across right-side-out PM vesicles obtained by aqueous two-phase partitioning by porcine pancreas phospholipase A(2) assay. (31)P-NMR assay showed that the amount of PLs is about equal in the outer and the inner leaflets of the right-side-out PM vesicles. These results suggest that the topography of PM PLs might not contribute to well-known asymmetrical properties of the outer and inner surfaces of higher plant PMs. It is also indicated that inside-out PM vesicles created by Brij 58-treatment do not retain the native PL topography on dithionate reduction of 7-nitro-2,1,3-benzoxadiazol-4-yl-labeled PLs incorporated in the PM vesicles.

If phosphatidylserine is the death knell, a new phosphatidylserine-specific receptor is the bellringer

Recognition of phosphatidylserine (PtdSer) is essential for engulfment of apoptotic cells by mammalian phagocytes. Engagement of a new phosphatidylserine-specific receptor (PtdSerR) appears to be necessary for uptake of apoptotic cells. Many other mammalian receptors have been described to function in the clearance of apoptotic cells. The emerging picture is that many of these receptors may provide the strong adhesion needed to increase the likelihood of contact between the PtdSerR and its phospholipid ligand, which is required for uptake. Furthermore, stimulation of this receptor on different types of phagocytes by apoptotic cells, PtdSer-containing liposomes or an IgM monoclonal anti-PtdSer antibody initiates release of TGFbeta, known to be involved in the anti-inflammatory effects of apoptotic cells. Although highly homologous genes exist in C. elegans and Drosophila melanogaster, their role in engulfment of apoptotic cells remains to be determined.

The cAMP-specific phosphodiesterase PDE4D3 is regulated by phosphatidic acid binding. Consequences for cAMP signaling pathway and characterization of a phosphatidic acid binding site

Hormones and growth factors induce in many cell types the production of phosphatidic acid (PA), which has been proposed to play a role as a second messenger. We have previously shown in an acellular system that PA selectively stimulates certain isoforms of type 4 cAMP-phosphodiesterases (PDE4). Here we studied the effect of endogenous PA on PDE activity of transiently transfected MA10 cells overexpressing the PA-sensitive isoform PDE4D3. Cell treatment with inhibitors of PA degradation, including propranolol, induced an accumulation of endogenous PA accompanied by a stimulation of PDE activity and a significant decrease in both cAMP levels and protein kinase A activity. Furthermore, in FRTL5 cells, which natively express PDE4D3, pretreatment with compounds inducing PA accumulation prevented both cAMP increase and cAMP-responsive element-binding protein phosphorylation triggered by thyroid-stimulating hormone. To determine the mechanism of PDE stimulation by PA, endogenous phospholipids were labeled by preincubating MA10 cells overexpressing PDE4D3 with [(32)P]orthophosphate. Immuno- precipitation experiments showed that PA was specifically bound to PDE4D3, supporting the hypothesis that PDE4D3 activation occurs through direct binding of PA to the protein. PA binding site on PDE4D3 was characterized by engineering deletions of selected regions in the N-terminal regulatory domain of the enzyme. Deletion of amino acid residues 31-59 suppressed both PA-activating effect and PA binding, suggesting that this region rich in basic and hydrophobic residues contains the PA binding site. These observations strongly suggest that endogenous PA can modulate cAMP levels in intact cells, through a direct activation of PDE4D3.

An essential role for a membrane lipid in cytokinesis. Regulation of contractile ring disassembly by redistribution of phosphatidylethanolamine

Phosphatidylethanolamine (PE) is a major membrane phospholipid that is mainly localized in the inner leaflet of the plasma membrane. We previously demonstrated that PE was exposed on the cell surface of the cleavage furrow during cytokinesis. Immobilization of cell surface PE by a PE-binding peptide inhibited disassembly of the contractile ring components, including myosin II and radixin, resulting in formation of a long cytoplasmic bridge between the daughter cells. This blockade of contractile ring disassembly was reversed by removal of the surface-bound peptide, suggesting that the PE exposure plays a crucial role in cytokinesis. To further examine the role of PE in cytokinesis, we established a mutant cell line with a specific decrease in the cellular PE level. On the culture condition in which the cell surface PE level was significantly reduced, the mutant ceased cell growth in cytokinesis, and the contractile ring remained in the cleavage furrow. Addition of PE or ethanolamine, a precursor of PE synthesis, restored the cell surface PE on the cleavage furrow and normal cytokinesis. These findings provide the first evidence that PE is required for completion of cytokinesis in mammalian cells, and suggest that redistribution of PE on the cleavage furrow may contribute to regulation of contractile ring disassembly.

A receptor for phosphatidylserine-specific clearance of apoptotic cells

cytosis of cellular corpses. During apoptosis, the asymmetry of plasma membrane phospholipids is lost, which exposes phosphatidylserine externally. The phagocytosis of apoptotic cells can be inhibited stereospecifically by phosphatidylserine and its structural analogues, but not by other anionic phospholipids, suggesting that phosphatidylserine is specifically recognized. Using phage display, we have cloned a gene that appears to recognize phosphatidylserine on apoptotic cells. Here we show that this gene, when transfected into B and T lymphocytes, enables them to recognize and engulf apoptotic cells in a phosphatidylserine-specific manner. Flow cytometric analysis using a monoclonal antibody suggested that the protein is expressed on the surface of macrophages, fibroblasts and epithelial cells; this antibody, like phosphatidylserine liposomes, inhibited the phagocytosis of apoptotic cells and, in macrophages, induced an anti-inflammatory state. This candidate phosphatidylserine receptor is highly homologous to genes of unknown function in Caenorhabditis elegans and Drosophila melanogaster, suggesting that phosphatidylserine recognition on apoptotic cells during their removal by phagocytes is highly conserved throughout phylogeny.

PS-liposome and ox-LDL bind to different sites of the immunodominant domain (#155-183) of CD36: a study with GS95, a new anti-CD36 monoclonal antibody

CD36, a multifunctional adhesive receptor on a variety of cells such as monocytes and platelets, has been implicated in clearance of modified LDL and in the removal of apoptotic or senescent cells. We recently developed a new anti-CD36 monoclonal antibody, GS95. We determined the binding site of phosphatidylserine (PS)-liposome on CD36 by flow cytometric analysis of competitive bindings between phospholipid-liposomes or synthetic CD36 peptides and FITC-labeled anti-CD36 antibodies (GS95, OKM5, and FA6-152). The epitope of GS95 was mapped to the amino acid sequence #162-183 of CD36 that was partially overlapped with, but distinct from, #155-183, which has been reported as the epitopes of two commercially available antibodies, OKM5 and FA6-152. Oxidized-LDL dose-dependently inhibited bindings of both GS95 and OKM5 antibodies to platelet CD36, while PS-liposome inhibited the binding of GS95 but not OKM5 or FA6-152. These results indicate that the binding site of PS-liposome on platelet CD36 is not identical to that of oxidized-LDL and may be located in the amino acid sequence #162-183.

Evidence of a phospholipid binding species within human fibrinogen preparations

Fibrinogen has been reported to interact with phospholipid; however, the properties of this binding interaction have not been characterized. Purified preparations of human fibrinogen bound to small unilamellar vesicles containing phosphatidylserine (PS) as measured by light scattering and radioisotope filtration. Binding to 100% PS was saturable (apparent Kd=5 microM, Bmax=1.9 g protein/g lipid), reversible, and involved a minor subfraction of the fibrinogen preparation (3-6% of total protein). Fibrinogen interacted minimally with phosphatidylinositol, and not at all with pure phosphatidylcholine (PC) or PC vesicles containing 5% glycosphingolipid (lactosylceramide, ganglioside GM3, ganglioside GD3). Binding efficiency decreased as the PS content of vesicles was diluted with PC. Calcium chloride (2 mM) enhanced protein binding to PS, which was reversed by EDTA. Fibrin clot formation almost quantitatively precipitated the PS binding activity. PS, but not PC, increased the final turbidity of fibrin clots. Computerized sequence analysis of fibrinogen revealed three candidate acidic phospholipid binding motifs located at position 143-210 in the alpha chain, and positions 59-77 and 101-139 in the beta chain. Further study of the PS binding activity of fibrinogen may lead to new insights about fibrinogen function.

Membrane phospholipid dynamics during cytokinesis: regulation of actin filament assembly by redistribution of membrane surface phospholipid

To study molecular motion and function of membrane phospholipids, we have developed various probes which bind specifically to certain phospholipids. Using a novel peptide probe, RoO9-0198, which binds specifically to phosphatidylethanolamine (PE) in biological membranes, we have analyzed the cell surface movement of PE in dividing CHO cells. We found that PE was exposed on the cell surface specifically at the cleavage furrow during the late telophase of cytokinesis. PE was exposed on the cell surface only during the late telophase and no alteration in the distribution of the plasma membranebound peptide was observed during the cytokinesis, suggesting that the surface exposure of PE reflects the enhanced transbilayer movement of PE at the cleavage furrow. Furthermore, cell surface immobilization of PE induced by adding of the cyclic peptide coupled with streptavidin to prometaphase cells effectively blocked the cytokinesis at late telophase. The peptide-streptavidin complex bound specifically to cleavage furrow and inhibited both actin filament disassembly at cleavage furrow and subsequent plasma membrane fusion. Binding of the peptide complex to interphase cells also induced immediate disassembly of stress fibers followed by assembly of cortical actin filaments to the local area of plasma membrane where the peptide complex bound. The cytoskeletal reorganizations caused by the peptide complex were fully reversible; removal of the surface-bound peptide complex by incubating with PE-containing liposome caused gradual disassembly of the cortical actin filaments and subsequent formation of stress fibers. These observations suggest that the redistribution of plasma membrane phospholipids act as a regulator of actin cytoskeleton organization and may play a crucial role in mediating a coordinate movement between plasma membrane and actin cytoskeleton to achieve successful cell division.

Lysenin, a novel sphingomyelin-specific binding protein

Lysenin, a novel 41-kDa protein purified from coelomic fluid of the earthworm Eisenia foetida, induced erythrocyte lysis. Preincubation of lysenin with vesicles containing sphingomyelin inhibited lysenin-induced hemolysis completely, whereas vesicles containing phospholipids other than sphingomyelin showed no inhibitory activity, suggesting that lysenin bound specifically to sphingomyelin on erythrocyte membranes. The specific binding of lysenin to sphingomyelin was confirmed by enzyme-linked immunosorbent assay, TLC immunostaining, and liposome lysis assay. In these assays, lysenin bound specifically to sphingomyelin and did not show any cross-reaction with other phospholipids including sphingomyelin analogs such as sphingosine, ceramide, and sphingosylphosphorylcholine, indicating that it recognized a precise molecular structure of sphingomyelin. Kinetic analysis of the lysenin-sphingomyelin interaction by surface plasmon resonance measurements using BIAcoreTM system showed that lysenin associated with membrane surfaces composed of sphingomyelin (kon = 3.2 x 10(4) M-1 s-1) and dissociated extremely slowly (koff = 1.7 x 10(-4) s-1), giving a low dissociation constant (KD = 5.3 x 10(-9) M). Incorporation of cholesterol into the sphingomyelin membrane significantly increased the total amount of lysenin bound to the membrane, whereas it did not change the kinetic parameters of the lysenin-membrane interaction, suggesting that lysenin specifically recognized sphingomyelin and cholesterol incorporation changed the topological distribution of sphingomyelin in the membranes, thereby increasing the accessibility of sphingomyelin to lysenin. Immunofluorescence staining of fibroblasts derived from a patient with Niemann-Pick disease type A showed that lysenin stained the surfaces of the fibroblasts uniformly, whereas intense lysosomal staining was observed when the cells were permeabilized by digitonin treatment. Preincubation of lysenin with vesicles containing sphingomyelin abolished lysenin immunostaining. This study demonstrated that lysenin bound specifically to sphingomyelin on cellular membranes and should be a useful tool to probe the molecular motion and function of sphingomyelin in biological membranes.