Intrinsic fluorescence study of the interaction of human apolipoprotein H with phospholipid vesicles

Atomistic characterization of β2-glycoprotein I domain V interaction with anionic membranes

BACKGROUND

Interaction of β2-glycoprotein I (β2GPI) with anionic membranes is crucial in antiphospholipid syndrome (APS), implicating the role of its membrane-binding domain, domain V (DV). The mechanism of DV binding to anionic lipids is not fully understood.

OBJECTIVES

This study aimed to elucidate the molecular details of β2GPI DV binding to anionic membranes.

METHODS

We utilized molecular dynamics simulations to investigate the structural basis of anionic lipid recognition by DV. To corroborate the membrane-binding mode identified in the highly mobile membrane mimetic simulations, we conducted additional simulations using a full membrane model.

RESULTS

The study identified critical regions in DV, namely the lysine-rich loop and the hydrophobic loop, which are essential for membrane association via electrostatic and hydrophobic interactions, respectively. A novel lysine pair contributing to membrane binding was also discovered, providing new insights into β2GPI's membrane interaction. Simulations revealed 2 distinct binding modes of DV to the membrane, with mode 1 characterized by the insertion of the hydrophobic loop into the lipid bilayer, suggesting a dominant mechanism for membrane association. This interaction is pivotal for the pathogenesis of APS, as it facilitates the recognition of β2GPI by antiphospholipid antibodies.

CONCLUSION

The study advances our understanding of the molecular interactions between β2GPI's DV and anionic membranes, which are crucial for APS pathogenesis. It highlights the importance of specific regions in DV for membrane binding and reveals a predominant binding mode. These findings have significant implications for APS diagnostics and therapeutics, offering a deeper insight into the molecular basis of the syndrome.

Atomistic Characterization of Beta-2-Glycoprotein I Domain V Interaction with Anionic Membranes

Background

Interaction of beta-2-glycoprotein I ( β 2 GPI) with anionic membranes is crucial in antiphospholipid syndrome (APS), implicating the role of it's membrane bind-ing domain, Domain V (DV). The mechanism of DV binding to anionic lipids is not fully understood.

Objectives

This study aims to elucidate the mechanism by which DV of β 2 GPI binds to anionic membranes.

Methods

We utilized molecular dynamics (MD) simulations to investigate the struc-tural basis of anionic lipid recognition by DV. To corroborate the membrane-binding mode identified in the HMMM simulations, we conducted additional simulations using a full mem-brane model.

Results

The study identified critical regions in DV, namely the lysine-rich loop and the hydrophobic loop, essential for membrane association via electrostatic and hydrophobic interactions, respectively. A novel lysine pair contributing to membrane binding was also discovered, providing new insights into β 2 GPI's membrane interaction. Simulations revealed two distinct binding modes of DV to the membrane, with mode 1 characterized by the insertion of the hydrophobic loop into the lipid bilayer, suggesting a dominant mechanism for membrane association. This interaction is pivotal for the pathogenesis of APS, as it facilitates the recognition of β 2 GPI by antiphospholipid antibodies.

Conclusion

The study advances our understanding of the molecular interactions be-tween β 2 GPI's DV and anionic membranes, crucial for APS pathogenesis. It highlights the importance of specific regions in DV for membrane binding and reveals a predominant bind-ing mode. These findings have significant implications for APS diagnostics and therapeutics, offering a deeper insight into the molecular basis of the syndrome.

Identification of Key Modules and Hub Genes Involved in Regulating the Color of Chicken Breast Meat Using WGCNA

Meat color is one of the most important economic traits in chickens. However, the gene network and regulatory mechanisms contributing to meat color traits in chickens remain largely unknown. In the present study, we performed weighted gene co-expression network analysis (WGCNA) based on RNA-Seq datasets of 16 pectoralis major muscle samples from two yellow-feather chicken breeds to identify the modules and hub genes related to meat color in chickens. A total of 18,821 genes were used to construct the weighted gene co-expression network, and 29 co-expression gene modules were identified. Among these modules, five modules including blue, brown, steel blue, paleturquoise and orange modules were found to be significantly correlated with meat color traits. Furthermore, several genes within the association module involved in the regulation of mitochondrial activity (e.g., ATP5L, UQCR10 and COX7C) and lipid oxidation (e.g., CAV3, RBP4A and APOH) were identified as hub genes that may play a crucial role in the regulation of meat color. These results provide valuable information to improve our understanding of gene expression and regulation in relation to meat color traits and contribute to future molecular breeding for improving meat color in chickens.

Periodic bilayer organization in the complexes of Beta-2 Glycoprotein I with anionic lipid membranes

β2 glycoprotein I (β2GPI) is a soluble protein that participates in blood coagulation, clearance of apoptotic bodies and generation of antigens in antiphospholipid syndrome among many other functions. We studied the aggregates formed by β2GPI with the anionic phospholipids palmitoyloleoylphosphatidyl glycerol, dimyristoylphosphatidyl glycerol, dipalmitoylphosphatidyl glycerol and cardiolipin using small angle X-ray scattering. The complexes obtained in a medium containing 0.01 M NaCl showed Bragg peaks up to the sixth order in a well-defined integer sequence indicating the presence of a lamellar stacking with a periodicity of 17.8 nm and with largely reduced membrane fluctuations. Modeling the complex signal allowed us to conclude that the coherence length was only two bilayers and that about 15% of the total surface was actually stacked. The space between bilayers allows accommodating an extended β2GPI molecule making a bridge between the interacting bilayers. The interactions between membranes mediated by β2GPI was favored when the membranes were in the liquid crystalline state.

Covalent attachment of functionalized cardiolipin on a biosensor gold surface allows repetitive measurements of anticardiolipin antibodies in serum

Antiphospholipid antibodies (aPL) are a relevant serological indicator of antiphospholipid syndrome (APS). A solid-state surface with covalently bound ω-amine-functionalized cardiolipin was established and the binding of β2-glycoprotein I (β2-GPI) was investigated either by use of surface plasmon resonance (SPR) biosensor, by electrically switchable DNA interfaces (switchSENSE) and by scanning tunneling microscopy (STM). STM could clearly visualize the attachment of β2-GPI to the cardiolipin surface. Using the switchSENSE sensor, β2-GPI as specific ligand could be identified by increased hydrodynamic friction. The binding of anti-cardiolipin antibodies (aCL) was detected against the ω-amine-functionalized cardiolipin-modified SPR biosensor (aCL biosensor) using sera from healthy donors, APS patients and syphilis patients. Our results showed that the aCL biosensor is a much more sensitive diagnostic device for APS patients compared to previous methods. The specificity between β2-GPI-dependent autoimmune- and β2-GPI-independent infection-associated types of aPLs was also studied and they can be distinguished by the different binding kinetics and patterns.

Insights into the pathophysiology of the antiphospholipid syndrome provided by atomic force microscopy

The antiphospholipid syndrome (APS) is an enigmatic autoimmune disorder in which patients present with thrombosis and/or recurrent pregnancy losses together with laboratory evidence for the presence of autoantibodies in the blood that recognize proteins that bind to anionic phospholipids - the most important of which is β(2)-glycoprotein I (β(2)GPI). Earlier, we hypothesized that the clinical manifestations arise from antibody-induced disruption of a two-dimensional anticoagulant crystal shield, composed of annexin A5, present on placental trophoblast plasma membranes. Accordingly, we reasoned that a high resolution imaging technology, such as atomic force microscopy could be used to investigate such molecular interactions at high resolution in a non-fixed hydrated environment. This review will focus on the contribution of this technique to the elucidation of the mechanism of APS.

Biosensor analyses of serum autoantibodies: application to antiphospholipid syndrome and systemic lupus erythematosus

Autoimmune disorders are rare human diseases characterized by the presence of circulating autoantibodies that bind the body's own structural compounds as target antigens. The detection of autoantibodies is important for the diagnostic process. Immunofluorescence and immunoassay methods do not allow a reliable characterization of binding characteristics. Therefore, novel analytical techniques should be considered. This review describes the application of surface plasmon resonance biosensor systems for the diagnosis of autoimmune disorders. The covalent attachment of native antigens to the sensor chip is a suitable method for obtaining highly reproducible analyses of autoantibodies, allowing the evaluation of kinetic rate and affinity constants, and it may enable the identification of disease-relevant autoantibodies linked to disease progression. The autoantibody microarray is another future-oriented technique. Patterns of differential antigen recognition should allow early diagnosis. This is due to the fact that a broad range of autoreactive B cell responses in autoimmune disorders can only be mirrored by including a sufficient number of antigens in a microarray format.

Plasmin-cleaved beta-2-glycoprotein 1 is an inhibitor of angiogenesis

beta-2-Glycoprotein 1, an abundant plasma glycoprotein, binds anionic cell surfaces and functions as a regulator of thrombosis. Here, we show that cleavage of the kringle domain at Lys317/Thr318 switches its function to a regulator of angiogenesis. In vitro, the cleaved protein specifically inhibited the proliferation and migration of endothelial cells. The protein was without effect on preformed endothelial cell tubes. In vivo, the cleaved protein inhibited neovascularization into subcutaneously implanted Matrigel and Gelfoam sponge implants and the growth of orthotopically injected tumors. Collectively, these data indicate that plasmin-cleaved beta-2-glycoprotein 1 is a potent antiangiogenic and antitumor molecule of potential therapeutic significance.

Antiphospholipid reactivity against cardiolipin metabolites occurring during endothelial cell apoptosis

We have recently shown that cardiolipin (CL) and its metabolites move from mitochondria to other cellular membranes during death receptor-mediated apoptosis. In this study, we investigate the immunoreactivity to CL derivatives occurring during endothelial apoptosis in patients with antiphospholipid syndrome (APS) and systemic lupus erythematosus (SLE). We compared the serum immunoreactivity to CL with that of its derivatives monolysocardiolipin (MCL), dilysocardiolipin (DCL), and hydrocardiolipin (HCL) by means of both enzyme-linked immunosorbent assay and thin-layer chromatography (TLC) immunostaining. In addition, we investigated the composition of phospholipid extracts from the plasma membrane of apoptotic endothelial cells and the binding of patients' sera to the surface of the same cells by using high-performance TLC and immunofluorescence analysis. The average reactivity to MCL was comparable with that of CL and significantly higher than that for DCL and HCL in patients studied, both in the presence or in the absence of beta₂-glycoprotein I. Of relevance for the pathogenic role of these autoantibodies, immunoglobulin G from patients' sera showed an increased focal reactivity with the plasma membrane of endothelial cells undergoing apoptosis. Interestingly, the phospholipid analysis of these light membrane fractions showed an accumulation of both CL and MCL. Our results demonstrated that a critical number of acyl chains in CL derivatives is important for the binding of antiphospholipid antibodies and that MCL is an antigenic target with immunoreactivity comparable with CL in APS and SLE. Our finding also suggests a link between apoptotic perturbation of CL metabolism and the production of these antibodies.

Budding, vesiculation and permeabilization of phospholipid membranes—evidence for a feasible physiologic role of β2-glycoprotein I and pathogenic actions of anti-β2-glycoprotein I antibodies

The in vivo physiologic role of beta2-glycoprotein I (beta2GPI) is presumed to be related to its interactions with negatively charged phospholipid membranes. Increased quantities of procoagulant microparticles derived by the vesiculation of blood cells have been detected in patients with antiphospholipid syndrome (APS) frequently associated with antibodies against beta2GPI (anti-beta2GPI). We investigated the influence of beta2GPI and anti-beta2GPI on giant phospholipid vesicles (GPVs). GPVs composed of phosphatidylserine and phosphatidylcholine were formed in an aqueous medium and individually transferred to a compartment containing either beta2GPI, anti-beta2GPI, or beta2GPI along with anti-beta2GPI. Shape changes of a single GPV were observed by a phase contrast microscope. Most GPVs transferred to the solution containing only beta2GPI budded moderately. Upon the transfer of GPVs to the solution containing beta2GPI and anti-beta2GPI either from patient with APS or mouse monoclonal anti-beta2GPI Cof-22, the budding was much more pronounced, generating also daughter vesicles. No such effects were seen when GPV was transferred to the solution containing anti-beta2GPI without beta2GPI. Our results suggest a significant physiologic role of beta2GPI in the budding of phospholipid membranes, which may be explained by the insertion of the C-terminal loop of beta2GPI into membranes, thus increasing the surface of the outer layer of a phospholipid bilayer. Anti-beta2GPI, recognizing domains I to IV of beta2GPI, enhanced the budding and vesiculation of GPVs in the presence of beta2GPI. This might be a novel pathogenic mechanism of anti-beta2GPI, promoting in vivo the expression of proadhesive and procoagulant phospholipid surfaces in APS.

Recruitment of beta-2-glycoprotein 1 to cell surfaces in extrinsic and intrinsic apoptosis

Apoptotic cells and phagocytes have developed a diverse array of distinct ligand-receptor systems that drive the recognition and uptake of dying cells. Phagocytes recognize apoptotic cells either directly, by binding to specific ligands at their cell surface, or indirectly, by binding to bridging proteins that bind these ligands. Previous observations showed that the plasma bridging protein beta2GP1, binds PS containing vesicles, and enhances their binding and engulfment by phagocytes in vitro. In this study we show that apoptotic cells injected intravenously and intraperitonealy into syngeneic mice recruited the PS binding protein, beta2GP1. Examination of peritoneal exudates and spleen thin sections showed that only the injected apoptotic cells picked up endogenous beta2GP1. Recovery of cells from the peritoneum showed that apoptotic cells bearing beta2GP1 were clustered around host peritoneal phagocytes. In addition, tissue sections from mice injected with Fas antibody showed colocalization of beta2GP1 with TUNEL-positive apoptotic cells. These results provide evidence that endogenous beta2GP1 binds apoptotic cells in vivo, suggesting that the protein plays an important physiologic role in the recognition of dying cells.

Aminophospholipid asymmetry: A matter of life and death

Maintenance of membrane lipid asymmetry is a dynamic process that influences many events over the lifespan of the cell. With few exceptions, most cells restrict the bulk of the aminophospholipids to the inner membrane leaflet by means of specific transporters. Working in concert with each other, these proteins correct for sporadic incursions of the aminophospholipids to the outer membrane leaflet as a result of bilayer imbalances created by various cellular events. A shift in the relative contribution in each of these activities can result in sustained exposure of the aminophospholipids at the cell surface, which allows capture of the cells by phagocytes before the integrity of the plasma membrane is compromised. The absence of an efficient recognition and elimination mechanism can result in uncontrolled and persistent presentation of self-antigens to the immune system, with development of autoimmune syndromes. To prevent this, phagocytes have developed a diverse array of distinct and redundant receptor systems that drive the postphagocytic events along pathways that facilitate cross-talk between the homeostatic and the immune systems. In this work, we review the basis for the proposed mechanism(s) by which apoptotic ligands appear on the target cell surface and the phagocyte receptors that recognize these moieties.

Human apolipoprotein H may have various orientations when attached to lipid layer

Apolipoprotein H (ApoH), also known as beta(2)-glycoprotein I, is a plasma glycoprotein with its in vivo physiological and pathogenic roles being closely related to its interaction with negatively charged membranes. Although the three-dimensional crystal structure of ApoH has been recently solved, direct evidence about the spatial state of ApoH on the membrane is still lacking. In this work, the interactions of ApoH with the lipid layer are studied by a combination of lipid monolayer approach and surface concentration determination. The spatial state of the orientation of ApoH on the lipid layer is investigated by analyzing the process of membrane-attached ApoH molecules being extruded out from the phospholipid monolayer by compression. The results show that on neutral lipid layer ApoH has an upright orientation, which is not sensitive to the phase state of the lipid layer. However, on acidic lipid layer, ApoH may have two forms of orientation. One is an upright orientation in the liquid phase region, and the other is flat orientation on the condensed domain region. The variation of the spatial state of ApoH on the lipid layer may relate to a variety of its physiological functions.

The omega-loop region of the human prothrombin gamma-carboxyglutamic acid domain penetrates anionic phospholipid membranes

The hydrophobic omega-loop within the prothrombin gamma-carboxyglutamic acid-rich (Gla) domain is important in membrane binding. The role of this region in membrane binding was investigated using a synthetic peptide, PT-(1-46)F4W, which includes the N-terminal 46 residues of human prothrombin with Phe-4 replaced by Trp providing a fluorescent probe. PT-(1-46)F4W and PT-(1-46) bind calcium ions and phospholipid membranes, and inhibit the prothrombinase complex. PT-(1-46)F4W, but not PT-(1-46), exhibits a blue shift (5 nm) and red-edge excitation shift (28 nm) in the presence of phosphatidylserine (PS)-containing vesicles, suggesting Trp-4 is located within the motionally restricted membrane interfacial region. PS-containing vesicles protect PT-(1-46)F4W, but not PT-(1-46), fluorescence from potassium iodide-induced quenching. Stern-Volmer analysis of the quenching of PT-(1-46)F4W in the presence and absence of 80% phosphatidylcholine/20% PS vesicles suggested that Trp-4 is positioned within the membrane and protected from aqueous quenching agents whereas Trp-41 remains solvent-accessible in the presence of PS-containing vesicles. Fluorescence quenching of membrane-bound PT-(1-46)F4W is optimal with 7- and 10-doxyl-labeled lipids, indicating that Trp-4 is inserted 5 to 7 A into the bilayer. This report demonstrates that the omega-loop region of prothrombin specifically interacts with PS-containing membranes within the interfacial membrane region.

beta(2)-glycoprotein I-dependent alterations in membrane properties

beta(2)-Glycoprotein I (beta(2)GP1), a 50 kDa serum glycoprotein, binds anionic phospholipids and plays a role in phosphatidylserine (PS)-dependent coagulation and apoptotic processes. To characterize the molecular consequences that occur to target membranes upon binding of beta(2)GP1, the interaction between beta(2)GP1 and PS-containing vesicles was investigated by fluorescent spectroscopy. Membranes containing pyrene-labeled lipid showed that binding of beta(2)GP1 induced a decrease in excimer/monomor ratios (E/M) of the target membrane. Although these membrane alterations occurred in isotonic buffer, the effects were greater in low ionic strength buffer and were coincident to membrane precipitation. In contrast, increases in membrane polarization were only seen in low ionic strength buffer. Analysis of beta(2)GP1 binding kinetics by resonance energy transfer between fluorescein-labeled beta(2)GP1 and rhodamine-containing PS vesicles revealed a two-component process: (1) a primary and rapid binding via the C-terminus that occurred <2 s in both isotonic and low ionic strength buffers, and (2) a sequential binding of the N-terminus that was approximately 100-fold slower in low ionic strength solution. Taken together, these data suggest that beta(2)GP1 alters the fluidity and membrane polarization of its target membrane, which in low ionic strength buffer is of sufficient magnitude to induce precipitation.

Raman spectroscopy of model membrane monolayers of dipalmitoylphosphatidic acid at the air-water interface using surface enhancement from buoyant thin silver films

A novel method for the acquisition of surface enhanced Raman (SER) spectra of model membranes of dipalmitoylphosphatidic acid (DPPA) in Langmuir layers at the air-water interface is reported. The approach is based on the electrochemical formation of a buoyant thin layer of coalesced silver colloids in the vicinity of the phosphatidic acid head groups at the interface. This Ag layer is an excellent platform for SER scattering, which shows the spectral features from all parts of the molecule and water between the Ag surface and the DPPA layer. The observation of the spectral response from the phosphatidic acid head groups is of particular significance, allowing insight into their chemical state and orientation at the air-water interface.