Cloning and expression of cDNA for human vascular anticoagulant, a Ca2+-dependent phospholipid-binding protein

New Therapeutic Strategies in Retinal Vascular Diseases: A Lipid Target, Phosphatidylserine, and Annexin A5-A Future Theranostic Pairing in Ophthalmology

Despite progress in the management of patients with retinal vascular and degenerative diseases, there is still an unmet clinical need for safe and effective therapeutic options with novel mechanisms of action. Recent mechanistic insights into the pathogenesis of retinal diseases with a prominent vascular component, such as retinal vein occlusion (RVO), diabetic retinopathy (DR) and wet age-related macular degeneration (AMD), may open up new treatment paradigms that reach beyond the inhibition of vascular endothelial growth factor (VEGF). Phosphatidylserine (PS) is a novel lipid target that is linked to the pathophysiology of several human diseases, including retinal diseases. PS acts upstream of VEGF and complement signaling pathways. Annexin A5 is a protein that targets PS and inhibits PS signaling. This review explores the current understanding of the potential roles of PS as a target and Annexin A5 as a therapeutic. The clinical development status of Annexin A5 as a therapeutic and the potential utility of PS-Annexin A5 as a theranostic pairing in retinal vascular conditions in particular is described.

An αIIbβ3- and phosphatidylserine (PS)-binding recombinant fusion protein promotes PS-dependent anticoagulation and integrin-dependent antithrombosis

Blood platelets are required for normal wound healing, but they are also involved in thrombotic diseases, which are usually managed with anticoagulant drugs. Here, using genetic engineering, we coupled the disintegrin protein echistatin, which specifically binds to the platelet integrin α_IIbβ₃ receptor, to annexin V, which binds platelet membrane-associated phosphatidylserine (PS), to create the bifunctional antithrombotic molecule recombinant echistatin-annexin V fusion protein (r-EchAV). Lipid binding and plasma coagulation studies revealed that r-EchAV dose-dependently binds PS and delays plasma clotting time. Moreover, r-EchAV inhibited ADP-induced platelet aggregation in a dose-dependent manner and exhibited potent antiplatelet aggregation effects. r-EchAV significantly prolonged activated partial thromboplastin time, suggesting that it primarily affects the in vivo coagulation pathway. Flow cytometry results indicated that r-EchAV could effectively bind to the platelet α_IIbβ₃ receptor, indicating that r-EchAV retains echistatin's receptor-recognition region. In vivo experiments in mice disclosed that r-EchAV significantly prolongs bleeding time, indicating a significant anticoagulant effect in vivo resulting from the joint binding of r-EchAV to both PS and the α_IIbβ₃ receptor. We also report optimization of the r-EchAV production steps and its purification for high purity and yield. Our findings indicate that r-EchAV retains the active structural regions of echistatin and annexin V and that the whole molecule exhibits multitarget-binding ability arising from the dual functions of echistatin and annexin V. Therefore, r-EchAV represents a new class of anticoagulant that specifically targets the anionic membrane-associated coagulation enzyme complexes at thrombogenesis sites and may be a potentially useful antithrombotic agent.

Effects of Different Zinc Species on Cellar Zinc Distribution, Cell Cycle, Apoptosis and Viability in MDAMB231 Cells

Intracellular metal elements exist in mammalian cells with the concentration range from picomoles per litre to micromoles per litre and play a considerable role in various biological procedures. Element provided by different species can influence the availability and distribution of the element in a cell and could lead to different biological effects on the cell's growth and function. Zinc as an abundant and widely distributed essential trace element, is involved in numerous and relevant physiological functions. Zinc homeostasis in cells, which is regulated by metallothioneins, zinc transporter/SLC30A, Zrt-/Irt-like proteins/SLC39A and metal-response element-binding transcription factor-1 (MTF-1), is crucial for normal cellular functioning. In this study, we investigated the influences of different zinc species, zinc sulphate, zinc gluconate and bacitracin zinc, which represented inorganic, organic and biological zinc species, respectively, on cell cycle, viability and apoptosis in MDAMB231 cells. It was found that the responses of cell cycle, apoptosis and death to different zinc species in MDAMB231 cells are different. Western blot analysis of the expression of several key proteins in regulating zinc-related transcription, cell cycle, apoptosis, including MTF-1, cyclin B1, cyclin D1, caspase-8 and caspase-9 in treated cells further confirmed the observed results on cell level.

Effects of exogenous zinc on cell cycle, apoptosis and viability of MDAMB231, HepG2 and 293 T cells

As a non-toxic metal to humans, zinc is essential for cell proliferation, differentiation, regulation of DNA synthesis, genomic stability and mitosis. Zinc homeostasis in cells, which is crucial for normal cellular functioning, is maintained by various protein families including ZnT (zinc transporter/SLC30A) and ZIP (Zrt-, Irt-like proteins/SLC39A) that decrease and increase cytosolic zinc availability, respectively. In this study, we investigated the influences of a specific concentration range of ZnSO4 on cell cycle and apoptosis by flow cytometry, and cell viability by MTT method in MDAMB231, HepG2 and 293 T cell lines. Fluorescent sensors NBD-TPEA and the counterstain for nuclei Hoechst 33342 were used to stain the treated cells for observing the localisation and amount of Zn(2+) via laser scanning confocal microscope. It was found that the influence manners of ZnSO4 on cell cycle, apoptosis and cell viability in various cell lines were different and corresponding to the changes of Zn(2+) content of the three cell lines, respectively. The significant increase on intracelluar zinc content of MDAMB231 cells resulted in cell death, G1 and G2/M cell cycle arrest and increased apoptotic fraction. Additionally, the mRNA expression levels of ZnT and ZIP families in the three cell lines, when treated with high concentration of ZnSO4, increased and decreased corresponding to their functions, respectively.

Effects of exogenous zinc on the cellular zinc distribution and cell cycle of A549 cells

As the second most abundant transition metal in humans, zinc plays essential roles in normal cellular biological functions, including metabolism, signalling, proliferation, gene expression and apoptosis. We use ZnSO(4) as a stressor in this study to investigate for the first time the effects of exogenous Zn(2+) on both the cellular distribution of zinc and zinc-related proteins and the cell cycle of human lung adenocarcinoma (A549) cells. The cellular distribution of zinc and soluble proteins was determined in the whole cell as well as in the cytoplasmic and nuclear fractions. Exogenous zinc in the tested exposure range (0-100 µM) resulted in an altered cellular distribution of both zinc and the soluble proteins, together with total glutathione (GSx), the ratio of glutathione (GSH) to glutathione disulfide (GSSG) and non-protein sulphydryl (NPSH). Surprisingly, a turning point was observed in the re-distribution trend at a concentration of approximately 50 µM ZnSO(4). It is concluded that there exists a regulatory system in A549 cells that maintains the cellular zinc content stable in the presence of a certain range of extracellular zinc concentration. In addition, an MTT assay and flow cytometric analysis showed that the ZnSO(4) treatment led to a bi-phasic variation in viability and a slight fluctuation in the apoptosis of A549 cells. Our results will help to further elucidate zinc-related cell biology and biochemistry.

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.

Annexin A5 uptake in ischemic myocardium: demonstration of reversible phosphatidylserine externalization and feasibility of radionuclide imaging

Ischemic insult to the myocardium is associated with cardiomyocyte apoptosis. Because apoptotic cell death is characterized by phosphatidylserine externalization on cell membrane and annexin-A5 (AA5) avidly binds to phosphatidylserine, we hypothesized that radiolabeled AA5 should be able to identify the regions of myocardial ischemia.

METHODS

Models of brief myocardial ischemia by the occlusion of the coronary artery for 10 min (I-10) and reperfusion for 180 min (R-180) for the detection of phosphatidylserine exteriorization using (99m)Tc-labeled AA5 and gamma-imaging were produced in rabbits. (99m)Tc-AA5 uptake after brief ischemia was compared with an I-40/R-180 infarct model. Histologic characterization of both myocardial necrosis and apoptosis was performed in ischemia and infarct models. Phosphatidylserine exteriorization was also studied in a mouse model, and the dynamics and kinetics of phosphatidylserine exposure were assessed using unlabeled recombinant AA5 and AA5 labeled with biotin, Oregon Green, or Alexa 568. Appropriate controls were established.

RESULTS

Phosphatidylserine exposure after ischemia in the rabbit heart could be detected by radionuclide imaging with (99m)Tc-AA5. Pathologic characterization of the explanted rabbit hearts did not show apoptosis or necrosis. Homogenization and ultracentrifugation of the ischemic myocardial tissue from rabbit hearts recovered two thirds of the radiolabeled AA5 from the cytoplasmic compartment. Murine experiments demonstrated that the cardiomyocytes expressed phosphatidylserine on their cell surface after an ischemic insult of 5 min. Phosphatidylserine exposure occurred continuously for at least 6 h after solitary ischemic insult. AA5 targeted the exposed phosphatidylserine on cardiomyocytes; AA5 was internalized into cytoplasmic vesicles within 10-30 min. Twenty-four hours after ischemia, cardiomyocytes with internalized AA5 had restored phosphatidylserine asymmetry of the sarcolemma, and no detectable phosphatidylserine remained on the cell surface. The preadministration of a pan-caspase inhibitor, zVAD-fmk, prevented phosphatidylserine exposure after ischemia.

CONCLUSIONS

After a single episode of ischemia, cardiomyocytes express phosphatidylserine, which is amenable to targeting by AA5, for at least 6 h. Phosphatidylserine exposure is transient and internalized in cytoplasmic vesicles after AA5 binding, indicating the reversibility of the apoptotic process.

Detection, purification and identification of an endogenous inhibitor of L-Dopa decarboxylase activity from human placenta

An endogenous inhibitor of L-Dopa decarboxylase activity was identified and purified from human placenta. The endogenous inhibitor of L-Dopa decarboxylase (Ddc) was localized in the membrane fraction of placental tissue. Treatment of membranes with phosphatidylinositol-specific phospholipase C or proteinase K did not affect membrane-associated Ddc inhibitory activity, suggesting that a population of the inhibitor is embedded within membranes. Purification was achieved by extraction from a nondenaturing polyacrylamide gel. The purification scheme resulted in the isolation of a single 35 kDa band, bearing L-Dopa decarboxylase inhibitory activity. The purified inhibitor was identified as Annexin V. The elucidation of the biological importance of the presence of an L-Dopa decarboxylase activity inhibitor in normal human tissues could provide us with new information leading to the better understanding of the biological pathways that Ddc is involved in.

99mTc-labeled annexin V fragments: a potential SPECT radiopharmaceutical for imaging cell death

INTRODUCTION

Annexin V is a protein that binds to phosphatidylserine exposed on dying cells. The phosphatidylserine-specific sequence is attributed to a chain on the N-terminal of annexin consisting of 13 amino acid sequence. Radiolabeled annexin V is used for imaging apoptosis.

METHODS

With an aim to synthesize a probe that can detect cell death akin to annexin V but smaller in size, annexin-13 fragments were derivatized to contain cysteine, cysteine-cysteine and histidine in their sequence at N terminal and were labeled with (99m)Tc via nitrido and carbonyl precursors. The (99m)Tc-labeled annexin-13 derivatives were characterized by HPLC and studied for their stability. In vitro and in vivo studies were carried out in apoptotic HL-60 cells and fibrosarcoma tumor-bearing Swiss mice, respectively.

RESULTS

The (99m)Tc complexes were formed in high yields and were found to be stable. HPLC pattern of (99m)Tc nitrido complex of cysteine-cysteine-annexine 13 (CC-Anx13) and (99m)Tc carbonyl complex of histdine-annexin 13 (H-Anx13) revealed the formation of single species. In vitro cell uptake studies with (99m)Tc nitrido complex of cysteine-cysteine-annexin 13 fragment showed 6.5% uptake in apoptotic HL-60 cells. The uptake was found to be specific on testing with apoptotic HL-60 cells. Biodistribution studies of (99m)Tc nitrido complex with CC-Anx13 in fibrosarcoma tumor-bearing Swiss mice revealed optimum tumor uptake of 0.52 (0.17) %ID/g at 1 h pi.

CONCLUSION

(99m)Tc(N)-CC-anx13 showed specific uptake in apoptotic tumor cells and warrants further evaluation.

Functional comparison of annexin V analogues labeled indirectly and directly with iodine-124

We are interested in imaging cell death in vivo using annexin V radiolabeled with (124)I. In this study, [(124)I]4IB-annexin V and [(124)I]4IB-ovalbumin were made using [(124)I]N-hydroxysuccinimidyl-4-iodobenzoate prepared by iododestannylation of N-hydroxysuccinimidyl-4-(tributylstannyl)benzoate. [(124)I]4IB-annexin V binds to phosphatidylserine-coated microtiter plates and apoptotic Jurkat cells and accumulates in hepatic apoptotic lesions in mice treated with anti-Fas antibody, while [(124)I]4IB-ovalbumin does not. In comparison with (124)I-annexin V, [(124)I]4IB-annexin V has a higher rate of binding to phosphatidylserine in vitro, a higher kidney and urine uptake, a lower thyroid and stomach content uptake, greater plasma stability and a lower rate of plasma clearance. Binding of radioactivity to apoptotic cells relative to normal cells in vitro and in vivo appears to be lower for [(124)I]4IB-annexin V than for (124)I-annexin V.

Annexin A5 down-regulates surface expression of tissue factor: a novel mechanism of regulating the membrane receptor repertoir

Phosphatidylserine (PtdSer) is exposed on the external leaflet of the plasma membrane during apoptosis. The protein annexin A5 (anxA5) shows high affinity for PtdSer. When anxA5 binds to the PtdSer-expressing membranes during apoptosis, it crystallizes as an extended two-dimensional network and activates thereby a novel portal of cell entry that results in the internalization of the PtdSer-expressing membrane patches. This novel pathway of cell entry is potentially involved in the regulation of the surface expression of membrane receptors. In this study we report the regulation of surface expression of the initiator of blood coagulation tissue factor (TF) by this novel pathway of cell entry. AnxA5 induces the internalization of tissue factor expressed on the surface of apoptotic THP-1 macrophages. This down-regulation depends on the abilities of anxA5 to bind to PtdSer and to form a two-dimensional crystal at the membrane. We furthermore show that THP-1 cells produce and externalize anxA5 that cause the internalization of TF in an autocrine type of mechanism. We extended our in vitro work to the in vivo situation and show in a mouse model that anxA5 causes the down-regulation of TF expression by smooth muscle cells of the media of the carotid artery that was mechanically injured. In conclusion, anxA5 down-regulates surface-expressed TF by activating the novel portal of cell entry. This mechanism may be part of a more general autocrine function of anxA5 to regulate the plasma membrane receptor repertoir under stress conditions associated with the surface expression of PtdSer.

Mechanisms of apoptosis: UVA1-induced immediate and UVB-induced delayed apoptosis in human T cells in vitro

OBJECTIVE

The decreased number of lymphocytes combined with the induction of apoptosis and necrosis seems to be the key mechanism of many phototherapeutic agents. The purpose of our study was to determine the regulating pathway, time course and dose dependence of UVA1- vs. UVB-induced cell death in human T lymphocytes.

METHODS

In our study we applied an in vitro method using single-laser flow cytometry differentiating between intact (Annexin V-FITC-/PI-), apoptotic (Annexin V-FITC+/PI-) and necrotic T cells (Annexin V-FITC+/PI+) following UVA1 (340-400 nm) or UVB (280-320 nm) irradiation. Additionally, fluorescence microscopy of apoptotic cells was performed using acridine orange and ethidium bromide.

RESULTS

Compared to DNA-binding fluorescent microscopy, the flow cytometric method revealed similar, but more precise, results concerning apoptosis and necrosis. Our data indicate that UVB irradiation exerts its effects by the induction of delayed apoptosis within 24-48 h. In contrast, UVA1 irradiation acts via the dose-dependent induction of immediate apoptosis and necrosis within 6 h.

CONCLUSIONS

Our findings demonstrate that UVA1 irradiation may effect structural and functional modifications leading to immediate initiation of apoptosis followed by early membrane rupture, whereas UVB irradiation leads to DNA damage followed by delayed apoptosis, obviously without initial membrane alteration.

Anti-annexin V antibodies in patients with early pregnancy loss or implantation failures

OBJECTIVE

To describe the prevalence of annexin V antibodies (aANX) in women with early recurrent pregnancy losses (RPLs) or recurrent IVF-ET failure.

DESIGN

Retrospective data analysis.

SETTING

Tokai University Hospital, Kanagawa, Japan.

PATIENT(S)

Two hundred thirty-eight patients with RPLs, 48 patients with recurrent IVF-ET failure and 179 nonpregnant and 120 pregnant control group women.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

ELISA was used to measure autoantibodies to annexin V.

RESULT(S)

An ELISA system developed for aANX revealed a dose-dependent relationship between annexin V and aANX. The positive/negative cutoff was set at 7 multiples of the median based on the 99th percentile of normal nonpregnant control group patients. Patients with RPLs (5.5%) or recurrent IVF-ET failure (8.3%) had a significantly increased incidence of aANX (IgG) compared with normal nonpregnant (1.1%) or pregnant control group women (0), whereas the prevalence of aANX between both patient groups or between both control groups was not statistically different. Specificity was confirmed by absorption studies using annexin V and by immunoblots.

CONCLUSION(S)

Our data show that aANX are associated with both RPLs and IVF-ET failure. We propose that anti-annexin V antibodies should be considered a risk factor for these reproductive failures.

Annexin A5 D226K structure and dynamics: identification of a molecular switch for the large-scale conformational change of domain III

The domain III of annexin 5 undergoes a Ca(2+)- and a pH-dependent conformational transition of large amplitude. Modeling of the transition pathway by computer simulations suggested that the interactions between D226 and T229 in the IIID-IIIE loop on the one hand and the H-bond interactions between W187 and T224 on the other hand, are important in this process [Sopkova et al. (2000) Biochemistry 39, 14065-14074]. In agreement with the modeling, we demonstrate in this work that the D226K mutation behaves as a molecular switch of the pH- and Ca(2+)-mediated conformational transition. In contrast, the hydrogen bonds between W187 and T224 seem marginal.

Development and characterization of annexin V mutants with endogenous chelation sites for (99m)Tc

[(99m)Tc]Annexin V can be used to image organs undergoing cell death during cancer chemotherapy and organ transplant rejection. To simplify the preparation and labeling of annexin V for nuclear-medicine studies, we have investigated the addition of peptide sequences that will directly form endogenous chelation sites for (99m)Tc. Three mutant molecules of annexin V, called annexin V-116, -117, and -118, were constructed with N-terminal extensions of seven amino acids containing either one or two cysteine residues. These molecules were expressed cytoplasmically in Escherichia coli and purified to homogeneity with a final yield of 10 mg of protein/L of culture. Analysis in a competitive binding assay showed that all three proteins retained full binding affinity for erythrocyte membranes with exposed phosphatidylserine. Using SnCl(2) as reducing agent and glucoheptonate as exchange agent, all three proteins could be labeled with (99m)Tc to specific activities of at least 50-100 microCi/microg. The proteins retained membrane binding activity after the radiolabeling procedure, and quantitative analysis indicated a dissociation constant (K(d)) of 7 nmol/L for the annexin V-117 mutant. The labeling reaction was rapid, reaching a maximum after 40 min at room temperature. The radiolabeled proteins were stable when incubated with phosphate-buffered saline or serum in vitro. Proteins labeled to a specific activity of 25-100 microCi/microg were injected intravenously in mice at a dose of 100 microg/kg, and biodistribution of radioactivity was determined at 60 min after injection. Uptake of radioactivity was highest in kidney and liver, consistent with previous results obtained with wild-type annexin V. Cyclophosphamide-induced apoptosis in vivo could be imaged with [(99m)Tc]annexin V-117. In conclusion, annexin V can be modified near its N-terminus to incorporate sequences that form specific chelation sites for (99m)Tc without altering its high affinity for cell membranes. These annexin V derivatives may be useful for in vivo imaging of cell death.

Competition of annexin V and anticardiolipin antibodies for binding to phosphatidylserine containing membranes

Annexin V, an intracellular protein with a calcium-dependent high affinity for anionic phospholipid membranes, acts as an inhibitor of lipid-dependent reactions of the blood coagulation. Antiphospholipid antibodies found in the plasma of patients with antiphospholipid syndrome generally do not interact with phospholipid membranes directly, but recognize (plasma) proteins associated with lipid membranes, mostly prothrombin or beta(2)-glycoprotein I (beta(2)GPI). Previously, it has been proposed that antiphospholipid antibodies may cause thrombosis by displacing annexin V from procoagulant cell surfaces. We used ellipsometry to study the binding of annexin V and of complexes of beta(2)GPI with patient-derived IgG antibodies to beta(2)GPI, commonly referred to as anticardiolipin antibodies (ACA), to phospholipid bilayers composed of phosphatidylcholine (PC) and 20% phosphatidylserine (PS). More specifically, we investigated the competition of these proteins for the binding sites at these bilayers. We show that ACA-beta(2)GPI complexes, adsorbed to PSPC bilayers, are displaced for more than 70% by annexin V and that annexin V binding is unaffected by the presence of ACA-beta(2)GPI complexes. Conversely, annexin V preadsorbed to these bilayers completely prevents adsorption of ACA-beta(2)GPI complexes, and none of the preadsorbed annexin V is displaced by ACA-beta(2)GPI complexes. Using ellipsometry, we also studied the effect of ACA-beta(2)GPI complexes on the interaction of annexin V with the membranes of ionophore-activated blood platelets as a more physiological relevant model of cell membranes. The experiments with blood platelets confirm the high-affinity binding of annexin V to these membranes and unequivocally show that annexin V binding is unaffected by the presence of ACA-beta(2)GPI. In conclusion, our data unambiguously show that ACA-beta(2)GPI complexes are unable to displace annexin V from procoagulant membranes to any significant extent, whereas annexin V does displace the majority of preadsorbed ACA-beta(2)GPI complexes from these membranes.

A new principle for rapid immunoassay of proteins based on in situ precipitate-enhanced ellipsometry

A new technique is presented that allows measurement of protein concentrations in the picomolar range with an assay time of only 10-20 min. The method is an enzyme-linked immunosorbent assay (ELISA), but uses in-situ ellipsometric measurement of a precipitating enzyme product instead of the usual colorimetric detection of accumulating enzyme product in solution. Quantitative validation was obtained by use of annexin V, a protein with high binding affinity for phosphatidylserine-containing phospholipid membranes, resulting in a transport-limited adsorption rate. This property was exploited to obtain a range of low surface concentrations of annexin V by timed exposures of phospholipid bilayers to known concentrations of annexin V. Using polyvinylchloride (PVC)-coated and silanized silicon slides, various versions of this technique were used for the rapid assay of fatty acid-binding protein (FABP), a recently introduced early marker for acute myocardial infarction with a normal plasma concentration below 1 nmol/l, interleukin 6 (IL-6), a cytokine with normal plasma concentrations below 1 pmol/l, and again, annexin V. A possible future application of the method in the development of a one-step ELISA is discussed.

Conformational flexibility of domain III of annexin V at membrane/water interfaces

The conformational dynamics of domain III in annexin V bound to negatively charged phospholipid vesicles of 1-palmitoyl-2-oleoyl-sn-glycerophosphocholine and 1-palmitoyl-2-oleoyl-sn-glycerophosphoserine or incorporated into reverse micelles of water/sodium bis(2-ethylhexyl) sulfosuccinate in isooctane, used to mimic the phospholipid/water interface, was studied by steady-state and time-resolved fluorescence of its single tryptophan residue (W187). Upon interaction with sonicated phospholipid vesicles in the presence of calcium, or upon incorporation into reverse micelles without calcium, a progressive 12-14 nm red shift of the fluorescence emission spectrum of W187 is observed. The indole environment becomes therefore more polar than in the unbound protein. Three major lifetime populations describe the fluorescence intensity decays of W187 in both systems. A long-lived excited-state population characterizes the membrane-bound state of the protein. The existence of local conformers with different subnanosecond mobility is suggested by specific association between lifetimes and correlation times both for the protein in buffer and in interaction with the membrane surface. The interaction of the protein with the membrane surface preserves the existence of a rapid unhindered rotational motion, which is coupled with all three lifetimes. The longest lifetime is coupled to restricted motions in subnanosecond and nanosecond time scales. The overall amplitude of rotation of the indole ring is increased in the membrane-bound conformation of the protein. In reverse micelles, the local dynamics reported by W187 is also considerably increased whereas the overall folding of the protein remains unaffected. The same conformational change of domain III can therefore be provoked by different conditions: calcium binding at high concentration, mild acidic pH [Sopkova, J., Vincent, M., Takahashi, M., Lewit-Bentley, A. , and Gallay, J. (1998) Biochemistry 37, 11962-11970] and the interaction of the protein with the membrane surface. The high flexibility of domain III in the membrane-bound protein suggests that this domain may not be crucial for the interaction of the protein with the membrane, in contrast with previous models. Our data are compatible with atomic force microscopy results which suggest that domain III of annexin V does not interact strongly with the membrane surface [Reviakine, I., Bergma-Schutter, W., and Brisson, A. (1998) J. Struct. Biol. 121, 356-361].

Functional identification of alpha 1-giardin as an annexin of Giardia lamblia

A protein with a relative molecular mass of 31 kDa was specifically extracted by EGTA from a detergent-insoluble fraction of Giardia lamblia. N-terminal sequencing showed this protein to be identical to alpha 1-giardin, a component of the ventral disc which, based on its predicted amino acid sequence, has been classified as annexin XIX. Purified alpha 1-giardin associated with multilamellar phosphatidyl serine-containing vesicles in a Ca(2+)-dependent manner, confirming that it is a functional annexin. Molecular modelling of the amino acid sequence of the giardial annexin into the X-ray structure of annexin V suggests that the Ca(2+)-binding sites, which, as in other annexins, are all located on the convex surface of the molecule, are of the low-affinity type III.

Generation of monoclonal antibodies to murine bile duct epithelial cells: identification of annexin V as a new marker of small intrahepatic bile ducts

Biliary epithelial cells (BECs) are distributed along the length of both the extrahepatic and intrahepatic biliary tree, but have distinctly different phenotypes and functions according to their anatomical location. It has been reasoned that the distinct appearance of pathogenic lesions in different biliary diseases may be associated with the expression of distinct proteins. These data prompted us to immunize rats with cultured murine BECs with the objective of determining if there are unique antigens on BECs. Of the 45 monoclonal antibodies (mAbs) produced, 12 mAbs (MBEC 1-12) were selected for detailed study based on their classification into three major groups. These groups included four antibodies (MBEC 1-4) that reacted in a staining pattern typical of mucin. A second group of mAbs, MBECs 5 to 8, reacted strongly along the biliary tract and by immunoblot analysis, reacted with several bands ranging from 44 kd to 64 kd. These antibodies were considered as markers of pan BECs and their staining pattern proved similar to that of a control polyclonal pan-cytokeratin. The final group of mAbs, MBECs 9 to 12, recognized a 36-kd antigen using lysates of murine BECs. These antibodies also predominantly stained small peripheral bile ducts. The reactive antigen was purified by immunoprecipitation and microsequenced; the peptides sequenced showed 100% homology with murine annexin V. The identification of annexin V with predominantly intrahepatic bile ducts, is of significant interest because of the multiple roles of annexin V, including that of membrane cytoskeletal interactions during transport and apoptosis.

Conformational flexibility of domain III of annexin V studied by fluorescence of tryptophan 187 and circular dichroism: the effect of pH

The conformation and dynamics of domain III of annexin V was studied by steady-state and time-resolved fluorescence of its single tryptophan residue (Trp187) as a function of pH in the absence of calcium. At neutral pH, the maximum of emission occurs at 326 nm, in agreement with the hydrophobic location of the tryptophan residue seen in the three-dimensional structure. Upon decreasing the pH, a progressive red-shift by about 12 nm of the fluorescence emission spectrum is observed. The effect is complete between pH 6 and 4.5, and most likely involves at least one and maybe two carboxylic group(s). Circular dichroism mesurements give evidence for a preservation of the native folding of the protein in these mild acidic conditions. A fluorescence red-shift of smaller amplitude is also observed at high pH (approximately 11). The aggregation state of the protein is affected by pH: while at neutral pH, the protein is monomeric (rotational correlation time = 14 ns); it forms aggregates larger than a dimer (rotational correlation time > 40 ns) in acidic pH conditions. These results suggest that electrostatic interactions are probably important for the stabilization of the folding of domain III without calcium. The conformational change may be related to the aggregation state of the molecule. Examination of the protein crystal structures with and without calcium ion in domain III shows an interplay of salt bridges implying charged amino acid side chains at the molecule surface of domain III. These observations may provide a further clue to the mechanism of the conformational change of domain III of annexin V induced by high calcium concentrations and interaction at the membrane/water interface.

Surface exposure of phosphatidylserine during apoptosis of rat thymocytes precedes nuclear changes

Cell surface exposure of phosphatidylserine (PS) during apoptosis serves recognition and removal of the dying cell by phagocytes. Loss of phospholipid asymmetry and PS exposure is investigated by immunocytochemistry and related to morphological changes. Loss of membrane asymmetry was determined on dexamethasone-treated rat thymocytes using the PS specific probe annexin V. Thymocytes incubated in the presence of dexamethasone were studied in time series during the execution of the apoptotic program. Thymocytes first start to expose PS at their cell surface. At this initial stage the barrier function of the plasma membrane remains intact. At a later stage the plasma membrane becomes leaky for compounds like propidium iodide and subsequently the cell disintegrates into apoptotic bodies. Microscopical evaluation of dexamethasone-treated thymocytes showed that the cells with an apoptotic morphology all bound annexin V. The cells with a normal viable morphology lacked annexin V binding except for those cells that started to shed small vesicles. These vesicles were positive for annexin V, indicating a local disturbance of the phospholipid asymmetry. The local exposure of PS is considered to be a very early event of apoptosis, preceding the full sequence of morphological changes at the ultrastructural level.

Calcium influx is a determining factor of calpain activation and microparticle formation in platelets

We have related the release of procoagulant microparticles from platelets to calcium movement and the activation of the Ca(2+)-dependent protease calpain. The effects of the Ca(2+)-ATPase inhibitors thapsigargin, cyclopiazonic acid and 2.5-di-(t-butyl)-1,4-benzohydroquinone were compared with those of the Ca2+ ionophore A23187. Whereas all three Ca(2+)-ATPase inhibitors induced aminophospholipid exposure on platelets, only thapsigargin and cyclopiazonic acid promoted microparticle formation and only when strong Ca2+ influx, calpain activation and proteolysis of cytoskeletal proteins occurred concomitantly. Preincubation with dibutylbenzohydroquinone inhibited the responses to thapsigargin and cyclopiazonic acid but not to A23187. When platelets were suspended in a Ca(2+)-free medium, calpain activation and microparticle formation were not observed, even with maximum mobilisation of internal Ca2+ stores by A23187. Incubation of fluo-3-loaded plateters with A23187 in 0.1 mM EGTA followed by the sequential addition of 25 microM Ca2+ increments to the medium showed that calpain activation occurred when the intraplatelet [Ca2+] reached 3-8 microM. To assess the physiologic significance of these results, the subpopulation of platelets that expressed procoagulant activity after stimulation by a thrombin/collagen mixture was isolated by means of annexin-V-coupled magnetic beads. Subsequent western blotting experiments confirmed that this subpopulation contained activated calpain. Overall, our results provide evidence that microparticle formation and calpain activation require an elevated intraplatelet [Ca2+] that is brought about by influx across the plasma membrane.

Measurement of plasma annexin V by ELISA in the early detection of acute myocardial infarction

Annexin V is a calcium binding protein which is widely present in various cells and tissues. Using annexin V which we isolated and purified from human cardiac muscle, we prepared an anti-human cardiac annexin V monoclonal antibody. Identification of annexin V was made by means of partial amino acid sequences. An enzyme-linked immunosorbent assay (ELISA) was developed using this monoclonal antibody and anti-canine cardiac annexin V polyclonal antibody. With this ELISA, plasma annexin V concentration was measured in 196 normal healthy individuals, 23 acute myocardial infarction (AMI) patients who were hospitalized within 6 h after the onset of chest pain, and 130 patients with other diseases, including lung, liver and kidney disease. The plasma annexin V concentration in normal healthy individuals was 1.7 +/- 0.6 ng/ml (mean +/- S.D.), while that in AMI patients was elevated to 13.2 +/- 6.8 ng/ml (P < 0.0001) at the time of initial blood drawing, 3.2 +/- 1.5 h after onset of pain, and these values were higher than normal in 21 out of 23 cases (91.3%) of AMI. In all cases excepting 3, annexin V concentration immediately decreased after the onset of pain. The annexin V concentration in patients with old myocardial infarction, chest pain syndrome, valvular heart disease, lung disease and kidney disease was 1.8 +/- 0.8, 2.0 +/- 0.7, 1.7 +/- 1.1, 2.3 +/- 1.4 and 2.1 +/- 1.2 ng/ml, respectively, being within normal limits. The values in liver disease patients and trauma patients were 3.7 +/- 2.7 (P < 0.05) and 3.3 +/- 2.4 (P < 0.05) ng/ml, respectively, being slightly higher than that in normal healthy individuals.

Procoagulant activity and active calpain in platelet-derived microparticles

Microparticles are released during in vitro platelet activation and have been detected in vivo in several pathologies. Their characterization is of interest as they may play a potential role in hemostasis. Here, we report the formation of microparticles as the result of increases in intracellular Ca2+ brought about by inhibition of Ca(2+)-ATPases. They were isolated following centrifugation of the activated platelet suspension over a sucrose layer. Flow cytometric studies using annexin V-FITC as a probe for aminophospholipids, prothrombinase activity measurements and annexin V inhibition experiments enabled us to evaluate the procoagulant activity of microparticles prepared in this way. The efficiency of the annexin V inhibition (IC50 = 10-20 nM) of this activity confirmed significant anticoagulant properties for this protein. Microparticles also contained the glycoprotein IIb-IIIa complex, detected in flow cytometry at a density higher than on the remnant platelets. The activation of calpain, a Ca(2+)-dependent protease, in platelets was shown to be more efficient under conditions of a sudden Ca2+ influx. The microparticles contained only the active form of calpain detected by Western blotting using a monoclonal antibody able to recognize both the unactivated and the activated catalytic subunit of the enzyme. However, flow cytometry failed to find significant amounts of active calpain on the microparticle or on the platelet surface. Our results, while confirming the procoagulant activity of microparticles, also document for the first time the exclusive presence of the activated form of calpain, inferring a possible role for this protease in microparticle-mediated functions.

Surface blebs on apoptotic cells are sites of enhanced procoagulant activity: implications for coagulation events and antigenic spread in systemic lupus erythematosus

The restriction of phosphatidylserine (PtdSer) to the inner surface of the plasma membrane bilayer is lost early during apoptosis. Since PtdSer is a potent surface procoagulant, and since there is an increased incidence of coagulation events in patients with systemic lupus erythematosus (SLE) who have anti-phospholipid antibodies, we addressed whether apoptotic cells are procoagulant and whether anti-phospholipid antibodies influence this. Apoptotic HeLa cells, human endothelial cells, and a murine pre-B-cell line were markedly procoagulant in a modified Russell viper venom assay. This procoagulant effect was entirely abolished by addition of the PtdSer-binding protein, annexin V, confirming that it was PtdSer-dependent. The procoagulant effect was also abolished by addition of IgG purified from the plasma of three patients with anti-phospholipid antibody syndrome, but not IgG from normal controls. Confocal microscopy of apoptotic cells stained with fluorescein-isothiocyanate-conjugated-annexin V demonstrated (Ca2+)-dependent binding to the surface of membrane blebs o apoptotic cells, but not to intracellular membranes. Recent data indicate that the surface blebs of apoptotic cells constitute an important immunogenic particle in SLE. We propose that the PtdSer exposed on the outside of these blebs can induce the production of anti-phospholipid antibodies, which might also enhance the immunogenicity of the bleb contents. When apoptosis occurs in a microenvironment in direct contact with circulating plasma, the unique procoagulant consequences of the apoptotic surface may additionally be expressed. This might explain the increased incidence of pathological intravascular coagulation events that occur in some lupus patients who have anti-phospholipid antibodies.

Apoptosis of cultured rat glomerular mesangial cells induced by IgG2a monoclonal anti-Thy-1 antibodies

Anti-Thy-1 nephritis is a model of mesangial proliferative glomerulonephritis. It has been suggested that apoptosis, which is a counteracting regulatory mechanism against undesired cell proliferation, is involved in sequential histological changes in this model. In the present study, we investigated whether IgG2a anti-Thy-1 monoclonal antibody (ER4) or its F(ab')2 fragments are able to induce apoptosis of rat glomerular mesangial cells (GMC) in vitro. After co-culture with ER4 or its F(ab')2 fragments, apoptosis was assessed by morphological studies with Hoechst 33258 stain and FITC-annexin V. The latter detects the dislocation of negatively charged phospholipid, phosphatidylserine, from the inner to the outer leaflet of the membrane during apoptosis. This is a sensitive method for the detection of apoptosis. Under fluorescent microscopy, distinct nuclear condensation and positive reactivity with FITC-annexin V were observed in cells co-cultured with ER4 or its F(ab')2 fragments. The results obtained by FACS analysis with annexin V showed a direct correlation with the detection of apoptosis with the terminal deoxynucleotidyl transferase reaction (TDT). Up to 19% and 23% of rat GMC, which were co-cultured for 24 hours with 1 microgram/ml (0.5 microgram/l x 10(5) cells) of ER4 or its F(ab')2 fragments, were labeled by TDT, respectively. With annexin V, up to 34% and 31% cells displaying apoptosis were seen. The degree of apoptosis as measured by the annexin V method was dependent on the concentration of ER4 and time of incubation in the presence of ER4. Finally, apoptosis was confirmed by gel electrophoresis of DNA isolated from the cells co-cultured with each monoclonal antibody (MAb). DNA extracts from cells co-cultured with ER4 or its F(ab')2 fragments demonstrated typical internucleosomal DNA fragmentation. Medium alone, controls of anti-human C3bi receptor MAb (IB4) and anti-rat MHC class I MAb (OX18) showed neither nuclear changes nor significant labeling of the cells with the TDT reaction or with the annexin V. Taken together, these results demonstrate for the first time that anti-Thy-1 MAb is able to induce apoptosis of rat GMC in vitro. The Thy-1 antigen on rat GMC, therefore, seems to function as one of the molecules regulating cell death and thereby may determine the degree of mesangial alteration in Thy-1 nephritis.

Early redistribution of plasma membrane phosphatidylserine is a general feature of apoptosis regardless of the initiating stimulus: inhibition by overexpression of Bcl-2 and Abl

A critical event during programmed cell death (PCD) appears to be the acquisition of plasma membrane (PM) changes that allows phagocytes to recognize and engulf these cells before they rupture. The majority of PCD seen in higher organisms exhibits strikingly similar morphological features, and this form of PCD has been termed apoptosis. The nature of the PM changes that occur on apoptotic cells remains poorly defined. In this study, we have used a phosphatidylserine (PS)-binding protein (annexin V) as a specific probe to detect redistribution of this phospholipid, which is normally confined to the inner PM leaflet, during apoptosis. Here we show that PS externalization is an early and widespread event during apoptosis of a variety of murine and human cell types, regardless of the initiating stimulus, and precedes several other events normally associated with this mode of cell death. We also report that, under conditions in which the morphological features of apoptosis were prevented (macromolecular synthesis inhibition, overexpression of Bcl-2 or Abl), the appearance of PS on the external leaflet of the PM was similarly prevented. These data are compatible with the notion that activation of an inside-outside PS translocase is an early and widespread event during apoptosis.

A novel assay for apoptosis. Flow cytometric detection of phosphatidylserine expression on early apoptotic cells using fluorescein labelled Annexin V

In the early stages of apoptosis changes occur at the cell surface, which until now have remained difficult to recognize. One of these plasma membrane alterations is the translocation of phosphatidylserine (PS) from the inner side of the plasma membrane to the outer layer, by which PS becomes exposed at the external surface of the cell. Annexin V is a Ca2+ dependent phospholipid-binding protein with high affinity for PS. Hence this protein can be used as a sensitive probe for PS exposure upon the cell membrane. Translocation of PS to the external cell surface is not unique to apoptosis, but occurs also during cell necrosis. The difference between these two forms of cell death is that during the initial stages of apoptosis the cell membrane remains intact, while at the very moment that necrosis occurs the cell membrane looses its integrity and becomes leaky. Therefore the measurement of Annexin V binding to the cell surface as indicative for apoptosis has to be performed in conjunction with a dye exclusion test to establish integrity of the cell membrane. This paper describes the results of such an assay, as obtained in cultured HSB-2 cells, rendered apoptotic by irradiation and in human lymphocytes, following dexamethasone treatment. Untreated and treated cells were evaluated for apoptosis by light microscopy, by measuring the amount of hypo-diploid cells using of DNA flow cytometry (FCM) and by DNA electrophoresis to establish whether or not DNA fragmentation had occurred. Annexin V binding was assessed using bivariate FCM, and cell staining was evaluated with fluorescein isothiocyanate (FITC)-labelled Annexin V (green fluorescence), simultaneously with dye exclusion of propidium iodide (PI) (negative for red fluorescence). The test described, discriminates intact cells (FITC-/PI-), apoptotic cells (FITC+/PI-) and necrotic cells (FITC+/PI+). In comparison with existing traditional tests the Annexin V assay is sensitive and easy to perform. The Annexin V assay offers the possibility of detecting early phases of apoptosis before the loss of cell membrane integrity and permits measurements of the kinetics of apoptotic death in relation to the cell cycle. More extensive FCM will allow discrimination between different cell subpopulations, that may or may not be involved in the apoptotic process.

Novel antithrombotic drugs in development

Platelet activation plays a critical role in thromboembolic disorders, and aspirin remains a keystone in preventive strategies. This remarkable efficacy is rather unexpected, as aspirin selectively inhibits platelet aggregation mediated through activation of the arachidonic-thromboxane pathway, but not platelet aggregation induced by adenosine diphosphate (ADP), collagen and low levels of thrombin. This apparent paradox has stimulated investigations on the effect of aspirin on eicosanoid-independent effects of aspirin on cellular signalling. It has also fostered the search for antiplatelet drugs inhibiting platelet aggregation at other levels than the acetylation of platelet cyclo-oxygenase, such as thromboxane synthase inhibitors and thromboxane receptor antagonists. The final step of all platelet agonists is the functional expression of glycoprotein (GP) IIb/IIIa on the platelet surface, which ligates fibrinogen to link platelets together as part of the aggregation process. Agents that interact between GPIIb/IIIa and fibrinogen have been developed, which block GPIIb/IIIa, such as monoclonal antibodies to GPIIb/IIIa, and natural and synthetic peptides (disintegrins) containing the Arg-Gly-Asp (RGD) recognition sequence in fibrinogen and other adhesion macromolecules. Also, some non-peptide RGD mimetics have been developed which are orally active prodrugs. Stable analogues of prostacyclin, some of which are orally active, are also available. Thrombin has a pivotal role in both platelet activation and fibrin generation. In addition to natural and recombinant human antithrombin III, direct antithrombin III-independent thrombin inhibitors have been developed as recombinant hirudin, hirulog, argatroban, boroarginine derivatives and single stranded DNA oligonucleotides (aptanes). Direct thrombin inhibitors do not affect thrombin generation and may leave some 'escaping' thrombin molecules unaffected. Inhibition of factor Xa can prevent thrombin generation and disrupt the thrombin feedback loop that amplifies thrombin production.

Annexin V perturbs or stabilises phospholipid membranes in a calcium-dependent manner

The potency of annexin V to transport Ca2+ ions across phospholipid membranes was investigated, using large unilamellar phospholipid vesicles loaded with the Ca2+ indicator fura-2. It was demonstrated that annexin V leaves the vesicle membranes intact when added in the presence of 1 mM Ca2+. However, if the vesicles were first incubated with annexin V in the absence of Ca2+, subsequent addition of Ca2+ produced a fluorescence signal due to binding of Ca2+ to fura-2. Centrifugation of the vesicle suspension immediately thereafter showed that this signal originated from the supernatant and not from the sedimented vesicles. Our results show that annexin V causes loss of vesicle integrity in the absence of Ca2+, and leakage of trapped fura-2, rather than inward Ca2+ transport. Bovine serum albumin or Ca2+ concentrations higher than 2.5 mM also caused such fura-2 leakage. Apparently, calcium-dependent binding of annexin V to the membrane prevents aspecific membrane damage caused by this protein.

Levels and localization of group II phospholipase A2 and annexin I in interleukin- and dexamethasone-treated rat mesangial cells: evidence against annexin mediation of the dexamethasone-induced inhibition of group II phospholipases A2

The mechanism by which glucocorticosteroids inhibit the synthesis and secretion of pro-inflammatory arachidonate metabolites is still controversial. Initially it was postulated that glucocorticoids can induce the formation of PLA2 inhibitory proteins termed annexins. We have previously shown that the cytokine-induced 14 kDa PLA2 activity and the synthesis of prostaglandin E2 in rat mesangial cells is dose-dependently blocked by pretreatment of the cells with dexamethasone (Schalkwijk et al. (1991) Biochem. Biophys. Res. Commun. 180, 46-52). Concurrently, the synthesis of 14 kDa group II PLA2 is suppressed. The regulation of PLA2 activity is complex and may well involve superimposable mechanisms. Thus, although the decrease in PLA2 protein levels could in itself explain the dexamethasone-induced decrease in PLA2 activity, a contribution of the glucocorticoid-induced anti-phospholipase A2 protein annexin cannot be ruled out a priori. To investigate this possibility we analyzed the level of annexin I by Western blotting and immunostaining in mesangial cells treated with interleukin-1 beta and/or dexamethasone. Under conditions where 14 kDa group II PLA2 activity and protein levels were dramatically affected by interleukin-1 and dexamethasone, the level of annexin I in the cells remained constant. Dexamethasone also did not induce the secretion of annexin I. In addition, no evidence for dexamethasone-induced translocation of annexin I from the cytosol to membranes, thereby possibly sequestering the substrates for PLA2, was obtained. Immunofluorescence studies localized the cytokine-induced PLA2 to the Golgi area and punctate structures in the cytoplasm. We have also studied the subcellular localization of annexin I in rat mesangial cells using confocal microscopy. These studies located annexin I mainly in the cytoplasma and the nucleus. We conclude from these experiments that the dexamethasone-induced inhibition of 14 kDa group II PLA2 in rat mesangial cells is not mediated by annexin I and is solely due to the suppression of PLA2 gene expression.

Calcium-induced changes in annexin V behaviour in solution as seen by proton NMR spectroscopy

The behaviour of human annexin V in the presence of calcium was studied by NMR. We observe the formation of well defined dimers, as well as a change in the local dynamics of one His side chain. We assign the observed changes to either His98 or His267 residues and conclude that they could be related either to the hinge-bending motion reported from crystal structures, or to a local side chain rearrangement within the calcium-binding loops concerned. Dimerization was also confirmed by a small-angle neutron-scattering experiment. Under the experimental conditions used, we do not observe the conformational change involving Trp187 seen in previous studies, which occurs at higher relative calcium concentrations.

Isolation and characterization of recombinant annexin V expressed in Saccharomyces cerevisiae

The cloning, purification and characterization of full-length annexin V, expressed intracellularly in Saccharomyces cerevisiae is detailed. Following homogenization in a glass bead mill, clarification by ultracentrifugation and fractional ammonium sulfate precipitation, the 319 amino acid protein was purified by column chromatography on phenyl-Sepharose and heparin-Sepharose. Annexin V elutes on reverse phase C4 silica as a single peak with greater than 97% homogeneity and is further characterized by a molecular mass of 34 kDa from electrophoresis under reducing conditions on SDS gels. Dynamic light scattering experiments reveal annexin V exists as a monomer in solution. Amino terminal Edman degradation afforded no sequence, therefore the carbamidomethylated protein was chemically cleaved with cyanogen bromide. Separation of the resulting peptide fragments on reverse phase HPLC followed by N-terminal sequencing and electrospray mass spectrometry supported the correct sequence as well as the existence of an acetyl blocking group on the N-terminus. The protein exhibits an isoelectric point of 4.73 by column chromatofocusing. Secondary structure predictions from CD spectroscopy indicate that the molecule is correctly folded. In anticoagulant assays, the purified protein exhibits dose-response effects in activated partial thromboplastin time (APTT) prolongation and doubles the clotting time of control human plasma at 70 micrograms ml-1. More specifically, in a factor Xa inhibition assay in which the activation of factor X via the tissue factor-factor VIIa complex is monitored by the cleavage of a factor Xa chromogenic substrate, recombinant annexin V exhibits a 50% inhibitory concentration (IC50) in the low nanomolar range.

Binding of recombinant annexin V to endothelial cells: effect of annexin V binding on endothelial-cell-mediated thrombin formation

Annexin V binds with high affinity to procoagulant phospholipid vesicles and thereby inhibits the procoagulant reactions catalysed by these surfaces in vitro. In vivo, vascular endothelial cells are known to catalyse the formation of thrombin by the expression of binding sites at which procoagulant complexes can assemble. Here, we have studied the binding capacity of recombinant annexin V (rANV) to quiescent, phorbol 12-myristate 13-acetate (PMA)- and tumour necrosis factor alpha (TNF-alpha)-stimulated cultured human umbilical-vein endothelial cells (HUVEC). The dissociation constant (Kd) was 15.5 +/- 3.3 nM and the number of binding sites was 8.8 (+/- 3.9) x 10(6)/cell. These binding parameters did not change significantly during a 30 h incubation period with PMA or TNF-alpha. rANV inhibited HUVEC-mediated factor Xa formation via the extrinsic as well as the intrinsic route. Activation of factor X by the tissue factor-factor VII-factor X complex and tenase complex was inhibited with IC50 values of 43 +/- 30 nM and 33 +/- 24 nM respectively. Endothelial-cell-mediated generation of thrombin by the prothrombinase complex was inhibited by rANV with an IC50 of 16 +/- 12 nM. Preincubation of rANV with the endothelial cells did not significantly influence the IC50 values. These results show that rANV binds to the same extent to quiescent, PMA- and TNF-stimulated HUVEC, and, as a result of this binding, rANV efficiently inhibits endothelial-cell-mediated thrombin formation.

Differential tissue expression of Annexin VIII in human

The expression of Annexins V and VIII by human lung, liver, kidney, skin, heart, uterus, spleen and skeletal muscle was investigated by ELISA. All investigated tissues contained Annexin V. Its level varied with the tissue from around 5 microgram (skin) to approximately 120 micrograms (spleen) per g of wet tissue. Contradistinctionally Annexin VIII expression was less ubiquitous and less abundant. Only lung, skin, liver, and kidney expressed Annexin VIII. Its levels were approximately 100-fold less then the Annexin V levels. Immunohistochemical analysis of lung sections revealed Annexin VIII presence exclusively in the endothelia. Annexin V and VIII levels of cultured human umbilical vein endothelial cells, human arterial smooth muscle cells, human lung fibroblasts and HeLa cells were measured by ELISA. All cell types expressed Annexin V whereas only HeLa cells had detectable levels of Annexin VIII. The results indicate a tissue specific expression of Annexin VIII by lung endothelium, suggesting a highly specialised function.

The interaction of metal ions with annexin V: a crystallographic study

Three closely related rhombohedral crystal structures of human annexin V have been analysed and compared: a low-calcium, a high-calcium and an ytterbium-soaked crystal. The occupancy of the calcium sites increases at higher calcium concentrations, but the calcium is removed rather than replaced during soaking in the ytterbium solution. Instead, other sites are substituted at high calcium concentrations as well as in the presence of ytterbium. Furthermore, a new site is revealed in the ytterbium-soaked crystal which may give a clue to the mechanism of conformational change that takes place in the third domain of annexin V in the presence of very high calcium concentrations and of phospholipids.

Expression of the annexin VIII gene in acute promyelocytic leukemia

Annexin VIII is preferentially expressed in APL, but its level of expression in other subtypes of AML is much lower. Annexin VIII was originally found to be a vascular anticoagulant, but evidence obtained from our recent studies suggests that it does not play a role in hemorrhage diathesis in APL. The specific expression of annexin VIII in APL may relate to its possible role in hematopoietic cell differentiation. The expression of annexin VIII is developmentally regulated in APL-derived NB4 cells. It can be downregulated as a response to induction by ATRA, an agent which is also capable of inducing maturation of NB4 cells. Our current understanding is that annexin VIII is most likely involved in signal transduction and may have a role as a modulator of PKC. A change in cellular PKC activity is expected to have a significant impact on cell differentiation and proliferation. The biological function of annexin VIII is currently unknown, but its expression in APL and its possible role in differentiation and proliferation of the leukemia cells would provide an excellent model system to study and elucidate this intriguing question.

Purification of cardiac annexin V from the beagle dog heart and changes in its localization in the ischemic rat heart

We isolated and purified 35 kDa protein from the myocardium of the beagle dog and identified it to be annexin V from partial amino acid sequence determination. It was confirmed that anticanine cardiac annexin V rabbit polyclonal antibody, which was produced using the 35 kDa protein, cross-reacts with annexin V of the myocardium, lung, liver, kidney, and brain of the rat. The localization of cardiac annexin V and the effect of ischemia for 30-180 min in the rat were immunohistochemically studied with the use of the Langendorff perfusion heart. In the normal myocardium, annexin V, accompanied by cross-striation, was observed throughout the cell. In ischemia of 30 min, extracellular leakage of annexin V was observed with uneven staining in the cytoplasm. When the ischemic time exceeded 60 min, annexin V was observed in the cell membrane with a decrease of annexin V in the cytoplasm. Also, extracellular leakage of annexin V was observed prominently. In ischemia for 180 min, almost all the annexin V in the cytoplasm disappeared. These results suggest that the level of ischemia can be estimated from the changes in localization of annexin V.

Annexin V inhibits the procoagulant activity of matrices of TNF-stimulated endothelium under blood flow conditions

A human ex vivo thrombosis model was used to investigate whether recombinant annexin V (rANV) can prevent thrombus formation under venous and arterial blood flow conditions. In this model, blood from an antecubital vein of healthy donors was allowed to flow directly over the extracellular matrix of tumor necrosis factor-stimulated endothelial cells (TNF-ECMs). TNF-ECMs were preincubated with rANV (2.9 mumol/L) for 30 minutes. With this rANV concentration all binding sites present on TNF-ECMs (1.6 +/- 0.5 x 10(12)/cm2) are occupied, and a maximal inhibition was observed in a tissue factor-dependent clotting assay. Fibrin deposition and platelet and leukocyte adhesion were measured on the rANV-treated and nontreated TNF-ECMs. Nontreated TNF-ECMs were used as controls. rANV inhibited fibrin deposition by 81% at a wall shear rate of 100 s-1. A nonsignificant inhibition was also observed at 650 s-1. Platelet-matrix adhesion, which is more prominent at higher shear rates, was significantly decreased by 60% at 100 s-1 but not at 650 s-1. The average leukocyte adherence was nonsignificantly lowered at 100 s-1. Virtually no leukocytes adhered at 650 s-1. The results demonstrated that rANV can inhibit blood coagulation under venous blood flow conditions and may serve as an antithrombotic drug.

The dynamic behavior of annexin V as a function of calcium ion binding: a circular dichroism, UV absorption, and steady-state and time-resolved fluorescence study

The binding of calcium ions to annexin V in the absence of phospholipids has been studied by UV-difference spectroscopy, circular dichroism, and steady-state and time-resolved fluorescence. In the absence of calcium, the unique tryptophan 187, located in domain III of annexin V, is surrounded by a strongly hydrophobic environment, as indicated by its "blue" fluorescence emission maximum (325 nm). This corresponds well with the description of the structure determined by X-ray crystallography of several crystal forms. The Trp187 time-resolved fluorescence decay shows the existence of a fast (picosecond) excited-state reaction which can involve the formation of an H-bond between the indole NH group and the proximate epsilon-OH and/or alpha-carbonyl groups of Thr224. Titration with calcium tends to stabilize the overall structure, as shown by circular dichroism, while leading to large modifications of the local structure around Trp187 making it accessible to the solvent as shown by UV-difference spectra, circular dichroism spectra, and the displacement of its fluorescence emission maximum at saturating concentrations of calcium (350 nm). A rapid (picosecond) formation of an excited-state complex, probably involving one or a few water molecules of the solvation shell, is observed. These observations correlate well with the conformational change observed in crystal structures obtained in high calcium concentrations, involving the removal of Trp187 from the buried position to the surface of the molecule [Sopkova, J., Renouard, M., & Lewit-Bentley, A. (1993) J. Mol. Biol. 234, 816-825; Concha, N. O., Head, J. F., Kaetzel, M. A., Dedman, J. R., & Seaton, B. A. (1993) Science 261, 1321-1324]. In the solvent-exposed conformation, the indole ring becomes mobile in the subnanosecond and nanosecond time range. This conformational change and the increase in local flexibility can be important for the accommodation of the protein on the surface of phospholipid membranes.

Annexin 5 as a potential regulator of annexin 1 phosphorylation by protein kinase C. In vitro inhibition compared with quantitative data on annexin distribution in human endothelial cells

In vitro phosphorylation of annexin 1 by purified rat brain protein kinase C (PKC) has been studied in the presence of annexin 5, which is not a substrate for PKC. Annexin 5 promoted a dose-dependent inhibition of annexin 1 phosphorylation, which could be overcome by increasing the concentration of phosphatidylserine (PtdSer). In addition, a close relationship was found between the amount of PtdSer uncovered by annexin 5 and the residual phosphorylation of annexin 1. These data fit with the 'surface depletion model' explaining the antiphospholipase activity of annexins. In order to check the possibility that the in vitro effect of annexin 5 could be of some physiological relevance, annexins 1, 2, and 5, as well as the light chain of calpactin 1 (p11), have been quantified in human endothelial cells by measuring the radioactivity bound to the proteins after Western blotting with specific antibodies and 125I-labelled secondary antibody. Our data indicate that annexins 1 and 5, PKC and PtdSer are present in human endothelial cells in relative amounts very similar to those used in vitro under conditions permitting the detection of the inhibitory effect of annexin 5. Since annexin 1 remained refractory to PKC-dependent phosphorylation in intact cells, we suggest that annexin 5 might exert its inhibitory effect towards PKC in vivo, provided that its binding to phospholipids can occur at physiological (micromolar) concentrations of Ca2+. This was previously shown to occur in vitro using phosphatidylethanolamine/phosphatidic acid vesicles [Blackwood and Ernst (1990) Biochem. J. 266, 195-200]. Using identical assay conditions, which also allowed expression of PKC activity, annexin 5 again inhibited annexin 1 phosphorylation without interfering with PKC autophosphorylation. These data suggest that annexins 1 and 5 might interact with each other on the lipid surface, resulting in a specific inhibition of annexin 1 phosphorylation by PKC. Whether a similar mechanism also occurs in vivo remains to be determined.

The sub-cellular localization of annexin V in cultured chick-embryo fibroblasts

The Ca(2+)- and phospholipid-binding protein, annexin V, has been shown by an immune assay to represent 0.4% of total cell protein in cultured chick-embryo fibroblasts. Immunofluorescent localization studies indicate that in primary cultures the protein is abundant in the cytoplasm of the cells and also extends into the nucleus. Nuclear staining is no longer detectable, however, in approx. 25% of the cells following sub-culture. Sub-populations of annexin V are associated with cytoskeletal structures and with the inner face of the plasma membrane in a Ca(2+)-independent manner. In addition, we report results indicating the secretion of annexin V from this cell type.

The effect of annexin IV and VI on the fluidity of phosphatidylserine/phosphatidylcholine bilayers studied with the use of 5-deoxylstearate spin label

An effect of annexin IV and VI on the fluidity of phosphatidylserine/phosphatidylcholine (PS/PC) membranes was studied by spin labeling technique with the use of 5-doxylstearic acid. It was found that calcium ions at micromolar concentrations induced a marked decrease in the order parameter of PS/PC membranes. This effect was completely abolished by both annexins. The influence of annexins on the mobility of fatty acid chains in the studied region in PE/PC membranes was insignificant.

Changes in annexin (lipocortin) content in human amnion and chorion at parturition

Arachidonic acid is mobilized from fetal membrane phospholipids at parturition leading to increased production of oxytocic prostaglandins which may initiate or maintain myometrial contractions. Phospholipid mobilization requires activation of phospholipase A2 or C, both of which require calcium for activity. The annexins (lipocortins) are a superfamily of proteins which bind to calcium and phospholipids and thereby may alter phospholipase activity through two mechanisms: modulation of intracellular free Ca2+ concentrations or regulation of the accessibility of phospholipids to hydrolyzing enzymes. Using Western immunoblotting with monospecific polyclonal antibodies, annexins I-VI were identified in human amnion and chorion/decidua at term in tissues obtained from patients in labor or not in labor. Each annexin was present in two distinct pools: a pool which only associated with the membrane in the presence of calcium (calcium-dependent pool) and a calcium-independent pool that remained membrane bound in the presence of calcium chelators. Annexin I was present as two species, resolving at 36 kDa and 68 kDa. The total concentration of annexin I in both amnion and chorion/decidua was significantly decreased with labor, while the total concentration of annexin V in chorion significantly increased with labor. The size of individual pools of annexins also changed with labor: the calcium-dependent pool of annexins I and II in both amnion and chorion significantly decreased; the calcium-dependent pool of annexin V increased in chorion; and calcium-independent pools of annexin I in amnion and annexins I, II, and V in chorion significantly decreased with labor. The decrease in total annexin I concentration with labor in amnion reflects a substantial decrease (80-90%) in the pool tightly bound to the membrane in a calcium-independent manner. This striking change distinguishes annexin I as a potential candidate inhibitor which is specifically downregulated at parturition, potentially leading to increased access of phospholipases to substrate phospholipids and increased prostaglandin production at labor.

The effect of metal binding on the structure of annexin V and implications for membrane binding

The structure of annexin V, crystallised in the presence of two calcium or barium ions for each protein molecule, was solved by molecular replacement to 0.24 nm resolution. The two metal ions are found in domains I and IV, i.e. on the same side of the channel that lies in the centre of the molecule. The structures of the barium and calcium form are extremely close, the only differences localised in the metal-binding sites that lie on the surface of the molecule. The occupancies of the metal ions, however, are lower for barium than for calcium, expressing the lower affinity of the protein for the former. The packing of the annexin molecules in the crystal asymmetric unit may represent a model for the calcium driven association of membrane-bound annexins that leads to membrane fusion.

Annexins in cardiac tissue: cellular localization and effect on phospholipase activity

Stimulation of cardiac phospholipid metabolism has diverse biological effects, ranging from subtle changes in cellular function to severe cellular damage. Accordingly, knowledge of the factors governing the activity of cardiac phospholipases is of great biological importance. A possible role of annexins, intracellular proteins that bind to membranes in a calcium dependent manner, as modulators of phospholipase activity has been proposed. In this study we investigated the cell type specific distribution of Annexin V and VIII in the heart. Recombinant Annexin V was used to examine the effect of this type of Annexin on cardiac phospholipase activity. Western blot analysis shows that annexin V is abundantly present in the heart. Using isolated myocytes and cultured cardiac endothelial and fibroblast-like cells, it is demonstrated that the localization of Annexin V is confined to non-myocytes. No positive bands matching the Mw of recombinant Annexin VIII are found in any of the cell types examined. In vitro studies demonstrate that recombinant Annexin V potently inhibits the activity of cardiac membrane-bound phospholipases, acting on their natural surrounding substrate, in a calcium dependent manner. Interestingly, annexin V also inhibits triacylglycerol hydrolysis. In conclusion, the expression of annexins is cell-type specific and suggests a cell-type specific function of these proteins in the heart. The absence of Annexin V in cardiac myocytes dismisses involvement of this annexin in cardiomyocyte phospholipid metabolism. The presence of Annexin V in cardiac endothelial and fibroblasts suggests a regulating role in the phospholipid homeostasis of non-myocyte cell types in the heart.