Inhibition of arterial thrombosis by recombinant annexin V in a rabbit carotid artery injury model

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.

TMEM16E regulates endothelial cell procoagulant activity and thrombosis

Endothelial cells (ECs) normally form an anticoagulant surface under physiological conditions, but switch to support coagulation following pathogenic stimuli. This switch promotes thrombotic cardiovascular disease. To generate thrombin at physiologic rates, coagulation proteins assemble on a membrane containing anionic phospholipid, most notably phosphatidylserine (PS). PS can be rapidly externalized to the outer cell membrane leaflet by phospholipid "scramblases," such as TMEM16F. TMEM16F-dependent PS externalization is well characterized in platelets. In contrast, how ECs externalize phospholipids to support coagulation is not understood. We employed a focused genetic screen to evaluate the contribution of transmembrane phospholipid transport on EC procoagulant activity. We identified 2 TMEM16 family members, TMEM16F and its closest paralog, TMEM16E, which were both required to support coagulation on ECs via PS externalization. Applying an intravital laser-injury model of thrombosis, we observed, unexpectedly, that PS externalization was concentrated at the vessel wall, not on platelets. TMEM16E-null mice demonstrated reduced vessel-wall-dependent fibrin formation. The TMEM16 inhibitor benzbromarone prevented PS externalization and EC procoagulant activity and protected mice from thrombosis without increasing bleeding following tail transection. These findings indicate the activated endothelial surface is a source of procoagulant phospholipid contributing to thrombus formation. TMEM16 phospholipid scramblases may be a therapeutic target for thrombotic cardiovascular disease.

Recombinant protein diannexin prevents preeclampsia-like symptoms in a pregnant mouse model via reducing the release of microparticles

Preeclampsia (PE) is characterized by placenta-mediated pregnancy complication. The only effective treatment for PE is the delivery of the placenta. However, this treatment may cause preterm birth and neonatal death. Therefore, preventing PE is needed. The mechanism of PE involves abnormal placentation, which leads to the release of anti-angiogenic and inflammatory mediators into maternal circulation. These mediators contribute to systemic vascular dysfunction, inflammatory responses, and excessive thrombin generation. Microparticles (MPs) are reportedly involved in PE by promoting the thromboinflammatory response. This study describes a strategy to prevent PE by reducing MP release using the recombinant protein, diannexin. Results showed that the patients with PE had elevated MP number and procoagulant activity and increased NLRP3 inflammasome activation. Additionally, diannexin remarkably reduced the release of MPs from activated cells by binding to phosphatidylserine exposed on the surface of activated cells. Moreover, in vivo results showed that diannexin could prevent PE-like symptoms by decreasing MPs and NLRP3 inflammasome activation in pregnant mice. Furthermore, diannexin effectively inhibited trophoblast cell activation and NLRP3 inflammasome activation in vitro. These findings suggested that diannexin inhibited MP release and might be an effective therapeutic strategy for preventing PE.

Annexin V-Modified Platelet-Biomimetic Nanomedicine for Targeted Therapy of Acute Ischemic Stroke

Thromboembolic stroke is typically characterized by the activation of platelets, resulting in thrombus in the cerebral vascular system, leading to high morbidity and mortality globally. Intravenous thrombolysis by tissue plasminogen activator (tPA) administration within 4.5 h from the onset of symptoms is providing a standard therapeutic strategy for ischemic stroke, but this reagent simultaneously shows potential serious adverse effects, e.g., hemorrhagic transformation. Herein, a novel delivery platform based on Annexin V and platelet membrane is developed for tPA (APLT-PA) to enhance targeting efficiency, therapeutic effects, and reduce the risk of intracerebral hemorrhage in acute ischemic stroke. After preparation by extrusion of platelet membrane and subsequent insertion of Annexin V to liposomes, APLT-PA exhibits a high targeting efficiency to activated platelet in vitro and thrombosis site in vivo, due to the binding to phosphatidylserine (PS) and activated platelet membrane proteins. One dose of APLT-PA leads to obvious thrombolysis and significant improvement of neurological function within 7 days in mice with photochemically induced acute ischemic stroke. This study provides a novel, safe platelet-biomimetic nanomedicine for precise thrombolytic treatment of acute ischemic stroke, and offers new theories for the design and exploitation of cell-mimetic nanomedicine for diverse biomedical applications.

Annexin A Protein Family in Atherosclerosis

Atherosclerosis, a silent chronic vascular pathology, is the cause of the majority of cardiovascular ischaemic events. Atherosclerosis is characterized by a series of deleterious changes in cellularity, including endothelial dysfunction, transmigration of circulating inflammatory cells into the arterial wall, pro-inflammatory cytokines production, lipid accumulation in the intima, vascular local inflammatory response, atherosclerosis-related cells apoptosis and autophagy. Proteins of Annexin A (AnxA) family, the well-known Ca²⁺ phospholipid-binding protein, have many functions in regulating inflammation-related enzymes and cell signaling transduction, thus influencing cell adhesion, migration, differentiation, proliferation and apoptosis. There is now accumulating evidence that some members of the AnxA family, such as AnxA1, AnxA2, AnxA5 and AnxA7, play major roles in the development of atherosclerosis. This article discusses the major roles of AnxA1, AnxA2, AnxA5 and AnxA7, and the multifaceted mechanisms of the main biological process in which they are involved in atherosclerosis. Considering these evidences, it has been proposed that AnxA are drivers- and not merely participator- on the road to atherosclerosis, thus the progression of atherosclerosis may be prevented by targeting the expression or function of the AnxA family proteins.

Anticoagulation targeting membrane-bound anionic phospholipids improves outcomes of traumatic brain injury in mice

Severe traumatic brain injury (TBI) often causes an acute systemic hypercoagulable state that rapidly develops into consumptive coagulopathy. We have recently demonstrated that TBI-induced coagulopathy (TBI-IC) is initiated and disseminated by brain-derived extracellular vesicles (BDEVs) and propagated by extracellular vesicles (EVs) from endothelial cells and platelets. Here, we present results from a study designed to test the hypothesis that anticoagulation targeting anionic phospholipid-expressing EVs prevents TBI-IC and improves the outcomes of mice subjected to severe TBI. We evaluated the effects of a fusion protein (ANV-6L15) for improving the outcomes of TBI in mouse models combined with in vitro experiments. ANV-6L15 combines the phosphatidylserine (PS)-binding annexin V (ANV) with a peptide anticoagulant modified to preferentially target extrinsic coagulation. We found that ANV-6L15 reduced intracranial hematoma by 70.2%, improved neurological function, and reduced death by 56.8% in mice subjected to fluid percussion injury at 1.9 atm. It protected the TBI mice by preventing vascular leakage, tissue edema, and the TBI-induced hypercoagulable state. We further showed that the extrinsic tenase complex was formed on the surfaces of circulating EVs, with the highest level found on BDEVs. The phospholipidomic analysis detected the highest levels of PS on BDEVs, as compared with EVs from endothelial cells and platelets (79.1, 15.2, and 3.5 nM/mg of protein, respectively). These findings demonstrate that TBI-IC results from a trauma-induced hypercoagulable state and may be treated by anticoagulation targeting on the anionic phospholipid-expressing membrane of EVs from the brain and other cells.

Procoagulant Phosphatidylserine-Exposing Platelets in vitro and in vivo

The physiological heterogeneity of platelets leads to diverse responses and the formation of discrete subpopulations upon platelet stimulation. Procoagulant platelets are an example of such subpopulations, a key characteristic of which is exposure either of the anionic aminophospholipid phosphatidylserine (PS) or of tissue factor on the activated platelet surface. This review focuses on the former, in which PS exposure on a subpopulation of platelets facilitates assembly of the intrinsic tenase and prothrombinase complexes, thereby accelerating thrombin generation on the activated platelet surface, contributing importantly to the hemostatic process. Mechanisms involved in platelet PS exposure, and accompanying events, induced by physiologically relevant agonists are considered then contrasted with PS exposure resulting from intrinsic pathway-mediated apoptosis in platelets. Pathologies of PS exposure, both inherited and acquired, are described. A consideration of platelet PS exposure as an antithrombotic target concludes the review.

Serum Annexin V and Anti-Annexin V levels and their relationship with metabolic parameters in patients with type 2 diabetes

BACKGROUND

We investigated the serum annexin V and anti-annexin V levels and their relationship with metabolic parameters in patients recently diagnosed type 2 diabetic.

METHODS

A total of 143 patients recently diagnosed type 2 diabetes and 133 control subjects were included in the study. Body mass index (BMI), hs-CRP, HOMA-IR, carotid intima-media thickness, and serum levels of annexin V and anti-annexin V were investigated.

RESULTS

HOMA-IR, serum hs-CRP, and carotid intima-media thickness were found to be statistically significant. The Pearson correlation analysis revealed a statistically significant positive relationship between the carotid intima-media thickness and the annexin V level (r=0.29, p=0.006*). A statistically significant positive relationship was also detected between the Annexin V level and level of serum hs-CRP (r=0.29 p=0.006*).

CONCLUSION

A positive relationship was observed between the carotid intima-media thickness and annexin V at the end of our investigation. In this regard, we also believe that serum levels of annexin V may be increased for cardiovascular protection in the elevation of carotid intima-media thickness.

Annexin A5 reduces infarct size and improves cardiac function after myocardial ischemia-reperfusion injury by suppression of the cardiac inflammatory response

Annexin A5 (AnxA5) is known to have anti-inflammatory and anti-apoptotic properties. Inflammation and apoptosis are key processes in post-ischemic cardiac remodeling. In this study, we investigated the effect of AnxA5 on left ventricular (LV) function and remodeling three weeks after myocardial ischemia-reperfusion (MI-R) injury in hypercholesterolemic ApoE*3-Leiden mice. Using a mouse model for MI-R injury, we demonstrate AnxA5 treatment resulted in a 27% reduction of contrast-enhanced MRI assessed infarct size (IS). End-diastolic and end-systolic volumes were decreased by 22% and 38%, respectively. LV ejection fraction was increased by 29% in the AnxA5 group compared to vehicle. Following AnxA5 treatment LV fibrous content after three weeks was reduced by 42%, which was accompanied by an increase in LV wall thickness of the infarcted area by 17%. Two days and three weeks after MI-R injury the number of cardiac macrophages was significantly reduced in both the infarct area and border zones following AnxA5 treatment compared to vehicle treatment. Finally, we found that AnxA5 stimulation leads to a reduction of IL-6 production in bone-marrow derived macrophages in vitro. AnxA5 treatment attenuates the post-ischemic inflammatory response and ameliorates LV remodeling which improves cardiac function three weeks after MI-R injury in hypercholesterolemic ApoE*3-Leiden mice.

Domain IV of Annexin A5 Is Critical for Binding Calcium and Guarantees Its Maximum Binding to the Phosphatidylserine Membrane

Background: Although domain IV of annexin A5 (anxA5) may be less effective in binding phosphatidylserine (PS), the four domains together may guarantee the maximum binding of anxA5 to the PS membrane. Additionally, previous research has shown that annexin mutants lacking one or more domain(s) have different biological activities compared to the wild-type. The present research mainly aims to study the role of domain IV in the crucial PS-binding function of anxA5. Methods: The domain IV-truncated anxA5 protein was constructed and purified. Isothermal titration calorimetry, flow cytometry and activated partial thromboplastin time were adopted to examine the function of domain IV in anxA5-PS binding directly or indirectly. Results: The domain IV-truncated form of anxA5 is impaired in binding PS liposome and apoptotic cells, and anticoagulation activity. The mutant cannot bind calcium, but binds PS only in the presence of calcium. Conclusions: Truncation of domain IV of anxA5 destroys its calcium-binding ability and impairs its PS-binding activity. Truncation of domain IV may induce conformation change of anxA5 or reduce the hydrophobic interactions between protein and membrane, which may explain the decrease of PS-binding affinity of the mutant.

Peptides derived from MARCKS block coagulation complex assembly on phosphatidylserine

Blood coagulation involves activation of platelets and coagulation factors. At the interface of these two processes resides the lipid phosphatidylserine. Activated platelets expose phosphatidylserine on their outer membrane leaflet and activated clotting factors assemble into enzymatically active complexes on the exposed lipid, ultimately leading to the formation of fibrin. Here, we describe how small peptide and peptidomimetic probes derived from the lipid binding domain of the protein myristoylated alanine-rich C-kinase substrate (MARCKS) bind to phosphatidylserine exposed on activated platelets and thereby inhibit fibrin formation. The MARCKS peptides antagonize the binding of factor Xa to phosphatidylserine and inhibit the enzymatic activity of prothrombinase. In whole blood under flow, the MARCKS peptides colocalize with, and inhibit fibrin cross-linking, of adherent platelets. In vivo, we find that the MARCKS peptides circulate to remote injuries and bind to activated platelets in the inner core of developing thrombi.

Membrane Repair: Mechanisms and Pathophysiology

Eukaryotic cells have been confronted throughout their evolution with potentially lethal plasma membrane injuries, including those caused by osmotic stress, by infection from bacterial toxins and parasites, and by mechanical and ischemic stress. The wounded cell can survive if a rapid repair response is mounted that restores boundary integrity. Calcium has been identified as the key trigger to activate an effective membrane repair response that utilizes exocytosis and endocytosis to repair a membrane tear, or remove a membrane pore. We here review what is known about the cellular and molecular mechanisms of membrane repair, with particular emphasis on the relevance of repair as it relates to disease pathologies. Collective evidence reveals membrane repair employs primitive yet robust molecular machinery, such as vesicle fusion and contractile rings, processes evolutionarily honed for simplicity and success. Yet to be fully understood is whether core membrane repair machinery exists in all cells, or whether evolutionary adaptation has resulted in multiple compensatory repair pathways that specialize in different tissues and cells within our body.

Brain-derived microparticles induce systemic coagulation in a murine model of traumatic brain injury

Traumatic brain injury (TBI) is associated with coagulopathy, although it often lacks 2 key risk factors: severe bleeding and significant fluid resuscitation associated with hemorrhagic shock. The pathogenesis of TBI-associated coagulopathy remains poorly understood. We tested the hypothesis that brain-derived microparticles (BDMPs) released from an injured brain induce a hypercoagulable state that rapidly turns into consumptive coagulopathy. Here, we report that mice subjected to fluid percussion injury (1.9 ± 0.1 atm) developed a BDMP-dependent hypercoagulable state, with peak levels of plasma glial cell and neuronal BDMPs reaching 17 496 ± 4833/μL and 18 388 ± 3657/μL 3 hours after TBI, respectively. Uninjured mice injected with BDMPs developed a dose-dependent hyper-turned hypocoagulable state measured by a progressively prolonged clotting time, fibrinogen depletion, and microvascular fibrin deposition in multiple organs. The BDMPs were 50 to 300 nm with intact membranes, expressing neuronal or glial cell markers and procoagulant phosphatidylserine and tissue factor. Their procoagulant activity was greater than platelet microparticles and was dose-dependently blocked by lactadherin. Microparticles were produced from injured hippocampal cells, transmigrated through the disrupted endothelial barrier in a platelet-dependent manner, and activated platelets. These data define a novel mechanism of TBI-associated coagulopathy in mice, identify early predictive markers, and provide alternative therapeutic targets.

Attenuating a sickle cell crisis with annexin V

A sickle cell crisis is a painful and dangerous condition that defies effective treatment but fortunately it usually terminates spontaneously and patients spend far more time crisis free than in its painful throes. This suggests that an unstable physiologic balance exists between steady state sickle cell disease (SCD) and the crisis state and if this is so a therapeutic nudge during a crisis may help to terminate it. Annexin V may be able to provide this push. The phosphatidylserine (PS) molecules normally appear on the surface of senescent erythrocytes where they are recognized by macrophages and rapidly removed so that normally only about 1% are present in the circulation but in SCD 30-40% are prematurely senescent and their removal is delayed. The PS+ sickle erythrocytes remaining in the circulation adhere to the endothelium and their exposed PS acts as a platform for the initiation of the coagulation cascade that is responsible for clot propagation. Annexin V's great affinity for PS allows it to bond to it forming a shield that blocks both of these actions suggesting that its therapeutic administration during a sickle crisis may be able to hasten its termination.

Annexin A5 haplotypes in familial hypercholesterolemia: lack of association with carotid intima-media thickness and cardiovascular disease risk

OBJECTIVE

Annexin A5 (ANXA5) has been suggested to possess antiatherogenic properties. We investigated whether ANXA5 genetic variations and plasma ANXA5 levels were associated with carotid atherosclerosis and contributed to cardiovascular disease (CVD) risk in patients with familial hypercholesterolemia (FH).

METHODS

We sequenced the promoter region and exon 2 of ANXA5 in 284 FH patients from the ASAP (Atorvastatin versus Simvastatin on Atherosclerosis Progression) trial. Common haplotypes (H) were constructed based on seven single nucleotide polymorphisms (SNPs). We studied whether plasma ANXA5 levels or ANXA5 haplotypes were associated with the extent of atherosclerosis (evaluated by carotid intima-media thickness (IMT). The association between ANXA5 haplotypes and the risk for CVD events was investigated in 1730 FH patients from the GIRaFH (Genetic Identification of Risk factors in Familial Hypercholesterolemia) study.

RESULTS

In ASAP, individuals carrying the ANXA5 haplotype H2 exhibited lower plasma ANXA5 levels, whereas H4 carriers had increased levels of circulating ANXA5 compared to non-carriers. Plasma ANXA5 levels were not associated with carotid IMT. None of the four ANXA5 haplotypes correlated with the age-related IMT progression (ASAP study) or contributed to CVD risk (GIRaFH cohort).

CONCLUSIONS

Both ANXA5 haplotypes and plasma ANXA5 levels were not associated with carotid IMT progression or CVD risk in FH patients.

Oxidized but not native cardiolipin has pro-inflammatory effects, which are inhibited by Annexin A5

OBJECTIVE

Cardiolipin (CL) is a phospholipid with an unusual dimeric structure containing four double-bonds and is easily oxidized. CL is present in mitochondria. Here we explored potential pro-inflammatory properties implicated in cardiovascular disease (CVD): activation of endothelial cells, 5-lipoxygenase (5-LOX) and leukotriene B4 (LTB4), by oxidized CL (oxCL) and inhibitory effects of Annexin A5, an antithrombotic and antiinflammatory plasma protein.

METHODS

In monocytes/macrophages and neutrophils, calcium mobilization was monitored spectrophotometrically with Fura-2 and synthesis of LTB4 was analyzed by EIA. Expression of adhesion molecules on endothelial cells was studied by FACScan. Binding of Annexin A5 were analyzed by ELISA. The mRNA expression of 5-LOX and cyclooxygenase-2 was assessed by Real-Time PCR.

RESULTS

We demonstrate that oxCL but not its non-oxidized counterpart CL induces biosynthesis of LTB4 and increases intracellular concentrations of calcium in monocytes/macrophages and neutrophils. oxCL rather than CL selectively elevates gene expression of 5-LOX but not COX-2 in human macrophages. Furthermore, oxCL but not CL raises levels of adhesion molecules ICAM-1 and VCAM-1 in endothelial cells. Annexin A5 can bind oxCL to abolish all these oxCL-induced effects.

CONCLUSIONS

oxCL may promote inflammation and related diseases especially in conditions involving unresolved apoptosis and necrosis, such as atherosclerosis, where free oxCL is likely to be released from liberated mitochondria. Increased intracellular calcium could activate 5-LOX to produce Leukotriene B4 (LTB4). Annexin A5 inhibits the pro-inflammatory effects of oxCL and its potential therapeutic use when oxCL is implicated in inflammation could be of interest.

Annexin A5 inhibits atherogenic and pro-inflammatory effects of lysophosphatidylcholine

OBJECTIVE

Atherosclerosis is an inflammatory condition, and rupture of atherosclerotic plaques is a major cause of cardiovascular disease (CVD). Lysophosphatidylcholine (LPC) is generated in low-density lipoprotein (LDL) during oxidation and/or enzymatic modification and has been implicated in atherosclerosis. Annexin A5 (ANXA5) is an antithrombotic and atheroprotective plasma protein. Here, we demonstrate novel pro-inflammatory and atherogenic properties of LPC, and inhibitory effects of ANXA5.

METHODS

Endothelial cells and macrophages (differentiated from, THP-1 a monocytic cell line) were co-cultured. Expression of MMP-9 and OxLDL uptake by macrophages were studied by flow cytometry. The effect of LPC on leukotriene B4 (LTB4) synthesis in macrophages was studied by enzyme immunoassay (EIA). Chemotactic properties of LPC were investigated using a mouse intra-peritoneal recruitment model.

RESULTS

Co-culture of macrophages and endothelial cells enhanced MMP-9 expression in both cell types. This effect was increased by LPC and diminished by ANXA5. Likewise, LPC induced LTB4 production by macrophages, whereas native LDL or phosphatidylcholine (PTC) had no effect. ANXA5 inhibited uptake of OxLDL in macrophages. LPC induced cell infiltration in vivo, as determined by increased cell count in mouse peritoneal exudates, and this effect was inhibited by ANXA5.

CONCLUSIONS

ANXA5 could potentially play an important protective role in both atherogenesis and atherosclerotic plaque rupture by reducing pro-inflammatory effects of OxLDL and LPC as well as inhibiting OxLDL binding and uptake by macrophages. The possibility that ANXA5 could be developed into a novel therapy against CVD deserves further study.

Diannexin, an annexin A5 homodimer, binds phosphatidylserine with high affinity and is a potent inhibitor of platelet-mediated events during thrombus formation

BACKGROUND

Shielding of procoagulant phosphatidylserine (PS) with annexin A5 attenuates thrombosis, but annexin A5 (35.7 kDa) is rapidly cleared from the circulation. In contrast, Diannexin, a 73.1 kDa homodimer of annexin A5, has an extended half-life.

OBJECTIVES

To quantify the affinity of Diannexin for PS, examine its interaction with activated platelets and determine its effects on platelet-mediated events during thrombus formation.

METHODS

The affinities of Diannexin and annexin A5 for PS-containing lipid bilayers were compared using surface plasmon resonance, and binding to activated platelets was assessed by flow cytometry. Calibrated automated thrombography and thromboelastography were employed to study the effects of Diannexin on thrombin generation and platelet-fibrin clot formation, respectively, whereas intravital videomicroscopy was used to examine its effect on platelet accumulation and activation after laser-induced injury to murine cremaster arterioles, and a tail tip bleeding model was used to explore its effects on hemostasis.

RESULTS

Diannexin and annexin A5 bind PS with K(D) values of 0.6 and 5 nm, respectively, and both bind to the same subpopulation of PS-exposing platelets. Diannexin inhibited thrombin generation and platelet-fibrin clot formation in vitro at 10 nm (P<0.05-0.001 compared with control), and reduced platelet accumulation at 1 μg g(-1) (P<0.05) and activation at 0.25 μg g(-1) (P<0.001) in experimentally induced arterial thrombi in mice while increasing blood loss at 1 μg g(-1) (P<0.01).

CONCLUSIONS

Diannexin binds to PS with high affinity and is a potent inhibitor of platelet-mediated events during thrombus formation.

Annexin A5 prevents post-interventional accelerated atherosclerosis development in a dose-dependent fashion in mice

BACKGROUND

Activated cells in atherosclerotic lesions expose phosphatidylserine (PS) on their surface. Annexin A5 (AnxA5) binds to PS and is used for imaging atherosclerotic lesions. Recently, AnxA5 was shown to inhibit vascular inflammatory processes after vein grafting. Here, we report a therapeutic role for AnxA5 in post-interventional vascular remodeling in a mouse model mimicking percutaneous coronary intervention (PCI).

METHODS AND RESULTS

Associations between the rs4833229 (OR = 1.29 (CI 95%), p(allelic) = 0.011) and rs6830321 (OR = 1.35 (CI 95%), p(allelic) = 0.003) SNPs in the AnxA5 gene and increased restenosis-risk in patients undergoing PCI were found in the GENDER study. To evaluate AnxA5 effects on post-interventional vascular remodeling and accelerated atherosclerosis development in vivo, hypercholesterolemic ApoE(-/-) mice underwent femoral arterial cuff placement to induce intimal thickening. Dose-dependent effects were investigated after 3 days (effects on inflammation and leukocyte recruitment) or 14 days (effects on remodeling) after cuff placement. Systemically administered AnxA5 in doses of 0.1, 0.3 and 1.0mg/kg compared to vehicle reduced early leukocyte and macrophage adherence up to 48.3% (p = 0.001) and diminished atherosclerosis development by 71.2% (p = 0.012) with a reduction in macrophage/foam cell presence. Moreover, it reduced the expression of the endoplasmic reticulum stress marker GRP78/BiP, indicating lower inflammatory activity of the cells present.

CONCLUSIONS

AnxA5 SNPs could serve as markers for restenosis after PCI and AnxA5 therapeutically prevents vascular remodeling in a dose-dependent fashion, together indicating clinical potential for AnxA5 against post-interventional remodeling.

Tirofiban versus abciximab: tirofiban is administered at suboptimal dosages when evaluated in an arterial thrombosis model in non-human primates

To prevent thrombosis in high-risk acute coronary syndrome patients undergoing percutaneous coronary intervention for re-vascularisation, concomitant administration of a glycoprotein IIb/IIIa inhibitor, such as abciximab, tirofiban or eptifibatide, is recommended. Abciximab and eptifibatide are mostly preferred over tirofiban, which is less effective in preventing ischaemic events. We compared the efficacy and bleeding potential of escalating doses of tirofiban and abciximab in non-human primates. The efficacy of tirofiban and abciximab in inhibiting cyclic flow reductions (CFRs) was tested in a high shear arterial thrombosis model. Bleeding was evaluated with the template bleeding time and an incision bleeding model. Abciximab completely inhibited arterial thrombosis after injection of its therapeutic bolus dose. With tirofiban, a dose three times higher than the recommended therapeutic dose caused weak inhibition characterised by a return of CFRs after re-injury. At nine times the recommended therapeutic dose, complete inhibition was observed, and the efficacy of tirofiban was comparable to abciximab at its therapeutic bolus dose. Blood loss was less than with abciximab at its effective dose. In this model, tirofiban compared favourably with abciximab, although only at a dose of 3-9 times the therapeutic dose, and caused less bleeding than abciximab.

Diannexin protects against renal ischemia reperfusion injury and targets phosphatidylserines in ischemic tissue

Renal ischemia/reperfusion injury (IRI) frequently complicates shock, renal transplantation and cardiac and aortic surgery, and has prognostic significance. The translocation of phosphatidylserines to cell surfaces is an important pro-inflammatory signal for cell-stress after IRI. We hypothesized that shielding of exposed phosphatidylserines by the annexin A5 (ANXA5) homodimer Diannexin protects against renal IRI. Protective effects of Diannexin on the kidney were studied in a mouse model of mild renal IRI. Diannexin treatment before renal IRI decreased proximal tubule damage and leukocyte influx, decreased transcription and expression of renal injury markers Neutrophil Gelatinase Associated Lipocalin and Kidney Injury Molecule-1 and improved renal function. A mouse model of ischemic hind limb exercise was used to assess Diannexin biodistribution and targeting. When comparing its biodistribution and elimination to ANXA5, Diannexin was found to have a distinct distribution pattern and longer blood half-life. Diannexin targeted specifically to the ischemic muscle and its affinity exceeded that of ANXA5. Targeting of both proteins was inhibited by pre-treatment with unlabeled ANXA5, suggesting that Diannexin targets specifically to ischemic tissues via phosphatidylserine-binding. This study emphasizes the importance of phosphatidylserine translocation in the pathophysiology of IRI. We show for the first time that Diannexin protects against renal IRI, making it a promising therapeutic tool to prevent IRI in a clinical setting. Our results indicate that Diannexin is a potential new imaging agent for the study of phosphatidylserine-exposing organs in vivo.

Antithrombotic drug candidate ALX-0081 shows superior preclinical efficacy and safety compared with currently marketed antiplatelet drugs

Neutralizing the interaction of the platelet receptor gpIb with VWF is an attractive strategy to treat and prevent thrombotic complications. ALX-0081 is a bivalent Nanobody which specifically targets the gpIb-binding site of VWF and interacts avidly with VWF. Nanobodies are therapeutic proteins derived from naturally occurring heavy-chain-only Abs and combine a small molecular size with a high inherent stability. ALX-0081 exerts potent activity in vitro and in vivo. Perfusion experiments with blood from patients with acute coronary syndrome on standard antithrombotics demonstrated complete inhibition of platelet adhesion after addition of ALX-0081, while in the absence of ALX-0081 residual adhesion was observed. In a baboon efficacy and safety model measuring acute thrombosis and surgical bleeding, ALX-0081 showed a superior therapeutic window compared with marketed antithrombotics. Pharmacokinetic and biodistribution experiments demonstrated target-mediated clearance of ALX-0081, which leads to a self-regulating disposition behavior. In conclusion, these preclinical data demonstrate that ALX-0081 combines a high efficacy with an improved safety profile compared with currently marketed antithrombotics. ALX-0081 has entered clinical development.

Characterization of a recombinant form of annexin VI for detection of apoptosis

We developed a recombinant form of human annexin VI called annexin VI-601 (M(r) 76,224) with the N-terminal extension of Ala-Gly-Gly-Cys-Gly-His to allow ready attachment of fluorescent or radioactive labels. The protein was produced by expression in E. coli and was purified by calcium-dependent membrane binding, anion-exchange chromatography, and heparin-Sepharose affinity chromatography. The protein could be readily labeled with iodoacetamidofluorescein and with (99m)Tc. The protein bound with high affinity to PS-containing phospholipid vesicles and to erythrocytes with exposed phosphatidylserine. Fluorescent annexin VI-601 readily detected apoptosis of Jurkat cells by flow cytometry at much lower calcium concentrations than those required for equivalent detection by annexin V. In vivo administration of radiolabeled protein showed that blood clearance was much slower than annexin V. In conclusion, annexin VI may have advantages over annexin V in certain situations for both in vitro and in vivo detection of apoptosis and therapeutic targeting of PS due to its lower calcium requirement for membrane binding and its higher molecular weight.

A highly constrained cyclic (S,S)-CDC- peptide is a potent inhibitor of carotid artery thrombosis in rabbits

Inhibition of platelet aggregation is indispensable for the treatment of acute arterial thrombotic episodes. We have previously reported the synthesis of a highly constrained cyclic peptide, that incorporates the -CDC- sequence, (S,S) PSRCDCR-NH(2), which potently inhibits aggregation and fibrinogen binding to human platelets in vitro. We have tested the safety and efficacy of the peptide on the electrically induced carotid artery thrombosis experimental rabbit model. The peptide's effects on carotid blood flow, thrombus weight, in vitro and ex vivo platelet aggregation, and bleeding and hemostatic parameters were evaluated. The peptide was administered via the femoral vein. Carotid blood flow was continuously monitored for 90 min after electrical thrombus formation. The peptide, at 12 mg/kg, prevented total artery occlusion and significantly preserved carotid artery's patency compared with placebo and eptifibatide. Furthermore, (S,S) PSRCDCR-NH(2) administration at 12 mg/kg reduced thrombus weight, whereas it inhibited ex vivo ADP, arachidonic acid (AA) and collagen-induced platelet aggregation. Moreover (S,S) PSRCDCR-NH(2) at 12 mg/kg presented significantly higher inhibitory effects on AA and collagen-induced ex vivo platelet aggregation compared to eptifibatide. The peptide at any dose did not affect the coagulation cascade, the bleeding times or the hemostatic response of the animals. Thus highly constrained cyclic peptides like (S,S) PSRCDCR-NH(2) that incorporate the -CDC- motif and fulfil certain conformational criteria represent novel compounds that potently inhibit thrombus formation, ex vivo platelet aggregation and carotid artery occlusion superiorly to other non-RGD peptides, such as YMESRADR, without causing hemorrhagic complications in a rabbit model of arterial thrombosis.

Genetic polymorphisms and atrial fibrillation: Insights into the prothrombotic state and thromboembolic risk

The pathophysiology of thromboembolism in atrial fibrillation (AF) is a multifactorial and complex process. Abnormalities of haemostasis, fibrinolysis, endothelium, and platelets have all been described in AF. This prothrombotic state observed in AF appears to be additive to the presence of clinical and echocardiography risk factors for thromboembolism. Nonetheless, the precise mechanistic pathway(s) leading to the prothrombotic state in AF remain to be elucidated. Of note, there are limited data on the influence of genetic polymorphisms in thromboembolic risk associated with AF. On the other hand, the response to coumarin derivatives depends on several factors, such as sex, age, diet, or interacting drugs. Optimal anticoagulation control is usually hampered by significant interindividual variability in dose requirements for a given target level of anticoagulation. There is increasing evidence that interindividual sensitivity and side-effects to coumarinics may be largely determined genetically. Thus, genetic polymorphisms could explain the individual risk of developing an adverse drug reaction (bleeding) or drug inefficacy (thrombosis) with oral anticoagulation. In this article, we provide an overview of the limited data about the possible influence of genetic polymorphisms on thromboembolic risk in AF, as well as the genetic influences on anticoagulant drug responsiveness.

Effects of anti-cardiolipin antibodies and IVIg on annexin A5 binding to endothelial cells: implications for cardiovascular disease

OBJECTIVES

Anti-phospholipid antibodies (aPL), including anti-cardiolipin antibodies (aCL), are risk factors for cardiovascular disease (CVD) in the general population and in patients with the anti-phospholipid syndrome (APS; Hughes syndrome). APS may be primary but is also common in patients with systemic lupus erythematosus (SLE). The anti-coagulant protein annexin A5 (ANXA5) is implicated in CVD by interfering with phospholipids and aPL.

METHODS

ANXA5 binding to human umbilical venous endothelial cells (HUVECs) was determined by flow cytometry.

RESULTS

When cells were cultured in serum from APS patients with a high aPL titre (aPL-S), binding of ANXA5 to HUVECs was reduced. Monoclonal immunoglobulin (Ig)G aPL against cardiolipin (mAb-CL) dose-dependently reduced ANXA5 binding to endothelium. Preincubation of intravenous (IV)Ig at therapeutically relevant doses with aPL-S and mAb-aCL restored ANXA5 binding to comparable levels when normal healthy serum (NHS) was used. By contrast, IVIg per se had the capacity to reduce ANXA5 binding to endothelium when added to NHS (but not to aPL-S).

CONCLUSIONS

Decreased ANXA5 binding to endothelium, mediated by aPL, is a novel mechanism of atherothrombosis that can be countered by IVIg in vitro. IVIg per se could, to a lesser degree, cause decreased ANXA5 binding in NHS, which raises the possibility that some antibodies in IVIg can be involved in a side-effect reported in IVIg treatment, namely atherothrombosis and CVD. Increasing ANXA5 binding, either by addition of ANXA5 or by use of neutralizing antibodies in IVIg, represents a possible therapeutic strategy that deserves further study.

Annexin V and anti-Annexin V antibodies: two interesting aspects in acute myocardial infarction

Background

Myocardial infarction is the combined result of environmental factors and personal predispositions. Prothrombotic factors might play an important role in this phenomenon. Annexin V (ANV) is a calcium-dependent glycoprotein widely present in various tissues exerting a potent anticoagulant effect in vitro by reducing plaque adhesion and aggregation. Anti-annexin V antibodies (aANVAs) are detected in various diseases like rheumatoid arthritis, systemic lupus erythematosus and anti-phospholipid antibody syndrome. The study of ANV in Acute Myocardial Infarction (AMI) might shed light on hypercoagulability mechanisms in the pathogenesis of acute coronary syndromes. This study was conducted to investigate the association of plasma ANV, aANVAs and anti-cardiolipin antibodies (aCLAs) with AMI.

Methods

This study recruited 45 patients with the diagnosis of AMI according to WHO criteria in their first 24 hours of admission. 36 matched individuals were studied as the control group with normal coronary artery angiography. Plasma levels of ANV, aANVAs and aCLAs were determined by enzyme-linked immunosorbent assay and the results were compared.

Results

Plasma ANV levels in the patients with AMI on admission were significantly lower than those in the control group (p = 0.002). Positive test for aANVAs were found to be present in a significant number of our patients (p = 0.004). The studied groups were similar in their rate of patients with positive aCLAs tests. ANV, aANVAs and aCLAs were not correlated with hypertension, diabetes mellitus, hyperlipidemia, sex, age and smoking.

Conclusion

Our findings suggest that low plasma ANV levels along with positive aANVAs tests in patients with AMI are indicative of hypercoagulable state that is not related to the traditional cardiovascular risk factors.

Assessment of the Thrombogenic Effect of Fibrin Sealant Dressing in a Vascular Surgery Model in Rabbits

This study's objective was to investigate the potential thrombogenic effects of thrombin-containing fibrin sealant dressings (FSD) in a vascular repair model. Oval-shaped pieces of the rabbit abdominal aorta and vena cava were excised, the injuries were repaired with FSD, and animals were allowed to recover. Thrombus formation was examined by (1) an infusion of indium-labeled platelets into the rabbits following FSD application and estimation of total number of platelets attached to the wounds at 2, 4, and 6 h later (short-term effect, n = 12); and by (2) morphological and histological examinations of the vessels and dressings on days 1, 3, and 7 after repair operation in another group of rabbits (long-term effect, n = 12). Application of FSD sealed the vascular injures and produced immediate hemostasis that was stable up to 1 week. The highest numbers of platelets (both native and labeled) adhered to the arterial and venous repair sites were 6.5 x 106 and 4.4 x 107, respectively, 6 h after operation. The adhered platelets, however, did not form a visible and clinically significant thrombus. In long-term experiments, no evidence of thrombus was found in the lumens of the repaired vessels or on the dressings, and no microthrombi were detected histologically in other tissues at any time point. Although vena caval injuries showed signs of healing at day 7 postoperatively, the aortic wounds expanded progressively (pseudoaneurysm) and were prone to rupture at later times. Thus, direct exposure of FSD does not cause intravascular thrombosis or thrombotic events in rabbits. The dressing appears to be safe and effective for short-term repair of vascular injuries. It may also allow healing of minor venous defects, but cannot replace conventional surgical techniques (suturing) for permanent repair of arterial damages.

Procoagulant potential of platelet α granules

In this brief review of the literature it is pointed out that during platelet activation and degranulation platelet alpha granules leave the platelet interior through blebs in platelet plasma membrane and through the tips of the pseudopods, and then accumulate in the external milieu. There they undergo disintegration and secondary adhesion to the platelet plasma membranes. During their disintegration they expose their tightly packed GPIIb-IIIa complexes, annexin V stainable aminophospholipids, factor V, and the membrane markers CD62 and CD63. There is also demasking of lysosomal acid phosphatase activity and microvesicle formation. Lysosomal nature of platelet alpha granules is mentioned. It is suggested that platelet alpha granules are the sole source of platelet procoagulant activity and platelet microparticles (MP) or microvesicles (MV). The implications of this concept for antiplatelet therapy are discussed. A relationship of this process to tissue factor exposure and apoptosis is suggested.

Lactadherin and clearance of platelet-derived microvesicles

The transbilayer movement of phosphatidylserine from the inner to the outer leaflet of the membrane bilayer during platelet activation is associated with the release of procoagulant phosphatidylserine-rich small membrane vesicles called platelet-derived microvesicles. We tested the effect of lactadherin, which promotes the phagocytosis of phosphatidylserine-expressing lymphocytes and red blood cells, in the clearance of platelet microvesicles. Platelet-derived microvesicles were labeled with BODIPY-maleimide and incubated with THP-1-derived macrophages. The extent of phagocytosis was quantified by flow cytometry. Lactadherin promoted phagocytosis in a concentration-dependent manner with a half-maximal effect at approximately 5 ng/mL. Lactadherin-deficient mice had increased number of platelet-derived microvesicles in their plasma compared with their wild-type littermates (950 +/- 165 vs 4760 +/- 650; P = .02) and generated 2-fold more thrombin. In addition, splenic macrophages from lactadherin-deficient mice showed decreased capacity to phagocytose platelet-derived microvesicles. In an in vivo model of light/dye-induced endothelial injury/thrombosis in the cremasteric venules, lactadherin-deficient mice had significantly shorter time for occlusion compared with their wild-type littermate controls (5.93 +/- 0.43 minutes vs 9.80 +/- 1.14 minutes;P = .01). These studies show that lactadherin mediates the clearance of phosphatidylserine-expressing platelet-derived microvesicles from the circulation and that a defective clearance can induce a hypercoagulable state.

Lactadherin blocks thrombosis and hemostasis in vivo: correlation with platelet phosphatidylserine exposure

BACKGROUND

Platelet membrane phosphatidylserine (PS) is considered to be essential for hemostasis and thrombosis, but the in vivo topography of platelet PS has not been characterized. We hypothesized that platelet PS exposure would be identified on adherent platelets at the site of vascular injury and that blockade of PS would impede hemostasis and thrombosis.

OBJECTIVE

To localize and estimate the extent of platelet PS exposure and evaluate the impact of PS blockade in vivo.

METHODS

Lactadherin, a PS-binding milk protein, was utilized together with annexin V to detect both partial and complete membrane PS exposure on platelets in a mouse model of thrombosis and to evaluate the functional need for PS. Preliminary experiments were performed with synthetic membranes and with purified platelets.

RESULTS

The number of lactadherin-binding sites on synthetic membranes was proportional to PS content, whereas annexin V required a threshold of 2.5-8% PS. Approximately 95% of thrombin-stimulated platelets exposed PS, but the quantity was below the threshold for annexin V binding at physiologic Ca(2+) concentrations. In mice, most adherent and aggregated platelets on the walls of ferric chloride-treated mesenteric veins exposed low levels of PS, rather than having complete exposure. In mice, blockade of PS with lactadherin inhibited platelet prothrombinase and factor Xase activity, and prolonged tail bleeding time and the time to carotid artery thrombosis.

CONCLUSIONS

In vivo PS exposure contributes to both hemostasis and thrombosis. In this model of vascular injury, most platelets exhibit partial rather than complete PS exposure.

Synthesis and application of [18F]FDG-maleimidehexyloxime ([18F]FDG-MHO): a [18F]FDG-based prosthetic group for the chemoselective 18F-labeling of peptides and proteins

2-[(18)F]Fluoro-2-deoxy-D-glucose ([(18)F]FDG) as the most important PET radiotracer is available in almost every PET center. However, there are only very few examples using [(18)F]FDG as a building block for the synthesis of (18)F-labeled compounds. The present study describes the use of [(18)F]FDG as a building block for the synthesis of (18)F-labeled peptides and proteins. [(18)F]FDG was converted into [(18)F]FDG-maleimidehexyloxime ([(18)F]FDG-MHO), a novel [(18)F]FDG-based prosthetic group for the mild and thiol group-specific (18)F labeling of peptides and proteins. The reaction was performed at 100 degrees C for 15 min in a sealed vial containing [(18)F]FDG and N-(6-aminoxy-hexyl)maleimide in 80% ethanol. [(18)F]FDG-MHO was obtained in 45-69% radiochemical yield (based upon [(18)F]FDG) after HPLC purification in a total synthesis time of 45 min. Chemoselecetive conjugation of [(18)F]FDG-MHO to thiol groups was investigated by the reaction with the tripeptide glutathione (GSH) and the single cysteine containing protein annexin A5 (anxA5). Radiolabeled annexin A5 ([(18)F]FDG-MHO-anxA5) was obtained in 43-58% radiochemical yield (based upon [(18)F]FDG-MHO, n = 6), and [(18)F]FDG-MHO-anxA5 was used for a pilot small animal PET study to assess in vivo biodistribution and kinetics in a HT-29 murine xenograft model.

Diannexin, a novel annexin V homodimer, protects rat liver transplants against cold ischemia-reperfusion injury

Ischemia/reperfusion injury (IRI) remains an important problem in clinical transplantation. Following ischemia, phosphatidylserine (PS) translocates to surfaces of endothelial cells (ECs) and promotes the early attachment of leukocytes/platelets, impairing microvascular blood flow. Diannexin, a 73 KD homodimer of human annexin V, binds to PS, prevents attachment of leukocytes/platelets to EC, and maintains sinusoidal blood flow. This study analyzes whether Diannexin treatment can prevent cold IRI in liver transplantation. Rat livers were stored at 4 degrees C in UW solution for 24 h, and then transplanted orthotopically (OLT) into syngeneic recipients. Diannexin (200 microg/kg) was infused into: (i) donor livers after recovering and before reperfusion, (ii) OLT recipients at reperfusion and day +2. Controls consisted of untreated OLTs. Both Diannexin regimens increased OLT survival from 40% to 100%, depressed sALT levels, and decreased hepatic histological injury. Diannexin treatment decreased TNF-alpha, IL-1beta, IP-10 expression, diminished expression of P-selectin, endothelial ICAM-1, and attenuated OLT infiltration by macrophages, CD4 cells and PMNs. Diannexin increased expression of HO-1/Bcl-2/Bcl-xl, and reduced Caspase-3/TUNEL+ apoptotic cells. Thus, by modulating leukocyte/platelet trafficking and EC activation in OLTs, Diannexin suppressed vascular inflammatory responses and decreased apoptosis. Diannexin deserves further exploration as a novel agent to attenuate IRI, and thereby improve OLT function/increase organ donor pool.

Annexin A5 as a novel player in prevention of atherothrombosis in SLE and in the general population

During recent years it has become evident that atherosclerosis is an inflammatory disease. Furthermore, immune reactions and especially autoimmunity, were demonstrated to modulate atherosclerosis in animal experiments. An interesting example of how autoimmune reactions can influence atherosclerosis and consequences thereafter, is systemic lupus erythematosus (SLE)-associated cardiovascular disease (CVD). Antithrombotic effect exerted by Annexin A5 (ANXA5) is thought to be mediated mainly by forming a mechanical shield over phospholipids (PLs) reducing availability of PLs for coagulation reactions. However, more specific properties of ANXA5 might be of importance for its antithrombotic function. Such examples include downregulation of surface-expressed tissue factor (TF), as well as upregulation of urokinase-type plasminogen activator (uPA) by ANXA5. Also, interaction of ANXA5 with ligands involved in hemostasis, such as sulfatide and heparin, has been demonstrated. We have recently described a novel mechanism potentially contributing to atherothrombosis in SLE, with ANXA5 binding to endothelium decreased in SLE, an effect caused by antiphospholipid antibodies (aPL). It may be hypothesized that ANXA5 can be effective as a treatment to prevent plaque rupture and atherothrombosis not only in SLE, but also in the general population prone to CVD. Antiatherothrombotic potential of ANXA5 deserves further attention and careful studies as the mechanism behind the majority of clinically significant cardiovascular ischemic disease is atherothrombosis, formed on an underlying vulnerable atherosclerotic lesion. It may be hypothesized that ANXA5 can be effective as a treatment to prevent plaque rupture and atherothrombosis not only in SLE, but also in a general population prone to CVD.

Diannexin, a novel annexin V homodimer, provides prolonged protection against hepatic ischemia-reperfusion injury in mice

BACKGROUND AND AIMS

Ischemia-reperfusion injury (IRI) remains an important cause of liver failure after hepatic surgery or transplantation. The mechanism seems to originate within the hepatic sinusoid, with damage to endothelial cells, an early, reproducible finding. Sinusoidal endothelial cells (SECs), damaged during reperfusion, activate and recruit inflammatory cells and platelets. We hypothesized that a recombinant human annexin V homodimer, Diannexin, would protect SECs from reperfusion injury.

METHODS

We tested this proposal in a well-characterized in vivo murine partial hepatic IRI model.

RESULTS

Whether administered 5 minutes or 24 hours before or 1 hour after ischemia-reperfusion, Diannexin (100-1000 microg/kg) almost completely protected against liver injury. The protective efficacy conferred by Diannexin was highly visible at the microcirculatory level. Thus, although IR in this model is associated with early swelling and gap formation in SECs, Diannexin ameliorated these effects as shown by >80% reduction in alanine aminotransferase values during the early phase of reperfusion injury (2 hours) and near normalization of liver necrosis and inflammation in the late phase of inflammatory recruitment (24 hours). Consistent with the proposed role of SEC injury in hepatic IRI, Diannexin also decreased hepatic expression of proinflammatory molecules (MIP-2, ICAM-1, VCAM), abolished leukocyte and platelet adherence to damaged SECs, and, by in vivo microscopy, Diannexin preserved microcirculatory blood flow and hepatocyte integrity during reperfusion.

CONCLUSIONS

Diannexin is an apparently safe therapeutic protein that provides prolonged protection against hepatic IRI via cytoprotection of SECs, thereby interrupting secondary microcirculatory inflammation and coagulation.

Antithrombotic action of annexin V proved as efficient as direct inhibition of tissue factor or thrombin

The role of phospholipid platelet membrane and tissue factor in thrombin generation and thrombus formation is accepted. In the present study we have explored antithrombotic action of strategies aimed to block exposure of negatively charged phospholipids and we compared effects with those obtained through tissue factor or a direct thrombin inhibition. Type III collagen was exposed to flowing blood (5 min, 300 s(-1)). Effects of inhibition of platelet deposition by annexin A5 (ANXA5), hirudin (HIR) or by an antibody against tissue factor (TF) were evaluated. Prothrombin fragment F1 + 2 (F1 + 2) was monitored. Pre-incubation of whole blood with HIR or ANXA5 resulted in a statistically significant reduction of platelet deposition (12.2 +/- 0.6% in control experiments vs. 8.3 +/- 0.4% and 8.5 +/- 0.5%, respectively, P < 0.05). A similar decrease was found when blood was incubated with an antibody against TF. Furthermore, ANXA5 and HIR inhibited the recruitment of platelets into forming aggregates. The height of platelet aggregates generated was decreased in the presence of HIR or ANXA5, but only incubation with both inhibitors reached levels of statistical significance. The presence of ANXA5 or HIR decreased levels of F1 + 2 suggesting a reduced activation of the coagulation system. In our experimental studies, the inhibitory potential of ANXA5 on platelet-thrombus formation was as effective as that of a direct thrombin inhibitor, as HIR, or an antibody against TF. Negatively charged phospholipids exposed on activated platelets potentiate the formation of platelet aggregates on a collagen surface and further suggest that inhibition of platelet procoagulant activity might be a specific target for antithrombotic drugs.

Lactadherin binding and phosphatidylserine expression on cell surface-comparison with annexin A5

BACKGROUND

Transbilayer movement of anionic phospholipids from the inner to the outer leaflet of the plasma membrane occurs during platelet activation, red cell senescence, and apoptosis. The anionic phospholipid-binding protein, annexin A5, has been used to detect the presence of phosphatidylserine on the outer leaflet of the cell membrane. Lactadherin, a glycoprotein secreted by macrophages, binds to phosphatidylserine on apoptotic cells and promote their clearance by macrophages.

METHODS

The authors isolated and labeled lactadherin and annexin A5 with FITC and compared their ability to detect phosphatidylserine expression by flow cytometry.

RESULTS

FITC-lactadherin induced greater shift in the histogram and a higher mean fluorescence intensity than FITC-annexin A5 when platelets were activated with thrombin (0.1 unit/mL) or Ca(2+) ionophore A23187 (1 microM). Similarly, lactadherin was more sensitive in detecting phosphatidylserine in red cells induced to express phosphatidylserine. Also, in HL 60 cells undergoing apoptosis, lactadherin detected phosphatidylserine expression earlier than annexin A5. In patients with disseminated intravascular coagulation, lactadherin detected phosphatidylserine-expressing platelets in most patients, whereas under similar conditions, FITC-annexin A5 could not.

CONCLUSIONS

The authors' studies show that FITC-lactadherin is a better probe than annexin A5 in detecting phosphatidylserine-expressing activated platelets, red cells, and apoptotic cells.

Thrombin overcomes the thrombosis defect associated with platelet GPVI/FcRgamma deficiency

Fibrillar collagens are among the most potent activators of platelets and play an important role in the initiation of thrombosis. The glycoprotein VI (GPVI)/FcRgamma-chain complex is a central collagen receptor and inhibitors of GPVI produce a major defect in arterial thrombogenesis. In this study we have examined arterial thrombus formation in mice lacking the GPVI/FcRgamma-chain complex (FcRgamma(-/-)). Using 3 distinct arterial thrombosis models involving deep vascular injury, we demonstrate that deficiency of GPVI/FcRgamma is not associated with a major defect in arterial thrombus formation. In contrast, with milder vascular injury deficiency of GPVI/FcRgamma was associated with a 30% reduction in thrombus growth. Analysis of FcRgamma(-/-) platelets in vitro, using thrombin-dependent and -independent thrombosis models, demonstrated a major role for thrombin in overcoming the thrombosis defect associated with GPVI/FcRgamma deficiency. Inhibition of thrombin in vivo produced a much greater defect in thrombus formation in mice lacking GPVI/FcRgamma compared with normal controls. Similarly, thrombin inhibition produced a marked prolongation in bleeding time in FcRgamma(-/-) mice relative to wild-type mice. Our studies define an important role for thrombin in overcoming the hemostatic and thrombotic defect associated with GPVI/FcRgamma deficiency. Moreover, they raise the interesting possibility that the full antithrombotic potential of GPVI receptor antagonists may only be realized through the concurrent administration of anticoagulant agents.

Annexin A5 in cardiovascular disease and systemic lupus erythematosus

Atherosclerosis, a major cause of disease and death from cardiovascular disease (CVD), is an inflammatory disease characterized by T cell and monocyte/macrophage infiltration in the intima of large arteries. During recent years and with improved treatment of acute disease manifestations, it has become clear that the risk of CVD is very high in systemic lupus erythematosus (SLE), often considered a prototypic autoimmune disease. A combination of traditional and non-traditional risk factors, including dyslipidemia, inflammation, antiphospholipid antibodies (aPL) and lipid oxidation are related to CVD in SLE. aPL are highly thrombogenic, and possible mechanisms include direct effects of aPL on endothelial and other cells, and interference with coagulation reactions. More than a thousand proteins of the annexin-superfamily are expressed in eukaryotes. Annexins are ubiquitous, highly conserved, predominantly intracellular proteins, widely distributed in tissues. Annexin A5 (ANXA5) is an important member of the annexin family due to its antithrombotic properties. These are believed to be caused by it forming a two-dimensional protective shield, covering exposed potentially thrombogenic cell surfaces. Recently, ANXA5 has been implicated in SLE since aPL interfere with ANXA5 binding to placental trophoblasts, causing microthrombosis and miscarriage, a rather common complication in SLE. We recently demonstrated that ANXA5 may play a role in CVD and is abundant in late-stage atherosclerotic lesions. Sera from SLE-patients with a history of CVD inhibited ANXA5 binding to endothelium, caused by IgG antibodies, to a significant degree aPL. This review will focus on potential involvement of ANXA5 in pathogenesis of CVD, particularly caused by underlying atherosclerosis and atherothrombosis.

Fusion proteins comprising annexin V and Kunitz protease inhibitors are highly potent thrombogenic site-directed anticoagulants

The anionic phospholipid, phosphatidyl-L-serine (PS), is sequestered in the inner layer of the plasma membrane in normal cells. Upon injury, activation, and apoptosis, PS becomes exposed on the surfaces of cells and sheds microparticles, which are procoagulant. Coagulation is initiated by formation of a tissue factor/factor VIIa complex on PS-exposed membranes and propagated through the assembly of intrinsic tenase (factor VIIIa/factor IXa), prothrombinase (factor Va/factor Xa), and factor XIa complexes on PS-exposed activated platelets. We constructed a novel series of recombinant anticoagulant fusion proteins by linking annexin V (ANV), a PS-binding protein, to the Kunitz-type protease inhibitor (KPI) domain of tick anticoagulant protein, an aprotinin mutant (6L15), amyloid beta-protein precursor, or tissue factor pathway inhibitor. The resulting ANV-KPI fusion proteins were 6- to 86-fold more active than recombinant tissue factor pathway inhibitor and tick anticoagulant protein in an in vitro tissue factor-initiated clotting assay. The in vivo antithrombotic activities of the most active constructs were 3- to 10-fold higher than that of ANV in a mouse arterial thrombosis model. ANV-KPI fusion proteins represent a new class of anticoagulants that specifically target the anionic membrane-associated coagulation enzyme complexes present at sites of thrombogenesis and are potentially useful as antithrombotic agents.

Tissue-factor-bearing microvesicles arise from lipid rafts and fuse with activated platelets to initiate coagulation

Tissue factor (TF) circulates in plasma, largely on monocyte/macrophage-derived microvesicles that can bind activated platelets through a mechanism involving P-selectin glycoprotein ligand-1 (PSGL-1) on the microvesicles and P-selectin on the platelets. We found these microvesicles to be selectively enriched in both TF and PSGL-1, and deficient in CD45, suggesting that they arise from distinct membrane microdomains. We investigated the possibility that microvesicles arise from cholesterol-rich lipid rafts and found that both TF and PSGL-1, but not CD45, localize to lipid rafts in blood monocytes and in the monocytic cell line THP-1. Consistent with a raft origin of TF-bearing microvesicles, their shedding was significantly reduced with depletion of membrane cholesterol. We also evaluated the interaction between TF-bearing microvesicles and platelets. Microvesicles bound only activated platelets, and required PSGL-1 to do so. The microvesicles not only bound the activated platelets, they fused with them, transferring both proteins and lipid to the platelet membrane. Fusion was blocked by either annexin V or an antibody to PSGL-1 and had an important functional consequence: increasing the proteolytic activity of the TF-VIIa complex. These findings suggest a mechanism by which all of the membrane-bound reactions of the coagulation system can be localized to the surface of activated platelets.