Physiology of membrane lipid asymmetry

Tissue factor: mechanisms of decryption

It is generally believed that only a small fraction of the tissue factor (TF) found on cell surfaces is active whereas the vast majority is cryptic in coagulation. It is unclear how cryptic TF differs from the coagulant active TF or potential mechanisms involved in transformation of cryptic TF to the coagulant active form. Exposure of phosphatidylserine (PS) in response to various chemical or pathophysiological stimuli has been considered as the most potent inducer of TF decryption. In addition to PS, TF self-association and association with specialized membrane domains may also play a role in TF decryption. It has been suggested recently that protein disulfide isomerase regulates TF decryption through its oxidoreductase activity by targeting Cys186-Cys209 disulfide bond in TF extracellular domain or regulating the PS equilibrium at the plasma membrane. However, this hypothesis requires further validation to become an accepted mechanism. In this article, we critically review literature on TF encryption/decryption with specific emphasis on recently published data and provide our perspective on this subject.

Antiphospholipid antibodies (APLA) in immune thrombocytopenic purpura (ITP) and antiphospholipid syndrome (APS)

Antiphospholipid antibodies (APLA) are associated with anti-phospholipid syndrome (APS), a thrombotic disorder, but they are also frequently detected in immune thrombocytopenic purpura (ITP), a bleeding disorder. To investigate possible differences of APLA between these two disorders, we assayed IgG and IgM APLA by ELISA in 21 patients with ITP and 33 with APS. The APLA reacting against two protein target antigens, beta(2)-glycoprotein 1 (beta2GP1) and FVII/VIIa, and four phospholipids [cardiolipin (CL), phosphatidylcholine (PC), phosphatidylserine (PS), and phosphatidylethanolamine (PE)] as well as lupus anticoagulant (LA) were analyzed. We made the following observations: (i) IgG and IgM antibodies to beta2GP1 and IgM antibodies to FVII/VIIa were more common in APS than ITP, P < 0.05, while IgG antibodies against the phospholipids (aCL, aPC, aPS, aPE) were more common in ITP than APS, P < 0.05; (ii) multiple APLA > or =3 antigens) were more frequent in APS than ITP, P < 0.05; (iii) LA was frequently associated with APS but was absent in ITP; (iv) APLA is quite common in ITP: two-thirds were positive for at least one APLA. In summary, APLA are prevalent in ITP but their profile differs from APS. In APS, antibodies were predominantly against beta2GP1 and 80% had positive LA, while in ITP the APLA reacted most often with the phospholipids without LA. The difference in APLA may result in opposite clinical manifestations in two disorders.

Regulation of tissue factor-factor VIIa expression on cell surfaces: a role for tissue factor-factor VIIa endocytosis

Tissue factor (TF) is the cellular receptor for plasma clotting factor VIIa (FVIIa) and the formation of TF-VIIa complexes on cell surfaces triggers the coagulation cascade. Further, TF-VIIa, either directly or indirectly, influences various biological processes, such as development, inflammation and tumor metastasis. Therefore, a proper regulation of TF-VIIa expression is critical for the maintenance of hemostatic balance and health in general. TF-VIIa functional expression on cell surfaces is regulated primarily by transcriptional regulation of TF gene or by specific plasma inhibitors, particularly tissue factor pathway inhibitor (TFPI). However, a number of other mechanisms that are yet to be well defined also regulate TF-VIIa functional expression. One such mechanism is the endocytosis of TF-VIIa. In this article, we provide a comprehensive review on the regulation of TF-VIIa functional expression by these other mechanisms, with a particular emphasis on TF-VIIa endocytosis, and our perspective of these studies.

Hypercoagulability in sickle cell disease: a curious paradox

There is evidence of activation of both blood coagulation and platelets in sickle cell disease. For example, plasma samples obtained in the steady state and during painful crisis demonstrate high levels of thrombin generation, depletion of anticoagulant proteins, and abnormal activation of the fibrinolytic system. Similarly, exposure of surface markers such as CD62P and CD40L, along with increased circulating levels of thrombospondin, signal platelet activation. In addition to its effects on the cleavage of fibrinogen and its ability to activate platelets, the increase in circulating thrombin levels, with its wide-ranging effects on endothelial cells and blood vessels, may be important in the pathophysiology of sickle cell disease. Therefore, treatments that could decrease thrombin generation or platelet activation may be beneficial in both the treatment of sickle cell disease and the prevention of complications that characterize this genetic disorder. This review discusses hypercoagulability in the various forms of sickle cell disease, including homozygous sickle cell anemia, hemoglobin SC disease, hemoglobin SD disease, and sickle cell-beta-thalassemia.

Identification and quantification of phosphatidylethanolamine-derived glucosylamines and aminoketoses from human erythrocytes--influence of glycation products on lipid peroxidation

While the Maillard reaction of amino acids and proteins as well as its consequences in vivo has been thoroughly investigated, little attention has so far been paid to the glycation of aminophospholipids such as phosphatidylethanolamine (PE) or phosphatidylserine (PS), which are essential for structure and functionality of biological membranes. PE-derived glucosylamines (Schiff-PEs) and aminoketoses (Amadori-PEs) have now for the first time been simultaneously identified and quantified in erythrocytes from diabetics and healthy individuals by liquid chromatography-electrospray mass spectrometry (LC-(ESI)MS). The amounts of glycated PE (gPE) were significantly higher in diabetics (0.18-34.2 mol% Schiff-PE and 0.047-0.375 mol% Amadori-PE) than in controls (0.12-3.99 mol% Schiff-PE and 0.018-0.055 mol% Amadori-PE). A positive correlation between fructosylated hemoglobin (HbA(1c)) and the gPE levels was established. No advanced glycation endproducts (AGEs) like 5-hydroxymethylpyrrole-2-carbaldehyde (pyrrole-PE), carboxymethyl (CM-PE), or carboxyethyl (CE-PE) derivatives were detected. To investigate the influence of gPE on lipid peroxidation of biological membranes, liposomes consisting of soy-PE and synthetically prepared Amadori-PE (16:0-16:0) were incubated for several days and the formation of oxidation products was monitored. It could be shown that Amadori-PE extensively promotes lipid peroxidation even in the absence of transition metal ions like Cu(2+) and Fe(3+). Oxidative damage to membrane lipids therefore is supposed to be at least partially caused by the glycation of aminophospholipids.

Antiphospholipid antibodies, proteins C and S, and coagulation changes in sickle cell disease

The significance, interactions, and sources of coagulation abnormalities and their relationship to clinical severity and painful episodes in sickle cell disease are not clear. To evaluate this, we have examined various measures of coagulation in 37 patients with sickle cell disease (20 patients with HbSS disease and 17 patients with HbSC disease). Measurements have included isotypes of antiphospholipid antibodies (IgG, IgM, IgA) to specific phospholipids; proteins C (activity, total antigen) and S (activity, total and free antigen); measures of coagulation activation (prothrombin fragment 1.2, thrombin-antithrombin, fibrinopeptide A, d-dimers); indicators of clinical severity; and studies obtained during steady states and painful episodes. Results in HbSS disease showed that antiphospholipid antibodies were increased, with IgG phosphatidylserine showing the highest and most frequently increased levels (37% of patients). Protein C (activity) and protein S (activity, total, free antigen) were decreased (P<.01), and all measures of coagulation activation were increased (P<.001). In HbSC disease, antiphospholipid antibodies were normal, protein C (activity) and protein S (free antigen) were decreased (P<.001), and all measures of coagulation activation were increased (P<.02). A strong correlation was observed in HbSS disease between IgG-PS and d-dimers. Moderate correlations occurred between protein C activity and thrombin-antithrombin and fibrinopeptide A, between protein S activity and prothrombin fragment 1.2 and d-dimers, and between protein C and protein S activity. In HbSC disease, moderate and fewer correlations occurred. Significant differences between HbSS disease and HbSC disease were observed in aPLs, proteins C and S, and measures of coagulation activation. Measurements during steady states and during painful episodes were not significantly different. We conclude that the antiphospholipid antibody IgG-PS may contribute to coagulation activation in HbSS disease and that IgG-PS, protein C, and protein S relate to each other and jointly to measures of coagulation activation. The increased level of IgG-PS in HbSS disease most likely reflects exposure of the procoagulant phosphatidylserine on the surfaces of red cell-shed vesicles and sickle red cells, which would further affect coagulation activation. The significant differences in coagulation measures between HbSS disease and HbSC disease are consistent with differences in clinical severity between the diseases. The development of painful episodes does not appear to be related to the coagulation changes.