Is atrial fibrillation associated with pulmonary embolism?

A pulmonary embolism (PE) is thought to be associated with atrial fibrillation (AF). Nevertheless, this association is based on weak data.

Impact of atrial fibrillation on clinical outcomes among patients with coronary artery disease undergoing revascularisation with drug-eluting stents

Coronary artery disease (CAD) and atrial fibrillation (AF) are major determinants of morbidity and mortality. A combined treatment with antiplatelet agents and vitamin K antagonists limits the risk of stent thrombosis and stroke while increasing the rate of bleeding. The objective of this study was to investigate the impact of atrial fibrillation (AF) on long-term clinical outcomes in patients with CAD undergoing percutaneous coronary intervention (PCI) with drug-eluting stents (DES).

A common origin of the 4143insA ADAMTS13 mutation

Severely deficient activity of the von Willebrand Factor (VWF) cleaving metalloprotease, ADAMTS13, is associated with thrombotic thrombocytopenic purpura (TTP). The mutation spectrum ofADAMTS13 is rather heterogeneous, and numerous mutations spread across the gene have been described in association with congenital TTP. The 4143insA mutation is unusual with respect to its geographic concentration. Following the initial report from Germany in which the 4143insA mutation was detected in four apparently unrelated families, we have now identified this mutation in a further eleven patients from Norway, Sweden, Poland, Germany, the Czech Republic and Australia. Confirmation that the Australian patient is of German ancestry, together with the Northern and Central European origin of most of the other patients, suggests that the 4143insA mutation has a common genetic background. We established ADAMTS13 haplotypes by analyzing 17 polymorphic intragenic markers. The haplotypes linked to 4143insA were identical in all informative families. Three novel candidate mutations, C347S, P671L and R1060W, as well as the known mutation R507Q, were also identified during the course of the study. We conclude that 4143insA has a common genetic background and is frequent among patients with hereditary ADAMTS13 deficiency in Northern and Central European countries.

Stejnulxin, a novel snake C-type lectin-like protein from Trimeresurus stejnegeri venom is a potent platelet agonist acting specifically via GPVI

Stejnulxin, a novel snake C-type lectin-like protein with potent platelet activating activity, was purified and characterized from Trimeresurus stejnegeri venom. Under non-reducing conditions, it migrated on a SDS-polyacrylamide gel with an apparent molecular mass of 120 kDa. On reduction, it separated into three polypeptide subunits with apparent molecular masses of 16 kDa (alpha), 20 kDa (beta1) and 22 kDa (beta2), respectively. The complete amino acid sequences of its subunits were deduced from cloned cDNAs. The N-terminal sequencing and cDNA cloning indicated that beta1 and beta2 subunits of stejnulxin have identical amino acid sequences and each contains two N-glycosylation sites. Accordingly, the molecular mass difference between beta1 and beta2 is caused by glycosylation heterogenity. The subunit amino acid sequences of stejnulxin are similar to those of convulxin, with sequence identities of 52.6% and 66.4% for the alpha and beta, respectively. Stejnulxin induced human platelet aggregation in a dose-dependent manner. Antibodies against alphaIIbbeta3 inhibited the aggregation response to stejnulxin, indicating that activation of alphaIIbbeta3 and binding of fibrinogen are involved in stejnulxin-induced platelet aggregation. Antibodies against GPIbalpha or alpha2beta1 as well as echicetin or rhodocetin had no significant effect on stejnulxin-induced platelet aggregation. However, platelet activation induced by stejnulxin was blocked by anti-GPVI antibodies. In addition, stejnulxin induced a tyrosine phosphorylation profile in platelets that resembled that produced by convulxin. Biotinylated stejnulxin bound specifically to platelet membrane GPVI.

European Working Group on Clinical Cell Analysis: Consensus protocol for the flow cytometric characterisation of platelet function

An increased or disturbed activation and aggregation of platelets plays a major role in the pathophysiology of thrombosis and haemostasis and is related to cardiovascular disease processes. In addition to qualitative disturbances of platelet function, changes in thrombopoiesis or an increased elimination of platelets, (e. g., in autoimmune thrombocytopenia), are also of major clinical relevance. Flow cytometry is increasingly used for the specific characterisation of phenotypic alterations of platelets which are related to cellular activation, haemostatic function and to maturation of precursor cells. These new techniques also allow the study of the in vitro response of platelets to stimuli and the modification thereof under platelet-targeted therapy as well as the characterisation of platelet-specific antibodies. In this protocol, specific flow cytometric techniques for platelet analysis are recommended based on a description of the current state of flow cytometric methodology. These recommendations are an attempt to promote the use of these new techniques which are at present broadly evaluated for diagnostic purposes. Furthermore, the definition of the still open questions primarily related to the technical details of the method should help to promote the multi-center evaluation of procedures with the goal to finally develop standardized operation procedures as the basis of interlaboratory reproducibility when applied to diagnostic testing.

Collagen receptors as potential targets for novel anti-platelet agents

Platelets have important roles in atherosclerosis and thrombosis and their inhibition reduces the risk of these disorders. There is still a need for platelet inhibitors affecting pathways that reduce thrombosis and atherosclerosis while leaving normal hemostasis relatively unaffected, thus reducing possible bleeding complications. Although combinations show progress in achieving these goals none of the present inhibitors completely fulfill these requirements. Collagen receptors offer attractive possibilities as alternative targets at early stages in platelet activation. Three major collagen receptors are assessed in this review; the alpha2beta1 integrin, responsible primarily for platelet adhesion to collagen; GPVI, the major signaling receptor for collagen; and GPIb-V-IX, which is indirectly a collagen receptor via von Willebrand factor. Several thrombosis models and experimental approaches suggest that all three are interesting targets and merit further investigation.

GPIb is involved in platelet aggregation induced by mucetin, a snake C-type lectin protein from Chinese habu (Trimeresurus mucrosquamatus) venom

Mucetin (Trimeresurus mucrosquamatus venom activator, TMVA) is a potent platelet activator purified from Chinese habu (Trimeresurus mucrosquamatus) venom. It belongs to the snake venom heterodimeric C-type lectin family and exists in several multimeric forms. We now show that binding to platelet glycoprotein (GP) Ib is involved in mucetin-induced platelet aggregation. Antibodies against GPIb as well as the GPIb-blocking C-type lectin echicetin inhibited mucetin-induced platelet aggregation. Binding of GPIb was confirmed by affinity chromatography and Western blotting. Antibodies against GPVI inhibited convulxin- but not mucetin-induced aggregation. Signalling by mucetin involved rapid tyrosine phosphorylation of a number of proteins including Syk, Src, LAT and PLC gamma 2. Mucetin-induced phosphorylation of the Fc gamma chain of platelet was greatly promoted by inhibition of alpha(IIb)beta(3) by the peptidomimetic EMD 132338, suggesting that phosphatases downstream of alpha(IIb)beta(3) activation are involved in dephosphorylation of Fc gamma. Unlike other multimeric snake C-type lectins that act via GPIb and only agglutinate platelets, mucetin activates alpha(IIb)beta(3). Inhibition of alpha(IIb)beta(3) strongly reduced the aggregation response to mucetin, indicating that activation of alpha(IIb)beta(3) and binding of fibrinogen are involved in mucetin-induced platelet aggregation. Apyrase and aspirin also inhibit platelet aggregation induced by mucetin, suggesting that ADP and thromboxane A2 are involved in autocrine feedback. Sequence and structural comparison with closely related members of this protein family point to features that may be responsible for the functional differences.

Variation in human platelet glycoprotein VI content modulates glycoprotein VI-specific prothrombinase activity

- Glycoprotein VI (GPVI) is a platelet-specific receptor for collagen that figures prominently in signal transduction. An addition to binding to type I and III collagens, GPVI is also bound specifically by collagen-related peptide and convulxin (CVX), a snake venom protein. We developed a quantitative assay of platelet GPVI in which biotin-conjugated CVX binds selectively to GPVI in separated total platelet proteins by a ligand blot procedure. Using this approach, we have documented a 5-fold range in platelet GPVI content among 23 normal healthy subjects. In addition, we have determined that CVX-induced or collagen-related peptide-induced prothrombinase activity is directly proportional to the platelet content of GPVI. A statistically significant correlation was observed at 2 CVX concentrations: 14.7 ng/mL (R(2)=0.854 and P<0.001, n=11) and 22 ng/mL (R(2)=0.776 and P<0.001, n=12). In previous studies, we established a similar range of expression of the integrin collagen receptor alpha(2)beta(1) on platelets of normal subjects. Among 15 donors, there is a direct correlation between platelet alpha(2)beta(1) density and GPVI content (R(2)=0.475 and P=0.004). In view of the well-documented association of GPVI with platelet procoagulant activity, this study suggests that the variation in GPVI content is a potential risk factor that may predispose individuals to hemorrhagic or thromboembolic disorders.

Platelet collagen receptors

Collagens are important platelet activators in the vascular subendothelium and vessel wall. Since the regulation of platelet activation is a key step in distinguishing normal haemostasis from pathological thrombosis, collagen interactions with platelets are important targets for pharmacological control. Platelets have two major receptors for collagens, the integrin alpha2beta1, with a major role in adhesion and platelet anchoring and the Ig superfamily member, GPVI, principally responsible for signalling and platelet activation. In addition, GPIb-V-IX, can be considered as an indirect collagen receptor acting via von Willebrand factor as bridging molecule and is essential for platelet interactions with collagen at high shear rates. There is some evidence for additional receptors, which may regulate the response to individual collagen types. This review discusses how these receptors work separately with specific agonists and proposes possible mechanisms for how they work together to regulate platelet activation by collagen, which remains controversial and poorly understood.