Thromboxane A2 is a key regulator of pathogenesis during Trypanosoma cruzi infection

Chagas' disease is caused by infection with the parasite Trypanosoma cruzi. We report that infected, but not uninfected, human endothelial cells (ECs) released thromboxane A2 (TXA2). Physical chromatography and liquid chromatography-tandem mass spectrometry revealed that TXA2 is the predominant eicosanoid present in all life stages of T. cruzi. Parasite-derived TXA2 accounts for up to 90% of the circulating levels of TXA2 in infected wild-type mice, and perturbs host physiology. Mice in which the gene for the TXA2 receptor (TP) has been deleted, exhibited higher mortality and more severe cardiac pathology and parasitism (fourfold) than WT mice after infection. Conversely, deletion of the TXA2 synthase gene had no effect on survival or disease severity. TP expression on somatic cells, but not cells involved in either acquired or innate immunity, was the primary determinant of disease progression. The higher intracellular parasitism observed in TP-null ECs was ablated upon restoration of TP expression. We conclude that the host response to parasite-derived TXA2 in T. cruzi infection is possibly an important determinant of mortality and parasitism. A deeper understanding of the role of TXA2 may result in novel therapeutic targets for a disease with limited treatment options.

Bruton’s Tyrosine Kinase Is Essential for Botrocetin/vWf-Induced Signaling and GPIb-Dependent Thrombus Formation In Vivo.

Botrocetin (bt) facilitated binding of von Willebrand factor (vWf) to the platelet membrane glycoprotein (GP) Ib-IX-V complex on platelets in suspension initiates a signaling cascade that causes αIIbβ3 activation and platelet aggregation. Previous work has demonstrated that bt/vWf-mediated agglutination activates αIIbβ3 and elicits ATP secretion in a thromboxane A2 (TxA2)-dependent manner. The signaling that results in TxA2 production is initiated by Lyn, enhanced by Src and propagated through Syk, SLP-76, PI3K, PLCγ2 and PKC. Here, we demonstrate that the signaling elicited by GPIb-mediated agglutination that results in TxA2 production is dependent on Bruton’s tyrosine kinase (Btk). The results demonstrate that Btk is downstream of Lyn, Syk, SLP-76 and PI3K, upstream of ERK1/2, PLCγ2 and PKC, and greatly enhances Akt phosphorylation. The relationship(s), if any between ERK1/2, PLCγ2 and PKC were not elucidated. The requirement for Btk and TxA2 receptor function in GPIb-dependent arterial thrombosis was confirmed in vivo by characterizing blood flow in ferric chloride treated mouse carotid arteries. These results demonstrate that the Btk family kinase, Tec, cannot provide the function(s) missing because of the absence of normal Btk function, and that Btk is essential for both bt/vWf-mediated agglutination-induced TxA2 production and GPIb-dependent stable arterial thrombus formation in vivo. Finally, consistent with the TxA2 receptor requirement for stable arterial thrombus formation in vivo, aspirin (at the appropriate concentration) prevented GPIb-dependent stable thrombus formation in vivo on ferric chloride damaged arterial endothelium.

Ketamine Suppresses Platelet Aggregation Possibly by Suppressed Inositol Triphosphate Formation and Subsequent Suppression of Cytosolic Calcium Increase

Background Ketamine has been shown to suppress platelet aggregation, but its mechanisms of action have not been defined. The purpose of the current study is to clarify the effects of ketamine on human platelet aggregation and to elucidate the underlying mechanisms of its action. Methods Platelet aggregation was measured using an eight-channel aggregometer, and cytosolic free calcium concentration was measured in Fura-2/AM-loaded platelets using a fluorometer. Inositol 1,4,5-triphosphate (IP3) was measured with use of a commercially available IP3 assay kit. To estimate thromboxane A2 (TXA2) receptor binding affinity and expression, Scatchard analysis was performed using [3H]S145, a specific TXA2 receptor antagonist. TXA2 agonist binding assay was also performed. The membrane-bound guanosine 5'-triphosphatase activity was determined using [gamma-32P]guanosine triphosphate by liquid scintillation analyzer. Results Ketamine (500 microm) suppressed aggregation induced by adenosine diphosphate (0.5 microm), epinephrine (1 microm), (+)-9,11-epithia-11,12-methano-TXA2 (STA2) (0.5 microm), and thrombin (0.02 U/ml) to 39.1 +/- 30.9, 46.3 +/- 4.3, -2.0 +/- 16.8, and 86.6 +/- 1.4% of zero-control, respectively. Ketamine (250 microm-1 mm) also suppressed thrombin- and STA2-induced cytosolic free calcium concentration increase dose dependently. Although ketamine (2 mm) had no effect on TXA2 receptor expression and its binding affinity, it (1 mm) suppressed intracellular peak IP3 concentrations induced by thrombin and STA2 from 6.60 +/- 1.82 and 4.39 +/- 2.41 to 2.41 +/- 0.98 and 1.90 +/- 0.86 pmol/109 platelets, respectively, and it suppressed guanosine triphosphate hydrolysis induced by thrombin (0.02 units/ml) and STA2 (0.5 microm) to 50.3 +/- 3.2 and 67.5 +/- 5.5% versus zero-control, respectively. Conclusion Ketamine inhibits human platelet aggregation possibly by suppressed IP3 formation and subsequent suppression of cytosolic free calcium concentration. The site of action of ketamine is neither TXA2 nor thrombin binding sites but possibly receptor-coupled mechanisms, including G-protein.