Platelet Dense Bodies Loaded with Mepacrine Study in Chronic Idiopathic Thrombocytopenic Purpura (ITP)

Platelet dense granules begin to selectively accumulate mepacrine during proplatelet formation

Platelet dense granules (DGs) are storage organelles for calcium ions, small organic molecules such as ADP and serotonin, and larger polyphosphates that are secreted upon platelet stimulation to enhance platelet activation, adhesion, and stabilization at sites of vascular damage. DGs are thought to fully mature within megakaryocytes (MKs) prior to platelet formation. Here we challenge this notion by exploiting vital fluorescent dyes to distinguish mildly acidic DGs from highly acidic compartments by microscopy in platelets and MKs. In isolated primary mouse platelets, compartments labeled by mepacrine - a fluorescent weak base that accumulates in DGs - are readily distinguishable from highly acidic compartments, likely lysosomes, that are labeled by the acidic pH indicator, LysoTracker, and from endolysosomes and alpha granules labeled by internalized and partially digested DQ™ BSA. By contrast, in murine fetal liver- and human CD34⁺ cell-derived MKs and the megakaryocytoid cell lines, MEG-01 and differentiated G1ME2, labeling by mepacrine overlapped nearly completely with labeling by LysoTracker and partially with labeling by DQ™ BSA. Mepacrine labeling in G1ME2-derived MKs was fully sensitive to proton ATPase inhibitors, but was only partially sensitive in platelets. These data indicate that mepacrine in MKs accumulates as a weak base in endolysosomes but is likely pumped into or retained in separate DGs in platelets. Fluorescent puncta that labeled uniquely for mepacrine were first evident in G1ME2-derived proplatelets, suggesting that DGs undergo a maturation step that initiates in the final stages of MK differentiation.

Specific inhibiting characteristics of tetramethylpyrazine, one of the active ingredients of the Chinese herbal medicine 'Chuanxiong,' on platelet thrombus formation under high shear rates

We have investigated the effects of tetramethylpyrazine, one of the active ingredients of the Chinese herbal medicine Chuanxiong, on platelet thrombus formation under flow conditions. We demonstrate herein that tetramethylpyrazine inhibits shear-induced platelet aggregation under relatively high shear rate of 10,800 s(-1) with modest inhibition of those occurring under relatively low shear rate of 1200 s(-1) by using optically modified cone-plate viscometer. We also demonstrate that platelet activation induced by shearing in the absence of exogenous platelet-activating agents such as ADP as evidenced by P-selectin surface expression and microparticle release detected by quantitative flow cytometry was also inhibited by tetramethylpyrazine. Moreover, we also demonstrate platelet thrombus formation on the collagen and von Willebrand factor (vWF) surface at high shear rates without significant influences on those occurring under relatively low shear rates. Because platelet thrombus formation occurring under high shear rates is known to be mediated by the vWF interaction with platelet receptor proteins GP Ibalpha and GP IIb/IIIa, we speculated that tetramethylpyrazine exerts antiplatelet effects by inhibiting the vWF-mediated process of platelet thrombus formation. Our findings, indicating the unique antiplatelet characteristics of tetramethylpyrazine, selectively inhibiting the platelet thrombus formation under high shear rates, provide good reasons for developing chemical analogs having biological functions similar to or more potent than those of tetramethylpyrazine as antiplatelet agents having unique biological functions.

A flow cytometric assay using mepacrine for study of uptake and release of platelet dense granule contents

Diagnosis of platelet dense granule storage pool disease and release defects at present requires a combination of studies including lumiaggregometry, conventional platelet aggregation, radioactive serotonin uptake and release, and electron microscopy. Flow cytometric methods have been developed to study platelet activation, aggregation, and alpha-granule protein release. Here, we have investigated the use of flow cytometry for analysis of platelet dense granule content uptake and release using mepacrine as a fluorescent marker. Mepacrine (quinacrine) is rapidly taken up and localized in dense granules of platelets. For the assay, as little as 20 microliters of blood from a fingerstick collected without anticoagulant or venous blood collected in 3.8% sodium citrate were diluted 1:40 with 2 ml Hanks balanced salt solution (BSS). 300 microliters of this cell suspension were incubated with mepacrine alone, or simultaneously with a mouse monoclonal antibody to human platelet glycoprotein IIb (Tab), used as a platelet-specific marker. The bound monoclonal antibody was then indirectly labelled with the fluorochrome, RED670. 100 microliters of the sample were further diluted with Hanks BSS for one- or two-colour flow cytometric analysis. To verify that mepacrine uptake was related to platelet dense granule content, platelets of beige mice, a strain with dense granule deficiency, were examined. Their mepacrine uptake was substantially decreased compared to that of normal mice. Decreased mepacrine uptake also was demonstrated in platelets of a patient with Hermansky-Pudlak syndrome in which a deficiency of platelet dense granules is characteristic. In both human and mouse platelets, mepacrine uptake was proportional to platelet size. Thrombin induced mepacrine release in a dose-dependent manner from 0.003 to 0.4 U/ml. Therefore both platelet uptake and release of mepacrine can be readily detected by flow cytometry. Flow cytometry provides an attractive alternative to aggregation and radioactive serotonin as methods to study defects in platelet dense granule function.

Selective decrease in platelet dense granule adenine nucleotides during recovery from acute experimental thrombocytopenia and ensuing thrombocytosis in baboons

Serial measurements of platelet volume, platelet content of adenine nucleotides, beta-thromboglobulin (beta-TG), platelet factor 4 (PF4) and ex vivo platelet aggregation were made in baboons under basal, steady-state conditions of normal platelet production, and during recovery from acute thrombocytopenia induced by the exposure of flowing blood to spherical glass microbeads. The mean basal platelet count of 509 +/- 107 X 10(9)/l (+/- 1 SD; n = 4) fell acutely to 36.8 +/- 12.6 X 10(9)/l after the insertion of glass bead columns and blood filters, placed distally, for 60 min into surgically implanted arteriovenous-shunts in heparinized baboons. After the irreversible removal of up to 90% of the baseline circulating platelet population, recovery from thrombocytopenia was characterized by a constant rate of increase in circulating platelet counts (115 +/- 11 X 10(9)/l/d) and a rebound thrombocytosis to 1.5 times the basal platelet count after 7 d. Steadystate thrombocytopoiesis was achieved by 3-4 weeks after the onset of thrombocytopenia. Platelet dense granule ADP and ATP decreased significantly from 3.89 +/- 0.20 and 2.33 +/- 0.25 mumol/10(11) platelets respectively at baseline to 2.17 +/- 0.37 and 1.68 +/- 0.37 mumol/10(11) platelets respectively after 7 d (P less than 0.001 in both cases) and normalization was achieved only after 4 weeks. By contrast, the mean platelet volume and platelet content of beta-TG and PF4 did not change significantly throughout the course of study (P greater than 0.1 in both cases). Platelet function, assessed by platelet aggregation ex vivo, demonstrated that platelet function was not impaired despite the significant decrease in dense granule ADP. We conclude that a selective, temporal reduction in platelet dense granule adenine nucleotides reflects changes in the thrombocytopoietic control mechanism secondary to induction of acute thrombocytopenia.

Bleeding on patients with autoimmune thrombocytopenic purpura and normal platelet count

80% of patients with chronic autoimmune thrombocytopenic purpura (ATP) may reach normalization of platelet count after steroids or splenectomy. In some of these cases a bleeding tendency may still persist and this has been attributed to abnormalities of platelet function resulting from the effect of platelet-bound antibodies. We have studied 49 ATP patients in order to assess the frequency and the pattern of this immune thrombopathia, and the correlation with the levels of platelet associated IgG (PAIgG). Only 3 patients (6%) had prolonged bleeding time and persistence of mucocutaneous haemorrhages, whereas 36% presented an altered platelet aggregation pattern. Platelet serotonin content, platelet and plasma beta-thromboglobulin concentration, and malondialdehyde generation after thrombin stimulus were found also altered, but the findings were not clearly correlated with the concentration of PAIgG.