Anita Eckly

Defective platelet aggregation and increased resistance to thrombosis in purinergic P2Y(1) receptor-null mice.

ADP is a key agonist in hemostasis and thrombosis. ADP-induced platelet activation involves the purinergic P2Y(1) receptor, which is responsible for shape change through intracellular calcium mobilization. This process also depends on an unidentified P2 receptor (P2cyc) that leads to adenylyl cyclase inhibition and promotes the completion and amplification of the platelet response. P2Y(1)-null mice were generated to define the role of the P2Y(1) receptor and to determine whether the unidentified P2cyc receptor is distinct from P2Y(1). These mice are viable with no apparent abnormalities affecting their development, survival, reproduction, or the morphology of their platelets, and the platelet count in these animals is identical to that of wild-type mice. However, platelets from P2Y(1)-deficient mice are unable to aggregate in response to usual concentrations of ADP and display impaired aggregation to other agonists, while high concentrations of ADP induce platelet aggregation without shape change. In addition, ADP-induced inhibition of adenylyl cyclase still occurs, demonstrating the existence of an ADP receptor distinct from P2Y(1). P2Y(1)-null mice have no spontaneous bleeding tendency but are resistant to thromboembolism induced by intravenous injection of ADP or collagen and adrenaline. Hence, the P2Y(1) receptor plays an essential role in thrombotic states and represents a potential target for antithrombotic drugs.

The P2Y1 receptor, necessary but not sufficient to support full ADP-induced platelet aggregation, is not the target of the drug clopidogrel

Recently we showed that the P2Y1 receptor coupled to calcium mobilization is necessary to initiate ADP‐induced human platelet aggregation. Since the thienopyridine compound clopidogrel specifically inhibits ADP‐induced platelet aggregation, it was of interest to determine whether the P2Y1 receptor was the target of this drug. Therefore we studied the effects of clopidogrel and of the two specific P2Y1 antagonists A2P5P and A3P5P on ADP‐induced platelet events in rats. Although clopidogrel treatment (50u2003mg/kg) greatly reduced platelet aggregation in response to ADP as compared to untreated platelets, some residual aggregation was still detectable. In contrast, A2P5P and A3P5P totally abolished ADP‐induced shape change and aggregation in platelets from both control and clopidogrel‐treated rats. A2P5P and A3P5P (100u2003μM) totally inhibited the [Ca2+]i rise induced by ADP (0.1u2003μM) in control and clopidogrel‐treated platelets, whereas clopidogrel treatment had no effect. Conversely, the inhibition of adenylyl cyclase induced by ADP (5u2003μM) was completely blocked by clopidogrel but not modified by A2P5P or A3P5P (100u2003μM). A3P5P (1u2003m M) reduced the number of [33P]2MeSADP binding sites on control rat platelets from 907u2003±u200350 to 611u2003±u200325 per platelet. After clopidogrel treatment, binding of [33P]2MeSADP decreased to 505u2003±u200368 sites per platelet and further decreased to 55u2003±u200312 sites in the presence of A3P5P (1u2003m M). In summary, these results demonstrate that the platelet P2Y1 receptor responsible for the initiation of aggregation in response to ADP is not the target of clopidogrel. Platelets may express another, as yet unidentified, P2Y receptor, specifically coupled to the inhibition of adenylyl cyclase and necessary to induce full platelet aggregation, which could be the target of this drug.

Preparation of washed platelet suspensions from human and rodent blood.

Citrate is the preferred anticoagulant for blood collection, as EDTA damages platelets and heparin modifies their function (1). Citrate allows the rapid generation of plateletrich plasma (PRP), with a high yield of platelets; however, this method has certain disadvantages. In particular, the PRP preparation has a limited stability (no longer than 2 h) and contains plasma proteins, including enzymes. In addition, human platelet-rich plasma (PRP) prepared from blood collected into trisodium citrate (3.8% w/v) has a depressed ionic calcium concentration, which can cause platelet aggregation and release of substances during centrifugation (2). To overcome these different problems, a centrifugation technique has been developed for the isolation and washing of platelets from human or rodent blood anticoagulated with acid-citrate-dextrose (ACD). The cells are resuspended in a physiological buffer under well-defined conditions, notably the presence of plasmatic ionic calcium concentrations (2 mM) and the absence of coagulation factors or other plasma components. The method for isolation of human platelets by centrifugation and washing described by Cazenave et al. (3) is derived directly from the technique of Mustard et al. (4). Blood collected into ACD is used to prepare PRP, from which the platelets are isolated by successive centrifugation steps and resuspended in Tyrode’s buffer, an iso-osmotic phosphate buffer at pH 7.35 containing glucose (0.1%, w/v), human serum albumin (HSA) (0.35%, w/v), calcium (2 mM), and magnesium (1 mM). Prostacyclin (PGI2) is used to prevent transitory platelet activation during the preparation. Addition of apyrase (adenosine 5′-triphosphate diphosphohydrolase, EC 3.6.1.5) to the final suspending medium prevents the cells from becoming refractory to ADP and maintains their discoid shape (5). Suspensions of washed platelets prepared by this method are stable for 5–8 h at 37°C, compared with citrated PRP preparations, which are stable for no more than 2 h.

The P2Y12 receptor induces platelet aggregation through weak activation of the αIIbβ3 integrin – a phosphoinositide 3-kinase-dependent mechanism

High concentrations of adenosine‐5′‐diphosphate ADP are able to induce partial aggregation without shape change of P2Y1 receptor‐deficient mouse platelets through activation of the P2Y12 receptor. In the present work we studied the transduction pathways selectively involved in this phenomenon. Flow cytometric analyses using R‐phycoerythrin‐conjugated JON/A antibody (JON/A‐PE), an antibody which recognizes activated mouse αIIbβ3 integrin, revealed a low level activation of αIIbβ3 in P2Y1 receptor‐deficient platelets in response to 100 μM ADP or 1 μM 2MeS‐ADP. Adrenaline induced no such activation but strongly potentiated the effect of ADP in a dose‐dependent manner. Global phosphorylation of 32P‐labeled platelets showed that P2Y12‐mediated aggregation was not accompanied by an increase in the phosphorylation of myosin light chain (P20) or pleckstrin (P47) and was not affected by the protein kinase C (PKC) inhibitor staurosporine. On the other hand, two unrelated phosphoinositide 3‐kinase inhibitors, wortmannin and LY294002, inhibited this aggregation. Our results indicate that (i) the P2Y12 receptor is able to trigger a P2Y1 receptor‐independent inside‐out signal leading to αIIbβ3 integrin activation and platelet aggregation, (ii) ADP and adrenaline use different signaling pathways which synergize to activate the αIIbβ3 integrin, and (iii) the transduction pathway triggered by the P2Y12 receptor is independent of PKC but dependent on phosphoinositide 3‐kinase.

Multiple alterations of platelet functions dominated by increased secretion in mice lacking Cdc42 in platelets

Platelet activation at sites of vascular injury is crucial for hemostasis, but it may also cause myocardial infarction or stroke. Cytoskeletal reorganization is essential for platelet activation and secretion. The small GTPase Cdc42 has been implicated as an important mediator of filopodia formation and exocytosis in various cell types, but its exact function in platelets is not established. Here, we show that the megakaryocyte/platelet-specific loss of Cdc42 leads to mild thrombocytopenia and a small increase in platelet size in mice. Unexpectedly, Cdc42-deficient platelets were able to form normally shaped filopodia and spread fully on fibrinogen upon activation, whereas filopodia formation upon selective induction of GPIb signaling was reduced compared with wild-type platelets. Furthermore, Cdc42-deficient platelets showed enhanced secretion of alpha granules, a higher adenosine diphosphate (ADP)/adenosine triphosphate (ATP) content, increased aggregation at low agonist concentrations, and enhanced aggregate formation on collagen under flow. In vivo, lack of Cdc42 resulted in faster occlusion of ferric chloride-injured arterioles. The life span of Cdc42-deficient platelets was markedly reduced, suggesting increased clearing of the cells under physiologic conditions. These data point to novel multiple functions of Cdc42 in the regulation of platelet activation, granule organization, degranulation, and a specific role in GPIb signaling.

Abnormal megakaryocyte morphology and proplatelet formation in mice with megakaryocyte-restricted MYH9 inactivation

Mutations in the MYH9 gene encoding nonmuscle myosin IIA lead to macrothrombocytopenia as observed in MYH9-related disorders. We used mice with megakaryocyte-restricted MYH9 inactivation to explore the role of myosin in thrombopoiesis. In situ, bone marrow MYH9Delta megakaryocytes were irregularly shaped, appearing leaky with poorly defined limits. The demarcation membranes were abnormally organized and poorly developed, pointing to an insufficient reservoir for the future formation of platelets. The cytoskeletal-rich peripheral zone was lacking due to the absence of the myosin filament network that normally surrounds the granular zone in wild-type cells. In vitro studies of cultured cells showed that MYH9Delta megakaryocytes were unable to form stress fibers upon adhesion to collagen, suggesting that the leaky shape results from defects in internal tension and anchorage to the extracellular environment. Surprisingly, the proportion of cells extending proplatelets was increased in MYH9Delta megakaryocytes and the proplatelet buds were larger. Overall, this study provides evidence for a role of myosin in different steps of megakaryocyte development through its participation in the maintenance of cell shape, formation and organization of the demarcation membranes and the peripheral zone, anchorage to the extracellular matrix, and proplatelet formation.

Defective tubulin organization and proplatelet formation in murine megakaryocytes lacking Rac1 and Cdc42

Blood platelets are anuclear cell fragments that are essential for blood clotting. Platelets are produced by bone marrow megakaryocytes (MKs), which extend protrusions, or so-called proplatelets, into bone marrow sinusoids. Proplatelet formation requires a profound reorganization of the MK actin and tubulin cytoskeleton. Rho GTPases, such as RhoA, Rac1, and Cdc42, are important regulators of cytoskeletal rearrangements in platelets; however, the specific roles of these proteins during platelet production have not been established. Using conditional knockout mice, we show here that Rac1 and Cdc42 possess redundant functions in platelet production and function. In contrast to a single-deficiency of either protein, a double-deficiency of Rac1 and Cdc42 in MKs resulted in macrothrombocytopenia, abnormal platelet morphology, and impaired platelet function. Double-deficient bone marrow MKs matured normally in vivo but displayed highly abnormal morphology and uncontrolled fragmentation. Consistently, a lack of Rac1/Cdc42 virtually abrogated proplatelet formation in vitro. Strikingly, this phenotype was associated with severely defective tubulin organization, whereas actin assembly and structure were barely affected. Together, these results suggest that the combined action of Rac1 and Cdc42 is crucial for platelet production, particularly by regulating microtubule dynamics.

Intrinsic impaired proplatelet formation and microtubule coil assembly of megakaryocytes in a mouse model of Bernard-Soulier syndrome

Bernard-Soulier syndrome is caused by a deficiency of the platelet surface glycoprotein Ib-IX-V complex resulting in a severe platelet function defect. However this is compounded by a reduction in platelet numbers and the platelets in the circulation are abnormally large. As for many other platelet disorders, recent evidence and findings of this study suggest that the defective morphology has its origins in megakaryocyte function. See related perspective article on page 756. Background Giant platelets and thrombocytopenia are invariable defects in the Bernard-Soulier syndrome caused by deficiency of the GPIb-V-IX complex, a receptor for von Willebrand factor supporting platelet adhesion to the damaged arterial wall. Various properties of this receptor may be considered potential determinants of the macrothrombocytopenia. Design and Methods To explore the underlying mechanisms of the disease, megakaryopoiesis was studied in a mouse model deficient in GPIbβ. Megakaryocytes were initially characterized in situ in the bone marrow of adult mice, after which their capacity to differentiate into proplatelet-bearing cells was evaluated in cultured fetal liver cells. Results The number of megakaryocyte progenitors, their differentiation and progressive maturation into distinct classes and their level of endoreplication were normal in GPIbβ−/− bone marrow. However, the more mature cells exhibited ultrastructural anomalies with a thicker peripheral zone and a less well developed demarcation membrane system. GPIbβ−/− megakaryocytes could be differentiated in culture from Lin− fetal liver cells in normal amounts but the proportion of cells able to extend proplatelets was decreased by 41%. Moreover, the GPIbβ−/− cells extending proplatelets displayed an abnormal morphology characterized by fewer pseudopodial extensions with thicker shaft sections and an increased diameter of the terminal coiled elements. GPIbβ−/− released platelets were larger but retained a typical discoid shape. Proplatelet formation was similarly affected in bone marrow explants from adult mice examined by videomicroscopy. The marginal microtubular ring contained twice as many tubulin fibers in GPIbβ−/− proplatelet buds in cultured and circulating platelets. Conclusions Altogether, these findings point to a role of the GPIb-V-IX complex intrinsic to megakaryocytes at the stage of proplatelet formation and suggest a functional link with the underlying microtubular cytoskeleton in platelet biogenesis.

Biogenesis of the demarcation membrane system (DMS) in megakaryocytes

The demarcation membrane system (DMS) in megakaryocytes forms the plasma membrane (PM) of future platelets. Using confocal microscopy, electron tomography, and large volume focused ion beam/scanning electron microscopy (FIB/SEM), we determined the sequential steps of DMS formation. We identified a pre-DMS that initiated at the cell periphery and was precisely located between the nuclear lobes. At all developmental stages, the DMS remained continuous with the cell surface. The number of these connections correlated well with the nuclear lobulation, suggesting a relationship with cleavage furrow formation and abortive cytokinesis. On DMS expansion, Golgi complexes assembled around the pre-DMS, and fusion profiles between trans-golgi network-derived vesicles and the DMS were observed. Brefeldin-A reduced DMS expansion, indicating that the exocytic pathway is essential for DMS biogenesis. Close contacts between the endoplasmic reticulum (ER) and the DMS were detected, suggesting physical interaction between the 2 membrane systems. FIB/SEM revealed that the DMS forms an intertwined tubular membrane network resembling the platelet open canalicular system. We thus propose the following steps in DMS biogenesis: (1) focal membrane assembly at the cell periphery; (2) PM invagination and formation of a perinuclear pre-DMS; (3) expansion through membrane delivery from Golgi complexes; and (4) ER-mediated lipid transfer.

The P2Y1 receptor plays an essential role in the platelet shape change induced by collagen when TxA2 formation is prevented

Summary.u2002 ADP and TxA2 are secondary agonists which play an important role as cofactors when platelets are activated by agonists such as collagen or thrombin. The aim of the present study was to characterize the role of the ADP receptor P2Y1 in collagen‐induced activation of washed platelets. Inhibition of P2Y1 alone with the selective antagonist MRS2179 prolonged the lag phase preceding aggregation in response to low or high concentrations of fibrillar collagen, without affecting the maximum amplitude of aggregation or secretion. A combination of MRS2179 and aspirin resulted in complete inhibition of platelet shape change at low and high collagen concentrations, together with a profound decrease in aggregation and secretion. Scanning electron microscopy showed that these platelets had conserved the discoid morphology typical of the resting state. A lack of shape change was also observed in aspirin‐treated P2Y1‐ and Gαq‐deficient mouse platelets and in δ‐storage pool‐deficient platelets from Fawn Hooded rats. In contrast, when the second ADP receptor P2Y12 was inhibited with AR‐C69931MX, aspirin‐treated platelets were still able to change shape and displayed only a moderate decrease in aggregation and secretion. In conclusion, this study provides evidence that collagen requires not only the TxA2 receptor Tpα, but also P2Y1, to induce platelet shape change.