Collagen Response and Glycoprotein VI Function Decline Progressively as Canine Platelets Age in vivo

Two novel platelet biotinylation methods and their impact on stored platelet concentrates in a blood bank environment

BACKGROUND

Storage of platelet concentrates (PCs) has an impact on platelet quality and possibly affects their functions after transfusion. The influence of processing and storage conditions of PCs on their in vivo function upon transfusion is unknown. One option for investigating this question is to implement an ex vivo labeling of human platelets, to analyze them after transfusion into heathy volunteers and/or patients. In this study, we developed two labeling methods employing biotin.

METHODS

Two methods of biotinylation were compared to a control (standard PC). The "Bio-Wash" process used washing steps to label all platelets within the PC; for the other method, "Bio-Direct," one fifth of the PC were directly labeled without washing steps. The control and the two biotinylated PCs were analyzed over 7 days of storage. Labeling efficiency, platelet counts, phenotypes, and functions, along with time and costs, were evaluated to select the best process.

RESULTS

Both methods achieved a stable labeling through the storage, with similar platelet counts and metabolism in comparison to control PCs. Bio-Wash showed higher activation phenotype and lower aggregation response in comparison to the Bio-Direct method. The Bio-Direct was performed within 1.5 h versus 3 h for the Bio-Wash. However, the Bio-Direct required 12 mg of biotin instead of 8 mg for the other process.

CONCLUSION

We set up two methods of biotinylation that can be easily implemented in a blood bank environment. The Bio-Direct process was preferred to the Bio-Wash because of its similarity, from a functional and phenotypic point of view, with standard PCs.

Mechanisms Underlying Dichotomous Procoagulant COAT Platelet Generation-A Conceptual Review Summarizing Current Knowledge

Procoagulant platelets are a subtype of activated platelets that sustains thrombin generation in order to consolidate the clot and stop bleeding. This aspect of platelet activation is gaining more and more recognition and interest. In fact, next to aggregating platelets, procoagulant platelets are key regulators of thrombus formation. Imbalance of both subpopulations can lead to undesired thrombotic or bleeding events. COAT platelets derive from a common pro-aggregatory phenotype in cells capable of accumulating enough cytosolic calcium to trigger specific pathways that mediate the loss of their aggregating properties and the development of new adhesive and procoagulant characteristics. Complex cascades of signaling events are involved and this may explain why an inter-individual variability exists in procoagulant potential. Nowadays, we know the key agonists and mediators underlying the generation of a procoagulant platelet response. However, we still lack insight into the actual mechanisms controlling this dichotomous pattern (i.e., procoagulant versus aggregating phenotype). In this review, we describe the phenotypic characteristics of procoagulant COAT platelets, we detail the current knowledge on the mechanisms of the procoagulant response, and discuss possible drivers of this dichotomous diversification, in particular addressing the impact of the platelet environment during in vivo thrombus formation.

Molecular Aspects of Pathophysiology of Platelet Receptors

Receptor is a dynamic instrumental surface protein that helps to interact with specific molecules to respond accordingly. Platelet is the smallest in size among the blood components, but it plays many pivotal roles to maintain hemostasis involving its surface receptors. It (platelet) has cell adhesion receptors (e.g., integrins and glycoproteins), leucine-rich repeats receptors (e.g., TLRs, glycoprotein complex, and MMPs), selectins (e.g., CLEC, P-selectin, and CD), tetraspanins (e.g., CD and LAMP), transmembrane receptors (e.g., purinergic—P2Y and P2X1), prostaglandin receptors (e.g., TxA2, PGH2, and PGI2), immunoglobulin superfamily receptors (e.g., FcRγ and FcεR), etc. on its surface. The platelet receptors (e.g., glycoproteins, protease-activated receptors, and GPCRs) during platelet activation are over expressed and their granule contents are secreted (including neurotransmitters, cytokines, and chemokines) into circulation, which are found to be correlated with different physiological conditions. Interestingly, platelets promote metastasis through circulation protecting from cytolysis and endogenous immune surveillance involving several platelets receptors. The updated knowledge about different types of platelet receptors in all probable aspects, including their inter- and intra-signaling mechanisms, are discussed with respect to not only its (platelets) receptor type but also under different pathophysiological conditions.

Cell surface expression of HLA I molecules as a marker of young platelets

BACKGROUND

Accurate identification of the proportion of young platelets is important to distinguish peripheral thrombocytopenia from a deficit in platelet production. Young platelets are defined by their higher RNA content and are often assessed as thiazole orange bright (TObright ) by flow cytometry. In clinical practice, their proportion is estimated by automatic blood counter according to their greater RNA content, which identifies a so-called immature platelet fraction (IPF). However, the detected IPFs are not strictly identical to the young TObright platelet population observed by flow cytometry.

OBJECTIVES

The aim of this study was to assess the reliability of HLA I/major histocompatibility I (MHC I) cell surface expression as a marker of young platelets.

METHODS

The HLA I/MHC I expression was evaluated by flow cytometry after costaining blood with TO and antibodies directed against HLA I/MHC I molecules.

RESULTS

We found that platelets with a higher expression of plasma membrane-localized MHC I molecules displayed an increased TO staining and a higher content in ribosomal P-antigen. Transfusion experiments in mice showed that the number of MHC I molecules expressed on the cell surface of young murine platelets decreased during platelet aging, reaching basal levels within 24 h. Finally, we demonstrated that for patients with thrombocytopenias, the identification of young platelets is better assessed by the flow cytometric determination of the level of HLA I expression than by TO staining or the use of hematological blood counter.

CONCLUSION

Overall, our results highlight the relevance of MHC I/HLA I expression as a valuable parameter to identify young platelets.

Delayed-onset of procoagulant signalling revealed by kinetic analysis of COAT platelet formation

The combined action of collagen and thrombin induces the formation of COAT platelets, which are characterised by a coat of procoagulant and adhesive molecules on their surface. Although recent work has started to highlight their clinical relevance, the exact mechanisms regulating the formation of procoagulant COAT platelets remain unclear. Therefore, we employed flow cytometry in order to visualise in real time surface and intracellular events following simultaneous platelet activation with convulxin and thrombin. After a rapid initial response pattern characterised by the homogenous activation of the fibrinogen receptor glycoprotein IIb/IIIa in all platelets, starting with a delay of about 2 minutes an increasing fraction transforms to procoagulant COAT platelets. Their surface is characterised by progressive loss of PAC-1 binding, expression of negative phospholipids and retention of α-granule von Willebrand factor. Intracellular events in procoagulant COAT platelets are a marked increase of free calcium into the low micromolar range, concomitantly with early depolarisation of the mitochondrial membrane and activation of caspase-3, while non-COAT platelets keep the intracellular free calcium in the nanomolar range and maintain an intact mitochondrial membrane. We show for the first time that the flow-cytometrically distinct fractions of COAT and non-COAT platelets differentially phosphorylate two signalling proteins, PKCα and p38MAPK, which may be involved in the regulation of the different calcium fluxes observed in COAT versus non-COAT platelets. This study demonstrates the utility of concomitant cellular and signalling evaluation using flow cytometry in order to further dissect the mechanisms underlying the dichotomous platelet response observed after collagen/thrombin stimulation.

Supplementary Material to this article is available online at www.thrombosis-online.com.

Clinical significance of receptor shedding-platelet GPVI as an emerging diagnostic and therapeutic tool

Platelet membrane bedecked with a wide array of receptors offers a platform to regulate platelet responsiveness, thrombotic propensity, inflammatory disposition, and immune reactivity under diverse pathophysiological conditions. Ectopic proteolytic cleavage of such receptors irreversibly inactivates receptor-mediated intracellular signaling governing cellular functions, further releases soluble fragments into circulation which might modulate functions of target cells. Glycoprotein VI-(GPVI) is a membrane glycoprotein expressed in platelets and megakaryocytes. Platelet GPVI surface expression is enhanced following acute ischemic events like myocardial infarction and cerebral stroke, serves as an imminent diagnostic tool independent of markers of tissue necrosis, and is associated with poor prognosis. Platelets undergo GPVI shedding and thereby contribute to soluble plasma levels of sGPVI, with distinct diagnostic and prognostic attributes. This review summarizes the functional significance and mechanistic basis whereby GPVI surface availability is up- or downregulated on platelets and the impact of GPVI in diagnostic, prognostic, and therapeutic strategies in diseases where platelets play a regulatory role. Further, we also highlight how novel non-invasive platelet-based diagnostic and therapeutic strategies have evolved utilizing GPVI for lesion-directed antithrombotic therapy or to counteract atherosclerotic disposition to ameliorate care of patients particularly in the context of cardio-cerebro-vascular medicine.

Thrombocytopenia in the Intensive Care Unit

Thrombocytopenia is a common laboratory finding in critically ill patients admitted to the intensive care unit. Potential etiologies of thrombocytopenia are myriad, ranging from acute disease processes and concomitant conditions to exposures and drugs. The mechanism of decreased platelet counts can also be varied: laboratory measurement may be spurious, platelet production may be decreased, or platelet destruction or sequestration may be increased. In addition to evaluation for the cause of thrombocytopenia, the clinician must also guard against spontaneous bleeding due to thrombocytopenia, prophylax against bleeding resulting from an invasive procedure performed in the setting of thrombocytopenia, and treat active bleeding related to thrombocytopenia.

BCL2/BCL-X(L) inhibition induces apoptosis, disrupts cellular calcium homeostasis, and prevents platelet activation

Apoptosis in megakaryocytes results in the formation of platelets. The role of apoptotic pathways in platelet turnover and in the apoptotic-like changes seen after platelet activation is poorly understood. ABT-263 (Navitoclax), a specific inhibitor of antiapoptotic BCL2 proteins, which is currently being evaluated in clinical trials for the treatment of leukemia and other malignancies, induces a dose-limiting thrombocytopenia. In this study, the relationship between BCL2/BCL-X(L) inhibition, apoptosis, and platelet activation was investigated. Exposure to ABT-263 induced apoptosis but repressed platelet activation by physiologic agonists. Notably, ABT-263 induced an immediate calcium response in platelets and the depletion of intracellular calcium stores, indicating that on BCL2/BCL-X(L) inhibition platelet activation is abrogated because of a diminished calcium signaling. By comparing the effects of ABT-263 and its analog ABT-737 on platelets and leukemia cells from the same donor, we show, for the first time, that these BCL2/BCL-X(L) inhibitors do not offer any selective toxicity but induce apoptosis at similar concentrations in leukemia cells and platelets. However, reticulated platelets are less sensitive to apoptosis, supporting the hypothesis that treatment with ABT-263 induces a selective loss of older platelets and providing an explanation for the transient thrombocytopenia observed on ABT-263 treatment.

Adverse Events After Stopping Clopidogrel in Post–Acute Coronary Syndrome Patients

Background— A prior study from the Veterans Health Administration found a clustering of cardiovascular events after clopidogrel cessation. We sought to confirm and expand these findings.

Methods and Results— This was a retrospective cohort study of 2017 patients with acute coronary syndrome discharged on clopidogrel from an integrated health care delivery system. Rates of all-cause mortality or acute myocardial infarction (MI) within 1 year after stopping clopidogrel were assessed among patients who did not have an event before stopping clopidogrel. Death/MI occurred in 4.3% (n=71) of patients. The rates of death/MI were 3.07, 1.62, 0.70, and 0.95 per 10 000 patient-days for the time intervals of 0 to 90, 91 to 180, 181 to 270, and 271 to 360 days after stopping clopidogrel. In multivariable analysis, the 0- to 90-day interval after stopping clopidogrel was associated with higher risk of death/MI (incidence rate ratio, 2.74; 95% confidence interval, 1.69 to 4.44) compared with 91- to 360-day interval. There was a similar trend of increased events after stopping clopidogrel for various subgroups (women versus men, medical therapy versus percutaneous coronary intervention, stent type, and ≥6 months or <6 months of clopidogrel treatment). Among patients taking clopidogrel but stopping ACE inhibitor medications, the event rates were similar in the 0- to 90-day versus the 91- to 360-day interval (2.67 versus 2.91 per 10 000 patient-days; P =0.91).

Conclusions— We observed a clustering of adverse events in the 0 to 90 days after stopping clopidogrel. This clustering of events was not present among patients stopping ACE inhibitors. These findings are consistent with a possible rebound platelet hyper-reactivity after stopping clopidogrel and additional platelet studies are needed to confirm this effect.

Measuring soluble platelet glycoprotein VI in human plasma by ELISA

Recent experimental evidence demonstrates that the platelet-specific collagen receptor, glycoprotein (GP)VI is essentially all uncleaved on normal circulating platelets, but is shed from the platelet surface in a metalloproteinase-dependent manner in response to GPVI ligands (including collagen), anti-GPVI antibodies or activation at the platelet Fc receptor, FcgammaRIIa. This raises the question of whether shed ectodomain fragment in plasma could be a useful biomarker of thrombotic risk and/or autoimmune thrombocytopenia. In this study, we developed a sandwich enzyme-linked immunosorbent assay (ELISA) for measuring soluble GPVI in human plasma, using rabbit anti-GPVI polyclonal antibody in the solid-phase, murine anti-GPVI monoclonal antibody (1A12) in the fluid-phase and horseradish peroxidase (HRP)-coupled anti-mouse antibody and enhanced chemiluminescence (ECL) for detection. The ELISA was optimized for sensitivity, reproducibility, inter- and intra-assay precision, addition and recovery and detected GPVI in plasma with a lower detection limit of approximately 1 ng/mL. Effects of different anti-coagulants (trisodium citrate, acid-citrate-dextrose or EDTA) were negligible. In ten healthy donors, soluble plasma GPVI levels were 18.9 +/- 4.1 ng/mL. Treating normal platelet-rich plasma with a GPVI ligand (collagen-related peptide, CRP), calmodulin inhibitor W7 (that induces GPVI shedding without platelet activation) or N-ethylmaleimide (that directly activates platelet sheddases), under conditions previously shown to induce GPVI shedding, also increased plasma GPVI levels by up to approximately 7-fold, compared to previously reported autoimmune (anti-GPVI) patient plasma where soluble GPVI was approximately 10-fold higher than normal. Characterization of this sensitive ELISA should facilitate analysis of functional/diagnostic role(s) for soluble GPVI in human plasma associated with thrombotic/immune dysfunction.

Impairment of the hemostatic potential of platelets during storage as evaluated by flow cytometry, thrombin generation, and thrombelastography under conditions promoting formation of coated platelets

BACKGROUND

The increasing demand for platelet (PLT) transfusions has focused attention on appropriate use. Coated PLTs are a subpopulation of highly procoagulant PLTs formed by simultaneous stimulation by the agonist's collagen and thrombin hypothesized to drive clot formation at the site of vascular injury. Prolonged storage of PLTs may reduce their ability to support optimal hemostasis upon transfusion.

STUDY DESIGN AND METHODS

PLT concentrates (PCs) stored for 1, 4, 6, and 8 days were costimulated with thrombin and the collagen glycoprotein VI (GPVI) receptor agonist convulxin, and their ability to form coated PLTs was determined by flow cytometry. Further, a plasma-based thrombin generation assay and thrombelastography were used to evaluate the aged PCs' capacity to support thrombin generation and clot formation, respectively. The stored PCs were additionally tested by standard quality control methods.

RESULTS

PLT quality as measured by standard analyses was acceptable according to current practice. The hemostatic potential, however, was impaired with increasing storage time. The formation of coated PLTs decreased significantly from approximately 85 to 55 percent with increasing storage time (p<0.05). The velocity of clot formation was significantly increased from Day 4 (p<0.05). The velocity of thrombin generation and resistance against fibrinolysis were significantly reduced on Day 8 compared to Day 1 of storage (p<0.05).

CONCLUSION

Data in the present study suggest that storage significantly reduced the stored PLTs' ability to respond to conditions expected to exist at the site of vascular injury and that storage-induced reduction in PLT activation sensitivity correlated with a loss of hemostatic potential.

Platelet Receptor Proteolysis

The platelet plasma membrane is literally at the cutting-edge of recent research into proteolytic regulation of the function and surface expression of platelet receptors, revealing new mechanisms for how the thrombotic propensity of platelets is controlled in health and disease. Extracellular proteolysis of receptors irreversibly inactivates receptor-mediated adhesion and signaling, as well as releasing soluble fragments into the plasma where they act as potential markers or modulators. Platelet-surface sheddases, particularly of the metalloproteinase-disintegrin (ADAM) family, can be regulated by many of the same mechanisms that control receptor function, such as calmodulin association or activation of signaling pathways. This provides layers of regulation (proteinase and receptor), and a higher order of control of cellular function. Activation of pathways leading to extracellular shedding is concomitant with activation of intracellular proteinases such as calpain, which may also irreversibly deactivate receptors. In this review, platelet receptor shedding will be discussed in terms of (1) the identity of proteinases involved in receptor proteolysis, (2) key platelet receptors regulated by proteolytic pathways, and (3) how shedding might be regulated in normal physiology or future therapeutics. In particular, a focus on proteolytic regulation of the platelet collagen receptor, glycoprotein (GP)VI, illustrates many of the key biochemical, cellular, and clinical implications of current research in this area.

Aspirin Induces Platelet Receptor Shedding via ADAM17 (TACE)

Aspirin is effective in the therapy of cardiovascular diseases, because it causes acetylation of cyclooxygenase 1 (COX-1) leading to irreversible inhibition of platelets. Additional mechanisms can be suspected, because patients treated with other platelet COX inhibitors such as indomethacin do not display an increased bleeding tendency as observed for aspirin-treated patients. Recently, aspirin and other anti-inflammatory drugs were shown to induce shedding of L-selectin in neutrophils in a metalloproteinase-dependent manner. Therefore, we investigated the effects of aspirin on the von Willebrand Factor receptor complex glycoprotein (GP) Ib-V-IX, whose lack or dysfunction causes bleeding in patients. As quantified by fluorescence-activated cell sorting analysis in whole blood, aspirin, but not its metabolite salicylic acid, induced dose-dependent shedding of human and murine GPIbalpha and GPV from the platelet surface, whereas other glycoproteins remained unaffected by this treatment. Biotinylated fragments of GPV were detected by immunoprecipitation in the supernatant of washed mouse platelets, and the expression level of GPIbalpha was decreased in these platelets as measured by Western blot analysis. Although shedding occurred normally in COX-1-deficient murine platelets, shedding was completely blocked by a broad-range metalloproteinase inhibitor and, more importantly, in mouse platelets expressing an inactive form of ADAM17. Shed fragments of GPIbalpha and GPV were elevated in the plasma of aspirin-injected mice compared with animals injected with control buffer. These data demonstrate that aspirin at high concentrations induces shedding of GPIbalpha and GPV by an ADAM17-dependent mechanism and that this process can occur in vivo.

Young thrombocytes initiate the formation of arterial thrombi in zebrafish

The zebrafish system is an excellent vertebrate genetic model to study hemostasis and thrombosis because saturation mutagenesis screens can identify novel genes that play a role in this vital physiologic pathway. To study hemostatic mutations, it is important to understand the physiology of zebrafish hemostasis and thrombosis. Previously, we identified zebrafish thrombocytes and have shown that they participate in arterial thrombus formation. Here, we recognized 2 populations of thrombocytes distinguishable by DiI-C18 (DiI) staining. DiI+ thrombocytes have a high density of adhesive receptors and are functionally more active than DiI- thrombocytes. We classified DiI+ thrombocytes as young and DiI- thrombocytes as mature thrombocytes. We found young and mature thrombocytes each formed independent clusters and that young thrombocytes clustered first. We have also shown that young thrombocytes initiate arterial thrombus formation. We propose that due to the increased adhesive receptor density on young thrombocytes, they adhere first to the subendothelial matrix, get activated rapidly, release agonists, and recruit more young thrombocytes, which further release more agonists. This increase in agonists activates the less active mature thrombocytes, drawing them to the growing thrombus. Since arterial thrombus formation is a fundamental hemostatic event, this mechanism may be conserved in mammals and may open new avenues for prevention of arterial thrombosis.

Platelet activation induces metalloproteinase-dependent GP VI cleavage to down-regulate platelet reactivity to collagen

Glycoprotein (GP) VI, the primary collagen receptor on platelets, has been shown to have variable expression, possibly as a consequence of immune modulation. The present study was designed to determine the mechanism by which GP VI clearance occurs. We found that direct activation of GP VI both by a GP VI-specific antibody and by GP VI ligands (collagen and convulxin) reduced binding of biotinylated convulxin to the stimulated platelets. Analysis of immunoblots of platelets and supernatants showed that the stimulated platelets contained less GP VI, while the soluble fraction contained a 57-kDa cleavage product. Stimulation of platelets with PAR-1 agonists (TRAP peptide and thrombin) also caused GP VI cleavage, although the amount of GP VI loss was less than that observed with direct GP VI ligands. The metalloproteinase (MMP) inhibitors GM6001 and TAPI prevented both the clearance of GP VI from the platelet surface and the appearance of the soluble cleavage product. Induction of GP VI cleavage caused specific down-regulation of collagen-induced platelet aggregation, providing a mechanism for the modulation of platelet responsiveness to this important platelet agonist.

Procoagulant surface exposure and apoptosis in rabbit platelets: association with shortened survival and steady-state senescence

BACKGROUND

The signal(s) for removal of senescent platelets from the circulation are not fully understood; phosphatidylserine (PS) expression on platelets and another marker of apoptosis, loss of mitochondrial inner membrane potential (DeltaPsim), have been implicated in platelet clearance.

OBJECTIVE

To investigate whether shortened platelet survival and steady-state platelet senescence are associated with increased surface exposure of PS and DeltaPsim collapse.

METHODS

Survival of in-vitro biotinylated rabbit platelets treated with thrombin or Ca(2+)-ionophore A23187 was tracked by flow cytometry after injection. Steady-state platelet senescence was investigated by infusing biotin to label a platelet cohort. PS expression and DeltaPsim of in-vitro biotinylated platelets and of the aging platelet cohort biotinylated in-vivo were measured by flow cytometry using annexin V-FLUOS and the DeltaPsim-sensitive dye CMXRos, respectively.

RESULTS

Although PS expression, DeltaPsim and survival of thrombin-degranulated platelets were similar to those of control platelets, increasing concentrations of A23187 caused increased surface exposure of PS and progressive shortening of platelet survival; only one-sixth of PS-expressing platelets also exhibited DeltaPsim loss. The cohort of senescent, biotinylated platelets remaining in the circulation at 96 h had increased exposure of PS and collapsed DeltaPsim; of the 17% of PS-expressing platelets, one-third did not exhibit DeltaPsim loss. There was also an increase in platelets with collapsed DeltaPsim but not expressing PS.

CONCLUSIONS

Platelets with shortened survival and senescent platelets have increased surface exposure of PS, that may be involved in their clearance. PS expression can occur independently of DeltaPsim collapse and conversely, in aged platelets, DeltaPsim loss can occur independently of PS expression.

COAT platelets

PURPOSE OF REVIEW

COAT platelets are a recently described subpopulation of cells resulting from simultaneous activation with collagen and thrombin. The complete process by which COAT platelets are produced is still not clear, although significant recent progress has been made.

RECENT FINDINGS

COAT platelets retain several procoagulant proteins on their surface by a previously unrecognized mechanism involving transglutaminase mediated conjugation of serotonin to released alpha-granule proteins. Fibrinogen and thrombospondin have been found to bind serotonin-conjugated proteins and thereby provide the requisite link for stabilization of serotonin-derivatized, procoagulant proteins on COAT platelets.

SUMMARY

Multivalent interactions, resulting from traditional receptor interactions and binding of conjugated serotonin by fibrinogen and thrombospondin, result in exceptionally strong retention of procoagulant alpha-granule proteins on the surface of COAT platelets. The physiologic significance of this new subclass of platelets remains to be determined.