Alec A. Schmaier

Platelets regulate lymphatic vascular development through CLEC-2–SLP-76 signaling

Although platelets appear by embryonic day 10.5 in the developing mouse, an embryonic role for these cells has not been identified. The SYK-SLP-76 signaling pathway is required in blood cells to regulate embryonic blood-lymphatic vascular separation, but the cell type and molecular mechanism underlying this regulatory pathway are not known. In the present study we demonstrate that platelets regulate lymphatic vascular development by directly interacting with lymphatic endothelial cells through C-type lectin-like receptor 2 (CLEC-2) receptors. PODOPLANIN (PDPN), a transmembrane protein expressed on the surface of lymphatic endothelial cells, is required in nonhematopoietic cells for blood-lymphatic separation. Genetic loss of the PDPN receptor CLEC-2 ablates PDPN binding by platelets and confers embryonic lymphatic vascular defects like those seen in animals lacking PDPN or SLP-76. Platelet factor 4-Cre-mediated deletion of Slp-76 is sufficient to confer lymphatic vascular defects, identifying platelets as the cell type in which SLP-76 signaling is required to regulate lymphatic vascular development. Consistent with these genetic findings, we observe SLP-76-dependent platelet aggregate formation on the surface of lymphatic endothelial cells in vivo and ex vivo. These studies identify a nonhemostatic pathway in which platelet CLEC-2 receptors bind lymphatic endothelial PDPN and activate SLP-76 signaling to regulate embryonic vascular development.

Microfluidic focal thrombosis model for measuring murine platelet deposition and stability: PAR4 signaling enhances shear‐resistance of platelet aggregates

Summary.  Background: Flow chambers allow the ex vivo study of platelet response to defined surfaces at controlled wall shear stresses. However, most assays require 1–10 mL of blood and are poorly suited for murine whole blood experiments. Objective: To measure murine platelet deposition and stability in response to focal zones of prothrombotic stimuli using 100 μL of whole blood and controlled flow exposure. Methods: Microfluidic methods were used for patterning acid‐soluble collagen in 100 μm × 100 μm patches and creating flow channels with a volume of 150 nL. Within 1 min of collection into PPACK and fluorescent anti‐mouse CD41 mAb, whole blood from normal mice or from mice deficient in the integrin α2 subunit was perfused for 5 min over the patterned collagen. Platelet accumulation was measured at venous and arterial wall shear rates. After 5 min, thrombus stability was measured with a ‘shear step‐up’ to 8000 s−1. Results: Wild‐type murine platelets adhered and aggregated on collagen in a biphasic shear‐dependent manner with increased deposition from 100 to 400 s−1, but decreased deposition at 1000 s−1. Adhesion to patterned collagen was severely diminished for platelets lacking a functional α2β1 integrin. Those integrin α2‐deficient platelets that did adhere were removed from the surface when challenged to shear step‐up. PAR4 agonist (AYPGKF) treatment of the thrombus at 5 min enhanced aggregate stability during the shear step‐up. Conclusions: PAR4 signaling enhances aggregate stability by mechanisms independent of other thrombin‐dependent pathways such as fibrin formation.

MicroRNA expression in maturing murine megakaryocytes

MicroRNAs are small noncoding RNAs that regulate cellular development by interfering with mRNA stability and translation. We examined global microRNA expression during the differentiation of murine hematopoietic progenitors into megakaryocytes. Of 435 miRNAs analyzed, 13 were up-regulated and 81 were down-regulated. Many of these changes are consistent with miRNA profiling studies of human megakaryocytes and platelets, although new patterns also emerged. Among 7 conserved miRNAs that were up-regulated most strongly in murine megakaryocytes, 6 were also induced in the related erythroid lineage. MiR-146a was strongly up-regulated during mouse and human megakaryopoiesis but not erythropoiesis. However, overexpression of miR-146a in mouse bone marrow hematopoietic progenitor populations produced no detectable alterations in megakaryocyte development or platelet production in vivo or in colony assays. Our findings extend the repertoire of differentially regulated miRNAs during murine megakaryopoiesis and provide a useful new dataset for hematopoiesis research. In addition, we show that enforced hematopoietic expression of miR-146a has minimal effects on megakaryopoiesis. These results are compatible with prior studies indicating that miR-146a inhibits megakaryocyte production indirectly by suppressing inflammatory cytokine production from innate immune cells, but cast doubt on a different study, which suggests that this miRNA inhibits megakaryopoiesis cell-autonomously.

Molecular priming of Lyn by GPVI enables an immune receptor to adopt a hemostatic role

The immune receptor signaling pathway is used by nonimmune cells, but the molecular adaptations that underlie its functional diversification are not known. Circulating platelets use the immune receptor homologue glycoprotein VI (GPVI) to respond to collagen exposed at sites of vessel injury. In contrast to immune cell responses, platelet activation must take place within seconds to successfully form thrombi in flowing blood. Here, we show that the GPVI receptor utilizes a unique intracellular proline-rich domain (PRD) to accelerate platelet activation, a requirement for efficient platelet adhesion to collagen under flow. The GPVI PRD specifically binds the Src-family kinase Lyn and directly activates it, presumably through SH3 displacement. In resting platelets, Lyn is constitutively bound to GPVI in an activated state and platelets lacking Lyn exhibit defective collagen adhesion like that of platelets with GPVI receptors lacking the PRD. These findings define a molecular priming mechanism that enables an immune-type receptor to adopt a hemostatic function. These studies also demonstrate that active kinases can constitutively associate with immune-type receptors without initiating signal transduction before receptor ligation, consistent with a recent molecular model of immune receptor signaling in which receptor ligation is required to bring active kinases to their receptor substrates.

Thrombosis in flowing blood

In this issue of Blood, van der Meijden and colleagues report on the mechanisms by which collagen exposure in flow-dependent circulation contributes to thrombus formation.

Occlusive thrombi arise in mammals but not birds in response to arterial injury: evolutionary insight into human cardiovascular disease

Mammalian platelets are small, anuclear circulating cells that form tightly adherent, shear-resistant thrombi to prevent blood loss after vessel injury. Platelet thrombi that form in coronary and carotid arteries also underlie common vascular diseases such as myocardial infarction and stroke and are the target of drugs used to treat these diseases. Birds have high-pressure cardiovascular systems like mammals but generate nucleated thrombocytes rather than platelets. Here, we show that avian thrombocytes respond to many of the same activating stimuli as mammalian platelets but are unable to form shear-resistant aggregates ex vivo. Avian thrombocytes are larger than mammalian platelets, spread less efficiently on collagen, and express much lower levels of the α(₂b)β₃ integrin required for aggregate formation, features predicted to make thrombocyte aggregates less resistant than platelets are to the high fluid shear forces of the arterial vasculature. In vivo carotid vessel injury stimulates the formation of occlusive platelet thrombi in mice but not in the size- and flow-matched carotid artery of the Australian budgerigar. These studies indicate that unique physical and molecular features of mammalian platelets enable them to form shear-resistant arterial thrombi, an essential element in the pathogenesis of human cardiovascular diseases.

Negative regulation of activated α2 integrins during thrombopoiesis

Circulating platelets exhibit rapid signaling and adhesive responses to collagen that facilitate hemostasis at sites of vessel injury. Because platelets are anuclear, their collagen receptors must be expressed by megakaryocytes, platelet precursors that arise in the collagen-rich environment of the bone marrow. Whether and how megakaryocytes regulate collagen adhesion during their development in the bone marrow are unknown. We find that surface expression of activated, but not wild-type, alpha2 integrins in hematopoietic cells in vivo results in the generation of platelets that lack surface alpha2 receptors. Culture of hematopoietic progenitor cells ex vivo reveals that surface levels of activated, but not wild-type, alpha2 integrin receptors are rapidly down-regulated during cell growth on collagen but reach wild-type levels when cells are grown in the absence of collagen. Progenitor cells that express activated alpha2 integrins are normally distributed in the bone marrow in vivo and exhibit normal migration across a collagen-coated membrane ex vivo. This migration is accompanied by rapid down-regulation of activated surface integrins. These studies identify ligand-dependent removal of activated alpha2 receptors from the cell surface as a mechanism by which integrin function can be negatively regulated in hematopoietic cells during migration between the adhesive environment of the bone marrow and the nonadhesive environment of the circulating blood.

Human platelets as a platform to monitor metabolic biomarkers using stable isotopes and LC-MS.

BACKGROUND Intracellular metabolites such as CoA thioesters are modulated in a number of clinical settings. Their accurate measurement from surrogate tissues such as platelets may provide additional information to current serum and urinary biomarkers. METHODS Freshly isolated platelets from healthy volunteers were treated with rotenone, propionate or isotopically labeled metabolic tracers. Using a recently developed LC-MS-based methodology, absolute changes in short-chain acyl-CoA thioesters were monitored, as well as relative metabolic labeling using isotopomer distribution analysis. RESULTS Consistent with in vitro experiments, isolated platelets treated with rotenone showed decreased intracellular succinyl-CoA and increased β-hydroxybutyryl-CoA, while propionate treatment resulted in increased propionyl-CoA. In addition, isotopomers of the CoAs were readily detected in platelets treated with the [(13)C]- or [(13)C(15)N]-labeled metabolic precursors. CONCLUSION Here, we show that human platelets can provide a powerful ex vivo challenge platform with potential clinical diagnostic and biomarker discovery applications.

Are Patients Getting Their Aspirin's Worth in Ischemic Stroke?

Given its long history, established safety profile, and efficacy, low‐dose aspirin remains the major antiplatelet therapy utilized in secondary prevention. Recurrent ischemic events occur despite prescription of antiplatelet therapy. Residual platelet activation following agonist stimulation can

Venous Thromboembolism and Cancer

Purpose of ReviewThis review aims to summarize the epidemiology, current pathophysiologic understanding, and state-of-the-art treatment of venous thromboembolism (VTE) in cancer patients.Recent FindingsThe risk of VTE varies among cancer patients. Recently introduced prediction models better identify those at high risk of VTE. New mechanisms underlying hypercoagulability in cancer have been uncovered. Initial data on the efficacy of direct oral anticoagulants (DOACs) compared with low-molecular weight heparin to treat VTE in patients with cancer are promising. However, they may be associated with higher risk of gastrointestinal bleeding.SummaryVTE causes significant morbidity and mortality in cancer patients. Our understanding of the mechanisms of VTE, including those associated with cancer treatments, has significantly grown. The assessment of the benefit/risk balance of VTE treatment remains challenging in many patients with cancer. The introduction of DOACs has expanded treatment options, but knowledge on their efficacy and safety is incomplete.