Factor VIIa releases phosphatidylserine-enriched extracellular vesicles from endothelial cells by activating acid sphingomyelinase

BACKGROUND

Our recent studies showed that activated factor (F) VII (FVIIa) releases extracellular vesicles (EVs) from the endothelium. FVIIa-released EVs were found to be enriched with phosphatidylserine (PS) and contribute to the hemostatic effect of FVIIa in thrombocytopenia and hemophilia.

OBJECTIVE

To investigate mechanisms by which FVIIa induces EV biogenesis and enriches EVs with PS.

METHODS

FVIIa activation of acid sphingomyelinase (aSMase) was evaluated by its translocation to the cell surface. The role of aSMase in the biogenesis of FVIIa-induced EVs and their enrichment with PS was investigated using specific siRNAs and inhibitors of aSMase and its downstream metabolites. Wild-type and aSMase-/- mice were injected with a control vehicle or FVIIa. EVs released into circulation were quantified by nanoparticle tracking analysis. EVs hemostatic potential was assessed in a murine thrombocytopenia model.

RESULTS

FVIIa activation of aSMase is responsible for both the externalization of PS and the release of EVs in endothelial cells. FVIIa-induced aSMase activation led to ceramide generation and de novo expression of transmembrane protein 16F. Inhibitors of ceramidases, sphingosine kinase, or sphingosine-1-phosphate receptor modulator blocked FVIIa-induced expression of transmembrane protein 16F and PS externalization without interfering with FVIIa release of EVs. In vivo, FVIIa release of EVs was markedly impaired in aSMase-/- mice compared with wild-type mice. Administration of a low dose of FVIIa, sufficient to induce EVs release, corrected bleeding associated with thrombocytopenia in wild-type mice but not in aSMase-/- mice.

CONCLUSION

Our study identifies a novel mechanism by which FVIIa induces PS externalization and releases PS-enriched EVs.

My patient is thrombocytopenic! Is (s)he? Why? And what shall I do?

Solving the riddle of a thrombocytopenic patient is a difficult and fascinating task. The spectrum of possible aetiologies is wide, ranging from an in vitro artefact to severe treatment-resistant thrombocytopenic bleeding conditions, or even life-threatening prothrombotic states. Moreover, thrombocytopenia by itself does not protect from thrombosis and sometimes a patient with a low platelet count requires concomitant antithrombotic treatment as well. In order to identify and treat the cause and the effects of the thrombocytopenia, you have to put together several pieces of information, solving a unique jig-jaw puzzle.

The present work is not a textbook article about thrombocytopenia, rather a collection of differential diagnostic thoughts, treatment concepts, and some basic knowledge, that you can retrieve when facing your next thrombocytopenic patient. Enjoy reading it, but most importantly enjoy taking care of patients with a low platelet count. I bet the present work will assist you in this challenging and rewarding clinical task.

Adjuncts to Resuscitation

Damage control resuscitation has been increasingly adopted and practiced over the last decade. The concepts used are not new to this era of medicine but are novel in combination. This chapter will focus on adjuncts to damage control resuscitation (DCR) including massive transfusion protocols, the “other” tenets of damage control resuscitation, hypertonic saline, tranexamic acid, pharmacologic resuscitation, Factor VIIa, and prothrombin complex, and viscoelastic testing.

Damage Control Resuscitation

Resuscitation of the hemorrhaging patient has undergone significant changes in the last decade resulting in the concept of damage control resuscitation (DCR). Hemostatic resuscitation aims to address the physiologic derangements found in the hemorrhaging patient, namely coagulopathy, acidosis, and hypothermia. Strategies to achieve this are permissive hypotension, high ratio of plasma and platelet transfusion to packed red blood cell transfusion, and limitation of crystalloid administration. Damage control surgery aims for early hemorrhage control and minimizing operative time by delaying definitive repair until the patient's physiologic status has normalized. Together these strategies constitute DCR and have led to improved outcomes for hemorrhaging patients over the last 2 decades. Recently, DCR has been augmented by both pharmacologic and laboratory adjuncts to improve the care of the hemorrhaging patient. These include thrombelastography as a detailed measure of the clotting cascade, tranexamic acid as an antifibrinolytic, and the procoagulant activated factor VII. In this review, we discuss the strategies that makeup DCR, their adjuncts, and how they fit into the care of the hemorrhaging patient.

Inherited platelet disorders and oral health

Platelets play a key role in thrombosis and hemostasis. Accumulation of platelets at the site of vascular injury is the first step in the formation of hemostatic plugs, which play a pivotal role in preventing blood loss after injury. Platelet adhesion at sites of injury results in spreading, secretion, recruitment of additional platelets, and formation of platelet aggregates. Inherited platelet disorders are rare causes of bleeding syndromes, ranging from mild bruising to severe hemorrhage. The defects can reflect deficiency or dysfunction of platelet surface glycoproteins, granule contents, cytoskeletal proteins, platelet pro-coagulant function, and signaling pathways. For instance, Bernard-Soulier syndrome and Glanzmann thrombasthenia are attributed to deficiencies of glycoprotein Ib/IX/V and GPIIb/IIIa, respectively, and are rare but severe platelet disorders. Inherited defects that impair platelet secretion and/or signal transduction are among the most common forms of mild platelet disorders and include gray platelet syndrome, Hermansky-Pudlak syndrome, and Chediak-Higashi syndrome. When necessary, desmopressin, antifibrinolytic agents, and transfusion of platelets remain the most common treatment of inherited platelet disorders. Alternative therapies such as recombinant activated factor VII are also available for a limited number of situations. In this review, we will discuss the management of patients with inherited platelet disorders in various clinical situations related to dental cares, including surgical intervention.

Laboratory assessment and perioperative management of patients on antiplatelet therapy: from the bench to the bedside

The contribution of platelets in the pathophysiology of thromboses has established antiplatelet therapy as a cornerstone for prevention or treatment of these disorders. However, patients on antiplatelet drugs undergoing surgery face the life-threatening dilemma between the risk of perioperative thrombosis by ceasing therapy and restoring platelet function versus the risk of surgical bleeding by its continuation. According to their mechanism of action, antiplatelet drugs can be conventionally classified as agents that inhibit cyclooxygenase, block the platelet adenosine diphosphate P2Y12 receptor, inhibit phosphodiesterase, or block platelet glycoprotein IIb/IIIa. Although several tests have been developed to assess platelet inhibition by most of these compounds, studies to date have not been able to reliably evaluate the diagnostic efficiency of these tests to predict hemorrhage and/or blood loss, and accordingly perioperative assessment of drug-induced platelet inhibition cannot be recommended as yet. Although several management options are available to counteract the hemorrhagic risk of surgical patients using antiplatelet agents, perioperative discontinuation of these drugs is the preferable choice wherever possible. The use of platelet transfusions should be limited where necessary to the treatment of major, life-threatening bleeding. The contribution of newer hemostatic agents, such as desmopressin and recombinant activated factor VII, is yet to be fully determined, and there remain many challenges and unresolved issues in the clinical care of these patients.