Platelets and megakaryocytes are critical players in immune responses. Recent reports suggest infection and inflammation alter the megakaryocyte and platelet transcriptome to induce altered platelet reactivity. We examined if non-viral sepsis induces differential platelet gene expression and reactivity. Non-viral sepsis upregulated IFITM3, an interferon responsive gene that restricts viral replication. As IFITM3 has been linked to clathrin-mediated endocytosis, we examined if IFITM3 promoted endocytosis of alpha granule proteins.Interferon stimulation enhanced fibrinogen endocytosis in megakaryocytes and platelets from Ifitm +/+ mice, but not Ifitm -/mice. IFITM3 overexpression or deletion in megakaryocytes demonstrated IFITM3 was necessary and sufficient to regulate fibrinogen endocytosis. Mechanistically, IFITM3 interacts with clathrin and αIIb and altered their plasma membrane localization into lipid rafts. In vivo interferon administration increased fibrinogen endocytosis, platelet reactivity, and thrombosis in an IFITM-dependent manner. In contrast, Ifitm -/mice were completely rescued from interferon-induced platelet hyperreactivity and thrombosis. During murine sepsis, platelets from Ifitm +/+ mice demonstrated increased fibrinogen content and platelet reactivity, which was dependent on interferon-alpha and IFITMs. Platelets from patients with non-viral sepsis had increases in platelet IFITM3 expression, fibrinogen content, and hyperreactivity. These data identify IFITM3 as a regulator of platelet endocytosis, hyperreactivity, and thrombosis during inflammatory stress.

The MAPK-interacting kinase (Mnk) family includes Mnk1 and Mnk2, which are phosphorylated and activated in response to extracellular stimuli. Mnk1 contributes to cellular responses by regulating mRNA translation and mRNA translation influences platelet production and function. However, the role of Mnk1 in megakaryocytes and platelets has not previously been studied. The present study investigated Mnk1 in megakaryocytes and platelets using both pharmacological and genetic approaches. We demonstrate that Mnk1, but not Mnk2, is expressed and active in human and murine megakaryocytes and platelets. Stimulating human and murine megakaryocytes and platelets induced Mnk1 activation and phosphorylation of eIF4E, a downstream target of activated Mnk1 that triggers mRNA translation. Mnk1 inhibition or deletion significantly diminished protein synthesis in megakaryocytes, as measured by polysome profiling and [35S]-methionine incorporation assays. Depletion of Mnk1 also reduced megakaryocyte ploidy and proplatelet forming megakaryocytes in vitro, and resulted in thrombocytopenia. However, Mnk1 deletion did not affect the half-life of circulating platelets. Platelets from Mnk1 knockout mice exhibited reduced platelet aggregation, alpha granule secretion, and integrin aIIbb3 activation. Ribosomal footprint sequencing indicated that Mnk1 regulates the translation of Pla2g4a mRNA (which encodes cPLA2) in megakaryocytes. Consistent with this, Mnk1 ablation reduced cPLA2 activity and thromboxane generation in platelets and megakaryocytes. In vivo, Mnk1 ablation in platelets protected against thromboembolism. These results provide previously unrecognized evidence that Mnk1 regulates mRNA translation and cellular activation in platelets and megakaryocytes, endomitosis and thrombopoiesis, and thrombosis.

Coagulopathy and platelet hyperactivity are hallmarks of coronavirus disease 2019 (COVID-19) and contribute to an increased thrombotic burden observed in many COVID-19 patients. 1 Anticoagulation and/or antiplatelet drugs are actively pursued in clinical trials aimed at improving outcomes in COVID-19 patients. However, while a consensus exists on the benefit of early anticoagulation, no consensus has been reached for antiplatelet drugs. A recent study, published in Nature Communications, described a novel technique for imaging and quantifying platelet aggregates in the circulation of COVID-19 patients longitudinally. 2 This technique could potentially help stratify patients who would benefit the most from antiplatelet drugs in COVID-19. Moreover, this high-throughput microscopy technique could have applications in other thrombotic disorders in which platelets play a key role in thrombotic complications.

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Purpose of review Platelet mitochondrial dysfunction is both caused by, as well as a source of oxidative stress. Oxidative stress is a key hallmark of metabolic disorders such as dyslipidemia and diabetes, which are known to have higher risks for thrombotic complications. Recent findings Increasing evidence supports a critical role for platelet mitochondria beyond energy production and apoptosis. Mitochondria are key regulators of reactive oxygen species and procoagulant platelets, which both contribute to pathological thrombosis. Studies targeting platelet mitochondrial pathways have reported promising results suggesting antithrombotic effects with limited impact on hemostasis in animal models. Summary Targeting platelet mitochondria holds promise for the reduction of thrombotic complications in patients with metabolic disorders. Future studies should aim at validating these preclinical findings and translate them to the clinic.