Aging-related alterations in mechanistic target of rapamycin signaling promote platelet hyperreactivity and thrombosis.

Background: Aging is an independent risk factor for the development of cardiovascular, thrombotic, and other chronic diseases. However, mechanisms of platelet hyperactivation in aging remain poorly understood.

Objectives: Here, we examine whether and how aging alters intracellular signaling in platelets to support platelet hyperactivity and thrombosis.

IFITM3 regulates fibrinogen endocytosis and platelet reactivity in nonviral sepsis.

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 determined whether nonviral sepsis induces differential platelet gene expression and reactivity.

MAPK-interacting kinase 1 regulates platelet production, activation, and thrombosis.

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 messenger RNA (mRNA) translation, and mRNA translation influences platelet production and function. However, the role of Mnk1 in megakaryocytes and platelets has not previously been studied.

COVID-19 generates hyaluronan fragments that directly induce endothelial barrier dysfunction.

Vascular injury has emerged as a complication contributing to morbidity in coronavirus disease 2019 (COVID-19). The glycosaminoglycan hyaluronan (HA) is a major component of the glycocalyx, a protective layer of glycoconjugates that lines the vascular lumen and regulates key endothelial cell functions. During critical illness, as in the case of sepsis, enzymes degrade the glycocalyx, releasing fragments with pathologic activities into circulation and thereby exacerbating disease.

Platelet MHC class I mediates CD8+ T-cell suppression during sepsis.

Circulating platelets interact with leukocytes to modulate host immune and thrombotic responses. In sepsis, platelet-leukocyte interactions are increased and have been associated with adverse clinical events, including increased platelet-T-cell interactions. Sepsis is associated with reduced CD8+ T-cell numbers and functional responses, but whether platelets regulate CD8+ T-cell responses during sepsis remains unknown.

Different glycoforms of alpha-1-acid glycoprotein contribute to its functional alterations in platelets and neutrophils.

Alpha-1-acid glycoprotein (AGP-1) is a positive acute phase glycoprotein with uncertain functions. Serum AGP-1 (sAGP-1) is primarily derived from hepatocytes and circulates as 12-20 different glycoforms. We isolated a glycoform secreted from platelet-activating factor (PAF)-stimulated human neutrophils (nAGP-1).

COVID-19 patients exhibit reduced procoagulant platelet responses.

Background: Emerging evidence implicates dysfunctional platelet responses in thrombotic complications in COVID-19 patients. Platelets are important players in inflammation-induced thrombosis. In particular, procoagulant platelets support thrombin generation and mediate thromboinflammation.

Neutrophil extracellular traps contribute to immunothrombosis in COVID-19 acute respiratory distress syndrome.

COVID-19 affects millions of patients worldwide, with clinical presentation ranging from isolated thrombosis to acute respiratory distress syndrome (ARDS) requiring ventilator support. Neutrophil extracellular traps (NETs) originate from decondensed chromatin released to immobilize pathogens, and they can trigger immunothrombosis. We studied the connection between NETs and COVID-19 severity and progression.

Platelet gene expression and function in patients with COVID-19.

There is an urgent need to understand the pathogenesis of coronavirus disease 2019 (COVID-19). In particular, thrombotic complications in patients with COVID-19 are common and contribute to organ failure and mortality. Patients with severe COVID-19 present with hemostatic abnormalities that mimic disseminated intravascular coagulopathy associated with sepsis, with the major difference being increased risk of thrombosis rather than bleeding.

Phospho-inositide-dependent kinase 1 regulates signal dependent translation in megakaryocytes and platelets.

Background: Regulated protein synthesis is essential for megakaryocyte (MK) and platelet functions, including platelet production and activation. PDK1 (phosphoinositide-dependent kinase 1) regulates platelet functional responses and has been associated with circulating platelet counts. Whether PDK1 also directly regulates protein synthetic responses in MKs and platelets, and platelet production by MKs, remains unknown.

Platelet necrosis mediates ischemic stroke outcome in mice.

Dysregulated platelet functions contribute to the development and progression of ischemic stroke. Utilizing mice with a platelet-specific deletion of cyclophilin D (CypD), a mediator of necrosis, we found that platelet necrosis regulates tissue damage and outcomes during ischemic stroke in vivo. Mice with loss of CypD in platelets (CypDplt-/-mice) exhibited significantly enhanced cerebral blood flow, improved neurological and motor functions, and reduced ischemic stroke infarct volume after cerebral ischemia-reperfusion injury.

TLT-1-CONTROLS EARLY THROMBUS FORMATION AND STABILITY BY FACILITATING AIIBB3 OUTSIDE-IN SIGNALING IN MICE.

Platelets regulate inflammation as well as hemostasis. Inflammatory insults often induce hemostatic function through mechanisms that are not always understood. The triggering receptor expressed in myeloid cells (TREM)-like transcript 1 (TLT-1) is an abundantly expressed platelet receptor and its deletion leads to hemorrhage and edema after lipopolysaccharide and TNF-α treatment.

Endogenous LINE-1 (Long Interspersed Nuclear Element-1) Reverse Transcriptase Activity in Platelets Controls Translational Events Through RNA-DNA Hybrids.

Objective: One source of endogenous reverse transcriptase (eRT) activity in nucleated cells is the LINE-1/L1 (long interspersed nuclear element-1), a non-LTR retrotransposon that is implicated in the regulation of gene expression. Nevertheless, the presence and function of eRT activity and LINE-1 in human platelets, an anucleate cell, has not previously been determined.

Approach And Results: We demonstrate that human and murine platelets possess robust eRT activity and identify the source as being LINE-1 ribonucleoprotein particles.

Clots Are Potent Triggers of Inflammatory Cell Gene Expression: Indications for Timely Fibrinolysis.

Objective: Blood vessel wall damage often results in the formation of a fibrin clot that traps inflammatory cells, including monocytes. The effect of clot formation and subsequent lysis on the expression of monocyte-derived genes involved in the development and progression of ischemic stroke and other vascular diseases, however, is unknown. Determine whether clot formation and lysis regulates the expression of human monocyte-derived genes that modulate vascular diseases.

G pathway regulates proximal C-type lectin-like receptor-2 (CLEC-2) signaling in platelets.

Platelets play a key role in the physiological hemostasis or pathological process of thrombosis. Rhodocytin, an agonist of the C-type lectin-like receptor-2 (CLEC-2), elicits powerful platelet activation signals in conjunction with Src family kinases (SFKs), spleen tyrosine kinase (Syk), and phospholipase γ2 (PLCγ2). Previous reports have shown that rhodocytin-induced platelet aggregation depends on secondary mediators such as thromboxane A2 (TxA2) and ADP, which are agonists for G-protein-coupled receptors (GPCRs) on platelets.

Identification of novel Syk-independent functional roles of FcγRIIa in platelet outside-in signaling using transgenic mice expressing human FcγRIIa.

Platelet outside-in signaling regulates important morphological changes such as cell spreading and retraction. A role for FcγRIIa in outside-in signaling has been recently described. Using murine platelets expressing human transgenic FcγRIIa we expand the evidence for the role of human FcγRIIa in platelet outside-in signaling.

Dicer1-mediated miRNA processing shapes the mRNA profile and function of murine platelets.

Human platelets contain microRNAs (miRNAs) and miRNA processing machinery, but their contribution to platelet function remains incompletely understood. Here, we show that murine megakaryocyte (MK)-specific knockdown of Dicer1, the ribonuclease that cleaves miRNA precursors into mature miRNAs, reduces the level of the majority of miRNAs in platelets. This leads to altered platelet messenger RNA (mRNA) expression profiles and mild thrombocytopenia.

Cross talk between serine/threonine and tyrosine kinases regulates ADP-induced thromboxane generation in platelets.

ADP-induced thromboxane generation depends on Src family kinases (SFKs) and is enhanced with pan-protein kinase C (PKC) inhibitors, but it is not clear how these two events are linked. The aim of the current study is to investigate the role of Y311 phosphorylated PKCδ in regulating ADP-induced platelet activation. In the current study, we employed various inhibitors and murine platelets from mice deficient in specific molecules to evaluate the role of PKCδ in ADP-induced platelet responses.

Distinct pathways regulate Syk protein activation downstream of immune tyrosine activation motif (ITAM) and hemITAM receptors in platelets.

Tyrosine kinase pathways are known to play an important role in the activation of platelets. In particular, the GPVI and CLEC-2 receptors are known to activate Syk upon tyrosine phosphorylation of an immune tyrosine activation motif (ITAM) and hemITAM, respectively. However, unlike GPVI, the CLEC-2 receptor contains only one tyrosine motif in the intracellular domain.

Characterization of UBO-QIC as a Gαq inhibitor in platelets.

Gαq plays an important role in platelet activation by agonists such as thrombin, adenosine diphosphate (ADP) and thromboxane. The significance of Gαq signaling in platelets was established using YM254890, a Gαq/11-specific inhibitor and Gαq knockout murine platelets. However, YM-254890 is no longer available for investigators and there is a need to characterize other Gαq inhibitors.

C-type lectin like receptor 2 (CLEC-2) signals independently of lipid raft microdomains in platelets.

C-type lectin like receptor 2 (CLEC-2) has been reported to activate platelets through a lipid raft-dependent manner. Secreted ADP potentiates CLEC-2-mediated platelet aggregation. We have investigated whether the decrease in CLEC-2-mediated platelet aggregation, previously reported in platelets with disrupted rafts, is a result of the loss of agonist potentiation by ADP.