Gray platelet syndrome and defective thrombo-inflammation in Nbeal2-deficient mice

Platelets: Orchestrators of immunity in host defense and beyond

Platelets prevent blood loss during vascular injury and contribute to thrombus formation in cardiovascular disease. Beyond these classical roles, platelets are critical for the host immune response. They guard the vasculature against pathogens via specialized receptors, intracellular signaling cascades, and effector functions. Platelets also skew inflammatory responses by instructing innate immune cells, support adaptive immunosurveillance, and influence antibody production and T cell polarization. Concomitantly, platelets contribute to tissue reconstitution and maintain vascular function after inflammatory challenges. However, dysregulated activation of these multitalented cells exacerbates immunopathology with ensuing microvascular clotting, excessive inflammation, and elevated risk of macrovascular thrombosis. This dichotomy underscores the critical importance of precisely defining and potentially modulating platelet function in immunity.

The Neurobeachin-like 2 protein (NBEAL2) controls the homeostatic level of the ribosomal protein RPS6 in mast cells

The Beige and Chediak-Higashi (BEACH) domain-containing, Neurobeachin-like 2 (NBEAL2) protein is a molecule with a molecular weight of 300 kDa. Inactivation of NBEAL2 by loss-of-function mutations in humans as well as deletion of the Nbeal2 gene in mice results in functional defects in cells of the innate immune system such as neutrophils, NK-cells, megakaryocytes, platelets and of mast cells (MCs). To investigate the detailed function of NBEAL2 in murine MCs we generated MCs from wild type (wt) and Nbeal2-/- mice, and deleted Nbeal2 by CRISPR/Cas9 technology in the murine mast cell line MC/9. We also predicted the structure of NBEAL2 to infer its function and to examine potential mechanisms for its association with interaction partners by using the deep learning-based method RoseTTAFold and the Pymol© software. The function of NBEAL2 was analysed by molecular and immunological techniques such as co-immunoprecipitation (co-IP) experiments, western blotting, enzyme-linked immunosorbent assay and flow cytometry. We identified RPS6 as an interaction partner of NBEAL2. Thereby, the NBEAL2/RPS6 complex formation is probably required to control the protein homeostasis of RPS6 in MCs. Consequently, inactivation of NBEAL2 leads to accumulation of strongly p90RSK-phosphorylated RPS6 molecules which results in the development of an abnormal MC phenotype characterised by prolonged growth factor-independent survival and in a pro-inflammatory MC-phenotype.

A role of platelet C-type lectin-like receptor-2 and its ligand podoplanin in vascular biology

PURPOSE OF REVIEW

Platelets are essential for hemostasis and are also vital in lymphatic and lung development and the maintenance of vascular integrity. Platelet activation receptor C-type lectin-like receptor 2 (CLEC-2) and its endogenous ligand podoplanin (PDPN) in lymphatic endothelial cells (LECs) and other cells regulate these processes. This review aims to comprehensively summarize the roles of platelet CLEC-2 and PDPN. This review also focuses on discussing the underlying mechanisms by which platelet CLEC-2 and PDPN mediate blood/lymphatic separation.

FINDINGS

CLEC-2/PDPN-induced platelet activation in the primary lymph sacs, developmental lymphovenous junctions, neonatal mesentery, and the site of tumor lymphangiogenesis prevents blood/lymphatic vessel misconnection. Further, CLEC-2/PDPN-induced platelet activation is essential for lung development. Mice deficient in CLEC-2 or PDPN show blood-filled lymphatics, lung malformations, and cerebrovascular abnormalities. CLEC-2 deletion in steady-state adult mice did not result in blood/lymphatic vessel mixing. In adulthood, CLEC-2 maintains vascular integrity and that of high endothelial venules in lymph nodes. CLEC-2 deletion in adulthood results in hemorrhage under inflammatory conditions, and hemolymph nodes.

SUMMARY

The platelet CLEC-2/LEC PDPN interaction prevents blood/lymphatic vessel mixing at active remodeling sites of the blood/lymphatic system, but not in steady-state adult mice. This interaction also regulates vascular integrity when vascular permeability increases before and after birth.

Immunothrombosis versus thrombo-inflammation: platelets in cerebrovascular complications

A State-of-the Art lecture titled "Thrombo-Neuroinflammatory Disease" was presented at the International Society on Thrombosis and Haemostasis Congress in 2023. First, we would like to advocate for discrimination between immunothrombosis and thrombo-inflammation, as immunothrombosis describes an overshooting inflammatory reaction that results in detrimental thrombotic activity. In contrast, thrombo-inflammation describes the interplay of platelets and coagulation with the immunovascular system, resulting in the recruitment of immune cells and loss of barrier function (hence, hallmarks of inflammation). Both processes can be observed in the brain, with cerebral venous thrombosis being a prime example of immunothrombosis, while infarct progression in response to ischemic stroke is a paradigmatic example of thrombo-inflammation. Here, we review the pathomechanisms underlying cerebral venous thrombosis and ischemic stroke from a platelet-centric perspective and discuss translational implications. Finally, we summarize relevant new data on this topic presented during the 2023 International Society on Thrombosis and Haemostasis Congress.

Intravital Imaging of Thrombosis Models in Mice

Intravital microscopy is a powerful tool to study thrombosis in real time. The kinetics of thrombus formation and progression in vivo is studied after inflicting damage to the endothelium through mechanical, chemical, or laser injury. Mouse models of atherosclerosis are also used to induce thrombus formation. Vessels of different sizes and from different vascular beds such as carotid artery or vena cava, mesenteric or cremaster arterioles, can be targeted. Using fluorescent dyes, antibodies, or reporter mouse strains allows to visualize key cells and factors mediating the thrombotic processes. Here, we review the latest literature on using intravital microscopy to study thrombosis as well as thromboinflammation following transient middle cerebral artery occlusion, infection-induced immunothrombosis, and liver ischemia reperfusion.

Impact of Hemostasis on the Lymphatic System in Development and Disease

Lymphatic vessels form a systemic network that maintains interstitial fluid homeostasis and regulates immune responses and is strictly separated from the circulatory system. During embryonic development, lymphatic endothelial cells originate from blood vascular endothelial cells in the cardinal veins and form lymph sacs. Platelets are critical for separating lymph sacs from the cardinal veins through interactions between CLEC-2 (C-type lectin-like receptor-2) and PDPN (podoplanin) in lymphatic endothelial cells. Therefore, deficiencies of these genes cause blood-filled lymphatic vessels, leading to abnormal lymphatic vessel maturation. The junction between the thoracic duct and the subclavian vein has valves and forms physiological thrombi dependent on CLEC-2/PDPN signaling to prevent blood backflow into the thoracic duct. In addition, platelets regulate lymphangiogenesis and maintain blood/lymphatic separation in pathological conditions, such as wound healing and inflammatory diseases. More recently, it was reported that the entire hemostatic system is involved in lymphangiogenesis. Thus, the hemostatic system plays a crucial role in the establishment, maintenance, and rearrangement of lymphatic networks and contributes to body fluid homeostasis, which suggests that the hemostatic system is a potential target for treating lymphatic disorders. This review comprehensively summarizes the role of the hemostatic system in lymphangiogenesis and lymphatic vessel function and discusses challenges and future perspectives.

Platelet glycoprotein V spatio-temporally controls fibrin formation

The activation of platelets and coagulation at vascular injury sites is crucial for haemostasis but can promote thrombosis and inflammation in vascular pathologies. Here, we delineate an unexpected spatio-temporal control mechanism of thrombin activity that is platelet orchestrated and locally limits excessive fibrin formation after initial haemostatic platelet deposition. During platelet activation, the abundant platelet glycoprotein (GP) V is cleaved by thrombin. We demonstrate with genetic and pharmacological approaches that thrombin-mediated shedding of GPV does not primarily regulate platelet activation in thrombus formation, but rather has a distinct function after platelet deposition and specifically limits thrombin-dependent generation of fibrin, a crucial mediator of vascular thrombo-inflammation. Genetic or pharmacologic defects in haemostatic platelet function are unexpectedly attenuated by specific blockade of GPV shedding, indicating that the spatio-temporal control of thrombin-dependent fibrin generation also represents a potential therapeutic target to improve haemostasis.

Platelets and ductus arteriosus closure in neonates

Platelet plug formation is critically involved in murine ductus arteriosus closure and thrombocytopenia in preterm infants seems to negatively affect spontaneous and pharmacologically induced ductal closure. Furthermore, platelet dysfunction may contribute to ductal patency, especially in extremely immature infants. Neonatal platelets likely have multifaceted roles during ductal closure, such as secretion of several signaling molecules and facilitation of specific cell-cell interactions. The only available randomized-controlled trial on platelet transfusions in preterm infants with patent ductus arteriosus demonstrated that a liberal transfusion regimen did not promote ductal closure, but was associated with an increased rate of intraventricular hemorrhage. Herein, we discuss the available mechanistic evidence on the role of platelets in ductus arteriosus closure and their potential clinical implications in preterm infants. We further briefly outline future research directions aimed at a better understanding of platelet-endothelial interactions in neonatal health and disease.

Mutations in Neurobeachin-like 2 do not impact Weibel-Palade body biogenesis and von Willebrand factor secretion in gray platelet syndrome Endothelial Colony Forming Cells

Background

Patients with gray platelet syndrome (GPS) and Neurobeachin-like 2 (NBEAL2) deficiency produce platelets lacking alpha-granules (AGs) and present with lifelong bleeding symptoms. AGs are lysosome-related organelles and store the hemostatic protein von Willebrand factor (VWF) and the transmembrane protein P-selectin. Weibel-Palade bodies (WPBs) are lysosome-related organelles of endothelial cells and also store VWF and P-selectin. In megakaryocytes, NBEAL2 links P-selectin on AGs to the SNARE protein SEC22B on the endoplasmic reticulum, thereby preventing premature release of cargo from AG precursors. In endothelial cells, SEC22B drives VWF trafficking from the endoplasmic reticulum to Golgi and promotes the formation of elongated WPBs, but it is unclear whether this requires NBEAL2.

Objectives

To investigate a potential role for NBEAL2 in WPB biogenesis and VWF secretion using NBEAL2-deficient endothelial cells.

Methods

The interaction of SEC22B with NBEAL2 in endothelial cells was investigated by interatomic mass spectrometry and pull-down analysis. Endothelial colony forming cells were isolated from healthy controls and 3 unrelated patients with GPS and mutations in NBEAL2.

Results

We showed that SEC22B binds to NBEAL2 in ECs. Endothelial colony forming cells derived from a patient with GPS are deficient in NBEAL2 but reveal normal formation and maturation of WPBs and normal WPB cargo recruitment. Neither basal nor histamine-induced VWF secretion is altered in the absence of NBEAL2.

Conclusions

Although NBEAL2 deficiency causes the absence of AGs in patients with GPS, it does not impact WPB functionality in ECs. Our data highlight the differences in the regulatory mechanisms between these 2 hemostatic storage compartments.

Procoagulant platelets: novel players in thromboinflammation

Platelets play a key role in maintaining hemostasis. However, dysregulated platelet activation can lead to pathological thrombosis or bleeding. Once a platelet gets activated, it will either become an aggregatory platelet or eventually a procoagulant platelet with both types playing distinct roles in thrombosis and hemostasis. Although aggregatory platelets have been extensively studied, procoagulant platelets have only recently come into the spotlight. Procoagulant platelets are a subpopulation of highly activated platelets that express phosphatidylserine and P-selectin on their surface, allowing for coagulation factors to bind and thrombin to be generated. In recent years, novel roles for procoagulant platelets have been identified and they have increasingly been implicated in thromboinflammatory diseases. Here, we provide an up-to-date review on the mechanisms resulting in the formation of procoagulant platelets and how they contribute to hemostasis, thrombosis, and thromboinflammation.

An intravascular perspective on hyper-acute neutrophil, T-cell and platelet responses: Similarities between human and experimental stroke

In stroke patients, local sampling of pial blood within the occluded vasculature before recanalization by mechanical thrombectomy emerged as powerful tool enabling insights into ultra-early stroke pathophysiology. Thereby, a strong intravascular inflammatory response hallmarked by hyper-acute neutrophil recruitment, altered lymphocyte composition and platelet activation could be observed. These human findings mirror experimental stroke. Here, neutrophil and T-cell activation are driven by platelets involving engagement of platelet glycoprotein receptor (GP)Ib, GPVI and CD84 as well as α-granule release orchestrating infarct progression. Thus, targeting of early intravascular inflammation may evolve as a new therapeutic strategy to augment the effects of recanalization.

Evolutionary Insight into Immunothrombosis as a Healing Mechanism

Both invertebrates and vertebrates possess a cluster of immediate and local wound-sealing, pathogen-killing, and tissue healing responses known as immunoclotting and immunothrombosis, respectively, to cope with two life-threatening emergencies, namely, bleeding and microbial invasion. Despite their convergence in function, immunoclotting and immunothrombosis are deployed by different blood cells and intravascular multidomain proteins. In vertebrates, these proteins share some domains with intrinsic chemical affinities useful in generating cooperative networks such as pathogen and damage pattern recognition molecules. Moreover, many of the proteins involved in coagulation and fibrinolysis in humans are multifunctional molecules playing roles in other processes from inflammation to healing and beyond. In our modern society, however, the interaction of activated intravascular allosteric proteins with one another and with blood cells entails vulnerabilities posing a biological paradox: intravascular proteins that locally operate as tissue repair enhancers can nevertheless generate pathogenic processes by acting systemically. In this manuscript, we contextualize and frame the coagulation system and hemostasis through an evolutionary time scale, illustrating their role as dual players in the defense against exsanguination and pathogens while significantly influencing wound healing.

Beyond Hemostasis: Platelet Innate Immune Interactions and Thromboinflammation

There is accumulating evidence that platelets play roles beyond their traditional functions in thrombosis and hemostasis, e.g., in inflammatory processes, infection and cancer, and that they interact, stimulate and regulate cells of the innate immune system such as neutrophils, monocytes and macrophages. In this review, we will focus on platelet activation in hemostatic and inflammatory processes, as well as platelet interactions with neutrophils and monocytes/macrophages. We take a closer look at the contributions of major platelet receptors GPIb, α_IIbβ₃, TLT-1, CLEC-2 and Toll-like receptors (TLRs) as well as secretions from platelet granules on platelet-neutrophil aggregate and neutrophil extracellular trap (NET) formation in atherosclerosis, transfusion-related acute lung injury (TRALI) and COVID-19. Further, we will address platelet-monocyte and macrophage interactions during cancer metastasis, infection, sepsis and platelet clearance.

The RNA-binding protein SRSF3 has an essential role in megakaryocyte maturation and platelet production

RNA processing is increasingly recognized as a critical control point in the regulation of different hematopoietic lineages including megakaryocytes responsible for the production of platelets. Platelets are anucleate cytoplasts that contain a rich repertoire of RNAs encoding proteins with essential platelet functions derived from the parent megakaryocyte. It is largely unknown how RNA binding proteins contribute to the development and functions of megakaryocytes and platelets. We show that serine-arginine-rich splicing factor 3 (SRSF3) is essential for megakaryocyte maturation and generation of functional platelets. Megakaryocyte-specific deletion of Srsf3 in mice led to macrothrombocytopenia characterized by megakaryocyte maturation arrest, dramatically reduced platelet counts, and abnormally large functionally compromised platelets. SRSF3 deficient megakaryocytes failed to reprogram their transcriptome during maturation and to load platelets with RNAs required for normal platelet function. SRSF3 depletion led to nuclear accumulation of megakaryocyte mRNAs, demonstrating that SRSF3 deploys similar RNA regulatory mechanisms in megakaryocytes as in other cell types. Our study further suggests that SRSF3 plays a role in sorting cytoplasmic megakaryocyte RNAs into platelets and demonstrates how SRSF3-mediated RNA processing forms a central part of megakaryocyte gene regulation. Understanding SRSF3 functions in megakaryocytes and platelets provides key insights into normal thrombopoiesis and platelet pathologies as SRSF3 RNA targets in megakaryocytes are associated with platelet diseases.

Transcriptomic Profiling of Mouse Mast Cells upon Pathogenic Avian H5N1 and Pandemic H1N1 Influenza A Virus Infection

Mast cells, widely residing in connective tissues and on mucosal surfaces, play significant roles in battling against influenza A viruses. To gain further insights into the host cellular responses of mouse mast cells with influenza A virus infection, such as the highly pathogenic avian influenza A virus H5N1 and the human pandemic influenza A H1N1, we employed high-throughput RNA sequencing to identify differentially expressed genes (DEGs) and related signaling pathways. Our data revealed that H1N1-infected mouse mast P815 cells presented more up- and down-regulated genes compared with H5N1-infected cells. Gene ontology analysis showed that the up-regulated genes in H1N1 infection were enriched for more degranulation-related cellular component terms and immune recognition-related molecular functions terms, while the up-regulated genes in H5N1 infection were enriched for more immune-response-related biological processes. Network enrichment of the KEGG pathway analysis showed that DEGs in H1N1 infection were specifically enriched for the FoxO and autophagy pathways. In contrast, DEGs in H5N1 infection were specifically enriched for the NF-κB and necroptosis pathways. Interestingly, we found that Nbeal2 could be preferentially activated in H5N1-infected P815 cells, where the level of Nbeal2 increased dramatically but decreased in HIN1-infected P815 cells. Nbeal2 knockdown facilitated inflammatory cytokine release in both H1N1- and H5N1-infected P815 cells and aggravated the apoptosis of pulmonary epithelial cells. In summary, our data described a transcriptomic profile and bioinformatic characterization of H1N-1 or H5N1-infected mast cells and, for the first time, established the crucial role of Nbeal2 during influenza A virus infection.

Polylysine for skin regeneration: A review of recent advances and future perspectives

There have been several attempts to find promising biomaterials for skin regeneration, among which polylysine (a homopolypeptide) has shown benefits in the regeneration and treatment of skin disorders. This class of biomaterials has shown exceptional abilities due to their macromolecular structure. Polylysine-based biomaterials can be used as tissue engineering scaffolds for skin regeneration, and as drug carriers or even gene delivery vectors for the treatment of skin diseases. In addition, polylysine can play a preservative role in extending the lifetime of skin tissue by minimizing the appearance of photodamaged skin. Research on polylysine is growing today, opening new scenarios that expand the potential of these biomaterials from traditional treatments to a new era of tissue regeneration. This review aims to address the basic concepts, recent trends, and prospects of polylysine-based biomaterials for skin regeneration. Undoubtedly, this class of biomaterials needs further evaluations and explorations, and many critical questions have yet to be answered.

The Role of Extracellular Matrix in Skin Wound Healing

Impaired wound healing is one of the unsolved problems of modern medicine, affecting patients’ quality of life and causing serious economic losses. Impaired wound healing can manifest itself in the form of chronic skin wounds or hypertrophic scars. Research on the biology and physiology of skin wound healing disorders is actively continuing, but, unfortunately, a single understanding has not been developed. The attention of clinicians to the biological and physiological aspects of wound healing in the skin is necessary for the search for new and effective methods of prevention and treatment of its consequences. In addition, it is important to update knowledge about genetic and non-genetic factors predisposing to impaired wound healing in order to identify risk levels and develop personalized strategies for managing such patients. Wound healing is a very complex process involving several overlapping stages and involving many factors. This thematic review focuses on the extracellular matrix of the skin, in particular its role in wound healing. The authors analyzed the results of fundamental research in recent years, finding promising potential for their transition into real clinical practice.

Healing through the lens of immunothrombosis: Biology-inspired, evolution-tailored, and human-engineered biomimetic therapies

Evolution, from invertebrates to mammals, has yielded and shaped immunoclotting as a defense and repair response against trauma and infection. This mosaic of immediate and local wound-sealing and pathogen-killing mechanisms results in survival, restoration of homeostasis, and tissue repair. In mammals, immunoclotting has been complemented with the neuroendocrine system, platelets, and contact system among other embellishments, adding layers of complexity through interconnecting blood-born proteolytic cascades, blood cells, and the neuroendocrine system. In doing so, immunothrombosis endows humans with survival advantages, but entails vulnerabilities in the current unprecedented and increasingly challenging environment. Immunothrombosis and tissue repair appear to go hand in hand with common mechanisms mediating both processes, a fact that is underlined by recent advances that are deciphering the mechanisms of the repair process and of the biochemical pathways that underpins coagulation, hemostasis and thrombosis. This review is intended to frame both the universal aspects of tissue repair and the therapeutic use of autologous fibrin matrix as a biology-as-a-drug approach in the context of the evolutionary changes in coagulation and hemostasis. In addition, we will try to shed some light on the molecular mechanisms underlying the use of the autologous fibrin matrix as a biology-inspired, evolution-tailored, and human-engineered biomimetic therapy.

Changes in Maternal Platelet Physiology during Gestation and Their Interaction with Trophoblasts

Upon activation, maternal platelets provide a source of proinflammatory mediators in the intervillous space of the placenta. Therefore, platelet-derived factors may interfere with different trophoblast subtypes of the developing human placenta and might cause altered hormone secretion and placental dysfunction later on in pregnancy. Increased platelet activation, and the subsequent occurrence of placental fibrinoid deposition, are linked to placenta pathologies such as preeclampsia. The composition and release of platelet-derived factors change over gestation and provide a potential source of predicting biomarkers for the developing fetus and the mother. This review indicates possible mechanisms of platelet-trophoblast interactions and discusses the effect of increased platelet activation on placenta development.

Release of α-granule contents during platelet activation

Upon activation, platelets release a plethora of factors which help to mediate their dynamic functions in hemostasis, inflammation, wound healing, tumor metastasis and angiogenesis. The majority of these bioactive molecules are released from α-granules, which are unique to platelets, and contain an incredibly diverse repertoire of cargo including; integral membrane proteins, pro-coagulant molecules, chemokines, mitogenic, growth and angiogenic factors, adhesion proteins, and microbicidal proteins. Clinically, activation of circulating platelets has increasingly been associated with various disease states. Biomarkers indicating the level of platelet activation in patients can therefore be useful tools to evaluate risk factors to predict future complications and determine treatment strategies or evaluate antiplatelet therapy. The irreversible nature of α-granule secretion makes it ideally suited as a marker of platelet activation. This review outlines the release and contents of platelet α-granules, as well as the membrane bound, and soluble α-granule cargo proteins that can be used as biomarkers of platelet activation.

Platelet Activation and Chemokine Release Are Related to Local Neutrophil-Dominant Inflammation During Hyperacute Human Stroke

Experimental evidence has emerged that local platelet activation contributes to inflammation and infarct formation in acute ischemic stroke (AIS) which awaits confirmation in human studies. We conducted a prospective observational study on 258 consecutive patients undergoing mechanical thrombectomy (MT) due to large-vessel-occlusion stroke of the anterior circulation (08/2018–05/2020). Intraprocedural microcatheter aspiration of 1 ml of local (occlusion condition) and systemic arterial blood samples (self-control) was performed according to a prespecified protocol. The samples were analyzed for differential leukocyte counts, platelet counts, and plasma levels of the platelet-derived neutrophil-activating chemokine C-X-C-motif ligand (CXCL) 4 (PF-4), the neutrophil attractant CXCL7 (NAP-2), and myeloperoxidase (MPO). The clinical-biological relevance of these variables was corroborated by specific associations with molecular-cellular, structural-radiological, hemodynamic, and clinical-functional parameters. Seventy consecutive patients fulfilling all predefined criteria entered analysis. Mean local CXCL4 (+ 39%: 571 vs 410 ng/ml, P = .0095) and CXCL7 (+ 9%: 693 vs 636 ng/ml, P = .013) concentrations were higher compared with self-controls. Local platelet counts were lower (− 10%: 347,582 vs 383,284/µl, P = .0052), whereas neutrophil counts were elevated (+ 10%: 6022 vs 5485/µl, P = 0.0027). Correlation analyses revealed associations between local platelet and neutrophil counts (r = 0.27, P = .034), and between CXCL7 and MPO (r = 0.24, P = .048). Local CXCL4 was associated with the angiographic degree of reperfusion following recanalization (r =  − 0.2523, P = .0479). Functional outcome at discharge correlated with local MPO concentrations (r = 0.3832, P = .0014) and platelet counts (r = 0.288, P = .0181). This study provides human evidence of cerebral platelet activation and platelet-neutrophil interactions during AIS and points to the relevance of per-ischemic thrombo-inflammatory mechanisms to impaired reperfusion and worse functional outcome following recanalization.

A Deep Dive into the Pathology of Gray Platelet Syndrome: New Insights on Immune Dysregulation

The gray platelet syndrome (GPS) is a rare platelet disorder, characterized by impaired alpha-granule biogenesis in megakaryocytes and platelets due to NBEAL2 mutations. Typical clinical features include macrothrombocytopenia, bleeding and elevated vitamin B12 levels, while bone marrow fibrosis and splenomegaly may develop during disease progression. Recently, the involvement of other blood lineages has been highlighted, revealing the role of NBEAL2 outside the megakaryocyte-platelet axis. Low leukocyte counts, decreased neutrophil granulation and impaired neutrophil extracellular trap formation represent prominent findings in GPS patients, reflecting deranged innate immunity and associated with an increased susceptibility to infection. In addition, low numbers and impaired degranulation of NK cells have been demonstrated in animal models. Autoimmune diseases involving different organs and a spectrum of autoantibodies are present in a substantial proportion of GPS patients, expanding the syndromic spectrum of this disorder and pointing to dysregulation of the adaptive immune response. Low-grade inflammation, as evidenced by elevation of liver-derived acute-phase reactants, is another previously unrecognized feature of GPS which may contribute to disease manifestations. This review will focus on the mechanisms underlying the pathogenesis of blood cell abnormalities in human GPS patients and NBEAL2-null animal models, providing insight into the effects of NBEAL2 in hemostasis, inflammation and immunity.

Gray Platelet Syndrome Presenting With Pancytopenia, Splenomegaly, and Bone Marrow Fibrosis

OBJECTIVES

Gray platelet syndrome (GPS) is a rare platelet storage pool disorder associated with a marked decrease or absence of platelet α-granules and their contents. It is characterized clinically by mild to moderate bleeding; moderate macrothrombocytopenia with large, agranular platelets; splenomegaly; and bone marrow fibrosis. Electron microscopy confirms markedly reduced or absent α-granules in platelets and megakaryocytes. The classic description of GPS is caused by homozygous mutations in NBEAL2 (neurobeachinlike 2).

METHODS

A 28-year-old Hispanic man with a history of easy bruising and occasional episodes of epistaxis sought treatment for pancytopenia and splenomegaly. Peripheral blood smear and bone marrow analysis, electron microscopy, and next-generation sequencing were performed.

RESULTS

Large and agranular platelets were present in the peripheral blood. There was bone marrow fibrosis. Electron microscopy of the platelets showed absence of α-granules. Next-generation sequencing revealed a germline apparently homozygous nonsense variant in the NBEAL2 gene: c.5674C>T, p.Gln1892X (p.Q1829X).

CONCLUSIONS

The differential diagnosis of GPS includes a myeloid neoplasm such as myelodysplastic syndrome with bone marrow fibrosis. The availability of diagnostic genetic panels for hereditable platelet disorders can assist in the recognition of GPS and other platelet disorders. We also describe a previously unreported pathogenic germline homozygous nonsense variant in the NBEAL2 gene: c.5674C>T, p.Gln1892X (p.Q1829X) in a patient with GPS.

Platelets as drivers of ischemia/reperfusion injury after stroke

Ischemic stroke is a leading cause of morbidity and mortality worldwide and, despite reperfusion either via thrombolysis or thrombectomy, stroke patients often suffer from lifelong disabilities. These persistent neurological deficits may be improved by treating the ischemia/reperfusion (I/R) injury that occurs following ischemic stroke. There are currently no approved therapies to treat I/R injury, and thus it is imperative to find new targets to decrease the burden of ischemic stroke and related diseases. Platelets, cell fragments from megakaryocytes, are primarily known for their role in hemostasis. More recently, investigators have studied the nonhemostatic role of platelets in inflammatory pathologies, such as I/R injury after ischemic stroke. In this review, we seek to provide an overview of how I/R can lead to platelet activation and how activated platelets, in turn, can exacerbate I/R injury after stroke. We will also discuss potential mechanisms by which platelets may ameliorate I/R injury.

Esomeprazole attenuates inflammatory and fibrotic response in lung cells through the MAPK/Nrf2/HO1 pathway

Introduction

Idiopathic pulmonary fibrosis (IPF) is an orphan disease characterized by progressive loss of lung function resulting in shortness of breath and often death within 3–4 years of diagnosis. Repetitive lung injury in susceptible individuals is believed to promote chronic oxidative stress, inflammation, and uncontrolled collagen deposition. Several preclinical and retrospective clinical studies in IPF have reported beneficial outcomes associated with the use of proton pump inhibitors (PPIs) such as esomeprazole. Accordingly, we sought to investigate molecular mechanism(s) by which PPIs favorably regulate the disease process.

Methods

We stimulated oxidative stress, pro-inflammatory and profibrotic phenotypes in primary human lung epithelial cells and fibroblasts upon treatment with bleomycin or transforming growth factor β (TGFβ) and assessed the effect of a prototype PPI, esomeprazole, in regulating these processes.

Results

Our study shows that esomeprazole controls pro-inflammatory and profibrotic molecules through nuclear translocation of the transcription factor nuclear factor-like 2 (Nrf2) and induction of the cytoprotective molecule heme oxygenase 1 (HO1). Genetic deletion of Nrf2 or pharmacological inhibition of HO1 impaired esomeprazole-mediated regulation of proinflammatory and profibrotic molecules. Additional studies indicate that activation of Mitogen Activated Protein Kinase (MAPK) pathway is involved in the process. Our experimental data was corroborated by bioinformatics studies of an NIH chemical library which hosts gene expression profiles of IPF lung fibroblasts treated with over 20,000 compounds including esomeprazole. Intriguingly, we found 45 genes that are upregulated in IPF but downregulated by esomeprazole. Pathway analysis showed that these genes are enriched for profibrotic processes. Unbiased high throughput RNA-seq study supported antifibrotic effect of esomeprazole and revealed several novel targets.

Conclusions

Taken together, PPIs may play antifibrotic role in IPF through direct regulation of the MAPK/Nrf2/HO1 pathway to favorably influence the disease process in IPF.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12950-021-00284-6.

Platelet α-granules contribute to organ-specific pathologies in a mouse model of severe malaria

Nbeal2 deficiency leads to significantly reduced lung and brain pathology and enhanced survival in a mouse model of malaria. Both antibody-dependent and antibody-independent platelet depletion in mice recapitulate the findings observed in Nbeal2−/− mice.

Primordial GATA6 macrophages function as extravascular platelets in sterile injury

Most multicellular organisms have a major body cavity that harbors immune cells. In primordial species such as purple sea urchins, these cells perform phagocytic functions but are also crucial in repairing injuries. In mammals, the peritoneal cavity contains large numbers of resident GATA6⁺ macrophages, which may function similarly. However, it is unclear how cavity macrophages suspended in the fluid phase (peritoneal fluid) identify and migrate toward injuries. In this study, we used intravital microscopy to show that cavity macrophages in fluid rapidly form thrombus-like structures in response to injury by means of primordial scavenger receptor cysteine-rich domains. Aggregates of cavity macrophages physically sealed injuries and promoted rapid repair of focal lesions. In iatrogenic surgical situations, these cavity macrophages formed extensive aggregates that promoted the growth of intra-abdominal scar tissue known as peritoneal adhesions.

The endoplasmic reticulum protein SEC22B interacts with NBEAL2 and is required for megakaryocyte α-granule biogenesis

Studies of inherited platelet disorders have provided many insights into platelet development and function. Loss of function of neurobeachin-like 2 (NBEAL2) causes gray platelet syndrome (GPS), where the absence of platelet α-granules indicates NBEAL2 is required for their production by precursor megakaryocytes. The endoplasmic reticulum is a dynamic network that interacts with numerous intracellular vesicles and organelles and plays key roles in their development. The megakaryocyte endoplasmic reticulum is extensive, and in this study we investigated its role in the biogenesis of α-granules by focusing on the membrane-resident trafficking protein SEC22B. Coimmunoprecipitation (co-IP) experiments using tagged proteins expressed in human HEK293 and megakaryocytic immortalized megakaryocyte progenitor (imMKCL) cells established binding of NBEAL2 with SEC22B, and demonstrated that NBEAL2 can simultaneously bind SEC22B and P-selectin. NBEAL2-SEC22B binding was also observed for endogenous proteins in human megakaryocytes using co-IP, and immunofluorescence microscopy detected substantial overlap. SEC22B binding was localized to a region of NBEAL2 spanning amino acids 1798 to 1903, where 2 GPS-associated missense variants have been reported: E1833K and R1839C. NBEAL2 containing either variant did not bind SEC22B coexpressed in HEK293 cells. CRISPR/Cas9-mediated knockout of SEC22B in imMKCL cells resulted in decreased NBEAL2, but not vice versa. Loss of either SEC22B or NBEAL2 expression resulted in failure of α-granule production and reduced granule proteins in imMKCL cells. We conclude that SEC22B is required for α-granule biogenesis in megakaryocytes, and that interactions with SEC22B and P-selectin facilitate the essential role of NBEAL2 in granule development and cargo stability.

Thrombocytopathies: Not Just Aggregation Defects-The Clinical Relevance of Procoagulant Platelets

Platelets are active key players in haemostasis. Qualitative platelet dysfunctions result in thrombocytopathies variously characterized by defects of their adhesive and procoagulant activation endpoints. In this review, we summarize the traditional platelet defects in adhesion, secretion, and aggregation. In addition, we review the current knowledge about procoagulant platelets, focusing on their role in bleeding or thrombotic pathologies and their pharmaceutical modulation. Procoagulant activity is an important feature of platelet activation, which should be specifically evaluated during the investigation of a suspected thrombocytopathy.

Platelets and lymphocytes drive progressive penumbral tissue loss during middle cerebral artery occlusion in mice

Background

In acute ischemic stroke, cessation of blood flow causes immediate tissue necrosis within the center of the ischemic brain region accompanied by functional failure in the surrounding brain tissue designated the penumbra. The penumbra can be salvaged by timely thrombolysis/thrombectomy, the only available acute stroke treatment to date, but is progressively destroyed by the expansion of infarction. The underlying mechanisms of progressive infarction are not fully understood.

Methods

To address mechanisms, mice underwent filament occlusion of the middle cerebral artery (MCAO) for up to 4 h. Infarct development was compared between mice treated with antigen-binding fragments (Fab) against the platelet surface molecules GPIb (p0p/B Fab) or rat immunoglobulin G (IgG) Fab as control treatment. Moreover, Rag1^−/− mice lacking T-cells underwent the same procedures. Infarct volumes as well as the local inflammatory response were determined during vessel occlusion.

Results

We show that blocking of the platelet adhesion receptor, glycoprotein (GP) Ibα in mice, delays cerebral infarct progression already during occlusion and thus before recanalization/reperfusion. This therapeutic effect was accompanied by decreased T-cell infiltration, particularly at the infarct border zone, which during occlusion is supplied by collateral blood flow. Accordingly, mice lacking T-cells were likewise protected from infarct progression under occlusion.

Conclusions

Progressive brain infarction can be delayed by blocking detrimental lymphocyte/platelet responses already during occlusion paving the way for ultra-early treatment strategies in hyper-acute stroke before recanalization.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-021-02095-1.

Variation in platelet-rich plasma compositions used for wound healing indications

At present, the role of platelet-rich plasma (PRP) in burn and wound management is undefined. We present some of the evidence for PRP in wound healing and other medical fields. Currently, the high variation in product composition, mode and timing of application prevent a clear definition of the position of PRP in wound healing. Perspectives on solving these issues are described.

Novel manifestations of immune dysregulation and granule defects in gray platelet syndrome

Gray platelet syndrome (GPS) is a rare recessive disorder caused by biallelic variants in NBEAL2 and characterized by bleeding symptoms, the absence of platelet α-granules, splenomegaly, and bone marrow (BM) fibrosis. Due to the rarity of GPS, it has been difficult to fully understand the pathogenic processes that lead to these clinical sequelae. To discern the spectrum of pathologic features, we performed a detailed clinical genotypic and phenotypic study of 47 patients with GPS and identified 32 new etiologic variants in NBEAL2. The GPS patient cohort exhibited known phenotypes, including macrothrombocytopenia, BM fibrosis, megakaryocyte emperipolesis of neutrophils, splenomegaly, and elevated serum vitamin B12 levels. Novel clinical phenotypes were also observed, including reduced leukocyte counts and increased presence of autoimmune disease and positive autoantibodies. There were widespread differences in the transcriptome and proteome of GPS platelets, neutrophils, monocytes, and CD4 lymphocytes. Proteins less abundant in these cells were enriched for constituents of granules, supporting a role for Nbeal2 in the function of these organelles across a wide range of blood cells. Proteomic analysis of GPS plasma showed increased levels of proteins associated with inflammation and immune response. One-quarter of plasma proteins increased in GPS are known to be synthesized outside of hematopoietic cells, predominantly in the liver. In summary, our data show that, in addition to the well-described platelet defects in GPS, there are immune defects. The abnormal immune cells may be the drivers of systemic abnormalities such as autoimmune disease.

Platelets as Mediators of Neuroinflammation and Thrombosis

Beyond platelets function in hemostasis, there is emerging evidence to suggest that platelets contribute crucially to inflammation and immune responses. Therefore, considering the detrimental role of inflammatory conditions in severe neurological disorders such as multiple sclerosis or stroke, this review outlines platelets involvement in neuroinflammation. For this, distinct mechanisms of platelet-mediated thrombosis and inflammation are portrayed, focusing on the interaction of platelet receptors with other immune cells as well as brain endothelial cells. Furthermore, we draw attention to the intimate interplay between platelets and the complement system as well as between platelets and plasmatic coagulation factors in the course of neuroinflammation. Following the thorough exposition of preclinical approaches which aim at ameliorating disease severity after inducing experimental autoimmune encephalomyelitis (a counterpart of multiple sclerosis in mice) or brain ischemia-reperfusion injury, the clinical relevance of platelet-mediated neuroinflammation is addressed. Thus, current as well as future propitious translational and clinical strategies for the treatment of neuro-inflammatory diseases by affecting platelet function are illustrated, emphasizing that targeting platelet-mediated neuroinflammation could become an efficient adjunct therapy to mitigate disease severity of multiple sclerosis or stroke associated brain injury.

Dominant negative Gfi1b mutations cause moderate thrombocytopenia and an impaired stress thrombopoiesis associated with mild erythropoietic abnormalities in mice

GFI1B-related thrombocytopenia (GFI1B-RT) is a rare bleeding disorder mainly caused by the presence of truncated GFI1B proteins with dominant-negative properties. The disease is characterized by low platelet counts, the presence of abnormal platelets, a megakaryocytic expansion and mild erythroid defects. However, no animal models faithfully reproducing the GFI1B-RT phenotype observed in patients exist. We had previously generated mice with floxed Gfi1b alleles that can be eliminated by Cre recombinase, but those animals developed a much more severe phenotype than GFI1B-RT patients and were of limited interest in assessing the disease. Using CRISPR/Cas9 technology, we have now established three independent mouse lines that carry mutated Gfi1b alleles producing proteins lacking DNA binding zinc fingers and thereby acting in a dominant negative (DN) manner. Mice heterozygous for these Gfi1b-DN alleles show reduced platelet counts and an expansion of megakaryocytes similar to features of human GFI1B-RT but lacking the distinctively large agranular platelets. In addition, Gfi1b-DN mice exhibit an expansion of erythroid precursors indicative of a mildly abnormal erythropoiesis but without noticeable red blood cell defects. When associated with megakaryocyte-specific ablation of the remaining allele, the Gfi1b-DN alleles triggered erythroid-specific deleterious defects. Gfi1b-DN mice also showed a delayed recovery from platelet depletion, indicating a defect in stress thrombopoiesis. However, injecting Gfi1b-DN mice with romiplostim, a thrombopoietin receptor super agonist, increased platelet numbers even beyond normal levels. Thus, our data support a causal link between DN mutations in GFI1B and thrombocytopenia and suggest that patients with GFI1B-RT could be treated successfully with thrombopoietin agonists.

Role of Kv1.3 Channels in Platelet Functions and Thrombus Formation

Objective:

Platelet activation by stimulatory factors leads to an increase in intracellular calcium concentration ([Ca 2+ ] i ), which is essential for almost all platelet functions. Modulation of Ca 2+ influx and [Ca 2+ ] i in platelets has been emerging as a possible strategy for preventing and treating platelet-dependent thrombosis. Voltage-gated potassium 1.3 channels (Kv1.3) are highly expressed in platelets and able to regulate agonist-evoked [Ca 2+ ] i increase. However, the role of Kv1.3 channels in regulating platelet functions and thrombosis has not yet been elucidated. In addition, it is difficult to obtain a specific blocker for this channel, since Kv1.3 shares identical drug-binding sites with other K + channels. Here, we investigate whether specific blockade of Kv1.3 channels by monoclonal antibodies affects platelet functions and thrombosis.

Approach and Results:

In this study, we produced the anti-Kv1.3 monoclonal antibody 6E12#15, which could specifically recognize both human and mouse Kv1.3 proteins and sufficiently block Kv1.3 channel currents. We found Kv1.3 blockade by 6E12#15 inhibited platelet aggregation, adhesion, and activation upon agonist stimulation. In vivo treatment with 6E12#15 alleviated thrombus formation in a mesenteric arteriole thrombosis mouse model and protected mice from collagen/epinephrine-induced pulmonary thromboembolism. Furthermore, we observed Kv1.3 regulated platelet functions by modulating Ca 2+ influx and [Ca 2+ ] i elevation, and that this is mediated in part by P2X 1 . Interestingly, Kv1.3 −/− mice showed impaired platelet aggregation while displayed no abnormalities in in vivo thrombus formation. This phenomenon was related to more megakaryocytes and platelets produced in Kv1.3 −/− mice compared with wild-type mice.

Conclusions:

We showed specific inhibition of Kv1.3 by the novel monoclonal antibody 6E12#15 suppressed platelet functions and platelet-dependent thrombosis through modulating platelet [Ca 2+ ] i elevation. These results indicate that Kv1.3 could act as a promising therapeutic target for antiplatelet therapies.

Hemostasis vs. homeostasis: Platelets are essential for preserving vascular barrier function in the absence of injury or inflammation

Platelets are best known for their vasoprotective responses to injury and inflammation. Here, we have asked whether they also support vascular integrity when neither injury nor inflammation is present. Changes in vascular barrier function in dermal and meningeal vessels were measured in real time in mouse models using the differential extravasation of fluorescent tracers as a biomarker. Severe thrombocytopenia produced by two distinct methods caused increased extravasation of 40-kDa dextran from capillaries and postcapillary venules but had no effect on extravasation of 70-kDa dextran or albumin. This reduction in barrier function required more than 4 h to emerge after thrombocytopenia was established, reverting to normal as the platelet count recovered. Barrier dysfunction was also observed in mice that lacked platelet-dense granules, dense granule secretion machinery, glycoprotein (GP) VI, or the GPVI signaling effector phospholipase C (PLC) γ2. It did not occur in mice lacking α-granules, C type lectin receptor-2 (CLEC-2), or protease activated receptor 4 (PAR4). Notably, although both meningeal and dermal vessels were affected, intracerebral vessels, which are known for their tighter junctions between endothelial cells, were not. Collectively, these observations 1) highlight a role for platelets in maintaining vascular homeostasis in the absence of injury or inflammation, 2) provide a sensitive biomarker for detecting changes in platelet-dependent barrier function, 3) identify which platelet processes are required, and 4) suggest that the absence of competent platelets causes changes in the vessel wall itself, accounting for the time required for dysfunction to emerge.

Platelet α-granules are required for occlusive high-shear-rate thrombosis

von Willebrand factor (VWF) is essential for the induction of arterial thrombosis. In this study, we investigated the critical role of platelet VWF in occlusive thrombosis formation at high shear in mice that do not express platelet VWF (Nbeal2-/-). Using in silico modeling, in vitro high-shear microfluidics, and an in vivo Folts model of arterial thrombosis we reproduced the platelet dynamics that occur under pathological flow in a stenosed vessel. Computational fluid dynamics (CFDs) simulated local hemodynamics in a stenosis based on arterial geometries. The model predicted shear rates, time course of platelet adhesion, and time to occlusion. These predictions were validated in vitro and in vivo. Occlusive thrombosis developed in wild-type control mice that had normal levels of plasma VWF and platelet VWF in vitro and in vivo. Occlusive thrombosis did not form in the Nbeal2-/- mice that had normal plasma VWF and an absence of platelet VWF. Occlusive thrombosis was corrected in Nbeal2-/- microfluidic assays by the addition of exogenous normal platelets with VWF. Combining model and experimental data, we demonstrated the necessary requirement of platelet VWF in α-granules in forming an occlusive thrombus under high shear. These results could inspire new pharmacological targets specific to pathological conditions and prevent arterial thrombosis.

The road to lysosome-related organelles: Insights from Hermansky-Pudlak syndrome and other rare diseases

Lysosome-related organelles (LROs) comprise a diverse group of cell type-specific, membrane-bound subcellular organelles that derive at least in part from the endolysosomal system but that have unique contents, morphologies and functions to support specific physiological roles. They include: melanosomes that provide pigment to our eyes and skin; alpha and dense granules in platelets, and lytic granules in cytotoxic T cells and natural killer cells, which release effectors to regulate hemostasis and immunity; and distinct classes of lamellar bodies in lung epithelial cells and keratinocytes that support lung plasticity and skin lubrication. The formation, maturation and/or secretion of subsets of LROs are dysfunctional or entirely absent in a number of hereditary syndromic disorders, including in particular the Hermansky-Pudlak syndromes. This review provides a comprehensive overview of LROs in humans and model organisms and presents our current understanding of how the products of genes that are defective in heritable diseases impact their formation, motility and ultimate secretion.

Inherited thrombocytopenias: history, advances and perspectives

Over the last 100 years the role of platelets in hemostatic events and their production by megakaryocytes have gradually been defined. Progressively, thrombocytopenia was recognized as a cause of bleeding, first through an acquired immune disorder; then, since 1948, when Bernard-Soulier syndrome was first described, inherited thrombocytopenia became a fascinating example of Mendelian disease. The platelet count is often severely decreased and platelet size variable; associated platelet function defects frequently aggravate bleeding. Macrothrombocytopenia with variable proportions of enlarged platelets is common. The number of circulating platelets will depend on platelet production, consumption and lifespan. The bulk of macrothrombocytopenias arise from defects in megakaryopoiesis with causal variants in transcription factor genes giving rise to altered stem cell differentiation and changes in early megakaryocyte development and maturation. Genes encoding surface receptors, cytoskeletal and signaling proteins also feature prominently and Sanger sequencing associated with careful phenotyping has allowed their early classification. It quickly became apparent that many inherited thrombocytopenias are syndromic while others are linked to an increased risk of hematologic malignancies. In the last decade, the application of next-generation sequencing, including whole exome sequencing, and the use of gene platforms for rapid testing have greatly accelerated the discovery of causal genes and extended the list of variants in more common disorders. Genes linked to an increased platelet turnover and apoptosis have also been identified. The current challenges are now to use next-generation sequencing in first-step screening and to define bleeding risk and treatment better.

Zebrafish for thrombocytopoiesis- and hemostasis-related researches and disorders

Platelets play vital roles in hemostasis, inflammation, and vascular biology. Platelets are also active participants in the immune responses. As vertebrates, zebrafish have a highly conserved hematopoietic system in the developmental, cellular, functional, biochemical, and genetic levels with mammals. Thrombocytes in zebrafish are functional homologs of mammalian platelets. Here, we summarized thrombocyte development, function, and related research techniques in zebrafish, and reviewed available zebrafish models of platelet-associated disorders, including congenital amegakaryocytic thrombocytopenia, inherited thrombocytopenia, essential thrombocythemia, and blood coagulation disorders such as gray platelet syndrome. These elegant zebrafish models and methods are crucial for understanding the molecular and genetic mechanisms of thrombocyte development and function, and provide deep insights into related human disease pathophysiology and drug development.

Orchestration of Primary Hemostasis by Platelet and Endothelial Lysosome-Related Organelles

Megakaryocyte-derived platelets and endothelial cells store their hemostatic cargo in α- and δ-granules and Weibel-Palade bodies, respectively. These storage granules belong to the lysosome-related organelles (LROs), a heterogeneous group of organelles that are rapidly released following agonist-induced triggering of intracellular signaling pathways. Following vascular injury, endothelial Weibel-Palade bodies release their content into the vascular lumen and promote the formation of long VWF (von Willebrand factor) strings that form an adhesive platform for platelets. Binding to VWF strings as well as exposed subendothelial collagen activates platelets resulting in the release of α- and δ-granules, which are crucial events in formation of a primary hemostatic plug. Biogenesis and secretion of these LROs are pivotal for the maintenance of proper hemostasis. Several bleeding disorders have been linked to abnormal generation of LROs in megakaryocytes and endothelial cells. Recent reviews have emphasized common pathways in the biogenesis and biological properties of LROs, focusing mainly on melanosomes. Despite many similarities, LROs in platelet and endothelial cells clearly possess distinct properties that allow them to provide a highly coordinated and synergistic contribution to primary hemostasis by sequentially releasing hemostatic cargo. In this brief review, we discuss in depth the known regulators of α- and δ-granules in megakaryocytes/platelets and Weibel-Palade bodies in endothelial cells, starting from transcription factors that have been associated with granule formation to protein complexes that promote granule maturation. In addition, we provide a detailed view on the interplay between platelet and endothelial LROs in controlling hemostasis as well as their dysfunction in LRO related bleeding disorders.

Red blood cell-derived semaphorin 7A promotes thrombo-inflammation in myocardial ischemia-reperfusion injury through platelet GPIb

Myocardial ischemia is one of the leading health problems worldwide. Therapy consists of the restitution of coronary perfusion which is followed by myocardial inflammation. Platelet–neutrophil interaction is a crucial process during inflammation, yet its consequences are not fully understood. Here, we show that platelet–neutrophil complexes (PNCs) are increased in patients with acute myocardial infarction and that this is associated with increased levels of neuronal guidance protein semaphorin 7A (SEMA7A). To investigate this further, we injected WT animals with Sema7a and found increased infarct size with increased numbers of PNCs. Experiments in genetically modified animals identify Sema7a on red blood cells to be crucial for this condition. Further studies revealed that Sema7a interacts with the platelet receptor glycoprotein Ib (GPIb). Treatment with anti-Sema7a antibody protected from myocardial tissue injury. In summary, we show that Sema7a binds to platelet GPIb and enhances platelet thrombo-inflammatory activity, aggravating post-ischemic myocardial tissue injury.

Reperfusion injury following myocardial ischemia is aggravated by inflammation and platelet–neutrophil complex formation. Here the authors show that semaphorin 7A binds to platelet GPIb, enhancing platelet–neutrophil interaction and increasing post-ischemic myocardial tissue injury, and that blockage of semaphorin 7A is protective.

Platelet biology and functions: new concepts and clinical perspectives

Platelets - blood cells continuously produced from megakaryocytes mainly in the bone marrow - are implicated not only in haemostasis and arterial thrombosis, but also in other physiological and pathophysiological processes. This Review describes current evidence for the heterogeneity in platelet structure, age, and activation properties, with consequences for a diversity of platelet functions. Signalling processes of platelet populations involved in thrombus formation with ongoing coagulation are well understood. Genetic approaches have provided information on multiple genes related to normal haemostasis, such as those encoding receptors and signalling or secretory proteins, that determine platelet count and/or responsiveness. As highly responsive and secretory cells, platelets can alter the environment through the release of growth factors, chemokines, coagulant factors, RNA species, and extracellular vesicles. Conversely, platelets will also adapt to their environment. In disease states, platelets can be positively primed to reach a pre-activated condition. At the inflamed vessel wall, platelets interact with leukocytes and the coagulation system, interactions mediating thromboinflammation. With current antiplatelet therapies invariably causing bleeding as an undesired adverse effect, novel therapies can be more beneficial if directed against specific platelet responses, populations, interactions, or priming conditions. On the basis of these novel concepts and processes, we discuss several initiatives to target platelets therapeutically.

Megakaryocyte emperipolesis: a new frontier in cell-in-cell interaction

Histology of bone marrow routinely identifies megakaryocytes that enclose neutrophils and other hematopoietic cells, a phenomenon termed emperipolesis. Preserved across mammalian species and enhanced with systemic inflammation and platelet demand, the nature and significance of emperipolesis remain largely unexplored. Recent advances demonstrate that emperipolesis is in fact a distinct form of cell-in-cell interaction. Following integrin-mediated attachment, megakaryocytes and neutrophils both actively drive entry via cytoskeletal rearrangement. Neutrophils enter a vacuole termed the emperisome which then releases them directly into the megakaryocyte cytoplasm. From this surprising location, neutrophils fuse with the demarcation membrane system to pass membrane to circulating platelets, enhancing the efficiency of thrombocytogenesis. Neutrophils then egress intact, carrying megakaryocyte membrane and potentially other cell components along with them. In this review, we summarize what is known about this intriguing cell-in-cell interaction and discuss potential roles for emperipolesis in megakaryocyte, platelet and neutrophil biology.

Platelets as Modulators of Cerebral Ischemia/Reperfusion Injury

Ischemic stroke is among the leading causes of disability and death worldwide. In acute ischemic stroke, the rapid recanalization of occluded cranial vessels is the primary therapeutic aim. However, experimental data (obtained using mostly the transient middle cerebral artery occlusion model) indicates that progressive stroke can still develop despite successful recanalization, a process termed “reperfusion injury.” Mounting experimental evidence suggests that platelets and T cells contribute to cerebral ischemia/reperfusion injury, and ischemic stroke is increasingly considered a thrombo-inflammatory disease. The interaction of von Willebrand factor and its receptor on the platelet surface, glycoprotein Ib, as well as many activatory platelet receptors and platelet degranulation contribute to secondary infarct growth in this setting. In contrast, interference with GPIIb/IIIa-dependent platelet aggregation and thrombus formation does not improve the outcome of acute brain ischemia but dramatically increases the susceptibility to intracranial hemorrhage. Here, we summarize the current understanding of the mechanisms and the potential translational impact of platelet contributions to cerebral ischemia/reperfusion injury.

Zinc Homeostasis in Platelet-Related Diseases

Zn²⁺ deficiency in the human population is frequent in underdeveloped countries. Worldwide, approximatively 2 billion people consume Zn²⁺-deficient diets, accounting for 1-4% of deaths each year, mainly in infants with a compromised immune system. Depending on the severity of Zn²⁺ deficiency, clinical symptoms are associated with impaired wound healing, alopecia, diarrhea, poor growth, dysfunction of the immune and nervous system with congenital abnormalities and bleeding disorders. Poor nutritional Zn²⁺ status in patients with metastatic squamous cell carcinoma or with advanced non-Hodgkin lymphoma, was accompanied by cutaneous bleeding and platelet dysfunction. Forcing Zn²⁺ uptake in the gut using different nutritional supplementation of Zn²⁺ could ameliorate many of these pathological symptoms in humans. Feeding adult rodents with a low Zn²⁺ diet caused poor platelet aggregation and increased bleeding tendency, thereby attracting great scientific interest in investigating the role of Zn²⁺ in hemostasis. Storage protein metallothionein maintains or releases Zn²⁺ in the cytoplasm, and the dynamic change of this cytoplasmic Zn²⁺ pool is regulated by the redox status of the cell. An increase of labile Zn²⁺ pool can be toxic for the cells, and therefore cytoplasmic Zn²⁺ levels are tightly regulated by several Zn²⁺ transporters located on the cell surface and also on the intracellular membrane of Zn²⁺ storage organelles, such as secretory vesicles, endoplasmic reticulum or Golgi apparatus. Although Zn²⁺ is a critical cofactor for more than 2000 transcription factors and 300 enzymes, regulating cell differentiation, proliferation, and basic metabolic functions of the cells, the molecular mechanisms of Zn²⁺ transport and the physiological role of Zn²⁺ store in megakaryocyte and platelet function remain elusive. In this review, we summarize the contribution of extracellular or intracellular Zn²⁺ to megakaryocyte and platelet function and discuss the consequences of dysregulated Zn²⁺ homeostasis in platelet-related diseases by focusing on thrombosis, ischemic stroke and storage pool diseases.

Description of a Novel Phosphodiesterase (PDE)-3 Inhibitor Protecting Mice From Ischemic Stroke Independent From Platelet Function

Supplemental Digital Content is available in the text.

Background and Purpose—

Acetylsalicylic acid and clopidogrel are the 2 main antithrombotic drugs for secondary prevention in patients with ischemic stroke (IS) without indication for anticoagulation. Because of their limited efficacy and potential side effects, novel antiplatelet agents are urgently needed. Cilostazol, a specific phosphodiesterase (PDE)-3 inhibitor, protected from IS in clinical studies comprising mainly Asian populations. Nevertheless, the detailed mechanistic role of PDE-3 inhibitors in IS pathophysiology is hardly understood. In this project, we analyzed the efficacy and pathophysiologic mechanisms of a novel and only recently described PDE-3 inhibitor (substance V) in a mouse model of focal cerebral ischemia.

Methods—

Focal cerebral ischemia was induced by transient middle cerebral artery occlusion in 6- to 8-week-old male C57Bl/6 wild-type mice receiving substance V or vehicle 1 hour after ischemia induction. Infarct volumes and functional outcomes were assessed between day 1 and day 7, and findings were validated by magnetic resonance imaging. Blood-brain barrier damage, as well as the extent of local inflammatory response and cell death, was determined.

Results—

Inhibition of PDE-3 by pharmacological blockade with substance V significantly reduced infarct volumes and improved neurological outcome on day 1 and 7 after experimental cerebral ischemia. Reduced blood-brain barrier damage, attenuated brain tissue inflammation, and decreased local cell death could be identified as potential mechanisms. PDE-3 inhibitor treatment did neither increase the number of intracerebral hemorrhages nor affect platelet function.

Conclusions—

The novel PDE-3 inhibitor substance V protected mice from IS independent from platelet function. Pharmaceutical inactivation of PDE-3 might become a promising therapeutic approach to combat IS via inhibition of thromboinflammatory mechanisms and stabilization of the blood-brain barrier.

A Case of Chronic Thrombocytopenia in a 17-Year-Old Female

Storage pool deficiency (SPD) is a group of rare platelet disorders that result from deficiencies in α-granules, δ-granules, or both. One type of α-SPD is gray platelet syndrome (GPS), caused by mutations in the neurobeachin-like 2 (NBEAL2) gene that results in a bleeding diathesis, thrombocytopenia, splenomegaly, and progressive myelofibrosis. Due to the lack of α-granules, platelets have a gray and degranulated appearance by light microscopy. However, definitive diagnosis of GPS requires confirmation of α-granule deficiency by electron microscopy. Treatment is nonspecific, with the conservative utilization of platelet transfusions being the most important form of therapy. We present a case of a 17-year-old female with a past medical history of thrombocytopenia, first identified at the age of five. Her clinical symptomatology included chronic fatigue, gingival bleeding, bruising, menorrhagia, and leg pain. This report will discuss both the clinical and the pathophysiologic aspects of this rare platelet disorder.