A comprehensive proteomics and genomics analysis reveals novel transmembrane proteins in human platelets and mouse megakaryocytes including G6b-B, a novel immunoreceptor tyrosine-based inhibitory motif protein

Probing the Interactions of Thiazole Abietane Inhibitors with the Human Serine Hydrolases ABHD16A and ABHD12

12-Thiazole abietanes are highly selective reversible inhibitors of hABHD16A that could potentially alleviate neuroinflammation. In this study, we used synthetic chemistry, competitive activity-based protein profiling, and computational methodologies to try to establish relevant structural determinants of activity and selectivity of this class of compounds for inhibiting ABHD16A over ABHD12. Five compounds significantly inhibited hABHD16A but also very efficiently discriminated between inhibition of hABHD16A and hABHD12, with compound 35 being the most effective, at 100 μM (55.1 ± 8.7%; p < 0.0001). However, an outstanding switch in the selectivity toward ABHD12 was observed in the presence of a ring A ester, if the C2' position of the thiazole ring possessed a 1-hydroxyethyl group, as in compound 28. Although our data were inconclusive as to whether the observed enzyme inhibition is allosteric or not, we anticipate that the structure-activity relationships presented herein will inspire future drug discovery efforts in this field.

Lost in traffic: consequences of altered palmitoylation in neurodegeneration

One of the first molecular events in neurodegenerative diseases, regardless of etiology, is protein mislocalization. Protein mislocalization in neurons is often linked to proteostasis deficiencies leading to the build-up of misfolded proteins and/or organelles that contributes to cellular toxicity and cell death. By understanding how proteins mislocalize in neurons, we can develop novel therapeutics that target the earliest stages of neurodegeneration. A critical mechanism regulating protein localization and proteostasis in neurons is the protein-lipid modification S-acylation, the reversible addition of fatty acids to cysteine residues. S-acylation is more commonly referred to as S-palmitoylation or simply palmitoylation, which is the addition of the 16-carbon fatty acid palmitate to proteins. Like phosphorylation, palmitoylation is highly dynamic and tightly regulated by writers (i.e., palmitoyl acyltransferases) and erasers (i.e., depalmitoylating enzymes). The hydrophobic fatty acid anchors proteins to membranes; thus, the reversibility allows proteins to be re-directed to and from membranes based on local signaling factors. This is particularly important in the nervous system, where axons (output projections) can be meters long. Any disturbance in protein trafficking can have dire consequences. Indeed, many proteins involved in neurodegenerative diseases are palmitoylated, and many more have been identified in palmitoyl-proteomic studies. It follows that palmitoyl acyl transferase enzymes have also been implicated in numerous diseases. In addition, palmitoylation can work in concert with cellular mechanisms, like autophagy, to affect cell health and protein modifications, such as acetylation, nitrosylation, and ubiquitination, to affect protein function and turnover. Limited studies have further revealed a sexually dimorphic pattern of protein palmitoylation. Therefore, palmitoylation can have wide-reaching consequences in neurodegenerative diseases.

Species Differences in Platelet Protease-Activated Receptors

Protease-activated receptors (PARs) are a class of integral membrane proteins that are cleaved by a variety of proteases, most notably thrombin, to reveal a tethered ligand and promote activation. PARs are critical mediators of platelet function in hemostasis and thrombosis, and therefore are attractive targets for anti-platelet therapies. Animal models studying platelet PAR physiology have relied heavily on genetically modified mouse strains, which have provided ample insight but have some inherent limitations. The current review aims to summarize the notable PAR expression and functional differences between the mouse and human, in addition to highlighting some recently developed tools to further study human physiology in mouse models.

Platelet-derived TLT-1 promotes tumor progression by suppressing CD8+ T cells

Current understanding of tumor immunosuppressive mechanisms forms the basis for modern day immunotherapies. Immunoregulatory role of platelets in cancer remains largely elusive. Platelets from non-small cell lung cancer (NSCLC) patients revealed a distinct activation phenotype. TREM-like transcript 1 (TLT-1), a platelet protein, was increased along with enhanced extracellular release from NSCLC platelets. The increased platelet TLT-1 was also evident in humanized mice with patient-derived tumors. In immunocompetent mice with syngeneic tumors, TLT-1 binding to T cells, in vivo, led to suppression of CD8 T cells, promoting tumor growth. We identified direct interaction between TLT-1 and CD3ε on T cells, implicating the NF-κB pathway in CD8 T cell suppression. Anti-TLT-1 antibody rescued patients' T cells from platelet-induced suppression ex vivo and reduced tumors in mice in vivo. Clinically, higher TLT-1 correlated with reduced survival of NSCLC patients. Our findings thus identify TLT-1 as a platelet-derived immunosuppressor that suppresses CD8 T cells and demonstrate its therapeutic and prognostic significance in cancer.

Inhibition of platelet aggregation by activation of platelet intermediate conductance Ca2+ -activated potassium channels

BACKGROUND

Within the vasculature platelets and endothelial cells play crucial roles in hemostasis and thrombosis. Platelets, like endothelial cells, possess intermediate conductance Ca2+ -activated K+ (IKCa ) channels and generate nitric oxide (NO). Although NO limits platelet aggregation, the role of IKCa channels in platelet function and NO generation has not yet been explored.

OBJECTIVES

We investigated whether IKCa channel activation inhibits platelet aggregation, and per endothelial cells, enhances platelet NO production.

METHODS

Platelets were isolated from human volunteers. Aggregometry, confocal microscopy, and a novel flow chamber model, the Quartz Crystal Microbalance (QCM) were used to assess platelet function. Flow cytometry was used to measure platelet NO production, calcium signaling, membrane potential, integrin αIIb /β3 activation, granule release, and procoagulant platelet formation.

RESULTS

Platelet IKCa channel activation with SKA-31 inhibited aggregation in a concentration-dependent manner, an effect reversed by the selective IKCa channel blocker TRAM-34. The QCM model along with confocal microscopy demonstrated that SKA-31 inhibited platelet aggregation under flow conditions. Surprisingly, IKCa activation by SKA-31 inhibited platelet NO generation, but this could be explained by a concomitant reduction in platelet calcium signaling. IKCa activation by SKA-31 also inhibited dense and alpha-granule secretion and integrin αIIb /β3 activation, but maintained platelet phosphatidylserine surface exposure as a measure of procoagulant response.

CONCLUSIONS

Platelet IKCa channel activation inhibits aggregation by reducing calcium-signaling and granule secretion, but not by enhancing platelet NO generation. IKCa channels may be novel targets for the development of antiplatelet drugs that limit atherothrombosis, but not coagulation.

A Novel Morphological Parameter Predicting Fibrotic Evolution in Myeloproliferative Neoplasms: New Evidence and Molecular Insights

Philadelphia-negative chronic myeloproliferative neoplasms (MPNs) represent a group of hematological disorders that are traditionally considered as indistinct slow progressing conditions; still, a subset of cases shows a rapid evolution towards myelofibrotic bone marrow failure. Specific abnormalities in the megakaryocyte lineage seem to play a central role in this evolution, especially in the bone marrow fibrosis but also in the induction of myeloproliferation. In this review, we analyze the current knowledge of prognostic factors of MPNs related to their evolution to myelofibrotic bone marrow failure. Moreover, we focused the role of the megakaryocytic lineage in the various stages of MPNs, with updated examples of MPNs in vitro and in vivo models and new therapeutic implications.

G6b-B regulates an essential step in megakaryocyte maturation

Loss of G6b-B leads to an unexpected megakaryocyte development defect resulting in severe macrothrombocytopenia.

G6b-B–deficient mice display reduced levels of MK-specific transcripts, surface receptors, GATA-1, and thrombopoietin signaling.

G6b-B is a megakaryocyte lineage-specific immunoreceptor tyrosine-based inhibition motif–containing receptor, essential for platelet homeostasis. Mice with a genomic deletion of the entire Mpig6b locus develop severe macrothrombocytopenia and myelofibrosis, which is reflected in humans with null mutations in MPIG6B. The current model proposes that megakaryocytes lacking G6b-B develop normally, whereas proplatelet release is hampered, but the underlying molecular mechanism remains unclear. We report on a spontaneous recessive single nucleotide mutation in C57BL/6 mice, localized within the intronic region of the Mpig6b locus that abolishes G6b-B expression and reproduces macrothrombocytopenia, myelofibrosis, and osteosclerosis. As the mutation is based on a single-nucleotide exchange, Mpig6b^mut mice represent an ideal model to study the role of G6b-B. Megakaryocytes from these mice were smaller, displayed a less-developed demarcation membrane system, and had a reduced expression of receptors. RNA sequencing revealed a striking global reduction in the level of megakaryocyte-specific transcripts, in conjunction with decreased protein levels of the transcription factor GATA-1 and impaired thrombopoietin signaling. The reduced number of mature MKs in the bone marrow was corroborated on a newly developed Mpig6b-null mouse strain. Our findings highlight an unexpected essential role of G6b-B in the early differentiation within the megakaryocytic lineage.

Conditional CRISPR-mediated deletion of Lyn kinase enhances differentiation and function of iPSC-derived megakaryocytes

BACKGROUND

Thrombocytopenia leading to life-threatening excessive bleeding can be treated by platelet transfusion. Currently, such treatments are totally dependent on donor-derived platelets. To support future applications in the use of in vitro-derived platelets, we sought to identify genes whose manipulation might improve the efficiency of megakaryocyte production and resulting hemostatic effectiveness. Disruption of Lyn kinase has previously been shown to improve cell survival, megakaryocyte ploidy and TPO-mediated activation in mice, but its role in human megakaryocytes and platelets has not been examined.

METHODS

To analyze the role of Lyn at defined differentiation stages during human megakaryocyte differentiation, conditional Lyn-deficient cells were generated using CRISPR/Cas9 technology in iPS cells. The efficiency of Lyn-deficient megakaryocytes to differentiate and become activated in response to a range of platelet agonists was analyzed in iPSC-derived megakaryocytes.

RESULTS

Temporally controlled deletion of Lyn improved the in vitro differentiation of hematopoietic progenitor cells into mature megakaryocytes, as measured by the rate and extent of appearance of CD41+ CD42+ cells. Lyn-deficient megakaryocytes also demonstrated improved hemostatic effectiveness, as reported by their ability to mediate clot formation in rotational thromboelastometry. Finally, Lyn-deficient megakaryocytes produced increased numbers of platelet-like particles (PLP) in vitro.

CONCLUSIONS

Conditional deletion of Lyn kinase increases the hemostatic effectiveness of megakaryocytes and their progeny as well as improving their yield. Adoption of this system during generation of in vitro-derived platelets may contribute to both their efficiency of production and their ability to support hemostasis.

A dry immersion model of microgravity modulates platelet phenotype, miRNA signature, and circulating plasma protein biomarker profile

Ground based research modalities of microgravity have been proposed as innovative methods to investigate the aetiology of chronic age-related conditions such as cardiovascular disease. Dry Immersion (DI), has been effectively used to interrogate the sequelae of physical inactivity (PI) and microgravity on multiple physiological systems. Herein we look at the causa et effectus of 3-day DI on platelet phenotype, and correlate with both miRomic and circulating biomarker expression. The miRomic profile of platelets is reflective of phenotype, which itself is sensitive and malleable to the exposome, undergoing responsive transitions in order to fulfil platelets role in thrombosis and haemostasis. Heterogeneous platelet subpopulations circulate at any given time, with varying degrees of sensitivity to activation. Employing a DI model, we investigate the effect of acute PI on platelet function in 12 healthy males. 3-day DI resulted in a significant increase in platelet count, plateletcrit, platelet adhesion, aggregation, and a modest elevation of platelet reactivity index (PRI). We identified 15 protein biomarkers and 22 miRNA whose expression levels were altered after DI. A 3-day DI model of microgravity/physical inactivity induced a prothrombotic platelet phenotype with an unique platelet miRNA signature, increased platelet count and plateletcrit. This correlated with a unique circulating protein biomarker signature. Taken together, these findings highlight platelets as sensitive adaptive sentinels and functional biomarkers of epigenetic drift within the cardiovascular compartment.

The structure of CLEC-2: mechanisms of dimerization and higher-order clustering

The platelet C-type lectin-like receptor CLEC-2 drives inflammation-driven venous thrombosis in mouse models of thrombo-inflammatory disease with a minimal effect on hemostasis identifying it as a target for a new class of antiplatelet agent. Here, we discuss how the protein structure and dynamic arrangement of CLEC-2 on the platelet membrane helps the receptor, which has a single YxxL motif (known as a hemITAM), to trigger intracellular signaling. CLEC-2 exists as a monomer and homo-dimer within resting platelets and forms higher-order oligomers following ligand activation, a process that is mediated by the multivalent nature of its ligands and the binding of the tandem SH2 domains of Syk to the phosphorylated hemITAM and concomitantly to PIP2 or PIP3 to localize it to the membrane. We propose that a low level of active Syk is present at the membrane in resting platelets due to phosphorylation by Src family kinases and that clustering of receptors disturbs the equilibrium between kinases and phosphatases, triggering phosphorylation of the CLEC-2 hemITAM and recruitment of Syk. Knowledge of the structure of CLEC-2 and the mechanism of platelet activation has important implications for development of therapeutics.

α/β-Hydrolase Domain (ABHD) Inhibitors as New Potential Therapeutic Options against Lipid-Related Diseases

Much of the experimental evidence in the literature has linked altered lipid metabolism to severe diseases such as cancer, obesity, cardiovascular pathologies, diabetes, and neurodegenerative diseases. Therefore, targeting key effectors of the dysregulated lipid metabolism may represent an effective strategy to counteract these pathological conditions. In this context, α/β-hydrolase domain (ABHD) enzymes represent an important and diversified family of proteins, which are involved in the complex environment of lipid signaling, metabolism, and regulation. Moreover, some members of the ABHD family play an important role in the endocannabinoid system, being designated to terminate the signaling of the key endocannabinoid regulator 2-arachidonoylglycerol. This Perspective summarizes the research progress in the development of ABHD inhibitors and modulators: design strategies, structure-activity relationships, action mechanisms, and biological studies of the main ABHD ligands will be highlighted.

Antiplatelet properties of Pim kinase inhibition are mediated through disruption of thromboxane A2 receptor signaling

Pim kinases are upregulated in several forms of cancer, contributing to cell survival and tumor development, but their role in platelet function and thrombotic disease has not been explored. We report for the first time that Pim-1 kinase is expressed in human and mouse platelets. Genetic deletion or pharmacological inhibition of Pim kinase results in reduced thrombus formation but is not associated with impaired hemostasis. Attenuation of thrombus formation was found to be due to inhibition of the thromboxane A2 receptor as effects on platelet function were non-additive to inhibition caused by the cyclo-oxygenase inhibitor indomethacin or the thromboxane A2 receptor antagonist GR32191. Treatment with Pim kinase inhibitors caused reduced surface expression of the thromboxane A2 receptor and resulted in reduced responses to thromboxane A2 receptor agonists, indicating a role for Pim kinase in the regulation of thromboxane A2 receptor function. Our research identifies a novel, Pim kinase-dependent regulatory mechanism for the thromboxane A2 receptor and represents a new targeting strategy that is independent of cyclo-oxygenase-1 inhibition or direct antagonism of the thromboxane A2 receptor that, while attenuating thrombosis, does not increase bleeding.

Aspirin and antiplatelet treatments in cancer

Platelets have been hypothesized to promote certain neoplastic malignancies; however, antiplatelet drugs are still not part of routine pharmacological cancer prevention and treatment protocols. Paracrine interactions between platelets and cancer cells have been implicated in potentiating the dissemination, survival within the circulation, and extravasation of cancer cells at distant sites of metastasis. Signals from platelets have also been suggested to confer epigenetic alterations, including upregulating oncoproteins in circulating tumor cells, and secretion of potent growth factors may play roles in promoting mitogenesis, angiogenesis, and metastatic outgrowth. Thrombocytosis remains a marker of poor prognosis in patients with solid tumors. Experimental data suggest that lowering of platelet count may reduce tumor growth and metastasis. On the basis of the mechanisms by which platelets could contribute to cancer growth and metastasis, it is conceivable that drugs reducing platelet count or platelet activation might attenuate cancer progression and improve outcomes. We will review select pharmacological approaches that inhibit platelets and may affect cancer development and propagation. We begin by presenting an overview of clinical cancer prevention and outcome studies with low-dose aspirin. We then review current nonclinical development of drugs targeted to platelet binding, activation, and count as potential mitigating agents in cancer.

Bioactive potential of natural biomaterials: identification, retention and assessment of biological properties

Biomaterials have had an increasingly important role in recent decades, in biomedical device design and the development of tissue engineering solutions for cell delivery, drug delivery, device integration, tissue replacement, and more. There is an increasing trend in tissue engineering to use natural substrates, such as macromolecules native to plants and animals to improve the biocompatibility and biodegradability of delivered materials. At the same time, these materials have favourable mechanical properties and often considered to be biologically inert. More importantly, these macromolecules possess innate functions and properties due to their unique chemical composition and structure, which increase their bioactivity and therapeutic potential in a wide range of applications. While much focus has been on integrating these materials into these devices via a spectrum of cross-linking mechanisms, little attention is drawn to residual bioactivity that is often hampered during isolation, purification, and production processes. Herein, we discuss methods of initial material characterisation to determine innate bioactivity, means of material processing including cross-linking, decellularisation, and purification techniques and finally, a biological assessment of retained bioactivity of a final product. This review aims to address considerations for biomaterials design from natural polymers, through the optimisation and preservation of bioactive components that maximise the inherent bioactive potency of the substrate to promote tissue regeneration.

Targeting platelet inhibition receptors for novel therapies: PECAM-1 and G6b-B

While current oral antiplatelet therapies benefit many patients, they deregulate the hemostatic balance leaving patients at risk of systemic side-effects such as hemorrhage. Dual antiplatelet treatment is the standard approach, combining aspirin with P2Y₁₂ blockers. These therapies mainly target autocrine activation mechanisms (TxA₂, ADP) and, more recently, the use of thrombin or thrombin receptor antagonists have been added to the available approaches. Recent efforts to develop new classes of anti-platelet drugs have begun to focus on primary platelet activation pathways such as through the immunoreceptor tyrosine-based activation motif (ITAM)-containing collagen receptor GPVI/FcRγ-chain complex. There are already encouraging results from targeting GPVI, with reduced aggregation and smaller arterial thrombi, without major bleeding complications, likely due to overlapping activation signaling pathways with other receptors such as the GPIb-V-IX complex. An alternative approach to reduce platelet activation could be to inhibit this signaling pathway by targeting the inhibitory pathways intrinsic to platelets. Stimulation of endogenous negative modulators could provide more specific inhibition of platelet function, but is this feasible? In this review, we explore the potential of the two major platelet immunoreceptor tyrosine-based inhibitory motif (ITIM)-containing inhibitory receptors, G6b-B and PECAM-1, as antithrombotic targets.

The role of phospho-tyrosine signaling in platelet biology and hemostasis

Platelets are small enucleated cell fragments specialized in the control of hemostasis, but also playing a role in angiogenesis, inflammation and immunity. This plasticity demands a broad range of physiological processes. Platelet functions are mediated through a variety of receptors, the concerted action of which must be tightly regulated, in order to allow specific and timely responses to different stimuli. Protein phosphorylation is one of the main key regulatory mechanisms by which extracellular signals are conveyed. Despite the importance of platelets in health and disease, the molecular pathways underlying the activation of these cells are still under investigation. Here, we review current literature on signaling platelet biology and in particular emphasize the newly emerging role of phosphatases in these processes.

Recent advances in inherited platelet disorders

PURPOSE OF REVIEW

The increasing use of high throughput sequencing and genomic analysis has facilitated the discovery of new causes of inherited platelet disorders. Studies of these disorders and their respective mouse models have been central to understanding their biology, and also in revealing new aspects of platelet function and production. This review covers recent contributions to the identification of genes, proteins and variants associated with inherited platelet defects, and highlights how these studies have provided insights into platelet development and function.

RECENT FINDINGS

Novel genes recently implicated in human platelet dysfunction include the galactose metabolism enzyme UDP-galactose-4-epimerase in macrothrombocytopenia, and erythropoietin-producing hepatoma-amplified sequence receptor transmembrane tyrosine kinase EPHB2 in a severe bleeding disorder with deficiencies in platelet agonist response and granule secretion. Recent studies of disease-associated variants established or clarified roles in platelet function and/or production for the membrane receptor G6b-B, the FYN-binding protein FYB1/ADAP, the RAS guanyl-releasing protein RASGRP2/CalDAG-GEFI and the receptor-like protein tyrosine phosphatase PTPRJ/CD148. Studies of genes associated with platelet disorders advanced understanding of the cellular roles of neurobeachin-like 2, as well as several genes influenced by the transcription regulator RUNT-related transcription factor 1 (RUNX1), including NOTCH4.

SUMMARY

The molecular bases of many hereditary platelet disorders have been elucidated by the application of recent advances in cell imaging and manipulation, genomics and protein function analysis. These techniques have also aided the detection of new disorders, and enabled studies of disease-associated genes and variants to enhance understanding of platelet development and function.

Perlecan, a modular instructive proteoglycan with diverse functional properties

This study reviewed some new aspects of the modular proteoglycan perlecan, a colossal proteoglycan with a 467 kDa core protein and five distinct functional domains. Perlecan is a heparan sulphate proteoglycan that transiently displays native CS sulphation motifs 4-C-3 and 7-D-4 during tissue morphogenesis these are expressed by progenitor cell populations during tissue development. Perlecan is susceptible to fragmentation by proteases during tissue development and in pathological tissues particularly in domains IV and V. The fragmentation pattern of domain IV has been suggested as a means of grading prostate cancer. Domain V of perlecan is of interest due to its interactive properties with integrin α5β1 that promotes pericyte migration enhancing PDGF-BB-induced phosphorylation of PDGFRβ, Src homology region 2 domain-containing phosphatase-2, and focal adhesion kinase supporting the repair of the blood brain barrier following ischaemic stroke. Fragments of domain V can also interact with α2β1 integrin disrupting tube formation by endothelial cells. LG1-LG2, LG3 fragments can antagonise VEGFR2, and α2β1 integrin interactions preventing angiogenesis by endothelial cells. These domain V fragments are of interest as potential anti-tumour agents. Perlecan attached to the luminal surfaces of endothelial cells in blood vessels acts as a flow sensor that signals back to endothelial and smooth muscle cells to regulate vascular tone and blood pressure. Perlecan also acts as a flow sensor in the lacuno-canalicular space regulating osteocytes and bone homeostasis. Along with its biomechanical regulatory properties in cartilaginous tissues this further extends the functional repertoire of this amazingly diverse functional proteoglycan.

Remodeling the Bone Marrow Microenvironment - A Proposal for Targeting Pro-inflammatory Contributors in MPN

Philadelphia-negative myeloproliferative neoplasms (MPN) are malignant bone marrow (BM) disorders, typically arising from a single somatically mutated hematopoietic stem cell. The most commonly mutated genes, JAK2, CALR, and MPL lead to constitutively active JAK-STAT signaling. Common clinical features include myeloproliferation, splenomegaly and constitutional symptoms. This review covers the contributions of cellular components of MPN pathology (e.g., monocytes, megakaryocytes, and mesenchymal stromal cells) as well as cytokines and soluble mediators to the development of myelofibrosis (MF) and highlights recent therapeutic advances. These findings outline the importance of malignant and non-malignant BM constituents to the pathogenesis and treatment of MF.

Platelets in Healthy and Disease States: From Biomarkers Discovery to Drug Targets Identification by Proteomics

Platelets are a heterogeneous small anucleate blood cell population with a central role both in physiological haemostasis and in pathological states, spanning from thrombosis to inflammation, and cancer. Recent advances in proteomic studies provided additional important information concerning the platelet biology and the response of platelets to several pathophysiological pathways. Platelets circulate systemically and can be easily isolated from human samples, making proteomic application very interesting for characterizing the complexity of platelet functions in health and disease as well as for identifying and quantifying potential platelet proteins as biomarkers and novel antiplatelet therapeutic targets. To date, the highly dynamic protein content of platelets has been studied in resting and activated platelets, and several subproteomes have been characterized including platelet-derived microparticles, platelet granules, platelet releasates, platelet membrane proteins, and specific platelet post-translational modifications. In this review, a critical overview is provided on principal platelet proteomic studies focused on platelet biology from signaling to granules content, platelet proteome changes in several diseases, and the impact of drugs on platelet functions. Moreover, recent advances in quantitative platelet proteomics are discussed, emphasizing the importance of targeted quantification methods for more precise, robust and accurate quantification of selected proteins, which might be used as biomarkers for disease diagnosis, prognosis and therapy, and their strong clinical impact in the near future.

Single-Cell Analyses Reveal Megakaryocyte-Biased Hematopoiesis in Myelofibrosis and Identify Mutant Clone-Specific Targets

Myelofibrosis is a severe myeloproliferative neoplasm characterized by increased numbers of abnormal bone marrow megakaryocytes that induce fibrosis, destroying the hematopoietic microenvironment. To determine the cellular and molecular basis for aberrant megakaryopoiesis in myelofibrosis, we performed single-cell transcriptome profiling of 135,929 CD34+ lineage- hematopoietic stem and progenitor cells (HSPCs), single-cell proteomics, genomics, and functional assays. We identified a bias toward megakaryocyte differentiation apparent from early multipotent stem cells in myelofibrosis and associated aberrant molecular signatures. A sub-fraction of myelofibrosis megakaryocyte progenitors (MkPs) are transcriptionally similar to healthy-donor MkPs, but the majority are disease specific, with distinct populations expressing fibrosis- and proliferation-associated genes. Mutant-clone HSPCs have increased expression of megakaryocyte-associated genes compared to wild-type HSPCs, and we provide early validation of G6B as a potential immunotherapy target. Our study paves the way for selective targeting of the myelofibrosis clone and illustrates the power of single-cell multi-omics to discover tumor-specific therapeutic targets and mediators of tissue fibrosis.

Modulation of Immune Responses by Platelet-Derived ADAM10

Platelets have a crucial function in maintaining hemostasis. However, beyond their role in coagulation and thrombus formation, platelets have been implicated to affect various pathophysiological conditions such as infectious diseases, autoimmune disorders, and cancer. It is well-established that platelets aid local cancer growth by providing growth factors or contributing to cancer angiogenesis. In addition, they promote metastasis, among others by facilitation of tumor cell-extravasation and epithelial-to-mesenchymal-like transition as well as protecting metastasizing cancer cells from immunosurveillance. A variety of membrane-bound and soluble platelet-derived factors are involved in these processes, and many aspects of platelet biology in both health and disease are regulated by platelet-associated metalloproteinases and their inhibitors. Platelets synthesize (i) members of the matrix metalloproteinase (MMP) family and also inhibitors of MMPs such as members of the "tissue inhibitor of metalloproteinases" (TIMP) family as well as (ii) members of the "a disintegrin and metalloproteinase" (ADAM) family including ADAM10. Notably, platelet-associated metalloproteinase activity not only influences functions of platelets themselves: platelets can also induce expression and/or release of metalloproteinases e.g., in leukocytes or cancer cells, and ADAMs are emerging as important components by which platelets directly affect other cell types and function. This review outlines the function of metalloproteinases in platelet biology with a focus on ADAM10 and discusses the role of platelet-derived metalloproteinases in the interaction of platelets with components of the immune system and/or cancer cells.

Proteomic profiling of the thrombin-activated canine platelet secretome (CAPS)

Domestic dogs share the same environment as humans, and they represent a valuable animal model to study naturally-occurring human disease. Platelet proteomics holds promise for the discovery of biomarkers that capture the contribution of platelets to the pathophysiology of many disease states, however, canine platelet proteomic studies are lacking. Our study objectives were to establish a protocol for proteomic identification and quantification of the thrombin-activated canine platelet secretome (CAPS), and to compare the CAPS proteins to human and murine platelet proteomic data. Washed platelets were isolated from healthy dogs, and stimulated with saline (control) or gamma-thrombin (releasate). Proteins were separated by SDS-page, trypsin-digested and analyzed by liquid chromatography and tandem mass spectrometry (MS). CAPS proteins were defined as those with a MS1-abundance ratio of two or more for releasate vs. unstimulated saline control. A total of 1,918 proteins were identified, with 908 proteins common to all dogs and 693 characterized as CAPS proteins. CAPS proteins were similar to human and murine platelet secretomes and were highly represented in hemostatic pathways. Differences unique to CAPS included replacement of platelet factor 4 with other cleavage products of platelet basic protein (e.g. interleukin-8), novel proteins (e.g. C-C motif chemokine 14), and proteins in relatively high (e.g. protease nexin-1) or low (e.g. von Willebrand factor) abundance. This study establishes the first in-depth platelet releasate proteome from healthy dogs with a reference database of 693 CAPS proteins. Similarities between CAPS and the human secretome confirm the utility of dogs as translational models of human disease, but we also identify differences unique to canine platelets. Our findings provide a resource for further investigations into disease-related CAPS profiles, and for comparative pathway analyses of platelet activation among species.

Advances in Platelet Subpopulation Research

Although lacking a nucleus, platelets are increasingly recognized not only for their complexity, but also for their diversity. Some 50 years ago platelet subpopulations were characterized by size and density, and these characteristics were thought to reflect platelet aging. Since, our knowledge of platelet heterogeneity has grown to recognize that differences in platelet biochemistry and function exist. This includes the identification of vanguard and follower platelets, platelets with differing procoagulant ability including "COAT-platelets" which enhance procoagulant protein retention on their surface, and most recently, the identification of platelet subpopulations with a differential ability to generate and respond to nitric oxide. Hence, in this mini-review, we summarize the current knowledge of platelet subpopulation diversity focusing on their physical, biochemical, and functional heterogeneity. In addition, we review how platelet subpopulations may change between health and disease and how differences among platelets may influence response to anti-platelet therapy. Finally, we look forward and discuss some of the future directions and challenges for this growing field of platelet research.

Heparan sulfates are critical regulators of the inhibitory megakaryocyte-platelet receptor G6b-B

The immunoreceptor tyrosine-based inhibition motif (ITIM)-containing receptor G6b-B is critical for platelet production and activation. Loss of G6b-B results in severe macrothrombocytopenia, myelofibrosis and aberrant platelet function in mice and humans. Using a combination of immunohistochemistry, affinity chromatography and proteomics, we identified the extracellular matrix heparan sulfate (HS) proteoglycan perlecan as a G6b-B binding partner. Subsequent in vitro biochemical studies and a cell-based genetic screen demonstrated that the interaction is specifically mediated by the HS chains of perlecan. Biophysical analysis revealed that heparin forms a high-affinity complex with G6b-B and mediates dimerization. Using platelets from humans and genetically modified mice, we demonstrate that binding of G6b-B to HS and multivalent heparin inhibits platelet and megakaryocyte function by inducing downstream signaling via the tyrosine phosphatases Shp1 and Shp2. Our findings provide novel insights into how G6b-B is regulated and contribute to our understanding of the interaction of megakaryocytes and platelets with glycans.

G6b-B: the "Y's" and wherefores

In this issue of Blood, Hofmann et al and Geer et al describe signal-transducing properties of G6b-B that are required for normal platelet production by megakaryocytes in both humans and mice.1,2

Differential eNOS-signalling by platelet subpopulations regulates adhesion and aggregation

Aims

In addition to maintaining haemostasis, circulating blood platelets are the cellular culprits that form occlusive thrombi in arteries and veins. Compared to blood leucocytes, which exist as functionally distinct subtypes, platelets are considered to be relatively simple cell fragments that form vascular system plugs without a differentially regulated cellular response. Hence, investigation into platelet subpopulations with distinct functional roles in haemostasis/thrombosis has been limited. In our present study, we investigated whether functionally distinct platelet subpopulations exist based on their ability to generate and respond to nitric oxide (NO), an endogenous platelet inhibitor.

Methods and results

Utilizing highly sensitive and selective flow cytometry protocols, we demonstrate that human platelet subpopulations exist based on the presence and absence of endothelial nitric oxide synthase (eNOS). Platelets lacking eNOS (approximately 20% of total platelets) fail to produce NO and have a down-regulated soluble guanylate cyclase-protein kinase G (sGC-PKG)-signalling pathway. In flow chamber and aggregation experiments eNOS-negative platelets primarily initiate adhesion to collagen, more readily activate integrin αIIbβ3 and secrete matrix metalloproteinase-2, and form larger aggregates than their eNOS-positive counterparts. Conversely, platelets having an intact eNOS-sGC-PKG-signalling pathway (approximately 80% of total platelets) form the bulk of an aggregate via increased thromboxane synthesis and ultimately limit its size via NO generation.

Conclusion

These findings reveal previously unrecognized characteristics and complexity of platelets and their regulation of adhesion/aggregation. The identification of platelet subpopulations also has potentially important consequences to human health and disease as impaired platelet NO-signalling has been identified in patients with coronary artery disease.

Congenital macrothrombocytopenia with focal myelofibrosis due to mutations in human G6b-B is rescued in humanized mice

Unlike primary myelofibrosis (PMF) in adults, myelofibrosis in children is rare. Congenital (inherited) forms of myelofibrosis (cMF) have been described, but the underlying genetic mechanisms remain elusive. Here we describe 4 families with autosomal recessive inherited macrothrombocytopenia with focal myelofibrosis due to germ line loss-of-function mutations in the megakaryocyte-specific immunoreceptor tyrosine-based inhibitory motif (ITIM)-containing receptor G6b-B (G6b, C6orf25, or MPIG6B). Patients presented with a mild-to-moderate bleeding diathesis, macrothrombocytopenia, anemia, leukocytosis and atypical megakaryocytes associated with a distinctive, focal, perimegakaryocytic pattern of bone marrow fibrosis. In addition to identifying the responsible gene, the description of G6b-B as the mutated protein potentially implicates aberrant G6b-B megakaryocytic signaling and activation in the pathogenesis of myelofibrosis. Targeted insertion of human G6b in mice rescued the knockout phenotype and a copy number effect of human G6b-B expression was observed. Homozygous knockin mice expressed 25% of human G6b-B and exhibited a marginal reduction in platelet count and mild alterations in platelet function; these phenotypes were more severe in heterozygous mice that expressed only 12% of human G6b-B. This study establishes G6b-B as a critical regulator of platelet homeostasis in humans and mice. In addition, the humanized G6b mouse will provide an invaluable tool for further investigating the physiological functions of human G6b-B as well as testing the efficacy of drugs targeting this receptor.

Uncoupling ITIM receptor G6b-B from tyrosine phosphatases Shp1 and Shp2 disrupts murine platelet homeostasis

The immunoreceptor tyrosine-based inhibitory motif (ITIM)-containing receptor G6b-B has emerged as a key regulator of platelet homeostasis. However, it remains unclear how it mediates its effects. Tyrosine phosphorylation of ITIM and immunoreceptor tyrosine-based switch motif (ITSM) within the cytoplasmic tail of G6b-B provides a docking site for Src homology 2 domain-containing protein-tyrosine phosphatases Shp1 and Shp2, which are also critical regulators of platelet production and function. In this study, we investigate the physiological consequences of uncoupling G6b-B from Shp1 and Shp2. To address this, we generated a transgenic mouse model expressing a mutant form of G6b-B in which tyrosine residues 212 and 238 within ITIM and ITSM were mutated to phenylalanine. Mice homozygous for the mutation (G6b-B diY/F) were macrothrombocytopenic, as a result of the reduction in platelet production, and had large clusters of megakaryocytes and myelofibrosis at sites of hematopoiesis, similar to those observed in G6b-deficient mice and patients. Platelets from G6b-B diY/F mice were hyporesponsive to collagen, as a result of the significant reduction in the expression of the immunoreceptor tyrosine-based activation motif (ITAM)-containing collagen receptor complex GPVI-FcR γ-chain, as well as thrombin, which could be partially rescued by costimulating the platelets with adenosine diphosphate. In contrast, platelets from G6b-B diY/F, G6b KO, and megakaryocyte-specific Shp2 KO mice were hyperresponsive to antibody-mediated cross-linking of the hemi-ITAM-containing podoplanin receptor CLEC-2, suggesting that G6b-B inhibits CLEC-2-mediated platelet activation through Shp2. Findings from this study demonstrate that G6b-B must engage with Shp1 and Shp2 to mediate its regulatory effects on platelet homeostasis.

Beyond COX-1: the effects of aspirin on platelet biology and potential mechanisms of chemoprevention

After more than a century, aspirin remains one of the most commonly used drugs in western medicine. Although mainly used for its anti-thrombotic, anti-pyretic, and analgesic properties, a multitude of clinical studies have provided convincing evidence that regular, low-dose aspirin use dramatically lowers the risk of cancer. These observations coincide with recent studies showing a functional relationship between platelets and tumors, suggesting that aspirin's chemopreventive properties may result, in part, from direct modulation of platelet biology and biochemistry. Here, we present a review of the biochemistry and pharmacology of aspirin with particular emphasis on its cyclooxygenase-dependent and cyclooxygenase-independent effects in platelets. We also correlate the results of proteomic-based studies of aspirin acetylation in eukaryotic cells with recent developments in platelet proteomics to identify non-cyclooxygenase targets of aspirin-mediated acetylation in platelets that may play a role in its chemopreventive mechanism.

Negative regulators of platelet activation and adhesion

Platelets are small anucleated cells that constantly patrol the cardiovascular system to preserve its integrity and prevent excessive blood loss where the vessel lining is breached. Their key challenge is to form a hemostatic plug under conditions of high shear forces. To do so, platelets have evolved a molecular machinery that enables them to sense trace amounts of signals at the site of damage and to rapidly shift from a non-adhesive to a pro-adhesive state. However, this highly efficient molecular machinery can also lead to unintended platelet activation and cause clinical complications such as thrombocytopenia and thrombosis. Thus, several checkpoints are in place to tightly control platelet activation and adhesiveness in space and time. In this review, we will discuss select negative regulators of platelet activation, which are critical to maintain patrolling platelets in a quiescent, non-adhesive state and/or to limit platelet adhesion to sites of injury.

Targeting platelet receptors in thrombotic and thrombo-inflammatory disorders

Platelet activation at sites of vascular injury is critical for the formation of a hemostatic plug which limits excessive blood loss, but also represents a major pathomechanism of ischemic cardio- and cerebrovascular diseases. Although currently available antiplatelet therapies have proved beneficial in preventing the recurrence of vascular events, their adverse effects on primary hemostasis emphasize the necessity to identify and characterize novel pharmacological targets for platelet inhibition. Increasing experimental evidence has suggested that several major platelet surface receptors which regulate initial steps of platelet adhesion and activation may become promising new targets for anti-platelet drugs due to their involvement in thrombotic and thrombo-inflammatory signaling cascades.

This review summarizes recent developments in understanding the function of glycoprotein (GP)Ib, GPVI and the C-type lectin-like receptor 2 (CLEC-2) in hemostasis, arterial thrombosis and thrombo-inflammation and will discuss the suitability of the receptors as novel targets to treat these diseases in humans.

Dextran sulphate induces fibrinogen receptor activation through a novel Syk-independent PI-3 kinase-mediated tyrosine kinase pathway in platelets

In our attempt to find a physiological agonist that activates PAR3 receptors, we screened several coagulation proteases using PAR4 null platelets. We observed that FXIIa and heat inactivated FXIIa, but not FXII, caused platelet aggregation. We have identified a contaminant activating factor in FXIIa preparation as dextran sulfate (DxS), which caused aggregation of both human and mouse platelets. DxS-induced platelet aggregation was unaffected by YM254890, a Gq inhibitor, but abolished by pan-Src family kinase (SFK) inhibitor PP2, suggesting a role for SFKs in this pathway. However, DxS-induced platelet aggregation was unaffected in FcRγ-chain null murine platelets, ruling out the possibility of glycoprotein VI-mediated events. More interesting, OXSI-2 and Go6976, two structurally unrelated inhibitors shown to affect Syk, had only a partial effect on DxS-induced PAC-1 binding. DxS-induced platelet aggregation and intracellular calcium increases were abolished by the pan PI-3 kinase inhibitor LY294002, or an isoform-specific PI-3 kinase β inhibitor TGX-221. Pretreatment of platelets with Syk inhibitors or ADP receptor antagonists had little effect on Akt phosphorylation following DxS stimulation. These results, for the first time, establish a novel tyrosine kinase pathway in platelets that causes fibrinogen receptor activation in a PI-3 kinase-dependent manner without a crucial role for Syk.

The actin binding proteins cortactin and HS1 are dispensable for platelet actin nodule and megakaryocyte podosome formation

A dynamic, properly organised actin cytoskeleton is critical for the production and haemostatic function of platelets. The Wiskott Aldrich Syndrome protein (WASp) and Actin-Related Proteins 2 & 3 Complex (Arp2/3 complex) are critical mediators of actin polymerisation and organisation in many cell types. In platelets and megakaryocytes, these proteins have been shown to be important for proper platelet production and function. The cortactin family of proteins (Cttn & HS1) are known to regulate WASp-Arp2/3-mediated actin polymerisation in other cell types and so here we address the role of these proteins in platelets using knockout mouse models. We generated mice lacking Cttn and HS1 in the megakaryocyte/platelet lineage. These mice had normal platelet production, with platelet number, size and surface receptor profile comparable to controls. Platelet function was also unaffected by loss of Cttn/HS1 with no differences observed in a range of platelet function assays including aggregation, secretion, spreading, clot retraction or tyrosine phosphorylation. No effect on tail bleeding time or in thrombosis models was observed. In addition, platelet actin nodules, and megakaryocyte podosomes, actin-based structures known to be dependent on WASp and the Arp2/3 complex, formed normally. We conclude that despite the importance of WASp and the Arp2/3 complex in regulating F-actin dynamics in many cells types, the role of cortactin in their regulation appears to be fulfilled by other proteins in platelets.

ITIM receptors: more than just inhibitors of platelet activation

Since their discovery, immunoreceptor tyrosine-based inhibition motif (ITIM)-containing receptors have been shown to inhibit signaling from immunoreceptor tyrosine-based activation motif (ITAM)-containing receptors in almost all hematopoietic cells, including platelets. However, a growing body of evidence has emerged demonstrating that this is an oversimplification, and that ITIM-containing receptors are versatile regulators of platelet signal transduction, with functions beyond inhibiting ITAM-mediated platelet activation. PECAM-1 was the first ITIM-containing receptor identified in platelets and appeared to conform to the established model of ITIM-mediated attenuation of ITAM-driven activation. PECAM-1 was therefore widely accepted as a major negative regulator of platelet activation and thrombosis for many years, but more recent findings suggest a more complex role for this receptor, including the facilitation of α_IIbβ₃-mediated platelet functions. Since the identification of PECAM-1, several other ITIM-containing platelet receptors have been discovered. These include G6b-B, a critical regulator of platelet reactivity and production, and the noncanonical ITIM-containing receptor TREM-like transcript-1, which is localized to α-granules in resting platelets, binds fibrinogen, and acts as a positive regulator of platelet activation. Despite structural similarities and shared binding partners, including the Src homology 2 domain-containing protein-tyrosine phosphatases Shp1 and Shp2, knockout and transgenic mouse models have revealed distinct phenotypes and nonredundant functions for each ITIM-containing receptor in the context of platelet homeostasis. These roles are likely influenced by receptor density, compartmentalization, and as-yet unknown binding partners. In this review, we discuss the diverse repertoire of ITIM-containing receptors in platelets, highlighting intriguing new functions, controversies, and future areas of investigation.

Mice Lacking the Inhibitory Collagen Receptor LAIR-1 Exhibit a Mild Thrombocytosis and Hyperactive Platelets

OBJECTIVE

Leukocyte-associated immunoglobulin-like receptor-1 (LAIR-1) is a collagen receptor that belongs to the inhibitory immunoreceptor tyrosine-based inhibition motif-containing receptor family. It is an inhibitor of signaling via the immunoreceptor tyrosine-based activation motif-containing collagen receptor complex, glycoprotein VI-FcRγ-chain. It is expressed on hematopoietic cells, including immature megakaryocytes, but is not detectable on platelets. Although the inhibitory function of LAIR-1 has been described in leukocytes, its physiological role in megakaryocytes and in particular in platelet formation has not been explored. In this study, we investigate the role of LAIR-1 in megakaryocyte development and platelet production by generating LAIR-1-deficient mice.

APPROACH AND RESULTS

Mice lacking LAIR-1 exhibit a significant increase in platelet counts, a prolonged platelet half-life in vivo, and increased proplatelet formation in vitro. Interestingly, platelets from LAIR-1-deficient mice exhibit an enhanced reactivity to collagen and the glycoprotein VI-specific agonist collagen-related peptide despite not expressing LAIR-1, and mice showed enhanced thrombus formation in the carotid artery after ferric chloride injury. Targeted deletion of LAIR-1 in mice results in an increase in signaling downstream of the glycoprotein VI-FcRγ-chain and integrin αIIbβ3 in megakaryocytes because of enhanced Src family kinase activity.

CONCLUSIONS

Findings from this study demonstrate that ablation of LAIR-1 in megakaryocytes leads to increased Src family kinase activity and downstream signaling in response to collagen that is transmitted to platelets, rendering them hyper-reactive specifically to agonists that signal through Syk tyrosine kinases, but not to G-protein-coupled receptors.

Integration of polycarbonate membranes in microfluidic free-flow electrophoresis

A general difficulty in the miniaturization of free-flow electrophoresis relates to the need to separate electrodes and separation bed compartments.

Risk factors for heparin-induced thrombocytopenia: Focus on Fcγ receptors

Fcγ receptors have critical roles in the pathophysiology of heparin-induced thrombocytopenia (HIT), a severe immune-mediated complication of heparin treatment. Activation of platelets, monocytes and neutrophils by platelet-activating anti-PF4/heparin IgG antibodies results in thrombocytopenia, hypercoagulability and thrombosis in susceptible patients, effects that depend on FcγRIIA. In addition, FcγRIIIA receptors probably contribute to clearance of platelets sensitised by HIT immune complexes. FcγRI has also been reported to be involved in monocyte activation by HIT IgG antibodies and synthesis of tissue factor. This review focuses on the role of these FcγRs in HIT pathophysiology, including the potential influence of several gene variations associated with variable risk of HIT and related thrombosis. In particular, the 276P and 326Q alleles of CD148, a protein tyrosine phosphatase that regulates FcγRIIA signalling, are associated with a lower risk of HIT, and platelets from healthy donors expressing these alleles are hyporesponsive to anti-PF4/H antibodies. It was also recently demonstrated that the risk of thrombosis is higher in HIT patients expressing the R isoform of the FcγRIIA H131R polymorphism, with HIT antibodies shown to activate RR platelets more efficiently, mainly explained by an inhibitory effect of normal IgG2, which bound to the FcγRIIA 131H isoform more efficiently. Environmental risk factors probably interact with these gene polymorphisms affecting FcγRs, thereby increasing thrombosis risk in HIT.

Erythrocyte and platelet proteomics in hematological disorders

Erythrocytes undergo ineffective erythropoesis, hemolysis, and premature eryptosis in sickle cell disease and thalassemia. Abnormal hemoglobin variants associated with hemoglobinopathy lead to vesiculation, membrane instability, and loss of membrane asymmetry with exposal of phosphatidylserine. This potentiates thrombin generation resulting in activation of the coagulation cascade responsible for subclinical phenotypes. Platelet activation also results in the release of microparticles, which express and transfer functional receptors from platelet membrane, playing key roles in vascular reactivity and activation of intracellular signaling pathways. Over the last decade, proteomics had proven to be an important field of research in studies of blood and blood diseases. Blood cells and its fluidic components have been proven to be easy systems for studying differential expressions of proteins in hematological diseases encompassing hemoglobinopathies, different types of anemias, myeloproliferative disorders, and coagulopathies. Proteomic studies of erythrocytes and platelets reported from several groups have highlighted various factors that intersect the signaling networks in these anucleate systems. In this review, we have elaborated on the current scenario of anucleate blood cell proteomes in normal and diseased individuals and the cross-talk between the two major constituent cell types of circulating blood.

CLEC-2 in megakaryocytes is critical for maintenance of hematopoietic stem cells in the bone marrow

Nakamura-Ishizu et al. report that megakaryocytes function as a niche to maintain HSC quiescence through CLEC-2–mediated production of Thpo and other key regulators of HSC function. These findings could enable manipulation of HSCs for clinical application.

Hematopoietic stem cells (HSCs) depend on the bone marrow (BM) niche for their maintenance, proliferation, and differentiation. The BM niche is composed of nonhematopoietic and mature hematopoietic cells, including megakaryocytes (Mks). Thrombopoietin (Thpo) is a crucial cytokine produced by BM niche cells. However, the cellular source of Thpo, upon which HSCs primarily depend, is unclear. Moreover, no specific molecular pathway for the regulation of Thpo production in the BM has been identified. Here, we demonstrate that the membrane protein C-type lectin-like receptor-2 (CLEC-2) mediates the production of Thpo and other factors in Mks. Mice conditionally deleted for CLEC-2 in Mks (Clec2^MkΔ/Δ) produced lower levels of Thpo in Mks. CLEC-2–deficient Mks showed down-regulation of CLEC-2–related signaling molecules Syk, Lcp2, and Plcg2. Knockdown of these molecules in cultured Mks decreased expression of Thpo. Clec2^MkΔ/Δ mice exhibited reduced BM HSC quiescence and repopulation potential, along with extramedullary hematopoiesis. The low level of Thpo production may account for the decline in HSC potential in Clec2^MkΔ/Δ mice, as administration of recombinant Thpo to Clec2^MkΔ/Δ mice restored stem cell potential. Our study identifies CLEC-2 signaling as a novel molecular mechanism mediating the production of Thpo and other factors for the maintenance of HSCs.

The N-terminal SH2 domain of Syk is required for (hem)ITAM, but not integrin, signaling in mouse platelets

We have used a novel knockin mouse to investigate the effect of disruption of phosphotyrosine binding of the N-terminal SH2 domain of Syk on platelet activation by GPVI, CLEC-2, and integrin αIIbβ3. The Syk(R41Afl/fl) mouse was crossed to a PF4-Cre(+) mouse to induce expression of the Syk mutant in the megakaryocyte/platelet lineage. Syk(R41Afl/fl;PF4-Cre) mice are born at approximately 50% of the expected frequency and have a similar phenotype to Syk(fl/fl;PF4-Cre) mice, including blood-lymphatic mixing and chyloascites. Anastomosis of the venous and lymphatic vasculatures can be seen in the mesenteric circulation accounting for rapid and continuous mixing of the 2 vasculatures. Platelet activation by CLEC-2 and GPVI is abolished in Syk(R41Afl/fl;PF4-Cre) platelets. Syk phosphorylation on Tyr519/20 is blocked in CLEC-2-stimulated platelets, suggesting a model in which binding of Syk via its N-terminal SH2 domain regulates autophosphorylation. In contrast, outside-in signaling by integrin αIIbβ3 is not altered, but it is inhibited in the presence of inhibitors of Src and Syk tyrosine kinases. These results demonstrate that αIIbβ3 regulates Syk through an ITAM-independent pathway in mice and provide novel insight into the course of events underlying Syk activation and hemITAM phosphorylation by CLEC-2.

SLAP/SLAP2 prevent excessive platelet (hem)ITAM signaling in thrombosis and ischemic stroke in mice

Glycoprotein VI and C-type lectin-like receptor 2 are essential platelet activating receptors in hemostasis and thrombo-inflammatory disease, which signal through a (hem)immunoreceptor tyrosine-based activation motif (ITAM)-dependent pathway. The adapter molecules Src-like adapter proteins (SLAP and SLAP2) are involved in the regulation of immune cell surface expression and signaling, but their function in platelets is unknown. In this study, we show that platelets expressed both SLAP isoforms and that overexpression of either protein in a heterologous cell line almost completely inhibited glycoprotein VI and C-type lectin-like receptor 2 signaling. In mice, single deficiency of SLAP or SLAP2 had only moderate effects on platelet function, whereas double deficiency of both adapters resulted in markedly increased signal transduction, integrin activation, granule release, aggregation, procoagulant activity, and thrombin generation in response to (hem)ITAM-coupled, but not G protein-coupled, receptor activation. In vivo, constitutive SLAP/SLAP2 knockout mice displayed accelerated occlusive arterial thrombus formation and a dramatically worsened outcome after focal cerebral ischemia. This was attributed to the absence of both adapter proteins in platelets, as demonstrated by adoptive transfer of Slap(-/-)/Slap2(-/-) platelets into wild-type mice. Our results establish SLAP and SLAP2 as critical inhibitors of platelet (hem)ITAM signaling in the setting of arterial thrombosis and ischemic stroke.

Emerging Roles for Platelets in Inflammation and Disease

Platelets and their interaction with cells of the immune system contribute through a variety of molecular mechanisms to support hemostasis and inflammation. These simple yet essential cells exert their effects in lymphocytes, monocytes, and neutrophils, both recruiting and modulating their function after activation. Emerging evidence is starting to define the mechanisms that allow platelets to also play pivotal roles in host defense. For example, platelet cell-surface expression of toll-like receptors allows platelets to direct neutrophil activation toward extracellular trap formation and facilitate the elimination of blood pathogens. In addition to these well-known receptors, two of the most recently discovered platelet receptors, C-type lectin receptor 2 (CLEC-2), and TREM-like transcript-1 (TLT-1), have been shown to modulate hemostatic and inflammation-related roles in platelets. This review will discuss the evolution of our understanding of platelet functions from hemostasis to inflammation, and highlight novel mechanisms that platelets use to mediate hemostasis under inflammatory pressure.

What can proteomics tell us about platelets?

More than 130 years ago, it was recognized that platelets are key mediators of hemostasis. Nowadays, it is established that platelets participate in additional physiological processes and contribute to the genesis and progression of cardiovascular diseases. Recent data indicate that the platelet proteome, defined as the complete set of expressed proteins, comprises >5000 proteins and is highly similar between different healthy individuals. Owing to their anucleate nature, platelets have limited protein synthesis. By implication, in patients experiencing platelet disorders, platelet (dys)function is almost completely attributable to alterations in protein expression and dynamic differences in post-translational modifications. Modern platelet proteomics approaches can reveal (1) quantitative changes in the abundance of thousands of proteins, (2) post-translational modifications, (3) protein-protein interactions, and (4) protein localization, while requiring only small blood donations in the range of a few milliliters. Consequently, platelet proteomics will represent an invaluable tool for characterizing the fundamental processes that affect platelet homeostasis and thus determine the roles of platelets in health and disease. In this article we provide a critical overview on the achievements, the current possibilities, and the future perspectives of platelet proteomics to study patients experiencing cardiovascular, inflammatory, and bleeding disorders.