Thrombopoietin levels in sepsis and septic shock - a systematic review and meta-analysis

OBJECTIVES

Sepsis is a life-threatening condition implicating an inadequate activation of the immune system. Platelets act as modulators and contributors to immune processes. Indeed, altered platelet turnover, thrombotic events, and changes in thrombopoietin levels in systemic inflammation have been reported, but thrombopoietin-levels in sepsis and septic-shock have not yet been systematically evaluated. We therefore performed a meta-analysis of thrombopoietin (TPO)-levels in patients with sepsis.

METHODS

Two independent reviewers screened records and full-text articles for inclusion. Scientific databases were searched for studies examining thrombopoietin levels in adult sepsis and septic-shock patients until August 1st 2022.

RESULTS

Of 95 items screened, six studies met the inclusion criteria, including 598 subjects. Both sepsis and severe sepsis were associated with increased levels of thrombopoietin (sepsis vs. control: standardized mean difference 3.06, 95 % CI 1.35-4.77; Z=3.50, p=0.0005) (sepsis vs. severe sepsis: standardized mean difference -1.67, 95 % CI -2.46 to -0.88; Z=4.14, p<0.0001). TPO-levels did not show significant differences between severe sepsis and septic shock patients but differed between sepsis and inflammation-associated non-septic controls. Overall, high heterogeneity and low sample size could be noted.

CONCLUSIONS

Concluding, increased levels of thrombopoietin appear to be present both in sepsis and severe sepsis with high heterogeneity but thrombopoietin does not allow to differentiate between severe sepsis and septic-shock. TPO may potentially serve to differentiate sepsis from non-septic trauma and/or tissue damage related (systemic) inflammation. Usage of different assays and high heterogeneity demand standardization of methods and further large multicenter trials.

Dynamic actin/septin network in megakaryocytes coordinates proplatelet elaboration

Megakaryocytes (MK) undergo extensive cytoskeletal rearrangements as they give rise to platelets. While cortical microtubule sliding has been implicated in proplatelet formation, the role of the actin cytoskeleton in proplatelet elongation is less understood. It is assumed that actin filament reorganization is important for platelet generation given that mouse models with mutations in actin-associated proteins exhibit thrombocytopenia. However, due to the essential role of the actin network during MK development, a differential understanding of the contribution of the actin cytoskeleton on proplatelet release is lacking. Here, we reveal that inhibition of actin polymerization impairs the formation of elaborate proplatelets by hampering proplatelet extension and bead formation along the proplatelet shaft, which was mostly independent of changes in cortical microtubule sliding. We identify Cdc42 and its downstream effectors, septins, as critical regulators of intracellular actin dynamics in MK, inhibition of which, similarly to inhibition of actin polymerization, impairs proplatelet movement and beading. Super-resolution microscopy revealed a differential association of distinctive septins with the actin and microtubule cytoskeleton, respectively, which was disrupted upon septin inhibition and diminished intracellular filamentous actin dynamics. In vivo, septins, similarly to F-actin, were subject to changes in expression upon enforcing proplatelet formation through prior platelet depletion. In summary, we demonstrate that a Cdc42/septin axis is not only important for MK maturation and polarization, but is further required for intracellular actin dynamics during proplatelet formation.

Evidence for a cytoplasmic proplatelet promoting factor that triggers platelet production

Not available.

The bone marrow is the primary site of thrombopoiesis

ABSTRACT

Megakaryocytes (MKs) generate thousands of platelets over their lifespan. The roles of platelets in infection and inflammation has guided an interest to the study of extramedullary thrombopoiesis and therefore MKs have been increasingly reported within the spleen and lung. However, the relative abundance of MKs in these organs compared to the bone marrow and the scale of their contribution to the platelet pool in a steady state remain controversial. We investigated the relative abundance of MKs in the adult murine bone marrow, spleen, and lung using whole-mount light-sheet and quantitative histological imaging, flow cytometry, intravital imaging, and an assessment of single-cell RNA sequencing (scRNA-seq) repositories. Flow cytometry revealed significantly higher numbers of hematopoietic stem and progenitor cells and MKs in the murine bone marrow than in spleens or perfused lungs. Two-photon intravital and light-sheet microscopy, as well as quantitative histological imaging, confirmed these findings. Moreover, ex vivo cultured MKs from the bone marrow subjected to static or microfluidic platelet production assays had a higher capacity for proplatelet formation than MKs from other organs. Analysis of previously published murine and human scRNA-seq data sets revealed that only a marginal fraction of MK-like cells can be found within the lung and most likely only marginally contribute to platelet production in the steady state.

Sodium bicarbonate as a local adjunctive agent for limiting platelet activation, aggregation, and adhesion within cardiovascular therapeutic devices

Cardiovascular therapeutic devices (CTDs) remain limited by thrombotic adverse events. Current antithrombotic agents limit thrombosis partially, often adding to bleeding. The Impella® blood pump utilizes heparin in 5% dextrose (D5W) as an internal purge to limit thrombosis. While effective, exogenous heparin often complicates overall anticoagulation management, increasing bleeding tendency. Recent clinical studies suggest sodium bicarbonate (bicarb) may be an effective alternative to heparin for local anti-thrombosis. We examined the effect of sodium bicarbonate on human platelet morphology and function to better understand its translational utility. Human platelets were incubated (60:40) with D5W + 25 mEq/L, 50 mEq/L, or 100 mEq/L sodium bicarbonate versus D5W or D5W + Heparin 50 U/mL as controls. pH of platelet-bicarbonate solutions mixtures was measured. Platelet morphology was examined via transmission electron microscopy; activation assessed via P-selectin expression, phosphatidylserine exposure and thrombin generation; and aggregation with TRAP-6, calcium ionophore, ADP and collagen quantified; adhesion to glass measured via fluorescence microscopy. Sodium bicarbonate did not alter platelet morphology but did significantly inhibit activation, aggregation, and adhesion. Phosphatidylserine exposure and thrombin generation were both reduced in a concentration-dependent manner-between 26.6 ± 8.2% (p = 0.01) and 70.7 ± 5.6% (p < 0.0001); and 14.0 ± 6.2% (p = 0.15) and 41.7 ± 6.8% (p = 0.03), respectively, compared to D5W control. Platelet aggregation via all agonists was also reduced, particularly at higher concentrations of bicarb. Platelet adhesion to glass was similarly reduced, between 0.04 ± 0.03% (p = 0.61) and 0.11 ± 0.04% (p = 0.05). Sodium bicarbonate has direct, local, dose-dependent effects limiting platelet activation and adhesion. Our results highlight the potential utility of sodium bicarbonate as a locally acting agent to limit device thrombosis.

A Critical Role for ERO1α in Arterial Thrombosis and Ischemic Stroke

BACKGROUND

Platelet adhesion and aggregation play a crucial role in arterial thrombosis and ischemic stroke. Here, we identify platelet ERO1α (endoplasmic reticulum oxidoreductase 1α) as a novel regulator of Ca2+ signaling and a potential pharmacological target for treating thrombotic diseases.

METHODS

Intravital microscopy, animal disease models, and a wide range of cell biological studies were utilized to demonstrate the pathophysiological role of ERO1α in arteriolar and arterial thrombosis and to prove the importance of platelet ERO1α in platelet activation and aggregation. Mass spectrometry, electron microscopy, and biochemical studies were used to investigate the molecular mechanism. We used novel blocking antibodies and small-molecule inhibitors to study whether ERO1α can be targeted to attenuate thrombotic conditions.

RESULTS

Megakaryocyte-specific or global deletion of Ero1α in mice similarly reduced platelet thrombus formation in arteriolar and arterial thrombosis without affecting tail bleeding times and blood loss following vascular injury. We observed that platelet ERO1α localized exclusively in the dense tubular system and promoted Ca2+ mobilization, platelet activation, and aggregation. Platelet ERO1α directly interacted with STIM1 (stromal interaction molecule 1) and SERCA2 (sarco/endoplasmic reticulum Ca2+-ATPase 2) and regulated their functions. Such interactions were impaired in mutant STIM1-Cys49/56Ser and mutant SERCA2-Cys875/887Ser. We found that ERO1α modified an allosteric Cys49-Cys56 disulfide bond in STIM1 and a Cys875-Cys887 disulfide bond in SERCA2, contributing to Ca2+ store content and increasing cytosolic Ca2+ levels during platelet activation. Inhibition of Ero1α with small-molecule inhibitors but not blocking antibodies attenuated arteriolar and arterial thrombosis and reduced infarct volume following focal brain ischemia in mice.

CONCLUSIONS

Our results suggest that ERO1α acts as a thiol oxidase for Ca2+ signaling molecules, STIM1 and SERCA2, and enhances cytosolic Ca2+ levels, promoting platelet activation and aggregation. Our study provides evidence that ERO1α may be a potential target to reduce thrombotic events.

Spatial transcriptomics of murine bone marrow megakaryocytes at single-cell resolution

Background

While megakaryocytes are known for making platelets, recent single-cell RNA sequencing data have revealed subpopulations of megakaryocytes with predicted immunoregulatory and bone marrow niche-supporting roles. Although these studies uncovered interesting information regarding the transcriptional variation of megakaryocytes, the generation, localization, and regulation of these subsets have not yet been studied and therefore remain incompletely understood. Considering the complex organization of the bone marrow, we reasoned that the application of spatial transcriptomic approaches could help dissect megakaryocyte heterogeneity within a spatiotemporal context.

Objectives

The aim of this study was to combine spatial context and transcriptomics to assess the heterogeneity of murine bone marrow megakaryocytes in situ at a single-cell level.

Methods

Bone marrow sections were obtained from femurs of C57BL/6J mice. Using the murine whole transcriptome array on the Nanostring GeoMx digital spatial profiling platform, we profiled 44 individual megakaryocytes (CD41+ by immunofluorescence) in situ throughout the bone marrow, both adjacent and nonadjacent to the endothelium (directly in contact with vascular endothelial-cadherin-positive cells).

Results

Principal component analysis revealed no association between transcriptomic profile and adjacency to the vasculature. However, there was a significant effect of proximal vs distal regions of the bone. Two and 3 genes were found overexpressed in the proximal and distal sides, respectively. Of note, proplatelet basic protein and platelet factor 4, 2 genes associated with platelet production, had higher expression in proximal megakaryocytes.

Conclusion

This study indicates a possible effect of spatial location on megakaryocyte heterogeneity and substantiate further interest in investigating megakaryocyte subpopulations in the context of their spatial orientation.

Shear-Mediated Platelet Microparticles Demonstrate Phenotypic Heterogeneity as to Morphology, Receptor Distribution, and Hemostatic Function

Implantable Cardiovascular Therapeutic Devices (CTD), while lifesaving, impart supraphysiologic shear stress to platelets, resulting in thrombotic and bleeding coagulopathy. We previously demonstrated that shear-mediated platelet dysfunction is associated with downregulation of platelet GPIb-IX-V and αIIbβ3 receptors via generation of Platelet-Derived MicroParticles (PDMPs). Here, we test the hypothesis that sheared PDMPs manifest phenotypical heterogeneity of morphology and receptor surface expression and modulate platelet hemostatic function. Human gel-filtered platelets were exposed to continuous shear stress. Alterations of platelet morphology were visualized using transmission electron microscopy. Surface expression of platelet receptors and PDMP generation were quantified by flow cytometry. Thrombin generation was quantified spectrophotometrically, and platelet aggregation was measured by optical aggregometry. Shear stress promotes notable alterations in platelet morphology and ejection of distinctive types of PDMPs. Shear-mediated microvesiculation is associated with the remodeling of platelet receptors, with PDMPs expressing significantly higher levels of adhesion receptors (αIIbβ3, GPIX, PECAM-1, P-selectin, and PSGL-1) and agonist receptors (P2Y12 and PAR1). Sheared PDMPs promote thrombin generation and inhibit platelet aggregation induced by collagen and ADP. Sheared PDMPs demonstrate phenotypic heterogeneity as to morphology and defined patterns of surface receptors and impose a bidirectional effect on platelet hemostatic function. PDMP heterogeneity suggests that a range of mechanisms are operative in the microvesiculation process, contributing to CTD coagulopathy and posing opportunities for therapeutic manipulation.

Efficient megakaryopoiesis and platelet production require phospholipid remodeling and PUFA uptake through CD36

Lipids contribute to hematopoiesis and membrane properties and dynamics, however, little is known about the role of lipids in megakaryopoiesis. Here, a lipidomic analysis of megakaryocyte progenitors, megakaryocytes, and platelets revealed a unique lipidome progressively enriched in polyunsaturated fatty acid (PUFA)-containing phospholipids. In vitro, inhibition of both exogenous fatty acid functionalization and uptake and de novo lipogenesis impaired megakaryocyte differentiation and proplatelet production. In vivo, mice on a high saturated fatty acid diet had significantly lower platelet counts, which was prevented by eating a PUFA-enriched diet. Fatty acid uptake was largely dependent on CD36, and its deletion in mice resulted in thrombocytopenia. Moreover, patients with a CD36 loss-of-function mutation exhibited thrombocytopenia and increased bleeding. Our results suggest that fatty acid uptake and regulation is essential for megakaryocyte maturation and platelet production, and that changes in dietary fatty acids may be a novel and viable target to modulate platelet counts.

Shear-Mediated Platelet Microparticles Demonstrate Phenotypic Heterogeneity as to Morphology, Receptor Distribution, and Hemostatic Function

Objective

Implantable cardiovascular therapeutic devices (CTD) including stents, percutaneous heart valves and ventricular assist devices, while lifesaving, impart supraphysiologic shear stress to platelets resulting in thrombotic and bleeding device-related coagulopathy. We previously demonstrated that shear-mediated platelet dysfunction is associated with downregulation of platelet GPIb-IX-V and αIIbβ3 receptors via generation of platelet-derived microparticles (PDMPs). Here, we test the hypothesis that shear-generated PDMPs manifest phenotypical heterogeneity of their morphology and surface expression of platelet receptors, and modulate platelet hemostatic function.

Approach and Results

Human gel-filtered platelets were exposed to continuous shear stress and sonication. Alterations of platelet morphology were visualized using transmission electron microscopy. Surface expression of platelet receptors and PDMP generation were quantified by flow cytometry. Thrombin generation was quantified spectrophotometrically, and platelet aggregation in plasma was measured by optical aggregometry. We demonstrate that platelet exposure to shear stress promotes notable alterations in platelet morphology and ejection of several distinctive types of PDMPs. Shear-mediated microvesiculation is associated with the differential remodeling of platelet receptors with PDMPs expressing significantly higher levels of both adhesion (α _IIb β ₃ , GPIX, PECAM-1, P-selectin, and PSGL-1) and agonist-evoked receptors (P ₂ Y ₁₂ & PAR1). Shear-mediated PDMPs have a bidirectional effect on platelet hemostatic function, promoting thrombin generation and inhibiting platelet aggregation induced by collagen and ADP.

Conclusions

Shear-generated PDMPs demonstrate phenotypic heterogeneity as to morphologic features and defined patterns of surface receptor alteration, and impose a bidirectional effect on platelet hemostatic function. PDMP heterogeneity suggests that a range of mechanisms are operative in the microvesiculation process, contributing to CTD coagulopathy and posing opportunities for therapeutic manipulation.

Pro-inflammatory megakaryocyte gene expression in murine models of breast cancer

Despite abundant research demonstrating that platelets can promote tumor cell metastasis, whether primary tumors affect platelet-producing megakaryocytes remains understudied. In this study, we used a spontaneous murine model of breast cancer to show that tumor burden reduced megakaryocyte number and size and disrupted polyploidization. Single-cell RNA sequencing demonstrated that megakaryocytes from tumor-bearing mice exhibit a pro-inflammatory phenotype, epitomized by increased Ctsg, Lcn2, S100a8, and S100a9 transcripts. Protein S100A8/A9 and lipocalin-2 levels were also increased in platelets, suggesting that tumor-induced alterations to megakaryocytes are passed on to their platelet progeny, which promoted in vitro tumor cell invasion and tumor cell lung colonization to a greater extent than platelets from wild-type animals. Our study is the first to demonstrate breast cancer-induced alterations in megakaryocytes, leading to qualitative changes in platelet content that may feedback to promote tumor metastasis.

DNA Origami-Platelet Adducts: Nanoconstruct Binding without Platelet Activation

Objective. Platelets are small, mechanosensitive blood cells responsible for maintaining vascular integrity and activatable on demand to limit bleeding and facilitate thrombosis. While circulating in the blood, platelets are exposed to a range of mechanical and chemical stimuli, with the platelet membrane being the primary interface and transducer of outside-in signaling. Sensing and modulating these interface signals would be useful to study mechanochemical interactions; yet, to date, no methods have been defined to attach adducts for sensor fabrication to platelets without triggering platelet activation. We hypothesized that DNA origami, and methods for its attachment, could be optimized to enable nonactivating instrumentation of the platelet membrane. Approach and Results. We designed and fabricated multivalent DNA origami nanotile constructs to investigate nanotile hybridization to membrane-embedded single-stranded DNA-tetraethylene glycol cholesteryl linkers. Two hybridization protocols were developed and validated (Methods I and II) for rendering high-density binding of DNA origami nanotiles to human platelets. Using quantitative flow cytometry, we showed that DNA origami binding efficacy was significantly improved when the number of binding overhangs was increased from two to six. However, no additional binding benefit was observed when increasing the number of nanotile overhangs further to 12. Using flow cytometry and transmission electron microscopy, we verified that hybridization with DNA origami constructs did not cause alterations in the platelet morphology, activation, aggregation, or generation of platelet-derived microparticles. Conclusions. Herein, we demonstrate that platelets can be successfully instrumented with DNA origami constructs with no or minimal effect on the platelet morphology and function. Our protocol allows for efficient high-density binding of DNA origami to platelets using low quantities of the DNA material to label a large number of platelets in a timely manner. Nonactivating platelet-nanotile adducts afford a path for advancing the development of DNA origami nanoconstructs for cell-adherent mechanosensing and therapeutic agent delivery.

Don't you forget about me(gakaryocytes)

Platelets (small, anucleate cell fragments) derive from large precursor cells, megakaryocytes (MKs), that reside in the bone marrow. MKs emerge from hematopoietic stem cells in a complex differentiation process that involves cytoplasmic maturation, including the formation of the demarcation membrane system, and polyploidization. The main function of MKs is the generation of platelets, which predominantly occurs through the release of long, microtubule-rich proplatelets into vessel sinusoids. However, the idea of a 1-dimensional role of MKs as platelet precursors is currently being questioned because of advances in high-resolution microscopy and single-cell omics. On the one hand, recent findings suggest that proplatelet formation from bone marrow-derived MKs is not the only mechanism of platelet production, but that it may also occur through budding of the plasma membrane and in distant organs such as lung or liver. On the other hand, novel evidence suggests that MKs not only maintain physiological platelet levels but further contribute to bone marrow homeostasis through the release of extracellular vesicles or cytokines, such as transforming growth factor β1 or platelet factor 4. The notion of multitasking MKs was reinforced in recent studies by using single-cell RNA sequencing approaches on MKs derived from adult and fetal bone marrow and lungs, leading to the identification of different MK subsets that appeared to exhibit immunomodulatory or secretory roles. In the following article, novel insights into the mechanisms leading to proplatelet formation in vitro and in vivo will be reviewed and the hypothesis of MKs as immunoregulatory cells will be critically discussed.

The secreted tyrosine kinase VLK is essential for normal platelet activation and thrombus formation

Tyrosine phosphorylation of extracellular proteins is observed in cell cultures and in vivo, but little is known about the functional roles of tyrosine phosphorylation of extracellular proteins. Vertebrate lonesome kinase (VLK) is a broadly expressed secretory pathway tyrosine kinase present in platelet α-granules. It is released from platelets upon activation and phosphorylates substrates extracellularly. Its role in platelet function, however, has not been previously studied. In human platelets, we identified phosphorylated tyrosines mapped to luminal or extracellular domains of transmembrane and secreted proteins implicated in the regulation of platelet activation. To determine the role of VLK in extracellular tyrosine phosphorylation and platelet function, we generated mice with a megakaryocyte/platelet-specific deficiency of VLK. Platelets from these mice are normal in abundance and morphology but have significant changes in function both in vitro and in vivo. Resting and thrombin-stimulated VLK-deficient platelets exhibit a significant decrease in several tyrosine phosphobands. Results of functional testing of VLK-deficient platelets show decreased protease-activated receptor 4-mediated and collagen-mediated platelet aggregation but normal responses to adenosine 5'-diphosphate. Dense granule and α-granule release are reduced in these platelets. Furthermore, VLK-deficient platelets exhibit decreased protease-activated receptor 4-mediated Akt (S473) and Erk1/2 (T202/Y204) phosphorylation, indicating altered proximal signaling. In vivo, mice lacking VLK in megakaryocytes/platelets display strongly reduced platelet accumulation and fibrin formation after laser-induced injury of cremaster arterioles compared with control mice but with normal bleeding times. These studies show that the secretory pathway tyrosine kinase VLK is critical for stimulus-dependent platelet activation and thrombus formation, providing the first evidence that a secreted protein kinase is required for normal platelet function.

Platelet Dysfunction During Mechanical Circulatory Support: Elevated Shear Stress Promotes Downregulation of αIIbβ3 and GPIb via Microparticle Shedding Decreasing Platelet Aggregability

[Figure: see text].

Sequence-specific 2'-O-methoxyethyl antisense oligonucleotides activate human platelets through glycoprotein VI, triggering formation of platelet-leukocyte aggregates

Antisense oligonucleotides (ASO) are DNA-based, disease-modifying drugs. Clinical trials with 2'-O-methoxyethyl (2’MOE) ASO have shown dose- and sequence-specific lowering of platelet counts according to two phenotypes. Phenotype 1 is a moderate (but not clinically severe) drop in platelet count. Phenotype 2 is rare, severe thrombocytopenia. This article focuses on the underlying cause of the more common phenotype 1, investigating the effects of ASO on platelet production and platelet function. Five phosphorothioate ASO were studied: three 2’MOE sequences; 487660 (no effects on platelet count), 104838 (associated with phenotype 1), and 501861 (effects unknown) and two CpG sequences; 120704 and ODN 2395 (known to activate platelets). Human cord bloodderived megakaryocytes were treated with these ASO to study their effects on proplatelet production. Platelet activation (determined by surface P-selectin) and platelet-leukocyte aggregates were analyzed in ASO-treated blood from healthy human volunteers. None of the ASO inhibited proplatelet production by human megakaryocytes. All the ASO were shown to bind to the platelet receptor glycoprotein VI (KD ~0.2-1.5 μM). CpG ASO had the highest affinity to glycoprotein VI, the most potent platelet-activating effects and led to the greatest formation of platelet-leukocyte aggregates. 2’MOE ASO 487660 had no detectable platelet effects, while 2’MOE ASOs 104838 and 501861 triggered moderate platelet activation and SYKdependent formation of platelet-leukocyte aggregates. Donors with higher platelet glycoprotein VI levels had greater ASO-induced platelet activation. Sequence-dependent ASO-induced platelet activation and platelet-leukocyte aggregates may explain phenotype 1 (moderate drops in platelet count). Platelet glycoprotein VI levels could be useful as a screening tool to identify patients at higher risk of ASO-induced platelet side effects.

VWF maturation and release are controlled by 2 regulators of Weibel-Palade body biogenesis: exocyst and BLOC-2

von Willebrand factor (VWF) is an essential hemostatic protein that is synthesized in endothelial cells and stored in Weibel-Palade bodies (WPBs). Understanding the mechanisms underlying WPB biogenesis and exocytosis could enable therapeutic modulation of endogenous VWF, yet optimal targets for modulating VWF release have not been established. Because biogenesis of lysosomal related organelle-2 (BLOC-2) functions in the biogenesis of platelet dense granules and melanosomes, which like WPBs are lysosome-related organelles, we hypothesized that BLOC-2-dependent endolysosomal trafficking is essential for WPB biogenesis and sought to identify BLOC-2-interacting proteins. Depletion of BLOC-2 caused misdirection of cargo-carrying transport tubules from endosomes, resulting in immature WPBs that lack endosomal input. Immunoprecipitation of BLOC-2 identified the exocyst complex as a binding partner. Depletion of the exocyst complex phenocopied BLOC-2 depletion, resulting in immature WPBs. Furthermore, releasates of immature WPBs from either BLOC-2 or exocyst-depleted endothelial cells lacked high-molecular weight (HMW) forms of VWF, demonstrating the importance of BLOC-2/exocyst-mediated endosomal input during VWF maturation. However, BLOC-2 and exocyst showed very different effects on VWF release. Although BLOC-2 depletion impaired exocytosis, exocyst depletion augmented WPB exocytosis, indicating that it acts as a clamp. Exposure of endothelial cells to a small molecule inhibitor of exocyst, Endosidin2, reversibly augmented secretion of mature WPBs containing HMW forms of VWF. These studies show that, although BLOC-2 and exocyst cooperate in WPB formation, only exocyst serves to clamp WPB release. Exocyst function in VWF maturation and release are separable, a feature that can be exploited to enhance VWF release.

High-content, label-free analysis of proplatelet production from megakaryocytes

BACKGROUND

The mechanisms that regulate platelet biogenesis remain unclear; factors that trigger megakaryocytes (MKs) to initiate platelet production are poorly understood. Platelet formation begins with proplatelets, which are cellular extensions originating from the MK cell body.

OBJECTIVES

Proplatelet formation is an asynchronous and dynamic process that poses unique challenges for researchers to accurately capture and analyze. We have designed an open-source, high-content, high-throughput, label-free analysis platform.

METHODS

Phase-contrast images of live, primary MKs are captured over a 24-hour period. Pixel-based machine-learning classification done by ilastik generates probability maps of key cellular features (circular MKs and branching proplatelets), which are processed by a customized CellProfiler pipeline to identify and filter structures of interest based on morphology. A subsequent reinforcement classification, by CellProfiler Analyst, improves the detection of cellular structures.

RESULTS

This workflow yields the percent of proplatelet production, area, count of proplatelets and MKs, and other statistics including skeletonization information for measuring proplatelet branching and length. We propose using a combination of these analyzed metrics, in particular the area measurements of MKs and proplatelets, when assessing in vitro proplatelet production. Accuracy was validated against manually counted images and an existing algorithm. We then used the new platform to test compounds known to cause thrombocytopenia, including bromodomain inhibitors, and uncovered previously unrecognized effects of drugs on proplatelet formation, thus demonstrating the utility of our analysis platform.

CONCLUSION

This advance in creating unbiased data analysis will increase the scale and scope of proplatelet production studies and potentially serve as a valuable resource for investigating molecular mechanisms of thrombocytopenia.

Platelet Activation via Shear Stress Exposure Induces a Differing Pattern of Biomarkers of Activation versus Biochemical Agonists

BACKGROUND

 Implantable cardiovascular therapeutic devices, while hemodynamically effective, remain limited by thrombosis. A driver of device-associated thrombosis is shear-mediated platelet activation (SMPA). Underlying mechanisms of SMPA, as well as useful biomarkers able to detect and discriminate mechanical versus biochemical platelet activation, are poorly defined. We hypothesized that SMPA induces a differing pattern of biomarkers compared with biochemical agonists.

METHODS

 Gel-filtered human platelets were subjected to mechanical activation via either uniform constant or dynamic shear; or to biochemical activation by adenosine diphosphate (ADP), thrombin receptor-activating peptide 6 (TRAP-6), thrombin, collagen, epinephrine, or arachidonic acid. Markers of platelet activation (P-selectin, integrin αIIbβ3 activation) and apoptosis (mitochondrial membrane potential, caspase 3 activation, and phosphatidylserine externalization [PSE]) were examined using flow cytometry. Platelet procoagulant activity was detected by chromogenic assay measuring thrombin generation. Contribution of platelet calcium flux in SMPA was tested employing calcium chelators, ethylenediaminetetraacetic acid (EDTA), and BAPTA-AM.

RESULTS

 Platelet exposure to continuous shear stress, but not biochemical agonists, resulted in a dramatic increase of PSE and procoagulant activity, while no integrin αIIbβ3 activation occurred, and P-selectin levels remained barely elevated. SMPA was associated with dissipation of mitochondrial membrane potential, but no caspase 3 activation was observed. Shear-mediated PSE was significantly decreased by chelation of extracellular calcium with EDTA, while intracellular calcium depletion with BAPTA-AM had no significant effect. In contrast, biochemical agonists ADP, TRAP-6, arachidonic acid, and thrombin were potent inducers of αIIbβ3 activation and/or P-selectin exposure. This differing pattern of biomarkers seen for SMPA for continuous uniform shear was replicated in platelets exposed to dynamic shear stress via circulation through a ventricular assist device-propelled circulatory loop.

CONCLUSION

 Elevated shear stress, but not biochemical agonists, induces a differing pattern of platelet biomarkers-with enhanced PSE and thrombin generation on the platelet surface. This differential biomarker phenotype of SMPA offers the potential for early detection and discrimination from that mediated by biochemical agonists.

Anti-apoptotic BCL2L2 increases megakaryocyte proplatelet formation in cultures of human cord blood

Apoptosis is a recognized limitation to generating large numbers of megakaryocytes in culture. The genes responsible have been rigorously studied in vivo in mice, but are poorly characterized in human culture systems. As CD34-positive (+) cells isolated from human umbilical vein cord blood were differentiated into megakaryocytes in culture, two distinct cell populations were identified by flow cytometric forward and side scatter: larger size, lower granularity (LLG), and smaller size, higher granularity (SHG). The LLG cells were CD41aHigh CD42aHigh phosphatidylserineLow, had an electron microscopic morphology similar to mature bone marrow megakaryocytes, developed proplatelets, and displayed a signaling response to platelet agonists. The SHG cells were CD41aLowCD42aLowphosphatidylserineHigh, had a distinctly apoptotic morphology, were unable to develop proplatelets, and showed no signaling response. Screens of differentiating megakaryocytes for expression of 24 apoptosis genes identified BCL2L2 as a novel candidate megakaryocyte apoptosis regulator. Lentiviral BCL2L2 overexpression decreased megakaryocyte apoptosis, increased CD41a+ LLG cells, and increased proplatelet formation by 58%. An association study in 154 healthy donors identified a significant positive correlation between platelet number and platelet BCL2L2 mRNA levels. This finding was consistent with the observed increase in platelet-like particles derived from cultured megakaryocytes over-expressing BCL2L2 BCL2L2 also induced small, but significant increases in thrombin-induced platelet-like particle αIIbβ3 activation and P-selectin expression. Thus, BCL2L2 restrains apoptosis in cultured megakaryocytes, promotes proplatelet formation, and is associated with platelet number. BCL2L2 is a novel target for improving megakaryocyte and platelet yields in in vitro culture systems.

Megakaryocyte emperipolesis mediates membrane transfer from intracytoplasmic neutrophils to platelets

Bone marrow megakaryocytes engulf neutrophils in a phenomenon termed emperipolesis. We show here that emperipolesis is a dynamic process mediated actively by both lineages, in part through the β2-integrin/ICAM-1/ezrin pathway. Tethered neutrophils enter in membrane-bound vesicles before penetrating into the megakaryocyte cytoplasm. Intracytoplasmic neutrophils develop membrane contiguity with the demarcation membrane system, thereby transferring membrane to the megakaryocyte and to daughter platelets. This phenomenon occurs in otherwise unmanipulated murine marrow in vivo, resulting in circulating platelets that bear membrane from non-megakaryocytic hematopoietic donors. Transit through megakaryocytes can be completed as rapidly as minutes, after which neutrophils egress intact. Emperipolesis is amplified in models of murine inflammation associated with platelet overproduction, contributing to platelet production in vitro and in vivo. These findings identify emperipolesis as a new cell-in-cell interaction that enables neutrophils and potentially other cells passing through the megakaryocyte cytoplasm to modulate the production and membrane content of platelets.

Abstract 37: Centrosome Destabilization Through the Ubiquitin-Proteasome Pathway Regulates Proplatelet Production

Background: Proteasome inhibitors such as bortezomib, a chemotherapeutic used to treat multiple myeloma, induce thrombocytopenia within days of initiation. The mechanism for this thrombocytopenia has been tied to data revealing that proteasome activity is essential for platelet formation. The major pathway of selective protein degradation uses ubiquitin as a marker that targets proteins for proteolysis by the proteasome. This pathway is previously unexplored in megakaryocytes (MKs).

Objectives: We aim to define the mechanism by which the ubiquitin-proteasome pathway affects MK maturation and platelet production.

Results: Pharmacologic inhibition of proteasome activity blocks proplatelet formation in megakaryocytes. To further characterize how this degradation was occurring, we probed distinct ubiquitin pathways. Inhibition of the ubiquitin-activating enzyme E1 significantly inhibited proplatelet formation up to 73%. In addition, inhibition of the deubiquitinase proteins UCHL5 and USP14 significantly inhibited proplatelet formation up to 83%. These data suggest that an intact ubiquitin pathway is necessary for proplatelet formation.

Proteomic and polysome analyses of MKs undergoing proplatelet formation revealed a subset of proteins decreased in proplatelet-producing megakaryocytes, consistent with data showing that protein degradation is necessary for proplatelet formation. Specifically, the centrosome stabilizing proteins Aurora kinase (Aurk) A/B, Tpx2, Cdk1, and Plk1 were decreased in proplatelet-producing MKs. Furthermore, inhibition of AurkA and Plk1, but not Cdk1, significantly inhibited proplatelet formation in vitro over 83%.

Conclusions: We hypothesize that proplatelet formation is triggered by centrosome destabilization and disassembly, and that the ubiquitin-proteasome pathway plays a crucial role in this transformation. Specifically, regulation of the AurkA/Plk1/Tpx2 pathway may be key in centrosome integrity and initiation of proplatelet formation. Determination of the mechanism by which the ubiquitin-proteasome pathway regulates the centrosome and facilitates proplatelet formation will allow us to design better strategies to target and reverse thrombocytopenia.

Megakaryocytes compensate for Kit insufficiency in murine arthritis

The growth factor receptor Kit is involved in hematopoietic and nonhematopoietic development. Mice bearing Kit defects lack mast cells; however, strains bearing different Kit alleles exhibit diverse phenotypes. Herein, we investigated factors underlying differential sensitivity to IgG-mediated arthritis in 2 mast cell-deficient murine lines: KitWsh/Wsh, which develops robust arthritis, and KitW/Wv, which does not. Reciprocal bone marrow transplantation between KitW/Wv and KitWsh/Wsh mice revealed that arthritis resistance reflects a hematopoietic defect in addition to mast cell deficiency. In KitW/Wv mice, restoration of susceptibility to IgG-mediated arthritis was neutrophil independent but required IL-1 and the platelet/megakaryocyte markers NF-E2 and glycoprotein VI. In KitW/Wv mice, platelets were present in numbers similar to those in WT animals and functionally intact, and transfer of WT platelets did not restore arthritis susceptibility. These data implicated a platelet-independent role for the megakaryocyte, a Kit-dependent lineage that is selectively deficient in KitW/Wv mice. Megakaryocytes secreted IL-1 directly and as a component of circulating microparticles, which activated synovial fibroblasts in an IL-1-dependent manner. Transfer of WT but not IL-1-deficient megakaryocytes restored arthritis susceptibility to KitW/Wv mice. These findings identify functional redundancy among Kit-dependent hematopoietic lineages and establish an unanticipated capacity of megakaryocytes to mediate IL-1-driven systemic inflammatory disease.

Tamoxifen Directly Inhibits Platelet Angiogenic Potential and Platelet-Mediated Metastasis

OBJECTIVE

Platelets, which are mainly known for their role in hemostasis, are now known to play a crucial role in metastasis. Tamoxifen is a selective estrogen receptor modulator that is widely used for the treatment of breast cancer. Tamoxifen and its metabolites have been shown to directly impact platelet function, suggesting that this drug has additional mechanisms of action. The purpose of this study was to determine whether tamoxifen exerts antitumor effects through direct platelet inhibition.

APPROACH AND RESULTS

This study found that pretreatment with tamoxifen leads to a significant inhibition of platelet activation. Platelets exposed to tamoxifen released significantly lower amounts of proangiogenic regulator vascular endothelial growth factor. In vitro angiogenesis assays confirmed that tamoxifen pretreatment led to diminished capillary tube formation and decreased endothelial migration. Tamoxifen and its metabolite, 4-hydroxytamoxifen, also significantly inhibited the ability of platelets to promote metastasis in vitro. Using a membrane-based array, we identified several proteins associated with angiogenesis metastasis that were lower in activated releasate from tamoxifen-treated platelets, including angiogenin, chemokine (C-X-C motif) ligand 1, chemokine (C-C motif) ligand 5, epidermal growth factor, chemokine (C-X-C motif) ligand 5, platelet-derived growth factor dimeric isoform BB, whereas antiangiogenic angiopoietin-1 was elevated. Platelets isolated from patients on tamoxifen maintenance therapy were also found to have decreased activation responses, diminished vascular endothelial growth factor release, and lower angiogenic and metastatic potential.

CONCLUSIONS

We demonstrate that tamoxifen and its metabolite 4-hydroxytamoxifen directly alter platelet function leading to decreased angiogenic and metastatic potential. Furthermore, this study supports the idea of utilizing targeted platelet therapies to inhibit the platelet's role in angiogenesis and malignancy.

Abstract C12: Platelets promote breast cancer metastasis by reprogramming tumor cells to produce IL-8

Platelets, primarily known for their role in hemostasis, are now recognized to play an integral role in cancer progression and metastasis. Recent evidence has established that platelets are activated by tumor cells, including breast cancer cells, leading to the release of hundreds of growth factors, cytokines, chemokines and angiogenesis mediators that could influence tumor growth and metastasis. Indeed, work from our group has demonstrated that factors released from activated platelets promote both metastasis and angiogenesis. However, little is known about the specific factors and signaling pathways that mediate this critical platelet-tumor cell crosstalk. To address this question, we performed an angiogenesis array (Ray Biotech) to identify specific pro-angiogenic and pro-metastatic factors released by tumor cells during platelet-tumor cell interactions. We identified several factors that were secreted by MCF-7 breast tumor cells in response to activated platelet releasate, including high levels of interleukin 8 (IL-8, CXCL8). IL-8 is a cytokine known to play a critical role in metastasis and angiogenesis and is elevated in the serum and tumor tissue of breast cancer patients. We confirmed that exposure to platelets strongly induced the production of IL-8 in several human breast cancer cell lines (MDA-MB-231, BT-20, SKBR-3 and MCF-7) by ELISA and found that platelets themselves do not contain detectable levels of IL-8. Furthermore, IL-8 production was highest in the more aggressive, triple negative MDA-MB-231 and BT-20 lines, suggesting a link between platelet-induced IL-8 and tumor subtype. Next we sought to determine the role of platelet-induced IL-8 in metastasis. We performed standard invasion assays using MDA-MB-231 cells transfected with IL-8shRNA or control cells. Platelets were able to increase the invasion of control MDA-MD-231 cells by 5 fold, while IL-8 knockdown reduced the effect by 50%. Furthermore, the ability of platelets to promote tumor cell migration across an endothelialized membrane was reduced 87% in IL-8 knockdown MDA-MB-231s compared to controls in standard transendothelial migration assays. These results suggest that platelets promote metastasis, in part, by driving tumor cell IL-8. To identify the specific component or components of platelet releasate responsible for driving tumor cell IL-8, we first characterized the contents of activated platelet releasate by array (Ray Biotech) and found an abundance of both chemokine (C-C motif) ligand 5 (CCL5, RANTES) and epidermal growth factor (EGF). Next, we treated breast tumor cell lines directly with recombinant CCL5 or EGF and observed an increase in IL-8 production; however, sensitivity to CCL5, EGF or the combination varied among the cell lines tested. We found that cell lines MCF-7 and MDA-MB-231, which express the CCL5 receptor CCR5, produced IL-8 in response to CCL5 while BT-20 and SKBR-3 cells produce IL-8 in response to EGF and express high levels of EGFR. To determine if platelet-derived CCL5 drives tumor cell IL-8 in MDA-MD-231 and MCF-7 cells, tumor cells were pretreated with the CCR5 blocker maraviroc and then exposed to platelets. CCR5 blockade abrogated the induction of IL-8 in response to platelets and decreased platelet-induced invasion. Similarly EGFR blockage with AG-1478 reduced IL-8 production in platelet-treated BT-20 and SKBR-3 tumor cells. Furthermore, pre-treatment of platelets with aspirin, an irreversible platelet inhibitor, diminished their ability to drive tumor cell IL-8 and to enhance invasion. Taken together, these results suggest that platelets, through release of soluble factors, drive tumor cells to produce IL-8 and that blocking this communication can disrupt the pro-metastatic potential of platelets. Ultimately, these studies support targeting specific platelet-tumor cell interactions as a novel means of limiting disease progression in breast cancer.

Citation Format: Kelly E. Johnson, Kellie R. Machlus, Jodi A. Forward, Mason D. Tippy, Saleh A. El-Husayni, Joseph E. Italiano, Jr., Elisabeth M. Battinelli. Platelets promote breast cancer metastasis by reprogramming tumor cells to produce IL-8. [abstract]. In: Proceedings of the AACR Special Conference: Function of Tumor Microenvironment in Cancer Progression; 2016 Jan 7–10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2016;76(15 Suppl):Abstract nr C12.

Abstract 41: Platelet Dream Plays a Critical Role During Thrombogenesis in Mice

Downstream regulatory element antagonist modulator (DREAM), a transcriptional repressor, is known to modulate pain. Using intravital microscopy with DREAM-null mice and their bone marrow chimeras, we demonstrated that hematopoietic and endothelial cell DREAM are required for platelet thrombus formation following arteriolar injury. DREAM deletion also prolonged tail bleeding times. In vitro flow chamber assays and in vivo adoptive transfer experiments indicated the importance of platelet DREAM in thrombogenesis. We found that deletion of platelet DREAM does not alter ultrastructural features but significantly impaired aggregation and ATP secretion induced by collagen-related peptide (CRP), ADP, or A23187, but not thrombin or U46619. Biochemical studies showed that platelet DREAM is required for phosphoinositide 3-kinase (PI3K) activation induced by GPVI-, A23187-, or integrin-mediated signaling. Studies using DREAM-null platelets and isoform-specific PI3K inhibitors revealed that platelet DREAM positively regulates granule secretion, Ca2+ mobilization, and aggregation induced by CRP or A23187 through PI3K class Iβ (PI3K-Iβ) activity. Genetic and pharmacological studies in megakaryoblastic MEG-01 cells showed that DREAM regulates A23187-induced Ca2+ mobilization and that the regulatory function of DREAM requires Ca2+ binding and PI3K-Iβ activity. Taken together, we have identified platelet DREAM as a novel regulator of PI3K-Iβ activity during thrombus formation.

Human thrombopoiesis depends on Protein kinase Cδ/protein kinase Cε functional couple

A deeper understanding of the molecular events driving megakaryocytopoiesis and thrombopoiesis is essential to regulate in vitro and in vivo platelet production for clinical applications. We previously documented the crucial role of PKCε in the regulation of human and mouse megakaryocyte maturation and platelet release. However, since several data show that different PKC isoforms fulfill complementary functions, we targeted PKCε and PKCδ, which show functional and phenotypical reciprocity, at the same time as boosting platelet production in vitro. Results show that PKCδ, contrary to PKCε, is persistently expressed during megakaryocytic differentiation, and a forced PKCδ down-modulation impairs megakaryocyte maturation and platelet production. PKCδ and PKCε work as a functional couple with opposite roles on thrombopoiesis, and the modulation of their balance strongly impacts platelet production. Indeed, we show an imbalance of PKCδ/PKCε ratio both in primary myelofibrosis and essential thrombocythemia, featured by impaired megakaryocyte differentiation and increased platelet production, respectively. Finally, we demonstrate that concurrent molecular targeting of both PKCδ and PKCε represents a strategy for in vitro platelet factories.

Cytoskeletal perturbation leads to platelet dysfunction and thrombocytopenia in variant forms of Glanzmann thrombasthenia

Several patients have been reported to have variant dominant forms of Glanzmann thrombasthenia, associated with macrothrombocytopenia and caused by gain-of-function mutations of ITGB3 or ITGA2B leading to reduced surface expression and constitutive activation of integrin αIIbβ3. The mechanisms leading to a bleeding phenotype of these patients have never been addressed. The aim of this study was to unravel the mechanism by which ITGB3 mutations causing activation of αIIbβ3 lead to platelet dysfunction and macrothrombocytopenia. Using platelets from two patients carrying the β3 del647-686 mutation and Chinese hamster ovary cells expressing different αIIbβ3-activating mutations, we showed that reduced surface expression of αIIbβ3 is due to receptor internalization. Moreover, we demonstrated that permanent triggering of αIIbβ3-mediated outside-in signaling causes an impairment of cytoskeletal reorganization arresting actin turnover at the stage of polymerization. The induction of actin polymerization by jasplakinolide, a natural toxin that promotes actin nucleation and prevents depolymerization of stress fibers, in control platelets produced an impairment of platelet function similar to that of patients with variant forms of dominant Glanzmann thrombasthenia. del647-686β3-transduced murine megakaryocytes generated proplatelets with a reduced number of large tips and asymmetric barbell-proplatelets, suggesting that impaired cytoskeletal rearrangement is the cause of macrothrombocytopenia. These data show that impaired cytoskeletal remodeling caused by a constitutively activated αIIbβ3 is the main effector of platelet dysfunction and macrothrombocytopenia, and thus of bleeding, in variant forms of dominant Glanzmann thrombasthenia.

Abnormal megakaryopoiesis and platelet function in cyclooxygenase-2-deficient mice

Previous studies suggest that cyclooxygenase-2 (COX-2) might influence megakaryocyte (MK) maturation and platelet production in vitro. Using a gene deletion model, we analysed the effect of COX-2 deficiency on megakaryopoiesis and platelet function. COX-2-/- mice (10-12 weeks old) have hyper-responsive platelets as suggested by their enhanced aggregation, TXA2 biosynthesis, CD62P and CD41/CD61 expression, platelet-fibrinogen binding, and increased thromboembolic death after collagen/epinephrine injection compared to wild-type (WT). Moreover, increased platelet COX-1 expression and reticulated platelet fraction were observed in COX-2-/- mice while platelet count was similar to WT. MKs were significantly reduced in COX-2-/- bone marrows (BMs), with high nuclear/cytoplasmic ratios, low ploidy and poor expression of lineage markers of maturation (CD42d, CD49b). However, MKs were significantly increased in COX-2-/- spleens, with features of MK maturation markers which were not observed in MKs of WT spleens. Interestingly, the expression of COX-1, prostacyclin and PGE2 synthases and prostanoid pattern were modified in BMs and spleens of COX-2-/- mice. Moreover, COX-2 ablation reduced the percentage of CD49b+ cells, the platelet formation and the haematopoietic stem cells in bone marrow and increased their accumulation in the spleen. Splenectomy decreased peripheral platelet number, reverted their hyper-responsive phenotype and protected COX-2-/- mice from thromboembolism. Interestingly, fibrosis was observed in spleens of old COX-2-/- mice (28 weeks old). In conclusion, COX-2 deletion delays BM megakaryopoiesis promoting a compensatory splenic MK hyperplasia, with a release of hyper-responsive platelets and increased thrombogenicity in vivo. COX-2 seems to contribute to physiological MK maturation and pro-platelet formation.

IL-1α induces thrombopoiesis through megakaryocyte rupture in response to acute platelet needs

An alternative pathway triggering enhanced platelet release from bone marrow megakaryocytes via a rupture-based mechanism is regulated by IL-1α in response to acute platelet requirements.

Intravital visualization of thrombopoiesis revealed that formation of proplatelets, which are cytoplasmic protrusions in bone marrow megakaryocytes (MKs), is dominant in the steady state. However, it was unclear whether this is the only path to platelet biogenesis. We have identified an alternative MK rupture, which entails rapid cytoplasmic fragmentation and release of much larger numbers of platelets, primarily into blood vessels, which is morphologically and temporally different than typical FasL-induced apoptosis. Serum levels of the inflammatory cytokine IL-1α were acutely elevated after platelet loss or administration of an inflammatory stimulus to mice, whereas the MK-regulator thrombopoietin (TPO) was not elevated. Moreover, IL-1α administration rapidly induced MK rupture–dependent thrombopoiesis and increased platelet counts. IL-1α–IL-1R1 signaling activated caspase-3, which reduced plasma membrane stability and appeared to inhibit regulated tubulin expression and proplatelet formation, and ultimately led to MK rupture. Collectively, it appears the balance between TPO and IL-1α determines the MK cellular programming for thrombopoiesis in response to acute and chronic platelet needs.

Road blocks in making platelets for transfusion

The production of laboratory-generated human platelets is necessary to meet present and future transfusion needs. This manuscript will identify and define the major roadblocks that must be overcome to make human platelet production possible for clinical use, and propose solutions necessary to accelerate development of laboratory-generated human platelets to market.

Inducible Gata1 suppression expands megakaryocyte-erythroid progenitors from embryonic stem cells

Transfusion of donor-derived platelets is commonly used for thrombocytopenia, which results from a variety of clinical conditions and relies on a constant donor supply due to the limited shelf life of these cells. Embryonic stem (ES) and induced pluripotent stem (iPS) cells represent a potential source of megakaryocytes and platelets for transfusion therapies; however, the majority of current ES/iPS cell differentiation protocols are limited by low yields of hematopoietic progeny. In both mice and humans, mutations in the gene-encoding transcription factor GATA1 cause an accumulation of proliferating, developmentally arrested megakaryocytes, suggesting that GATA1 suppression in ES and iPS cell-derived hematopoietic progenitors may enhance megakaryocyte production. Here, we engineered ES cells from WT mice to express a doxycycline-regulated (dox-regulated) shRNA that targets Gata1 transcripts for degradation. Differentiation of these cells in the presence of dox and thrombopoietin (TPO) resulted in an exponential (at least 10¹³-fold) expansion of immature hematopoietic progenitors. Dox withdrawal in combination with multilineage cytokines restored GATA1 expression, resulting in differentiation into erythroblasts and megakaryocytes. Following transfusion into recipient animals, these dox-deprived mature megakaryocytes generated functional platelets. Our findings provide a readily reproducible strategy to exponentially expand ES cell-derived megakaryocyte-erythroid progenitors that have the capacity to differentiate into functional platelet-producing megakaryocytes.

A secreted tyrosine kinase acts in the extracellular environment

Although tyrosine phosphorylation of extracellular proteins has been reported to occur extensively in vivo, no secreted protein tyrosine kinase has been identified. As a result, investigation of the potential role of extracellular tyrosine phosphorylation in physiological and pathological tissue regulation has not been possible. Here, we show that VLK, a putative protein kinase previously shown to be essential in embryonic development, is a secreted protein kinase, with preference for tyrosine, that phosphorylates a broad range of secreted and ER-resident substrate proteins. We find that VLK is rapidly and quantitatively secreted from platelets in response to stimuli and can tyrosine phosphorylate coreleased proteins utilizing endogenous as well as exogenous ATP sources. We propose that discovery of VLK activity provides an explanation for the extensive and conserved pattern of extracellular tyrosine phosphophorylation seen in vivo, and extends the importance of regulated tyrosine phosphorylation into the extracellular environment.

Microtubule sliding drives proplatelet elongation and is dependent on cytoplasmic dynein

Bone marrow megakaryocytes produce platelets by extending long cytoplasmic protrusions, designated proplatelets, into sinusoidal blood vessels. Although microtubules are known to regulate platelet production, the underlying mechanism of proplatelet elongation has yet to be resolved. Here we report that proplatelet formation is a process that can be divided into repetitive phases (extension, pause, and retraction), as revealed by differential interference contrast and fluorescence loss after photoconversion time-lapse microscopy. Furthermore, we show that microtubule sliding drives proplatelet elongation and is dependent on cytoplasmic dynein under static and physiological shear stress by using fluorescence recovery after photobleaching in proplatelets with fluorescence-tagged β1-tubulin. A refined understanding of the specific mechanisms regulating platelet production will yield strategies to treat patients with thrombocythemia or thrombocytopenia.

Proteasome function is required for platelet production

The proteasome inhibiter bortezomib has been successfully used to treat patients with relapsed multiple myeloma; however, many of these patients become thrombocytopenic, and it is not clear how the proteasome influences platelet production. Here we determined that pharmacologic inhibition of proteasome activity blocks proplatelet formation in human and mouse megakaryocytes. We also found that megakaryocytes isolated from mice deficient for PSMC1, an essential subunit of the 26S proteasome, fail to produce proplatelets. Consistent with decreased proplatelet formation, mice lacking PSMC1 in platelets (Psmc1(fl/fl) Pf4-Cre mice) exhibited severe thrombocytopenia and died shortly after birth. The failure to produce proplatelets in proteasome-inhibited megakaryocytes was due to upregulation and hyperactivation of the small GTPase, RhoA, rather than NF-κB, as has been previously suggested. Inhibition of RhoA or its downstream target, Rho-associated protein kinase (ROCK), restored megakaryocyte proplatelet formation in the setting of proteasome inhibition in vitro. Similarly, fasudil, a ROCK inhibitor used clinically to treat cerebral vasospasm, restored platelet counts in adult mice that were made thrombocytopenic by tamoxifen-induced suppression of proteasome activity in megakaryocytes and platelets (Psmc1(fl/fl) Pdgf-Cre-ER mice). These results indicate that proteasome function is critical for thrombopoiesis, and suggest inhibition of RhoA signaling as a potential strategy to treat thrombocytopenia in bortezomib-treated multiple myeloma patients.

Interpreting the developmental dance of the megakaryocyte: a review of the cellular and molecular processes mediating platelet formation

Platelets are essential for haemostasis, and thrombocytopenia (platelet counts <150 × 10(9) /l) is a major clinical problem encountered across a number of conditions, including immune thrombocytopenic purpura, myelodysplastic syndromes, chemotherapy, aplastic anaemia, human immunodeficiency virus infection, complications during pregnancy and delivery, and surgery. Circulating blood platelets are specialized cells that function to prevent bleeding and minimize blood vessel injury. Platelets circulate in their quiescent form, and upon stimulation, activate to release their granule contents and spread on the affected tissue to create a physical barrier that prevents blood loss. The current model of platelet formation states that large progenitor cells in the bone marrow, called megakaryocytes, release platelets by extending long, branching processes, designated proplatelets, into sinusoidal blood vessels. This review will focus on different factors that impact megakaryocyte development, proplatelet formation and platelet release. It will highlight recent studies on thrombopoeitin-dependent megakaryocyte maturation, endomitosis and granule formation, cytoskeletal contributions to proplatelet formation, the role of apoptosis, and terminal platelet formation and release.

Macrocytic anemia and mitochondriopathy resulting from a defect in sideroflexin 4

We used exome sequencing to identify mutations in sideroflexin 4 (SFXN4) in two children with mitochondrial disease (the more severe case also presented with macrocytic anemia). SFXN4 is an uncharacterized mitochondrial protein that localizes to the mitochondrial inner membrane. sfxn4 knockdown in zebrafish recapitulated the mitochondrial respiratory defect observed in both individuals and the macrocytic anemia with megaloblastic features of the more severe case. In vitro and in vivo complementation studies with fibroblasts from the affected individuals and zebrafish demonstrated the requirement of SFXN4 for mitochondrial respiratory homeostasis and erythropoiesis. Our findings establish mutations in SFXN4 as a cause of mitochondriopathy and macrocytic anemia.

Animating platelet production adds physiological context

Animating complex biological processes contextualizes them within their underlying physiology, identifies gaps in our mechanistic understanding, affirms the importance of continued research, and provides a bridge between academic scientists and the general public. Here, two videos illustrate the clinical value of and translate state-of-the-art research in platelet production.

The incredible journey: From megakaryocyte development to platelet formation

Circulating blood platelets are specialized cells that prevent bleeding and minimize blood vessel injury. Large progenitor cells in the bone marrow called megakaryocytes (MKs) are the source of platelets. MKs release platelets through a series of fascinating cell biological events. During maturation, they become polyploid and accumulate massive amounts of protein and membrane. Then, in a cytoskeletal-driven process, they extend long branching processes, designated proplatelets, into sinusoidal blood vessels where they undergo fission to release platelets. Given the need for platelets in many pathological situations, understanding how this process occurs is an active area of research with important clinical applications.

Dynamin-related protein-1 controls fusion pore dynamics during platelet granule exocytosis

OBJECTIVE

Platelet granule exocytosis serves a central role in hemostasis and thrombosis. Recently, single-cell amperometry has shown that platelet membrane fusion during granule exocytosis results in the formation of a fusion pore that subsequently expands to enable the extrusion of granule contents. However, the molecular mechanisms that control platelet fusion pore expansion and collapse are not known.

METHODS AND RESULTS

We identified dynamin-related protein-1 (Drp1) in platelets and found that an inhibitor of Drp1, mdivi-1, blocked exocytosis of both platelet dense and α-granules. We used single-cell amperometry to monitor serotonin release from individual dense granules and, thereby, measured the effect of Drp1 inhibition on fusion pore dynamics. Inhibition of Drp1 increased spike width and decreased prespike foot events, indicating that Drp1 influences fusion pore formation and expansion. Platelet-mediated thrombus formation in vivo after laser-induced injury of mouse cremaster arterioles was impaired after infusion of mdivi-1.

CONCLUSIONS

These results demonstrate that inhibition of Drp1 disrupts platelet fusion pore dynamics and indicate that Drp1 can be targeted to control thrombus formation in vivo.

Unraveling mechanisms that control platelet production

Platelets are formed by giant precursor cells called megakaryocytes that reside within the bone marrow. The generation of platelets, and their release into the bloodstream by megakaryocytes, requires a complex series of remodeling events powered by the cytoskeleton to result in the release of many platelets from a single megakaryocyte. Abnormalities in this process can result in thrombocytopenia (low platelet count) and can lead to increased risk of bleeding. This review describes the process of platelet production in detail and discusses new insights into novel platelet biology.

Netrin-1 promotes glioblastoma cell invasiveness and angiogenesis by multiple pathways including activation of RhoA, cathepsin B, and cAMP-response element-binding protein

Glioblastomas are very difficult tumors to treat because they are highly invasive and disseminate within the normal brain, resulting in newly growing tumors. We have identified netrin-1 as a molecule that promotes glioblastoma invasiveness. As evidence, netrin-1 stimulates glioblastoma cell invasion directly through Matrigel-coated transwells, promotes tumor cell sprouting and enhances metastasis to lymph nodes in vivo. Furthermore, netrin-1 regulates angiogenesis as shown in specific angiogenesis assays such as enhanced capillary endothelial cells (EC) sprouting and by increased EC infiltration into Matrigel plugs in vivo, as does VEGF-A. This netrin-1 signaling pathway in glioblastoma cells includes activation of RhoA and cyclic AMP response element-binding protein (CREB). A novel finding is that netrin-1-induced glioblastoma invasiveness and angiogenesis are mediated by activated cathepsin B (CatB), a cysteine protease that translocates to the cell surface as an active enzyme and co-localizes with cell surface annexin A2 (ANXA2). The specific CatB inhibitor CA-074Me inhibits netrin-1-induced cell invasion, sprouting, and Matrigel plug angiogenesis. Silencing of CREB suppresses netrin-1-induced glioblastoma cell invasion, sprouting, and CatB expression. It is concluded that netrin-1 plays an important dual role in glioblastoma progression by promoting both glioblastoma cell invasiveness and angiogenesis in a RhoA-, CREB-, and CatB-dependent manner. Targeting netrin-1 pathways may be a promising strategy for brain cancer therapy.