Background The functional diversity of microtubules is regulated through the expression of distinct α- and β-tubulin isotypes together with several posttranslational modifications, a concept known as tubulin code. Tubulin detyrosination is a reversible posttranslational modification that consists of the removal of the genetically encoded C-terminal tyrosine residue of most α-tubulins. While this modification has been observed in the megakaryocyte lineage, its importance remains poorly understood in platelet biogenesis. Objectives To assess the role of α-tubulin detyrosination in platelet biogenesis. Methods The responsible enzymes and the relative abundance of detyrosinated α-tubulins were monitored by quantitative reverse transcription-polymerase chain reaction and Western blotting, respectively, in human cultured megakaryocytes and platelets differentiated from CD34+ hematopoietic stem and progenitor cells. The function of α-tubulin detyrosination was assessed in human cultured megakaryocytes treated with the VASH-SVBP inhibitor EpoY, and in mice constitutively inactivated for Svbp (which encodes the cofactor of the VASH detyrosinases). Results Transcriptional analysis identified VASH1-SVBP and MATCAP as the predominant detyrosinases in the megakaryocyte lineage. During megakaryocyte maturation, their transcript levels progressively increased and correlated with the accumulation of detyrosinated α-tubulins. Remarkably, inhibition of VASH1-SVBP by EpoY abolished tubulin detyrosination, establishing VASH1-SVBP as the main functional detyrosinase in megakaryocytes. More importantly, EpoY enhanced proplatelet formation and platelet production in vitro. These in vitro data were confirmed in vivo in SVBP-deficient mice, which exhibited an increase in platelet counts. Conclusion These findings reveal, for the first time, a role for tubulin detyrosination in proplatelet formation, thereby expanding our understanding of the megakaryocyte tubulin code beyond tubulin isotypes.

Different approaches are being developed to efficiently produce in vitro platelets from cultured megakaryocytes to meet the constant demand of platelet transfusion and serve for research purposes. Recent works have shown that turbulence and periodic stress can significantly enhance platelet yield. Here we have developed and characterized a platelet production device that takes in account these properties. This device is based on the Taylor-Couette reactor in which a suspension is confined and sheared between two concentric cylinders. We have demonstrated that such a system allows obtaining high number of in vitro platelets per megakaryocyte with native-like morphology and functional properties. Using the combination of in silico and in vitro techniques, we claimed that overall turbulent conditions are not sufficient for efficient platelet release, and highlighted the importance of the uniform impact of flow on each megakaryocyte, a property that must be taken into account along with general flow characteristics when designing platelet release bioreactors. In addition, we have demonstrated that our system can be scaled up to large volumes without loss of efficiency, a significant advantage for the industrialization of platelet culture. In conclusion, we have developed a platelet production device with a predictable and highly precise effect on each megakaryocyte.

BACKGROUND:Platelets prevent bleeding in a variety of inflammatory settings, the adhesion receptors and activation pathways involved being highly context-dependent and functionally redundant. In some situations, platelets recruited to inflammatory sites act independently of aggregation. The mechanisms underlying stable platelet adhesion in inflamed microvessels remain incompletely understood, in particular, whether and if so, how β1 and β3 integrins are involved. METHODS:The impact of isolated or combined platelet deficiency in β1 and β3 integrins on inflammation-associated hemostasis was investigated in 3 models of acute inflammation: immune complex-based cutaneous reverse passive Arthus reaction, intranasal lipopolysaccharide-induced lung inflammation, and cerebral ischemia-reperfusion following transient (2-hour) occlusion of the middle cerebral artery. RESULTS:Mice with platelet-directed inactivation of Itgb1 (PF4Cre-β1-/-) displayed no bleeding in any of the inflammation models, while mice defective in platelet Itgb3 (PF4Cre-β3-/-) exhibited bleeding in all 3 models. Remarkably, the bleeding phenotype of PF4Cre-β3-/- mice was exacerbated in the reverse passive Arthus model by the concomitant deletion of β1 integrins, PF4Cre-β1-/-/β3-/- animals presenting increased bleeding. Intravital microscopy in reverse passive Arthus experiments highlighted a major defect in the adhesion of PF4Cre-β1-/-/β3-/- platelets to inflamed microvessels. Unlike PF4Cre-β1-/- and PF4Cre-β3-/- mice, PF4Cre-β1-/-/β3-/- animals developed early hemorrhagic transformation 6 hours after transient middle cerebral artery occlusion. PF4Cre-β1-/-/β3-/- mice displayed no more bleeding in lipopolysaccharide-induced lung inflammation than PF4Cre-β3-/- animals. CONCLUSIONS:Altogether, these results show that the requirement for and degree of functional redundancy between platelet β1 and β3 integrins in inflammation-associated hemostasis vary with the inflammatory situation.

ABSTRACT:Glycoprotein Ibα (GPIbα) is expressed on the surface of platelets and megakaryocytes (MKs) and anchored to the membrane skeleton by filamin A (flnA). Although GPIb and flnA have fundamental roles in platelet biogenesis, the nature of this interaction in megakaryocyte biology remains ill-defined. We generated a mouse model expressing either human wild-type (WT) GPIbα (hGPIbαWT) or a flnA-binding mutant (hGPIbαFW) and lacking endogenous mouse GPIbα. Mice expressing the mutant GPIbα transgene exhibited macrothrombocytopenia with preserved GPIb surface expression. Platelet clearance was normal and differentiation of MKs to proplatelets was unimpaired in hGPIbαFW mice. The most striking abnormalities in hGPIbαFW MKs were the defective formation of the demarcation membrane system (DMS) and the redistribution of flnA from the cytoplasm to the peripheral margin of MKs. These abnormalities led to disorganized internal MK membranes and the generation of enlarged megakaryocyte membrane buds. The defective flnA-GPIbα interaction also resulted in misdirected release of buds away from the vasculature into bone marrow interstitium. Restoring the linkage between flnA and GPIbα corrected the flnA redistribution within MKs and DMS ultrastructural defects as well as restored normal bud size and release into sinusoids. These studies define a new mechanism of macrothrombocytopenia resulting from dysregulated MK budding. The link between flnA and GPIbα is not essential for the MK budding process, however, it plays a major role in regulating the structure of the DMS, bud morphogenesis, and the localized release of buds into the circulation.

Zebrafish is widely adopted as a grafting model for studying human development and diseases. Current zebrafish xenotransplantations are performed using embryo recipients, as the adaptive immune system, responsible for host versus graft rejection, only reaches maturity at juvenile stage. However, transplanted primary human hematopoietic stem/progenitor cells (HSC) rapidly disappear even in zebrafish embryos, suggesting that another barrier to transplantation exists before the onset of adaptive immunity. Here, using a labelled macrophage zebrafish line, we demonstrated that engraftment of human HSC induces a massive recruitment of macrophages which rapidly phagocyte transplanted cells. Macrophages depletion, by chemical or pharmacological treatments, significantly improved the uptake and survival of transplanted cells, demonstrating the crucial implication of these innate immune cells for the successful engraftment of human cells in zebrafish. Beyond identifying the reasons for human hematopoietic cell engraftment failure, this work images the fate of human cells in real time over several days in macrophage-depleted zebrafish embryos.

Introduction. Microtubules are hollow cylindrical polymers that are made of α/β-tubulin heterodimers. They are involved in a broad range of essential functions such as cell division, intracellular trafficking or cell motility. Their functional diversity is thought to be regulated through the expression of distinct α- and β-tubulin isotypes together with several post-translational modifications (PTMs); a concept known as tubulin code. In the megakaryocytic lineage, while tubulin isotypes are well-known regulators of platelet biogenesis (TUBB1, TUBA4A, TUBA8) 1, the temporal regulation and biological significance of tubulin PTMs remain mostly unknown. Aims. This project aims to characterize the kinetics and to evaluate the functional importance of tubulin detyrosination, one of the main α-tubulin PTM, in platelet biogenesis. Methods. Analyzes were performed in megakaryocytes and platelets obtained from cultured human CD34 + hematopoietic stem and progenitor cells. To characterize the kinetics of tubulin detyrosination, the PTM and its modifying enzymes were monitored at the proteomic (Western Blot) and transcriptional (qRT-PCR) levels, at several defined stages of megakaryocyte differentiation. To evaluate the function of tubulin detyrosination, differentiation of CD34 + hematopoietic stem and progenitor cells was performed in the presence of the SVBP/VASH-detyrosinase inhibitor EpoY 2 or following CRISPR-mediated inactivation of the VASH detyrosinase co-factor SVBP. Results. Tubulin detyrosination levels progressively increased as megakaryocytes matured. This was accompanied by a gradual increase in the transcript levels of the detyrosinase VASH1, while those of its cofactor SVBP remained stable. Transcripts of the newly identified detyrosinase MATCAP 3 mirrored those of VASH1. Together, these results suggest that tubulin detyrosination occurring during platelet biogenesis could be mediated by both the SVBP-VASH1 complex and/or MATCAP, and prompt the question of their functional importance in the process. To evaluate this, we first focused on the SVBP-VASH1 complex with the inhibitor EpoY. Interestingly, while this compound had no impact on early megakaryocyte differentiation, the final stage of platelet biogenesis, proplatelet formation, was perturbed. In particular, proplatelets obtained in the presence of EpoY showed morphological alterations with thinner unbranched shafts, suggesting potential repercussions on platelet production. Preliminary data obtained with SVBP-inactivated megakaryocytes confirmed these observations. Follow-up work will aim to evaluate the downstream consequences on platelet production as well as the relative contribution of MATCAP. Conclusion. These results suggest that tubulin detyrosination could act as a potential regulator of terminal megakaryocyte maturation, and further highlight the importance of the microtubule cytoskeleton and the tubulin code in platelet biogenesis. References. 1. Kimmerlin, Q., Strassel, C., Eckly, A., and Lanza, F. (2023). The tubulin code in platelet biogenesis. Semin Cell Dev Biol 137, 63-73. 10.1016/j.semcdb.2022.01.010. 2. Hotta, T., Haynes, S.E., Blasius, T.L., Gebbie, M., Eberhardt, E.L., Sept, D., Cianfrocco, M., Verhey, K.J., Nesvizhskii, A.I., and Ohi, R. (2021). Parthenolide Destabilizes Microtubules by Covalently Modifying Tubulin. Curr Biol 31, 900-907 e906. 10.1016/j.cub.2020.11.055. 3. Landskron, L., Bak, J., Adamopoulos, A., Kaplani, K., Moraiti, M., van den Hengel, L.G., Song, J.Y., Bleijerveld, O.B., Nieuwenhuis, J., Heidebrecht, T., et al. (2022). Posttranslational modification of microtubules by the MATCAP detyrosinase. Science 376, eabn6020. 10.1126/science.abn6020.

Blood platelets are small non-nucleated cellular fragments that prevent and stop hemorrhages. They are produced in the bone marrow by megakaryocytes through megakaryopoiesis. This intricate process involves profound microtubule rearrangements culminating in the formation of a unique circular sub-membranous microtubule array, the marginal band, which supports the typical disc-shaped morphology of platelets. Mechanistically, these processes are thought to be controlled by a specific tubulin code. In this review, we summarize the current knowledge on the key isotypes, notably β1-, α4A- and α8-tubulin, and putative post-translational modifications, involved in platelet and marginal band formation. Additionally, we provide a provisional list of microtubule-associated proteins (MAPs) involved in these processes and a survey of tubulin variants identified in patients presenting defective platelet production. A comprehensive characterization of the platelet tubulin code and the identification of essential MAPs may be expected in the near future to shed new light on a very specialized microtubule assembly process with applications in platelet diseases and transfusion.

Megakaryocytes (MKs) are the precursor cells of platelets, located in the bone marrow (BM). Once mature, they extend elongated projections named proplatelets through sinusoid vessels, emerging from the marrow stroma into the circulating blood. Not all signals from the microenvironment that regulate proplatelet formation are understood, particularly those from the BM biomechanics. We sought to investigate how MKs perceive and adapt to modifications of the stiffness of their environment. Although the BM is one of the softest tissue of the body, its rigidification results from excess fibronectin (FN), and other matrix protein deposition occur upon myelofibrosis. Here, we have shown that mouse MKs are able to detect the stiffness of a FN-coated substrate and adapt their morphology accordingly. Using a polydimethylsiloxane substrate with stiffness varying from physiological to pathological marrow, we found that a stiff matrix favors spreading, intracellular contractility, and FN fibrils assembly at the expense of proplatelet formation. Itgb3, but not Itgb1, is required for stiffness sensing, whereas both integrins are involved in fibrils assembly. In contrast, soft substrates promote proplatelet formation in an Itgb3-dependent manner, consistent with the ex vivo decrease in proplatelet formation and the in vivo decrease in platelet number in Itgb3-deficient mice. Our findings demonstrate the importance of environmental stiffness for MK functions with potential pathophysiological implications during pathologies that deregulate FN deposition and modulate stiffness in the marrow.

In vitro production of blood platelets for transfusion purposes is gaining interest. While platelet production is now possible on a laboratory scale, the challenge is to move towards industrial production. Attaining this goal calls for the development of platelet release devices capable of producing large quantities of platelets. To this end, we have developed a continuous-flow platelet release device composed of five spherical chambers each containing two calibrated cones placed in a staggered configuration. Following perfusion of proplatelet-bearing cultured megakaryocytes, the device achieves a high yield of about 100 bona-fide platelets/megakaryocyte, at a flow rate of ∼80 mL/min. Performances and operating conditions comply with the requirements of large-scale platelet production. Moreover, this device enabled an in-depth analysis of the flow regimes through Computational Fluid Dynamics (CFD). This revealed two new universal parameters to be taken into account for an optimal platelet release: i.e. a periodic hydrodynamic load and a sufficient accumulation of shear stress. An efficient 16 Pa.s shear stress accumulation is obtained in our system at a flow rate of 80 mL/min.

Native circulating blood platelets present with a discoid flat morphology maintained by a submembranous peripheral ring of microtubules, named marginal band. The functional importance of this particular shape is still debated, but it was initially hypothesized to facilitate platelet interaction with the injured vessel wall , to contribute to hemostasis. The importance of the platelet discoid morphology has since been questioned on the absence of clear bleeding tendency in mice lacking the platelet-specific beta 1-tubulin isotype, which exhibits platelets with a thinner marginal band and an ovoid shape. Here, we generated a mouse model inactivated for beta 1-tubulin and alpha 4A-tubulin, an alpha-tubulin isotype strongly enriched in platelets. These mice present with fully spherical platelets completely devoid of a marginal band. In contrast to the single knockouts, the double deletion resulted in a severe bleeding defect in a tail-clipping assay, which was not corrected by increasing the platelet count to normal values by the thrombopoietin-analog romiplostim. In vivo, thrombus formation was almost abolished in a ferric chloride & ndash;injury model, with a thin of and mice were death in a model of thromboembolism. In vitro, platelets adhered less efficiently , formed smaller-sized and loosely assembled aggregates when perfused over von Willebrand factor and collagen matrices. In conclusion, this study shows that blood platelets require 2 unique alpha- and beta-tubulin isotypes to acquire their characteristic discoid morphology. Lack of these 2 isotypes has a deleterious effect on flow-dependent aggregate formation and stability, leading to a severe bleeding disorder.

•Platelets lacking α4A- and β1-tubulins are devoid of marginal band and fully spherical and inefficiently interact with matrices under flow.•Mice deficient for α4A- and β1-tubulins exhibit severe defects in hemostatic and thrombotic responses in vivo.

Glycoprotein V (GPV) is a highly expressed 82 KDa platelet surface transmembrane protein which is loosely attached to the GPIb-IX complex. Despite remaining questions concerning its function, GPV presents several unique features which have repercussions in hematology, atherothrombosis, immunology and transfusion. GPV is specifically expressed in platelets and megakaryocytes and is an ideal marker and reporter gene for the late stages of megakaryopoiesis. The ectodomain of GPV can be released by a number of proteases, namely thrombin, elastase and ADAM10 and 17. Although it was originally proposed as a thrombin receptor, this hypothesis was abandoned since thrombin activation was preserved after blockade of GPV cleavage and in Gp5 knockout mice. The combined potential of GPV to reflect the direct action of thrombin, platelet exposure to strong agonists and inflammatory conditions has led one to evaluate its utility as a marker in the context of atherothrombosis. Increased plasma levels of soluble GPV have notably been recorded in myocardial infarction, stroke and venous thromboembolism. It is also highly valued in transfusion to monitor platelet storage lesions. GPV presents several polymorphisms, which are a possible source of alloantibodies, while autoantibodies have been frequently detected in immune thrombocytopenia. The real biological function of this glycoprotein nevertheless remains an enigma, despite the respectively decreased and increased responses to low concentrations of collagen and thrombin observed in Gp5 knockout mice. Current studies are exploring its role in modulating general or VWF-induced platelet signaling, which could bear relevance in thrombosis and platelet clearance.

BACKGROUND:Glycoprotein (GP)Ibα plays a critical role in regulating platelet clearance. Recently, we identified the mechanosensory domain (MSD) in GPIbα and reported evidence to suggest that unfolding of the GPIbα MSD induces exposure of the Trigger sequence therein and subsequent GPIb-IX signaling that accelerates platelet clearance. In a commonly used transgenic mouse model, IL4R-IbαTg, where the Trigger sequence is constitutively exposed, constitutive GPIb-IX-mediated cellular signals are present. Clearance of their platelets is also significantly faster than that of wild-type mice. Previously, an anti-GPIbβ antibody RAM.1 was developed. RAM.1 inhibits GPIbα-dependent platelet signaling and activation. Further, RAM.1 also inhibits anti-GPIbα antibody-mediated filopodia formation.OBJECTIVE:To investigate whether RAM.1 can ameliorate trigger sequence exposure-mediated platelet clearance.METHODS:Spontaneous filopodia were measured by confocal microscopy. Other platelet signaling events were measured by flow cytometry. Endogenous platelet life span was tracked by Alexa 488-labeled anti-mouse GPIX antibody.RESULT:Transfected Chinese hamster ovary cells stably expressing the same chimeric IL4R-Ibα protein complex as in IL4R-IbαTg mice also constitutively exhibit filopodia, and that such filopodia could be abolished by treatment of RAM.1. Further, transfusion of a recombinant RAM.1 derivative that is devoid of its Fc portion significantly extends the endogenous life span of IL4R-IbαTg platelets.CONCLUSION:These results provide the key evidence supporting the causative link of Trigger sequence exposure to accelerated platelet clearance, and suggest that a RAM.1 derivative may be therapeutically developed to treat GPIb-IX-mediated thrombocytopenia.

During infection neuraminidase desialylates platelets and induces their rapid clearance from circulation. The underlying molecular basis, particularly the role of platelet glycoprotein (GP)Ibα therein, is not clear. Utilizing genetically altered mice we report that the extracellular domain of GPIbα, but neither von Willebrand factor nor ADAM17 (a disintegrin and metalloprotease 17), is required for platelet clearance induced by intravenous injection of neuraminidase. Lectin binding to platelets following neuraminidase injection over time revealed that the extent of desialylation of O-glycans correlates with the decrease of platelet count in mice. Injection of α2,3-neuraminidase reduces platelet counts in wild-type but not in transgenic mice expressing only a chimeric GPIbα that misses most of its extracellular domain. Neuraminidase treatment induces unfolding of the O-glycosylated mechanosensory domain in GPIbα as monitored by single-molecule force spectroscopy, increases the exposure of the ADAM17 shedding cleavage site in the mechanosensory domain on the platelet surface, and induces ligand-independent GPIb-IX signaling in human and murine platelets. These results suggest that desialylation of O-glycans of GPIbα induces unfolding of the mechanosensory domain, subsequent GPIb-IX signaling including amplified desialylation of N-glycans, and eventually rapid platelet clearance. This new molecular mechanism of GPIbα-facilitated clearance could potentially resolve many puzzling and seemingly contradicting observations associated with clearance of desialylated or hyposialylated platelets.

In vitro, the differentiation of megakaryocytes (MKs) is improved by aryl-hydrocarbon receptor (AHR) antagonists such as StemRegenin 1 (SR1), an effect physiologically recapitulated by the presence of stromal mesenchymal cells (MSC). This inhibition promotes the amplification of a CD34(+)CD41(low) population able to mature as MKs with a high capacity for platelet production. In this short report, we showed that the emergence of the thrombocytogenic precursors and the enhancement of platelet production triggered by SR1 involved IKAROS. The downregulation/inhibition of IKAROS (shRNA or lenalidomide) significantly reduced the emergence of SR1-induced thrombocytogenic population, suggesting a crosstalk between AHR and IKAROS. Interestingly, using a proximity ligation assay, we could demonstrate a physical interaction between AHR and IKAROS. This interaction was also observed in the megakaryocytic cells differentiated in the presence of MSCs. In conclusion, our study revealed a previously unknown AHR/ IKAROS -dependent pathway which prompted the expansion of the thrombocytogenic precursors. This AHR- IKAROS dependent checkpoint controlling MK maturation opens new perspectives to platelet production engineering.

Platelets are produced by megakaryocytes, specialized cells located in the bone marrow. The possibility to image megakaryocytes in real time and their native environment was described more than 10 years ago and sheds new light on the process of platelet formation. Megakaryocytes extend elongated protrusions, called proplatelets, through the endothelial lining of sinusoid vessels. This paper presents a protocol to simultaneously image in real time fluorescently labeled megakaryocytes in the skull bone marrow and sinusoid vessels. This technique relies on a minor surgery that keeps the skull intact to limit inflammatory reactions. The mouse head is immobilized with a ring glued to the skull to prevent movements from breathing. Using two-photon microscopy, megakaryocytes can be visualized for up to a few hours, enabling the observation of cell protrusions and proplatelets in the process of elongation inside sinusoid vessels. This allows the quantification of several parameters related to the morphology of the protrusions (width, length, presence of constriction areas) and their elongation behavior (velocity, regularity, or presence of pauses or retraction phases). This technique also allows simultaneous recording of circulating platelets in sinusoid vessels to determine platelet velocity and blood flow direction. This method is particularly useful to study the role of genes of interest in platelet formation using genetically modified mice and is also amenable to pharmacological testing (study the mechanisms, evaluating drugs in the treatment of platelet production disorders). It has become an invaluable tool, especially to complement in vitro studies as it is now known that in vivo and in vitro proplatelet formation rely on different mechanisms. It has been shown, for example, that in vitro microtubules are required for proplatelet elongation per se. However, in vivo, they rather serve as a scaffold, elongation being mainly promoted by blood flow forces.

The in vitro expansion and differentiation of human hematopoietic progenitors into megakaryocytes capable of elongating proplatelets and releasing platelets allows an in-depth study of the mechanisms underlying platelet biogenesis. Available culture protocols are mostly based on hematopoietic progenitors derived from bone marrow or cord blood raising a number of ethical, technical, and economic concerns. If there are already available protocols for obtaining CD34 cells from peripheral blood, this manuscript proposes a straightforward and optimized protocol for obtaining CD34+ cells from leukodepletion filters readily available in blood centers. These cells are isolated from leukodepletion filters used in the preparation of blood transfusion products, corresponding to eight blood donations. These filters are meant to be discarded. A detailed procedure to collect hematopoietic progenitors identified as CD34+ cells from these filters is described. The method to obtain mature megakaryocytes extending proplatelets while discussing their phenotypic evolution is also detailed. Finally, the protocol present a calibrated pipetting method, to efficiently release platelets that are morphologically and functionally similar to native ones. This protocol can serve as a basis for evaluating pharmacological compounds acting at various steps of the process to dissect the underlying mechanisms and approach the in vivo platelet yields.

Abstract Background: Glycoprotein (GP)Ib-IX complex plays a critical role in mediating platelet activation and platelet clearance. Recently, we identified the mechanosensory domain (MSD) in the GPIbα subunit, and demonstrated that unfolding of the MSD and subsequent exposure of the Trigger sequence (residues 473-483) therein activates GPIb-IX and induces rapid platelet clearance. This mechanism could explain acute thrombocytopenia induced by activated VWF, anti-GPIbα antibodies, neuraminidase, and ectodomain shedding of GPIbα. Consistently, platelets in IL4R-IbαTg mice, a transgenic strain in which the entire extracellular domain of human GPIbα except the Trigger sequence was replaced with that of the α-subunit of interleukin-4 receptor, exhibit constitutively more filopodia and are cleared much faster than the wild type. Previously, an anti-GPIbβ antibody RAM.1 was developed. RAM.1 significantly inhibits GPIb-IX-mediated filopodia formation and Ca 2+ signaling in platelets. In addition, it could inhibit GPIb-dependent thrombin generation. These results suggest that targeting GPIbβ could inhibit activation of GPIb-IX induced by MSD unfolding. Objectives: To investigate whether targeting GPIbβ with RAM.1 can impede rapid platelet clearance induced by exposed Trigger sequence and ameliorate related thrombocytopenia. Methods: Spontaneous filopodia in platelets and transfected Chinese hamster ovary (CHO) cells were visualized by fluorescence staining of actin and confocal microscopy. Images were quantified by ImageJ. Platelet signaling events, like P-selectin exposure, β-galactose exposure, and Ca 2+ influx, were measured by flow cytometry. Endogenous platelet life span was tracked by Alexa 488-labeled anti-mouse GPIX antibody. Results: CHO cells stably expressing the same mutant GPIb-IX complex in IL4R-IbαTg mouse platelets have been successfully obtained. Like IL4R-IbαTg platelets, these IL4R-IbαTg CHO cells exhibited spontaneous filopodia in the absence of any GPIbα ligands. RAM.1 could inhibit spontaneous filopodia formation in these CHO cells and IL4R-IbαTg platelets (Fig. 1, 2). Compared to wild-type mouse platelets, IL4R-IbαTg platelets constitutively exhibited increased P-selectin exposure, increased β-galactose exposure, and elevated intracellular Ca 2+, all of which could be inhibited by treatment of RAM.1 (Fig. 3). Recombinant RAM.1-GCN4 protein (rRAM.1-GCN4), in which the Fc region of RAM.1 heavy chain was replaced with the GCN4 coiled coil dimerizing sequence, has been generated and used as an alternative of the divalent RAM.1-Fab2. It retained the ability of RAM.1 antibody to inhibit GPIb-IX signaling. Injecting rRAM.1-GCN4 into IL4R-IbαTg mice dramatically improved the life span of endogenous IL4R-IbαTg platelets (Fig. 4). Conclusion: These results demonstrate that the exposed Trigger sequence is sufficient to activate GPIb-IX in transfected CHO cells, and that RAM.1 derivatives can impede GPIbα-mediated rapid platelet clearance. Targeting GPIbβ may be a novel approach to treat GPIb-related thrombocytopenia. Figure 1 Figure 1. Disclosures No relevant conflicts of interest to declare.

Differentiation and maturation of megakaryocytes occur in close association with the cellular and extracellular components of the bone marrow. These processes are characterized by the gradual appearance of essential structures in the megakaryocyte cytoplasm such as a polyploid and polylobulated nucleus, an internal membrane network called demarcation membrane system (DMS) and the dense and alpha granules that will be found in circulating platelets. In this article, we describe a standardized protocol for the in situ ultrastructural study of murine megakaryocytes using transmission electron microscopy (TEM), allowing for the identification of key characteristics defining their maturation stage and cellular density in the bone marrow. Bone marrows are flushed, fixed, dehydrated in ethanol, embedded in plastic resin, and mounted for generating cross-sections. Semi-thin and thin sections are prepared for histological and TEM observations, respectively. This method can be used for any bone marrow cell, in any EM facility and has the advantage of using small sample sizes allowing for the combination of several imaging approaches on the same mouse.