STEM tomography reveals that the canalicular system and α-granules remain separate compartments during early secretion stages in blood platelets

New insights of platelet endocytosis and its implication for platelet function

Endocytosis constitutes a cellular process in which cells selectively encapsulate surface substances into endocytic vesicles, also known as endosomes, thereby modulating their interaction with the environment. Platelets, as pivotal hematologic elements, play a crucial role not only in regulating coagulation and thrombus formation but also in facilitating tumor invasion and metastasis. Functioning as critical components in the circulatory system, platelets can internalize various endosomal compartments, such as surface receptors, extracellular proteins, small molecules, and pathogens, from the extracellular environment through diverse endocytic pathways, including pinocytosis, phagocytosis, and receptor-mediated endocytosis. We summarize recent advancements in platelet endocytosis, encompassing the catalog of cargoes, regulatory mechanisms, and internal trafficking routes. Furthermore, we describe the influence of endocytosis on platelet regulatory functions and related physiological and pathological processes, aiming to offer foundational insights for future research into platelet endocytosis.

Platelet-Rich Plasma in Dermatology: New Insights on the Cellular Mechanism of Skin Repair and Regeneration

The skin's recognised functions may undergo physiological alterations due to ageing, manifesting as varying degrees of facial wrinkles, diminished tautness, density, and volume. Additionally, these functions can be disrupted (patho)physiologically through various physical and chemical injuries, including surgical trauma, accidents, or chronic conditions like ulcers associated with diabetes mellitus, venous insufficiency, or obesity. Advancements in therapeutic interventions that boost the skin's innate regenerative abilities could significantly enhance patient care protocols. The application of Platelet-Rich Plasma (PRP) is widely recognized for its aesthetic and functional benefits to the skin. Yet, the endorsement of PRP's advantages often borders on the dogmatic, with its efficacy commonly ascribed solely to the activation of fibroblasts by the factors contained within platelet granules. PRP therapy is a cornerstone of regenerative medicine which involves the autologous delivery of conditioned plasma enriched by platelets. This is achieved by centrifugation, removing erythrocytes while retaining platelets and their granules. Despite its widespread use, the precise sequences of cellular activation, the specific cellular players, and the molecular machinery that drive PRP-facilitated healing are still enigmatic. There is still a paucity of definitive and robust studies elucidating these mechanisms. In recent years, telocytes (TCs)-a unique dermal cell population-have shown promising potential for tissue regeneration in various organs, including the dermis. TCs' participation in neo-angiogenesis, akin to that attributed to PRP, and their role in tissue remodelling and repair processes within the interstitia of several organs (including the dermis), offer intriguing insights. Their potential to contribute to, or possibly orchestrate, the skin regeneration process following PRP treatment has elicited considerable interest. Therefore, pursuing a comprehensive understanding of the cellular and molecular mechanisms at work, particularly those involving TCs, their temporal involvement in structural recovery following injury, and the interconnected biological events in skin wound healing and regeneration represents a compelling field of study.

Deep learning, 3D ultrastructural analysis reveals quantitative differences in platelet and organelle packing in COVID-19/SARSCoV2 patient-derived platelets

Platelets contribute to COVID-19 clinical manifestations, of which microclotting in the pulmonary vasculature has been a prominent symptom. To investigate the potential diagnostic contributions of overall platelet morphology and their α-granules and mitochondria to the understanding of platelet hyperactivation and micro-clotting, we undertook a 3D ultrastructural approach. Because differences might be small, we used the high-contrast, high-resolution technique of focused ion beam scanning EM (FIB-SEM) and employed deep learning computational methods to evaluate nearly 600 individual platelets and 30 000 included organelles within three healthy controls and three severely ill COVID-19 patients. Statistical analysis reveals that the α-granule/mitochondrion-to-plateletvolume ratio is significantly greater in COVID-19 patient platelets indicating a denser packing of organelles, and a more compact platelet. The COVID-19 patient platelets were significantly smaller -by 35% in volume - with most of the difference in organelle packing density being due to decreased platelet size. There was little to no 3D ultrastructural evidence for differential activation of the platelets from COVID-19 patients. Though limited by sample size, our studies suggest that factors outside of the platelets themselves are likely responsible for COVID-19 complications. Our studies show how deep learning 3D methodology can become the gold standard for 3D ultrastructural studies of platelets.

Autophagy and its consequences for platelet biology

Autophagy, the continuous recycling of intracellular building blocks, molecules, and organelles is necessary to preserve cellular function and homeostasis. In this context, it was demonstrated that autophagy plays an important role in megakaryopoiesis, the development and differentiation of hematopoietic progenitor cells into megakaryocytes. Furthermore, in recent years, autophagic proteins were detected in platelets, anucleate cells generated by megakaryocytes, responsible for hemostasis, thrombosis, and a key cell in inflammation and host immune responses. In the last decade studies have indicated the occurrence of autophagy in platelets. Moreover, autophagy in platelets was subsequently demonstrated to be involved in platelet aggregation, adhesion, and thrombus formation. Here, we review the current knowledge about autophagy in platelets, its function, and clinical implications. However, at the advent of platelet autophagy research, additional discoveries derived from evolving work will be required to precisely define the contributions of autophagy in platelets, and to expand the ever increasing physiologic and pathologic roles these remarkable and versatile blood cells play.

Preeclampsia: Platelet procoagulant membrane dynamics and critical biomarkers

A state-of-the-art lecture titled "Preeclampsia and Platelet Procoagulant Membrane Dynamics" was presented at the International Society on Thrombosis and Haemostasis (ISTH) Congress in 2022. Platelet activation is involved in the pathophysiology of preeclampsia and contributes to the prothrombotic state of the disorder. Still, it remains unclear what mechanisms initiate and sustain platelet activation in preeclampsia and how platelets drive the thrombo-hemorrhagic abnormalities in preeclampsia. Here, we highlight our findings that platelets in preeclampsia are preactivated possibly by plasma procoagulant agonist(s) and overexpress facilitative glucose transporter-3 (GLUT3) in addition to GLUT1. Preeclampsia platelets are also partially degranulated, procoagulant, and proaggregatory and can circulate as microaggregates/microthrombi. However, in response to exposed subendothelial collagen, such as in injured vessels during cesarean sections, preeclampsia platelets are unable to mount a full procoagulant response, contributing to blood loss perioperatively. The overexpression of GLUT3 or GLUT1 may be monitored alone or in combination (GLUT1/GLUT3 ratio) as a biomarker for preeclampsia onset, phenotype, and progression. Studies to further understand the mediators of the platelet activation and procoagulant membrane dynamics in preeclampsia can reveal novel drug targets and suitable alternatives to aspirin for the management of prothrombotic tendencies in preeclampsia. Finally, we summarize relevant new data on this topic presented during the 2022 ISTH Congress.

Tethered platelet capture provides a mechanism for restricting circulating platelet activation to the wound site

Background

Puncture wounding is a longstanding challenge to human health for which understanding is limited, in part, by a lack of detailed morphological data on how the circulating platelet capture to the vessel matrix leads to sustained, self-limiting platelet accumulation.

Objectives

The objective of this study was to produce a paradigm for self-limiting thrombus growth in a mouse jugular vein model.

Methods

Data mining of advanced electron microscopy images was performed from authors' laboratories.

Results

Wide-area transmission electron mcrographs revealed initial platelet capture to the exposed adventitia resulted in localized patches of degranulated, procoagulant-like platelets. Platelet activation to a procoagulant state was sensitive to dabigatran, a direct-acting PAR receptor inhibitor, but not to cangrelor, a P2Y₁₂ receptor inhibitor. Subsequent thrombus growth was sensitive to both cangrelor and dabigatran and sustained by the capture of discoid platelet strings first to collagen-anchored platelets and later to loosely adherent peripheral platelets. Spatial examination indicated that staged platelet activation resulted in a discoid platelet tethering zone that was pushed progressively outward as platelets converted from one activation state to another. As thrombus growth slowed, discoid platelet recruitment became rare and loosely adherent intravascular platelets failed to convert to tightly adherent platelets.

Conclusions

In summary, the data support a model that we term Capture and Activate, in which the initial high platelet activation is directly linked to the exposed adventitia, all subsequent tethering of discoid platelets is to loosely adherent platelets that convert to tightly adherent platelets, and self-limiting, intravascular platelet activation over time is the result of decreased signaling intensity.

Targeting Platelet Activation Pathways to Limit Tumour Progression: Current State of Affairs

The association between cancer and a hypercoagulatory environment is well described. Thrombotic complications serve not only as a major mortality risk but the underlying molecular structure and function play significant roles in enhancing tumour progression, which is defined as the tumour's capacity to survive, invade and metastasise, amongst other hallmarks of the disease. The use of anticoagulant or antiplatelet drugs in cardiovascular disease lessens thrombotic effects, but the consequences on tumour progression require interrogation. Therefore, this review considered developments in the management of platelet activation pathways (thromboxane, ADP and thrombin), focusing on the use of Aspirin, Clopidogrel and Atopaxar, and their potential impacts on tumour progression. Published data suggested a cautionary tale in ensuring we adequately investigate not only drug-drug interactions but also those unforeseen reciprocal interactions between drugs and their targets within the tumour microenvironment that may act as selective pressures, enhancing tumour survival and progression.

Platelet transmission electron microscopy for the assessment of poor biological response to antiplatelet agent: pilot descriptive and prospective study - ELECTROSTROKE

INTRODUCTION

Ischaemic stroke is the leading cause of adult disability. Thus, a strategy based on an efficient antiplatelet therapy has been developed. The monitoring of antiplatelet therapy is now limited to high risk and poor prognosis patients. Indeed, the biological monitoring of the antiplatelet therapy with available platelet function assays do not provide a global integrative approach. Platelet transmission electron microscopy, recently validated for assessing distinct ultrastructural abnormalities is a reliable morphological platelet structural analysis tool which could be used to collect all the ultrastructural platelet characteristics and assess the degree of activation of platelets.

METHODS AND ANALYSIS

Our pilot prospective and descriptive study will include 50 consecutive patients hospitalized for an ischaemic stroke. We expect to identify ultrastructural characteristics that will be correlated with the degree of platelet activation to guide clinicians in decision making regarding the antiplatelet therapy strategy.

ETHICS AND DISSEMINATION

The French Ethics Committee (comité de protection des personnes d'Ile-de-France VII) approved the information notice that will be given to participants and the protocol of this trial (protocol No 21-031).The results of the trial will be disseminated through scientific publications.

TRIAL REGISTRATION NUMBER

NCT05004233.

Platelet Membrane: An Outstanding Factor in Cancer Metastasis

In addition to being biological barriers where the internalization or release of biomolecules is decided, cell membranes are contact structures between the interior and exterior of the cell. Here, the processes of cell signaling mediated by receptors, ions, hormones, cytokines, enzymes, growth factors, extracellular matrix (ECM), and vesicles begin. They triggering several responses from the cell membrane that include rearranging its components according to the immediate needs of the cell, for example, in the membrane of platelets, the formation of filopodia and lamellipodia as a tissue repair response. In cancer, the cancer cells must adapt to the new tumor microenvironment (TME) and acquire capacities in the cell membrane to transform their shape, such as in the case of epithelial−mesenchymal transition (EMT) in the metastatic process. The cancer cells must also attract allies in this challenging process, such as platelets, fibroblasts associated with cancer (CAF), stromal cells, adipocytes, and the extracellular matrix itself, which limits tumor growth. The platelets are enucleated cells with fairly interesting growth factors, proangiogenic factors, cytokines, mRNA, and proteins, which support the development of a tumor microenvironment and support the metastatic process. This review will discuss the different actions that platelet membranes and cancer cell membranes carry out during their relationship in the tumor microenvironment and metastasis.

Release of α-granule contents during platelet activation

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

Venous puncture wound hemostasis results in a vaulted thrombus structured by locally nucleated platelet aggregates

Primary hemostasis results in a platelet-rich thrombus that has long been assumed to form a solid plug. Unexpectedly, our 3-dimensional (3D) electron microscopy of mouse jugular vein puncture wounds revealed that the resulting thrombi were structured about localized, nucleated platelet aggregates, pedestals and columns, that produced a vaulted thrombus capped by extravascular platelet adherence. Pedestal and column surfaces were lined by procoagulant platelets. Furthermore, early steps in thrombus assembly were sensitive to P2Y₁₂ inhibition and late steps to thrombin inhibition. Based on these results, we propose a Cap and Build, puncture wound paradigm that should have translational implications for bleeding control and hemostasis.

Rhee, Pokrovskaya et al. utilize 3D electron microscopy of mouse jugular vein puncture wounds to reveal thrombi structured around localized, nucleated platelet aggregates that produced a vaulted thrombus capped by extravascular platelet adherence. As a result, the authors propose a “Cap and Build” paradigm of primary hemostasis.

Super-resolution imaging of platelet-activation process and its quantitative analysis

Understanding the platelet activation molecular pathways by characterizing specific protein clusters within platelets is essential to identify the platelet activation state and improve the existing therapies for hemostatic disorders. Here, we employed various state-of-the-art super-resolution imaging and quantification methods to characterize the platelet spatiotemporal ultrastructural change during the activation process due to phorbol 12-myristate 13-acetate (PMA) stimuli by observing the cytoskeletal elements and various organelles at nanoscale, which cannot be done using conventional microscopy. Platelets could be spread out with the guidance of actin and microtubules, and most organelles were centralized probably due to the limited space of the peripheral thin regions or the close association with the open canalicular system (OCS). Among the centralized organelles, we provided evidence that granules are fused with the OCS to release their cargo through enlarged OCS. These findings highlight the concerted ultrastructural reorganization and relative arrangements of various organelles upon activation and call for a reassessment of previously unresolved complex and multi-factorial activation processes.

FIB-SEM as a Volume Electron Microscopy Approach to Study Cellular Architectures in SARS-CoV-2 and Other Viral Infections: A Practical Primer for a Virologist

The visualization of cellular ultrastructure over a wide range of volumes is becoming possible by increasingly powerful techniques grouped under the rubric “volume electron microscopy” or volume EM (vEM). Focused ion beam scanning electron microscopy (FIB-SEM) occupies a “Goldilocks zone” in vEM: iterative and automated cycles of milling and imaging allow the interrogation of microns-thick specimens in 3-D at resolutions of tens of nanometers or less. This bestows on FIB-SEM the unique ability to aid the accurate and precise study of architectures of virus-cell interactions. Here we give the virologist or cell biologist a primer on FIB-SEM imaging in the context of vEM and discuss practical aspects of a room temperature FIB-SEM experiment. In an in vitro study of SARS-CoV-2 infection, we show that accurate quantitation of viral densities and surface curvatures enabled by FIB-SEM imaging reveals SARS-CoV-2 viruses preferentially located at areas of plasma membrane that have positive mean curvatures.

Platelet α-granule cargo packaging and release are affected by the luminal proteoglycan, serglycin

BACKGROUND

Serglycin (SRGN) is an intragranular, sulfated proteoglycan in hematopoietic cells that affects granule composition and function.

OBJECTIVE

To understand how SRGN affects platelet granule packaging, cargo release, and extra-platelet microenvironments.

METHODS

Platelets and megakaryocytes from SRGN-/- mice were assayed for secretion kinetics, cargo levels, granule morphology upon activation, and receptor shedding.

RESULTS

Metabolic, 35 SO4 labeling identified SRGN as a major sulfated macromolecule in megakaryocytes. SRGN colocalized with α-granule markers (platelet factor 4 [PF4], von Willebrand factor [VWF], and P-selectin), but its deletion did not affect α-granule morphology or number. Platelet α-granule composition was altered, with a reduction in basic proteins (pI ≥8; e.g., PF4, SDF-1, angiogenin) and constitutive release of PF4 from SRGN-/- megakaryocytes. P-Selectin, VWF, and fibrinogen were unaffected. Serotonin (5-HT) uptake and β-hexosaminidase (HEXB) were slightly elevated. Thrombin-induced exocytosis of PF4 from platelets was defective; however, release of RANTES/CCL5 was normal and osteopontin secretion was more rapid. Release of 5-HT and HEXB (from dense granules and lysosomes, respectively) were unaffected. Ultrastructural studies showed distinct morphologies in activated platelets. The α-granule lumen of SRGN-/- platelet had a grainy staining pattern, whereas that of wild-type granules had only fibrous material remaining. α-Granule swelling and decondensation were reduced in SRGN-/- platelets. Upon stimulation of platelets, a SRGN/PF4 complex was released in a time- and agonist-dependent manner. Shedding of GPVI from SRGN-/- platelets was modestly enhanced. Shedding of GP1b was unaffected.

CONCLUSION

The polyanionic proteoglycan SRGN influences α-granule packaging, cargo release, and shedding of platelet membrane proteins.

Dense cellular segmentation for EM using 2D-3D neural network ensembles

Biologists who use electron microscopy (EM) images to build nanoscale 3D models of whole cells and their organelles have historically been limited to small numbers of cells and cellular features due to constraints in imaging and analysis. This has been a major factor limiting insight into the complex variability of cellular environments. Modern EM can produce gigavoxel image volumes containing large numbers of cells, but accurate manual segmentation of image features is slow and limits the creation of cell models. Segmentation algorithms based on convolutional neural networks can process large volumes quickly, but achieving EM task accuracy goals often challenges current techniques. Here, we define dense cellular segmentation as a multiclass semantic segmentation task for modeling cells and large numbers of their organelles, and give an example in human blood platelets. We present an algorithm using novel hybrid 2D–3D segmentation networks to produce dense cellular segmentations with accuracy levels that outperform baseline methods and approach those of human annotators. To our knowledge, this work represents the first published approach to automating the creation of cell models with this level of structural detail.

Time-resolved MIET measurements of blood platelet spreading and adhesion

Human blood platelets are non-nucleated fragments of megakaryocytes and of high importance for early hemostasis. To form a blood clot, platelets adhere to the blood vessel wall, spread and attract other platelets. Despite the importance for biomedicine, the exact mechanism of platelet spreading and adhesion to surfaces remains elusive. Here, we employ metal-induced energy transfer (MIET) imaging with a leaflet-specific fluorescent membrane probe to quantitatively determine, with nanometer resolution and in a time-resolved manner, the height profile of the basal and the apical platelet membrane above a rigid substrate during platelet spreading. We observe areas, where the platelet membrane approaches the substrate particularly closely and these areas are stable on a time scale of minutes. Time-resolved MIET measurements reveal distinct behaviors of the outermost rim and the central part of the platelets, respectively. Our findings quantify platelet adhesion and spreading and improve our understanding of early steps in blood clotting. Furthermore, the results of this study demonstrate the potential of MIET for simultaneous imaging of two close-by membranes and thus three-dimensional reconstruction of the cell shape.

Structural analysis of resting mouse platelets by 3D-EM reveals an unexpected variation in α-granule shape

Mice and mouse platelets are major experimental models for hemostasis and thrombosis; however, important physiological data from this model has received little to no quantitative, 3D ultrastructural analysis. We used state-of-the-art, serial block imaging scanning electron microscopy (SBF-SEM, nominal Z-step size was 35 nm) to image resting platelets from C57BL/6 mice. α-Granules were identified morphologically and rendered in 3D space. The quantitative analysis revealed that mouse α-granules typically had a variable, elongated, rod shape, different from the round/ovoid shape of human α-granules. This variation in length was confirmed qualitatively by higher-resolution, focused ion beam (FIB) SEM at a nominal 5 nm Z-step size. The unexpected α-granule shape raises novel questions regarding α-granule biogenesis and dynamics. Does the variation arise at the level of the megakaryocyte and α-granule biogenesis or from differences in α-granule dynamics and organelle fusion/fission events within circulating platelets? Further quantitative analysis revealed that the two major organelles in circulating platelets, α-granules and mitochondria, displayed a stronger linear relationship between organelle number/volume and platelet size, i.e., a scaling in number and volume to platelet size, than found in human platelets suggestive of a tighter mechanistic regulation of their inclusion during platelet biogenesis. In conclusion, the overall spatial arrangement of organelles within mouse platelets was similar to that of resting human platelets, with mouse α-granules clustered closely together with little space for interdigitation of other organelles.

Quantification of heparin's antimetastatic effect by single-cell force spectroscopy

In circulation, cancer cells induce platelet activation, leading to the formation of a cancer cell-encircling platelet cloak which facilitates each step of the metastatic cascade. Since cancer patients treated with the anticoagulant heparin showed reduced metastasis rates and improved survival, it is supposed that heparin suppresses the cloak's formation by inhibiting the interaction between platelet's adhesion molecule P-selectin with its ligands on cancer cells. To quantify this heparin effect, we developed a single-cell force spectroscopy approach and quantified the adhesion (maximum adhesion force [F_A ] and detachment work [W_D ]) between platelets and human non-small cell lung cancer cells (A549). A configuration was used in which A549 cells were glued to tipless cantilevers and force-distance (F-D) curves were recorded on a layer of activated platelets. The concentration-response relationship was determined for heparin at concentrations between 1 and 100 U/mL. Sigmoid dose-response fit revealed half-maximal inhibitory concentration (IC₅₀ ) values of 8.01 U/mL (F_A ) and 6.46 U/mL (W_D ) and a maximum decrease of the adhesion by 37.5% (F_A ) and 38.42% (W_D ). The effect of heparin on P-selectin was tested using anti-P-selectin antibodies alone and in combination with heparin. Adding heparin after antibody treatment resulted in an additional reduction of 9.52% (F_A ) and 7.12% (W_D ). Together, we quantified heparin's antimetastatic effect and proved that it predominantly is related to the blockage of P-selectin. Our approach represents a valuable method to investigate the adhesion of platelets to cancer cells and the efficiency of substances to block this interaction.

Quantitative method for estimating stain density in electron microscopy of conventionally prepared biological specimens

Stain density is an important parameter for optimising the quality of ultrastructural data obtained from several types of 3D electron microscopy techniques, including serial block-face electron microscopy (SBEM), and focused ion beam scanning electron microscopy (FIB-SEM). Here, we show how some straightforward measurements in the TEM can be used to determine the stain density based on a simple expression that we derive. Numbers of stain atoms per unit volume are determined from the measured ratio of the bright-field intensities from regions of the specimen that contain both pure embedding material and the embedded biological structures of interest. The determination only requires knowledge of the section thickness, which can either be estimated from the microtome setting, or from low-dose electron tomography, and the elastic scattering cross section for the heavy atoms used to stain the specimen. The method is tested on specimens of embedded blood platelets, brain tissue and liver tissue.

In systemic lupus erythematosus anti-dsDNA antibodies can promote thrombosis through direct platelet activation

Systemic lupus erythematosus (SLE) is associated with a high risk of venous and arterial thrombosis, not necessarily associated with prothrombotic antiphospholipid antibodies (Abs). Alternatively, thrombosis may be due to an increased titer of anti-dsDNA Abs that presumably promote thrombosis via direct platelet activation. Here, we investigated effects of purified anti-dsDNA Abs from the blood of SLE patients, alone or in a complex with dsDNA, on isolated normal human platelets. We showed that anti-dsDNA Abs and anti-dsDNA Ab/dsDNA complexes induced strong platelet activation assessed by enhanced P-selectin expression and dramatic morphological and ultrastructural changes. Electron microscopy revealed a significantly higher percentage of platelets that lost their discoid shape, formed multiple filopodia and had a shrunken body when treated with anti-dsDNA Abs or anti-dsDNA Ab/dsDNA complexes compared with control samples. In addition, these platelets activated with anti-dsDNA Ab/dsDNA complexes typically contained a reduced number of secretory α-granules that grouped in the middle and often merged into a solid electron dense area. Many activated platelets released plasma membrane-derived microvesicles and/or fell apart into subcellular cytoplasmic fragments. Confocal microscopy revealed that platelets treated with anti-dsDNA Ab/dsDNA complex had a heterogeneous distribution of septin2 compared with the homogeneous distribution in control platelets. Structural perturbations were concomitant with mitochondrial depolarization and a decreased content of platelet ATP, indicating energetic exhaustion. Most of the biochemical and morphological changes in platelets induced by anti-dsDNA Abs and anti-dsDNA Ab/dsDNA complexes were prevented by pre-treatment with a monoclonal mAb against FcγRIIA. The aggregate of data indicates that anti-dsDNA Abs alone or in a complex with dsDNA strongly affect platelets via the FcγRIIA receptor. The immune activation of platelets with antinuclear Abs may comprise a prothrombotic mechanism underlying a high risk of thrombotic complications in patients with SLE.

Canalicular system reorganization during mouse platelet activation as revealed by 3D ultrastructural analysis

The canalicular system (CS) has been defined as: 1) an inward, invaginated membrane connector that supports entry into and exit from the platelet; 2) a static structure stable during platelet isolation; and 3) the major source of plasma membrane (PM) for surface area expansion during activation. Recent analysis from STEM tomography and serial block face electron microscopy has challenged the relative importance of CS as the route for granule secretion. Here, We used 3D ultrastructural imaging to reexamine the CS in mouse platelets by generating high-resolution 3D reconstructions to test assumptions 2 and 3. Qualitative and quantitative analysis of whole platelet reconstructions, obtained from immediately fixed or washed platelets fixed post-washing, indicated that CS, even in the presence of activation inhibitors, reorganized during platelet isolation to generate a more interconnected network. Further, CS redistribution into the PM at different times, post-activation, appeared to account for only about half the PM expansion seen in thrombin-activated platelets, in vitro, suggesting that CS reorganization is not sufficient to serve as a dominant membrane reservoir for activated platelets. In sum, our analysis highlights the need to revisit past assumptions about the platelet CS to better understand how this membrane system contributes to platelet function.

3D ultrastructural analysis of α-granule, dense granule, mitochondria, and canalicular system arrangement in resting human platelets

BACKGROUND

State-of-the-art 3-dimensional (3D) electron microscopy approaches provide a new standard for the visualization of human platelet ultrastructure. Application of these approaches to platelets rapidly fixed prior to purification to minimize activation should provide new insights into resting platelet ultrastructure.

OBJECTIVES

Our goal was to determine the 3D organization of α-granules, dense granules, mitochondria, and canalicular system in resting human platelets and map their spatial relationships.

METHODS

We used serial block face-scanning electron microscopy images to render the 3D ultrastructure of α-granules, dense granules, mitochondria, canalicular system, and plasma membrane for 30 human platelets, 10 each from 3 donors. α-Granule compositional data were assessed by sequential, serial section cryo-immunogold electron microscopy and by immunofluorescence (structured illumination microscopy).

RESULTS AND CONCLUSIONS

α-Granule number correlated linearly with platelet size, while dense granule and mitochondria number had little correlation with platelet size. For all subcellular compartments, individual organelle parameters varied considerably and organelle volume fraction had little correlation with platelet size. Three-dimensional data from 30 platelets indicated only limited spatial intermixing of the different organelle classes. Interestingly, almost 70% of α-granules came within ≤35 nm of each other, a distance associated in other cell systems with protein-mediated contact sites. Size and shape analysis of the 1488 α-granules analyzed revealed no more variation than that expected for a Gaussian distribution. Protein distribution data indicated that all α-granules likely contained the same major set of proteins, albeit at varying amounts and varying distribution within the granule matrix.

Alterations to the contents of plasma membrane structural lipids are associated with structural changes and compartmentalization in platelets in hypertension

Arterial hypertension (HTN) can lead to serious organ damage. Several mechanisms have been implicated in the pathogenesis of HTN including constitutive activation of platelets, which increases the risk of aggregation and clot formation. We recently demonstrated the plasma membranes of platelets from patients with HTN exhibit modified structural and physicochemical properties; Raman and Fourier transform infrared by attenuated total reflectance (FTIR-ATR) spectroscopy also indicated lipid content and protein structure alterations. This study aimed to precisely quantify the constituents of the main structural phospholipids and cholesterol in the plasma membranes of platelets from patients with HTN and normotensive individuals. We also assessed the consequence of these alterations on platelet structure and function. Liquid chromatography coupled to triple quadrupole mass spectrometry revealed the plasma membranes of HTN platelets contained less cholesterol and phosphatidylcholine, more phosphatidylserine and phosphatidylethanolamine and had similar sphingosine contents. Atomic force microscopy revealed HTN platelets exhibited increased surface roughness and more pleats. Transmission electron microscopy revealed diminution of the internal membranous structures in HTN platelets. Our findings strongly suggest plasma membrane lipid content alterations-including cholesterol depletion-occur in HTN, and these alterations may induce morphological and physiological abnormalities that participate in the functional changes associated with hypertension.

Platelets modulate multiple markers of neutrophil function in response to in vitro Toll-like receptor stimulation

INTRODUCTION

In addition to their role in facilitating leukocyte-mediated inflammation, platelets can dampen leukocyte pro-inflammatory responses in some contexts. Consequently, platelets are increasingly appreciated as regulators of inflammation. Together, platelets and neutrophils play a role in inflammation through Toll-like receptor (TLR) expression, although we do not fully understand how platelets shape neutrophil responses to TLR stimulation. Here, we aimed to determine the extent to which platelets can modulate neutrophil function in response to in vitro stimulation with TLR4, TLR2/1, and TLR2/6 agonists.

METHODS

Neutrophils from 10 healthy individuals were cultured alone or with autologous platelets. Neutrophils ± platelets were left unstimulated or were stimulated with 1 or 100 ng/mL lipopolysaccharide (LPS; a TLR4 agonist), Pam3CSK4 (a TLR2/1 agonist) and fibroblast-stimulating lipopeptide (FSL)-1 (a TLR2/6 agonist). Neutrophil activation and phagocytic activity were assessed by flow cytometry, and elastase and interleukin-8 secretion were assessed by ELISA.

RESULTS

The addition of platelets attenuated neutrophil CD66b and CD11b expression in response to various doses of Pam3CSK4 and FSL-1. Furthermore, platelet co-culture was associated with higher CD62L expression (indicating reduced CD62L shedding) in response to these TLR agonists. Platelets also reduced elastase secretion in unstimulated cultures and in response to low-dose TLR stimulation. Conversely, platelet co-culture increased neutrophil phagocytosis in unstimulated cultures and in response to low-dose Pam3CSK4 and FSL-1. Platelets also increased IL-8 secretion in response to low-dose LPS.

CONCLUSION

Platelets are complex immunomodulators that can attenuate some, and simultaneously augment other, neutrophil functions. This modulation can occur both in the absence and presence of TLR stimulation.

SNARE-dependent membrane fusion initiates α-granule matrix decondensation in mouse platelets

Platelet α-granule cargo release is fundamental to both hemostasis and thrombosis. Granule matrix hydration is a key regulated step in this process, yet its mechanism is poorly understood. In endothelial cells, there is evidence for 2 modes of cargo release: a jack-in-the-box mechanism of hydration-dependent protein phase transitions and an actin-driven granule constriction/extrusion mechanism. The third alternative considered is a prefusion, channel-mediated granule swelling, analogous to the membrane "ballooning" seen in procoagulant platelets. Using thrombin-stimulated platelets from a set of secretion-deficient, soluble N-ethylmaleimide factor attachment protein receptor (SNARE) mutant mice and various ultrastructural approaches, we tested predictions of these mechanisms to distinguish which best explains the α-granule release process. We found that the granule decondensation/hydration required for cargo expulsion was (1) blocked in fusion-protein-deficient platelets; (2) characterized by a fusion-dependent transition in granule size in contrast to a preswollen intermediate; (3) determined spatially with α-granules located close to the plasma membrane (PM) decondensing more readily; (4) propagated from the site of granule fusion; and (5) traced, in 3-dimensional space, to individual granule fusion events at the PM or less commonly at the canalicular system. In sum, the properties of α-granule decondensation/matrix hydration strongly indicate that α-granule cargo expulsion is likely by a jack-in-the-box mechanism rather than by gradual channel-regulated water influx or by a granule-constriction mechanism. These experiments, in providing a structural and mechanistic basis for cargo expulsion, should be informative in understanding the α-granule release reaction in the context of hemostasis and thrombosis.

Autophagy in Platelets

Anucleate platelets are produced by fragmentation of megakaryocytes. Platelets circulate in the bloodstream for a finite period: upon vessel injury, they are activated to participate in hemostasis; upon senescence, unused platelets are cleared. Platelet hypofunction leads to bleeding. Conversely, pathogenic platelet activation leads to occlusive events that precipitate strokes and heart attacks. Recently, we and others have shown that autophagy occurs in platelets and is important for platelet production and normal functions including hemostasis and thrombosis. Due to the unique properties of platelets, such as their lack of nuclei and their propensity for activation, methods for studying platelet autophagy must be specifically tailored. Here, we describe useful methods for examining autophagy in both human and mouse platelets.

The life cycle of platelet granules

Platelet granules are unique among secretory vesicles in both their content and their life cycle. Platelets contain three major granule types—dense granules, α-granules, and lysosomes—although other granule types have been reported. Dense granules and α-granules are the most well-studied and the most physiologically important. Platelet granules are formed in large, multilobulated cells, termed megakaryocytes, prior to transport into platelets. The biogenesis of dense granules and α-granules involves common but also distinct pathways. Both are formed from the trans-Golgi network and early endosomes and mature in multivesicular bodies, but the formation of dense granules requires trafficking machinery different from that of α-granules. Following formation in the megakaryocyte body, both granule types are transported through and mature in long proplatelet extensions prior to the release of nascent platelets into the bloodstream. Granules remain stored in circulating platelets until platelet activation triggers the exocytosis of their contents. Soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins, located on both the granules and target membranes, provide the mechanical energy that enables membrane fusion during both granulogenesis and exocytosis. The function of these core fusion engines is controlled by SNARE regulators, which direct the site, timing, and extent to which these SNAREs interact and consequently the resulting membrane fusion. In this review, we assess new developments in the study of platelet granules, from their generation to their exocytosis.

Comparison of 3D cellular imaging techniques based on scanned electron probes: Serial block face SEM vs. Axial bright-field STEM tomography

Microscopies based on focused electron probes allow the cell biologist to image the 3D ultrastructure of eukaryotic cells and tissues extending over large volumes, thus providing new insight into the relationship between cellular architecture and function of organelles. Here we compare two such techniques: electron tomography in conjunction with axial bright-field scanning transmission electron microscopy (BF-STEM), and serial block face scanning electron microscopy (SBF-SEM). The advantages and limitations of each technique are illustrated by their application to determining the 3D ultrastructure of human blood platelets, by considering specimen geometry, specimen preparation, beam damage and image processing methods. Many features of the complex membranes composing the platelet organelles can be determined from both approaches, although STEM tomography offers a higher ∼3 nm isotropic pixel size, compared with ∼5 nm for SBF-SEM in the plane of the block face and ∼30 nm in the perpendicular direction. In this regard, we demonstrate that STEM tomography is advantageous for visualizing the platelet canalicular system, which consists of an interconnected network of narrow (∼50-100 nm) membranous cisternae. In contrast, SBF-SEM enables visualization of complete platelets, each of which extends ∼2 µm in minimum dimension, whereas BF-STEM tomography can typically only visualize approximately half of the platelet volume due to a rapid non-linear loss of signal in specimens of thickness greater than ∼1.5 µm. We also show that the limitations of each approach can be ameliorated by combining 3D and 2D measurements using a stereological approach.

The ins and outs of endocytic trafficking in platelet functions

PURPOSE OF REVIEW

Although platelet endocytosis has been recognized in granule cargo loading and the trafficking of several platelet surface receptors, its acute physiological relevance is poorly understood as is its mechanism. The present review discusses the current understanding of platelet endocytosis and its implications for platelet function.

RECENT FINDINGS

Recent studies are beginning to identify and define the proteins that mediate platelet endocytosis. These studies have shown that platelets contain different endosomal compartments and may use multiple endocytic routes to take in circulating molecules and surface proteins. The studies have also shown that platelet endocytosis is involved in several aspects of platelet function such as signaling, spreading, and granule cargo loading.

SUMMARY

Mechanistic studies of platelet endocytosis have shown it to be not only involved in granule cargo loading but also in various other platelet functions important for hemostasis and beyond.

Small but mighty: Platelets as central effectors of host defense

Platelets actively participate in inflammatory processes and drive diseases such as atherosclerosis, rheumatoid arthritis and cancer metastasis. However, platelets also have anti-inflammatory and anti-infective properties, which have received less consideration in the past. In this review, we highlight recent findings on the role of platelets in host defense and describe regulatory pathways modulating immuneresponses. Furthermore, we discuss the role of platelets for the resolution of inflammation and tissue repair. These conceptual changes contribute to our understanding of platelet biology in disease.

Cellubrevin/vesicle-associated membrane protein-3-mediated endocytosis and trafficking regulate platelet functions

Endocytosis is key to fibrinogen (Fg) uptake, trafficking of integrins (αIIbβ₃, α_vβ₃), and purinergic receptors (P2Y₁, P2Y₁₂), and thus normal platelet function. However, the molecular machinery required and possible trafficking routes are still ill-defined. To further identify elements of the platelet endocytic machinery, we examined the role of a vesicle-residing, soluble N-ethylmaleimide factor attachment protein receptor (v-SNARE) called cellubrevin/vesicle-associated membrane protein-3 (VAMP-3) in platelet function. Although not required for normal platelet exocytosis or hemostasis, VAMP-3^-/- mice had less platelet-associated Fg, indicating a defect in Fg uptake/storage. Other granule markers were unaffected. Direct experiments, both in vitro and in vivo, showed that loss of VAMP-3 led to a robust defect in uptake/storage of Fg in platelets and cultured megakaryocytes. Uptake of the fluid-phase marker, dextran, was only modestly affected. Time-dependent uptake and endocytic trafficking of Fg and dextran were followed using 3-dimensional-structured illumination microscopy. Dextran uptake was rapid compared with Fg, but both cargoes progressed through Rab4⁺, Rab11⁺, and von Willebrand factor (VWF)⁺ compartments in wild-type platelets in a time-dependent manner. In VAMP-3^-/- platelets, the 2 cargoes showed limited colocalization with Rab4, Rab11, or VWF. Loss of VAMP-3 also affected some acute platelet functions, causing enhanced spreading on Fg and fibronectin and faster clot retraction compared with wild-type. In addition, the rate of Janus kinase 2 phosphorylation, initiated through the thrombopoietin receptor (TPOR/Mpl) activation, was affected in VAMP-3^-/- platelets. Collectively, our studies show that platelets are capable of a range of endocytosis steps, with VAMP-3 being pivotal in these processes.

Platelet dense granules begin to selectively accumulate mepacrine during proplatelet formation

Platelet dense granules (DGs) are storage organelles for calcium ions, small organic molecules such as ADP and serotonin, and larger polyphosphates that are secreted upon platelet stimulation to enhance platelet activation, adhesion, and stabilization at sites of vascular damage. DGs are thought to fully mature within megakaryocytes (MKs) prior to platelet formation. Here we challenge this notion by exploiting vital fluorescent dyes to distinguish mildly acidic DGs from highly acidic compartments by microscopy in platelets and MKs. In isolated primary mouse platelets, compartments labeled by mepacrine - a fluorescent weak base that accumulates in DGs - are readily distinguishable from highly acidic compartments, likely lysosomes, that are labeled by the acidic pH indicator, LysoTracker, and from endolysosomes and alpha granules labeled by internalized and partially digested DQ™ BSA. By contrast, in murine fetal liver- and human CD34⁺ cell-derived MKs and the megakaryocytoid cell lines, MEG-01 and differentiated G1ME2, labeling by mepacrine overlapped nearly completely with labeling by LysoTracker and partially with labeling by DQ™ BSA. Mepacrine labeling in G1ME2-derived MKs was fully sensitive to proton ATPase inhibitors, but was only partially sensitive in platelets. These data indicate that mepacrine in MKs accumulates as a weak base in endolysosomes but is likely pumped into or retained in separate DGs in platelets. Fluorescent puncta that labeled uniquely for mepacrine were first evident in G1ME2-derived proplatelets, suggesting that DGs undergo a maturation step that initiates in the final stages of MK differentiation.

Glucose Transporter 3 Potentiates Degranulation and Is Required for Platelet Activation

OBJECTIVE

On activation, platelets increase glucose uptake, glycolysis, and glucose oxidation and consume stored glycogen. This correlation between glucose metabolism and platelet function is not well understood and even less is known about the role of glucose metabolism on platelet function in vivo. For glucose to enter a cell, it must be transported through glucose transporters. Here we evaluate the contribution of GLUT3 (glucose transporter 3) to platelet function to better understand glucose metabolism in platelets.

APPROACH AND RESULTS

Platelet-specific knockout of GLUT3 was generated by crossing mice harboring GLUT3 floxed allele to a PF4 (platelet factor 4)-driven Cre recombinase. In platelets, GLUT3 is localized primarily on α-granule membranes and under basal conditions facilitates glucose uptake into α-granules to be used for glycolysis. After activation, platelets degranulate and GLUT3 translocates to the plasma membrane, which is responsible for activation-mediated increased glucose uptake. In vivo, loss of GLUT3 in platelets increased survival in a collagen/epinephrine model of pulmonary embolism, and in a K/BxN model of autoimmune inflammatory disease, platelet-specific GLUT3 knockout mice display decreased disease progression. Mechanistically, loss of GLUT3 decreased platelet degranulation, spreading, and clot retraction. Decreased α-granule degranulation is due in part to an impaired ability of GLUT3 to potentiate exocytosis.

CONCLUSIONS

GLUT3-mediated glucose utilization and glycogenolysis in platelets promotes α-granule release, platelet activation, and postactivation functions.

The cellular basis of platelet secretion: Emerging structure/function relationships

Platelet activation has long been known to be accompanied by secretion from at least three types of compartments. These include dense granules, the major source of small molecules; α-granules, the major protein storage organelle; and lysosomes, the site of acid hydrolase storage. Despite ~60 years of research, there are still many unanswered questions about the cell biology of platelet secretion: for example, how are these secretory organelles organized to support cargo release and what are the key routes of cargo release, granule to plasma membrane or granule to canalicular system. Moreover, in recent years, increasing evidence points to the platelet being organized for secretion of the contents from other organelles, namely the dense tubular system (endoplasmic reticulum) and the Golgi apparatus. Conceivably, protein secretion is a widespread property of the platelet and its organelles. In this review, we concentrate on the cell biology of the α-granule and its structure/function relationships. We both review the literature and discuss the wide array of 3-dimensional, high-resolution structural approaches that have emerged in the last few years. These have begun to reveal new and unanticipated outcomes and some of these are discussed. We are hopeful that the next several years will bring rapid advances to this field that will resolve past controversies and be clinically relevant.

Golgi proteins in circulating human platelets are distributed across non-stacked, scattered structures

Platelets are small, anucleate cell fragments that are central to hemostasis, thrombosis, and inflammation. They are derived from megakaryocytes from which they inherit their organelles. As platelets can synthesize proteins and contain many of the enzymes of the secretory pathway, one might expect all mature human platelets to contain a stacked Golgi apparatus, the central organelle of the secretory pathway. By thin section electron microscopy, stacked membranes resembling the stacked Golgi compartment in megakaryocytes and other nucleated cells can be detected in both proplatelets and platelets. However, the incidence of such structures is low and whether each and every platelet contains such a structure remains an open question. By single-label, immunofluorescence staining, Golgi glycosyltransferases are found within each platelet and map to scattered structures. Whether these structures are positive for marker proteins from multiple Golgi subcompartments remains unknown. Here, we have applied state-of-the-art techniques to probe the organization state of the Golgi apparatus in resting human platelets. By the whole cell volume technique of serial-block-face scanning electron microscopy (SBF-SEM), we failed to observe stacked, Golgi-like structures in any of the 65 platelets scored. When antibodies directed against Golgi proteins were tested against HeLa cells, labeling was restricted to an elongated juxtanuclear ribbon characteristic of a stacked Golgi apparatus. By multi-label immunofluorescence microscopy, we found that each and every resting human platelet was positive for cis, trans, and trans Golgi network (TGN) proteins. However, in each case, the proteins were found in small puncta scattered about the platelet. At the resolution of deconvolved, widefield fluorescence microscopy, these proteins had limited tendency to map adjacent to one another. When the results of 3D structured illumination microscopy (3D SIM), a super resolution technique, were scored quantitatively, the Golgi marker proteins failed to map together indicating at the protein level considerable degeneration of the platelet Golgi apparatus relative to the layered stack as seen in the megakaryocyte. In conclusion, we suggest that these results have important implications for organelle structure/function relationships in the mature platelet and the extent to which Golgi apparatus organization is maintained in platelets. Our results suggest that Golgi proteins in circulating platelets are present within a series of scattered, separated structures. As separate elements, selective sets of Golgi enzymes or sugar nucleotides could be secreted during platelet activation. The establishment of the functional importance, if any, of these scattered structures in sequential protein modification in circulating platelets will require further research.

Respective contributions of single and compound granule fusion to secretion by activated platelets

Although granule secretion is pivotal in many platelet responses, the fusion routes of α and δ granule release remain uncertain. We used a 3D reconstruction approach based on electron microscopy to visualize the spatial organization of granules in unstimulated and activated platelets. Two modes of exocytosis were identified: a single mode that leads to release of the contents of individual granules and a compound mode that leads to the formation of granule-to-granule fusion, resulting in the formation of large multigranular compartments. Both modes occur during the course of platelet secretion. Single fusion events are more visible at lower levels of stimulation and early time points, whereas large multigranular compartments are present at higher levels of agonist and at later time points. Although α granules released their contents through both modes of exocytosis, δ granules underwent only single exocytosis. To define the underlying molecular mechanisms, we examined platelets from vesicle-associated membrane protein 8 (VAMP8) null mice. After weak stimulation, compound exocytosis was abolished and single exocytosis decreased in VAMP8 null platelets. Higher concentrations of thrombin bypassed the VAMP8 requirement, indicating that this isoform is a key but not a required factor for single and/or compound exocytosis. Concerning the biological relevance of our findings, compound exocytosis was observed in thrombi formed after severe laser injury of the vessel wall with thrombin generation. After superficial injury without thrombin generation, no multigranular compartments were detected. Our studies suggest that platelets use both modes of membrane fusion to control the extent of agonist-induced exocytosis.