The cytoskeleton of the resting human blood platelet: structure of the membrane skeleton and its attachment to actin filaments

Platelet Physiology

Platelets are the smallest blood cells, numbering 150 to 350 × 109/L in healthy individuals. The ability of activated platelets to adhere to an injured vessel wall and form aggregates was first described in the 19th century. Besides their long-established roles in thrombosis and hemostasis, platelets are increasingly recognized as pivotal players in numerous other pathophysiological processes including inflammation and atherogenesis, antimicrobial host defense, and tumor growth and metastasis. Consequently, profound knowledge of platelet structure and function is becoming more important in research and in many fields of modern medicine. This review provides an overview of platelet physiology focusing particularly on the structure, granules, surface glycoproteins, and activation pathways of platelets.

New insights of glycoprotein Ib-IX-V complex organization and glycoprotein Ibα in platelet biogenesis

PURPOSE OF REVIEW

Glycoprotein (GP) Ib-IX-V, a platelet surface receptor that plays a critical role in platelet adhesion and platelet-mediated immune responses, consists of GPIbα, GPIbβ, GPIX, and GPV in a stoichiometry of 2 : 4 : 2 : 1. Forming a complex is essential for GPIb-IX-V to function. GPIb-IX-V also plays an important role in platelet biogenesis by regulating the number and size of platelets. Yet how GPIb-IX-V regulates platelet biogenesis remains elusive. This review will summarize recent findings in the complex organization of GPIb-IX-V and its role in platelet biogenesis.

RECENT FINDINGS

Proteomics studies suggest that GPIbα, GPIbβ, GPIX, and GPV form the complex in a ratio of 1 : 2 : 1 : 1, which is supported by analysis of molecular weight of GPIb-IX-V and GPIb-IX and the structure of entire GPIb-IX-V. To activate platelets, GPIbα requires binding of CLEC-2 to trigger signals. Furthermore, disrupting the GPIbα anchorage to filamin A causes defects in platelet budding away from proplatelets leading to giant platelets and a low platelet count.

SUMMARY

New studies challenge the traditional model for the organization of GPIb-IX-V as a complex and indicate the role of GPIb-IX-V in platelet production. Those studies provide insights for GPIb-IX-V in the regulation of platelet activation and platelet biogenesis.

Inhibition of RHOA activity preserves the survival and hemostasis function of long-term cold-stored platelets

ABSTRACT

Patients with thrombocytopenia require platelet transfusion to prevent and stop hemorrhage. Cold storage of platelets results in complex molecular lesions, including changes in membrane microdomains that are recognized by host macrophages and hepatocyte counter-receptors, resulting in phagocytosis and clearance upon transfusion. For this reason, platelets are stored at room temperature, a method that confers increased risk of bacterial contamination. By applying signaling analysis and genetic and pharmacological approaches, we identified that cold-induced activation of RAS homolog family, member A (RHOA) GTPase causes the major hallmarks of platelet cold storage lesions. RHOA deficiency renders murine platelets insensitive to cold storage-induced damage, and pharmacological inhibition by a RHOA activation inhibitor, R-G04, can prevent the cold storage-induced lesions. RHOA inhibition prevents myosin activation and clathrin-independent formation and internalization of lipid rafts enriched in active glycosyltransferases as well as abnormal distribution of GPIbα. RHOA inhibition further prevents the metabolic reprogramming of cold storage-induced lesions and allows the maintenance of glycolytic flux and mitochondria-dependent respiration. Importantly, human platelets transfused in mice after cold storage, in the presence of R-G04 or its more potent enantiomer S-G04, can circulate in vivo at similar levels as room temperature-stored platelets while retaining their hemostatic activity in vivo, as assessed by bleeding time correction in aspirin-treated mice. Our studies provide a mechanism-based translational approach to prevent cold storage-induced damage, which is useful for human platelet transfusion in patients with thrombocytopenia.

Platelet Biorheology and Mechanobiology in Thrombosis and Hemostasis: Perspectives from Multiscale Computation

Thrombosis is the pathological clot formation under abnormal hemodynamic conditions, which can result in vascular obstruction, causing ischemic strokes and myocardial infarction. Thrombus growth under moderate to low shear (<1000 s-1) relies on platelet activation and coagulation. Thrombosis at elevated high shear rates (>10,000 s-1) is predominantly driven by unactivated platelet binding and aggregating mediated by von Willebrand factor (VWF), while platelet activation and coagulation are secondary in supporting and reinforcing the thrombus. Given the molecular and cellular level information it can access, multiscale computational modeling informed by biology can provide new pathophysiological mechanisms that are otherwise not accessible experimentally, holding promise for novel first-principle-based therapeutics. In this review, we summarize the key aspects of platelet biorheology and mechanobiology, focusing on the molecular and cellular scale events and how they build up to thrombosis through platelet adhesion and aggregation in the presence or absence of platelet activation. In particular, we highlight recent advancements in multiscale modeling of platelet biorheology and mechanobiology and how they can lead to the better prediction and quantification of thrombus formation, exemplifying the exciting paradigm of digital medicine.

Platelet Proteome Reveals Novel Targets for Hypercoagulation in Pseudoexfoliation Syndrome

Pseudoexfoliation syndrome (PEX) is characterized by the accumulation of abnormal extracellular matrix material in ocular and non-ocular tissues, including blood vessel walls. Clot-forming dysfunction might be responsible for venous thrombosis in PEX. We investigated global coagulation, the proteome, and functions of platelets in PEX patients and aimed to determine prognostic biomarkers for thrombosis risk in PEX. Peripheral blood was collected from PEX and retinal vein occlusion (RVO) patients, and age-sex matched controls. Viscoelastic hemostasis was evaluated by rotational thromboelastometry (ROTEM). Platelet markers (CD41, CD42, CD61, and CD62p) and endothelial markers (P-selectin, E-selectin, and von Willebrand factor) were investigated by flow cytometry and ELISA, respectively. The platelet proteome was analyzed by 2D fluorescence difference gel electrophoresis followed by mass spectrometry. Clot formation time (CFT) is significantly reduced in PEX patients compared to the controls (p < 0.05). P-selectin levels were higher in PEX patients than in controls (p < 0.05); E-selectin and von Willebrand factor remained unchanged. The monitorization of CFT by ROTEM, and soluble P-selectin, may help assess thrombotic risk in PEX patients. Proteomic analysis revealed differential expression of Profilin-1 in platelets. Profilin-1 regulates the stability of actin-cytoskeleton and may contribute to impaired platelet hemostatic functions. Increased P-selectin levels together with impaired coagulation dynamics might be responsible for the thrombotic events in PEX disease.

A mechanism of platelet integrin αIIbβ3 outside-in signaling through a novel integrin αIIb subunit-filamin-actin linkage

The communication of talin-activated integrin αIIbβ3 with the cytoskeleton (integrin outside-in signaling) is essential for platelet aggregation, wound healing, and hemostasis. Filamin, a large actin crosslinker and integrin binding partner critical for cell spreading and migration, is implicated as a key regulator of integrin outside-in signaling. However, the current dogma is that filamin, which stabilizes inactive αIIbβ3, is displaced from αIIbβ3 by talin to promote the integrin activation (inside-out signaling), and how filamin further functions remains unresolved. Here, we show that while associating with the inactive αIIbβ3, filamin also associates with the talin-bound active αIIbβ3 to mediate platelet spreading. Fluorescence resonance energy transfer-based analysis reveals that while associating with both αIIb and β3 cytoplasmic tails (CTs) to maintain the inactive αIIbβ3, filamin is spatiotemporally rearranged to associate with αIIb CT alone on activated αIIbβ3. Consistently, confocal cell imaging indicates that integrin α CT-linked filamin gradually delocalizes from the β CT-linked focal adhesion marker-vinculin likely because of the separation of integrin α/β CTs occurring during integrin activation. High-resolution crystal and nuclear magnetic resonance structure determinations unravel that the activated integrin αIIb CT binds to filamin via a striking α-helix→β-strand transition with a strengthened affinity that is dependent on the integrin-activating membrane environment containing enriched phosphatidylinositol 4,5-bisphosphate. These data suggest a novel integrin αIIb CT-filamin-actin linkage that promotes integrin outside-in signaling. Consistently, disruption of such linkage impairs the activation state of αIIbβ3, phosphorylation of focal adhesion kinase/proto-oncogene tyrosine kinase Src, and cell migration. Together, our findings advance the fundamental understanding of integrin outside-in signaling with broad implications in blood physiology and pathology.

Dematin Regulates Calcium Mobilization, Thrombosis, and Early Akt Activation in Platelets

The complex intrinsic and extrinsic pathways contributing to platelet activation profoundly impact hemostasis and thrombosis. Detailed cellular mechanisms that regulate calcium mobilization, Akt activation, and integrin signaling in platelets remain incompletely understood. Dematin is a broadly expressed actin binding and bundling cytoskeletal adaptor protein regulated by phosphorylation via cAMP-dependent protein kinase. Here, we report the development of a conditional mouse model specifically lacking dematin in platelets. Using the new mouse model termed PDKO, we provide direct evidence that dematin is a major regulator of calcium mobilization, and its genetic deletion inhibits the early phase of Akt activation in response to collagen and thrombin agonists in platelets. The aberrant platelet shape change, clot retraction, and in vivo thrombosis observed in PDKO mice will enable future characterization of dematin-mediated integrin activation mechanisms in thrombogenic as well as nonvascular pathologies.

SHIP1 Controls Internal Platelet Contraction and αIIbβ3 Integrin Dynamics in Early Platelet Activation

The Src homology 2 domain-containing inositol 5-phosphatase 1 (SHIP1) is known to dephosphorylate PtdIns(3,4,5)P₃ into PtdIns(3,4)P₂ and to interact with several signaling proteins though its docking functions. It has been shown to negatively regulate platelet adhesion and spreading on a fibrinogen surface and to positively regulate thrombus growth. In the present study, we have investigated its role during the early phase of platelet activation. Using confocal-based morphometric analysis, we found that SHIP1 is involved in the regulation of cytoskeletal organization and internal contractile activity in thrombin-activated platelets. The absence of SHIP1 has no significant impact on thrombin-induced Akt or Erk1/2 activation, but it selectively affects the RhoA/Rho-kinase pathway and myosin IIA relocalization to the cytoskeleton. SHIP1 interacts with the spectrin-based membrane skeleton, and its absence induces a loss of sustained association of integrins to this network together with a decrease in α_IIbβ₃ integrin clustering following thrombin stimulation. This α_IIbβ₃ integrin dynamics requires the contractile cytoskeleton under the control of SHIP1. RhoA activation, internal platelet contraction, and membrane skeleton integrin association were insensitive to the inhibition of PtdIns(3,4,5)P₃ synthesis or SHIP1 phosphatase activity, indicating a role for the docking properties of SHIP1 in these processes. Altogether, our data reveal a lipid-independent function for SHIP1 in the regulation of the contractile cytoskeleton and integrin dynamics in platelets.

The GPIb-IX complex on platelets: insight into its novel physiological functions affecting immune surveillance, hepatic thrombopoietin generation, platelet clearance and its relevance for cancer development and metastasis

The glycoprotein (GP) Ib-IX complex is a platelet receptor that mediates the initial interaction with subendothelial von Willebrand factor (VWF) causing platelet arrest at sites of vascular injury even under conditions of high shear. GPIb-IX dysfunction or deficiency is the reason for the rare but severe Bernard-Soulier syndrome (BSS), a congenital bleeding disorder. Although knowledge on GPIb-IX structure, its basic functions, ligands, and intracellular signaling cascades have been well established, several advances in GPIb-IX biology have been made in the recent years. Thus, two mechanosensitive domains and a trigger sequence in GPIb were characterized and its role as a thrombin receptor was deciphered. Furthermore, it became clear that GPIb-IX is involved in the regulation of platelet production, clearance and thrombopoietin secretion. GPIb is deemed to contribute to liver cancer development and metastasis. This review recapitulates these novel findings highlighting GPIb-IX in its multiple functions as a key for immune regulation, host defense, and liver cancer development.

A multiscale model for multiple platelet aggregation in shear flow

We developed a multiscale model for simulating aggregation of multiple, free-flowing platelets in low-intermediate shear viscous flow, in which aggregation is mediated by the interaction of αIIbβ3 receptors on the platelet membrane and fibrinogen (Fg). This multiscale model uses coarse grained molecular dynamics (CGMD) for platelets at the microscales and dissipative particle dynamics (DPD) for the shear flow at the macroscales, employing our hybrid aggregation force field for modeling molecular level receptor ligand bonds. We define an aggregation tensor and use it to quantify the molecular level contact characteristics between platelets in an aggregate. We perform numerical studies under different flow conditions for platelet doublets and triplets and evaluate the contact area, detaching force and minimum distance between different pairs of platelets in an aggregate. We also present the dynamics of applied stress and velocity magnitude distributions on the platelet membrane during aggregation and quantify the increase in stress in the contact region under different flow conditions. Integrating the knowledge from our previously validated models, together with new aggregation scenarios, our model can dynamically quantify aggregation characteristics and map stress and velocity distribution on the platelet membrane which are difficult to measure in vitro, thus providing an insight into mechanotransduction bond formation response of platelets to flow-induced shear stresses. This modeling framework, together with the tensor method for quantifying inter-platelet contact, can be extended to simulate and analyze larger aggregates and their adhesive properties.

Rapid analysis of streaming platelet images by semi-unsupervised learning

We developed a fast and accurate deep learning approach employing a semi-unsupervised learning system (SULS) for capturing the real-time noisy, sparse, and ambiguous images of platelet activation. Outperforming several leading supervised learning methods when applied to segment various platelet morphologies, the SULS detects their complex boundaries at submicron resolutions and it massively decreases to only a few hours for segmenting streaming images of 45 million platelets that would have taken 40 years to annotate manually. For the first time, the fast dynamics of pseudopod formation and platelet morphological changes including membrane tethers and transient tethering to vessels are accurately captured.

Platelet Shape Changes during Thrombus Formation: Role of Actin-Based Protrusions

Platelet activation and aggregation are essential to limit blood loss at sites of vascular injury but may also lead to occlusion of diseased vessels. The platelet cytoskeleton is a critical component for proper hemostatic function. Platelets change their shape after activation and their contractile machinery mediates thrombus stabilization and clot retraction. In vitro studies have shown that platelets, which come into contact with proteins such as fibrinogen, spread and first form filopodia and then lamellipodia, the latter being plate-like protrusions with branched actin filaments. However, the role of platelet lamellipodia in hemostasis and thrombus formation has been unclear until recently. This short review will briefly summarize the recent findings on the contribution of the actin cytoskeleton and lamellipodial structures to platelet function.

A predictive multiscale model for simulating flow-induced platelet activation: Correlating in silico results with in vitro results

Flow-induced platelet activation prompts complex filopodial formation. Continuum methods fail to capture such molecular-scale mechanisms. A multiscale numerical model was developed to simulate this activation process, where a Dissipative Particle Dynamics (DPD) model of viscous blood flow is interfaced with a Coarse Grained Molecular Dynamics (CGMD) platelet model. Embedded in DPD blood flow, the macroscopic dynamic stresses are interactively transferred to the CGMD model, inducing intra-platelet associated events. The platelets activate by a biomechanical transductive linkage chain and dynamically change their shape in response. The models are fully coupled via a hybrid-potential interface and multiple time-stepping (MTS) schemes for handling the disparity between the spatiotemporal scales. Cumulative hemodynamic stresses that may lead to platelet activation are mapped on the surface membrane and simultaneously transmitted to the cytoplasm and cytoskeleton. Upon activation, the flowing platelets lose their quiescent discoid shape and evolve by forming filopodia. The model predictions were validated by a set of in vitro experiments, Platelets were exposed to various combinations of shear stresses and durations in our programmable hemodynamic shearing device (HSD). Their shape change was measured at multiple time points using scanning electron microscopy (SEM). The CGMD model parameters were fine-tuned by interrogating a parameter space established in these experiments. Segmentation of the SEM imaging streams was conducted by a deep machine learning system. This model can be further employed to simulate shear mediated platelet activation thrombosis initiation and to study the effects of modulating platelet properties to enhance their shear resistance via mechanotransduction pathways.

Use of electron microscopy to study platelets and thrombi

Electron microscopy has been a valuable tool for the study of platelet biology and thrombosis for more than 70 years. Early studies using conventional transmission and scanning electron microscopy (EM) provided a foundation for our initial understanding of platelet structure and how it changes upon platelet activation. EM approaches have since been utilized to study platelets and thrombi in the context of basic, translational and clinical research, and they are instrumental in the diagnosis of multiple platelet function disorders. In this brief review, we provide a sampling of the many contributions EM based studies have made to the field, including both historical highlights and contemporary applications. We will also discuss exciting new imaging modalities based on EM and their utility for the study of platelets, hemostasis and thrombosis into the future.

Localization Microscopy of Actin Cytoskeleton in Human Platelets

Here, we measure the actin cytoskeleton arrangement of different morphological states of human platelets using a new protocol for photo-switching of rhodamine class fluorophores. A new medium composition was established for imaging the cytoskeleton using Alexa Fluor 488 conjugated to phalloidin. Morphological states of platelets bound to a glass substrate are visualized and quantified by two-dimensional localization microscopy at nanoscopic resolution. Marker-less drift correction yields localization of individual Alexa 488 conjugated to phalloidin with a positional accuracy of 12 nm.

Platelets and cancer angiogenesis nexus

There has been remarkable insight into the importance of platelets in a wide range of pathophysiologic events, including inflammation and cancer progression. Thrombocytosis in cancer patients is a common finding. Tumor cells induce platelet activation and subsequent aggregation through direct and indirect mechanisms. Platelets are recognized to contribute to metastatic dissemination. There is plenty of evidence that components of the hemostatic system contribute to the process of angiogenesis. Furthermore, there are accumulated data on the substantial influence of blood platelets in the process of blood vessel formation during malignancy. Platelets appear to be the main physiologic transporters of proangiogenic and antiangiogenic factors. Moreover, they influence the process of angiogenesis through platelet-derived microparticles, microRNA, lipids, and variety of surface receptors. Platelets contribute to early and late stages of angiogenesis. Available data support the overall stimulatory effect of platelets on tumor angiogenesis. It raises the possibility that interfering with platelet function may be an effective antineoplastic treatment strategy.

Preactivated and Disaggregated Shape-Changed Platelets Protected Against Acute Respiratory Distress Syndrome Complicated by Sepsis Through Inflammation Suppression

BACKGROUND

This study tested the hypothesis that preactivated and disaggregated shape-changed platelet (PreD-SCP) therapy attenuates lung injury from acute respiratory distress syndrome (ARDS) induced by 100% oxygen inhalation and complicated by sepsis through peritoneal administration of 1.5 mg/kg lipopolysaccharide (LPS).

METHODS

Adult male Sprague-Dawley rats, weighing 325 to 350 g, were randomized into group 1 (normal controls [NC]), group 2 (NC + PreD-SCP [3.0 × 10, intravenous administration]), group 3 (ARDS-LPS), and group 4 (ARDS-LPS + PreD-SCP), and sacrificed by 72 h after ARDS induction.

RESULTS

The lung injury score was significantly higher in group 3 than that in other groups, and significantly higher in group 4 than that in groups 1 and 2, whereas the numbers of alveolar sacs and oxygen saturation (%) showed a reversed pattern compared with that of lung injury score among the four groups (all P < 0.0001) without significant difference between groups 1 and 2. The expressions of proinflammatory cells (CD11+, CD14+, CD68+) and proteins (tumor necrosis factor [TNF]-α, nuclear factor [NF]-κB, interleukin [IL]-1ββ, matrix metalloproteinase [MMP]-9, inducible nitric oxide synthase, intercellular adhesion molecule-1) exhibited a pattern identical to the lung injury score. Circulating levels of white blood cell, IL-6, TNF-α, myeloperoxidase and CCL5, and pulmonary protein expressions of oxidative stress (NOX-1/NOX-2, oxidized protein), apoptotic (Bax, cleaved caspase 3/poly (ADP-ribose) polymerase), fibrotic (Smad3, transforming growth factor [TGF]-β), and DNA damage (γ-H2AX) biomarkers showed an identical pattern, whereas protein expressions of antifibrotic (Smad1/5, bone morphogenetic protein [BMP]-2) and anti-inflammatory (Bcl-2) biomarkers demonstrated an opposite pattern compared with the proinflammatory indices among the four groups (all P < 0.001).

CONCLUSIONS

PreD-SCP therapy effectively improved lung injury in ARDS complicated by sepsis.

Balance of microtubule stiffness and cortical tension determines the size of blood cells with marginal band across species

The fast bloodstream of animals is associated with large shear stresses. To withstand these conditions, blood cells have evolved a special morphology and a specific internal architecture to maintain their integrity over several weeks. For instance, nonmammalian red blood cells, mammalian erythroblasts, and platelets have a peripheral ring of microtubules, called the marginal band, that flattens the overall cell morphology by pushing on the cell cortex. In this work, we model how the shape of these cells stems from the balance between marginal band rigidity and cortical tension. We predict that the diameter of the cell scales with the total microtubule polymer and verify the predicted law across a wide range of species. Our analysis also shows that the combination of the marginal band rigidity and cortical tension increases the ability of the cell to withstand forces without deformation. Finally, we model the marginal band coiling that occurs during the disk-to-sphere transition observed, for instance, at the onset of blood platelet activation. We show that when cortical tension increases faster than cross-linkers can unbind, the marginal band will coil, whereas if the tension increases more slowly, the marginal band may shorten as microtubules slide relative to each other.

A multiscale biomechanical model of platelets: Correlating with in-vitro results

Using dissipative particle dynamics (DPD) combined with coarse grained molecular dynamics (CGMD) approaches, we developed a multiscale deformable platelet model to accurately describe the molecular-scale intra-platelet constituents and biomechanical properties of platelets in blood flow. Our model includes the platelet bilayer membrane, cytoplasm and an elaborate elastic cytoskeleton. Correlating numerical simulations with published in-vitro experiments, we validated the biorheology of the cytoplasm, the elastic response of membrane to external stresses, and the stiffness of the cytoskeleton actin filaments, resulting in an accurate representation of the molecular-level biomechanical microstructures of platelets. This enabled us to study the mechanotransduction process of the hemodynamic stresses acting onto the platelet membrane and transmitted to these intracellular constituents. The platelets constituents continuously deform in response to the flow induced stresses. To the best of our knowledge, this is the first molecular-scale platelet model that can be used to accurately predict platelets activation mechanism leading to thrombus formation in prosthetic cardiovascular devices and in vascular disease processes. This model can be further employed to study the effects of novel therapeutic approaches of modulating platelet properties to enhance their shear resistance via mechanotransduction pathways.

Glutamate Receptor Interacting Protein 1 Mediates Platelet Adhesion and Thrombus Formation

Thrombosis-associated pathologies, such as myocardial infarction and stroke, are major causes of morbidity and mortality worldwide. Because platelets are necessary for hemostasis and thrombosis, platelet directed therapies must balance inhibiting platelet function with bleeding risk. Glutamate receptor interacting protein 1 (GRIP1) is a large scaffolding protein that localizes and organizes interacting proteins in other cells, such as neurons. We have investigated the role of GRIP1 in platelet function to determine its role as a molecular scaffold in thrombus formation. Platelet-specific GRIP1^-/- mice were used to determine the role of GRIP1 in platelets. GRIP1^-/- mice had normal platelet counts, but a prolonged bleeding time and delayed thrombus formation in a FeCl₃-induced vessel injury model. In vitro stimulation of WT and GRIP1^-/- platelets with multiple agonists showed no difference in platelet activation. However, in vivo platelet rolling velocity after endothelial stimulation was significantly greater in GRIP1^-/- platelets compared to WT platelets, indicating a potential platelet adhesion defect. Mass spectrometry analysis of GRIP1 platelet immunoprecipitation revealed enrichment of GRIP1 binding to GPIb-IX complex proteins. Western blots confirmed the mass spectrometry findings that GRIP1 interacts with GPIbα, GPIbβ, and 14-3-3. Additionally, in resting GRIP1^-/- platelets, GPIbα and 14-3-3 have increased interaction compared to WT platelets. GRIP1 interactions with the GPIb-IX binding complex are necessary for normal platelet adhesion to a stimulated endothelium.

Calpain-controlled detachment of major glycoproteins from the cytoskeleton regulates adhesive properties of activated phosphatidylserine-positive platelets

In resting platelets, adhesive membrane glycoproteins are attached to the cytoskeleton. On strong activation, phosphatidylserine(PS)-positive and -negative platelet subpopulations are formed. Platelet activation is accompanied by cytoskeletal rearrangement, although the glycoprotein attachment status in these two subpopulations is not clear. We developed a new, flow cytometry-based, single-cell approach to investigate attachment of membrane glycoproteins to the cytoskeleton in cell subpopulations. In PS-negative platelets, adhesive glycoproteins integrin αIIbβ3, glycoprotein Ib and, as shown for the first time, P-selectin were associated with the cytoskeleton. In contrast, this attachment was disrupted in PS-positive platelets; it was retained to some extent only in the small convex regions or 'caps'. It correlated with the degradation of talin and filamin observed only in PS-positive platelets. Calpain inhibitors essentially prevented the disruption of membrane glycoprotein attachment in PS-positive platelets, as well as talin and filamin degradation. With the suggestion that detachment of glycoproteins from the cytoskeleton may affect platelet adhesive properties, we investigated the ability of PS-positive platelets to resist shear-induced breakaway from the immobilized fibrinogen. Shear rates of 500/s caused PS-positive platelet breakaway, but their adhesion stability increased more than 10-fold after pretreatment of the platelets with calpain inhibitor. In contrast, the ability of PS-positive platelets to adhere to immobilized von Willebrand's factor at 100/s was low, but this was not affected by the preincubation of platelets with a calpain inhibitor. Our data suggest that calpain-controlled detachment of membrane glycoproteins is a new mechanism that is responsible for the loss of ability of the procoagulant platelets to resist detachment from thrombi by high shear stress.

Cytoskeletal Network Morphology Regulates Intracellular Transport Dynamics

Intracellular transport is essential for maintaining proper cellular function in most eukaryotic cells, with perturbations in active transport resulting in several types of disease. Efficient delivery of critical cargos to specific locations is accomplished through a combination of passive diffusion and active transport by molecular motors that ballistically move along a network of cytoskeletal filaments. Although motor-based transport is known to be necessary to overcome cytoplasmic crowding and the limited range of diffusion within reasonable timescales, the topological features of the cytoskeletal network that regulate transport efficiency and robustness have not been established. Using a continuum diffusion model, we observed that the time required for cellular transport was minimized when the network was localized near the nucleus. In simulations that explicitly incorporated network spatial architectures, total filament mass was the primary driver of network transit times. However, filament traps that redirect cargo back to the nucleus caused large variations in network transport. Filament polarity was more important than filament orientation in reducing average transit times, and transport properties were optimized in networks with intermediate motor on and off rates. Our results provide important insights into the functional constraints on intracellular transport under which cells have evolved cytoskeletal structures, and have potential applications for enhancing reactions in biomimetic systems through rational transport network design.

Platelet actin nodules are podosome-like structures dependent on Wiskott-Aldrich syndrome protein and ARP2/3 complex

The actin nodule is a novel F-actin structure present in platelets during early spreading. However, only limited detail is known regarding nodule organization and function. Here we use electron microscopy, SIM and dSTORM super-resolution, and live-cell TIRF microscopy to characterize the structural organization and signalling pathways associated with nodule formation. Nodules are composed of up to four actin-rich structures linked together by actin bundles. They are enriched in the adhesion-related proteins talin and vinculin, have a central core of tyrosine phosphorylated proteins and are depleted of integrins at the plasma membrane. Nodule formation is dependent on Wiskott–Aldrich syndrome protein (WASp) and the ARP2/3 complex. WASp^−/− mouse blood displays impaired platelet aggregate formation at arteriolar shear rates. We propose actin nodules are platelet podosome-related structures required for platelet–platelet interaction and their absence contributes to the bleeding diathesis of Wiskott–Aldrich syndrome.

During early platelet spreading a novel F-actin structure forms, called the actin nodule. Here Poulter et al. demonstrate that actin nodule formation depends on WASp and the Arp2/3 complex, and using super-resolution microscopy they show that nodules bear a structural resemblance to podosomes.

Regulation of actin polymerization by tropomodulin-3 controls megakaryocyte actin organization and platelet biogenesis

The actin cytoskeleton is important for platelet biogenesis. Tropomodulin-3 (Tmod3), the only Tmod isoform detected in platelets and megakaryocytes (MKs), caps actin filament (F-actin) pointed ends and binds tropomyosins (TMs), regulating actin polymerization and stability. To determine the function of Tmod3 in platelet biogenesis, we studied Tmod3(-/-) embryos, which are embryonic lethal by E18.5. Tmod3(-/-) embryos often show hemorrhaging at E14.5 with fewer and larger platelets, indicating impaired platelet biogenesis. MK numbers are moderately increased in Tmod3(-/-) fetal livers, with only a slight increase in the 8N population, suggesting that MK differentiation is not significantly affected. However, Tmod3(-/-) MKs fail to develop a normal demarcation membrane system (DMS), and cytoplasmic organelle distribution is abnormal. Moreover, cultured Tmod3(-/-) MKs exhibit impaired proplatelet formation with a wide range of proplatelet bud sizes, including abnormally large proplatelet buds containing incorrect numbers of von Willebrand factor-positive granules. Tmod3(-/-) MKs exhibit F-actin disturbances, and Tmod3(-/-) MKs spreading on collagen fail to polymerize F-actin into actomyosin contractile bundles. Tmod3 associates with TM4 and the F-actin cytoskeleton in wild-type MKs, and confocal microscopy reveals that Tmod3, TM4, and F-actin partially colocalize near the membrane of proplatelet buds. In contrast, the abnormally large proplatelets from Tmod3(-/-) MKs show increased F-actin and redistribution of F-actin and TM4 from the cortex to the cytoplasm, but normal microtubule coil organization. We conclude that F-actin capping by Tmod3 regulates F-actin organization in mouse fetal liver-derived MKs, thereby controlling MK cytoplasmic morphogenesis, including DMS formation and organelle distribution, as well as proplatelet formation and sizing.

Dances with Membranes: Breakthroughs from Super-resolution Imaging

Biological membrane organization mediates numerous cellular functions and has also been connected with an immense number of human diseases. However, until recently, experimental methodologies have been unable to directly visualize the nanoscale details of biological membranes, particularly in intact living cells. Numerous models explaining membrane organization have been proposed, but testing those models has required indirect methods; the desire to directly image proteins and lipids in living cell membranes is a strong motivation for the advancement of technology. The development of super-resolution microscopy has provided powerful tools for quantification of membrane organization at the level of individual proteins and lipids, and many of these tools are compatible with living cells. Previously inaccessible questions are now being addressed, and the field of membrane biology is developing rapidly. This chapter discusses how the development of super-resolution microscopy has led to fundamental advances in the field of biological membrane organization. We summarize the history and some models explaining how proteins are organized in cell membranes, and give an overview of various super-resolution techniques and methods of quantifying super-resolution data. We discuss the application of super-resolution techniques to membrane biology in general, and also with specific reference to the fields of actin and actin-binding proteins, virus infection, mitochondria, immune cell biology, and phosphoinositide signaling. Finally, we present our hopes and expectations for the future of super-resolution microscopy in the field of membrane biology.

A phenomenological particle-based platelet model for simulating filopodia formation during early activation

We developed a phenomenological three-dimensional platelet model to characterize the filopodia formation observed during early stage platelet activation. Departing from continuum mechanics based approaches, this coarse-grained molecular dynamics (CGMD) particle-based model can deform to emulate the complex shape change and filopodia formation that platelets undergo during activation. The platelet peripheral zone is modeled with a two-layer homogeneous elastic structure represented by spring-connected particles. The structural zone is represented by a cytoskeletal assembly comprising of a filamentous core and filament bundles supporting the platelet's discoid shape, also modeled by spring-connected particles. The interior organelle zone is modeled by homogeneous cytoplasm particles that facilitate the platelet deformation. Nonbonded interactions among the discrete particles of the membrane, the cytoskeletal assembly, and the cytoplasm are described using the Lennard-Jones potential with empirical constants. By exploring the parameter space of this CGMD model, we have successfully simulated the dynamics of varied filopodia formations. Comparative analyses of length and thickness of filopodia show that our numerical simulations are in agreement with experimental measurements of flow-induced activated platelets. Copyright © 2015 John Wiley & Sons, Ltd.

Roll, adhere, spread and contract: structural mechanics of platelet function

Platelets are involved in life-sustaining processes such as hemostasis, wound healing, atherothrombosis and angiogenesis. Mechanical trauma to blood vessels causes platelet activation resulting in their adherence and clot formation at the damaged site, culminating in clot retraction and tissue repair. Two of the major players underlying this process are the cytoskeleton, i.e., actin and microtubules, and the membrane integrin receptors. Rare congenital bleeding disorders such as Glanzmann thrombasthenia and Bernard-Soulier syndrome are associated with genetic alterations of platelet surface receptors, also affecting the platelet cytoskeletal structure. In this review, we summarize the current knowledge about platelet structure and adhesion, and delve into the mechanical aspects of platelet function. Platelets lack a nucleus, and can thus provide a minimal model of a biological cell. New biophysical tools may help to scrutinize platelets anew and to extend the existing knowledge on cell biology.

Multiscale Particle-Based Modeling of Flowing Platelets in Blood Plasma Using Dissipative Particle Dynamics and Coarse Grained Molecular Dynamics

We developed a multiscale particle-based model of platelets, to study the transport dynamics of shear stresses between the surrounding fluid and the platelet membrane. This model facilitates a more accurate prediction of the activation potential of platelets by viscous shear stresses - one of the major mechanisms leading to thrombus formation in cardiovascular diseases and in prosthetic cardiovascular devices. The interface of the model couples coarse-grained molecular dynamics (CGMD) with dissipative particle dynamics (DPD). The CGMD handles individual platelets while the DPD models the macroscopic transport of blood plasma in vessels. A hybrid force field is formulated for establishing a functional interface between the platelet membrane and the surrounding fluid, in which the microstructural changes of platelets may respond to the extracellular viscous shear stresses transferred to them. The interaction between the two systems preserves dynamic properties of the flowing platelets, such as the flipping motion. Using this multiscale particle-based approach, we have further studied the effects of the platelet elastic modulus by comparing the action of the flow-induced shear stresses on rigid and deformable platelet models. The results indicate that neglecting the platelet deformability may overestimate the stress on the platelet membrane, which in turn may lead to erroneous predictions of the platelet activation under viscous shear flow conditions. This particle-based fluid-structure interaction multiscale model offers for the first time a computationally feasible approach for simulating deformable platelets interacting with viscous blood flow, aimed at predicting flow induced platelet activation by using a highly resolved mapping of the stress distribution on the platelet membrane under dynamic flow conditions.

Axon initial segment cytoskeleton comprises a multiprotein submembranous coat containing sparse actin filaments

The axon initial segment of differentiated neurons contains a dense submembranous cytoskeleton that overlays microtubule bundles and includes two sparse actin populations: short, stable actin filaments and longer, dynamic non-oriented filaments.

The axon initial segment (AIS) of differentiated neurons regulates action potential initiation and axon–dendritic polarity. The latter function depends on actin dynamics, but actin structure and functions at the AIS remain unclear. Using platinum replica electron microscopy (PREM), we have characterized the architecture of the AIS cytoskeleton in mature and developing hippocampal neurons. The AIS cytoskeleton assembly begins with bundling of microtubules and culminates in formation of a dense, fibrillar–globular coat over microtubule bundles. Immunogold PREM revealed that the coat contains a network of known AIS proteins, including ankyrin G, spectrin βIV, neurofascin, neuronal cell adhesion molecule, voltage-gated sodium channels, and actin filaments. Contrary to existing models, we find neither polarized actin arrays, nor dense actin meshworks in the AIS. Instead, the AIS contains two populations of sparse actin filaments: short, stable filaments and slightly longer dynamic filaments. We propose that stable actin filaments play a structural role for formation of the AIS diffusion barrier, whereas dynamic actin may promote AIS coat remodeling.

Platelets and cancer: a casual or causal relationship: revisited

Human platelets arise as subcellular fragments of megakaryocytes in bone marrow. The physiologic demand, presence of disease such as cancer, or drug effects can regulate the production circulating platelets. Platelet biology is essential to hemostasis, vascular integrity, angiogenesis, inflammation, innate immunity, wound healing, and cancer biology. The most critical biological platelet response is serving as "First Responders" during the wounding process. The exposure of extracellular matrix proteins and intracellular components occurs after wounding. Numerous platelet receptors recognize matrix proteins that trigger platelet activation, adhesion, aggregation, and stabilization. Once activated, platelets change shape and degranulate to release growth factors and bioactive lipids into the blood stream. This cyclic process recruits and aggregates platelets along with thrombogenesis. This process facilitates wound closure or can recognize circulating pathologic bodies. Cancer cell entry into the blood stream triggers platelet-mediated recognition and is amplified by cell surface receptors, cellular products, extracellular factors, and immune cells. In some cases, these interactions suppress immune recognition and elimination of cancer cells or promote arrest at the endothelium, or entrapment in the microvasculature, and survival. This supports survival and spread of cancer cells and the establishment of secondary lesions to serve as important targets for prevention and therapy.

Wiskott-Aldrich syndrome protein (WASp) controls the delivery of platelet transforming growth factor-β1

Platelets are immunologically competent cells containing cytokines such as TGF-β1 that regulate cell-mediated immunity. However, the mechanisms underlying cytokine secretion from platelets are undefined. The Wiskott-Aldrich syndrome protein (WASp) regulates actin polymerization in nucleated hematopoietic cells but has other role(s) in platelets. WASp-null (WASp(-/-)) platelets stimulated with a PAR-4 receptor agonist had increased TGF-β1 release compared with WT platelets; inhibiting WASp function with wiskostatin augmented TRAP-induced TGF-β1 release in human platelets. TGF-β1 release is dissociated from α-granule secretion (P-selectin up-regulation) and occurs more gradually, with ∼10-15% released after 30-60 min. Blockade of Src family kinase-mediated WASp Tyr-291/Tyr-293 phosphorylation increased TGF-β1 release, with no additive effect in WASp(-/-) platelets, signifying that phosphorylation is critical for WASp-limited TGF-β1 secretion. Inhibiting F-actin assembly with cytochalasin D enhanced secretion in WT platelets and further increased TGF-β1 release in WASp(-/-) platelets, indicating that WASp and actin assembly independently regulate TGF-β1 release. A permeabilized platelet model was used to test the role of upstream small GTPases in TGF-β1 release. N17Cdc42, but not Rac1 mutants, increased TGF-β1 secretion and abrogated WASp phosphorylation. We conclude that WASp function restricts TGF-β1 secretion in a Cdc42- and Src family kinase-dependent manner and independently of actin assembly.

Comparison of platelet ultrastructure and elastic properties in thrombo-embolic ischemic stroke and smoking using atomic force and scanning electron microscopy

Thrombo-embolic ischemic stroke is a serious and debilitating disease, and it remains the second most common cause of death worldwide. Tobacco smoke exposure continues to be responsible for preventable deaths around the world, and is a major risk factor for stroke. Platelets play a fundamental role in clotting, and their pathophysiological functioning is present in smokers and stroke patients, resulting in a pro-thrombotic state. In the current manuscript, atomic force and scanning electron microscopy were used to compare the platelets of smokers, stroke patients and healthy individuals. Results showed that the elastic modulus of stroke platelets is decreased by up to 40%, whereas there is an elasticity decrease of up to 20% in smokers' platelets. This indicates a biophysical alteration of the platelets. Ultrastructurally, both the stroke patients and smokers' platelets are more activated than the healthy control group, with prominent cytoskeletal rearrangement involved; but to a more severe extent in the stroke group than in the smokers. Importantly, this is a confirmation of the extent of smoking as risk factor for stroke. We conclude by suggesting that the combined AFM and SEM analyses of platelets might give valuable information about the disease status of patients. Efficacy of treatment regimes on the integrity, cell shape, roughness and health status of platelets may be tracked, as this cell's health status is crucial in the over-activated coagulation system of conditions like stroke.

Platelet physiology

Platelets are cell fragments which circulate in blood. They are of pivotal importance in blood clot formation, affecting thrombosis and haemostasis. By rapidly altering the activation and expression of surface receptors, platelets are able to quickly undergo structural and phenotypic changes in response to stimulation, such as collagen exposure on injured vascular endothelium. This response to stimulation allows platelets to become adhesive, aggregate to form a thrombus, and release a variety of mediators affecting coagulation, inflammation, and chemotaxis at the site of injury. Therefore, in addition to their critical role in thrombosis and haemostasis, platelets also play a role in immunity, inflammation, wound healing, haematologic malignancies, and metabolic disorders. The role of platelets in disease, particularly in atherothrombosis, is increasingly the focus of current research and antiplatelet therapy plays a significant role in the prevention and treatment of atherothrombotic and inflammatory diseases.

T granules in human platelets function in TLR9 organization and signaling

TLR9 localizes to a novel intracellular compartment called the T granule to promote immune signaling by platelets.

Human and murine platelets (PLTs) variably express toll-like receptors (TLRs), which link the innate and adaptive immune responses during infectious inflammation and atherosclerotic vascular disease. In this paper, we show that the TLR9 transcript is specifically up-regulated during pro-PLT production and is distributed to a novel electron-dense tubular system-related compartment we have named the T granule. TLR9 colocalizes with protein disulfide isomerase and is associated with either VAMP 7 or VAMP 8, which regulates its distribution in PLTs on contact activation (spreading). Preincubation of PLTs with type IV collagen specifically increased TLR9 and CD62P surface expression and augmented oligodeoxynucleotide (ODN) sequestration and PLT clumping upon addition of bacterial/viral ODNs. Collectively, this paper (a) tracks TLR9 to a new intracellular compartment in PLTs and (b) describes a novel mechanism of TLR9 organization and signaling in human PLTs.

Actin polymerization stabilizes α4β1 integrin anchors that mediate monocyte adhesion

Rapid signaling and structural adaptations to the actin cytoskeleton enable leukocytes to stabilize α4 integrin–mediated adhesion and resist detachment from inflamed endothelium.

Leukocytes arrested on inflamed endothelium via integrins are subjected to force imparted by flowing blood. How leukocytes respond to this force and resist detachment is poorly understood. Live-cell imaging with Lifeact-transfected U937 cells revealed that force triggers actin polymerization at upstream α4β1 integrin adhesion sites and the adjacent cortical cytoskeleton. Scanning electron microscopy revealed that this culminates in the formation of structures that anchor monocyte adhesion. Inhibition of actin polymerization resulted in cell deformation, displacement, and detachment. Transfection of dominant-negative constructs and inhibition of function or expression revealed key signaling steps required for upstream actin polymerization and adhesion stabilization. These included activation of Rap1, phosphoinositide 3-kinase γ isoform, and Rac but not Cdc42. Thus, rapid signaling and structural adaptations enable leukocytes to stabilize adhesion and resist detachment forces.

Does size matter in platelet production?

Platelet (PLT) production represents the final stage of megakaryocyte (MK) development. During differentiation, bone marrow MKs extend and release long, branched proPLTs into sinusoidal blood vessels, which undergo repeated abscissions to yield circulating PLTs. Circular-prePLTs are dynamic intermediate structures in this sequence that have the capacity to reversibly convert into barbell-proPLTs and may be related to "young PLTs" and "large PLTs" of both inherited and acquired macrothrombocytopenias. Conversion is regulated by the diameter and thickness of the peripheral microtubule coil, and PLTs are capable of enlarging in culture to generate barbell-proPLTs that divide to yield 2 smaller PLT products. Because PLT number and size are inversely proportional, this raises the question: do macrothrombocytopenias represent a failure in the intermediate stages of PLT production? This review aims to bring together and contextualize our current understanding of terminal PLT production against the backdrop of human macrothrombocytopenias to establish how "large PLTs" observed in both conditions are similar, how they are different, and what they can teach us about PLT formation. A better understanding of the cytoskeletal mechanisms that regulate PLT formation and determine PLT size offers the promise of improved therapies for clinical disorders of PLT production and an important source of PLTs for infusion.

High-content live-cell imaging assay used to establish mechanism of trastuzumab emtansine (T-DM1)--mediated inhibition of platelet production

Proplatelet production represents a terminal stage of megakaryocyte development during which long, branching processes composed of platelet-sized swellings are extended and released into the surrounding culture. Whereas the cytoskeletal mechanics driving these transformations have been the focus of many studies, significant limitations in our ability to quantify the rate and extent of proplatelet production have restricted the field to qualitative analyses of a limited number of cells over short intervals. A novel high-content, quantitative, live-cell imaging assay using the IncuCyte system (Essen BioScience) was therefore developed to measure the rate and extent of megakaryocyte maturation and proplatelet production under live culture conditions for extended periods of time. As proof of concept, we used this system in the present study to establish a mechanism by which trastuzumab emtansine (T-DM1), an Ab-drug conjugate currently in clinical development for cancer, affects platelet production. High-content analysis of primary cell cultures revealed that T-DM1 is taken up by mouse megakaryocytes, inhibits megakaryocyte differentiation, and disrupts proplatelet formation by inducing abnormal tubulin organization and suppressing microtubule dynamic instability. Defining the pathways by which therapeutics such as T-DM1 affect megakaryocyte differentiation and proplatelet production may yield strategies to manage drug-induced thrombocytopenias.

Platelets: production, morphology and ultrastructure

Platelets are anucleate, discoid cells, roughly 2-3 μm in diameter that function primarily as regulators of hemostasis, but also play secondary roles in angiogensis and innate immunity. Although human adults contain nearly one trillion platelets in circulation that are turned over every 8-10 days, our understanding of the mechanisms involved in platelet production is still incomplete. Platelets stem from large (30-100 μm) nucleated cells called megakaryocytes that reside primarily in the bone marrow. During maturation megakaryocytes extend long proplatelet elongations into sinusoidal blood vessels from which platelets ultimately release. During this process, platelets develop a number of distinguishable structural elements including: a delimited plasma membrane; invaginations of the surface membrane that form the open canalicular system (OCS); a closed-channel network of residual endoplasmic reticulum that form the dense tubular system (DTS); a spectrin-based membrane skeleton; an actin-based cytoskeletal network; a peripheral band of microtubules; and numerous organelles including α-granules, dense-granules, peroxisomes, lysosomes, and mitochondria. Proplatelet elongation and platelet production is an elaborate and complex process that defines the morphology and ultrastructure of circulating platelets, and is critical in understanding their increasingly numerous and varied biological functions.

GPIbα regulates platelet size by controlling the subcellular localization of filamin

Interaction between the cytoplasmic domain of GPIbα with its cytoskeletal binding partner, filamin, is a major determinant of platelet size, and deficiency of either protein results in macrothrombocytopenia. To clarify the mechanism by which GPIbα-filamin interactions regulate platelet production, we manipulated the expression levels of filamin and GPIb in cultured embryonic stem cells (ESCs) that were subsequently differentiated into platelets. Knocking down filamins A and B resulted in the production of ESC-derived proplatelets with abnormally large swellings and proplatelet shafts that generated giant platelets in culture. Large platelets could also be generated by overexpressing GPIbα in ESCs, or by overexpressing in vivo a transgene encoding a chimeric protein containing the cytoplasmic domain of GPIbα. To identify the mechanism by which the GPIb:filamin ratio regulates platelet size, we manipulated filamin and GPIbα levels in HEK293T cells and examined the effects of overexpressing either protein on their ability to traffic to the cell periphery. Accumulation of either protein within the endoplasmic reticulum resulted in trapping of the other. Taken together, these data demonstrate that coordinated expression of GPIbα and filamin is required for efficient trafficking of either protein to the cell surface, and for production of normal-sized platelets.

Remodelling of cortical actin where lytic granules dock at natural killer cell immune synapses revealed by super-resolution microscopy

Natural Killer (NK) cells are innate immune cells that secrete lytic granules to directly kill virus-infected or transformed cells across an immune synapse. However, a major gap in understanding this process is in establishing how lytic granules pass through the mesh of cortical actin known to underlie the NK cell membrane. Research has been hampered by the resolution of conventional light microscopy, which is too low to resolve cortical actin during lytic granule secretion. Here we use two high-resolution imaging techniques to probe the synaptic organisation of NK cell receptors and filamentous (F)-actin. A combination of optical tweezers and live cell confocal microscopy reveals that microclusters of NKG2D assemble into a ring-shaped structure at the centre of intercellular synapses, where Vav1 and Grb2 also accumulate. Within this ring-shaped organisation of NK cell proteins, lytic granules accumulate for secretion. Using 3D-structured illumination microscopy (3D-SIM) to gain super-resolution of ~100 nm, cortical actin was detected in a central region of the NK cell synapse irrespective of whether activating or inhibitory signals dominate. Strikingly, the periodicity of the cortical actin mesh increased in specific domains at the synapse when the NK cell was activated. Two-colour super-resolution imaging revealed that lytic granules docked precisely in these domains which were also proximal to where the microtubule-organising centre (MTOC) polarised. Together, these data demonstrate that remodelling of the cortical actin mesh occurs at the central region of the cytolytic NK cell immune synapse. This is likely to occur for other types of cell secretion and also emphasises the importance of emerging super-resolution imaging technology for revealing new biology.

The spectrin-based membrane skeleton stabilizes mouse megakaryocyte membrane systems and is essential for proplatelet and platelet formation

Megakaryocytes generate platelets by remodeling their cytoplasm first into proplatelets and then into preplatelets, which undergo fission to generate platelets. Although the functions of microtubules and actin during platelet biogenesis have been defined, the role of the spectrin cytoskeleton is unknown. We investigated the function of the spectrin-based membrane skeleton in proplatelet and platelet production in murine megakaryocytes. Electron microscopy revealed that, like circulating platelets, proplatelets have a dense membrane skeleton, the main fibrous component of which is spectrin. Unlike other cells, megakaryocytes and their progeny express both erythroid and nonerythroid spectrins. Assembly of spectrin into tetramers is required for invaginated membrane system maturation and proplatelet extension, because expression of a spectrin tetramer-disrupting construct in megakaryocytes inhibits both processes. Incorporation of this spectrin-disrupting fragment into a novel permeabilized proplatelet system rapidly destabilizes proplatelets, causing blebbing and swelling. Spectrin tetramers also stabilize the "barbell shapes" of the penultimate stage in platelet production, because addition of the tetramer-disrupting construct converts these barbell shapes to spheres, demonstrating that membrane skeletal continuity maintains the elongated, pre-fission shape. The results of this study provide evidence for a role for spectrin in different steps of megakaryocyte development through its participation in the formation of invaginated membranes and in the maintenance of proplatelet structure.

FLNA p.V528M substitution is neither associated with bilateral periventricular nodular heterotopia nor with macrothrombocytopenia

Filamin A is encoded by the FLNA gene on chromosome Xq28 and functions in cross-linking actin filaments into orthogonal networks in the cortical cytoplasm. FLNA p.V528M was initially detected in a female autopsy case of X-linked bilateral periventricular nodular heterotopia (BPNH), a neuronal migration disorder characterized by subependymal nodules of gray matter. During our mutation analysis of FLNA in a boy with apparent X-linked thrombocytopenia, we detected the p.V528M variant. The patient, mother and sister, who were heterozygous for the substitution, did not have BPNH. We observed an allele frequency of 4.8% in healthy control Japanese, but did not observe the variant in Caucasian subjects. Hemizygous controls had a normal platelet count and size. We suggest that p.V528M is neither associated with BPNH nor with thrombocytopenia and giant platelets, and represents a functional polymorphism.

Tropomodulin 1-null mice have a mild spherocytic elliptocytosis with appearance of tropomodulin 3 in red blood cells and disruption of the membrane skeleton

The short actin filaments in the red blood cell (RBC) membrane skeleton are capped at their pointed ends by tropomodulin 1 (Tmod1) and coated with tropomyosin (TM) along their length. Tmod1-TM control of actin filament length is hypothesized to regulate spectrin-actin lattice organization and membrane stability. We used a Tmod1 knockout mouse to investigate the in vivo role of Tmod1 in the RBC membrane skeleton. Western blots of Tmod1-null RBCs confirm the absence of Tmod1 and show the presence of Tmod3, which is normally not present in RBCs. Tmod3 is present at only one-fifth levels of Tmod1 present on wild-type membranes, but levels of actin, TMs, adducins, and other membrane skeleton proteins remain unchanged. Electron microscopy shows that actin filament lengths are more variable with spectrin-actin lattices displaying abnormally large and more variable pore sizes. Tmod1-null mice display a mild anemia with features resembling hereditary spherocytic elliptocytosis, including decreased RBC mean corpuscular volume, cellular dehydration, increased osmotic fragility, reduced deformability, and heterogeneity in osmotic ektacytometry. Insufficient capping of actin filaments by Tmod3 may allow greater actin dynamics at pointed ends, resulting in filament length redistribution, leading to irregular and attenuated spectrin-actin lattice connectivity, and concomitant RBC membrane instability.

A comparative optical aggregometry study of antiplatelet activity of taxanes from Taxus cuspidata

Platelets are highly reactive components of the circulatory system. The cytoskeleton of a platelet is an important structure for platelet aggregation as stimulated by several agonists. An anticancer agent, taxol, has been suggested to exert platelet anti-aggregating activity by stabilizing microtubules during the aggregation process. An activity-guided fractionation was performed with a methanol extract of the leaves and twigs of Taxus cuspidata to isolate taxanes with platelet anti-aggregating effects. Compounds 1 to 7 - taxinine (1), taxinine A (2), taxinine B (3), 2-deacetoxytaxinine B (4), taxacin (5), taxchinin B (6), and taxol (7) - were obtained as the antiplatelet components of this plant. These taxane compounds present the possibility of securing new antiplatelet compounds which differ from currently available antiplatelet agents in chemical structure and possibly in mechanisms of action. All compounds showed stronger inhibitory effects than acetylsalicylic acid (ASA) on platelet aggregation induced by arachidonic acid (AA) (IC(50): 14.4, 64.5, 35.5, 16.0, 21.9, 28.6 and 48.2 versus 63.0microM) or U46619 (IC(50): 34.8, 24.9, 36.2, 35.0, 46.9, 71.9 and 68.7 versus 340microM). Compounds 1, 3, 4 and 5, with a cinnamoyl group at the C(5) position, showed strong inhibitory effects against AA-induced aggregation compared to compound 2 (with an -OH group at C(5)) or compounds with an oxetane ring at C(4),(5), such as compounds 6 and 7. All of the seven compounds were 5-13-fold more strongly inhibitory than ASA against U46619-induced aggregation.

The platelet glycoprotein Ib-IX-V complex anchors lipid rafts to the membrane skeleton: implications for activation-dependent cytoskeletal translocation of signaling molecules

BACKGROUND

The glycoprotein (GP) Ib-IX-V complex attaches platelets to areas of endothelial damage by binding von Willebrand factor (VWF), an interaction that transmits intracellular activation signals. These signals require that the complex associates with both lipid rafts and the membrane cytoskeleton, but it is not clear whether the same GPIb-IX-V subpopulation associates with both structures.

OBJECTIVES

To determine which subpopulation of GPIb-IX-V associates with lipid rafts, and the consequences of that interaction.

METHODS

We analyzed the content of proteins (particularly the GPIb-IX-V complex) and lipids in rafts from detergent lysates of platelets before and after removal of the actin cytoskeleton alone or both the actin cytoskeleton and membrane skeleton (by successive centrifugations of 15,800 x g and 100,000 x g).

RESULTS

In unstimulated platelets, little raft-associated GPIb-IX-V sedimented with the actin skeleton; most was removed by sedimentation of the membrane skeleton. The Src family kinase Lyn followed the same pattern. In VWF-activated platelets, almost all of the GPIb-IX-V complex and Lyn in rafts sedimented with the actin cytoskeleton, consistent with a previously described crosslinking of the membrane and actin skeletal structures following platelet activation. Disruption of the GPIbalpha-filamin linkage with N-ethylmaleimide prevented depletion of raft-associated GPIb-IX-V by skeletal sedimentation. Not all raft-associated proteins and lipids followed this pattern.

CONCLUSION

These results suggest that the raft association and cytoskeletal linkage of the GPIb-IX-V complex are interrelated, and both are required for optimal receptor function, perhaps because raft association attracts signaling proteins and membrane skeletal association allows these proteins to move en masse to new locations.