The structure and function of platelet integrins

Integrins in Cardiovascular Health and Disease: Molecular Mechanisms and Therapeutic Opportunities

Cardiovascular diseases, including atherosclerosis, hypertension, and heart failure, remain the leading cause of global mortality, with endothelial dysfunction and vascular remodeling as critical contributors. Integrins, as transmembrane adhesion proteins, are central regulators of cell adhesion, migration, and signaling, playing a pivotal role in maintaining vascular homeostasis and mediating pathological processes such as inflammation, angiogenesis, and extracellular matrix remodeling. This article comprehensively examines the role of integrins in the pathogenesis of cardiovascular diseases, focusing on their dysfunction in endothelial cells and interactions with inflammatory mediators, such as TNF-α. Molecular mechanisms of integrin action are discussed, including their involvement in mechanotransduction, leukocyte adhesion, and signaling pathways that regulate vascular integrity. The review also highlights experimental findings, such as the use of specific integrin-targeting plasmids and immunofluorescence to elucidate integrin functions under inflammatory conditions. Additionally, potential therapeutic strategies are explored, including the development of integrin inhibitors, monoclonal antibodies, and their application in regenerative medicine. These approaches aim not only to mitigate pathological vascular remodeling but also to promote tissue repair and angiogenesis. By bridging insights from molecular studies with their translational potential, this work underscores the promise of integrin-based therapies in advancing the management and treatment of cardiovascular diseases.

Conformational response of αIIbβ3 and αVβ3 integrins to force

As major adhesion receptors, integrins transmit biochemical and mechanical signals across the plasma membrane. These functions are regulated by transitions between bent and extended conformations and modulated by force. To understand how force on integrins mediates cellular mechanosensing, we compared two highly homologous integrins, αIIbβ3 and αVβ3. These integrins, expressed in circulating platelets vs. solid tissues, respectively, share the β3 subunit, bind similar ligands and have similar bent and extended conformations. Here, we report that in cells expressing equivalent levels of each integrin, αIIbβ3 mediates spreading on softer substrates than αVβ3. These effects correlate with differences in structural dynamics of the two integrins under force. All-atom simulations show that αIIbβ3 is more flexible than αVβ3 due to correlated residue motions within the α subunit domains. Single molecule measurements confirm that αIIbβ3 extends faster than αVβ3. These results reveal a fundamental relationship between protein function and structural dynamics in cell mechanosensing.

Exploiting the Molecular Properties of Fibrinogen to Control Bleeding Following Vascular Injury

The plasma protein fibrinogen is critical for haemostasis and wound healing, serving as the structural foundation of the blood clot. Through a complex interaction between coagulation factors, the soluble plasma fibrinogen is converted to insoluble fibrin networks, which form the skeleton of the blood clot, an essential step to limit blood loss after vascular trauma. This review examines the molecular mechanisms by which fibrinogen modulates bleeding, focusing on its interactions with other proteins that maintain fibrin network stability and prevent premature breakdown. Moreover, we also cover the role of fibrinogen in ensuring clot stability through the physiological interaction with platelets. We address the therapeutic applications of fibrinogen across various clinical contexts, including trauma-induced coagulopathy, postpartum haemorrhage, and cardiac surgery. Importantly, a full understanding of protein function will allow the development of new therapeutics to limit blood loss following vascular trauma, which remains a key cause of mortality worldwide. While current management strategies help with blood loss following vascular injury, they are far from perfect and future research should prioritise refining fibrinogen replacement strategies and developing novel agents to stabilise the fibrin network. Exploiting fibrinogen's molecular properties holds significant potential for improving outcomes in trauma care, surgical interventions and obstetric haemorrhage.

Integrative Quantitative Analysis of Platelet Proteome and Site-Specific Glycoproteome Reveals Diagnostic Potential of Platelet Glycoproteins for Liver Cancer

The role of peripheral blood platelets as indicators of cancer progression is increasingly recognized, and the significance of abnormal glycosylation in platelet function and related disorders is gaining attention. However, the potential of platelets as a source of protein site-specific glycosylation for cancer diagnosis remains underexplored. In this study, we proposed a general pipeline that integrates quantitative proteomics with site-specific glycoproteomics, allowing for an in-depth investigation of the platelet glycoproteome. With this pipeline, we generated a data set comprising 3,466 proteins with qualitative information, 3,199 proteins with quantitative information, 3,419 site-specific glycans with qualitative information and 3,377 site-specific glycans with quantitative information from peripheral blood platelets of hepatocellular carcinoma (HCC) patients, metastatic liver cancer (mLC) patients, and healthy controls. The integrated analysis revealed significant changes in platelet protein N-glycosylation in liver cancer patients. Further systems biology analysis and lectin pull-down-coupled ELISA assays in independent clinical samples confirmed two N-glycoproteins with specific glycan types, complement C3 (C3) with oligomannose modification and integrin β-3 (ITGB3) with sialylation, as potential biomarkers distinguishing liver cancer patients from healthy individuals, without differentiating between HCC and mLC patient group. These findings highlight the potential of platelet protein glycosylation as biomarkers.

The Influence of Physiological Blood Clot on Osteoblastic Cell Response to a Chitosan-Based 3D Scaffold-A Pilot Investigation

BACKGROUND

The use of ex vivo assays associated with biomaterials may allow the short-term visualization of a specific cell type response inserted in a local microenvironment. Blood is the first component to come into contact with biomaterials, providing blood clot formation, being substantial in new tissue formation. Thus, this research investigated the physiological blood clot (PhC) patterns formed in 3D scaffolds (SCAs), based on chitosan and 20% beta-tricalcium phosphate and its effect on osteogenesis. Initially, SCA were inserted for 16 h in rats calvaria defects, and, after that, osteoblasts cells (OSB; UMR-106 lineage) were seeded on the substrate formed. The groups tested were SCA + OSB and SCA + PhC + OSB. Cell viability was checked by MTT and mineralized matrix formation in OSB using alizarin red (ARS). The alkaline phosphatase (ALP) and bone sialoprotein (BSP) expression in OSB was investigated by indirect immunofluorescence (IF). The OSB and PhC morphology was verified by scanning electron microscopy (SEM).

RESULTS

The SCA + PhC + OSB group showed greater cell viability (p = 0.0169). After 10 days, there was more mineralized matrix deposition (p = 0.0365) and high ALP immunostaining (p = 0.0021) in the SCA + OSB group. In contrast, BSP was more expressed in OSB seeded on SCA with PhC (p = 0.0033).

CONCLUSIONS

These findings show the feasibility of using PhC in ex vivo assays. Additionally, its inclusion in the experiments resulted in a change in OSB behavior when compared to in vitro assays. This "closer to nature" environment can completely change the scenario of a study.

Advances in mechanotransduction and sonobiology: effects of audible acoustic waves and low-vibration stimulations on mammalian cells

In recent decades, research on mechanotransduction has advanced considerably, focusing on the effects of audible acoustic waves (AAWs) and low-vibration stimulation (LVS), which has propelled the field of sonobiology forward. Taken together, the current evidence demonstrates the influence of these biosignals on key cellular processes, such as growth, differentiation and migration in mammalian cells, emphasizing the determining role of specific physical parameters during stimulation, such as frequency, sound pressure level/amplitude and exposure time. These mechanical waves interact with various cellular elements, including ion channels, primary cilia, cell-cell adhesion receptors, cell-matrix and extracellular matrix proteins, and focal adhesion complexes. These components connect with the cytoskeletal fibre network, enabling the transmission of mechanical stimuli towards the nucleus. The nucleus, in turn, linked to the cytoskeleton via the linkers of the nucleoskeleton and cytoskeleton complex, acts as a mechanosensitive centre, not only responding to changes in cytoskeletal stiffness and nuclear tension but also regulating gene expression through the transcriptional co-activator YAP/TAZ and interactions between chromatin and the nuclear envelope. This intricate chain of mechanisms highlights the potential of sonobiology in various fields, including dentistry, regenerative medicine, tissue engineering and cancer research. However, progress in these fields requires the establishment of standardized measurement methodologies and biocompatible experimental setups to ensure the reproducibility of results.

Bone Disease in Primary Hyperparathyroidism-Changes Occurring in Bone Metabolism and New Potential Treatment Strategies

Primary hyperparathyroidism (PHPT) is a common endocrinopathy, predominantly caused by a single parathyroid adenoma that is responsible for the excessive secretion of parathyroid hormone (PTH)-the hallmark of disease. Excess of this hormone causes remarkable changes in bone metabolism, including an increased level of bone remodeling with a predominance of bone resorption. Those changes lead to deterioration of bone structure and density, especially in cortical bone. The main treatment for PHPT is surgical removal of the adenoma, which normalizes PTH levels and terminates the progression of bone disease and leads to its regeneration. However, because not all the patients are suitable candidates for surgery, alternative therapies are needed. Current non-surgical treatments targeting bone disease secondary to PHPT include bisphosphonates and denosumab. Those antiresorptives prevent further bone loss, but they lack the ability to regenerate already degraded bone. There is ongoing research to find targeted drugs capable of halting resorption alongside stimulating bone formation. This review presents the advancements in understanding the molecular mechanisms responsible for bone disease in PHPT and assesses the efficacy of new potential therapeutic approaches (e.g., allosteric inhibitors of the PTH receptor, V-ATPase, or cathepsin inhibitors) aimed at mitigating bone loss and enhancing bone regeneration in affected patients.

Immunity and Coagulation in COVID-19

Discovered in late 2019, the SARS-CoV-2 coronavirus has caused the largest pandemic of the 21st century, claiming more than seven million lives. In most cases, the COVID-19 disease caused by the SARS-CoV-2 virus is relatively mild and affects only the upper respiratory tract; it most often manifests itself with fever, chills, cough, and sore throat, but also has less-common mild symptoms. In most cases, patients do not require hospitalization, and fully recover. However, in some cases, infection with the SARS-CoV-2 virus leads to the development of a severe form of COVID-19, which is characterized by the development of life-threatening complications affecting not only the lungs, but also other organs and systems. In particular, various forms of thrombotic complications are common among patients with a severe form of COVID-19. The mechanisms for the development of thrombotic complications in COVID-19 remain unclear. Accumulated data indicate that the pathogenesis of severe COVID-19 is based on disruptions in the functioning of various innate immune systems. The key role in the primary response to a viral infection is assigned to two systems. These are the pattern recognition receptors, primarily members of the toll-like receptor (TLR) family, and the complement system. Both systems are the first to engage in the fight against the virus and launch a whole range of mechanisms aimed at its rapid elimination. Normally, their joint activity leads to the destruction of the pathogen and recovery. However, disruptions in the functioning of these innate immune systems in COVID-19 can cause the development of an excessive inflammatory response that is dangerous for the body. In turn, excessive inflammation entails activation of and damage to the vascular endothelium, as well as the development of the hypercoagulable state observed in patients seriously ill with COVID-19. Activation of the endothelium and hypercoagulation lead to the development of thrombosis and, as a result, damage to organs and tissues. Immune-mediated thrombotic complications are termed "immunothrombosis". In this review, we discuss in detail the features of immunothrombosis associated with SARS-CoV-2 infection and its potential underlying mechanisms.

One immune cell to bind them all: platelet contribution to neurodegenerative disease

Alzheimer's disease (AD) and related dementias (ADRD) collectively affect a significant portion of the aging population worldwide. The pathological progression of AD involves not only the classical hallmarks of amyloid beta (Aβ) plaque buildup and neurofibrillary tangle development but also the effects of vasculature and chronic inflammatory processes. Recently, platelets have emerged as central players in systemic and neuroinflammation. Studies have shown that patients with altered platelet receptor expression exhibit accelerated cognitive decline independent of traditional risk factors. Additionally, platelets from AD patients exhibit heightened unstimulated activation compared to control groups. Platelet granules contain crucial AD-related proteins like tau and amyloid precursor protein (APP). Dysregulation of platelet exocytosis contributes to disease phenotypes characterized by increased bleeding, stroke, and cognitive decline risk. Recent studies have indicated that these effects are not associated with the quantity of platelets present in circulation. This underscores the hypothesis that disruptions in platelet-mediated inflammation and healing processes may play a crucial role in the development of ADRD. A thorough look at platelets, encompassing their receptors, secreted molecules, and diverse roles in inflammatory interactions with other cells in the circulatory system in AD and ADRD, holds promising prospects for disease management and intervention. This review discusses the pivotal roles of platelets in ADRD.

Contributions of the individual domains of αIIbβ3 integrin to its extension: Insights from multiscale modeling

The platelet integrin αIIbβ3 undergoes long-range conformational transitions between bent and extended conformations to regulate platelet aggregation during hemostasis and thrombosis. However, how exactly αIIbβ3 transitions between conformations remains largely elusive. Here, we studied how transitions across bent and extended-closed conformations of αIIbβ3 integrin are regulated by effective interactions between its functional domains. We first carried out μs-long equilibrium molecular dynamics (MD) simulations of full-length αIIbβ3 integrins in bent and intermediate conformations, the latter characterized by an extended headpiece and closed legs. Then, we built heterogeneous elastic network models, perturbed inter-domain interactions, and evaluated their relative contributions to the energy barriers between conformations. Results showed that integrin extension emerges from: (i) changes in interfaces between functional domains; (ii) allosteric coupling of the head and upper leg domains with flexible lower leg domains. Collectively, these results provide new insights into integrin conformational activation based on short- and long-range interactions between its functional domains and highlight the importance of the lower legs in the regulation of integrin allostery.

A molecular arm: the molecular bending-unbending mechanism of integrin

The balance of integrin activation and deactivation regulates its function and mediates cell behaviors. Mechanical force triggers the unbending and activation of integrin. However, how an activated and extended integrin spontaneously bends back is unclear. I performed all-atom molecular dynamics simulations on an integrin or its subunits to reveal the bending-unbending mechanism of integrin. According to the simulations, the integrin structure works like a human arm. The integrin α subunit serves as the bones, while the β leg serves as the bicep. The integrin extension results in the stretching of the β leg, and the extended integrin spontaneously bends as a consequence of the contraction of the β leg. This study provides new insights into the mechanism of how the integrin secures in the bent inactivated state and sheds light on how the integrin could achieve a stable extended state.

Tea polyphenol-derived nanomedicine for targeted photothermal thrombolysis and inflammation suppression

Thrombotic diseases impose a significant global health burden, and conventional drug-based thrombolytic therapies are encumbered by the risk of bleeding complications. In this study, we introduce a novel drug-free nanomedicine founded on tea polyphenols nanoparticles (TPNs), which exhibits multifaceted capabilities for localized photothermal thrombolysis. TPNs were synthesized through a one-pot process under mild conditions, deriving from the monomeric epigallocatechin-3-gallate (EGCG). Within this process, indocyanine green (ICG) was effectively encapsulated, exploiting multiple intermolecular interactions between EGCG and ICG. While both TPNs and ICG inherently possessed photothermal potential, their synergy significantly enhanced photothermal conversion and stability. Furthermore, the nanomedicine was functionalized with cRGD for targeted delivery to activated platelets within thrombus sites, eliciting robust thrombolysis upon laser irradiation across diverse thrombus types. Importantly, the nanomedicine's potent free radical scavenging abilities concurrently mitigated vascular inflammation, thus diminishing the risk of disease recurrence. In summary, this highly biocompatible multifunctional nanomaterial holds promise as a comprehensive approach that combines thrombolysis with anti-inflammatory actions, offering precision in thrombosis treatment.

Soluble endoglin reduces thrombus formation and platelet aggregation via interaction with αIIbβ3 integrin

BACKGROUND

The circulating form of human endoglin (sEng) is a cleavage product of membrane-bound endoglin present on endothelial cells. Because sEng encompasses an RGD motif involved in integrin binding, we hypothesized that sEng would be able to bind integrin αIIbβ3, thereby compromising platelet binding to fibrinogen and thrombus stability.

METHODS

In vitro human platelet aggregation, thrombus retraction, and secretion-competition assays were performed in the presence of sEng. Surface plasmon resonance (SPR) binding and computational (docking) analyses were carried out to evaluate protein-protein interactions. A transgenic mouse overexpressing human sEng (hsEng+) was used to measure bleeding/rebleeding, prothrombin time (PT), blood stream, and embolus formation after FeCl3-induced injury of the carotid artery.

RESULTS

Under flow conditions, supplementation of human whole blood with sEng led to a smaller thrombus size. sEng inhibited platelet aggregation and thrombus retraction, interfering with fibrinogen binding, but did not affect platelet activation. SPR binding studies demonstrated that the specific interaction between αIIbβ3 and sEng and molecular modeling showed a good fitting between αIIbβ3 and sEng structures involving the endoglin RGD motif, suggesting the possible formation of a highly stable αIIbβ3/sEng. hsEng+ mice showed increased bleeding time and number of rebleedings compared to wild-type mice. No differences in PT were denoted between genotypes. After FeCl3 injury, the number of released emboli in hsEng+ mice was higher and the occlusion was slower compared to controls.

CONCLUSIONS

Our results demonstrate that sEng interferes with thrombus formation and stabilization, likely via its binding to platelet αIIbβ3, suggesting its involvement in primary hemostasis control.

Matricellular proteins in atherosclerosis development

The extracellular matrix (ECM) is an intricate network composed of various multi-domain macromolecules like collagen, proteoglycans, and fibronectin, etc., that form a structurally stable composite, contributing to the mechanical properties of tissue. However, matricellular proteins are non-structural, secretory extracellular matrix proteins, which modulate various cellular functions via interacting with cell surface receptors, proteases, hormones, and cell-matrix. They play essential roles in maintaining tissue homeostasis by regulating cell differentiation, proliferation, adhesion, migration, and several signal transduction pathways. Matricellular proteins display a broad functionality regulated by their multiple structural domains and their ability to interact with different extracellular substrates and/or cell surface receptors. The expression of these proteins is low in adults, however, gets upregulated following injuries, inflammation, and during tumor growth. The marked elevation in the expression of these proteins during atherosclerosis suggests a positive association between their expression and atherosclerotic lesion formation. The role of matricellular proteins in atherosclerosis development has remained an area of research interest in the last two decades and studies revealed these proteins as important players in governing vascular function, remodeling, and plaque formation. Despite extensive research, many aspects of the matrix protein biology in atherosclerosis are still unknown and future studies are required to investigate whether targeting pathways stimulated by these proteins represent viable therapeutic approaches for patients with atherosclerotic vascular diseases. This review summarizes the characteristics of distinct matricellular proteins, discusses the available literature on the involvement of matrix proteins in the pathogenesis of atherosclerosis and suggests new avenues for future research.

The Tissue Factor Pathway in Cancer: Overview and Role of Heparan Sulfate Proteoglycans

Historically, the only focus on tissue factor (TF) in clinical pathophysiology has been on its function as the initiation of the extrinsic coagulation cascade. This obsolete vessel-wall TF dogma is now being challenged by the findings that TF circulates throughout the body as a soluble form, a cell-associated protein, and a binding microparticle. Furthermore, it has been observed that TF is expressed by various cell types, including T-lymphocytes and platelets, and that certain pathological situations, such as chronic and acute inflammatory states, and cancer, may increase its expression and activity. Transmembrane G protein-coupled protease-activated receptors can be proteolytically cleaved by the TF:FVIIa complex that develops when TF binds to Factor VII (PARs). The TF:FVIIa complex can activate integrins, receptor tyrosine kinases (RTKs), and PARs in addition to PARs. Cancer cells use these signaling pathways to promote cell division, angiogenesis, metastasis, and the maintenance of cancer stem-like cells. Proteoglycans play a crucial role in the biochemical and mechanical properties of the cellular extracellular matrix, where they control cellular behavior via interacting with transmembrane receptors. For TFPI.fXa complexes, heparan sulfate proteoglycans (HSPGs) may serve as the primary receptor for uptake and degradation. The regulation of TF expression, TF signaling mechanisms, their pathogenic effects, and their therapeutic targeting in cancer are all covered in detail here.

Molecular Modeling Insights into the Structure and Behavior of Integrins: A Review

Integrins are heterodimeric glycoproteins crucial to the physiology and pathology of many biological functions. As adhesion molecules, they mediate immune cell trafficking, migration, and immunological synapse formation during inflammation and cancer. The recognition of the vital roles of integrins in various diseases revealed their therapeutic potential. Despite the great effort in the last thirty years, up to now, only seven integrin-based drugs have entered the market. Recent progress in deciphering integrin functions, signaling, and interactions with ligands, along with advancement in rational drug design strategies, provide an opportunity to exploit their therapeutic potential and discover novel agents. This review will discuss the molecular modeling methods used in determining integrins' dynamic properties and in providing information toward understanding their properties and function at the atomic level. Then, we will survey the relevant contributions and the current understanding of integrin structure, activation, the binding of essential ligands, and the role of molecular modeling methods in the rational design of antagonists. We will emphasize the role played by molecular modeling methods in progress in these areas and the designing of integrin antagonists.

Filamin A in platelets: Bridging the (signaling) gap between the plasma membrane and the actin cytoskeleton

Platelets are anucleate cells that are essential for hemostasis and wound healing. Upon activation of the cell surface receptors by their corresponding extracellular ligands, platelets undergo rapid shape change driven by the actin cytoskeleton; this shape change reaction is modulated by a diverse array of actin-binding proteins. One actin-binding protein, filamin A (FLNA), cross-links and stabilizes subcortical actin filaments thus providing stability to the cell membrane. In addition, FLNA binds the intracellular portion of multiple cell surface receptors and acts as a critical intracellular signaling scaffold that integrates signals between the platelet's plasma membrane and the actin cytoskeleton. This mini-review summarizes how FLNA transduces critical cell signals to the platelet cytoskeleton.

Posttranslational modifications of platelet adhesion receptors

Platelets play a key role in normal hemostasis, whereas pathological platelet adhesion is involved in various cardiovascular events. The underlying cause in cardiovascular events involves plaque rupture leading to subsequent platelet adhesion, activation, release, and eventual thrombosis. Traditional antithrombotic drugs often target the signal transduction process of platelet adhesion receptors by influencing the synthesis of some key molecules, and their effects are limited. Posttranslational modifications (PTMs) of platelet adhesion receptors increase the functional diversity of the receptors and affect platelet physiological and pathological processes. Antithrombotic drugs targeting PTMs of platelet adhesion receptors may represent a new therapeutic idea. In this review, various PTMs, including phosphorylation, glycosylation, ubiquitination, nitrosylation, methylation, lipidation, and proteolysis, of three platelet adhesion receptors, glycoprotein Ib-IX-V (GPIb-IX-V), glycoprotein VI (GPVI), and integrin α_IIbβ_3, are reviewed. It is important to comprehensively understand the PTMs process of platelet adhesion receptors.

Novel Functions of Integrins as Receptors of CD154: Their Role in Inflammation and Apoptosis

CD154, an inflammatory mediator also known as CD40 ligand, has been identified as a novel binding partner for some members of the integrin family. The αIIbβ3, specifically expressed on platelets, was the first integrin to be described as a receptor for CD154 after CD40. Its interaction with soluble CD154 (sCD154) highly contributes to thrombus formation and stability. Identifying αIIbβ3 opened the door for investigating other integrins as partners of CD154. The αMβ2 expressed on myeloid cells was shown capable of binding CD154 and contributing as such to cell activation, adhesion, and release of proinflammatory mediators. In parallel, α5β1 communicates with sCD154, inducing pro-inflammatory responses. Additional pathogenic effects involving apoptosis-preventing functions were exhibited by the CD154–α5β1 dyad in T cells, conferring a role for such interaction in the survival of malignant cells, as well as the persistence of autoreactive T cells. More recently, CD154 receptors integrated two new integrin members, αvβ3 and α4β1, with little known as to their biological significance in this context. This article provides an overview of the novel role of integrins as receptors of CD154 and as critical players in pro-inflammatory and apoptotic responses.

Platelets, Bacterial Adhesins and the Pneumococcus

Systemic infections with pathogenic or facultative pathogenic bacteria are associated with activation and aggregation of platelets leading to thrombocytopenia and activation of the clotting system. Bacterial proteins leading to platelet activation and aggregation have been identified, and while platelet receptors are recognized, induced signal transduction cascades are still often unknown. In addition to proteinaceous adhesins, pathogenic bacteria such as Staphylococcus aureus and Streptococcus pneumoniae also produce toxins such as pneumolysin and alpha-hemolysin. They bind to cellular receptors or form pores, which can result in disturbance of physiological functions of platelets. Here, we discuss the bacteria-platelet interplay in the context of adhesin–receptor interactions and platelet-activating bacterial proteins, with a main emphasis on S. aureus and S. pneumoniae. More importantly, we summarize recent findings of how S. aureus toxins and the pore-forming toxin pneumolysin of S. pneumoniae interfere with platelet function. Finally, the relevance of platelet dysfunction due to killing by toxins and potential treatment interventions protecting platelets against cell death are summarized.

Force-enhanced biophysical connectivity of platelet β3 integrin signaling through Talin is predicted by steered molecular dynamics simulations

Platelet β3-integrin signaling through Talin is crucial in platelet transmembrane signaling, activation, adhesion, spreading and aggregation, and remains unclear in mechano-microenvironments. In order to examine Talin-β3 integrin biophysical connectivity, a series of “ramp-clamp” steered molecular dynamics (SMD) simulations were performed on complex of F3 domain of Talin and cytoplasmic tail of β3 integrin to imitate different force-loads in platelet. Pull-induced allostery of the hydrophobic pocket in F3 domain might markedly enhance complex rupture-force (> 150pN) and slow down breakage of the complex; the complex should mechano-stable for its conformational conservation under loads (≤ 80pN); increasing force below 60pN would decrease the complex dissociation probability, and force-induced extension of β5 strand on Talin and binding site residues, ASP⁷⁴⁰ and ALA⁷⁴² as well as Asn⁷⁴⁴, on β3-integrin were responsible for the force-enhanced linkage of the Talin-β3 integrin. Force might enhance biophysical connectivity of β3-integrin signaling through Talin by a catch bond mechanism, which be mediated by the force-induced allostery of complex at clamped stage. This work provides a novel insight into the force-regulated transmembrane β3-integrin signaling and its molecular basis for platelet activation, and exhibited a potential power of the present computer strategy in predicting mechanical regulation on ligand-receptor interaction under loads.

Arenaviral infection causes bleeding in mice due to reduced serotonin release from platelets

Bleeding correlates with disease severity in viral hemorrhagic fevers. We found that the increase in type I interferon (IFN-I) in mice caused by infection with the Armstrong strain of lymphocytic choriomeningitis virus (LCMV; an arenavirus) reduced the megakaryocytic expression of genes encoding enzymes involved in lipid biosynthesis (cyclooxygenase 1 and thromboxane A synthase 1) and a thrombopoietic transcription factor (Nf-e2). The decreased expression of these genes was associated with reduced numbers of circulating platelets and defects in the arachidonic acid synthetic pathway, thereby suppressing serotonin release from δ-granules in platelets. Bleeding resulted when severe thrombocytopenia and altered platelet function reduced the amount of platelet-derived serotonin below a critical threshold. Bleeding was facilitated by the absence of the activity of the kinase Lyn or the administration of aspirin, an inhibitor of arachidonic acid synthesis. Mouse platelets were not directly affected by IFN-I because they lack the receptor for the cytokine (IFNAR1), suggesting that transfusion of normal platelets into LCMV-infected mice could increase the amount of platelet-released serotonin and help to control hemorrhage.

Thy-1 (CD90), Integrins and Syndecan 4 are Key Regulators of Skin Wound Healing

Acute skin wound healing is a multistage process consisting of a plethora of tightly regulated signaling events in specialized cells. The Thy-1 (CD90) glycoprotein interacts with integrins and the heparan sulfate proteoglycan syndecan 4, generating a trimolecular complex that triggers bi-directional signaling to regulate diverse aspects of the wound healing process. These proteins can act either as ligands or receptors, and they are critical for the successful progression of wound healing. The expression of Thy-1, integrins, and syndecan 4 is controlled during the healing process, and the lack of expression of any of these proteins results in delayed wound healing. Here, we review and discuss the roles and regulatory events along the stages of wound healing that support the relevance of Thy-1, integrins, and syndecan 4 as crucial regulators of skin wound healing.

The choice of targets and ligands for site-specific delivery of nanomedicine to atherosclerosis

As nanotechnologies advance into clinical medicine, novel methods for applying nanomedicine to cardiovascular diseases are emerging. Extensive research has been undertaken to unlock the complex pathogenesis of atherosclerosis. However, this complexity presents challenges to develop effective imaging and therapeutic modalities for early diagnosis and acute intervention. The choice of ligand-receptor system vastly influences the effectiveness of nanomedicine. This review collates current ligand-receptor systems used in targeting functionalized nanoparticles for diagnosis and treatment of atherosclerosis. Our focus is on the binding affinity and selectivity of ligand-receptor systems, as well as the relative abundance of targets throughout the development and progression of atherosclerosis. Antibody-based targeting systems are currently the most commonly researched due to their high binding affinities when compared with other ligands, such as antibody fragments, peptides, and other small molecules. However, antibodies tend to be immunogenic due to their size. Engineering antibody fragments can address this issue but will compromise their binding affinity. Peptides are promising ligands due to their synthetic flexibility and low production costs. Alongside the aforementioned binding affinity of ligands, the choice of target and its abundance throughout distinct stages of atherosclerosis and thrombosis is relevant to the intended purpose of the nanomedicine. Further studies to investigate the components of atherosclerotic plaques are required as their cellular and molecular profile shifts over time.

Microfluidic chip grafted with integrin tension sensors for evaluating the effects of flowing shear stress and ROCK inhibitor on platelets

Integrins are key players in platelet adhesion and aggregation. Integrin molecular tensions, the forces transmitted by integrin molecules, are regulated by both mechanical and biochemical cues, and the outside-in and inside-out signaling has been extensively studied. While the mechanical properties of platelets at static status have been studied by atomic force microscopy, traction force microscopy and tension sensors, the biomechanical properties of flowing platelets remain elusive. Herein, we report microfluidic chips grafted with integrin tension sensors for microfluidic-force mapping in platelets. Specifically, the process of integrin αIIbβ3 mediating tension transmission and platelet adhesion under low flow rates has been obtained, and the process of platelet clustering at post-stenotic regions has been demonstrated. We found that flowing shear force can postpone the integrin-mediated tension transmission and platelet adhesion. We further evaluated the effect of Y-27632, a ROCK inhibitor that has been proven to reduce integrin-mediated platelet adhesion, at a series of concentrations and demonstrated that microfluidic chips with integrin tension sensors are sensitive to the concentration-dependent effects of Y-27632. Given their low cost and scalable throughput, these chips are ideal technical platforms for biological studies of platelets at flowing status and for platelet inhibitor or potential antiplatelet drug screening.

Comparative Evaluation on Impacts of Fibronectin, Heparin-Chitosan, and Albumin Coating of Bacterial Nanocellulose Small-Diameter Vascular Grafts on Endothelialization In Vitro

In this study, we contrast the impacts of surface coating bacterial nanocellulose small-diameter vascular grafts (BNC-SDVGs) with human albumin, fibronectin, or heparin–chitosan upon endothelialization with human saphenous vein endothelial cells (VEC) or endothelial progenitor cells (EPC) in vitro. In one scenario, coated grafts were cut into 2D circular patches for static colonization of a defined inner surface area; in another scenario, they were mounted on a customized bioreactor and subsequently perfused for cell seeding. We evaluated the colonization by emerging metabolic activity and the preservation of endothelial functionality by water soluble tetrazolium salts (WST-1), acetylated low-density lipoprotein (AcLDL) uptake assays, and immune fluorescence staining. Uncoated BNC scaffolds served as controls. The fibronectin coating significantly promoted adhesion and growth of VECs and EPCs, while albumin only promoted adhesion of VECs, but here, the cells were functionally impaired as indicated by missing AcLDL uptake. The heparin–chitosan coating led to significantly improved adhesion of EPCs, but not VECs. In summary, both fibronectin and heparin–chitosan coatings could beneficially impact the endothelialization of BNC-SDVGs and might therefore represent promising approaches to help improve the longevity and reduce the thrombogenicity of BNC-SDVGs in the future.

From Classical to Unconventional: The Immune Receptors Facilitating Platelet Responses to Infection and Inflammation

Platelets have long been recognized for their role in maintaining the balance between hemostasis and thrombosis. While their contributions to blood clotting have been well established, it has been increasingly evident that their roles extend to both innate and adaptive immune functions during infection and inflammation. In this comprehensive review, we describe the various ways in which platelets interact with different microbes and elicit immune responses either directly, or through modulation of leukocyte behaviors.

Bioactive Fatty Acids in the Resolution of Chronic Inflammation in Skin Wounds

Significance: Optimal skin wound healing is crucial for maintaining tissue homeostasis, particularly in response to an injury. The skin immune system is under regulation of mediators such as bioactive lipids and cytokines that can initiate an immune response with controlled inflammation, followed by efficient resolution. However, nutritional deficiency impacts wound healing by hindering fibroblast proliferation, collagen synthesis, and epithelialization, among other crucial functions. In this way, the correct nutritional support of bioactive lipids and of other essential nutrients plays an important role in the outcome of the wound healing process. Recent Advances and Critical Issues: Several studies have revealed the potential role of lipids as a treatment for the healing of skin wounds. Unsaturated fatty acids such as linoleic acid, α-linolenic acid, oleic acid, and most of their bioactive products have shown an effective role as a topical treatment of chronic skin wounds. Their effect, when the treatment starts at day 0, has been observed mainly in the inflammatory phase of the wound healing process. Moreover, some of them were associated with different dressings and were tested for clinical purposes, including pluronic gel, nanocapsules, collagen films and matrices, and polymeric bandages. Therefore, future research is still needed to evaluate these dressing technologies in association with different bioactive fatty acids in a wound healing context. Future Directions: This review summarizes the main results of the available clinical trials and basic research studies and provides evidence-based conclusions. Together, current data encourage the use of bioactive fatty acids for an optimal wound healing resolution.

Optical uncaging of ADP reveals the early calcium dynamics in single, freely moving platelets

Platelet activation is considered to be a cornerstone in pathogenesis of cardiovascular disease. The assessment of platelet activation at the single-cell level is a promising approach for the research of platelet function in physiological and pathological conditions. Previous studies used the immobilization of platelets on the surface, which significantly alters the activation signaling. Here we show that the use of photolabile "caged" analog of ADP allows one to track the very early stage of platelet activation in single, freely moving cells. In this approach, the diffusion step and ADP receptor ligation are separated in time, and a millisecond-timescale optical pulse may trigger the activation. The technique allows us to measure the delay (lag time) between the stimulus and calcium response in platelets. We also propose a simple model function for calcium peaks, which is in good agreement with the measured data. The proposed technique and model function can be used for in-depth studies of platelet physiology.

Differential dynamics of early stages of platelet adhesion and spreading on collagen IV- and fibrinogen-coated surfaces

Background: Upon wound formation, platelets adhere to the neighboring extracellular matrix and spread on it, a process which is critical for physiological wound healing. Multiple external factors, such as the molecular composition of the environment and its mechanical properties, play a key role in this process and direct its speed and outcome. Methods: We combined live cell imaging, quantitative interference reflection microscopy and cryo-electron tomography to characterize, at a single platelet level, the differential spatiotemporal dynamics of the adhesion process to fibrinogen- and collagen IV-functionalized surfaces. Results: Initially, platelets sense both substrates by transient rapid extensions of filopodia. On collagen IV, a short-term phase of filopodial extension is followed by lamellipodia-based spreading. This transition is preceded by the extension of a single or couple of microtubules into the platelet's periphery and their apparent insertion into the core of the filopodia. On fibrinogen surfaces, the filopodia-to-lamellipodia transition was partial and microtubule extension was not observed leading to limited spreading, which could be restored by manganese or thrombin. Conclusions: Based on these results, we propose that interaction with collagen IV stimulate platelets to extend microtubules to peripheral filopodia, which in turn, enhances filopodial-to-lamellipodial transition and overall lamellipodia-based spreading. Fibrinogen, on the other hand, fails to induce these early microtubule extensions, leading to full lamellipodia spreading in only a fraction of the seeded platelets. We further suggest that activation of integrin αIIbβ3 is essential for filopodial-to-lamellipodial transition, based on the capacity of integrin activators to enhance lamellipodia spreading on fibrinogen.

Defining the TLT-1 interactome from resting and activated human platelets

The triggering receptor expressed on myeloid cells (TREM) protein family forms a class of type I transmembrane proteins expressed in immune cells that play important roles in innate and adaptive immune responses. The TREM family member TREM-like transcript 1 (TLT-1, also TREML1) is expressed in megakaryocytes and packaged into platelet granules. TLT-1 binds fibrinogen and plays a role in bleeding initiated by inflammatory insults. Here, we describe a proteomics screen that maps the TLT-1 interactome in resting and activated human platelets. Several identified TLT-1 interactors are involved in cell adhesion and migration, as well as platelet activation. Select interactors, including β3-integrin, RACK1, GRB2, and Rabs 5A, 7, and 11A, were additionally characterized in co-immunoprecipitation/immunoblotting experiments. Finally, several phosphorylation sites were found on immunoprecipitated TLT-1, including Thr280, a novel, regulated site on a conserved residue near the TLT-1 ITIM regulatory sequence. SIGNIFICANCE: Platelet function relies on the secretion of active molecules from intracellular vesicles, or granules, which contain soluble and membrane-bound proteins that are essential for platelet aggregation, coagulation reactions, and pathogen defense mechanisms. TLT-1 is sequestered in α-granules and transported to the plasma membrane, where it plays a unique role in hemostasis after inflammatory insults. Despite the known importance of TLT-1 in platelet biology, our knowledge of TLT-1 mechanistic signaling is limited. This study defines the TLT-1 interactome in resting and active human platelets, identifying several novel TLT-1 interactors, as well as TLT-1 phosphorylation sites, all with likely signaling implications in platelet aggregation dynamics.

Cross-Talk between Transforming Growth Factor-β and Periostin Can Be Targeted for Pulmonary Fibrosis

Idiopathic pulmonary fibrosis (IPF) is a devastating disease characterized as progressive and irreversible fibrosis in the interstitium of lung tissues. There is still an unmet need to develop a novel therapeutic drug for IPF. We have previously demonstrated that periostin, a matricellular protein, plays an important role in the pathogenesis of pulmonary fibrosis. However, the underlying mechanism of how periostin causes pulmonary fibrosis remains unclear. In this study, we sought to learn whether the cross-talk between TGF-β (transforming growth factor-β), a central mediator in pulmonary fibrosis, and periostin in lung fibroblasts leads to generation of pulmonary fibrosis and whether inhibitors for integrin αVβ3, a periostin receptor, can block pulmonary fibrosis in model mice and the TGF-β signals in fibroblasts from patients with IPF. We found that cross-talk exists between TGF-β and periostin signals via αVβ3/β5 converging into Smad3. This cross-talk is necessary for the expression of TGF-β downstream effector molecules important for pulmonary fibrosis. Moreover, we identified several potent integrin low-molecular-weight inhibitors capable of blocking cross-talk with TGF-β signaling. One of the compounds, CP4715, attenuated bleomycin-induced pulmonary fibrosis in vivo in mice and the TGF-β signals in vitro in fibroblasts from patients with IPF. These results suggest that the cross-talk between TGF-β and periostin can be targeted for pulmonary fibrosis and that CP4715 can be a potential therapeutic agent to block this cross-talk.

Prospects for risk stratification of anti-HPA-1a alloimmunized pregnant women

A diagnosis of fetal/neonatal alloimmune thrombocytopenia (FNAIT) is made if a platelet-specific antibody is detected in the mother and the fetus or newborn carries the cognate antigen. Some children will experience very low platelet counts or even intracranial hemorrhage with devastating consequences, whereas others are largely unaffected. At the moment, predictive tools to forecast the severity of FNAIT during pregnancy are not available and over- or under-treatment may put the mother or the fetus at risk. A number of potential modulators of FNAIT severity have been reported. Maternal immune responses differ in respect to the IgG subtype composition, the glycosylation pattern of the antibodies, their fine specificity, and their functional effects on platelets, the trophoblast, and endothelial cells. In addition, antibody levels are variable. The efficacy of IgG transfer and, on the fetal side, gender and inflammatory responses, were also investigated for their potential impact on FNAIT severity. These potential risk modulators are scrutinized for available experimental and clinical evidence. Antibody glycosylation and anti-endothelial activity are hot candidates which, most likely in conjunction with the antibody level, should be explored further as tools to stratify fetal risk.

Stimulation of mTORC2 by integrin αIIbβ3 is required for PI3Kβ-dependent activation of Akt but is dispensable for platelet spreading on fibrinogen

Phosphatidylinositol 3 kinase (PI3K) is a major player in platelet activation and regulates thrombus formation and stabilization. The β isoform of PI3K is implicated in integrin αIIbβ3 outside-in signaling, is required for the phosphorylation of Akt, and controls efficient platelet spreading upon adhesion to fibrinogen. In this study we found that during integrin αIIbβ3 outside-in signaling PI3Kβ-dependent phosphorylation of Akt on Serine473 is mediated by the mammalian target of rapamycin complex 2 (mTORC2). The activity of mTORC2 is stimulated upon platelet adhesion to fibrinogen, as documented by increased autophosphorylation. However, mTORC2 activation downstream of integrin αIIbβ3 is PI3Kβ-independent. Inhibition of mTORC2, but not mTORC1, also prevents Akt phosphorylation of Threonine308 and affects Akt activity, resulting in the inhibition of GSK3α/β phosphorylation. Nevertheless, mTORC2 or Akt inhibition does not alter PI3Kβ-dependent platelet spreading on fibrinogen. The activation of the small GTPase Rap1b downstream of integrin αIIbβ3 is regulated by PI3Kβ but is not affected upon inhibition of either mTORC2 or Akt. Altogether, these results demonstrate for the first time the activation of mTORC2 and its involvement in Akt phosphorylation and stimulation during integrin αIIbβ3 outside-in signaling. Moreover, the results demonstrate that the mTORC2/Akt pathway is dispensable for PI3Kβ-regulated platelet spreading on fibrinogen.

An integrin αIIbβ3 intermediate affinity state mediates biomechanical platelet aggregation

Integrins are membrane receptors that mediate cell adhesion and mechanosensing. The structure-function relationship of integrins remains incompletely understood, despite the extensive studies carried out because of its importance to basic cell biology and translational medicine. Using a fluorescence dual biomembrane force probe, microfluidics and cone-and-plate rheometry, we applied precisely controlled mechanical stimulations to platelets and identified an intermediate state of integrin α_IIbβ₃ that is characterized by an ectodomain conformation, ligand affinity and bond lifetimes that are all intermediate between the well-known inactive and active states. This intermediate state is induced by ligand engagement of glycoprotein (GP) Ibα via a mechanosignalling pathway and potentiates the outside-in mechanosignalling of α_IIbβ₃ for further transition to the active state during integrin mechanical affinity maturation. Our work reveals distinct α_IIbβ₃ state transitions in response to biomechanical and biochemical stimuli, and identifies a role for the α_IIbβ₃ intermediate state in promoting biomechanical platelet aggregation.

Platelet Inflammatory Response to Stress

Blood platelets play a central hemostatic role, (i) as they repair vascular epithelial damage, and (ii) they play immune defense roles, as they have the capacity to produce and secrete various cytokines, chemokines, and related products. Platelets sense and respond to local dangers (infectious or not). Platelets, therefore, mediate inflammation, express and use receptors to bind infectious pathogen moieties and endogenous ligands, among other components. Platelets contribute to effective pathogen clearance. Damage-associated molecular patterns (DAMPs) are danger signals released during inflammatory stress, such as burns, trauma and infection. Each pathogen is recognized by its specific molecular signature or pathogen-associated molecular pattern (PAMP). Recent data demonstrate that platelets have the capacity to sense external danger signals (DAMPs or PAMPs) differentially through a distinct type of pathogen recognition receptor (such as Toll-like receptors). Platelets regulate the innate immune response to pathogens and/or endogenous molecules, presenting several types of “danger” signals using a complete signalosome. Platelets, therefore, use complex tools to mediate a wide range of functions from danger sensing to tissue repair. Moreover, we noted that the secretory capacity of stored platelets over time and the development of stress lesions by platelets upon collection, processing, and storage are considered stress signals. The key message of this review is the “inflammatory response to stress” function of platelets in an infectious or non-infectious context.

From Snake Venom's Disintegrins and C-Type Lectins to Anti-Platelet Drugs

Snake venoms are attractive natural sources for drug discovery and development, with a number of substances either in clinical use or in research and development. These drugs were developed based on RGD-containing snake venom disintegrins, which efficiently antagonize fibrinogen activation of αIIbβ3 integrin (glycoprotein GP IIb/IIIa). Typical examples of anti-platelet drugs found in clinics are Integrilin (Eptifibatide), a heptapeptide derived from Barbourin, a protein found in the venom of the American Southeastern pygmy rattlesnake and Aggrastat (Tirofiban), a small molecule based on the structure of Echistatin, and a protein found in the venom of the saw-scaled viper. Using a similar drug discovery approach, linear and cyclic peptides containing the sequence K(R)TS derived from VP12, a C-type lectin protein found in the venom of Israeli viper venom, were used as a template to synthesize Vipegitide, a novel peptidomimetic antagonist of α2β1 integrin, with anti-platelet activity. This review focus on drug discovery of these anti-platelet agents, their indications for clinical use in acute coronary syndromes and percutaneous coronary intervention based on several clinical trials, as well as their adverse effects.

There Is Selective Increase in Pro-thrombotic Circulating Extracellular Vesicles in Acute Ischemic Stroke and Transient Ischemic Attack: A Study of Patients From the Middle East and Southeast Asia

Stroke attacks were found to be present at a younger age in patients from Southeast Asia (SE) and the Middle East (ME) resident in the state of Qatar. Extracellular vesicles (EVs), which are small membrane vesicles with pro-thrombotic properties, may contribute to the high risk of stroke in this population. Thus, total and cell-specific medium size EVs were counted by flow cytometry in platelet-free plasma from healthy volunteers and patients with transient ischemic attacks (TIA) and acute ischemic stroke (AIS) from SE and ME. Acutely, within 48 h of attacks, there was an increase in total endothelial EVs in TIA (6.73 ± 1.77; P = 0.0156; n = 21) and AIS (11.23 ± 1.95; P = 0.0007; n = 66) patients compared to controls (2.04 ± 0.78; n = 24). Similar increases were also evident in EVs originating from platelets, erythrocytes, granulocytes, and leukocytes. Compared to controls, there was also an increase in EVs derived from activated endothelial cells, platelets, granulocytes, leukocytes, and pro-coagulant EVs (Annexin V⁺) at 5 and 30-days following the acute events, while a decrease was observed in erythrocyte-derived EVs. This is the first study characterizing EVs in TIA and AIS patients from ME and SE showing an increase in EVs associated with endothelial and platelet cell activation, which may contribute to the elevated risk of stroke at a younger age in this population.

Coarse-Grained Simulation of Full-Length Integrin Activation

Integrin conformational dynamics are critical to their receptor and signaling functions in many cellular processes, including spreading, adhesion, and migration. However, assessing integrin conformations is both experimentally and computationally challenging because of limitations in resolution and dynamic sampling. Thus, structural changes that underlie transitions between conformations are largely unknown. Here, focusing on integrin αvβ₃, we developed a modified form of the coarse-grained heterogeneous elastic network model (hENM), which allows sampling conformations at the onset of activation by formally separating local fluctuations from global motions. Both local fluctuations and global motions are extracted from all-atom molecular dynamics simulations of the full-length αvβ₃ bent integrin conformer, but whereas the former are incorporated in the hENM as effective harmonic interactions between groups of residues, the latter emerge by systematically identifying and treating weak interactions between long-distance domains with flexible and anharmonic connections. The new hENM model allows integrins and single-point mutant integrins to explore various conformational states, including the initiation of separation between α- and β-subunit cytoplasmic regions, headpiece extension, and legs opening.

In Vitro Methods to Characterize the Effects of Tobacco and Nontobacco Products on Human Platelet Function

In this document, we describe methods for the isolation, treatment, and functional testing of human blood platelets in vitro. Functional assays for inflammatory function include flow cytometry and immunoassays for platelet release of platelet factor 4, soluble CD40L, prostaglandin E₂ , and thromboxane. Assays for platelet hemostatic function described here examine platelet spreading, aggregation using platelet-rich plasma, and thromboelastography. Also described here are methods for testing cigarette smoke on primary human platelets in vitro, which our lab developed to address a major knowledge gap regarding how cigarette smoke dysregulates platelets and how this platelet dysregulation contributes to cardiovascular disease. Some of these protocols may be repurposed for investigation of the toxicity potential of other tobacco products and environmental insults. © 2018 by John Wiley & Sons, Inc.

Obstructive Sleep Apnea: From Intermittent Hypoxia to Cardiovascular Complications via Blood Platelets

Obstructive sleep apnea is a chronic condition characterized by recurrent episodes of apneas or hypopneas during sleep leading to intermittent hypoxemia and arousals. The prevalence of the sleep disordered breathing is estimated that almost 50% of men and 24% of women suffer from moderate to severe form of the disorder. Snoring, collapse of upper airways and intermittent hypoxia are main causes of smoldering systemic inflammation in patients suffering from obstructive sleep apnea. The systematic inflammation is considered one of the key mechanisms leading to significant cardiovascular complications. Blood platelets, formerly not even recognized as cells, are currently gaining attention as crucial players in the immune continuum. Platelet surface is endowed with receptors characteristic for cells classically belonging to the immune system, which enables them to recognize pathogens, immune complexes, and interact in a homo- and heterotypic aggregates. Platelets participate in the process of transcellular production of bioactive lipids by delivering both specific enzymes and substrate molecules. Despite their lack of nucleus, platelets synthetize proteins in a stimuli-dependent manner. Atherosclerosis and consequent cardiovascular complications result from disruption in homeostasis of both of the platelet roles: blood coagulation and inflammatory processes modulation. Platelet parameters, routinely evaluated as a part of complete blood count test, were proposed as markers of cardiovascular comorbidity in patients with obstructive sleep apnea. Platelets were found to be excessively activated in this group of patients, especially in obese subjects. Persistent activation results in enhanced spontaneous aggregability and change in cytokine production. Platelet-lymphocyte ratio was suggested as an independent marker for cardiovascular disease in obstructive sleep apnea syndrome and continuous positive air pressure therapy was found to have an impact on platelet parameters and phenotype. In this literature review we summarize the current knowledge on the subject of platelets involvement in obstructive sleep apnea syndrome and consider the possible pathways in which they contribute to cardiovascular comorbidity.

From blood coagulation to innate and adaptive immunity: the role of platelets in the physiology and pathology of autoimmune disorders

Thrombosis and cardiovascular complications are common manifestations of a variety of pathological conditions, including infections and chronic inflammatory diseases. Hence, there is great interest in determining the hitherto unforeseen immune role of the main blood coagulation executor-the platelet. Platelets store and release a plethora of immunoactive molecules, generate microparticles, and interact with cells classically belonging to the immune system. The observed effects of platelet involvement in immune processes, especially in autoimmune diseases, are conflicting-from inciting inflammation to mediating its resolution. An in-depth understanding of the role of platelets in inflammation and immunity could open new therapeutic pathways for patients with autoimmune disorders. This review aims to summarize the current knowledge on the role of platelets in the patomechanisms of autoimmune disorders and suggests directions for future research.

Immobilized fibrinogen activates human platelets through glycoprotein VI

Glycoprotein VI, a major platelet activation receptor for collagen and fibrin, is considered a particularly promising, safe antithrombotic target. In this study, we show that human glycoprotein VI signals upon platelet adhesion to fibrinogen. Full spreading of human platelets on fibrinogen was abolished in platelets from glycoprotein VI- deficient patients suggesting that fibrinogen activates platelets through glycoprotein VI. While mouse platelets failed to spread on fibrinogen, human-glycoprotein VI-transgenic mouse platelets showed full spreading and increased Ca²⁺ signaling through the tyrosine kinase Syk. Direct binding of fibrinogen to human glycoprotein VI was shown by surface plasmon resonance and by increased adhesion to fibrinogen of human glycoprotein VI-transfected RBL-2H3 cells relative to mock-transfected cells. Blockade of human glycoprotein VI with the Fab of the monoclonal antibody 9O12 impaired platelet aggregation on preformed platelet aggregates in flowing blood independent of collagen and fibrin exposure. These results demonstrate that human glycoprotein VI binds to immobilized fibrinogen and show that this contributes to platelet spreading and platelet aggregation under flow.

The human platelet antigen-1b (Pro33) variant of αIIbβ3 allosterically shifts the dynamic conformational equilibrium of this integrin toward the active state

Integrins are heterodimeric cell-adhesion receptors comprising α and β subunits that transmit signals allosterically in both directions across the membrane by binding to intra- and extracellular components. The human platelet antigen-1 (HPA-1) polymorphism in α_IIbβ₃ arises from a Leu → Pro exchange at residue 33 in the genu of the β₃ subunit, resulting in Leu³³ (HPA-1a) or Pro³³ (HPA-1b) isoforms. Although clinical investigations have provided conflicting results, some studies have suggested that Pro³³ platelets exhibit increased thrombogenicity. Under flow-dynamic conditions, the Pro³³ variant displays prothrombotic properties, characterized by increased platelet adhesion, aggregate/thrombus formation, and outside-in signaling. However, the molecular events underlying this prothrombotic phenotype have remained elusive. As residue 33 is located >80 Å away from extracellular binding sites or transmembrane domains, we hypothesized that the Leu → Pro exchange allosterically shifts the dynamic conformational equilibrium of α_IIbβ₃ toward an active state. Multiple microsecond-long, all-atom molecular dynamics simulations of the ectodomain of the Leu³³ and Pro³³ isoforms provided evidence that the Leu → Pro exchange weakens interdomain interactions at the genu and alters the structural dynamics of the integrin to a more unbent and splayed state. Using FRET analysis of fluorescent proteins fused with α_IIbβ₃ in transfected HEK293 cells, we found that the Pro³³ variant in its resting state displays a lower energy transfer than the Leu³³ isoform. This finding indicated a larger spatial separation of the cytoplasmic tails in the Pro³³ variant. Together, our results indicate that the Leu → Pro exchange allosterically shifts the dynamic conformational equilibrium of α_IIbβ₃ to a structural state closer to the active one, promoting the fully active state and fostering the prothrombotic phenotype of Pro³³ platelets.

The roles of cellular and molecular components of a hematoma at early stage of bone healing

Bone healing is a complex repair process that commences with the formation of a blood clot at the injured bone, termed hematoma. It has evidenced that a lack of a stable hematoma causes delayed bone healing or non-union. The hematoma at the injured bone constitutes the early healing microenvironment. It appears to dictate healing pathways that ends in a regenerative bone. However, the hematoma is often clinically removed from the damaged site. Conversely, blood-derived products have been used in bone tissue engineering for treating critical sized defects, including fibrin gels and platelet-rich plasma. A second generation of platelet concentrate that is based on leukocyte and fibrin content has also been developed and introduced in market. Conflicting effect of these products in bone repair are reported. We propose that the bone healing response becomes dysregulated if the blood response and subsequent formation and properties of a hematoma are altered. This review focuses on the central structural, cellular, and molecular components of a fracture hematoma, with a major emphasis on their roles in regulating bone healing mechanism, and their interactions with mesenchymal stem cells. New angles towards a better understanding of these factors and relevant mechanisms involved at the beginning of bone healing may help to clarify limited or adverse effects of blood-derived products on bone repair. We emphasize that the recreation of an early hematoma niche with critical compositions might emerge as a viable therapeutic strategy for enhanced skeletal tissue engineering.

Merkel Cell Polyomavirus Small T Antigen Drives Cell Motility via Rho-GTPase-Induced Filopodium Formation

Cell motility and migration is a complex, multistep, and multicomponent process intrinsic to progression and metastasis. Motility is dependent on the activities of integrin receptors and Rho family GTPases, resulting in the remodeling of the actin cytoskeleton and formation of various motile actin-based protrusions. Merkel cell carcinoma (MCC) is an aggressive skin cancer with a high likelihood of recurrence and metastasis. Merkel cell polyomavirus (MCPyV) is associated with the majority of MCC cases, and MCPyV-induced tumorigenesis largely depends on the expression of the small tumor antigen (ST). Since the discovery of MCPyV, a number of mechanisms have been suggested to account for replication and tumorigenesis, but to date, little is known about potential links between MCPyV T antigen expression and the metastatic nature of MCC. Previously, we described the action of MCPyV ST on the microtubule network and how it impacts cell motility and migration. Here, we demonstrate that MCPyV ST affects the actin cytoskeleton to promote the formation of filopodia through a mechanism involving the catalytic subunit of protein phosphatase 4 (PP4C). We also show that MCPyV ST-induced cell motility is dependent upon the activities of the Rho family GTPases Cdc42 and RhoA. In addition, our results indicate that the MCPyV ST-PP4C interaction results in the dephosphorylation of β₁ integrin, likely driving the cell motility pathway. These findings describe a novel mechanism by which a tumor virus induces cell motility, which may ultimately lead to cancer metastasis, and provides opportunities and strategies for targeted interventions for disseminated MCC.

IMPORTANCE Merkel cell polyomavirus (MCPyV) is the most recently discovered human tumor virus. It causes the majority of cases of Merkel cell carcinoma (MCC), an aggressive skin cancer. However, the molecular mechanisms implicating MCPyV-encoded proteins in cancer development are yet to be fully elucidated. This study builds upon our previous observations, which demonstrated that the MCPyV ST antigen enhances cell motility, providing a potential link between MCPyV protein expression and the highly metastatic nature of MCC. Here, we show that MCPyV ST remodels the actin cytoskeleton, promoting the formation of filopodia, which is essential for MCPyV ST-induced cell motility, and we also implicate the activity of specific Rho family GTPases, Cdc42 and RhoA, in these processes. Moreover, we describe a novel mechanism for the activation of Rho-GTPases and the cell motility pathway due to the interaction between MCPyV ST and the cellular phosphatase catalytic subunit PP4C, which leads to the specific dephosphorylation of β1 integrin. These findings may therefore provide novel strategies for therapeutic intervention for disseminated MCC.

Platelet adhesion on endothelium early after vein grafting mediates leukocyte recruitment and intimal hyperplasia in a murine model

Intimal hyperplasia (IH) is the substrate for accelerated atherosclerosis and limited patency of vein grafts. However, there is still no specific treatment targeting IH following graft surgery. In this study, we used a mouse model of vein grafting to investigate the potential for early intervention with platelet function for later development of graft IH. We transferred the inferior vena cava (IVC) from donor C57BL/6 mice to the carotid artery in recipients using a cuff technique. We found extensive endothelial injury and platelet adhesion one hour following grafting. Adhesion of leukocytes was distinct in areas of platelet adhesion. Platelet and leukocyte adhesion was strongly reduced in mice receiving a function-blocking antibody against the integrin αIIbβ3. This was followed by a reduction of IH one month following grafting. Depletion of platelets using antiserum also reduced IH at later time points. These findings indicate platelets as pivotal to leukocyte recruitment to the wall of vein grafts. In conclusion, the data also highlight early intervention of platelets and inflammation as potential treatment for later formation of IH and accelerated atherosclerosis following bypass surgery.

Role of platelets in immune system and inflammation

Platelets have significant role in modulating clot formation. Additionally, emerging data indicates that platelets have considerable roles in inflammation and immune response. Platelets gather at the damaged cite and adhere to white blood cells. Subsequently, they release cytokines and chemokines which are chemotactic for neutrophils and monocytes. Therefore, platelets are necessary for targeting lymphocytes, neutrophils and monocytes to inflammation site. Those interactions enhance inflammation. Moreover, platelets serve as an immune cell by engulfing microbes. Presence of platelets affect prognosis in some bacterial or viral infection and several other diseases.