Integrin α6β1 is the main receptor for vascular laminins and plays a role in platelet adhesion, activation, and arterial thrombosis

Exploring the Emerging Association Between Immune Checkpoint Inhibitors and Thrombosis

Immune checkpoint inhibitors (ICIs) have revolutionized cancer treatment, but their association with thrombosis presents significant clinical challenges. Patients with cancer already exhibit elevated risks for venous thromboembolism and arterial thrombosis, with treatment modalities like chemotherapy further exacerbating this risk. Emerging evidence suggests that ICIs contribute to thrombotic events through multifactorial mechanisms, including immune dysregulation, T cell activation, endothelial dysfunction, elevated tissue factor expression, and impaired fibrinolysis. Additional risk factors such as obesity, smoking, prior thrombotic events, and combination ICI therapy further increase thrombosis susceptibility. The literature reports varying incidence rates of ICI-associated thrombosis, with some studies indicating comparable risks to chemotherapy, while others highlight higher rates, particularly during the initial treatment phase. Management aligns with standard protocols for cancer-associated thrombosis, using low-molecular-weight heparin or direct oral anticoagulants, though optimal treatment duration and the role of prophylactic anticoagulation require further investigation. This review provides a comprehensive overview of the mechanisms, incidence rates, and clinical management strategies of ICI-associated thrombosis, emphasizing the importance of proactive risk assessment to optimize patient outcomes.

The critical role of platelets in venous thromboembolism: Pathogenesis, clinical status, and emerging therapeutic strategies

Venous thromboembolism (VTE), encompassing deep vein thrombosis (DVT) and pulmonary embolism (PE), is a complex vascular disorder with high morbidity and mortality, driven by Virchow's Triad: blood stasis, hypercoagulability, and endothelial injury. VTE is now recognized as an inflammatory process involving multiple components. Platelets are involved in the process of VTE, contributing to thrombosis initiation, progression, resolution and recurrence through coagulation activation, and interactions with immune and endothelial cells. Anticoagulation remains the cornerstone of VTE treatment; however, antiplatelet agents like aspirin have demonstrated therapeutic potential, particularly following major orthopedic surgeries. Furthermore, emerging platelet-targeted therapies and biomarkers offer new opportunities for improving VTE diagnosis and treatment. This review explores the evolving role of platelets in VTE pathophysiology, assesses current antiplatelet strategies, and highlights novel therapeutic approaches. Advancing platelet research in VTE may lead to safer, more effective interventions, optimizing outcomes for patients with this life-threatening condition.

LAMA4+ CD90+ eCAFs provide immunosuppressive microenvironment for liver cancer through induction of CD8+ T cell senescence

Despite significant advances in cancer biology research and treatment, clinical outcomes for patients with liver cancer remain unsatisfactory. The biological and molecular mechanisms underlying the bidirectional signaling between tumor cells and the tumor microenvironment (TME), which promotes tumor progression in the liver, remain to be elucidated. Fibroblasts are crucial regulators of tumor progression and response to therapy; however, our understanding of their roles remains limited. Here, we integrated single-cell RNA sequencing and spatial transcriptomic data of pan-liver cancers to characterize the different subtypes of cancer-associated fibroblasts (CAFs). siRNA transfection was used for knockdown the expression of LAMA4. Western blot assay was used for gene expression analysis. Flow cytometry was used to detect proliferation, toxicity and cytolytic capacity of CD8+ T cells. To establish a spontaneous murine hepatocellular carcinoma (HCC) model, a combined DEN and CCL4 approach was performed. Notably, we identified CD90+ extracellular matrix CAFs (eCAFs) associated with poor prognosis. These CD90+ eCAFs, located distal to the tumor nest, overlapped with the distribution of CD8+ T cells. Functional experiments demonstrated that CD90+ eCAFs recruited CD8+ T cells and inhibited their function through secretion of LAMA4. Further investigation revealed that LAMA4 induced the CD8+ T cell senescence through a DNA damage signaling pathway mediated by the receptor ITGA6. In a mouse model of spontaneous HCC, targeting LAMA4 can inhibit the progression of malignant transformation and synergize with anti-PD-1 therapy. Our study reveals the function of specific CAFs subtypes and highlights the importance of interactions with the immune system.

Thrombotic events associated with immune checkpoint inhibitors and novel antithrombotic strategies to mitigate bleeding risk

Although immunotherapy is expanding treatment options for cancer patients, the prognosis of advanced cancer remains poor, and these patients must contend with both cancers and cancer-related thrombotic events. In particular, immune checkpoint inhibitors are associated with an increased risk of atherosclerotic thrombotic events. Given the fundamental role of platelets in atherothrombosis, co-administration of antiplatelet agents is always indicated. Platelets are also involved in all steps of cancer progression. Classical antithrombotic drugs can cause inevitable hemorrhagic side effects due to blocking integrin β3 bidirectional signaling, which regulates simultaneously thrombosis and hemostasis. Meanwhile, many promising new targets are emerging with minimal bleeding risk and desirable anti-tumor effects. This review will focus on the issue of thrombosis during immune checkpoint inhibitor treatment and the role of platelet activation in cancer progression as well as explore the mechanisms by which novel antiplatelet therapies may exert both antithrombotic and antitumor effects without excessive bleeding risk.

TNF Induces Laminin-332-Encoding Genes in Endothelial Cells and Laminin-332 Promotes an Atherogenic Endothelial Phenotype

Laminins are essential components of the basement membranes, expressed in a tissue- and cell-specific manner under physiological conditions. During inflammatory circumstances, such as atherosclerosis, alterations in laminin composition within vessels have been observed. Our study aimed to assess the influence of tumor necrosis factor-alpha (TNF), a proinflammatory cytokine abundantly found in atherosclerotic lesions, on endothelial laminin gene expression and the effects of laminin-332 (LN332) on endothelial cells' behavior. We also evaluated the expression of LN332-encoding genes in human carotid atherosclerotic plaques. Our findings demonstrate that TNF induces upregulation of LAMB3 and LAMC2, which, along with LAMA3, encode the LN332 isoform. Endothelial cells cultured on recombinant LN332 exhibit decreased claudin-5 expression and display a loosely connected phenotype, with an elevated expression of chemokines and leukocyte adhesion molecules, enhancing their attractiveness and adhesion to leukocytes in vitro. Furthermore, LAMB3 and LAMC2 are upregulated in human carotid plaques and show a positive correlation with TNF expression. In summary, TNF stimulates the expression of LN332-encoding genes in human endothelial cells and LN332 promotes an endothelial phenotype characterized by compromised junctional integrity and increased leukocyte interaction. These findings highlight the importance of basement membrane proteins for endothelial integrity and the potential role of LN332 in atherosclerosis.

HCMV US2 co-opts TRC8 to degrade the endoplasmic reticulum-resident protein LMAN2L

The human cytomegalovirus (HCMV) pUS2 glycoprotein exploits the host's endoplasmic reticulum (ER)-associated degradation (ERAD) pathway to degrade major histocompatibility complex class I (MHC-I) and prevent antigen presentation. Beyond MHC-I, pUS2 has been shown to target a range of cellular proteins for degradation, preventing their cell surface expression. Here we have identified a novel pUS2 target, ER-resident protein lectin mannose binding 2 like (LMAN2L). pUS2 expression was both necessary and sufficient for the downregulation of LMAN2L, which was dependent on the cellular E3 ligase TRC8. Given the hypothesized role of LMAN2L in the trafficking of glycoproteins, we employed proteomic plasma membrane profiling to measure LMAN2L-dependent changes at the cell surface. A known pUS2 target, integrin alpha-6 (ITGA6), was downregulated from the surface of LMAN2L-deficient cells, but not other integrins. Overall, these results suggest a novel strategy of pUS2-mediated protein degradation whereby pUS2 targets LMAN2L to impair trafficking of ITGA6. Given that pUS2 can directly target other integrins, we propose that this single viral protein may exhibit both direct and indirect mechanisms to downregulate key cell surface molecules.

Identification of direct connections between the dura and the brain

The arachnoid barrier delineates the border between the central nervous system and dura mater. Although the arachnoid barrier creates a partition, communication between the central nervous system and the dura mater is crucial for waste clearance and immune surveillance1,2. How the arachnoid barrier balances separation and communication is poorly understood. Here, using transcriptomic data, we developed transgenic mice to examine specific anatomical structures that function as routes across the arachnoid barrier. Bridging veins create discontinuities where they cross the arachnoid barrier, forming structures that we termed arachnoid cuff exit (ACE) points. The openings that ACE points create allow the exchange of fluids and molecules between the subarachnoid space and the dura, enabling the drainage of cerebrospinal fluid and limited entry of molecules from the dura to the subarachnoid space. In healthy human volunteers, magnetic resonance imaging tracers transit along bridging veins in a similar manner to access the subarachnoid space. Notably, in neuroinflammatory conditions such as experimental autoimmune encephalomyelitis, ACE points also enable cellular trafficking, representing a route for immune cells to directly enter the subarachnoid space from the dura mater. Collectively, our results indicate that ACE points are a critical part of the anatomy of neuroimmune communication in both mice and humans that link the central nervous system with the dura and its immunological diversity and waste clearance systems.

Modelling arterial thrombus formation in vitro

PURPOSE OF REVIEW

Models of arterial thrombus formation represent a vital experimental tool to investigate platelet function and test novel antithrombotic drugs. This review highlights some of the recent advances in modelling thrombus formation in vitro and suggests potential future directions.

RECENT FINDINGS

Microfluidic devices and the availability of commercial chips in addition to enhanced accessibility of 3D printing has facilitated a rapid surge in the development of novel in-vitro thrombosis models. These include progression towards more sophisticated, 'vessel on a chip' models which incorporate vascular endothelial cells and smooth muscle cells. Other approaches include the addition of branches to the traditional single channel to yield an occlusive model; and developments in the adhesive coating of microfluidic chambers to better mimic the thrombogenic surface exposed following plaque rupture. Future developments in the drive to create more biologically relevant chambers could see a move towards the use of human placental vessels, perfused ex-vivo. However, further work is required to determine the feasibility and validity of this approach.

SUMMARY

Recent advances in thrombus formation models have significantly improved the pathophysiological relevance of in-vitro flow chambers to better reflect the in-vivo environment and provide a more translational platform to test novel antithrombotics.

Cancer Cell Targeting, Magnetic Sorting, and SERS Detection through Cell Surface Receptors

Integrins are cellular surface receptors responsible for the activation of many cellular pathways in cancer. These integrin proteins can be specifically targeted by small peptide sequences that offer the potential for the differentiation of cellular subpopulations by using magnetically assisted cellular sorting techniques. By adding a gold shell to the magnetic nanoparticles, these integrin-peptide interactions can be differentiated by surface-enhanced Raman spectroscopy (SERS), providing a quick and reliable method for on-target binding. In this paper, we demonstrate the ability to differentiate the peptide-protein interactions of the small peptides CDPGYIGSR and cyclic RGDfC functionalized on gold-coated magnetic nanoparticles with the integrins they are known to bind to using their SERS signal. SW480 and SW620 colorectal cancer cells known to have the integrins of interest were then magnetically sorted using these functionalized nanoparticles, suggesting differentiation between the sorted populations and integrin populations among the two cell lines.

Illustrated Abstracts of the 5th EUPLAN International Conference

These illustrated capsules have been prepared by some speakers of State-of-the-Art talks and of original investigations, presented at the 5th European Platelet Network (EUPLAN) International Conference, which was held at the Università degli Studi di Milano (Italy) on September 28-30, 2022. The programme featured various state-of-the-art lectures and a selection of oral presentations covering a broad range of topics in platelet and megakaryocyte biology, from basic science to recent advances in clinical studies. As usual, the meeting brought together senior scientists and trainees in an informal atmosphere to discuss platelet science in person.

The relative importance of platelet integrins in hemostasis, thrombosis and beyond

Integrins are heterodimeric transmembrane receptors composed of α and β chains, with an N-terminal extracellular domain forming a globular head corresponding to the ligand binding site. Integrins regulate various cellular functions including adhesion, migration, proliferation, spreading and apoptosis. On platelets, integrins play a central role in adhesion and aggregation on subendothelial matrix proteins of the vascular wall, thereby ensuring hemostasis. Platelet integrins belong either to the β1 family (α2β1, α5β1 and α6β1) or to the β3 family (αIIbβ3 and αvβ3). On resting platelets, integrins can engage their ligands when the latter are immobilized but not in their soluble form. The effects of various agonists promote an inside-out signal in platelets, increasing the affinity of integrins for their ligands and conveying a modest signal reinforcing platelet activation, called outside-in signaling. This outside-in signal ensures platelet adhesion, shape change, granule secretion and aggregation. In this review, we examine the role of each platelet integrin in hemostatic plug formation, hemostasis and arterial thrombosis and also beyond these classical functions, notably in tumor metastasis and sepsis.

Glycoprotein VI interplay with fibrin(ogen) in thrombosis

Platelets play a central role in the arrest of bleeding. The ability of platelets to engage with extracellular matrix proteins of the subendothelium has long been recognized as a pivotal platelet attribute, underpinning adequate hemostasis. The propensity of platelets to rapidly bind and functionally respond to collagen was one of the earliest documented events in platelet biology. The receptor primarily responsible for mediating platelet/collagen responses was identified as glycoprotein (GP) VI and successfully cloned in 1999. Since that time, this receptor has held the attention of many research groups, and through these efforts, we now have an excellent understanding of the roles of GPVI as a platelet- and megakaryocyte-specific adheso-signaling receptor in platelet biology. GPVI is considered a viable antithrombotic target, as data obtained from groups across the world is consistent with GPVI being less involved in physiological hemostatic processes but participating in arterial thrombosis. This review will highlight the key aspects of GPVI contributions to platelet biology and concentrate on the interaction with recently identified ligands, with a focus on fibrin and fibrinogen, discussing the role of these interactions in the growth and stability of thrombi. We will also discuss important therapeutic developments that target GPVI to modulate platelet function while minimizing bleeding outcomes.

Biomechanical activation of blood platelets via adhesion to von Willebrand factor studied with mesoscopic simulations

Platelet adhesion and activation are essential initial processes of arterial and microvascular hemostasis, where high hydrodynamic forces from the bloodflow impede coagulation. The process relies on von Willebrand factor (VWF)-a linear multimeric protein of blood plasma plays a pivotal role in mechanochemical regulation of shear-induced platelet aggregation (SIPA). Adhesive interactions between VWF and glycoprotein receptors GPIb are crucial for platelet recruitment under high shear stress in fluid. Recent advances in experimental studies revealed that mechanical tension on the extracellular part of GPIb may trigger a cascade of biochemical reactions in platelets leading to activation of integrins [Formula: see text] (also known as GPIIb/IIIa) and strengthening of the adhesion. The present paper is aimed at investigation of this process by three-dimensional computer simulations of platelet adhesion to surface-grafted VWF multimers in pressure-driven flow of platelet-rich plasma. The simulations demonstrate that GPIb-mediated mechanotransduction is a feasible way of platelet activation and stabilization of platelet aggregates under high shear stress. Quantitative understanding of mechanochemical processes involved in SIPA would potentially promote the discovery of new anti-platelet medication and the development of multiscale numerical models of platelet thrombosis and hemostasis.

The Platelet Aggregation Inhibition Activity of Polyphenols can be Mediated by 67kda Laminin Receptor: A New Therapeutic Strategy For the Treatment of Venous Thromboembolism

BACKGROUND

Thrombotic disease is still a major killer. Aspirin, Ticagrelor, Clopidogrel, etc. are the most widely used conventional antiplatelet drugs. The significant number of patients who are resistant to this drug shows a poor outcome.

OBJECTIVE

Developing a new antiplatelet agent with a stable antiplatelet effect and minimal bleeding risk is required for a patient who is resistant to antiplatelet drugs.

METHODS

Protein-ligand docking was performed using Autodock Vina 1.1.2 to study the interaction of 67LR with different Polyphenols.

RESULTS

Among the 18 polyphenols, thearubigin has the highest binding affinity towards 67LR and gallic acid shows the lowest binding affinity. Among the 18 molecules, the top 4 molecules from the highest to lowest binding affinity range from-10.6 (thearubigin) to -6.5 (Epigallocatechin).

CONCLUSION

Polyphenols may inhibit platelet aggregation through 67 LR and can be an alternative treatment for Thrombotic Disease. Moreover, it will be interesting to know whether polyphenols interfere with the same pathways as aspirin and clopidogrel. Effective polyphenols could help prototype the compound development of novel antiplatelet agents.

Junctional epithelium and hemidesmosomes: Tape and rivets for solving the "percutaneous device dilemma" in dental and other permanent implants

The percutaneous device dilemma describes etiological factors, centered around the disrupted epithelial tissue surrounding non-remodelable devices, that contribute to rampant percutaneous device infection. Natural percutaneous organs, in particular their extracellular matrix mediating the "device"/epithelium interface, serve as exquisite examples to inspire longer lasting long-term percutaneous device design. For example, the tooth's imperviousness to infection is mediated by the epithelium directly surrounding it, the junctional epithelium (JE). The hallmark feature of JE is formation of hemidesmosomes, cell/matrix adhesive structures that attach surrounding oral gingiva to the tooth's enamel through a basement membrane. Here, the authors survey the multifaceted functions of the JE, emphasizing the role of the matrix, with a particular focus on hemidesmosomes and their five main components. The authors highlight the known (and unknown) effects dental implant - as a model percutaneous device - placement has on JE regeneration and synthesize this information for application to other percutaneous devices. The authors conclude with a summary of bioengineering strategies aimed at solving the percutaneous device dilemma and invigorating greater collaboration between clinicians, bioengineers, and matrix biologists.

Reversible Platelet Integrin αIIbβ3 Activation and Thrombus Instability

Integrin αIIbβ3 activation is essential for platelet aggregation and, accordingly, for hemostasis and arterial thrombosis. The αIIbβ3 integrin is highly expressed on platelets and requires an activation step for binding to fibrinogen, fibrin or von Willebrand factor (VWF). A current model assumes that the process of integrin activation relies on actomyosin force-dependent molecular changes from a bent-closed and extended-closed to an extended-open conformation. In this paper we review the pathways that point to a functional reversibility of platelet αIIbβ3 activation and transient aggregation. Furthermore, we refer to mouse models indicating that genetic defects that lead to reversible platelet aggregation can also cause instable thrombus formation. We discuss the platelet agonists and signaling pathways that lead to a transient binding of ligands to integrin αIIbβ3. Our analysis points to the (autocrine) ADP P2Y₁ and P2Y₁₂ receptor signaling via phosphoinositide 3-kinases and Akt as principal pathways linked to reversible integrin activation. Downstream signaling events by protein kinase C, CalDAG-GEFI and Rap1b have not been linked to transient integrin activation. Insight into the functional reversibility of integrin activation pathways will help to better understand the effects of antiplatelet agents.

Galectin-9 activates platelet ITAM receptors glycoprotein VI and C-type lectin-like receptor-2

BACKGROUND

Platelets are multifunctional cellular mediators in many physiological and pathophysiological processes such as thrombosis, angiogenesis, and inflammation. Several members of galectins, a family of carbohydrate-binding proteins with a broad range of immunomodulatory actions, have been reported to activate platelets.

OBJECTIVE

In this study, we investigated the role of galectin-9 (Gal-9) as a novel ligand for platelet glycoprotein VI (GPVI) and C-type lectin-like receptor 2 (CLEC-2).

METHODS

Platelet spreading, aggregation, and P-selectin expression in response to Gal-9 were measured in washed platelet suspensions via static adhesion assay, light transmission aggregometry, and flow cytometry, respectively. Solid-phase binding assay and protein phosphorylation studies were utilized to validate the interaction between Gal-9 and GPVI, and immunoprecipitation for detecting CLEC-2 phosphorylation. Wild-type (WT), GPVI-knockout (Gp6-/- ), and GPVI and CLEC-2-double knockout (Gp6-/- /Gp1ba-Cre-Clec1bfl/fl ) mice were used.

RESULTS

We have shown that recombinant Gal-9 stimulates aggregation in human and mouse washed platelets dose-dependently. Platelets from both species adhere and spread on immobilized Gal-9 and express P-selectin. Gal-9 competitively inhibited the binding of human recombinant D1 and D2 domains of GPVI to collagen. Gal-9 stimulated tyrosine phosphorylation of CLEC-2 and proteins known to lie downstream of GPVI and CLEC-2 including spleen tyrosine kinase and linker of activated T cells in human platelets. GPVI-deficient murine platelets exhibited significantly impaired aggregation in response to Gal-9, which was further abrogated in GPVI and CLEC-2-double-deficient platelets.

CONCLUSIONS

We have identified Gal-9 as a novel platelet agonist that induces activation through interaction with GPVI and CLEC-2.

Proteomics and enriched biological processes in Antiphospholipid syndrome: A systematic review

Identification of differentially expressed proteins in antiphospholipid syndrome (APS) is a developing area of research for unique profiles of this pathology. Advances in technologies of mass spectrometry brings improvements in proteomics and results in assessment of soluble or cellular proteins which could be candidates for clinical biomarkers of primary APS. The use of blood as a source of proteins ease the acquisition of samples for proteomics analyses and later for disease diagnosis. We performed a systematic review to explore the proteomics studies carried out in circulating released proteins (serum, plasma) or cellular proteins (monocytes and platelets) of APS patients. The study groups differentiate among clinical APS cases with the aim to translate molecular findings to disease stratification and to improve APS diagnosis and prognosis. These studies also include the unravelling of new autoantibodies in non-criteria APS or how post-translational protein modifications provides clues about the pathological mechanisms of antigen-autoantibody recognition. Herein, we identified 82 proteins that were dysregulated in APS across eleven studies. Enrichment analysis revealed its connection to cellular activation and degranulation that eventually leads to thrombosis as the main biological process highlighted by these studies. Validation of APS-relevant proteins by functional and mechanistic studies will be essential for patient stratification and the development of targeted therapies for every clinical subtype of APS.

Characterization of the Role of Integrin α5β1 in Platelet Function, Hemostasis, and Experimental Thrombosis

Objective  Integrins are key regulators of various platelet functions. The pathophysiological importance of most platelet integrins has been investigated, with the exception of α5β1, a receptor for fibronectin. The aim of this study was to characterize the role of α5β1 in megakaryopoiesis, platelet function, and to determine its importance in hemostasis and arterial thrombosis.

Approach and Results  We generated a mouse strain deficient for integrin α5β1 on megakaryocytes and platelets (PF4Cre-α5 ^−/− ). PF4Cre-α5 ^−/− mice were viable, fertile, and presented no apparent signs of abnormality. Megakaryopoiesis appears unaltered as evidence by a normal megakaryocyte morphology and development, which is in agreement with a normal platelet count. Expression of the main platelet receptors and the response of PF4Cre-α5 ^−/− platelets to a series of agonists were all completely normal. Adhesion and aggregation of PF4Cre-α5 ^−/− platelets under shear flow on fibrinogen, laminin, or von Willebrand factor were unimpaired. In contrast, PF4Cre-α5 ^−/− platelets displayed a marked decrease in adhesion, activation, and aggregation on fibrillar cellular fibronectin and collagen. PF4Cre-α5 ^−/− mice presented no defect in a tail-bleeding time assay and no increase in inflammatory bleeding in a reverse passive Arthus model and a lipopolysaccharide pulmonary inflammation model. Finally, no defects were observed in three distinct experimental models of arterial thrombosis based on ferric chloride-induced injury of the carotid artery, mechanical injury of the abdominal aorta, or laser-induced injury of mesenteric vessels.

Conclusion  In summary, this study shows that platelet integrin α5β1 is a key receptor for fibrillar cellular fibronectin but is dispensable in hemostasis and arterial thrombosis.

Platelet GPVI (Glycoprotein VI) and Thrombotic Complications in the Venous System

The immunoglobulin receptor GPVI (glycoprotein VI) is selectively expressed on megakaryocytes and platelets and is currently recognized as a receptor for not only collagen but also a variety of plasma and vascular proteins, including fibrin, fibrinogen, laminin, fibronectin, and galectin-3. Deficiency of GPVI is protective in mouse models of experimental thrombosis, pulmonary thromboembolism as well as in thromboinflammation, suggesting a role of GPVI in arterial and venous thrombus formation. In humans, platelet GPVI deficiency is associated with a mild bleeding phenotype, whereas a common variant rs1613662 in the GP6 gene is considered a risk factor for venous thromboembolism. However, preclinical studies on the inhibition of GPVI-ligand interactions are focused on arterial thrombotic complications. In this review we discuss the emerging evidence for GPVI in venous thrombus formation and leukocyte-dependent thromboinflammation, extending to venous thromboembolism, pulmonary thromboembolism, and cancer metastasis. We also recapitulate indications for circulating soluble GPVI as a biomarker of thrombosis-related complications. Collectively, we conclude that the current evidence suggests that platelet GPVI is also a suitable cotarget in the prevention of venous thrombosis due to its role in thrombus consolidation and platelet-leukocyte complex formation.

Complement, inflammation and thrombosis

A mutual relationship exists between immune activation and mechanisms of thrombus formation. In particular, elements of the innate immune response such as the complement system can modulate platelet activation and subsequently thrombus formation. Several components of the complement system including C3 or the membrane attack complex have been reported to be associated with platelets and become functionally active in the micromilieu of platelet activation. The exact mechanisms how this interplay is regulated and its consequences for tissue inflammation, damage or recovery remain to be defined. This review addresses the current state of knowledge on this topic and puts it into context with diseases featuring both thrombosis and complement activation. LINKED ARTICLES: This article is part of a themed issue on Canonical and non-canonical functions of the complement system in health and disease. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v178.14/issuetoc.

In vitro flow-based assay: From simple toward more sophisticated models for mimicking hemostasis and thrombosis

In vitro flow-based assays are widely used to investigate the role of platelets and coagulation in hemostasis and thrombosis. Their main advantage over other assays relies on the fact that they integrate blood flow that regulates many aspects of platelet function, including adhesion, activation, and aggregation. Blood flow is also central in the regulation of coagulation through its ability to modulate the local concentrations of coagulation factors within and around thrombi. The most broadly used assay to study thrombus formation consists in perfusing whole blood over immobilized fibrillar collagen through a single channel, which helps to reproduce thrombus formation as it occurs in vivo after vascular injury, with platelets adhering, becoming activated, and forming a mural thrombus. This process can also be studied under conditions of thrombin generation, notably by recalcifying blood collected in sodium citrate. In this manuscript, we briefly discuss the advantages and limits of this broadly used "in vitro thrombus formation model." The main emphasis is on the description of the most recent developments regarding design of new flow models and new techniques, and how these may advance the landscape of in vitro studies into the formation of physiological or pathophysiological thrombi. Challenges linked to mimicking the formation of a hemostatic plug in a healthy vessel or a thrombus in diseased arteries and the complexity of reproducing the various aspects of venous thrombosis are discussed. Future directions are proposed to improve the physiological or pathophysiological relevance of current flow-based assays.

Platelets as Mediators of Neuroinflammation and Thrombosis

Beyond platelets function in hemostasis, there is emerging evidence to suggest that platelets contribute crucially to inflammation and immune responses. Therefore, considering the detrimental role of inflammatory conditions in severe neurological disorders such as multiple sclerosis or stroke, this review outlines platelets involvement in neuroinflammation. For this, distinct mechanisms of platelet-mediated thrombosis and inflammation are portrayed, focusing on the interaction of platelet receptors with other immune cells as well as brain endothelial cells. Furthermore, we draw attention to the intimate interplay between platelets and the complement system as well as between platelets and plasmatic coagulation factors in the course of neuroinflammation. Following the thorough exposition of preclinical approaches which aim at ameliorating disease severity after inducing experimental autoimmune encephalomyelitis (a counterpart of multiple sclerosis in mice) or brain ischemia-reperfusion injury, the clinical relevance of platelet-mediated neuroinflammation is addressed. Thus, current as well as future propitious translational and clinical strategies for the treatment of neuro-inflammatory diseases by affecting platelet function are illustrated, emphasizing that targeting platelet-mediated neuroinflammation could become an efficient adjunct therapy to mitigate disease severity of multiple sclerosis or stroke associated brain injury.

Drug Targeting via Platelet Membrane-Coated Nanoparticles

Platelets possess distinct surface moieties responsible for modulating their adhesion to various disease-relevant substrates involving vascular damage, immune evasion, and pathogen interactions. Such broad biointerfacing capabilities of platelets have inspired the development of platelet-mimicking drug carriers that preferentially target drug payloads to disease sites for enhanced therapeutic efficacy. Among these carriers, platelet membrane-coated nanoparticles (denoted 'PNPs') made by cloaking synthetic substrates with the plasma membrane of platelets have emerged recently. Their 'top-down' design combines the functionalities of natural platelet membrane and the engineering flexibility of synthetic nanomaterials, which together create synergy for effective drug delivery and novel therapeutics. Herein, we review the recent progress of engineering PNPs with different structures for targeted drug delivery, focusing on three areas, including targeting injured blood vessels to treat vascular diseases, targeting cancer cells for cancer treatment and detection, and targeting drug-resistant bacteria to treat infectious diseases. Overall, current studies have established PNPs as versatile nanotherapeutics for drug targeting with strong potentials to improve the treatment of various diseases.

Pharmacological Blockade of Glycoprotein VI Promotes Thrombus Disaggregation in the Absence of Thrombin

OBJECTIVE

Atherothrombosis occurs upon rupture of an atherosclerotic plaque and leads to the formation of a mural thrombus. Computational fluid dynamics and numerical models indicated that the mechanical stress applied to a thrombus increases dramatically as a thrombus grows, and that strong inter-platelet interactions are essential to maintain its stability. We investigated whether GPVI (glycoprotein VI)-mediated platelet activation helps to maintain thrombus stability by using real-time video-microscopy. Approach and Results: We showed that GPVI blockade with 2 distinct Fab fragments promoted efficient disaggregation of human thrombi preformed on collagen or on human atherosclerotic plaque material in the absence of thrombin. ACT017-induced disaggregation was achieved under arterial blood flow conditions, and its effect increased with wall shear rate. GPVI regulated platelet activation within a growing thrombus as evidenced by the loss in thrombus contraction when GPVI was blocked, and the absence of the disaggregating effect of an anti-GPVI agent when the thrombi were fully activated with soluble agonists. The GPVI-dependent thrombus stabilizing effect was further supported by the fact that inhibition of any of the 4 key immunoreceptor tyrosine-based motif signalling molecules, src-kinases, Syk, PI3Kβ, or phospholipase C, resulted in kinetics of thrombus disaggregation similar to ACT017. The absence of ACT017-induced disaggregation of thrombi from 2 afibrinogenemic patients suggests that the role of GPVI requires interaction with fibrinogen. Finally, platelet disaggregation of fibrin-rich thrombi was also promoted by ACT017 in combination with r-tPA (recombinant tissue plasminogen activator).

CONCLUSIONS

This work identifies an unrecognized role for GPVI in maintaining thrombus stability and suggests that targeting GPVI could dissolve platelet aggregates with a poor fibrin content.

In vitro flow based systems to study platelet function and thrombus formation: Recommendations for standardization: Communication from the SSC on Biorheology of the ISTH

Experimental videomicroscopic in vitro assays of thrombus formation based on blood perfusion are instrumental in a wide range of basic studies in thrombosis, screening for hereditary or acquired plateletrelated pathologies, and assessing the effectiveness of novel anti-platelet therapies. Here, we discuss application of the broadly used "in vitro thrombosis model": a frequently used assay to study the formation of 3D aggregates under flow, which involves perfusing anticoagulated whole blood over fibrillar collagen in a flow geometry of rectangular cross-section, such as glass microcapillaries or parallel-plate flow chambers. Major advantaged of this assay are simplicity and ability to reproduce the four main stages of platelet thrombus formation, i.e. platelet tethering, adhesion, activation and aggregation under a wide range of hemodynamic conditions. On the other hand, these devices represent, at best, simple reductive models of thrombosis. We also describe how blood flow assays can be used to study various aspects of platelet function on adhesive proteins and discuss the relevance of such flow models. Finally, we propose recommendations for standardization related to the use of this assay that cover collagen source, coating methods, micropatterning, sample composition, anticoagulation, choice of flow device, hemodynamic conditions, quantification challenges, variability, pre-analytical conditions and other issues.

Platelet biology and functions: new concepts and clinical perspectives

Platelets - blood cells continuously produced from megakaryocytes mainly in the bone marrow - are implicated not only in haemostasis and arterial thrombosis, but also in other physiological and pathophysiological processes. This Review describes current evidence for the heterogeneity in platelet structure, age, and activation properties, with consequences for a diversity of platelet functions. Signalling processes of platelet populations involved in thrombus formation with ongoing coagulation are well understood. Genetic approaches have provided information on multiple genes related to normal haemostasis, such as those encoding receptors and signalling or secretory proteins, that determine platelet count and/or responsiveness. As highly responsive and secretory cells, platelets can alter the environment through the release of growth factors, chemokines, coagulant factors, RNA species, and extracellular vesicles. Conversely, platelets will also adapt to their environment. In disease states, platelets can be positively primed to reach a pre-activated condition. At the inflamed vessel wall, platelets interact with leukocytes and the coagulation system, interactions mediating thromboinflammation. With current antiplatelet therapies invariably causing bleeding as an undesired adverse effect, novel therapies can be more beneficial if directed against specific platelet responses, populations, interactions, or priming conditions. On the basis of these novel concepts and processes, we discuss several initiatives to target platelets therapeutically.

Altered platelet proteome in lupus anticoagulant (LA)-positive patients-protein disulfide isomerase and NETosis as new players in LA-related thrombosis

Patients with antiphospholipid syndrome (APS) are at high risk of developing venous and arterial thromboembolism (TE). The role of platelets in the pathogenesis of these prothrombotic conditions is not yet fully understood. The aim of this study was to gain mechanistic insights into the role of platelets in APS by comparing the platelet proteome between lupus anticoagulant (LA)-positive patients with (LA+ TE+) and without a history of TE (LA+ TE-) and healthy controls. The platelet proteome of 47 patients with LA, 31 with a history of TE and 16 without thrombotic history, and 47 healthy controls was analyzed by two-dimensional differential in-gel electrophoresis and mass spectrometry to identify disease-related proteins. Afterward, selected LA-related platelet proteins were validated by western blot and ELISA. Alterations of 25 proteins were observed between the study groups. STRING pathway analysis showed that LA-related protein profiles were involved in platelet activation, aggregation, and degranulation. For example, protein disulfide isomerase family members, enzymes that promote thrombosis, were upregulated in platelets and plasma of LA+ TE+ patients. Leukocyte elastase inhibitor (SERPINB1), an antagonist of neutrophil extracellular trap (NET) formation, was decreased in platelets of LA+ TE+ patients compared to healthy controls. Additionally, citrullinated histone H3, a NET-specific marker, was increased in plasma of LA+ TE+ patients. These findings suggest that decreased platelet SERPINB1 levels favor prothrombotic NETosis, especially in LA+ TE+ patients. Our findings reveal protein abundance changes connected to altered platelet function in LA-positive patients, thus suggesting a pathogenic role of platelets in thrombotic complications in APS.

β4GALT1 controls β1 integrin function to govern thrombopoiesis and hematopoietic stem cell homeostasis

Glycosylation is critical to megakaryocyte (MK) and thrombopoiesis in the context of gene mutations that affect sialylation and galactosylation. Here, we identify the conserved B4galt1 gene as a critical regulator of thrombopoiesis in MKs. β4GalT1 deficiency increases the number of fully differentiated MKs. However, the resulting lack of glycosylation enhances β1 integrin signaling leading to dysplastic MKs with severely impaired demarcation system formation and thrombopoiesis. Platelets lacking β4GalT1 adhere avidly to β1 integrin ligands laminin, fibronectin, and collagen, while other platelet functions are normal. Impaired thrombopoiesis leads to increased plasma thrombopoietin (TPO) levels and perturbed hematopoietic stem cells (HSCs). Remarkably, β1 integrin deletion, specifically in MKs, restores thrombopoiesis. TPO and CXCL12 regulate β4GalT1 in the MK lineage. Thus, our findings establish a non-redundant role for β4GalT1 in the regulation of β1 integrin function and signaling during thrombopoiesis. Defective thrombopoiesis and lack of β4GalT1 further affect HSC homeostasis.

Platelets Are at the Nexus of Vascular Diseases

Platelets are important actors of cardiovascular diseases (CVD). Current antiplatelet drugs that inhibit platelet aggregation have been shown to be effective in CVD treatment. However, the management of bleeding complications is still an issue in vascular diseases. While platelets can act individually, they interact with vascular cells and leukocytes at sites of vascular injury and inflammation. The main goal remains to better understand platelet mechanisms in thrombo-inflammatory diseases and provide new lines of safe treatments. Beyond their role in hemostasis and thrombosis, recent studies have reported the role of several aspects of platelet functions in CVD progression. In this review, we will provide a comprehensive overview of platelet mechanisms involved in several vascular diseases.

Comparative Analysis of Microfluidics Thrombus Formation in Multiple Genetically Modified Mice: Link to Thrombosis and Hemostasis

Genetically modified mice are indispensable for establishing the roles of platelets in arterial thrombosis and hemostasis. Microfluidics assays using anticoagulated whole blood are commonly used as integrative proxy tests for platelet function in mice. In the present study, we quantified the changes in collagen-dependent thrombus formation for 38 different strains of (genetically) modified mice, all measured with the same microfluidics chamber. The mice included were deficient in platelet receptors, protein kinases or phosphatases, small GTPases or other signaling or scaffold proteins. By standardized re-analysis of high-resolution microscopic images, detailed information was obtained on altered platelet adhesion, aggregation and/or activation. For a subset of 11 mouse strains, these platelet functions were further evaluated in rhodocytin- and laminin-dependent thrombus formation, thus allowing a comparison of glycoprotein VI (GPVI), C-type lectin-like receptor 2 (CLEC2) and integrin α₆β₁ pathways. High homogeneity was found between wild-type mice datasets concerning adhesion and aggregation parameters. Quantitative comparison for the 38 modified mouse strains resulted in a matrix visualizing the impact of the respective (genetic) deficiency on thrombus formation with detailed insight into the type and extent of altered thrombus signatures. Network analysis revealed strong clusters of genes involved in GPVI signaling and Ca²⁺ homeostasis. The majority of mice demonstrating an antithrombotic phenotype in vivo displayed with a larger or smaller reduction in multi-parameter analysis of collagen-dependent thrombus formation in vitro. Remarkably, in only approximately half of the mouse strains that displayed reduced arterial thrombosis in vivo, this was accompanied by impaired hemostasis. This was also reflected by comparing in vitro thrombus formation (by microfluidics) with alterations in in vivo bleeding time. In conclusion, the presently developed multi-parameter analysis of thrombus formation using microfluidics can be used to: (i) determine the severity of platelet abnormalities; (ii) distinguish between altered platelet adhesion, aggregation and activation; and (iii) elucidate both collagen and non-collagen dependent alterations of thrombus formation. This approach may thereby aid in the better understanding and better assessment of genetic variation that affect in vivo arterial thrombosis and hemostasis.

Functional significance of the platelet immune receptors GPVI and CLEC-2

Although platelets are best known for their role in hemostasis, they are also crucial in development, host defense, inflammation, and tissue repair. Many of these roles are regulated by the immune-like receptors glycoprotein VI (GPVI) and C-type lectin receptor 2 (CLEC-2), which signal through an immunoreceptor tyrosine-based activation motif (ITAM). GPVI is activated by collagen in the subendothelial matrix, by fibrin and fibrinogen in the thrombus, and by a remarkable number of other ligands. CLEC-2 is activated by the transmembrane protein podoplanin, which is found outside of the vasculature and is upregulated in development, inflammation, and cancer, but there is also evidence for additional ligands. In this Review, we discuss the physiological and pathological roles of CLEC-2 and GPVI and their potential as targets in thrombosis and thrombo-inflammatory disorders (i.e., disorders in which inflammation plays a critical role in the ensuing thrombosis) relative to current antiplatelet drugs.

Clot Contraction Drives the Translocation of Procoagulant Platelets to Thrombus Surface

Objective—

After activation at the site of vascular injury, platelets differentiate into 2 subpopulations, exhibiting either proaggregatory or procoagulant phenotype. Although the functional role of proaggregatory platelets is well established, the physiological significance of procoagulant platelets, the dynamics of their formation, and spatial distribution in thrombus remain elusive.

Approach and Results—

Using transmission electron microscopy and fluorescence microscopy of arterial thrombi formed in vivo after ferric chloride–induced injury of carotid artery or mechanical injury of abdominal aorta in mice, we demonstrate that procoagulant platelets are located at the periphery of the formed thrombi. Real-time cell tracking during thrombus formation ex vivo revealed that procoagulant platelets originate from different locations within the thrombus and subsequently translocate towards its periphery. Such redistribution of procoagulant platelets was followed by generation of fibrin at thrombus surface. Using in silico model, we show that the outward translocation of procoagulant platelets can be driven by the contraction of the forming thrombi, which mechanically expels these nonaggregating cells to thrombus periphery. In line with the suggested mechanism, procoagulant platelets failed to translocate and remained inside the thrombi formed ex vivo in blood derived from nonmuscle myosin ( MYH9 )-deficient mice. Ring-like distribution of procoagulant platelets and fibrin around the thrombus observed with blood of humans and wild-type mice was not present in thrombi of MYH9 -knockout mice, confirming a major role of thrombus contraction in this phenomenon.

Conclusions—

Contraction of arterial thrombus is responsible for the mechanical extrusion of procoagulant platelets to its periphery, leading to heterogeneous structure of thrombus exterior.

Anti-platelet drugs and their necessary interaction with endothelial mediators and platelet cyclic nucleotides for therapeutic efficacy

For many millions of patients at secondary risk of coronary thrombosis pharmaceutical protection is supplied by dual anti-platelet therapy. Despite substantial therapeutic developments over the last decade recurrent thrombotic events occur, highlighting the need for further optimisation of therapies. Importantly, but often ignored, anti-platelet drugs interact with cyclic nucleotide systems in platelets and these are the same systems that mediate key endogenous pathways of platelet regulation, notably those dependent upon the vascular endothelium. The aim of this review is to highlight interactions between the anti-platelet drugs, aspirin and P2Y₁₂ receptor antagonists and endogenous pathways of platelet regulation at the level of cyclic nucleotides. These considerations are key to concepts such as anti-platelet drug resistance and individualized anti-platelet therapy which cannot be understood by study of platelets in isolation from the circulatory environment. We also explore novel and emerging therapies that focus on preserving haemostasis and how the concepts outlined in this review could be exploited therapeutically to improve anti-thrombotic efficacy whilst reducing bleeding risk.

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.

Maintenance of murine platelet homeostasis by the kinase Csk and phosphatase CD148

Src family kinases (SFKs) coordinate the initiating and propagating activation signals in platelets, but it remains unclear how they are regulated. Here, we show that ablation of C-terminal Src kinase (Csk) and receptor-like protein tyrosine-phosphatase CD148 in mice results in a dramatic increase in platelet SFK activity, demonstrating that these proteins are essential regulators of platelet reactivity. Paradoxically, Csk/CD148-deficient mice exhibit reduced in vivo and ex vivo thrombus formation and increased bleeding following injury rather than a prothrombotic phenotype. This is a consequence of multiple negative feedback mechanisms, including downregulation of the immunoreceptor tyrosine-based activation motif (ITAM)- and hemi-ITAM-containing receptors glycoprotein VI (GPVI)-Fc receptor (FcR) γ-chain and CLEC-2, respectively and upregulation of the immunoreceptor tyrosine-based inhibition motif (ITIM)-containing receptor G6b-B and its interaction with the tyrosine phosphatases Shp1 and Shp2. Results from an analog-sensitive Csk mouse model demonstrate the unconventional role of SFKs in activating ITIM signaling. This study establishes Csk and CD148 as critical molecular switches controlling the thrombotic and hemostatic capacity of platelets and reveals cell-intrinsic mechanisms that prevent pathological thrombosis from occurring.

An update on factor XI structure and function

Factor XI (FXI) is the zymogen of a plasma protease, factor XIa (FXIa), that contributes to thrombin generation during blood coagulation by proteolytic activation of several coagulation factors, most notably factor IX (FIX). FXI is a homolog of prekallikrein (PK), a component of the plasma kallikrein-kinin system. While sharing structural and functional features with PK, FXI has undergone adaptive changes that allow it to contribute to blood coagulation. Here we review current understanding of the biology and enzymology of FXI, with an emphasis on structural features of the protein as they relate to protease function.

Platelets and vascular integrity: how platelets prevent bleeding in inflammation

Platelets play a central role in primary hemostasis by forming aggregates that plug holes in injured vessels. Half a century ago, detailed studies of the microvasculature by electron microscopy revealed that under inflammatory conditions that do not induce major disruption to vascular structure, individual platelets are mobilized to the vessel wall, where they interact with leukocytes and appear to seal gaps that arise between endothelial cells. Recent developments in genetic engineering and intravital microscopy have allowed further molecular and temporal characterization of these events. Surprisingly, it turns out that platelets support the recruitment of leukocytes to sites of inflammation. In parallel, however, they exercise their hemostatic function by securing the integrity of inflamed blood vessels to prevent bleeding from sites of leukocyte infiltration. It thus appears that platelets not only serve in concert as building blocks of the hemostatic plug but also act individually as gatekeepers of the vascular wall to help preserve vascular integrity while coordinating host defense. Variants of this recently appreciated hemostatic function of platelets that we refer to as "inflammation-associated hemostasis" are engaged in different contexts in which the endothelium is challenged or dysfunctional. Although the distinguishing characteristics of these variants and the underlying mechanisms of inflammation-associated hemostasis remain to be fully elucidated, they can differ notably from those supporting thrombosis, thus presenting therapeutic opportunities.

Platelet glycoprotein VI aids in local immunity during pneumonia-derived sepsis caused by gram-negative bacteria

Platelet collagen receptor glycoprotein VI (GPVI) and podoplanin receptor C-type lectin-like receptor 2 (CLEC2) are receptors implicated in platelet activation that both signal via an immunoreceptor tyrosine-based activation motif. Platelets are necessary for host defense and prevention of hemorrhage during sepsis, but the role of platelet GPVI and CLEC2 herein is unknown. To investigate this, we infected mice depleted of platelet GPVI or CLEC2 by antibody treatment or GPVI^-/- mice with the common human sepsis pathogen Klebsiella pneumoniae via the airways to induce pneumonia-derived sepsis. The GPVI ligand collagen and the CLEC2 ligand podoplanin were constitutively present in the lung, whereas the GPVI ligands fibrin and histone were induced during pneumonia. During late-stage infection, both mice depleted of GPVI and GPVI^-/- mice showed increased bacterial growth in lungs, and GPVI^-/- mice also showed increased bacterial growth in distant body sites. Despite higher bacterial loads, GPVI-depleted mice showed reduced platelet numbers, platelet activation, and platelet-leukocyte complex formation in the bronchoalveolar space. Consistently, in human whole blood, GPVI stimulation of platelets increased platelet-leukocyte complex formation and leukocyte activation, which was accompanied by enhanced phagocytosis of Klebsiella GPVI-depleted mice showed increased lung hemorrhage during infection, but not to the extent observed in platelet-depleted mice, and lung bleeding was not significantly different between GPVI^-/- and wild-type mice. CLEC2 depletion did not affect any of the responses during pneumonia. These results suggest that platelet GPVI, but not CLEC2, contributes to local host defense during pneumonia-derived sepsis by enhancing leukocyte function.

An astrocyte derived extracellular matrix coating reduces astrogliosis surrounding chronically implanted microelectrode arrays in rat cortex

Available evidence suggests that the magnitude of the foreign body response (FBR) to implants placed in cortical brain tissue is affected by the extent of vasculature damage following device insertion and the magnitude of the ensuing macrophage response. Since the extracellular matrix (ECM) serves as a natural hemostatic and immunomodulatory agent, we examined the ability of an FDA-approved neurosurgical hemostatic coating and an ECM coating derived from primary rat astrocytes to reduce the FBR surrounding a penetrating microelectrode array chronically implanted in rat cortex. Using quantitative methods, we examined various components of the FBR in vitro and after implantation. In vitro assays showed that both coatings accelerated coagulation in a similar fashion but only the astrocyte-derived material suppressed macrophage activation. In addition, the ECM coating derived from astrocytes, also decreased the astrogliotic response 8 weeks after implantation. Neither coating had a significant influence on the intensity or spatial distribution of FBR biomarkers 1 week after implantation or on degree of macrophage activation or neuronal survival at the later time point. The results show that microelectrode coatings with similar hemostatic properties but different immunomodulatory characteristics differentially affect the FBR to an anchored, single-shank, silicon microelectrode array. The results also support the concept that divergent biological pathways affect the various components of the FBR in the CNS and suggests that decreasing its impact will require a multifaceted approach.

Platelet Integrins in Tumor Metastasis: Do They Represent a Therapeutic Target?

Platelets are small anucleated cell fragments that ensure the arrest of bleeding after a vessel wall injury. They are also involved in non-hemostatic function such as development, immunity, inflammation, and in the hematogeneous phase of metastasis. While the role of platelets in tumor metastasis has been recognized for 60 years, the molecular mechanism underlying this process remains largely unclear. Platelets physically and functionally interact with various tumor cells through surface receptors including integrins. Platelets express five integrins at their surface, namely α2β1, α5β1, α6β1, αvβ3, and αIIbβ3, which bind preferentially to collagen, fibronectin, laminin, vitronectin, and fibrinogen, respectively. The main role of platelet integrins is to ensure platelet adhesion and aggregation at sites of vascular injury. Two of these, α6β1 and αIIbβ3, were proposed to participate in platelet–tumor cell interaction and in tumor metastasis. It has also been reported that pharmacological agents targeting both integrins efficiently reduce experimental metastasis, suggesting that platelet integrins may represent new anti-metastatic targets. This review focuses on the role of platelet integrins in tumor metastasis and discusses whether these receptors may represent new potential targets for novel anti-metastatic approaches.

Role of P2Y12 Receptor in Thrombosis

P2Y₁₂ receptor is a 342 amino acid Gi-coupled receptor predominantly expressed on platelets. P2Y₁₂ receptor is physiologically activated by ADP and inhibits adenyl cyclase (AC) to decrease cyclic AMP (cAMP) level, resulting in platelet aggregation. It also activates PI3 kinase (PI3K) pathway leading to fibrinogen receptor activation, and may protect platelets from apoptosis. Abnormalities of P2Y₁₂ receptor include congenital deficiencies or high activity in diseases like diabetes mellitus (DM) and chronic kidney disease (CKD), exposing such patients to a prothrombotic condition. A series of clinical antiplatelet drugs, such as clopidogrel and ticagrelor, are designed as indirect or direct antagonists of P2Y₁₂ receptor to reduce incidence of thrombosis mainly for patients of acute coronary syndrome (ACS) who are at high risk of thrombotic events. Studies on novel dual-/multi-target antiplatelet agents consider P2Y₁₂ receptor as a promising part in combined targets. However, the clinical practical phenomena, such as "clopidogrel resistance" due to gene variations of cytochrome P450 or P2Y₁₂ receptor constitutive activation, call for better antiplatelet agents. Researches also showed inverse agonist of P2Y₁₂ receptor could play a better role over neutral antagonists. Personalized antiplatelet therapy is the most ideal destination for antiplatelet therapy in ACS patients with or without other underlying diseases like DM or CKD, however, there is still a long way to go.

Platelet "first responders" in wound response, cancer, and metastasis

Platelets serve as "first responders" during normal wounding and homeostasis. Arising from bone marrow stem cell lineage megakaryocytes, anucleate platelets can influence inflammation and immune regulation. Biophysically, platelets are optimized due to size and discoid morphology to distribute near vessel walls, monitor vascular integrity, and initiate quick responses to vascular lesions. Adhesion receptors linked to a highly reactive filopodia-generating cytoskeleton maximizes their vascular surface contact allowing rapid response capabilities. Functionally, platelets normally initiate rapid clotting, vasoconstriction, inflammation, and wound biology that leads to sterilization, tissue repair, and resolution. Platelets also are among the first to sense, phagocytize, decorate, or react to pathogens in the circulation. These platelet first responder properties are commandeered during chronic inflammation, cancer progression, and metastasis. Leaky or inflammatory reaction blood vessel genesis during carcinogenesis provides opportunities for platelet invasion into tumors. Cancer is thought of as a non-healing or chronic wound that can be actively aided by platelet mitogenic properties to stimulate tumor growth. This growth ultimately outstrips circulatory support leads to angiogenesis and intravasation of tumor cells into the blood stream. Circulating tumor cells reengage additional platelets, which facilitates tumor cell adhesion, arrest and extravasation, and metastasis. This process, along with the hypercoagulable states associated with malignancy, is amplified by IL6 production in tumors that stimulate liver thrombopoietin production and elevates circulating platelet numbers by thrombopoiesis in the bone marrow. These complex interactions and the "first responder" role of platelets during diverse physiologic stresses provide a useful therapeutic target that deserves further exploration.